months of defective castings in mere minutes.
Follow the Rules Te biggest difference between
iron and other alloys is the expansion of iron during graphite precipita- tion in solidification. Tis difference means iron castings can become self-feeding after the onset of expan- sion 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 expan- sion. Once the expansion begins, a well-designed feeding system should contain the pressure so the casting is self-feeding during the remainder of solidification. Tis 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 dur- ing 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. Te ductile iron control arm, shown
in Figure 1a, is an example of an iron casting with an incorrectly designed feeding system. Te metalcasting facil-
Fig. 4. The casting has two areas of high modu- lus value, but there’s only one feeding zone.
Fig. 2. Porosity at one of the two feeders was common.
determine the location and size of the casting’s feed zones. Understanding the transfer modulus (MTR
), a calcula-
tion 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
ity originally approached the feeding design for this iron casting by placing two symmetrical feeders, shown in Figure 1b. Tis approach was under- standable, 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 Fig- ure 2. Te porosity was not always at the same contact, but on a large major- ity 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
to surface area in various areas of the casting. Te 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
location to another, each feeding zone may require its own feeder (but no more than one). Te casting modulus (MC
) represents the ratio of volume
simulate solidification in a few minutes, and the resulting data then can be converted to casting modu- lus values. A casting with a higher modulus (heavy section castings) will begin to expand earlier and will undergo more expansion than a cast- ing with a low modulus (light section castings). Te 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. Tis modulus value is known as the MTR
casting where liquid metal transfer is possible. Te calculation of MTR
MTR
Fig. 3. Engineers first ran a solidification simulation without gating or feeders.
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). Te number of feeding
= SQR (ST/100) * MC By plotting the MTR
in a casting
, because it defines the areas of the is:
ONLINE RESOURCE
Find the original research paper at
www.moderncasting.com
October 2013 MODERN CASTING | 35
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