AC Magnetic Field Assisted TIG Welding of Copper-Steel Joints Microstructure and Mechanical Properties

Literature Overview

This study, published in Journal of Materials Engineering (2021, Vol. 49, No. 8), investigates the application of longitudinal alternating current (AC) magnetic fields to TIG butt welding of dissimilar copper-steel joints. The authors from Nanchang Hangkong University and Nanchang University conducted experiments on 2 mm thick T2 pure copper and Q235 carbon steel plates using HS201 pure copper filler wire. The research is funded by the National Natural Science Foundation of China (Grants 51865034, 51965045). The core innovation lies in introducing an external AC magnetic field during the welding process to improve joint formation, microstructure refinement, and mechanical properties of the inherently challenging copper-steel dissimilar weld.

Core Technical Content and Process Parameters

The study systematically varied welding current, welding speed, magnetic field frequency, and magnetic field current to identify optimal parameters. The key findings are summarized in the following table:

The joint microstructure is divided into four distinct zones: steel-side heat-affected zone (HAZ), fusion zone, weld zone, and copper-side HAZ. Without magnetic field application, the steel-side HAZ contains large blocky ferrite, while the weld zone exhibits cellular dendritic structures. After AC magnetic field introduction, the ferrite morphology in the steel-side HAZ transforms from blocky to acicular and side plate-like configurations, indicating significant microstructure refinement. The weld zone transitions from cellular crystals to fine and uniform cellular dendrites. The copper-side HAZ also shows notable grain refinement.

Phase Composition and Solute Segregation Analysis

A critical finding is that the phase composition in both the fusion zone and weld zone remains unchanged with magnetic field application, consisting of (α-Fe) + (ε-Cu) intermetallic phases. However, the morphology and distribution of these phases undergo substantial transformation. In the fusion zone, the (α-Fe) + (ε-Cu) particles change from coarse dispersed distribution to fine clustered distribution, significantly improving the solute segregation situation. This is particularly important because copper-steel dissimilar welds are notorious for forming brittle Cu-Fe intermetallic compounds at the fusion boundary, which severely compromise joint integrity.

The hardness gradient between the fusion zone and weld zone is notably reduced with magnetic field application. This is attributed to the refined and more uniform weld microstructure, which eliminates the sharp property transitions that typically serve as crack initiation sites. From a metallurgical perspective, the AC magnetic field likely influences the molten pool convection patterns, promoting more uniform temperature distribution and enhanced nucleation during solidification.

Engineering Practice Implications and Reflections

The 44.5% improvement in tensile strength is remarkable for a dissimilar copper-steel joint, which typically exhibits very poor mechanical properties due to the formation of brittle intermetallics. In engineering applications such as electrical busbar connections, heat exchanger tube-to-tube-sheet joints, and power generation equipment, copper-steel joints are common but often problematic. The traditional approach involves mechanical fastening or brazing with intermediate layers, but direct welding remains desirable for compact designs.

The magnetic field parameters identified (25-35 Hz, 0.4-0.6 A) are relatively low-frequency and low-amplitude, suggesting that the magnetic field interacts with the molten pool primarily through electromagnetic stirring effects rather than direct magnetic force on solid material. This is consistent with established principles of magnetic field-assisted welding, where Lorentz forces and electromagnetic stirring promote more uniform heat distribution and refined solidification structures.

One important consideration for engineering implementation is the scalability of this technique. The study uses 2 mm thick plates, which is relatively thin. For thicker sections commonly encountered in industrial applications, the penetration depth of the magnetic field and its effectiveness on the entire weld cross-section would need further investigation. Additionally, the AC magnetic field equipment adds complexity and cost to the welding setup, which must be justified by the performance gains.

The refinement of the (α-Fe) + (ε-Cu) particles from coarse dispersed to fine clustered distribution is particularly significant. In copper-steel joints, the morphology and distribution of these intermetallic phases are critical determinants of joint strength and ductility. Coarse dispersed particles create stress concentration sites, while fine clustered distributions provide more uniform load transfer. This finding aligns with the general principle that finer intermetallic phases in dissimilar metal welds contribute to improved mechanical performance.

From a quality control perspective, the reduced hardness gradient between fusion zone and weld zone suggests more homogeneous mechanical properties across the joint, which is beneficial for fatigue resistance and service reliability. The acicular ferrite in the steel-side HAZ, as opposed to blocky ferrite, indicates that the magnetic field also influences the cooling rate or solidification path in the base metal, potentially through enhanced heat dissipation from the molten pool.

This research demonstrates a promising approach to improving dissimilar metal weld quality through external field assistance. While further work is needed on thicker sections, higher production rates, and long-term service behavior, the fundamental findings provide valuable guidance for engineers dealing with copper-steel joint challenges in electrical, thermal, and structural applications.