Regardless of the technological problems and the choice of the welding method, one of the main problems that arise in the welding of titanium and its alloys concerns the thermal effect that the welding itself produces in the thermally altered area.
In a welded joint, with particular reference to the metallurgy of titanium and its alloys, it is necessary to distinguish the following zones:
1. Unaltered base material. The unaltered base material is the material that is not altered by the thermal welding cycles. In the case of commercially pure titanium and its α alloys, the material that has not reached the α → β transition temperature is considered unchanged.
In the case of α-β or β alloys, it is more prudent to consider base material unaltered the part of junction that has not reached the temperature of 540 ° C, since already above this temperature there are phenomena of structural changes.
2. Thermally altered area. In α alloys and in commercially pure titanium, in the thermally altered area the α structure, transformed into β in heating, resumes, upon cooling, the α lattice in an almost accicular configuration, according to the cooling speed depending on the thickness. The extension of the thermally altered zone is limited due to the high temperature of the α → β transition zone.
In the α-β and β alloys one must distinguish instead:
· The area where the temperature is closest to that of solidification; this area has a structure similar to that obtainable by tempering from the solubilization temperature (no transformation but only swelling of the grain of the β-alloys);
· The area where the temperature remained above 540 ° C allowing a further variation of the state of participation of the α phase in the β matrix (aging);
1. Molten zone. In α alloys and in commercially pure titanium, the β grains that form upon solidification extend perpendicularly to the isotherms and are transformed into α grains with an almost accicular conformation. In α-β alloys we obtain an α structure with “martensite” and a β structure in the α matrix.
In β-alloys with a more or less large percentage of fine structure α in the β matrix.