Microstructure And Properties Of Magnesium Alloy Surface Alloyed By Electron Beam

The poor corrosion resistance and low hardness of magnesium alloys limit the application of magnesium alloys in aerospace and other high-precision fields to a large extent.In this study,a combination of electron beam vacuum evaporation and scanning electron beam surface alloying was used to perform composite strengthening treatment on magnesium alloys,and the surface microstructure,properties and changes of AZ91D magnesium alloy surface after Cu alloying treatment were explored.This study on industrial pure copper as the target material,AZ91D as matrix,under the high vacuum environment copper substrate to make use of electron beam evaporation experiments,through orthogonal experiment design scheme explores the steamed copperplating layer under different process parameters,the change rule of surface quality for the following scanning electron beam evaporation of alloying test for reasonable process parameters.The temperature field simulation model of the surface alloying process of copper-plated AZ91D magnesium alloy by scanning electron beam was established,and the temperature field simulation of copper-plated magnesium alloy was carried out to explore the influence of scanning electron beam on the variation law of the Cu alloying temperature field of AZ91D magnesium alloy.Combined with the temperature field distribution law in the scanning electron beam alloying simulation and the results of the electron beam evaporation test,the control variable method was used to conduct the scanning electron beam AZ91D magnesium alloy surface alloying test under different process parameters.The influence of the microstructure,element distribution and mechanical properties of the alloy layer was investigated,and the formation mechanism of Nano-grains in the strengthening layer and the principle of electrochemical self-corrosion of magnesium alloys were explored.The results show that AZ91D magnesium alloy can obtain copper-colored,uniform and tightly bonded thin film on the surface by electron beam evaporation.The increase of electron beam current will increase the adhesion between the coating and the substrate.The increase of beam current will decrease the uniformity of the coating surface.The thickness of the coating is proportional to the evaporation time.The long-term energy accumulation of the target will decrease the uniformity of the coating surface.The experimental results of electron beam vacuum evaporation are as follows:beam current 25 m A,duration 125 s.The temperature field simulation of scanning electron beam alloying process shows that the surface temperature reaches more than1200°C in the stable stage of alloying process,and the surface cooling speed reaches510°C/s in the beam cooling stage,reaching the conditions of alloying and grain refinement.The scanning electron beam can fuse the AZ91D substrate with the copper coating on the surface,forming a good metallurgical bond.The surface microstructure of AZ91D after Cu alloying is divided into alloying zone,heat affected zone and matrix zone.The alloy layer area is mainly composed of:α-Mg,hard phaseβ-Mg₁₇Al₁₂,Nano-sized Al enriched particles and non-equilibrium phases Cu Mg,Al Cu and other alloy phases;Mg in the strengthening layer will splash and evaporate due to electron beam bombardment while the content decreased.When the beam current of scanning electron beam is 15 m A,the microhardness increases most obviously,which is about2.4 times of the original material.When the beam current is 10 m A,the wear resistance is optimal,and the wear loss is reduced by 87.5%compared with the original material.When the beam current is 9 m A,the corrosion resistance of magnesium alloy is the best,and the surface corrosion rate is reduced to 4%of the matrix.The depth of the strengthened layer increases with the increase of electron beam current,and the grain size in the cross-section becomes finer with the increase of electron beam current.The surface hardness of AZ91D magnesium alloy increases first and then decreases with the increase of beam current;the surface wear resistance increases first and then decreases with the increase of beam current;the average corrosion resistance of AZ91D magnesium alloy is greatly improved by scanning electron beam alloying energy,and increases first and then decreases with the increase of beam current.