Solution treatment and age hardening normally cannot be applied to aluminum high pressure die castings, due to heating at high temperature leads to pores or blisters from entrapped air and hydrogen gases, and these defects cause the deterioration of mechanical properties and appearance quality. However, it can improve mechanical properties if the most effective solution treatment conditions for preventing pores by increasing solid solution amount such as Cu can be determined. This study focuses on rapid solution treatment until five minutes with the high temperature rising rate of 50℃/sec using high frequency induction heating. The effects of rapid solution treatment on the microstructure and hardness of JIS ADC12 high pressure die castings were investigated in terms of the formation of pores and progress of solid solution amount. The results were as follows. It was found that the area fraction of pores significantly increased when treated for more than five minutes at 490℃, three minutes at 500℃, and one minute at 510℃. The shape of the pores was round until 500℃, whereas crack shapes were observed at 510℃. Eutectic silicon was refined and spheroidized when solution treatment was carried out for one minute. However, treatment exceeding one minute resulted in eutectic silicon growth. Based on changes in the electric conductivity and Vickers hardness during solution treatment, the solid solution amount gradually increased till one minute and then saturated after that. From these results, the recommended solution treatment conditions for increasing hardness with reduced pores are treatment for one to three minutes at 490℃, or treatment within one minute at 500℃.
It has been reported that the elimination of trace phosphor changes eutectic Si phases from a coarse plate-like structure to a fine structure in Al-Si alloys, thereby enhancing their mechanical properties, particularly ductility. In the present study, the damage initiation and growth behavior in Al-7%Si alloys with and without trace phosphor of 10ppm were investigated by employing an in-situ X-ray microtomography technique. High resolution 3D/4D observation and subsequent image-based analyses enabled the detailed assessment of the damage evolution behavior.
From the results, it was concluded that both initiation and growth of micro voids originating from particle fractures are effectively suppressed in phosphor-free alloys, allowing drastic improvement in both the strength and ductility. By eliminating trace phosphor, dominant particle fracture is effectively postponed to later loading steps. It can be inferred that mechanical properties can further be improved by controlling the existence and geometrical distribution of hydrogen micro pores in phosphor-free alloys.
Core pins and core blocks are frequently used for aluminum alloy die-casting to produce net-shape parts and/or to prevent the parts from shrinkage in thick casting sections. However, for large casting, some other problems such as soldering of core pin due to insufficient cooling, leakage, etc. may also arise. The authors have studied the heat flow from casting to core pin under various casting conditions and developed an efficient cooling system. This paper reports the optimization of thin wall core pin shape from the perspective of heat flow and mechanicals of core pin. The optimum cooling channel diameter of the core pin should be 70 percent of the outer diameter of the core pin. This thin wall core pin in combination with high pressure water cooling would eliminate soldering and would give a longer core pin life than that of conventional solid core pin without internal cooling.
In order to improve the ductility of AC2B aluminum alloy, which is generally applied to automotive cylinder-heads, microstructure and tensile properties were studied using rheocast disc samples with sono-solidified slurry. Rheocast AC2B alloy with sono-solidified slurry shows superior tensile properties compared to those with normally solidified slurry. However, eutectic silicon morphology becomes slightly coarser in rheocast AC2B alloy with sono-solidified slurry, although sono-solidification provides the required granular primary α-Al grains for rheocasting. Primary silicon particles grown as the nucleus of AlP are recognized in the eutectic region of rheocast AC2B alloy with sono-solidified slurry, and they cause the coarse eutectic silicon morphology. Such coarse eutectic silicon morphology can be modified by the addition of strontium, which remarkably improves ductility. Impurity of iron in rheocast AC2B alloy reduces ductility owing to the easy crack initiation at the plate-like intermetallic compound of β-Al9Fe2Si2. The direct observation of sono-solidifying model liquid revealed that new crystals are generated at the collapsed sites of cavitation, requiring ultrasound to be irradiated to molten metal above the melting point for the effective grain refinement. Hence, by applying ultrasonic vibration to molten AC2B alloy above the melting point of α-Al8Fe2Si, tensile properties of rheocast alloy are improved by the appearance of granular intermetallic compound of α-Al8Fe2Si, instead of plate-like β-Al9Fe2Si2.