Effect of pressurizing speed on filling behavior of gradual expansion structure in low pressure casting of ZL205A alloy

The mold filling behavior of gradual expansion structure in low pressure casting was studied by two phase flow model using the Volume of Fluid method, and was verified by water simulation with a Plexiglas mold. To get smooth mold filling process and provide a guide for the pressurizing speed design in the producing practice, the mathematical model with the pressurizing speed, expansion angle and height of the gradual expansion structure was established. For validation experiments, ZL205 A alloy castings were produced under two different pressurizing speeds. Weibull probability plots were used to assess the fracture mechanisms under different pressurizing speeds. Mechanical properties of ZL205 A alloy were applied to assess the entrainment of oxide film. The results show that the filling process of a gradual expansion structure in a low pressure casting can be divided into the spreading stage and filling stage by gate velocity. The gate velocity continues to increase in the gradual expansion structure, and increases with the increase of pressurizing speed or expansion angle. Under the effect of the falling fluid raised by the jet flow along the sidewall, the fluid velocity decreases in the jet zone from ingate to free surface. As such, oxide film entrainment does not occur when the gate velocity is greater than the critical velocity, andthe gate velocity no longer reflects the real state of the free surface. The scatter of the mechanical properties is strongly affected by the entrainment of oxide films.

挤压铸造态6082铝合金的加工图及热变形行为

研究在不同压力下由挤压铸造工艺制备的6082铝合金的热变形行为和显微组织演化。对合金进行热压缩实验并建立3D热加工图。采用光学显微镜、扫描电子显微镜以及背散射电子衍射技术研究合金的显微组织演化。结果表明,在较高挤压铸造压力下得到的试样在热变形过程中产生较多动态再结晶晶粒。在高温下由于第二相颗粒的溶解,挤压铸造压力对热变形后显微组织的影响减弱。此外,当较高挤压铸造压力下制备的样品在低温及中等应变速率下变形时,不均匀第二相分布容易导致流变失稳的发生。最后,得到挤压铸造态6082铝合金的最优加工区域为430~500℃和0.01~1s^(-1)。