Study On Compound Fabrication Technology Of Semisolid Billet Of AlSi30 Alloy Based On SIMA Method

Hypereutectic aluminum-silicon alloys (Si>12.6wt%), which have many merits such as low density and thermal expansion, high wear resistance, are well-known as wear resistant material, and thus are widely used in making automobile and compressor parts, especially are the ideal material for making automobile engine plunger. The main forming methods of hypereutectic Aluminum-Silicon alloys are Ingot Metallurgy (IM) and Powder Metallurgy (PM) at the present time, but both of them have autologous deficiencies.Semi-Solid Metal Forming (SSMF) is a new forming technique with many merits such as net-shape forming and high quality. Because of the primary phase is spheroidism, many problems of conventional methods can be solved commendably by this forming technique. Strain-induced melt activation (SIMA) technique is a very important route for making semi-solid billet in solid state. However, due to the complex technological process and finite deformation, which greatly confine its extension and application. The main object of this dissertation is to introduce high-pressure torsion (HPT) to strain-induced melt activation technique,? as a substitute for conventional thermal and cold plastic deformation. This can simplify conventional process flow notablely, and at the same time can introduce more deformation,?refine the primary phases, create a favorable strain inducement condition for subsequent isothermal treatment in semisolid state.Firstly, the influences of high pressure torsion applied load and number of rotations on microstructure and performance of powder performs are investigated in this work.?The results show that the primary Si particles of the bulk disks, which processed by cold consolidation of AlSi30 powder using HPT, are refined and rounded very significantly, and the?maximum of microhardness is 144.56kg/mm2, which is much larger than AlSi30 alloy produced by conventional casting method. The greater the high pressure torsion load and the more turns applied,? the more obvious refinement effect and the higher the microhardness value. The refinement and rounding of grains are mainly completed by the interaction of internal phases, while the densification of the powder specimen is achieved by the interaction of the powder particles. Secondly, the influences of high pressure torsion applied load, number of rotations and semi-solid isothermal treatment holding temperature, holding time on the semisolid microstructure evolution at different positions on the sample are also investigated. The results indicate that because of the different deformation which caused different accumulated strain energy, for the microstructure in different radius of the sample, the best matched semi-solid isothermal heat treatment process parameters are different too. For r=5mm, the best match process parameters are high pressure torsion applied load 0.2GPa, 5 number of rotations, semi-solid isothermal treatment temperature 645℃and holding time 50min; for r=10mm, the best match process parameters are high pressure torsion applied load 0.2GPa, 5 number of rotations, semisolid isothermal treatment temperature 615℃and holding time 20min. The semisolid microstructure evolution of AlSi30 alloy is mainly accomplished by two processes. One is precipitation, amalgamation and growing of eutectic Si in eutectic matrix. The other is Si particles, which is around the primary Si, segregate to the primary Si and resulting in the coarsening of primary Si.The compound of HPT into SIMA technique, from which can obtain more excellent semisolid billet, and greatly simplifies the traditional process at the same time, has practical significance to the traditional semisolid billet preparation technology from single to compound and from experimental research to practical application.