Study On Laser Cladding The High-Entropy Alloy Composite Layer On The AZ91D Magnesium Alloy

The low strength and hardness and poor corrosion resistance of magnesium and its alloys have seriously hindered its application in industrial fields.High entropy alloys,as a new structural material,have the advantages of high strength,high hardness,wear resistance,corrosion resistance and heat resistance.The use of high entropy alloy to improve the surface properties of magnesium alloy has an important role in expanding the industrial applications of magnesium alloy.The LDF4000-100 fiber-coupled semiconductor laser was used to improve the surface properties of magnesium alloy by melting and coating the high-entropy alloy powder on the surface of magnesium alloy.The difference between the thermophysical properties of AZ91D magnesium alloy and high entropy alloy is too large,and there are some difficulties in laser melting high entropy alloy on the surface of magnesium alloy,only the good wettability and self-melting of the molten material and the base material can get the ideal metallurgical bond.In this paper,based on the research on the surface modification material system of magnesium alloy and the formation mechanism of high entropy alloy phase,the gradient composite cladding layer alloy system applicable to the surface of AZ91D magnesium alloy was designed,and the system was further optimized by conducting laser cladding experiments,and the gradient composite cladding layer alloy system was selected--Al₄Ni/Al2Cr Cu2Fe Ni2Ti and Al₄Ni/Al2Cr Cu Fe Ni2Ti The gradient composite cladding layers Al₄Ni/Al2Cr Cu2Fe Ni2Ti and Al₄Ni/Al2Cr Cu Fe Ni2Ti were successfully prepared on AZ91D magnesium alloy substrate by laser cladding method.The microhardness and corrosion resistance of the gradient composite cladding layer at room temperature were investigated.The experimental results show that the laser power and scanning speed have a direct effect on the quality of the cladding layer.High power or low speed may lead to surface evaporation and hole formation due to excessive energy.Conversely,low power or high speed can lead to insufficient melting and mixing of the powder,resulting in inhomogeneous distribution of the clad layer or failure of metallurgical bonding.When the laser power is constant,the height H,depth h and dilution rate?of the cladding layer all decrease gradually with the increase of scanning speed.After considering the surface morphology of the molten layer,pores,cracks and other defects,finally,the process parameters of the intermediate molten layer were chosen as P=900 W,v=10 mm·s^-1;the process parameters of the high entropy alloy molten layers Al₂Cr Cu Fe Ni₂Ti and Al₂Cr Cu₂Fe Ni₂Ti were P=1500 W,v=17 mm·s^-1 and P=1400 W,v=17mm·s^-1.The intermediate melt layer Al₄Ni is mainly composed of-7),Al₃Ni₂,Al₂Mg,Al₁₂Mg₁₇ and Al₃Mg₂.The high-entropy alloy melt layer Al₂Cr Cu Fe Ni₂Ti and Al₂Cr Cu Fe Ni₂Ti are both composed of BCC phase,FCC phase and Laves phase.The high entropy alloy cladding layer,intermediate cladding layer Al₄Ni and AZ91D magnesium substrate have good metallurgical bonding.The average hardness of the intermediate melt layer Al₄Ni(269.25 HV_0.2)is about 4.9 times that of the AZ91D magnesium alloy matrix(54.96 HV_0.2);the average hardness of the high entropy alloy melt layer Al₂Cr Cu Fe Ni₂Ti(677.67 HV_0.2)and Al₂Cr Cu₂Fe Ni₂Ti(678.57 HV_0.2)is about 12 times of AZ91D magnesium alloy matrix(54.96 HV_0.2),the gradient composite cladding layer effectively improves the surface hardness of AZ91D magnesium alloy.The corrosion resistance of gradient composite fusion cladding layers Al₄Ni/Al₂Cr Cu₂Fe Ni₂Ti and Al₄Ni/Al₂Cr Cu Fe Ni₂Ti in 3.5 wt%Na Cl solution is better than that of AZ91D magnesium alloy matrix,and the gradient composite fusion cladding layer Al₄Ni/Al₂Cr Cu Fe Ni₂Ti has the best corrosion resistance with corrosion self-corrosion current density of 3.002×10^-7 and corrosion voltage of-0.639V.