HELIUM-ASSISTED SAND CASTING Abstract
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43International Journal of Metalcasting/Fall 2012
HELIUM-ASSISTED SAND CASTING
M. Saleem University of Engineering and Technology, Lahore, Pakistan
M. Makhlouf Worcester Polytechnic Institute, Worcester, MA, USA
Copyright 2012 American Foundry Society
This paper reports a novel approach to enhance the rate of heat extraction from sand castings by flowing helium gas through the sand mold during the course of the process. The effect of helium flow rate, flow direction, and mold design on the thermal profile, grain size, secondary dendrite arm spacing, and tensile properties of the castings is investigated. An optimum set of process parameters is identified and a performance index for assessing the efficiency of the helium-assisted sand casting process
is defined. It is found that when the helium-assisted aluminum sand casting process is performed with close to the optimum parameters, it produces castings that exhibit a 22 percent increase in ultimate tensile strength and a 34 percent increase in yield strength with no significant loss of ductility and no degradation in the quality of the as-cast surfaces.
Keywords: helium, sand casting
Sand casting is the most widely used casting process for both ferrous and non-ferrous alloys,1 however, the process is marred by large grain size structures and long solidifica-tion times. The coarser microstructure has a negative effect on the mechanical properties of the cast components and the long processing time affects the overall productivity of the process. The research reported herein addresses these prob-lems by enhancing the rate of heat extraction from the cast-ing by replacing air, which is typically present in the pores of the sand mold and has a relatively low thermal conductiv-ity2 by helium gas, which has a thermal conductivity that is at least five times that of air in the temperature range of in-terest.3 Various studies have established an inverse relation-ship between the cooling rate and the coarseness of the mi-crostructure of the cast component.4,5 Dendrite arm spacing and grain size are two microstructure features that become refined with an increased cooling rate;5-7 and the quantita-tive characterization of these two parameters is often used to gauge the coarseness (or fineness) of the microstructure.
The secondary dendrite arm spacing (SDAS) is related to the mean cooling rate, during solidification (R) as shown in Equation (1)8
SDAS = CRm Eqn.1
In Equation 1, C is a constant, R is in C/sec and SDAS is in m. Extensive studies have been performed to investigate the effect of SDAS on mechanical properties of aluminum al-
loys, especially their tensile properties.4,5,9,10 It is an accepted conclusion that, with other factors kept constant, a smaller SDAS results in better tensile properties.4,5,9-11 Similarly, a fine grain structure results in enhanced tensile properties, less tendency towards hot tearing, better pressure tightness, consistent properties after heat treatment, as well as a finer distribution of secondary phases and pores.7,12
Typically, a sand mold is a porous medium that consists of an aggregate of sand particles with air occupying the voids between the sand particles. Therefore, the type of sand and binder, as well as the particle size of the sand, the volume fraction of voids, the type of gas in the voids, as well as the temperature of the mold all affect the thermal conductivity of the mold.13 Keeping everything else constant, the extrac-tion of heat from the mold may be enhanced by replacing the air in the voids between the sand particles by another gas that has a higher thermal conductivity than air. Helium and hydrogen are two common gases whose thermal con-ductivity is appreciably higher than that of air.6,14,15 There are serious safety issues associated with hydrogen which prevent its use in sand molds.14,15 Helium, on the other hand, is a non-flammable and non-toxic gas that is readily avail-able.3,14,15 In the temperature range between 25C (77F) and 500C (932F), the thermal conductivity of helium is at least five times that of air.3 Therefore, if the air present within the voids of sand molds is replaced by helium during sand casting of aluminum alloys, then the rate of heat extraction from the casting would significantly increase resulting in a faster cooling rate with all its associated benefits including microstructure refinement and improved tensile properties.16
44 International Journal of Metalcasting/Fall 2012
The use of helium to enhance the rate of heat extraction from metal castings has been reported by many researchers with encouraging results.3,6,14,15,17 Doutre14,15 injected helium at the interface of metallic molds during solidification of the casting and concentrated on cooling time reduction and productivity aspects. He reported reductions in cooling time ranging between 30% and 50% for a range of aluminum al-loys as well as a 29% increase in the production rate when helium was used in making a complex cored casting.14,15 Wan and Pehlke6 used heat transfer equations to show that using helium with metallic molds can result in an increase of 2.3-4.3 times in the interfacial heat transfer coefficient across air gaps of varying thicknesses.6 Argyropoulos and Carletti3 measured the increased interfacial heat transfer co-efficient in metallic molds resulting from the use of helium at the metal-mold interface. They reported a 48% increase in the average interfacial heat transfer coefficient in the ini-tial phase of solidification (i.e., from the beginning of the casting operation to the onset of the air gap) when helium was used in a metallic mold.3 Griffiths17 made castings in a controlled helium environment within an enclosed chamber and reported that the interfacial heat transfer coefficient in-creased by 70% for permanent mold castings and 20% for sand castings. Griffiths also reported a 50-90% increase in cooling rate along with 20% refine-ment in the SDAS, as well as 10-20% improvement in yield strength and ultimate tensile strength of castings in their as-cast condition.17 However Griffiths observed little improvement in the heat treated condition and reported that the mold gases diluted the effect of helium at the mold/casting interface.17
The work presented herein investi-gates the possibility of using helium in a continuous flow mode with the understanding that in addition to its better thermal properties compared to air, the forced flow of gas be-tween the sand particles introduces convection, which further improves the rate of heat extraction from the casting, and helps in expelling the process-generated mold gases. The effect of the flow rate of helium, the flow direction, and the mold design on the average as-cast grain size, the average as-cast SDAS, and the room temperature tensile properties of castings was investigated and com-pared to their counterparts produced in a typical sand casting process. In addition, a cost analysis of the he-
lium-assisted sand casting process was performed16 and an optimum set of parameters are identified.
Apparatus, Materials, and Procedures
Figure 1 is a schematic representation of the apparatus used in this work.
Three different modes of supplying helium to the mold were investigated. These are:
(i) Cross flow in a partially encapsulated mold(ii) Cross flow in an un-encapsulated mold(iii) Parallel flow in an un-encapsulated mold
Figure 2 is a schematic representation that illustrates the dif-ference between cross flow and parallel flow. The following modifications were done to the apparatus shown schemati-cally in Figure 1 in order to accommodate the different he-lium supply modes.
(i) Cross flow in a partially encapsulated moldA partial encapsulation was designed such that it could accommodate sand molds up to a maximum size of 17.5 inches (444.5 mm) x 11.5 inches (292.1 mm) with a fixed height of 11 inches (279.4 mm). Figure 3 shows the main components of this ap-
Figure 2. Schematic representation illustrating the difference between cross flow and parallel flow of helium.
Figure 1. Schematic representation of the general apparatus.
45International Journal of Metalcasting/Fall 2012
paratus. The design consists of a base plate that provides a pedestal on which the mold rests and through which helium is supplied to the drag, an encapsulation case that provides par-tial encapsulation of the mold and a top sealing assembly that seals the peripheral gap which forms because of mismatch be-tween the sand mold and the encapsulating case. The helium was supplied at the base of the mold by means of a rectangular recess in the base plate. The recess ensured that helium was not supplied at one point, but rather it covered more than the area of the cast parts surface. An epoxy sealant was used to seal the peripheral interface between the drag and the base plate in order to ensure that helium supplied at the base of the mold did not leak through this interface (a good mechanical seal should nullify the need for this chemical sealant). The gas was thus forced to pass through the mold in the upward direc-tion under the pressure of the incoming helium supply which was allowed to escape through the only exposed surface (top) of the mold.
(ii) Cross flow in an un-encapsu-lated mold - This mode of helium supply requires only the base of the device that is used with the cross flow in the partially encapsulated mold (Figure 3). Though helium was sup-plied in the same manner as for the partially encapsulated mold, more system losses could be expected due to the absence of mold encapsulation.
(iii) Parallel flow in an un-encapsu-lated mold - Two supply plates were designed for supplying helium in this mode