Research And Optimize Of Heat Treatment Technique Of ZM61Magnesium Alloys

In this paper, the effect of heat treatment techniques on microstructure andproperties of Mg-6Zn-1Mn wrought magnesium alloys were investigated systematicallyby using microstructure observation, scanning electron microscopy （SEM）, EDS, tensiletest and hardness test. The necessarily of solution before aging of aging-strengthen alloyis testified and the solution technique was researched. The2-step aging technique werebe optimized through changing the temperatures and time of pre-aging. Additionally, theinterrupted multi-step aging was researched exploratory and the effect ofpre-deformation on microstructure and properties of ZM61were investigated by usingEDS, XRD and TEM. The results as follows:（1） The microstructure of Mg-6wt.%Zn-1wt.%Mn cast is branch grain and Mg₇Zn₃dispersed at boundaries. Cast segregation disappeared by homogenized of330℃×24hand microstructures became uniform. Mg₇Zn₃diffused gradually and dispersion newcompound MgZn again in the codling phase. During this process, a little α-Mn appearedand acted as dividing mixed. Wrought magnesium alloy could be extruded at320～420℃while complete dynamic recrystallization took place during extruding.Microstructure of alloy prefer to large and unusual size grains if extruded at420℃and as-extruded alloy hasn’t fine properties. As well as, extruding at320℃got finemicrostructures. MgZn comes from Mg₇Zn₃didn’t dissolve in matrix completely duringextruding but left behind and the high temperature, the lower left.（2） Refer to ZM61alloy, if it’s intensity were be improved by heat-treatment, thesolution before aging is necessary and the best solution technique is420℃×2h.Compared with T5（including Single aging and Double aging）,“Solution+aging”（especially T4+2-step-aging） can improved intensity of alloy, UTS was362MPa.The fine grains grow up after solution, while alloy extruded in high temperature hascompleted partly solution. Aging treatment has little effect on the size of grains. Gonealong420℃×2h, the un-dissolve Mg-Zn compound in extruded alloy dissolved inmatrix drastically and the α-Mn particles began to precipitate to make the preparationfor following aging. During the pre-aging of “2-step-aging”, dispersion α-Mn and G.P.zones appeared, dispersion β’ precipitated in peak-aged.（3） In the “2-step-aging” of ZM61, the time of pre-aging longer, the hardness afterlast-aging higher, and the time of peak during last-aging almost4h. The UTS reaches 370MPa through90℃×32h+180℃×4h.110℃×5h+180℃×4h can shrink theaging time largely when the needed intensity lower350Map. Considering all of this,90℃×12h+180℃×4h is the best technique, after16h heat treatment, alloy has perfectproperties: YS330MPa, UTS350MPa and5.5%. After different pre-aging, themicrostructures have same characteristics: the clean grains and fine structure. Thenumber of G.P. zones after90℃×32h is larger than110℃×5h or90℃×12h. Theresult is dispersion rod-shaped β1’ precipitated to strengthen alloy.（4） The hardness of ZM61alloy reached peak when last-aging2h of T6I6techniques（T6I6-1:180℃×10min+90℃×32h+180℃×x h; T6I6-2:180℃×10min+70℃×60h+180℃×x h）, the peak is95HV. The UTS is365MPa and YS is352MPa after T6I6third step-12h. Compared with T6peak-aging, the intensities of ZM61alloy improved significantly after T6I6, while T6I6wouldn’t increased mechanicalproperties higher to the fine2-step-aging, only the YS increased again. T6I6hasn’tsuperiority at the sight of technique-optimizing. During the first step and second step ofT6I6, only Mn element continued to precipitate and appeared to the more α-Mnparticles and large numbers of G.P. zones （especially the second step）. The speed ofstrengthen improved sharply in third-step aging because they acted as the heterogeneousnuclei towards metastable particles, especially rod-like β1’. It leads to accelerating theprecipitation kinetics and improved intensity of alloy.（5） Pre-deformation can significantly accelerate the precipitation kinetics of ZM61alloy aging, short the aging time of peak hardness and approved the ratio of YS andUTS near to1, which enlarged range of actual application of ZM61alloy. In the termsof microstructures, many twins and dislocations appeared after pre-deformation and thegreater the deformation, the more twins. Dislocations could act as the nucleus for β’ toaccelerate the speed of precipitate. During the aging at some temperature, alloy withdifferent deformation has assimilated rise of intensity, deformation by5%a little higherthan deformation3%. The tenacity of pre-deformation alloy decreased a lot, and3%deformation has made the strengthen effect exerted as large as possible. When alloyaging at different temperatures after typical deformation, the strengthen effect almostequal.