Study of Mechanical Properties of AZ91 Magnesium Alloy Welded by Laser Process Taking into Account the Anisotropy Microhardness and Residual Stresses by X-Ray Diffraction

The objective of this investigation was to study the mechanical properties of a magnesium alloy welded by a CO₂ laser. Residual stresses were measured by X-ray diffraction. They were calculated by the classic sin² ψ method in the isotropic zones by using the orientation distribution function (ODF) in the textured zones. The results demonstrated that laser welding results in the formation of several different zones with different microstructural and mechanical properties. Welding principally leads to a reduction in grain size and a new distribution of phases. The most remarkable observation was that of a superficial layer on the surface of the welded zone. This layer has a marked crystallographic texture, a reduction in the level of aluminum, and an elevated microhardness. These characteristics disappear at a depth of 200 μm under the welded zone. These modifications can be explained by the nature of the solidification, which occurs under nonequilibrium conditions resulting in an equiaxial columnar transition. This transition is evident also within the profile of residual tensile stresses, which are at their maximum at the interface between the superficial layer and the rest of the welded zone. These results are explained by the anisotropic properties of the textured layer in relation to the plasticity.