Research On Microstructure And Wear Resistance Of AZ91D Magnesium Alloy With Surface Nanocrystallization And Recovery Treatment

At present,the vehicle population in China ranks in the forefront of the world.In the face of a series of problems such as climate warming and lack of resources,the "lightweight" of automobile components can reduce emissions,protect the environment and save resources.Based on its low density,magnesium alloy can effectively reduce the weight of automobiles,so its large application in automobile components has become an inevitable trend.However,the wear resistance of magnesium alloy is poor,which is not conducive to its wide application.In view of ultrasonic surface rolling process(USRP)can significantly improve surface properties of alloys,AZ91 D magnesium alloy was treated by USRP in this paper,and its surface nanocrystallization mechanism,mechanical properties and wear resistance were studied.At the same time,in order to further improve wear resistance of AZ91 D magnesium alloy,recovery treatment at different temperatures was ultized.The effects of recovery temperature on the microstructure,mechanical properties and wear resistance of the USRP samples were studied.The main conclusions are as follows:(1)The mechanism of surface nanocrystallization of AZ91 D magnesium alloy after USRP was studied.The results showed that a 190 ?m thick plastic deformation layer was formed on the surface of USRP sample,and its surface microhardness was increased by 64.9% compared with that of the original sample.In addition,at the beginning of plastic deformation behavior,the deformation twins dominated the process.With the increase of strain energy,dislocation slip system was activated and dislocation began to slip.When dislocation movement was hindered,dislocation wall was formed by dislocation pile-ups,and coarse grains were refined into sub-grains.Meanwhile,dynamic recrystallization occurred near the surface of the USRP sample.Finally,the surface nanocrystallization of AZ91 D magnesium alloy was realized through a variety of plastic deformation mechanisms.(2)The friction and wear behavior of AZ91 D magnesium alloy before and after USRP under different loads and frequencies was studied.The results showed that the wear volume loss and friction coefficient of USRP sample were lower than those of the original sample under the same load or frequency.Meanwhile,compared with the original sample,the wear of the sample after USRP was lighter.(3)The microstructure and mechanical properties for AZ91 D magnesium alloy after recovery treatment were studied.It was found that the thickness of plastic deformation layer of USRP-R sample was about 160 ?m when the recovery treatment temperature was 150 ?.With the increase of temperature,the thickness of deformation layer decreased gradually,and the grains in deformation layer expanded gradually.At the same time,the surface microhardness of USRP-R sample decreased,the plasticity enhanced,with the increase of temperature.(4)The friction and wear behavior of USRP sample before and after recovery treatment was studied.The results showed that the wear volume loss,friction coefficient,wear rate and cross-section profile of wear scar for AZ91 D magnesium alloy after recovery treatment at 150 ? were the smallest compared with the samples after recovery treatment at other temperatures.Meanwhile,the wear of the worn surface for USRP-R-150 sample was the lightest under the same experimental condition.