| Hypereutectic Al-Si alloys is a kind of excellent wear-resistant material which has various advantages such as low density, low coefficient of thermal expansion, good thermal stability, wear resistance, widely used in the manufacture of automotive and compression machine parts, particularly perfect for the automobile engine piston material. However, since its wide crystallinity temperature range and the huge latent heat during the forming process, the primary silicon phase can easily grow into large plate or strip which can seriously fragmented substrate brittleness, causes the mechanical properties castings reduce significantly and cutting performance deteriorates.As a newly proposed material forming new technology, spray Conform has many advantages of spray deposition and Conform. It is suitable for high strength and low plasticity alloy and difficult forming materials of continuous production of small and medium size tube, rod, section bar and wire rod. This technology has many characteristics, such as high efficiency, energy saving, near net shape, short process, and so on. The products have high density, small and uniform microstructure, no macro-segregation, good performance, etc.In this thesis, preparation of hyper-eutectic Al-20Si alloy was produced by spray Conform, and its microstructure, tensile property, density, hardness and wear resistance were tested and analyzed. In addition, combined the microstructure of2A12aluminum alloy during spray Conform and physical simulation, the microstructure evolution of2A12aluminum alloy during spray Conform was studied. The specific contents are as follows:(1)Equipment of spray Conform is modified in the following working, such as support system of extrusion shoe, limit device of extrusion shoe, hydraulic systems of extrusion shoe. The Φ9mm2A12alloy and the Φ9mm Al-20Si alloy were produced by the improved equipment of spray Conform.(2) The microstructure of different state Al-20Si alloy was analyzed by Image-Pro software, results show that comparing with the casting,the primary silicon phase of spray Conformed Al-20Si alloy is smaller,only3～3.4μm, more spheroidization (roundness is1.2), and more uniform (diffusion factor is2.3μm). And the density of casting and spray Conformed Al-20Si alloy was tested, results show that the former can be up to99.3%, higher than the latter(97.4%) obviously.(3) The tensile property of spray Conformed Al-20Si alloy was tested by AG-X100KN electronic universal testing machine, results show that the average of elongation (A) of all spray Conformed2A12alloy is7.3%, its plastic was improved obviously. And the morphology of tensile fracture was observed by XL30ESEM-TEP SEM, results show that the fracture mechanism is typical microvoid accumulation fracture.(4) The wear resistance of casting and spray Conformed Al-20Si alloy was researched by MMU-5G friction and wearing testing machine, results show that in the same conditions, the wear loss of the latter is only the45%of the former, namely wear resistance is improved by more than one time.(5) Metal flow mechanism during spray Conform is that the alloy rod is composed of alternately hat-like coarse-grained layer and fine-grained layer. On the basis of the microstructure of spray preform and physical simulation, the formation model of this structure is proposed. In front of the abutment, the spray preform bent into an S-shaped billet in the container. The external zones of the S-shaped billet are held back by the friction between the billet and the working face&the abutment of the continuous extrusion chamber and, consequently, the billet core flows forward. |
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