When the wall thickness of high-temperature alloy castings such as adjusting piece and sealing piece is changed from 1.1mm to 0.8mm, the solidification speed of castings will be accelerated. The original casting process can not effectively fill and supplement the edges of castings, resulting in under casting and loose defects. Therefore, on the basis of the original process plan, the method of wrapping asbestos in the thin-walled area is adopted to slow down the temperature of the die shell, prolong the solidification time in the thin-walled area, effectively fill the mold with alloy liquid and supplement the edge of the casting, so as to eliminate the under cast and loose defects.
The wall thickness of over 90% of the high-temperature alloy castings, such as adjusting and sealing pieces, is required to be 0.8mm. The wall thickness of the castings is cast according to the upper deviation (+ 0.3). The casting molding effect and metallurgical quality are good. The process plan is basically mature, but the weight exceeds the design requirements. After the wall thickness is changed to 0.8mm, the castings are still produced according to the original process plan. Almost 100% of the castings are cast In addition, more than 40% of the castings are under cast, and slag inclusions are also increased.
The sectional view of the edge on both sides of the bottom plate of the casting is shown in Figure 3. There is a stiffener plate on the edge. The design requirements for the wall thickness of the stiffener plate and the wall thickness of the bottom plate are shown in Figure 3. In the initial design of the mold, the wall thickness of the rib plate and the wall thickness of the base plate are all 1.1mm, that is, the equal wall thickness. The joint between the rib plate and the bottom plate wall forms the hot joint of casting. Compared with the bottom plate, the modulus of the hot joint has little difference. In addition, the hot joint is on the edge of the casting, and the cooling and solidification are relatively fast. The alloy liquid can supplement the hot joint through the bottom plate without loose defects.
When the thickness of the bottom plate is changed to 0.8 mm, the modulus of the bottom plate is reduced by nearly 1/3, and the cooling and solidification speed of the bottom plate is accelerated; while the thickness of the reinforced plate is not changed, the modulus of the hot node is basically unchanged, and the cooling and solidification speed is basically unchanged, which leads to the failure of the alloy liquid to effectively shrink the hot node through the bottom plate, resulting in porosity.
The whole casting is of plate structure, with thin wall and large area. In order to ensure casting forming, the shell needs to be packed with sand for preheating. In the preheating process, the shell is softened by heat and the strength is reduced. Under the sand extrusion, there will be a certain amount of deformation, resulting in narrower inner cavity, slightly smaller wall thickness of the casting. After the wall thickness of the bottom plate is changed to 0.8mm, the actual wall thickness will be smaller. In addition The cooling and solidification speed is accelerated, and the alloy liquid has solidified before it flows to some places, so the under casting defects appear.
When the wall thickness of the bottom plate is changed to 0.8 mm, the cooling and solidification speed will be accelerated. Before the slag inclusions in the alloy liquid float or sink, the alloy liquid will be solidified, thus remaining in the casting, forming slag inclusion defects.