Introduction: High power laser welding produces large weld pools and deep penetrations, resulting in spatially variable thermal histories, solidification conditions, and stress states, which enhance solidification cracking susceptibility of alloys that are readily weldable under traditional arc welding or low power laser welding. For example, in high power laser welding of a creep resistant nickel alloy, Inconel 740H, horizontal solidification cracking was consistently observed across the width of the fusion zones at depths between 70 and 80% of the overall weld root. To elucidate the mechanisms driving these unique solidification cracking behaviors under high power laser welding, a comprehensive understanding of the role of composition, process condition, and stress state is required. Existing models almost exclusively focus on the effect of alloy composition and are unable to capture the effect of different welding conditions on solidification cracking behaviors. Validated numerical thermo-mechanical models were employed to capture the effect of welding conditions on the spatially variable thermal history, solidification conditions, and stress state under high power laser welding. The integrated composition-process-stress model provides a comprehensive understanding of solidification cracking under different welding conditions.
Experimental Procedures: High power keyhole mode bead-on-plate laser welding was conducted on 12.7 mm thick plates of Inconel 740H and Inconel 690 under welding speed of 12.7 mm/s and laser powers of 2.5 kW, 5 kW, 7.5 kW, and 10 kW, and the laser radius was 0.524 mm. Transverse cross sections of the laser welds were prepared using standard metallographic techniques to reveal the microstructure of the fusion zone and the base metal and characterized using standard microscopy techniques. Computational thermodynamic simulations were conducted to investigate the effect of alloying elements on the solidification paths of the alloys. A well tested three-dimensional heat transfer and fluid flow model was coupled with another well-tested keyhole model to calculate the temperature fields and solidification conditions during high power laser welding. These data were then imported to the stress calculation model in Abaqus® to calculate the stress states at locations which were most susceptible to solidification cracking.
Results and Discussion: Horizontal solidification cracking across the fusion zone was observed in Inconel 740H welds at depths between 70 and 80% of weld root, while no such defect was found in Inconel 690 welds. Scheil simulations indicated that Inconel 690 has a smaller solidification temperature range, which is around 77 K compared with 244 K for Inconel 740H. The more than three times smaller solidification temperature range of Inconel 690 determined it has a much smaller liquid/solid two phases region, which increases the solidification cracking resistance. A well accepted solidification cracking susceptibility criterion dT/d (fs1/2) near fs1/2=1 also predicted a higher solidification susceptibility of Inconel 740H.
The calculated solidification parameters showed significant variations under different welding conditions. Thermal gradient generally increased from top to bottom of the weld pool and showed lower values under higher laser powers. Solidification rate was largely determined by the local solidification direction, which can vary significantly with the local solidification front profile. The nearly vertical solidification front unique to high power conditions resulted in local high solidification rates close to welding speed, which can enhance elemental segregation and increase solidification cracking susceptibility under high power conditions.
To understand the occurrence of horizontal cracks, vertical stress state along locations with solid fraction of 0.9 was extracted from the stress calculation model. The vertical strain rate generally increased from top to bottom of the weld due to the increasing temperature gradient. An experimentally determined critical strain rate, which represents the minimum strain rate to initiate solidification cracking for nickel alloys, was attained at specific depths. Vertical stress was found to transit from tensile to compressive at some other depths. These two depths defined a range of depths where solidification cracking is predicted to occur, which matched well with experimentally measured crack depths.
1. High power laser welding produced deep penetrations and unique weld pool profiles, leading to variable thermal histories and solidification conditions particularly along the weld pool depth, which enhanced the solidification cracking susceptibility of a creep resistance nickel alloy, Inconel 740H, and promoted horizontal cracking at depths between 70 and 80% of weld root.
2. Driven by the variable process conditions, the vertical stress state, which is responsible for the horizontal cracks, displayed spatially variable distribution under high power laser welding. The vertical strain rate along solid fraction of 0.9 generally increases from weld top to bottom due to the increasing temperature gradient. The vertical stress along the same location transits from tensile to compressive near the bottom of the weld caused by the constraint from the cold base metal.
3. The complex stress state distribution under high power laser welding can vary with welding conditions, which impacted the crack susceptibility, locations, and orientations. Horizontal solidification cracking was predicted to occur at depths with vertical strain rate higher than the critical strain rate (0.012 1/s) and vertical stress simultaneously in tension at solid fraction of 0.9, which was validated by the experimentally determined cracking depths.