"Pre-Peening" as Applied in the Welding of Copper Alloys

Abstract

In the welding of copper and its alloys we encounter several difficulties of somewhat different nature from those of ferritic steels. Weld metal cracking is one of these, which generally is hot cracking. This is largely due to the grain growth of crystals during welding. Refinement of them by heat treatment can not be expected, because most of the copper alloys undergo no trasformation of crystal structures. The cracking was found to be largely affected by alloy content. It was also shown that in TIG-welding, the controllable range of heat input was very narrow. Considering these, we may say that the measures to be taken for the prevention of cracking are (1) employment of filler material of dissimilar composition ; (2) lowering the amount of impurity in the base metal and the filler ; and (3) employment of other welding process. (1) is most effective and widely adopted in practice. In the case where the chemical analysis is to be the same for base and filler metals, measures (2) or (3) should be considered.
Besides these, there should be another means. Grain refinement of weld metal which solidifies from the fusion zone of base metal would be effective, if a certain measure were taken to make the grain size fine in the base metal adjacent to the fusion line. Cold working of welding groove by, for example, peening prior to welding would make the base metal recry stallize by subsequent heating of welding, make its grain size small.
In order to distinguish this process from the usually employed peening process, we christened it "Pre-peening", from which refinement can be expected not ont only in the heat affected zone of base metal, but also in the first loyer weld metal which is adjacent to the former.
The effect of Pre-peening on the reduction of cracking susceptibility as well as ductility in weldments was noticed in Cu-Zn, Cu-Si, Cu-Ni and Cu-Sn alloys. For alloys for which high preheat temperature was needed, this effect seemed to be canceled by increased heat input, which gave rise to grain growth before welding.
We may expect similar improvement in weld metal properties by use of Pre-peening in metals other than copper of similar metallurgical features.