Study On Low Power Laser Modification Of AZ31 Magnesium Alloy Surface

Magnesium alloy has wide application prospects in the aerospace,transportation and defense fields because of its high specific strength,low density,good vibration damping,and excellent electromagnetic shielding properties.However,magnesium alloys cannot be widely used in engineering applications due to magnesium alloys have low hardness,poor wear resistance,and susceptibility to corrosion in media.In order to improve the above-mentioned defects in magnesium alloys,the Yb-fibre solid fiber laser was used to laser surface melting and laser surface cladding of AZ31 magnesium alloys.The microstructure,hardness,wear resistance and corrosion resistance of laser modified layers were studied in different process parameters for the above two kinds of surface modification.In addition,the laser surface modification mechanism was also discussed.The research mainly included the following main contents:The surface of AZ31 magnesium alloy was melted at different laser powers.Studies have shown that when the laser power was 104W,the absorption rate of the laser energy on the surface of the magnesium alloy increases sharply and the microstructure of laser melted layer was the most dense,and the penetration depth can reach 125?m.The Al element had higher solid solubility in the laser melted layer than the untreated magnesium alloy and the surface microhardness was also higher than that of the untreated magnesium alloy.Theoretical analysis showed that the strengthening mechanism of the laser melted layer was fine grain strengthening,solid solution strengthening and dispersion strengthening.The frictional wear and corrosion electrochemical tests of the laser melted layer showed that the wear of the laser melted layer was 66.4%of that of the untreated magnesium alloy,and the wear resistance of the laser melted layer was significantly higher than that of the untreated magnesium alloy.The corrosion current density of the laser melted layer was reduced by an order of magnitude compared to the untreated magnesium alloy,indicating that the corrosion resistance of the laser melted layer was significantly improved.Laser remelting experiments were performed on the surface of AZ31 magnesium alloy at different laser powers.The effects of laser remelting times on the microstructure,surface Al content and the equivalent of the melted layer were studied.The results showed that the microstructure of the laser remelting layer formed at 56W laser power was the most dense,and the number of cracks and holes were the least.Combining the results of multiple remelting under three laser power parameters,it was found that as the number of laser remelting increases,the Al content and the surface roughness of the laser remelted layer gradually increase,while the thickness of the laser remelting layer gradually decreases.After repeated remelting with laser,both the remelting layer and the untreated magnesium alloy consisted of?-Mg phase and a small amount of?-Mg₁₇Al₁₂ phase,and the phase composition of the remelting layer did not change.The content of?-Mg₁₇Al₁₂ phase in the microstructure of the remelting layer was lower than that of the untreated magnesium alloy after one laser fusion treatment.However,as the number of remeltings continued to increase,the content of?-Mg₁₇Al₁₂ in the remeltings layer gradually increased.Laser cladding technology was used to prepare Ti₆Al₄V coating on the surface of magnesium alloy.The effects of different laser energy density on the microstructure,hardness,corrosion resistance and wear resistance of the Ti₆Al₄V coating were investigated.The results show that expected dense Ti₆Al₄V coating was formed under the optimized parameters.The Ti₆Al₄V coating connect with AZ31 magnesium alloy substrate by partially metallurgical bond.The microhardness of Ti₆Al₄V coating?374±4 HV?was nearly 6 times that of the base magnesium alloy.Corrosion electrochemical analysis shows that the corrosion potential of Ti₆Al₄V coating was increased by 1.01V and the corrosion current density was reduced by two orders of magnitude compared with the untreated magnesium alloy.Friction and wear experiments show that both the wear mechanism of the laser cladding layer and the untreated magnesium alloy were abrasive wear,and the wear resistance of the laser cladding layer was significantly better than that of the matrix magnesium alloy.