Weldability of AZ91D Magnesium Alloy Using Aluminum-Based Filler Wire with Zinc Addition

Literature Overview

This paper by Mao Jin and colleagues from North University of China, published in Special Casting and Nonferrous Alloys (2018, Vol. 38, No. 4, pp. 425–428), investigates the weldability of AZ91D magnesium alloy using aluminum-based filler wires (S301 φ1.2 mm and ER5356 φ1.6 mm) and evaluates the effect of zinc alloy addition on weld metal microstructure and mechanical properties. This research addresses a significant challenge in magnesium alloy welding: the formation of brittle intermetallic compounds at the weld/base metal interface.

Material System and Welding Challenge

AZ91D is a widely used wrought magnesium alloy (Mg-9Al-1Zn) in automotive and aerospace applications due to its excellent strength-to-weight ratio. The fundamental challenge in welding this alloy lies in the thermodynamic tendency for Mg-Al intermetallic compounds to form when aluminum-containing filler metals are used. The Mg₁₇Al₁₂ phase (also written as Mg₁₇Al₁₂ or β-phase) is a hard, brittle intermetallic that significantly degrades the ductility and fracture toughness of the weld metal.

Experimental Design and Key Findings

Welding Parameters and Wire Selection

Parameter	Configuration 1	Configuration 2
Filler wire	S301	ER5356
Wire diameter	1.2 mm	1.6 mm
Process	MIG (GMAW)	MIG (GMAW)
Base metal	AZ91D	AZ91D
Shielding gas	Argon	Argon

Both wire configurations achieved valid welds within their respective parameter ranges, demonstrating that aluminum-based filler wires are viable for AZ91D welding despite the intermetallic formation concern.

Microstructural Analysis

The weld metal microstructure consists of two primary phases:

Mg-based solid solution (α-phase): Provides ductility and toughness.
Mg₁₇Al₁₂ intermetallic (β-phase): Hard and brittle, detrimental to mechanical properties.

The relative proportion of these two phases depends on the local Mg and Al content in the weld pool, which is determined by the dilution ratio between base metal and filler metal.

Effect of Zinc Addition

The most significant finding of this study is the effect of zinc alloy addition on the weld microstructure:

Condition	Mg₁₇Al₁₂ Content	MgZn₂ Phase	Mg Solid Solution	Mechanical Properties
Without Zn addition	High	Absent	Reduced	Poor ductility
With Zn addition	Significantly reduced	Present	Promoted growth	Improved comprehensive properties

The zinc addition mechanism operates through several pathways:

Competitive phase formation: Zn preferentially combines with Mg to form MgZn₂, reducing the available Mg for Mg₁₇Al₁₂ formation.
Solid solution modification: Zn atoms dissolved in the Mg matrix alter the thermodynamic driving force for Mg₁₇Al₁₂ precipitation.
Microstructural refinement: The presence of MgZn₂ particles may act as nucleation sites for the Mg solid solution, promoting grain refinement.

Thermodynamic and Metallurgical Analysis

The formation of Mg₁₇Al₁₂ is governed by the equilibrium phase diagram of the Mg-Al binary system. At welding temperatures (approximately 650–750°C for magnesium alloys), the Mg₁₇Al₁₂ phase is thermodynamically stable. However, the introduction of zinc creates a ternary Mg-Al-Zn system where MgZn₂ becomes a competing stable phase.

The equilibrium phase diagram of the Mg-Zn system shows that MgZn₂ is stable up to approximately 450°C, well above the service temperatures of most magnesium alloy applications. This makes MgZn₂ a viable strengthening phase that does not suffer from the extreme brittleness of Mg₁₇Al₁₂.

Engineering Practice and FMEA Considerations

Failure Mode Analysis

Failure Mode	Cause	Detection Method	Countermeasure
Brittle fracture	Excessive Mg₁₇Al₁₂	Metallography, hardness test	Add Zn to filler wire
Hot cracking	Rapid solidification	Visual inspection, X-ray	Preheat, adjust parameters
Porosity	Hydrogen absorption	X-ray, ultrasonic testing	Dry shielding gas, clean surface
Oxidation	Mg reactivity	Visual inspection	Inert atmosphere, flux

Practical Recommendations

For production welding of AZ91D magnesium alloy components:

Preheat the base metal to 150–200°C to reduce thermal gradients and cracking susceptibility.
Use high-purity argon shielding gas (≥99.995%) to minimize porosity formation.
Apply zinc-containing filler wire or add zinc foil to the weld pool to suppress Mg₁₇Al₁₂ formation.
Control welding speed to maintain appropriate dilution ratio (target: 30–50% base metal dilution).
Apply post-weld heat treatment (solution treatment at 415°C for 2–4 hours) to dissolve any remaining intermetallics.

Study Insights and Implications

This research demonstrates a practical metallurgical solution to the fundamental challenge of magnesium alloy welding with aluminum-based filler metals. The zinc addition strategy is particularly valuable because it requires minimal modification to existing welding equipment and procedures—only the filler material composition needs adjustment.

The finding that Zn can "promote the growth of Mg-based solid solution" suggests a beneficial modification of the weld microstructure that goes beyond simple intermetallic suppression. This has implications for fatigue performance, as a ductile matrix surrounding dispersed strengthening phases typically provides superior crack resistance compared to a coarse intermetallic network.

For engineering applications in automotive lightweighting and aerospace structural components, this work provides a pathway to achieving acceptable weld quality in AZ91D without resorting to exotic or expensive filler metals. The approach is scalable to production welding and compatible with standard MIG welding equipment.