CLADDING TECHNOLOGY SHANXI CO., LTDTIG Cladding of Copper on Q235 Steel Plate Microstructure and Mechanical Properties
Literature Overview
This paper, published in the Journal of Aeronautical Materials in 2014 by researchers from Nanchang University's Key Laboratory of Robotics and Welding Automation, investigates the microstructural evolution and mechanical behavior of copper weld-overlay joints produced on Q235 carbon steel substrates using gas tungsten arc welding (GTAW / TIG). The work was supported by the Jiangxi Provincial Youth Scientists Funding Project (2010DQ01000) and the Jiangxi Provincial Department of Education Science and Technology Project (GJJ13064). The study is particularly relevant to engineers dealing with bimetallic components where electrical conductivity, corrosion resistance, or thermal management is required on low-cost carbon steel substrates.
Core Technical Content
The fundamental challenge in copper-on-steel TIG cladding lies in the large difference in thermal expansion coefficients and melting points between copper (melting point approximately 1085 °C, thermal expansion coefficient approximately 17 × 10⁻⁶ /K) and Q235 steel (melting point approximately 1510 °C, thermal expansion coefficient approximately 12 × 10⁻⁶ /K). This mismatch leads to significant residual stresses and potential intermetallic compound (IMC) formation at the interface, which can severely degrade bond strength and ductility.
The authors employed a systematic approach to optimize the TIG cladding parameters, including welding current, travel speed, arc voltage, and filler wire feed rate. The microstructural analysis revealed that the interface between the copper overlay and the Q235 steel substrate exhibited a distinct diffusion zone characterized by the formation of intermetallic phases such as CuFe, CuFe₂, and Cu₂Fe. The thickness of this IMC layer was found to be strongly dependent on the heat input and welding speed.
Key Microstructural Observations
| Zone | Microstructural Features | Typical Thickness |
|---|---|---|
| Copper overlay | Recrystallized equiaxed grains, some dendritic structure near interface | Varies with pass number |
| Diffusion/IMC layer | CuFe, CuFe₂, Cu₂Fe intermetallic compounds | 10–50 μm (optimized) |
| Heat-affected zone (HAZ) | Ferrite-pearlite with some grain growth | 100–300 μm |
| Base metal (Q235) | Ferrite-pearlite, unchanged microstructure | — |
The study demonstrated that excessive heat input leads to over-diffusion and thickening of the brittle IMC layer, which acts as a crack initiation site during mechanical loading. Conversely, insufficient heat input results in incomplete bonding and porosity at the interface.
Process Parameter Optimization
The research identified optimal TIG cladding parameter windows that balance bonding quality with minimal IMC formation:
| Parameter | Optimized Range | Effect of Deviation |
|---|---|---|
| Welding current | 120–180 A | Too low: incomplete melt; Too high: thick IMC layer |
| Travel speed | 150–250 mm/min | Too slow: excessive heat input; Too fast: poor penetration |
| Arc voltage | 12–16 V | Correlates with arc stability and heat input |
| Filler wire diameter | 1.6–2.4 mm | Affects dilution ratio and deposition efficiency |
| Preheating temperature | 150–250 °C | Reduces thermal gradient and residual stress |
The dilution ratio between the copper filler and the Q235 base metal was found to be a critical factor. A dilution ratio of approximately 30–40% was considered acceptable for maintaining good copper properties while ensuring adequate bond strength. Higher dilution ratios resulted in the copper layer losing its characteristic electrical and corrosion resistance properties.
Mechanical Property Analysis
The mechanical characterization included tensile tests, microhardness profiling across the joint, and bond strength measurements. The results showed that:
- The microhardness of the copper overlay decreased from approximately 70 HV at the surface to approximately 120 HV near the interface due to dilution with steel.
- The IMC layer exhibited the highest microhardness values, reaching 200–300 HV, confirming its brittle nature.
- The tensile strength of the optimized joints reached 300–400 MPa, with fracture typically occurring in the base metal or near the HAZ rather than at the interface.
- The elongation was significantly lower than that of the base metal, indicating reduced ductility due to the presence of the brittle IMC layer.
Defect Analysis and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Cracking at interface | Thick IMC layer, high residual stress | Reduce heat input, preheat substrate, use multiple thin passes |
| Porosity | Hydrogen absorption from flux contamination | Clean filler wire, use dry shielding gas, preheat |
| Incomplete bonding | Insufficient penetration, low current | Increase current, improve arc stability |
| Excessive dilution | High heat input, slow travel speed | Increase travel speed, reduce current, use smaller filler wire |
| Surface oxidation | Inadequate shielding gas coverage | Increase gas flow rate, use trailing shield |
Integration with Engineering Practice
In practical bimetal product manufacturing, copper cladding on carbon steel is commonly used for electrical busbars, heat exchanger tubes, and corrosion-resistant linings in chemical processing equipment. The findings from this study have direct implications for process qualification under standards such as NB/T 47014 and ASME IX. Engineers must pay particular attention to:
- Bond strength verification: The peel test or bond tensile test should be performed to ensure the interface can withstand operational stresses.
- Intermetallic compound control: Metallographic examination of cross-sections is essential to verify that the IMC layer remains within acceptable thickness limits.
- Residual stress management: Stress-relief annealing at 400–500 °C for 2 hours may be necessary to reduce residual stresses that could lead to delayed cracking.
- Non-destructive testing: Ultrasonic testing (UT) and magnetic particle testing (MT) should be applied to detect subsurface cracks and lack of fusion at the interface.
Key Questions and Reflections
One of the most thought-provoking aspects of this research is the fundamental trade-off between bonding quality and property retention. The more thoroughly the copper and steel are metallurgically bonded, the more extensive the intermetallic compound formation becomes, which inherently degrades the ductility and corrosion resistance of the copper layer. This is a classic engineering compromise that requires careful process design.
Another important consideration is the effect of multi-pass cladding. The study suggests that using multiple thin passes rather than a single thick deposit can help control the IMC layer thickness by limiting the thermal cycle at each pass. However, this increases production time and cost, which must be balanced against the criticality of the application.
Study Insights and Implications
The research by Yu Yexiao and colleagues provides valuable quantitative data for engineers designing copper-on-steel bimetallic components. The identification of optimal parameter windows and the systematic characterization of interface microstructures offer a solid foundation for process development and qualification. For pressure vessel engineers working with clad-plate components where copper or copper alloys are specified for corrosion resistance, this study reinforces the importance of controlling heat input and understanding the metallurgical interactions at dissimilar metal interfaces. The work also highlights the need for thorough non-destructive testing protocols to ensure the integrity of the bond interface throughout the service life of the component.