Surface Modification Of Mg Alloy ZK60 And Hypereutectic Al-Si Alloys By High Current Pulsed Electron Beam

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 last few decades. It has been confirmed that the properties of modified surface layers of pure Al and mold steels treated by HCPEB such as wear and corrosion resistance could be improved significantly in the previous works. As a consequent, HCPEB treatment was selected to improve the surface properties of ZK60 Mg alloy because of the inferior corrosion and wear resistance with an aim of expanding its scope of application in this paper. While the characteristics of the modified surface and optimum parameters for the treatment operation were studied. Then HCPEB technology was compounded with another surface modification technology micro-arc oxidation (MAO) as a new technology for the Mg alloy surface modification treatment basing on the available study with the purpose of developing a new surface treatment technology of Mg alloy.The microstructure characteristic of modified surface of Mg alloy ZK60 after HCPEB and/or MAO complex treatment were analysis by XRD and SEM, and corrosion and wear resistance were tested. The results were as following:(1) There was a remelted layer with the thickness of 4-8μm on the top surface of the alloy irradiated by HCPEB characterized with a few of cracks and craters. As the temperature of the melting layer was much higher than that of evaporation of Mg, particles of pure Mg were found on the surface of the treated sample. The grain of alloy surface was refined, and the degree of segregation phenomenon of alloying agents was reduced significantly. Solid solubility of alloying element in Mg base was increased as the result of the rapid solidification process. Effects of HCPEB surface treatment on the corrosion resistance of Mg alloy ZK60 was investigated by means of potentiodynamic polarization curves in 3.5% NaCl solution. It can be concluded that the treated samples exhibited an improved corrosion resistance as the result of the refined grain and homogeneous distribution of alloying agents. The sample treated with 10 pulses at the accelerating voltage of 23KV performed better than any other treated samples as the corrosion current decreased 86%, from 311μA/cm2 to 42.2μA/cm2, and corrosion potential increased from-1312mV to-1220mV. The friction and wear test results showed that the friction coefficients of treated alloys surfaces were decreased, of which was treated with 15 pulses at the accelerating voltage of 27KV represented lower wear rate as it was just 80%.(2) ZK60 Mg alloy treated by MAO showed that a ceramic coating formed on the sample surface. The average thickness of this layer was about 18μm. The ceramic layer comprised two layers, an outer looser layer and inner denser layer, the thickness of the former was 90% compared with that of the whole coating. The pores and cracks were observed on the outer layer. After MAO then HCPEB treatments, the sample surface became rougher with a melted layer of 3-4μm. While with the converse process, the sample surface was formed a similar ceramic coating as the sample after MAO treatment. The model for the modified layer with HCPEB then MAO treatments was established. The potentiodynamic polarization curves in 3.5% NaCl solution indicated that the corrosion resistance of alloys treated with MAO and/or HCPEB treatment were improved significantly. Especially that, the sample modified by HCPEB then MAO exhibited better corrosion resistance than any other samples and the corrosion current density decreased 3 orders, from 311μA/cm2 to 0.2μA/cm2. The frictional wear test was showed that the friction coefficients and wear rates of all treated samples were decreased. The wear rate of alloy treated by MAO then HCPEB decreased 80% compared with that of initial sample. The result of the complex treatment with MAO then HCPEB illustrated that the superfast fused process decreased the porosity of ceramic layer similar as a plugging agent.(3) HCBEB bombardment induces hypereutectic Al-Si alloys surface rapid melting and solidification. As a result, grains of remelted layer have been significantly refined, content of Si in a(Al) solid solutions was improved and Si element distribution in the alloys intended to be homogenous.The typical surface morphology-crater and a few crack are observed on the surface layer after the bombardment of HCPEB. It is confirmed that temperature rises faster at a sublayer instead of on the top surface. Such a special sub-layer heating and melting mode causes eruptions of the sub-layer liquid through the outer surface and produces the typical surface crater morphology and this process can purify the matrix. The micro-cracks are only on the top surface by cross-section SEM analysis and the mechanical performances of alloys have been little influenced.The friction coefficient decreases and wear resistance was improved of the alloys after HCPEB treatment. According to potential-dynamic polarization curves measurement, the treated samples exhibit an improved corrosion resistance in 3.5%NaCl solution compared with the initial samples. In the weight-loss experiment, the corrosion rate of the treated alloys significantly decreased Corrosion resistance in 3.5%NaCl solution of all the modified alloys was improved.