The Effect Of Melt Overheating Treatment On The Microstructure And Wear Resistance Of AL-Si Hypereutectic Alloys

Al-Si hypereutectic alloys are widely used in industries and Hi-Tech areas, butcoarse primary Si phase in casting is detrimental to the mechanical properties of thealloy. Therefore, refinement of primary silicon have been received great interestfrom metallurgists. In this thesis, the dissolution and growth behavior of primarysilicon phase in hypereutectic Al- 17% Si alloy melt is studied systematically bymeans of melt overheating treatment. The quench interrupting and rapidsolidification technique has been used in order to control primary silicon. Thequenched microstructure was obtained from the melt of different holding times withthe overheating temperature ranging from 7000C to 1 1000C. The dynamical modelsfor dissolution and growth of primary silicon were established based on currentsolidification models. The interrelations between cooling rate and solidificationstructures were examined under different melt treatment. The effects of melttreatment on the wear resistance were finally investigated. Based on the experimentsand theoretical analysis, the following main results were obtained.1.The variation of primary silicon and eutectic structure with overheating processfrom 700 to 1 1000C overheating and with different cooling rate (1O5-1O0~C/sec)is systematically studied. The results show that the solidification microstructurecan be controlled by adjusting cooling rate and parameter of melt treatment. Themeasurements of improving morphology and size of primary silicon areproposed:a) Higher cooling rate during solidification;b) Higher overheating temperature of the melt;c) Heating and cooling between liquidus cyclically.?III ?2.The study on the processes and dynamics of dissolution and growth of primarysilicon in the melt reveals that primary silicon is of high heat stability in themelt and its dissolution is controlled both by interface reaction between siliconand the melt and by diffusion in the bulk liquid. By combination of dissolutionand growth model, a more precise model to describe the size and morphology ofprimary silicon in the quenched samples is proposed. The calculated resultsfrom this model are basically agreed with experiment.3.The relationship between melt overheating treatment parameters and wearresistance of the alloy is established. It is concluded that the finer the primarysilicon phase is, the more uniformly distribute in the matrix, the higher the wearresistance. The hardness of matrix increases with increasing of melt overheatingtemperature, which improves the deformation consistency. Therefore, theinitiation and propagation of cracks will be prevented or slowed down, resultingin higher wear resistance.