| High strength aluminum alloy castings used for aviation, aerospace and other industrial integral structure parts, require advanced casting technology to realise: integration of casting, high-precision, high performance and high reliability. This has promoted the basic researches of solidification principle and the development of advanced casting technologies. The further study of aluminum alloys solidification behaviors, controlling casting defects and exploring new casting technology are still of great importance. In this paper, we concentrated on the theoretical analysis of aluminum alloys solidification process, the influence of casting parameters on solidified microstructure, mechanical properties of aluminum alloys, and numerical simulation of solidification process of real castings. Based on the achievement of aluminum alloys solidification theory, a new net shape casting technology of complex structural components with low defects, high precision and high performance has been developed, using the laser solid forming technology(3D printing) and plaster mold precision casting technology. The main results are as follows.(1) A mathematic model to evaluate microporosity of aluminum alloys has been established for both the columnar dendrite and equiaxed dendrite microstructural solidification respectively, with the consideration of the dissolved hydrogen, solidification shrinkage and interface energy. For columnar dendrite solidification, Darcy’s law was adopted to calculate the pressure drop in mush zone induced by shrinkage. For equiaxed dendrite solidification, the pressure drop was calculated by Hagen-Poiseuille’s law before mass flow stopped. The critical solid fraction fsc of microporosity formation in aluminum alloys was obtained by the mathematical model. The results show that higher hydrogen concentration will result in higher amount for microporosity formation. With the increase of secondary dendrite arm spacing(SDAS) and grain size, the tendency to form microporosity will be postponed. On the other hand, microporosity is not easy to form. The effect of mass flow passage on microporosity formation is tiny when the passage is large. However, once the passage is lower than 3 mm, the effect increases significantly. The results calculated from the present model shows the same tendency with Poirier and Piwonka-Flemings’ s models and in agreement with experimental data.(2) The solidification microstructures of Al-4.5Cu–0.55 Mn alloys cast by sand mold and metal mold were compared, and typical hot tearing was found in the samples of metal mould. The studies show that a few liquid was isolated between dendrite arm spacing after coalescences of solid particles at the last stage of solidification. With the decrease of temperature, the hole induced by solidification shrinkage will be filled up if the external liquid can flow through interdendritic channels. Otherwise, the porosity will form. If the suffered stress σf applied by external surrounding exceed the limit of semisolid strength, cracks will initiate at the location of porosity, while eventually lead to hot tearing.(3) The solidification microstructure of A357 and ZL205 A alloys were analyzed casted in metal mold, sand mold and plaster mold respectively. The results show that, the samples have the minimum SDAS or equivalent grain diameter in the metal mold castings and the maximal in plaster mold castings. As for the same mold, the SDAS or grain equivalent diameter of the same distance from the wall in the radial directional also increased with the increase of sample diameters. The experimental results suggested the relationships between cooling rates V and SDAS λ2 or equivalent grain diameter L0 to be 332802822541.V.-(28)l for A357 alloy and 245100 17380.0V.L-(28) for ZL205 A alloy. SDAS and the grain equivalent diameter were only related to the specific cooling rate, but had little to do with the specific reasons generating the different cooling rates.(4) A356 alloy samples were obtained by vacuum pressurizing and plaster precision casting technology, where the vacuum degree is 0.5 atm, pressure is 3 atm. After T6 heat treatment, the tensile strength σb reaches 280 MPa, elongation δ is 4%. The S-N curve of rotary bending fatigue of A356 alloys was obtained, which shows the limiting fatigue stress is no less than 90 MPa up to the life of 107 cycles. The fractography, microstructure and mechanical properties indicate that the porosity defects in the samples introduce the fatigue crack-initiations and promote the crack propagations. Quantitative relationship between the fatigue life, initial porosity size and applied stress is obtained. It shows that the fatigue life will decrease with the increase of initial crack size under a constant applied stress. Two constants in Pairs-Erdogan’s law of aluminum alloy A356-T6 are calculated based on the experimental data, C=0.626×10-14 and m=4.009. The results show that, with the increase of cycles, the critical size of porosity becomes smaller which will cause cracks in samples and lead to rupture, as keeping the applied stress content.(5) Casting technology for structural component of aircraft was designed based on 3D models. A new concept instead of ingate with convex plate was proposed. The filling and solidification process were successfully simulated, temperature fields and flow fields of filling and solidification were obtained. Through analyzing on the calculated results and solidification characteristics of aluminium alloys, gating system and technology parameters of plaster casting were optimized; the material database of commercial software was extended.(6) A new net shape casting technology for complicated structural components has been deveploped based on the 3D printing mold making and plaster mold precision casting technology. A complex structure component of aircraft with low defects, high precision and high performance was successfully casted through this researches. The size of the components is 2830 mm long, 1255 mm wide, 420 mm high with the minimum wall thickness as large as of 3 mm. The actual casting results in conformity with the simulation results. |
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