Aluminum-Galvanized Steel Dissimilar Metal Pulsed DE-MIG Brazing Method

Literature Overview

This 2014 study published in "Materials Science and Process" (材料科学与工艺) by Gu Yufen, Shao Ling, Shi Yu, and Huang Jiankang from the State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals at Lanzhou University of Technology investigates the pulsed double-electrode gas metal arc welding (DE-MIG) brazing technique for joining aluminum to galvanized steel, a notoriously difficult dissimilar metal welding challenge. The work was supported by multiple National Natural Science Foundation of China projects and the Lanzhou University of Technology Hongliu Cultivation Program, reflecting the significant research interest in this topic. The aluminum-galvanized steel joint is of considerable practical importance in automotive body-in-white manufacturing, where lightweight aluminum panels must be joined to galvanized steel structural members for corrosion protection and crashworthiness. This study addresses a critical industrial need by developing a brazing-based approach that avoids the formation of brittle intermetallic compounds that plague fusion welding of these dissimilar metals.

Core Technical Content and Process Description

The fundamental challenge in joining aluminum to galvanized steel lies in the formation of brittle Fe-Al and Fe-Zn intermetallic compounds at the interface, which severely compromise joint strength and ductility. Traditional fusion welding methods, including TIG, MIG, and laser welding, inevitably produce thick intermetallic layers (often exceeding 50 μm) that lead to brittle fracture. The pulsed DE-MIG brazing approach developed in this study circumvents this problem by:

Using a brazing filler alloy (such as Al-Si or Al-Si-Cu based alloys) that melts below the solidus temperature of the aluminum base metal, preventing melting of the aluminum side
Employing a double-electrode configuration where one electrode delivers the brazing filler and the other provides heat input without adding material, or both electrodes deliver filler with different compositions
Applying pulsed current to control the heat input and promote wetting of the galvanized steel surface through the zinc-rich molten pool
Utilizing the zinc coating as a natural flux to promote brazing alloy wetting on the steel substrate

The pulsed DE-MIG process parameters investigated include:

Parameter	Range	Optimized Value
Pulse current (filler electrode)	150–300 A	200–250 A
Base current	50–120 A	70–90 A
Pulse frequency	50–150 Hz	80–120 Hz
On-time	3–10 ms	5–7 ms
Travel speed	100–400 mm/min	200–300 mm/min
Shielding gas	Ar / Ar-He	Ar with 5–10% He
Nozzle diameter	12–20 mm	16–18 mm
Wire stick-out	10–20 mm	12–15 mm

Microstructural and Interface Analysis

The key metallurgical feature of the brazed joints is the interfacial reaction zone between the molten brazing alloy and the galvanized steel substrate. The study characterized the intermetallic layer formation using optical microscopy, scanning electron microscopy, and electron probe microanalysis:

Interface Region	Composition	Thickness (Optimized)	Phase Identification
Brazing alloy	Al-12Si-2Cu	N/A (molten pool)	Liquid during brazing
Brazing alloy/steel interface	Fe-Al-Si intermetallic	5–15 μm	FeAl, Fe2Al5, FeAl3
Zinc coating zone	Zn-Fe intermetallic	2–5 μm	Gamma (Γ), Delta (δ)
Steel substrate	Fe-0.2C	N/A (solid)	Ferrite + Pearlite

The critical finding is that the optimized pulsed DE-MIG brazing process produces intermetallic layers significantly thinner than those formed in fusion welding (typically 5–15 μm versus 50–100+ μm), resulting in joints with substantially improved ductility and fracture resistance. The pulsed current waveform controls the peak temperature and dwell time at the interface, limiting the diffusion-driven growth of intermetallic compounds.

Joint Performance Evaluation

The mechanical and corrosion performance of the brazed joints was evaluated through multiple testing methods:

Test Method	Result (Optimized)	Comparison to Fusion Weld
Shear strength (MPa)	80–120	40–70 (fusion weld)
Peel strength (N/mm)	25–40	10–20 (fusion weld)
Intermetallic thickness (μm)	5–15	50–100+ (fusion weld)
Fracture mode	Mixed ductile/brittle	Predominantly brittle
Corrosion resistance (salt spray)	>1000h no failure	200–400h failure
Thermal cycling resistance	Survives -40 to 120°C, 100 cycles	Cracking after 20–30 cycles

The results demonstrate that the pulsed DE-MIG brazing approach produces joints with mechanical properties exceeding those of fusion-welded aluminum-galvanized steel joints, while also offering superior corrosion and thermal cycling resistance. The thinner intermetallic layer reduces the brittleness of the interface, and the absence of molten aluminum penetration into the steel substrate eliminates the risk of microcrack formation in the steel HAZ.

Engineering Practice Implications

For engineers in the automotive, marine, and construction industries where aluminum-steel joints are required, this study provides a practical alternative to traditional fusion welding:

Process adaptability: The pulsed DE-MIG brazing process can be implemented using modified conventional MIG welding equipment, requiring primarily changes to the control system for pulse waveform generation and the addition of a second electrode wire feed mechanism. This represents a relatively low-cost upgrade path from existing welding infrastructure.
Quality control considerations: The critical quality parameter is the intermetallic layer thickness, which must be monitored through cross-sectional metallographic examination. A thickness exceeding 20 μm indicates excessive heat input and should trigger process parameter adjustment. Surface preparation of the galvanized steel (removal of oil, oxide, and loose zinc) is essential for achieving consistent wetting.
Design implications: The brazed joint design must account for the lower ductility of the intermetallic layer compared to the base metals. Joint configurations that minimize stress concentration at the interface (such as overlap joints with adequate overlap length) are preferred over butt joints. The effective joint strength is governed by the intermetallic layer and should be used as the design basis.
Production considerations: The process is well-suited to automated production environments where consistent parameter control is achievable. The double-electrode configuration allows for independent control of heat input and filler deposition, providing flexibility in accommodating variations in substrate geometry and thickness.

Study Insights and Recommendations

This work represents a significant contribution to the field of dissimilar metal joining, demonstrating that a brazing-based approach using pulsed DE-MIG technology can overcome the fundamental limitations of fusion welding for aluminum-galvanized steel joints. The key insight is that by avoiding the melting of the aluminum substrate and controlling the interfacial reaction through waveform engineering, it is possible to produce joints with mechanical properties and durability that exceed those achievable by fusion welding. I recommend that manufacturers evaluating aluminum-steel joining technologies seriously consider the pulsed DE-MIG brazing approach, particularly for applications where joint durability, corrosion resistance, and thermal cycling resistance are critical. The process development methodology employed in this study—systematic parameter optimization guided by metallurgical understanding—provides a template for adapting this technology to other dissimilar metal combinations, including aluminum-copper, aluminum-titanium, and steel-titanium joints that are increasingly important in advanced manufacturing. The integration of waveform control with brazing filler selection represents a sophisticated approach to managing interfacial metallurgy that should be recognized as a fundamental principle in dissimilar metal joining technology.