Bypass-Current MIG Welded-Brazing of Aluminum to Dissimilar Steel Substrates

Literature Overview

The paper by Yu-Gang Miao and colleagues from Harbin Engineering University's National Key Laboratory of Science and Technology on Underwater Vehicle, published in Acta Metallurgica Sinica (English Letters) (2017, Vol. 30, No. 8, pp. 721–730), presents a systematic comparative study of bypass-current MIG welded-brazing technology applied to two critical dissimilar material combinations: aluminum to galvanized steel and aluminum to stainless steel. Funded by the National Natural Science Foundation of China (No. 51005049), this research addresses a persistent challenge in marine and automotive engineering where lightweight aluminum structures must be joined to steel components.

Core Technical Content

The bypass-current MIG welded-brazing technique represents an elegant solution to the fundamental incompatibility of aluminum and steel in conventional fusion welding. The core principle involves directing the welding arc primarily toward the aluminum side while using a separate electrical current path (bypass current) to heat the steel substrate without melting it. This allows aluminum to melt and flow onto the heated steel surface, forming a metallurgical bond through interfacial reaction rather than full fusion of both materials.

Microstructural Comparison of the Two Joint Types

Intermetallic Compound Analysis

The identification of distinct intermetallic compound systems in each joint type is of paramount importance for predicting long-term joint performance. In the aluminum/galvanized steel joint, the zinc coating introduces Zn into the reaction zone, forming Fe-Al-Si-Zn complex phases. These phases, while hard and brittle, form in a controlled, thin layer (2–4 μm) due to the galvanizing layer's moderate reactivity with aluminum.

In contrast, the aluminum/stainless steel joint produces Fe-Al-Cr-Ni complex phases with significantly greater thickness variation (5–45 μm). The chromium and nickel in stainless steel create a more complex thermodynamic landscape at the interface, leading to non-uniform reaction kinetics and uneven intermetallic growth. This thickness variation is a critical concern because regions with thicker intermetallic layers represent potential crack initiation sites under mechanical or thermal loading.

Hardness Distribution and Mechanical Implications

The microhardness measurements reveal a consistent pattern: hardness increases rapidly within the interfacial reaction layer due to the formation of brittle intermetallic compounds. This creates a hard, brittle zone adjacent to relatively ductile aluminum and steel substrates. The mechanical implications are significant:

• Brittleness concern: Intermetallic phases such as FeAl, Fe₂Al₅, and FeAl₂ are inherently brittle with limited ductility. Under cyclic loading or impact conditions, these layers can act as crack initiation and propagation paths.
• Thermal cycling vulnerability: Differential thermal expansion between aluminum (23 × 10⁻⁶/°C) and steel (~12 × 10⁻⁶/°C) creates cyclic stresses at the interface during thermal cycling, potentially accelerating intermetallic growth and crack formation.
• Galvanized steel advantage: The thinner, more uniform reaction layer in the galvanized steel joint suggests better fatigue resistance and more predictable mechanical behavior compared to the stainless steel joint.

Engineering Practice Integration

For marine engineering applications—particularly underwater vehicle construction where this research originated—the bypass-current MIG welded-brazing technique offers several practical advantages:

1. Weight reduction: Joining aluminum superstructures to steel pressure hulls achieves significant weight savings while maintaining structural integrity.
2. Corrosion management: The galvanized steel variant provides an additional corrosion barrier at the interface, which is beneficial in marine environments.
3. Process reproducibility: The aluminum/galvanized steel combination demonstrates superior process stability, making it more suitable for automated production welding.

FMEA Considerations for Production Implementation

Applying Failure Mode and Effects Analysis to this technology reveals the following critical failure modes:

Key Questions and Reflections

The uneven interfacial reaction layer in the aluminum/stainless steel joint (5–45 μm) raises fundamental concerns about the reliability of this joint type for structural applications. The thickness variation of nearly an order of magnitude suggests that local process conditions—such as arc positioning accuracy, current stability, and surface cleanliness—have outsized influence on the final joint quality. In production environments where these parameters cannot be perfectly controlled, the aluminum/stainless steel variant may require additional process controls or post-weld treatments to ensure acceptable quality.

The study does not address long-term aging behavior of the intermetallic compounds, which is a critical gap for engineering qualification. Intermetallic layers can continue to grow under sustained elevated temperatures, potentially degrading joint properties over time.

Study Insights and Implications

This comparative study provides valuable guidance for selecting appropriate material combinations in dissimilar welding applications. The clear superiority of the aluminum/galvanized steel combination in terms of process stability and interface uniformity makes it the preferred choice for production applications. For cases where stainless steel must be joined to aluminum, the bypass-current technique remains viable but demands stricter process control and potentially supplementary treatments such as intermediate coating layers or post-weld stress relief. The work underscores that dissimilar material joining is fundamentally an interface engineering challenge, where the quality of the interfacial reaction zone determines the entire joint's performance envelope.