A properly designed feeding system for iron castings (both gray and ductile) requires an understanding of how these alloys differ from others, such as steel. If these differences are not considered, feeding systems may be less than adequate and casting quality will suffer. In many cases, feeders designed essentially for steel castings lead to production defects when applied to iron. Such misunderstandings lead to suggested solutions that often worsen the situation.
Understanding design properties specific to iron, when applied in conjunction with simulation software, can lower scrap rates and result in quality castings. Additionally, running computer simulations prior to initial production can help avoid weeks and months of defective castings in mere minutes.
Follow the Rules
The biggest difference between iron and other alloys is the expansion of iron during graphite precipitation in solidification. This difference means iron castings can become self-feeding after the onset of expansion in most situations, so no further feeding is required.
Appropriately, a feeding system for iron castings should provide metal only for the contraction of the liquid alloy and solidifying iron prior to expansion. Once the expansion begins, a well-designed feeding system should contain the pressure so the casting is self-feeding during the remainder of solidification. This principle is in direct contrast to other alloys, such as steel, where there is no expansion and feed metal must be supplied during most or all of solidification.
Another major difference between iron and other alloys has to do with the mechanism involved in piping. Iron alloys (particularly ductile iron) do not readily form a solid skin during solidification. For feeders to pipe effectively, atmospheric pressure must be able to collapse the weak plastic skin when the internal pressure drops. After a feeder punctures the skin, the internal pressure then is equalized within a feeding zone (the area within a casting where liquid metal can flow from one point to another in response to expansion pressures). Only one feeder should be used for each feeding zone. If multiple feeders are placed on the same zone, one feeder will begin piping while the others will not. Often, porosity will be seen at contact points of the non-piping feeders.
Iron’s requirement for a single feeder within a single zone is the design rule that is violated most often. When porosity is found at a feeder contact point, the tendency of many engineers is to add more feeders; this is exactly the wrong approach to take and will worsen the situation.
The ductile iron control arm, shown in Figure 1a, is an example of an iron casting with an incorrectly designed feeding system. The metalcasting facility originally approached the feeding design for this iron casting by placing two symmetrical feeders, shown in Figure 1b.
This approach was understandable, because the feeders were attached to the heaviest sections of the casting. During initial production, porosity occurred at one feeder contact on a consistent basis, as shown in Figure 2. The porosity was not always at the same contact, but on a large majority of castings, one contact showed evidence of porosity while the other did not. As a result, the metalcasting facility could not produce a quality casting with this pattern design.
Design With Data
To correctly design feeder systems for iron castings, it is necessary to determine the location and size of the casting’s feed zones. Understanding the transfer modulus (MTR), a calculation relating to metal flow within a casting, can help determine if a casting has one or more feeding zones.
If metal cannot flow from one location to another, each feeding zone may require its own feeder (but no more than one). The casting modulus (MC) represents the ratio of volume to surface area in various areas of the casting. The modulus is used to estimate the order of solidification by allowing engineers to estimate the progress of solidification in a casting. In iron castings, the modulus is used to estimate when expansion will begin and is expressed as a percentage of complete solidification.
Modern software programs can simulate solidification in a few minutes, and the resulting data then can be converted to casting modulus values. A casting with a higher modulus (heavy section castings) will begin to expand earlier and will undergo more expansion than a casting with a low modulus (light section castings). The point when expansion begins is referred to as the shrinkage time point.
Knowing the shrinkage time point allows the calculation of an equivalent modulus value that corresponds to the modulus at which contraction of the iron stops and expansion begins. This modulus value is known as the MTR, because it defines the areas of the casting where liquid metal transfer is possible. The calculation of MTR is:
MTR = SQR (ST/100) * MC
By plotting the MTR in a casting simulation, one can determine whether the entire casting is a single feed zone (the modulus transfer is continuous throughout the casting) or contains multiple zones (modulus transfer is discontinuous). The number of feeding zones then determines the number of required feeders, using one feeder per zone.
The value of transfer modulus can be understood as representing the casting modulus value below which feeding from risers is no longer effective and the iron becomes self-feeding due to expansion. The expansion pressure must be controlled, which means, assuming the mold is rigid enough, all contacts with the casting (gates and riser contacts) should be solid enough to ensure the expansion pressure is contained within the casting after the onset of the graphite expansion. This also means the modulus of the feeder contact neck should be equal to transfer modulus to ensure the feeding of the liquid contraction will be able to occur and also that the expansion pressure will be contained within the casting due to freezing of the feeder contact at the correct point in solidification.
To resolve this problem in the ductile iron control arm example, the casting was analyzed to determine feeding requirements. First, a solidification simulation of the casting without gating or feeders was performed. The results of this simulation are shown in the plot of solidification time (in minutes) in Figure 3.
The data from the simulation was converted to modulus data so the feeding calculations could be performed. It is tempting to conclude the original feeder design was correct, because the two areas of high modulus value in the casting were in close proximity to the feeder contacts in the original design.
However, it is necessary to analyze this casting further to determine the shrinkage time and MTR to understand the location and size of the feeding zone(s). Analysis of the iron characteristics indicates the value of the MTR is 0.254 in. (0.645 cm). Creating a plot of this value within the casting will indicate the location of feed zone(s), shown in Figure 4.
The entire casting is actually a single feed zone. The areas of higher modulus are connected by a section in which the modulus is above the transfer modulus value, thus allowing liquid transport for feeding throughout the casting. Only a single feeder should be used to avoid potential pososity at a non-piping feeder.
The computer simulation in this case took 16 minutes to perform, and after calculating the shrinkage time and transfer modulus, the plot was created in 5 minutes. After about 20 minutes of analysis, the correct feeder design was determined. Had this been done before the original pattern equipment was created, several months dealing with defective castings could have been avoided. The associated costs were far greater than the upfront investment for the simulation software and training to perform the analysis.
The pattern was revised to reflect a single feeder, shown in Figures 5a-b. The feeder in this case is not connected to an area of high modulus. In iron castings, the location of the feeder is not as critical as with steel castings because of the expansion pressure’s effects throughout the casting after graphite precipitation begins.
No porosity was discovered at the casting’s contact area with the single feeder, shown in Figure 6, or elsewhere. A simple, quick analysis of the casting produced the correct feeder design that resulted in a sound casting. Computer simulation offers engineers an effective tool to design a production process, thereby avoiding potential costs involved with defective castings. ■
This article is based on a paper (13-1261) from the AFS Metalcasting Congress Proceedings 2013.