Heat Treatment Research Of 7A52 Aluminum Alloy Plate With High Strength And Weldability And Its Related Fundamental

In the present work, the foundational science issues of the heat-treatment process optimization and the welding joint structure evolution in the process of the preparation of 7A52 aluminum alloy have been extensively studied. Major attention have been paid to the effects of homogenization treatments process on the 7A52 alloy cast structure and properties and the effects of the process of solid solution and ageing treatment on the 7A52 alloy plate structure and properties by means of the modern physical research methods. In addition, the welding procedure and welding joint microstructure and properties of 7A52 aluminum alloy plate have also been investigated. The main conclusions were as follows:1. There was very severity dendrite segregation in the ingots of 7A52 aluminum alloy prepared by half-continuous casting method, it was necessary to deal with the ingots using homogenization treatment. In the homogenization process, at the temperature lower than 400℃, theη-phases were precipitated from the metastable super-solubility aluminum solid solution matrix; and at the temperature higher than 400℃, the dendrite segregations were gradually eliminated, theη-phases precipitated from metastable super-solubility aluminum solid solution matrix were re-dissolved into aluminum matrix. At the same time, the Al₆Mn primary crystals were precipitated from solid solution.2. In the homogenization process, the variations of hardness and specific conductivity of the alloys were regularity with the increase of homogenization temperature and time, which were mainly controlled by that theηphase precipitated from the super solubility aluminum solid solution matrix and theηphases re-dissolved into aluminum matrix. It has been found that the aluminum matrix lattice constant was closely connected with solid solubility, casting macro-stress and micro-strain. The homogenization process could be monitored by means of the measurement of aluminum matrix lattice constant.3. Annealing at 470℃for 24h was a suitable homogenization process for the 7A52 aluminum alloy ingots prepared by semi-continuous casting method. Under this condition, the dendrite segregations were eliminated, theη-phases were re-dissolved into aluminum matrix, the Al₆Mn primary crystals were completely precipitated from the matrix, the aluminum matrix lattice constant became stable and the casting macro-stress and micro-strain were basically eliminated.4. The solid solution temperature of 7A52 aluminum alloy could be enhanced effectively by two-stage strengthening solid solution treatment, which was higher than the multi-phase eutectic temperature; the remainder eutectic structure could be decreased effectively and the super-saturation degree of 7A52 alloy solid solution could be increased after quenching because of the increase of solid solution temperature of the alloy. Annealing at 460℃for 2h and then 480℃for 1h was a suitable two-stage solution treatment condition for the alloy.5. 7A52 aluminum alloy showed remarkable ageing hardening characteristic. the alloy plate, subjected to strengthening solid solution-treatment at 460℃for 2h and 480℃for 1h followed by ageing at 120℃for 24h, had a tensile strength of 495MPa, yield strength of 438MPa, elongation percent of 11.3% and specific conductivity of 29.16% IACS. However, the stress corrosion properties of the plates were poor, the reason was that theηphases precipitated continuously along the grain boundaries, the grain boundaries became the corrosion channels and the crack would be easy to initiate and grow in the grain boundary under an external stress.6. The strengthening solid solution treated at 460℃for 2h and 480℃for 1h followed by two-stage ageing at 105℃for 8h and 130℃for 14h was the best heat treatment condition for 7A52 aluminum alloy. Under this condition, the alloy had a tensile strength of 500MPa, yield strength of 444MPa, elongation percentage of 11.1 %, HB of 157and specific conductivity of 31.9%IACS.7. The corrosion properties of 7A52 alloy were controlled by G.P. zone,η',η, and PFZ. The electrode potential ofη' andηwere lower than that of the aluminum matrix and PFZ of the alloy, the micro-battery could be formed between theη' (orη) and aluminum matrix (or PFZ), andη' (orη) phase was dissolved as the anode. The corrosion resistance of the alloy was good as the precipitations along grain boundaries were discontinuous and coarse; otherwise, corrosion resistance of the alloy was poor as it was continuous and fine. The corrosion resistance of the alloy could be enhanced noticeably by two-stage aging.8. The solid solution-aged state 7A52 aluminum alloy plates were welded using argon-arc welding and Al-Mg welding wires with Sc, Zr and Ti additions, the welding joint had a tensile strength of 358MPa, yield strength of 238MPa, and elongation percentage of 6.6%. The welding coefficients reached to 0.72, which achieved the design objective of 7A52 aluminum alloy welding plate.9. The weld joint of the 7A52 aluminum alloy plate was composed of weld seam zone, semi fusion zone, heat-affection zone and base-material zone. The heat-affection zone could be divided into two zones, quenching zone and softening zone. The strength of seam was attributed to the fine-crystal and dispersion strengthening. The crystal grains of the seam could be refined by the Sc, Zr and Ti in the weld and the fine Al₃SC, Al₃Zr and TiAl₃ primary crystal phases could be dispersion precipitated during solidifying of seam. The soften zone was due to the coarsening of precipitation phase caused by the welding heating.