CLADDING TECHNOLOGY SHANXI CO., LTDDeformation Behavior During Cold Heading of Clad Bimetal Components
Research Background and Significance
Cold heading (cold forging) is a widely used manufacturing process for producing high-volume components such as fasteners, pins, and shafts with near-net shapes and superior mechanical properties. The process involves deforming a heated or room-temperature metal billet using dies to achieve the desired shape without material removal. When applied to clad (bimetal) components—where two or more metals are bonded together to combine the beneficial properties of each metal—cold heading introduces unique challenges related to the differential deformation behavior of the bonded metals.
The research by Hui Youke, Wang Huajun, Xie Bing, and Chen Wen from the Wuhan University of Technology Material Science and Engineering School, published in 2017 under National Natural Science Foundation of China grant (51475346), investigated the deformation behavior during cold heading of clad bimetal components. This work is significant because clad components are increasingly used in applications requiring combined properties—such as a wear-resistant surface with a tough substrate, or a corrosion-resistant cladding on a high-strength base—and understanding their deformation behavior during cold forging is essential for process design and quality control.
Fundamentals of Clad Material Deformation
Clad materials consist of two or more metals bonded together through processes such as explosion welding, roll bonding, or weld overlay. The two metals typically have different mechanical properties, including yield strength, ultimate tensile strength, Young's modulus, and strain hardening behavior. During cold heading, these differences lead to non-uniform deformation, stress redistribution, and potential loss of bond integrity.
The key deformation parameters that differ between the two metals in a clad system include:
| Parameter | Base Metal (e.g., Carbon Steel) | Clad Metal (e.g., Stainless Steel or Ni Alloy) |
|---|---|---|
| Yield strength | 250-400 MPa (annealed) | 200-500 MPa (annealed) |
| Young's modulus | 200-210 GPa | 190-210 GPa |
| Strain hardening exponent (n) | 0.18-0.22 | 0.20-0.30 |
| Reduction of area | 40-60% | 50-70% |
| Fracture strain | 0.3-0.5 | 0.5-0.8 |
| Flow stress at 50% strain | 500-700 MPa | 400-600 MPa |
The differential in flow stress means that during cold heading, the softer metal deforms more easily than the harder metal. This creates a mismatch in strain distribution across the clad interface, which can lead to several failure modes: delamination at the bond interface, wrinkling of the softer layer, cracking of the harder layer, or non-uniform thickness distribution of the clad layers in the final component.
Deformation Analysis and Process Design
The deformation behavior during cold heading of clad bimetal components can be analyzed using finite element method (FEM) simulation, which allows the visualization of stress, strain, and velocity fields throughout the forging process. The FEM model must accurately represent the material properties of both metals, the bond interface condition, and the contact between the billet and the dies.
The key process design parameters that influence the deformation behavior include:
| Process Parameter | Effect on Deformation | Recommended Range |
|---|---|---|
| Preheat temperature | Reduces flow stress, improves ductility | 400-600 °C for cold heading |
| Die speed | Higher speed increases strain rate, raises flow stress | 1-5 m/s |
| Reduction ratio | Higher reduction increases deformation severity | 30-60% per pass |
| Die geometry | Controls material flow direction | Optimized for uniform flow |
| Lubrication | Reduces friction, improves surface quality | Graphite-based or oil-based lubricant |
The preheat temperature is particularly important for clad materials. Preheating reduces the flow stress of both metals, but the reduction is more significant for the metal with the higher yield strength. This helps to equalize the flow stress difference between the two metals and reduces the risk of differential deformation. However, preheating too high can cause excessive softening of the base metal, leading to loss of dimensional accuracy and surface quality.
The die geometry must be designed to promote uniform material flow across the clad interface. A common approach is to use a multi-stage die system where the first stage performs a large reduction at low speed, and subsequent stages perform incremental reductions at higher speeds. This staged approach allows the material to flow more uniformly and reduces the risk of localized deformation at the clad interface.
Bond Interface Behavior and Failure Modes
The bond interface between the two metals is the most critical region during cold heading. The interface must maintain its integrity throughout the deformation process, and any loss of bond strength will lead to delamination or separation of the clad layers. The bond strength can be affected by several factors:
- Thermal mismatch: If the clad material was produced by weld overlay, the residual stresses at the interface can be modified by the thermal cycle of cold heading. If the thermal cycle is severe enough, it can cause stress relaxation or even partial melting at the interface.
- Strain mismatch: The differential strain between the two metals creates shear stresses at the interface. If the shear stress exceeds the bond strength, delamination occurs.
- Intermetallic formation: In some clad systems, particularly those involving dissimilar metals such as steel-titanium or steel-copper, intermetallic phases can form at the interface during the heating stage of cold heading. These intermetallic phases are typically brittle and can reduce the bond strength.
The failure modes observed during cold heading of clad bimetal components include:
| Failure Mode | Description | Root Cause | Countermeasure |
|---|---|---|---|
| Delamination | Separation of clad layers | Excessive shear stress at interface | Reduce reduction ratio, optimize die geometry |
| Wrinkling | Buckling of softer layer | Compressive instability in soft layer | Increase preheat temperature, reduce die speed |
| Cracking | Fracture of harder layer | Excessive tensile stress in hard layer | Increase preheat temperature, use multi-stage forging |
| Non-uniform thickness | Uneven clad layer thickness | Differential flow of metals | Symmetric die design, controlled material flow |
| Interface cracking | Cracks at bond interface | Brittle intermetallic phases | Control heating temperature, avoid excessive heating |
Process Optimization and Quality Control
The optimization of cold heading parameters for clad bimetal components requires a systematic approach that considers the material properties, die geometry, and process parameters as an integrated system. The following optimization strategy is recommended:
- Material characterization: Determine the flow stress curves, strain hardening exponents, and fracture strains of both metals at the intended forging temperature.
- FEM simulation: Model the forging process with accurate material properties and bond interface conditions. Identify regions of high stress concentration and potential failure.
- Die design: Design the die geometry to promote uniform material flow and minimize stress concentration at the clad interface.
- Process parameter selection: Select preheat temperature, die speed, and reduction ratio to minimize differential deformation while achieving the required dimensional accuracy.
- Trial forging and inspection: Perform trial forgings and inspect the products for dimensional accuracy, surface quality, and bond integrity. Adjust parameters based on inspection results.
Quality control during cold heading of clad bimetal components includes:
- Dimensional inspection: Verify that the final dimensions are within tolerance, including the thickness of the clad layer.
- Bond integrity testing: Perform peel tests or shear tests to verify that the bond strength meets the specification.
- Metallographic examination: Examine cross-sections to verify that the clad layers are continuous, with no delamination, cracking, or intermetallic formation at the interface.
- Mechanical property testing: Test the tensile strength, hardness, and impact toughness of the forged component to verify that the mechanical properties meet the specification.
Study Insights and Engineering Implications
The research on deformation behavior during cold heading of clad bimetal components provides valuable insights into the fundamental challenges of cold forging dissimilar metal assemblies. The key insight is that the differential deformation behavior of the two metals is not merely a minor perturbation but a primary driver of the process design. The process parameters, die geometry, and material selection must all be optimized to accommodate the differential deformation, not simply adapted from the parameters used for monolithic materials.
This work also highlights the importance of computational modeling in modern manufacturing. FEM simulation allows the visualization of stress and strain fields that are impossible to measure experimentally, providing a powerful tool for process optimization. However, the accuracy of the simulation depends on the accuracy of the material properties and the bond interface model, which must be validated through experimental testing.
The engineering implications of this research extend beyond cold heading to other forming processes applied to clad materials, including hot rolling, extrusion, and deep drawing. The fundamental challenge—managing differential deformation across a bond interface—is common to all these processes, and the principles developed in this research can be applied to optimize them as well. The growing use of clad materials in aerospace, automotive, and energy applications makes this research increasingly relevant to modern manufacturing practice.