Research On Microstructure And Property Of Slow Shot Aluminium Alloy Die Casting

In conventional die casting process, liquid melts are injected into die cavities at a high speed. The turbulent filling usually causes gas entrapment, which finally results in porosities in the castings. The porosities are detrimental to casting tensile properties and may induce blister defects during high temperature solution treatment. To avoid the gas entrapment and increase the casting integrity and mechanical properties, several new technologies of die casting forming have been proposed and one of them is slow shot die casting. During this process, the shot speed is kept at a lower value through the whole cavity filling, and the further improvement on tensile properties of the castings manufactured by this new approach can be achieved using high temperature solution treatment without any blister defects. However, the microstructure, mechanical properties and fracture behavior of the slow shot die casting have not yet been reported.In this paper, microstructure, tensile properties and impact properties of slow shot hypoeutectic A356 die casting with partial squeeze have been investigated. The results show that as for the as-cast castings, eutectic Si particles are characterized as fibrous or long acicular shape as well as unhomogenously distribution; after T6 heat treatment, eutectic Si particles were changed to small granular or short bacilliform, and Si particle aspect ratio and area fraction decreased evidently. Effect of the microstructural characteristics on tensile properties and fracture behavior of partial squeeze added slow shot die-cast A356 alloy die casting in the as-cast and T6 heat-treated conditions was studied. The results show that inferior tensile properties of the casting partial squeeze part are caused by the heterogeneity ofα-Al cells with fragment, rosette, angular and globular shapes, while finer dendrites with smaller secondary dendrite arm spacing and more rounded silicon particles correspond to higher tensile properties. After T6 treatment, tensile properties increase significantly, due to the spheroidization of silicon particle and consequently the reduction of stress concentration at silicon/eutectic matrix interface. Differences observed in the tensile fracture path are attributed to microstructural changes as well as morphological aspects of silicon phase. It is shown that the impact absorbed energy of the as-cast castings varies from 1.17J to 2.35J and the energy of the T6-treated castings is between 1.45J and 3.80J. Furthermore, fracture mechanism and mode of the casting different regions are analyzed and discussed. The mathematical regression analysis indicates that the impact property is dependent on both Si particle aspect ratio and secondary dendrite arm spacing.The microstructure and mechanical properties of samples sectioned from regions with different wallthickness in slow shot hypereutectic ADC14 die castings were investigated. The results show that as the wall thicknesses increase and consequently the cooling rates decrease, the microstructure becomes coarser and the dendriticα-Al grain decreases significantly, while eutectic Si particles transform from granular to acicular shape, and the amount and size of primary Si phase increase remarkably. Moreover, lots of fishbone, Chinese script and flower shapedα-Fe phases and fine Al2Cu metallic compound were found in the microstructure. The relationship between primary Si phase and mechanical properties were investigated, which indicated that with the increase of equivalent circle diameter of primary Si phase, the tensile strength and elongation decrease but the hardness increases. The fractography of tensile samples from different regions was analysed using SEM. On the basis of in situ tensile observation, the crack initiation and propagation mechanics were investigated. The results show that the crack initiates in the aforehand nicks, interfaces between metallic compounds andα-Al matrix as well as coarse brittle phases, and then they grow along theα-Al grain boundaries by shearing encountered primary Si phase, brittle metallic compounds and Al-Si eutectic phase, and that if the crack meets the Al matrix in its propagation, the extending crack stops and a slight deflection occurs to enable the crack to continuously grow until the final fracture.