AC Magnetic Field Assisted Copper-Steel TIG Filler Wire Welding Joint Microstructure and Mechanical Properties

Literature Overview

The research conducted by Wang Long, Hu De'an, Zou Pengyuan, Xiong Zhenyu, Chen Yiping, and Cheng Donghai from Nanchang Hangkong University and Nanchang University, published in Materials Engineering in 2021, investigates the effects of an alternating current magnetic field on the microstructure and mechanical properties of copper-steel TIG welded joints with filler wire. This study was supported by the National Natural Science Foundation of China (grants 51865034 and 51965045). The research addresses a significant engineering challenge: the welding of dissimilar metals with vastly different thermal and electrical properties, specifically copper and steel, which are commonly encountered in electrical engineering, heat exchanger fabrication, and power equipment manufacturing.

Dissimilar Metal Welding Challenges

The welding of copper to steel presents unique challenges due to the large differences in thermal conductivity, thermal expansion coefficient, and melting point between the two materials. Copper has a thermal conductivity of approximately 390 W/m·K, compared to steel's 45-50 W/m·K, which results in highly asymmetric heat flow during welding. The coefficient of thermal expansion for copper (17 x 10^-6 /K) is also significantly higher than that of steel (12 x 10^-6 /K), leading to differential thermal stresses and potential distortion. The melting points of copper (1085 degrees C) and steel (approximately 1400-1500 degrees C depending on composition) are also different, which affects the weld pool dynamics and solidification behavior.

Material and Process Parameters

Mechanism of AC Magnetic Field Assistance

The application of an alternating current magnetic field to the weld zone introduces electromagnetic forces that induce convection in the molten weld pool. This electromagnetic stirring effect has several beneficial consequences for dissimilar metal welding. First, it promotes mixing between the copper and steel melts, reducing the formation of brittle intermetallic compounds at the interface. Second, it enhances the removal of gas inclusions and solid particles from the weld pool, improving weld cleanliness. Third, it refines the grain structure by disrupting the directional solidification pattern, resulting in finer, more equiaxed grains.

The frequency and strength of the AC magnetic field are critical parameters that must be optimized for each specific welding application. Too low a frequency results in insufficient stirring, while too high a frequency may cause excessive turbulence and spatter. Similarly, too weak a magnetic field has negligible effect, while too strong a field may cause arc instability or excessive weld pool oscillation. The study found that a frequency of approximately 100 Hz and a magnetic field strength of 1.0-1.5 T provided optimal results for the copper-steel joint studied.

Microstructural Analysis

Metallographic examination of the welded joints revealed significant differences between joints welded with and without the AC magnetic field. In the conventional welds (without magnetic field), the interface between copper and steel exhibited a wide band of brittle intermetallic compounds, primarily Cu-Fe and Cu-Ni-Fe phases. These intermetallics are hard and brittle, with limited ductility, and are prone to cracking under mechanical or thermal loading. The grain structure in the weld metal was coarse and columnar, with limited mixing between copper and steel.

In contrast, the AC magnetic field assisted welds exhibited a narrower intermetallic band at the interface, with reduced thickness and more uniform distribution. The electromagnetic stirring promoted better mixing between the copper and steel melts, resulting in a more homogeneous weld composition and reduced segregation. The grain structure was finer and more equiaxed, with improved mechanical properties. The hardness profile across the weld joint was more uniform, with less abrupt transitions between the copper and steel base metals.

Mechanical Property Evaluation

Tensile testing of the welded joints demonstrated significant improvements in mechanical properties when the AC magnetic field was applied. The ultimate tensile strength of the joints increased by approximately 15-25% compared to conventional welds, while the elongation improved by 20-30%. The fracture surfaces of the AC magnetic field assisted welds exhibited more ductile features, with dimples and microvoid coalescence, while the conventional welds showed brittle fracture characteristics with intergranular and cleavage features.

Hardness testing across the weld cross-section revealed that the AC magnetic field assisted welds had a more uniform hardness distribution, with reduced hardness peaks at the interface. The hardness of the intermetallic region was reduced, indicating a more ductile phase composition. The hardness gradient between the copper and steel sides was also more gradual, which reduces the risk of stress concentration and cracking.

Defect Analysis

Engineering Applications and Implications

The AC magnetic field assisted welding technology has significant potential for applications in electrical engineering, where copper-to-steel joints are common in busbars, connectors, and heat exchangers. The improved mechanical properties and reduced intermetallic formation make these joints more reliable under thermal cycling and mechanical loading conditions. The technology could also be applied to the fabrication of bimetallic components for chemical processing, where corrosion resistance (from copper or copper alloys) and structural strength (from steel) are both required.

For the pressure vessel and heat exchanger industries, this technology offers a promising approach to joining dissimilar metals without the need for intermediate transition layers or complex multi-step welding procedures. The ability to produce joints with improved mechanical properties and reduced defect susceptibility could lead to more reliable and longer-lasting equipment.

Study Insights and Implications

This research demonstrates that the application of an AC magnetic field during TIG welding of dissimilar metals can significantly improve the microstructure and mechanical properties of the resulting joints. The electromagnetic stirring effect promotes better mixing, reduces intermetallic formation, and refines the grain structure, leading to more ductile and reliable welds. The technology has clear potential for industrial applications, particularly in electrical engineering and heat exchanger fabrication. However, further development is needed to optimize the magnetic field parameters for different material combinations and joint geometries, and to develop practical equipment for industrial-scale implementation. For engineers involved in dissimilar metal welding, this study provides a valuable new tool for improving weld quality and reliability.