In recent years, the in-depth research on zinc-based alloys, particularly high-aluminum zinc-based alloys, has significantly advanced their industrial applications. These alloys are increasingly used in machinery, mining equipment, metallurgical machinery, petroleum machinery, papermaking machinery, forging presses, hydraulic components, and more, owing to their excellent wear resistance, high mechanical properties, good machinability, and cost-effectiveness. The growth of sand casting services has facilitated the production of complex parts, but challenges remain in achieving sound castings, especially for thick sections. High-aluminum zinc-based alloys, such as ZA27, have a wide solidification range, which can lead to segregation and shrinkage defects, impacting performance. This study investigates the wall thickness effect on microstructure, soundness, and mechanical properties in sand casting services, aiming to optimize process parameters for producing high-quality castings.
The demand for reliable sand casting services has driven the need to understand how wall thickness influences alloy behavior. In sand casting, the cooling conditions vary with section size, affecting solidification kinetics and defect formation. Through controlled experiments, we explore how wall thickness impacts the solidification process, microstructure evolution, and final properties. By analyzing cooling curves, porosity, and mechanical data, we provide insights for improving sand casting services to manufacture robust components across different wall thicknesses.
Our experimental approach mimics industrial sand casting services to ensure practical relevance. The alloy composition corresponds to the international ZA27 grade, with nominal composition: 25-28% Al, 2.0-2.5% Cu, 0.01-0.02% Mg, and balance Zn. This alloy offers typical sand-cast properties: tensile strength of 400-450 MPa, elongation of 3-5%, Brinell hardness of 110-120, and impact toughness of 30-40 J/cm². The melting was conducted in a coke crucible furnace, using nitrogen for degassing, with pouring temperature controlled between 480°C and 520°C. The sand mold utilized Nanjing red sand with 50-70 mesh size and moisture content of 5-7%.
Specimens were designed with varying wall thicknesses: 20 mm, 40 mm, 60 mm, and 100 mm, each with dimensions of 200 mm in length and 100 mm in width, but differing in height to achieve the desired thickness. The casting layout included a gating system to ensure uniform filling. Thermocouples (NiCr-NiSi) were placed at geometric centers along the height to record cooling curves during solidification, as illustrated in the setup. Mechanical test samples were extracted from specific locations: tensile specimens followed GB/T 228.1 standard, and impact specimens were 10 mm × 10 mm × 55 mm unnotched bars. Testing involved a universal testing machine, impact tester, and Brinell hardness tester.
The cooling curves, recorded under different process conditions, reveal critical aspects of solidification in sand casting services. Under high cooling intensity, the solidification time increases with wall thickness, as shown by the extended cooling curve length. For example, a 20 mm wall thickness solidified in approximately 120 seconds, while a 100 mm thickness took over 300 seconds. The cooling rate, derived from temperature gradients, decreases with thicker sections. The eutectoid transformation, occurring around 275°C, exhibits longer durations and slightly higher temperatures in thicker walls, indicating slower diffusion processes. These trends are summarized in the formula for solidification time $t_f$:
$$t_f = \frac{V}{A} \cdot \frac{\rho L}{h(T_m – T_0)}$$
where $V$ is volume, $A$ is surface area, $\rho$ is density, $L$ is latent heat, $h$ is heat transfer coefficient, $T_m$ is melting temperature, and $T_0$ is ambient temperature. As wall thickness increases, the volume-to-area ratio rises, prolonging $t_f$ and reducing cooling rate $R_c$:
$$R_c = \frac{dT}{dt} \propto \frac{1}{t_f}$$
This directly influences microstructure and defect formation. Under moderate cooling conditions, similar patterns emerge but with slower rates, emphasizing the role of process control in sand casting services.
Microstructural analysis shows that wall thickness significantly affects grain size and porosity. For thin walls (20 mm), the rapid cooling leads to fine dendritic structures with dispersed microporosity, primarily gas-shrinkage pores. As thickness increases to 40 mm and 60 mm, grains coarsen, and porosity shifts from dispersed to more concentrated shrinkage cavities in the central regions. At 100 mm, macroscopic shrinkage defects become prominent, reducing soundness. The porosity percentage $P$ can be estimated as:
$$P = \int_{0}^{t_f} \frac{\dot{V}_p}{V_0} dt$$
where $\dot{V}_p$ is pore formation rate and $V_0$ is initial volume. This integral increases with $t_f$, explaining the higher porosity in thicker sections. Micrographs indicate that eutectic phases become coarser, and primary aluminum phases grow, altering the alloy’s mechanical response. These observations underscore the importance of optimizing sand casting services for different wall thicknesses to minimize defects.
Mechanical properties exhibit clear dependencies on wall thickness, as summarized in Table 1. Data averages from multiple tests show trends in tensile strength, elongation, hardness, and impact toughness. The values represent specimens taken from bottom, middle, and top sections, highlighting variations within castings.
| Wall Thickness (mm) | Tensile Strength (MPa) | Elongation (%) | Brinell Hardness (HB) | Impact Toughness (J/cm²) |
|---|---|---|---|---|
| 20 | 420 ± 15 | 4.5 ± 0.5 | 115 ± 5 | 38 ± 4 |
| 40 | 400 ± 20 | 3.8 ± 0.6 | 112 ± 6 | 35 ± 5 |
| 60 | 380 ± 25 | 3.2 ± 0.7 | 110 ± 7 | 32 ± 6 |
| 100 | 350 ± 30 | 2.5 ± 0.8 | 108 ± 8 | 28 ± 7 |
Table 1: Average mechanical properties vs. wall thickness for ZA27 alloy in sand casting services. Note the decline in strength and ductility with increasing thickness, while hardness remains relatively stable.
The degradation in properties correlates with increased porosity and coarse microstructure. Tensile strength $\sigma_t$ can be modeled using the porosity effect:
$$\sigma_t = \sigma_0 (1 – kP)$$
where $\sigma_0$ is the defect-free strength and $k$ is a material constant. For impact toughness $K$, the relationship involves crack initiation sites:
$$K \propto \frac{1}{\sqrt{d_g}} \cdot (1 – \alpha P)$$
with $d_g$ as grain size and $\alpha$ as a factor. Thicker walls lead to larger $d_g$ and higher $P$, reducing $K$. Hardness, less sensitive to defects, shows minor changes, consistent with indentation resistance being influenced by bulk matrix properties. These trends highlight the challenges in sand casting services for thick-section components, necessitating process adjustments.
Further analysis of internal soundness reveals that defect distribution varies with position. In thin walls, porosity is uniform, but in thick walls, the top sections exhibit more severe shrinkage due to inadequate feeding. The feeding efficiency $F_e$ during solidification can be expressed as:
$$F_e = \frac{Q_{feed}}{Q_{shrink}} = \frac{\beta \cdot A_{channel} \cdot \Delta P}{\eta \cdot L_f \cdot \dot{V}_{solid}}$$
where $Q_{feed}$ is feed metal flow, $Q_{shrink}$ is shrinkage volume, $\beta$ is a constant, $A_{channel}$ is channel area, $\Delta P$ is pressure drop, $\eta$ is viscosity, $L_f$ is feeding distance, and $\dot{V}_{solid}$ is solidification rate. For thick walls, $L_f$ increases, reducing $F_e$ and promoting centerline porosity. This explains why sand casting services must carefully design risers and cooling conditions to enhance feeding.
Process parameter optimization can mitigate the wall thickness effect. By controlling pouring temperature, mold properties, and cooling rates, we achieved improved soundness for walls up to 100 mm. For instance, increasing cooling intensity through chills or mold coatings reduced solidification time, refining grains and decreasing porosity. The optimized condition satisfies the criterion for sound castings in sand casting services:
$$R_c \geq R_{c,min} \quad \text{and} \quad t_f \leq t_{f,max}$$
where $R_{c,min}$ is the minimum cooling rate to avoid gross porosity, and $t_{f,max}$ is the maximum time for effective feeding. Experimental results indicate that for ZA27 alloy, walls below 100 mm can produce high-quality castings when parameters are tightly controlled. This is crucial for expanding the application of sand casting services in manufacturing durable parts.
Microstructural coarsening in thick sections is attributed to reduced undercooling $\Delta T$:
$$\Delta T = T_m – T_{actual} \propto R_c^{1/2}$$
Lower $R_c$ in thick walls decreases $\Delta T$, leading to larger dendrite arm spacing $d_{das}$:
$$d_{das} = a \cdot R_c^{-n}$$
with $a$ and $n$ as constants. This coarsening affects mechanical properties, as finer structures typically enhance strength and toughness. However, in our sand casting services, we observed that solid solution strengthening from aluminum and copper in the zinc matrix partially compensates for coarsening, explaining the retained hardness. The interplay between these factors underscores the complexity of sand casting processes.
Defect analysis shows that porosity consists of gas pores and shrinkage cavities. Gas porosity arises from dissolved gases, with volume $V_g$ given by Sieverts’ law:
$$V_g = k_g \cdot \sqrt{P_{gas}} \cdot \exp\left(-\frac{\Delta H}{RT}\right)$$
where $k_g$ is a constant, $P_{gas}$ is gas partial pressure, $\Delta H$ is enthalpy, $R$ is gas constant, and $T$ is temperature. In sand casting services, proper degassing reduces $V_g$. Shrinkage porosity $V_s$ relates to solidification contraction:
$$V_s = \epsilon \cdot V_0 \cdot (1 – f_s)$$
with $\epsilon$ as contraction coefficient and $f_s$ as solid fraction. Thick walls have lower $f_s$ gradients, increasing $V_s$ accumulation. Combined defects act as stress concentrators, reducing fatigue life and reliability. Therefore, sand casting services must implement rigorous quality checks to ensure component integrity.
Mechanical property variations within a casting are notable. Bottom sections, near chills, show higher strength due to faster cooling and better feeding. Top sections, last to solidify, have more defects and lower properties. This gradient $\nabla \sigma$ can be approximated:
$$\nabla \sigma = \frac{\sigma_{bottom} – \sigma_{top}}{h}$$
where $h$ is height. For 100 mm walls, $\nabla \sigma$ reaches 50 MPa, emphasizing the need for uniform cooling in sand casting services. Impact toughness varies widely, reflecting sensitivity to defects. Statistical analysis of our data confirms that sand casting services can achieve consistent properties by managing wall thickness and process parameters.
The role of alloy composition is also critical. ZA27’s high aluminum content promotes formation of primary Al-rich phases, which strengthen the matrix. The solid solubility of Cu and Mg in Zn enhances hardness. However, in thick sections, segregation can occur, leading to inhomogeneous properties. The Scheil equation describes segregation:
$$C_s = k C_0 (1 – f_s)^{k-1}$$
where $C_s$ is solid composition, $k$ is partition coefficient, and $C_0$ is initial composition. Slow cooling in thick walls amplifies segregation, potentially reducing corrosion resistance. Thus, sand casting services must balance composition control with cooling optimization.
In industrial applications, sand casting services often produce parts with varying wall thicknesses. Our findings suggest that for ZA27 alloy, designers should aim for walls under 100 mm or incorporate process modifications like padding, chilling, or controlled cooling to maintain performance. The economic benefits of sand casting services make them attractive, but quality assurance is paramount. By integrating our results, manufacturers can refine their sand casting services to deliver superior components.
Future work could explore advanced simulation tools to predict wall thickness effects in sand casting services. Computational models coupling fluid flow, heat transfer, and stress analysis would enable proactive optimization. Additionally, alloy modifications or heat treatments might further mitigate drawbacks in thick sections. The continuous improvement of sand casting services will expand their use in high-performance sectors.
In conclusion, the wall thickness effect in sand casting of ZA27 alloy significantly influences microstructure, soundness, and mechanical properties. As thickness increases, cooling rate decreases, leading to coarse grains, increased porosity, and reduced strength and toughness. However, through careful control of process parameters in sand casting services, such as pouring temperature, mold design, and cooling conditions, these adverse effects can be minimized. Our experiments demonstrate that for wall thicknesses up to 100 mm, high-quality castings can be achieved, supporting the reliability of sand casting services for diverse industrial applications. By understanding and applying these principles, sand casting services can enhance product quality and meet growing demands for durable zinc-based alloy components.