Microstructure and Mechanical Properties of DP-MIG Welding and Rolling Composite Joint of 6061-T6 Aluminum Alloy

Literature Overview

This paper, published in the Transactions of the Welding Journal (Vol. 44, No. 4, 2023, pp. 1-6) by Song Gang, Dong Xiaonan, Cheng Jiwen, Wang Zeli, and Liu Liming from Dalian University of Technology and the Liaoning Key Laboratory of Advanced Joining Technology, presents innovative research on combining DP-MIG welding with synchronous dual-roller rolling to address the heat-affected zone softening problem in 6061-T6 aluminum alloy. Funded by the National Natural Science Foundation (U1960111) and the Central Universities Basic Research Business Fee Special Fund (DUT21LAB133), this work represents a significant advancement in weld joint strengthening technology.

Core Technical Problem and Solution

The fundamental challenge addressed is the inevitable strength loss in the heat-affected zone (HAZ) of 6061-T6 aluminum alloy welds. The T6 temper condition, achieved through solution treatment and artificial aging, provides the alloy with its optimal strength through precipitation hardening. During welding, the thermal cycle causes over-aging and dissolution of strengthening precipitates in the HAZ, resulting in significant softening and reduced mechanical properties.

The proposed solution is innovative: using a dual-roller system to perform synchronous double-sided rolling of the DP-MIG weld joint while the material is still at elevated temperature. This approach exploits the weld bead's excess height as a deformation source, using the rolling force to plastically deform the weld metal and the adjacent HAZ, thereby strengthening the over-aged softened region through work hardening.

Process Description and Mechanism

The following table summarizes the key aspects of the welding-rolling composite process:

The deformation mechanism operates through a cascading effect. The weld metal undergoes large plastic deformation in the vertical direction under the rolling force, causing pore elimination, weld width increase, and grain refinement. The adjacent HAZ experiences compressive stress transmitted from the weld metal, resulting in coordinated macroscopic deformation. The over-aged softened region farther from the weld receives compressive stress from the weld area, producing partial coordinated deformation strengthening along the direction perpendicular to the weld.

Mechanical Property Results

The most significant result is the improvement in HAZ hardness from 60-70 HV to 80-90 HV, representing a 20-25% increase. More importantly, the tensile strength of the joint achieves 83.6% of the base metal strength at a welding speed of 800 mm/min, which represents a 19.4% improvement over the unwelded joint. These results demonstrate that the welding-rolling composite process can substantially mitigate the HAZ softening problem that has long plagued aluminum alloy welding.

The improvement is particularly notable because 6061-T6 aluminum alloy is one of the most widely used structural aluminum alloys, and the HAZ softening problem has historically limited its application in high-strength structural welding. The ability to recover 83.6% of base metal strength represents a significant practical improvement.

Engineering Practice Implications

The welding-rolling composite process has significant potential for applications where aluminum alloy strength retention is critical. Potential applications include:

• Aerospace structural components requiring high strength-to-weight ratio
• Automotive lightweight structures where joint strength is critical
• Marine structures exposed to fatigue loading
• Energy storage containers requiring high integrity

The process requires additional equipment (dual roller system) and process integration, but the strength improvement may justify the added complexity in critical applications. The DP-MIG welding process itself provides advantages over conventional MIG welding, including reduced heat input and improved weld geometry, which complement the rolling operation.

Key Reflections

This research represents a paradigm shift in aluminum alloy welding from a purely thermal process to a thermo-mechanical process. The concept of using plastic deformation to strengthen the HAZ is fundamentally different from traditional approaches that focus on minimizing thermal damage through reduced heat input or post-weld heat treatment.

The approach also addresses a practical limitation of post-weld heat treatment: not all welded structures can be easily heat treated due to size, complexity, or application constraints. The welding-rolling composite process provides an in-situ strengthening solution that can be integrated into the production line without requiring separate heat treatment operations.

The use of the weld bead's excess height as the deformation source is particularly elegant, as it simultaneously eliminates the need for post-weld grinding of the weld reinforcement and provides the material for HAZ strengthening. This dual benefit improves both mechanical properties and dimensional accuracy.

Summary

This paper presents a breakthrough approach to solving the HAZ softening problem in 6061-T6 aluminum alloy welding through the innovative combination of DP-MIG welding and synchronous dual-roller rolling. The resulting joint achieves 83.6% of base metal tensile strength, with HAZ hardness improvement from 60-70 HV to 80-90 HV, representing a 19.4% improvement over conventional weld joints. The approach transforms aluminum alloy welding from a purely thermal process to a thermo-mechanical process, offering a viable solution for high-strength structural applications where joint strength is critical.