Subproject B6 Arc Welding of Titanium-Alloys

In the aerospace industry today, the welding of aircraft engines is partly carried out by fusion welding. TIG welding, as well as beam-based processes such as laser and electron beam processes are used for repairs. Fusion welding technology is mainly used in fan and compressor units.

If the development of repair technology is considered separately from the entire regeneration process of the engine, the best results have so far been achieved with beam welding processes. In the entire process chain of the regeneration of the investment goods, however, the use of the vacuum electron beam process, for example, leads to immense investment and manufacturing costs, which can increase the repair costs up to 65% of the new part price. By contrast, arc welding processes are much cheaper and can therefore make repairs more attractive.

A certain minimum amount of thermal energy must be introduced into the material in order to produce a cohesive joint connection. This thermal energy is pent-up in the component by the low thermal conductivity of 22 W/mK of titanium (steel ~50 W/mK, copper ~401 W/mK). The significant heating of the base material in the accumulation areas leads to structural changes. This means that the property profile of a welded joint is formed from different areas. The fusion zone (FZ) is characterized by a solidification structure mostly oriented towards the weld seam centre, which is often expressed by a so-called "overmatching" of the material properties. This leads to a higher tensile strength with a simultaneous strong reduction in ductility. In the heat-affected zone (HAZ), on the other hand, heat is introduced into the base material through heat dissipation and thus recrystallization processes occur in the material, which in its initial state is characterized by a mostly very fine-grained basic structure. This process is associated with grain coarsening, which leads to a deterioration of the mechanical properties and thus constitutes the actual weak point of the joint.

To improve arc welding processes, the solidification structure in the melting zone must be optimized and the heat-affected zone minimized. Here, the inoculation and flux effect should be exploited.

Patch repair welding of a titanium blisk

Despite the use of inoculants and fluxes, the mechanical properties of the weld seam are below those of the rest of the component. Currently, further approaches are being pursued to post-treat the weld seam, such as introducing Nitrogen  into the grid by plasma nitriding. This creates compressive stress, which has a positive effect on the durability under cyclic loading.

In the second funding period, the repair was carried out with a patch. To do this, the blisk must be milled off in a defined manner and then measured, the patch manufactured, welded on and then the blisk re-contoured to restore the desired geometry.

By substituting the patch repair with arc deposition welding (3d printing), the production of the patch is omitted and thus one work step.