Interface heat transfer in investment casting of aluminum alloys
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INTRODUCTION
WHEN liquid metal spreads over a solid surface, a perfect thermal contact cannot be achieved. This is a result of such factors as roughness of the solid surface, the surface tension of the melt, impurities on the surface, and gas entrapment. If the surface temperature is below the liquidus of the metal alloy, then after the alloy has partially solidified, other factors that influence the thermal contact include interface geometry and the pressure applied to the solidifying metal. The nature of the thermal contact between solidifying metal and mold is characterized by a heat transfer coefficient (HTC), the determination of which is often achieved by inverse techniques. It is critical for the proper numerical modeling of solidification that this boundary condition is known accurately. The interface HTC (hint) is given by hint ⫽
qint ⌬Tint
[1]
Here, ⌬Tint is the temperature drop across the interface, and qint the interfacial heat flux per unit area. In general, hint is not constant but varies during solidification and depends upon a number of factors.[1] For a given mold, the main variable to consider in interface heat transfer analysis is the alloy composition. This determines such factors as surface tension (of the liquid) and freezing range, both of which can have a significant effect on the interface heat transfer.[2] As the surface tension increases, the alloy will be less able to conform to the shape of the mold, thus reducing the effective contact area and increasing the resistance to heat flow. Alloys with a short freezing range will form a well-defined solid shell at the outer surface and may tend to contract away from the interface. This will reduce the contact pressure and the HTC. As
DAVID J. BROWNE, Lecturer, is with the Department of Mechanical Engineering, University College Dublin, Dublin 4, Ireland. DENIS O’MAHONEY, Research Scientist, is with the National Microelectronics Research Centre, University College Cork, Cork, Ireland. Manuscript submitted February 6, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
the freezing range increases, the alloy will tend to solidify in a less directional manner. Increasing the depth to which the alloy is poured increases the pressure of the liquid metal at the interface, helping to overcome surface-tension forces. It may also help the alloy to resist thermal contraction of the solid shell next to the interface, thus maintaining good thermal contact. The pouring temperature for most aluminum alloys is typically 700 ⬚C. Above this temperature there is a risk of burning off some of the alloying elements (Mg in particular). The amount of shrinkage and the solidification time also increase with pouring temperature. Low initial superheat can lead to incomplete filling. Thus, there is little to be gained by altering the pouring temperature. For aluminum investment casting, the mold preheat is usually between 300 ⬚C and 500 ⬚C. Increasing preheat reduces the amount of heat a mold can absorb, the thermal gradient across the mold, and the amount by which i
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