Process Improvement For Loose Thin-Walled Castings And Under-Casting Defects

Publish Time: 06/02 2021 Author: Site Editor Visit: 17315

The wall thickness of the regulating piece and sealing piece of high-temperature alloy castings is changed from 1.1mm to 0.8mm, which leads to faster solidification of the castings. The original casting process cannot effectively fill and feed the edges of the castings, resulting in insufficient casting and looseness. defect. In this regard, on the basis of the original process plan, the method of wrapping asbestos in the thin-walled area is adopted to slow down the cooling of the mold shell and prolong the solidification time of the thin-walled area, so that the alloy liquid can effectively fill the mold and feed the edge of the casting to eliminate undercasting, Loose defects.

High-temperature alloy castings such as adjusting piece and sealing piece

Regulating piece and sealing piece high-temperature alloy castings (as shown in the figure above), the wall thickness of more than 90% of the area is required to be 0.8mm, and the wall thickness of the castings is cast according to the upper deviation (+0.3), and 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 original process plan is still produced. Almost 100% of the casted castings are loose, and the loose defects are concentrated on the edges of both sides of the casting floor. Location (as shown in the figure below). In addition, more than 40% of the castings are under-casting, and slag inclusion defects have also increased significantly.

Loose defects are concentrated on the edges of both sides of the casting floor

The Analysis Of Porosity Defects

The cross-sectional view of the edges on both sides of the casting bottom plate is shown in the figure below. There are reinforcing ribs on the edges. The design requirements for the wall thickness of the ribs and the bottom plate are shown in the figure below. In the initial design of the mold, the wall thickness of the rib and the bottom plate shall be 1.1mm, that is, equal wall thickness. The joint between the rib plate and the bottom plate wall forms a casting hot node; compared to the bottom plate, the modulus of the hot node is not much different. In addition, the hot node is on the edge of the casting, cooling and solidifying relatively fast, and the alloy liquid can heat through the bottom plate The nodes are fed without loose defects.

Design requirements for rib wall thickness and bottom plate wall thickness

After the wall thickness of the bottom plate was changed to 0.8 mm, the modulus of the bottom plate was reduced by nearly 1/3, and the cooling and solidification speed of the bottom plate was accelerated; while the wall thickness of the ribs did not change, the modulus of the hot node remained basically unchanged, and the cooling and solidification speeds were basically unchanged. As a result, the alloy liquid cannot effectively feed the hot nodes through the bottom plate, resulting in looseness.

The Analysis Of Undercasting Defects

The casting is a plate-like structure as a whole, with thin wall thickness and large area. In order to ensure the casting molding, the mold shell needs to be filled with sand and preheated. During the preheating process, the mold shell is heated and softened and its strength is reduced. As shown in the figure below, the inner cavity becomes narrower, and the wall thickness of the casting is slightly smaller. After the bottom plate wall thickness is changed to 0.8 mm, the actual wall thickness will be smaller. In addition, the cooling and solidification speed will increase. The location alloy liquid has solidified before it can flow, resulting in under-casting defects.

Before and after deformation

After the bottom plate wall thickness is changed to 0.8 mm, the cooling and solidification speed is accelerated, and the slag inclusions in the alloy liquid have no time to float or sink, and the alloy liquid has solidified and stayed in the casting to form slag inclusion defects.

Improvement Measures For Loose And Under-Casting Defects Of Regulating Piece And Sealing Piece Castings

In the process, asbestos is wrapped on the mold shell (as shown in following figure), that is, asbestos is pasted on the bottom plate (preserving the hot node), and the casting is poured out, and the hot node is placed.

No porosity was found, and no porosity was seen on the bottom plate, and the lack of casting defects was also increased by 80%.

In the process, measures are taken to wrap asbestos on the mold shell

Process Improvement Effect Analysis

When smelting pouring castings, during the vacuuming process, the temperature of the mold shell in the melting furnace will drop. After the bottom plate is wrapped with asbestos, the asbestos will heat the mold shell, slowing down the cooling rate of the bottom plate and prolonging the solidification time of the bottom plate, thereby ensuring the alloy The liquid effectively feeds the hot nodes through the bottom plate, and also effectively ensures the bottom plate filling.

Asbestos is a soft material. After asbestos is wrapped, asbestos slows down the extrusion of sand on the mold shell, reduces the deformation of the mold shell, indirectly increases the wall thickness of the bottom plate, and prolongs the filling time of the bottom plate.

After the asbestos is wrapped, the solidification time of the bottom plate is prolonged, so that the inclusions in the alloy liquid can fully float or sink, and the tendency of casting slag inclusion is virtually reduced.

Conclusion

The method of using the mold shell to wrap the asbestos can hinder the extrusion of the mold shell by the sand, reduce the cooling rate of the mold shell, and extend the filling and solidification time of the bottom plate, thereby effectively feeding the hot nodes at the edge of the casting and improving the porosity tendency.

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