Basic Research For Application Of Hot Forming Process With Synchronous Cooling Of AA2024 Aluminum Alloy

As a kind of lightweight material,heat-treatable,high-strength aluminum alloys show great potential application in aerospace and auto industries.However,sheets of these alloys are difficult to be formed into complex contoured parts by traditional cold stamping processes because of their narrow plastic deformation range,low cracking resistance,and Young's modulus.Elevated forming temperature in warm/hot forming process can improve their formability,whereas bring drawbacks like grain growth,high energy consumption,and low productivity.Moreover,after a forming process,the formed parts also need solution and aging treatments to achieve the performance requirements,so the possibility of warp distortion during solution treatment exists.In this dissertation,based on the formability and microstructure evolution of heat-treatable,high-strength aluminum alloys during solution treatment,a novel technique—hot forming process with synchronous cooling?HFSC?was presented to solve the complicated shape formation of such alloys.And then the formability,instability criteria,forming limit,microstructures and strengthening mechanism of the heat-treatable,high-strength aluminum alloys formed by HFCS were investigated and analyzed both theoretically and experimentally taking AA2024 aluminium alloy as subject which has been widely used in aerospace manufacturing.The main work and the results obtained in this dissertation are summarized as follows:?1?Based on the hot stamping technological principle of high strength steel and the formability evolution of heat-treatable,high-strength aluminum alloys at elevated temperature,a novel forming process HFSC was brought up to solve the complicated shape formation of heat-treatable,high-strength aluminum alloys.?2?The rheological properties of AA2024 aluminium alloy under HFSC process were investigated with a Gleeble-3800 Thermal-Mechanical Simulation Tester.The influence of HFSC process and its parameter on the rheological properties of AA2024 aluminum alloy and the softening mechanism of this alloy under HFSC process were obtained.The results provide deformation characteristics foundation for application and technological parameter selection of HFSC process.?3?A viscoplasticity constitutive equation was obtained by multiple linear regression analysis and modification of Inoue's constitutive relation model based on the comprehensive analysis of the rheological properties of AA2024 aluminium alloy under HFSC process and a comparison of existing constitutive relation,which provides theoretical calculation foundation for simulation and optimization of HFSC process according to the theoretical calculating results are agree well with experimental dates.?4?The forming limit diagram?FLD?of AA2024 aluminum alloy under HFSC process was obtained based on theoretical analysis and Nakazima's experiment.The fracture mechanism of this alloy under HFSC process was investigated and discussed by SEM experiment.And then the influence of HFSC process and its parameters on the forming limit and the fracture mechanism of AA2024 aluminum alloy under HFSC process were discussed.The results shown that the HFSC can improve the heat-treatable aluminum alloy forming limit,in this condition the main fracture failure is plastic damage and the fracture mechanism is intercrystalline fracture.The above results provide the forming limit,instability criterion and fracture mechanism foundation for application of HFSC process.?5?Based on the comprehensive analysis of the microstructure evolvement and its influence factors of heat-treatable aluminum alloys at elevated temperature,the grain morphology,second phases and dislocation of AA2024 aluminium alloy after HFSC process were studyed by sheet forming,OEM and TEM experiment.The increasing of dislocation density and precipitation of some fine acicular particles in form of CuMgAl₂ were found in grain of AA2024 aluminium alloy underwent HFSC.The cause of dislocation density increasing and precipitation,as well as the influence of HFSC process and its parameters on the microstructure of AA2024 aluminium alloy was discussed.It provides the microstructure foundation for application of HFSC process.?6?Material property tests showed that the material strength of AA2024 aluminium alloy underwent HFSC increased with forming temperature.After natural ageing heat treatment,the yield strength and ultimate strength were increased 7%and 5%,respectively,due to the increased dislocation density and the appearance of fine needle CuMgAl₂ in AA2024 aluminium alloy.And then the hardening model of AA2024 aluminium alloy underwent HFSC was discussed based on analysis of the relationship between the strength of aluminium alloy and the second phase particle.?7?Based on analysis of the heat treatable aluminum alloy microstructure and strengthening mechanism,combined with microstructure observation and material property tests,the strengthening mechanism of AA2024 aluminium alloy underwent HFSC and natural ageing were obtained:the G.P.zone impediment effects on dislocation movement;dispersed phase Cu₂Mn₃Al₂₀0 and precipitated phase CuMgAl₂ pinning effects on dislocation and subboundary;interaction effects of dislocations.And the strengthening mechanism of AA2024 aluminium alloy underwent HFSC and artificial ageing were:dispersed phase Cu₂Mn₃Al₂₀ and precipitated phase CuMgAl₂ pinning effects on dislocation and subboundary and the interaction effects of dislocations.?8?According to the temperature variation of blank and molds during HFSC and the rheological properties and forming limit of AA2024 aluminium alloy obtain by experiments,a finite element?FE?model was established for HFSC process.The effects of multi-field coupling,stress field caused by inhomogeneous temperature distribution and change of strain field on forming were discussed.It provided a basis for forming tool design and forming parameters selection.And the practicability of computer aided FE modeling application for HFSC process was verified.?9?The feasibility and reliability of HFSC in production has been verified by the AA2024HFSC experiments.The HFSC process parameters effects on the AA2024 forming quality were investigated by dimension measurement of the formed parts.The feasibility basis of HFSC application in industry was proved,and the thermal mechanical coupled FE model was verified by comparing the simulation results with test results.The research results in this work show that HFSC can improve the formability of heat-treatable,high-strength aluminum alloy,simplify the production process of parts,improve the dimensional accuracy of the final products with reduced springback and no warp distortion,and produce parts with effective mechanical properties.Therefore this technique has a great aplication prospect in aviation,aerospace,transportation and other fields.