| As a kind of important casting alloy, hypereutectic 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 coarse primary Si phase in hypereutectic 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 tradditional surface treatment methods.In the paper, cast hypereutectic Al-Si alloys (Al-15Si, Al-17.5Si, Al-20Si) were first treated by high current pulsed electron beam with an aim of refining massive coarse primary Si and needle eutectic Si. The main research contents were the micro structure variations 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.The surface morphologies and microstructures of HCPEB-treated hypereutectic Al-Si alloys were investigated by means of SEM, EPMA, metallograph and XRD. The obtained results are as follows:1) After HCPEB treatment, coarse primary Si melts and forms a typical "halo" structure. The "halo" structure becomes more obvious with the increase of pulse number, thus, there is a better mutual fusion between primary Si and Al matrix;2) Many circular pure Al particles are distributed on Al-17.5Si alloy surface. The size of pure Al particles decreases gradually and becomes more uniform with increasing pulse number. After5pulses, oc(Al) dendrites on Al-15Si alloy surface are refined obviously;3) The diffraction peaks of Al and Si in the XRD patterns of Al-15Si alloy broadens and shifts toward to high angles after HCPEB treatment. The d(111) values of Al(111) and Si(111) crystal planes are reduced, and the lattice parameters of Al are also decreased, indicating a slight distortion of crystal lattices. After25pulses, the modified surface presents a preferred orientation of Al(200) crystal plane;4) HCPEB irradiation leads to the formation of remelted layer with a thickness of several microns on the alloy surface, and it possesses the fine-grained structure and homogeneous composition. The heat-affected zone (HAZ) and unaffected matrix are located under the remelted layer. The thickness of remelted layer for Al-15Si alloy increases slightly from4.4μm (5pulses) to5.6μm (25pulses), but remelted layer of Al-20Si alloy has a thickness of about4μm after25pulses.The formation of metastable structures on HCPEB-treated hypereutectic Al-Si alloy surfaces were confirmed by both TEM and Raman spectrum, and the results are summarized as follows:1) There are two types of nano-silicon grains existed on the treated surface, one is free nano-silicon grains with an average size of30-100nm. But another is fine and equiaxed nano-silicon grains dispersively distributed in Al matrx, having an average size of5to20nm, which are preferentially precipitated in grain boundaries and subgrain boundaries with high defect densities;2) By TEM observation, a large number of Si atoms are aggregated in certain areas of Al grains, in particular on the edge. Si atoms dissolve in the Al lattice forming the supersaturated solid solution. The average solid solubilities of Si in Al matrix for15-pulse treated Al-15Si alloy and25-pulse bombarded Al-20Si alloy are respectively8.54%and8.9%by EDS analysis;3) TEM analysis shows that amorphous alumina is formed in top surface layer of Al-17.5Si alloy after15pulses. Through Raman analysis, amorphous silicon (a-Si) is generated on the edge of "halo" structure for Al-15Si alloy after15and25pulses, whose characteristic broadband peaks are respectively located in468cm"1and478cm"1in Raman spectra;4) After5pulses, the equiaxed dislocation cells with size of about200nm are formed in α(Al) phase on Al-15Si alloy surface, and cell walls are high-density dislocation tangles having thickness difference. When the number of pulses is increased from5pulses to25pulses, high density dislocations are generated in the local area of Al-20Si surface, and the dislocations are tangled with each other. Besides, Al lattices have a serious distoration in the dislocation tangle area, therefore diffraction spots are elongated.Finally the micro-Vickers hardness tester and pin-on-disc sliding friction and wear tester were conducted to measure surface hardness and wear resistance of hypereutectic Al-Si alloys before and after HCPEB treatment, the main results are as follows:1) The microhardness of eutectic structure and α(Al) for Al-15Si, Al-17.5Si and Al-20Si alloys are gradually increased with increasing pulse number. Compared with initial sample, the microhardness of eutectic structure for three alloys are respectively enhanced by factors of about0.7,1and0.7after25pulses, while that of α(Al) are respectively increased by factors of about0.7,1and1;2) The microhardness of primary Si for Al-20Si alloy does not show a big fluctuation, and its average value is8514.7MPa. However, the microhardness of "halo" structure emerges a descending gradient variation from the center of "halo" to the edge;3) The microhardness of remelted layer for three alloys have an obvious enhancement after25pulses. Their values are1049.3MPa,1147.4MPa and1216MPa in sequence, which are enhanced by more than50%compared with those of the matrix at140μm far from the surface;4) Compared with initial sample, three hypereutectic Al-Si alloys have a significant increase in wear resistance after HCPEB treatment. Wear loss is first decreased and then increased with the increase of pulse number, so wear resistance is first enhanced and then reduced. Best wear resistance of three alloys after HCPEB treatment are respectively9times,6.5times and2times of initial samples. By analyzing the friction coefficients of Al-15Si alloy, the same result concerning wear resistance is obtained.In short, HCPEB can induce the formation of non-equilibrium structure on hypereutectic Al-Si alloy surface, and achieve the improvement of surface hardness and wear resistance, so it is a simple, efficient and novel surface modification technology. |
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