CLADDING TECHNOLOGY SHANXI CO., LTDGeneration and Characteristic Analysis of Welding Cracks in AZ31B Magnesium Alloy TIG Welding
Literature Overview
The research by Liu Jinhua, Wang Wenxian, Zhang Hongxia, Li Jinyong, and Xu Bingshe from the School of Materials Science and Engineering at Taiyuan University of Technology (2008), supported by the National Natural Science Foundation of China (Grant 50675148) and Taiyuan University Student Innovation Project (07010745), investigates the formation mechanisms and characteristics of welding cracks in AZ31B magnesium alloy during GTAW/TIG welding. Magnesium alloys are increasingly important for lightweight structural applications in automotive, aerospace, and defense industries, yet their susceptibility to welding cracks presents significant manufacturing challenges.
AZ31B Magnesium Alloy Characteristics
AZ31B is a widely used wrought magnesium alloy containing approximately 3.0 percent aluminum, 1.0 percent zinc, and minor amounts of iron, silicon, and other trace elements. The B designation indicates a wrought product with specific processing conditions. Key metallurgical characteristics relevant to weldability include:
| Property | Value |
|---|---|
| Melting point | 450-480 °C (solidus-liquidus) |
| Thermal conductivity | 70-80 W/(m·K) |
| Coefficient of thermal expansion | 26 × 10⁻⁶ /K |
| Yield strength (as-received) | 130-170 MPa |
| Tensile strength (as-received) | 200-240 MPa |
| Elongation (as-received) | 10-15% |
| Maximum safe temperature | 150-200 °C |
| Oxide melting point (MgO) | 2852 °C |
The extremely low melting range of AZ31B (only 30 °C between solidus and liquidus) creates a narrow solidification interval, which paradoxically increases susceptibility to both solidification cracks and hot cracks. The high thermal conductivity distributes heat rapidly, creating steep thermal gradients at the weld pool boundary.
Crack Formation Mechanisms
The authors identify three primary crack types in AZ31B TIG welds:
1. Solidification Cracks (Hot Cracks)
These form during the final stages of solidification when liquid films between solidifying dendrites are pulled apart by thermal contraction stresses. The narrow solidification range of AZ31B means that a large fraction of the weld pool solidifies simultaneously over a narrow temperature range, creating conditions favorable for crack formation. The aluminum and zinc segregation at dendrite boundaries further weakens the interdendritic regions.
2. Hot Cracks (Liquid Film Cracks)
These develop in the HAZ during cooling when low-melting-point intermetallic phases (Mg₁₇Al₁₂, MgZn₂) form continuous networks at grain boundaries. As the surrounding matrix contracts during cooling, these liquid films rupture, producing intergranular cracks. The presence of iron impurities (>0.005 percent) significantly exacerbates this mechanism by forming iron-magnesium intermetallics with particularly low melting points.
3. Cold Cracks (Delayed Cracks)
Although less common in magnesium alloys than in steels, cold cracks can form when hydrogen from moisture contamination combines with residual stresses in the weld zone. The high diffusivity of hydrogen in magnesium and the relatively low hydrogen embrittlement threshold make this mechanism relevant, particularly when welding in high-humidity environments.
TIG Welding Parameters and Crack Susceptibility
The study examines the effect of various TIG welding parameters on crack formation:
| Parameter | Crack-Free Range | Crack-Prone Range | Mechanism |
|---|---|---|---|
| Current | 80-120 A | >140 A | Excessive heat input increases HAZ |
| Voltage | 10-14 V | >16 V | Wider weld pool, more dilution |
| Travel speed | 150-250 mm/min | <100 mm/min | Low speed increases thermal cycle |
| Electrode angle | 0-15° | >25° | Off-axis heat distribution |
| Shielding gas | Pure Ar (15-20 L/min) | Ar + 5% H₂ | Hydrogen pickup risk |
| Joint gap | <0.5 mm | >1.5 mm | Excessive pool volume |
The optimal welding window for crack-free AZ31B TIG welds is narrow: current of 90-110 A, voltage of 11-13 V, travel speed of 180-220 mm/min, with pure argon shielding at 18-20 L/min. Outside this window, crack susceptibility increases dramatically.
Metallurgical Analysis
Metallographic examination reveals that solidification cracks follow interdendritic paths with smooth, oxide-covered surfaces characteristic of hot cracking. The cracks are typically 0.05-0.3 mm wide and extend 2-5 mm in length, oriented perpendicular to the weld direction. Scanning electron microscopy shows that crack surfaces contain intermetallic phases of Mg₁₇Al₁₂ and MgZn₂, confirming the hot crack mechanism.
The HAZ exhibits a band of recrystallized grains adjacent to the fusion line, followed by a region of partially recrystallized grains. The recrystallized zone is where hot cracks preferentially initiate due to the presence of coarse grain boundaries with segregated low-melting-point phases. The thermal gradient in this region is approximately 50-100 °C/mm, creating significant residual stresses that promote crack propagation.
Countermeasures and Process Optimization
Based on the crack mechanism analysis, the authors recommend the following countermeasures:
- Preheating to 150-200 °C to reduce thermal gradients and slow cooling rates in the HAZ
- Using AC TIG with appropriate balance ratio (40-50 percent positive time) to provide cathodic cleaning without excessive hydrogen pickup
- Employing pulsed TIG with peak current of 120-150 A and background current of 30-50 A to control instantaneous heat input
- Applying external constriction (e.g., ceramic nozzle with reduced orifice) to concentrate the arc and reduce HAZ width
- Using filler metal with slightly higher aluminum content (e.g., AZ91 wire) to reduce segregation tendencies
- Implementing post-weld heat treatment at 175-200 °C for 2-4 hours to relieve residual stresses before crack initiation can occur
- Strictly controlling iron content in base material to below 0.002 percent to minimize iron-rich intermetallic formation
Engineering Practice Implications
For aerospace and automotive manufacturers using AZ31B magnesium alloy components, this research provides essential guidance for developing qualified welding procedures. The narrow process window means that welding parameter control must be more stringent than for aluminum alloys, requiring stable power sources with tight current regulation and precise travel speed control.
The crack susceptibility of AZ31B also has implications for repair welding. Unlike aluminum alloys where repair is relatively straightforward, magnesium alloy repair requires careful management of the thermal cycle to avoid introducing new cracks in the already-stressed component. The authors recommend that repair procedures should include preheating, reduced current, increased travel speed, and post-weld stress relief.
For pressure vessel applications involving magnesium alloy components (such as hydrogen storage vessels), the crack-free requirement is absolute. Any cracking in the weld zone compromises pressure containment and must be detected during inspection. The study reinforces the need for comprehensive NDT including both surface methods (PT, MT) and volumetric methods (UT, RT) for magnesium alloy welds.
Study Insights and Reflections
This paper provides a comprehensive understanding of the fundamental metallurgical mechanisms governing crack formation in AZ31B magnesium alloy TIG welds. The systematic approach—combining parameter variation, metallographic analysis, and SEM examination—establishes clear cause-effect relationships that can guide welding procedure development.
One particularly important finding is the sensitivity of crack formation to iron contamination. In industrial environments where magnesium alloys are processed alongside steel components, even trace amounts of iron transfer can dramatically increase crack susceptibility. This has direct implications for workshop organization and material handling practices.
The research also highlights the importance of post-weld heat treatment as a crack prevention measure. For critical applications, a low-temperature stress relief treatment at 175-200 °C can eliminate the residual stresses that drive cold crack formation without significantly affecting the mechanical properties of the AZ31B alloy. This treatment should be considered a mandatory step in the welding procedure specification for any AZ31B application where crack-free integrity is required.