In a welding operation involving a titanium based material, the fusion zone and part of the heat affected zone are heated to temperatures above the beta transformation temperature and on completion of the welding cycle these cool through the beta transus to ambient temperature. On cooling, transformation of the beta phase present in the weld and heat affected zones takes place and it is the form and consequently the properties of the transformation product that dictate whether or not the material can be regarded as fully weldable. The form of the transformation product also determines the extent to which weld zone properties can be improved by subsequent heat treatment.
During welding of alloys containing only alpha stabilisers and/or neutral elements, the beta phase transforms to alpha having weld zone hardness and tensile strength similar to that of the parent metal (assuming minimal contamination by atmospheric gases during welding). Minor variations in ductility and bend performance tend only to reflect the different forms of alpha phase present in the weld and parent metal respectively. Similar observations apply to alloys containing modest amounts of beta stabilisers. Martensitic alpha developed as a result of such additions will exhibit little supersaturation with respect to beta stabilising elements and hence little increase in hardness relative to the parent metal.
In view of the generally similar level of properties that exists between weld and parent metal of the materials described, the only heat treatment normally applied is a post weld stress relief which is carried out as and when appropriate to a particular application. Stress relieving does not improve weld zone properties but may be necessary to avoid undesirable effects such as overload stress failure and corrosion associated with high residual stress.
By contrast, alloys of higher beta stabiliser content, i.e. those of the alpha‑beta type, are not normally regarded as weldable. Because of their higher beta stabiliser content, the transformation product developed in the weld zone is a supersaturated martensite having properties which, depending on composition, can differ markedly from those of the parent metal. Hardness of the weld zone relative to the parent metal is increased with increasing supersaturation, a change accompanied by a corresponding deterioration in weld ductility, toughness and bend performance. Thus weldability depends largely on the composition controlled differential in hardness between weld zone and parent metal.
Whilst it is possible to reduce weld zone hardness by post weld heat treatment and thereby improve ductility and toughness, such treatments cannot normally be applied as they reduce parent metal properties to an unacceptable level. An alternative and viable approach for certain compositions is use of commercially pure titanium filler wire to effectively create a dilute alloy joint of adequate ductility and toughness. In this instance it may of course be necessary to build up the weld zone thickness to compensate for loss in strength caused by weld dilution.
Electron Beam Welding
The advantages of the electron beam welding process which stem from the concentrated beam and small heat affected zone significantly ease the problem of welding the more difficult alloys.