The Research Of Manufacturing Process And Microstructure Evolution Of Micro-nanostructured High-silicon Aluminium Alloys

In order to enhance the operational properties of the automobiles, karts greatly, the power density should be improved and the weight of the engine should be lightened. As the development of the automobiles, the power increase with the increasing of the weight and volume of the engine more and more, which result in overweight and severe weakening operational properties of the automobiles. The efficient way of improving properties of the engine is to manufacture the parts of the engine with new and high-properties material, which can lighten the mass of the moving parts and reduce the power transmission resistance of the engine.As a wear-resistant material, the A1-Si alloy was used in the mechanical industry extensively. Being the low density and coefficient of thermal expansion, the high coefficient of heat conductivity , high-temperature strength and well wear-resisting property, the hypereutectic A1-Si alloys, especially the alloys with the percent of Si is 18%-26%, were the ideal light and wear-resisting materials of the engine. But with the ordinary casting process, the properties of the alloys were weakened greatly by the coarsened primary Si phases which make the matrix of the hypereutectic Al-Si alloy be rent severely. When the percent of Si exceed 14%, the properties of the alloys could not be improved even through the modification.As the development of modem industry, especially the needs of the motor and aerospace industry ask the A1-Si alloys be improved in the properties of wear resistance, heat resistance, linear expansion coefficient and density, then the composition of the alloys should be high-Si content and high-Alloying. Evidently, the ordinary alloy and casting process could not meet the needs. The rapid solidification materials developed in the recent years offer a new way to meet the needs for the development of the motor and aerospace industry.In this paper, the high-silicon aluminum alloys which have the properties of well wear resistance and low-thermal expansion coefficient were prepared by RS/PM. Through systematic research and investigation the regularity knowledge on the alloys was acquired. And the alloy was successfully used to manufacture the cylinder sleeve of the high power engine.The gas-atomizer arrangement was designed and manufactured, the influence of the parameters on the character and microstructures of the powder particles was studied with orthogonal experiments. It was discovered that the slot width of the gas nozzle was the key factor to influence quality of the powder particles, the gas flow rate was the second and the degree of the superheat was the third factor. The best combination of the atomization parameters is that——the diameter of the ceramic nozzle was 6.4mm, the angle of gas nozzle was 25°, the gas flow rate was about 34m³/h and the slot width of the gas nozzle was 0.55mm, the superheat was 100℃. Based on the population dynamics model considering the continuous varieties of thermo-physical parameters, supersaturation, nucleation rate, a model which was compiled with both the droplets heat transfer controlling equation and the droplets motion controlling equation has been developed to describe the microstructure evolution of hypereutectic A1-Si alloy during rapid solidification. The results of the solution to the model for the A390 alloy show that with decreasing droplet size, the average cooling rate increases rapidly. And when size of a droplet arrives at enough small, its temperature and microstructure varieties break out. The results also show that the primary Si phases of the powder particles in the microstructure have not been extinct until its size is less than a critical size, which is d_lim=[6.Nu.k_g(T_x-T_g)/ρ.L.(df)／(dt)]^1／2. With increasing the initial gas velocity and decreasing the superheat of the melt droplet, the nucleation and growth of primary phases are suppressed and the microstructure becomes into the metastable state.Meanwhile the results of atomization experiments of hypereutectic Al-Si alloys also show a good agreement of the experiments with the theoretical calculations. That is to say, the model can be used to predict satisfactorily microstructures evolution of the hypereutectic A1-Si alloys and the model would be useful for the microstructure predictions to other eutectic alloys.On the basis of the model for rapid growth of dendrite and eutectic crystals and the criterion of the highest temperature at the interface growth, a interface response function IRF(v)=max(T_pn.(v),T_eut(v)) for the growth of a crystal of a eutectic alloys during equiaxed rapid solidification was established. With the IRF, the competitive growth between the primary and eutectic phases of Al-Si alloys was investigated. Then a microstructure-selection map of Al-Si during non-equilibrium solidification was drawn. The map shows that there are three growth zone (primaryα-Al,primary Si phase and (α+Si) eutectic zone) in the Al-Si alloy. When the percentage of the Si phase is 12%-25%, the metastable microstructures wereα-Al plus (α+Si), and when the percentage of the Si phase below 12% or exceed 25%, the metastable microstructure was the eutectic of (α+Si) only. The calculation results was shown a good agreement with that of atomization experiments. The IRF model can be used to predict satisfactorily the microstructure selection and the evolution of the microstructures of the Al-Si alloy system during non-equilibrium solidification. Meanwhile the IRF model would be of benefit for the microstructure predictions during the non-equilibrium solidification of other eutectic alloys.With moulds of cold compacting and extrusion the extrusion processes of the specimen were simulated. And the regular principles of the densification and rheidity of the powders during the extrusion process were studied. The results show that the extrusion processes of the ingot include three stages, which are the packing densification stage, the stable extrusion stage and the turbulent extrusion stage. The bigger the powder particles, the better the performance of cold compacting. At the meantime, the quality of the alloy bar extruded from bigger powder particles was good enough without cracking. From the point of density, the smaller the powder particles, the higher the density of the alloy bar. On the other hand, the microstructures and mechanical properties of the small powder particles were better than that of the bigger particles.For practice application, mixed powders of various sizes (＜147μm) were better than the powders of single size. In order to improve the quality of alloy bar, some methods should be used in hot extrusion, such as coating with aluminum film, lubricating, increasing temperature of extrusion, increasing the extrusion area ratio (＞16) and so on.The experiments show that influence of the extrusion temperatures on microstructures of alloy bar were more greater and the growth tendency of primary Si phases increased with the increasing percent of Si phases. The results of measurements proved that the growth law of the dispersed primary Si phases conformed to the LSW dynamics theory. The extrusion area ratio and the shape of transverse section can change the microstructures and mechanical properties of alloy bar by influencing deforming and combination extent of powder particles. When the extrusion area ratio was 16, most of the porosities among the powder particles were meanly disappeared and the combination of the powders was firm. So the greater extrusion area ratio should be necessary for refine microstructures. For the mixed powders of A1-30%Si alloy, the best extrusion temperature , the extrusion area ratio and the angle of mould were 520℃,16 and 90°respectively.To sum up, compared with the alloy prepared by normal casting without any modification the mechanical properties of the alloy prepared by RS/PM increased greatly. The tensile strength, hardness, and the wear resistance of the alloy increased and the extensibility of the alloy decreased with the increasing percentage of Si phases for the alloy prepared by RS/PM. It is by reason of refining and improving morphology of primary Si phases to increase the mechanical properties of the alloy and the lower extensibility of the Si phases.The high-silicon aluminum alloy prepared by RS/PM was used to manufacture cylinder sleeve of high power engine. The test measurements show the various properties of the cylinder sleeve including the circumferential reinforcement and the radial restraint could meet the needs of the actual work of the automobile. The tensile strength was over 400N/mm2, high temperature tensile strength was over 300N/mm², the mass loss of alloys bar extrusion at 520℃was only 2mg. Its coefficient of friction was 0.3, the thermal conductivity and coefficient of thermal expansion at room temperature were 130 W/m/k and 1.5×10^-5K^-1 respectively. As you seen, the mechanical properties of the alloy were better than that reported by foreign countries and the synthesis properties of the high silicon aluminum alloy prepared by RS/PM were better than that of cast iron and steel materials.