Low weld strength can be caused by anything which retards weld nugget growth or formation. It can also be caused by the condition of the materials being welded.
The size 4 and 5 designate the body size of the electrode. A #4 electrode has a ½” body diameter with a 3/16” face diameter. A #5 electrode has a 5/8” body diameter with a ¼” face diameter. These are sizes established by the Resistance Welding Manufacturers Alliance (RWMA) and are published in Bulletin 16 Section 18. All manufacturers produce to these standards here in the USA. There also is a size 6 and 7. Size 7 is rarely used in cap form. The charts below show data for RWMA caps and full size electrodes.
Generally all spot welding electrodes are made from a copper alloy. During the resistance welding operations if their tensile strength is exceeded they will bend or collapse and fail from the mechanical force being exerted on them. RWMA Fig 18.3 in the RWMA Manual 4th Edition, shows the relationship of electrode weld face size, applied force and maximum capability.
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