Effect Of Mg And Nd On Low Temperature Properties Of Cast Al-Si Alloys

With the thriving development of high-speed rail industry in China,higher requirements have been placed on the performance of materials at low temperature.In China,the high-speed rails operating in northern regions and plateaus may face the temperature down to-40°C.Hence,it is necessary to study the performance of Al-Si alloys bolster in low-temperature environments.In this paper,the effect of Mg element and rare earth element Nd on the tensile properties of Al-Si alloys at low temperature was studied.This effect was analyzed based on the varieties of alloys morphology,phase composition and tensile fracture behavior after adding alloy elements mentioned above.This paper also studied the low-temperature tensile properties of Al-Si with different alloy compositions after T6 heat treatment.The effect of T6 heat treatment on the low-temperature tensile properties of Al-Si alloys with different alloy compositions was studied.Furthermore,the effect of Mg element and Nd element on the fatigue performance of Al-7Si-0.3Mg alloy at low temperature was studied.The experimental results showed that for the Al-7Si-x Mg alloys(x = 0 / 0.3 / 0.6 / 1.0),with the increase of Mg content,the yield strength and tensile strength of the alloy increased,while the elongation decreased.Compared with room temperature,the strength rose at low temperature(-60°C)but the elongation reduced.When the Al-7Si-0.6Mg alloy experienced8 h solution treatment and 6h aging treatment,the tensile strength,yield strength and elongation at low temperature(-60°C)reached 252 MPa,227MPa and 6.15% respectively.Observation of the microstructure near the alloy fracture showed that at low temperatures(-60°C),there are many micro-cracks near the tensile fracture of the alloys.The microcracks are mainly due to the fracture of the Si phase or peeling off from the substrate.The micro-cracks gradually expand and communicate with each other,eventually caused the alloys to fracture.After adding Nd element to Al-7Si-0.3Mg alloy,the yield strength,tensile strength and elongation of the alloy at low temperature(-60°C)showed a tendency that rising first then declining.The addition of Nd element made the original thick long strip or lamellar Si phase in the Al-Si alloy into a small short rod and the structure of the alloy was also significantly refined.Compared with room temperature,under the same strain condition,the times of low-temperature fatigue cycle of Al-7Si-0.3Mg alloy increased significantly,which was closely related to the rise in the crack initiation threshold and the decline in the propagation rate at low temperature.At the same temperature,the material life decreased with the increase in strain amplitude.Besides,the addition of alloy elements could also promote the improvement of the low-temperature fatigue performance of the Al-7Si-0.3Mg alloy.When the Mg element content was 0.6wt.%,the fatigue cycle of the alloy reached 4983 times,while when the Nd element content was 0.2wt.%,the fatigue cycle of the alloy reached 4169 times,which was increased by 62.84% and 36.24% compared with the Al-7Si-0.3Mg alloy,respectively.