DEVELOPING A LOCALIZED SQUEEZE COOLING TECHNIQUE FOR IMPROVED CASTING SOLIDIFICATION
D.Singh, V. Navaneeth and J. Lee
Centre for Advanced Manufacturing Technology (CAMTEC), Auckland University of Technology, New Zealand Copyright © 2011 American Foundry Society
Abstract
In permanent mold casting, die design for cast aluminum alloy and magnesium alloy products includes a number of high conductivity material cooling blocks (also called cooling circuits or chills) that are aimed to extract heat away from molten metal through direct conduction heat transfer and freeze the casting as quickly as possible in a directional manner. One of the biggest problems during this solidification process occurs when the molten metal naturally shrinks away from the mould as it solidifies. This makes it increasingly difficult to efficiently and effectively cool targeted areas in the casting through conduction, because the direct contact between the solidifying casting and the cooling block is significantly reduced or even lost. This can create millions of dollars of waste in terms of casting rejects, lost productivity, and reworks for a given manufacturing company. A movable water cooled copper chill was designed and built to push into an insulated mold during the solidification process. The experiments were carried out using commercially available A356 aluminum alloy. The chill was squeezed into the casting at various
Introduction
The use of cooling circuits or chills that are embed- ded in permanent molds not only regulate and provide some control of the solidification rate and solidification direction in a localized section of a casting, but also assist in achieving rapid solidification (i.e. increased productivity).2, 7
that can take up to ten minutes to naturally solidify can be frozen in less than one third the time if properly designed with cooling circuits in the mold.8
While this research reveals that
this is still not optimal, the typical approach for forced cooling using “chills” in industry is accepted best practice.
International Journal of Metalcasting/Spring 11
The need for lighter and more efficiently designed casting components for industries such as automotive, aerospace and marine is creating an even larger and more immediate de- mand within manufacturing plants to maintain productivity and quality at the same time. This has made the issue of rapid directional solidification particularly important especially at thick-thin (TT) junctions in castings where most of the qual- ity and structural integrity issues are prone to occur. Previ- ously, studies have been carried out to effectively control the solidification direction and rate in the case of permanent mold casting.1-6
A casting
times during the solidification cycle. Studies were carried out to understand the effect of a movable water–cooled chill with different cooling conditions compared to a typical fixed chill. Numerical simulations were conducted using developed boundary conditions in a commercial casting solidification package (ProCASTTM
). The resulting
sensitivities of the casting due to different chill conditions (i.e. fixed vs. moving) are described through physical phenomenon, metallographic analysis and computational modeling. Experimental results indicate that the air gap formation can be reduced using a movable chill and a higher heat removal rate can be achieved, thereby reducing the solidification time and hence improving productivity of a given casting. The effective solidification time with a movable water-cooled chill has decreased by over four times compared to a fixed water cooled chill which is at present being used in the industry.
Keywords: cooling, air gap, casting productivity, casting simulation and inverse modeling
The limiting factor in heat transfer currently is the contin- ued presence of an air gap between the casting/mold in- terface. These air gaps significantly reduce the heat trans- fer from the casting to the mold, causing cooling circuits to be ineffective and overall solidification/manufacturing cycles to be less than optimal. This can make a difference of millions of dollars in lost productivity and quality.8 The importance of maintaining optimal heat transfer at the casting/mold interface to drive the solidification rate and microstructure is well published.9, 10, 11 & 12
Previous
presented two independent means for deter- mining the time varying interfacial heat transfer coeffi- cient. These two methods are; (1) computer solution of the inverse heat conduction problem using thermocouple measurement at selected locations; (2) measurement of the variation of interfacial gap size with time, and deriv- ing interfacial heat transfer coefficient from heat trans- fer data across an air gap. A number of investigators 16- 23 have studied interfacial heat transfer under various condi-
65
studies have confirmed that the interfacial heat transfer coefficients between the casting and the mold are dictated by a growing air gap during solidification.10, 11 &13 Pehlke 14, 15
Ho and
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