In recent years, the demand for high-performance stainless steel components with complex geometries has driven advancements in precision investment casting. This study investigates the development of magnesium-based ceramic cores enhanced with CaCO3 mineralizers and wood flour additives to optimize sintering behavior, mechanical strength, and dissolution characteristics. The experimental framework and results are summarized below.
Experimental Methodology
Materials and Core Fabrication
The ceramic cores were formulated using magnesium oxide (MgO, 400 mesh, >98% purity) as the base material, with CaCO3 (reagent-grade) as a sintering aid and wood flour (200 mesh) as a pore-forming agent. The powders were blended in a planetary ball mill (350 rpm, 2 h) and mixed with a 5 wt% polyvinyl alcohol (PVA) solution. The mixture was pressed into 60 mm × 10 mm × 6 mm green bodies under 6 MPa and sintered at 1,340°C for 30 min using the thermal profile shown in Figure 1.
Performance Evaluation
1. Mechanical Strength: Three-point bending tests were conducted using a universal testing machine (CMT-4503) with a 30 mm span and 0.5 mm/min loading rate. The bending strength (σ) was calculated as:
$$σ = \frac{3FL}{2bh^2}$$
where \(F\) is the fracture load, \(L\) is the span, and \(b\) and \(h\) are the width and height of the specimen, respectively.
2. Porosity: Apparent porosity (\(P\)) was measured via the Archimedes method:
$$P = \frac{m_{\text{wet}} – m_{\text{dry}}}{m_{\text{wet}} – m_{\text{suspended}}} \times 100\%$$
3. Dissolution Kinetics: Cores were immersed in 40% acetic acid at 80°C for 10 h. The mass loss ratio (\(K\)) was determined as:
$$K = \frac{m_0 – m_1}{m_0} \times 100\%$$
where \(m_0\) and \(m_1\) are the initial and post-dissolution masses.
Results and Discussion
Effect of CaCO3 Content
Table 1 summarizes the influence of CaCO3 addition on core properties. At 5 wt% CaCO3, the bending strength peaked at 24.03 MPa due to enhanced sintering via CaO-MgO solid solution formation. Higher CaCO3 concentrations (>10 wt%) introduced excessive porosity from CO2 decomposition, reducing strength and slowing dissolution.
| CaCO3 (wt%) | Bending Strength (MPa) | Porosity (%) | Shrinkage (%) | Mass Loss (%) |
|---|---|---|---|---|
| 0 | 7.57 | 35.2 | 0.22 | 100 |
| 5 | 24.03 | 28.1 | 0.97 | 50.2 |
| 10 | 18.41 | 30.5 | 0.89 | 45.2 |
| 15 | 14.76 | 32.8 | 0.91 | 52.5 |
| 20 | 11.25 | 34.6 | 0.88 | 55.3 |
Effect of Wood Flour Additives
Introducing wood flour (1–5 wt%) increased porosity but compromised mechanical integrity (Table 2). At 5 wt% wood flour, porosity reached 39.8%, accelerating dissolution (70.1% mass loss) but reducing bending strength to 4.73 MPa.
| Wood Flour (wt%) | Bending Strength (MPa) | Porosity (%) | Shrinkage (%) | Mass Loss (%) |
|---|---|---|---|---|
| 0 | 22.19 | 28.1 | 0.97 | 50.2 |
| 1 | 10.56 | 31.3 | 0.46 | 57.7 |
| 3 | 7.03 | 35.2 | 0.77 | 63.5 |
| 5 | 4.73 | 39.8 | 1.58 | 70.1 |
Casting Validation
CF8 stainless steel test castings were produced using precision investment casting. Cores with 5 wt% wood flour exhibited the fastest dissolution rate (0.78 mm/h) in acetic acid, whereas 3 wt% wood flour cores dissolved at 0.49 mm/h (Figure 2). Post-casting vibratory descaling and shot blasting confirmed the feasibility of core removal without damaging the castings.
Conclusions
1. A 5 wt% CaCO3 addition optimized sintering, achieving 24.03 MPa bending strength and 50.2% dissolution mass loss. Excessive CaCO3 (>10 wt%) degraded performance due to porosity.
2. Wood flour additives (5 wt%) enhanced porosity (39.8%) and dissolution (70.1% mass loss) but reduced bending strength by 78%.
3. In casting trials, 5 wt% wood flour cores dissolved 59% faster than unmodified cores, demonstrating viability for precision investment casting of intricate stainless steel components.
4. Agitation during core dissolution can further accelerate removal by improving acid circulation.
This study establishes a foundation for designing MgO-based ceramic cores tailored for high-temperature precision investment casting applications, balancing mechanical integrity and post-casting removability.