7xxx系铝合金半连续铸造开裂缺陷的研究

发布时间：2018-07-03 18:53

本文选题：7×××系铝合金 + 半连续铸造　；参考：《北京科技大学》2016年博士论文

【摘要】：高强度7×××系铝合金因其优异的性能在航空航天等工业领域得到广泛的应用,然而该系铝合金在半连续铸造时往往会面临开裂缺陷这一严重问题的挑战。热裂是合金在凝固时形成的裂纹,与糊状区中热应力应变、液相补缩及半固态合金微观组织有关。冷裂是铸锭冷却到低温时发生的脆性开裂,是铸锭中残余应力和铸态合金塑性的恶化导致的。本文结合Gleeble拉伸实验、约束杆铸造、热收缩测量以及计算机模拟等方法对半连续铸造开裂现象及相关判据进行了系统研究。分别采用连续冷却法和原位凝固法测量了铸态7050铝合金在固态和半固态的机械性能,研究了合金强度和塑性随温度的变化趋势以及应变速率的作用,并根据所得实验数据拟合了扩展Ludwik方程和高温蠕变方程所需的参数。在此基础上建立了模拟铝合金半连续铸造的有限元模型,详细研究了大尺寸的圆铸锭和扁铸锭中温度场、流场和应力场分布以及工艺参数的变化所带来的影响。以Suyitno的SKK判据为基础,提出了一个适用于7×x×系铝合金半连续铸造的改进型热裂判据。新判据引入了两个重要的参数,分别是描述凝固过程中晶粒聚合情况的参数fGBWL和热裂纹在沿聚合晶界扩展时所消耗能量的固相能项Us。新热裂判据预测的糊状区热裂敏感性曲线与铸造经验比较一致。测量了几种具有不同Zn含量和Zn/Mg比的7×××系铝合金的热裂敏感性及其在凝固过程中的热收缩行为。研究发现,合金热裂倾向并不完全取决于其在凝固过程中的热收缩量,凝固时形成的非平衡共晶含量也对合金的热裂敏感性有深刻影响。晶粒细化可以显著降低7×××系铝合金的热裂倾向,但是过量加入细化剂又会使合金热裂敏感性出现明显上升。后者据推测可能是由于晶粒过度细化和在晶界形成大量含Ti一次析出相引起的液相补缩能力降低所致。根据实验结果和有限元模拟提出了一个冷裂指数,发现7×××系铝合金半连续铸造时,铸锭冷裂倾向在250。C以上很低,随着温度降至200。C以下开始迅速上升。在相同的铸造工艺条件下,铸锭半连铸冷裂倾向大体上与合金的非平衡共晶含量一致。这很可能是因为,低温下大量沿晶界分布的共晶极大地削弱了铸态合金的塑性,使铸锭更容易因应力集中而开裂。根据冷裂指数可知,采用刮水处理使铸锭中的残余应力降低,可以有效地减小合金在半连续铸造时发生冷裂的倾向。
[Abstract]:The high strength 7 脳脳脳 series aluminum alloy has been widely used in aerospace and other industrial fields because of its excellent properties. However, it often faces the challenge of cracking defect in semi-continuous casting. Hot cracking is a kind of crack formed during solidification of the alloy, which is related to the thermal stress and strain in the paste zone, liquid phase feeding and microstructure of the semi-solid alloy. Cold cracking is the brittle cracking of ingot when it is cooled to low temperature. It is caused by the deterioration of residual stress in ingot and the plasticity of as-cast alloy. In this paper, Gleeble tensile experiment, casting of restrained rod, thermal shrinkage measurement and computer simulation are used to study the cracking phenomenon and related criteria of semi-continuous casting. The mechanical properties of as-cast 7050 aluminum alloy in solid and semi-solid state were measured by continuous cooling method and in situ solidification method, respectively. The variation trend of strength and plasticity with temperature and the effect of strain rate on the strength and plasticity of the alloy were studied. The parameters of the extended Ludwik equation and the high temperature creep equation are fitted according to the experimental data. A finite element model for simulating semi-continuous casting of aluminum alloy is established, and the influence of temperature field, flow field and stress field distribution and technological parameters on the temperature field, flow field and stress field in large scale round ingot and flat ingot are studied in detail. Based on Suyitno's SKK criterion, an improved hot cracking criterion for 7 脳 x 脳 series aluminum alloy semi-continuous casting is proposed. The new criterion introduces two important parameters, fGBWL, which describe the state of grain polymerization during solidification, and the solid energy term, U _ s, of the energy consumed by the thermal crack propagating along the polymerized grain boundary. The hot crack sensitivity curve predicted by the new hot crack criterion is in good agreement with the casting experience. The hot cracking susceptibility and thermal shrinkage behavior of several 7 脳脳脳 series aluminum alloys with different Zn content and Zn / mg ratio during solidification were measured. It is found that the hot cracking tendency of the alloy does not depend entirely on its thermal shrinkage during solidification, and the non-equilibrium eutectic content formed during solidification also has a profound influence on the hot cracking sensitivity of the alloy. Grain refinement can significantly reduce the hot cracking tendency of 7 脳脳脳 series aluminum alloy, but excessive addition of refiner can obviously increase the hot cracking sensitivity of the alloy. The latter may be due to excessive grain refinement and the reduction of the liquid phase feeding capacity due to the formation of a large number of Ti containing primary precipitates at grain boundaries. Based on the experimental results and the finite element simulation, a cold cracking index is proposed. It is found that the cold cracking tendency of the ingot is very low above 250.C when the casting temperature of 7 脳脳脳 system aluminum alloy is semicontinuous casting, and it begins to rise rapidly with the temperature falling below 200.C. Under the same casting conditions, the cold cracking tendency of the ingot is approximately the same as the non-equilibrium eutectic content of the alloy. This is probably because at low temperature a large number of eutectic particles distributed along grain boundaries greatly weaken the plasticity of as-cast alloys and make ingots more susceptible to cracking due to stress concentration. According to the cold crack index, the residual stress in the ingot can be reduced by water scraping treatment, which can effectively reduce the cold cracking tendency of the alloy during semi-continuous casting.
【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG292

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