Fundamental Research Related To The Fabrication Technology For High Quality Thick Plates Of 7×75 Series Aluminum Alloys

The thick plates of aluminum alloys with high strength and toughness,excellent corrosion resistance and low residual stress are of great importance asstructural materials in the industry of aerospace, vehicles and armors.Compared with the developed countries, China has a long way to go in theaspect of fabrication technology of high-strength aluminum alloys thick plates,not only because of lacking special huge equipments for heavy strain on thethick plates, but because of weak basic research on the theory and technologyfor processing of aluminum alloy thick plates. It is the less understanding andinferior control on the structures evolution in the deformation, residual stress inthe thick plates, damage and fracture during service conditions, that make us lagbehind in this area. Nowadays, most of the high quality aluminum alloy thickplates are imported, seriously retarding our development in aerospace andtransport industry and national defence. Therefore, it is of far-reachingsignificance on the development of Chinese aluminum manufacturing industryto make deep and systematical and successive theory and technology researchon the high quality aluminum alloys and their thick plates.The present dissertation is supported by the National Key FundamentalResearch and Development Program of China (G1999064908). A series basicresearch on some key procedures of the fabrication processes for thick plates ofthe high quality 7x75 series aluminum alloys is carried out with the method ofcombination of physical simulation and numerical simulation. The primaryinvestigation contents of this paper include three main parts: study on the flowstress and structures evolution during hot deformation, research on thestructures refining by severe plastic strain and investigation on thequenching-induced residual stress. The main conclusions are as follows:1. Study on the hot deformation behaviors of high-strengthaluminum alloysThe flow stress behaviors and structures evolution during single-stage,double-stage and four-stage hot deformation are studied by isothermalcompression test, using 7075 aluminum alloy as the test material.(1) The flow stress curves of 7075 aluminum alloy are measured. The results show that the flow stress increases rapidly with the increase of strain, and afterreach a peak value it decreases or hold at a constant. The peak stress and flowstress rise with decreasing the deformation temperature and increasing the swainrote. The flow stress equation and peak stress equation are established, asshowed in formulas (3-14) and (3-15) respectively.(2) The effects of hot deformation conditions on the re-crystallized grainsizes of 7075 aluminum alloy are obtained, the results indicate that with theincrease of parameter Z, the re-crystallized grain sizes decrease. Thequantitative relation between parameter Z and average re-crystallized grain sizeD_r is set up in formula (3-16).(3) The softening laws and mechanism of 7075 aluminum alloy during thedouble-stage hot deformation gap are studied and verified. The results showthat the softening fractions are improved by increasing the holding time andtemperature of hot deformation and deformation gap. Softening at lowtemperature is resulted from recovery only, while at high temperature it isresulted from both recovery and re-crystallization.(4) The flow stress and structures evolution laws of 7075 aluminum alloyduring multi-stage hot deformation are gained by tests with two deformingprocedures. It is discovered that the flow stress during the multi-stagedeformation decrease with increase the deforming passes and decline thetemperature, because of the dynamic recovery and re-crystallization duringdeforming, and of the static recovery and re-crystallization during thedeformation gaps, and especially of the second phases precipitate from thematrix at high temperature.2. Study on structures refining of high quality aluminum alloysthick plates(1) An Intermediate Thermal-Mechanical Treatment (ITMT), which includesreinforced solid solution and heavy strain by combination of multi-directionalforging and rolling at warm temperature is proposed and studied for structuresrefining of high-strength aluminum alloy thick plates. The results of simulatedfabrication test of thick plates using the above procedures show that, thestructures of 7075 aluminum alloy thick plates with thickness more than 6mmare effectively refined, though the rolling reduction is not more than 80%. The average re-crystallized grain size is refined to around 8μm in the shorttransverse direction and 12μm in the long transverse direction and longitudinaldirection, and most of the second particles are refined to less than 5μm. Thisresearch provides a very important new method for fabrication of high qualityaluminum alloys thick plates in China.(2) A new process for fabrication of 7075 aluminum alloys thick plates isproposed which includes multi-directional forging for cogging, intermediatethermal-mechanical treatment (ITMT) and aging. The results of the tensile testshow that, with the combination of precipitation hardening and fine-grainedhardening, the tensile strengthσ_b and yield strengthσ_0.2 of 7075 aluminumalloy thick plates in T6 temper from the ITMT route of (LD+QA) are 15.7%and 11.6% higher respectively than that of the plates in T6 temper from theroute of hot rolling followed by solution and aging (HR+QA), and theelongationδ₅ of the former plate sample is also slightly enhanced from 10.4%to 11.2%.(3) The mechanism of grain refining with the above ITMT is broughtto light, which can be shortly summarized as: the fine, homogenous andequi-axial grains are refined by discontinuous re-crystallization, makingfull use of the heavy strain energy formed by the severe plasticdeformation and the feasible size and distribution of second particlesformed chiefly in the over-aging.(4) It is discovered that the coarse particles in the 7075 aluminumalloy ingot are often clustered by several second phases, which are formedduring melting and casting and are hard to solve in the matrix. Thesebrittle and hard coarse particles are often the origination of micro-cracksduring loading and deforming.3. Study on the quenching-induced residual stress of high qualityaluminum alloys thick plates(1) The effects of processes on the residual stress in quenched 7075aluminum alloy thick plates are systematically studied, the stresses aremeasured experimentally by the hole drilling method. For the first time,the cooling curves of the alloy in the quenchants with differenttemperature are obtained by quenching simulated tests. Processingparameters for 7075 aluminum alloy thick plates with high mechanical properties and low residual stress are optimized as: quenching in warmwater at 40～60℃, pre-stretching with plastic deformation of 2～2.5%,and aging at 140℃for 15 hours.(2) Besides the water quenchant, the influences of other twoquenchants, emulsion and polymer AQ251, on the quenching processand residual stress in 7075 aluminum alloy thick plates are investigatedby systematical experiments. For the first time, the cooling curves of7075 aluminum alloy in these two quenchants are measured at differenttemperature and different concentration, and the influent mechanism onthe residual stress are revealed. The optimized quenching parametersfor these two quenchants to reach low residual stress and excellentstructures and properties of the thick plates are obtained as: for AQ251polymer quenchant, the concentration is 30%, quenching at 40℃; foremulsion quenchant, the optimized concentration is 20～30% andquenching at room temperature.(3) The temperature fields, strain fields and stress fields in thealuminum alloy thick plates during quenching are studied by FEMnumerical simulation. For the first time, the critical conditions at whichthe stresses in the quenching plates change their symbols (at 7 secondafter quenching for the surface layer and at 13 second for the core part,respectively) are determined by point tracing method. A local balancedstress and strain shift model for explaining the formation mechanismand distribution laws of quenching-induced residual stress in thealuminum alloy thick plates is proposed in the present investigation.