| With the rapid development of automobile industry,requirement for improved wear resistance of key parts of automobile is increased.Hypereutectic Al-Si alloys are becoming an ideal material for automobile wear-resistant parts and have a very broad application due to their the advantages of small density,good wear resistance and corrosion resistance,high strength and small thermal expansion coefficient.However,with increase of silicon contents,the strength will decrease and the brittleness will increase,the mechanical properties of the material will deteriorate,which is due to the existence of coarse primary silicon phase in the hypereutectic Al-Si alloy structure.At present,the commonly ways to refine the coarse primary silicon phase include modification agent treatment,semi-solid forming technology,pressure casting method,rapid solidification method and so on.As a kind of rapid solidification method,high current pulsed electron beam(HCPEB)has attracted attentions from researchers for its advantages of high energy utilization rate,environmental protection,simple operation and small deformation of workpieces.Our previous research found that Mg modifier can refine the coarse primary silicon phase and improve the microcrack structure on the sample surface after electron beam treatment.Therefore,in this paper,the hypereutectic Al-15Si alloy is modified by high current pulsed electron beam,and the effect of electron beam on coarse primary silicon is analyzed.Then,the Mg element was added to hypereutectic Al-Si alloy to further refine the coarse primary silicon during process of HCPEB treatment and improve the microcrack and crater structure.At the same time,the properties of the alloy surface after adding Mg element were studied in detail,and the mechanism was analyzed.Firstly,the microstructure of Al-15Si alloy surface modified by electron beam was analyzed by means of SEM,TEM,EBSD and XRD,and the hardness,wear resistance and strip tension tests were carried out.The main conclusions are as follows:(1)The coarse primary silicon phase was refined,and microcracks appeared on the alloy surface,and the microcrack structure became more and more serious with the increased pulse number of HCPEB;(2)Through TEM and EBSD techniques,it was found that the shock wave generated during forming of the crater,indicating that the shock wave can refine the primary silicon phase which is the center of the crater.(3)Hardness test results show that the hardness of aluminum matrix on the top surface of sample was increased by 170%,wear resistance test results show that the friction coefficient was decreased by 51.9%,the wear rate was decreased by 64.0%,mechanical property tests show that the tensile strength was increased by 41.4%and this proves that the properties of modified sample are significantly improved.Secondly,the microstructure of Al-15Si-5Mg alloy surface modified by electron beam was analyzed by means of SEM,TEM and XRD,and the hardness and wear resistance tests were carried out.The main conclusions are as follows:(1)Through analyzing SEM and XRD results,it was found that Mg element can refine the size of primary silicon phase,while HCPEB modification can further refine coarse primary silicon and improve the micro-crack structure on the alloy surface.And decrease the density of craters structure with the increase of electron beam treatment times.(2)By TEM analysis,it was found that fine primary silicon with a size of 50?250 nm is formed in the melted layer of HCPEB-treated Al-15Si-5Mg alloy.(3)Hardness and wear resistance tests show that the hardness of aluminum matrix on Al-15Si-5Mg alloy surface was increased by 146.4%,friction coefficient decreased by 40.8%,and wear rate decreased by 69.9%after HCPEB treatment.Compared with the sample without Mg element adding,the hardness of aluminum matrix was increased by 28.6%,friction coefficient and wear rate decreased by 6.23%and 22.2%respectively,indicating that the composite treatment with Mg element and electron beam further improved the hardness and wear resistance of the alloy surface.In a word,the surface modification of HCPEB and the addition of Mg modifier can both refine the coarse primary silicon on the surface of hypereutectic Al-Si alloy.At the same time,when Mg element is added,the surface treatment of electron beam not only further refines the primary silicon on the alloy surface,but also improves the microcrack defect on the alloy surface after electron beam treatment,which significantly improves the hardness and wear resistance of the alloy surface. |
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