TWI Industrial Member Report Summary 848/2006
By S A Lockyer, L S Smith and P J Tubby
Titanium alloys are finding increasing application outside the aerospace sector. One of the difficulties facing new users is the lack of available mechanical data for welded joints. While there is a wide range of titanium alloys, more than 90% of all titanium used falls into one of two categories: commercially pure material, or Ti-6Al-4V. Gas shielded welding is the most common process employed, particularly gas tungsten arc welding (GTAW) or plasma arc welding (PAW). GTAW is flexible and suitable for manual welding, but is slow and requires many passes for thicker material. PAW is typically employed in keyhole mode, for which single pass autogeneous welding of up to 15mm thick material is possible. Commercially pure weldments are rarely stress-relieved after fabrication, but weldments in Ti-6Al-4V alloys are, due to lower toughness coupled with more demanding service loading.
Fatigue design guidance is readily available for welded joints in steel structures and aluminium alloy structures in the form of stress-life (S-N) curves for typical structural joints between plates. Typical 'rule of thumb' guidance for alloys other than steel, requires the allowable stresses for steel joints to be factored by the ratio of the elastic modulus for the alloy of interest to that of steel. For titanium alloys this typically reduces the allowable stresses for joints in steel by a factor of about 2. The increasing use of titanium in offshore risers led to publication of design guidance for butt welds by DNV (i) based on published data for girth welds, but this is for welds ground flush with the parent metal. Given the general paucity of fatigue data for welded titanium in general engineering applications (i.e. bead profile in the as-welded condition and welded using more common workshop practice), the present fatigue testing programme was initiated.
- Determine fatigue properties for GTAW and PAW joints in the two most commonly used titanium alloys.
- To compare the results with current fatigue design guidance