technical review & discussion
Heat Flux transients and Casting surface Macro- Profile during downward solidification of al-12% si alloy against Chills
K. N. Prabhu, K. Sharath and G. Ramesh; National Institute of Technology Karnataka, Surathkal, Mangalore, India
Reviewer: Can the authors comment about the ability to de- rive a heat transfer co-efficient/contact conductance value (HTC) from their work? The reviewers feel this would be an important addition.
Authors: Heat transfer coefficients could be estimated using the relation:
h = q/(Tcasting – Tchill
surface temperatures respectively. In the present work the inverse model adopted directly yields the heat flux as well as the chill surface temperature. The surface temperature of the casting in contact with the chill cannot be easily es- timated or measured. One of the techniques for measuring the casting surface temperature is to apply the Inverse Heat Conduction Problem (IHCP) for both the chill as well the casting simultaneously. This method has several limitations that are given below: ·
where q, Tcasting and Tchill
The precise determination of the location of the hot junction of thermocouple inside the solidifying casting is difficult because of the possibility of the shift of thermocouple junctions during the flow of liquid metal. This may also affect the values of heat flux estimated by inverse analysis.
·
The numerical simulation of thermal history inside the casting part requires modeling of latent heat evolved during solidification. This aspect further complicates the problem.
The other technique is to use the heat flux transients estimat- ed as boundary condition for predicting the casting surface temperatures which again requires a reliable model for sim- ulation of casting solidification. However this technique is only an indirect method of estimating ‘h’ and is not reliable.
Alternatively the heat transfer coefficients could be estimat- ed by assuming that the temperatures in the solidifying cast- ing measured at locations very near to the interface nearly approximates the casting surface temperature (see Figure below). This figure is not included in the main manuscript.
The authors believe that ‘heat flux transients’ are better es- timators of casting/mold(chill) interfacial heat transfer as
Heat Transfer Coefficients during solidification against Al and graphite chills
) are the heat flux, casting and chill
compared to ‘heat transfer coefficients’. However in the revised paper we have calculated the heat transfer coeffi- cients corresponding to the peak heat flux assuming that the occurrence of the peak is due to the formation of a stable solid shell. The casting surface temperature corresponding to the peak in the heat flux transients was taken the eutectic temperature for 413 alloy.
Reviewer: I wish the researchers had included more use- ful chill materials than just graphite and aluminum, such as cast iron and tool steel, as well as more common mold coatings, that would have had greater relevance to operat- ing foundries and provide more useful guidance to pro- cess modelers.
Authors: We have used graphite and Al since graphite molds are generally used in D.C. casting and aluminium has a high thermal conductivity of about 234 W/mK. The methodology could be easily adopted for other materials also including cast iron and tool steels with different mold coatings.
Reviewers: Downward solidification will automati- cally cause the solid shell to pull away from the chill sur- face because of increased aluminum density (smaller volume). The concept of surface re-melting due to con- vection in the liquid alloy could have been confirmed by micro analysis (incipient melting would be eutec- tic only) except for the alloy being 413 (all eutectic).
Authors: We fully agree with the reviewer that the re-melt- ing step could be confirmed by micro-analysis. However the processes involving the formation of unstable solid shell, melting, thin solid skin formation, re-melting and subsequent solidification take place within a very short period of time and it is quite difficult to freeze any of the intermediate steps.
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International Journal of Metalcasting/Fall 2011
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