Modification Effects of Lanthanum-Rich Mischmetal on A357 Alloy

A357 alloy was modified with lanthanum -rich mischmetal, and modification effects of the mischmetal on its as-cast and heat-treated structures, mechanical properties and hydrogen content were investigated and compared with that of strontium metal. It is shown that the modification effect of La-rich mischmetal is better than that of strontium in the aspects of refining eutectic silicon, ejecting hydrogen from the melt and improving comprehensive mechanical properties of the alloy. Therefore Al-Si-Mg alloy with high strength and improved toughness can be obtained by modification with La-rich mischmetal.

Effect of Solidification Condition on Secondary Dendrite Arm Spacing of the A357 Alloy under Counter-pressure Casting

The effect of counter-pressure casting parameters on secondary dendrite arm spacing （SDAS） of A357 alloy under different process parameters was studied. Quartz sand mould with chill can strongly decrease the SDAS. Reduced SDAS close to the mould bottom because of chilling was obtained. Pressure seems to have no apparent effect on the SDAS. In order to obtain casts with UTS ≥320 MPa, SDAS must be less than 55 μm, which means a local cooling rate VL≥0.23 ℃ /s.

Effects of beryllium and iron additions on iron-bearing phase in A357 aluminum alloys

Iron is the most deleterious impurity in the Al-Si-Mg casting alloys and can easily form inter-metallic compounds that can significantly affect the subsequent behavior of material properties.Using differential scanning calorimetry (DSC) and microstructural analysis, how the Be and Fe additions affect the iron-bearing phase in A357 alloys was investigated.The results show that the iron-bearing phase in A357 alloy comprises mainly the plate-like β-Al5FeSi and a small quantity of the script-type π-Al8FeMg3Si6; and that the plate-like β-Al5FeSi proportion increases with increasing iron content in the alloy.The iron-bearing phase is mostly transformed from the plate-like β-Al5FeSi to the script-type π-Al8FeMg3Si6 with the addition of Be in the alloy.The hardness of alloy samples was also tested.The results show that both the increasing iron content and Be content can increase the hardness of the alloy.This may be contributed to the change of morphology and distribution of the iron-bearing phase in A357 alloy with the addition of iron or Be to the alloy.

In-situ Observation of Tensile Fracture in A357 Casting Alloys

The mechanism of damage evolution and fracture in A357 casting alloys was investigated by in-situ scanning electron microscopy （SEM） tensile testing. Different microstructures of A357 casting alloys were produced by eutectic Si modification and T6 heat treatment. It is shown that microcracks in these alloys are predominantly formed in eutectic Si particles. Large and elongated eutectic Si particles in unmodified alloy show the greater tendency to cracking, whereas cracking of small and round eutectic Si particles in Sr modified and T6 heat treated alloys is relatively lag. The crack mainly propagates along the broken eutectic Si particles in unmodified and Sr modified alloys or along the deepened shear bands in T6 heat treated alloy with accumulating the applied strain. The results were discussed in terms of Weibull statistics and the fracture models were established.