The addition of B to titanium alloys leads to a reduction in grain and α-colony size as well as to a weakening of the texture in the as-cast state, whereby energy- and cost-intensive hot forming steps can be partially or completely eliminated. Since these process steps lead to scaling and so-called α-case formation, microstructural modification with B can reduce material consumption and save time. Through the reduction of manufacturing costs, it could be possible to replace competing materials with titanium alloys. This results in an expansion of the range of applications.
In the course of the present work the microstructure of a B-modified (α+β)-titanium alloy was characterized and the high-temperature forming behavior was investigated. The alloy is available with three different hypoeutectic B contents, while all alloy variants were investigated using the same experimental methods. The most commonly used (α+β)-alloy Ti-6Al-4V serves as a reference material. The material characterization includes the investigation of the microstructures, the determination of the β transus temperature using differential scanning calorimetry and high temperature laser scanning confocal microscopy, a phase analysis by means of backscattered electron diffraction analysis, contrast analysis and X-ray diffraction and the verification of the observed grain refinement using high energy X-ray diffraction. Furthermore, this work deals with the high temperature forming behavior. Thereby, step upsetting tests were performed on a forming dilatometer, with the true strain and forming temperature mimicking a typical rolling process in the (α+β)-two-phase region. In order to investigate the dependance of the microstructure and the mechanical properties on deformation strain, each alloy variant was characterized in three- and fivefold upset condition. The evaluation also includes flow curve interpretation, texture analysis and hardness measurements in the forming zone.
The B-modification results in a significantly reduced as-cast prior β grain size, which leads to a homogeneous distribution of the crystallographic orientations in the as-cast state. All alloy variants, which were alloyed with B, could be formed without deformation-induced deterioration and significantly more homogeneously than the alloys free of B.