Boeing has had seven years of high rate production of SPF titanium parts. The effect of 900°C forming temperature on die life, part surface finish and alpha case formation has been documented, which has shown that there would be a great advantage in forming at a lower temperature. This paper investigates in detail the SPF properties of three new alloy developments, SP700 from NKK/RMI (USA), fine grain Ti-6-4 from VSMPO (Russia) and sub-microcrystalline Ti-6-4 from IMSP (Russia). All these alloys are formable at around 760°C. The relationship between temperature and SPF stress, elongation, oxygen contamination and formability is examined and compared between the alloys. In addition, the probability of the alloy being implemented is discussed.

The determination of the superplastic properties of a material, just like any mechanical property, is highly dependent on the test method, coupon geometry, and analysis of the raw data from the test. Thus the published properties of a material from one source will differ from that of another source unless a common test method is employed. The ASTM E2448 Standard Test Method for Determining the Superplastic Properties of Metallic Sheet Materials has been written to provide a common platform for testing, evaluating, and publishing superplastic properties to a uniform format, useful for both academia and industry. The Boeing Company is now using ASTM 2448 to quantify the superplastic properties of fine grain Ti-6Al-4V alloy, and is specifying it to qualify production material to the Boeing Material Standard BMS7-385. The standard includes specimen geometry and testing conditions, the test machine requirements, and how to analyze the data, including the basic stress vs. strain curve and determination of ÔmÕ value.

Most parts made by superplastic forming (SPF) have been formed at an optimum strain rate. The rate is selected to give the best SPF properties of the material. However, it has been proposed that multi-rate forming, where an initial high strain rate is successively reduced as the part is strained, can be used to make high strain parts in a much shorter time than traditional SPF forming. This paper examines the performance of fine grain Ti-6Al-4V alloy at very high initial strain rates, from 10-30 times faster than usual, with step reductions at prescribed levels of strain that still enables a total strain of over 2.1 (800%) to be achieved without degradation of the material. The paper also shows that the forming time to 100% deformation can be reduced from 55 min to 9 min. This technique can be used by industry to enable faster flow times and lower production costs of SPF parts.The superplastic properties of SPF materials vary with the strain rate imposed on them. In general, the faster the strain rate, the lower the total elongation to failure. However, very slow strain rates also have lower elongations due to grain growth and other factors. Thus, there is an optimum strain rate for maximum elongation. There is a perception that this coincides with the best SPF properties for any particular material. The strain rate is different for different materials, and for the Ti-6Al-4V titanium alloy, it is around 1 · 10 )4 s )1 . This strain rate is rather slow for most industrial applications, and the aerospace industry generally forms parts in the 2 · 10 )4 s )1 to 3 · 10 )4 s )1 range. To predict how to form a sheet of SPF material, most users have a Finite Element Analysis software package that breaks down a sheet of material into small elements, then analyses the stress and strain conditions on each element. A maximum strain rate is imposed on the fastest forming elements, and a gas pressure/time profile can be generated to make the part. This represents the fastest time a part can be formed, as allowed by the imposition of a constant strain rate. Non-Optimum Strain RatesAs stated above, SPF materials can be deformed at higher strain rates than the optimum. Figure 1 shows a series of curves representing the forming stress required to SPF fine grain Ti6Al-4V at certain constant strain rates at 775°C. The coupons were pulled at BatelleÕs PNNL facility, with careful attention paid to testing conditions, coupon geometry and application of strain rate. The strain rates vary from 3 · 10 )5 s )1 to 1 · 10 )2 s )1 . It can be seen that the stress needs to be increased to achieve higher strain rates, and the higher strain rates achieve less total superplastic elongation. A maximum strain of 2.2 (800%) is obtained at 1 · 10 )4 s )1 strain rate.A curve can be fitted against the set of different strain rate curves that defines a 'Maximum Stress/StrainÕ boundary. To the left of the boundary, material can be formed without failure, to the right it will fail prematurely. Figure 2 shows such a boundary. Kraisheh (Ref 1) has proposed...