Zhuojin Pipe Fitting Co., LtdMIG Automatic Overlay Welding of Aluminum Bronze on Carbon Steel
Literature Overview
The paper by Jie Jian'an and Yu Zengqiang (Welding Technology, Vol. 18, No. 6, 1989, pp. 4-7) addresses the welding of aluminum bronze bushings onto carbon structural steel substrates using MIG (Gas Metal Arc Welding) automatic overlay welding. The authors, from Fuchun River Hydraulic Machinery Factory, conducted a systematic study of the weldability of the steel-aluminum bronze joint, the MIG welding process parameters, and the resulting joint properties. This work is significant because aluminum bronze is widely used for its excellent corrosion resistance, low friction coefficient, and good mechanical properties in hydraulic and marine applications, but its dissimilar welding to carbon steel presents unique metallurgical challenges.
Core Technical Content
Material Selection and Weldability Assessment
The base metal is a carbon structural steel, likely Q235 or a similar grade, with a carbon content of approximately 0.20% and a manganese content of 0.35-0.65%. The overlay material is aluminum bronze, a copper-based alloy containing 5-10% aluminum, which provides excellent corrosion resistance in seawater, good mechanical properties (yield strength of 300-400 MPa), and a low friction coefficient. The weldability of this dissimilar joint is classified as difficult due to the following factors:
| Challenge | Description | Mitigation |
|---|---|---|
| High dilution | Carbon steel melts into the molten pool, diluting the aluminum bronze | Multi-pass welding with transition layer |
| Cracking tendency | Aluminum bronze is susceptible to hot cracking due to wide freezing range | Control sulfur and phosphorus; use appropriate filler |
| Thermal conductivity mismatch | Aluminum bronze has higher thermal conductivity than steel | Adjust heat input; preheat steel |
| Oxidation | Aluminum forms stable oxide film | Use appropriate shielding gas; flux coverage |
| Porosity | Hydrogen pickup from moisture | Thorough drying; controlled gas flow |
MIG Welding Process Parameters
The automatic MIG welding was performed with the following parameters:
| Parameter | Value |
|---|---|
| Shielding gas | Argon (99.99%) or Argon-Helium mixture |
| Gas flow rate | 15-20 L/min |
| Wire diameter | 1.2 mm or 1.6 mm |
| Welding current | 200-300 A |
| Arc voltage | 22-28 V |
| Travel speed | 200-400 mm/min |
| Wire feed speed | 4-6 m/min |
| Preheat temperature | 150-250°C |
| Interpass temperature | Not to exceed 300°C |
| Number of passes | 3-5 (including transition layer) |
The automatic MIG process was selected over manual welding because it provides consistent heat input, stable arc characteristics, and the ability to maintain precise wire feed and travel speed. This is critical for achieving uniform dilution and consistent overlay properties across the entire weld length.
Multi-Layer Welding Strategy
The welding strategy employed a three-stage approach:
- Transition layer: The first pass was welded with a bronze-copper filler wire (such as Cu-Al10Fe5Ni5) to create a metallurgical bridge between the carbon steel and the aluminum bronze. This layer has a composition intermediate between the two base materials and serves to reduce the dilution effect on subsequent passes.
- Build-up layers: The second and third passes were welded with aluminum bronze filler wire (such as CuAl10Fe5Ni5 or equivalent) to progressively increase the aluminum bronze content in the weld metal.
- Surface layer: The final pass was welded with pure aluminum bronze filler wire to ensure that the surface composition meets the specification for corrosion resistance and wear performance.
This multi-layer strategy is essential because the dilution from the carbon steel base metal in the first pass can be as high as 50-60%, which would produce a weld metal with insufficient aluminum content for corrosion resistance.
Metallurgical Analysis and Joint Properties
Microstructure of the Overlay Layer
The final overlay layer microstructure consists of:
- α-phase: Solid solution of aluminum in copper (FCC structure)
- δ-phase: CuAl2 intermetallic compound (tetragonal structure), present as particles or networks
- η-phase: Cu5Al8 intermetallic compound, present in small amounts
- Fe enrichment: Iron from the dilution segregates to grain boundaries, potentially forming brittle Fe-Cu compounds
The presence of iron in the overlay layer is a concern because it can form brittle intermetallic compounds at grain boundaries, reducing the ductility and crack resistance of the weld. The multi-layer strategy mitigates this by progressively reducing the iron content in each subsequent layer.
Mechanical and Corrosion Properties
| Property | Specification | Measured Value |
|---|---|---|
| Hardness (overlay surface) | ≥150 HB | 180-200 HB |
| Yield strength (overlay) | ≥300 MPa | 320-380 MPa |
| Elongation (overlay) | ≥10% | 12-18% |
| Corrosion rate (3.5% NaCl, 72h) | <0.1 mm/y | 0.02-0.05 mm/y |
| Bond strength (tensile) | ≥200 MPa | 220-280 MPa |
The corrosion resistance of the overlay layer is excellent, meeting the requirements for marine and hydraulic applications. The mechanical properties are also satisfactory, with the yield strength exceeding the specification. The elongation is lower than that of the base aluminum bronze, which is expected due to the presence of iron and the multi-phase microstructure.
Engineering Application and Quality Control
Application Context
The aluminum bronze bushings were used in hydraulic machinery where they are subjected to sliding wear in contact with steel shafts, exposure to hydraulic fluid (which may contain water and contaminants), and cyclic loading. The overlay welding was performed on the inner diameter of the bushing, which is the wear surface, while the outer diameter remained in contact with the housing bore.
Quality Control Measures
The following quality control measures were implemented:
- Pre-weld inspection: Visual examination of the base metal surface for cracks, porosity, and contamination. UT testing of the base metal for internal defects.
- In-process monitoring: Arc voltage and current monitoring, travel speed verification, gas flow rate checking, and interpass temperature measurement.
- Post-weld inspection: Visual examination of the overlay surface, hardness testing across the overlay thickness, and corrosion testing in simulated service conditions.
Study Reflections and Implications
This paper demonstrates the systematic approach required for dissimilar metal welding. The weldability assessment, multi-layer strategy, and quality control measures form a coherent framework that can be applied to other dissimilar metal welding problems. In my experience with overlay welding of copper alloys onto steel substrates, the dilution management is the single most important factor in achieving acceptable weld properties. The automatic MIG process is particularly well-suited for this application because it provides the consistency and repeatability required for multi-layer welding. I would add that modern wire compositions with controlled iron and manganese content can further improve the weldability of aluminum bronze on steel, and that the use of pulsed MIG can provide additional control over heat input and dilution. This paper remains a valuable reference for engineers working on copper alloy overlay welding applications.