Effects Of Heat Treatment On Microstructure And Properties Of Mg-7Zn-0.3Mn-xCu Magnesium Alloy

Magnesium alloy parts are often made by die casting method.Liquid die castings cannot be further improved by heat treatment,while semi-solid die castings can be heat treated,which provides a new way to solve production problem of magnesium alloy workpieces.However,the heat treatment strengthening effect of various kinds of magnesium alloys used in semi-solid forming is poor,which can not give full play to the advantage of heat treatment.Mg-Zn-Mn base alloy is a kind of magnesium alloy with good age-strengthening effect.In this work,Cu element was added on the basis of Mg-7Zn-0.3Mn alloy.It is expected that the new alloys not only have better heat treatment effect but also can meet the requirements of microstructure in semi-solid forming.In order to achieve this goal,the following conclusions are obtained by means of methods such as OM,XRD,SEM,TEM and hardness test.Firstly,the microstructure and hardness of as-cast Mg-7Zn-0.3Mn-x Cu alloy were studied.The results show that there is a new CuMgZn phase after adding Cu element,and the three experimental alloys containing Cu are composed of?-Mg and eutectic microstructure??-Mg+CuMgZn+MgZn₂+Mg₄Zn₇?.When the Cu content is1.0%,the grain refinement effect and the microstructure improvement effect of the alloy is the best,the hardness is the highest,reaches 54.3.Secondly,the evolution of non-dendritic microstructure of Mg-7Zn-0.3Mn-x Cu alloy was studied by semi-solid isothermal heat treatment.The results show that the addition of Cu element can obviously accelerate the separation and spheroidization of non-dendritic structure.The average size,shape factor and solid fraction of the primary particles were 38.85?m,1.39 and 53.38%,respectively,when the Cu content was 1.0%and the optimum processing condition was 585°C for 20 min.The semi-solid non-dendritic structure obtained by water quenching consists of solid-liquid two phases,the solid phase containing?₁-Mg and?₂-Mg phases.The liquid phase consists of the liquid phase between the solid particles,the"small liquid pool"in the particles,and the liquid film.Then,the effects of heat treatment of T4 and T6 on the microstructure and hardness of as-cast Mg-7Zn-0.3Mn-1Cu alloy was studied.The results show that a large amount of CuMgZn and MgZn2 precipitates distribute in the grain boundaries and grains from as-cast Mg-7Zn-0.3Mn-1Cu alloy after solution treatment at 410°C for 32h and aging treatment at 160°C for 15h,reaches its peak aging state,and the peak hardness is 88.2HV.Compared with the Mg-7Zn-0.3Mn alloy,reduce 5h time to reach the peak-aged and hardness increase 7.9 HV under the same heat treatment.Finally,the solid solution and aging of semi-solid microstructure of the Mg-7Zn-0.3Mn-1Cu alloy after holding at 585°C for 20 min were studied.The results show that the eutectic phase of non-dendritic Mg-7Zn-0.3Mn-1Cu alloy is gradually dissolved into secondary solidified area and primary?-Mg particles during solution treatment,the primary solid particles tend to merge and grow up in the same time.The atomic cluster or G.P.and a small amount of precipitated phases can be observed in the non-dendritic Mg-7Zn-0.3Mn-1Cu alloy in the early stage of aging.At the peak-aged,the distribution of the two types of strengthening phases is more uniform and the number increases obviously.One type is a long rod-like?₁'phase which has obvious strengthening effect on the alloy and is perpendicular to the base plane of{0001}_Mg?Another type is the plate-like?₂'phase with weak strengthening effect and parallel to the base plane of{0001}_Mg.