A Research On Microstructure And Mechanical Properties Of 6063 Aluminum Alloy Prepared By Hot Extrusion Process

The greenhouse effect and energy shortage have become urgent problems to be solved worldwide.In automobile industry,using aluminum alloys to replace steels is an important means to reduce the weight of vehicle and save energy.6063 is a heat-treatable wrought aluminum alloy,which has been widely used in automobile manufacturing due to its low density,high specific strength,excellent extrusion formability and good corrosion resistance.Hot extrusion is one of the commonly used techniques of aluminum profiles production.In order to improve the mechanical properties and production efficiency of 6063 aluminum alloy,microstructure evolution and mechanical properties of the alloy at high extrusion ratio and high extrusion speed were studied.Firstly,the microstructure characteristics of as-cast 6063aluminum alloy were analyzed,and the appropriate parameters of homogenization treatment were determined.Secondly,the thermal deformation behavior of 6063 aluminum alloy under different conditions was studied by hot compression test,and the processing maps were constructed in order to obtain the optimum processing parameter.Then the effect of extrusion ratio,especially the high extrusion ratio,and extrusion speed on the microstructure and mechanical properties of the alloy was studied by hot extrusion process.Finally,the extruded rod with optimum properties was slected and subjected to heat treatment.The effect of aging time on the microstructure and properties of the alloy were analyzed.The results of hot compression tests illustrate that the flow stress of 6063 aluminum alloy is affected by strain,deformation temperature and strain rate during the hot deformation.At the initial stage of deformation,the flow stress increases sharply with the increasing strain,and then shows two kinds of trend:when the strain rate is 1-10 s^-1,the flow stress changes little with the increase of strain;when the strain rate is 20 s^-1,the flow stress decreases with increasing strain after reaching the peak stress.From the processing maps,it is found that the instability zone expand to high temperature and high strain rate region with the increase of strain.Microstructure observation shows that shear bands and flow localization exist in the instability region,while the microstructure in the safe region is more homogenous.Dynamic recrystallization occurres at high temperature and low strain rate condition.According to the processing maps and microstructure analysis,the optimized deformation temperature of the alloy is 460-490?and the strain rate is 2-6.3 s^-1.The activation energy of the alloy is 161.51k J/mol and the Arrhenius-type constitutive equation can be expressed as:The results of hot extrusion process shows that surface defect-free extruded rods of 6063aluminum alloy can be obtained under high extrusion ratio and different extrusion speed.Dynamic recrystallization occurres during hot extrusion.The grains size are decreased continuously with the extrusion ratio increases.The microstructures are transformed from elongated grains along the extrusion direction to completely equiaxed recrystallized grains.The mechanical properties of the alloy increase with the increasing extrusion ratio due to grain refinement.When extrusion ratio is 156,the tensile strength and elongation of the alloy reach the maximum,and the values are 239 MPa and 27.4%,respectively.The as-extruded tensile morphologies composed of a large number of dimples and the fracture mechanism is ductile fracture.The grain size increases slightly and mechanical properties decrease with the extrusion speed increasing.The alloy has apparent preferential orientation after extrusion.The texture transform to<100>fiber texture and the intensity increases with the increasing extrusion ratio.The increase of extrusion speed has little effect on the alloy texture.The heat treatment experimental works indicate that,after heat treatment,the initial equaixed grains grow coarse.Most of the small precipitations are dissolved into matrix and only some coarse impurity phase remain after 530?50 min solution treatment.The tensile strength and elongation has been increased obviously,which the values are 264 MPa and30.3%,respectively.After artificial aging treatment,the fine needle-like?"phase re-precipitate from the matrix and gradually transform to?'phase and??Mg₂Si?phase with increasing aging time.The microhardness curve shows a significant"bimodal"characteristic and reaches its peak at 6 h and 15 h,which the values were 87.2 HV and 94.9 HV,respectively.The tensile strengths of the samples aging for 6 h and 15 h are 301 MPa and 339MPa,while the elongations are 16.7%and 16.4%,respectively.The fracture mechanism of solution state is ductile fracture,while aging-state is brittle fracture accompanied with some ductile fracture.