Pulsed TIG Welding-Brazing of Aluminum-Stainless Steel Thin Plates with Different Fillers

Literature Overview

This study by He Huan and colleagues from the State Key Laboratory of Advanced Welding and Joining at Harbin Institute of Technology, published in China Welding (2014, Vol. 23, No. 1, pp. 59-63), investigates the pulsed TIG welding-brazing process for joining 1.5 mm thick aluminum and stainless steel plates using three different aluminum-based filler metals: ER1100, ER4043, and ER2319. The research is supported by the National Natural Science Foundation of China (Grant No. 50874033) and addresses a critical challenge in dissimilar metal joining: the formation and control of intermetallic compounds (IMCs) at the aluminum-steel interface.

Background and Technical Challenge

The direct welding of aluminum to steel is notoriously difficult due to the formation of brittle intermetallic compounds (Fe-Al intermetallics such as FeAl, Fe₂Al₅, Fe₃Al, and FeAl₂) at the joint interface. These IMCs are hard, brittle, and can easily exceed a thickness of 5 μm, at which point the joint becomes susceptible to cracking during cooling or under mechanical loading. Traditional fusion welding approaches often result in excessive IMC growth, while pure brazing may not provide sufficient joint strength for structural applications.

The pulsed TIG welding-brazing process represents a hybrid approach that combines the advantages of both fusion welding and brazing. By carefully controlling the pulse parameters, the process achieves a molten weld pool on the aluminum side while maintaining the steel side in a semi-solid or liquidus temperature range, thereby limiting IMC formation while ensuring adequate wetting and bonding.

Experimental Setup and Parameters

The pulse parameters—peak current, base current, pulse frequency, and pulse duration ratio—are critical in controlling the thermal cycle and, consequently, the IMC thickness. The authors state that appropriate pulse parameter adjustment yields good weld formation, suggesting that a stable, controllable process window was identified.

Intermetallic Compound Analysis

The SEM results reveal significant differences in IMC thickness depending on the filler metal used:

The ER2319 filler, which contains 2.6% silicon and 0.45% copper, produces the thinnest IMC layer. This is attributed to the silicon content, which can react with iron to form FeSi compounds, thereby reducing the availability of iron for IMC formation at the interface. The copper in ER2319 may also contribute to a more complex local chemistry at the interface, potentially altering the nucleation and growth kinetics of the IMCs.

The ER4043 filler, with 5% silicon and no copper, produces the thickest IMC layer. While silicon content is higher than in ER2319, the absence of copper and the different solidification behavior may result in less effective suppression of IMC growth. The ER1100 filler, being pure aluminum, produces an intermediate IMC thickness, serving as a baseline for comparison.

The element distribution analysis (EDS) confirms that the IMC layers and weld seams exhibit different elemental profiles depending on the filler composition. This is expected, as the local chemistry at the interface is influenced by the filler metal composition, which affects diffusion rates and phase stability.

Mechanical Performance

The mechanical testing results present an interesting trade-off between strength and toughness:

The ER2319 filler produces the highest tensile strength but the lowest impact energy. This is consistent with the metallurgical understanding that copper-containing aluminum alloys have higher strength but reduced ductility and toughness. The thin IMC layer contributes to high strength by minimizing the weak interface, but the overall joint toughness is limited by the inherent brittleness of the copper-containing weld metal.

The ER1100 filler, despite producing a thicker IMC layer, delivers the best comprehensive mechanical performance. This is because the pure aluminum weld metal has excellent ductility and toughness, which compensates for the slightly thicker IMC layer. The joint can absorb more energy before failure, making it more suitable for applications subject to impact or cyclic loading.

The ER4043 filler produces intermediate results, with the thicker IMC layer slightly reducing both strength and toughness compared to ER1100.

Engineering Practice Implications

For engineering applications involving aluminum-steel joints, the selection of filler metal must be based on the specific loading conditions and service environment:

1. High-stress, static loading: ER2319 is preferred for maximum joint strength, provided that the impact loading is minimal.
2. Dynamic or impact loading: ER1100 is the optimal choice for its superior toughness and energy absorption capacity.
3. General-purpose applications: ER1100 offers the best balance of strength and toughness, making it the most versatile option.

The pulsed TIG welding-brazing process itself is well-suited for thin plate applications (≤2 mm) where precise heat control is essential. The process can be automated for production environments, and the pulse parameters can be optimized for specific material thicknesses and compositions.

Study Insights and Reflections

This study provides valuable quantitative data on the relationship between filler composition, IMC thickness, and mechanical performance in aluminum-steel joints. The finding that ER2319 produces the thinnest IMC layer but the lowest impact energy challenges the common assumption that thinner IMC always leads to better overall performance. In practice, the selection of filler metal must consider the full spectrum of mechanical requirements, not just strength. The pulsed TIG welding-brazing process demonstrates its potential as a practical solution for dissimilar metal joining in lightweight structural applications, particularly in the automotive and aerospace industries where weight reduction is critical.