The Challenges of Welding Titanium
Titanium is a light weight metal with excellent corrosion resistance and the highest strength-to-density ratio of any metallic element. It’s a great material for many applications, from super light weight aerospace parts to artificial joints to implanted medical devices.
Titanium has two properties that greatly influence its weldability: 1) Titanium has a great chemical affinity for combining with oxygen; and 2) Titanium doesn’t have a great affinity for combining with any other chemicals.
In open air, freshly machined or cleaned titanium quickly forms a microscopic layer of oxides. This formation of oxides creates a natural passivity that inhibits the reactions with other chemicals, such as salt or oxidizing acid solutions. The result is that titanium has superior corrosion resistance. However, when heated for welding, these oxides form even faster, and as the temperature reaches titanium’s melting point (1668 °C, 3034 °F), the oxides dissolve into solution and contaminate the weld pool, causing an impure and very weak weld. For this reason, special care must be taken to minimize the weld piece’s exposure to oxygen after cleaning and during welding. Generally, a shield gas such as argon or helium is used to protect the part, and special care must be taken to make sure the gas completely covers the heat affected area including the back side and/or interior of the part. One option that provides exceptional gas coverage would be to weld the part in a laser welding glove box filled with pure gas. Parts can also be reliably welded in a full vacuum (as is the case with electron beam welding), however, laser welding in a vacuum not widely available.
Since titanium is a very nonreactive substance, it is an excellent material for use in medical applications in which it will be in extended contact with human tissue, because chemicals and compounds within the body simply don’t bond with titanium. However, this characteristic also means that titanium does not easily alloy with other materials, which makes it almost impossible to weld titanium to any other metal. Hence, it is not recommended to use titanium in any dissimilar material welding application. If bonding titanium to another metal is essential to an application, explosion bonding or certain types of brazing would be the only viable options.