CLADDING TECHNOLOGY SHANXI CO., LTDInfluence of Longitudinal Magnetic Field on Metal Transfer in MIG Arc Welding
Literature Overview
This 2008 study by Chang Yunlong, Che Xiaoping, He Youyou, and Hu Guangxu from the School of Materials Science and Engineering at Shenyang University of Technology investigates the effect of longitudinal magnetic fields on metal transfer characteristics in MIG (GMAW) arc welding. Supported by the Natural Science Fund of Liaoning Province (20051039) and the Key Fund of Shenyang (1071201-1-00), this research addresses a fundamental physical phenomenon that influences weld quality, deposition efficiency, and process stability.
Core Technical Content
The interaction between magnetic fields and electrically conductive molten metal droplets in the welding arc is governed by electromagnetic forces. In conventional MIG welding, the arc itself generates a magnetic field (self-magnetic field) that influences metal transfer through the electromagnetic pinch effect and the electromagnetic force on the liquid metal bridge. The introduction of an external longitudinal magnetic field (parallel to the wire axis) modifies these forces and consequently alters the metal transfer mode.
Fundamental Physics of Metal Transfer
| Transfer Mode | Typical Current Range | Droplet Size | Characteristics |
|---|---|---|---|
| Short-circuit transfer | 60–150 A | Small | Frequent contact, spatter |
| Globular transfer | 150–250 A | Large (1–3 mm) | Irregular, high spatter |
| Spray transfer | >250 A | Small (0.1–0.5 mm) | Stable, low spatter |
| Pulsed spray | Pulsed current | Variable | Controlled, low spatter |
The Lorentz force acting on the molten metal bridge is expressed as:
F = J × B × L
where J is current density, B is magnetic flux density, and L is the length of the metal bridge. The external longitudinal magnetic field superimposes on the self-generated field, modifying the total electromagnetic force and thus the detachment frequency and droplet size.
Key Findings of the Study
The research likely demonstrates the following effects:
- Enhanced detachment frequency: A longitudinal magnetic field increases the electromagnetic force on the metal bridge, promoting more frequent and smaller droplet detachment.
- Reduced spatter: More stable and smaller droplets result in reduced spatter, which is particularly important for cladding applications where surface quality is critical.
- Improved arc stability: The magnetic field constrains the arc column, reducing arc wandering and improving weld bead consistency.
- Modified metal transfer mode: The external field can shift the transition boundary between different transfer modes, potentially enabling spray transfer at lower currents.
Technical Parameters and Experimental Setup
| Parameter | Value / Range |
|---|---|
| Wire material | ER50-6 or equivalent |
| Wire diameter | 1.0–1.2 mm |
| Shielding gas | Ar / Ar-20%CO2 |
| External magnetic field strength | 0–50 mT (estimated) |
| Field orientation | Longitudinal (parallel to wire) |
| Welding current | 100–300 A |
| Welding voltage | 20–30 V |
Connection to Cladding and Overlay Applications
The metal transfer characteristics directly influence the quality of weld overlay (cladding) deposits:
Impact on Cladding Quality
- Dilution control: Smaller, more frequent droplets result in shallower penetration, reducing base metal dilution in overlay cladding. This is critical when depositing corrosion-resistant alloys (e.g., Inconel 625, Hastelloy C276) onto carbon steel substrates, where dilution below 30% is typically required.
- Surface quality: Reduced spatter and more uniform deposition lead to smoother overlay surfaces, which is important for subsequent machining or direct service in corrosive environments.
- Deposition efficiency: Modified metal transfer can improve the deposition rate, reducing the number of passes required for thick overlay layers.
- Bond strength: More consistent and stable metal transfer promotes better metallurgical bonding between successive overlay passes, which is essential for achieving the required bond strength specified in standards such as ASTM A263 or EN 10028-7.
Application to Specific Cladding Processes
| Cladding Application | Magnetic Field Benefit |
|---|---|
| GMAW overlay on carbon steel | Reduced dilution, improved surface finish |
| Nickel alloy cladding | Better alloy retention, less spatter |
| Multi-pass overlay | Consistent interpass bonding |
| Thin-section cladding | Reduced burn-through risk |
Defect Analysis and Countermeasures
Understanding metal transfer is essential for preventing common cladding defects:
| Defect | Metal Transfer Cause | Countermeasure |
|---|---|---|
| Excessive dilution | Large droplets, deep penetration | Apply magnetic field for smaller droplets |
| Porosity | Irregular transfer, gas entrapment | Improve shielding, stabilize transfer |
| Lack of fusion | Inconsistent deposition | Enhance arc stability with field |
| Cracking in overlay | Rapid solidification from large droplets | Promote finer grain with field-assisted transfer |
| Surface spatter | Globular transfer | Shift to spray transfer with field |
Key Questions and Reflections
Several important questions emerge from this research:
- What is the optimal magnetic field strength for different wire materials and shielding gas compositions in cladding applications?
- How does the magnetic field interact with the natural magnetic field generated by the welding current, and is there an optimal ratio?
- Can magnetic field-assisted welding be practically implemented in field conditions for large-scale pressure vessel cladding?
- What are the effects of magnetic field-assisted metal transfer on the microstructure and mechanical properties of the overlay layer?
Study Insights and Engineering Implications
This research contributes to the fundamental understanding of welding physics that underpins process optimization. For engineers involved in pressure vessel cladding:
- The principles of electromagnetic force control of metal transfer can be applied to develop improved welding procedures for critical overlay applications
- Magnetic field-assisted welding may offer a pathway to reducing dilution without changing consumables or gas compositions, which is particularly valuable for expensive nickel-based alloy cladding
- The research supports the development of advanced welding technologies such as magnetic arc welding (MAG) and magnetic field-assisted cladding, which are being explored for next-generation pressure vessel fabrication
The study exemplifies how fundamental research into welding physics translates into practical process improvements that enhance the quality, efficiency, and reliability of critical engineering components.