Figure 1 shows the metallographic structure of different parts of 7075 alloy wheel extrusion casting. It can be seen from the figure that:
① On the whole, the microstructure of squeeze casting is dense and uniform, the grain is fine, and there is no coarse dendrite like that of ordinary casting. The fine non dendrite structure can be obtained by rheo squeeze casting.
② According to the statistics of metallographic analysis software, the average grain size of each position is almost the same, and positions 2 and 5 are relatively small.
③ The solid fraction of position 4 is the highest, and that of position 1 is the lowest, with some liquid segregation. The liquid component in the semi-solid slurry flows around under the effect of filling pressure, resulting in the relatively large liquid phase ratio at the top edge position 1 of the indirect extrusion casting.
④ Squeeze casting combines the advantages of die casting and die forging, which can make the semi-solid slurry solidify under pressure, and the solid composition will produce a small part of plastic deformation, which makes the internal structure of the material more compact, so it has better performance than liquid squeeze casting.
⑤ More fine grains appear in positions 4 and 5, which may be caused by the recrystallization of these two parts during the later filling and holding period.
The appearance of these characteristics is related to the die structure of rheo squeeze casting and the process characteristics of indirect squeeze casting. The primary α – al of 7075 alloy is nearly spherical in semi-solid slurry, which is non dendritic structure. The liquid phase surrounds the primary α – Al, so the alloy can flow smoothly and fill the mold completely under the reasonable process parameters (pressure, mold temperature, etc.). In the indirect squeeze casting process, the semi-solid slurry is directly connected with the bottom of the die by the barrel with heating and insulation through the translation device. In the forming process, the extrusion head overcomes the resistance of gravity and gate from bottom to top, and finally completes the mold filling. Due to the design characteristics of the wheel shaped part mold structure, the size of the bottom gate is large, and the pressure is directly pressed on the metal surface through the end face of the injection punch below. All the energy of the equipment is basically used to make the metal melt obtain isostatic pressure and keep it until the end of pressure relief. Therefore, rheo squeeze casting is to make the semi-solid metal flow under pressure until the alloy melt is completely solidified and a complete part is obtained with dense structure and fine grains.
The semi-solid metal flows into the annular channel composed of punch and die under the action of punch end face of indirect squeeze casting to realize flow filling. After the filling process starts, the semi-solid metal contacts the upper surface of the die bore and produces upsetting flow under the action of extrusion pressure. Then the semi-solid metal expands radially and produces shear deformation. At this time, the extrusion resistance increases continuously and “shear thinning” filling occurs, which makes the liquid part flow to the edge part. Therefore, the liquid phase rate of position 1 is large, and the liquid phase segregation is easy to occur in the upper corner.
The metal in position 2 is extruded in the opposite direction and flows downward with the upward movement of the extrusion punch. The deformation force at this position can be analyzed by the approximate back extrusion model. Position 3 is mainly subject to shear deformation, which is at the turning point of the thickness change of the part shape, so that the primary α – Al in this position will undergo plastic deformation and elongate along the slurry flow direction.
The irregular grains and fine grains in positions 4 and 5 may be due to the great extrusion force on the semi-solid slurry in positions 4 and 5, and recrystallization will occur during the pressure holding period and late filling period, resulting in some very fine grains and primary α – al Welding will take place under great extrusion, and some irregular non spherical grains will be formed.
The typical microstructure of 7075 alloy by rheo squeeze casting is shown in Fig. 2. The semi-solid microstructure is obvious, and the microstructure grains are round and fine without dendrite.