Effects Of Complex Modification On The Microstructures And Properties Of The Hypereutectic Al-Si Alloy

There are a series of advantages of the hypereutectic Al-Si alloy, for example, highabrasion resistance, light weight, high strength, low thermal expansion and high heatresistance, so it is attracted people’s widespread attentions, especially in the field ofautomobile. However, there are bulky five star shape, octahedral shape and plate-likemorphology of primary silicon in the microstructures of hypereutectic of Al-Si alloy, thesegross primary silicon tips and edges and hole defect are easy to cause local stressconcentration which seriously split the alloy matrix, reduced the mechanical properties,reduced the plasticity and strength of the alloy by external force. In addition, in the processof machining, the hard and brittle primary silicon will speed up the tool wear, makes theworkpiece surface finish poor and unable to meet the needs of practical application.Studies have shown that if small primary silicon uniformly distributed in thehypereutetic Al-Si alloy matrix, the alloy performance will be improved significantly.Therefore, it is very necessary to refining the primary silicon of the hypereutetic Al-Si alloy.There are many methods to refine primary silicon, such as, metamorphic process,electromagnetic stirring, ultrasonic vibration and rapid solidification, etc. But metamorphicprocess,which is join foreign nucleation agent to the melt in order to form the core ofprimary silicon nucleation, is the most obvious and the most simple method.In this paper, the influence of microstucture and property of hypereutectic Al-20Si andAl-20Si-4Cu alloy under the condition of different modifications is studied, at the same time,we get the following conclusions:(1) Complex modification has good refining effect on microstructures of hypereutectic Al-Sialloy. To the hypereutectic Al-20Si alloy, complex modification of Al-P, Y2O3and TiC can make primary silicon refined to15μm, meanwhile, eutectic silicon refined obviously,the long needle eutectic silicon turns to short needle or dot eutectic silicon. To thehypereutectic Al-20Si-4Cu alloy, complex modification of Al-P and Y2O3can refineprimary silicon and eutectic silicon simultaneously, the average size of primary siliconcan be refined from85μm to16μm and the long needle eutectic silicon turns to shortneedle or dot eutectic silicon; the edges of primary silicon become obtuse without changeof everage size, eutectic silicon refined obviously and become dot morphology.(2) We find the rule and mechanism of modification treatment increase the tensile strength ofhypereutectic Al-Si alloy. To the hypereutectic Al-20Si alloy, the tensile strengthincreased by52%from127MPa to193MPa, meanwhile, the elongation increased by147%from1.7%to4.2%; To the hypereutectic Al-20Si-4Cu alloy, after T6heattreatment, the tensile strength increased by57%from209MPa to329MPa, meanwhile,the elongation increased by47%from4.0%to5.9%. The size of the primary siliconplays a main effect on the tensile strength, meanwhile, the morphology of eutecticsilicon has little effect on the tensile strength of the alloy. It reveals the fracture mode isthe combination of ductile fracture and brittle fracture. The primary silicon is refinedobviously after modification in the hypereutectic Al-20Si and Al-20Si-4Cu alloy, thenumber of defects which exists in primary silicon greatly reduced, the crack initiationand propagation are restrained in stretch, so the tensile strength increased; To thehypereutectic Al-20Si-4Cu alloy, the movement of dislocation is restrained in stretchbecause there exists Cu2Al in the alloy, so its tensile strength overtop the hypereutecticAl-20Si alloy’s.(3) We find the change rule about the relation of the volume abrasion rate and the grain grit,volume abrasion rate and load of hypereutectic Al-20Si and Al-20Si-4Cu alloy: with theincreasing of the grain grit and the load, the volume abrasion rate of hypereutectic Al-Sialloy is gradually increased; Wear resistance is slightly decreased after modification,approximately about5%~10%. It reveals the abrasive wear mechanism of hypereutecticAl-Si alloy, which is the joint action of micro cutting, micro furrows and microscopicpeeling, but the micro cutting is the most important. (4) The surface finish of hypereutectic Al-20Si alloy after modification treatment is betterthan before modification treatment, the surface roughness reduce from2.83μm to1.33μm. It reveals the machining mechanism of the advancement of machinability ofhypereutectic Al-20Si alloy, the small primary silicon can be coordinated the deformationof the substrate in the process of machining to avoid collisions with the coarse primarysilicon.