Research On Fabrication And Modification Of In-situ A356-TiB₂ Composite

The in-situ A356-TiB2 composites are widely used in the automotive, chemicals and aerospace fields due to their high specific strength and excellent thermo-stability. Among various methods to fabricate in situ A356-TiB2 composites, the Flux-assisted synthesis (FAS) approach which involves the halide salt reactions is the most popular.In this paper, in-situ A356-TiB2 composites were fabricated via a re-melting and diluting (RD) approach which is based on the FAS approach. The process of RD approach includes two steps. First, high quality Al-5wt.% TiB2 master alloy is fabricated via the FAS approach in which the residual salts are cleaned out completely. Second, the prepared Al-5wt.% TiB2 master alloy is re-melted and then diluted by high-purity Al, Si and Mg according to the proportion of the A356-x wt.% TiB2 composites. It is feasible to control the TiB2 content in the final composites by adjusting the addition rate during RD process. Low content of TiB2 (-3wt.%) in the composites can lead to a favorable strengthening effect. The ultimate tensile strength of the A356-3wt.% TiB2 composites is 313.7 MPa in the T6-tempered state,35.4% higher than the A356 matrix alloy while the elongation is decreased from 5% to 3.5%. The comprehensive mechanical quality value Q is 17.5% higher than the matrix.A further study was carried out on the effect of Sr acting as a modifier to treat with the prepared in-situ A356-TiB2 composites fabricated via RD and FAS approaches, respectively. Addition of 0.03wt.% Sr is sufficient to achieve full modification for A356-3wt.% TiB2 composite via RD approach, which displays a microstructure with globular-fibrous eutectic Si particles. On the contrary, the modification efficiency of the composite fabricated via FAS approach is very poor when treated with the same level of Sr, in which the eutectic Si particles appear as irregular polygon blocks. In the FAS process, there exist lots of B-rich residual salts inevitably, which will lead to the Sr-B interaction and take disadvantage of the modification efficiency. However, the residual salts are cleaned out completely when preparing the Al-5wt.% TiB2 master alloy. As a result, the RD process can minimize the Sr-B interaction so as to achieve full modification on eutectic Si of the A356-TiB2 composites. The addition of Sr greatly improves the mechanical properties (especially the elongation) of thus prepared composites, only when the composites are in a fully modified state. The elongation of the 0.03wt.% Sr modified RD A356-3wt.% TiB2 composite is 5.6% in T6 states,60% higher than the unmodified composite. Moreover, further elevating the Sr addition rate to 0.1wt.% results in over modification for the RD composite, which is detrimental to mechanical properties of the composite.