Cooperation within MEBSP on the Subject of Air Gap Formation During a Casting Process

Abstract

Some work has been done over the years to achieve understanding of the mechanisms behind air gap formation in casting processes. Experimental work has been done as well as work within the subject field of numerical modeling of air gap formation during solidification. It is, however, quite rare to find comparisons between experimental measurements and simulation results with commercial codes.There are a number of different commercial codes available on the market useful for example for qualitative identifications of various problem areas in complex casting geometries. But to get reliable quantitative predictions of the growth of an air gap in a casting process there still remains a lot of development and research to be done.To be able to quantitatively check the results from a simulation it is necessary to have measured data from an experimental procedure that is simple to model in a program. Within the MEBSP subgroup IV members such experiments are made. There are also four different simulation programs available within the group for calculating solidification as well as thermal stresses and strains during a casting process. The aim of the work of MEBSP subgroup IV within this area is to get a better understanding of the models implemented in the codes and see how well the existing models fit to experimental data. It is also of interest to see if the different models give quantitatively the same results and whether any of the four programs have special suitability for certain purposes.At the Division of Casting of Metals at the Royal Institute of Technology, KTH, a lot of experiment based research is taking place within the subject field of casting. In the area of air gap formation the work is focused on the behavior and properties of a solidifying metal during solidification and the influence these have on the shrinkage behavior, the solidification process and the air gap formation.In order to study the solidification process coupled with the air gap formation an experimental set-up has been constructed. It consists of a cylindrical mould made from low alloy steel which is insulated at the bottom and the top so that solidification occurs axisymmetric. In the center of the cylinder a cylindrical core is placed, made from quartz glass filled with oil bound sand. Figure 1 shows a schematic drawing of the set-up.The radial temperature distribution is recorded during cooling and solidification via thermocouples placed in the mould and the casting. Simultaneously, the movements of the mould walls and the outer surface of the casting are measured with linear variable differential transducers (LVDT). Figure 2 and 3 show an example of results from this kind of experiments. These particular curves are taken from experi-COMMUNICATIONS

Theory of hot crack formation

“…Some of the results are presented elsewhere. 18,19 In this report we discuss the results from tests on pure Al, pure Cu, and binary Fe-10%Ni and Fe-2%Ni. The compositions of the alloys used, are presented in Table 1 except for Cu, which was highly pure Cu used for electrodes.…”

“…The furnace was used in previous work by Rogberg, 10 Hansson 18 and Fredriksson, and Kron and Fredriksson. 19 The filaments are placed in one focal point, with the sample in the other. This gives a distinct zone of concentrated heat.…”

“…By maximising the growth rate, the tip radius of the crack can be determined. This value was put into equation (19) to find the growth rate, which is expressed as…”

“…A crack in a sample which is under a tensile stressstrain is influenced by stress in the same way as was discussed for the nucleation of a crack. A stress-strain term must be added to the right hand side of equation (19) in order to describe the elastic energy released during the growth of the crack. This term can be described in the following way…”

“…Incorporation of this relation into equation (19) and maximising the growth rate gives iron as an example of the case where Ds is 2?1 J m 22 and 0?1 J m 22 . The effect is rather small in Fig.…”