Session on Thursday, April 10, 2014 from 8:30 a.m.-10:00 a.m.; the background for the project, the test sampling approach, me- chanical testing, chemistry, hardness and microstructure results, including evaluation for chill depth and analysis thermal analysis curves, along with the key findings will be presented. The work is being monitored by the AFS Ductile Iron, CG Iron & Gray Iron Research Committee (5-R) plus a group of sponsoring companies. Those wishing more information about the project or how to participate as a sponsor should contact the Steering Committee chair Leonard Winardi at LWinardi@charlottepipe. com or Rick Gundlach at rick.gundlach@element.com.


Helium-Enhanced Semi-Permanent Mold Aluminum Casting (12-13#05)


Coordinator: Prof. Paul Sanders, Michigan Tech University; Prof. Kyle Metzloff, UW-Platteville and AFS Aluminum Per- manent Mold Committee (2-E)


The effect of helium injection in aluminum permanent mold casting has been investigated by Doutre (2000), Wan and Pehlke (2004), and Metzloff (2009). Filling the air gap that forms be- tween the solidifying metal and permanent mold with helium increases the heat transfer coefficient and casting cooling rate. Higher cooling rates decrease the time to ejection resulting in throughput improvements. Doutre measured the effect of he- lium on the cooling rate of several aluminum alloys using cylin- drical and plate molds and found a 30-50% reduction in time to ejection temperature. Doutre found that helium-enhanced cooling improved commercial semi-permanent mold intake manifold casting productivity by 29%, but the details of the he- lium injection process (injection time, location related to cores, etc.) and the resulting microstructure and mechanical proper- ties were not discussed.


Wan and Pehlke performed both modeling and experiments on helium injection on permanent molds. They found that injec- tion of helium (as compared to air) improved cooling times to 400°C by 37% with conductive mold coatings and 48% with insulating coatings. Metzloff examined the effects of helium- enhanced cooling in a production environment with conduc- tive and insulating mold coatings and the effect of external mold cooling. The helium injection was most beneficial with a standard insulating coating and external cooling, yielding a 33% reduction in cycle time over the baseline production practice and a 10% reduction over an optimized cycle without helium injection. The die in this study had a large internal metal core through which helium was injected. The benefit of helium was likely minimized as the casting shrunk onto the metal core, de- creasing the air gap in the core area. It was thought that the helium injection would have a greater effect if the air gap was larger, especially in semi-permanent mold castings that have poor thermal conductivity in the sand core regions.


Saleem (2012) studied the effect of helium on the cooling rate and resulting properties of sand castings. This study found a 43-100% increase in cooling rate with a corresponding decrease in SDAS leading to a 34% increase in yield strength and a 22% increase in ultimate strength with no significant loss in ductility


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or increase in cost. Argyropoulos (2008) found that helium in- jection into a refractory mold made the gap develop up to 34% faster compared to air injection, but the heat transfer rate was higher by up to 48%.


The cost of helium would suggest that gas mixtures should be investigated. A 1992 U.S. Patent by Air Products (5,173,124) provides evidence that a 80%He-20%Ar gas mixture has a 12% higher convective heat transfer coefficient in turbulent flow. It was also noted that a 59%He-41%Ar mixture had the same convective heat transfer coefficient in turbulent flow. How- ever, helium injection into the mold-casting shrinkage gap is not thought to provide turbulent flow (Wan and Pehlke). In this case, the more relevant parameter is likely thermal diffusiv- ity. Sevast’yanov (1985) showed a quadratic decrease (decaying faster than a linear rate) in thermal diffusivity as argon was sub- stituted for helium in the range of 20-80% at 27°C. Addition- ally, Purohit (1979) has shown a quadratic decrease in thermal conductivity with argon additions to helium at 727°C.


The objective of the project is to develop and demonstrate a method for improved productivity and properties of semi-per- manent mold aluminum castings using helium-assisted cooling. A semi-permanent mold will be designed to produce a pipe with three section thicknesses using a cylindrical sand core. This simple casting geometry will be used to evaluate the effect of helium injection through a core, and allow for the character- ization of a sand core and permanent mold within the same casting. A proposed CAD model has been completed by Carley Foundry (see figure at end). Andrei Starobin at Mold Dynamics will model core outgassing due to binder loss and the pressure head required for helium gas delivery. MAGMA modeling of the mold design will be done as a cost-share in collaboration with MAGMA. MAGMA will be used to optimize the feed- ing system to produce a sound casting and provide an initial estimate of cooling rates based on literature heat transfer coef- ficients. The deliverable will be a CAD model and process gas flow (Starobin) and casting parameters (MAGMA).


During mold design, consideration will be given to the require- ments necessary for a proposed semi-permanent mold core di- mensional study. This research mold is expected to be utilized in several AFS-sponsored projects.


The experience gained during the 2009 Metzloff work for he- lium injection and temperature collection will be utilized. Co-PI Metzloff will lead the helium injection system and temperature data collection specifications. Thermocouples (1/16 in. diameter to improve response time) will be placed in the mold, core, and casting cavity. The mold thermocouples will be placed as near to the cavity surface as possible and spring loaded to maintain con- tact with the casting. The helium injection port will be placed in the core print to allow helium flow through the core and into the air gap between the solidifying metal and mold in the remainder of the casting. The mold will be fabricated in the Michigan Tech School of Technology Machine Shop and assembled for casting on a permanent mold machine at a participating member found- ry. The temperature data collection and helium injection system will be assembled and tested prior to the casting trials.


International Journal of Metalcasting/Volume 8, Issue 2, 2014


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