Study On Microstructure And Properties Of 7075 Aluminum Alloy With The Contact Solid-solution Treatment

Lightweight and safety have always been two important goals for developing the automobile industry.Lightweight of the car can effectively reduce weight,thereby reducing fuel consumption and improving energy efficiency.In recent years,the research and application of aluminum alloys instead of steel in automotive parts materials is of great significance for the development of automobile lightweight.Among them,7075 aluminum alloy is gradually becoming an ideal choice for automotive parts due to its high specific strength,strong impact resistance,and good corrosion resistance.At present,the hot stamping process of aluminum alloy can be used in the process of 7075 aluminum alloy automotive parts.Its process is to first treat aluminum alloy sheet with solid solution,then transfer it to the low-temperature die quickly to form and quench,and finally increase the strength of the part through aging treatment.At present,heating furnaces with forced air convection are widely used in the solid-solution treatment of 7075 aluminum alloy sheet metal,which needs 15-30min;however,the subsequent forming and quenching only needs 20 s.The long solution treatment in the early stage cannot synchronize with forming and quenching,which severely restricts the production and application of the hot stamping technology.Combined with low reflectivity and high thermal conductivity of aluminum alloy,this paper proposed the aluminum alloy contact solid-solution technology,which can shorten the solution time within 30 s.The contact solid-solution device can be installed on the same press with the forming and quenching die in order to facilitate sheet transfer,improve productive efficiency,and save cost of production with huge market potential.The influence of contact solid-solution technology on the microstructures and properties of 7075 aluminum alloy is mainly studied in this paper.First,through the optimized distribution design of the electric heating tube in the contact body,a self-made contact solid-solution experimental device was made;Secondly,through a contrast experiment between the contact solid-solution method and the furnace solid-solution method,the feasibility and advancement of the contact solid-solution method was explored;Then,the effect of contact body temperature and holding time on the mechanical properties and microstructure of 7075 aluminum alloy were investigated,and the second phase evolution mechanism was analyzed to obtain a reasonable contact solid-solution system;Finally,the effect of contact solid-solution treatment on the peak aging of 7075 aluminum alloy was studied.The main conclusions are as follows:1.Compared with the average heating rate of the furnace solid-solution treatment,which is 0.4?/s,the heating rate of the contact solid-solution treatment is 15.8?/s.Its high heating rates results in a large heat input per unit time and a narrow phase dissolution temperature range,which accelerates the dissolution rate of the secondary phase and improve the solid solubility and complete the solid solution in 30s.After T6aging treatment,the precipitated phase in the sample with the contact solid-solution treatment is uniformly dispersed,and the grain size is basically the same as the sample with the furnace solid-solution treatment.The main strengthening mechanism is precipitation strengthening;2.In the contact solid-solution treatment,the temperature of the contact body ranges from 465-495? and the holding time ranges from 10-50 s.With the increase of the contact body temperature and the holding time,the solubility of the secondary phase increased and the grain size varied little with different solid solution parameters.The secondary phase dissolved most at 495? for 30 s.After aging,the microstructure of this sample showed that the Orowan strengthening mechanism could increase the strength by 442 MPa,which accounts for 78.2%of the ultratensile strength of 565 MPa.At 495? for 30 s,the insoluble phase mainly composed of Mg₂Si in the alloy is coarsened,so that the solute atoms are agglomerated and the strength after aging is reduced;3.When the temperature of the contact body is 530?,the contact solid-solution treatment can be completed within 10 s.The material temperature reaches 450? in 5s during the ultra-high temperature solid-solution process,which avoids the accumulation of the hardly soluble phases such as Al₂CuMg in the low temperature range.The increase of the diffusion coefficient accelerates the dissolution of the hardly soluble phases at high temperature.The shorter holding time can also prevent the tissue from over-burning and coarsening of the insoluble phase.After artificial aging,the ultratensile strength of the material reaches 545 MPa;4.Compared with the peak aging of the sample with the furnace solid-solution treatment for 24 h,the contact solid-solution treatment can shorten the peak aging time to 18 h.The high solubility of the sample with the contact solid-solution treatment causes the temperature range of the phase to be large and accelerates the second phase precipitation.