| In-situ A356-TiB2 composites have been found a great deal of application in the automotive and aerospace industries by virtue of combining excellent properties. As the matrix alloy of the composites, components made of A356 alloy are normally formed by shape casting and hence their mechanical properties depend primarily on as-cast microstructures, which consists of undesirable coarse a-Al dendrites and platelet-like Si particles. It is of great significance to further improve the mechanical properties of the composites by improving the as-cast structures of the A356 matrix alloy via grain refinement and eutectic modification.In this paper, a re-melting and diluting(RD)approach was adopted to fabricate the in-situ A356-2.5 wt.% TiB2 composites. The effects of La on the grain refinement and eutectic modification were investigated. The results demonstrated that the addition of La can not only refine the second dendrite arm space (SDAS) of the a-Al grains, but also modify the eutectic Si particles into fibrous ones. Due to the distinct addition of La for grain refinement and modification, it shows a bimodal effect on the mechanical properties of the composites. The optimum La addition for grain refinement and eutectic modification is 0.10 wt.% La and 0.80 wt.%, respectively. With 0.10 wt.% La refined, the SDAS of the a-Al grains was reduced from 31.53 ?m to 20.74 ?m, recording a 34.20% reduction. On the other hand, with 0.80 wt.% La modified, the coarse lamellar eutectic Si was changed into fine, globular/fibrous particles, achieving full modification. With full modification, the mean area and aspect ratio of the eutectic Si particles was reduced from 6.45?m2 and 3.39 to 2.61 ?m2 and 1.82, respectively. After T6 temper, the eutectic Si particles were coarsened and spheroidized and the mean area and aspect ratio was decreased from 9.61 ?m2 and 2.51 to 4.62 ?m2 and 1.62, respectively. Consequently, the variation in the mechanical properties of the A356-2.5 wt.% TiB2 composites as a function of the La content is a result of the combination of grain refinement and modification and exhibits a bimodal distribution, peaking at 0.10 wt.% La and 0.80 wt.% La, respectively. Compared with the A356-TiB2 without La, the mechanical properties at the peaks are significantly improved, especially the elongation which has been nearly doubled.As a kind of aging-hardenable alloy, change in microstructures can significantly influence the aging behavior, affecting the final mechanical properties of the A356 alloy. Therefore, the aging behavior of the in-situ A356-TiB2-La composites was also investigated in this paper. Via the comparison of the microstructures and age-hardening curves, the combining effects of TiB2 and La on the aging behavior of A356 alloy were investigated. According to the TEM results, with the addition of TiB2, the density of precipitates during the aging process is increased because of the increased dislocation density caused by the coefficient mismatch of thermal expansion between the matrix and reinforcements. Besides, the size of the precipitates was also reduced, which can significantly increase the aging hardness of the matrix. The results also show that La can help to refine the SDAS of the a-Al grains and disperse the TiB2 particles, both of which can facilitate the aging process. Moreover, the addition of La promotes the diffusion of Mg and Si in the matrix and decreases the conglomeration of impurities in unit area and grain boundaries, which provides the increased number of nucleation sites for the subsequent precipitates and improves the precipitation rate. As a result, La accelerates the precipitation process in the aging kinetics which shortens the peak-aged time by some 30 min based on the existence of TiB2. Therefore, the aging precipitation hardening and mechanical properties of the composite was further improved. |
PDF Full Text Request