Microstructure And Properties Research Of In Situ TiB₂/BH135 Composites

Piston is the heart of the engine, which is under high alternating load and thermal load in the running process of engine. Under this hard condition, the piston works circularly and bears the continuous impact of high pressure and high temperature. Therefore, the alloy must be with high tensile strength at high and room temperature, high wear resistance and less coefficient of thermal expansion. The performance, especially under the condition of high temperature of material BH135 and the descendible material that were developed on the basis of ZL109 has reached the limit. It is necessary to research a new material to make up for the inadequacy of traditional piston material, meet the increasing requirement of peak pressure increasing because of more and more stringent emission standard. Particle reinforced Al composite is well known for its high specific strength, specific stiffness, high wear resistance and fatigue resistance, great high temperature mechanical properties and dimensional stability. Therefore, it is expected to become a new generation of piston material.In this paper, Ti B2 particle reinforced BH135 composite materials are formed by using mixed reaction in situ synthesis. Modern analysis and testing methods like scanning electron microscopy(SEM), X-ray diffraction analysis(XRD), universal material test machine, thermal dilatometer and friction-abrasion testing machine are used to study the heat treatment process, tensile strength at high temperature and thermal expansion properties. The main conclusions are as follows:1. The in-suit synthesis TiB2 particles were small and formed in rodand hexagonal shape. The average particle size was about 200 nm. TheTiB2 particle combined with Al matrix closely and the TiB2 particledistributed uniformly in the matrix material. Therefore, defects like porosity and crack will not be happened. The combination is directly combined interface. The TiB2 particles distribute around the α-Al grain boundary and enrich in the grain boundary ofα-Al matrix finally.2. Based on the research, the proper heat treatment of the TiB2/BH135 composites is that solution at 525℃ for 1.5 hrs and aging at 170℃ for 8hrs.3. The composites have good high temperature mechanical properties. The reinforcement effect of particle will be the best at the temperature of 60℃. In this case, the tensile properties were increased by 44.6% and 63.3% for 1.9wt.%TiB2/BH135 and 8.3wt.%TiB2/BH135 composite respectively than that of BH135 alloy.4. The thermal expansion coefficient of in situ TiB2/BH135 composites is lower than matrix alloy. The thermal cycle curve exhibits only very small contraction at ambient temperature. The CTEs of 1.9wt.%TiB2/BH135 and 8.3wt.%TiB2/BH135 are decreased by 16.2% and 12.8% respectively than that of BH135 alloy.