Study On Welding Process, Microstructure And Properties Of Mg-RE-Zr Alloy

In this paper, we mainly used tungsten inert gas welding(TIG welding for short) to realize the welding of squeeze state EW75(Mg-7Gd-5Y-1Nd-0.5Zr) Magnesium alloy. By using the method of optical microscopy(OM), scanning electron microscopy(SEM), X-ray diffraction(XRD) and other analytical tools, we researched the microstructure of EW75 magnesium alloy welded joints, and we tested the properties of welded joints by hardness test and universal tensile test. We introduced the preparatory work before weling as well as the selection of welding parameters, and studied the effect of welding parameters and postweld heat treatment(PWHT) system on microstructure and properties of EW75 magnesium alloy welded joints.It makes great effect on the quality of welded joints of the welding groove design, choice of wire materials and pre-weld cleaning work. In this paper, we chose V-shaped groove, and use butt method to realize the welding of EW75 magnesium alloy. The welding we use is the EW75 magnesium alloy extruded wire which has the same component as base material. Before welding it is necessary to remove oil and oxide attached to the groove and the wire surface, in order to avoid welding process being affected.The selection of appropriate welding parameters has a great influence on the welded joint microstructure and mechanical properties, in this paper we study the effect of welding current, welding speed and flow rate of argon on the microstructure and mechanical properties of EW75 magnesium alloy, and mainly focus on the welding current. The results show that EW75 magnesium alloy TIG welded joint consists of the base metal(BM), heat affected zone(HAZ) and welding zone(WZ). finer equiaxed recrystallized grains are distributed among BM, whose grain size is uniform, Compared with BM, the grain size of HAZ increases, while the WZ is quenched casting organizations, composed of fine grain size, and eutectic structure distributed on the grain boundary as network structure; The hardness curve presented typical “W” shape, and the microhardness of the FZ was the highest, then was the BM, while theHAZ was the lowest; with the welding current of 125 A, the welding speed of 160mm/min, the argon flow rate of 8L/min, the tensile strength of EW75 magnesium alloy welded joints reaches the higest, which is 89% of the base material strength. The fracture occurs at the WZ, and present to be intergranular fracture.We observe the tensile fracture of EW75 magnesium alloy and find that fracture generally occurs at grain boundaries, it mainly due to the grain boundary eutectic structure, therefore we decide to do solution treatment after welding. It can be see that coarse eutectic structure at weld zone could melt back after solution treatment, as time goes on and the solution temperature increases, the eutectic structure melt back more thoroughly, and at the same time, the BM grains clearly grow up.Aging treatment was carried out after solution. When the treatment is at 220℃ for 10 h, the hardness of the alloy reaches the maximum value and fine precipitates appear in the grain and grain boundaries. Comparing the mechanical properties of the two alloys in different treatment, the tensile strength of the alloy after direct aging is higher than that of aging after solution.