Abstract:
Some metals, such as Ti-6Al-4V, have a high elongation to failure when strained at certain
rates and temperatures. Superplastic forming is the utilisation of this property, and it can be
used to form thin, geometrically complex components. Superplastic forming is a slow process,
and this is one of the reasons why it is an expensive manufacturing process. Localised thinning
occurs if the specimen is strained too quickly, and components with locally thin wall thickness
fail prematurely. The goal of this study is to find a technique that can be used to minimise
the forming time while limiting the minimum final thickness.
The superplastic forming process is investigated with the finite element method. The finite
element method requires a material model which describes the superplastic behaviour of the
metal. Several material models are investigated in order to select a material model that
can show localised thinning at higher strain rates. The material models are calibrated with
stress-strain data, grain size-time data and strain rate sensitivity-strain data. The digitised
data from literature is for Ti-6Al-4V with three different initial grain sizes strained at different
strain rates at 927 C.
The optimisation of the forming time is done with an approximate optimisation algorithm.
This algorithm involves fitting a metamodel to simulated data, and using the metamodels
to find the optimum instead of using the finite element model directly. One metamodel is
fitted to the final forming time results, and another metamodel is fitted to the final minimum
thickness results.
A regressive radial basis function method is used to construct the metamodels. The
interpolating radial basis function method proved to be unreliable at the design space
boundaries due to non-smooth finite element results. The non-smooth results are due to
the problem being path dependent.
The final forming time of the superplastic forming of a rectangular box was successfully
minimised while limiting the final minimum thickness. The metamodels predicted that
allowing a 4% decrease in the minimum allowable thickness (1.0 mm to 0.96 mm) and a
1 mm gap between the sheet and the die corner the forming time is decreased by 28.84%.
The finite element verification indicates that the final minimum thickness reduced by 3.8%
and that the gap between the sheet and the die corner is less than 1 mm, resulting in the
forming time being reduced by 28.81%.
