Prediction of Shrinkage Defects During Investment Casting Process
This article was contributed by Dr. S. Savithri, Senior Principal Scientist at CSIR-NIIST
Investment casting process is one of the oldest casting processes was prevalent since about 4000 B.C. It involves pouring liquid metal into a ceramic shell mold created around an expendable (wax) pattern. Earlier it was used to produce jewellery and idols in gold, silver, copper and bronze alloys. Investment casting process came into use as a modern industrial process in the late 19th century, when dentists began using it to make crowns and inlays, as described by Barnabas Frederick Philbrook of Council Bluffs, Iowa in 1897. In the 1940s, during World War II, the use of investment casting process increased due to the demand for precision net shape manufacturing techniques of specialized alloys that could not be shaped by traditional methods, or that required too much machining. Today investment casting process is usually employed for producing small industrial parts of ferrous, nonferrous and super alloys in near net shape with good surface finish and dimensional accuracy.
The investment casting process consists of four major steps:
- Creation of a wax pattern, followed by cleaning and assembling with the gating system to make the pattern cluster, or ‘tree’
- The tree is alternatively coated with the slurry of fine and coarse sand particles to obtain a ceramic shell
- The shell is dried, heated to melt the wax then preheated to increase its strength and prepare for pouring
- Finally the cast alloy is melted and poured into the preheated shell; after the solidification the shell is broken to obtain the cast part
The parts obtained from investment casting process are used in many critical applications and hence they need to be free of internal defects. The main defects that occur during the investment casting process are ceramic inclusion, crack, distortion, flash, misrun, shrinkage, slag inclusion and cold shut. In order to predict the quality of the obtained casting, it is necessary to study the effects of various casting process parameters such as the metal–mold heat transfer coefficient, pouring temperature, shell thickness and the shell heat transfer coefficient. With the advent of modern computer systems and simulation software, simulation of mold filling and solidification is being increasingly used in foundries to predict casting defects and optimize the design to obtain maximum output.
The main purpose of this work is to investigate whether radiation heat transfer that is a predominant factor in investment casting process, and shell molds that are unique to investment casting process can be effectively implemented in FLOW-3D. The different effects of these two components of the process are also investigated by carrying out mold filling and solidification simulations of the investment casting process for a simple geometry using FLOW-3D. The numerical values of the temperature obtained at various locations are validated with the experimental results reported in the literature [1]. The effect of the radiation heat transfer coefficient, shell mold thickness, and location of sprue and in-gates were also investigated.
Methodology
The computational geometry used in the present investigation is shown in Figure 1. A shell mold was created using the following steps:
- Import the geometry as component 1 into FLOW-3D and create a mesh block around the imported geometry with a specified cell size.
- Make the first sub-component of component1 of type “complement” to make everything outside of the subcomponent solid to the extents of the mesh.
- Define the mold material properties for this solid block from the solids database.
- There is an option to define “Thermal penetration depth” under component properties in solid properties GUI. There the shell thickness value can be defined.
- Now run the preprocessor.
- Go to Analyze tab> 3D tab then open the prpgrf file created in the previous step. Under both ‘Iso-surface’ and ‘color variable’ select “thermally active component volume” and select “Render”.
- Now in Display it should display only the shell part of the geometry.
- Save this surface as an STL file by selecting “component 1” in the object list (left side, bottom of the window), right-clicking on “component 1” and selecting “export to stl”.
After creating the STL file for the shell mold, this file is imported into a new simulation as component 1, the casting geometry created earlier is imported as a subcomponent and the type is chosen as ‘hole’. The casting geometry along with the shell mold is shown in Figure 2. This serves as our computational domain. The next task is to create a mesh to discretize the computational domain into cubical/rectangular cells. The mesh is generated in FLOW-3D by creating a mesh block. For the current work we have chosen the uniform mesh option shown in Figure 3 where a fixed cell size of 2.5 mm is chosen. Two mesh blocks were created for the current simulation where mesh block 2 is used around the inlet location. A void region is defined around the shell to account for the heat transfer between the shell and ambient air at 30 °C. This is chosen as a void region with ‘heat transfer type 1’ and a heat transfer coefficient value between the shell and ambient air is assigned. The heat transfer type 1 will be a lumped heat transfer coefficient including the radiation.
The material chosen for shell mold is zircon and the thermal properties are obtained from experiments conducted by Sabau and Vishwanathan [2]. Table 1 shows the values assigned for the materials used in the study.
| MATERIAL | PROPERTY | VALUE | UNIT |
|---|---|---|---|
| Fluid –Aluminium A356 alloy | Density Thermal conductivity Specific heat Latent heat Liquidus temperature Solidus temperature | 2437 116.8 1074 433.22 608 552.4 | kg/m³ W/(m K) J/(kg K) kJ/m³ 0C 0C |
| Zircon Mold | Thermal conductivity Specific heat* Density | 1.09 1.63E+06 | W/(m K) J/( m³ K) |
| Table 2. Initial and boundary conditions used for the simulation | |
|---|---|
| Mold temperature Melt pouring temperature Filling time Interface heat transfer coefficient Heat transfer coefficient between ambient and mold (radiation effect) | 430°C 680°C 7 s 850 W/m2K 30 -100 W/m2K |
The initial velocity and temperature of the melt entering the sprue basin is given as the velocity boundary condition at the top boundary of the mesh block 2. By default all the other boundaries are set to the symmetry type.
Four locations that were chosen by Sabuet.al [1] in their experiments for obtaining cooling curves during filling and solidification were used for validation purposes. They are referred to as C1, C2 and S11, S12 and S21. The points C1 and C2 are at the center of the two-plate casting and S11, S12 and S21 are all located in the shell. The evolution of temperature at these locations is shown in Figure 4.
It can be seen that the comparison of numerical and experimental results of the temperature profiles is within acceptable limits. For the probe points C1 and C2, the variations between numerical and experimental results are within 5% during solidification and 12% for cooling after solidification. For the points in the shell, the numerical results are higher than the experimental results by around 5%. This may be due to the assumptions made in assigning thermophysical properties to the shell material and the value of the shell heat transfer coefficient.
Fill sequence & solidification pattern for two different sprue locations
The fill sequences during mold filling for two different sprue locations are shown in Figures 5a & 5b. It can be observed that the end sprue creates more splashing which can lead to inclusion type defects. When the sprue is placed in the middle, the flow is more uniform and shows similar temperature distributions in both casting sections. The 2D view of the temperature profile after 50% solidification is shown in Figures 5c & 5d for both cases. From the shrinkage location, it is very clear that both sprue locations will give rise to defects.
The fill sequences during mold filling for two different sprue locations are shown in Figures 5a & 5b. It can be observed that the end sprue creates more splashing which can lead to inclusion type defects. When the sprue is placed in the middle, the flow is more uniform and shows similar temperature distributions in both casting sections. The 2D view of the temperature profile after 50% solidification is shown in Figures 5c & 5d for both cases. From the shrinkage location, it is very clear that both sprue locations will give rise to defects.
Effect of shell thickness
In order to study the effect of shell thickness on investment casting, castings with shell thickness of 7.2, 10, 15 and 20 mm were considered. Figures 6a & 6b show the cooling curve at a particular location in the casting that is depicted as C1 and at a particular location in the shell mold, which is depicted as S11 during solidification. It can be observed that the increase in thickness of the ceramic shell from 7.2 mm to 15 mm decreases the rate of cooling, and hence, leads to longer solidification times.
Effect of shell heat transfer coefficient
The shell heat transfer coefficient, ha, represents the rate at which heat is dissipated from the outer wall of the shell mold to the surrounding air through radiation. To investigate this effect, the value of the heat transfer coefficient was varied from 20 to 80 W/m2K . It can be seen from Figures 7a & 7b that the change in ha has a significant effect on cooling rate of the cast material and shell. When the heat transfer coefficient was increased from 20 to 80 W/m2K, it was seen that the solidification time at C1 was reduced from 812 s to 334 s (by approximately 44%). Therefore, changing the value of ha will have a bearing on the microstructure of the cast product.
Conclusions
Mold filling and solidification simulations of the investment casting process were carried out using FLOW-3D. Parametric studies have been conducted to study the effect of casting parameters on the casting process. The following conclusions may be drawn from the present study:
- FLOW-3D is capable of modeling filling and solidification in a multi-cavity mold for investment casting process. The predicted temperature profiles at the probe locations were within acceptable limits of the experimental data.
- For shell thickness, it was seen that in both cases there is a critical thickness of the shell, beyond which the heat transfer characteristics reverse. As the shell thickness increases, it was seen that the solidification time increased, up to the critical thickness and then started decreasing. For the original geometry, the critical thickness lies between 15 to 20 mm, whereas for the modified geometry it lies between 10 and 15 mm.
- The heat transfer coefficient between the shell and the ambient air, ha, was found to have the most significant effect on the heat transfer characteristics. When ha was increased by a factor of 4, from 20 to 80 W/m2K, the solidification time at the center of the sprue decreased by more than 40%.
References
Sabau, A.S., Numerical Simulation of the Investment Casting Process, Transactions of the American Foundry Society, vol. 113, Paper No. 05-160, 2005.
Sabau, A.S., and Viswanathan, S., Thermophysical Properties of Zircon and Fused Silica-based Shells used in the Investment Casting Process, Transactions of the American Foundry Society, vol. 112, Paper No. 04-081, 2004.
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