CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Preheat Temperature on Microstructure and Mechanical Properties of Copper MIG Welding Joints
Literature Overview
This study by Shi Xiaoyue, Tian Wei, and Chen Linling from Nantong Runbang Heavy Machinery Co., Ltd. (2023), published in Hot Working Technology, investigates the influence of preheat temperature on the microstructure and mechanical performance of pure copper MIG (GMAW) welding joints. Copper welding remains one of the most challenging joining operations in heavy machinery fabrication, particularly in power equipment and electrical contact manufacturing. The work addresses a practical problem faced by manufacturers dealing with large copper components where thermal cracking and porosity are persistent quality concerns.
Core Technical Content
The study systematically examines preheat temperatures ranging from room temperature to elevated levels, correlating the thermal input with resulting weld microstructure, hardness distribution, and tensile performance. Pure copper (Cu-ETP or equivalent grades) possesses exceptionally high thermal conductivity (approximately 399 W/m·K at 20°C), which causes rapid heat dissipation from the weld zone during MIG welding. This characteristic leads to incomplete penetration, excessive solidification rates, and susceptibility to hot cracking due to the wide solidification range and high surface tension of liquid copper.
Preheat Temperature Windows and Their Effects
| Preheat Temperature (°C) | Solidification Rate | Grain Structure | Hardness (HV) | Tensile Strength (MPa) | Crack Susceptibility |
|---|---|---|---|---|---|
| 0 (as-welded) | Very high | Fine equiaxed + columnar | 70–85 | 210–230 | High |
| 100 | High | Moderately fine | 60–75 | 220–240 | Moderate |
| 200 | Moderate | Coarse equiaxed | 50–65 | 230–250 | Low |
| 300 | Low | Very coarse | 45–55 | 225–245 | Very low |
| 400 | Very low | Excessive coarsening | 40–50 | 200–220 | Very low (but oxidation risk) |
Key Findings and Technical Interpretation
- Optimal preheat range: The study identifies approximately 200–300°C as the optimal preheat window for pure copper MIG welding, balancing crack suppression against excessive grain coarsening and oxidation.
- Microstructural evolution: Without preheat, rapid solidification produces a dendritic microstructure with interdendritic segregation of impurities, creating hot crack paths. Preheating reduces the cooling rate, allowing more uniform solidification and suppressing the formation of liquid films at grain boundaries.
- Oxidation concerns: At preheat temperatures exceeding 350°C, copper oxide (Cu₂O and CuO) formation accelerates significantly. These oxides are denser than copper and cannot float to the weld surface, becoming trapped as inclusions that degrade mechanical properties and cause spatter.
- Mechanical property optimization: Maximum tensile strength is achieved at moderate preheat (200–300°C), where the combination of crack-free weld metal and moderate grain size provides the best strength-ductility balance.
Engineering Practice Integration
In heavy machinery fabrication, copper welding is critical for electrical busbars, transformer windings, and specialized heat exchanger components. The practical implications of this study are significant:
- Welding procedure specification: Preheat at 200–300°C using induction heating or gas flame should be mandatory in WPS for thick copper sections (>10 mm).
- Flux protection: When preheat exceeds 250°C, active flux or controlled atmosphere protection becomes essential to prevent oxide inclusion formation.
- Post-weld treatment: Light annealing at 400–500°C after welding can relieve residual stresses without causing significant grain growth.
- Welding parameters: With preheat applied, welding current can be reduced by 10–15% while maintaining adequate penetration, further reducing thermal distortion.
Key Questions and Reflections
The study raises important questions for engineering practice. First, the transition from copper welding to copper-clad or copper-bimetal applications (such as copper-clad steel for electrical applications) introduces additional complexity—the thermal mismatch between copper and steel creates significant residual stresses that preheat alone cannot resolve. Second, the study focuses on pure copper; industrial applications often involve copper alloys (CuCrZr, CuBe, CuNi) where preheat requirements differ substantially. Third, the interaction between preheat temperature and shielding gas composition (Ar vs. He vs. Ar-He mixtures) remains an area requiring further investigation, as helium-rich mixtures increase arc temperature and may shift the optimal preheat window upward.
Study Insights and Implications
This research provides a clear, practical framework for copper MIG welding procedure development. The systematic approach of correlating preheat temperature with microstructural and mechanical outcomes is directly applicable to welding procedure qualification under standards such as ISO 15614-1 or AWS D10.6. For engineers involved in bimetal pressure vessel fabrication where copper-alloyed cladding layers are specified, understanding the fundamental role of preheat in copper welding becomes essential when developing overlay procedures for copper-containing alloys. The work reinforces the principle that in welding high-thermal-conductivity materials, thermal management through preheat is not merely a distortion control measure but a fundamental metallurgical requirement for crack-free weld integrity.