Research On HFQ^? Technology Of Automotive Cold-Rolled High-Strength Al Alloy And Its Verification In Engineering

The development of high-strength lightweight body materials is critical to vigorously promoting energy conservation and pollution reduction in the automobile industry.The 7xxx-series aluminum alloy exhibits significant lightweight advantages due to its relatively high specific strength and ductility,as well as its good toughness and corrosion resistance.However,it is practically impossible to form a 7xxx-series aluminum alloy sheet at room temperature.Hot forming and in-die quenching(HFQ^?)process is a technically feasible approach for producing 7xxx-series aluminum alloy components.As the research target,this paper employed automotive cold-rolled 7075aluminum alloy for the first time,which are suitable for industrial application of HFQ^?process.The evolution of microstructures and properties,hot tensile deformation behavior,the thermal forming limit,and quench sensitivity of cold-rolled7075 aluminum alloy are systematically studied by employing a variety of research analysis techniques such as microstructure and thermodynamic analysis,macroscopic mechanical property testing,theoretical modeling,and numerical calculation.The main associated accomplishments are as follows:（1）Starting with the evolution of microstructures and properties during the solution heat treatment process,this paper establishes qualitative and quantitative relationships between"solution parameters-microstructure-properties".The existence of a typical cold-rolled sheet texture distributed along theβ-fiber is revealed in the cold-rolled 7075 aluminum alloy sheet.Two distinct precipitates,i.e.,insoluble Al₂₃Cu Fe₄ and Mg₂Si particles,and solubleη-Mg Zn₂ particles are distributed in the matrix.For the solubleηparticles,a dissolution kinetic model is established,and the dissolution process is predicted.The research demonstrates that the cold-rolled sheet microstructure is completely softened by static recovery and static recrystallization after the solution heat treatment,forming a weak texture solid solution with randomly dispersed crystal orientation and that the content of static softened microstructure and grain size of sheets exhibit a strong correlation to the solution parameters.Furthermore,using the response surface methodology,the optimized process windows that balance the strength and toughness of sheets for the solution heat treatment are established from the perspective of strengthening and toughening the alloys.（2）The plastic flow behavior and microstructure evolution of the cold-rolled7075 aluminum alloy under HFQ^?process conditions are investigated in detail.The former is employed to correctly detect defect formation using numerical simulation,while the latter is utilized to support the regulation of microstructures and properties of hot-formed components.According to the results,the flow stress of cold-rolled7075 aluminum alloy is relatively sensitive to deformation temperature and strain rate.The plastic flow of cold-rolled 7075 aluminum alloy is successfully predicted using three constitutive models based on the plastic deformation characteristics of cold-rolled 7075 aluminum alloy.In addition,the HFQ^?process characteristics are employed to establish the hot processing maps based on the thermal tensile data.T>350℃is the temperature at which the HFQ^?process produces components without microstructure defects.It should be noted that the hot deformation process of cold-rolled 7075 aluminum alloy is essentially a competitive process of dynamic strengthening and softening.The softening mechanism is a mainly dynamic recovery and dynamic recrystallization,and the critical condition for dynamic recrystallization is T>350℃and ln Z<19.9.The crystal orientation of the<001>fiber texture is determined by texture analysis to be compatible with the stretching axis induced by the hot stretching of sheets.（3）Two theoretical models are developed based on M-K plastic instability theory and GTN meso-damage theory to estimate the thermal forming limit of cold-rolled 7075 aluminum alloy,thereby establishing the failure assessment criterion of cold-rolled 7075 aluminum alloy under HFQ^?process conditions.The results reveal that as the temperature or strain rate is increased,the thermal forming limit curves of cold-rolled 7075 aluminum alloy are lifted,and the lowest point of the thermal forming limit diagram shifts to the right region（i.e.,the positive strain ratio area）rather than the plane strain point.The established M-K model properly predicts the left half of the thermal forming limit of cold-rolled 7075 aluminum alloy,while the right half is conservatively predicted.The established GTN meso-damage model predicts the plastic damage behavior of cold-rolled 7075 aluminum alloy,enabling the complete thermal forming limits of cold-rolled 7075 aluminum alloy to be achieved under HFQ^?process conditions.（4）The quench sensitivity of cold-rolled 7075 aluminum alloy is investigated in detail to precisely regulate the cooling path of components during cold-die quenching,achieving accurate control of the microstructures and properties of hot-formed components.The crucial transfer time of cold-rolled 7075 aluminum alloy in HFQ^?process is presented as 12 s.The results demonstrate that the cold-rolled 7075aluminum alloy has a high quench sensitivity.The isothermal transformation C-curves exhibit a tip temperature of 350℃,an incubation period of 0.23 s,and a quenching sensitive temperature range of 271℃to 404℃.The critical cooling rate of cold-rolled 7075 aluminum alloy under a continuous cooling process is 65℃/s.（5）The HFQ^?process of B-pillar and anti-collision beam is investigated in detail by applying the research results of cold-rolled 7075 aluminum alloy in key technologies such as solution heat treatment,thermoplastic deformation,thermal forming limit and quench sensitivity.The results demonstrate that the established solution process windows,that balance the strength and toughness are suitable for the HFQ^?process of practical components.The established improved Arrhenius model,Johnson-Cook model,and Zerilli-Armstrong model precisely predict the plastic deformation behavior of actual components in HFQ^?process.The calculated thermal forming limits based on M-K plastic instability theory and GTN meso-damage theory successfully predict the forming defects of practical components.The obtained quench sensitivity data and critical cooling conditions of cold-rolled 7075 aluminum alloy are suitable for HFQ^?process of practical components.The hot-formed components without forming defects and with good strength and toughness are successfully manufactured.This part of work has comprehensively examined the above research conclusions on solution heat treatment,thermoplastic deformation,thermal forming limit and quench sensitivity,and can provide important theoretical support and systematic guidance scheme for the industrial application of HFQ^?process of automotive cold-rolled 7075 aluminum alloy.