Research On Ultrasonic Assisted Squeeze Casting And Strengthening Mechanism Of Al₃Ti/2024Al Composites

Particle reinforced aluminum matrix composites is one of the main research and development directions of metal matrix composites due to the excellent performance, low preparation costs and processing costs. In situ synthesis solves the problem of wettability between reinforcement and matrix, resulting in a strong interface bonding, which can yield better mechanical properties, but the processing characteristics and fabricating route of in situ preparation are still not well established. In this background, the research on the preparation of Al₃Ti particles reinforced 2024Al composites by in situ casting method was carried out. The concrete research include ultrasonic direct melt reaction preparation of Al₃Ti/2024Al in situ composite method, effects of squeeze casting on microstructure and mechanical properties of Al₃Ti/2024Al composites, research on heat treatment process of Al₃Ti/2024 composites, and the strengthening mechanism of the Al₃Ti/2024Al composite.The study adopted the ultrasound assisted to produce the Al₃Ti reinforcing particles by in situ synthesis inside the 2024Al melt, what is more, the phase was determined and the structure was observed by using the advanced materials analysis and measurement technology; by squeeze casting and heat treatment process to improve the composite macrostructure and microstructure, enhancing the properties of materials; by discussing the strengthening mechanism of the Al₃Ti/2024Al composites, confirm the main factors which have a great impact on the properties of composites; by comparing the different theoretical results which were calculated by diverse models with the experimental value, the modified theoretical model in condition of experimental phenomenon was proposed. The main results and conclusions are follows:In situ Al₃Ti generated by Ti powder with average size of 20 μm are most ranged from 4 to 6 μm. Al₃Ti particles are mainly distributed inside α-Al grain. Oversized Ti powder may lead to an incomplete response or Al₃Ti particles agglomeration. On the other hand, undersized Ti powder（10μm） did not obtain the more fine Al₃Ti particles. The tensile yield strength, hardness and high temperature compression performance of composite material are improved.Insufficient squeeze casting pressure could not guarantee the packing efficiency of the composite. Too large pressure would transform continuous network of the eutectic structures in the matrix into discontinuous state. It would achieve the same result with the increasing content of Al₃Ti particles under fixed pressure（150MPa）, which is related with non-equilibrium solidification in as cast microstructure.Composites aging sensitivity changed because of Al₃Ti particles. The higher content of Al₃Ti particles was, the shorter time to reach the peak-aged state needed. When Al₃Ti/2024Al composites produced by squeeze casting gone through solution and aging heat treatment, the mechanical properties in peak-aged state were improved obviously at room temperature. In addition, the ductility of materials decreased with the increase of the content of Al₃Ti.The main strengthening mechanisms of Al₃Ti particles in the composite include thermal mismatch strengthening, work hardening, grain refinement strengthening and load transfer strengthening. And the reinforcement effect was enhanced with decreasing size of Al₃Ti particles or the increasing of the content of Al₃Ti particles. However, the existing strengthening prediction model could not accurately predict yield strength of Al₃Ti/2024Al composites, this study proposed a modified model in condition of uniformity of particle distribution and the strengthening cooperative coefficient, in which the predicted value is fitted well to the experimental value.