This term usually refers to dilution of weld metal with metal melted from the base metal. To make a good weld, the joint edges of the base metal must be melted and intentionally mixed with the weld metal. This melted weld metal “picks up” metal from the base metal.
This term also applies to resistance welding, in which the electrode tips partially melt and weld to the base metal, and pick up metal from the other. This type of pickup results in degraded welds and can be avoided by correct control of the welding parameters, and by reshaping worn electrode tips to their original dimensions.

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