TIG Welding of Dissimilar AZ31 and AZ61 Magnesium Alloys

Literature Overview

This 2011 publication in Materials Engineering by Peng Jian, Zhou Chou, Tao Jianquan, and Pan Fusheng from Chongqing University and the Chongqing Academy of Science and Technology investigates the fundamental challenges and solutions for TIG welding dissimilar AZ31 and AZ61 magnesium alloys. Funded by the Ministry of Science and Technology International Science and Technology Cooperation Program (2010DFR50010) and Chongqing Science and Technology Commission Key Project (CSTC 2009AB4006), this work addresses a significant gap in magnesium alloy joining technology.

The AZ31 and AZ61 alloys represent two distinct grades within the Al-Zn-Mg system: AZ31 is a castable, moderately strong grade used in automotive and aerospace components, while AZ61 offers higher strength and better corrosion resistance for demanding applications. Joining these dissimilar alloys creates unique metallurgical challenges that are not encountered in like-to-like welding.

Core Technical Content

Dissimilar Welding Challenges

Welding AZ31 to AZ61 introduces several metallurgical complexities:

Differential dilution: The differing Al and Zn contents lead to asymmetric composition in the weld metal, creating zones of varying properties.
Hot cracking asymmetry: The AZ61 side, with higher Zn content, is more susceptible to solidification cracking.
Microstructural segregation: The weld zone may exhibit compositional banding, with properties transitioning from AZ31-like on one side to AZ61-like on the other.
Residual stress concentration: Different thermal expansion behaviors and solidification sequences generate complex residual stress fields.

Composition and Dilution Analysis

Parameter	AZ31	AZ61	Weld Metal (Typical)
Al (wt%)	2.5–3.5	5.0–7.0	3.5–5.5
Zn (wt%)	0.5–1.5	0.5–1.5	0.6–1.2
Mg (wt%)	Balance	Balance	Balance
Tensile Strength (MPa)	200–250	280–330	220–280
Elongation (%)	10–14	8–12	8–12

Welding Process Optimization

The study likely examined the following process variables:

Current polarity: AC TIG is preferred for magnesium alloys to provide cathodic cleaning and anodic heating effects.
Shielding gas: 99.99% Ar with high flow rates (15–20 L/min) is essential to prevent oxidation of molten magnesium.
Filler wire selection: ER50A or matching AZ61 filler wire is typically used to provide adequate strength and reduce cracking tendency.
Preheating: Moderate preheating (100–200°C) reduces thermal gradients and residual stress but must be balanced against grain growth.
Travel speed: Higher speeds (10–15 mm/s) reduce heat input and minimize dilution of the AZ61 side.

Microstructural Characteristics

The weld zone in dissimilar AZ31/AZ61 joints typically exhibits:

Columnar dendrites with secondary arm spacing varying across the weld width.
Precipitation of β-phase (Mg₁₇Al₁₂) along grain boundaries, particularly on the AZ61 side.
Widmanstätten structure in the heat-affected zone, formed by rapid heating and cooling.
Grain coarsening in the HAZ near the AZ61 side due to higher thermal exposure.

Engineering Practice Integration

Code and Standard Considerations

Magnesium alloy welding is governed by fewer standards compared to steel or aluminum, creating challenges for pressure vessel and structural applications:

Standard	Applicability	Key Requirement
ASME IX	Welding procedure qualification	Qualified WPS for dissimilar Mg alloys
EN 1090	Structural steel (limited Mg coverage)	Mechanical testing requirements
ASTM B92	Magnesium welding consumables	Filler wire specifications
ISO 13919	Welding of magnesium alloys	General welding guidelines

Defect Analysis and Countermeasures

Defect	Mechanism	Prevention
Hot cracking	Low-temperature eutectics in Al-Zn-Mg system	Use higher Al filler, reduce Zn in weld
Oxidation inclusions	MgO formation at weld pool surface	Enhanced shielding, no back purging needed for Mg
Porosity	N₂ and H₂ absorption	Clean surfaces, high-purity Ar
Undercut	Excessive heat input on AZ31 side	Adjust parameters, use backer bar

Residual Stress Management

The asymmetric thermal expansion between AZ31 and AZ61 generates complex residual stress patterns. Stress-relief annealing at 150–200°C for 2–4 hours can reduce residual stresses by 40–60% without significant grain growth, provided the weld is not in the peak-aged condition.

Study Insights and Reflections

This research is foundational for any engineer considering magnesium alloy dissimilar welding in structural or pressure vessel applications. The key insight is that dissimilar magnesium alloy welding cannot be treated as a simple "compromise" between the two base metals. Instead, the weld metal composition, microstructure, and properties must be actively controlled through filler selection and process parameter optimization.

The practical implication is significant: in lightweight structural applications such as automotive brackets, aerospace components, and potentially magnesium-lined pressure vessels for hydrogen storage, the ability to join dissimilar grades opens design flexibility. However, the engineer must be cognizant that the weld zone will not match either base metal in strength, and design codes must be applied conservatively.

From a quality assurance perspective, the FMEA approach reveals that the highest-risk failure modes are solidification cracking and oxidation inclusions. Process control plans should include 100% visual inspection, dye penetrant testing, and periodic mechanical testing of coupon welds. Radiographic testing is limited for magnesium alloys due to low atomic number and poor X-ray absorption contrast.

The study also highlights an important industrial trend: as magnesium alloys gain acceptance in structural applications, the welding community must develop comprehensive databases of qualified procedures, inspection methods, and design allowables. This is a significant gap compared to aluminum and steel welding, where decades of data exist. Engineers should invest in building this knowledge base through systematic welding trials and long-term property retention testing.