| Due to the excellent physical, chemical and mechanical performances, magnesium alloys, the so called "21st century green engineering materials", have been widely used in aerospace, automotive, electronic, medical and other fields. However, the low hardness and electrode potential lead to the poor wear performance and corrosion resistance of Mg alloys. In order to make full use of the magnesium alloys, scientists and engineers have done a lot of experiments to improve their wear resistance and corrosion resistance. In this paper, we select the Mg6Zn1 Ca and AZ31 magnesium alloys as substrate materials to be treated by using laser surface treatment so as to increase its hardness and corrosion resistance of these two magnesium alloys.As Mg-Zn-Ca alloy could be biomedical degradable materials, we have done the surface modification research of Mg6Zn1 Ca alloy by laser remelting. And we have characterized the surface morphology, section hardness and corrosion resistance of the modified samples. The results showed that: the laser remelted layer consisted of Mg,MgZn2 and Ca2Mg6Zn3 phases. The Ca2Mg6Zn3 phase was segregated on the grain boundary or precipitated from the grain as particles within the remelt layer. Comparing with the matrix(55.3HV0.01), the average hardness of the coating has increased by more than 10%. The electrochemical test of Mg6Zn1 Ca alloy matrix and laser remelting specimens in Hank’s simulated body fluids revealed that the low frequency impedance of laser remelted samples was higher than that of the substrate. The corrosion current density was 4.78×10-7A/cm2 when the laser power was 3500 W and the scanning speed was 6mm/s,which is one-fifth of the substrate(2.33×10-6A/cm2).Through the study of Mg6Zn1 Ca alloy surface laser cladding with Al-Si powders, we found that: the cladding layer was mainly consisted of α-Mg solid solution, β-Mg17Al12,Mg2 Si dendrite and Al3Mg2 phase; The dendrite Mg2 Si was uniformly distributed in the middle and top layer of the coating. The matrix and the cladding layer have formed a good metallurgical bonding. The average hardness of the treated layer was more than four times higher than that of the matrix(55.3 HV0.025). The electrochemical tests of as-received Mg6Zn1 Ca and laser cladding specimens in 3.5 wt.% NaCl aqueous solution revealed that:the low frequency impedance of the treated samples was higher than that of the matrix.When the output power was 2500 W, the scanning speed was 4mm/s, the specimen’corrosion potential was-0.129 V, and the corrosion current density was 6.72×10-6A /cm2,which was one twenty-fifth of the matrix(1.7×10-4A/cm2).We have also investigated the surface modification of the AZ31 alloy by laser cladding with Al-Ni and Al-Ni/TiC composite coating, the results indicated that: The cladding layer was mostly composes of α-Mg, β-Mg17Al12 and Al3Ni2 phases. The Al3Ni2 phase has a morphology like short sticks and they distributed in the middle and top of the coating. The TiC particles were mainly distributed in the middle of the cladding layer. The average hardness of the samples was 3.5~4 times higher than the matrix(54.6HV0.05). The electrochemical test of untreated AZ31 substrate and treated samples in 3.5 wt.% NaCl aqueous solution illustrated that: the low frequency impedance of the treated specimens were higher than that of the matrix. The sample treated with the output power of 3000 W and the scanning speed of 4mm/s has a corrosion current density of 1.96×10-5A/cm2,which is one order of magnitude lower than that of the matrix(1.66×10-4A/cm2). |
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