Porosity, Element Loss, and Strength Model on Softening Behavior of Hybrid Laser Arc Welded Al-Zn-Mg-Cu Alloy with Synchrotron Radiation Analysis

The severe strength loss of hybrid welded Al-Zn-Mg-Cu alloy joints such as 7075-T6 has been characterized using high-resolution synchrotron radiation X-rays and theoretical modeling. To elucidate the physical causes of this static strength change, the distribution of strengthening elements such as Zn and Cu and the three-dimensional gas porosity were mapped. The interesting findings included the following: 1) Hybrid welds only reach approximately 53% of the ultimate tensile strength of the base phase, and the lowest strength is situated in the central fusion zone; 2) because of the excessive evaporation of elemental Zn and the significant inverse segregation of elemental Cu in cenral fusion welds, the major strengthening elements gather near the heat-affected zone; 3) the elastic modulus of hybrid welds is slightly larger than that of their base alloys, probably as a result of the inhomogeneity in their chemical composition, microstructure, geometry, and residual stress; 4) the pore size ranges from below 0.01 mu m to approximately 107 mu m, and it is modeled by the Schwartz-Saltykov method to reveal its damaging effects on the static strength; 5) a strength model of 7075-T6 welded butt joints is established to correlate with the elastic modulus, porosity, and heat inputs, and it coincides well with the computational and experimental results; and 6) in principle, the porosity has little influence on the static strength of hybrid welds, and the modified strengthening structure markedly dominates the overall mechanical properties.