Fins vs. Chills: solidifying an answer.

Inside This Story:

* Two of the most widely used methods to control nonferrous casting solidification are cooling fins and external chills.

* Although these methods are applied differently, their functions provide for a more sufficient solidification process.

* This article compares the effectiveness of the two cooling aids and develops practical recommendations on their applications.

As a metalcaster, a key to casting a perfect component is to prevent internal porosity errors. A way to achieve this is through a beneficial solidification process that avoids such errors. Besides the proper design of gating systems, several methods exist to control and modify the solidification behavior of cast alloys:

* cooling fins--thin segments attached to problematic areas of a casting;

* external chills--metal inserts placed into mold walls to accelerate heat transfer from potential hot spots.

Despite completely different mechanisms, both chills and fins serve to reach the same goal: to locally increase the solidification rate at the problematic areas of a casting. Both approaches have technological and economical advantages and disadvantages.

An investigation was conducted to determine if one aid is more beneficial than the other, and this investigation had two objectives:

1. Find the relationships between cooling means parameters--the effectiveness of the number of attached fins of contact surface area and volume of a chill;

2. Develop the methodology to select either fins of chills with the same or similar effectiveness to the aluminum sand casting process.

Solidify Digitally

The testing process consisted of two phases. Phase 1 compared the effectiveness of cooling fins and external chills through computer-aided solidification modeling. Phase 2 compared the results from actual chills and fins used in aluminum sand castings.

In Phase 1, simulations of mold filling and casting solidification were conducted on a 4 x 4 x 0.75-in test plate casting of aluminum alloy A206.

The thickness of fins attached to the casting was 0.1 in. and the length of fins was 1.5 in. Fourteen different configurations of fins were studied. Sets of one, two, three, four, five, six and seven fins were attached to one or to both sides of the casting. In all sets having more than one fin on each side, the distance between the next fins was kept constant at half an inch.

Two different materials of external chills, copper and iron, were studied. Chills were embedded into the wall of the mold facing the mold cavity. Thus, the only significant variables of chills, besides their material properties, were their volume, representing the heat capacity and contact surface area, which influenced heat exchange between the chill and the casting.

For both chill materials, five contact surfaces sizes (1, 1.5, 2, 2.5, and 3-sq.-in.) and five corresponding volumes (1, 1.5, 2, 2.5, and 3 cubic-in.) were studied.

The first series of solidification analysis of the test casting was conducted without fins or chills to establish baseline data for further comparisons. In the first series analysis, it was determined that the thermal center (last freezing point) was insignificantly shifted from the geometrical center of the casting due to the influence of the attached gating system. Solidification time observed in the thermal center was 2.61 min.

When fins were added to the test casting, the difference in solidification behavior appeared to be significant only when solidification time in the center of the casting is considered (Fig. 1). In this case, fins attached to both sides allowed greater improvement than the same number of fins attached to one side. This possibly occurred because their junctions were closer to the test plate thermal center.

[FIGURE 1 OMITTED]

While the area of the iron chills' contact surface practically had no influence on the solidification time of the test casting, the volume, representing heat capacity of the chill, was the only significant factor (Fig. 2). By reducing the contact surface area along with keeping constant volume of the insert, it was possible to achieve a substantial increase of chill efficiency in the local area of its attachment.

[FIGURE 2 OMITTED]

As shown in Fig. 2a, the copper chills significantly changed the solidification parameters in the center and last freezing point of the test casting.

In contrast to copper chills, iron chills' volume and contact surface area variations did not significantly change the solidification time (Fig 2b).

This difference between copper and iron chills is due to the different thermal conductivity of the two materials. In the case of copper, due to high thermal conductivity, the whole chill worked, regardless of its shape. On the contrary, iron's relatively low thermal conductivity causes the cross-section of the chill, through which heat is transferred, to absorb most of the heat.

For example, if the chill had a large volume, but small contact surface area, only the part of the chill that is closest to the casting will transfer heat from the solidifying casting, while the other end of the chill remains ineffective. In order to increase iron chill effectiveness, its contact surface must be relatively large with a volume-to-contact area ratio of 1:1.

Differences in the chills' thermal conductivity also influenced better solidification rates found in the central area of the test casting. For copper chills, the contact surface area influenced solidification time in the center. In other words, if contact surface of a copper chill were reduced while its volume remained the same, the cooling effect would become stronger in the local area of chill's location. This, however, could likely reduce effectiveness of the chill overall.

Although they might not be as effective in particular applications, chills still might be better utilized than fins. Incorporating cooling fins into casting design requires additional cleaning and finishing operations for their removal, reduces casting yield and increases cost. Analysis of experimental results shows that by applying chills with different contact surface areas, different heat transfer effects can be achieved--concentrated of distributed. At the same time, it was not possible to achieve strong concentrated effects by applying cooling fins (Fig. 3). The only way to increase the effectiveness of cooling fins is to increase the overall number of fins; however they must be uniformly located.

[FIGURE 3 OMITTED]

Neither iron chills, nor copper chills have an absolute advantage. Copper has greater thermal conductivity (223 vs. iron's 26 BTU/hr x ft. x F), but iron has greater specific heat (0.11 vs. copper's 0.092 BTU/1bm x F). Thus, iron chills likely have a greater heat capacity than ones made of copper with equal volume. However, because of the relatively low thermal conductivity of iron that will result in relatively slow heat transfer through the chill cross-section, not all the heat transfer potential of an iron chill can be realized.

For relatively small volume chills, iron may be the more appropriate material. Using copper appeared to have been beneficial only in cases when it was necessary and possible to use relatively large volume chills, especially when a strong and concentrated heat transfer effect was required by placing large volume chills with a relatively small contact surface.

Hands-on Applications

The Phase 2 process incorporated a test casting that had two identical wages connected together. The ingates were symmetrically attached to thin sections of the casting, and no risers were used.

This impractical gating system design allowed the generation of a hot spot in the center of the casting in order to concentrate shrinkage in that area. A pressurized gating system with ratios approximately 2:3:1 was chosen in order to maintain a relatively small cross-section of the gates. A pattern for nobake molding was generated via the same rapid prototyping technique model as previously used for the solidification modeling.

To use a single pattern to make molds with different sets of attached fins (and no fins), a specially designed composite pattern was developed. The pattern consisted of three parts: a main body with attached gates, runners and bottom sprue, and two interchangeable pieces inserted in the center of the main body. A total of three variants of inserts were made: plain (for making the test casting with no fins), one with one fin attached, and one with three fins attached.

Experiments using aluminum alloy A206 were repeated via solidification modeling using the same approach as described in Phase 1. All simulation parameters (filling time, initial temperature, etc.) were adjusted to provide the most accurate representation of actual conditions of the physical experiment. In order to record the cooling curve during the simulation, pseudo-thermocouples were embedded into the model at the same point as in the actual experiments.

The application of fins led to a significant shift of the solidus point on the cooling curve in direction of time axis. Experimental and modeled cooling curves had different shapes; however, the locations of solidification points are close (Fig. 4), and the deviation was inside 10% tolerance. The data found in the physical experiments was similar to that discovered with the computer-animated investigations. Thus, it can be concluded that results gained from the simulations were reliable.

[FIGURE 4 OMITTED]

Fins vs. Chills

Based on this investigation, the following conclusions can be drawn.

In the sand casting investigations. both cooling fins and external chills were found to be partially interchangeable. The digitally simulated examinations, however, revealed that chills were more flexible because by varying their geometrical parameters they can achieve different cooling effects (distributed, as well as concentrated). At the same time, it is not possible to reach a highly concentrated heat transfer effect by applying cooling fins. Copper chills tend to be more expensive and have no significant advantages in comparison with iron ones, unless the use of large chills is recommended. For smaller chills, iron is more ideal.

Although these examinations were performed using aluminum, they can be used for other alloys, as well. The results found here may have been due to the properties of aluminum and could differ greatly when used in other sand casting designs and metals. However, the aluminum experiments revealed that, though chills and fins are similar in effectiveness, chills may be a better option for more optimal solidification.

For More Information

Proceedings of the 5th AFS International Conference on Permanent Mold Casting of Aluminum, Y.S. Lerner, Milwaukee, 2000, p. 82-94.

"Enhanced Solidification Rate in Castings By Use of Cooling Fins," Wright T.C., & Campbell J., AFS Transactions, 1997, p. 639-644.

"Modeling the Effectiveness of Chills During Solidification," Huang H., Lodhia A.V., Berry J. T., AFS Transactions, 1990, p.547-552.

Visit www.moderncasting.com for the entire report, "Cooling Aids Selection for Sand Mold Casting of Aluminum Alloys."

Yury S. Lerner is FEF Key Professor at the University of Northern Iowa. Vladimir E. Kouznetsov is Research Associate at the Moscow State Steel and Alloys University (Russia).