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

Literature Overview

This paper by He Huan, Fan Chenglei, Lin Sanbao, Yang Chunli, and Zhang Yuqi 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. Three filler metals were evaluated: ER1100 (pure aluminum), ER4043 (Al-Si 5%), and ER2319 (Al-Cu 4.5%). The study focuses on the thickness of intermetallic compounds (IMCs), tensile strength, and impact energy as functions of filler metal composition and pulse parameters.

Core Technical Challenge

Joining aluminum to stainless steel is one of the most challenging dissimilar metal welding problems in engineering. The fundamental difficulty arises from the formation of brittle intermetallic compounds at the interface, particularly FeAl₂, Fe₂Al₅, and FeAl₆, which have limited ductility and can easily fracture under mechanical loading. The large difference in thermal conductivity between aluminum (approximately 237 W/m·K) and stainless steel (approximately 16 W/m·K) creates asymmetric heat flow, making it difficult to achieve uniform melting and solidification. The melting points also differ significantly: aluminum at 660°C versus typical austenitic stainless steel at approximately 1400°C.

Welding-Brazing Process Fundamentals

The welding-brazing process is a hybrid technique that combines the advantages of both welding and brazing. Unlike conventional welding, which fully melts both base metals, welding-brazing selectively melts the aluminum side while the stainless steel side remains in the solid state but is heated above its recrystallization temperature. The filler metal flows onto the heated stainless steel surface by capillary action and wetting, creating a brazed joint on the stainless side and a welded joint on the aluminum side. This approach minimizes the thickness of the intermetallic layer because the reaction between aluminum and iron is kinetically limited by the absence of a fully molten stainless steel pool.

Pulsed TIG welding provides the additional advantage of precise heat input control through modulation of the current waveform. The pulse parameters — base current, peak current, pulse frequency, and duty cycle — can be adjusted to maintain the stainless steel surface at a temperature sufficient for wetting (typically 600-700°C) without causing excessive melting or excessive IMC growth.

Results and Analysis

The SEM results reveal that the IMC layer thickness is strongly dependent on the filler metal composition. ER2319, containing 4.5% copper, produces the thinnest IMC layer at approximately 2 μm, while ER4043 with 5% silicon produces the thickest at 3.5 μm. This difference is attributed to the different diffusion rates and interfacial reaction kinetics of copper and silicon in the aluminum-iron system. Copper tends to form more stable phases that limit further iron diffusion into the aluminum matrix, whereas silicon promotes more rapid interfacial reactions.

The mechanical property results present an interesting trade-off. The ER2319 filler achieves the highest tensile strength, which is logically consistent with its thinnest IMC layer and the strengthening effect of copper in the aluminum solid solution. However, the impact energy is the lowest, indicating reduced ductility and fracture toughness. The ER1100 filler, despite having a thicker IMC layer, exhibits the best comprehensive mechanical performance because pure aluminum maintains high ductility and the absence of alloying elements reduces the risk of brittle fracture initiation at the interface.

Engineering Practice Implications

For engineering applications where joint strength is the primary requirement — such as structural brackets, heat exchanger plates, or pressure vessel components — ER2319 would be the preferred filler metal. However, for applications requiring high toughness and fatigue resistance — such as automotive body-in-white joints, aerospace structures, or cryogenic components — ER1100 would be more appropriate. The ER4043 filler, while offering good castability and fluidity for brazing applications, is generally less suitable for structural joints due to its thicker IMC layer and reduced mechanical properties.

The pulse parameter optimization is critical in practice. The authors demonstrate that appropriate pulse parameters can achieve good weld formation, but the specific values must be determined through systematic trials for each production setup. The pulse frequency should be high enough (typically 50-200 Hz) to maintain a stable arc and uniform heat input, while the duty cycle should be adjusted to prevent overheating of the stainless steel side.

Study Insights

This research provides valuable quantitative data on IMC thickness as a function of filler composition, which is essential for quality control in production environments. The finding that ER2319 produces the thinnest IMC layer but lowest impact energy is particularly instructive: it demonstrates that minimizing intermetallic thickness alone does not guarantee optimal joint performance. Engineers must balance strength, ductility, and toughness requirements based on the specific service conditions. The study also highlights the importance of filler metal selection as a design variable in dissimilar metal joining, which is often underappreciated in practice.