Lost Foam Casting Process: Part Two

요약:

The lost foam casting process is a niche casting method which can be ideal for complex geometries and quality requirements which cannot be achieved by more conventional processes.
One key environmental advantage when compared with traditional sand casting methods is that with LFC does not require chemically treated sand to be used meaning that it is can be reused and is completely safe to dispose of.

The Lost Foam Casting (LFC) was initially developed in 1962 and has been commercially available since the 1980s, providing a niche casting approach for products that are not ideal for other casting methods. LFC is a near-net-shape method because it can produce components with complex geometries and open cavities. As mentioned earlier, in this process, molten metal is poured over the expanded polymeric foam pattern, the heat from the molten metal degrades the foam into liquid and gas products by endothermic reaction. The degradation product escapes through foam’s ceramic coating to the surrounding sand. The molten metal fills the cavity and shape of the pattern as it solidifies; the metal velocity is an important parameter since it determines the overall casting quality.

With increased interests in developing lightweight and high-performance materials for structural applications, magnesium becomes a good candidate because of its lowest density among the structural metals. While much research has been done to develop the LFC technology for aluminum alloys, magnesium alloys have only been cast successfully in research environment.

The fillability of magnesium alloys with LFC becomes a challenge because of magnesium’s lower density and heat content compared to aluminum, which results in lower metal velocity during casting. Aluminum alloys offer high castability, while magnesium alloys may offer enhanced weight specific material properties.

A Master thesis S.S.Cho investigates the possibility of fabricating periodic cellular materials (PCMs) via the lost foam casting (LFC) process using aluminum alloy A356 and magnesium alloy AZ91. This approach combines the structural efficiency of PCM architectures with the processing advantages of near-net-shape LFC. An initial feasibility study fabricated corrugated A356 panels. This was followed by a study of casting variables such as pattern design, vacuum assistance, and alloying additions in order to improve the fillability of the small cross-section struts.

Finally, integrated pyramidal sandwich panels having different relative densities were subjected to artificial aging treatments and subsequently tested in uniaxial compression. The A356 PCMs experienced a continuous increase after yielding while the AZ91 PCMs exhibited strut fracture after peak strength. The results showed the compressive yield strengths of this study are comparable with those previously reported PCMs produced by different fabrication methods.

In paper M.Xie et al., interface elapse model is established base on gap pressure as boundary of front of melt flow. By validation and comparison of the basic model, the proposed model can reflect the disappearance of flow shape by melt filling. Under negative pressure environment by vapor pressure, the flow velocity and temperature of melt significantly reduced at front of melt flow. Meanwhile, the actual casting process of two different designs is simulated to investigate filling and solidification process and defect prediction.

Two designs could ensure the smooth flow of melt filling. Scheme 1 could ensure temperature balance of upper and lower part; Scheme 2 could be effectively reducing shrinkage defects. Thus, the simulation results could help design evaluating different process design, to do corresponding improvement measures, and ultimately to reduce the nu mb er of experiments and improve process design level.

It is well known that LFC is a relatively new process that offers several advantages over traditional green sand casting, including high dimensional accuracy, good surface finish, greater design flexibility, and reduced skilled labor requirements. From an environmental (and economic) perspective one of the principal advantages of LFC is the fact that the casting sand can be reused. Traditional green sand casting requires a chemically treated sand to maintain the part shape inside the casting flask.

Disposal of this contaminated sand is the greatest environmental obstacle associated with traditional green sand casting. In contrast, with LFC the foam pattern maintains the integrity of the part shape during pouring and therefore loose sand can be used to fill the casting flask. Unlike chemically treated sand, the loose sand is relatively clean and can be reused many times before disposal. This has led to the perception that LFC is more “environmentally friendly” than green sand casting.

However, the process does produce significant quantities of airborne emissions that are known to be toxic to humans. Many researchers have investigated the process phenomena related to mold filling and pattern degradation, but little work has been done to characterize the emissions from this process. Surprisingly, little is known about how individual process variables impact the quantity and makeup of these emissions. It is not only necessary to understand what the process waste streams are, but also how adjusting process settings can reduce (and perhaps minimize) the negative impacts of these waste streams. This fundamental knowledge may lead to a more environmentally responsible process by reducing the airborne emissions from lost foam casting operations.


References

1. Samson Shing Chung Ho: Lost foam casting of periodic cellular materials with aluminum and magnesium alloys, University of Toronto, Toronto, Canada, 2009;

2. Mingguo XIE, Changan ZHU, Jianxin ZHOU: Mold-filling and Solidification Simulation of Grey Iron in Lost-Foam Casting, 5th International Conference on Advanced Design and Manufacturing Engineering (ICADME 2015), p.387 – 394;

3. S. U. Behm K. L. Gunter J. W. Sutherland: An Investigation into the Effect of Process Parameter Settings on Air Emission Characteristics in the Lost Foam Casting Process, 2003 American Foundry Society

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