Quality control of 7075 alloy by rheo squeeze casting

Through lspsf process, connecting with the special semi-solid vertical squeeze casting equipment developed by the laboratory, and adopting reasonable process parameters for rheo squeeze casting, the parts with the appearance, microstructure and properties meeting the requirements can be obtained. The parts are shown in Figure 1. From the surface quality point of view, the surface is smooth, the forming size is accurate, and the forming quality is good. In the process of rheo squeeze casting, the control of process parameters is affected by many factors, which will lead to a certain deviation and eventually lead to some defects. Due to the overflow tank with exhaust and impurity discharge, no shrinkage defects are found in the parts when they solidify under pressure. The defects found from the microstructure are mainly cold interlayer, and the other kind of defects are mainly cold barrier on the surface.

It can be seen from Figure 2 that the microstructure of the cold sandwich is an abnormal structure, and there is an obvious dividing line between the cold sandwich and the normal structure. The cold sandwich generally appears at Y in Figure 3 and 1 in Figure 4 of the wheel parts. It can also be seen from Fig. 2 that the primary solid in the cold sandwich is much less than that in the normal structure, and the secondary dendrite spacing of the crystal in the cold sandwich is smaller than that in the normal structure, which indicates that the cold sandwich is easy to form in the melt of rheo squeeze casting under the condition of chilling. Based on the above analysis, combined with the characteristics of rheo indirect squeeze casting process with transverse moving feed, the alloy melt in the barrel flows into the mold cavity under the extrusion of the injection hammer, and the top of the alloy melt is strongly cooled by the top wall of the mold to produce a chilling layer. In addition, there are some oxide scales on the top surface of the alloy melt during the transverse movement of the barrel. Because the injection head is moving vertically upward, the alloy melt changes the direction of movement and flows around when it touches the top of the mold cavity. There is a deviation in the movement speed between the chilling layer and part of the oxide scale and the alloy melt, which makes them friction with each other during the flow The quench layer is broken and enters into the inner layer of the alloy melt, forming cold interlayer defects. Because of the difference between the cold interlayer and the normal solidification structure, there are obvious boundaries in the microstructure, and the microstructure morphology is also different. Because the chilling layer and the surface oxidation of the barrel are the main factors for the formation of the cold interlayer, higher heat preservation temperature of the barrel, appropriate preheating temperature of the die and faster filling speed are beneficial to eliminate and inhibit the formation of the cold interlayer.

Cold shut defect is a kind of surface defect, which mainly refers to the gap caused by incomplete fusion or discontinuous metal melt at the confluence intersection. The reason for the cold shut defect is that the molten metal in the semi-solid casting is unstable and continuous, and the filling time is too long, which makes the alloy melt which has been injected into the mold cavity and the alloy slurry which has been injected into the mold cavity can not be fused together, forming a fusion interface. In the actual production, it is necessary to adjust and match the mold preheating temperature, pouring interval, pouring speed, barrel insulation temperature and other process parameters, which will effectively eliminate or reduce the cold shut defects.

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