CLADDING TECHNOLOGY SHANXI CO., LTDDesign of IGBT Inverter DC and TIG Dual-Purpose Welding Machine
Literature Overview
This research by Wu Xianping, Hong Bo, and Hu Yajun, published in 1996, addresses the design of an IGBT inverter welding machine capable of both shielded metal arc welding (SMAW/DC) and gas tungsten arc welding (TIG/DC) operations. The collaboration between Xiangtan University and Guangzhou Electric Welding Machine Factory reflects the integration of academic research with industrial manufacturing. This study is particularly relevant to cladding and overlay applications where both SMAW and TIG processes may be required for different stages of the welding operation.
Core Technical Content
Power Electronics Architecture
The IGBT inverter welding machine employs a high-frequency switching topology to convert AC mains power into a regulated DC output suitable for welding. The key components include:
| Component | Function | Typical Specification |
|---|---|---|
| IGBT inverter module | High-frequency switching | 600–1200 V, 100–400 A |
| High-frequency transformer | Voltage transformation and isolation | 20–40 kHz switching frequency |
| Rectifier bridge | AC to DC conversion | 1000 V, 400 A |
| LC filter | Output smoothing | 10–20 mH inductor, 1000–5000 μF capacitor |
| Control circuit | Parameter regulation and protection | Microprocessor-based |
The dual-purpose design requires the machine to switch between SMAW and TIG operating modes, which have different electrical characteristics:
- SMAW mode: Constant voltage (CV) or constant current (CC) characteristic with a high open-circuit voltage (60–80 V) for arc striking. The current range is typically 20–400 A with a steep voltage-current characteristic for arc stability.
- TIG mode: Constant current (CC) characteristic with a lower open-circuit voltage (50–70 V) for pilot arc initiation. The current range is typically 5–200 A with precise current regulation to maintain a stable arc.
Control Strategy
The control circuit implements several key functions:
- Mode selection: A relay or electronic switch selects between SMAW and TIG output characteristics.
- Current regulation: The welding current is regulated by adjusting the duty cycle of the IGBT switching, with feedback from a current transducer.
- Arc voltage feedback: In SMAW mode, the arc voltage is monitored to adjust the output impedance for arc stability. In TIG mode, the current is held constant regardless of arc voltage variations.
- Pilot arc control: For TIG operation, a high-frequency (20–100 kHz) or high-voltage (500–1000 V) pilot arc circuit is provided to initiate the arc without touching the tungsten electrode to the workpiece.
- Post-flow timer: A programmable timer extends the shielding gas flow after the welding current is terminated to protect the cooling weld pool and tungsten electrode.
Performance Characteristics
| Parameter | SMAW Mode | TIG Mode |
|---|---|---|
| Current range | 20–400 A | 5–200 A |
| Voltage range | 16–32 V | 8–20 V |
| Power factor | >0.95 | >0.95 |
| Efficiency | >85% | >85% |
| Duty cycle | 60% at rated current | 100% at low current |
| Response time | <10 ms | <5 ms |
| Current regulation | ±5% | ±2% |
The high-frequency switching (20–40 kHz) provides several advantages over conventional transformer-based welders:
- Reduced size and weight: The high-frequency transformer is significantly smaller than a 50/60 Hz transformer, making the machine more portable.
- Improved efficiency: Switching losses are lower than copper losses in conventional transformers at high frequencies.
- Precise control: The duty cycle can be adjusted rapidly to respond to arc conditions, improving arc stability and weld quality.
- Soft start: The output voltage and current can be ramped up gradually to reduce mechanical stress on the arc and workpiece.
Engineering Practice Integration
Application in Cladding and Overlay
In cladding and overlay applications, the dual-purpose welding machine offers several advantages:
- Flexibility: The ability to switch between SMAW and TIG allows the operator to use the most appropriate process for each stage of the cladding operation. SMAW may be used for building up thick overlay layers, while TIG is used for the final surface finish or for welding thin overlay layers.
- Portability: The compact size and light weight of the inverter machine make it suitable for field applications where conventional welders are impractical, such as overlay repair on large pressure vessels or heat exchangers.
- Energy efficiency: The high efficiency of the inverter machine reduces operating costs, which is significant for high-volume cladding operations.
- Precise current control: The TIG mode provides precise current regulation that is essential for maintaining consistent weld pool geometry and heat input in overlay applications.
Welding Procedure Development
For cladding applications, the welding procedure must be qualified in accordance with ASME IX or NB/T 47014. The dual-purpose machine must be included in the qualification procedure, and the welding parameters must be verified for both SMAW and TIG modes.
Key parameters for procedure qualification include:
- Base metal: Material grade, thickness, and heat number.
- Filler metal: Electrode type (for SMAW) or wire type (for TIG), including chemical composition and mechanical properties.
- Shielding gas: Type and flow rate (for TIG).
- Current and voltage: Range of acceptable values for each mode.
- Travel speed: Range of acceptable values.
- Preheat and interpass temperature: Maximum allowable values.
- Post-weld heat treatment: Required or not required.
Common Issues and Troubleshooting
| Issue | Possible Cause | Solution |
|---|---|---|
| Arc instability | Incorrect output characteristic | Verify mode selection and adjust impedance |
| Tungsten erosion | Excessive current or incorrect polarity | Reduce current, verify DCEN polarity |
| Porosity | Inadequate gas shielding | Check gas flow, nozzle condition, and air drafts |
| Overheating | Excessive duty cycle | Allow cooling period, reduce current |
| Pilot arc failure | Faulty high-frequency circuit | Inspect and replace high-frequency components |
Key Insights and Reflections
The design of an IGBT inverter dual-purpose welding machine represents a significant advancement in welding power source technology. The integration of SMAW and TIG capabilities in a single machine provides practical benefits for cladding and overlay operations, where both processes may be required.
One of the most important insights is the role of the control circuit in determining weld quality. The response time and regulation accuracy of the control system directly affect arc stability, which in turn affects weld pool geometry, penetration, and defect formation. For TIG overlay applications, a current regulation accuracy of ±2% is essential to maintain consistent weld bead geometry, particularly when building up multiple layers.
Another important consideration is the duty cycle rating of the machine. Inverter machines can achieve high duty cycles at lower currents but may be limited at higher currents due to thermal management constraints. For cladding operations that require extended welding times, the duty cycle rating must be carefully evaluated to prevent overheating and component failure.
The study also highlights the importance of the pilot arc circuit in TIG operation. A reliable pilot arc is essential for consistent arc initiation, particularly in automated or semi-automated TIG welding. The high-frequency pilot arc circuit must be designed to provide sufficient energy to ionize the gas between the tungsten electrode and the workpiece without causing tungsten erosion or contamination.
For engineers involved in cladding and bimetal product manufacturing, the dual-purpose inverter welding machine offers a practical solution for operations that require both SMAW and TIG processes. The key to successful application is the proper selection of welding parameters, thorough procedure qualification, and ongoing maintenance of the machine to ensure consistent performance.
In conclusion, the IGBT inverter dual-purpose welding machine is a versatile and efficient power source for cladding and overlay applications. Its compact size, high efficiency, and precise control make it suitable for both shop and field applications. Engineers must understand the electrical characteristics of both SMAW and TIG modes, the role of the control circuit in weld quality, and the importance of proper maintenance to ensure reliable operation over the service life of the machine.