S. Kou, Y. Yang, University of Wisconsin, Madison, WI
Filler metals different from the base metal in composition, called dissimilar filler metals, are often used to reduce cracking or develop desired weld-metal properties. Macrosegregation in welds made with dissimilar filler metals can degrade the weld quality. In the present study the basic concepts of heat transfer, fluid flow and mass transfer leading to macrosegregation in arc welds made with dissimilar filler metals are discussed first. Four different mechanisms of macrosegregation in such welds are then presented. Macrosegregation can occur near the fusion boundary even when the dissimilar filler metal mixes completely with the bulk weld pool. Mechanism 1 is for TLW < TLB and Mechanism 2 for TLW > TLB, where TLW is the liquidus temperature of the weld metal and TLB that of the base metal. These two different mechanisms can result in two distinctly different forms of macrosegregation. With TLW < TLB a filler-deficient beach can form along the fusion boundary and the beach is often discontinuous. With TLW > TLB, on the other hand, a filler-deficient beach can form along the fusion boundary and the beach is often continuous and intruded by the weld metal. Macrosegregation can occur near the weld bottom well within the fusion boundary when the dissimilar filler metal mixes partially with the bulk weld pool. Mechanism 3 is for TLW < TLB and Mechanism 4 for TLW > TLB. These two different mechanisms can also result in two distinctly different forms of macrosegregation. With TLW < TLB a filler-rich metal can exist near the weld bottom, with numerous islands of partially mixed base metal scattered in it. With TLW > TLB, on the other hand, a filler-rich zone can form near the weld bottom and intrude into the continuous filler-deficient beach along the fusion boundary. Evidence supporting these mechanisms will be presented.
Summary: Filler metals different (dissimilar) from the base metal in composition are often used but can cause macrosegregation and degrade the weld quality. Basic concepts of heat transfer, fluid flow and mass transfer leading to macrosegregation in such welds are discussed. Four mechanisms of macrosegregation are presented and verified.
