Study on controlling thermal expansion coefficient of ZrO₂-TiO₂ ceramic die for superplastic blow-forming high accuracy Ti-6Al-4V component

The high temperatures and extended times involved with superplastic forming titanium mean that conventional die-steels are not suitable. Although dies manufactured from nickel-based alloys are used frequently, these are expensive, and more importantly, the coefficient of thermal expansion (CTE) of this materials are bigger than that of Ti-6Al-4V, which causes the dimensional inaccuracy between the dies and workpieces. The aim of this paper is to control the CTE of ZrO₂-TiO₂ ceramic die by adjusting the volume fraction of TiO₂ and the relative density, and then controlling its' CTE to be similar to Ti-6Al-4V alloy. In this paper, firstly, the relation between the expansion coefficient and the volume fraction of TiO₂ and ceramic relative density was investigated. The results showed that with the increment of the volume fraction of TiO₂, the expansion coefficient decreased. And with the rising of the relative density, the expansion coefficient increased. Secondly, based on the Turner Equation, a modified theoretical model was proposed in which the relative density of ceramic was considered. Using the model, the comparison was made between the experimental data and calculated data. The result showed that the linear CTE of ZrO₂-TiO₂ can be controlled by adjusting the volume fraction of TiO₂ and corresponding relative density. Thirdly, to evaluate the practicability of the ZrO₂-TiO₂ ceramic die on superplastic forming, the ZrO₂-TiO₂ ceramic die (volume fraction of TiO₂ is 27%, relative density is 93%) with the expansion coefficient (8.92 × 10^-6 °C^-1) similar to Ti-6Al-4V was fabricated by sintering at 1520 °C for 2 h. The experiment of superplastically forming Ti-6Al-4V using ZrO₂-TiO₂ cylinder ceramic die was carried out, also. The Ti-6Al-4V cylinder shows better shape retention, surface quality and high dimensional accuracy, and the ceramic die seems adequate for superplastically forming the high accurate Ti-6Al-4V.