Crack-free pulsed laser welding of 7075 aluminum alloy: Microstructure, crack prediction, and mechanical properties

Abstract

The 7xxx series aluminum alloy, renowned for its ultra-high specific strength, has become a crucial material in the structures of aircraft, high-speed trains, and vehicles. However, the welding of 7xxx aluminum alloys presents significant challenges, primarily due to the occurrence of severe weld cracks and softening behavior. Pulsed laser welding with a low duty cycle has emerged as an effective technique to address these issues, due to its flexible parameters and low heat input. Nevertheless, the microstructure evolution, the generation and propagation of cracks in 7xxx aluminum alloy during pulsed laser welding is still unclear. In this study, pulsed laser welding of 7075 aluminum alloy with varying welding parameters was conducted and the typical microstructure of the welded joint was characterized. A new method to quantify cracking susceptibility was proposed based on a non-steady cracking model, facilitating the analysis on the relationship among crack shapes, cracking susceptibility, and welding parameters. By calculating cracking susceptibility, the condition of cracks can be predicted, aligning closely with experimental results. Crack-free welded joints were obtained from the welding with filler strips cut from the same 7075 sheet. X-ray computed tomography confirmed the quality of these joints, demonstrating a strong correlation with the calculated cracking susceptibility. Mechanical property tests indicated that the average microhardness of the weld reached 134.0 ± 2.1 HV_0.2, and the ultimate tensile strength of the welded joint reached up to 444 ± 16 MPa (76% of the base metal), highlighting the potential of pulsed laser welding for 7075 aluminum alloys.