Effects Of Welding Heat Input On The Microstructures And Mechanical Properties Of Tungsten Inert Gas AZ Magnesium Alloys Joints

The magnesium alloy has a lot of merits such as big elastic modulus, low density, high specific strength, good thermal diffusivity, good cushioning effect and excellent capability of withstanding dynamic impact loading. Therefore, it has a extensive application prospect in automobile industry, aircraft industry and electronic informatio industry. However, the magnesium alloy has very limited slip systems in the room temperature because it has a hexagonal close packed crystal structure, which leads to a very poor elastic deformation ability. It leads the plasticity forming process for fabricating complicated magnesium alloys components to be diffcult. Therfore, the welding technology becomes an effective method to fabricate the complicated magnesium alloys components with low cost. However, as the mangesium alloy has some drawbacks such as low melting point, being easy to burning loss and large deformation under heating, the welding technology of magnesium alloys has always been a research hotspot and a diffcult subject in both the academia and the manufacturing industry. The AZ series deformation magnesium alloys were the research subject in this work. Different TIG welding procedures were made for magnesium alloys plates with different thicknesses. The mircostruture characterisation, tensile experiments and microhardness tests were conducted to systematically investigate the influence of welding heat input on the macro morphology, the microstructure and the mechanical properties of TIG welded magnesium alloys joints. Besides, the software SYSWELD was applied for the finite element analysis for the AZ61 magnesium alloys TIG welding process, clarifying the internal relationships between the welding heat input and welding defects in double-sided TIG welding. The analysation was given on the basis of experimental results, the main conclusions are as follows:A double side gas tungsten arc welding(GTA) of AZ61 magnesium alloy plate and finite element method(FEM) were conducted to investigate the effect of partial melting on the formation of stress cracking in the welded joint. The surface microstructure of cracking formed in the welded seam was characterized by scanning electron microscopy, and the software SYSWELD was used to simulate the thermal history and residual stresses distribution of the weldment. In the test process, under a relatively low residual stress(simulated as 45 MPa in the simulation) compared with the ultimate tensile strength of AZ91 D, a cracking was formed in the welded seam at a high temperature(simulated as 500 °C in the simulation). The experimental and simulation results suggest that the fusion of the low melting point eutectic compound β-Mg17Al12 in welded seam can decrease the tensile strength of magnesium alloy weldment, promoting the generation of stress cracking under significantly low residual stresses. Cracking is a major reason for the failure of magnesium alloy joint. It would be often caused by stress concentration.The effects of welding current on macro-morphologies, microstructures and mechanical properties of nano-particles strengthening activating flux tungsten inert gas(NSA-TIG) welded AZ31 magnesium alloy joints were investigated by scanning electron microscope(SEM), energy dispersive x-ray(DRX) spectrometer observations, microhardness and tensile tests. Activating flux Ti O2 and nano-sized Si C particles were employed in this study. With the same welding parameters, the NSA-TIG welded AZ31 magnesium alloy joints had deeper weld penetrations, narrower weld widths, less welding defects and smaller α-Mg grains in FZ than those produced by O-TIG welding. This effect was intensified by increasing welding current. Obeying the Stokes’ s Law, the nano-Si C particles were driven to move along the path of the liquid metal flow(Maragoni Flow) in the welding pool during the NSA-TIG welding process. As a result, they were mainly distributed in the middle and bottom of welding pool after cooling to the room temperature. The Si C particles strengthened the NSA-TIG welded AZ31 magnesium alloy joints through grain refinement strengthening and dispersion strengthening. Higher welding currents increased the Si C particles amount in welding pool and therefore improved the joint strength further except for the case when welding current exceeded 115 A.