The Study On Dispersion-Type Al-P Master Alloys And Their Applications

Hypereutectic Al-Si alloys have obtained wide application for combustion engine parts owing to their favorable properties, including high strength-weight-ratio, good abrasion resistance and low expansion coefficient. However, the traditional refinement process cannot be suitable in die casting industry due to required high melt temperature for high P absorptivity. Therefore, it is necessary to research the low-temperature refinement technology to optimize microstructure and improve products quality.In this work, Al-P and Al-Fe-P master alloys were studied adopting deformation and in situ synthesis to optimize the size and distribution of AlP particles and make them dispersive to be the potential seed crystals of primary Si, thus improving the heterogeneous nucleation efficiency and realizing the low-temperature refinement. The influence of deformation on the microstructure and low-temperature refining behavior of Al-P master alloy was investigated and an AlP/Al interface transformation mechanism was proposed. Meanwhile, the microstructure evolution, phase constituent, thermodynamic and kinetic conditions of Al-Fe-P were studied and solid-solid, liquid-solid gradual reaction mechanism were put forward. Moreover, the refining behaviors of dispersion-type Al-P master alloys on hypereutectic Al-Si alloys were carried out in squeeze casting and die casting production. The major research results are as follows:(1) Influence of deformation on low-temperature refinement of Al-P master alloy and AlP/Al interface transformation mechanismAs the deformation temperature rises, the size of AlP particles can be decreased significantly and the distribution becomes more dispersive. Deformation temperature has significant influence on activity of AlP/Al interface, thus affecting its refinement performance:deformation at room temperature compels the AlP/Al interface destroyed and transforms it into free surface, hence decreasing the number of effective nuclei and fading the refining performance; while high temperature deformation drives the surface to transform to new high efficient interface through atom diffusion on AlP/Al interface, thus achieving low-temperature refinement. When Al-P master alloy was deformed at 550℃, primary Si can be refined from 137pm to 21μm at 730℃ and the tensile strength of A390 alloy can be increased by 9.9%.(2) Synthesis and evolution of dispersive AlP in Al-Fe-P systemThe effect of temperature, size of Fe-P particles, holding time on the diffusion reaction rate of Al-Fe-P system was systematically studied and the thermodynamic and kinetic conditions were analyzed. The microstructure of FexP phases evolves from the "egg-type", the "sponge-type" to the "sesame-cake" structure by controlling the reaction parameter.The solid-solid reaction can be described as the unidirectional diffusion of Al atom and gradual transformation. As the atom percentage of Al diffusing into the (Al, Fe, P) intermediate phase increases, AlP generated accompanying with AlxFey in which the ratio of Al and Fe increases until the reaction reaches to the final stage with final products of AlP and Al13Fe4. The liquid-solid mechanism can be described as the double-directional diffusion of Al and Fe atoms. AlP and Al13Fe4 generate at the interface due to the reaction between liquid Al and Fe-P particles. Meanwhile, dynamic dissolution-precipitation of Al13Fe4 phase lead to the uniform distribution of AlP on Al melt, while Fe-riched phases aggregate and evolve to ring-like structure due to the obstruction of Al13Fe4 to the diffusion of Fe atoms.(3) Industrial applications of Al-(Fe)-P master alloys on hypereutectic Al-Si alloy.A dispersion-type Al-Fe-P master alloy which contains nano AlP particles has been synthesized. The average size of primary Si in A390 alloy can be decreased to 25 μm under the optimal conditions determined by orthogonal test with master alloy prepared at 550℃, holding time of 30min and addition of 1%.Dispersion-type Al-(Fe)-P master alloys can significantly improve the microstructure, macrohardness and wear resistance of hypereutectic Al-Si alloys under squeeze casting condition. The average size of primary Si can be refined to 20μn by deformed Al-P master alloy and the wear resistance can be increased by 16.9%, while the average size of primary Si can be refined to 23μn by Al-Fe-P master alloy and the wear resistance can be increased by 12%. When Al-Fe-P master alloys are applied for refining die casting ADC 14 alloys, the average size of primary Si can be refined to 5μm and the macrohardness improves significantly.