Microstructure and Mechanical Properties of Laser Welded Al–Mg–Si Alloy Joints

Abstract

In this paper, Al–Mg–Si alloy with the thickness of 2.5 mm was overlap welded by fiber laser. Optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, hardness tester, and electro-hydraulic servo testing machine were used for the microstructure observation and mechanical property examination. Specimens extracted from the welded plates were tested at room temperature for the determination of the tensile and fatigue properties of the welded joints. Results showed the columnar grains were formed in the fusion boundary, while equiaxed grains were formed in the fusion zone; the lowest hardness in the fusion zone is 72.0 HV, gradually increasing from the heat-affected zone to the BM. Tensile shear strength of the welded joint is 96.0 Mpa, which is about 25% of the tensile strength of base metal. The fatigue limit at 1 × 10⁶ conditional cycles is 22.5 MPa. The fractography showed intergranular and quasi-cleavage fracture. Bending deformation is related to the magnitude of the force. As the stress lower, the deformation become more severe.

Fig. 7 SEM images of fatigue fracture of WJ (a) rock candy pattern, (b) cleavage terrace and river pattern, (c) and (d) crack propagation pattern at different magnification.