| In recent years,with the rapid development of aerospace and other fields,aluminum alloy has been widely used because of its low density,high strength and great corrosion resistance.At present,the method of improving the properties of aluminum alloy mainly includes adjusting the composition of alloy or heat treatment.Regression re-aging(RRA)is a method of heat treatment that can improve the comprehensive properties of aluminum alloy obviously.However,due to the limitation heat treatment of furnace,the shortest heat treatment time also requires 30 s and the temperature is no higher than 300?.The laser has the characteristics of high energy and high efficiency,which can make up for the deficiency of traditional heat treatment,shorten the regression time greatly and raise the regression temperature.In addition,the application environment of alloy becomes more and more complex.This will lead to higher and higher requirements for the high and low temperature properties of the alloy.The laser heat treatment is used to replace the furnace heat treatment of the traditional RRA in the regression stage.The best heat treatment process was studied by adjusting the laser parameters.The laser power was divided into 550 W,600W,650 W and 700 W,and the scanning speed was 1mm/s,2mm/s,3mm/s and 4mm/s respectively.The mechanical properties of the alloy after heat treatment were tested at room temperature.The fracture and microstructure of the alloy were observed by transmission electron microscope(TEM)and scanning electron microscope(TEM).The tensile properties of the alloy at high and low temperature were studied with the best process.The results show that 120?×16h+650W×2mm/s+120?×24h was the best parameter of laser regression re-aging(LRRA).The properties of the alloy were improved after heat treatment,which was slightly higher than that of the traditional RRA treatment.The tensile strength and elongation of the alloy was 552.3MPa and 10.3% respectively at room temperature.The performance loss of the alloy at high temperatures is severe,and the elongation is greatly increased.The toughness and plasticity of the alloy are improved at low temperature.The performance of the alloy after LRRA heat treatment was improved mainly due tothe existence of undissolved GP zones and ? 'phase in the matrix during the early stage of regression,which results the nucleation core of ?' phase increase greatly in the subsequent heat treatment.As the effectively dissolute of larger phases during the regression treatment,the smaller size of GP zones and ?' phase were precipitated again in the re-aging stage.The number of strengthening phases was increased.On the other hand,when the regression temperature is higher than the re-ageing temperature,the amount of Zn and Mg atoms dissolved in the matrix were increases.The amount of precipitation of ?' phase and ? phase was increased during re-aging treatment.Therefore,there is still a large number of ?' phase in the final matrix after heat treatment.The property of the alloy was improved.The loss of properties at high temperature is due to the appearance of necking and voids.The fracture form of aluminum alloy at high temperature tensile is still ductile fracture.The alloy experienced more deformation before fracture at low temperature.Besides the increasing number of dimples,layered fracture occurs at the fracture surface.All of these will causes the strength increase.The growth rate of alloy crack will slow down at low temperature.It is difficult for the fracture to occur.The strength and elongation of the alloy are improved. |
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