Interface Analysis and Mechanical Properties of Aluminium Alloy and Low Carbon Steel TIG Brazed Joints

Literature Overview

This paper, authored by Sun Yuping, Sun Yuchong, and Li Xianfen from the School of Materials Science and Engineering at Hefei University of Technology, was published in the Welding (焊接) journal, Issue 5, 2013, pages 39–42. The study investigates the TIG brazing (also referred to as fusion-brazing or semi-solid brazing) of 5052 aluminium alloy to Q235 low carbon steel using 4043 Al-Si filler wire. The research employs optical microscopy (OM), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) to characterise the interface layer morphology and intermetallic compound composition, supplemented by tensile testing to evaluate joint strength. This work addresses a practically significant challenge in the fabrication of dissimilar metal joints, particularly relevant to pipe fitting applications where aluminium and steel components may need to be joined.

Core Technical Content

The Challenge of Dissimilar Metal Joining

The joining of aluminium alloys to carbon steels presents a fundamental metallurgical challenge. The large difference in melting points (approximately 660°C for aluminium versus 1500°C for steel), thermal expansion coefficients, and chemical affinity between the two metals makes conventional fusion welding impractical. In fusion welding, the high melting point of steel results in excessive heat input to the aluminium side, leading to over-melting and cracking. Conversely, the low melting point of aluminium results in poor wetting of the steel surface and weak mechanical bonding.

TIG brazing (or fusion-brazing) offers a viable alternative by controlling the heat input to keep the steel base metal below its melting point while allowing the filler metal to melt and flow. In this approach, the aluminium alloy is melted and flows onto the preheated steel surface, where a controlled reaction layer of intermetallic compounds forms at the interface. The mechanical integrity of the joint depends critically on the thickness, composition, and morphology of this reaction layer.

Experimental Methodology

The authors used the following experimental parameters:

The key experimental variable is the welding heat input, which controls the thickness and composition of the intermetallic reaction layer. Excessive heat input leads to thick, brittle intermetallic layers that act as crack initiation sites, while insufficient heat input results in poor wetting and inadequate bonding.

Interface Layer Characterisation

Microstructural Features

The most significant finding of this study is the detailed characterisation of the interface layer at the weld/steel boundary. The authors identified a non-uniform interface layer with a thickness ranging from 8 to 11 micrometres. The thickness varies along the interface, being thicker at the upper portion of the steel bevel and thinner at the lower portion. This non-uniformity is attributed to the local variations in heat input and molten metal flow during the welding process.

The interface layer was further divided into two distinct sub-layers based on their microstructural morphology and phase composition:

This two-layer structure is characteristic of intermetallic compound formation during the reaction between molten aluminium (containing Si from the 4043 filler) and iron from the steel substrate. The reaction proceeds from the steel side outward, with the iron-rich phases (θ-FeAl₃, η-Fe₂Al₅) forming first at the steel interface and the silicon-containing phases (τ-Al₇.₄Fe₂Si, τ₆-FeAl₄Si) forming closer to the weld metal.

Phase Formation Mechanism

The formation of the intermetallic phases follows a well-established sequence in Al-Fe-Si systems:

1. Initial wetting: Molten Al-Si alloy wets the preheated steel surface, forming a thin initial reaction layer of iron-rich phases (θ-FeAl₃, η-Fe₂Al₅).
2. Diffusion-controlled growth: Iron diffuses from the steel into the molten aluminium, while aluminium and silicon diffuse into the steel. The reaction front advances as the intermetallic layer grows.
3. Secondary phase formation: As the reaction proceeds, silicon-containing phases (τ-Al₇.₄Fe₂Si, τ₆-FeAl₄Si) form at the interface between the primary intermetallic layer and the weld metal.
4. Layer stabilisation: The reaction eventually slows as the intermetallic layer becomes thick enough to impede further diffusion, resulting in a stable two-layer structure.

The interlocking finger-like morphology of Layer I is particularly important for joint strength, as it creates a mechanical interlock between the weld metal and the intermetallic layer, enhancing resistance to interfacial delamination.

Mechanical Properties and Failure Analysis

Tensile Strength

The tensile strength of the brazed joints was measured to be approximately 84 MPa. This value is significantly lower than the tensile strength of either the 5052 aluminium alloy base metal (approximately 220 MPa) or the Q235 steel base metal (approximately 370 MPa). However, for brazed dissimilar metal joints, this strength level is considered acceptable for many engineering applications, particularly where the joint is not subjected to high tensile loads.

Fracture Mode

The fracture analysis revealed that the lower portion of the weld/steel interface layer was the weakest link in the joint, serving as the crack initiation site during tensile testing. This finding is consistent with the observation that the interface layer is thinner at the lower portion of the bevel, resulting in a smaller reaction zone and potentially weaker bonding. The fracture likely initiates at the interface between Layer II (the dense plate-like structure adjacent to the steel) and the steel substrate, where the brittle intermetallic phases (θ-FeAl₃, η-Fe₂Al₅) are most concentrated.

Integration with Engineering Practice

Relevance to Pipe Fitting Applications

While the specific application studied in this paper is not pipe fitting fabrication, the underlying metallurgical principles are directly relevant to several pipe and fitting applications:

1. Aluminium-to-steel transitions in heat exchangers: In chemical processing and power generation, aluminium-to-steel brazed joints are used in heat exchanger tubesheets and channel covers. The interface layer characteristics described in this study directly affect the long-term reliability of these joints.
2. Corrosion-resistant pipe systems: In environments where galvanic corrosion between aluminium and steel is a concern, the intermetallic layer composition and thickness influence the galvanic couple behaviour. Iron-rich intermetallic phases are generally more cathodic than the aluminium matrix, which can accelerate localised corrosion at the interface.
3. Repair welding: In the field of pipe repair, TIG brazing may be used to join aluminium components to existing steel piping. Understanding the interface layer characteristics is essential for predicting the service life of such repairs.

Process Optimisation Considerations

Based on the findings of this study, several process optimisation strategies can be identified:

• Heat input control: Minimising the welding heat input reduces the thickness of the intermetallic layer, thereby limiting the volume of brittle phases. However, excessive reduction in heat input may compromise wetting and bonding.
• Filler wire selection: The 4043 Al-Si filler wire was used in this study. Alternative filler wires with different Si and Mg contents may produce different intermetallic phase compositions and thicknesses.
• Joint geometry: The bevel angle and root gap influence the molten metal flow and heat distribution, which in turn affect the uniformity of the interface layer thickness.

Key Questions and Reflections

Several important questions emerge from this study. First, the non-uniformity of the interface layer thickness (8–11 μm) raises concerns about the consistency of joint properties along the weld length. In pipe fitting applications, where joints are often circumferential, this non-uniformity could lead to localised weakness at specific clock positions around the pipe circumference.

Second, the study does not address the long-term stability of the intermetallic layer under thermal cycling or creep conditions. In service, the intermetallic phases may continue to grow or transform, potentially degrading the joint properties over time.

Third, the role of the 4043 filler wire composition in controlling the intermetallic layer is not fully explored. The Si content of the filler wire influences the formation of silicon-containing intermetallic phases, which may be less brittle than the iron-rich phases. Optimising the filler wire composition could potentially reduce the volume fraction of brittle phases and improve joint ductility.

Study Insights and Implications

This study provides a valuable contribution to the understanding of TIG brazed dissimilar metal joints between aluminium alloys and carbon steels. The detailed characterisation of the two-layer intermetallic structure, with its distinct phase compositions and morphologies, offers practical guidance for process parameter selection and joint design. For engineers working on pipe and fitting applications involving dissimilar metal joints, the key takeaway is that the interface layer thickness and composition are the primary determinants of joint strength and reliability. The non-uniformity of the interface layer along the weld length is a particular concern that warrants further investigation through process optimisation and joint geometry design.

The work also underscores the importance of microstructural characterisation in evaluating brazed joint quality. Conventional non-destructive testing methods such as radiographic testing or ultrasonic testing may not detect the intermetallic layer thickness variations or phase composition differences that govern joint performance. Metallographic examination and SEM-EDS analysis remain essential tools for the quality assessment of brazed dissimilar metal joints.