Study on controlling thermal expansion coefficient of ZrO_2-TiO_2 ceramic die for superplastic blow-forming high accuracy Ti-6Al-4V component

摘要

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 ZrO_2-TiO_2 ceramic die by adjusting the volume fraction of TiO_2 and the relative density, and then controlling its' CTE to be similar to Ti-6A1-4V alloy. In this paper, firstly, the relation between the expansion coefficient and the volume fraction of TiO_2 and ceramic relative density was investigated. The results showed that with the increment of the volume fraction of TiO_2, 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 ZrO_2-TiO_2 can be controlled by adjusting the volume fraction of TiO_2 and corresponding relative density. Thirdly, to evaluate the practicability of the ZrO_2-TiO_2 ceramic die on superplastic forming, the ZrO_2-TiO_2 ceramic die (volume fraction of TiO_2 is 27%, relative density is 93%) with the expansion coefficient (8.92 × 10~(-6)℃~(-1)) similar to Ti-6Al-4V was fabricated by sintering at 1520 ℃ for 2 h. The experiment of superplastically forming Ti-6Al-4V using ZrO_2-TiO_2 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.