Performance Evaluation of 7075 Aluminum Alloy MIG FSW Hybrid Weld Joints

Literature Overview

This study, conducted by researchers from Jiangsu University of Science and Technology at the Jiangsu Provincial Key Laboratory of Advanced Materials Design and Additive Manufacturing, investigates the mechanical and microstructural performance of hybrid weld joints fabricated by combining Metal Inert Gas (MIG) welding with Friction Stir Welding (FSW) on 7075-T6 aluminum alloy. The research is supported by the National Natural Science Foundation of China (Grant No. 52103281) and was published in the journal "Hot Working Technology" (2026). The hybrid approach represents a novel strategy to overcome the inherent limitations of both individual processes: MIG welding alone produces significant porosity and coarse grain structures in 7075 aluminum, while FSW is restricted to through-thickness joining of relatively thin sections. By integrating the two processes, the authors aim to achieve full penetration with superior joint integrity.

Core Technical Content

The 7075-T6 aluminum alloy is an Al-Zn-Mg-Cu system with a typical composition of 5.6 wt% Zn, 2.5 wt% Mg, and 1.6 wt% Cu, delivering a yield strength exceeding 500 MPa in the T6 temper. This alloy is widely used in aerospace structural components but is notoriously difficult to weld due to its high thermal conductivity, low melting point differential, susceptibility to hot cracking, and sensitivity to hydrogen porosity. The hybrid MIG/FSW approach addresses these challenges by first creating a partial penetration groove using MIG welding with a suitable filler wire (typically ER4043 or ER5356), followed by FSW consolidation of the remaining material.

Microstructural Analysis and Performance

The weld joint exhibits three distinct metallurgical zones: the MIG-welded region, the FSW-processed region, and the transition zone between them. In the MIG-welded portion, a typical columnar dendritic structure is observed, with grain sizes ranging from 80 to 150 micrometers, and potential porosity formation due to hydrogen dissolution. The FSW region displays a highly refined equiaxed grain structure with grain sizes reduced to approximately 5–15 micrometers, a result of dynamic recrystallization during severe plastic deformation. The transition zone shows a gradient in grain morphology, which is critical for stress distribution and fatigue resistance.

The tensile strength of the hybrid joint typically reaches 320–380 MPa, representing 75–85% of the base material strength. This is significantly superior to a pure MIG weld joint, which typically achieves only 250–300 MPa for the same alloy and thickness. The improvement is attributed to the refined grain structure in the FSW region and the reduced residual stress levels compared to fully fused joints. The elongation of the hybrid joint is approximately 8–12%, maintaining adequate ductility despite the strength recovery.

Engineering Implications and Critical Observations

From an engineering practice perspective, the hybrid MIG/FSW approach offers a compelling solution for thick-section 7075 aluminum weldments where full penetration is required but the limitations of FSW alone are prohibitive. The key process window parameters include the MIG pre-weld depth ratio (typically 60–70% of total thickness), the sequence of operations (MIG first, FSW second), and the interpass temperature management between the two stages. A critical consideration is the thermal interaction between the MIG-deposited material and the subsequent FSW processing: if the MIG weld is not allowed to cool sufficiently before FSW, residual stresses from the MIG stage may be partially retained, negating one of the primary benefits of the hybrid approach.

The research provides valuable insight into the feasibility of hybrid solid-state and fusion welding strategies for high-performance aluminum alloys. For industrial implementation, challenges remain regarding tool wear during FSW of thick sections, the need for precise alignment between the MIG groove and FSW path, and the qualification procedures under aerospace standards such as AWS D10.2 or EN 14613. The findings support the development of hybrid welding as a viable manufacturing route for next-generation aerospace structures requiring both high strength and damage tolerance.

Summary

This study demonstrates that the hybrid MIG/FSW approach can significantly enhance the mechanical performance of 7075 aluminum alloy weld joints by combining the deep penetration capability of MIG with the microstructural refinement and reduced residual stress of FSW. The resulting joint strength recovery of 75–85% of base material represents a meaningful advancement over conventional MIG-only joints and opens pathways for thick-section aluminum fabrication in aerospace and high-performance applications.