Welding Parameters Influence on Microstructure and Properties of Aluminum-Steel TIG Braze-Welding Joints

Literature Overview and Dissimilar Metal Joining Challenge

This paper, published in 2014 in Welding (Issue 12, pp. 20-24) by Song Yang, Dong Honggang, Guo Xin, and Zhang Xiaosheng from Dalian University of Technology and China FAW Group Corporation, investigates the effect of welding parameters on the microstructure and mechanical properties of aluminum-to-steel TIG braze-welding joints. The study was funded by the National Natural Science Foundation of China (Grants 50904012 and 51374048) and the Central University Basic Research Business Fee Special Fund (DUT13ZD209).

The joining of aluminum and steel is a significant challenge in modern manufacturing, particularly in automotive lightweighting where aluminum body-in-white structures must be joined to steel components for crashworthiness and cost efficiency. The fundamental challenge lies in the formation of brittle intermetallic compounds (IMCs) at the aluminum-steel interface, which severely degrade the joint strength and ductility. The TIG braze-welding technique (also known as hot-braze welding) offers a promising approach by using a low-melting-point filler alloy that wetts and bonds to both base metals without full melting of the steel, thereby minimizing IMC formation.

Technical Approach and Experimental Design

The researchers used AlSi12 flux-cored wire as the filler material for TIG braze-welding of pure aluminum to TV stainless steel. The flux-cored wire contains a flux core that generates a reducing atmosphere during welding, promoting wetting of the steel surface by the aluminum filler. Multiple welding parameter combinations were investigated, varying current (30-70 A) and travel speed (30-100 mm/min) to determine the optimal parameter window for joint strength and microstructure.

The microstructural analysis employed optical microscopy (OM), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) to characterize the weld metal, interface layer, and base metal microstructures. Mechanical testing included tensile tests to determine joint strength and fracture analysis to identify failure locations and mechanisms.

Microstructural Analysis and Interface Characterization

The SEM and EDS analysis revealed that the interface layer between the aluminum filler and the steel base metal consists of intermetallic phases identified as T5-Al7.4Fe2Si (also known as Al-Fe-Si ternary phase). This phase forms at the steel-aluminum interface during the braze-welding process and is a critical determinant of joint strength. The morphology and thickness of this intermetallic layer are directly influenced by the welding parameters, particularly the heat input (which is a function of current and travel speed).

The study identified two distinct interface layer morphologies:

1. Uniform morphology at the root and middle sections: In regions where the heat input is moderate and the arc force is stable, the interface layer is relatively uniform and continuous, with a thickness of approximately 5-15 micrometers. This uniform layer provides good bonding strength and is less prone to crack initiation.
2. Sawtooth morphology at the tail section: In the trailing edge of the weld, where the arc force and heat distribution are less stable, the interface layer exhibits a jagged, sawtooth-like morphology. This irregular morphology creates stress concentrations and weak points that can serve as crack initiation sites.

Within the weld metal itself, the AlSi12 filler solidifies into a eutectic microstructure consisting of alpha-Al matrix and alpha-Al-Fe-Si intermetallic phases. The study identified two morphologies of these phases: plate-like and Chinese-character-shaped (referred to as "hanzi-shaped" in the original text, describing the characteristic angular morphology of alpha-Al-Fe-Si particles). Both morphologies are identified as the T5-Al7.4Fe2Si phase, confirming that the same intermetallic compound forms both at the interface and within the weld metal.

Mechanical Properties and Parameter Optimization

The tensile strength results show that the optimal welding parameters (50 A, 60 mm/min) produce a joint strength of 92 MPa, which represents 73% of the aluminum parent material strength. This is a notable achievement for aluminum-steel braze-welding, where joint strengths of 50-70% of the weaker parent material are typical. The fracture consistently occurred in the aluminum heat-affected zone (HAZ), indicating that the interface and weld metal are stronger than the aluminum HAZ, which is the weakest link in the joint.

The parameter sensitivity analysis reveals that:

• Current effect: Higher currents increase heat input, leading to thicker intermetallic layers and reduced joint strength. However, excessively low currents result in insufficient wetting and incomplete bonding.
• Travel speed effect: Higher travel speeds reduce heat input per unit length, producing thinner intermetallic layers and potentially better joint strength, but may lead to poor wetting and lack of fusion.
• Heat input optimization: The optimal heat input window balances sufficient wetting and bonding against excessive intermetallic growth. For this material combination, a heat input of approximately 0.2-0.3 kJ/mm represents the optimal range.

The fracture analysis confirms that the aluminum HAZ is the critical region for joint failure. The aluminum HAZ experiences grain growth due to the thermal cycle, and the resulting coarse grains have lower strength and ductility than the base metal. This is a common limitation of aluminum-steel braze-welding joints and represents a potential area for future improvement through filler alloy design or process modification.

Engineering Practice Implications and Quality Control

The TIG braze-welding technique for aluminum-steel joining has significant applications in automotive lightweighting, where the replacement of steel components with aluminum can reduce vehicle weight by 15-25% while maintaining structural integrity. The study's findings provide a practical parameter window for production implementation, with the optimal conditions at 50 A and 60 mm/min serving as a starting point for process development.

Key quality control considerations for this process include:

• Surface preparation: The steel surface must be thoroughly cleaned to remove oxides, oils, and contaminants that inhibit wetting. Mechanical cleaning (grinding, brushing) followed by chemical cleaning (acid pickling or alkaline etching) is recommended.
• Flux management: The flux-cored wire must be stored and handled to prevent moisture absorption, which can lead to porosity and reduced wetting. Flux activity should be verified periodically.
• Parameter consistency: Automated torch tracking and wire feeding are essential for maintaining consistent heat input and travel speed, particularly for production applications.
• Post-weld inspection: Visual inspection, X-ray radiography, and ultrasonic testing should be used to detect lack of fusion, porosity, and interfacial defects.
• Strength verification: Periodic tensile testing of coupon specimens should be performed to verify joint strength meets specifications.

The sawtooth morphology observed at the tail section of the interface layer is a particular concern for production quality. This morphology is associated with arc instability at the end of the weld and can be mitigated by using a trailing arc or post-heat treatment to homogenize the interface. In production applications, weld end caps or backing materials can be used to minimize tail-end defects.

Study Insights and Future Directions

This research contributes valuable data to the growing body of knowledge on aluminum-steel joining technologies. The identification of the T5-Al7.4Fe2Si phase as the dominant intermetallic compound at both the interface and within the weld metal provides a clear metallurgical target for process optimization. Future research should focus on filler alloy development to suppress or modify this phase, potentially through the addition of elements such as copper, magnesium, or manganese that alter the phase diagram and intermetallic formation kinetics.

The observation that the fracture occurs in the aluminum HAZ rather than at the interface suggests that the interface strength is adequate for most applications, and that the overall joint strength is limited by the aluminum-side properties. This insight is important for design engineers, as it indicates that improving the aluminum-side properties (through alloy selection, preheat, or post-weld treatment) may be more effective than optimizing the interface.

The parameter window identified in this study (50 A, 60 mm/min) should be considered as a starting point for process development, with further optimization tailored to specific production conditions, joint geometries, and quality requirements. The integration of real-time monitoring systems (such as arc voltage/current monitoring and thermal imaging) with automated parameter adjustment would enhance process consistency and quality in production environments.