稀土Er对A356铝合金组织和性能的影响

发布时间：2018-05-21 00:11

  本文选题：A356合金 + Er　； 参考：《江苏理工学院》2017年硕士论文

【摘要】：A356铝合金具有良好的综合性能,合金的流动性好、气密性小、收缩率小以及热裂倾向性小。A356铝合金具有亚共晶成分和较宽的固液共存区,广泛应用在航空、汽车等领域。在铸造条件下,A356铝合金中初生α-Al枝晶较粗大,共晶Si容易形成粗大的针片状,粗大的共晶Si使基体被严重割裂,合金的强度、塑性大大降低。因此为了提高合金的力学性能,通常采用变质处理的方法。本文选用稀土Er作为细化剂,通过示差扫描量热分析(DSC)、X射线衍射(XRD)、扫描电镜(SEM)等分析手段来研究稀土Er对铸态A356铝合金组织和性能的影响。在分析出最佳稀土添加量的基础上,对变质后的合金进行不同的热处理,研究传统T6工艺对合金的影响,并优化工艺参数,进一步研究双级时效对合金的影响。在经稀土Er变质的基础上,添加微量元素Zr,研究复合变质对铸态合金组织和性能的影响。研究结果显示:稀土Er可细化A356合金的初生α-Al相,而且可使共晶硅发生变质。DSC分析可知稀土与铝发生共晶反应,生成了α-Al和Al3Er化合物。而稀土共晶反应先析出的α-Al可作为合金初生相的形核核心,细化晶粒,缩小二次枝晶间距。在硅相前沿,稀土Er元素容易形成成分过冷,使得硅相的原生长方向发生改变;部分吸附在硅相孪晶沟槽中的稀土原子嵌入了硅相晶格中,成为异类原子缺陷,引起晶格畸变,使硅在更多的方向产生孪晶,共晶硅的形貌发生改变,由粗大的板条状转变为细小的粒状、短棒状,具有明显的变质作用。当稀土Er元素含量为0.4%时,铸态合金组织得到明显改善,力学性能得到提高。A356+0.4%Er合金在T6工艺参数530℃×4h+175℃×6h下综合性能较优。同时研究了T6I6双级时效工艺对A356+0.4%Er合金组织和性能的影响,优化了工艺参数。当预时效为110℃×2h,再时效为200℃×4h时,合金的抗拉强度、延伸率和硬度比T6工艺处理分别提高了11.9%、4.4%、18.2%。在A356+0.4%Er合金中添加微量的Zr,α-Al细化更显著,硅相变质效果更好。尤其当Zr含量为0.2%,细化效果最佳,合金的抗拉强度可达225MPa,伸长率为6.4%,硬度为75.3HV,与单一Er元素变质相比,分别提高了32.7%、4.9%、23.8%。
[Abstract]:A356 aluminum alloy has good comprehensive properties, good fluidity, low gas tightness, low shrinkage and small hot cracking tendency. A356 aluminum alloy has hypoeutectic composition and wide solid-liquid coexistence zone, which is widely used in aviation, automobile and other fields. The primary 伪 -Al dendrites in A356 aluminum alloy are coarse under casting conditions. The eutectic Si is easy to form coarse needle flake, the coarse eutectic Si makes the matrix be severed seriously, and the strength and plasticity of the alloy are greatly reduced. Therefore, in order to improve the mechanical properties of the alloy, the modification method is usually adopted. In this paper, rare earth er was used as the refiner, and the effect of rare earth er on the microstructure and properties of as-cast A356 aluminum alloy was studied by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM) and so on. On the basis of analyzing the optimum amount of rare earth, different heat treatment was carried out on the modified alloy, the influence of traditional T6 process on the alloy was studied, and the technological parameters were optimized, and the effect of two-stage aging on the alloy was further studied. On the basis of rare earth er modification, the effect of complex modification on the microstructure and properties of as-cast alloys was studied by adding the trace element Zr. The results show that rare earth er can refine the primary 伪 -Al phase of A356 alloy, and the eutectic silicon can be modified by DSC. The results show that the rare earth reacts with aluminum to form 伪 -Al and Al3Er compounds. The 伪 -Al precipitated by the rare earth eutectic reaction can be used as the nucleation core of the primary phase of the alloy, refine the grain size and reduce the secondary dendrite spacing. At the front of silicon phase, rare earth er elements are easy to form subcooling elements, which make the original growth direction of silicon phase change, and some rare earth atoms adsorbed in silicon twin grooves are embedded in silicon phase lattice, resulting in defects of heterogeneous atoms, resulting in lattice distortion. The morphology of eutectic silicon is changed from coarse plate strip to fine grain shape, short bar shape and obvious metamorphism. When the content of rare earth er is 0.4, the microstructure of as-cast alloy is obviously improved, and the mechanical properties of .A356 0.4%Er alloy are improved at 530 鈩，

本文编号：1916855