Mechanical Properties and Tensile Fracture Mechanism of Rheocast A356 Al Alloy Using Cooling Slope

Abstract

A356 aluminium alloy has a wide applicability in the manufacturing of automotive parts. Cooling slope (CS) rheocasting process has been used in the present work to produce A356 billets having near spherical morphology of primary Al phase. Absence of dendritic primary phase, observed in case conventionally cast A356 alloy, and finer distribution of secondary eutectic Si phase within the matrix establishes the usefulness of the CS casting route. Near spherical primary phase in the rheocast alloy ensures better strength, elongation properties and structural integrity in the produced billets. The liquid melt is allowed to flow through the cooling slope after pouring at 6500C. Rapid heat exchange between the flowing melt and slope wall and the atmosphere facilitates heterogeneous nucleation of α-Al phase on the cooling slope wall. Shear driven flow of the solidifying melt is found responsible for separation of α-Al phase from the slope wall and generation of nearly spherical morphology of the primary phase in the microstructure. Grain refiner addition in the melt leads to enhance the primary α-Al percentage in the microstructure and also aids to the improvement of degree of sphericity and reduction of spheroid size. So, grain refining helps to improve the strength, elongation and fracture properties of rheocast billets further.

Effect of Cooling Slope Angle on Microstructure of Al-7Si Alloy

“…The present finding is in contradiction with most of the previous studies (Table 1) which shows a minimum grain size and maximum shape factor at higher cooling slope angles [4,[11][12][13][14][15]. The experimental investigations by Taghavi et al [11] and Das et al [4,12,15] concluded 40°(slope length of 400 mm) and 60°angle (slope length of 500 mm) to be the optimum cooling slope angle respectively. Also, Nourouzi et al [13] reported 60°t o be the optimum angle by using genetic algorithm.…”

“…Figure 3 shows the variation of grain size and shape factor in Al-7Si alloy with respect to cooling slope angle. The average grain size (CD = 2 ffiffi ffi A p q ) and shape factor (SF = 4pA P 2 ) were calculated using equations mentioned [12], where, A and P represent the area and perimeter of aAl phase, respectively.…”

Production of semi-solid feedstock of A356 alloy and A356-5TiB2 in-situ composite by cooling slope casting

“…The solidified strip and the morphology of A356 alloy on CS strip at exit end, poured at 60° angle is shown in figure 6. (2), reported elsewhere [11,12]. The variation of Grain size and S.F of α-Al phase of A356 alloy at different processing conditions is shown in figure 8.…”

“…Their results indicated that 40° and 40 cm condition is optimum inclined plate angle and length presenting the minimum grain size of 28μm and maximum shape factor of 0.70 with highest uniformity. Das et al [12] has conducted experiments and studied the effects of slope angle and addition of grain refiner on microstructure evolution of the solidifying melt through rheocasting process employing cooling slope (CS) casting route. They reported that there is a significant increase in the tensile properties of the rheocast feed stock processed by CS casting route.…”

Thixoforming and Rheo-Die-Casting of A356/SiC Composite

“…It is reported that hardness can be improved by decreasing the re-heating temperature of semi-solid A356 [19]. In addition, increase in cooling rate by cooling slope of a rheo-cast A356 alloy also produced near spherical structures, which enhance tensile strength compared to dendritic structures produced by gravity casting [20]. Figure 8b,c shows thixoformed samples and microstructures at the center of a thixoformed slug of A356/15%SiC-37µm and A356/15%SiC-15µm, respectively, indicating increased particle size and decreased segregation of the solid phase.…”