新型7A56铝合金半连续铸锭组织及其在均匀化过程中的演化

发布时间：2018-09-01 15:29

【摘要】：追求更高的强度级别和更加优异的综合性能是7xxx系铝合金材料永恒的发展方向。提高合金化程度正成为7xxx系铝合金成分设计的主要趋势。然而,合金化程度提高会使得在制备的过程中产生更多的一次非平衡凝固析出相,促进第二相的回溶的难度会增大。本文以工业化条件下制备的高合金化新型7A56铝合金大规格半连续铸锭为研究对象,通过系统研究均匀化热处理过程中非平衡凝固析出相的回溶规律,为高合金化的7xxx系铝合金成分设计和热处理技术创新提供理论依据和技术支撑。本论文的研究重点是,研究铸锭中非平衡凝固析出相的回溶规律,及其对铝合金基体组织产生的影响。首先对合金铸锭横截面不同位置的微观组织、第二相及铝基体组织特征进行对比研究,研究掌握新型7A56铝合金大规格尺寸铸锭组织结构特点,评估组织均匀性对后续实验的可能影响。在此基础上,对均匀化过程中,合金中的非平衡凝固析出相的演化规律及其对铝基体组织的影响规律进行系统研究。研究发现,7A56铝合金铸锭的中心位置的合金元素质量分数较低,中心和中部硬度值较高,电导率较低,平均晶粒尺寸较大;铸锭边缘的硬度值较低,电导率高,平均晶粒尺寸细小。相对而言,铸锭中心和中部区域的均匀性较好。铸锭组织中有大量的非平衡凝固析出相,不同位置处的第二相类型基本相同。第二相主要包括低熔点富AlZnMgCu元素的相(T相),少量的Al2Cu相(θ相)和难溶的针状Al7Cu2Fe相,其中T相是主要的非平衡共晶相。中部典型位置处,铸态7A56合金的晶格常数平均为4.051 78A,硬度为102HV,电导率为22.8Ms/m。对7A56铝合金的单级均匀化热处理过程研究发现,温度会影响T相的转化以及回溶限度。在380℃下进行均匀化热处理,T相可以部分转化为Al2CuMg相(S相),大部分的T相仍然保留原有的共晶网状结构不变;T相的回溶量低于5%。在430℃下进行均匀化热处理,T相直接回溶,未观察到转化为S相的现象;残余的T相共晶结构内部发生溶解,边缘变得不连续;T相回溶量约50%-60%。在470℃下进行均匀化热处理,T相直接回溶,未观察到转化为S相的现象;残余的T相共晶结构被完全破坏,T相回溶量在90%以上。T相的回溶进程对铝基体组织会产生较为显著地影响,伴随着T相回溶量的不断增加,合金的硬度上升、电导率下降,晶格常数出现增大现象。对7A56铝合金的双级均匀化热处理过程研究发现,低温阶段(430℃/12h)处理后基体内残留的T相数量较多,继续通过高温阶段(470℃)处理T相可以快速回溶,在整个过程中未观察到T相向S相转化的现象发生。T相回溶对基体组织特征的影响规律和单级均匀化热处理过程基本类似。双级均匀化处理的第一级温度会影响纳米级析出相Al3Zr粒子的形貌,随着第一级处理温度的升高,方形的Al3Zr粒子数量呈现逐渐增加的趋势:经过400℃/12h+470℃/24h处理后,合金中析出的Al3Zr粒子呈现圆球状或花瓣状;经过430℃/12h+470℃/24h处理后,出现了少量的方形的Al3Zr粒子;经过450℃/12h+470℃/24h处理后,方形Al3Zr粒子的比例明显提高。圆形和方形的Al3Zr粒子的电子衍射花样相同,表明方形粒子和圆形粒子具有相似的晶体结构。
[Abstract]:The pursuit of higher strength grade and better comprehensive properties is the eternal development direction of 7xxx series aluminum alloys. Increasing alloying degree is becoming the main trend of composition design of 7xxx series aluminum alloys. In this paper, a new type of highly alloyed 7A56 aluminum alloy ingot with large size and semi-continuous casting was prepared under industrial conditions. The regularity of non-equilibrium solidified precipitates during homogenization heat treatment was studied systematically, which provided a basis for composition design and heat treatment technology innovation of highly alloyed 7xxx aluminum alloy. The research emphasis of this paper is to study the dissolution law of the non-equilibrium solidified precipitate phase in the ingot and its effect on the matrix structure of aluminum alloy. Based on the microstructure characteristics of the specimens and sizes of ingots, the possible effects of microstructure homogeneity on subsequent experiments were evaluated. The evolution of non-equilibrium solidified precipitates and their effects on the microstructure of aluminum matrix during homogenization were systematically studied. Low mass fraction, high hardness at center and center, low electrical conductivity and large average grain size; low hardness at the edge of ingot, high electrical conductivity and fine average grain size. Relatively speaking, the uniformity of the center and middle of ingot is good. There are a large number of non-equilibrium solidified precipitates in ingot structure, and the second phase at different positions. The second phase mainly consists of low melting point Al-Zn-Mg-Cu-rich phase (T phase), a small amount of Al_2Cu phase (theta phase) and insoluble acicular Al_7Cu_2Fe phase, in which T phase is the main non-equilibrium eutectic phase. The average lattice constant of as-cast 7A56 alloy is 4.051-78 A, the hardness is 102 HV, and the conductivity is 22.8 Ms/m. The single-stage homogenization heat treatment process showed that the temperature affected the transformation of T phase and the limit of solubilization. The T phase could be partially transformed into Al2CuMg phase (S phase) at 380 C. Most of the T phase still retained the original eutectic network structure; the solubilization of T phase was less than 5%. After homogenization heat treatment at 430 C, the T phase could be partially transformed into Al2CuMg phase (S phase). The residual T-phase eutectic structure was dissolved in the interior and the edge became discontinuous. The T-phase was about 50%-60%. Homogenization heat treatment at 470 C resulted in the T-phase directly dissolving and no S-phase transformation was observed. The residual T-phase eutectic structure was completely destroyed and the T-phase solubility was more than 90%. The hardness of the alloy increases, the conductivity decreases, and the lattice constant increases with the increase of the amount of T-phase. The two-stage homogenization heat treatment process of 7A56 aluminum alloy shows that the amount of residual T-phase in the matrix after low temperature treatment (430 C / 12h). In addition, the T-phase can be rapidly dissolved by continuous treatment at high temperature (470 C), and the transformation of T-phase to S-phase has not been observed during the whole process. With the increase of the first treatment temperature, the number of square Al_3Zr particles increased gradually. After 400 C/12h+470 C/24h treatment, the Al_3Zr particles precipitated from the alloy presented spherical or petal shape; after 430 C/12h+470 C/24h treatment, a small amount of square Al_3Zr particles appeared; after 450 C/12h+470 C/2 h treatment, a small amount of square Al_3Zr particles appeared. After 4 h treatment, the proportion of square Al_3Zr particles is obviously increased. The electron diffraction patterns of round and square Al_3Zr particles are the same, indicating that square particles and round particles have similar crystal structures.
【学位授予单位】：北京有色金属研究总院
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG146.21;TG292

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