Microstructure and Properties of PLC-Controlled TIG Welded Joints in AZ31 Magnesium Alloy

Literature Overview

This study by Zhang Ying, published in Hot Working Technology (热加工工艺) in 2016, investigates the microstructure and mechanical properties of TIG welded joints in AZ31 magnesium alloy, with emphasis on PLC (Programmable Logic Controller) controlled welding parameters. The research originates from the Department of Information and Management Engineering at Inner Mongolia Institute of Mechanical and Electrical Vocational Technology. AZ31 is one of the most widely used wrought magnesium alloys, valued for its excellent combination of low density, good formability, and reasonable corrosion resistance.

Core Technical Content

AZ31 magnesium alloy (containing approximately 3% aluminum and 1% zinc) presents unique welding challenges due to:

Extremely low melting point (650°C) compared to steel (1500°C+)
High reactivity with oxygen and nitrogen at elevated temperatures
Limited solid solubility of aluminum in magnesium matrix
Susceptibility to hot cracking during solidification
Rapid solidification leading to coarse grain structures in the heat-affected zone

PLC-Controlled Welding Parameters

The use of PLC control in TIG welding allows for precise, programmable control of welding parameters throughout the weld sequence. This is particularly valuable for magnesium alloys where:

Precise heat input control is essential to prevent burning
Parameter ramping during start and end sequences prevents crater defects
Consistent parameter maintenance throughout the weld ensures uniform properties
Automated parameter adjustment compensates for process variations

PLC-Controlled Parameter	Typical Range for AZ31	Control Objective
Welding current (A)	80–150	Control heat input and penetration
Travel speed (mm/min)	100–300	Control cooling rate and grain size
Pulse frequency (Hz)	10–50	Control pool dynamics and grain refinement
Pulse width (%)	20–60	Control peak and background current
Gas flow rate (L/min)	10–20	Ensure complete shielding
Interpass temperature (°C)	<150	Prevent grain coarsening

Microstructural Characteristics

The welded joint in AZ31 typically exhibits distinct microstructural zones:

Weld metal: Fine equiaxed grains formed during rapid solidification, potentially with eutectic phases (β-Mg₁₇Al₁₂) at grain boundaries
Heat-affected zone (HAZ): Coarse grains with precipitation of β-phase along grain boundaries, potentially leading to intergranular fracture
Thermo-mechanically affected zone (TMAZ): Partially recrystallized grains with modified precipitation distribution
Base metal: Unaffected AZ31 microstructure with typical wrought grain structure

The HAZ is particularly critical because the β-phase precipitation along grain boundaries significantly reduces ductility and creates preferential paths for intergranular corrosion and fracture.

Interpretation of Technical Points

Solidification Behavior of AZ31 Welds

The solidification of AZ31 weld metal follows a hypoeutectic solidification path. The primary phase is α-Mg (Mg solid solution), with eutectic β-Mg₁₇Al₁₂ forming at the final stages of solidification. The cooling rate in TIG welding (typically 10–100 K/s) produces:

Fine primary α-Mg dendrites with spacing of 20–50 μm
Small amounts of eutectic β-phase at dendrite tips and grain boundaries
Potential for columnar grain growth in the center of the weld

The cooling rate is directly influenced by welding parameters and can be optimized through PLC-controlled parameter scheduling. Higher cooling rates (achieved by lower current, higher travel speed) produce finer microstructures but may increase residual stress.

PLC Control Advantages for Magnesium Alloys

The PLC control system provides several specific advantages for AZ31 welding:

Start/end sequence control: Current ramping prevents crater formation and arc instability at weld initiation and termination
Parameter consistency: Maintains precise current and travel speed throughout the weld, critical for uniform properties
Multi-stage welding: Enables different parameter sets for root, fill, and cap passes
Real-time monitoring: Detects and responds to process anomalies such as arc wandering or shielding gas interruption
Reproducibility: Ensures consistent weld quality across multiple welds and production runs

Mechanical Property Distribution

The mechanical properties of AZ31 TIG welded joints typically show significant variation across the weld cross-section:

Zone	Tensile Strength (MPa)	Yield Strength (MPa)	Elongation (%)	Hardness (HV)
Base metal	200–230	100–120	15–20	55–65
Weld metal	180–210	90–110	12–18	50–60
HAZ (critical)	150–180	70–90	5–10	45–55
TMAZ	170–200	80–100	10–15	50–60

The HAZ typically represents the weakest zone due to grain coarsening and β-phase precipitation, making it the critical region for fracture initiation under both static and fatigue loading.

Engineering Practice Integration

Magnesium Alloy Pressure Vessel Considerations

While magnesium alloys are not commonly used for pressure vessel construction due to corrosion and creep concerns, they find application in:

Lightweight pressure-containing components in aerospace applications
Cryogenic pressure vessels where low weight is critical
Specialized chemical processing equipment where magnesium alloys offer specific advantages
Research and development applications for novel pressure vessel concepts

Cladding Application Relevance

The welding knowledge gained from AZ31 studies has relevance to:

Welding of magnesium alloy components that may be clad with more corrosion-resistant materials
Understanding of light metal welding challenges that apply to aluminum and titanium welding
Development of welding procedures for dissimilar metal joints involving magnesium alloys

Process Optimization for AZ31 Welding

Based on the research findings, the following process recommendations apply:

Shielding gas: Use high-purity argon (99.99%) with pre-flow of 15–30 seconds and post-flow of 30–60 seconds
Current selection: Use AC TIG with appropriate balance (60–70% positive time) for cathode cleaning and penetration
Filler metal: Use AZ91 or AZ61 filler to minimize dilution effects and improve weld metal properties
Preheat: Apply moderate preheat (100–150°C) to reduce thermal stress and prevent cold cracking
Post-weld treatment: Consider solution treatment and aging to restore HAZ properties

Common Defects and Countermeasures

Defect	Cause in AZ31 Welding	Prevention
Hot cracking	Low melting point eutectic at grain boundaries	Reduce travel speed, use appropriate filler
Oxidation	High reactivity of Mg at welding temperatures	Maximize shielding gas coverage, pre-flow/post-flow
HAZ softening	Grain coarsening and β-phase precipitation	Control heat input, consider PWHT
Porosity	Hydrogen absorption from moisture	Dry base metal, use dry shielding gas
Burn-through	Low melting point and thin sections	Reduce current, use backing material

Key Questions and Reflections

The PLC control aspect of this research raises an important question about the role of automation in achieving consistent weld quality for difficult-to-weld materials. While the research focuses on parameter control, the broader implication is that precision control systems are essential for materials with narrow process windows—such as magnesium alloys, where small parameter variations can lead to significant quality differences.

The HAZ weakness identified in this study is a fundamental challenge for magnesium alloy welding that cannot be fully eliminated through process optimization alone. For critical applications, the combination of optimized welding parameters, appropriate filler metal selection, and post-weld heat treatment is necessary to achieve acceptable joint properties.

Study Insights and Implications

This research demonstrates that PLC-controlled TIG welding can significantly improve the consistency and quality of AZ31 magnesium alloy welds through precise parameter management. The key finding is that the HAZ remains the critical zone for joint strength, and while welding parameters can influence HAZ properties, they cannot fully eliminate the grain coarsening and phase precipitation that occur during welding.

For engineers working with light metal alloys in pressure-containing applications, the research reinforces the importance of:

Precise heat input control to minimize HAZ degradation
Comprehensive shielding gas protection to prevent oxidation
Post-weld heat treatment to restore HAZ properties where required
Thorough NDE to detect hot cracks and porosity that are common in magnesium welds

The integration of PLC control with TIG welding represents a significant advancement in welding process control, enabling the automation of parameter scheduling that would be impossible to maintain manually. As the industry moves toward more automated welding operations, this approach offers a pathway to achieving the consistent quality required for safety-critical pressure vessel applications.