Microstructure and Mechanical Properties of Aluminum-Stainless Steel Joints Welded by TIG Brazing with Mixed Filler Wire

Literature Overview

The paper by Huang Xuebin, published in Welding (2014, Issue 3, pages 56-59), investigates the microstructure and mechanical properties of aluminum alloy to stainless steel joints produced by TIG brazing (melting-brazing) using mixed filler wires. Three combinations of filler wires were evaluated: ER1100/ER2319, ER2319/ER1100, and ER2319/ER2319. The study employed SEM and EDS for microstructural analysis and tensile and impact testing for mechanical property evaluation. The results provide valuable insights into the relationship between intermetallic compound (IMC) formation, filler wire composition, and joint performance in dissimilar aluminum-to-steel joints.

Technical Background

Joining aluminum alloys to stainless steel is a significant challenge in manufacturing, particularly in automotive, aerospace, and marine applications where weight reduction and corrosion resistance are critical design requirements. The fundamental difficulty lies in the formation of brittle intermetallic compounds (IMCs) at the aluminum-steel interface, which severely degrade the joint's mechanical properties and corrosion resistance.

TIG brazing (also known as melting-brazing or semi-solid brazing) offers a solution by maintaining the base metals in a solid state while melting only the filler metal. This approach minimizes the extent of IMC formation compared to full penetration welding, but IMC formation cannot be completely eliminated. The choice of filler wire composition directly influences the type, thickness, and distribution of IMCs, which in turn govern the joint's strength and toughness.

Filler Wire Combinations and IMC Analysis

The following table summarizes the three filler wire combinations and their resulting IMC characteristics:

A key finding is that the IMC layer thickness is remarkably consistent across all three combinations (3-4 μm), suggesting that the IMC thickness is primarily governed by the brazing temperature and time rather than the filler wire composition. However, the type of IMC formed varies significantly, with direct implications for joint performance.

IMC Type and Properties

• Al13Fe4: A relatively brittle iron-rich phase with limited ductility
• θ-Al13(Fe,Cu)4: A ternary phase containing both iron and copper, exhibiting higher crack resistance than Al13Fe4 due to the copper content, which modifies the phase's fracture behavior

The presence of Al2Cu in the weld metal also plays a significant role in joint performance. Al2Cu is a strengthening phase that improves weld strength but can reduce toughness if present in excessive amounts. The optimal balance between strength and toughness is achieved when Al2Cu is present in moderate quantities.

Mechanical Property Results

The following table presents the tensile strength and impact energy results for each filler wire combination:

The ER2319/ER1100 combination achieved the highest tensile strength (184 MPa) and impact energy (1.83 J), representing an 18 MPa improvement in strength over ER1100/ER2319 and nearly a 4× improvement in impact energy over ER2319/ER2319.

Microstructure-Property Relationship

The superior performance of the ER2319/ER1100 combination is attributed to two factors:

1. IMC type: The θ-Al13(Fe,Cu)4 phase exhibits higher crack resistance than Al13Fe4, delaying crack initiation and propagation at the interface
2. Al2Cu content: The moderate amount of Al2Cu in the weld metal provides strengthening without significantly reducing toughness

The ER2319/ER2319 combination, while producing the same IMC type as ER2319/ER1100, contains excessive Al2Cu, which strengthens the weld but severely reduces toughness. The ER1100/ER2319 combination produces the less favorable Al13Fe4 IMC, resulting in lower overall performance.

Process Parameters

TIG brazing requires careful control of thermal input to maintain the base metals in a solid state. Typical parameters for aluminum-to-steel brazing include:

The travel speed is a critical parameter: higher speeds reduce the time available for IMC growth, resulting in thinner IMC layers. However, excessively high travel speeds may result in incomplete wetting and poor joint formation.

Study Insights and Engineering Implications

This paper provides a clear demonstration of how filler wire selection can be used to optimize the mechanical properties of dissimilar metal joints without changing the base metals or process parameters. The finding that IMC thickness is relatively insensitive to filler wire composition (3-4 μm across all combinations) while IMC type is highly sensitive is particularly valuable for process design.

For engineers working on aluminum-to-steel joining, the key takeaway is that the ER2319/ER1100 combination offers the best balance of strength and toughness for TIG brazing applications. The moderate Al2Cu content and the crack-resistant θ-Al13(Fe,Cu)4 IMC phase work synergistically to produce a joint with superior mechanical performance.

The study also highlights the importance of microstructural analysis in understanding joint performance. Without SEM and EDS characterization, the relationship between filler wire composition and joint properties would remain unclear, and the selection of the optimal filler wire combination would be purely empirical.