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TESTING 1-2-3


Helium-Assisted Sand Casting


Researchers studied how replacing the air in sand molds with helium could increase cooling rates to achieve better casting microstructure and improved properties. A MODERN CASTING STAFF REPORT


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lthough it is the most widely used casting process for ferrous and nonferrous alloys, sand casting is inhibited by large


grain size structures and long solidi- fication times. Metalcasters, research- ers, engineers and metallurgists have investigated and developed various methods to improve cooling rates to achieve better microstructure in the casting over the years. Most recently, research has been conducted to look at the possibility of using helium to increase cooling rates. A sand mold is porous, consisting of sand particles with air occupying the voids between the particles. Te type of sand and binder, sand particle size, volume fraction of the voids between the particles and the type of gas in the voids affect the thermal conductivity of the mold. Research- ers have posited that replacing the air in the voids with another gas with higher thermal conductivity, such as hydrogen or helium, could enhance the extraction of heat from the mold. Because hydrogen brings safety and health issues, helium was the focus of a research paper, “Helium-Assisted Sand Casting,” by Muhammad Q. Saleem and Makhlouf M. Makhlouf, both with the Advanced Casting Research Center of Worcester Poly- technic Institute, Worcester, Mass. Te thermal conductivity of helium is at least five times that of air in the temperature range between 77-932F (25-500C).


40 | MODERN CASTING December 2012 Question


Would flowing helium through a sand mold during the course of the casting process enhance the rate of heat extraction and improve grain size, sec- ondary dendrite arm spacing (SDAS) and tensile properties of the casting?


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Background Using helium to improve


the rate of heat extraction from metal castings, mostly in the permanent mold process,


has been investigated in the past with encouraging results. Reductions in cool- ing time ranged between 30% and 50% for aluminum as well as a 29% increase in production rate, according to one study.


ADDING IT ALL UP Breaking down the latest research is as easy as 1-2-3.


“Helium-Assisted Sand Casting” Muhammad Saleem, Univ. of Engineering and Technology, Lahore, Pakistan; Makhlouf Makhlouf, Worcester Polytechnic Institute, Worcester, Mass.


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Background—Te study investigates the possibility of using helium in sand casting to improve the rate of heat extraction from the casting and help expel the process-generated mold gases to achieve better


secondary dendrite arm spacing (SDAS) in the microstructure. Procedure—Tree modes of supplying helium to the mold were inves- tigated, cross flow in a partially encapsulated mold, cross flow in an unencapsulated mold, and parallel flow in an un-encapsulated mold. Te effect on the average as-cast grain size, as-cast SDAS and room


temperature tensile properties were investigated. Results and Conclusions—Helium-assisted sand casting with a cross flow of helium increased porosity in the cast component and was deemed not viable. Helium-assisted casting with a parallel flow, however, showed improved tensile properties and reduced SDAS by 23% com- pared to the baseline sand casting.


Another report indicated using helium in metal molds can increase interfacial heat transfer coefficient across air gaps by 2.3 to 4.3 times. In another experiment, a 48% increase in the average interfacial heat transfer coefficient in the initial phase of solidification was recorded. In their study, Saleem and Makhlouf


investigated the possibility of using helium in a continuous flow mode in a sand mold with the understanding that, “In addition to its better thermal properties compared to air, the forced flow of gas between 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.


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