Thermoelastic Stability Analysis of Solidification of Pure Metals on a Coated Planar Mold of Finite Thickness
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HANICAL instability during the solidification process has a very important role in manufacturing process such as casting and welding due to its undesirable effects on the final products such as cracks. The cracks occur due to the uneven shell growth in the solidified shell and cause to reduce the quality and usability of the casting. Growth instability during the solidification of metals is investigated experimentally in References 1 through 3 and theoretically in References 4 through 8. The authors indicated that sinusoidal periodic variations are shown at the moving interface due to the non-uniform heat extraction at the shell/mold interface. There is some thermal contact resistance at this interface due to surface roughness and other contaminant films. The thermal contact resistance resists the heat extraction at this interface and it is changed as a function of contact pressure. If the thermal contact resistance is smaller at certain regions of the shell/mold interface than other regions, the heat flux MEHMET HAKAN DEMIR is with Hooketech Mak. Mek. ve Med. Tek. San. Tic. Ltd. Sti., Yildiz Technical University Incubation Center, 34220 Istanbul, Turkey. FARUK YIGIT is with the Department of Mechatronics Engineering, Yildiz Technical University, 34349 Istanbul, Turkey. Contact e-mail: [email protected] Manuscript submitted June 29, 2016. Article published online December 21, 2016. 966—VOLUME 48B, APRIL 2017
increases and the liquid shell material solidifies faster above those regions due to higher cooling rate. As a result, the solidified shell thickness above these regions becomes thicker than the other regions in which the thermal contact resistance is greater due to low cooling rate. For this reason, undulatory structure is observed at freezing front between liquid and solid phases of solidified material. Controlling the heat transfer rate is a very effective technique to minimize or eliminate these undesirable periodic variations at the moving interface. There are two commonly used methods in the industrial applications of casting to control the cooling rate: Changing the surface topography of the mold is one of these methods for adjusting heat transfer. Some of the experimental studies about the effects of mold surface topography on the growth instability are listed in References 9 through 11. They declare that grooved mold leads to more uniform contact along the shell/mold interface and the quality of the final cast improves due to more uniform heat extraction from the process. Some theoretical works about the heat controlling method[12-15] indicated that gap nucleation occurs when the evolving distortions of shell and mold cause the contact pressure falls to zero at shell/mold interface. Nucleation of the air gaps at the lowest points of the mold surface troughs leads to increase in the growth instability during the process. On the other hand, if the air gaps occur at the crests of the mold surface, more uniform shell growth is observed during the METALLURGICAL AND MATERIALS TRANSACTIONS B
solidification. The second
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