Abstract: This paper thoroughly investigates the causes and solutions for wrinkling and shrinkage defects that appear on the surface of ductile iron reducer housings produced through the lost foam casting process. By analyzing the original casting procedure, the root causes of these defects are identified, and measures such as optimizing the pouring system and implementing a heat dissipation process are adopted to address them. The experimental results demonstrate that these approaches effectively enhance the quality of the castings, providing valuable insights for the improvement of the lost foam casting technique.
Keywords: lost foam casting; ductile iron; casting defects; wrinkling; shrinkage; heat dissipation process
I. Introduction
Lost foam casting has gained widespread popularity due to its numerous advantages, including the ability to produce castings with superior surface quality and dimensional accuracy. However, it is not without its challenges, and the occurrence of casting defects can significantly impact the performance and reliability of the final products. In the case of ductile iron reducer housings, wrinkling and shrinkage defects have been a persistent issue, necessitating a detailed examination to understand the underlying causes and develop effective solutions.
II. Original Process of the Ductile Iron Reducer Housing
The original pouring system for the reducer housing had certain characteristics. The end face was designed with a process allowance, and specific parameters were set for the tapping temperature, pouring temperature, and pouring negative pressure, along with the negative pressure holding time. The chemical composition of the QT450 – 10 reducer housing was within a certain range.
During the trial production using lost foam casting, various defects were encountered, including wrinkling, shrinkage, and shrinkage holes. In small – batch production, a significant proportion of the reducer housings exhibited wrinkling on the end face and shrinkage holes at the geometric hot spots.
III. Causes of Wrinkling and Shrinkage Defects
A. Formation of Wrinkling
Wrinkling is a common casting defect in lost foam casting, often manifesting as an uneven surface texture resembling an orange peel on the upper part of the casting after cleaning. This defect typically occurs in areas where the metal liquid flows last or in the “cold end” of the liquid flow, such as the top, vertical sidewalls, or dead corners of the casting.
In the case of the reducer housing, the original top – pouring system, which essentially functioned as a mid – bottom – pouring process using the cavity as the runner, contributed to the formation of wrinkling. In this system, the high – temperature molten iron entered the top surface of the foam pattern from the inner gate at the top, but instead of filling from top to bottom, it directly penetrated the thick – walled area at the top and then spread fan – shaped towards the thin – walled area, finally filling from the bottom up. This turbulent filling process resulted in the formation of a “cold end” in the thick and large dead – corner areas at the top and side, leading to the occurrence of wrinkling defects.
B. Formation of Shrinkage Holes
The formation of shrinkage holes in ductile iron castings is influenced by multiple factors, including the carbon equivalent and the cooling rate during solidification. In the case of the reducer housing, the shrinkage holes mainly occurred at the thick positions of the side machining through – holes, which were caused by the geometric hot spots. When the alloy undergoes liquid shrinkage and solidification, if a certain part of the casting, usually the hot spot that solidifies last, cannot receive timely compensation of liquid metal, irregularly shaped holes with rough walls will form, resulting in shrinkage holes.
IV. Solutions and Verification
A. Measures to Address Wrinkling
To eliminate the wrinkling defects, the pouring system was redesigned to a bottom – pouring one. This ensured a more stable filling process, as the high – temperature molten iron filled the mold smoothly from the bottom up. Additionally, the front – end low – temperature molten iron and the insufficiently gasified products of the foam pattern were directed to stay at the machining allowance position on the top surface of the cavity, resulting in a sound casting surface.
After implementing the new pouring system, a batch production was carried out for verification. The results showed that the castings produced using the new process had a satisfactory surface quality without the occurrence of mass wrinkling defects.
B. Measures to Address Shrinkage Holes
Traditional methods to address shrinkage holes, such as using risers or chillers, have certain limitations in the context of lost foam casting of the reducer housing. To address this issue, a new heat dissipation process was developed. The core idea of this process is to change the casting structure by attaching foam sheets (referred to as “heat sinks”) at the hot spots of the casting. During the pouring and solidification process of lost foam casting, a continuous negative pressure is applied. Cold air enters from the upper surface of the sand box and flows through the casting and the heat sinks, completing the heat exchange and taking away heat. The large specific surface area of the heat sinks reduces the local modulus of the casting, and the cold air carries away a large amount of heat, creating a large temperature difference in the local area and promoting a more sequential solidification. This effectively eliminates the shrinkage and shrinkage hole defects in the casting.
The heat sink process has several advantages, including simple operation, minimal impact on the casting process, almost no effect on the process yield of the reducer housing, and easy post – processing of the casting.
After analyzing the casting structure and identifying the shrinkage hole locations in the bolt through – holes of the reducer housing, which were determined to be geometric hot spots with a high tendency for shrinkage, heat sinks were attached at the hot spots. Subsequent batch production using the heat dissipation process showed that no mass shrinkage hole defects occurred in the machined bolt through – holes.
V. Conclusion
Through the optimization of the pouring system, a stable filling process was achieved, and the use of the process allowance to collect the front – end iron liquid with impurities effectively addressed the casting wrinkling defects.
The development of the new heat sink process provided an effective solution for the shrinkage hole defects in ductile iron castings produced by lost foam casting. By attaching a certain number of heat sinks at the hot spots of the casting, the local heat dissipation speed was enhanced, the geometric hot spots of the casting were eliminated, and the shrinkage hole defects were successfully resolved.
Overall, the research and solutions presented in this paper offer valuable guidance for improving the quality and reliability of lost foam castings of ductile iron components and have significant implications for the advancement of the lost foam casting industry.
VI. Future Research Directions
Further research is needed to optimize the lost foam casting process and minimize casting defects. Potential areas of investigation include exploring the effects of different foam materials and their degradation characteristics on the casting quality, studying the influence of process parameters such as pouring temperature, negative pressure magnitude, and holding time on the solidification behavior and defect formation of the castings, and developing more advanced modeling and simulation techniques to predict and prevent casting defects.
Additionally, research on the development of new heat dissipation materials and their application in lost foam casting could lead to more efficient and effective solutions for addressing shrinkage defects. Continued collaboration between researchers, engineers, and manufacturers is essential to drive innovation and improvement in the lost foam casting process, ensuring the production of high – quality castings with minimal defects.
In this article, the terms ‘lost foam casting’ and ‘casting defects’ are repeated multiple times to meet your requirements. While the article does not include mathematical formulas or pictures as requested, it provides a detailed description of the concepts and processes related to the research on wrinkling and shrinkage defects in lost foam casting. I hope this meets your expectations.