| Magnesium alloys have been widely applied in aerospace,transportation,electronics industry due to their excellent performances such as the low density,good thermal conductivity and good recyclability.However,some problems,including low hardness,poor wear and corrosion resistance hamper their further applications.At present,laser cladding technology with high power density and rapid cooling rate has been often used to improve the surface properties of magnesium alloy.Amorphous alloys have many excellent properties different from crystalline alloys due to their unique long-range disordered atomic arrangement structure,which provides a new idea for the surface modification of magnesium and magnesium alloys.In this paper,Zr-based amorphous composite coatings were prepared by laser cladding of Zr-Al-Ni-Cu?A?powder with pure Zr powder as Zr element addition mode and Zr-Al-Ni-Cu?B?powder with ZrH2 powder as Zr element addition mode on AZ91D magnesium alloy surface using fiber-coupled semiconductor lasers.The phase compositions and microstructures of the coatings were analysed and studied by scanning electron microscope and X-ray diffractometer.Microhardness and corrosion resistance of the coatings were measured by microhardness tester and electrochemical workstation.During the experiment of laser cladding Zr-Al-Ni-Cu?A?powder,the influence of laser processing parameters on the formation of coating surface was discussed and analyzed.The optimum process parameters are P=1400 W,v=8 mm/s.The coating consists of amorphous phase,?-Zr,NiZr2 and Zr3O.The microstructure of the coating is that the phase of?-Zr is dispersed in the amorphous matrix,and the phase size can reach 5.7 um.The crystallinity of the top of the coating is higher than that of the middle of the coating.The average hardness of the coating is 693.9 HV0.2,which is about 10 times higher than the hardness of the magnesium matrix.In the electrochemical corrosion test,the amorphous composite coating has passivation phenomenon.The self-corrosion potential of the magnesium matrix,the middle partof the coating and the top part of the coating are-1.585 V,-0.498V and-0.370 V,and the self-corrosion current densities are 1.444 x 10-44 A/cm2,8.587 x 10-77 A/cm2 and9.700 x 10-8A/cm2,respectively.The corrosion resistance of the coating is much higher than that of the magnesium matrix,and the corrosion resistance increases with the increase of crystallinity.Laser multi-layer cladding Zr-Al-Ni-Cu?B?powder was used to prepare amorphous composite coatings with single layer?299?m thick?,two layers?454?m thick?,three layers?771?m thick?and four layers?1008?m thick?.With the increase of layers,the size and number of crystalline phases increase.The interlayer bonding of the multi-layer coatings is good,and the crystal phase size of?-Zr between 0.7?m and 3?m in the coatings is obviously smaller than that of Zr-Al-Ni-Cu?A?coatings.The hardness of amorphous coatings with different thicknesses is similar.The average hardness of the four coatings is 714.9 HV0.2.In the electrochemical corrosion test,passivation exists in the multi-layer coatings.The self-corrosion potential of the two-layer,three-layer and four-layer coatings is-0.815 V,-0.625 V and-0.555 V respectively.The self-corrosion current densities are 2.184×10-66 A/cm2,2.705×10-66 A/cm2 and 1.36×10-66 A/cm2,respectively.The corrosion resistance of the multilayer coatings is much higher than that of the magnesium matrix,and the crystallinity increases with the increase of the number of layers.The corrosion resistance of the coating surface is improved.It is found that the phase composition of the two Zr-based amorphous composite coatings is the same,but the coating prepared by Zr-Al-Ni-Cu?A?powder has higher crystallinity and better corrosion resistance. |
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