Microstructures and Properties of Laser Al Alloying on AZ31 Magnesium Alloy

In order to improve corrosion and wear resistance of magnesium alloy, Al coating was prepared on the AZ31 magnesium alloy by the thermal spraying, then the Al-rich layer was obtained by using a CO2 laser re-melting. The microstructures and phases of the alloying layer were analyzed by canning electron microscope (SEM) and X-ray diffraction apparatus (XRD). The mechanical properties were investigated by using hardness measurement and ring-on-flat apparatus. The corrosion behaviour was investigated in 3.5% (mass fraction) NaCl solution by electrochemical measurements. The results show that there are several different microstructures in the alloying layer, such as columnar, snowflake and network structure; the alloying layer consist of Mg2Al3, Mg17Al12 and alpha-Mg phases. The microhardness of alloy layer is about 170HV, higher than that of the AZ31 matrix (about 50HV). The wear tests show that the wear resistance of alloying layer is considerably improved comparing with the matrix. The potentiodynamic polarization results indicate that the corrosion resistance by laser alloying is enhanced. As the lightest metal structure of industrial materials, magnesium and its alloys are widely used because of their low densities, high specific strength, strong electromagnetic shielding and antiradiation, easy cutting and recycling, and is applied extensively on products of aerospace, automobile, motorcycle and electron industry [1]. However, the poor anticorrosion performance of magnesium alloys restricts their expanded applications. However, a number of undesirable properties including poor corrosion and wear resistance, and high chemical reactivity, limit their extensive use in many applications. Surface treatment has shown great potential in improving the resistance to corrosion and wear of Mg alloys, without affecting their bulk properties. In recent years, to improve the corrosion resistance of magnesium alloys, a lot of studies have been conducted primarily focusing on surface modification of the magnesium alloys, such as chemical conversion treatment, anodic oxidation treatment, chemical plating and laser surface treatment [2-4]. Maker and Kruger[5] found that Al is the only alloying element for magnesium alloys to cause a significant decrease in the corrosion rate when the Al concentration is over 28.8%. Due to Al and its alloys have the ability to self-repair corrosion, and aluminum element is one of the main compositions of the Mg alloy, Al coating is designed to protect magnesium alloy from corrosion resistance. There are many methods to prepare Al coating, such as PVD, CVD and high energy beam [6]. In this paper, the Al-alloy layer on the AZ31 magnesium alloy was prepared by using CO2 laser in order to improve the surface properties of magnesium alloy, the microstructures were observed. Wear and corrosion resistance of alloying layer were tested.