Effect Of Laser Heat Treatment On Microstructure And Impact Properties Of 7075 Aluminum Alloy

With the vigorous development and continuous progress of material science,many new engineering material emerges constantly,the application range of the traditional engineering material has been expanded constantly,and the requirements for conventional mechanical properties of material have become more and more stringent,for example,the aluminum alloy parts do not bear a large impact load generally.However,with the rapid development of automobile manufacturing industry,the demand for lightweight structure members is becoming more and more urgent.The aluminum alloy not only has to bear a certain static load,but also has to bear a certain amount of impact load in service.Therefore the conventional mechanical properties of aluminum alloys include not only tensile strength,elongation and hardness,but also impact resistance.But up to now,the effect of heat treatment on the impact properties of aluminum alloys has been rarely reported.In this paper,7075 alloy was heat treated by laser retrogression and re-aging,the laser power of laser retrogression and re-aging is 550 W,600 W,650 W,700 W,the scan speed of the laser are all 2 mm/s,the impact properties of the alloy after heat treatment were tested at room temperature and low temperature,the fracture surface of the alloy was observed by scanning electron microscope?SEM?,and the microstructure of the alloy was observed by transmission electron microscope?TEM?,the surface roughness of the alloy fracture surface was measured by the laser confocal microscope,and the impact properties of the alloy were studied at room temperature and low temperature under laser retrogression and re-aging.The results show that the optimum parameters of laser retrogressing and re-aging treatment are 120?×16 h+650 W×2 mm/s+120?×16 h,after laser retrogression and re-aging treatment,the mechanical properties of the alloy were improved greatly.After laser retrogression and re-aging,the impact energy of the alloy can reach 19.72 J,and the hardness of the alloy can reach 194 HBW.The improvement of the alloy properties after LRRA treatment is mainly due to the fact that LRRA treatment can make the large size precipitated phase of 7075 alloy resolute better,and the small size strengthening phase precipitated in the re-aging stage is smaller and the volume fraction of the strengthening phase is larger.Therefore,the mechanical properties of the alloy have been improved by a great extent.With the decrease of impact temperature,the impact energy of 7075 alloy first increased and then decreased,and the extreme point appeared at-50?.The average strength energy E_stt under high impact load was the main influencing factor of the impact energy change of7075 alloy.The average strength energy E_stt under high impact load is determined by the increase of the average impact load f_aveve and the change of the metastable expansion displacement d_st.