Laser Surface Modification Of AZ91HP Magnesium Alloy

Magnesium (Mg) alloys are regarded as the ideal engineering materials in the 21^st century because of their low density, high specific strength and rigidity, good heat and electrical conductivity. However, the poor corrosion resistance and mechanical properties of Mg alloys limit the advantages of their performance. Therefore, by taking the surface modification technology, the improvement of the chemical and mechanical properties of Mg alloys has the important practical significance. In this dissertation, the techniques, such as laser surface melting and laser surface cladding, are carried out to improve the surface properties of AZ91HP Mg alloy. The structure and properties of the melted layer, the low melting point Al alloy coating, Cu_58.1Zr_35.9Al₆ amorphous coating and high melting point Al₂O₃ ceramic coating, as well as the changing relations to the laser technology parameters, are studied systematically. It is expected to make a deeper research on the surface modification technology improving the wear and corrosion resistance of Mg alloys.The studied results of laser melting Mg alloy show that with the low energy density, the melted layers mainly consist of α-Mg dendrite and β-Mg₁₇Al₁₂ distributing in the inter-dendrite, and the content of β-Mg₁₇Al₁₂ increases with the increasing of the powers. Due to the fine-grained and precipitation strengthening, the microhardness, wear and corrosion resistance of the melted layers have the significant improvement; With the high energy density, the melted layers are also composed of a-Mg dendrite and β-Mg₁₇Al₁₂, but owing to the increasing of the dendritic dimension, modified effect of the high energy density is lower than that of the low energy density.In laser cladding Al-Si and Al-Cu eutectic alloy coatings with low melting point, the coatings form multiple Mg intermetallic compounds due to the dilution of Mg alloy. As for Al-Si coatings, when laser powers are 3.0kW and 3.5kW, the microstructure of the coatings is composed of Mg₂Si dendrite and white needle-like Mg₂Al₃ (Hex) distributing on the black matrix Mg₁₇Al₁₂. Besides the above three intermetallic compounds, the coating with laser power of 2.5kW forms the petal-like Mg₂Al₃ (Fcc) and the coating with laser power of 4kW appears a-Mg solid solution because of the difference of Mg dilution. Al-Cu coatings consist of the equiaxed crystal Mg₁₇Al₁₂ enclosed by black reticular AlMg, and different Cu intermetallic compounds with the increasing of the powers. With the composite action of the fine-grained and precipitation strengthening of multiple Mg intermetallic compounds, Al-Si and Al-Cu eutectic coatings exhibit better wear and corrosion resistance.