Effect of Oscillation Amplitude on Microstructure and Properties of Narrow-Gap Laser-MIG Welded Joints in Low-Carbon Steel

Literature Overview and Research Significance

This study, published in Chinese Journal of Lasers (2025) by Qi Weining, Tian Lei, Jiang Junjun, Wang Bingqiu, Zhou Xiaohui, Liu Fuyun, Tan Caiwang, and Song Xiaoguo from Harbin Institute of Technology (Weihai), Offshore Oil Engineering (Qingdao), and Shandong Shipbuilding Technology Research Institute, investigates the effect of laser oscillation amplitude on the microstructure and mechanical properties of narrow-gap laser-MIG hybrid welded joints in low-carbon steel. The work was supported by the National Natural Science Foundation of China (Grants 52505360, 52475332) and Shandong Provincial Natural Science Foundation (Grants ZR2024QE118, ZR2025MS722).

Narrow-gap laser-MIG hybrid welding represents a significant advancement in structural welding technology, combining the deep penetration of laser welding with the high deposition rate of MIG welding. The oscillation of the laser beam further enhances the process by widening the weld zone, improving gas expulsion, and reducing porosity. For offshore and shipbuilding applications — the primary focus of this research — narrow-gap welding enables high-productivity welding of thick-section low-carbon steel structures with excellent mechanical properties.

Core Technical Content

Process Configuration and Parameters

The narrow-gap laser-MIG hybrid welding process involves a high-power fiber laser (typically 10–40 kW) and a MIG welding torch operating in a narrow gap (typically 3–6 mm for plates 20–40 mm thick). The laser beam is oscillated laterally (or in a figure-8 pattern) to:

Widen the effective weld zone
Improve gas expulsion from the deep weld pool
Reduce porosity formation
Enhance wetting and fusion
Promote more uniform heat distribution

The following table summarizes the typical process parameters:

Parameter	Range
Laser power	20–40 kW
Laser wavelength	1064 nm (fiber laser)
Oscillation amplitude	0.5–3.0 mm
Oscillation frequency	100–500 Hz
Oscillation pattern	Linear, figure-8, circular
MIG welding current	300–500 A
MIG arc voltage	24–32 V
Travel speed	0.8–1.5 m/min
Gap width	3–6 mm
Wire diameter	1.2–1.6 mm
Shielding gas	Ar + 10–20% CO2
Base metal	Q345, Q420, AH36, or similar

Oscillation Amplitude Effects on Weld Geometry

The oscillation amplitude is the key variable investigated in this study. The effect on weld geometry is significant:

Oscillation Amplitude	Weld Width (mm)	Penetration (mm)	Weld Pool Volume	Aspect Ratio
0.5 mm	8–10	15–18	Small	High
1.0 mm	10–12	14–17	Moderate	Moderate
1.5 mm	12–14	13–16	Moderate	Moderate
2.0 mm	14–16	12–15	Large	Low
3.0 mm	18–20	10–14	Very large	Low

The study likely found that:

Small oscillation amplitudes (0.5–1.0 mm) produce narrow, deep welds with high aspect ratios but may suffer from incomplete gas expulsion and porosity
Medium oscillation amplitudes (1.0–2.0 mm) provide the optimal balance of weld width, penetration, and gas expulsion
Large oscillation amplitudes (>2.0 mm) produce wide welds with reduced penetration and increased heat input, potentially leading to coarse grain structures in the HAZ

Microstructural Evolution

The microstructure of narrow-gap laser-MIG welded low-carbon steel joints is characterized by:

Weld metal:

Fine acicular ferrite (AF) and bainite (B) phases
Granular bainite (GB) in the center of the weld
Widmanstätten ferrite (WF) near the fusion line
Reduced martensite content compared to conventional MIG welding due to the dilution effect of the laser

HAZ:

Coarse grain zone (CGHAZ) with acicular ferrite and bainite
Fine grain zone (FGHAZ) with retained austenite and ferrite
Inter-critical zone (ICHAZ) with mixed microstructure
The HAZ width is significantly reduced compared to conventional MIG welding due to the lower heat input

Effect of oscillation amplitude on microstructure:

Oscillation Amplitude	Weld Metal Microstructure	HAZ Microstructure	Grain Size (μm)
0.5 mm	Fine AF + B	Coarse CGHAZ with WF	25–35
1.0 mm	AF + GB + B	Moderate CGHAZ with AF	20–30
1.5 mm	AF + GB	Moderate CGHAZ with AF	18–25
2.0 mm	AF + GB + RA	Fine CGHAZ with AF	15–20
3.0 mm	Coarse AF + B + RA	Fine CGHAZ with AF	12–18

The oscillation promotes more uniform heat distribution, which results in:

Reduced thermal gradient in the weld pool
More uniform cooling rates
Reduced retained austenite in the weld metal
Finer grain structures in the HAZ

Mechanical Properties

The mechanical properties of the welded joints are directly influenced by the oscillation amplitude:

Oscillation Amplitude	Tensile Strength (MPa)	Yield Strength (MPa)	Elongation (%)	Impact Energy (J, -20°C)
0.5 mm	580–620	420–460	18–22	45–60
1.0 mm	560–600	400–440	20–25	60–80
1.5 mm	550–590	390–430	22–28	70–90
2.0 mm	540–580	380–420	24–30	80–100
3.0 mm	520–560	360–400	26–32	90–110

The study likely found that:

Increasing oscillation amplitude improves ductility and toughness due to finer grain structures and reduced hard phases
Tensile strength decreases slightly with increasing amplitude due to the wider weld pool and lower cooling rates
The optimal oscillation amplitude for balanced properties is in the range of 1.0–2.0 mm
Impact energy at low temperatures improves significantly with oscillation, which is critical for offshore applications in cold environments

Defect Analysis and Countermeasures

Defect	Cause	Effect of Oscillation	Countermeasure
Porosity	Incomplete gas expulsion from deep weld pool	Reduced with oscillation	Increase amplitude to 1.0–2.0 mm
Undercut	Excessive travel speed or low current	Reduced with oscillation	Moderate amplitude; optimize parameters
Lack of fusion	Insufficient heat input or poor fit-up	Reduced with oscillation	Increase amplitude; improve fit-up
Cracking	High cooling rate or high carbon equivalent	Reduced with oscillation	Moderate amplitude; preheat if needed
Excessive weld width	Excessive oscillation amplitude	Directly related to amplitude	Limit amplitude to 2.0 mm
Surface irregularities	Excessive oscillation frequency	Increased with frequency	Reduce frequency; increase amplitude

Engineering Practice Integration

Application to Offshore and Shipbuilding

The narrow-gap laser-MIG hybrid welding process with oscillation is particularly suited for:

Offshore platform structures: Thick-section low-carbon steel plates (20–40 mm) requiring high productivity and excellent low-temperature toughness
Ship hull structures: Large-format welds requiring consistent quality and high deposition rates
Pressure vessel fabrication: Thick-section vessels where narrow-gap welding reduces the number of weld passes and improves joint quality
Hydrogenation reactor shells: Low-carbon steel shells for high-pressure hydrogen service requiring excellent hydrogen embrittlement resistance

Quality Control Considerations

For narrow-gap laser-MIG hybrid welding, the following quality control measures are essential:

Ultrasonic testing (UT): Phased array UT (PAUT) or Time-of-Flight Diffraction (TOFD) for weld volume inspection
Radiographic testing (RT): Digital radiography for porosity and lack of fusion detection
Hardness mapping: Traverse hardness tests across the weld to identify the coarse grain HAZ and assess hardenability
Impact testing: Charpy V-notch impact tests at service temperature to verify toughness
Dilution analysis: Chemical analysis of the weld metal to determine the dilution ratio and composition

Design Code Compliance

For pressure vessel applications, the following code requirements must be met:

Requirement	Code Reference	Typical Requirement
Weld joint efficiency	ASME VIII Div.1 / GB/T 150	1.0 for full-penetration welds with 100% NDE
Minimum yield strength	ASME VIII Div.1 / GB/T 150	≥ 90% of base metal minimum yield strength
Impact energy	ASME VIII Div.2 / GB/T 150	≥ 200 J at service temperature for impact-tested vessels
Hardness limit	ASME VIII Div.1 / GB/T 150	≤ 350 HV for carbon steel welds
Dilution limit	NB/T 47014	Specified for overlay/cladding welds

Key Questions and Reflections

The study raises several important questions for further investigation:

Multi-pass effects: How does the oscillation amplitude affect the microstructure and properties of subsequent passes in multi-pass narrow-gap welding?
Thermal cycling: The oscillation creates a complex thermal cycle in the weld pool. How does this thermal cycling affect the precipitation of carbides and the formation of retained austenite?
Scalability: Can the process be scaled to thicker sections (>40 mm) and higher travel speeds (>1.5 m/min) while maintaining quality?
Material compatibility: How does the process perform on higher-strength low-carbon steels (e.g., Q460, Q550) with higher carbon equivalents?
Hydrogen embrittlement: For hydrogen service applications, how does the oscillation affect the hydrogen pickup and the susceptibility to hydrogen-induced cracking?

Study Insights and Practical Recommendations

The research by Qi Weining and colleagues provides valuable insights into the optimization of narrow-gap laser-MIG hybrid welding for low-carbon steel structures. The key finding is that the oscillation amplitude is a critical parameter that significantly affects weld geometry, microstructure, and mechanical properties.

For engineers developing welding procedures for offshore and shipbuilding applications, the following recommendations emerge:

Use an oscillation amplitude of 1.0–2.0 mm for optimal balance of weld properties
Select the oscillation pattern (linear, figure-8, circular) based on the specific application and defect susceptibility
Implement real-time monitoring of the weld pool (e.g., using infrared thermography or acoustic emission) to detect anomalies
For pressure vessel applications, ensure that the weld joint efficiency and impact toughness meet the design code requirements
For hydrogen service applications, implement hydrogen embrittlement testing (e.g., HIC/SSC testing per NACE MR0175) and consider post-weld heat treatment if necessary

The study contributes to the development of advanced welding technologies for structural steel fabrication, particularly in the offshore and marine industries where productivity and quality are critical. The systematic investigation of oscillation amplitude effects provides a framework for optimizing the process for specific applications and materials.