Surface Modification of Mg Alloys AZ31 and ZK60-1Y by High Current Pulsed Electron Beam

Abstract

The quality of Mg alloys with high specific modulus and specific strength is the lightest in the structural materials. Their density is about 2/3 of aluminium alloys and 1/4 of steels. The weight of whole structural materials is decreased drastically owing to some components or parts produced by Mg alloys. Thus, Mg alloys are widely used in aerospace, weapons, automobile and other fields [1-3]. Meanwhile, Mg alloys has many other advantages, such as excellent electromagnetic shielding performance, shock absorption ability, electric and heat conductivity, etc [4-6]. However, the chemical stability of Mg is very low, and its electrode potential is negative (-2.34V). As a result, the corrosion resistance of Mg alloys is poor in acid and neutral mediums. Furthermore, other properties of Mg alloys, wear resistance, hardness and resistance to high temperature, are also poor. Consequently, the superiority of Mg alloys in the application is restricted to some extent. Nowadays, the researches are concentrated on the improvement of hardness, wear and corrosion resistance of Mg alloys. Energy beam surface modification is an important developing direction, such as plasma micro-arc oxidation [7-11], laser surface treatment [12-16], ion beam surface modification [17-19], etc. A.V. Apelfeld et al [10] have studied oxide protective coatings on the surface of Mg alloys obtained by micro-arc oxidation (MAO). A model of micro-arc coating formation is proposed. For Mg alloys, the structure of MAO coating plays an important role in improvement of corrosion resistance. The research team (Y.M. Wang et al [11]) has investigated that dense oxide coatings formed in alkaline silicate electrolyte with and without titania sol addition are fabricated on AZ91D alloy using micro-arc oxidation. It reveals that the coating thickness decreases from 22μm to 18 μm with increasing concentration of titania sol from 0 to 10 vol. %. Electrochemical tests show that the Ecorr of Mg substrate positively shifts about 300-500 mV and Icorr lowers more than 100 times after micro-arc oxidation. The literature [15] (A.K. Mondal et al) has reported that Mg alloy ACM720 is subjected to laser surface treatment using Nd:YAG laser in argon atmosphere. This treatment is beneficial for enhancing the corrosion and wear resistance of the alloy. The improved corrosion resistance is attributed to the absence of second phase Al2Ca at the rain boundaries, microstructural refinement and extended solid solubility, particularly of Al, in (Mg) matrix owing to rapid solidification. The laser treatment also increases surface hardness two times and reduces the wear rate considerably due to grain refinement.