Participants: LTU Division of Materials Science and GKN Aerospace Engine Systems
This project ended on September 27, 2019, when Magnus Neikter graduated with a doctorate, after four years of collaboration. Below the abstract of his thesis “Microstructure and hydrogen embrittlement of additively manufactured Ti-6Al-4V”, which can be downloaded here.
Titanium is a biocompatible metal with high specific strength and excellent corrosion resistance. It is widely applied within additive manufacturing (AM). The thermal cycles and all the different process parameters in AM render different microstructures. This and the presence of texture, residual stresses and defects affect the mechanical properties. In this project, these issues of additively manufactured Ti-6Al-4V have been addressed, with a focus on microstructure. To deepen the understanding of AM built Ti-6Al-4V a wide collection of different AM processes have been investigated; three AM processes from the subgroup directed energy deposition (DED), namely laser metal wire deposition (LMwD), shaped metal deposition (SMD) and laser metal powder deposition (LMpD). Moreover, two AM processes belonging to the subgroup powder bed fusion (PBF) have been investigated as well, namely electron beam melting (EBM) and selective laser melting (SLM). Several characterization methods such as electron backscattered diffraction (EBSD), scanning electron microscope (SEM) and light optical microscope (LOM) and stereomicroscope have been used for characterizing the microstructures, whereas neutron time of flight (TOF) diffraction has been used for investigating the three-dimensional bulk texture. Moreover, for two-dimensional texture EBSD has been used. Some of the findings of this work are that LMwD had a stronger texture (both from the EBSD and neutron TOF diffraction data) than the two investigated PBF processes. This was further supported by the microstructural characterization that showed that LMwD had larger prior Ⱦ grains than both SLM and EBM built material. Both cast and EBM built Ti-6Al-4V were investigated using TOF diffraction. The results showed that the cast had more than twice the amount of texture than the EBM material, which was explained by the coarser microstructure of the cast material, with large Ƚ colonies where numerous Ƚ laths had the same crystal orientation. The binary microstructure pattern (BMP) that is found in SLM built material was furthermore characterized in detail. It was found that the BMP is composed of a fine microstructure zone (FMZ) that surrounds a coarse microstructure zone (CMZ). By performing a Ⱦ grain reconstruction it was shown that each FMZ and CMZ belonged to separate Ⱦ grains, explaining why the BMP does not disappear unless heat treatment is conducted above the Ⱦ transus temperature. A phenomenon that is known to degrade the mechanical properties of titanium is hydrogen embrittlement (HE). Titanium is a so-called hydride forming metal, where hydrogen reacts with titanium forming titanium hydrides. This formation cause reduced mechanical properties and is of wide interest, especially for conditions where titanium is exposed to excessive amounts of hydrogen. To investigate how hydrogen affects EBM built Ti-6Al-V fatigue crack growth (FCG) experiments were conducted in two different atmospheres; air and hydrogen. The FCG rates of the two scenarios were compared. From the experiments, it was shown that by introducing the EBM built Ti6Al-4V to a hydrogen-rich atmosphere the FCG rate was increased. Below ȟK 23 ܽܲܯξ݉, the average FCG rate was similar for the hydrogen and air tested materials. Above this ȟK the FCG rate accelerated for the hydrogen-tested material, reaching a maximum of 13 times faster FCG rate than the corresponding FCG rate in air. The EBM tested material was furthermore compared to cast and forged material that was previously tested using the same ȟK. From this comparison it was concluded that cast Ti-6Al-4V had the earliest onset of FCG rate acceleration, occurring at ȟK 17 ܽܲܯξ݉, while for the forged material the same acceleration onset occurred at 26 ܽܲܯξ݉. The reason for this was explained by the difference in microstructure, where the cast Ti-6Al-4V had the coarsest microstructure.
PhD. Magnus Neikter
Supervisor Prof. Marta-Lena Antti