Laser weld formation and microstructure evolution in stainless steel alloys

Variations in laser beam welding parameters can significantly influence weld pool shape and microstructure formation in stainless steel (SS) alloys. Fusion zone development using high travel speeds and low overall heat input promotes rapid solidification and high cooling rates. The difference in solidification rate within the weld pool is strongly influenced by changes in the weld pool geometry, which is directly dependent on process parameters. Therefore, a complete understanding of weld pool geometry as a function of process conditions is crucial to predicting solidification phenomena and avoiding weldability-related issues such as solidification cracking or an undesirable ferrite/austenite phase balance. In this work, the effect of laser weld parameters on the transition from conduction to keyhole mode welding was determined and related to microstructure formation in six SS alloys. The alloys had WRC-1992 Cr_eq/Ni_eq ratios ranging from 1.7 to 2.8. The 1.7 Cr_eq/Ni_eq alloy was a commercially available 304L SS, while the 2.7 Cr_eq/Ni_eq was a 2205 duplex stainless steel. Compositions were also tested between these extremes. Both transverse and longitudinal view sections were evaluated to determine solidification behavior and phase fractions. Electron backscatter diffraction patterns were used extensively to determine phase fractions. Relationships among penetration mode (conduction vs. keyhole), composition (Cr_eq/Ni_eq), and microstructure (solidification mode and phase fraction) are presented and compared to rapid weld solidification diagrams from the literature. The results presented here provide a more complete picture of rapid solidification phenomena associated with laser welds in SS alloys over a wide range of compositions.