Hybrid laser/arc girth welding of 304L stainless steel tubes, part 1 - Pore mitigation, thermal analysis and mechanical properties

Single-pass deep penetration hybrid laser arc welding (HLAW) of 8 mm thick AISI 304L stainless steel tubes with diameter of 280 mm was conducted by using a high-power disk laser combined with a gas metal arc welding machine. First, the effects of scanning speed as well as heat source arrangement on the weld integrity and pore mitigation were investigated. Then, the thermal simulation of the pipe girth welding using ANSYS was carried out to identify the heat distribution throughout the weld. Finally, the microstructural and mechanical attributes of the optimal weld were characterized by scanning electron microscopy (SEM), micro-hardness, and tensile test, respectively. The results showed that high welding speed along with the application of arc-push mode not only generated a visually sound weld but also significantly mitigated the number of keyhole-induced or gas-caused pores. The proposed heat source model for thermal simulation resulted in isotherms that were relatively consistent with the cross section of the weld for the arc-push and arc-pull modes. From the microstructural point of view, it was observed that the amount of δ-ferrite that was decomposed at the arc zone of the weld was proportional to the cooling rate. Therefore, the arc-pull mode with the higher cooling rate led to the formation of a large amount of δ-ferrite. Micro-hardness analysis of the optimal joint revealed neither softening nor hardening inside the FZ and HAZ. The optimal joint showed relatively uniform mechanical properties throughout the weld. Tensile test results showed that all joints failed in base metal confirming a stronger weld compared to the base metal.