| Because of low density,high specific strength,and good corrosion resistance,6××× series aluminum alloys are widely used in aerospace,automotive,and many other indusries.As the requirements for better performance aluminum alloys increase,people begin to explore new material processing methods to achieve this goal.In this paper,the homogenization and aging heat-treatments of a new 6××× aluminum alloy were studied,and the optimised homogenization and aging heat-treatments parameters were determined.The effects of pre-aging and deformation on the properties of the alloy were systematically studied,mainly from microhardness,electrical conductivity and tensile properties.The effects of heat treatment on hardness,electrical conductivity,tensile properties and dispersoids were analyzed in detail by means of metallographic microscope,differential calorimeter,scanning electron microscope,universal testing machine and transmission electron microscope.The conclusions are as following:(1)The study on the as-cast microstructure of the new 6××× aluminum alloy shows that there were a large amount of non-equilibrium constituent paticles distributed along grain boundary.Three types of constituent particles were found.Al(MnFe)Si partiles were white with fishbone morphology.Small bright paticles were Q phase.And primary Mg2Si phases were black.(2)The initial over-burning temperature of the new 6××× series aluminum alloy was found to be 567.4? by DSC test,and the peak temperature of over-burning was 581?;Through the study of homogenization of the alloy,it is found that as the homogenization temperature time increases,the non-equilibrium phase at the grain boundary gradually decreases,and the Q phase dissolves completely.The larger the size of the dispersoids,the smaller the density.The optimum heat treatment process is 550?×10h,under this heat-treatment condition,the primary Mg2Si phase in the alloy is substantially dissolved,leaving only a small amount of insoluble gray iron-containing phase;When the temperature is 560?,the alloy is over-burned.(3)Through the study of aging heat treatment,it is found that the aging treatment at 180?×8h,the alloy has the best strengthening effect.The hardness is 129.1HV,the electrical conductivity is 25.1MS/m,the tensile strength is 345.8MPa,the yield strength is 317MPa,and the elongation is 13.2%.(4)Pre-aging treatment at 120?×80min before aging can not only improve the strength and hardness of the peak aging of the alloy,but also improve the elongation.Under this heattreatment condition,the mechanical properties of the alloy were as following:hardness of 131.7HV,electrical conductivity of 25.2MS/m,tensile strength of 356.7MPa,yield strength of 324.7MPa,and elongation of 13.9%.20%deformation treatment before aging can improve the peak ageing strength and hardness of the new 6××× series aluminum alloy and reduce the elongation.At this time,the properties of the alloy were:hardness of 132.7HV,electrical conductivity of 25MS/m,tensile strength of 384.3MPa,yield strength of 359.8MPa,and elongation of 8.95%.(5)It was found through the analysis of the tensile fracture that the dimple size of the tensile fracture of the sample after the pre-aging treatment became larger and the depth of the dimple became deeper.After the pre-deformation treatment,the sample found that the number of tensile fracture dimples decreased,and the depth of the dimples became shallow,indicating that the pre-aging effect effectively improved the plasticity of the material,and the predeformation reduced the plasticity of the material.Observation by transmission electron microscopy,after the material is pre-deformed,a large number of dislocations are generated in the matrix.These dislocations provide the nucleation sites for the precipitation phase,and many precipitates will nucleate on the dislocations,which improves the alloy mechanical properties. |
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