Our work focuses on the production of NCu30-4-2-1 alloy bars using high precision investment casting, aiming to overcome the challenges associated with small batch sizes and diverse product types. This nickel-copper alloy, strengthened by the β-Ni₃Si phase, exhibits excellent hardness, wear resistance, and anti-galling properties, making it ideal for precision friction components in aerospace fuel systems. However, its wide solidification range, poor fluidity, and high susceptibility to gas absorption often lead to casting defects such as shrinkage porosity and hot tearing. Through careful design of the high precision investment casting process, including a bottom-gating system and gradient cooling, we successfully produced near-net-shape bars meeting mechanical property requirements. This paper details our methodology, the defect analysis of underperforming samples, and the subsequent improvements implemented to enhance casting quality.
Our experimental procedure began with vacuum melting to obtain high-purity master alloy ingots, minimizing oxygen content and inclusions. The high precision investment casting shell mold was designed with a riser section to compensate for the alloy’s significant volume contraction during solidification. The mold configuration, as shown below, utilized a bottom-fill approach to ensure smooth metal flow and promote the upward floatation of gases and slag.
To achieve directional solidification, we applied insulating material around the outer surface of the shell near the riser, creating a controlled temperature gradient along the height of the casting. This gradient facilitated sequential freezing from the bottom upward, ensuring that the riser could feed liquid metal to the solidifying front. The alloy’s nominal and measured chemical compositions are listed in the table below.
| Element | Cu | Si | Fe | Mn | C | Mg | Pb | Ni |
|---|---|---|---|---|---|---|---|---|
| Standard | 30–32 | 3.9–4.3 | 1.5–2.8 | 0.5–1.5 | ≤0.1 | ≤0.1 | ≤0.05 | Balance |
| Measured | 30.92 | 4.08 | 2.20 | 1.09 | 0.023 | 0.0005 | 0.0003 | Balance |
After high precision investment casting, the bars underwent ultrasonic inspection followed by a two-stage heat treatment: solution treatment to dissolve as-cast dendrites, followed by aging to precipitate fine, uniformly distributed β-Ni₃Si strengthening particles. The final mechanical property requirements stipulated a minimum tensile strength of 784 MPa and an elongation of at least 2%. While most specimens exceeded these targets, a few exhibited significantly lower strength and poor ductility. The measured properties of both normal and abnormal samples are compared in the following table.
| Sample Type | Tensile Strength (MPa) | Elongation (%) |
|---|---|---|
| Normal | 946 | 10 |
| Abnormal | 726 | 2 |
To understand the root cause of the low strength, we examined the fracture surfaces using scanning electron microscopy and energy dispersive spectroscopy. The abnormal sample revealed a distinct dark spot on the macro-fracture, which under higher magnification appeared as an interdendritic shrinkage porosity region. The dendrite arms were well-developed, and the gaps between them indicated insufficient liquid feeding during the final stages of solidification. This type of defect is typical in high precision investment casting when the alloy has a wide freezing range and a low nucleation rate, leading to coarse columnar dendrites that isolate small pools of remaining liquid.
We performed EDS analysis on the dark spot area to identify any compositional anomalies. The results, summarized in the table below, showed a significantly higher oxygen concentration along with enrichment of manganese and iron compared to the nominal alloy composition.
| Element | O | Mn | Fe | Ni | Cu | Si |
|---|---|---|---|---|---|---|
| Dark Spot | 12.4 | 4.8 | 3.2 | 50.1 | 24.5 | 5.0 |
| Nominal | – | 1.09 | 2.20 | Balance | 30.92 | 4.08 |
The presence of oxygen is particularly detrimental in high precision investment casting of nickel-copper alloys. Oxygen reacts with alloying elements to form low-melting-point oxides that segregate to the last solidifying liquid at the dendrite tips. These oxide inclusions not only reduce fluidity but also act as nucleation sites for microporosity. Furthermore, the coarse dendritic structure observed in the abnormal sample hindered inter-dendritic feeding, exacerbating shrinkage porosity. The relationship between the solidification shrinkage volume \( V_s \) and the porosity formation can be expressed as:
$$ V_s = V_L \cdot \beta \cdot (1 – f_s) $$
where \( V_L \) is the volume of liquid metal, \( \beta \) is the solidification shrinkage factor (approximately 4–5% for this alloy), and \( f_s \) is the fraction solid at the moment of feeding cutoff. When dendrites become fully bridged, \( f_s \) is high, and the remaining liquid cannot compensate for shrinkage, leading to voids. The tensile strength reduction due to such porosity can be approximated by the area fraction of defects:
$$ \sigma_f = \sigma_0 \cdot (1 – A_d / A_0) $$
Here, \( \sigma_f \) is the fracture strength, \( \sigma_0 \) is the strength of defect-free material, \( A_d \) is the total projected area of shrinkage pores on the fracture surface, and \( A_0 \) is the nominal cross-sectional area. In our abnormal sample, the dark spot covered approximately 5–8% of the fracture area, which would reduce the strength by a similar percentage, consistent with the observed drop from 946 MPa to 726 MPa.
To mitigate these defects in high precision investment casting, we implemented several modifications. First, we enhanced the vacuum refining process by extending the holding time and increasing the melt superheat to allow more complete degassing. Second, we introduced trace additions of rare earth elements (Ce) and boron (B) to the melt. These elements act as deoxidizers and grain refiners, reducing the oxygen content and promoting the formation of finer equiaxed dendrites rather than coarse columnar ones. The effect of boron on the secondary dendrite arm spacing (SDAS) can be described by the following empirical relationship:
$$ \lambda_2 = K \cdot (C_R)^{-1/3} $$
where \( \lambda_2 \) is the SDAS, \( K \) is a material constant influenced by alloy composition and refining additions, and \( C_R \) is the cooling rate. By refining the grain structure, the effective feeding distance increases, and the tendency for interdendritic shrinkage decreases significantly.
We also optimized the high precision investment casting thermal parameters, including preheating the shell mold to a higher temperature (950°C) and applying a slower initial cooling rate within the mold. These changes promoted a more directional solidification front and reduced the thermal gradient that drives dendritic growth. A summary of the optimized casting parameters is given in the table below.
| Parameter | Original | Optimized |
|---|---|---|
| Melt temperature (°C) | 1480 | 1500 |
| Shell mold temperature (°C) | 900 | 950 |
| Pouring rate (kg/s) | 0.5 | 0.3 |
| Insulation thickness on riser (mm) | 10 | 15 |
| Ce addition (wt%) | 0 | 0.02 |
| B addition (wt%) | 0 | 0.005 |
After implementing these improvements in subsequent batches of high precision investment casting, we re-examined the bars using ultrasonic testing and tensile testing. The porosity level dropped below the detection limit, and all samples achieved tensile strengths above 900 MPa with elongations exceeding 8%. The refined microstructure also exhibited more uniform hardness and improved fatigue resistance, which are critical for the intended aerospace applications. The success of this investigation underscores the importance of controlling melt quality and solidification conditions in high precision investment casting of complex nickel-copper alloys.
In conclusion, our work demonstrates that high precision investment casting can reliably produce NCu30-4-2-1 alloy bars meeting stringent mechanical requirements when proper process control is applied. The primary defect—shrinkage porosity—originates from a combination of high oxygen content and coarse dendritic growth. By enhancing vacuum refining, adding grain refiners, and optimizing thermal gradients, we eliminated these defects and achieved consistent quality. The knowledge gained here is directly transferable to other high-performance alloys processed via high precision investment casting, particularly those with wide solidification ranges and high sensitivity to gas absorption.