| Magnesium alloy is a lightest metal structure material in practicalapplication, and has increasingly become an important material for automotive,aerospace, military, electronic communications industrials. ZK60alloy has agood extensive application prospect as a high strength wrought magnesium alloy.Unfortunately, it is easy to form shrinkage and microsegregation during ZK60alloy solidification due to the fact that ZK60alloy has a wider solidificationrang，resulting in its low casting property and applying limitation.It was been achieved that the solidification microstructure, compositiondistribution and properties of ZK60alloy were controlled through applyinghelical magnetic field during the solidification, which can improve castingproperty, mechanical property and plastic forming capacity. The effect of helicalmagnetic field stirring on solidification microstructure and refining mechanismof ZK60alloy, microstructure evolution, refining mechanism, deformationbehavior and fracture mechanism of ZK60alloy prepared by differentialtemperature extrusion were emphatically been studied, and the strengthening andtoughening mechanism of ZK60alloy prepared by differential temperatureextrusion was revealed. It hoped to find an effective method and theory toimprove mechanical property and forming property.In this paper, the solidification process of ZK60alloy was been controlledfirstly by a self-design helical magnetic field stirring device. The effect ofdifferent Electromagnetic processes on solidification microstructure, phasecomposition, element distribution, microhardness, mechanical property of ZK60 alloy were studied by optical microscope, scanning electronic microscope, X-raydiffraction, and finning mechanism of microstructure are explored. At90V ofexcitation voltage, alloy microstructures were fine and homogenous, resulting ingood finning effect. Applied helical magnetic field exacerbated non-equilibriumfreezing of ZK60alloy, obstructed the continuous precipitation of eutectic phases,increased the element solid solubility in solid solution, and promoted the elementuniform distribution to reduce the segregation, resulting in significant effect ofsolid solution strengthening and refined grain strengthening and increase inmicrohardness and mechanical property.ZK60alloy ingots were heat treated by different homogenization process tooptimize the homogenization schedule. The main precipitate phase was MgZn2after homogenization treatment, and small amount MgZn phases in as-cast alloydissolved in matrix after homogenization treatment. New precipitate phases werebeen observed by TEM, and new precipitate phases were small and dispersivelydistributed with increasing excitation voltage. The mechanical property was beenimproved, which was caused by precipitation strengthening. The appropriatehomogenization schedule was380℃/12h for the ZK60alloy. Non-equilibriumphases on grain boundaries in ingot were been eliminated and elementdistribution tends to uniformity after this treatment, which was consisting withthe result of homogenization dynamics analysis.The effect of normal extrusion and differential temperature extrusion onmicrostructure evolution of ZK60alloy from different scales was been analyzedby optical microscope (OM), scanning electronic microscope (SEM), X-raydiffraction (XRD) and transmission electron microscope (TEM). Differentialtemperature extrusion has a good finning effect for the microstructure of ZK60alloy; the microstructure was fined and uniform. Complete recrystallization wasoccurred during differential temperature extrusion, most of grain boundary washigh angle grain boundary and grain size was fine, ultra-fine grain (<1μm) wasobserved in extruded alloy which melted at90V of excitation voltage. The mainfining mechanism of differential temperature extrusion is dynamicrecrystallization, which has different occurrence mechanism under differentprocess.The effect of differential temperature extrusion process on room temperature mechanical property of ZK60alloy was been analyzed via tensiletesting. The mechanical property of the alloy was been improved afterdifferential temperature extrusion, the alloy had20.8%elongation,287MPa ofyield strength and388MPa at90V, and deformation uniformity of the alloy wasimproved. At low differential temperature extrusion temperature, twinning wasthe main deformation mechanism. With increasing temperature, the deformationway converted into dislocation sliding. When the temperature further increased,recrystallization occurred obviously. Recrystallized grains grew with temperature.The optimal process of differential temperature extrusion was350℃ofextrusion billet surface and150℃of temperature difference at90V. Subgrainand recrystallization microstructures were typical TEM microstructure of ZK60alloy under appropriate differential temperature extrusion process.Strengthening and toughening mechanism of differential temperatureextrusion for ZK60alloy showed that the mechanical property of the alloy wasbeen improved due to the synergistic effect of refined grain strengthening,second phase strengthening, dislocation strengthens and grain boundarystrengthening. Refined grain strengthening was one of the main strengtheningand toughening mechanisms: grain finning activated non-basal sliding and grainboundary sliding and suppressed twinning deformation, resulting in increase inplasticity of the alloy. Second phase strengthening was another importanttoughening mechanism: the types, quantity and distribution of the second phasecontroled the effect of second phase strengthening. The second phase was beenfined and the uniformity was improved with excitation voltage, leading to thealteration of mechanical property. Therefore, strengthening and tougheningmechanism of differential temperature extrusion for ZK60alloy was a compositestrengthening mechanisms of refined grain strengthening and second phasestrengthening. |
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