Current knowledge of metallurgical and mechanical properties of FSW welds in titanium alloys is inadequate to predict their performance for industrial applications. The microstructures generated in friction stir welding are notably different from conventional welding techniques and, if properly understood and exploited, this appears to offer a considerable potential to tailor the weld properties, producing welds with enhanced strength, toughness, fatigue life and corrosion resistance.
The objectives of the proposed project are:
- Identify target applications for joining titanium alloys by FSW
- Find better tool materials for making longer welds and enhanced repeatability
- Develop an increased understanding of the FSW process parameters, microstructure and mechanical properties for welding titanium alloys
- Provide member companies with data on the mechanical, distortion and corrosion properties of FSW in titanium alloys that will allow them to begin planning for the industrial deployment of FSW technology
The concept behind this proposal is initially to identify potential commercial applications where joining titanium by FSW may be advantageous. Based on this investigation develop the tooling and weld procedures using the stationary shoulder FSW technique (SSFSW) developed, and patented by TWI for the alloy grades, thicknesses and joint configurations identified. These welds will then be characterised to produce data on performance and suitability for the application.
TWI has the expertise, experience and technical facilities for FSW of titanium alloys and the outcome of this project will establish the status of FSW as a reliable technique. The proposed CRP project will include:
- A literature study to investigate the titanium alloy grades and thicknesses, currently employed by different industrial sectors and state of the art for joining these alloys
- Investigation of different joint configurations that can be welded with an emphasis on industrially relevant geometries and the particular requirements for jigs and fixtures
- Investigation of FSW tool materials for longer tool life
- Tool design and process development for FSW of the alloy grades, thicknesses and joint configurations identified during the literature study
- Investigation of microstructure and mechanical properties of titanium alloy welds
- Demonstrate FSW process on an industry representative component
Relevant Industry Sectors
Titanium alloys due to their higher specific strength and superior mechanical properties are particularly attractive to the aerospace, chemical, petrochemical, naval and biomedical industries. However, when it comes to the weldability of titanium, there are a number of problems that make it challenging. Fusion welding techniques are currently used for joining thin section titanium (~2mm), however difficulties can arise when joining thicker material where multi pass welding is required which can produce heating cycles that may harm the weld properties. Titanium is also reactive and can easily be contaminated by atmospheric gases and therefore welding is performed in an inert atmosphere such as argon in a sealed chamber.
Friction stir welding (FSW) offers the potential for solid state joining of titanium, avoiding many of the technical limitations of fusion welding. Recent developments in FSW tool technology now make joining longer length components possible. This in combination with fast production rates in especially thick section material compared with fusion welding, has stimulated interest across a range of industries and applications in this technique.
- Aerospace industry:
- Welding 1-2mm thickness Ti-6Al-4V & Ti-15V-3Al-3Sn-3Cr along with joining of dissimilar titanium alloy grades for propellant tank applications
- Welding 4-8mm thickness Ti-6Al-4V for engine applications
- Welding and repair of components thickness over 10mm
- Oil & Gas sector showed interest in joining tube less than 10mm diameter