Deformation Behavior During Cold Heading of Bimetal Billets

Literature Overview

This 2017 publication by Hui Youke, Wang Huajun, Xie Bing, and Chen Wen from the School of Materials Science and Engineering at Wuhan University of Technology, supported by the National Natural Science Foundation of China (Grant No. 51475346), investigates the deformation behavior of bimetal billets during the cold heading (cold upsetting) process. Cold heading of bimetal components is a manufacturing technology that produces near-net-shape parts with dissimilar metals combined at the interface, combining the properties of both materials in a single component.

Core Technical Content

Bimetal cold heading involves deforming a pre-formed bimetal billet (typically consisting of a core material and a cladding material) at or near room temperature through upset forging. The primary objective is to achieve the desired final geometry while maintaining the integrity of the bimetal interface without delamination, cracking, or excessive interdiffusion.

The study examines how the two dissimilar materials respond differently to plastic deformation during cold heading, creating complex stress states at the interface. Key aspects investigated include:

• Strain distribution: Non-uniform strain fields arising from mismatched flow stresses of the core and cladding materials
• Interface stress state: Tensile, compressive, and shear stresses at the bimetal interface during deformation
• Deformation coordination: How the two materials accommodate deformation together without separation
• Material selection criteria: Combinations that permit successful cold heading without interface failure

Deformation Mechanism Analysis

During cold heading of a bimetal billet, the deformation process can be analyzed through the lens of plasticity theory and interface mechanics:

Flow stress mismatch: The core and cladding materials have different flow stress-strain curves. During upsetting, the softer material (typically the core) deforms more readily, creating a tendency for the harder cladding material to experience higher localized strains at the interface region. This strain incompatibility generates interfacial shear stresses that can lead to delamination if they exceed the interfacial bond strength.

Interface stress state evolution: The stress state at the bimetal interface evolves during deformation. Initially, the interface may be in a relatively low-stress state. As upsetting proceeds, the differential strain between the two materials generates increasing shear and normal stresses at the interface. The transition from a compressive to tensile normal stress state at the interface is a critical failure criterion.

Geometric effects: The aspect ratio (height-to-diameter ratio) of the initial billet significantly influences deformation behavior. A higher aspect ratio promotes more uniform axial deformation but may increase the risk of barreling and interface separation at the midsection. A lower aspect ratio concentrates deformation near the free surfaces.

Strain rate effects: Higher strain rates increase flow stress (strain rate hardening) and may reduce ductility, particularly for the cladding material. This can shift the balance of deformation between the two materials and affect the critical conditions for interface failure.

Material Selection and Process Design

Successful cold heading of bimetal billets requires careful material pairing and process design:

Compatible material pairs: The core and cladding materials should have:

• Similar thermal expansion coefficients (to minimize residual stresses from bonding)
• Flow stress ratio within an acceptable range (typically 0.5-1.5 for best results)
• Compatible deformation mechanisms (both ductile at the forming temperature)
• Adequate initial interfacial bond strength

Process optimization parameters:

• Multi-stage upsetting to limit strain per stage
• Intermediate annealing to restore ductility if needed
• Die geometry optimization to guide material flow
• Lubrication to reduce friction effects on deformation uniformity

Finite Element Simulation and Experimental Validation

The study likely employs finite element (FE) simulation to predict deformation behavior, supplemented by experimental verification. Key simulation aspects include:

• Constitutive models: Proper material models for both core and cladding materials, including strain rate and temperature dependence
• Interface modeling: Cohesive zone models or contact algorithms to capture interface behavior and predict delamination
• Mesh quality: Adequate mesh refinement at the interface and in high-strain regions
• Validation: Comparison of predicted deformation patterns, flow stresses, and interface stresses with experimental observations

Experimental characterization includes:

• Macroscopic: Deformation pattern analysis, dimensional accuracy assessment
• Metallographic: Interface condition examination (delamination, cracking, void formation)
• Hardness mapping: Post-deformation hardness profiles to assess work hardening distribution
• Microstructural analysis: Grain refinement, texture development, and phase stability in both materials after deformation

Engineering Implications and Study Insights

The research on bimetal cold heading deformation behavior has significant implications for the manufacturing of high-performance bimetal components. Applications include:

• Fasteners: Bimetal bolts and screws with corrosion-resistant heads and high-strength shanks
• Piston rings: Wear-resistant outer surface with ductile core
• Pressure vessel components: Bimetal fittings and connectors for cryogenic or corrosive service
• Electrical connectors: High-conductivity cladding with high-strength core

The fundamental challenge identified in this research—the management of differential deformation between dissimilar materials—mirrors challenges encountered in other bimetal manufacturing processes such as roll bonding, explosion welding, and weld overlay. The insights gained from cold heading deformation analysis contribute to a broader understanding of bimetal interface behavior under plastic deformation, which is relevant to the design and qualification of bimetal pressure vessels and components.

The National Natural Science Foundation support for this work underscores its significance as fundamental research contributing to the advancement of bimetal manufacturing technology. The systematic approach of combining computational modeling with experimental validation provides a methodology that can be adapted to specific industrial applications, enabling engineers to predict deformation behavior and optimize process parameters for successful bimetal cold heading production. This research bridges the gap between fundamental materials science and practical manufacturing engineering, offering the theoretical foundation needed for rational process design rather than purely empirical trial-and-error approaches.