Study On Microstrctures And Properties Of Al-Si Casting Alloys For Pressure Tanks

Near eutectic Al-Si alloys are suitable for complex castings due to their superior castability and low shrinkage. However, their poor tensile properties are becoming a bottleneck for their application in structural components. Numerous efforts, for instances, alloying, refinement and modification as well as heat treatment, are made to improve the tensile properties of near eutectic Al-Si alloys. It obviously has important benefits to develop theory and guide pratcices.In this thesis, the effects of Cu addition on the microstructures and tensile properties of near eutectic Al-11Si-0.3Mg alloy were investigated. The results show the UTS is improved and the El is decreased with the addition of Cu. With the addition of 1.5 wt% Cu, the UTS and El are of 270MPa and 4.2%, respectively. However, excessive Cu level of 2.0 wt% introduced an increase in microporosity resulting in distinct reduction of both UTS and El. Thus, the best tensile property can be found in Al-11Si-1.5Cu-0.3Mg alloy. The strengthening effect mainly depends on the precipitation of Al2Cu and Al5Cu2Mg8Si6.The influences of La additions on microstructures and tensile properties of Al-11Si-1.5Cu-0.3Mg alloy, as well as the interactions of B, La and Sr, are studied. The results show the depression of both modification and refinement can be found with the addition of 0.05 wt%B due to the formation of SrB6 compounds. But with 0.1wt%La addition, the a-Al phases are refined and the eutectic Si phases are modificated sufficiently. It can be contributed to the formation LaB6 compounds, which can be a kind of effective nuclei of Al, instead of SrB6 compounds.The influences of heat treatment on microstuctures and tensile properties of Al-11Si-1.5Cu-0.3Mg alloy with 0.1wt%La+0.05wt%B+0.03wt%Sr are investigated. After solution treatment of 490℃×4h+505℃×4h, the UTS and El increase to 343MPa and 10.2%, respectively. The UTS increases greatly to 399MPa, but El decreases to 5.0% in T6 treatment (490℃×4h+505℃×4h+170℃×6h). Finally, the influences of combinative addition of La and B on the artificial aging behavior are discussed. From age hardening curves, it can be concluded the time to age hardening peaks of the La-added alloys are shortened and their peak values are higher. The results of TEM also reveal the density of the precipitated θ’phases becomes larger and the average size of precipitation becomes smaller with the further refinement effect attributed to the combinative addition of B and La. The mechanism is related to the reduction of nucleation activation energy from 42kJ/mol in the B-refined alloys to 30KJ/mol in the combinative addition of B and La alloys.