Effect of Environmental Temperature and Humidity on Porosity and Mechanical Properties of MIG Welded Joints of 6082-T6 Aluminum Alloy Profiles

Literature Overview

This study investigates how ambient temperature and relative humidity affect the quality of MIG (Metal Inert Gas) welded joints in 6082-T6 aluminum alloy profiles, with particular focus on porosity formation and resulting mechanical property degradation. The research is significant because aluminum alloy welding is highly sensitive to environmental conditions, and understanding these sensitivities is essential for producing reliable weldments in outdoor or uncontrolled workshop environments. From a cladding and weld overlay perspective, the porosity mechanisms and their mitigation strategies are directly applicable to weld overlay processes used in pressure vessel fabrication.

Core Technical Approach

The study examines MIG welding of 6082-T6 aluminum alloy profiles under controlled environmental conditions, varying temperature from 5°C to 40°C and relative humidity from 30% to 90%. The welding parameters include a wire feed speed of 5–8 m/min, an arc voltage of 18–22 V, a travel speed of 300–500 mm/min, and a shielding gas flow rate of 15–20 L/min using pure argon. The 6082-T6 alloy is a precipitation-hardened aluminum-magnesium-silicon alloy with a yield strength of approximately 260 MPa and an ultimate tensile strength of 310 MPa in the as-received condition.

Porosity Mechanisms

Porosity in aluminum MIG welds arises primarily from hydrogen dissolution. Hydrogen is introduced into the weld pool from moisture on the base metal surface, contamination on the welding wire, and moisture in the shielding gas. The solubility of hydrogen in liquid aluminum is significantly higher than in solid aluminum—approximately 0.036 cm³/g at 700°C versus 0.0008 cm³/g at 20°C. As the weld pool solidifies, hydrogen is rejected from the solidifying grain boundaries and forms bubbles that become trapped as porosity. The study demonstrates that high humidity conditions (above 70%) increase porosity volume fraction from 1.5% to over 8% in severe cases, while low temperatures exacerbate the problem by increasing the cooling rate and reducing the time available for bubbles to escape.

The relationship between environmental conditions and porosity formation can be summarized as follows: high humidity increases hydrogen absorption from surface moisture, low temperature increases the cooling rate which reduces bubble escape time, and the combination of both factors produces a synergistic effect that dramatically increases porosity. This is directly analogous to the hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSC) mechanisms observed in carbon steel pressure vessels, where hydrogen ingress from the environment or from welding processes leads to brittle fracture.

Mechanical Property Degradation

The study quantifies the effect of porosity on the mechanical properties of the welded joints. Tensile tests reveal that joints with porosity volume fractions exceeding 5% show a 15–25% reduction in ultimate tensile strength compared to sound joints. The reduction is attributed to the effective cross-sectional area reduction caused by porosity and the stress concentration effect at pore boundaries. Hardness measurements show a 10–20% reduction in the heat-affected zone (HAZ) hardness under high-humidity conditions, likely due to the combined effects of porosity and altered solidification microstructure. The study also reports that the fatigue strength of the welded joints is significantly degraded by porosity, with a 30–50% reduction in fatigue life at stress levels below the yield strength.

Microstructural Analysis

Metallographic examination reveals that the weld microstructure in 6082-T6 MIG welds consists of a fine-grained equiaxed dendritic structure in the weld metal and a grain-refined but precipitate-free structure in the HAZ. The T6 temper is completely lost in the HAZ due to the welding heat input, resulting in a soft zone that is susceptible to cracking under stress. The study demonstrates that higher welding temperatures (i.e., higher heat input) reduce the HAZ width but increase the risk of hot cracking, while lower heat input produces a wider HAZ with greater property loss. The optimal balance is achieved at intermediate heat input levels, with a heat input of 15–20 kJ/mm recommended for 6082-T6 profiles of 6–10 mm thickness.

Countermeasures and Quality Control

The study recommends several countermeasures to mitigate porosity formation: pre-heating the base metal to 100–150°C to reduce hydrogen absorption and slow the cooling rate, using dry shielding gas with a dew point below -40°C, implementing a wire brush or solvent cleaning step before welding to remove surface contamination, and using a pulsed MIG welding mode to reduce heat input and improve gas coverage. From a quality control perspective, the study advocates for ultrasonic testing (UT) or X-ray testing (RT) of welds to detect porosity, with acceptance criteria based on GB/T 11345 or ASME V. The application of the 5W2H methodology is evident in the systematic investigation of how environmental factors affect weld quality.

Study Insights and Implications

This literature provides critical insights into the sensitivity of aluminum alloy welding to environmental conditions, with direct relevance to weld overlay processes used in pressure vessel fabrication. The porosity mechanisms, hydrogen control strategies, and quality assessment methods described in the study are transferable to the overlay welding of stainless steel and nickel-based alloy cladding layers on carbon steel pressure vessels. The study reinforces the importance of environmental control during welding operations and the need for rigorous non-destructive testing to ensure weld integrity—principles that are fundamental to the fabrication and inspection of bimetallic pressure vessels.