| 7075aluminium alloy is an age-hardening alloy with a series of advantages such as low density, high specific strength and toughness, good formability and processing property, lower cost, etc. And it has been widely used in aero field, transportation and other high stress structure parts requiring lightweight, high strength and good corrosion resistance.In this paper, the microstructure and mechanical properties were studied by using optical microscopy(OM), scanning electron microscopy(SEM), energy dispersive spectroscopy, tensile test and Vickers hardness.The strengthening mechanism has been discussed and optimized the heat treatment processing.The major conclusions are drawn as follows.Along with the solution temperature rising and solution time prolonging, the solid solubility gets higher, the number of coarse second phase reduces and the size reduces, too.EDS shows these coarse second phase are AlZnMgCuFe phase riching in Fe and Cu and Si-O phase riching in Si and O.The tensile strength, yield strength, elongation and Vickers hardness of the alloy increase at first and then decrease gradually. When the solution temperature too low or the solution time too short, the solute atoms can’t fully dissolve to matrix and can’t give full play to next ageing process.Conversely, the grain has grown up tendency which makes the properties decrease.The best solution process is (470±5)℃X60min.7075has a significant effect of ageing precipitation strengthening, and the major ageing precipitation phases include GP zones, η’phase (MgZn2) and η phase (MgZn2). The main strengthening phases are GP zones and η’phase. With the ageing temperature rising and ageing time prolonging, the GP zones transform to η’phase, and when the temperature is over120℃or the time is longer than24hours, some parts of r]’phase grow up and coarsen to η phase. Both the tensile strength and Vickers hardness increase to the peak values and then decrease by degrees, while the elongation decreases in early ageing stage and increases slightly in later ageing stage. The peak ageing process is120℃X24h, and under this process, the tensile strength is655MPa,yield strength is576MPa, elongation is11.4%and Vickers hardness is 205.6HV.According to the result of orthogonal test of double ageing, the ageing temperature of the second step ageing has the most significant influence on the tensile strength and Vickers hardness, and the first step ageing temperature has the most significant influence on the elongation. When the second step ageing temperature up to180℃, the ageing precipitation phase occurs seriously gathering, coarsening and growing, and more discontinuous distribution of grain boundaries precipitation and wider precipitation free zone of grain boundaries appeared, which greatly decrease the tensile strength and Vickers hardness but keep higher elongation. After double ageing process, the major strengthening phases are η’and η. The optimal double ageing process is110℃X5h+1500C X14h, at which the tensile strength is625MPa, the Vickers hardness is201.3HV and the elongation is12.0%. Compared with the peak ageing, the strength and hardness decrease but the elongation slightly increases.The fracture mechanism of the alloy after single ageing and double ageing both are the mixed type of transgranular fracture and intergranular fracture, and the trend of intergranular fracture increases under double ageing. The coarse second phases at the bottom of the dimples are AlCuFeSi and AlMgSi containing impurity element Fe and Si. These coarse second phases with irregular shape and a lot of volume distribute in the grain boundary and the grain, cause local plastic deformation ability to drop, produce dislocation pile-up and stress concertration at the interface of coarse second phases and matrix, generate pore and micro cracks. These micro cracks extend gradually, promoting matrix to fracture at local parts, and being harmful for plasticity and toughness of the alloy. |
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