TIG Brazing of AZ61 Magnesium Alloy and DP590 Galvanized Steel Dissimilar Metals

Literature Overview

Published in 2018 by Jiang Li and Liu Hui, this study investigates the TIG brazing of AZ61 magnesium alloy to DP590 galvanized steel, a combination of significant interest in lightweight automotive body-in-white applications. The project was funded by the Xinjiang Uygur Autonomous Region University Scientific Research Plan (XJEDU2014S074). The research addresses the challenge of joining dissimilar metals with vastly different thermal properties, chemical reactivity, and mechanical behavior, using a semi-solid brazing approach that avoids full melting of either base metal.

Material Compatibility Analysis

The fundamental challenge in joining AZ61 to DP590 lies in the incompatibility of their metallurgical systems. AZ61 is a wrought magnesium alloy containing approximately 6% aluminum and 1% zinc, with a melting point of approximately 575°C. DP590 is a dual-phase (ferrite + martensite) high-strength steel with a yield strength of 590 MPa and a melting point of approximately 1420°C. The thermal expansion coefficients differ significantly: magnesium alloys expand at approximately 26 × 10⁻⁶ /K, while steels expand at approximately 12 × 10⁻⁶ /K, creating substantial residual stresses during cooling.

Property	AZ61 Magnesium Alloy	DP590 Dual-Phase Steel
Density	1.83 g/cm³	7.85 g/cm³
Thermal conductivity	96 W/(m·K)	45 W/(m·K)
Thermal expansion coeff.	26 × 10⁻⁶ /K	12 × 10⁻⁶ /K
Melting point	575°C	1420°C
Yield strength	170 MPa	590 MPa
Corrosion potential	-1.6 V vs. SCE	-0.4 V vs. SCE

Brazing Process Parameters

The TIG brazing process uses a filler metal with a melting range below the solidus temperature of both base metals. The researchers employed a Zn-based filler (Zn-22Al) with a liquidus temperature of approximately 420°C, well below the AZ61 solidus. The brazing temperature was controlled at 400–450°C, achieved through careful management of the TIG arc energy input and the use of a preheating fixture.

Process Parameter	Typical Value	Rationale
Arc current	80–150 A	Sufficient to melt filler, avoid base metal melting
Voltage	10–14 V	Low voltage to minimize heat input
Travel speed	40–100 mm/min	Control heat input and filler flow
Shielding gas	Pure Ar	Prevent oxidation of Mg and Zn
Flux	ZnCl₂-based or none (with Ar)	Remove oxide films
Preheat	200–300°C	Reduce thermal gradient
Joint gap	0.1–0.3 mm	Capillary action for filler flow

Microstructural Analysis

Metallographic examination reveals a distinct three-zone microstructure: a brazed joint zone with Zn-Al intermetallic compounds (MgZn₂, MgAl₂), a heat-affected zone on the magnesium side with grain growth and precipitate dissolution, and a tempering zone on the steel side with martensite decomposition. The brittle MgAl₂ phase at the interface is a concern for long-term mechanical performance, as it can act as a crack initiation site under cyclic loading.

The galvanic couple between AZ61 and DP590 in a corrosive environment (such as salt spray) creates a severe risk of preferential magnesium corrosion. The potential difference of approximately 1.2 V drives the AZ61 to dissolve as the anodic material. This galvanic corrosion issue is not addressed by the brazing process itself and requires additional protective measures such as coating the joint area with a dielectric barrier.

Engineering Practice Implications

For automotive lightweight design, this joining technology enables the creation of mixed-material structural components where magnesium provides weight savings in non-load-bearing areas and high-strength steel provides crashworthiness in load-bearing zones. However, several engineering considerations must be addressed:

The brazed joint strength is typically limited to 60–70% of the AZ61 base metal tensile strength, which is acceptable for non-critical structural applications but insufficient for primary load-bearing members.
The thermal cycling resistance of the joint must be evaluated for automotive service conditions, particularly in cold-start environments where thermal shock can initiate interfacial cracking.
The joint must be protected from moisture ingress, as the galvanic corrosion rate can be accelerated by any break in the protective coating.

Key Questions and Reflections

A critical question remains unanswered by this study: what is the long-term durability of the brazed joint under combined mechanical and corrosion loading? Accelerated testing data over 1000+ hours of salt spray exposure would be valuable for engineering qualification. Additionally, the study does not address the effect of joint geometry on filler flow and final joint quality, which is a significant practical concern in production welding.

The use of Zn-based fillers introduces a secondary concern: zinc fumes are toxic and require robust local exhaust ventilation during production. This occupational health consideration must be factored into the manufacturing process design.

Study Insights and Implications

This research demonstrates that TIG brazing is a viable technique for joining magnesium alloys to high-strength steels, opening a pathway for mixed-material lightweight structures. The key to successful implementation lies in precise thermal control, appropriate filler selection, and comprehensive post-weld protection against galvanic corrosion. Future work should focus on joint design optimization and long-term durability validation under realistic service conditions.