Surface Modification Of Hypoeutectic And Eutectic Al-Si Alloys By High Current Pulsed Electron Beam

As a kind of important casting alloy, hypoeutectic and eutectic Al-Si alloys are widely applied in aerospace and automobile manufacturing fields due to their low thermal expansion coefficient, low density, high specific strength, good wear and corrosion resistance. However, the needle eutectic Si phase in hypoeutectic and eutectic Al-Si alloys under conventional casting condition severely dissevers the matrix and aggravates their properties, thus, the range of industrial application is restricted. High current pulsed electron beam (HCPEB) has been developed rapidly as a new high-power energetic beam used for surface modification of materials in recent years. The superfast heating and cooling process induced by the interaction of pulsed electron beam and material surface gives rise to the nonequilibrium solidification on material surface and achieves specific surface modification effect, unattainable in traditional surface treatment methods.In the paper, cast hypoeutectic and eutectic Al-Si alloys (Al-10Si, Al-12.6Si) were first treated by high current pulsed electron beam with an aim of refining massive needle eutectic Si. The main research contents were the microstructure. In surface layer of hypereutectic Al-Si alloys under the action of HCPEB and the effects of different pulses on surface hardness and wear resistance. Rapid melting and solidification was realized on the surface of Al-10Si and Al-12.6Si alloys by HCPEB treatment, and the obtained microstructure features were described as follows:The supersaturated solid solution was formed due to eutectic Si dissolving into Al matrix; Pure aluminum particles were appeared on the alloy surfaces; The melting phenomenon of alloy surfaces under multiple pulse treatment becomes very obvious, and "craters" morphology disappeared and surface structure was more compact; Cross-section structure analysis of Al-lOSi and Al-12.6Si showed that the thickness of remelted layer was increased with the increase in the number of pulses; XRD analysis indicated that the grains of modification surface were refined. The lattice parameters of Al were decreased, showing Al lattices had a slight distortion.The surface hardness of two alloys before and after HCPEB treatment was tested. The results showed that the hardness of Al-10Si and Al-12.6Si significantly was increased with the increase of pulse numbers. The hardness values of Al-10Si alloy varied from1454MPa of initial sample to1752MPa of35-pulsed sample, and its surface hardness was increased by20%after35pulses as compared to initial sample. However, surface hardness of Al-12.6Si alloy was enhanced by16%after35pulses, and the hardness value was1554MPa for untreated sample, whereas reached1804MPa for35-pulsed sample.The two alloys have a significant increase in wear resistance after HCPEB treatment, and wear resistance was gradually enhanced with increasing pulse numbers. The wear loss of Al-12.6Si alloy was respectively decreased by22%,44%and77%after5,15and25pulses. Friction coefficient analysis of Al-lOSi alloy showed that friction coefficients of the alloy were decreased after HCPEB treatment. The friction coefficients were minimal after25pulses, showing the best wear resistance of Al-10Si alloy. The obtained results are consistent with that of wear loss.In short, HCPEB can induce the formation of non-equilibrium structure on hypoeutectic and eutectic Al-Si alloy surfaces, and achieve the improvement of surface hardness and wear resistance, so it is a simple, efficient and novel surface modification technology.