Corrosion Behavior And Protection Of Several Different Welded Joints Of Magnesium Alloy

Magnesium is the most widely distributed element in nature,and has a high specific strength and stiffness, higher elastic modulus, good damping, good thermal conductivity, good electromagnetic shielding and anti-radiation, and good size stability. It is easy cutting and recycling, and does not pollute the environment. It was widely used in aerospace, petrochemicals, atomic energy, machine manufacturing, automobile and other industries. However, the welding of magnesium and magnesium alloy is more difficult and has become a bottleneck in the development of magnesium alloys. The traditional welding methods easily cause pores, cracks, inclusions, deformation, and greater residual stress shortcoming. In addition, magnesium has stronger chemical activity and electrochemical activity, so the oxide film formed on the surface has many porous and its corrosion resistance is poor, particularly in humid environments and the conditions of Cl - existence. The chemical composition and properties of welded joints are different from the base material, which makes the weld corrosion protection even more complex. Therefore, the welding process of different welding methods of the magnesium alloy, as well as the study on weld corrosion protection is particularly important.In this paper, MIG welding process, microstructure and mechanical properties of AZ31B magnesium alloy added with 0.5%Ce were systematically studied. The welding parameters were optimized with orthogonal method, and the best process parameters match was obtained. The main factors affecting MIG welding include the welding current, arc voltage and welding speed. When the arc voltage and welding speed are constant, with the increase of the welding current, the penetration depth increases, and the width and heigth of weld also increase. When the welding current is too small, the incomplete penetration defects will appear. When the welding speed and current are constant, the weld becomes wide and flat with the increase of the arc voltage. When the arc voltage is too high, the incomplete penetration and undercut easily formed, and the protective effect of argon gas is not well. When the arc voltage decreases, the weld becomes narrow and deep. When the arc voltage is too low, the wire is easy to run into tungsten and causes a short circuit, resulting in the burning of the tungsten electrode and cliping tungsten. When the welding current and arc voltage are constant, the welding speed is higher, and the droplet transition is quick, so that the width of the weld increases, the surface ripple of the weld is not uniform, the welding is instable, and the splash is bigger. When the welding speed is slow, the heat input is large, the overheating of the weld and heat affected zone and the burning of alloying elements are easy to produce. The welding parameters were validated with the process parameters obtained by orthogonal test. The tensile strength was 254.03MPa and reached 94.03% of the base metal; the elongation was 14.75% and reached 82.94% of the base material; the fracture position was in the heat-affected zone.The microstructure of MIG welding was studied. It can be found that the microstructures are different in different regions of the cross-section of a welded joint. The weld zone is small equiaxed grains, the heat-affected zone is coarse large overheating microstructure. After MIG welded joints were annealed. the grain sizes of the weld and heat affected zone were smaller than those before heat treatment.The mechanical properties of MIG welding were studied. it can be found that the fracture position of the defect-free welded joint is in the heat-affected zone, and the angle between the fracture surface and tensile direction is 45°. The tensile strength is more than 90% of base metal. After annealing, the tensile strength of the defect-free weld reduced, and the elongation increased.The corrosion resistance of magnesium alloy weld is poor. In this paper, the immersion corrosion and electrochemical corrosion are used to study the corrosion resistance of the welded joints of TIG welding, MIG welding and friction stir welding of AZ31B magnesium alloy. The micro-arc oxidation and heat treatment are used to improve the corrosion resistance of the welded joints.The corrosion rate of the weld was studied using immersion corrosion. It can be found that with the increase of time, the corrosion rate increased in each region of the welded joints of TIG welding, MIG welding and friction stir welding. For TIG welding, MIG welding and friction stir welding, the weld zone has the minimum corrosion rate, the heat affected zone has the highest corrosion rate, and the corrosion rate of the base material is between the weld zone and the heat affected zone. Compared the weld and the heat affected zone of TIG welding, MIG welding and friction stir welding, it can be found that the corrosion rate of the weld zone of MIG welding is the lowest, the corrosion rate of the weld zone of FSW is the highest, and the corrosion rate of the weld zone of TIG welding is between MIG and FSW. The corrosion rate of the heat affected zone of FSW is the lowest, the corrosion rates of the heat affected zones of TIG welding and MIG welding are basically the same.The electrochemical corrosion experiment was carried out. The results show that the corrosion potentials of the weld zones for TIG welding, MIG welding and FSW are all the highest, the corrosion potentials of the heat affected zones are the minimum, and the corrosion potentials of the base material are between the weld zoes and the heat affected zones. According to the polarization curves of the weld zone and heat affected zone of TIG, MIG and FSW, the corrosion resistance of the weld zones of TIG welding and MIG welding is slightly better than that of the friction stir welding. For the heat-affected zone, the corrosion potential of FSW is about 50mv higher than that of TIG welding and MIG welding.The heat treatment and micro-arc oxidation of the weld of TIG welding, MIG welding and FSW were carried out, and the polarization curves were measured. It can be seen that the corrosion potential of the weld is shifted to the right after the micro-arc oxidation and heat treatment were carried out. The corrosion potential increases about 100mv after micro-arc oxidation; the corrosion potential increases about 200mv after heat treatment. It indicates that the micro-arc oxidation and the heat treatment both can improve the corrosion resistance of the weld, and the effect of the heat treatment is better.