High strain rate, superplastic metal forming

Citation

Nieh, T.G. (2001), "High strain rate, superplastic metal forming", Assembly Automation, Vol. 21 No. 4. https://doi.org/10.1108/aa.2001.03321daa.002

High strain rate, superplastic metal forming

High strain rate, superplastic metal forming

T.G. Nieh

Keywords: Metal forming

Superplastic forming is a net-shape forming technology. Conventional superplastic forming is usually carried out at a strain rate of 10^–3 to 10^–4 per second. Under these strain rates, it would take about one hour to form a typical structural component (assume 100 per cent deformation strain). Thus, conventional superplastic forming may be useful for manufacturing aerospace structures but is not economically feasible for large-volume commercial production (for example, automotive parts). For this very reason, significant efforts have been made to improve the superplastic forming rate in the past decade. High strain rate forming is defined as when the forming rate is faster than 10^–2 per second. At this forming rate, a typical structural part can be formed in less than one minute. Although some alloys have been observed to exhibit superplasticity at a strain (or forming) rate faster than 10¹ per second, however, such high strain rate is impractical for production environments because it is difficult to control. In theory, a strain rate of 10^–2 to 10⁰ per second is probably the most desirable.

Resulting from the intensive recent efforts, many alloys and composites have been shown to have the superplastic forming capability at high strain rates (coined as high strain rate superplasticity, HSRS). These materials include aluminum, magnesium, nickel, titanium, and their composites. HSRS is usually observed in materials having a very fine grain size (~ 1µm or less). The phenomenon has also been found to be associated with the presence of liquid phases at grain boundaries in the materials. In the former case, the sliding of small grains on each other along grain boundaries is the controlling deformation mode. Constitutive equation describing superplastic deformation indicates that the deformation rate is proportional to the inverse of grain size raised to a second or third power; that is, the smaller the grain size, the faster the superplastic deformation rate. In the case of having a liquid phase at grain boundaries, testing temperature plays a major role and grain size is not important. With a proper amount of liquids at grain boundaries, grain boundaries are lubricated. This would ease the sliding process, in a manner similar to that of applying oil lubricant to reduce friction between two sliding metals. The presence of liquid can also assist the accommodation process at grain triple junction during grain-boundary sliding, to prevent void formation and sample fracturing.

The development of HSRS is expected to have a significant technological impact on the commercial applications of superplastic materials because an increase in forming rate results in a reduction of forming time. However, the implementation of this technology faces many technical and economic challenges. For example, in order to use high strain rate superplastic forming with a fine grain size alloy, extensive thermomechanical processing is usually employed on the alloy prior to the forming operation. The processing may include extrusion, forging, rolling, and torsion. This greatly increases the cost of the starting materials. However, from a technological point of view, fine-grained materials have a better chance of being adapted to automated manufacturing. On the other hand, in the case of having liquid phases at grain boundaries, the temperature window for superplasticity is usually quite narrow (~less than 20°C). This is because the amount of liquid is a function of forming temperature, and if the liquid exceeds a critical amount, grain boundary would no longer support tensile force and lead to fracture. It is considered challenging to control temperature within 20°C in a production environment, especially for automated manufacturing. Another concern for high strain rate superplastic forming is tooling. When the strain rate is high, the applied pressure is also high. This increases equipment requirement and accelerates the rate of die wear.