Rheo squeeze casting is to squeeze the semi-solid alloy melt from the bottom into the cavity which has been closed and locked by the injection hammer, and then continue to press until solidification. Its feature is that the casting is not affected by the pouring amount, and the dimensional accuracy is high, but only part of the part area is directly pressed, so the compression effect is limited. It is similar to die casting, and it is a continuous, low-speed and pressure transfer process Casting technology. According to the quantitative analysis of metallographic microstructure, the solid fraction of parts 1 and 4 is relatively low, which is mainly due to the liquid phase flowing to the four peripheral parts first under the pressure gradient, while the liquid phase failing to flow to the surrounding parts finally solidifies in part 4 due to the sequential solidification above the injection hammer. From the analysis of solidification behavior similar to die casting, when the semi-solid slurry prepared by lspsf process flows into the accumulator and is transferred to the horizontally moving barrel (with heater) on the equipment, it enters into the slow cooling stage, which is conducive to the formation of spherulite structure, high solid ratio is conducive to maintaining the stability of interface, and the solidification process of alloy melt is similar to equilibrium solidification, A356 The equilibrium microstructure of the alloy is primary α + eutectic (α + Si) + Mg2Si, which can effectively avoid a large number of impurity phases during solidification of non-equilibrium microstructure. The secondary solidification takes place in the mold cavity, and the solidification speed is very fast. In addition, the heat content of semi-solid metal melt with high solid phase is less. Under the condition of strong cooling, there is almost no more growth of grains, which makes the morphology of intermetallic compounds maintained and improved. According to the thermodynamic calculation, the solidification process can be divided into two stages, and the two-step thermodynamic calculation assumption can be carried out. The first stage solidification occurs in the slurry accumulator and barrel, which can be regarded as equilibrium state; the second stage solidification occurs in the mold, which can be regarded as non-equilibrium state under the condition of high pressure cooling, which can be calculated by Scheil model. The detailed solidification path and the relationship between phase formation and temperature during squeeze casting can be obtained by the two-stage thermodynamic calculation. Due to the forced feeding behavior of solid-liquid zone in squeeze casting under pressure, the indirect squeeze casting method is used in the experiment, and the pressure transmission is relatively not too large. Generally speaking, the wall thickness of the workpiece should not be too thin, otherwise it will bring some difficulties to the forming, and the quality of the parts will also be affected.
Fig. 1 shows the microstructure of cross section 1 ~ 4 of A356 alloy by liquid squeeze casting. The primary α – Al phases in parts 1, 2, 3 and 4 are coarse and developed Rosa crystal and a small part of dendrite. The results show that there are very fine rosette grains and some fine near spherical grains in parts 1, 2, 3 and 4, and the microstructure distribution is also uneven, which is formed by secondary solidification of residual liquid phase in slurry and recrystallization of some dendrites. The results show that the grain in position 3 is the densest, the solid phase is higher, the extrusion pressure is greater in this position, and the liquid phase flows preferentially when the pressure gradient is larger, while positions 1, 2 and 4 are more than the liquid phase. Compared with Fig. 1 and Fig. 2, the primary α – Al grains in liquid squeeze casting are obviously different from those in rheo squeeze casting, and their distribution is slightly different. The grains in rheo squeeze casting are spherical or nearly spherical with uniform distribution. From the distribution of liquid phase composition, the liquid phase ratio of liquid squeeze casting is less than that of rheo squeeze casting, which is mainly due to the fact that spherical grains make the liquid phase easier to flow under the same pressure gradient. For details, see the formula, FS is the grain shape factor coefficient, FS The smaller the grain size is, the less the liquid flow rate is, and the less the liquid flow rate is.
It can be seen from Figure 1 that the microstructure of A356 alloy formed by conventional liquid squeeze casting has obvious dendrite structure; after squeeze casting, the dendrite is more finely broken, and its position is 3 Because of the extrusion of reflux and hammer head, it is found that the grain boundary is denser than other parts, and the degree of extrusion is greater.