Frequently Asked Questions
Intermetallic phases are inevitable in brazing titanium alloys, even when using the conventional Ti(Zr)-Cu-Ni brazes that are employed for brazing titanium to itself. Thus, the practice is to choose braze alloys that result in ductile intermetallic phases or that ensure that the brittle intermetallic phases are discrete in a ductile matrix. It is also essential to choose an optimum braze temperature and minimise the brazing time in order to minimise the depth of diffusion into the parent metals. In addition, minimum joint clearance helps to prevent centreline intermetallic formation.
Ag-base vacuum brazes have been employed for brazing Ti to stainless steels or carbon steels. Ag-Al-Mn alloys (including Ag-5Al-0.5Mn) were developed for brazing titanium honeycomb structures in the late 1950s. Brazing temperatures are approximately in the range of 843°C and 904°C. A good history of this filler - including corrosion, strength, fatigue, hardness and structure - is provided in the Kaarlela and Margolis work [1], albeit for Ti-Ti joints.
More recent work [2] on brazing Ti to 304 stainless steel using Ag-28wt%Cu and Ag-46wt%Cu has demonstrated that discrete Ti 2Cu could be tolerated at the braze Ti metal interface as regards shear strength, with strengths in excess of 200MPa achievable employing shorter brazing times (about 15min). Indeed, the greatest hardness was on the stainless steel side of the joint, after prolonged brazing times. One caveat to this work was that the braze alloy was only 20microns thick.
The Ti to Ti joints situation is much more simple and a Ti-15Cu-15Ni (or Ti-20Cu-20Ni) braze metal, or similar, could be employed to vacuum braze the materials. This would probably result in a better joint, although particular care would have to be taken in ensuring that an optimum braze time/temperature/joint gap is employed in order to avoid formation of centreline intermetallics, which degrade performance.
References
1. Kaarlela W T and Margolis W S: 'Development of the Ag-Al-Mn brazing filler metal for titanium',Welding Journal, Vol.53 (10) Oct. 1974, pp. 629-636.
2. Caargo P R C, Trevisan R E and Liu S: 'Microstructural characterisation of titanium to 304 stainless steel brazed joints', Welding Journal, Vol.72(12). Dec.1993. pp.537s-544s.
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Welding titanium - a designer's and user's handbook