The behavior of Titanium Alpha Alloys during Hot Creep Forming

Hot creep forming (HCF) is an innovative sheet metal forming process used to obtain high demands on material properties along with high geometric accuracy in terms of dimensional and shape stability without any need for post-treatment. Such a manufacturing process has been used mainly for special aluminum alloys. The application on titanium alloys is not known so far.
Titanium alloys, on the other hand, have excellent density-specific mechanical properties combined with high chemical resistance, making them preferred in the aerospace industry. The mechanical properties such as the deformation behavior, are strongly determined by the existing microstructure and phase composition. In this context, the crystal modifications of α-titanium (hexagonal) and β-titanium (body-centered cubic) differ significantly.
A compromise is offered by the mixed form α +β titanium alloys, which are represented by the most common titanium alloy, Ti-6Al-4V. The latter alloy is used in many different areas, from aerospace to medical applications, and their usage has, therefore, already been widely explored. α-titanium alloys have so far been used in automobiles, sports goods, and biomedical applications, but rarely in the aerospace industry.
In this work, the behavior of several titanium alpha alloys has been investigated during the hot creep forming process. KS1.2ASNEX, Exhaust-XT®, and ASTM grade 04 were the spoken alloys owing to their excellent oxidation resistance at high temperatures. However, the pronounced springback behavior of titanium alloys poses a significant challenge for sheet metal forming in order to produce dimensionally accurate and stable components and should definitely be avoided. Since the springback effect is temperature dependent, the springback behavior at different temperature levels was investigated. To develop a better understanding of material behavior during hot creep forming, several factors were varied and analyzed for their influence. The relationships between the significant factors such as forming temperature, heating time, creep strain, strain rate and relaxation time were worked out by means of a statistical design of experiments, on the basis of which the HCF process was optimized. In order to analyze the influence of further parameters such as the friction coefficient, die temperature and blankholder force and to use them for process optimization, HCF simulations based on the finite element method were carried out. Based on the approach mentioned above, forming temperature and relaxation time are the most significant factors to impact the oxidation behavior and the residual stresses. The alloy KS1.2ASNEX exhibits the best behavior at 650°C with a relaxation time of 720s so that full stress relief can be achieved without any detrimental oxidation layers. The simulation also shows an improvement in results due to a small coefficient of friction in combination with increasing the blank holding force. The experimental research was accompanied by metallographic investigations, which were used to evaluate the process-microstructure-property relationships.