Raytron Technical Review RESEARCH ARTICLE WP-01-02

The Metallurgy of Clad Metals: Bond Formation Mechanisms

Q: Why does CCA need to control intermetallic compound thickness?

Intermetallic compounds (e.g., CuAl₂) have high hardness but high brittleness. Excessively thick IMC layer (>5μm) causes interface embrittlement and easy cracking under thermal cycling or mechanical stress. Controlling IMC thickness to <3μm ensures good ductility and fatigue performance.

Q: How to judge bond quality of clad metals?

Key indicators include: peel strength (>45MPa for CCA, >50MPa for CCS), interface resistance ( 95%). Assessment methods include peel testing, ultrasonic inspection, and metallographic analysis.

RAYTRON Technical Team¹ *

¹RAYTRON Group Technical Research Center, China

^*Corresponding author

Received: December 2025 Accepted: February 2026 Published: March 2026

DOI: 10.1234/raytron.2026.WP-01-02

Abstract

The performance of clad metal conductors fundamentally depends on the quality of the bond between constituent metals. This whitepaper investigates the metallurgical principles of bond formation in bimetallic conductors, including solid-state diffusion, mechanical interlocking, and intermetallic compound formation. We analyze process parameters affecting bond strength, introduce characterization techniques for interface evaluation, and provide guidelines for optimizing bond quality across different material combinations.

Key Findings

Three Bonding Mechanisms Synergize Solid-state diffusion, mechanical interlocking, and clean-surface metallic bonding contribute together to bond formation in clad metals. The relative contribution of each mechanism depends on material combination and processing conditions.
Temperature Control is Critical Optimal bonding temperature for CCA is 320-380°C. Excessive temperature (>450°C) leads to excessive intermetallic compound formation, while insufficient temperature results in inadequate bonding. Temperature window varies for each material combination.
Intermetallic Compounds Need Strict Control Cu-Al interface forms brittle intermetallic compounds such as CuAl₂ and Cu₉Al₄. IMC thickness should be controlled to <3μm to ensure good mechanical properties and long-term stability.
Ni-Cu System Most Stable Ni and Cu form a continuous solid solution without brittle intermetallic compounds, enabling the strongest metallurgical bond. NCC products exhibit excellent high-temperature stability and corrosion resistance.

Keywords

Clad Metallurgy;Metal Bonding;Solid-State Diffusion;Interface Phenomena;Bond Strength;Intermetallic Compounds;Bimetallic Interface

1. Introduction

1.1 Critical Role of Interface Quality

In bimetallic conductors, the interface between constituent metals is both the greatest challenge and the most critical factor determining product performance. High-quality bonding enables:

Efficient interlayer load transfer
Uniform current distribution across cross-section
Thermal stability during operation
Long-term reliability in service

Schematic showing current and stress transmission through bimetallic interface — **Fig. 1** Schematic of interface function showing current and stress transmission across the interface

1.2 Historical Development

Understanding of metal-metal bonding has significantly evolved:

Timeline showing evolution from ancient forge welding to modern solid-state bonding techniques — **Fig. 2** Timeline of metal bonding science development

2. Bond Formation Mechanisms

2.1 Overview of Bonding Mechanisms

Three primary mechanisms contribute to bond formation in clad metals:

Schematic showing solid-state diffusion, mechanical interlocking, and metallic bonding mechanisms — **Fig. 3** Schematic of three bonding mechanisms

Solid-state diffusion
Mechanical interlocking
Clean-surface metallic bonding

The relative contribution of each mechanism depends on material combination and processing conditions.

2.2 Solid-State Diffusion

Thermodynamic Basis:

Diffusion occurs to reduce the free energy of the system. The atomic flux across the interface follows Fick's first law:

J = -D (∂C/∂x)

(1)

Where:

J = Diffusion flux (atoms/m²·s)
D = Diffusion coefficient (m²/s)
∂C/∂x = Concentration gradient

Temperature Dependence:

The diffusion coefficient follows the Arrhenius relationship:

D = D₀ exp(-Q/RT)

(2)

2.3 Mechanical Interlocking

Mechanism Description:

At the microscopic scale, even polished metal surfaces exhibit roughness. When two surfaces are pressed together and deformed, asperities interlock:

Key Factors:

Surface roughness: Initial asperity height distribution
Deformation pressure: Contact stress magnitude
Material ductility: Ability to fill voids
Temperature: Softening for better conformity

Quantitative Model:

The ratio of actual contact area Aᵣ to apparent area Aₐ:

Aᵣ/Aₐ = P/H

(3)

Where P = applied pressure, H = hardness of the softer material.

2.4 Clean-Surface Metallic Bonding

Theoretical Basis:

When atomically clean metal surfaces contact, metallic bonds can form directly:

Schematic showing electron cloud overlap forming metallic bonds when two metal surfaces contact — **Fig. 4** Atomic-scale interface bonding schematic showing electron cloud overlap

Barrier: Oxide Films

In practice, oxide films prevent direct metal contact:

Aluminum oxide (Al₂O₃): 2-10 nm thick, hard, insulating
Copper oxide (Cu₂O, CuO): <2 nm, semiconducting
Nickel oxide (NiO): <1 nm, insulating

3. Interface Characterization

3.1 Microscopy Techniques

SEM image clearly showing diffusion layer and microstructure at copper-clad aluminum interface — **Fig. 5** SEM image of CCA interface with diffusion layer annotation

3.2 Chemical Analysis

EDS (Energy Dispersive Spectroscopy):

EDS line scan showing copper and aluminum concentration gradient variation across CCA interface — **Fig. 6** EDS line scan results showing Cu-Al concentration gradient

3.3 Mechanical Testing

Peel Testing:

Standard method for quantifying bond strength:

σ_peel = F/b

(4)

Where F = peel force, b = specimen width.

Peel test setup schematic and typical test result data curves — **Fig. 7** Peel test setup and typical results

4. Material-Specific Considerations

4.1 Copper-Aluminum System (CCA)

Phase Diagram Analysis:

The Cu-Al system contains multiple intermetallic phases:

Cu-Al binary phase diagram with key phase regions including intermetallic compound phases — **Fig. 8** Cu-Al phase diagram with key phase regions annotated

Formation Kinetics:

Intermetallic compound thickness x follows parabolic growth:

x² = kt

(5)

Where k = growth constant, t = time.

Optimization Strategies:

Minimize high-temperature dwell time
Control interface temperature during drawing
Target intermetallic compound thickness <3 μm

4.2 Copper-Steel System (CCS)

Diffusion Characteristics:

The Cu-Fe system has limited mutual solubility:

Simplified Cu-Fe binary phase diagram showing limited solubility regions — **Fig. 9** Simplified Cu-Fe phase diagram

Key Points:

Maximum solubility of Cu in Fe: ~8% (at 850°C)
Maximum solubility of Fe in Cu: ~4% (at 1100°C)
No significant intermetallic compound formation below 600°C

Bonding Mechanism: Primarily mechanical interlocking with minimal diffusion contribution; strong bonds achievable through cold welding

4.3 Nickel-Copper System (NCC)

Complete Solid Solution:

Ni and Cu form a continuous solid solution:

Ni-Cu binary phase diagram showing nickel and copper form continuous solid solution without intermetallic compounds — **Fig. 10** Ni-Cu phase diagram showing continuous solid solution

Significance:

Excellent diffusion compatibility
Gradual concentration gradient
No brittle intermetallic compounds
Strong metallurgical bonding achievable

4.4 Silver-Copper System (SCC)

Eutectic System:

The Ag-Cu system has a eutectic point at 780°C:

Ag-Cu binary phase diagram showing eutectic point and solid solution regions — **Fig. 11** Ag-Cu phase diagram

Processing Considerations:

Below 780°C: Limited diffusion
Low-temperature bonding is primarily mechanical
Higher temperatures enable diffusion bonding

5. Process Parameters

5.1 Temperature Effects

Temperature Window:

5.2 Pressure and Deformation

Clad Welding Process:

5.3 Time Considerations

Diffusion Time:

High-temperature dwell time affects intermetallic compound growth:

t_optimal = x_target²/k(T)

(6)

Practical Guidelines:

Minimize Cu-Al time above 300°C
Allow sufficient stress relief time during annealing
Balance diffusion requirements with IMC growth

5.4 Atmosphere Control

6. Quality Assessment

6.1 Non-Destructive Testing

Ultrasonic Testing:

Detection Capabilities:

Delamination >0.5 mm
Porosity >10% interface area
Inclusions >50 μm

Eddy Current Testing:

Effective detection: interface gaps, cladding thickness variation, localized bonding defects

6.2 Destructive Testing Protocols

Standard Test Sequence:

Standard destructive testing flowchart including peel testing, bend testing, metallographic analysis steps — **Fig. 15** Destructive testing flowchart

6.3 Statistical Process Control

Key Metrics:

Statistical process control (SPC) chart example showing peel strength monitoring data and control limits — **Fig. 16** SPC control chart example

7. Failure Modes

7.1 Interface-Related Failures

7.2 Failure Analysis Case Studies

Case Study 1: CCA Thermal Cycling Failure

Application: Automotive engine compartment wiring harness
Failure: Interface delamination after 500 thermal cycles
Root Cause: Intermetallic compound layer >5 μm causing brittleness
Solution: Reduce processing temperature, shorten annealing time

Case Study 2: CCS Grounding Conductor Failure

Application: Substation grounding
Failure: Interface corrosion after 10 years
Root Cause: Cladding damage during installation exposed steel core
Solution: Improve installation procedures, add protective coating

8. Optimization Strategies

8.1 Process Optimization Framework

Framework diagram for clad metal bond quality optimization including process parameters, material selection, and quality control systematic approach — **Fig. 17** Optimization strategy framework diagram

Design of Experiments (DOE) Approach:

8.2 Material Selection Guidelines

8.3 Advanced Bonding Technologies

Emerging Technologies:

Ultrasonic-assisted cladding: Enhanced bonding at low temperatures
Electromagnetic pulse bonding: Rapid solid-state bonding
Laser surface treatment: Controlled oxide removal
Nanostructured interfaces: Engineered interface properties

9. Conclusions

9.1 Key Findings

Bond quality is critical，determining bimetallic conductor performance
Three mechanisms contribute to bond formation: diffusion, interlocking, and metallic bonding
Temperature control is critical for managing intermetallic compound formation
Characterization methods enable quality verification and optimization
Material-specific considerations require tailored processing parameters

9.2 Recommendations

For Production: Control temperature window, optimize pressure profile, monitor atmosphere

For Quality: Implement SPC, standardize testing, track failure modes

For R&D: Study new material combinations, develop advanced characterization, model interface behavior

Frequently Asked Questions

What is metallurgical bonding? How does it differ from mechanical bonding?

Metallurgical bonding is a permanent bond formed through atomic diffusion, with bond strength exceeding 150MPa and inseparable interface. Mechanical bonding relies only on surface roughness interlocking, typically <50MPa strength, and may delaminate under thermal cycling or mechanical stress. Clad welding process produces metallurgical bonding.

Why does CCA need to control intermetallic compound thickness?

Intermetallic compounds (e.g., CuAl₂) have high hardness but high brittleness. Excessively thick IMC layer (>5μm) causes interface embrittlement and easy cracking under thermal cycling or mechanical stress. Controlling IMC thickness to <3μm ensures good ductility and fatigue performance.

How to judge bond quality of clad metals?

Key indicators include: peel strength (>45MPa for CCA, >50MPa for CCS), interface resistance (<0.5μΩ·cm²), IMC thickness (<3μm), and bond coverage (>95%). Assessment methods include peel testing, ultrasonic inspection, and metallographic analysis.

How do bonding difficulties differ among material combinations?

Ni-Cu is easiest to bond (complete solid solution, no IMC); Cu-steel has moderate difficulty (limited mutual solubility, mainly mechanical bonding); Cu-Al is most challenging (forms multiple IMCs, requires strict temperature control); Ag-Cu needs to avoid eutectic temperature (780°C).

Figures

**Fig. 1** Schematic of interface function showing current and stress transmission across the interface

**Fig. 2** Timeline of metal bonding science development

**Fig. 3** Schematic of three bonding mechanisms

**Fig. 4** Atomic-scale interface bonding schematic showing electron cloud overlap

**Fig. 5** SEM image of CCA interface with diffusion layer annotation

**Fig. 6** EDS line scan results showing Cu-Al concentration gradient

**Fig. 7** Peel test setup and typical results

**Fig. 8** Cu-Al phase diagram with key phase regions annotated

**Fig. 9** Simplified Cu-Fe phase diagram

**Fig. 10** Ni-Cu phase diagram showing continuous solid solution

**Fig. 12** Temperature effect on bond quality curve

Fig. 13 Clad welding process pressure profile

**Fig. 14** Ultrasonic testing principle and result images

**Fig. 15** Destructive testing flowchart

**Fig. 17** Optimization strategy framework diagram

Tables

Table 1 Effect of bond quality on performance

Bond Quality	Conductivity Efficiency	Tensile Strength	Fatigue Life
Excellent (>50 MPa)	95-100%	Optimal	Maximum
Good (30-50 MPa)	90-95%	Near optimal	Good
Fair (20-30 MPa)	80-90%	Reduced	Moderate
Poor (<20 MPa)	<80%	Compromised	Limited

Table 2 Diffusion parameters for common metal pairs

System	D₀ (m²/s)	Q (kJ/mol)	Temp at D=10⁻¹⁴ m²/s
Cu → Al	6.5 × 10⁻⁵	136	345°C
Al → Cu	1.7 × 10⁻⁴	157	385°C
Ni → Cu	2.7 × 10⁻⁵	236	520°C
Cu → Ni	2.3 × 10⁻⁴	228	485°C
Cu → Fe	3.0 × 10⁻⁴	240	530°C

Table 3 Interface characterization methods

Technique	Resolution	Information Obtained	Typical Application
Optical Microscopy	0.5 μm	Layer thickness, defects	Routine inspection
SEM	10 nm	Interface morphology	Quality control
TEM	0.1 nm	Atomic structure	Research
AFM	0.01 nm	Surface topography	Research

Table 4 Typical diffusion zone widths

Material Pair	Processing Temp	Time	Diffusion Zone Width
Cu/Al	350°C	1 hour	2-5 μm
Cu/Al	400°C	1 hour	5-10 μm
Ni/Cu	400°C	1 hour	<1 μm
Cu/Steel	500°C	1 hour	<0.5 μm

Table 5 Typical peel strength by material

Material	Peel Strength (MPa)	Failure Mode
CCA (Well bonded)	45-65	Al cohesive failure
CCA (Poorly bonded)	15-30	Interface failure
CCS	50-70	Steel cohesive failure
NCC	40-55	Interface or cohesive

Table 6 Cu-Al intermetallic phases

Phase	Composition	Crystal Structure	Hardness (HV)	Brittleness
θ	CuAl₂	Tetragonal	350-400	High
η₂	CuAl	Orthorhombic	400-450	Very high
γ₂	Cu₉Al₄	Cubic	300-350	Medium
γ₁	Cu₈Al₃	Cubic	250-300	Medium

Table 7 Intermetallic compound growth rates

Temperature	k (μm²/h)	Time to form 5 μm IMC
300°C	0.5	50 hours
350°C	2.0	12.5 hours
400°C	8.0	3 hours
450°C	30.0	1 hour

Table 8 Ni-Cu interface characteristics

Temperature	Time	Bond Strength	Failure Mode
350°C	1h	35-45 MPa	Interface
400°C	1h	45-55 MPa	Mixed
450°C	1h	55-65 MPa	Cohesive

Table 9 Optimal temperature ranges

Material	Min Bonding Temp	Optimal Range	Avoid Exceeding
Cu/Al (CCA)	250°C	320-380°C	450°C (Excess IMC)
Cu/Steel (CCS)	200°C	300-400°C	600°C
Ni/Cu (NCC)	300°C	350-450°C	600°C
Ag/Cu (SCC)	200°C	300-400°C	780°C (Melting)

Table 10 Atmosphere requirements

Material	Atmosphere	Purpose
CCA	Inert (N₂, Ar)	Prevent oxidation
CCS	Inert or reducing	Remove surface oxides
NCC	Inert	Maintain Ni surface quality
SCC	Inert	Prevent Ag tarnishing

Table 11 Common failure modes

Failure Mode	Cause	Appearance	Prevention
Delamination	Weak bonding	Layer separation	Optimize bonding parameters
Brittle fracture	Excess IMC	Clean fracture	Control time/temperature
Corrosion	Galvanic effect	Preferential attack	Correct material selection
Fatigue cracking	Cyclic loading	Interface cracks	Reduce stress concentration

Table 12 Bond quality material selection

Priority	Cu/Al	Cu/Steel	Ni/Cu	Ag/Cu
Highest bond strength	Medium	High	Very high	Medium
Process simplicity	Medium	High	High	Medium
Temperature stability	Medium	High	Very high	Medium
Cost effectiveness	High	Very high	Medium	Low

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Authoritative References

ASTM International IEC (International Electrotechnical Commission) ISO (International Organization for Standardization) SAE International IEEE