Study On Microstructure And Properties Of Mg/Al Joints By Filler Wire GTAW

Magnesium alloy has huge reserves in the earth, and some excellent performences such as lower density, higher intensity, environmental protection and other advantages. It is already widely applied in automobile, military, electronics and other fields in the current, and has an excellent development potential in the furure. The development of magnesium alloy is determined by the study of connection of Magnesium alloy to itself and other materials. If it can be realized that magnesium and aluminum alloys are well welded, it can reduce structural parts’weight, give full play to the two light metals’ respective performance advantages, and promote the further development and application of magnesium and aluminum.AZ31Magnesium and7005aluminum alloy fittings are welded by means of argon tungsten arc welding with5filler wires and got some technology parameters in the paper. The microstructures of weld joints using different alloy welding wires and the influence on the properties by different alloy composition are analyzed. The cause of precipitated phase and welding cracks as well as their effects on welding joints are studied, and preventive measures of cracks are given.The microstructures of joints are observed by optical microscope and scanning electron microscope, and the energy spectrum analyzer was used for composition analysis. Mixed dendrite grid distribution microstructure of alpha Al, Mg2Al3and Mg17Al12are found in weld area on magnesium side by pure aluminum wire ER1100and Al-Mg alloy wire (ER5183and ER5356), and microhardness value is more than200HV50g; columnar crystal, primary Si and Mg2Si eutectic are found on the magnesium side weld zone by the Al-Si alloy wire (ER4043and ER4047) and the hardness value is about40-80HV50g. The microstructure in weld center is mainly equiaxed dendrite with a mixed eutectic structure of a-Al, Mg3Zn3Al2and CuMgAl2supposedly, the intergranular precipitation phase may be AlMnFeSi and FeMnAl6, and the microhardness is100-200HV50g; the aluminum side weld zone is mainly columnar crystal, aluminum solid solution, and the microhardness is60-100HV50g. The weld zone shows uniformly distributed tissue and excellent mechanical properties using Al-Si alloy wires.The phenomenon that the grains’ size grows significantly and the precipiates increased was found on both sides of the heat affected zone of Mg/Al welded joints. The magnesium side heat affected zone’s precipitates are β-Mg17Al12and Mg2Si and the aluminum side heat affected zone’s precipitates are AlZnMgCu and CuMgAl2. Precipitated phase has retarding effect on dislacation in alloy plastic deformation process, improves the strength of the alloy significantly, plays an important role in the increasement of the Mg, Al alloy and their weld seam’s strength.The hardness of Mg/Al welded joints using5kind of wires were measured with microhandness tester and the results show that magnesium side’s fusion zone is the weak part of the joint:hardness increased significantly, the tendency of hardness and brittleness is obvious, and the area is prone to some weld defects such as cracks and pores; the fusion zone on the aluminum side is in good conditions. The Magnesium lateral fusion zone’s hardness becomes bigger with the increasement of Mg and Si elements’content, and more attention should be paid to the Mg and Si elements’content in the wire selection process. Priority should be given to ER5183wire because it’s magnesium side weld fusion zone shows lowest microhardness and medium width,Line scanning was done using electron probe micro-analysis (EPMA) on magnesium side fusion zone with different fillers and the results demonstrated a similar regulation of the element distribution. Magnesium side heat affect zone is mainly Mg element, and Al element is rarely diffused to magnesium base material. On the interface of this area, Mg element reduces sharply with Al element significantly increases; and both elements remaine stable in the fusion zone:means that a stable Mg-Al intermetallic compound is generated. The element contents gradually tranfered to the weld seam’s level as the test area passed over the fusion zone.The coordination to magnesium base material should be considered in the weld wire selection process. The weld wire should have a higher Si content to make sure that the weld area can form ample Mg2Si strengthening phase without over Mg element, so the weld wire with a low content of magnesium is preferred for the brittle tendency’s reduction in the fusion zone. In order to optimize the Mg/Al joint property, a transition layer, like Zn element, between two base metals, and other technology means such as the magnetic pulse current and laser-GTAW hybrid power are considered.Heat cracks generated near the magnesium side weld fusion zone where lots of rigid Mg-Al intermetallic compounds generated with pure aluminum and Al-Mg alloy wires. Crack initiation point locates in the junction between intermetallic compounds and eutectic structure, which has a relatively lower hardness, and the carak extends along the direction of hardness increases with the area near the fusion zone softer than the weld zone, this has a restriction on the mode of the crack’s expansion.