After The Eutectic Al-20wt% Si Alloy Composite Metamorphic Study

In the present study, the complex modification of hypereutectic Al-20wt.%Si alloy is conducted by adding different contents of P and RE. The effect of P-RE complex modification is investigated on the microstructure and properties. Besides, the modification mechanism and strengthening mechanism of hypereutectic Al-Si alloy are discussed.The experimental results show that the refinement effect of P on the primary silicon is distinct. The modification of P makes the coarse block primary silicon be refined to granular ones and the edges and angles of primary silicon are passivated. The optimal refinement effect of primary silicon is obtained when the alloy contains 0.08wt.%P. The primary silicon particles are refined to 28.4μm from 66.4μm of the unmodified alloys. Excess P is unfavorable to the refinement of primary silicon. It can be seen from the profile of DSC that the crystallizing temperature of primary silicon increased about 10℃after the modification of P. Under the same cooling condition, the undercooling of hypereutectic Al-20wt.%Si alloy was improved. The modification of primary silicon is attributed to heterogeneous nucleation of A1P and the increment of crystallizing temperature after the modification of P.The complex modifications of P-Sr and P-Sb can't modify both primary and eutectic silicon simultaneously, while the complex modification of P-RE can well modify both primary and eutectic silicon at the same time. The P-RE complex modification makes the coarse block primary silicon be obviously refined to granular ones and the large needle eutectic silicon be modified to fibrous ones. When adding the content of P and RE to the alloy is 0.08wt.%P and 0.9wt.%RE, the modification of primary silicon particles is optimal and the primary silicon particles are refined from 66.4μm to 23.3μm. The best modification effect on the eutectic silicon particles can be obtained with the addition of 0.06wt.%P and 0.6wt.%RE. The size of eutectic silicon particles decreases from 8.34μm to 5.22μm and the roundness is improved from 0.41 to 0.58. P decreases the modification of RE on eutectic silicon in the P-RE complex modification. It may be related to the formation of CeP which results in the decrease of the effective RE content.The properties of hypereutectic Al-20wt.%Si alloys are improved obviously after the P-RE complex modification. When the tested alloys modified with 0.08wt.%P and 0.6wt.%RE, the optimal properties are obtained. The tensile strength is improved to 306 MPa from 256 MPa of the unmodified alloy and the ductility is improved to 0.48% from 0.35% of the unmodified alloy, at room temperature respectively. The tensile strength is improved to 187 MPa from 140 MPa and the ductility is improved to 1.58% from 0.84%, at 300℃respectively. Under the same condition, the weight loss is decreased to 4.225mg from 5.2mg of the unmodified alloy. The improvement of properties is related to the improvement of the fracture resistance of Si phase and the bonding force between Si phase and matrix, which is attributed to modification of primary and eutectic silicon in hypereutectic Al-Si alloy.