Technical Article


An Investigation into Misrun Defects in Investment Cast Stainless Steel Castings


by Gerald Richard, MAGMA Foundry Technologies, Inc. and Anthony Geiger, Stainless Foundry & Engineering Abstract


T


he Investment Casting industry produces to


near net


complex shape.


castings Regularly,


these precision components either have thin walls or thin features that are challenging to fill, which result in misrun defects. Producing these defects is especially problematic because they will likely result in scrap castings due to their inability to be repaired, leading to lost time and high cost. Methods used to mitigate this challenge such as increasing shell and pouring temperatures can be effective, however, changing those thermal conditions are subject to process variation or could even produce defects of their own. Understanding how the melt front cools as it flows through the investment shell cavities, analyzing the temperature and velocity profiles of the flow front while the shell is cooling, and knowing the fluidity of an alloy’s impact on the length it can flow given a section thickness will provide the tools needed to gain insight as to why these defects develop and how to eliminate them. In this paper, the various parameters needed to understand and combat misrun defects will be discussed.


Introduction There is increasing demand from casting designers to produce lightweight, thin- walled castings which is incentivized by one major factor with a few different implications. The major factor is that thin-walled castings are lighter weight. If the end component is going into an application where fuel consumption is used or there are loading limits that are imposed, light weight become paramount. There are cost implications associated with casting weight as well. Castings are often sold on a per pound basis and so lighter castings are cheaper and use less materials to produce, and if there are shipping cost or any other


20 ❘ November 2022 ®


Figure 1: Effects of changing pouring temperature.


cost aligned with weight, castings are better lighter.


Even so, there are many


complicated technical challenges involved in producing castings with smaller and smaller section sizes. Misrun and other non-fill defects can impose limits to how thin we can make a casting given a specific process. When those challenges arise in the foundry, engineers have to understand all of the relevant effects that lead to challenges with filling a casting and how to combat them. Misrun is a complex defect with physical, chemical, and thermal causes. A challenge for the foundry engineer is to determine the root cause of the defect and execute necessary measures to combat the defect while also minimizing any negative effects produced by the solution. With looming deadlines for casting shipments and incentives to find quick fixes for defects that result in scrap castings, a typical approach of steadily increasing the pouring temperature or telling the pourer to “pour it faster” becomes


the strategy for many.


However, understanding the cause of the defect will allow us to analyze and weigh potential solutions that are more effective and suffer less shortfalls in the


long run. Before we get into temperature, velocity and other parameters, we must begin with the physical characteristic of the material itself that will dictate flow, fluidity.


Fluidity In physics, fluidity is defined as the inverse of viscosity, however in casting, fluidity is described as the ability of a given material to flow a certain length given a certain section thickness. Many factors influence how far metal can flow including temperature, velocity, local air


pressure, metal chemistry, among


other factors. Let’s first turn to the alloy composition and note the effects it plays on fluidity. Alloy composition plays an


important role in the fluidity of the material. Different alloys have different fluidity profiles depending on alloy makeup. Pure metals or alloys closer to the eutectic composition tend to have a higher fluidity than their larger solidification range counterparts due to the nature of their solidifying front (1). Even within a particular alloy, elemental changes can have an effect on fluidity. For example, when pouring certain steel


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