This comprehensive article explores the intricacies of wrinkling and shrinkage defects encountered in the production of ductile iron reducer housings via lost foam casting. Through a detailed analysis of the casting process, the study identifies the primary causes of these defects and proposes effective solutions. Specifically, the article outlines the optimization of the pouring system from top pouring to bottom pouring, which significantly reduces iron liquid turbulence and eliminates surface wrinkling. Additionally, a novel heat dissipation technique is introduced to tackle shrinkage holes in the casting’s hot zones. Experimental results demonstrate the efficacy of these measures, showcasing a complete resolution of surface wrinkling and a substantial improvement in shrinkage defect mitigation. The simplicity and high production yield of the proposed methods underscore their practical applicability in the manufacturing industry.
1. Introduction
Lost foam casting (LFC), also known as evaporative pattern casting (EPC), has gained significant traction in the foundry industry due to its advantages such as excellent surface quality, high dimensional accuracy, and increased process yield. This process is particularly suitable for producing complex-shaped castings with intricate geometries, making it an ideal choice for manufacturing components like ductile iron reducer housings. However, despite its numerous benefits, LFC is prone to certain defects, including wrinkling and shrinkage holes, which can significantly compromise the quality and performance of the final product.
This article delves into the study of wrinkling and shrinkage defects encountered during the production of ductile iron reducer housings using the lost foam casting method. Through a thorough examination of the casting process and its parameters, the study aims to identify the root causes of these defects and develop practical solutions to mitigate them.
2. Background and Problem Statement
Ductile iron reducer housings, with their demanding requirements for high strength, toughness, wear resistance, and shock absorption, pose significant challenges in the casting process. The reducer housing studied in this research weighs 112 kg, with a maximum wall thickness of 54 mm and a minimum thickness of 14 mm, resulting in a concentrated geometric hot spot. To meet these requirements, LFC was chosen as the casting method due to its ability to produce high-quality castings with complex geometries.
However, during the trial production of the reducer housing using LFC, defects such as wrinkling and shrinkage holes were observed. Wrinkling, characterized by an uneven, rough surface resembling an orange peel, was predominantly found on the top and vertical sidewalls of the casting. Shrinkage holes, on the other hand, were more prevalent in the thick and geometrically complex regions of the casting, particularly around the bolt holes.
3. Causes of Wrinkling and Shrinkage Defects
3.1 Causes of Wrinkling
Wrinkling in LFC is primarily attributed to the irregular distribution of carbon-containing residues generated during the thermal decomposition of the foam pattern. These residues, in the form of gaseous, liquid, and solid products, can react with the molten metal, leading to the formation of carbon defects on the casting surface.
In the case of the reducer housing, the original pouring system employed a top pouring method, where the molten metal entered the mold through the top surface. This resulted in a turbulent flow pattern, with the metal initially passing through the thick upper section before spreading downwards and then rising back up to fill the remainder of the mold. This flow pattern created cold zones in the thick and dead-end regions, where the molten metal cooled rapidly and solidified, entrapping carbon-containing residues and causing wrinkling.
| Element | C | Si | Mn | P | S | Mg | RE |
|---|---|---|---|---|---|---|---|
| Content | 3.5-4.0 | 2.0-3.0 | ≤0.45 | ≤0.05 | ≤0.025 | 0.02-0.06 | 0.015-0.04 |
3.2 Causes of Shrinkage Holes
Shrinkage holes in castings arise due to insufficient metal replenishment during the liquid shrinkage and solidification phases. In ductile iron castings, these defects are often found in the last-to-solidify regions, known as hot spots or geometric hot zones.
For the reducer housing, the shrinkage holes were Certainly, here’s the continuation of the previous output in English, based on the reference material provided:
3.2 Measures and Verification of Shrinkage Defects
The traditional approaches to addressing shrinkage defects in castings include using risers to compensate for the metal volume change during solidification or employing chilling systems, such as using chillers to create an artificial freeze zone at hot spots to promote directional solidification. However, both of these methods are not feasible for the ductile iron reducer housing in lost foam casting due to factors like reduced process yield and increased complexity. The use of chillers is also challenging in lost foam casting as they tend to fall off during molding, leading to deformation and inconsistencies in casting quality.
To address the issue of shrinkage holes in the geometric hot spots of the reducer housing, a novel heat dissipation process was developed specifically for lost foam casting. The core objective of this process is to alter the casting’s structure, enhancing the heat dissipation surface area and reducing the modulus of the hot spots. During pouring and solidification, the constant evacuation of vacuum draws significant heat away, creating a chilling effect.
The specific method involves adhering foam sheets, termed “heat dissipation sheets” (see Figure 4), to the hot spot regions of the casting. Following the coating, drying, molding, and pouring procedures, qualified castings are produced. During the pouring and solidification process, the vacuum pump continuously operates, allowing cool air to enter through the top of the sand box, flow through the casting and heat dissipation sheets, and complete heat exchange to remove heat.
The large specific surface area of the heat dissipation sheets reduces the local modulus of the casting. The cool air removes substantial heat, and the local sand in contact with the casting and sheets forms a microchannel flow heat exchange, creating a chilled zone with a significant temperature difference. The heat dissipation sheets act as chillers, transforming the local solidification mode into a similar directional solidification, effectively eliminating shrinkage holes and porosity defects. This principle is illustrated in Figure 5.
The advantages of the heat dissipation sheet process are its simplicity, convenience, minimal impact on the casting process, and negligible effect on the process yield of the reducer housing. Additionally, the post-processing of the reducer housing is straightforward.
Initial tests on reducer housings produced with the original process revealed shrinkage defects in the bolt holes (see Figure 6). Upon analysis, it was identified that these defects originated from the geometric hot spots, which have a high tendency for shrinkage. To address this, 12 heat dissipation sheets, each measuring 50 mm x 30 mm x 7 mm, were attached to the hot spot regions during the cutting and bonding process. The remaining steps of the process remained unchanged. After trial production and machining, the bolt holes of the reducer housings were verified to be free from defects (see Figure 7). Subsequent batch production of 2,000 reducer housings using this heat dissipation process confirmed that no shrinkage defects were present in the bolt holes.
4. Conclusion
(1) By optimizing the pouring system from top pouring to bottom pouring, the turbulent flow of iron liquid during pouring was effectively reduced, and the surface wrinkling of the reducer housing was eliminated by utilizing the process allowance to collect the front-end iron liquid contaminated with impurities.
(2) Traditional methods such as risers and chillers are challenging to implement for ductile iron castings produced by lost foam casting due to hot spots causing shrinkage holes. To overcome this, a novel heat dissipation sheet process was designed. By placing a certain number of heat dissipation sheets at the hot spots of the casting, the local heat dissipation rate was enhanced, eliminating the geometric hot spots and thereby resolving the shrinkage hole defects. This heat dissipation sheet process has been successfully verified through the production of reducer housing castings, demonstrating its ability to meet design requirements and address shrinkage defects. It represents a new approach for solving shrinkage defects in lost foam casting.
Introduction
Lost mold casting, renowned for its superior surface quality, high dimensional accuracy, and high process yield, has been applied successfully in the production of ductile iron components. One such component is the reducer housing, which demands high strength, toughness, wear resistance, and shock absorption capabilities. This study focuses on the defects of wrinkling and shrinkage holes encountered during the production of QT450-10 ductile iron reducer housings using lost mold casting.
Original Process and Defects
The original pouring system for the reducer housing was designed with a top-pouring approach, featuring a 4 mm process allowance on the end face, an exit temperature range of 1580-1600°C, a pouring temperature range of 1370-1440°C, a pouring negative pressure of -0.06 to -0.04 MPa, and a negative pressure holding time of 900 seconds. Despite the chemical composition and mechanical properties of the Y-type sample meeting the QT450-10 requirements, defects such as wrinkling, porosity, and shrinkage holes were observed during trial production, particularly on the end face and in the geometrically complex hot spots.
Analysis of Defect Formation
2.1 Wrinkling
Wrinkling, characterized by an uneven, orange-peel-like surface texture, is commonly associated with carbon defects in lost mold casting. The formation of wrinkles is often attributed to the uneven flow of molten metal and the resulting cold zones, where inadequate foam vaporization occurs. In the original process, the top-pouring design caused turbulent flow of molten iron, directly penetrating the thick top section and fanning out downwards towards the thinner sections. This led to the formation of cold zones in the top and thick-walled corners, ultimately resulting in wrinkling.
2.2 Shrinkage Holes
Shrinkage holes are irregularly shaped pores that form in the last-to-solidify areas of castings, usually the hot spots. In the case of the reducer housing, these defects were found primarily in the thick sections near the machined holes. The root cause of shrinkage holes lies in the inability of liquid metal to replenish the shrinkage during solidification. Therefore, modifying the cooling structure and eliminating hot spots through innovative casting techniques was essential.
Solutions and Verification
3.1 Solution for Wrinkling
To address the wrinkling issue, the pouring system was redesigned from top-pouring to bottom-pouring. This change ensured a smooth and controlled flow of molten iron from the bottom upwards, allowing for the accumulation of impurities and incompletely vaporized foam residues in the process allowance area. The theoretical calculations for the pouring time, head pressure, and gate cross-section were performed, resulting in a center-closed bottom-pouring system with four inlets, each with a cross-section of 70×40 mm. The modified pouring system was then validated through the production of 2,000 castings, all of which exhibited satisfactory surface quality with no instances of wrinkling.
3.2 Solution for Shrinkage Holes
Traditional methods such as the use of risers and chillers were deemed impractical for the reducer housing due to their negative impact on process yield and casting quality. Instead, a novel heat dissipation process was developed. This process involved the application of foam sheets, referred to as “heat sinks,” on the hot spots of the casting. During pouring and solidification, the continuous application of negative pressure facilitated the flow of cool air through the heat sinks, significantly enhancing heat dissipation and promoting directional solidification.
The heat sinks, with dimensions of 50x30x7 mm, were bonded to the hot spots of the casting prior to coating, drying, and pouring. The cooling effect of the heat sinks, coupled with the microchannel heat transfer mechanism between the casting, heat sinks, and the sand mold, resulted in the elimination of shrinkage holes. This method was validated through the production of 2,000 castings, all of which exhibited satisfactory quality in the bolt holes, previously prone to shrinkage holes.
Conclusion
This study has successfully addressed the issues of wrinkling and shrinkage holes in QT450-10 ductile iron reducer housings produced via lost mold casting. By optimizing the pouring system to a bottom-pouring configuration, the turbulent flow of molten iron was minimized, resulting in the elimination of wrinkling defects. Furthermore, the introduction of a novel heat dissipation process utilizing foam heat sinks effectively addressed the problem of shrinkage holes in geometrically complex hot spots.
