Effect of Root Gap on Chemical Composition and Microstructure Evolution in 6061-T6 Aluminum Alloy MIG Welds

Literature Overview

This 2020 publication from Harbin Welding Research Institute and Central South University, authored by Fang Naiwen, Liu Xinyu, Huang Ruisheng, Yang Yicheng, Ma Yiming, and Xu Kai, investigates the influence of root gap on the chemical composition and microstructure evolution in MIG welds of 6061-T6 aluminum alloy. The research was supported by the Mechanical Science Research Institute Technology Development Fund (201810903-2) and the Heilongjiang Province Energy Equipment Advanced Welding Technology Innovation Team Fund (201910312). The work addresses a practical manufacturing issue: how root gap variation, which is inevitable in production welding, affects weld quality and properties.

Core Technical Content

Root gap is one of the most critical fit-up parameters in welding, yet it is also one of the most difficult to control in production environments. In MIG welding of aluminum alloys, root gap directly affects heat input distribution, weld pool geometry, and solidification conditions, all of which influence the final weld composition and microstructure. The study systematically investigated the effect of root gap variation from 0 to 3 mm on the chemical composition, microstructure, and mechanical properties of 6061-T6 aluminum alloy MIG welds.

The root gap was controlled using precision fixtures and verified by visual and optical measurement before welding. Welding parameters were held constant throughout the study to isolate the effect of root gap. The welding current was set at 200 A, travel speed at 550 mm/min, arc length at 3 mm, and shielding gas flow at 15 L/min of pure argon. The filler wire was ER4043 aluminum alloy, which is the standard filler for 6061 welding.

The results showed that a moderate root gap of 1 to 2 mm produced the best combination of mechanical properties and weld quality. At zero gap, the heat input was lowest and the weld bead was narrow, resulting in incomplete fusion and reduced mechanical properties. At excessive gap (3 mm), the heat input increased significantly, leading to excessive dilution, coarse grain structure, and reduced mechanical properties.

Chemical Composition Analysis

The chemical composition of the weld metal varied systematically with root gap due to changes in dilution ratio. At zero gap, the dilution ratio was approximately 35%, meaning that 35% of the weld metal was base material and 65% was filler metal. At 3 mm gap, the dilution ratio increased to 50%, with equal contributions from base material and filler metal.

The Mg and Si content in the weld metal increased with root gap, reflecting the higher dilution ratio. At 0 mm gap, the Mg content was approximately 0.35% and Si content was 0.55%.