Pulsed MIG Welding Process and Joint Properties of Magnesium Alloy

Literature Overview

This paper by Wang Peng, Song Gang, and Liu Liming from Dalian University of Technology, published in the Transactions of the China Welding Institution (2009, Vol. 30, Issue 12, pp. 109-112), presents a systematic study of pulsed MIG welding applied to AZ31B magnesium alloy sheet. The research demonstrates that through optimized process parameters, single-sided welding with double-sided penetration can be achieved without groove preparation or back-side gas shielding, which represents a significant advance for magnesium alloy fabrication.

Welding Process Parameters

Pulsed MIG welding is particularly well-suited for magnesium alloys due to its ability to control heat input through pulse parameters. The key process parameters studied include:

Parameter	Typical Value	Purpose
Pulse current	180-250 A	Controls droplet detachment
Background current	60-100 A	Maintains arc stability
Pulse frequency	50-100 Hz	Controls heat input
Travel speed	200-400 mm/min	Controls weld geometry
Shielding gas	Ar + 5-10% CO2 or pure Ar	Protects molten pool

The dual-staircase external characteristic of the pulsed MIG power source allows precise control of the pulse current and background current independently, enabling optimization of both arc stability and droplet transfer.

Microstructural and Mechanical Properties

The welded joints were characterized using optical microscopy, scanning electron microscopy, tensile testing, and microhardness profiling:

Zone	Grain Structure	Hardness	Notes
Weld metal	Fine, uniform grains	Higher than base metal	Strengthened by rapid solidification
HAZ	Slightly coarsened grains	Moderate	Narrow HAZ due to low heat input
Base metal	AZ31B as-received	Reference	Reference condition

The tensile strength of the welded joint reached over 95% of the base metal strength, which is an excellent result for magnesium alloy welding. The narrow HAZ is a direct consequence of the pulsed welding process, which limits the total heat input while maintaining adequate penetration through controlled droplet transfer.

Comparison with Conventional MIG Welding

Conventional (non-pulsed) MIG welding of magnesium alloys typically requires:

Pre-grooving of the base metal to ensure penetration
Back-side gas shielding to prevent oxide formation
Higher heat input, leading to wider HAZ and more grain coarsening

The pulsed MIG approach eliminates these requirements, offering significant advantages in production efficiency and joint quality. The single-sided welding capability is particularly valuable for pipe fabrication where access to the interior of the pipe is limited.

Engineering Applications

Magnesium alloys such as AZ31B are increasingly used in automotive, aerospace, and consumer electronics due to their low density (approximately 1.8 g/cm³) and good mechanical properties. The ability to weld these alloys without groove preparation and back-side shielding opens up new fabrication possibilities:

Pipe and tubing fabrication for lightweight structural applications
Sheet metal forming and joining for automotive body panels
Aerospace structural components where weight reduction is critical

Key Technical Challenges

Despite the promising results, several challenges remain in magnesium alloy welding:

Oxide formation (MgO) is highly detrimental to weld quality and requires rigorous gas shielding
Hydrogen porosity is common due to the affinity of magnesium for hydrogen at elevated temperatures
Hot cracking susceptibility requires careful control of solidification rate and composition

The pulsed MIG process addresses these challenges through controlled heat input and stable arc conditions, but the study does not extensively discuss the metallurgical mechanisms behind the improved weld quality.

Study Insights and Implications

This work demonstrates that pulsed MIG welding is a viable and efficient process for magnesium alloy fabrication. The achievement of over 95% joint strength relative to base metal, combined with the elimination of groove preparation and back-side shielding, represents a practical advancement for industrial applications. For engineers working with magnesium alloy components, this study provides confidence that pulsed MIG welding can produce high-quality joints with minimal pre-processing. The narrow HAZ and fine grain weld metal are particularly beneficial for applications where fatigue resistance and corrosion performance are critical. Future work should address long-term durability testing, including stress corrosion cracking resistance and fatigue life, to fully characterize the service performance of pulsed MIG welded magnesium alloy joints.