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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.

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