Influence Of Non-isothermal Heat Treatment On Microstructure And Properties Of Al-Zn-Mg Alloy

Al-Zn-Mg alloy has been widely used in equipment manufacturing industries,however,the degradation of service performance of this alloy after flame correction limits its application.Although some work aiming at the problem had been made,there is still a lack of a comprehensive and clear understanding on effect of the flame correction on microstructure and properties of Al-Zn-Mg alloy.In this dissertation,the influence of peak temperature,quenching medium and cycle numbers of the non-isothermal heat treatment based on flame correction on the mechanical properties and corrosion properties of Al-4.7Zn-1.2Mg(wt.%)alloy in T4 state were respectively investigated by tensile tests,micro-hardness measurements,differential scanning calorimetry(DSC)measurements,scanning electron microscopy(SEM)observation,and transmission electron microscopy(TEM)observation in order to provide some technical guidance and theoretical basis for industrial community.The main conclusions can be drawn as follows:The main strengthening precipitation,that is to say GP zones,within Al-Zn-Mg alloy of T4 state dissolved into matrix during the heating process of the non-isothermal heat treatment.Micro-hardness of the heat treated samples increased significantly as time went on at room temperature,which denoted natural aging could take place in the heat treated samples.When using water as quenching medium,tensile strength of samples after non-isothermal heat treatment all drop comparing with the base metal,although tensile strength of the samples after non-isothermal heat treatment at 350 ? is just a little higher than that at 300 ?.The greater vacancy concentration and the greater supersaturated solid solubility in the samples after non-isothermal heat treatment at 350 ?,which induce the precipitation of GP zones,are responsible for the strength difference.The exfoliation corrosion resistance,intergranular corrosion resistance,electrical conductivity of samples after non-isothermal heat treatment at 350 ? all decrease more noticeable than those at 300 ?,which is related to the coarser precipitations at grain boundaries and more even distribution of the solute atoms.When using water,oil and air as quenching medium respectively after heating samples to 300 ?,the microstructure of the corresponding samples display remarkable difference under TEM observation.GP zones occurred in all sorts of samples,but different level of ? phases could only be observed in samples quenched by oil and air.What should be noted is that precipitations at grain boundaries grow to unusually large in samples quenched by oil,which may be the main reason for the serious deterioration in intergranular corrosion resistance for the samples.The order of tensile strength for samples is water quenching> oil quenching> air quenching.Moreover,oil quenching leads to a more serious intergranular corrosion and a worse electrochemical properties compared with other quenching medium.Different cool rate is the reason for the above differences,that is to say,slow quenching decreases solutes and vacancy level in the solid solution,and thus lowers hardening potential,mechanical properties and corrosion properties,which arise quench sensitivity.When samples were subjected to non-isothermal heat treatment at 350 ? using water as quenching medium,the mechanical properties and corrosion properties both decreased with the increasing numbers of non-isothermal heat treatment.The increase of Mg segregation at grain boundary provides an explanation for the decrease in tensile strength and fracture morphology which transforms from ductile-brittle mixed to intergranular embrittlement.