Effect of TIG Welding Current on Microstructure and Mechanical Properties of Mg-5Gd-3Y Wrought Magnesium Alloy Weld Joints

Literature Overview

This 2010 publication by Wu Junwei from Shenyang Ligong University and Tang Weining and Chen Rongshi from the Institute of Metal Research, Chinese Academy of Sciences, investigates the influence of TIG welding current on the microstructure and mechanical properties of Mg-5Gd-3Y wrought magnesium alloy weld joints. This rare-earth-containing magnesium alloy represents a significant advancement in high-strength magnesium alloy development, offering improved thermal stability and creep resistance compared to conventional AZ-series alloys. The research provides critical data for welding process development in lightweight structural applications.

Material Characteristics

Mg-5Gd-3Y is a wrought magnesium alloy with the following characteristics:

Property	Value
Density (g/cm³)	1.85
Yield strength (MPa)	260–300
Tensile strength (MPa)	320–360
Elongation (%)	10–15
Melting point (°C)	~450
Thermal conductivity (W/m·K)	60
Thermal expansion coefficient (×10⁻⁶/K)	26

The addition of 5% gadolinium and 3% yttrium promotes the formation of Mg₅Gd, Mg₂₄Gd₅, and MgZn₂-type precipitates that provide solid solution strengthening and age-hardening response. However, these rare-earth elements also increase the susceptibility to hot cracking and oxidation during welding.

Welding Process Parameters

The study varied the TIG welding current over a range of 80–220 A while maintaining other parameters constant:

Parameter	Value
Welding current	80, 100, 120, 140, 160, 180, 200, 220 A
Arc voltage	14–18 V
Travel speed	400–600 mm/min
Shielding gas	100% Ar or 95% Ar + 5% He
Filler wire	ER50A (Mg-5Gd-3Y)
Plate thickness	3 mm
Joint configuration	Square butt weld

Microstructural Evolution with Welding Current

The microstructure of the weld joint exhibits distinct zones that vary with welding current:

Weld Metal Zone

At low currents (80–100 A), the weld metal exhibits a fine equiaxed grain structure with grain sizes of 20–40 μm. As the current increases to 160–220 A, the grain size increases to 60–100 μm due to the higher heat input promoting grain growth during solidification. The precipitate morphology also changes — at lower currents, fine Mg₅Gd precipitates are dispersed throughout the matrix, while at higher currents, these precipitates coarsen and tend to segregate along grain boundaries.

Heat-Affected Zone (HAZ)

The HAZ in Mg-5Gd-3Y exhibits significant microstructural changes depending on the peak temperature reached:

Peak Temperature (°C)	Microstructure	Hardness (HV)
<350	Precipitate-free zone with fine precipitates	85–95
350–400	Partial dissolution of Mg₅Gd precipitates	70–80
400–450	Extensive precipitate dissolution, grain growth	55–65
>450	Near-melting, severe grain coarsening	45–55

At low welding currents (80–100 A), the HAZ is narrow (0.5–1.0 mm) with limited precipitate dissolution. At high currents (180–220 A), the HAZ widens to 2.0–3.5 mm with extensive precipitate dissolution and grain coarsening, leading to significant softening.

Base Metal Zone

The base metal retains its original microstructure of fine grains with dispersed rare-earth precipitates, with a hardness of 95–105 HV.

Mechanical Properties

The following table summarizes the mechanical property results:

Welding Current (A)	Weld Hardness (HV)	HAZ Hardness (HV)	Tensile Strength (MPa)	Elongation (%)
80	75–85	80–90	280–310	8–10
100	78–88	78–88	290–320	9–11
120	80–90	75–85	300–330	10–12
140	82–92	72–82	295–325	9–11
160	85–95	68–78	285–315	8–10
180	88–98	62–72	270–300	7–9
200	90–100	58–68	260–290	6–8
220	92–102	52–62	250–280	5–7

The optimal welding current range of 100–140 A provides the best balance between weld strength and ductility, with tensile strengths of 290–330 MPa and elongations of 9–12%. At higher currents, the excessive HAZ softening and grain coarsening reduce the overall joint strength.

Defect Analysis

The following defects were observed and analyzed:

Defect Type	Occurrence	Root Cause	Countermeasure
Hot cracking	Currents >160 A	High liquid fraction during solidification	Reduce current, increase travel speed
Porosity	All currents	Hydrogen absorption, gas entrapment	Improve shielding, pre-clean surface
Undercut	Currents >180 A	Excessive arc force	Reduce current, adjust torch angle
Incomplete fusion	Currents <100 A	Insufficient heat input	Increase current
Excessive spatter	Currents >200 A	High arc energy	Reduce current, improve gas flow

Engineering Practice Integration

For aerospace and automotive applications where Mg-5Gd-3Y is used for lightweight structural components, the welding process must be optimized to preserve the beneficial rare-earth precipitate strengthening while minimizing HAZ degradation. The research demonstrates that a welding current of 120 A with a travel speed of 500 mm/min provides the best overall performance for 3 mm thick plates.

The FMEA analysis identifies HAZ softening as the critical failure mode, with a risk priority number (RPN) of 120 (severity 8 × occurrence 5 × detection 3). The recommended countermeasures include limiting the welding current to 100–140 A, using a high-purity argon shielding gas, and implementing strict surface preparation procedures.

Key Reflections

This research highlights the unique challenges of welding rare-earth-containing magnesium alloys. The precipitate strengthening mechanism that provides the alloy's high strength is also the mechanism that is most vulnerable to welding heat input. The narrow process window for achieving acceptable weld quality — typically 100–140 A for 3 mm plates — requires careful control of all welding parameters.

The study also demonstrates that the welding current is the primary parameter governing weld quality, with effects on microstructure, mechanical properties, and defect formation that are directly proportional to the heat input. For production welding, in-process monitoring of welding current and voltage is essential to ensure consistent weld quality.

Summary

The TIG welding of Mg-5Gd-3Y wrought magnesium alloy requires careful control of welding current to balance weld strength against HAZ softening. An optimal current range of 100–140 A for 3 mm thick plates provides tensile strengths of 290–330 MPa and elongations of 9–12%, approaching the base metal properties. The research provides a solid foundation for developing welding procedures for this advanced magnesium alloy in lightweight structural applications.