CLADDING TECHNOLOGY SHANXI CO., LTDCold Roll Bonding of Copper Stainless Steel Bimetallic Composite and Its Microstructure and Properties
Literature Overview
Cold roll bonding (CRB) represents an alternative to explosive cladding and weld overlay for producing copper-stainless steel bimetallic composites. This method relies on the plastic deformation of two metal strips during rolling to achieve intimate surface contact and mechanical interlocking, followed by optional low-temperature annealing to promote metallurgical bonding. The resulting composite combines the electrical conductivity of copper with the corrosion resistance of austenitic stainless steel, making it suitable for electrical contact materials, heat exchanger tubes, and specialized structural applications. This study investigates the rolling parameters, interfacial microstructure, and resulting mechanical and physical properties of the bonded composite.
Rolling Process Parameters and Bond Mechanism
The cold rolling process for copper-stainless steel bonding involves several critical parameters that must be optimized simultaneously. The pre-rolling reduction ratio, rolling temperature, rolling speed, and number of passes all influence the quality of the bond. Typically, a total thickness reduction of 40-70% is required to achieve metallurgical bonding, with intermediate annealing steps to prevent excessive work hardening and cracking.
| Rolling Parameter | Typical Range | Effect on Bond Quality |
|---|---|---|
| Total Reduction (%) | 40-70 | Insufficient reduction leads to mechanical-only bonding |
| Rolling Temperature (°C) | 20-400 | Higher temperatures promote diffusion bonding |
| Number of Passes | 3-8 | More passes increase contact area |
| Rolling Speed (m/min) | 10-50 | Affects heat generation and deformation uniformity |
| Strip Surface Roughness (Ra) | 0.2-1.6 μm | Smoother surfaces improve initial contact |
The bonding mechanism in cold roll bonding operates through three stages: initial mechanical interlocking at asperity contacts, progressive plastic deformation creating additional contact area, and finally diffusion bonding at elevated temperatures where atomic diffusion across the interface creates true metallurgical bonds. The transition from mechanical to metallurgical bonding is critical for achieving the required bond strength in engineering applications.
Microstructural Characterization
The interface region of cold roll bonded copper-stainless steel composites shows distinct features compared to explosive cladding. The interface is typically flat without the characteristic wave pattern seen in explosive bonds, and the interfacial layer consists of a narrow zone of mixed copper and iron with limited intermetallic formation. The grain structure adjacent to the interface is severely deformed in the as-rolled condition, with elongated grains oriented in the rolling direction. After annealing at 500-700°C, partial recrystallization occurs, producing equiaxed grains and promoting diffusion bonding.
The hardness profile across the composite shows a gradient from the copper face (~60-80 HV) through the interface (~100-150 HV) to the stainless steel substrate (~200-250 HV). The interface hardness is elevated due to work hardening and the presence of fine precipitates at the contact zone. Importantly, the interfacial layer in cold roll bonded composites is generally thinner and more uniform than in explosive cladding, which can be advantageous for applications requiring consistent interfacial properties.
Mechanical and Physical Property Evaluation
Tensile testing of transverse specimens demonstrates that properly bonded cold roll composites achieve bond strengths exceeding 250 MPa, with fracture occurring in the copper layer rather than at the interface. The electrical conductivity of the composite, measured through the copper face, retains approximately 70-80% of pure copper conductivity, which is adequate for most electrical contact applications. The corrosion resistance of the composite in chloride-containing environments is dominated by the stainless steel layer, with pitting potential values consistent with 304 or 316 stainless steel depending on the specific grade used.
Cyclic loading tests reveal that the bond strength decreases with increasing number of load cycles, with significant degradation observed beyond 10^5 cycles at stress levels above 150 MPa. This fatigue behavior is attributed to microcrack initiation at the interface asperities and progressive interfacial delamination. The rate of degradation is influenced by the rolling parameters, with higher reduction ratios and more passes generally improving fatigue resistance.
Engineering Practice Considerations
For pressure vessel applications using cold roll bonded copper-stainless steel composites, several additional considerations arise:
- Dimensional accuracy: The rolling process introduces residual stresses and may cause slight thickness variation across the width of the strip, requiring final machining to achieve specified dimensions.
- Weldability: The composite must be evaluated for weldability per NB/T 47014, with particular attention to the behavior of the interfacial layer during welding heat input.
- Post-weld heat treatment: Any stress-relief or solution treatment required for the pressure vessel must be verified not to degrade the interfacial bond. Temperatures above 500°C for extended periods may promote excessive intermetallic growth.
The composite plate must also satisfy the requirements of applicable standards such as ASTM A263 for clad plate specification, including proof of bond strength, absence of delamination, and acceptable thickness uniformity. Non-destructive testing by ultrasonic methods is recommended to verify the absence of internal voids or incomplete bonding throughout the production lot.
Study Insights and Conclusions
The cold roll bonding method offers a practical alternative to explosive cladding for copper-stainless steel composites, particularly for applications requiring flat plate or strip forms. The key advantage is the ability to produce large-area composites with uniform bond quality, while the primary challenge is controlling the transition from mechanical to metallurgical bonding through appropriate rolling and annealing parameters.
The most significant finding from this study is that the rolling reduction ratio is the dominant parameter governing bond quality. Below 40% reduction, the bond is predominantly mechanical and susceptible to delamination under cyclic loading. Above 60% reduction, excessive work hardening can lead to cracking during rolling, requiring intermediate annealing steps. The optimal window of 45-55% total reduction, achieved in 4-6 passes with intermediate annealing, produces composites with excellent bond strength, uniform microstructure, and acceptable mechanical properties.
In conclusion, cold roll bonded copper-stainless steel composites represent a viable manufacturing route for specialized applications requiring combined electrical and corrosion resistance properties, but their successful implementation requires careful control of rolling parameters, thorough quality verification, and appropriate consideration of service conditions to ensure long-term structural integrity.