CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Gradient Functional Layer by CO2 Welding with Electromagnetic Stirring
Literature Overview and Core Concept
This study explores the application of electromagnetic stirring (EMS) as an auxiliary technique during CO2 gas metal arc welding (GMAW) to fabricate a gradient functional layer. The electromagnetic stirring field is imposed on the weld pool during deposition, modifying the solidification behavior, fluid flow patterns, and ultimately the microstructure and properties of the cladding layer. The concept of a gradient functional layer means that the composition, microstructure, and mechanical properties vary gradually from the surface to the interface with the base material, creating a transition zone that mitigates the mismatch between the hard, wear-resistant surface layer and the ductile substrate.
This research is significant because it addresses two persistent challenges in cladding technology: (1) the formation of brittle intermetallic compounds and microcracks at the interface between dissimilar materials, and (2) the difficulty of achieving a uniform yet gradient-controlled microstructure in weld overlay deposits. Electromagnetic stirring offers a non-contact, controllable means of influencing the weld pool dynamics without modifying the filler metal or shielding gas composition.
Electromagnetic Stirring Mechanism and Process Parameters
Electromagnetic Stirring Principle
Electromagnetic stirring works by inducing eddy currents in the molten weld pool through an alternating magnetic field. These eddy currents interact with the magnetic field to produce Lorentz forces that drive the molten metal in a controlled manner. The effect on the weld pool includes:
- Enhanced heat transfer – Forced convection increases the heat dissipation rate, resulting in a smaller weld pool volume and faster cooling rate.
- Modified solidification front – The imposed flow disrupts the natural convection patterns, leading to directional solidification and modified dendrite morphology.
- Inclusion modification – Non-metallic inclusions are entrained and redistributed, potentially reducing their detrimental effect on mechanical properties.
- Stress relief – The fluid motion helps relieve thermal stresses that develop during solidification, reducing the tendency for hot cracking.
Process Parameters
| Parameter | Typical Value | Effect on Cladding Layer |
|---|---|---|
| CO2 shielding gas flow rate | 15–25 L/min | Adequate protection; slight carbonization effect |
| Welding current | 200–350 A | Penetration depth and deposition rate |
| Arc voltage | 22–30 V | Weld bead width and profile |
| Travel speed | 200–400 mm/min | Heat input and dilution |
| Electromagnetic stirring frequency | 50–1000 Hz | Stirring intensity and penetration pattern |
| Electromagnetic stirring current | 50–200 A | Lorentz force magnitude |
| Electromagnetic stirring coil distance | 10–30 mm from weld pool | Field intensity at weld pool |
| Number of cladding layers | 3–5 | Build up gradient from surface to interface |
The CO2 shielding gas is chosen specifically for its cost-effectiveness and the mild carburizing effect it can impart to the weld pool. Carbon dissolution in the molten metal can promote the formation of carbide hard phases, which is beneficial for wear-resistant applications. However, excessive carbon pickup can increase the risk of porosity due to CO gas formation, so the balance must be carefully managed.
Microstructural Analysis
The electromagnetic stirring significantly modifies the microstructure of the cladding layer compared to conventional CO2 welding without stirring.
Without Electromagnetic Stirring
The conventional CO2 GMAW cladding layer typically exhibits:
- Columnar dendritic structure with primary dendrite arm spacing of 20–50 μm
- Segregation of alloying elements at dendrite boundaries
- Possible formation of brittle intermetallic phases (Fe3C, Fe2B if boron is present) at the interface
- Microcracks along grain boundaries due to thermal stress concentration
With Electromagnetic Stirring
The EMS-treated cladding layer shows:
- Equiaxed grain structure with grain size of 5–20 μm (reduced by 50–70% compared to unstirred)
- Refined dendrite arm spacing of 5–15 μm
- More uniform distribution of alloying elements with reduced microsegregation
- Reduced volume fraction of brittle intermetallic phases at the interface
- Elimination or significant reduction of microcracks
The grain refinement mechanism is attributed to the combination of increased nucleation sites (due to thermal oscillation from the stirring-induced temperature fluctuations) and the suppression of competitive grain growth (due to the modified thermal gradient and solidification velocity).
Gradient Layer Formation
The gradient functional layer is achieved through multiple cladding passes with progressively varying filler metal composition or welding parameters:
| Layer (from surface) | Filler Composition | Hardness (HV) | Purpose |
|---|---|---|---|
| Layer 1 (surface) | High alloy content | 600–800 | Wear/corrosion resistance |
| Layer 2 | Medium alloy content | 400–550 | Transition zone |
| Layer 3 | Low alloy content | 250–350 | Bonding to substrate |
| Substrate | Base material | 150–250 | Structural support |
The electromagnetic stirring enhances the quality of each layer and particularly improves the transition between adjacent layers by promoting better mixing and more uniform solidification.
Mechanical Properties and Performance Evaluation
| Property | Conventional CO2 Welding | CO2 + EMS | Improvement |
|---|---|---|---|
| Surface hardness (HV) | 450–550 | 550–680 | 20–30% |
| Wear resistance (mm³ loss) | 1.0 (reference) | 1.4–1.8 | 40–80% |
| Impact energy (J, at 25°C) | 15–25 | 25–40 | 50–100% |
| Interfacial bond strength (MPa) | 100–150 | 160–220 | 50–80% |
| Fatigue strength (MPa, 10⁶ cycles) | 180–220 | 240–300 | 30–40% |
The improvement in impact energy and interfacial bond strength is particularly notable, as these properties are typically sacrificed when depositing hard, wear-resistant cladding layers. The electromagnetic stirring achieves a rare combination of enhanced hardness without proportional loss of toughness, which is critical for engineering applications where the component must withstand both wear and impact loading.
Common Defects and Process Optimization
| Defect | Cause | EMS Effect | Additional Countermeasure |
|---|---|---|---|
| Porosity | CO gas formation from C + O reaction | Reduced (better mixing) | Control carbon activity; increase gas flow |
| Cracking | Thermal stress; brittle phases | Significantly reduced | Preheat; control cooling rate |
| Dilution | Excessive base metal melting | Reduced (smaller pool) | Lower current; increase travel speed |
| Uneven hardness | Microsegregation | Reduced (uniform solidification) | Multi-layer deposition |
| Interface segregation | Slow solidification at interface | Reduced (enhanced convection) | Optimize EMS frequency |
Engineering Practice and Application Prospects
The CO2 + EMS composite welding process has clear advantages for several industrial applications:
- Hydrogenation reactor internals – Where nickel-based alloy cladding is required on low-alloy steel substrates, EMS can improve the quality of the transition zone and reduce the risk of hydrogen embrittlement cracking.
- Heat exchanger tubes – Gradient layers can provide corrosion resistance on the process side while maintaining structural integrity on the shell side.
- Mining equipment – Wear plates with gradient hardness distribution can handle both abrasive wear at the surface and impact loading from the back.
From a standards perspective, the qualification of EMS-assisted welding processes requires additional consideration under NB/T 47014 and ASME IX. The electromagnetic stirring parameter (frequency, current, coil geometry) must be included in the procedure specification and qualification test, as it constitutes a supplementary welding technique that affects the weld properties. The current standards may not explicitly address EMS, so engineering judgment and additional qualification testing are necessary.
Study Insights and Implications
The most compelling finding of this study is that electromagnetic stirring can simultaneously improve multiple properties of the cladding layer — hardness, toughness, interfacial strength, and fatigue resistance — without requiring changes to the filler metal or shielding gas. This represents a paradigm shift from the conventional approach of property optimization through compositional modification alone.
The economic implications are significant. CO2 shielding gas is the most economical option in GMAW, and the electromagnetic stirring equipment, while requiring capital investment, can be integrated into existing welding setups with relatively low incremental cost. The combination of low-consumable-cost shielding gas with the quality-enhancing effect of EMS offers a cost-effective pathway to high-quality cladding.
However, several challenges remain for industrial scale-up. The electromagnetic stirring coil must be positioned accurately relative to the weld pool, which requires either fixed-position equipment or a coordinated motion system. For large components such as pressure vessel shells or heat exchanger tubes, the practical implementation of EMS requires careful engineering design of the stirring apparatus. Additionally, the long-term stability of the refined microstructure under thermal cycling needs to be evaluated, as grain coarsening during service could partially negate the benefits of EMS.
In conclusion, the CO2 + electromagnetic stirring composite welding process represents a promising advancement in cladding technology that achieves superior microstructural and mechanical properties through process innovation rather than material cost escalation, and its adoption in high-value applications such as pressure vessels and heat exchangers warrants further engineering development and standardization.