A method of superplastically forming a workpiece comprises placing the workpiece
(16) in a die (14) and heating the workpiece (16) to a temperature
at which the workpiece (16) is superplastically formable. Pressure is applied
to the workpiece (16) to superplastically form the workpiece (16)
to the shape of the die (14). The temperature of the workpiece (16)
is measured and is analysed to determine the measured temperature distribution
of the workpiece (16) at stages in the superplastic forming process. A database
of desired temperature distributions of the workpiece (16) at different
stages in the superplastic forming process is kept. The measured temperature distribution
of the workpiece (16) at each stage in the superplastic forming process
is compared with the desired temperature distribution of the workpiece (16)
at the stage in the superplastic forming process and heating means and/or cooling
means (36, 38, 40, 46, 48, 58, 60, 66, 68, 94, 96) are controlled such that
the temperature distribution of the workpiece (16) is adjusted to more closely
match the desired temperature distribution of the workpiece (16) to control
the thickness of the workpiece (16).
