Relationship Between Grain Size And Mechanical Properties Of Friction Stir Processing AZ31Magnesium Alloy

Friction stir processing (FSP) is a new method of micro structure modification that has advantages in terms of processing methods, environmental protection and energy use. FSP has been successfully used in aluminum alloy, magnesium alloy, copper alloy and fine grain materials to make the preparation of UFG. With the expansion of the field of application of magnesium alloy, deformation behavior of fine grain and ultrafine-grained magnesium alloy made by FSP is making people’s attention increasing.In this paper, AZ31magnesium alloy were carried out by FSP in air and circulation water cooling condition, The processing parameters are475-1500rpm of rotation speed and30-150mm/min of advancing speed. Microstructure and mechanical properties were studied by OM and TEM techniques. Annealing treatment is carried out in different temperature (150℃、200℃、250℃、300℃、350℃>400℃) with same time(40min). The major conclusions are as follow:The average grain size of AZ31magnesium alloy was refined to0.84-4.18μm, and the minimum grain size was0.84μm when the parameter is475/118in circulation water cooling condition. Relationship between grain size and hardness are Hv=74.14-43.5d-1/2and Hv=81.39-14.13d-1/2both in air and circulation water cooling condition, exhibit anti-Hall-petch relation. The relationship between yield strength and grain size exhibit different variation in the two media which areσ0.2=57.7+207.9d-1/2and σ0.2=202.3-72.96d-1/2. The strength and grain size follow the H-P relationship but has a low K value in air. The Annealing treatment show that the refine grain size AZ31magnesium alloy had good microstructure stabilitywhen the Annealing temperature was under250℃, however, hardness we relationshipbetween strength and grain size shows anti-hall-petch in water, The hardness exhibitstrengthened at low temperature after annealing treatment.