AZ31B Magnesium Alloy TIG Welding Crack Formation and Characteristic Analysis

Literature Overview

This 2008 publication from Taiyuan University of Science and Technology, published in Ordnance Materials and Materials and Engineering, provides a systematic investigation of cracking mechanisms in AZ31B magnesium alloy during TIG welding. The authors—Liu Jinhua, Wang Wenxian, Zhang Hongxia, Li Jinyong, and Xu Bingshe—examined the formation, morphology, and characteristics of welding cracks in this widely used wrought magnesium alloy. Supported by the National Natural Science Foundation of China (50675148), this work contributes valuable insights into the cracking behavior of magnesium alloys, which is critical for the reliable fabrication of magnesium components in automotive, aerospace, and consumer electronics industries.

AZ31B Magnesium Alloy Welding Characteristics

AZ31B is a wrought magnesium alloy with a nominal composition of 3.0% Al, 1.0% Zn, and the balance Mg. The addition of aluminum and zinc improves the strength and corrosion resistance of the alloy but also increases its susceptibility to hot cracking during welding. The high vapor pressure of magnesium and its alloys, combined with the low melting point (650°C) and high thermal conductivity, creates challenging welding conditions that predispose the material to cracking.

Welding Parameter	Typical Value	Effect on Cracking
Current (A)	80–150	Higher current increases cracking risk
Travel speed (mm/min)	300–600	Optimal speed minimizes thermal stress
Shielding gas	Ar / Ar-He	He increases penetration and thermal input
Heat input (kJ/mm)	1.5–4.0	Higher heat input promotes cracking
Preheat temperature (°C)	0–150	Moderate preheat reduces cracking
Joint design	Butt / Lap	Joint geometry affects constraint

Cracking Mechanisms and Classification

The cracking observed in AZ31B TIG welds can be classified into several categories, each with distinct formation mechanisms and morphological characteristics. Hot cracks form in the solidification range due to the formation of low-melting-point phases at grain boundaries, particularly Mg-Al eutectic phases with melting points below 450°C. These cracks typically appear as intergranular cracks in the weld centerline or near the fusion boundary.

Cold cracks, although less common in magnesium alloys than in steel, can form due to hydrogen embrittlement. Magnesium has a high affinity for hydrogen, which can dissolve in the liquid metal during welding and then precipitate during solidification, creating internal pressure that leads to cracking. The hydrogen can originate from moisture in the shielding gas, surface oxides, or contamination of the base metal.

Crack Type	Location	Morphology	Primary Cause
Hot crack	Weld centerline	Intergranular, branching	Low-melting Mg-Al eutectic
Hot crack	Fusion boundary	Intergranular	Thermal stress + segregation
Cold crack	HAZ / weld	Transgranular or intergranular	Hydrogen embrittlement
Undercut crack	Weld toe	Surface longitudinal	Thermal stress + constraint

Microstructural Analysis and Fractography

Metallographic examination of the cracked welds reveals distinct microstructural features associated with each crack type. Hot cracks are typically found to be associated with coarse grain boundaries in the weld, where liquid films persist during solidification and are unable to sustain the tensile stresses imposed by shrinkage. The presence of Mg₁₇Al₁₂ intermetallic phases at grain boundaries, which have a eutectic melting point of 449°C, is a key indicator of hot crack susceptibility.

Fractographic analysis using scanning electron microscopy (SEM) distinguishes between the different crack types. Hot crack surfaces exhibit a characteristic intergranular morphology with faceted features corresponding to the crystallographic planes of the magnesium grains. Cold crack surfaces show a more mixed morphology with features of both ductile and brittle fracture, often accompanied by hydrogen blistering or microvoid coalescence.

Countermeasures and Process Optimization

Several process modifications can reduce the cracking susceptibility of AZ31B TIG welds. Controlling the heat input within a narrow range (2.0–3.0 kJ/mm) minimizes the thermal gradient and reduces the tensile stresses that promote cracking. The use of a slightly constricted joint fit-up allows for some plastic deformation to accommodate shrinkage stresses. Preheating the base metal to 100–150°C reduces the cooling rate and the thermal gradient, while also driving off surface moisture that could contribute to hydrogen pickup.

The selection of filler metal is also critical. Using a filler with a slightly higher aluminum content than the base metal (e.g., ER40470 with 5% Al) can reduce the formation of low-melting-point phases at grain boundaries. Additionally, the use of helium-enriched shielding gas can increase the penetration depth and reduce the weld width, creating a more favorable solidification pattern that minimizes the formation of hot cracks.

Study Insights and Reflections

The systematic investigation of cracking in AZ31B magnesium alloy TIG welds provides a comprehensive understanding of the mechanisms, characteristics, and countermeasures for this critical welding defect. The work underscores the importance of controlling both the metallurgical factors (alloy composition, microstructure, low-melting-point phases) and the process factors (heat input, cooling rate, constraint) in achieving crack-free welds. For engineers involved in the fabrication of magnesium alloy components, including bimetallic assemblies where magnesium is used in combination with steel or aluminum, this knowledge is essential for developing robust welding procedures that ensure structural integrity and reliability. The findings also highlight the need for careful process qualification and quality control when welding magnesium alloys, given their inherent susceptibility to cracking and their sensitivity to contamination.