TA2 Titanium and 1060 Aluminum Dissimilar Metal Pulsed MIG Brazing-Fusion Welding

Literature Overview

The research by Wei Shouzheng, Han Yujie, Wang Jianguo, Li Zhiyong, and Zhang Yingqiao, published in the Journal of Material Heat Treatment (2018), addresses the challenging problem of joining TA2 titanium to 1060 aluminum using pulsed MIG brazing-fusion welding. This is a dissimilar metal joining problem of considerable practical importance in aerospace and transportation industries, where lightweight titanium structures must be joined to aluminum components for weight reduction. The study originates from the Welding Research Center of North University of China, the Beijing Institute of Aeronautical Manufacturing Technology under AVIC, and the experimental center of Taiyuan Heavy Industry Rail Transit Equipment Co., Ltd., reflecting a strong industry-academia collaboration.

Core Technical Points

Dissimilar Metal Joining Challenges

The joining of titanium to aluminum is inherently difficult due to the formation of brittle intermetallic compounds at the interface. The primary intermetallic phases that form include TiAl, TiAl2, Ti2Al, and Ti3Al, all of which are hard and brittle with poor ductility. The melting point difference between titanium (1668 degrees Celsius) and aluminum (660 degrees Celsius) is extreme, making conventional fusion welding impractical without significant dilution of the titanium side. The brazing-fusion welding approach addresses this by achieving a fusion bond on the aluminum side and a brazing bond on the titanium side, using a filler metal that is compatible with both base metals.

Pulsed MIG Process Parameters

The pulsed MIG process is selected for its ability to provide precise control over heat input and droplet transfer. The pulsing parameters are critical for achieving a stable weld pool on the aluminum side while maintaining a controlled brazing temperature on the titanium side. The pulse frequency, peak current, background current, and pulse duration must be optimized to prevent excessive heat input into the titanium, which could lead to excessive intermetallic growth and potential cracking.

Microstructural Analysis

The microstructure of the brazing-fusion weld joint exhibits distinct regions: the aluminum fusion zone, the brazing interface on the titanium side, the intermetallic layer, and the titanium heat-affected zone. In the aluminum fusion zone, a dendritic microstructure is observed with fine alpha-phase aluminum dendrites surrounded by eutectic Al-Si phases from the filler metal. The grain size in the fusion zone is refined by the rapid solidification rates associated with the pulsed MIG process.

At the titanium-aluminum interface, a thin intermetallic layer forms, consisting primarily of TiAl and Ti2Al phases. The thickness of this intermetallic layer is strongly dependent on the brazing temperature and holding time. The study demonstrates that the pulsed MIG process, with its controlled heat input, produces a thinner intermetallic layer compared to conventional welding methods, which is beneficial for joint strength and ductility. The titanium HAZ on the other side of the intermetallic layer shows minimal grain growth and no phase transformation, as the temperature remains below the beta-transus temperature of titanium.

Mechanical Properties

The mechanical properties of the brazing-fusion joint are evaluated through tensile testing, microhardness mapping, and fracture analysis. The tensile strength of the joint is typically governed by the weaker of the two base metals or the intermetallic layer, depending on the joint design. The study reports that the joint achieves a tensile strength of 150-200 MPa, which is acceptable for non-critical structural applications. The microhardness profile across the joint shows a peak at the intermetallic layer, with hardness values of 400-500 HV, decreasing to 100-150 HV in the aluminum fusion zone and 250-300 HV in the titanium base metal.

Engineering Practice Considerations

The brazing-fusion welding of titanium to aluminum is applicable in aerospace structures, automotive lightweighting, and rail transit equipment. In pressure vessel applications, while titanium-aluminum joints are less common than titanium-steel or aluminum-steel joints, they may be encountered in cryogenic service or specialized chemical processing equipment. The process must be qualified under relevant standards, and the intermetallic layer thickness must be controlled within acceptable limits to ensure joint integrity.

For non-destructive testing, the brazing interface on the titanium side may be difficult to detect by conventional ultrasonic testing due to the small acoustic impedance mismatch between the intermetallic layer and the titanium. Radiographic testing may be more effective for detecting lack of bond or voids at the brazing interface. The aluminum fusion zone can be inspected using standard ultrasonic and radiographic methods.

Key Questions and Reflections

The primary challenge in titanium-aluminum brazing-fusion welding is controlling the intermetallic layer thickness. Excessive intermetallic growth leads to brittle fracture at the interface, while insufficient bonding leads to lack of adhesion. The pulsed MIG process offers fine control over heat input, but the process parameters must be carefully optimized for each specific joint configuration. The study highlights the importance of understanding the thermodynamics and kinetics of intermetallic formation, as the intermetallic layer grows according to a parabolic kinetics law, with thickness proportional to the square root of time at the brazing temperature.

Another consideration is the thermal mismatch between titanium and aluminum during cooling. The coefficient of thermal expansion of aluminum (approximately 23 micrometers per meter per degree Celsius) is nearly twice that of titanium (approximately 8.6 micrometers per meter per degree Celsius). This mismatch can lead to residual stresses and potential cracking at the interface during cooling. The pulsed MIG process, with its lower heat input compared to continuous MIG, helps mitigate this problem by reducing the overall thermal distortion and residual stress.

Study Insights and Implications

This research provides a practical solution for joining titanium to aluminum in applications where weight reduction is critical. The brazing-fusion approach avoids the formation of a fully fused joint, which would require excessive heat input and lead to severe intermetallic embrittlement. The pulsed MIG process parameters identified in the study can serve as a starting point for process development in production environments.

For engineers working on bimetal products and pressure vessels, this research reinforces the principle that dissimilar metal joining requires a tailored approach that considers the metallurgical compatibility of the base metals, the formation of intermetallic compounds, and the residual stress state of the joint. The brazing-fusion welding technique represents a mature technology for dissimilar metal joining, and its application to titanium-aluminum combinations opens new possibilities for lightweight structural design.

In conclusion, the pulsed MIG brazing-fusion welding of TA2 titanium to 1060 aluminum is a viable process for producing joints with adequate strength and controlled intermetallic formation. The process parameters and microstructural characteristics identified in this study provide a solid foundation for further development and standardization of the technique for industrial applications.