| In the paper,the influence of the content of Cu on the microstructure and mechanical properties of cast Mg-6Al-2Sm-xCu alloy has been investigated.To improve and enhance the microstructure and mechanical properties of the experimental alloy,solid solution treatment + aging treatment was carried out on the alloy.The microstructure and mechanical properties of the heat treated alloy were investigated,and the changes of microstructure and mechanical properties of the cast alloy and the heat treated alloy were compared.In order to improve the corrosion resistance of the experimental alloy,the micro arc oxidation and electroless plating were performed on the cast Mg-6Al-2Sm-3Cu alloy.The microstructure and corrosion resistance of micro arc oxidation coating and electroless plating coating on the surface of the experimental alloy were investigated.The microstructure and corrosion resistance of both of them were analyzed and compared.The results are as follows:(1)For cast Mg-6Al-2Sm-xCu(x=1,3 and 5)alloy,with the increase of Cu content,the grain size decreases first and then increases while mechanical properties increase first and then decrease.The results of XRD show that all cast alloy consist of a-Mg matrix,?-Mg17Al12,Al4Cu9,Al3Sm,and Mg2Cu6Al5 phase.All fracture surfaces at room temperature show mixed fracture.(2)For T6 heat treated Mg-6Al-2Sm-xCu(x=1,3 and 5)alloy,the grain size decreases first and then increases while mechanical properties increase first and then decrease with the increase of Cu content.Compared with cast alloy,the microstructure after heat treatment is more homogeneous,and the mechanical properties are improved accordingly.The results of XRD show that the heat treated alloy consists of the phases of cast alloy and new phase Al2Cu.The fracture morphology shows that the quasi cleavage characteristics of the alloy were more obvious after heat treatment.In addition,compared with the fracture surface of the cast alloy,there are a lot of finer alloy phases distribute on the fracture surface.The fracture form is mixed fracture.(3)For cast Mg-6Al-2Sm-3Cu alloy,micro arc oxidation was performed on the alloy.The result of XRD shows that the phase in the micro arc oxidation coating mainly include MgO,Al2O3 and MgAl2O4 phase.The film thickness distribution of the micro arc oxidation film is more homogeneous,but the surface is not smooth enough,and some holes are distributed on the cross section of the film layer.There is a good bonding force between the microarc oxidation film and the matrix alloy.The electrochemical polarization curve and the results of full immersion corrosion test show that the corrosion resistance of the alloy with micro arc oxidation treatment was improved.The corrosion forms of the matrix alloy and the alloy with micro arc oxidation treatment are comprehensive corrosion.(4)For cast Mg-6Al-2Sm-3Cu alloy,electroless plating was performed on the alloy,and the obtained coating was Ni-P coating.The microstructure shows that the surface of Ni-P coating has a typical morphology of cellular nickel and phosphorus,and the size and distribution of the cell are basically uniform.Compared with the micro arc oxidation coating,the Ni-P coating is more compact,no defects,and the quality of the coating is much better.Combined with XRD result and EDS result,Ni-P coating is microcrystalline structure,and Ni and P element distribute mainly on the coating,and the distribution is uniform.The bonding force between Ni-P coating and matrix alloy is excellent,and the thickness of the coating is thicker,and the average roughness is higer than that of micro arc oxidation coating.The electrochemical polarization curve and the results of full immersion experiments show that the alloy has better corrosion resistance after electroless plating.Compared with the micro arc oxidation coating,the corrosion degree of Ni-P coating is lower than that of micro arc oxidation coating.The corrosion form of Ni-P coating is local corrosion. |
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