Effects Of Thermo-hydrogenation On The Diffusion Bonding Of Ti3AL Alloy In Electric Field

In this work the diffusion bonding of Ti-24Al-14Nb-3V(at %) alloys was carried out with the help of thermo-hydrogen treatment and electric field. The hydrogenation of Ti3 Al samples was conducted using a hydrogenation furnace conceived by Harbin Institute of Technology. A study was realized beforehand to control the hydrogenation process, evaluate the influence of the parameters on the resulting hydrogen content and establish the influence of hydrogen on Ti3 Al alloys. The influence of the hydrogen gas flowing rate, hydrogenation time, hydrogenation temperature and finally the shape of the samples were determined and the Ti3 Al samples were successfully hydrogenated at 0, 0.3, 0.5 and 0.7wt%H. The influence of hydrogen on the microstructure of Ti3 Al alloys was investigated via scanning electron microscopy and X-Ray diffraction using these samples with different hydrogen contents. The results show that the base material is composed of three phases: alpha2, beta2 and O phases. The addition of hydrogen greatly influenced the proportion of each phase and in particular the proportion of the O phase greatly increased with increasing hydrogen content. At high hydrogen content ε hydrides appeared in the material. In addition hot compressive tests were carried out at 990°C under a strain rate of 8.3x10-2 and it was found that the hydrogen treatment greatly improved the hot deformability of Ti3 Al alloys. In particular, the hot compressive strength of the material could be decreased by about 40% at 0.3wt%H.The diffusion bonding of Ti3 Al was successfully realized and the influences on the quality of the bond of hydrogen, electric field and both hydrogen and electric field were investigated via shear tests and scanning electron microscopy. The thermo-hydrogen treatment managed to greatly improve the shear strength of the bond. When the diffusion bonding was realized at 990°C, 90 min, 12 MPa, the she ar increased from 206 MPa for the sample without hydrogen to 302 MPa for the samples with 0.9wt% hydrogen. Correspondingly, some voids were observed along the interface for the basic diffusion bonding but disappeared when hydrogen was added. Furthermore a thicker layer of crystals formed along the interface of the joint with 0.6wt% and 0.9wt% hydrogen, respectively and the grain size of the microstructure decreased with increasing hydrogen content. In a similar fashion, the shear strength was found to increase when an electric field was applied during the bonding. The effect of the electric field was more potent at low voltage and decreased at highvoltages. Furthermore cumulating both hydrogen treatment and electric field resulted in higher shear strength than the ones obtained with the single action of thermohydrogen treatment or electric field. A maximum shear strength of 316 MPa w as reached when the samples hydrogenated with 0.3wt%H were bonded assisted by a voltage of 200 V, representing an increase of the shear strength of 54% compared to classic diffusion bonding. Finally, Vickers hardness tests were conducted to quantify the refinement of the microstructure and the hardness of joints increased with increasing hydrogen content. A maximum hardness of 856 HV was measured for the samples with 0.9wt%H as opposed to 336 HV for the base material.