Research Programs

Castability Control in Metal Casting via Fluidity Measures: Application of Error Analysis to Variations in Fluidity Testing

Research Team:

Brian Dewhirst
Diran Apelian

Introduction

At the surface, the question "what is fluidity" (to a metallurgist) is a relatively simple question. Having said that, the necessary caveat 'to a metallurgist' has already revealed one problem. Physicists define fluidity to be one over the viscosity. Metallurgists, on the other hand, refer to the ability of a molten metal to flow and fill a channel or cavity as fluidity. This is most often measured by the length metal can flow through a given mold before freezing.

The answer to the question 'why is fluidity important' is highly dependent on who is asking. There are at least three scenarios:

To a foundry worker, the answer is "because it is useful." Fluidity refers to a very important property of cast alloys. The more fluid an alloy is, the more easily it should be able to fill a given cavity. As the response of fluidity with increasing superheat is known to be linear, fluidity directly relates to the amount of superheat needed to fill a given cavity
Theorists express interest in fluidity as it relates to the study of solidification and interdendritic metal flow. The majority of fluidity investigations in the last 25 years have focused on maximizing fluidity with respect to precise alloy chemistry. The influence of minor alloy additions, however, is often slight when compared with that of superheat, head pressure, or melt cleanliness
A third answer, one which might satisfy an ambitious experimentalist, is that there are believed to be significant problems with the repeatability and precision of fluidity measurements. Surmounting these challenges so that more accurate and repeatable measurements of fluidity can be conducted would be an important contribution in the area of experimentation, and given the interest in fluidity by both theorists and industrialists, these accomplishments would receive praise beyond the scope of just the experimentalist community

All answers are equally correct, but each touches on a different aspect of the ways fluidity measurements are conducted and used. Our definition of fluidity shall be: Fluidity is a material's ability to flow into and fill a given cavity, as measured by the dimensions of that cavity under specified experimental conditions. As will be detailed in future work, fluidity is heavily dependent on heat flow during solidification.

Context

Since the earliest spiral castings by Saito and Hayaschi in 1919 [1], simple one-dimensional castings of metals have been conducted to determine how well a given metal can fill a cavity. Refinements on this technique by Ragone et. al. in 1956 [1,2], along with analytical solutions for pure metals, were a great leap forwards in our understanding of fluid length. Ragone's technique, employing Pyrex tubes to directly observe metal velocity and vacuum to draw the melt into a horizontal channel, reduced experimental error as compared with spiral castings. Over the next few years, the work was expanded by M.C. Flemings et. al. [3-6] to include multi-phase alloy systems. One key to this later development was micrographic investigations that led to conclusions regarding the solidification mechanisms at work. In brief, the flow of mostly-pure alloys stops by the growth of columnar grains near the entrance of the mold, while flow in multi-component systems is brought to a halt by nucleation of grains, often equi-axed dendrites, which halt flow at the tip after nucleating earlier in the casting and coarsening as they flow to the point of flow stoppage once a critical fraction solid is reached.

With this work as a foundation, investigations into the impact of foundry variables such as mold coatings, alloying additions, head pressure, and especially superheat have been investigated and correlated with mechanisms. For sand and permanent mold castings, it is abundantly clear that increasing solidification range results in decreasing fluidity (all other factors being equal). Specific investigations are often alloy or metal/mold/coating specific in scope, but very subtle influences of minor variations in alloy purity can be detected.

Past work in the field has focused on maximizing fluidity, however we believe that decreasing the variations in fluidity is as important as determining under which conditions fluidity is maximized. There are two main aspects to variation in fluidity:

One is the standard deviation of test methods used in the lab to determine fluidity
The other is the range over which fluidity values will vary in a real casting environment where alloy chemistry and temperature controls vary within some range

Objectives

As inspired by Ragone's [1] elegant statement of purpose, the purpose of this investigation is to quantitatively relate variations in fluidity to fundamental properties of metals, mold materials and test equipment design. Fluidity is a material's ability to flow into and fill a given cavity, as measured by the dimensions of that cavity under specified experimental conditions. As will be detailed in future work, fluidity is heavily dependent on heat flow during solidification. Specifically, the plan is to derive analytical equations relating the variation of fluidity of metals to the above-mentioned properties and to conduct controlled experiments to validate these relationships.

Methodology

The methods to determine the uncertainty of measurable quantities of interest are well understood. Both theoretical calculations and practical experiments will be used to determine the repeatability and reliability of fluidity tests.

Once this has been done, the existing body of data from the published literature will be examined in light of our formulations and further in-house fluidity experiments will be conducted to confirm our equations concerning expected variations in the fluidity of a melt as a function of experimental and alloy conditions.

The lessons learned are used to determine which experimental method is preferred for lab investigation of fluidity effects in general, and will be used to generate a set of guidelines for performing these experiments such that different groups can compare different experiments quantitatively. It is understood that based on different process needs different researchers will conduct investigations with different setups. Aerospace engine manufacturers are much more interested in the fluidity of engine-fin shaped molds, for example, while sand casters may have much more of an interest of the length of a sand spiral made from the same sand as their foundry castings, and rightly so. Work has already been done in the ACRC on relating different fluidity techniques and this dissertation will expand on that earlier work as well [7].

A large number of lab fluidity tests must be conducted to show that we have not just qualitative relations but quantitative predictive equations.

Outcomes

In increasing order of impact, and with prerequisite steps preceding anticipated later results, the expected outcomes of this research are:

Existing experimental methods to determine fluidity will be quantitatively analyzed
An improved procedure for conducting and discussing fluidity measurements will be implemented to further communications and comparisons between different research groups
Validated formulas that highlight the most important factors which impact variations in fluidity results
The factors impacting the variation of lab fluidity results also impact the variation (standard deviation) of the fluidity of actual industrial castings. Through the above, simple calculations will allow anticipation of variations in foundry practice from a small number of lab tests
Quantitative predictive ability of the impact of alloy chemistry on the variations in fluidity in foundry (sand casting, permanent mold, etc) casting will allow for process parameter (alloy, superheat, mold composition and coating etc) selection to not only maximize fluid length but to minimize variations in that length in foundry practice. This, in turn will help to reduce scrap rate. More consistent fluidity should lead to more consistent castings

References

Ragone, D. V., J. Adams, C.M.;, et al. (1956). "Some factors affecting fluidity of metals." Transactions of the American Foundrymen's Society 64: p640.
Ragone, D. V., J. Adams, C.M.;, et al. (1956). "A new method for determing the effect of solidification range on fluidity." Transactions of the American Foundrymen's Society 64: p653-7.
Niesse, J. E., M. C. Flemings, et al. (1959). ""Applications of Theory in Understanding Fluidity of Metals." Transactions of the American Foundrymen's Society 67: p685.
Flemings, M. C., E. Niyama, et al. (1961). "Fluidity of Aluminum Alloys: An experimental and quantitative evaluation." Transactions of the American Foundrymen's Society 69: 625-635.
Flemings, M. C. (1964). "Fluidity of metals-- techniques for producing ultra-thin section castings." Brit. Foundryman 57: 312.
Flemings, M. C. (1974). Solidification Processing. New York, McGraw-Hill.
Di Sabatino, M. (2005). PhD. Thesis in Materials Science and Engineering, Fluidity of Aluminum Foundry Alloys, NTNU, Trondheim, Norway.

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Last modified: September 22, 2008 10:07:13