Effect of Environmental Humidity on AM60 Magnesium Alloy TIG Welding Process and Joint Properties

Literature Overview

The 2008 study by Lu Sheng and colleagues from Jiangsu University (published in Welding, Vol. 2008, No. 4, pp. 22-25), funded by the Jiangsu Provincial Key Laboratory of Advanced Welding Technology (Grant JSAWT-06-08), systematically investigates the influence of ambient relative humidity (60% versus 85%) on the TIG welding of AM60 cast magnesium alloy. This research addresses a practical manufacturing challenge: the sensitivity of magnesium alloy welding to environmental conditions, particularly moisture content in the surrounding atmosphere.

Core Technical Content

Experimental Parameters

Parameter	Dry Condition	Humid Condition
Relative humidity	60%	85%
Base material	AM60 cast magnesium alloy	AM60 cast magnesium alloy
Welding process	TIG (GTAW)	TIG (GTAW)
Shielding gas	Argon	Argon
Welding current range	Variable	Variable
Argon flow rate range	Variable	Variable

Key Findings

Porosity Formation:

Under humid conditions (85% RH), porosity defects are readily formed in the weld zone
Under dry conditions (60% RH), porosity is not observed even with wide parameter variations
The moisture sensitivity of magnesium alloy welding is significantly higher than aluminum alloy welding

Process Parameter Optimization (Humid Conditions):

Higher welding current (170 A) reduces porosity formation
Higher argon flow rate (8-10 L/min) improves shielding effectiveness
Both parameters contribute to better joint mechanical properties under humid conditions

Process Parameter Optimization (Dry Conditions):

Wider parameter window available without porosity
Lower heat input achievable without defect formation
Finer weld microstructure obtained at lower heat input
Reduced HAZ grain coarsening at optimized lower heat input

Microstructural Implications

Condition	Weld Microstructure	HAZ Characteristics	Mechanical Properties
Dry (60% RH), low heat input	Fine grain, equiaxed	Moderate grain growth	Good strength and ductility
Dry (60% RH), high heat input	Coarser grain	Significant grain coarsening	Adequate strength, reduced ductility
Humid (85% RH), high current/high flow	Moderate grain, some porosity	Moderate grain growth	Acceptable properties with defects
Humid (85% RH), low current/low flow	Porous weld metal	Variable	Poor properties; unacceptable quality

Technical Analysis

Moisture Sensitivity Mechanism

The elevated porosity susceptibility of AM60 magnesium alloy under humid conditions arises from multiple mechanisms:

Atmospheric moisture dissolution — Magnesium readily dissolves hydrogen from atmospheric moisture, with solubility increasing with temperature
Shielding gas contamination — Even high-purity argon can absorb moisture from humid ambient air during transfer
Surface oxide hydration — Magnesium oxide/hydroxide layers on the base metal release moisture during heating
Hydrogen pickup — Dissolved hydrogen in the molten pool forms gas porosity upon solidification

Critical Process Parameters

The study identifies critical thresholds for defect-free welding:

Parameter	Minimum for Humid (85% RH)	Recommended for Dry (60% RH)
Welding current	≥ 170 A	120-170 A (flexible)
Argon flow rate	8-10 L/min	5-8 L/min (flexible)
Heat input	Higher (for porosity resistance)	Lower (for microstructure refinement)
Travel speed	Moderate	Moderate to high

The Heat Input Paradox

A notable engineering trade-off emerges from the study results:

Under humid conditions, higher heat input is required to achieve acceptable weld quality (reducing porosity through increased fluidity and gas escape time)
However, higher heat input causes coarser microstructure and potential HAZ softening
Under dry conditions, lower heat input can be used to achieve both defect-free welds and fine microstructure
This creates a fundamental tension between process robustness and microstructural optimization

Engineering Practice Recommendations

Environmental Control Requirements

Control Measure	Specification	Purpose
Workshop humidity	≤ 60% RH	Minimize atmospheric moisture contamination
Shielding gas storage	Dry gas cylinder storage	Prevent gas moisture absorption
Pre-weld cleaning	Solvent cleaning + mechanical preparation	Remove surface moisture and oxides
Gas delivery system	Desiccant filters	Remove moisture from gas supply
Wind protection	Enclosed welding stations	Prevent ambient air contamination of weld zone

Process Selection Matrix

Application Priority	Recommended Condition	Parameter Strategy
Maximum strength/ductility	Dry environment (≤60% RH)	Low heat input, moderate flow
Production robustness	Humid-tolerant parameters	High current, high flow rate
Thin-section welding	Dry environment mandatory	Low heat input, precise control
Thick-section welding	Either condition acceptable	Optimize for joint geometry
Critical service applications	Dry environment + full inspection	Conservative parameters, NDT verification

Study Insights and Outlook

This research provides valuable quantitative data on the environmental sensitivity of magnesium alloy TIG welding, a topic of increasing practical importance as magnesium alloys gain acceptance in lightweight structural applications. The finding that 85% relative humidity fundamentally alters the process window—requiring higher currents and flow rates to achieve acceptable quality—has direct implications for manufacturing facility design and process specification.

From a standards perspective, current welding procedure qualification standards (such as AWS D10.9M for magnesium alloys) do not explicitly address ambient humidity as a variable parameter. This study suggests that humidity should be incorporated as a qualified variable in magnesium alloy welding procedures, with specific limits established based on the production environment. For manufacturers operating in coastal or tropical regions where high humidity is unavoidable, the elevated parameter requirements must be factored into equipment capacity planning and production cost calculations.

The practical recommendation is clear: magnesium alloy welding operations should be conducted in environmentally controlled facilities with humidity maintained below 60% relative humidity. Where this is not feasible, process parameters must be elevated to compensate, with the understanding that this comes at the cost of microstructural quality and potentially reduced long-term performance. This trade-off between manufacturing robustness and material performance is a recurring theme in reactive metal welding and warrants careful consideration during process development for any production application involving magnesium alloys in piping, fittings, or structural components.