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Resistance Welding by David Steinmeier microJoining Solutions – microTips™ 5563 Hallowell Avenue • Arcadia, CA 91007 Phone: 626-444-9606 • Fax: 626-279-7450 • Email: [email protected] • Web: www.microjoining.com Electrode Design for Small and Miniature Scale Resistance Welding By David Steinmeier Introduction Proper electrode design is critical for achieving BASE ELECTRODE MATERIALS consistent weld quality in the world of small and MATERIAL RWMA COMPOSITION CONDUCTIVITY Be-Cu miniature resistance welding. Each welding Nickel -2 or Cu/Cr application requires the correct electrode material, tip Platinum -3 or Cu/Co/Be High Stainless-Steel Glidcop Cu/ Al2O3 profile, and shank profile to ensure consistent weld Titanium quality, minimum electrode sticking, and maximum Brass -11 Cu/W Moderate electrode life. Bronze Copper -13 or W Copper Alloys Low -14 Mo Begin with the Application Silver To create the proper electrode design, you must begin Al2O3 Aluminum Oxide Cr Chromium W Tungsten Be Beryllium Cu Copper by carefully reviewing your application. Three Co Cobalt Mo Molybdenum application factors determine your electrode design: • Part base metal Part Plating Type and Thickness • Part plating type and thickness Plating causes electrode tip sticking and reduces • Part weld geometry electrode life. Use the thinnest plating possible. Plating such as solder, tin, and zinc have low Part Base Metal electrical conductivity, which dictates the use of Most small and miniature scale resistance welding RWMA-2, -3, or Glidcop electrode materials. Over applications involve welding one metal alloy to a time, tin, solder, and zinc plating alloy with these different alloy. These “mixed metal” applications copper based electrode alloys, fouling the tips and may require the use of two different electrode degrading the weld quality. Reduce electrode tip materials. An additional problem arises when a part contamination from these plating materials by: is a “combination” material. Many axial electronic • Experimenting with Cu/W, Mo, and W electrode component leads have a steel core, followed by a tips. Mo and W do not easily alloy with most copper clad layer, followed by tin plating. In most plating materials. cases, the electrical conductivity of the part base • Making the electrode tip area large in relation to metal determines the electrode material. the actual weld area contact. A larger electrode tip mass operates at a lower average temperature, Low conductivity (high resistivity) metals such as thus reducing electrode tip sticking. beryllium-copper, platinum, nickel, and steel require high conductivity electrode materials made from Part Weld Geometry special copper alloys. The high electrical The electrode tip design should ensure a constant conductivity allows weld heat to build up at the part- contact area between the tip surface and the part to-part weld interface while minimizing heat build up regardless of the electrode-to-part position. in the electrode tip. It is best to use a High conductivity metals such as copper, some rectangular copper alloys, and silver require low conductivity electrode tip to electrode materials made from copper-tungsten, weld round-to- molybdenum, or tungsten. The electrode tip round or round-to- generates extra heat, which flows into the part-to-part flat parts. interface, to augment the low interface weld heat. Avoid using a round electrode tip since Here is a partial list of small and miniature scale the contact area will application metals and their recommended electrode change, depending materials. For a more detailed list, contact the on the electrode-to- American Welding Society (AWS) at www.aws.org. part position. microTips™ ©2006.07 microJoining Solutions – microTips™ 5563 Hallowell Avenue • Arcadia, CA 91007 Phone: 626-444-9606 • Fax: 626-279-7450 • Email: [email protected] • Web: www.microjoining.com Electrode Design Factors Electrode Tip Hardness Good electrode design requires optimizing the Utilize Cu/W, Mo, and W in place of RWMA-2, -3, following factors for maximum life in relation to the and Glidcop to improve overall mechanical strength actual part weld geometry. Minimal electrode-to-part and durability when designing very small electrode sticking correlates with maximum electrode life. tips. Welding round-to-round or round-to-flat parts Note that the relationship between electrode tip can be very brutal on electrode tips made from factors and electrode life is non-linear. RWMA-2 or Glidcop. These soft electrode materials grove rapidly, which changes weld quality. Consider using a pre-grooved electrode tip shape to reduce electrode wear and extend electrode life if your process control capability can maintain the groove. Electrode Tip Length Most electrodes are designed with long tips in relation to the tip area. This design logic assumes that the tip can be economically resurfaced many times before the entire electrode is replaced. In reality, weld Electrode Tip Area quality decreases very rapidly Use a tip area that is larger than the actual contact with increasing tip length. Keep the tip length less tip-to-part weld area. The non-contact tip area than two times the largest tip width or diameter. dissipates the waste weld heat, lowering the average tip temperature, which in turn increases tip life. Electrode Tip Weld Current Density An electrode tip can only accommodate a finite Electrode Tip Conductivity amount of weld current through its tip area. Severe When utilizing low conductivity electrode tips made electrode sticking, tip area deformation, and tip from Cu/W, Mo, and W, silver braze these tip length bending are obvious signs of excess weld materials to a RWMA-2 or Glidcop shank to increase current density in relation to the tip geometry. the absorption of waste weld heat. Avoid using long solid Electrode Tip Weld Pressure rods of Mo and W at weld There is an optimum weld pressure for each currents above 1 KA and application. Insufficient weld pressure causes high weld duty cycles. electrode degradation by spitting out pieces of the Long, small diameter, low electrode tip and parts. Excessive weld pressure conductivity electrode rods mechanically deforms the electrode tip. Too little or generate a lot of waste heat, too much weld pressure negatively affects weld which then flows into the tip quality. and reduces electrode life. Conclusion Also, avoid using Mo and Good electrode design is a series of compromises W tips that are pressed into that can only be verified by performing an electrode a RWMA-2 or Glidcop life test under real production operating conditions. shanks. Under high weld current and duty cycle Acknowledgments: operating conditions, the Computer models were provided by Steinmeier electrical resistance between Design, Hawthorne, California, USA the shank and tip increases over time, negatively affecting the weld quality. microTips™ ©2006.07 .
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