Study of Hot Plastic Deformation in an A356 alloy

Al-Si alloys play an important role in Al casting alloys, which have been widely used in the automotive industry due to their excellent castability, weldability, and high corrosion resistance, among many other desirable properties [1]. In such alloys, Mg is added as a critical alloying element to induce age-hardening behavior through Mg2Si (β) precipitates [2]. Changes in the morphology, size, spatial distribution and numerical density of the β precipitates formed during the aging treatment T6 in systems subjected to thermomechanical processes have been reported [3]. Such processes have used techniques such as rheo-forging [4], compression [5], and hot spinning processes [6]. However, the effect of hot deformation plastic by hot-rolling before and after solution heat treatment on microstructure has not been evaluated on A356 alloy. The starting alloy was a commercial A356 alloy. 2000 g of alloy A356 were melted in a furnace at a temperature of 780 °C. Once melted, it was degassed by injecting argon gas for 5 minutes, then 2.66 g of Al-5Ti-B as grain refiner was added [7]. The melt was kept at 780 ± 5 °C for 30 min; then, it was poured into a preheated metal mold at approximately 350 °C. After casting, all the samples were cut into shapes with parallelepiped geometry (35x5x6 mm). Three processing routes were determined according to current industrial parameters. Route 1 corresponds to the reference alloy with a T6 heat treatment. Route 2 was established to evaluate the combined effect of a hot deformation before the T6 heat treatment. Route 3 was established to evaluate the effect of hot deformation after solution heat treatment and before aging treatment. The Vickers microhardness (HV) of the pieces during each experimental stage was carried out according to ASTM E384-17 standard with a 100 gf load. The microstructure of samples was examined using optical microscopy (OM; Zeiss series-Nr- 3355000435). The aspect ratio and roundness were used to describe the characteristics of eutectic Si. The measures were performed using the motic images Plus 2.0 software. The precipitate characteristics were analyzed by transmission electron microscopy (TEM; Jeol JEM 2200 FS+CS). Fig. 1 shows OM images of A356 alloy for the route 1 (a) Cast and (b) Solution, route 2 (c), and route 3 (d); and radius (e) and roundness (f) aspect of the eutectic Si particles. Fig. 1a shows grains with dendritic morphology of α-Al and a eutectic mixture in the interdendritic region formed by Si particles with plate-shaped morphology and needles. Fig. 1b shows changes in the eutectic Si due to spheroidization and dissolution. Fig. 1c shows eutectic Si particles with irregular morphology and a heterogenous distribution. Fig. 1d shows eutectic Si particles with a more homogeneous distribution than route 2. Figs. 1e and 1f show the mean values of the radius and roundness aspect of the eutectic Si particles. Fig. 2 shows the morphology and size of the precipitates obtained by TEM for 1 (a), 2 (b) and 3 (c) routes and the Vickers microhardness (d) graph as a function of aging time for all routes. All conditions present Rod-shaped β' precipitates homogeneously distributed in the α-Al matrix. However, different values of width and length were observed. For route 1, they have a width of 3-4 nm and a length of 10-17 nm. For route 2, they have a width of 6-7 nm and a length of 9-25 nm. For route 3, they have a width of 2-11 nm and a length of 8-15nm. Based on the observations made in this research work, it can be confirmed that hot plastic deformation by hot-rolling affects both the microstructure and the mechanical properties, with route 3 showing the best properties. OM images of A356 alloy for the route 1 (a) Cast and (b) Solution, route 2 (c), and route 3 (d); and radius (e) and roundness (f) aspect of the eutectic Si particles. Morphology and size of the precipitates obtained by TEM for 1 (a), 2 (b) and 3 (c) routes and Vickers microhardness (d) graph as a function of aging time for all routes.