The intermetallic titanium based alloys mainly from the orthorhombic family have shown excellent properties including strength and oxidation resistance. In addition, they have also displayed good specific strength, remarkable specific resistance, splendid corrosion resistance and outstanding high temperature mechanical properties thus making them the best placed candidate to replace nickel-based alloys, for purposes such as aircraft engine discs and blades fabrication.Currently used nickel based alloys are expensive to fabricate.
Based on current technology, the optimal processing temperature for the orthorhombic titanium alloys is about 1060°C at which their deformation resistance is significantly low. Also at room temperatures, their formability is very low which means that they cannot form intricate shapes as desired. Therefore, increasing the intrinsic plastic deformation-ability of the orthorhombic titanium-based alloys can fundamentally improve their processing properties and significantly reduce difficulties in fabricating the titanium alloy based parts. Knowledge exists that plasticity of titanium alloys can be increased by adding hydrogen.
Researchers led by professor Yingying Zong from the National Key Laboratory for Precision Hot Processing of Metals & School of Materials Science and Engineering at Harbin Institute of Technology in China investigated whether hydrogen could be used to improve the plastic deformation ability of the orthorhombic titanium alloy (Ti–22Al–25Nb). They also hoped to unravel the influence of hydrogen content on the cylindrical drawing properties of the orthorhombic titanium alloy. Their work is now published in Advanced Engineering Materials.
The research team proposed to use 1 mm-thick orthorhombic titanium alloy sheets as the raw material. The rolled sheets were then placed in a hydrogen environment at 700 °C for about 2h and then allowed to cool down in air to room temperature, where pressure was controlled to produce varying hydrogen absorbed alloys. Tensile tests were then conducted on the specimens. Drawing tests closely followed after a 10 min thermal insulation procedure.
The authors were able to observe that the plasticizing effect of the hydrogen on the titanium alloy varied as the hydrogen content varied. Low hydrogen content yielded low plasticizing effect where the reverse is also true. They also realized that for the tensile experiment, the alloys directly softened after yielding and the tensile stress decreased. For the drawing test, the researchers observed that fractures occurred on all of the formed cylindrical work pieces at the round corners of the punch and no deformation occurred in most of the flanged area. The 0.2H alloy work piece was good quality and exhibited a smooth, wrinkle free surface indicating that the plastic deformation ability of the titanium alloy increased significantly after 0.2 wt% Hydrogen was added to the alloy.
The hydrogen induced plasticization effect of the orthorhombic titanium alloy is confirmed to be at its best at 0.2 wt% hydrogenation. This is due to the fact that the observed elongation is greatest here. The cylindrical drawing-formability of the alloy also is seen to increase significantly at the same hydrogenation level. A conclusion can therefore be drawn from their work that excellent forming performance of the orthorhombic titanium alloy can be attained via hydrogenation process which has been seen to induce desired plasticization effects. Titanium based alloys are significantly cheaper that nickel based alloys.
Bin Shao, Shengxiang Wan, Debin Shan, Bin Guo and Yingying Zong. Hydrogen-Induced Improvement of the Cylindrical Drawing Properties of a Ti–22Al–25Nb Alloy. Advanced Engineering Materials 2017, 19, volume number 3.Go To Advanced Engineering Materials