In the realm of advanced manufacturing, the investment casting process stands as a pivotal technique for producing complex, high-precision metal components with excellent surface finish and dimensional accuracy. Traditionally, this process involves multiple labor-intensive steps: creating a wax pattern, repeatedly coating it with ceramic slurries, stuccoing with refractory materials, drying, hardening, dewaxing, and finally high-temperature sintering to form the mold shell. This conventional approach is time-consuming, costly, and limited by the constraints of pattern fabrication, particularly for small-batch or customized parts. With the rise of additive manufacturing (AM) technologies, there is a growing interest in streamlining the investment casting process by directly fabricating shell molds through AM methods, thereby eliminating intermediate steps and enabling rapid prototyping and production. Among various AM techniques, ceramic slurry extrusion-based additive manufacturing offers a promising route due to its versatility in material selection, low equipment costs, and ability to create complex geometries without support structures. This study focuses on investigating the effect of sintering temperature on the mechanical and physical properties of precision casting shells fabricated via ceramic slurry extrusion additive manufacturing, aiming to optimize the investment casting process for enhanced efficiency and performance.
The core of the investment casting process lies in the ceramic shell, which must possess adequate strength to withstand handling and metal pouring, sufficient porosity for gas escape during casting, and minimal shrinkage to maintain dimensional fidelity. In traditional methods, these properties are achieved through multi-layer construction and controlled sintering. However, for additively manufactured shells, which are built layer-by-layer from a homogeneous ceramic paste, the sintering behavior becomes critical in determining final properties. Sintering temperature influences phase transformations, microstructural evolution, and densification, all of which directly impact shell integrity. Thus, understanding the relationship between sintering temperature and shell properties is essential for advancing AM in the investment casting process. In this work, we explore this relationship through experimental analysis, employing X-ray diffraction (XRD) and scanning electron microscopy (SEM) to elucidate phase composition and fracture morphology, while measuring key performance metrics such as bending strength, porosity, and shrinkage.
To begin, we formulated a ceramic slurry tailored for extrusion-based additive manufacturing. The composition was designed to meet the requirements of the investment casting process, balancing flowability for printing and green strength for handling. The slurry consisted of bauxite powder (average particle size 24 μm, purity ≥86%) as the primary refractory material, silica sol (30% SiO₂ content) as a binder, yttria powder (99% purity, 1 μm particle size) as a sintering aid, along with glycerol, sodium polyacrylate, and fatty alcohol polyoxyethylene ether (JFC) as dispersants and plasticizers. The precise mass percentages are summarized in Table 1. The formulation process involved initial mixing of liquid components under magnetic stirring, followed by ball-milling of all constituents for 12 hours to ensure homogeneity. The pH of the slurry was then adjusted using 10% dilute hydrochloric acid to achieve an optimal viscosity for extrusion, which is crucial for consistent layer deposition in the AM process.
| Component | Bauxite | Silica Sol | Yttria | Glycerol | Sodium Polyacrylate | JFC |
|---|---|---|---|---|---|---|
| Mass Percentage (%) | 65.50 | 29.00 | 3.00 | 1.70 | 0.55 | 0.25 |
The additive manufacturing workflow for shell fabrication commenced with computer-aided design (CAD). We designed specimen geometries with dimensions of 40 mm × 10 mm × 4 mm using Solidworks software, exporting them as STL files. These files were then processed in Repetier host software, where AM parameters were set: layer height of 0.8 mm, nozzle diameter of 1.2 mm, and 5 layers per specimen. The software performed slicing and toolpath planning, generating G-code that dictated the extrusion head’s motion. The G-code was transmitted to the control board of our custom-built extrusion AM device, which drove a stepper motor to push the ceramic slurry through a syringe-based extruder. The slurry was deposited layer-by-layer along the predefined paths, forming green bodies of the shell specimens. The toolpath strategy involved contour-following to ensure a closed outer surface while allowing internal micro-porosity from intersecting extrusion strands, which is beneficial for透气性 in the investment casting process. After printing, the green bodies underwent freeze-drying at -30°C under vacuum for 3 hours to remove moisture without causing cracking or distortion.
Subsequently, the dried specimens were sintered in a tube furnace under air atmosphere. We employed a heating rate of 10°C/min to reach target sintering temperatures, which were varied from 900°C to 1100°C in increments of 50°C (specifically, 900°C, 950°C, 1000°C, 1050°C, and 1100°C). At each temperature, the specimens were held for 3 hours to ensure complete phase transformations and sintering, followed by furnace cooling to room temperature. This range was selected based on preliminary studies and literature on bauxite-based ceramics, covering the critical transitions in the investment casting process shell materials. Post-sintering, we conducted comprehensive characterization to evaluate the effects of temperature on shell properties.
We measured porosity using the Archimedes principle, which involves weighing specimens in air and when immersed in a liquid to calculate open porosity. Shrinkage was determined by comparing pre-sintered and post-sintered dimensions along length, width, and height directions. Bending strength was assessed via three-point bending tests on a universal testing machine, with a span length of 30 mm, following standard HB5353.3-2004. The stress-strain behavior was recorded to understand fracture mechanics. For microstructural and phase analysis, we used scanning electron microscopy (SEM) to examine fracture surfaces and X-ray diffraction (XRD) to identify crystalline phases. These analyses provide insights into how sintering temperature alters the material’s internal structure, which is fundamental to optimizing the investment casting process for AM shells.
The results from bending tests revealed a distinct trend in mechanical strength. Figure 1 shows the typical stress-strain curve from a three-point bending test, which exhibits linear elastic behavior up to sudden brittle fracture, indicative of ceramic materials with minimal plastic deformation. This characteristic is crucial for shells in the investment casting process, as they must resist deformation under thermal and mechanical loads during metal pouring. Plotting bending strength against sintering temperature, as summarized in Table 2, demonstrates a rapid increase from 3.42 MPa at 900°C to 7.31 MPa at 1000°C, representing a 113.7% enhancement. Beyond 1000°C, strength gains plateau, reaching only 7.73 MPa at 1100°C (a mere 5.7% increase from 1000°C). This suggests that 1000°C marks a threshold where sintering reactions near completion, providing optimal strength for the investment casting process without excessive energy input.
| Sintering Temperature (°C) | Bending Strength (MPa) | Porosity (%) | Shrinkage in Length Direction (%) | Shrinkage in Width Direction (%) | Shrinkage in Height Direction (%) | Average Shrinkage (%) |
|---|---|---|---|---|---|---|
| 900 | 3.42 | 45.8 | 0.60 | 0.68 | 0.52 | 0.60 |
| 950 | 5.18 | 44.7 | 0.75 | 0.77 | 0.61 | 0.71 |
| 1000 | 7.31 | 43.6 | 0.83 | 0.85 | 0.69 | 0.79 |
| 1050 | 7.45 | 40.2 | 0.85 | 0.88 | 0.72 | 0.82 |
| 1100 | 7.73 | 36.8 | 0.89 | 0.92 | 0.74 | 0.85 |
Porosity, a key factor for gas permeability in the investment casting process, decreased monotonically with rising sintering temperature. As shown in Table 2, porosity dropped from 45.8% at 900°C to 43.6% at 1000°C, and then more sharply to 36.8% at 1100°C. This reduction is attributed to enhanced particle bonding and pore closure during sintering. For investment casting shells, a porosity range of 36–44% is generally desirable to balance strength and透气性; thus, 1000°C yields a value (43.6%) within this optimal window. Shrinkage, which affects dimensional accuracy, increased gradually with temperature. The average shrinkage, calculated from length, width, and height directions, rose from 0.60% at 900°C to 0.79% at 1000°C, and further to 0.85% at 1100°C. Minimal shrinkage is preferred in the investment casting process to maintain part precision, so 1000°C offers a compromise with manageable dimensional change.
To delve deeper into the mechanisms behind these property changes, we analyzed phase evolution using XRD. The bauxite powder primarily contains diaspore (Al2O3·H2O), kaolinite (Al2O3·2SiO2·2H2O), and pyrophyllite (Al2O3·4SiO2·H2O), which undergo sequential dehydration and phase transformations upon heating. The reactions can be represented by the following chemical equations, crucial for understanding the investment casting process shell sintering:
$$ \text{Al}_2\text{O}_3 \cdot \text{H}_2\text{O} \xrightarrow{450-800^\circ\text{C}} \text{Al}_2\text{O}_3 + \text{H}_2\text{O} $$
$$ \text{Al}_2\text{O}_3 \cdot 2\text{SiO}_2 \cdot 2\text{H}_2\text{O} \xrightarrow{400-700^\circ\text{C}} \text{Al}_2\text{O}_3 \cdot 2\text{SiO}_2 + 2\text{H}_2\text{O} $$
$$ \text{Al}_2\text{O}_3 \cdot 4\text{SiO}_2 \cdot \text{H}_2\text{O} \xrightarrow{>700^\circ\text{C}} 3\text{Al}_2\text{O}_3 \cdot 4\text{SiO}_2 + \text{H}_2\text{O} $$
$$ 3(\text{Al}_2\text{O}_3 \cdot 2\text{SiO}_2) \xrightarrow{>980^\circ\text{C}} 3\text{Al}_2\text{O}_3 \cdot 2\text{SiO}_2 + 4\text{SiO}_2 $$
$$ 3(\text{Al}_2\text{O}_3 \cdot 4\text{SiO}_2) \xrightarrow{>1080^\circ\text{C}} 3\text{Al}_2\text{O}_3 \cdot 2\text{SiO}_2 + 10\text{SiO}_2 $$
In these reactions, metakaolin and metastable phases form initially, eventually yielding mullite (3Al2O3·2SiO2) and alumina (Al2O3) at higher temperatures. The XRD patterns (Figure 2) confirm that at 900°C and 950°C, traces of mullite are present due to pre-existing mullite in the bauxite feedstock. At 1000°C, peaks for mullite, alumina, and yttria become dominant, indicating completion of major transformations. The silica sol binder also crystallizes into cristobalite or tridymite phases, contributing to strength. Beyond 1000°C, no new phases emerge, explaining the stabilized strength. This phase analysis is vital for tailoring the investment casting process to achieve desired shell properties.
SEM images of fracture surfaces provide microstructural insights. At 900°C (Image A), the structure shows layered AM deposition features with numerous pores and weak interparticle bridges, leading to low strength. Cracks are visible, likely from thermal stresses during cooling. At 1000°C (Image B), particles become rounded and sinter-necking is prominent, with reduced pore size and improved connectivity. This enhances mechanical integrity. At 1100°C (Image C), extensive fusion occurs, particles coalesce, and porosity diminishes significantly, but strength improvement is marginal due to limited further bonding. These observations align with the property trends and underscore the importance of microstructural control in the investment casting process.
Based on these findings, we determined that 1000°C is the optimal sintering temperature for our additively manufactured shells in the investment casting process. At this temperature, bending strength reaches 7.31 MPa, porosity is 43.6%, and average shrinkage is 0.79%, all meeting typical requirements for investment casting applications. To validate this, we fabricated prototype shell geometries, including cylindrical and pyramidal hollow structures, using the same slurry and AM process. After sintering at 1000°C, the shells retained shape integrity with clear contours and no cracking, demonstrating feasibility for complex part production. This success highlights the potential of AM to revolutionize the investment casting process by enabling direct, rapid shell fabrication.
Further discussion on the implications for the investment casting process reveals several advantages. Additive manufacturing eliminates the need for wax patterns and multiple coating cycles, reducing lead time and material waste. The ability to tailor slurry composition and sintering parameters allows for customization of shell properties for specific alloys or casting conditions. For instance, higher porosity might be desirable for gasesous metals, while stronger shells are needed for heavy sections. Moreover, AM facilitates the creation of integrated cores and shells, simplifying mold assembly. However, challenges remain, such as achieving uniform drying and sintering in thick sections, or scaling up for large parts. Future work could explore hybrid approaches combining AM with traditional methods, or developing new ceramic formulations for enhanced performance.
To quantify the relationship between sintering temperature and properties, we can propose empirical models. For bending strength (σ) as a function of temperature (T), a sigmoidal curve might fit:
$$ \sigma(T) = \sigma_{\text{max}} – \frac{\sigma_{\text{max}} – \sigma_{\text{min}}}{1 + e^{-k(T – T_0)}} $$
where σmax ≈ 7.7 MPa, σmin ≈ 3.4 MPa, T0 ≈ 1000°C is the inflection point, and k is a constant. Similarly, porosity (φ) may follow an exponential decay:
$$ \phi(T) = \phi_0 e^{-\alpha T} + \phi_{\infty} $$
with φ0, α, and φ∞ as fitting parameters. These models can guide process optimization in the investment casting process.
In conclusion, sintering temperature profoundly influences the properties of additively manufactured investment casting shells. Our systematic study shows that 1000°C yields an optimal balance of strength, porosity, and shrinkage, driven by mullite formation and silica binder crystallization. This work advances the integration of additive manufacturing into the investment casting process, offering a pathway to faster, more flexible production of precision metal parts. As AM technologies evolve, continued research into material systems and process parameters will further enhance the viability of this innovative approach, paving the way for next-generation foundry practices.
Expanding on the broader context, the investment casting process has historically relied on artisan skills and iterative refinement. With digitalization and AM, it is transitioning toward a data-driven, automated workflow. Key considerations include thermal management during sintering to avoid residual stresses, and post-processing like infiltrations to seal pores if needed. Additionally, environmental aspects are crucial; water-based slurries and reduced energy consumption from optimized sintering align with sustainable manufacturing goals. By embracing these advancements, the investment casting process can maintain its relevance in industries like aerospace, medical, and automotive, where complexity and quality are paramount.
Ultimately, the synergy between additive manufacturing and the investment casting process represents a paradigm shift. It enables on-demand production of molds, reduces tooling costs, and allows for geometric complexities unattainable with conventional methods. As we refine materials and processes, the dream of fully digital foundries—where CAD models directly become casting shells—becomes increasingly attainable. This study contributes a foundational understanding of sintering effects, serving as a stepping stone toward that future, where the investment casting process is more agile, efficient, and innovative than ever before.