Welding Current Effects on Microstructure and Mechanical Properties of Aluminum-Galvanized Steel TIG Welds

Literature Overview

This paper by Wu Kanglong, Yuan Xinjian, Wang Haodong, and Hu Zhan from the College of Materials Science and Engineering at Chongqing University, published in Chinese Journal of Electron Microscopy in 2017 (Volume 36, Issue 6, pages 552-555), investigates the effects of welding current on the microstructure and mechanical properties of aluminum-galvanized steel lap joints produced by TIG brazing. The study was supported by the National Natural Science Foundation of China (Grant No. 51205428). The authors used Al-Si12 filler material to join 6061 aluminum alloy to HSLA350 galvanized steel and characterized the joints using SEM, EBSD, XRD, and EDS.

Core Technical Content

The study examines three welding current levels: 40 A, 60 A, and 80 A. At 40 A, the joint is poorly bonded, indicating insufficient heat input to achieve metallurgical bonding. At 80 A, excessive intermetallic compound (IMC) formation degrades mechanical properties. At 60 A, the joint achieves optimal properties with a shear strength of 188.4 MPa, representing 71% of the 6061 aluminum base metal strength.

The microstructural analysis reveals a layered IMC structure at the interface. On the aluminum side, a relatively thick FeAl3 layer forms, while on the steel side, a thinner Fe2Al5 layer develops. Small amounts of Fe3Al are also present. The formation of these brittle intermetallic phases is the primary mechanism of joint strength degradation at high welding currents.

Microstructural Analysis

The interface microstructure is critical to joint performance. The FeAl3 phase, which forms preferentially on the aluminum side, is relatively thick and brittle. The Fe2Al5 phase on the steel side is thinner but equally brittle. These IMCs have poor ductility and are prone to cracking under mechanical loading. The presence of Fe3Al, while in small quantities, further contributes to interface embrittlement.

The EBSD analysis reveals that the IMC layers exhibit distinct crystallographic orientations, with FeAl3 showing a complex unit cell structure and Fe2Al5 exhibiting a hexagonal arrangement. The EDS analysis confirms the stoichiometry of these phases and provides quantitative data on their thickness and composition gradients. The XRD analysis identifies the crystalline phases present and provides information on phase purity and crystallite size.

The formation of IMCs is thermodynamically driven and kinetically controlled. Higher welding currents increase the interface temperature, accelerating the diffusion of iron and aluminum atoms across the interface. This promotes IMC nucleation and growth, leading to thicker layers. The growth kinetics follow a parabolic rate law, where the square of the layer thickness is proportional to time at temperature.

Mechanical Property Analysis

The shear strength of 188.4 MPa at 60 A represents a significant achievement for dissimilar metal joints. The failure mode at this current level likely involves cohesive failure within the aluminum side of the joint, rather than interfacial failure. This indicates that the IMC layer is thinner than the critical thickness for interfacial failure, and the joint strength is limited by the weaker aluminum base metal.

At 80 A, the increased IMC thickness promotes interfacial failure, where cracking initiates at the IMC/base metal interface and propagates along the interface. The reduced shear strength reflects the brittle nature of the IMC layer and its poor bonding to the adjacent base metals. The presence of porosity and lack of fusion, which may occur at excessive currents, further degrades mechanical properties.

Engineering Practice Integration

In pipeline and pipe fitting applications, dissimilar metal joints are common where aluminum components interface with steel structures. For example, aluminum alloy pipe supports may be welded to steel pipe frameworks, or aluminum alloy fittings may be joined to steel pipe assemblies. The TIG brazing approach described in this paper offers a viable solution for such applications, provided the welding parameters are carefully controlled.

The optimal current of 60 A for the specific joint configuration studied provides a baseline for welding procedure development. However, the optimal current will vary with joint geometry, filler material, base metal thickness, and thermal mass. Engineers developing welding procedures for dissimilar metal joints must conduct systematic parameter optimization studies similar to the one described in this paper.

The IMC formation issue is a fundamental challenge in aluminum-steel joining. Unlike similar metal joints, where the weld microstructure can be controlled through heat input management, dissimilar metal joints are inherently limited by the thermodynamic tendency to form brittle intermetallic phases. Process development must focus on minimizing IMC thickness while ensuring adequate bonding.

Key Questions and Reflections

A critical question is the long-term stability of the joint under thermal cycling or corrosion exposure. The IMC layers, while thin at the optimal current, may grow under sustained thermal exposure, particularly in high-temperature service environments. Corrosion may also preferentially attack the IMC/base metal interfaces, leading to joint degradation over time.

Another consideration is the effect of joint geometry on the parameter sensitivity. The lap joint configuration used in this study provides a relatively simple geometry, but actual pipeline and pipe fitting joints may involve more complex geometries with varying thermal masses and heat flow paths. The parameter optimization must account for these geometric effects.

Study Insights and Implications

This paper demonstrates that TIG brazing with Al-Si12 filler can produce functional aluminum-steel joints with acceptable mechanical properties. The identification of the optimal welding current and the characterization of the IMC formation mechanism provide valuable guidance for engineering applications. The systematic use of advanced characterization techniques (SEM, EBSD, XRD, EDS) to correlate microstructure with mechanical properties exemplifies best practices in welding metallurgy research.

For engineers working on dissimilar metal joining in pipeline and pipe fitting applications, this paper reinforces the importance of understanding the fundamental metallurgical processes governing joint formation. The IMC formation mechanism, while challenging, can be managed through careful parameter control. The key is to operate within the narrow window where bonding is sufficient but IMC growth is limited.

The paper also highlights the potential of TIG brazing as a viable alternative to solid-state joining methods such as friction stir welding for aluminum-steel joints. While friction stir welding offers superior mechanical properties, it requires specialized equipment and is limited to specific joint configurations. TIG brazing, with its lower equipment cost and greater geometric flexibility, may be preferable for many practical applications.