Filler-Deficient Zone in Arc Welds

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Dissimilar filler metals refer to filler metals that differ in composition from that of the workpiece. They are routinely used in arc welding to prevent cracking and to obtain desirable weld properties. The filler-metal droplets mix with the melted base metal by strong convection in the bulk weld pool and the resultant weld is uniform in composition except in the region near the base metal where the weld metal composition differs from that of the filler metal. The formation of this filler-deficient zone (FDZ) has been attributed to weak weld-pool convection and hence poor mixing near the pool boundary. Fundamental understanding is needed on how dissimilar filler metals affect the weld-pool solidification process and why the melted base metal can solidify quickly without mixing with the surrounding bulk weld pool. The objective of this research is to better understand the FDZ formation in arc welds. The tasks include: (1) proposing a mechanism for the FDZ formation by first clearly defining the pool boundary with the liquidus temperatures of the base metal (TLB) and the weld metal (TLW); (2) welding with selected dissimilar filler metals that can make TLW significantly different from TLB; (3) examining the microstructure and composition profiles across the resultant FDZ; (4) calculating TLB and TLW to help explain the FDZ formation observed; and (5) investigating the effect of the welding parameters on the FDZ formation, such as the heat input, the welding speed, and the shielding gas. Aluminum and Ni-Cu alloys are welded in the initial stage of the investigation. Aluminum alloys are selected because in view of the absence of solid-state transformations the solidification microstructure is much easier to understand than in steels or stainless steels and reliable database and computer software are available for calculating TLB and TLW in aluminum alloys. Ni-Cu alloys are selected because the simple isomorphous Ni-Cu phase diagram offers a wide range for varying in TLB and TLW. The intellectual merit of the study includes advancing the fundamental understanding of welding with dissimilar filler metals and providing a basis for better understanding of the FDZ formation in steels and stainless steels, which are susceptible to hydrogen cracking and stress corrosion cracking.The broader impact of the study includes improving weld homogeneity through a better understanding of FDZ formation. This is significant in view of the increasing use of aluminum in automobiles to reduce weight and air pollution. The educational components include the training of graduate students in welding science and the related use of solidification computer software and databases. Useful materials are provided for teaching welding metallurgy, involving undergraduate students in welding, and demonstrating the welding process to K-12 students and their parents.