Effect of Solidification Processing Parameters and Silicon Content on the Dendritic Spacing and Hardness in Hypoeutectic Al-Si Alloys

Aluminium with silicon as alloying element, form a class of material providing the most significant part of all material casting manufactured materials. These alloys have a wide range of applications in the automotive and aerospace industries. It is widely recognized that moderate addition of silicon to aluminum, significantly improves the resulting mechanical properties. Hypoeutectic Al-Si alloys were used in the present experimental study to investigate the solidification parameters effect and Si 3-5 wt% addition on the microstructural features and resulting hardness in a vertical directional solidification system. The hypoeutectic alloys were directionally solidified under transient heat flow conditions in a range of cooling rates from 0.3 to 4 ºC/s. Characterization analyses by optical microscopy indicate clearly that secondary dendritic arm spacing (λ2) increases significantly with the decrease in solidification speed (SP) and cooling rate (Ṫ). On the other hand, experimental growth laws relating the secondary dendritic arm spacing (λ2) to the solidification speed (SP) and cooling rate (Ṫ) indicate that Si addition from 3 wt% to 5 wt% has induced a thickening effect leading to increase in λ2 . While, experimental results have shown that the resulting hardness increase as solidification processing parameters (SP and Ṫ ) increase. Results of Vickers hardness test for Al-5 wt% Si alloy has hardness increase of 18 % from that of Al-3 wt% Si alloy with mean values of 26 and 22 HV, respectively.

以硅为合金元素的铝基合金，是铸造加工材料中占比最高的一类材料。此类合金在汽车与航空航天工业中拥有广泛的应用场景。众所周知，向铝中适量添加硅，可显著改善合金的力学性能。本实验研究采用亚共晶铝硅合金（Hypoeutectic Al-Si alloys），探究凝固参数以及3~5 wt%硅添加量对垂直定向凝固系统（vertical directional solidification system）内显微组织特征与最终硬度的影响。该类亚共晶合金在0.3~4 ℃/s的冷却速率范围内，于瞬态热流条件下完成定向凝固。光学显微镜表征分析结果清晰显示，二次枝晶臂间距（secondary dendritic arm spacing，λ2）随凝固速度（solidification speed，SP）与冷却速率（cooling rate，Ṫ）的降低而显著增大。另一方面，关联二次枝晶臂间距（λ2）与凝固速度（SP）、冷却速率（Ṫ）的实验生长规律表明，硅添加量从3 wt%提升至5 wt%会产生粗化效应，致使λ2增大。实验结果同时证实，合金的最终硬度随凝固工艺参数（SP与Ṫ）的升高而提升。Al-5 wt%硅合金的维氏硬度试验（Vickers hardness test）结果显示，其硬度较Al-3 wt%硅合金提升18%，二者的平均硬度分别为26 HV与22 HV。