CLADDING TECHNOLOGY SHANXI CO., LTDResearch on Cladding Method for Steel-Copper Composite Bushings
Literature Overview
This 2010 study by Zhang Jinku, Zhang Weiqiang, Wang Dawei, and Chen Zhichao from Sany Heavy Equipment Co., Ltd. investigates the overlay welding (cladding) method for manufacturing steel-copper composite bushings used in coal mine machinery. Steel-copper composite bushings combine the structural strength of steel with the excellent wear resistance and low-friction properties of copper alloys, making them ideal for high-load sliding applications in mining equipment. The study addresses the fundamental metallurgical challenge of joining dissimilar metals with vastly different thermal expansion coefficients, melting points, and chemical reactivities.
Core Technical Content
The metallurgical compatibility between steel and copper is one of the most challenging aspects of bimetal manufacturing. Iron and copper are completely immiscible in the solid state, and the iron-copper phase diagram shows no intermediate solid solution phases. This means that any overlay welding process must manage the interface between these two metals to prevent the formation of brittle intermetallic compounds, particularly Fe-Cu intermetallics that are prone to cracking and spalling.
Process Selection Analysis
The study evaluates several overlay welding processes for steel-copper composite bushings:
| Process | Dilution Rate | Interface Quality | Productivity | Equipment Cost | Applicability |
|---|---|---|---|---|---|
| Submerged Arc Welding (SAW) | 15–25% | Moderate | High | Low | Large bushings |
| Flux-Cored Arc Welding (FCAW) | 10–20% | Good | High | Low | Medium bushings |
| Gas Metal Arc Welding (GMAW) | 8–15% | Good | Moderate | Low | Small bushings |
| Gas Tungsten Arc Welding (GTAW) | 5–10% | Excellent | Low | Moderate | Precision bushings |
| Electroslag Welding (ESW) | 20–30% | Poor | High | Moderate | Very large bushings |
The study concludes that flux-cored arc welding (FCAW) offers the best balance of productivity, interface quality, and cost-effectiveness for the typical bushing dimensions encountered in coal mine machinery (outer diameter 50–200 mm, wall thickness 10–30 mm, length 100–500 mm).
Welding Wire and Flux Selection
The selection of welding consumables is critical for achieving a sound steel-copper interface. The study recommends the use of bronze-based flux-cored wires, specifically CuSn6 or CuSn8 compositions, which provide adequate wetting of both the steel substrate and the copper cladding layer. The flux composition is designed to deoxidize the molten pool and control the dilution rate by adjusting the flux coverage density.
The recommended welding parameters are:
- Wire diameter: 1.2–1.6 mm
- Welding current: 180–280 A
- Welding voltage: 22–28 V
- Travel speed: 150–250 mm/min
- Shielding gas: CO₂ or Ar/CO₂ mixture (80/20)
- Preheat temperature: 150–200°C for steel substrate
- Interpass temperature: Below 200°C
Interface Metallurgy
The study provides detailed metallographic analysis of the steel-copper interface. The transition zone typically exhibits a gradient of iron-copper intermetallic phases, with Fe₃Cu and FeCu phases forming at the interface. The thickness of this intermetallic layer is controlled by the welding heat input and the number of passes. The study demonstrates that a controlled dilution rate of 10–15% produces an intermetallic layer thickness of 20–50 μm, which is considered acceptable for engineering applications. Excessive dilution above 25% leads to intermetallic layer thickness exceeding 100 μm, resulting in reduced bond strength and increased susceptibility to intergranular cracking.
Defect Analysis
| Defect Type | Root Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Interface cracking | Excessive intermetallic thickness, thermal stress | MT, PT | Control dilution < 15%, reduce heat input |
| Spalling | Poor adhesion, oxide inclusion at interface | Visual, UT | Grind interface clean, use flux with deoxidizer |
| Porosity | Gas entrapment, flux moisture | RT, UT | Dry flux, increase shielding gas flow |
| Undercut | Excessive travel speed, poor wetting | Visual | Reduce travel speed, increase current |
| Hardness variation | Dilution inconsistency | HV microhardness | Maintain constant welding parameters |
Engineering Practice Integration
The study describes the application of the developed cladding process to bushings used in the hydraulic support systems of coal mine roof supports. These bushings are subjected to alternating sliding loads of 200–500 kN in abrasive coal dust environments. The steel-copper composite bushings produced using the recommended FCAW process demonstrated a service life of 18–24 months, compared to 6–8 months for solid copper bushings and 3–5 months for plain steel bushings.
The study also addresses the dimensional accuracy requirements of the composite bushings. After overlay welding, the copper cladding layer must be machined to achieve a surface roughness of Ra 0.8–1.6 μm and dimensional tolerance of IT7–IT8. The study recommends a minimum copper layer thickness of 3–5 mm to allow for machining allowance while maintaining adequate wear reserve.
Key Reflections
This study is particularly instructive for engineers dealing with dissimilar metal cladding applications. The emphasis on dilution control as the primary lever for managing interface metallurgy is a principle that extends well beyond steel-copper systems to other bimetal combinations such as titanium-steel and nickel alloy-carbon steel. The FCAW process selection is well-justified by the productivity requirements of mining equipment manufacturing, where bushing production volumes can be in the thousands per year. The study's practical focus on service life comparison provides a compelling economic case for the steel-copper composite approach.