Surface Modification Of Pure Mg, Mg Alloys AZ31 And AZ91HP By High Current Pulsed Electron Beam

In the last few decades, high current pulsed electron beam (HCPEB) has been developed intensively as a new high-power energetic beam used for surface modification of materials. In the previous works on HCPEB treatments of pure Al and mold steels, it was found that the properties of modified surface layers such as wear and corrosion resistance could be improved significantly. Consequently, Mg-based metal and alloys were selected in the present work to investigate the effect of HCPEB treatment on their wear and corrosion resistance and their specific characteristics.Based on the experiments of HCPEB treatment of pure Mg, wrought Mg alloy AZ31 and die-cast Mg alloy AZ91HP, it is verified that the wear and corrosion resistance of modified samples could be enhenced obviously under the optimum treating conditions of HCPEB irradiation.First, the morphological evolution and changes of structure-phase states induced by HCPEB irradiation were studied systematically. According to the range of incident energy fluence used in our experiments, the treating regimes were devided into two typical classes, pulsed melting and pulsed vaporizing treatment with correspondense to the energy density of 2J/cm2 and 3J/cm2. Under these two kinds of treating conditions, remelted layers appeared and could be measured with a depth of 3~8μm for Mg and 4-10 μm for Mg alloys. Moreover, severe plastic deformations of twinning characteristic were found on the top surface layers due to the large stress caused by superfast cooling rate at the end of pulsed energy input. In the HCPEB treatments of Mg alloys, the content of Al component increased obviously after multiple pulses processing, which is related to the selective evaporation effect of Mg over Al element. For AZ91HP, the particles of second phase Mg17Al12 could be dissolved completely in the near surface layers through liquid state diffusion processes and a supersaturated Al solid solution were formed on the top surface.Then, a physical model was proposed to describe the transient thermal process associated with the pulsed beam bombardment, and the melting and evaporation processes were simulated numerically. For the melting mode, the combination of subsurface melting and compression of quasi-static stress lead to the eruption of small droplets from subsurface layers, whichcorresponds to the formation of craters on the irradiated surface and an intense thermal shock stress propagating in the depth of sample. For the vaporizing mode, the boiling occurs simultaneously in a certain depth range, which is the main cause for the increased suface roughness and for the spillage of small droplets from the boiling liquid. These droplets are charged in the electron beam, and some of them are attracted back to the surface by electric field. This mechanism explains the presence, of,particles: on the surface observed on pure Mg treated under the vaporizing mode.Mechanical properties like microhardness, "friction and wear, corrosion behaviour were measured. Depth of the distribution of micro-hardness was measured on the cross-section of treated samples. Elevation of hardness in the range of several hundreds micrometers near the treated surface was found for all the samples. For pure Mg samples treated with 10 pluses at 3J/cm2, the peak value of hardness increased 80% from 55 HK to 100 HK. As to AZ31 and AZ91HP, the hardness value increased about 113% from 61 HK to 130 HK and 118% from 65HK to 142HK respectively. The distribution of hardness along the depth direction had a fluctuant shape and exceeded far beyond the heat-affected zone. Due to the increased microhardness after HCPEB treatment, the friction coefficient of treated sample reduces and the wear resistance increases.Effects of HCPEB surface treatment on the corrosion resistance of Mg and Mg alloys were investigated by means of potentiodynamic polarization curves, where 5% NaCl solution was used. It is tested that the treated samples exhibited an improved corrosion resistance as a result of the formation of nano-sized MgO on the top surface and the in...