The relentless advancement of modern manufacturing demands materials that offer superior performance under extreme conditions. In this context, metal matrix composites (MMCs) have emerged as indispensable materials, bridging the gap between traditional alloys and the stringent requirements of high-tech industries. Among these, particle-reinforced iron-based composites are particularly attractive for applications requiring exceptional wear resistance, such as in mining, mineral processing, and power generation equipment. High chromium cast iron (HCCI) is a benchmark material for abrasion-resistant components due to its network of hard carbides. However, its inherent brittleness and the potential for carbide detachment during service limit its performance envelope. Enhancing HCCI with ceramic particles presents a promising route to overcome these limitations, synergistically combining the matrix’s hardness with the reinforcement’s properties.
Conventional methods for fabricating particulate MMCs, such as powder metallurgy or hot pressing, often involve high costs and complex processing, making them less suitable for large-scale, near-net-shape production of industrial components. Stir casting, while more scalable, frequently struggles with the uniform distribution of reinforcement particles, leading to agglomeration that severely undermines the composite’s properties. This study explores the lost foam casting process as a viable and efficient alternative for manufacturing titanium carbide (TiC) particle-reinforced high chromium cast iron composites. The lost foam casting process offers unique advantages for this purpose: it allows for the precise pre-placement of reinforcements within the foam pattern, enables the production of complex geometries with excellent surface finish, and is highly amenable to industrial batch production. This investigation focuses on the critical influence of TiC particle size on the resultant hardness, relative wear resistance, and impact toughness of the composite, providing essential insights for tailoring properties in demanding applications.
Experimental Methodology: Integrating TiC into the Lost Foam Process
The core of this research involves adapting the standard lost foam casting process to incorporate fine ceramic reinforcements. The matrix material was a standard high chromium cast iron. The chosen reinforcement was titanium carbide (TiC) powder, known for its exceptional hardness, high melting point, and good thermodynamic stability with iron melts. Four different particle size distributions were investigated, corresponding to nominal mesh sizes of 600, 1000, 1500, and 2000, which translate to an approximate particle diameter range of 6.5 to 23.0 µm. The physical properties of TiC are summarized in Table 1.
| Crystal System | Density (g/cm³) | Microhardness (HV) | Elastic Modulus (GPa) | Melting Point (°C) | Thermal Expansion Coefficient (10⁻⁶/K) |
|---|---|---|---|---|---|
| Cubic | 4.92 | ~3000 | 269 | ~3300 | 7.40 |
The key innovation in the experimental procedure was the method of introducing the TiC particles into the mold. Instead of adding them to the molten metal, the particles were first integrated into the expendable foam pattern. A volume fraction of 8% TiC particles was uniformly mixed with expandable polystyrene (EPS) beads and a water-based binder in a mechanical mixer. This ensured that the TiC particles were effectively coated onto the surface of the EPS beads. This mixture was then used to create foam patterns for standard test blocks (30 mm x 30 mm x 60 mm) and a real industrial component—a slurry pump back plate (φ350 mm x 25 mm thick). The foam patterns were produced using steam molding in preheated dies.
A gating system was designed to minimize the冲刷 of particles during metal pouring. The prepared foam assembly, consisting of the pattern and gating, was invested in unbonded silica sand. The standard lost foam casting process was then followed: the molten high chromium iron was poured into the mold, vaporizing the EPS pattern. The metal front progressively replaced the foam, capturing the TiC particles that were originally on the pattern’s surface and distributing them within the casting. After solidification and cooling, the castings were extracted from the sand. All composite castings underwent a standard heat treatment (annealing at 850°C) to ensure consistent microstructure and relieve casting stresses before mechanical testing.
Hardness was measured on the Rockwell C scale. Impact toughness was determined using unnotched Charpy specimens. Wear resistance was evaluated using a custom-built slurry erosion tester designed to simulate the actual service conditions of a slurry pump component. The relative wear resistance, a key performance metric, was calculated using the following formula:
$$ \text{Relative Wear Resistance} = \frac{J_2 – J_1}{M_2 – M_1} $$
Where \( J_1 \) and \( J_2 \) are the masses of the base HCCI sample before and after wear, and \( M_1 \) and \( M_2 \) are the masses of the composite sample before and after wear, respectively. A value greater than 1 indicates superior performance compared to the unreinforced matrix.
Influence of TiC Particle Size on Microstructure and Mechanical Properties
The introduction of TiC particles via the lost foam casting process induced significant and systematic changes in the microstructure of the high chromium iron. In the unreinforced base metal, the microstructure was characterized by a continuous network of long, rod-like primary chromium carbides (M7C3), which impart hardness but also act as stress concentrators and crack initiation sites. With the addition of TiC particles, this carbide morphology was altered. For composites with coarser particles (600-1000 mesh), the matrix still contained rod-like carbides, but their quantity appeared reduced. At an optimal particle size (1500 mesh), the microstructure showed a noticeable refinement; the primary carbides became more hexagonal or blocky, and the overall matrix grain structure was finer. However, with the finest particles (2000 mesh), although carbide refinement was observed, optical microscopy revealed the onset of particle agglomeration—clusters of TiC particles—which is a critical defect in composites.
The mechanical property data for the test block castings, presented in Table 2, reveal clear trends correlated with particle size. The lost foam casting process successfully produced composites with enhanced properties across all particle sizes tested.
| Sample ID | TiC Particle Size (Mesh) | Hardness (HRC) | Relative Wear Resistance | Impact Toughness (J/cm²) |
|---|---|---|---|---|
| Base HCCI | – | 30.3 | 1.00 (Reference) | 6.7 |
| Composite A | 600 | 33.1 | 1.28 | 10.3 |
| Composite B | 1000 | 34.3 | 1.35 | 11.0 |
| Composite C | 1500 | 35.0 | 1.60 | 12.3 |
| Composite D | 2000 | 34.5 | 1.47 | 11.8 |
Hardness and Wear Resistance: The hardness showed a consistent increase over the base material, following a parabolic trend with particle size. It increased from 33.1 HRC with 600-mesh particles to a maximum of 35.0 HRC with 1500-mesh particles, representing a significant improvement. The 2000-mesh composite showed a slight decrease to 34.5 HRC. The relative wear resistance mirrored this trend precisely, peaking at 1.60 times that of the base iron for the 1500-mesh composite. This enhancement can be explained by multiple strengthening mechanisms. The primary mechanism is Orowan strengthening, where hard, non-deformable TiC particles act as obstacles to dislocation motion in the metallic matrix. The required shear stress \( \tau \) to bypass particles with an inter-particle spacing \( \lambda \) is given by:
$$ \tau = \frac{G b}{\lambda} $$
where \( G \) is the shear modulus and \( b \) is the Burgers vector. Finer, more uniformly distributed particles decrease \( \lambda \), thereby increasing \( \tau \) and the macroscopic yield strength/hardness. Furthermore, during wear, the hard TiC particles protrude as the softer matrix wears away, protecting the surface and reducing the wear rate. The decline in properties for the 2000-mesh composite is directly attributable to particle agglomeration, which increases the effective inter-particle spacing within clusters and creates local weak points.
Impact Toughness: Perhaps the most remarkable result is the dramatic improvement in impact toughness, a property typically very low in white cast irons. The toughness more than doubled, from 6.7 J/cm² for the base metal to a maximum of 12.3 J/cm² for the 1500-mesh composite. Fractographic analysis provided the explanation. The fracture surface of the base HCCI exhibited classic “river patterns” indicative of cleaved carbide networks and brittle intergranular fracture. In contrast, the composite fracture surfaces showed a more chaotic, “quasi-cleavage” appearance with tear ridges and dimples around particles. The TiC particles, especially when finely and uniformly dispersed, help to pin and blunt advancing cracks. More importantly, they modify the solidification process, refining and potentially breaking up the continuous, brittle carbide network of the HCCI matrix. This change in microstructure from a continuous brittle skeleton to a more discontinuous one dispersed with hard particles transforms the fracture mode, absorbing more energy during crack propagation. The slight drop in toughness for the finest particles again correlates with agglomeration, which can act as a large, pre-existing flaw.
Validation on an Industrial Component: The Slurry Pump Back Plate
To demonstrate the scalability and practical relevance of the lost foam casting process for manufacturing reinforced components, the study successfully produced and tested full-scale slurry pump back plates. The results, summarized in Table 3, confirmed that the trends observed in small test blocks translate directly to a complex, real-world casting.
| Back Plate ID | TiC Particle Size (Mesh) | Hardness (HRC) | Relative Wear Resistance | Impact Toughness (J/cm²) |
|---|---|---|---|---|
| Base HCCI Plate | – | 30.3 | 1.00 | 6.7 |
| Plate 1 | 600 | 43.1 | 1.47 | 11.8 |
| Plate 2 | 1000 | 44.7 | 1.60 | 12.3 |
| Plate 3 | 1500 | 47.1 | 1.80 | 12.8 |
| Plate 4 | 2000 | 45.9 | 1.68 | 12.6 |
The performance enhancements were even more pronounced in the full-scale component. The optimal 1500-mesh TiC composite back plate achieved a hardness of 47.1 HRC—a 55.4% increase over the base material. Its relative wear resistance was 1.80 times higher, and its impact toughness reached 12.8 J/cm², which is 1.91 times that of the standard HCCI back plate. This unequivocally proves the efficacy of the lost foam casting process for manufacturing high-performance, particle-reinforced metal matrix composites for heavy-duty industrial applications. The process successfully maintains a reasonably uniform distribution of particles even in a larger, more complex geometry, leading to property improvements throughout the component.
Discussion and Concluding Synthesis
This research successfully establishes a robust framework for enhancing high chromium cast iron using TiC ceramic reinforcements via the lost foam casting process. The process demonstrates distinct advantages: it is a near-net-shape manufacturing route suitable for complex parts, it allows for precise pre-positioning of the reinforcement phase, and it avoids the severe agitation that can lead to gas entrapment or particle segregation common in stir casting.
The particle size of the reinforcement emerges as a paramount factor controlling the final composite properties. There exists a critical optimum size, identified in this study as around 1500 mesh (approximately 6-10 µm). Particles of this size are effective in refining the matrix microstructure, providing a high number of Orowan strengthening sites due to a small inter-particle spacing \( \lambda \), and effectively modifying the fracture path without inducing significant agglomeration. The relationship between particle size \( d_p \), inter-particle spacing \( \lambda \), and volume fraction \( V_f \) for a uniform distribution can be approximated by:
$$ \lambda = d_p \left( \sqrt{\frac{\pi}{6 V_f}} – 1 \right) $$
This equation highlights that for a fixed \( V_f \) (8% in this case), decreasing \( d_p \) directly decreases \( \lambda \), leading to higher strength—up to the point where interfacial forces and processing limitations cause agglomeration, which effectively increases the local \( \lambda \) and creates stress concentrators.
The property trade-offs are clearly mapped: excessive particle coarseness (>600 mesh) provides limited hardening and microstructural refinement. Excessive fineness (<2000 mesh) leads to processing challenges within the lost foam casting process, specifically the agglomeration of particles during the foam coating and pyrolysis stages, which counteracts the potential benefits of a finer dispersion.
In conclusion, the lost foam casting process presents a highly viable and efficient manufacturing pathway for producing TiC particle-reinforced high chromium iron composites. By carefully selecting the reinforcement particle size—with an optimum found near 1500 mesh (6-10 µm)—it is possible to simultaneously and significantly improve hardness, wear resistance, and, most notably, impact toughness. This breakthrough combination of properties, validated on an industrial-scale component, opens new possibilities for extending the service life and reliability of critical wear-resistant parts in sectors like mining, cement production, and power generation. The lost foam casting process, therefore, stands out not only as a method for shape-making but as a powerful tool for microstructural and property engineering of metal matrix composites.