Part. 5: Weldability
Apart from the technological problems and the choice of welding method, one of the main problems you can meet while welding titanium and its alloys is the thermal effect that the welding itself produces in the heat altered zone.
In a welded joint, in reference to the metallurgy of the titanium and its alloys, you need to distinguish the following zones:
- Basic Unaltered material. The basic unaltered material is not altered by welding thermal cycles. In the case of Titanium commercially-pure and of its α alloys, you can consider as unaltered the material that has not reached the α→β transition temperature. In the case of α-β or β alloys, it’s better to consider as unaltered basic material the part of joint that hasn’t reach the temperature of 540°C, because over it, you meet with phenomenons of structural changes.
- Thermal-altered zone. In the case of α alloys and commercially pure titanium, in the thermal-altered zone, the α structure, changed into β when warming, resumes, when cooling, the reticulum α in an almost acicular configuration, according to the cooling speed, related to the thickness. The range of the thermal-altered zone is restricted because of the high temperature of the α→β transition zone.
In the α-β and β alloys you have to distinguish:
- The zone in which temperature gets closer to the solidification; this zone has a structure similar to the one resulting from the dissolution temperature through annealing process (no change but enlargement of the β alloys grain).
- The zone in which temperature has been over 540°C, allowing another variation in the state of participation of the α phase in the β matrix (ageing).
- Melted zone. In α alloys and commercially pure titanium, the β grains you find during the dissolution extend perpendicular to the isotherms and change into α grains with a configuration more or less acicular. In the α-β alloys you get an α structure with “martensite” and a β structure in the α matrix.
- With β alloys with a percentage more or less big of thin structure and with β matrix.