| The thick-walled parts of low-pressure die cast (LPDC) wheel are prone to porosity defects during solidification due to insufficient forming pressure and shrinkage feeding, which seriously influence its mechanical properties and service performance. Local squeeze method as a special technology, which applies pressure to thick part to decrease porosity of castings, is widely use in die casting process. This paper presents a wheel forming method of LPDC added local squeeze, and mainly studies the relationship of casting processing, micro structure, and mechanical properties.The general scheme of LPDC added local squeeze for aluminum wheel was designed. By analyzing the mold filling and solidification stages of the wheel LPDC process, the temperature variation with solidification time at the critical parts of wheel casting were studied. The results show that the solidification sequence of the casting are in rim, spoke and hub parts. Based on the simulation results, the local squeeze process parameters were determined, including the delay time in the range of 67.63～103.15 s, the squeeze speed divided into low-speed stage (0.1 mm/s<V1<0.2 mm/s) and high-speed stage (0.4 mm/s<V2<0.7 mm/s), and the maximum displacement of squeeze pin was 71 mm.A356 aluminum alloy wheel castings were prepared by the LPDC added local squeeze process, and the microstructures in the hub, spoke and rim parts of wheel casting under different process conditions were investigated. In addition, based on X-ray three dimensional tomography inspection technology, the influence of local squeeze on the porosity defects in spoke specimens was analyzed. The results show that the local squeeze brought about the change in hub and spoke parts. In the hub part, a large number of primary phase α-Al were formed as the direct result of squeeze force and high cooling rate. In the spoke part, the forced feeding flow resulted from the squeeze force made the Si-riched liquid flow from the hub to spoke part at the early solidification, and the feeding decreased with the solidification proceeding due to the increasing solids, which led to few porosity defects in the spoke.The tensile and impact properties of A356 alloy wheel castings with LPDC added local squeeze were studied, and the relationship between the microstructure and mechanical properties of castings was discussed. The results show that A356 aluminum alloy wheels produced by LPDC added local squeeze process have better mechanical properties compared with those by LPDC. As the squeeze speed V2 was 0.5 mm/s and the squeezing time T2 was 50 s, the wheel castings have the optimal tensile properties. Tensile strength and elongation of castings were mainly affected by porosity, oxide film and SDAS, while yield strength was mainly affected by the distribution, quantity and size of Mg2Si precipitation phase. The results of regression analysis show that porosity and oxide film defects were the main factor for the decrease of impact toughness of aluminum alloy wheel castings. In general, the impact toughness of specimens decreased with the increase of area fraction of the defects. |
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