| Magnesium alloys have many advantages such as small specific gravity,high specific strength,large modulus of elasticity,good heat dissipation,good shock absorption,non-toxicity and easy recycling.With the diversification of industrial environment,magnesium alloys can not meet the requirements of industrial applications because of their low hardness and poor corrosion resistance.Laser cladding technology can effectively improve the hardness and corrosion resistance of magnesium alloys,so it is widely used to improve the surface properties of magnesium alloys.However,at present,the laser cladding layers on the surface of magnesium alloys are generally thin and easy to be damaged in industrial applications,so that magnesium alloys are re-exposed to harsh environment,and again face the risk of corrosion damage.Although high laser power can be used to prepare thick cladding layers,the magnesium alloys matrix are easy to evaporate and deform during high power laser cladding process due to the low melting point and high temperature deformation of magnesium alloys.At the same time,some coatings with high hardness have higher melting points and also need high heat input.Moreover,the thermophysical properties of these high-hardness coatings and magnesium alloys are quite different,which makes cracks and other defects easily occur in the process of cladding,seriously affecting the service life and performance of the coatings.Therefore,there is a lack of a process to prepare a thicker protective coating on magnesium alloys and to significantly improve the properties of magnesium alloys.In this paper,a gradient modified layer was prepared on AZ91D magnesium alloy by DC-PMIG combined laser cladding.Firstly,the DC-PMIG welding was used to fabricate an interlayer with enough thickness on the surface of AZ91D magnesium alloy.After that,a pulsed YAG laser was applied to clad Ni-Cr-Al powders on the interlayer to form a top modified layer with higher hardness and better corrosion resistance.The SEM,EDS and XRD results show that the interlayer is mainly composed of?-Al,Al+Al3Mg2 and Mg2Si while the top layer is mainly composed of?-Al,Al3Ni,Al45Cr7,Mg2Si and Al+Al3Mg2.Furthermore,Y2O3 was added to ameliorate the microstructure and properties of the gradient layer.The results show that the addition of Y2O3 can refine the grain size,reduce the dilution rate and homogenize the structure.The micro-hardness of the top cladding layer without Y2O3 is about320HV0.2.2 and about 5 times higher than that of AZ91D matrix(about 65 HV0.2),and about 2times higher than that of the interlayer(150 HV0.2).The corrosion potential of the top cladding layer without Y2O3 is-1.180V,0.14V higher than that of the interlayer and 0.287V higher than that of the magnesium alloy matrix.The addition of Y2O3 improves the micro-hardness and corrosion resistance of the top cladding layer.When the content of Y2O3is 1%,the micro-hardness of the top cladding layer is the highest,which is about 400 HV0.2.When the content of Y2O3 is 1.5%,the corrosion current density of the top cladding layer is the lowest,which is 5.116×10-5A/cm2. |
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