Effect Of Si Phase And Heat Treatment On Fracture Behavior Of Cast Al-Si Alloy At Low Temperature

The bolster structure is a key component of high-speed railway,and the bolster structure is complex.Casting Al-Si alloy is used to replace the traditional welded steel plate bolster structure to lighten the body of high-speed motor vehicle.In northern China,plateau and other places,the ambient temperature of high-speed railway may reach to-40°C,so it is very important to study the performance of the bolster of Al-Si alloy in low temperature environment.In this paper,the effect of Na modification,solution treatment and cryogenic treatment on the size and morphology of Si phase was studied,and which can effect on the tensile properties of Al-Si alloy at low temperature.In addition,the effect of Si phase content and squeeze casting on the tensile properties of Al-Si alloy at low temperature was studied.The morphology of Si phases in Al-7Si-0.3Mg alloy had been changed from the thick strip or sheet to the fine fiber after Na modification treatment.Squeeze casting refined the grain size of Al-7Si-0.3Mg alloy,and the Si phase was fine and uniformly distributed.Compare with the strength at room temperature（20°C）,increases than it at low temperature（-60°C）,and the elongation decreases.For Al-xSi-0.3Mg alloy（x=1/2/3/4/7）,the strength of the alloy increased and the elongation decreased with the increase of Si content.When the content of Si was less than 4%,the Si phase breaked off and micro-voids were formed in the process of stretching.Compare with the room temperature,the strength and elongation were increased at low temperature.When the content of Si was more than 4%,the Si phase breaked primarily and micro-cracks were formed in the process of stretching.Compare with the room temperature,the strength were increased and elongation were decreased at low temperature.The tensile strength,yield strength and elongation of Al-7Si-0.3Mg alloy were 281MPa,265MPa and 10.04%respectively at low temperature（-60°C）after solution treatment for 8h and aging treatment for 6h.At this time,the average diameter of Si phase and the number of Si phase in unit area were the smallest,and the Si phase was round and uniformly distributed.The stress of dislocation plugging in the alloy decreased during plastic deformation.The fracture was not easy to occur,the micro-cracks produced during the fracture were smaller,and the strength and elongation of alloy increased.The tensile strength and elongation of Al-7Si-0.3Mg alloy were 185MPa and 18.00%at low temperature which had been cryogenically treated for 24h after solution treatment.The grain size can be refined after cryogenic treatment,while a large number of dislocations occured in the alloy.A large amount of Mg₂Si phase precipitated from the cryogenic treated alloy after aging treatment for 6h.The movement of dislocations are be hindered and a large number of dislocations accumulated near the Mg₂Si phase,and the mechanical properties of Al-7Si-0.3Mg alloy were improved.The tensile strength and elongation of Al-7Si-0.3Mg alloy were 295MPa and 15.25%at low temperature（-60°C）.For Al-7Si-0.3Mg alloy,the elongation can be increased while ensuring the strength with cryogenic treatment and aging treatment after solution treatment at low temperature.