| High-silicon aluminum alloys have the advantages of high specific strength,high hardness,good wear resistance,low thermal expansion coefficient,good thermal conductivity and low density.It is widely used in the field of automobile engine.However,the high-silicon aluminum alloy fabricated by the traditional cast has a coarse structure and low density due to the primary crystal silicon,which deteriorates the mechanical properties.This work used AlMgSi10 alloy powders and Si powders as raw materials to fabricate the AlMgSix high-silicon aluminum alloy via planetary ball milling combined with hot-press sintering,and optimized the structure and properties of the alloy through hot extrusion and heat treatment.The influence of Si content,hot extrusion and heat treatment on room temperature compression,room temperature bending,hardness,thermal expansion coefficient and thermal conductivity of high silicon aluminum alloy was studied,and the wear resistance and friction and wear mechanism of heat treated high silicon aluminum alloy were analyzed.Exploring the ball milling process,low-energy ball milling made the Si powders evenly distribute on the AlMgSi10 spherical powders,with less external scattering,obtaining the best powder mixing effect.The ball milling process is:ball-to-battery ratio at 6:1 and ball milling at 220 rpm for 6 h.The AlMgSix(x=20,30,40)alloy adopted a multi-stage liquid phase hot pressing sintering process,in which the sintering temperature was 10?above the liquidus,and the pressure was 25MPa,with the temperature holding for 90 min.The as-sintered high-silicon aluminum alloy mainly contained?-Al phases,primary Si phase and a very small amounts of Mg-Si phases.The primary silicon was in the shape of slabs and lapped together irregularly.The mechanical properties and thermophysical properties of the as-sintered high-silicon aluminum alloys were tested.The room temperature compression test results showed that the higher the Si content,the lower the compressive strength of the alloy,which is believed to be caused by the splitting of the matrix by the primary silicon.The compressive strength of AlMgSi20 alloy is up to 270.9MPa;the elastic modulus and hardness of the alloy increased with the Si content.Besides,the elastic modulus and hardness of AlMgSi40 alloy were the highest 103.76 GPa and HBW 67.07,respectively.The increase of the primary Si content inhibited the volume expansion of the?-Al phases and reduced the overall thermal expansion coefficient of the alloy.The thermal expansion coefficient of the AlMgSi40 alloy was as low as 12.2×10-6 K-1;the Si phases could block the Al phases.The interconnected structure of phonon and electron played a role in hindering the transmission of phonons and electrons,which reduced the thermal conductivity of the alloy.The as-sintered high-silicon aluminum alloy was subjected to hot extrusion deformation.The deformation temperature of AlMgSi10 and AlMgSi20 alloys was460?,and the deformation temperature of AlMgSi30 and AlMgSi40 alloys was 490?.After hot extrusion deformation,the primary crystal silicon was refined into small slabs and blocks,and the sharp corners were passivated,especially exhibiting a uniform distribution.However,the hard and brittle phases(primary crystal silicon)had a great deformation resistance,and small pores appeared in the alloy structure.Comparing the mechanical properties of the alloy before and after deformation,the compressive strength and hardness are improved,but the elastic modulus is slightly reduced,which is considered to be caused by the decrease in density.Hot extrusion deformation had an adverse effect on the thermal physical properties of the alloy.Compared with the as-sintered alloy,the thermal expansion coefficient of the as-extruded high-silicon aluminum alloy was slightly increased,and the thermal conductivity was slightly reduced.The as-extruded high-silicon aluminum alloy was subjected to diffusion heating and secondary heating treatment to further improve the morphology of primary silicon and the mechanical properties.After the first diffusion heating and then the secondary heating,the shape factor of the primary crystal silicon increased.There were a large number of dislocations in the?-Al phases,and a large number of Mg-Si phases were formed.The room temperature compression and bending tests of the heat-treated high-silicon aluminum alloy show that the compressive strength,bending strength and bending fracture work of the alloy decrease with the increase of Si content.It can be considered that the increase of hard and brittle phase Si makes the high-silicon aluminum alloy more effective.Both strength and toughness are reduced.Compared with the sintered and hot extruded high-silicon aluminum alloys,the compressive strength and flexural strength of the heat-treated high-silicon aluminum alloys are greatly improved,and the flexural strengths are increased by 157.8MPa and 87.3MPa,respectively.The combined effect of increased solubility,massive formation of Mg-Si phase and spheroidization of primary silicon improved the strength of the alloy.The wear resistance and friction and wear mechanism of the heat-treated high-silicon aluminum alloy were explored.During the friction and wear process,the primary Si protruded to support and protected the matrix Al.The higher the Si content,the higher the hardness of the alloy,and the better the wear resistance of the alloy.When the Si content was low,the friction and wear mechanism of the alloy was adhesive wear.By contrast,when the Si content increased,it transformed into abrasive wear and oxidative wear.Compared with the as-sintered and as-extruded AlMgSi40 alloys,the heat-treated AlMgSi40 alloy possessed the highest strength and hardness,so it had the best wear resistance. |
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