Macrosegregation in dissimilar-metal fusion welding

Solute segregation on a macroscopic scale in a weld between two dissimilar metals or alloys has long been recognized, but fundamental understanding of macrosegregation in dissimilar-metal welding is still lacking. Two mechanisms for macrosegregation were proposed based on the liquidus temperature of the bulk weld metal, T_LW, relative to the liquidus temperature of metal 1, T_L1, and the liquidus temperature of metal 2, T_L2. According to the mechanisms, two distinctly different macrosegregation features can form. A "peninsula" of an unmixed metal 1 can form if T_LW < T_L1. On the other hand, a "beach" of unmixed metal 2 irregular in shape can form if T_LW > T_L2. To verify the mechanisms, a pure Cu sheet was butt welded to a low carbon steel sheet by gas-tungsten arc welding without a filler metal. Composition measurements were conducted inside and across the weld metal. A peninsula of unmixed steel and an irregular-shaped beach of unmixed Cu were observed, which verified the mechanisms. In addition, the bulk weld metal exhibited a layered structure caused by undercooling of the bulk weld pool into a metastable miscibility gap in the Cu-Fe phase diagram. Macrosegregation in previous studies on laser- and electron-beam welding of Cu to steel or stainless steel was discussed in light of the findings in the present study. -- Graphical abstract: Solute segregation across a weld between two dissimilar metals or alloys has long been recognized, but fundamental understanding is still lacking. Two macrosegregation mechanisms were proposed based on the liquidus temperature of the bulk weld metal T_LW relative to those of metal 1 (or alloy 1) T_L1 and metal 2 (or alloy 2) T_L2. A "peninsula" of an unmixed metal 1 can form if T_LW < T_L1, while a "beach" of unmixed metal 2 irregular in shape can form if T_LW > T_L2. The mechanisms were verified in Cu-to-steel arc welding. The bulk weld metal showed a layered structure caused by undercooling of the bulk weld pool into a metastable miscibility gap in the Cu-Fe phase diagram. These findings were used to explain macrosegregation observed in laser- and electron-beam welding of Cu to steel or stainless steel. Display Omitted