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

Literature Overview

This paper by Zhang Ying, published in 2016 in the Hot Working Technology journal, investigates the microstructure and mechanical properties of AZ31 magnesium alloy welded joints produced using a PLC-controlled TIG welding process. Conducted at the Inner Mongolia Institute of Electromechanical Technology, the study systematically examines the effects of welding current, welding speed, and shielding gas flow rate on weld quality, and identifies optimal parameters through controlled experiments.

Core Technical Points

AZ31 is a widely used wrought magnesium alloy containing 3.0 wt% aluminum and 1.0 wt% zinc, known for its excellent specific strength, good corrosion resistance, and decent castability. However, magnesium alloys are notoriously difficult to weld due to:

Low melting point: 450°C, requiring careful heat input control to avoid excessive HAZ softening.
High vapor pressure: Magnesium readily evaporates at elevated temperatures, leading to gas porosity and material loss.
High reactivity: The oxide layer (MgO) is thermodynamically stable and difficult to break down, promoting oxide inclusions.
Susceptibility to hot cracking: The solidification range and low solid solubility of alloying elements promote hot cracking.

The PLC (Programmable Logic Controller) system used in this study provides precise and repeatable control of welding parameters, enabling systematic investigation of parameter effects.

Parameter Study Results

The study varies three key parameters while maintaining others constant:

Parameter	Range Studied	Optimal Value	Effect on Tensile Strength
Welding current	125–225 A	175 A	Increases then decreases
Welding speed	2–8 mm/s	5 mm/s	Minimal variation
Argon flow rate	5–15 L/min	11 L/min	Wave-like variation

Welding Current Effects

At low currents (125–150 A), the heat input is insufficient for complete fusion, resulting in lack of fusion defects and reduced mechanical properties. As current increases to 175 A, complete fusion is achieved and the weld metal has adequate dilution with the base metal, maximizing tensile and yield strength. Beyond 175 A, excessive heat input causes:

Coarsening of the β-phase (Mg₁₇Al₁₂) precipitates in the weld metal.
Increased porosity due to magnesium vaporization.
HAZ softening due to over-aging of the base metal microstructure.

Welding Speed Effects

Welding speed has a relatively minor effect on mechanical properties compared to current. This is because, for the parameter ranges studied, the linear heat input (Q = VI/v) is primarily controlled by current. However, at very low speeds (< 3 mm/s), excessive heat input leads to a wider weld bead and increased HAZ width, while at very high speeds (> 7 mm/s), incomplete fusion may occur.

Shielding Gas Flow Rate Effects

The wave-like variation in mechanical properties with increasing gas flow rate can be attributed to:

Low flow rates (5–7 L/min): Inadequate shielding allows atmospheric contamination, introducing oxide inclusions and porosity.
Optimal flow rate (11 L/min): Complete shielding prevents contamination while maintaining laminar flow.
High flow rates (13–15 L/min): Turbulent gas flow entrains atmospheric air, paradoxically increasing contamination.

Microstructural Analysis

The weld joint consists of four distinct zones with different microstructures:

Zone	Microstructure	Mechanical Behavior
Base metal	α-Mg matrix with β-phase precipitates	Best strength and ductility
HAZ	Coarsened β-phase, possible recrystallization	Slightly reduced strength
Fusion boundary	Fine-grained α-Mg with dispersed β-phase	Good strength
Weld metal	Columnar α-Mg grains with β-phase at grain boundaries	Lower strength, susceptible to hot cracking

The presence of coarse second-phase particles and oxide inclusions in the weld metal is identified as the primary reason for reduced mechanical properties compared to the base metal. The fracture morphology transitions from ductile dimple rupture in the base metal to a mixed ductile-brittle mode in the weld metal, with intergranular fracture along β-phase boundaries.

Engineering Practice Integration

For industrial production of AZ31 magnesium alloy welded components, the following recommendations emerge:

Pre-weld preparation: Thorough mechanical and chemical cleaning of the joint surfaces to remove the native oxide layer.
Shielding gas optimization: Use of high-purity argon (99.99%) at 10–12 L/min, with a properly designed gas cup to ensure laminar shielding.
Heat input control: Maintain linear heat input in the range of 0.5–0.8 kJ/mm to balance fusion quality with microstructural integrity.
Post-weld treatment: Solution heat treatment (520°C for 2–4 hours followed by water quenching and aging at 150°C for 6–8 hours) can restore mechanical properties by dissolving coarse β-phase and precipitating fine β-phase.
Filler wire selection: Use AZ91 or AZ92 filler wire to improve weld metal castability and reduce hot cracking susceptibility.

FMEA Analysis of Welding Defects

Based on the study findings, a Failure Mode and Effects Analysis (FMEA) for AZ31 TIG welding is presented:

Defect	Severity	Occurrence	Detection	RPN	Countermeasure
Gas porosity	8	6	7	336	Increase gas flow to 11 L/min, reduce current to 175 A
Oxide inclusion	9	5	6	270	Improve surface preparation, use flux if necessary
Hot cracking	10	4	5	200	Use AZ91 filler wire, reduce welding speed
Lack of fusion	9	3	6	162	Increase current to minimum 150 A, optimize travel speed
HAZ softening	6	7	5	210	Reduce heat input, apply post-weld heat treatment

Study Insights and Implications

This research demonstrates that PLC-controlled TIG welding can produce high-quality AZ31 magnesium alloy welds when parameters are carefully optimized. The identification of 175 A current, 5 mm/s speed, and 11 L/min gas flow as the optimal parameter combination provides a practical starting point for welding procedure development.

The wave-like behavior of mechanical properties with gas flow rate is an important finding that may not be immediately obvious to practitioners. It underscores the importance of gas flow optimization through experimental investigation rather than assuming that higher flow rates always improve shielding quality.

For the broader magnesium alloy welding community, this study reinforces the need for precise process control and systematic parameter optimization. As magnesium alloys find increasing application in automotive, aerospace, and electronics industries, reliable welding procedures become increasingly important for enabling lightweight structural designs. The combination of PLC control with systematic parameter studies represents a practical approach to achieving consistent weld quality in production environments.