In modern industrial applications, the demand for materials with superior comprehensive properties has driven significant interest in bimetal materials. These materials combine the advantages of two distinct metals, offering enhanced performance where single materials fall short. Among these, Al/Cu bimetal materials are particularly noteworthy due to their ability to merge the excellent electrical and thermal conductivity of copper with the low density of aluminum. This combination not only meets performance requirements but also contributes to weight reduction and cost savings, making them highly valuable in sectors such as automotive, power systems, heat exchangers, and mechanical engineering. For instance, Al/Cu bimetal transition joints are crucial in electrical systems, while heat exchange devices benefit from the superior thermal properties of both metals. Consequently, the development of reliable Al/Cu bimetal materials has become a focal point in materials science and engineering.
Traditional methods for fabricating Al/Cu bimetal materials include solid-solid bonding techniques such as rolling, extrusion, diffusion bonding, and friction stir welding. However, these methods often involve complex surface preparation, lengthy processes, and limitations in shape and size. In contrast, solid-liquid composite methods, particularly those integrated with casting processes like sand casting, permanent mold casting, and lost foam casting, offer distinct advantages. Lost foam casting, as a near-net-shape casting technology, is cost-effective, environmentally friendly, and capable of producing complex parts with low surface roughness without the need for sand cores. When combined with composite casting, the lost foam casting process allows for easy fixation of inserts and provides a protective atmosphere from the decomposition of the foam pattern, which helps prevent oxidation of the copper insert during pouring. This makes lost foam casting an attractive method for producing Al/Cu bimetals.
A critical parameter in the lost foam casting process for bimetal materials is the melt-to-solid volume ratio (VR), defined as the volume of molten metal to the volume of the solid insert. This ratio significantly influences the interfacial characteristics, including the formation of metallurgical bonds, the thickness and morphology of interfacial layers, and the resulting mechanical properties. Despite its importance, there is limited research on the effects of VR on Al/Cu bimetal interfaces fabricated via lost foam casting. This study aims to investigate how varying the melt-to-solid volume ratio impacts the microstructure and mechanical properties of Al/Cu bimetal interfaces produced through lost foam casting. By examining interfacial reactions, layer formation, hardness, and shear strength, we seek to provide insights into the optimal processing conditions for achieving robust Al/Cu bimetal components.
The interfacial bonding in Al/Cu bimetals involves complex diffusion and reaction processes due to the mutual solubility and formation of intermetallic compounds (IMCs) between aluminum and copper. These IMCs, while necessary for metallurgical bonding, are often brittle and can compromise mechanical integrity if excessively thick. Therefore, controlling the interfacial reaction layer through process parameters like VR is essential. In this work, we explore VR values of 3:1, 5:1, 7:1, and 9:1 to understand their effects on interface formation. We employ macroscopic and microscopic characterization, hardness testing, and shear strength evaluations to correlate VR with interfacial quality. Additionally, we discuss the formation mechanisms of the interfacial layers based on diffusion kinetics and phase transformation theories.
The experimental setup for the lost foam casting process involved preparing foam patterns with embedded T2 copper rods as solid inserts. The copper rods, with dimensions of 10 mm in diameter and 50 mm in length, were subjected to surface treatment to ensure a clean and active surface for bonding. This treatment included grinding with 2000-grit sandpaper, ultrasonic cleaning in acetone for 15 minutes, and pickling in a mixed acid solution (30% H2SO4, 40% HNO3, and 4 g/L HCl) at room temperature for 40 seconds to remove oxides and create a rough surface. The treated copper rods were then assembled with expandable polystyrene (EPS) foam patterns to achieve different melt-to-solid volume ratios: 3:1, 5:1, 7:1, and 9:1. The patterns were designed such that the volume of aluminum melt surrounding the copper rod corresponded to the specified VR.
A356 aluminum alloy was used as the molten metal, melted in a resistance furnace at 800°C. After degassing and refining, the molten aluminum was poured into the molds under a vacuum of -0.03 MPa. The vacuum assists in removing gases from the foam decomposition and enhances mold filling. After solidification, the Al/Cu bimetal castings were sectioned at the mid-length of the copper rod for analysis. Microstructural examination was conducted using optical microscopy and scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS). Phase identification was performed via X-ray diffraction (XRD). Hardness across the interface was measured using a Vickers microhardness tester with a load of 0.98 N and a dwell time of 15 s. Shear strength tests were carried out on a universal testing machine, with the shear strength calculated using the formula:
$$ \tau = \frac{F}{S} $$
where \( \tau \) is the shear strength in MPa, \( F \) is the maximum load in N, and \( S \) is the contact area between the aluminum and copper in mm². Fracture surfaces after shear testing were analyzed to determine the failure mode.
The lost foam casting process inherently involves the thermal decomposition of the foam pattern, which generates a reducing atmosphere that protects the copper insert from oxidation. This aspect is particularly beneficial in solid-liquid composite casting, as it promotes cleaner interfaces. By varying the VR, we alter the heat dissipation dynamics and the duration of molten metal contact with the solid insert, which in turn affects diffusion and reaction times. This study systematically evaluates these effects to guide process optimization in lost foam casting for bimetal fabrication.
Macroscopic examination of the Al/Cu bimetal samples revealed distinct interfacial characteristics depending on the melt-to-solid volume ratio. At VR = 3:1, no significant metallurgical bonding was observed; the copper rod surface remained largely intact without noticeable melting or deformation, and the interface appeared as a mere mechanical junction with gaps. As VR increased to 5:1, partial metallurgical bonding began to occur, evidenced by localized melting pits on the copper surface and the formation of a thin transition layer. At VR = 7:1 and 9:1, complete metallurgical bonding was achieved, with a continuous and visibly thicker interfacial transition zone. However, porosity defects were noted within the transition zone, particularly at VR = 9:1, where gas entrapment seemed more pronounced.
The average thickness of the interfacial transition zone and the melting rate of the copper substrate increased with higher VR, as summarized in Table 1. The thickening of the interface is attributed to prolonged contact time between the molten aluminum and solid copper, allowing for greater diffusion and dissolution. The melting rate, calculated as the percentage of copper mass dissolved into the aluminum melt, also rose due to enhanced heat input and longer exposure.
| Melt-to-Solid Volume Ratio (VR) | Average Interface Thickness (μm) | Copper Melting Rate (%) | Metallurgical Bonding |
|---|---|---|---|
| 3:1 | ~0 | < 1 | None (Mechanical only) |
| 5:1 | 50 ± 10 | 3 ± 1 | Partial |
| 7:1 | 120 ± 20 | 7 ± 2 | Complete |
| 9:1 | 180 ± 30 | 12 ± 3 | Complete |
Microstructural analysis of the interfacial regions showed a layered structure for samples with VR ≥ 5:1. The interface comprised two main zones: an intermetallic compound (IMC) layer adjacent to the copper substrate and a eutectic reaction layer toward the aluminum side. The IMC layer itself consisted of three distinct sub-layers, identified via EDS as Al4Cu9 (Layer I), AlCu (Layer II), and Al2Cu (Layer III). The eutectic reaction layer (Layer IV) was composed of α(Al), Al2Cu, and Si phases. The formation sequence of these layers is critical to understanding the bonding mechanism.
At VR = 5:1, Layer III (Al2Cu) exhibited an irregular, dendritic morphology with fine protrusions, indicative of faceted growth under high constitutional supercooling. As VR increased to 7:1, the Al2Cu layer transformed into a more cellular structure, suggesting reduced supercooling due to more extensive copper diffusion. At VR = 9:1, the Al2Cu layer thickened further, and the eutectic reaction layer showed refined α(Al)-Al2Cu eutectic spacing compared to VR = 7:1. This refinement is likely due to a composition closer to the eutectic point from increased copper dissolution, promoting finer eutectic solidification.
The effective heat of formation (EHF) model was applied to predict the formation sequence of IMCs. The EHF is given by:
$$ \Delta H_{\text{eff}} = \Delta H^0 \times \frac{c_e}{c_0} $$
where \( \Delta H^0 \) is the standard heat of formation, \( c_e \) is the effective concentration of the limiting element at the interface (taken as the concentration at the lowest eutectic temperature), and \( c_0 \) is the concentration of the limiting element in the IMC. Calculations for Al-Cu IMCs are presented in Table 2.
| IMC Phase | Composition (at.%) | ΔH⁰ (kJ/mol) | ΔHeff (kJ/mol) |
|---|---|---|---|
| Al2Cu | Al0.66Cu0.34 | -13.05 | -6.72 |
| AlCu | Al0.49Cu0.51 | -19.92 | -6.64 |
| Al4Cu9 | Al0.34Cu0.66 | -21.69 | -5.59 |
Based on the EHF values, Al2Cu has the most negative ΔHeff, indicating it forms first, followed by AlCu and then Al4Cu9. This sequence aligns with the observed microstructure, where Al2Cu nucleates directly on the copper substrate, and subsequent solid-state diffusion leads to the formation of AlCu and Al4Cu9 layers. Kirkendall voids were observed at the boundaries between these layers, particularly between Al2Cu and AlCu, due to unequal diffusion rates of Al and Cu atoms, which can inhibit layer growth and affect mechanical properties.
Vickers microhardness profiles across the interface revealed that the IMC and eutectic layers had significantly higher hardness (140-190 HV) compared to the aluminum matrix (~60 HV) and copper substrate (~80 HV). The hardness did not show a clear trend with VR, as variations in porosity and layer morphology influenced local hardness values. Representative hardness indentation images confirmed the brittle nature of the IMC layers.
Shear strength testing demonstrated a strong dependence on VR, as shown in Table 3. At VR = 5:1, the shear strength was low (~28 MPa) due to incomplete metallurgical bonding. At VR = 7:1, shear strength peaked at approximately 81 MPa, attributed to a well-bonded interface with an optimal IMC thickness. However, at VR = 9:1, shear strength dropped to ~52 MPa, likely because of excessive IMC thickness and increased porosity, both of which promote brittle fracture. Fracture surface analysis via XRD indicated that failure occurred within the IMC layer for all samples, confirming that the IMCs are the weak link in the joint.
| Melt-to-Solid Volume Ratio (VR) | Average Shear Strength (MPa) | Standard Deviation (MPa) | Fracture Location |
|---|---|---|---|
| 5:1 | 28 | ± 3 | IMC layer |
| 7:1 | 81 | ± 5 | IMC layer |
| 9:1 | 52 | ± 4 | IMC layer |
The interfacial formation process in lost foam casting of Al/Cu bimetals can be described in several stages. Initially, when molten aluminum contacts the solid copper, interdiffusion occurs rapidly at high temperature, creating a copper-rich liquid layer at the copper surface. This layer results from partial melting of the copper substrate due to the formation of low-melting-point compositions from Al-Cu interdiffusion. As diffusion continues, a concentration gradient of copper develops in the aluminum melt adjacent to the interface. Upon cooling, primary silicon particles may precipitate first due to the high melting point of silicon, followed by α(Al) dendrites in regions with lower copper concentration. Concurrently, Al2Cu nucleates on the copper substrate and grows into the melt. Subsequently, solid-state diffusion between the Al2Cu layer and the copper substrate leads to the formation of AlCu and Al4Cu9 layers through phase transformations. Finally, the remaining liquid solidifies as a eutectic mixture of α(Al), Al2Cu, and Si, completing the interfacial structure.
The melt-to-solid volume ratio profoundly affects this process by controlling the heat capacity and solidification time. A higher VR means more molten metal, which retains heat longer, extending the time for diffusion and reaction. This leads to thicker IMC layers and more copper dissolution. However, excessive VR can also promote defect formation, such as porosity from gas entrapment during foam decomposition in the lost foam casting process. Therefore, optimizing VR is crucial to balance bond strength and interface quality.
In conclusion, this study systematically investigates the influence of melt-to-solid volume ratio on the microstructure and mechanical properties of Al/Cu bimetal interfaces fabricated via lost foam casting. The findings highlight that a VR of 3:1 fails to produce metallurgical bonding, while VR values of 5:1 and above enable interfacial reaction layer formation. The interface consists of multiple IMC layers (Al4Cu9, AlCu, Al2Cu) and a eutectic reaction layer, with their morphology and thickness varying with VR. Hardness of the interface ranges from 140 to 190 HV, significantly higher than the base metals. Shear strength reaches a maximum at VR = 7:1 (81 MPa) but decreases at VR = 9:1 due to thickened IMCs and porosity. Fracture consistently occurs within the IMC layer, underscoring its brittleness.
These results provide valuable guidelines for selecting process parameters in lost foam casting for Al/Cu bimetal production. A melt-to-solid volume ratio around 7:1 appears optimal for achieving a strong metallurgical bond without excessive brittle phases. Future work could explore additional factors such as pouring temperature, cooling rate, and alloy composition to further enhance interface properties. The lost foam casting method, with its unique advantages, holds great potential for manufacturing high-performance Al/Cu bimetals for advanced engineering applications.