Effects of parameters on formation and microstructure of surface composite layer prepared by lost foam casting technique

Surface composite layer was fabricated on the AZ91D substrate using the lost foam casting (LFC) process. The pre-coating layer reacted with melt substrate and formed the composite layer, and the coating was mainly consist of alloying aluminum powder and low-temperature glass powder (PbO-ZnO-Na₂O). The vacuum degree, pouring temperature, mold filling process of melt, and pre-coating thickness played an important role during the formation process of composite layer. The results show that surface morphology of composite layer can be divided into three categories: alloying effect of bad and good ceramic layer, alloying effect of good and bad ceramic layer, composite layer of good quality. The main reason for bad alloying layer is that alloying pre-coating thickness is so thin that it is scoured easily and involved in the melt, in addition, it is difficult for melt to infiltrate into the alloying coating owing to the surface tension of coating when the vacuum degree is excessively low. Bad ceramic layer is because of somewhat lower pouring temperature and the thicker alloying coating, due to the absorption of heat from the melt, making low temperature glass powder pre-coating layer fuse inadequate. Thus, to get good quality composite layer, the process conditions must be appropriate, the result shows that the optimum process parameters are as follows: at a pouring temperature of 800 °C, vacuum degree of −0.06 MPa, alloying pre-coating thickness of 0.4 mm, and low glass powder pre-coating layer thickness of 1 mm.