| As light alloys, magnesium alloys can meet the light weight requirement and have an optimistic development potential. Compared with other metallic material, with the merits of low density, high specific stiffness and specific strength, good damping performance, good barrier of electromagnetic interference and machining property, magnesium alloys have a widespread availability in aerospace, automobile, electronics, nuclear and petrochemical industry. However, low strength and hardness, low wear resisting and corrosion resisting properties have restricted the application field of magnesium alloys.In this study, a high performance, economical and convenient for operation of surface modification process for Mg alloy has been established, i.e. the pulse current gas tungsten arc surface melting process was employed to prepare a surface melting layer on Mg alloy. In addition, Nd: YAG laser surface melting process was used to prepare a surface melting layer reinforced by ceramic particles. In this paper, the magnesium alloy AZ31 and ceramic particles were used as matrix and reinforcement, respectively. Optical microscope (OM), scanning electron microscope (SEM), electron spectrometer (EDS), transmission electron microscope (TEM) and electrochemical corrosion have been used to analysis the microstructure, mechanical property, wear and corrosion behavior of melting layer reinforced with SiCp, SiCp+Al, B4Cp.The process of ceramic particles coming into molten pool has been divided two steps: ceramic particles traverse the barrier of molten pool and the movement in molten pool. A simple model has been established and mathematically analyzed the two steps. Forming mechanism of GTA melting layer was analyzed. Under the optimized processing parameters (I=150A,v=200mm/min,f=8Hz and P=20.KW,v=12mm/s), the macro surface of melting layer was uniform and no defects such as porosities, slag and craters. Under the optimized processing parameters, the grain size of GTA melting layer and laser surface melting layer was 9μm and 4μm, respectively. A variety of driving force (such as buoyancy and surface tension of molten metal, electromagnetic force, pulse arc blow force) and drastic stirring generated by impulse current could provoke intensive convection and turbulence. Meanwhile, due to the sample was assembled on the copper plate, there was a large temperature gradient on the boundary between surface and substrate of the sample, so it caused the molten pool rapid solidification under high cooling rate. The time of dendrite breaking was shorter than plateau period, i.e. before all molten metal solidification, dendrite started to break, so that fine equiaxed grains have been achieved in melting layer.A defect-free and adherent SiCp/AZ31 or B4Cp/AZ31 interface formed in melting layer and there was no obvious melting or dissolution of ceramic particles and no obvious reactant at the interface. The content of Mg in surface of melting layer was decreased and Al content was increased, which can improved the wear and corrosion resisting of the melting layer.Compared with hardness of AZ31 matrix (approximately 55HV), the hardness of GTA surface melting layer reinforced by SiCp and B4Cp was approximately 120-160HV. And the hardness of laser surface melting layer reinforced by SiCp was approximately 150-185HV. The main reasons of improving in hardness of surface melting layer were presence of large numbers of ceramic particles in melting layer, fine grains and a mass ofβphase uniform distribution in melting layer. So it can effectively prevent dislocation migration and grain boundary sliding, limit deformability of melting layer.The yield strength (σ0.2) and ultimate tensile stress (σUTS) increased with volume fraction of SiCp ceramic particles. And the reinforcing range of melting layer reinforced with smaller size SiCp ceramic particles was higher than that reinforced with larger size ceramic particles. However, the changing tendency of elongation test result was inverse to the strength result. The fracture texture of melting layer reinforced with lower volume fraction of ceramic particles mainly was interfacial sticky point of ceramic particle and ductile fracture of AZ31 matrix. However, the fracture texture of melting layer reinforced with higher volume fraction of ceramic particles mainly was fracture of ceramic particle. TheσUTS of melting layer reinforced by B4Cp with volume fraction of 11% and 7% was increased by 14.8% and 6.1% as compared withσUTS of AZ31 matrix, respectively.The relative wear resistance (RWR1,2) of melting layer reinforced with the same particle size was proportion with particle volume fraction. On the other hand, if the particle volume fraction is kept constant and particle size varies, the wear resistance is a function of inversesquare of the particles size, which follows the equation of RWR1,2=(V1/V2)(G2/G1)2. The dominant mechanism of the GTA melting layer under the applied loads of 49N, 98N, 147N (particle size of 20μm, particle volume fraction of 12 vol. %, wear period of 2.0h) was oxidation type, delamination and adhesion, respectively. All wear rates of melting layer were lower than that of AZ31 base metal. Under the high temperature conditions (200℃, applied loads of 98N, wear period of 2.0h), melting layer had higher wear resistance than AZ31 base metal.The corrosion mass loss after immersion in 3.5% NaCl solution suggested that the corrosion rate of melting layer was lower than AZ31 base metal. The main reason of melting layer corrosion was galvanic corrosion betweenβphase andαphase. The resistance to local corrosion of melting layer was higher than that of AZ31 base metal. The Icorr of AZ31 base metal was higher than that of melting layer and the Ecorr of melting layer was increased by up to 0.073-0.153V, while the discrepancy of Epit was slight. The presence of ceramic particles in melting layer was not obvious effect on sensibility of pitting corrosion. The grain size in melting layer was obviously refined and massiveβphase was dissolved and refined. Al element uniformly distributed in melting layer and solid solubility of Al inαphase was increased, so that passivity ofαphase was enhanced. In melting layer, either the corrosion accelerating effect or blocking effect of galvanic corrosion acted as a leading role depended on the distribution and the amount ofβphase. As for the SiCp+Al reinforced melting layer, when the content of Al was lower, the amount ofβphase was also lower and discontinuously distributed in melting layer, so that the corrosion accelerating effect ofβphase galvanic corrosion acted as leading role. However, when the content of Al was higher, the amount ofβphase was also higher and continuously distributed in melting layer, so that theβphase acted as corrosion blocking effect and the corrosion of melting layer was inhibited at a certain extent. The corrosion rate was decreased with increasing Al content in melting layer. |
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