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Development of an automated capacitor discharge welding

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1. Welding head mounted on 2 axis NO DESCRIPTION QTY Automated Capacitor Discharge Welding System Dept of Industrial and Manufacturing Engr OSU OWG Srinivas Patnalk DATE 06 06 96 SHEET X X Not to Scale Figure B1 Welding head assembly mounted on Z axis Cylinder proJection bracket 1 foce plate Z Axis carriage clamps projection bracket 2 Vertical Head Asembly IRSE DN o osceno Aulomated Capacitor Discharge Welding System Dept of Industrial and Manufacturing Engr OSU DWG Srinivas Patnaik DATE 06 06 96 Figure B2 Welding head assembly igen 3 8 16 bene nee 0 75 r id IPIE atil 1 8 NPT T 1 Welding Ram Automated Capacitor Discharge Welding System TOLERANCES Dept of Industrial and Manufacturing Engr OSU XX 01 XXX 005 OWG Srini Potnai DATE 06 06 96 i dcos Not to scole SHEET X X Figure B3 Welding Ram Note Split in two Automated Capacitor Discharge Welding System Dept of Industrial and Manufacturing Engr OSU 005 _ ANGLES 2 DWG Srinivas Patnaik DATE 06 06 96 Not
2. procedure described in section 2 1 2 The parameters used are given in the table below Table 5 1 Parameters used in the experiment 5 2 Results CD welds were made on the manual machine at ALRC and on the automated system using the parameters given in Table 5 1 Metallographic examination was done to compare the welds between the automated unit and the manual unit as well as with resistance welding A cycle time comparison between the two machines was performed 5 2 1 Specimen Preparation Transverse cross sections of the joints were used for metallographic investigation A water cooled aluminum oxide abrasive wheel was used to section all specimens Ample cooling was provided during cutting to minimize the heat input and any alteration to the original structure of the specimen Kehl 1949 Since the specimen were too small to be handled conveniently they had to be appropriately mounted to facilitate the investigation 41 A thermosetting plastic bakelite was used to mount the specimen Green bakelite molding powder was used to make the mounts of uniform size 1 inch diameter and 3 4 inch length A Buehler mounting press was used to apply heat and pressure simultaneously The specimen was placed on the bottom of a greased crucible and bakelite was poured over the specimen This was then compressed on the press using a pressure of 8000 psi and a temperature of about 150 C After a short curing time it was cooled down
3. ACKNOWLEDGMENTS First I would like to thank my parents for their love support and more importantly for tolerating me and my long distance frustration No amount of words can express my gratitude to Dr Rick Wilson materials engineer at the DOE s Albany Research Center Rick has not only been my minor professor and guide on this project he has been so much more to me that I don t know where to start and how to start 1 am sure when all that extra allotment of ego with the Ph D degree was handed Rick was in the pool Rick has a good mind and a great heart I want to thank him for many things He spent so much time to help me not only with my graduate work but also to be a better individual He is the most memorable person I have ever met and our relationship is special he has been an advisor guide friend guardian and so much more to me I would like to thank my committee members Drs Terry Beaumariage Brian Paul and Antonio Torres for their time and help in reviewing my thesis Special thanks go to Terry Beaumariage for his help and encouragement during my academic pursuit and for being such a friendly advisor I would like to thank my hosts the Peter s family for their friendship and warmth during my stay in corvallis This thesis would not have been complete without the help and good wishes of my roommate Balbhim Mahurkar He has helped me during rough times and provided encouragement moral and financial support I would like to thank m
4. Coaxial cables are used to provide isolation and protection from electrical noise The operational amplifier attenuates the voltage by a magnitude of approximately 20 Channel 0 of the DAS 50 card is used on the card for measuring the voltage Current is measured by placing a shunt in the circuit The current shunt produces a low voltage signal proportional to the current passing through it The shunt used in this unit produces a voltage of 50 mV for 300 amps of current in the circuit This low power signal from the shunt is magnified by the circuit shown in Figure 4 7 b and input to channel 3 on the DAS 50 card The remaining two channels 1 amp 2 on the DAS 50 were used to obtain signals from a load cell and strobe pulses They were used primarily for earlier research work at ALRC and are not implemented in this system The load cell was used to calculate the 10 PF ta Cho win Sor 12W Voltage Pin 2 monitor ire 25 Pin3 inputs y 50K 15 0 1 yF a Voltage measurement Circuit 10 PF Ch3 Lin 50 1 2W Pin 8 Shunt monitor SED inputs b Current measurement Circuit 120VAC Dgl Trg 3 9K 2W aki Trigger Pin 10 Signal L Ol yF Inputs 2 asoy 392 Pin 11 c Digital Pulse Generator 15 VDC Regutated 120VAC YAmp fuses DC Supply a 15 VDC d DC power for amplifiers Figure 4 7 Signal Conditioning Circuitry ua weight required on the vertical head assemb
5. The welding time starts when the current changes from 0 and the cathode tip comes in contact with the anode This marks the beginning of the weld time t Current rapidly spikes to a maximum value when contact is made Figure 4 9 b The arc causes the discharge of energy and the tip starts melting The current drops down due to a change in the resistance of the circuit After the tip melts and the joint is made there is an increase in the current flow due to an increase in the area of contact The current through the circuit diminishes to 0 after the capacitors are discharged This marks the end of the weld time te The time between t and t gives the actual weld time 37 From the voltage and current data resistance and power are calculated These curves are shown in Figure 4 10 From the dynamic resistance curve a fluctuation in the resistance is observed as the tip makes contact arc begins During arcing it remains relatively constant 80000 01 0 08 2 g 006 5 5 004 0 02 0 3 DNE e E A _ ms a b Figure 4 10 Plots of a power vs time and b resistance vs time The total weld energy is calculated by finding the area under the power curve between t and t or by using numerical integration Total energy for weld watts 4 1 The weld energy calculated from equation 4 1 can be used to verify if enough energy was developed at the joint interface 4 6
6. A chamfered cavity was milled at the end of the welding ram for the silver contact Figure B3 Appendix B A solenoid activated vacuum generator capable of producing up to 20 inches vacuum was used to hold the silver contact within this cavity The welding ram is attached to a face plate with clamps Figure B4 and Figure B5 Appendix B A 1 8 thick bakelite collar is used to insulate the welding ram from the clamps The face plate and welding ram are attached to a high precision linear bearing The linear bearing allows the ram to be dropped upto 3 inches onto the tip of the brass bracket A pneumatic cylinder is used to trigger the drop of the ram assembly The welding head assembly is mounted on the Z axis Figure B1 Appendix B The Z axis unit consists of a Rhino Robots Inc model vertical slide It has one degree of freedom in the vertical direction and is used to move the welding head to pick up the silver button set the drop height and for retracting the welding ram assembly after a weld It consists of a carriage that travels up and down on a pair of polished round bars A DC servomotor drives the carriage through a gear and chain assembly The DC servomotor is connected to a motor port on the Mark IV controller Two projection brackets are used to extend the reach of the Z axis carriage Figure B6 and Figure B7 Appendix B 4 3 2 Fixture A custom fixture was fabricated for the cathodes This is a significant difference from the
7. molecular weight of Silver is 107 86 g mol Therefore the mass of silver flash m 15 m no of moles x molecular weight 5 16 x 10 x 107 86 005 gms c2 2 From tables density of Silver Pag 10 5 g cc Therefore theoretical volume of metal expelled in the form of flash from each corrugation V is V4 0 005 10 5 3 97 x 10 cc c2 3 75 R Figure C2 A hemispherical corrugation on the Silver contact Assuming a corrugation to be a perfect hemisphere the radius was measured to be 0 038cm This was measured from the photomicrographs using simple geometry The theoretical weld area available with the corrugated side being welded to brass is the area of the corrugation that is left after the flash is expelled Hence we need to find the section of the hemisphere Figure C2 whose volume is equal to the volume of flash V From Figure C2 the volume of segment V 1 3 nb 3R h 1 6 th 32 th c2 4 Therefore V 1 3 nh 3 x 0 038 h c2 5 Solving for h h3 0 114h 3 79 x 10 0 c2 6 Solving the Cubic equation we get three values for h One out of the three values is a feasible solution h 02 cm Now the value of a has to be calculated in order to find the circular cross sectional area that is available for weld Substituting the value of h in equation c2 4 3 79 x 10 1 6n 02 022 033 Weld area from one corrugation na 3 42 x 10 cm c2 7 76 T
8. 1992 Optimum welding conditions are set and a test specimen is welded The test specimen is subjected to quality tests and if found good parameter curve profiles of this specimen are stored as a master for the application Norrish 1992 The curves of the production welds are then compared with the master curve profile and if there is no similarity in the profile it is considered a bad weld The quality monitoring software was developed at Westinghouse Corporation Microsoft Quickbasic 1990 All the parameters for using the DAS 50 are set in the software This includes the sampling rate voltage ranges trigger mode channels etc The program activates the DAS 50 card using the digital pulse that is generated when the welding head is dropped and triggers the data collection It then transfers 200 values of current and voltage data in steps of 10 microseconds to the computer s memory The 200 values of voltage and current are stored in arrays and are plotted against time Figure 4 9 36 illustrates graphs of voltage and current against time from a sample CD weld between a silver contact and brass bracket The values are also stored in a data file which can be used for experimental investigation The voltage and current profiles are then compared and if there is no match a bad weld label is displayed on the screen volts amps a b Figure 4 9 Plots of a voltage vs time and b current vs time for a CD weld
9. ALRC unit In the ALRC unit the brass brackets are inserted in a thin slot cut on a copper cylinder This cylinder was then inserted in a collet chuck and tightened until the brass contact is gripped A matrix type fixture with cavities was designed for the brass contacts Two rows of five cavities were machined on an aluminum block Figure B8 Appendix B Aluminum was chosen as compared to copper because of the ease of machining the 26 fixture The brass contacts were to be inserted into these cavities However it was determined that there would not be positive contact between the cathode and the aluminum fixture as the parts rest freely on the fixture base Also any rough spot on the aluminum fixture would prevent surface contact with the brass plate The main issue was to increase the area of contact as much as possible for good conductivity To address this problem a mechanism to positively grip the parts to ensure good contact was designed A 1 4 thick aluminum plate was machined with the matrix cavities for the brass contacts Figure B9 Appendix B A dove tail rail was attached to this plate for sliding on the base fixture After the brass contacts are placed on this plate they are locked by sliding it on the dove tail The sliding plate is pulled by a pneumatic cylinder The end of the sliding plate is attached to this pneumatic cylinder and insulated by a bakelite wrapper so that the cylinder body does not conduct This fixture pla
10. Operation The first step involves switching on power 120 v ac to the main DC supply signal conditioner and other DC supplies associated with actuators The required voltage and capacitance is set manually A detailed list of operating instructions is 38 provided in Appendix A Robotalk software is loaded and the machine control software file is executed The welding head picks up the silver button from a fixed position on the XY table and moves to the position for weld The capacitors are charged and the DC supply is isolated The weld is performed by triggering the air cylinder The welding head is then retracted up and the XY table returns to the origin 4 7 Safety Safety is an important consideration of CD welding Although potential hazards are minimized by this automated CDW unit some precautions are still necessary Unlike conventional arc welding and gas welding there is no metal splatter or release of toxic gases in CDW However CDW involves very high currents and capacitance and good electrical safety must be provided The automated CDW equipment was installed in a clean and dry area adequately guarded from moisture Power cables and terminals were shielded from accidental contact by personnel or metal objects The frame and electrical circuitry were connected to good electrical ground the electrical connections are tight Insulation was provided between the fixture and the XY table and similarly between the welding ra
11. fo asian Recte eode cide sed 49 5 3 Issues and Limitations of the Automated CDW 50 6 CONCLUSIONS AND RECOMMENDATIONS ee 53 0 ECODSIUSIODSS 7 a ee eet ee 53 53 BIBLIOGRAPHY 55 APBPENDICES 57 Appendix A AUTOMATED CDW SYSTEM OPERATION 58 Appendix B MECHANICAL ASSEMBLY AND COMPONENT DBRASVINGIS EE e qn 61 Figure 4 3 4 4 4 5 4 6 4 7 4 8 4 9 4 10 5 1 5 2 5 3 5 4 5 5 LIST OF FIGURES Page Schematic of CDW DIOCSSS b oo Rute aes ut tee lem odode dinate 7 Electric schematic for CDW representing the primary process parameters 11 Photomicrograph of a resistance weld between silver top and brass on the bottom at a magnification of 50X aures 6 6 ah Moco cael cette belt um 20 CDW Automation Architecture 2 ety s A P pats 2 CDW Software DeSiPfians do eniti b Up Det ioi ete dece de 23 Picture of the automated CDW system 27 Main Circuit for CD Welding puce Mene sites eee ce ud 28 ooo oco dte 6 29 Signal Conditioning ito ec 32 Flowchart for Machine Control Software 34 Plots of a voltage vs time and b current vs time for a CD weld 36 Plots of a power vs
12. hence it can be directed and focused precisely This process does not require vacuum and can operate at atmospheric pressure However reflections can hamper the transfer of laser energy to the work piece The advantages of RSP are reduced grain size increased solute solubility and smaller heat affected zone 2 1 4 Advantages of CDW Because CDW is an RS process it leads to improved solubility and stability of metastable phases Large brittle intermetallic phases do not have time to form in the fusion 4 13 zone interface of the joints Because of rapid cooling rates little porosity develops at the weld interface resulting in a high strength joint CD welds do not penetrate through base metals so parts can be welded onto sealed containers The ALRC has welded a variety of dissimilar metals using the CDW process Joints between metals such as copper and nickel copper and aluminum aluminum and iron have been tested Ordinarily joints between dissimilar metals have to be prepared with a fluoride flux and then soldered or brazed The fluoride flux then has to be disposed of according to environmental regulations thus increasing the cost CD welds do not require substantial cleaning or finishing CDW is also an autogenous process and does not require any flux Since the weld time is short in CDW the heat input to the base metal is small compared to conventional arc welding Therefore the weld metal fusion zone and heat affected zo
13. nine capacitors the maximum capacitance that can be obtained is 90 000 uF 09 Disconnect the bleed resistor using the toggle switch Turn on the power 120v AC to the Kenwood dc power supply signal conditioner Rhino controller and the two dc power supplies that are used for the relays and actuators Set the required voltage for the weld on the Kenwood power supply using the voltage control knob A digital display of the voltage is provided on the front panel of the power supply This set voltage is then obtained at the terminals by pressing a push button on the front panel Switch on the air compressor The air compressor automatically stops after the pressure builds up to 8 bar The quality monitoring software is then executed typing weldgaf on the process monitoring computer Robotalk software is loaded on the machine control computer by typing rt and pressing enter until the menu of the software appears on the screen The machine control program autocdw is loaded 60 6 Place the silver contact on the small cavity milled on the fixture assembly and insert a brass bracket into the fixture The valve on the manifold of the compressed air line is opened and the machine control program autocdw is executed to perform a CD weld Steps 1 5 are the set up operations required before performing the weld To repeat the process step 6 has to be performed APPENDIX B MECHANICAL ASSEMBLY AND COMPONENT DRAWINGS 61
14. occurs during the solidification of the weld metal Solidification cracking also occurs due to the lack of ductility of the weld metal Lancaster 1993 2 2 CDW Automation A significant application of robots is in automated welding The automotive industry is the largest user of welding robots Lane 1987 Automation has been applied to many welding and interrelated processes including brazing soldering and thermal cutting Robotic arc welding operations are used in heavy industries like construction Robots are extensively used in resistance welding operations in the automotive industry Welding equipment is classified as manual mechanized or automatic depending on the level of sophistication Some limitations Malin 1985 of the welding robots in existence are expensive set up time changes in fixtures finite work envelopes that limit robot positioning tooling and software for changing parts and the need for high investment The CDW process does not require complex equipment for automation Some stud welding processes require a ferrule a ceramic ring shaped enclosure around the workpiece to confine the heat generated to the weld region and limit the spread of arc CDW does not require ferrules around the workpieces hence the parts can be easily fed 16 into automated tooling and fixturing CDW is an autogenous welding process thus there is no need for supplying filler metal during the process Also since the process is applied
15. significantly improve consistency and reduce the cost of inspection and rejection 1 1 Overview of Capacitor Discharge Welding Capacitor Discharge Welding CDW is a variation of stud welding Although it has long been in existence this process has not been extensively used primarily due to the lack of familiarity and understanding of the metallurgy and parameters of the process Researchers at the US Department of Energy s Albany Research Center ALRC have investigated the process and developed models to represent the process physics CDW has been found to be a rapid solidification process RSP with great benefits in terms of metallurgy and manufacturing Wilson 1994 The CDW equipment consists of a power supply capacitor bank and the fixtures to hold the electrodes The process is initiated by dropping the cathode onto the anode from a pre determined height The capacitor bank discharges as a tip on the cathode makes contact with the anode The tip presents a high resistance to the discharge current and is heated It disintegrates and generates a plasma that melts the surfaces to be joined The actual welding process takes from 1 to 3 ms using a current of about 10 A DC This makes the process cost effective for high volume production Some advantages of the process include a narrow heat affected zone thin weld zones on line quality monitoring short welding times and high cooling rates permitting the joining of dissim
16. time and b resistance vs 37 Front and Side views of a brass bracket b silver contact and c welded SS ETM o seen tse hM Ee sr CR dra ed 39 Photomicrograph of a resistance weld between silver top and brass on the bottom at a magnification of 50X 43 Photomicrographs of a CD weld performed on the manual machine with the flat side of the silver contact top welded to brass 44 50X photomicrograph of a weld performed on the automated system V 20V HIS mm Co tpa tante Ed 45 A 200X photomicrograph of a CD weld performed on the automated system at optimal parameters V 31V 25 mm C 09 45 LIST OF FIGURES continued Figure Page 5 6 Photomicrograph showing the entire weld interface The silver contact is on the top and the brass bracket is on the bottom 47 5 7 pick up of the silver contact 5 5 LIST OF TABLES Page Parameters used for Welds Wilson 1991 8 Parameters used in the 40 Distance from base of Ag corrugation to brass surface of the Automated Weld See Figure 5 6 4
17. to small parts the equipment can be compact and portable The CDW process does not require any complex operations like seam tracking in arc welding Further an automatic CDW unit is far less dangerous than other types of robotic welding equipment CDW has not been fully automated and implemented in a production line due to limited process knowledge However there have been attempts to mechanize or semi automate the process Erico Jones model of a PSW 100 capacitor discharge production stud welding system is equipped with microprocessor to control the electrical circuitry It brings the cathode into contact with the anode under the influence of gravity It consists of a pneumatically operated semi automatic collet clamping feature for gripping the anodes TRW Inc produces the Nelson stud welder The Nelson stud welder consists of a stud gun where the parts are loaded manually and tightened Electronic sequencing is used to trigger the process and after the weld the parts are unloaded manually by spring load An electrical controller box is used to control the voltage interrupt the weld current etc Neither of the above two machines have precise control of voltage current or any on line process monitoring capabilities The Bitterman stud welder was another commercial machine developed for stud welding based on the capacitor discharge principle In that machine pneumatic feed is used for stroking the welding head But fluctuation in the air press
18. 2 has not been implemented in this system The optimization program is essentially a database that consists of the melting points thermodynamic enthalpy values of the metals When the weld design information area of cross section material etc is input to this program it automatically generates the process parameters required V C L H for the weld Also since classical feedback control cannot be applied to CDW the machine controller can be programmed to analyze the data obtained from the data acquisition system and make on line adjustment of variable parameters like drop height and voltage setting This can be done by a custom data acquisition software or by an appropriate algorithm e g Neural Network that is capable of estimating the deviation and propagating the error A Neural Network program was developed at Eaton Westinghouse corporation to backward propagate the deviation of the CD weld parameters In conclusion this automated CDW system has provided a foundation for conducting further investigations especially in the level of automation and control of the CDW process BIBLIOGRAPHY American Welding society AWS Welding handbook vol 2 1991 Cary H B Arc Welding Automation Marcel Dekker 1995 Cary H B Modern Welding Technology Prentice Hall 1979 Eichhorn F Aspects of the Mechanization Automation and Utilisation of Robots in Welding Proceedings of the conference of the International Institute of Weldin
19. 8 Distance from base of Ag corrugation to brass surface of the Manual Weld See Figure 3 be 48 Empirical and Theoretical weld area comparison between automated and manual divenne e Dn 49 Figure Bl B3 B4 B5 B6 B7 B8 B9 CI C2 LIST OF APPENDIX FIGURES Page Welding head assembly mounted on Z axis s 62 Welding head assembly o sedat edet o DeL E cot 63 bosco 64 as n au M eti e 65 Face Mtas Ay th nd 66 Projection Bracket Posset sto pco bee ob ED 67 Projection ecce ione ee egt f 68 Fixture Base ie eMe stes 69 Fixt r Sliding Plate o eee rte ME 70 Free body diagram of the silver contact 72 DEVELOPMENT OF AN AUTOMATED CAPACITOR DISCHARGE WELDING SYSTEM 1 INTRODUCTION Automation is defined as the technology concerned with the application of mechanical electronic and computer based systems to operate and control production Groover 1987 This revolution started over three decades ago Robotic technology is at the leading edge of the current race towards productivity and improved product quality The advantages of automation include improved productivity better product quality and improvement in the status of workers Welding is an effective and economical method for joining most commer
20. AN ABSTRACT OF THE THESIS Srinivas M Patnaik for the degree of Master of Science in Industrial Engineerin presented on September 9 1996 Title Development of an Automated Capacitor Redacted for Privacy Terrence G Beaumariage Brian K Paul Abstract approved Automation of Capacitor Discharge Welding CDW has not been investigated in industry due to the limited understanding of the process The US Department of Energy s Albany Research Center ALRC has done an extensive study of the process and reported applications of welds in the manufacture of electro mechanical products such as relays and circuit breakers The main purpose of this work was to investigate automation of the load unload cycle of CDW An automatic CDW machine was built with real time quality monitoring capabilities Tooling and fixtures were fabricated for joining silver to brass components used in a circuit breaker assembly A machine controller was used to automatically sequence the process and the actuators Computer based instrumentation was developed for on line data acquisition and quality monitoring of the weld This demonstrational set up shows that the equipment can be compact and the process can be easily automated for high production rates The automation architecture developed can be used as a general model for automating CDW operations in other environments The automated CDW system was tested by welding silver contacts to brass bracket
21. APAS d ipi D MPH HERI 15 3 RESEARCH GOALS AND OBJECTIVES 18 4 SYSTEM DEVELOPMENT 85 int 19 4 1 Background of Application 19 4 2 System CV CIVIC WW ed ducti Raat oni uL a Ru refe cem ad 20 eetue meines Ea iM 24 4 3 1 Welding Head Assembly eive nti lue 24 N E tissus e e e esed er 25 Ax M n CALCU I m Hr 27 4 3 4 Pneumatic Circuit o oos ttt eli eese bun onde t i MA 29 4 4 Data Di c e 30 4 4 1 Signal Conditioner ederet oi atia en vede 31 x pius e RR 33 TABLE OF CONTENTS continued Page pios ai oC 33 4 5 1 cepe idet as ases cd dre 33 4 5 2 Quality MOOT DS iuris ra 35 ai cists dat ea epe back ut Led Ee UE 37 dd Sd IG D md UMS Ld 38 5 EXPERIMENTATION AND RESULTE cccccccsccssscecesesececsesessssesasssscevsccssveesesavoreses 39 5 1 Experimental Seb DB ce ppc e items a bcd teases fuit cod UT Rx 39 I2 ici MEC 40 5 2 1 Specimen Preparation messo ea acetate tr eei oot 40 5 2 2 Analysis of Results 3 oo teh 42 5 2 2 1 Resistance Welding vs Manual CD 1 42 5 2 2 2 Automatic CDW System vs Manual CD Welder 44 5 2 2 3 Cycle Time
22. Photomicrographs across the entire weld interface are shown in Figure 5 6 Higher energy densities high heat inputs also created welds of same thickness as of low heat input welds It appears that the extra energy was used in forming flash and did not increase the weld thickness or the width of the HAZ Continuous joints can be observed between silver corrugations and brass in both the welds However some voids are present in both the welds This is due to the corrugated side of the silver contact being welded to the brass bracket The quantity of molten metal was not sufficient to fill the spaces between the corrugations Some gas porosity can be observed due to the entrapment of oxygen from the disassociation of surface oxides In Figure 5 6 a it appears like the silver contact is misaligned Measurement of the angle between the brass bracket and the base of the silver corrugation revealed that there is a small difference between the misalignment in the automated and manual weld 0 23 and 0 7 respectively A careful examination of the photomicrograph of the automated weld Figure 5 6 a reveals that the silver contact was not centered on the tip 47 of the brass bracket This caused a lever arm effect on the silver button resulting in the momentary misalignment of the faying surfaces This resulted in an uneven discharge across the surfaces This is evidenced as more metal has been removed from the corrugation on the left of the photomicr
23. a magnification of 50X Because of the extreme mechanical load requirements the brass brackets required high modulus of elasticity and strength These materials are hardened and strengthened by cold rolling High heat input due to resistance welding anneals the base metal and reduces the yield strength in cold rolled copper In addition low melting impurities were produced near the WCL The diffusion of Zinc is also noticed at the interface On the manual CD welder at ALRC the silver contact was welded to the brass part with the flat side being welded Photomicrographs of a sample weld done at optimum conditions are shown Figure 5 3 The weld centerline is free of voids and inclusions 44 330 um 50pm a 30X b 200X Figure 5 3 Photomicrographs of a CD weld performed on the manual machine with the flat side of the silver contact top welded to brass bottom Continuous bonds are created because silver and copper readily form a solid solution as they have the same valence and a small difference in the atomic radii and electronegativity No oxide layers can be seen It can be observed that copper has infiltrated into the surface of the Ag W alloy forming an unmixed zone commonly referred to as marble cake structure 5 2 2 2 Automatic CDW System vs Manual CD Welder Sample welds were initially performed with the corrugated side of the Silver contact touching the welding ram The suction hole that was drilled on the weld
24. and data acquisition is explained A description of the machine control software quality monitoring software operation and safety of the system is also provided in this chapter Metallographic examination results are explained in Chapter 5 An analysis of the photomicrographs of the silver brass welds is presented Chapter 6 summarizes the research findings conclusions and recommendations for future work 2 LITERATURE REVIEW Welding is one of the most important aspects of manufacturing operations whether it involves joining small electronic components or large structures It is also complex since a number of metallurgical changes occur during the process There are a variety of welding processes each requiring a different source of energy Stud welding is a process for joining a metal stud or a similar part to a work piece In stud arc welding the base of the stud is joined to the other workpiece by heating the stud and the workpiece with an arc drawn between the two AWS 1991 If a capacitor storage bank is used to supply the arc power it is known as capacitor discharge welding 2 1 Capacitor Discharge Welding In CDW DC arc power is produced by discharging a large amount of electrical energy stored in capacitors CDW has been used at a limited level This is due to the lack of available process knowledge Arc stud welding is more widely used than the CDW process Cary 1979 CD welding can be done in one of the following t
25. anufacturing Engr OSU DWG Srinivas Patnoik DATE 10 09 95 Not to Scale SHEET X X Figure B9 Fixture Sliding Plate TOLERANCES XX 01 0L APPENDIX DESIGN CALCULATIONS 71 C1 CALCULATION OF THE SUCTION HOLE DIAMETER FOR THE WELDING RAM Since vacuum chucking was implemented for the silver contact pick up the hole drilled through the welding ram had to be large enough to provide good suction for the pick up while maximizing the area of contact between the welding ram and the silver contact for the conduction of high current Hence the suction hole diameter had to be optimized for these requirements Figure C1 is a free body diagram of the silver contact The suction force or force due to vacuum F should be greater than or equal to the force due to the weight F of the silver contact Land Figure C1 Free body diagram of the silver contact F Fy c1 1 F is calculated in the following way F x Py 7 4 d P c1 2 where A is the area of cross section of the hole P is the back pressure due to vacuum and d is the diameter of the hole Substituting the value of F in equation 1 1 1 4 d P F Therefore d Vz c1 3 b Let Pag represent the density of silver and Viver the volume of silver contact then F can be calculated as follows EQ Pag X V saver c1 4 and 0 4 disci contact t where contact and t are the diameter and thickness
26. asma and molten metal particles Wilson et al 1993 The arc spreads along the face of the electrodes and metal plasma moves away from the joint as shown in Figure 2 1 D The resistance of the circuit at this instant was found to be in the order of 2 x 10 The actual arcing time is on the order of 1 3 milliseconds with a current of about 10 amps drop height 09 tip length L Vapor wY reas EAM A B C Figure 2 1 Schematic of CDW process The super heated vapor cools and condenses into metal particles Plasma and solid particles move away from the weld joint according to the magnetic field A thin layer of liquid metal remains on the faying surfaces The arc is extinguished after the electrodes make full contact Figure 2 1 E leaving a fusion zone about 0 1 mm thick Wilson et al 1993b CD welding has been successfully applied to the joining of intermetallics Wilson et al 1995 An analysis of CD welded 1 intermetallic alloys was performed by Wilson 1991 The electrodes used in that analysis were made of FeAl intermetallic alloy cylinders with a diameter of 6 35 mm and a length of 16 5 mm The cathode had an ignition tip with the length ranging from 0 63 to 1 4 mm Ultra high speed photography was used to record the welds Welding conditions that were used in the analysis are shown in Table 2 1 Equations for arc force and radius of expelled plasma were developed but a discuss
27. bots 1994 The language has commands for moving motors activating output channels and other program control statements begin weld XY position cycle for drop XY position fo TERRE set drop height contact pickup Pa Place ram charge capacitors isolate DC supply drop welding ram and trigger DAQ End weld cycle over contact vacuum on retract head XY position for pressing the ram lower head to press the ram retract head 4 8 Flowchart for Machine Control Software Figure 4 8 represents the flowchart used for the machine control software for one weld cycle Lead through programming is used to program the machine control 35 software This consists of first moving the equipment to the required positions using the motor keys on the teach pendant The encoder counts at the various positions are recorded and later used with the motor move commands 4 5 2 Quality Monitori CD welds can be examined using destructive or nondestructive testing procedures Since the CDW process is applied to small parts sometimes it may not be feasible to conduct tensile tests Destructive tests do not immediately permit corrective action in production although they are valuable in research Also they are very expensive and time consuming In process quality assurance procedures have been successfully implemented for other welding processes including resistance spot welding Norrish
28. cial metals Welding processes have been found more difficult to automate and control compared to other manufacturing processes Lane 1987 However recent technological developments in the field of electronics computers and robotics have opened possibilities for the introduction of flexible low cost automation of welding processes Arc welding robots are used in large volume production lines for automobiles Norrish 1992 According to Eichhorn 1985 in the future automated welding will be associated with small mass produced units for which the loading of assembly parts to be assembled and unloading the finished assembly can be integrated relatively easily into the production line Many welding processes are labor intensive and an analysis of welding costs shows that some 70 to 80 of the total cost may be accounted for by direct labor Norrish 1992 Welding automation is a means for reducing the overall cost of welding to operations by replacing some or all of the manual effort with a mechanized or automated system Apart from reduced labor costs automated welding also improves safety health and product quality Most welding operations are hazardous due to toxic gases noise and electromagnetic radiation other risks include welding in confined spaces underwater or in radioactive environments Automation eliminates these risks and hazards Also high product quality is difficult to achieve with manual techniques Automation can
29. contact between the ram and the button for the conduction of high current during the weld The first criteria i e the pick up of the silver contact required the accurate positioning of the welding ram above the silver contact This was dependent on the tip of the ram and the accuracy of the XY table Figure 5 7 shows a misaligned pick up of the silver contact due to positioning error Figure 5 7 Misaligned pick up of the silver contact The repeatability of the XY table was approximately 1 mm To account for this repeatability a chamfer was provided at the end of the welding ram The welding ram _ was programmed to press against a flat surface which caused the silver contact to be forced into the cavity of the welding ram The second design criteria was the area of contact between the welding ram and the silver electrode Insufficient area of contact between the welding ram and the silver contact causes a capacitive discharge at the interface thus joining the silver contact to the ram Since vacuum chucking was implemented for pick up a suction hole was drilled through the welding ram Analysis of the vacuum gripping force and the hole diameter 22 is provided in Appendix CI This analysis shows that a smaller diameter hole will still provide enough suction for the contact while increasing the contact area between the silver contact and the ram Flash metal vapor that is formed during the weld deposits on the tip of the welding
30. ed Optimization program drop height machine controller motor position amp actuator commands hardware process monitoring QC good weld bad weld Figure 4 3 CDW Software Design 24 Figure 4 3 shows a structured analysis of the software design An optimization program under development is used to calculate the CDW parameters for the required joint This program essentially consists of a database of melting points and enthalpies of all the metals and performs the energy calculations as per the theory described in Section 2 1 2 The settings of voltage and capacitance are performed manually Control software is used to set the drop height and automate the process This software is described in Section 4 5 1 Voltage and current during the process are measured by the data acquisition system with the quality monitoring software is used to determine the quality of the weld based on this data 4 3 Hardware 4 3 1 Welding Head Assembly The welding head assembly is used to pick up a silver electrical contact and deliver it to a brass bracket for the CD weld under the influence of gravity It consists of the welding ram linear bearing and a cylinder to release the assembly so that it drops down due to gravity Figure B2 Appendix B The welding ram is an aluminum rod used to pick up the silver contacts Based on simplicity vacuum chucking was implemented for holding the silver contacts
31. ed CDW system This is because of the uneven discharge across the automated weld see explanation of Figure 5 6 a earlier in this section The uneven discharge is due to poor XY positioning in the automated system Overall the weld area of the automated system is within the range of the theoretical calculations This suggests that an explanation other than weld pick up misalignment is needed to explain the differences in weld area A plausible explanation is that the manual unit is more energy efficient than the automated unit This is consistent with observations of discharging and thus loss of efficiency both in the welding head and anode fixture of the automated system Table 5 4 Empirical and Theoretical weld area comparison between automated and manual system avg d h em V a based on theoretical 0 02 3 97x 10 0 033 0 041 mass expelled based on actual iud 0 015 0 034 5 2 2 3 Cycle Time Comparison weld area from all 12 2 corrugations cm mass expelled Cycle time comparison between the automated CDW system and the manual CDW unit was performed The total cycle time for performing a weld is the time from the 50 loading of the components to the unloading of the finished assembly The set up operations in both the machines include setting the values of voltage and capacitance required for the weld The welders activities in a weld cycle on the manual unit are loading the parts into t
32. enthalpy required for the molten metal as well as the flash The total enthalpy required for the weld is AH AH fusion AH nash 2 3 where AHfusion 15 the energy required for fusion i e for melting the base metals and AH fash is the energy required to vaporize the tip and the melt back zone The mass of molten metal for each element is calculated and converted to the equivalent number of moles The thermodynamic enthalpy values Pankratz 1982 for melting these metals is used to calculate the individual enthalpies and hence the total enthalpy for fusion The energy required for flash is also calculated similarly 11 correction factor of 1 5 is used to account for the efficiency of the machine due to extraneous electrical losses and the loss of energy in the form of sound and light Wilson et al 1995 An energy balance is made between this thermodynamic energy requirement for the weld and the energy E supplied by the capacitors E 1 2CV 2 4 With fixed system capacitance a value for the voltage can be determined In an earlier study Wilson 1991 it was determined that the highest energy density develops at the interface when the welding time was approximately twice the RC time constant Ty 2 RC L b 8 H 2 5 Since the capacitance is fixed equation 2 5 can be used to calculate the drop height and the tip length L The electrical schematic is shown in Figure 2 2 with the parameters of operation A DC
33. estigated CD welds for different stainless steels to find the optimum welding parameters that produce the best joint and rapid quenching rates and reported the following 1 Weld thickness decreased with increasing drop height and decreasing ignition tip length 2 Decreasing voltages on the capacitor also decreased weld thickness but the voltage for good joining was 90 V for stainless steels 3 Drop weight did not have a significant effect 4 Average weld thickness was proportional to arc time 5 Ignition tip and drop height were the most effective controllers of weld arc time 2 1 2 Process Parameter Calculations CDW process variables are those parameters that can be adjusted to control the weld After a CD weld joint is designed these parameters must be estimated in order to perform the weld The joint design provides information on the area of the weld material weld thickness etc This information has to be converted to the electrical energy requirements for the heat and the weld time to control the quality of the weld Hence the primary parameters required for satisfactorily performing CD welding are voltage V and capacitance C for the energy tip length L and drop height H for controlling the weld time The energy required for a CD weld is determined by calculating the thermodynamic weld energy required to melt the base metals and vaporize the ignition tip and melt back zone This is done by calculating the value of
34. g France pp 3 33 September 2 3 1985 Groover M P Automation Production Systems Computer Integrated manufacturing Prentice Hall 1987 Kehl G L The principles of Metallographic laboratory practice McGraw Hill 1949 Keithley MetraByte Corp DAS 50 User manual 1990 Lancaster J F Metallurgy of welding Chapman amp Hall 1993 Lane J D Robotic Welding IFS Publications UK Ltd 1987 Malin V Designer s guide to effective welding automation part I analysis of welding operations as objects for automation Welding Journal vol 64 no 11 pp 17 27 1985 Microsoft Corporation Microsoft Quick Basic Programming for DOS Systems Version 4 5 1990 Niebel B W A B Draper and R A Wysk Modern Manufacturing Process Engineering McGraw Hill 1989 Norrish J Advanced welding processes Institute of Physics Publishing 1992 Pankratz L B Thermodynamic properties of elements and oxides US Bureau of Mines Bulletin 672 1982 Rhino Robots Inc RoboTalk User s manual version 3 82 1994 Venkataraman S and J H Devletian Rapid solidification of stainless steels by capacitor discharge welding Welding Journal research supplement vol 67 no 6 pp 1115 1185 1988 56 Wilson R D A capacitor discharge weld microstructure model for iron aluminide Ph D Dissertation Oregon Graduate Institute OR 1991 Wilson R D Explore the potential of capacitor d
35. hat material is ejected as plasma from the weld area due to induced magnetic forces Joints between dissimilar metals have been studied at ALRC Wilson et al 1993a used ultra high speed photography to record the CD welding of similar metals with different tip lengths and CD welds between dissimilar metals copper and nickel copper and aluminum etc They found that CDW creates continuous metallurgical bonds between dissimilar metals with an unmixed microstructure 2 1 1 Physical Process CD welding requires a capacitor bank a power supply for charging the capacitors and fixtures to hold the electrodes Figure 2 1 is a schematic of the CDW cycle initial gap method The initial position of the cathode with the welding tip and the anode is shown in Figure 2 1 A The de power supply is used to charge the capacitor bank The circuit is open at this stage and the electrodes are separated by a distance known as the drop height After the capacitors are charged the welding head is dropped by gravity The electrodes come in contact Figure 2 1 B shows the tip making contact discharging the capacitance and initiating the arc The small projection on the cathode creates a high resistance to the discharge current causing it to heat up The tip rapidly disintegrates and generates an arc that melts the surfaces to be joined Figure 2 1 C The photographs revealed that surface layers of metal are expelled from the joint in the form of pl
36. he fixtures setting the drop height operating the electrical controls power on remove bleed resistor charge capacitors isolate the power supply triggering the release of welding head to perform the weld removing the assembly and resetting the electrical controller In the automated CDW system after the brass bracket is inserted into the fixture and the silver contact is place at the pick up point the machine control software has to be executed Table 5 5 Cycle time comparison between automated and manual system Cycle Time sec weld 1 weld 2 Average Manual 119 123 121 Automated 27 27 27 Cycle times for performing two consecutive welds were measured on both the machines The cycle time on the manual unit was measured when it was operated by a semi skilled worker The cycle time comparison is provided in Table 5 5 The automated CDW system has a superior cycle time and is approximately 4 5 121 27 times faster than the manual unit 5 3 Issues and Limitations of the Automated CDW System The welding head is the most critical design in the development of the automated CDW system Automation of loading the silver contact welding it to the brass bracket 51 and unloading the joined assembly were the core functions of the unit To perform these functions the two most important design requirements of the head were 1 to pick up the silver contact in a good orientation and ii to provide maximum surface area of
37. heoretical weld area obtained with the corrugated side down 12 x 3 42 x 10 041 cm The actual amount of flash expelled in a weld was found to be 006 gms This was obtained by measuring the difference in the weights of the sample before and after the weld Assuming equal masses removed from both metals the amount of flash expelled by silver is 0 003 gms The same weld area calculations were performed as above The weld area was found to be 034 cm which is nearly 83 of the theoretical weld area The excess energy must have dissipated in the form of light sound and electro magnetic forces
38. hree different ways initial contact method drawn arc method and initial gap method In the initial contact method the electrodes are first placed such that they are in contact The circuit is then connected electric power supplied and the projection on the cathode presents a high resistance to the high energy and initiates the arc The drawn arc method is complicated and uses electronic control to sequence the operation Initially the electrodes are in contact When the circuit is triggered the electrodes are separated by a small gap to draw the arc The electrodes are then brought back together and the drawn arc produces the 6 heat required for the weld In the initial gap method there is a gap between the two electrodes with the open circuit voltage applied One of the electrodes is then brought into contact with the other by gravity drop causing a high flash of current The initial gap method has been found to be the fastest of the three methods in terms of the actual weld time AWS 1991 ALRC has conducted extensive research into this method and identified the parameters that control the process Cylindrical geometries of CD welds were analyzed to develop models which were compared with high speed photographs of the process It was found that is a rapid solidification process RSP where high cooling rates 10 K s are obtained as a result of the large weld surface areas and small weld volumes Their analyses revealed t
39. ilar metals These features makes it ideal to automate the process 1 2 Investigative Summary Automation of CD welding operations can provide the benefits of speed and economy Although experimental work to develop models and understand the CDW process has been done there has not been any effort to automate the process In this thesis an automated CDW system was developed and investigated In particular the load unload cycle of a gravity fed CDW unit was automated and integrated with an on line quality monitoring system This automation lead to improvements in cycle time without detrimental effects to weld quality The process was applied to the joining of silver contacts to brass brackets The welded samples were compared with similar samples made by a manual CDW process and a resistance welding process 1 3 Thesis Organization Chapter 2 provides a detailed description of the CDW process and a review of the literature on CDW The literature review provides the models that have been developed for optimizing and controlling the process A detailed description of the mechanics of the process is also described Specific goals and objectives of this thesis are listed in Chapter 3 Automated CDW system development is described in Chapter 4 An automation architecture is developed Design and fabrication of the fixtures welding head electrical circuit and the associated accessories is described in this chapter Computer based instrumentation
40. ing a hardware system including tooling and actuators allowing automatic loading and welding of electrical silver contacts 2 Designing a software system which controls the process automatically 3 Integrating computer based instrumentation and software for data acquisition and quality monitoring 4 Conducting experimental investigation and comparison of manual system CD weld joints automated system CD weld joints and resistance weld joints 5 Comparing the cycle time between the manual and automated systems The project has been designed to promote the implementation of an automated CD welding unit in the industry The approach of the project has been to address the essential issues involved in the automation of CD welding 19 4 SYSTEM DEVELOPMENT This chapter describes in detail the design and construction of the various components of the automated CDW system An overview of the system is provided first followed by the hardware and data acquisition system Machine control software and quality monitoring software are explained in later sections 4 1 Background of Application The requirements of this set up were to automate the process for CD welding silver contacts to brass electrodes Figure 5 1 This assembly is used in 20 amp circuit breakers manufactured by Westinghouse Corporation The contacts are the most important constituent of relays circuit breakers and similar electromechanical products Their characteristic
41. ing ram further reduced the area of contact between the welding ram and the corrugated side of the silver contact Due to insufficient contact there was a capacitive discharge and the silver contact was fused to the welding ram To address this problem CD welds were performed with the corrugated side of the silver contact being welded to the brass bracket 45 silver tip brass 200 um Figure 5 4 50X photomicrograph of a weld performed on the automated system V 20V H 18 mm C 09 F Figure 5 4 shows a weld performed at low voltage on the automated unit Low energy density fails to create sufficient arc power between the electrodes Due to insufficient arcing the tip did not completely vaporize Silver corrugation Has Brass bracket 50 um Figure 5 5 A 200X photomicrograph of a CD weld performed on the automated system at optimal parameters 31 25 mm C 09 F 46 200X photomicrograph of a CD weld made at optimum conditions V 31 5V C 09F H 25mm L 508mm on the automated system is shown in Figure 5 5 corrugation on the silver button was continuously welded to brass No cracks voids or inclusions were observed The joint is thin and is less than 10um wide These optimal parameters for CD welding silver contacts to brass were determined in an earlier study Wilson 1996 The voltage was gradually increased in steps of 3 volts Higher voltages caused longer arcing time and more flash
42. ion of these equations is beyond the scope of this work Table 2 1 Parameters used for Fe Al Welds Wilson 1991 Capacitance 0 63 1 4 mm Wilson found that the welding time was controlled by tip length and drop height The welding time T was measured from the beginning of current flow to the current rise during electrode closure An equation to predict the welding time was developed T 1 2 1 where L is the tip length b is the distance the electrodes melt back is the drop height Venkataraman and Devletian 1988 developed an equation for the arc power density P to describe the cooling rate P CV 4aT 2 2 where C capacitance a area of cross section of the weld and V open circuit voltage Welding time was found to be very important because of it s effect on the metallurgical characteristics Welding time should be coordinated with the time to discharge the capacitors the resistor capacitor RC time constant of the circuit Shorter welding times cause pre solidification Longer welding times greater than the 2RC time constants allow the molten metal of the electrode surfaces to solidify before joint closure The melt back distance b was found to be negligible compared to the tip length Drop height has a lesser effect on the weld time compared to the tip length since weld time was inversely proportional to the square root of the drop height Venkataraman and Devletian 1988 inv
43. ischarge welding Advanced Materials and Processes vol 145 no 6 pp 93 94 1994 Wilson R D Rapid solidification joining of silver electrical contacts to copper conductors using the capacitor discharge welding process 11th annual Americ Welding research conference Advances in welding technology pp 203 211 1996 Wilson R D D E Alman and J A Hawk Rapid solidification joining of intermetallics using capacitor discharge welding Materials Research Society Symposium Proceedings vol 364 pp 237 242 1995 Wilson R D J A Hawk and J H Devletian Capacitor discharge weld modeling using ultra high speed photography Materials Research Society Symposium Proceedings vol 314 pp 151 162 1993a Wilson R D J R Woodyard and J H Devletian Capacitor discharge welding Analysis through ultrahigh speed photography Welding Journal research supplement vol 72 no 3 pp 101s 106s 1993b APPENDICES 57 APPENDIX AUTOMATED CDW SYSTEM OPERATION 58 59 Before operating the system a visual examination of the unit is required to check for any loose wires unprotected cables etc for safety purposes Operation of the unit is explained in the following steps 1 Set the required capacitance on the capacitor bank This is done by connecting the required number of copper bus bars of the positive terminals of the capacitors Each capacitor is rated for 10 000 uF Since there are
44. ly to overcome the Lorentzian forces The strobe pulse was used in high speed photography of the process 4 4 2 Trigger Figure 4 7 c shows the trigger circuit used to activate the DAS 50 card to collect analog signals When the card is set in the collection mode it samples data and discards it until it finds the trigger Upon finding the digital trigger data collection is done for the required number of samples set in the software Microsoft Quickbasic 1990 When the vertical head is released the relay supplies the 120V AC to this circuit The bridge circuit rectifies the 120V AC line and generates a DC pulse which is input to the digital trigger pin on the DAS 50 Data sampling is done after the trigger post trigger An opto isolator is provided in the circuit as a protection from any stray signals The signal conditioning circuit and the trigger circuit were built on a breadboard and assembled in a box with the 15V DC power supply Figure 4 7 d which is used to power the operational amplifiers Terminal strips are provided at the input and output of the circuit A 25 pin connector is used to connect the output of the signal conditioner to the DAS 50 4 5 Software 4 5 1 Machine Control The machine control software is used to position the hardware and control actuators that are used to sequence the process The software used for this purpose is 34 RoboTalk a robot control language provided with the controller Rhino Ro
45. m and vertical head assembly the equipment including cables wires and fuses were used within their current and duty cycles At the end of the weld cycle the circuit is disconnected Residual and excess voltage is drained by the bleed resistor An emergency button on the control pendant is used to stop the machine if a malfunction is detected 39 5 EXPERIMENTATION AND RESULTS 5 1 Experimental Set up The automated CDW system was set up to weld silver contacts to brass The contacts are made by compacting silver powders and sintering them to form a disk of 1 47 mm thickness and 3 91 mm diameter It has a face centered cubic FCC crystal structure The actual composition is 82 silver 11 tungsten 2 9 nickel and negligible amounts of other elements One side of it has corrugations for obtaining a high resistance for the required heat when used in the resistance welding process The brass conductor is a p phase mixture of Copper and Zinc It has a body centered cubic BCC crystal structure The commercial name for this brass is yellow brass ASTM B13437 It has a 508 mm ignition tip The front and side views of these parts are shown in Figure 5 1 a b BEI Figure 5 1 Front and Side views of a brass bracket b silver contact and c welded assembly 40 The welding parameters and energy requirements were made for a weld thickness of 100um with a flash of 01 grams These calculations were done according to the
46. nation was used to determine the presence of voids inclusions and to compare the welds made using the manual and automatic machine 5 2 2 Analysis of Results 5 2 2 1 Resistance Welding vs Manual CD Welder In resistance welding the heat required for welding is generated by the resistance to the flow of current through the parts to be joined A pair of electrodes one on either side of the parts to be joined conduct the electrical current through the joint interface The brass and silver contacts are currently resistance welded at Westinghouse with 20 volts AC and a current on the order of 100 200 amps A photomicrograph of a sample weld is shown in Figure 5 2 Several unwanted metallurgical defects can be observed near the weld center line WCL Resistance welding depends on the interface contact resistance It was found that the interface contact resistance between silver and copper is low Wilson 1996 which required a large heat input into the joint Since the resistance at the joint interface is low the heat needed to make the weld develops at the electrode interface and flows to the cold joint through the silver contact and then through copper brass bracket The parts are 43 over heated and the large heat input into the joint caused large amounts of oxygen porosity Silver voids center line Brass uet 200 um Figure 5 2 Photomicrograph of a resistance weld between silver top and brass on the bottom at
47. ne HAZ are narrow Distortion of the base metal is minimal CDW allows retention of stiffness of welded parts because of the small HAZ an extremely important feature for a satisfactory life span of parts made for contactors circuit breakers and relays In addition the CDW process was found to have several advantages over the other RSP techniques in terms of simple equipment parameter variation and ease of microstructural modeling Wilson 1991 The process is fast and cost effective and is suitable for high production rates 2 1 5 Limitations of CDW CDW requires a precision arc ignition tip to initiate the welding process The process can be only applied to electrically conductive materials This process is generally used for cylindrical geometries with diameters in the range of 1 8 to 3 8 inches AWS 14 1991 The limitation on the size is due to the finite amount of energy stored in the capacitors and the length of the ignition tip required to spread the arc between workpieces 2 1 6 Defects in CD welds CDW like any other welding process involves many metallurgical phenomena The weld arc time has a important effect on the quality of the welds In previous studies it was found that the optimum weld time T should equal twice the RC time constant of the circuit Wilson 1996 Wilson et al 1995 Longer welding time T gt 2 RC causes pre solidification of the joint because the capacitors discharge before complete joi
48. ning of the metals Shorter weld times T lt 2 RC causes a cold joint lack of fusion due to energy dissipation in the circuit capacitive discharge rather than at the weld interface The shape and geometry of the contact area between the surfaces also has an important effect on the quality of CD welds The surfaces should be flat and not misaligned when they are brought into contact A small bevel angle is required for the escape of flash metal vapor Surface oxides are also expelled with plasma and vapor due to the beveled geometry Any rough spots or corrugations on the surfaces to be joined causes discontinuities and voids The amount of molten metal may not be sufficient to fill the gaps between the rough spots This couid cause the entrapment of gases and creates voids However control of gas porosity is not required during the process as compared to some arc welding methods CDW suppresses diffusion which could cause the formation of deleterious intermetallic phases Residual stresses are induced some metals around the area adjacent to the weld interface Cracks occur due to instantaneous heating and uneven cooling rates When the thermostresses cross the yield strength of the material cracks are formed In very brittle materials underbead cracking occurs below the weld center line during heat up This is because the layer of metal adjacent to the weld center line is subject to tension Hot or solidification cracking
49. of the silver contact respectively From tables Pag 38 Ib in Therefore 1 4 154 058 1 1 x 10 in and F 38 L1 x10 4 1x 107 Ib Back pressure P is the difference between the atmospheric pressure P 29 9 in Hg and negative pressure due to vacuum P 20 in Hg Therefore P P P 29 9 20 9 9 in Hg 1 914 psi Substituting the value of F and P in equation c1 3 the value of d is obtained d 0 0165 in Assuming a safety factor of 3 the suction diameter in the welding ram 05 in 74 C2 CALCULATION OF THE THEORETICAL AND ACTUAL WELD AREAS OBTAINED FROM THE CORRUGATED SIDE OF SILVER CONTACT The weld area that is obtained when the corrugated side of the silver contact is welded to brass is the area of the corrugations that is available after flash is expelled i e area of the circle with radius a Figure C2 The thermodynamic weld energy calculations were performed for a theoretical loss of 8 6 moles m of metal in the form of flash Wilson 1996 This was used for the theoretical weld area calculation Assuming equal amounts from both metals the theoretical amount of flash from silver is 4 3 moles m The diameter of the silver contact is 3 91 x 10 m Therefore number of moles expelled in the form of flash M is 8 6 x 1 4 x 3 91 x 107 1 03 x 10 moles c2 1 Assuming equal amounts from both metals the number of moles from silver is 5 16 x 10 moles From tables
50. ograph The uneven discharge resulted in less efficient use of arc power and consequently less penetration of the button into the bracket a automated 200 um b manual Figure 5 6 Photomicrograph showing the entire weld interface The silver contact is on the top and the brass bracket is on the bottom The weld area obtained with the corrugated side of the silver contact is the area of the corrugations remaining after the metal vapor flash has escaped The theoretical and 45 actual weld area calculations are provided in Appendix C2 Weld areas obtained with the automated and manual systems were also calculated empirically This was done by measuring the distance d from the base of the corrugations to the brass surface These values are provided in Table 5 2 and 5 3 The value of h the thickness of spherical segment evaporated in the form of flash is the difference between the radius of the corrugation and the distance d Appendix C2 The empirical weld areas are calculated using equation C2 4 and C2 7 see Appendix C2 Table 5 2 Distance from base of Ag corrugation to brass surface of the Automated Weld See Figure 5 6 a Table 5 3 Distance from base of Ag corrugation to brass surface of the Manual Weld See Figure 5 6 b 49 The empirical and theoretical weld areas obtained are provided in Table 5 4 The empirical weld area obtained from the manual unit was larger than that obtained from the automat
51. power supply is used for charging the capacitors to the required voltage The DC capacitors are connected in parallel to form the capacitor bank A detailed description of the actual circuit developed in this thesis is provided in chapter 4 cathode tip length E capacitor 4 DC supply V bank drop height H C anode Figure 2 2 Electric schematic for CDW representing the primary process parameters 2 1 3 Rapid Solidification Rapid solidification is the process of cooling quenching a material from a very high temperature It causes the high temperature phase to be retained at the ambient temperature Electron beam welding and laser beam welding are examples of rapid solidification processing RSP CDW was also found to be a rapid solidification process with a cooling rate of about 10 K s Wilson 1991 All these RS processes have similar characteristics like high energy density high welding speed etc In electron beam welding Cary 1979 the parts to be welded require close tolerances as they have to be placed in a vacuum chamber Heat is generated by high velocity electrons in a narrow beam The kinetic energy of the electrons is converted into heat as they strike the work piece A major problem with this process is the possibility of the beam deflection due to stray magnetic effects In laser beam welding Cary 1979 a focused high power laser provides the heat for the weld The beam has deep penetration power
52. ram Over a period of time this accumulates to form a thin layer and increases the resistance between the welding ram and the silver contact The high resistance could cause loss of energy at this interface and fuse the silver contact to the weld ram Therefore the welding ram has to be maintained properly and the accumulated flash should be removed The tip of the welding ram can also be made insertable to address this problem Also the feeding of silver contacts and the brass brackets has not been automated in this unit The silver contact is manually positioned at a fixed point on the XY table and the brass bracket is loaded into the fixture manually An automatic escapement feeder mechanism can be designed for implementing the automated CDW system on a high volume production basis tod 6 CONCLUSIONS AND RECOMMENDATIONS 6 1 Conclusions Since the actual process time in CDW is very small the speed and accuracy of the system depends on the speed of the positioners and actuators The time taken for one complete weld cycle on the automated CDW system was approximately 4 5 times faster than for a weld cycle on the manual unit CD welds between brass and silver were found to be superior to resistance welds An unmixed microstructure of the cathode and anode materials was observed without any cracks voids or inclusions There was no significant difference in the metallurgical characteristics of the welds made by the manual unit and the a
53. s are significantly affected by material and the joint interface Currently they are joined by resistance welding at Westinghouse Corporation Resistance welding depends on the interface contact resistance It was found that the contact resistance between silver and copper was low requiring large heat input into the joint which caused several unwanted metallurgical changes Wilson 1996 High heat input at the silver copper joint caused large amounts of oxygen porosity In addition impurities that melt at low temperatures were produced on the weld center line WCL A photomicrograph of a resistance welded joint between brass and silver is shown in Figure 41 The life span of the circuit breaker was poor due to the fatigue and fracture of the metallurgical joint between the silver contact and brass bracket Silver voids center line fog 2 N A 1 25 Brass 200 u Figure 4 1 Photomicrograph of a resistance weld between silver top and brass on the bottom at a magnification of 50X Westinghouse Corporation has been investigating the practicality of replacing resistance welded joints with CDW joints to increase the electrical joint service reliability Also since a commercial automated CDW system is not currently available Westinghouse Corporation has been interested in examining the possibility of an automated CDW system with real time process monitoring capabilities 4 2 System Overview The overall a
54. s at different parameter settings Metallographic examination of the resultant capacitor discharge CD welds made by the automated CDW system revealed the formation of continuous metallurgical bonds between silver and brass A comparison of CD welds made by the manual and the automated system was performed to determine the effect of CDW automation on weld quality The welds were also found to be superior to resistance welds Copyright by Srinivas M Patnaik September 9 1996 All Rights Reserved Development of an Automated Capacitor Discharge Welding System by Srinivas M Patnaik A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Presented September 9 1996 Commencement June 1997 Master of Science Thesis of Srinivas M Patnaik presented on September 9 1996 APPROVED Redacted for Privacy Professor representing Industrial Engin ring Redacted for Privacy Co Major Professor representing Industrial Engineering Redacted for Privacy Head of Department of Industrial Manufacturing Engineering Redacted for Privacy Dean of School I understand that my Thesis will become part of the permanent collection of Oregon State University libraries My signature below authorizes release of my thesis to any reader upon request Redacted for Privacy Srinivas M Patnaik Author
55. sor A hose is used to connect this pressure source to port 1 on the vacuum generator The vacuum generator operates on the venturi principle and generates vacuum at port 2 which is connected to the welding ram through a hose The vacuum generator is solenoid activated 24V DC and is controlled by a relay through the Rhino controller vacuum Sa generator Spring retum air cylinder Normally closed xc valve PR Compressed air supply Figure 4 6 Pneumatic Circuit Compressed air is supplied to the air cylinder through a normally closed solenoid valve The solenoid valve also operates on 24V DC and is controlled by a relay through 30 the Rhino controller The air cylinder is single acting and has a spring return mechanism The air cylinder is mounted on the welding head assembly and is used to release the welding ram to perform the weld 4 4 Data Acquisition In a manufacturing process the outcome of operations should be within a defined range of objectives specifications and the process parameters must be within specified control limits In process measurement of varying parameters is required for analyzing the process and making corrections if deviations are present It is also fundamental to on line quality monitoring of the process CD welding can be controlled by establishing the satisfactory conditions and parameters based on the theory discussed in Chap
56. t the capacitor bank to the fixtures to allow the conduction of high current that flows during the weld cycle The anode terminal of the capacitor bank is directly connected to the welding ram The negative terminal of the capacitor bank is connected to the matrix fixture with shunt placed in series The shunt is used for current measurement explained in Section 4 4 1 The length of these cables is kept to a minimum to avoid any inductive losses due to the movement and positioning of the fixtures 30 KQ Arc 10 W i Figure 4 5 Main Circuit for CD Welding Because capacitors have the tendency of regenerating and building up residual energy a bleed resistor is used to discharge the energy that builds up when the machine is not operational A 30 resistor connected in parallel to the capacitor terminals was used to drain this residual energy The power supply has to be isolated after the capacitors are charged to prevent any feedback from the capacitors A double pole single throw relay is used to connect and disconnect the power supply to the capacitor bank This also protects the power supply from the arc that is created during welding 4 3 4 Pneumatic Circuit The pneumatics used in the system are 1 air cylinder to release the vertical head and 2 vacuum generator for the silver contact pick up Figure 4 6 represents the pneumatic circuit ISO symbols Compressed air 8 bar is supplied by the air compres
57. te and pneumatic cylinder assembly are mounted on a gray PVC plate for insulation The whole assembly is attached to the bed of the XY table The XY table serves as the machine bed and provides two dimensional positioning of the fixture The XY table used is a Rhino Robots Inc motorized orthogonal XY table It consists of a carriage that travels in two di ctitons on two pairs of polished round bars The drive in each direction is provided by a DC servomotor A gear and chain assembly is used to transmit the drive to the carriage which slides on the polished bars The two DC servomotors are controlled by the machine controller Rhino Mark IV A picture of the automated CDW system is shown in Figure 4 4 Figure 4 4 Picture of the automated CDW system 4 3 3 Main Circuit The main electric circuit used for CD welding is shown in Figure 4 5 The DC power supply is a special purpose supply rated for heavy duty applications It has voltage and current control features and is capable of producing an open circuit voltage of up to 125 volts DC The capacitor bank consists of nine capacitors with a capacitance of 01 F each They are mounted in a wooden box for insulation and are connected by copper bus bars Since parallel connection was required all the positive terminals were connected together to a common bus bar and similarly all the negative terminals to another bus bar 28 Flexible cables 4 AWG copper welding cable used to connec
58. ter 2 This involves weld parameter control and optimization and repeatability of the process itself The parameter settings required for CD welding of a joint can be optimized by setting an experimental design of the parameters at different levels and testing the quality of the welds Computer based instrumentation was used for monitoring the CD weld parameters CDW parameter profiles voltage and current are compared to determine the quality of the weld A data acquisition system was used to acquire the data process it to obtain the weld parameters and record the results for display and storage The plug in board used for this purpose is a high speed system Keithley Metrabyte DAS 50 with 4 multiplexed analog input channels and a sampling rate of 1 MHz 4 4 1 Signal Conditioner The current flow during a CD weld is on the order of 6 10 kiloamperes and the voltage used to charge the capacitor bank was about 30V DC These signals are not suitable for direct input to the DAS 50 card as it only operates within a range of 10V DC Hence the signals from the CDW process must be modified and made compatible for input to the DAS 50 A signal conditioning circuit is used to make this conversion from power signals from the process to low power signals for the card The signal conditioning circuitry used is shown in Figure 4 7 Figure 4 7 a shows the circuit used to attenuate voltage from the process obtained by direct connection across the electrodes
59. to Scale SHEET X X Figure B4 Clamps 3 4 20 UNC enm wo foescnnion jerk Automated Capacitor Discharge Welding System TOLERANCES XX 01 Dept of Industrial ond Manufacturing Engr OSU 005 Sri ANGLES 2 DWG Srinivas Patnalk DATE 06 06 96 Not to Scale SHEET X X Figure B5 Face Plate 99 7 16 20 UNF for mounting cylinder PROJECTION BRACKET 1 Aluminum Automated Capacitor Discharge Welding System QTY Dept of Industria and Manufacturing Engr OSU DWG Srinivas Patnaik DATE 06 06 96 Not to Scale SHEET X X Figure B6 Projection Bracket 1 DWG Srinivas Patnaik CHECKED Figure B7 Projection Bracket 2 PROJECTION BRACKET 2 DESCRIPTION MAT L Automated Capacitor Discharge Welding System Dept of Industrial and Manufacturing Engr OSU Mesum i DATE 06 06 96 SHEET X X 89 4 0 275 1 HS SON TOLERANCES XX 01 005 T vo pescano Automated Capacitor Discharge Welding System Dept of Industrial and Manufacturing Engr OSU DWG Srinivos Patnolk DATE 10 09 95 Figure B8 Fixture Base Plate 69 0 45 L Sliding Plate Aluminum f DESCRIPTION MAT L Automated Capacitor Dischorge Welding System Dept of Industria and M
60. to about 90 and the specimen was ejected from the molding die The next step was to obtain a flat semi polished surface of the specimen This was done by grinding the specimen on a series of abrasive grinding laps Rough intermediate and fine grinding was done using silicon carbide grinding laps of increasing grit size A Leco automatic polisher was used for this procedure The samples were mounted on a cartridge holder Circular cut silicon carbide paper was attached to the disk The disk was then rotated with the holder resting on it A constant load of 10 Ib was applied Excessive pressure was not applied to avoid deep scratches or the formation of disturbed metal The specimen was kept cool by a water spray Grinding was done for about 2 minutes The holder was then removed and was cleaned in an ultrasonic cleaner to remove any abrasive particles present on the holder or the specimen The specimen were then observed for scratches using a magnifying glass This procedure was repeated with increasing grit sizes for intermediate and fine grinding Four sets were performed using grit sizes of 120 240 320 400 and 600 42 Similar procedure was used for polishing the specimen with diamond dust as the abrasive Intermediate and fine polishing was done with particles sizes ranging from 6 0 25 microns All the specimens were photographed to a magnification of 50X and 200X using a Bausch amp Lomb Research II Metallograph This optical exami
61. ure stroking force was found to have caused cold welds that resulted in poor fusion conditions 17 None of the machines discussed above have automatic stud feed capability They are either manual or semi automatic An operator is required to run the process The load unload of the parts is not automated The operator s activities involve loading the parts into the fixtures and clamping them operating the electrical controller and triggering the weld After the weld the parts have to be manually unloaded The CDW machine at ALRC is based on the Erico Jones model and also uses gravity drop for bringing the electrodes together A high speed data acquisition system is used for monitoring the weld parameters and on line quality monitoring of the weld In CDW the actual weld time is a negligible fraction of the load unload cycle The manual tasks performed by the operator constitute most of the cycle time Hence an automated CDW system can also offer economic advantages by increasing productivity 3 RESEARCH GOALS AND OBJECTIVES Application of CD Welding to the joining of brass and silver electrical contacts will result in superior weld quality based on the quantity of cracks voids and inclusions as compared to resistance welding Also automated CDW will demonstrate a faster process cycle time as compared to the manual CDW system with no deleterious effects to weld quality This research has the following five main objectives l Design
62. utomated system However better weld areas were obtained with the manual unit Because clamping force in both the welding ram and the fixture is not great in the automated system the automated system is not very energy efficient Future methods of automated loading unloading should take this into consideration 6 2 Recommendations A system of higher speed and accuracy can shorten the process cycle time Positioners of higher speed and greater accuracy controlled by a sophisticated machine controller can be used to make a production machine The tooling and fixtures that were designed were for specifically handling the brass brackets and silver contacts The tooling and fixtures can be made modular and flexible to weld parts of differing geometry s and sizes Also copper tooling and fixtures can enhance the quality of welds made by the 54 automated system Semi automatic parts feeding was used in the set up The feeding of parts and unloading of the assembly can be fully automated The welding head developed was a gravity feed unit With the gravity feed unit the weld time can be calculated easily Pneumatic and spring feed mechanisms are used in other stud welders Bitterman Nelson etc DC motor powered ramming of the welding head has never been implemented Powered feed of the electrodes can be investigated for it s speed as well as it s impact on the weld time and the metallurgy The optimization program described in Section 4
63. utomation architecture for the process is shown in Figure 4 2 The machine control computer is used to control the process and the process monitoring computer is used for data acquisition and quality monitoring A serial communications port RS 232 on the machine control computer is used for communication with the Process Monitoring Machine Control 1 i Computer DAS 50 RS 232 motor position amp actuator commands DC Supply Mark IV Rhino Controller voltage current trigger Capacitors motor encoders output channels v P DC servo motors Cylinder Relays Z Axis and Pneumatics Main Circuit XY Table Vacuum Generator Signal conditioner T Figure 4 2 CDW Automation Architecture to controller The hardware and equipment associated with the automation is explained in Section 4 3 The controller is used to control the movement of the XY table Z axis and to interface with the actuators used to sequence the process The controller used for this automatic CDW system is Mark IV controller Rhino Robots Inc The DC motors on the Z axis and the XY table are directly connected to the motor ports on the controller The Mark IV unit controls the DC servomotors by sending encoder count for a required motor position The motor circuits include automatic o
64. verload protection and stall detection features Output channels are provided on the controller They are used for controlling the relays associated with the actuators Relays were used to control the pneumatics i e an air cylinder to drop the weld head and vacuum generator for button pick up The relay used for the air cylinder also triggers the digital pulse for the voltage and current measurement circuits The isolation contacts between the DC supply and the capacitors are also operated by a relay A 16 bit main processor is used for the overall operation of the system The controller has a built in teach pendant used to teach motor positions and is integrated with the operation of the main controller The teach pendant is used to teach the points of operation and is used to program the system It operates on an 8 bit controller and has an emergency button on it to halt the whole unit if any malfunction is detected 27 DAS 50 card on the process monitoring computer is used to measure the voltage and current values during the weld A detailed description of the computer based instrumentation and data acquisition is presented in Section 4 4 A signal conditioner is used to make the signals suitable for measurement by the DAS 50 Data collection is triggered when the welding head is released a digital pulse is generated vel ial i 7 weld manual setting of area of join voltage capacitance amp tip length fix
65. y fellow graduate student Rob Wilson for his friendship and the good times we had talking about football and the miseries of graduate school I appreciate the technical support from DOE s Albany Research Center and the administrative support from the staff of IME department OSU In addition 1 would like to thank the following persons who have helped me along the way Ramesh Dandu for his help with some drawings and during the preparation for the defense My cousin Venkat for his inspiration and support Bryan Feyerherm was always there to cheer me up whenever I was low Thank you all TABLE OF CONTENTS Page LTNTEODUC uuu eus arid i dro 1 1 1 Overview of Capacitor Discharge Welding 2 1 2 Investigative SUMMARY 42i oie detti bab edes Prata d 3 1 3 Thesis Organization nates Rata ie 3 2 LITERATURE REVIEW 525 555 QOEM descen ir tet inn 5 2 1 Capacitor Discharge Welding raices FU ERE pte E EH eau 5 Physical PTOCES Sis ete Wea ah eta 6 2 1 2 Process Parameter 1 10 2 1 3 Rapid noon 12 2 1 4 Advantages or CDW soie wor Ead tco aos ae 12 2 1 5 Limitations of CDW pt o 13 2 1 6 Defects in CD welds oret ette todo esee hei 14 P

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