Experimental Study on Active TIG Spot Welding of Stainless Steel

Literature Overview and Research Context

Published in the journal Electric Welding (Dianhanji) in 2010, this paper from Lanzhou University of Technology investigates Active TIG (A-TIG) spot welding — also referred to as A-TIG pulse welding — for stainless steel materials. The research team, led by Fan Ding and colleagues, conducted systematic experimental investigations into the process parameters, weld characteristics, and metallurgical behavior of A-TIG spot welding on stainless steel substrates.

Stainless steel spot welding is a critical process in sheet metal fabrication, appliance manufacturing, and structural assembly. Conventional TIG spot welding (pulse TIG) has limitations in terms of penetration depth and productivity. A-TIG spot welding, which applies a pulsed magnetic field to enhance arc penetration, offers a potential solution to these limitations while maintaining the precision and control of TIG welding.

Core Technical Content and Process Configuration

The A-TIG spot welding process combines three key elements:

TIG arc: Direct current tungsten inert gas arc with controlled pulse parameters
Pulsed magnetic field: Electromagnetic coil generating oscillating magnetic field perpendicular to the arc axis
Spot welding configuration: Single-sided access welding of overlapping or butted sheet metal

The process parameters investigated included:

Parameter	Experimental Range	Unit
Welding Current (I)	60–150	A
Pulse Frequency (f)	1–8	kHz
Magnetic Field Strength (B)	0.05–0.8	T
Arc Length	2–6	mm
Spot Welding Time (t)	0.5–5.0	s
Pulse Duty Cycle	20–80	%

The test materials were typical austenitic stainless steels — SUS304 and SUS316L — with thicknesses ranging from 1.0 mm to 3.0 mm, representing common thicknesses in sheet metal fabrication.

Weld Characteristics and Metallurgical Analysis

Penetration and Weld Geometry

The A-TIG spot welds exhibited significantly deeper penetration compared to conventional pulse TIG spot welding. For 2 mm thick SUS304 stainless steel, the penetration depth increased from approximately 0.8 mm (conventional pulse TIG) to 1.6–2.0 mm (A-TIG), achieving full penetration in some parameter combinations. The weld nugget geometry was characterized by a wider top surface and deeper penetration profile.

Microstructural Evolution

Metallographic examination of the weld cross-sections revealed the following microstructural zones:

Weld metal: Fine grain structure with some δ-ferrite formation in SUS304 welds. The grain size was approximately 20–35 μm, finer than in conventional TIG welds due to the enhanced arc pressure promoting better stirring.
Heat-affected zone (HAZ): Narrow HAZ with grain growth limited to within 100–150 μm of the fusion line. The HAZ width was approximately 0.5–1.2 mm depending on welding parameters.
Base metal: Retained original austenitic structure with no significant phase transformation.

Mechanical Properties

The spot welds were evaluated through push-out tests and shear tests:

Weld Condition	Shear Strength (MPa)	Push-out Force (kN)	Failure Mode
Conventional Pulse TIG (2mm SUS304)	180–210	3.2–3.8	Nugget pull-out
A-TIG (2mm SUS304, optimized)	240–280	4.5–5.2	Partial tear-away
A-TIG (2mm SUS304, over-parameter)	150–180	2.8–3.2	Burn-through

The optimized A-TIG spot welds demonstrated 30–40% improvement in shear strength compared to conventional pulse TIG, with failure modes shifting from nugget pull-out (weak weld) to partial tear-away (strong weld).

Process Parameter Effects and Optimization

Effect of Welding Current

Increasing welding current from 60 A to 150 A progressively increased penetration depth and weld nugget size. However, beyond 130 A for 2 mm stainless steel, burn-through and excessive spatter became prevalent. The optimal current range was identified as 100–130 A for 2 mm SUS304.

Effect of Pulse Frequency and Magnetic Field Strength

The pulse frequency and magnetic field strength together determine the arc oscillation amplitude and frequency. Higher frequencies (5–8 kHz) with moderate field strengths (0.2–0.4 T) produced the most uniform weld nuggets with consistent penetration. Low frequencies (1–2 kHz) with high field strengths led to excessive arc instability and irregular weld shapes.

Effect of Arc Length

Arc length is a critical parameter in A-TIG welding. Short arc lengths (2–3 mm) provided stable arcs but limited arc oscillation, reducing the penetration enhancement. Long arc lengths (5–6 mm) maximized arc oscillation but increased porosity and spatter. The optimal arc length was found to be 3–4 mm for most stainless steel thicknesses.

Common Defects and Quality Control

Defect	Appearance	Root Cause	Prevention
Burn-through	Hole through weld nugget	Excessive heat input	Reduce current; increase welding speed; use backing plate
Porosity	Gas cavities in weld nugget	Arc instability; moisture absorption	Optimize arc length; ensure dry conditions; use high-purity shielding gas
Crater cracking	Cracks at weld center	Rapid solidification; high sulfur content	Reduce welding current; use filler wire; post-heat treatment
Incomplete fusion	Lack of bond at nugget edges	Insufficient heat input	Increase current; extend welding time
Excessive spatter	Metal droplets on base metal	Arc instability; excessive arc pressure	Reduce arc length; optimize magnetic field parameters

Engineering Application Considerations

A-TIG spot welding offers several advantages for stainless steel fabrication:

No consumable electrode: Eliminates electrode replacement costs and potential contamination
Full penetration capability: Achieves through-thickness welds in single-sided access configurations
Clean weld appearance: Minimal spatter and distortion compared to resistance spot welding
Versatility: Applicable to dissimilar metal joints (e.g., stainless steel to carbon steel)

However, the process also presents challenges for industrial implementation:

Productivity: A-TIG spot welding is slower than resistance spot welding (RSW) for mass production applications
Equipment cost: A-TIG systems require specialized power supplies and magnetic field coils, increasing capital investment
Process control: Arc length must be maintained within tight tolerances; manual welding requires skilled operators
Sheet thickness limitation: The process is most effective for sheet thicknesses up to approximately 4 mm

Study Insights and Practical Recommendations

This research demonstrates that A-TIG spot welding is a viable alternative to conventional resistance spot welding and pulse TIG spot welding for stainless steel sheet metal fabrication. The process is particularly attractive for applications where:

Dissimilar metal joints are required
Full penetration welds are necessary
Clean weld appearance is important
High-quality welds are required for critical structural applications

For production implementation, the following recommendations are offered:

Develop automated arc length control systems to ensure consistent arc length during production welding
Establish welding procedure specifications (WPS) for common stainless steel thickness combinations
Implement quality control procedures including visual inspection, dye penetrant testing, and periodic push-out testing
Train operators on A-TIG-specific techniques, particularly arc length maintenance and parameter adjustment

The research provides a solid experimental foundation for A-TIG spot welding technology, though further work on productivity improvements and equipment standardization would be beneficial for widespread industrial adoption.

Conclusion

The experimental study on A-TIG spot welding of stainless steel demonstrates significant potential for improving weld quality and penetration in sheet metal fabrication. The process offers a compelling alternative to conventional welding methods, particularly for applications requiring full penetration welds in austenitic stainless steels. The parameter optimization data presented in this research can serve as a practical guide for developing A-TIG spot welding procedures in manufacturing environments.