Journal of Welding Science

Wire-arc additive manufacturing (WAAM) is a prominent technique for producing large metallic components due to its high deposition rate. Utilizing austenitic stainless steels in this process not only reduces production costs but also provides greater design freedom. Among these steels, SS310, known as heat-resistant steel in the industry, offers excellent oxidation resistance and high-temperature performance. However, it is highly susceptible to hot cracking during welding and additive manufacturing processes. In this study, the microstructure and mechanical properties of SS310 fabricated using WAAM with Cold Metal Transfer (CMT) and Gas Metal Arc Welding (GMAW) processes were compared. The results revealed that the CMT process, due to its lower heat input, effectively reduces the susceptibility of SS310 to hot cracking compared to the GMAW process. These findings emphasize the importance of selecting an appropriate process to achieve high-quality components and minimize structural defects.

In this study, 1 mm thick austenitic stainless steel 316L sheets were used for experimental testing. The experimental welding process was carried out using a Nd:YAG pulsed laser welding machine, and the welding simulation was performed using the SYSWELD software with a three-dimensional model for thermodynamic and mechanical analysis. The simulation results showed over 90% correlation with the experimental results. Analysis of experimental and numerical data revealed that at a constant voltage of 440 volts, decreasing the welding speed from 2 to 0.5 mm/s increased the overlap rate of pulses from 67% to 93% and the maximum average power density (EPPD) from 5963 to 21831 W/mm². Additionally, increasing the voltage from 440 to 480 volts at a constant speed of 1 mm/s raised the heat input from 114 to 138 J/mm and the weld depth from 0.56 to 0.66 mm. Due to the high cooling rate, the grain size of the weld metal became finer than the base metal (63% reduction in grain size). Two phases, austenite and ferrite, were observed in the weld metal, and the solidification mode was predicted to be FA.With an increase in welding speed from 0.5 mm/s to 2 mm/s at a constant voltage of 440 volts, the maximum tensile residual stress increased from 96 to 260 MPa due to reduced pulse overlap (from 93% to 67%), uneven heat distribution in the part, and the generation of thermal stresses. Furthermore, increasing the welding voltage from 440 to 480 volts at a constant speed of 1 mm/s caused the maximum tensile residual stress to rise from 124 to 152 MPa. The maximum hardness of the weld metal increased from 180 to 215 Vickers as the welding speed rose due to the prevention of carbon diffusion and an increased growth rate. However, with an increase in welding voltage and heat input (from 57 to 69 J/mm), the hardness decreased from 225 to 215 Vickers due to a reduction in thermal gradients and grain growth.

This research looks at how microstructure and mechanical properties change in resistance spot welds of QP980 advanced high-strength steel. It specifically focuses on the effects of zinc coating and how it influences weld nugget formation, mechanical properties, and fracture behavior. The study involved microscopic examinations, mechanical tests, and finite element simulations to determine the thermal history of different weld zones. A key finding was that rapid cooling during the welding process led to the formation of, metastable phases, such as martensite, in both the weld nugget and the heat-affected zone. A finite element model of the welding process was used to simulate heat distribution and analyze the microstructure in various weld regions. This model showed that reaching the peak temperature during four-pulse resistance spot welding is delayed. This delay, along with proper hold times, helps prevent the formation of voids. The simulated thermal history and the rapid heating/cooling conditions effectively predicted the evolution and transformation of the microstructure in different weld areas. It was found that the presence of a zinc coating, and the resulting reduction in electrical contact resistance, delayed the formation of the weld nugget at lower welding currents. However, at higher currents, the primary source of heat generation shifted from contact resistance to bulk resistance within the steel sheet. This led to larger weld nuggets in coated samples compared to uncoated ones. While uncoated samples showed higher weld nugget hardness (512 Vickers) and greater tensile-shear strength (with a maximum load-bearing capacity of 28.1 kN in uncoated samples versus 24 kN in coated samples), coated samples were able to achieve the critical weld nugget size for a change in fracture mode at lower welding currents (9 kA compared to 9.5 kA).

This study investigated the influence of resistance spot welding current intensity on the formation of liquid metal embrittlement (LME) cracks in galvanized advanced QP1180 steel. Galvanized steel sheets with a thickness of 1 mm were welded at currents of 6.5, 7, 7.5, and 8 kA. The results revealed that increasing the current significantly enlarged the weld nugget size, molten volume, electrode indentation, and the likelihood of LME crack formation. Microstructural analysis, elemental distribution, and crack characterization were conducted using optical and electron microscopy. The findings indicated that the weld zone microstructure primarily consisted of martensite, while the non-uniform distribution of zinc along grain boundaries facilitated the initiation and propagation of LME cracks. Cracks were predominantly observed at the periphery of the weld pool indentation and in the electrode-sheet contact area. This study demonstrates that controlling welding current intensity is a key factor in mitigating LME and improving the mechanical properties of joints in galvanized QP1180 steel. Optimizing welding parameters, particularly limiting current intensity, can prevent molten metal-induced cracking and enhance the durability and safety of automotive structures. Hardness profiling revealed peak hardness in the weld zone, followed by a gradual decrease toward the heat-affected zone (HAZ).