CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Mechanical Properties of TIG Cladding Copper Joint on Q235 Steel Plate Surface
Literature Overview
This study investigates the microstructure and mechanical properties of a TIG (gas tungsten arc welding) cladding copper joint on the surface of Q235 carbon steel plate. Copper cladding is widely used in electrical applications, heat exchangers, and corrosion-resistant linings where the combination of the strength of carbon steel and the conductivity or corrosion resistance of copper is required. The TIG process is chosen for its low heat input and precise control, which minimizes dilution and distortion. This topic is directly relevant to engineers designing bimetallic components for electrical contact applications, heat exchanger tubes, and corrosion-resistant linings in chemical processing equipment.
Core Technical Content and Process Parameters
The TIG cladding of copper on Q235 steel is a challenging process due to the significant differences in thermal conductivity, melting point, and thermal expansion coefficient between copper and steel. Copper has a thermal conductivity approximately 5 times that of carbon steel, which leads to asymmetric heat flow and potential lack of fusion at the interface. The study examined the effect of welding parameters on the joint quality and properties.
The following table presents the welding parameters investigated:
| Parameter | Range | Optimal Value |
|---|---|---|
| Welding current | 80–150 A | 100–120 A |
| Arc voltage | 14–20 V | 16–18 V |
| Welding speed | 5–15 mm/s | 8–12 mm/s |
| Shielding gas | Argon | 100% Ar |
| Gas flow rate | 10–20 L/min | 15 L/min |
| Filler wire | Pure copper or Cu-Si | Pure copper |
| Wire diameter | 1.6–2.4 mm | 2.0 mm |
| Travel angle | 70–85° | 75° |
| Stickout length | 8–15 mm | 10 mm |
The optimal parameters were determined by balancing penetration depth, dilution ratio, and joint quality. Higher currents and lower speeds increased penetration but also increased dilution and distortion. The optimal window provided sufficient penetration for bonding while maintaining a dilution ratio below 15%.
Microstructure Analysis
The TIG copper cladding joint on Q235 steel exhibits a complex microstructure with distinct zones: the copper cladding layer, the dilution zone (also called the transition zone or bonding zone), and the steel base metal. The dilution zone is the critical region where the copper and steel interact metallurgically, and its composition and microstructure determine the bonding strength and joint integrity.
The copper cladding layer itself is relatively homogeneous, consisting of equiaxed grains with an average size of 50–100 micrometers. The grain size increases with increasing heat input, as higher temperatures promote grain growth during solidification and subsequent cooling. The presence of residual stresses in the copper layer is significant due to the thermal mismatch between copper and steel during cooling.
The dilution zone is the most complex region, containing a mixture of copper and iron in varying proportions. The composition gradient extends from nearly pure copper at the cladding surface to nearly pure iron at the base metal, with an intermediate zone containing a copper-iron solid solution and intermetallic compounds such as Cu₄Fe, Cu₆Fe, and CuFe₂. These intermetallic phases are brittle and can act as crack initiation sites if present in excessive amounts.
The study found that the dilution ratio (defined as the fraction of base metal in the dilution zone) was approximately 10–20% at the optimal welding parameters. This dilution ratio was low enough to prevent excessive intermetallic formation while providing sufficient metallurgical bonding. Higher dilution ratios (above 25%) led to the formation of continuous intermetallic networks along grain boundaries, which severely reduced the ductility and fracture resistance of the joint.
Mechanical Properties and Bonding Strength
The mechanical properties of the TIG copper cladding joint were evaluated through tensile testing, shear testing, and hardness profiling. The results are summarized below:
| Test | Result |
|---|---|
| Shear strength of cladding layer | 180–220 MPa |
| Peel strength | 150–180 MPa |
| Hardness of copper layer (HV) | 80–100 |
| Hardness of dilution zone (HV) | 150–200 |
| Hardness of base metal (HV) | 120–140 |
| Dilution ratio (%) | 10–20% |
The shear strength of the cladding layer was in the range of 180–220 MPa, which is acceptable for most engineering applications. The hardness of the dilution zone was higher than both the copper layer and the base metal due to the presence of intermetallic phases and solid solution strengthening. However, the increased hardness was accompanied by reduced ductility, as evidenced by the brittle fracture mode observed in the dilution zone during tensile testing.
The bonding strength between the copper cladding and the steel base was found to be limited by the dilution zone, which was the weakest link in the joint. The fracture consistently initiated in the dilution zone and propagated along the interface between the intermetallic-rich region and the copper matrix. This indicates that the dilution zone composition and microstructure are the critical factors controlling the joint strength and durability.
Defect Analysis and Countermeasures
The following table summarizes the common defects in TIG copper cladding on steel and their countermeasures:
| Defect | Cause | Countermeasure |
|---|---|---|
| Lack of fusion | Insufficient heat input, poor wetting | Increase current, preheat base metal |
| Cracking in dilution zone | Excessive intermetallic formation | Control dilution ratio below 20% |
| Porosity | Gas entrapment, oxide inclusion | Improve shielding, clean surfaces |
| Excessive distortion | Thermal mismatch | Reduce heat input, use backing plate |
| Cold cracking | Hydrogen pickup, high restraint | Low-hydrogen process, post-weld bake |
The most critical defect is cracking in the dilution zone, which is caused by the formation of brittle intermetallic compounds. The countermeasure is to control the dilution ratio by adjusting the welding parameters and, if necessary, using a multi-pass approach where the first pass is a low-dilution transition layer and subsequent passes build up the copper cladding thickness.
Engineering Practice Integration
In the context of pressure vessel and heat exchanger fabrication, copper cladding is used for applications requiring both structural strength and corrosion resistance or electrical conductivity. For example, copper-clad steel tubes are used in condensers and coolers where the copper provides corrosion resistance to seawater while the steel provides structural strength. The TIG cladding process is particularly suitable for small-diameter tubes and thin-wall components where the low heat input minimizes distortion.
The following table compares the TIG cladding process with other cladding methods for copper on steel:
| Process | Dilution Control | Thickness Range | Surface Quality | Cost |
|---|---|---|---|---|
| TIG cladding | Good | 0.5–5 mm | Excellent | Moderate |
| GMAW cladding | Moderate | 1–10 mm | Good | Low |
| PTA cladding | Excellent | 0.5–3 mm | Excellent | High |
| Explosive cladding | None | 1–10 mm | Good | High |
| Roll bonding | None | 0.5–5 mm | Excellent | Moderate |
The TIG process offers a good balance of dilution control, surface quality, and cost for copper cladding applications. However, for thick cladding layers (above 5 mm), PTA or GMAW processes may be more efficient.
Key Reflections and Study Insights
This study underscores the importance of dilution control in dissimilar metal cladding applications. The dilution zone between copper and steel is a region of metallurgical complexity where the properties are highly sensitive to composition and microstructure. Engineers must carefully control the welding parameters to maintain the dilution ratio within the optimal window, and must be aware that the dilution zone is the critical region governing the joint strength and durability.
A significant insight is that the TIG process, while offering excellent control over heat input, is still limited by the fundamental metallurgical incompatibility between copper and iron. The formation of intermetallic compounds is inevitable to some extent, and the goal is not to eliminate them but to control their morphology and distribution to minimize their detrimental effect on mechanical properties.
Summary
The TIG cladding of copper on Q235 steel is a viable process for producing bimetallic components with acceptable bonding strength and surface quality. The key to success lies in controlling the dilution ratio below 20% to prevent excessive intermetallic formation, using adequate shielding to prevent oxidation and porosity, and managing the thermal input to minimize distortion. Engineers should perform thorough non-destructive testing and mechanical testing of the joints to ensure that the cladding meets the required performance criteria, and should consider the long-term effects of thermal cycling and corrosion on the dilution zone, which is the most vulnerable region in the joint.