Effects Of Process Parameters On The Microstructures And Mechanical Properties Of In-Situ Mg₂Si_p/AM60B Composite

In this paper, the in-situ Mg₂Si_p/AM60B composite has been prepared by thixoforming technology, which would broaden the research filed of semisolid processing. The main works focus on the relationshi_ps between technological parameter, microstructure and properties, and have been revealed, such as the effects of mould temperature, punch velocity, solidification pressure, reheating time and temperature on the microstructure and tensile properties of the thixoforged composite. The optimized parameters were determined by individual factor experiment. The results were comparing with the traditional as-cast composite and the thixoforged AM60B in order to explain the advantages of thixoforming technology and particle reinforced composite. In addition, the wear behaviors of the thixoforged composite were investigated. In comparison with the traditional as-cast composite and the thixoforged AM60B, the anti-attrition mechanisms were studied.The results indicated that:1. The semisolid microstructure of the refined in-situ Mg₂Si_p/AM60B composite was obtained after been partial remelting, which was expected that the thixoforging technology was suitable for this composite. The microstructural evolution during partial remelting could be divided into four stages, the initial coarsening, structural separation, spheroidization and final coarsening. The Mg₂Si particles had no effect on the microstructural evolution steps, but slower the evolution progress. Their morphologies first became blunt and then spheroidal because of melting of the edges and corners, and finally coarsened owing to Ostwald ri_pening.2. The optimized thixoforging technological parameters were mould temperature 300℃， punch velocity 60 mm/s, hold pressure 192 MPa, heating time 60 min, heating temperature 600℃. The main strengthening mechanism of the Mg₂Si particles was load transfer mechanism, which could effectively prevent from the cracks initiation in the matrix. The failure modes of the Mg₂Si particles were interfacial debonding, the fracture and fragmented of the particles. In order to take full advantages of the composite, it should avoid the Mg₂Si cluster and high residual stress concentration in the interface Mg₂Si_p/matrix. Of course, if the cracks easily initiated from the shrinkage porosities in the matrix, the Mg₂Si particles could not taken part in strengthening the matrix.3. The UTS and the elongation of the composite thixoforged under the optimized parameters were 209 MPa and 11.9%, respectively. In comparison with the tradition as-cast composite, the UTS was increased 93% and the elongation 138%. The super tensile properties of the thixoforged composite were attributed to the elimination of shrinkage porosities, the decrease in harmful eutectic β phases, the enhanced solution strengthening and working hardening roles. In meeting with the thixoforged AM60B, the UTS of the composite increased 36% at the cost of elongation, which resulted from the strengthen role of the Mg₂Si particles. However, the differences in thixoforged composite and AM60B were continually decreasing as the tensile testing temperature rises. Both of UTS decrease and the elongation increases as the tensile testing temperature rises, which was attributed to the enhanced dislocation motion ability, the softened eutectic β phase, the activated non-basal sli_pping and the dynamic recovery and recrystallization mechanisms. The tensile testing operated at 200℃ under different strain rate, the fracture of the composite transformed from a ductile fracture to a brittle fracture.4. The wear mechanisms of the thixoforged composite under different conditions were oxidative wear, abrasive wear, adhesive wear, de-lamination and fusion wear. Each wear mechanism was not independently from one another, but operated at the same time, and one or more of the wear mechanisms were dominant under a given condition. The enhanced wear properties of the thixoforged composite resulted from the Mg₂Si particles in compared with the thixoforged AM60B, which protected the fatigue cracks initiation in the matrix. The fewer amount of eutectic β phase in the thixoforged composite was responsible for its high wear properties in comparison with the as-cast composite.