铝合金轮毂低压加局部挤压铸造工艺与组织性能的研究

发布时间：2018-06-05 04:45

本文选题：A356铝合金轮毂 + 低压加局部挤压铸造　；参考：《华南理工大学》2015年硕士论文

【摘要】：由于低压铸造成形压力较小,凝固过程中轮毂铸件厚壁部位易产生缩孔／缩松缺陷,导致轮毂力学性能和服役使用性能降低。局部挤压法作为一种专门技术,可通过对铸件厚壁部位施加压力而改善其内部孔洞缺陷,近年来在压铸工艺上得到广泛应用。本文提出了低压加局部挤压铸造的轮毂成形方法,重点研究了该成形中“铸造工艺—微观组织—力学性能”三者之间的联系。首先设计了轮毂低压加局部挤压铸造总体方案,利用AnyCasting软件对轮毂低压铸造充型和凝固过程进行模拟,研究了铸件关键部位凝固温度场的变化规律。结果表明,铸件中轮辋部位冷却速度最快,辐条部位次之,轮芯部位最慢。基于模拟结果,确定了局部挤压工艺参数范围,其中挤压延迟时间范围为67.63～103.15 s；挤压速度分为两个阶段,低速阶段(0.1 mm/sV10.2 mm/s)和高速阶段(0.4 mm/sV20.7 mm/s);挤压杆的最大位移为71mm。采用低压加局部挤压铸造工艺制备了A356铝合金轮毂,分析了不同工艺条件下铸件关键部位的微观组织,并基于X射线三维断层扫描技术研究了工艺条件对辐条部位内部孔洞三维形貌与特征的影响。结果表明,局部挤压工艺对轮毂铸件的作用主要体现在轮芯和辐条部位。对于轮芯部位,由于受到挤压力和冷却速度的共同作用,心部尚未凝固的金属液发生非平衡凝固,组织中含有大量的α-Al初生相。对于辐条部位,凝固前期受挤压力作用明显,得到来自轮芯的富Si液相的强制流动补缩,辐条处Si粒子体积分数较高；随着凝固的进行,凝壳变形抗力和枝晶阻碍力不断增大,补缩作用下降,导致辐条部位仍存在少量孔洞缺陷。研究了低压加局部挤压A356铝合金轮毂的拉伸和冲击性能,探讨了铸件微观组织与力学性能之间的关系。结果表明,与低压铸造工艺相比,经局部挤压后轮毂铸件的拉伸性能得到了提高。其中当挤压速度V2为0.5 mm/s,挤压时间T2为50s时,轮毂铸件的综合拉伸性能最优。轮毂铸件的抗拉强度和伸长率主要受孔洞和氧化膜等缺陷以及二次枝晶间距的影响；而屈服强度主要受Mg2Si析出相的影响。回归分析表明,铝合金轮毂铸件的冲击韧性主要受孔洞和氧化膜缺陷影响,总体上试样冲击韧性随着断面缺陷总面积的增加而降低。
[Abstract]:Due to low pressure casting forming pressure, shrinkage hole / shrinkage defect is easy to occur in the thick wall of hub casting during solidification, which results in the decrease of mechanical properties and service performance of hub. As a special technique, local extrusion can improve the internal cavity defects by applying pressure on the thick wall of castings. It has been widely used in die casting technology in recent years. In this paper, the forming method of wheel hub with low pressure and local squeeze casting is put forward, and the relationship between "casting process, microstructure and mechanical properties" in the forming process is mainly studied. Firstly, the overall scheme of hub low pressure and local squeeze casting is designed, and the mold filling and solidification process of hub low pressure casting are simulated by AnyCasting software, and the variation law of solidification temperature field in key parts of the casting is studied. The results show that the cooling speed of the rim is the fastest, the spoke is the second, and the core is the slowest. Based on the simulation results, the range of local extrusion parameters is determined, in which the extrusion delay time range is 67.63 ~ 103.15 s, the extrusion speed is divided into two stages, the low speed stage is 0. 1 mm/sV10.2 / mm / s) and the high speed stage is 0. 4 mm/sV20.7 / mm / s, and the maximum displacement of the extruding rod is 71 mm. The A356 aluminum alloy wheel hub was prepared by low pressure and local squeeze casting. The microstructure of the key parts of the casting under different technological conditions was analyzed. The effect of the process conditions on the three-dimensional morphology and characteristics of the voids in the spokes was studied based on the X-ray 3D scanning technique. The results show that the effect of local extrusion on hub casting is mainly reflected in wheel core and spoke position. For the core, due to the joint action of extrusion pressure and cooling rate, non-equilibrium solidification occurs in the liquid metal which has not yet solidified in the center, and there are a large number of 伪 -Al primary phases in the microstructure. For the spokes, the extrusion force is obvious in the early stage of solidification, the forced flow of Si-rich liquid phase from the wheel core is obtained, the volume fraction of Si particles at the spokes is higher, and with the solidification, the deformation resistance and dendrite resistance of solidification increase continuously. The decrease of shrinkage results in a small number of holes in the spokes. The tensile and impact properties of A356 aluminum alloy wheel with low pressure and local extrusion were studied, and the relationship between microstructure and mechanical properties of the casting was discussed. The results show that the tensile properties of the hub castings after local extrusion are improved compared with the low pressure casting process. When the extrusion speed V2 is 0.5 mm / s and the extrusion time T2 is 50 s, the overall tensile properties of the hub castings are optimal. The tensile strength and elongation of hub castings are mainly affected by defects such as holes and oxide films and secondary dendrite spacing while the yield strength is mainly affected by Mg2Si precipitates. Regression analysis shows that the impact toughness of aluminum alloy wheel castings is mainly affected by holes and oxide film defects, and the impact toughness decreases with the increase of the total area of section defects.
【学位授予单位】：华南理工大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TG249.2;TG146.21

【参考文献】