【作者】 黄继武；

【导师】 尹志民；

【作者基本信息】 中南大学 ， 材料物理与化学， 2008， 博士

【摘要】 论文以7A52铝合金制备过程中的热处理工艺优化和焊接接头组织性能演变方面的基础科学问题为研究对象,采用近代物理研究方法着重研究了均匀化处理对7A52合金铸锭组织和性能的影响、固溶—时效处理方式和工艺对7A52合金板材组织与性能的影响,在此基础上还研究了7A52合金板材焊接及焊接接头组织与性能。主要结论如下:1.7A52合金半连续铸锭中存在比较严重的枝晶偏析,需要进行均匀化处理。铸锭均匀化过程中,400℃以下主要是亚稳过饱和铝基固溶体分解析出η相,400℃以上枝晶偏析逐渐消除、分解析出的η相重新溶入固溶体。与此同时,固溶体分解析出Al₆Mn初晶相。2.铸锭均匀化过程中,合金硬度和电导率随均匀化温度和时间的增加出现规律性的变化,这种变化主要由过饱和铝基固溶体分解析出η相和分解析出的η相重新溶入固溶体来控制。发现固溶体基体点阵常数与基体固溶度、宏观铸造应力和微观应力密切相关,可以通过点阵常数测量来监控铸锭均匀化过程。3.7A52合金半连续铸锭合宜的均匀化处理工艺为470℃保温24h。在此条件下,枝晶偏析消除、分解析出的η相重新溶入固溶体、Al₆Mn初晶相充分析出、固溶体基体点阵常数趋于稳定、宏观铸造应力和微观应力也基本消除。4.双级强化固溶能使7A52合金的固溶温度超过多相共晶点温度、更有效地减小和消除粗大的过剩相和增加淬火后固溶体的过饱和度。7A52合金板材合宜的双级固溶工艺为460℃/2h+480℃/1h。5.7A52铝合金有很强的时效强化效应,合金板材经460℃/2h+480℃/1h强化固溶之后的120℃/24h峰值时效,合金的抗拉强度、屈服强度、延伸率和电导率分别达到495MPa、438MPa、11.3%和29.16%IACS。但是,抗应力腐蚀性能差,主要原因是η相在晶界连续分布,晶界成为腐蚀通道,在外应力的作用下,裂纹容易在晶界处萌生和扩展。6.强化固溶-105℃/8h+130℃/14h双级时效是7A52铝合金最佳热处理工艺。在此条件下,合金的抗拉强度、屈服强度、延伸率、布氏硬度和电导率分别为500MPa、444MPa、11.1%、157和31.9%IACS。7.影响7A52铝合金腐蚀性能的主要因素为G.P.区、η′、η和无沉淀析出带（PFZ）。G.P.区、η′、η电极电位比较低,容易与铝基体和PFZ构成微电池成为阳极而溶解。晶界析出相尺寸越大分布越不连续,合金的抗蚀性能越好,反之晶界析出相尺寸越小且呈链状分布,则合金的抗蚀性能越差。双级时效处理后,合金的抗蚀性能显著改善。8.采用Sc、Zr和Ti复合微合金化的Al-Mg-Sc-Zr焊丝对固溶-时效态的7A52合金板材进行焊接,在氩弧焊条件下,焊接接头σ_b=358MPa,σ_0.2=238MPa,δ₅=6.6%,焊接系数为0.72,达到了7A52铝合金板材焊接设计的目标。9.7A52铝合金板材焊接接头上存在焊缝区、半熔化区、热影响区和基材区,其中热影响区又分为淬火区和软化区。焊缝的强度主要来源于Sc、Zr和Ti产生的晶粒细化强化以及焊缝凝固过程中形成的Al₃Sc、Al₃Zr和TiAl₃等初晶相的弥散强化。软化区是由于焊接加热时沉淀强化相粗化的结果。更多还原

【Abstract】 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.更多还原