CLADDING TECHNOLOGY SHANXI CO., LTDHeat Source Characteristics of Magnetically Controlled Plasma Arc Cladding
Literature Overview and Technical Context
This 1997 study published in the "Journal of Shenyang University of Technology" by Chang Yunlong and Chen Deshan investigates the heat source characteristics of magnetically controlled plasma arc cladding (MC-PAC), a specialized variant of plasma transferred arc (PTA) cladding that employs an external magnetic field to manipulate the arc shape, heat distribution, and molten pool geometry. The research addresses a persistent challenge in PTA cladding: the difficulty of achieving uniform, wide, shallow welds with controlled dilution on thick or large-diameter substrates. Magnetic control of the plasma arc offers a non-contact method to expand the arc footprint, reduce peak heat flux, and improve the geometric quality of the cladding layer without modifying the torch or consumable design.
Core Technical Content: Magnetic Arc Control Principle
The fundamental principle of magnetic arc control is based on the Lorentz force acting on the electrically conducting plasma arc. When an external magnetic field is applied perpendicular to the arc axis, the current-carrying plasma experiences a force F = J × B (where J is the current density and B is the magnetic flux density), causing the arc column to deflect, expand, or rotate. In the context of cladding, this magnetic manipulation achieves several beneficial effects:
- Arc spreading: A transverse magnetic field causes the arc to widen at the workpiece surface, distributing the heat input over a larger area and reducing the peak temperature gradient.
- Arc rotation: A circumferential magnetic field causes the arc to rotate, creating a circular molten pool that averages out directional solidification effects and produces a more isotropic microstructure.
- Arc stability: The magnetic field dampens arc oscillation and improves the consistency of the heat input, reducing the formation of surface defects such as ripples and cold shuts.
The researchers developed a magnetic coil configuration consisting of a single-turn or multi-turn water-cooled copper coil positioned around the plasma torch, generating a controlled magnetic field of 0.05–0.30 T at the arc location. The magnetic field strength was adjustable through the coil current, allowing real-time control of the arc geometry during cladding operations.
Heat Source Modeling and Measurement
The study employed a combination of analytical modeling and experimental measurement to characterize the heat source:
Analytical model: The plasma arc heat flux distribution was modeled using a modified double-ellipse heat source function, where the major and minor axes of the ellipse were adjusted to account for the magnetic spreading effect. The peak heat flux q_max was calculated as:
q_max = (0.8 × η × I × U) / (π × a × b)
where η is the arc efficiency (typically 0.75–0.85 for PTA), I is the welding current, U is the arc voltage, and a and b are the major and minor semi-axes of the heat distribution ellipse.
Experimental measurement: A thin-foil thermocouple method was used to measure the heat flux distribution on the substrate surface. A 0.1 mm thick copper foil embedded with a grid of thermocouples was placed at the substrate surface, and the heat flux was calculated from the measured temperature gradients using the Fourier heat conduction equation.
| Parameter | Without Magnetic Field | With Magnetic Field (0.15 T) | With Magnetic Field (0.25 T) |
|---|---|---|---|
| Arc current (A) | 200 | 200 | 200 |
| Arc voltage (V) | 38 | 38 | 38 |
| Travel speed (mm/min) | 300 | 300 | 300 |
| Peak heat flux (kW/cm²) | 18.5 | 12.3 | 8.7 |
| Heat source major axis (mm) | 2.8 | 4.5 | 6.2 |
| Heat source minor axis (mm) | 2.2 | 3.8 | 5.5 |
| Dilution rate (%) | 35–40 | 18–22 | 10–14 |
| Overlay width (mm) | 8.0 | 12.5 | 16.0 |
| Overlay depth (mm) | 1.8 | 1.2 | 0.8 |
The data clearly demonstrate that increasing the magnetic field strength progressively spreads the heat source, reduces the peak heat flux, decreases the dilution rate, and increases the overlay width while reducing the overlay depth. This is precisely the desired effect for cladding operations where low dilution and wide, shallow welds are required.
Microstructural and Mechanical Property Analysis
The reduced dilution and altered solidification conditions resulting from magnetic arc control significantly affect the cladding layer microstructure:
- Without magnetic field: The high dilution rate (35–40%) resulted in a cladding layer with significant base metal alloying, producing a microstructure dominated by ferrite with dispersed carbides. The hardness was 320–380 HV, and the composition deviated substantially from the intended overlay alloy.
- With 0.15 T magnetic field: The dilution rate dropped to 18–22%, and the cladding layer exhibited a more austenitic-ferritic microstructure with finer grain size. The hardness decreased to 280–320 HV, but the corrosion resistance improved markedly due to the higher chromium and nickel content in the deposit.
- With 0.25 T magnetic field: The dilution rate was reduced to 10–14%, and the cladding layer closely matched the intended alloy composition. The microstructure was predominantly austenitic with fine carbide precipitates, and the hardness was 250–290 HV. The intergranular corrosion resistance (ASTM A262 Practice E) was excellent, with no intergranular attack observed after 6 hours in 66% boiling HNO3.
The mechanical properties of the cladding-base metal interface also improved with magnetic arc control. The lower heat input and reduced dilution resulted in a narrower heat-affected zone (HAZ) and lower residual stresses. The HAZ width decreased from approximately 3.5 mm (without magnetic field) to 1.8 mm (with 0.25 T), reducing the risk of microcracking and distortion.
Process Optimization and Engineering Considerations
The study identified several key process windows for optimal magnetic arc cladding performance:
| Application | Recommended Magnetic Field (T) | Current (A) | Travel Speed (mm/min) | Target Dilution (%) |
|---|---|---|---|---|
| Stainless steel on carbon steel | 0.10–0.20 | 150–250 | 250–400 | 15–25 |
| Nickel alloy on stainless steel | 0.15–0.25 | 180–300 | 200–350 | 10–18 |
| Titanium alloy on steel | 0.20–0.30 | 120–200 | 200–300 | 8–15 |
| Copper alloy on steel | 0.08–0.15 | 150–250 | 300–450 | 20–30 |
The researchers also noted that the magnetic field configuration must be carefully designed to avoid arc instability. At very high magnetic field strengths (above 0.35 T), the arc can become detached from the substrate, leading to porosity and incomplete fusion. The optimal magnetic field strength is application-specific and should be determined through trial welding and microstructural evaluation.
Key Questions and Reflections
A significant question arising from this research is the scalability of magnetic arc control to industrial production environments. The laboratory-scale magnetic coil described in the study is relatively simple and requires a dedicated power supply for the coil current. In a production setting, the integration of magnetic control with existing PTA systems, particularly automated multi-wire or multi-torch configurations, presents additional engineering challenges. The cost-benefit analysis of adding magnetic control equipment must be evaluated against the value of reduced dilution and improved overlay quality.
Another important reflection is the interaction between magnetic arc control and substrate geometry. The study was conducted on flat plates, but in practice, cladding is often performed on curved surfaces, thick-walled vessels, and complex geometries. The magnetic field distribution around a curved surface differs significantly from a flat plate, and the arc spreading effect may be non-uniform. This geometric variability requires careful consideration in process development and may necessitate adaptive magnetic field control strategies.
Summary and Reference Value
This study demonstrates that magnetic arc control is a powerful and effective technique for optimizing PTA cladding processes, particularly for applications requiring low dilution and high overlay quality. The quantitative heat source data, dilution rate measurements, and microstructural analyses provide engineers with a solid technical foundation for implementing magnetic arc control in production environments. The key insight is that magnetic arc control is not merely an incremental improvement but a qualitative shift in cladding capability, enabling the deposition of high-value alloy overlays with minimal base metal dilution and superior corrosion and mechanical properties. Engineers working on critical cladding applications—such as nuclear components, aerospace engine parts, and chemical processing equipment—should seriously evaluate magnetic arc control as a process enhancement option, recognizing that the investment in magnetic control equipment is often justified by the reduction in overlay material consumption and the improvement in service life of the clad component.