Titanium Hot Stretch Forming: Experimental and modeling residual stress analysis

Titanium alloys, due to their high mechanical properties coupled with light weight and high
corrosion resistance, are finding a widespread use in the aeronautic industry. The use of titanium in
replacing the conventional alloys, such as aluminum alloys and steel, is reduced by both the high
cost of the raw material (it costs anywhere from 3 to 10 times as much as steel or aluminium) and
the machining costs (at least 10 times that to machine aluminium). For such a reason new
technologies have been studied and developed. In particular many researchers are searching for
technologies, such as the precision hot forming, that allows to obtain components with a low buy to
fly ratio. Many of the airframe component structures are designed to fit against the inside radius of
the fuselage curvature. By combining traditional stretch forming technology with hot titanium
forming techniques, the HSF guarantees a saving in material and machining time, which are two
serious cost issues for today’s aircraft manufacturers. In addition, the process allows for consistent
quality in a productively efficient manner, assuring the sustainable attainment of delivery and build
schedules. In order to develop and improve the HSF process a modeling of the process itself was
executed in order to study the stresses and strains undergone by the material among the
deformation. The FEM model was validated through the residual stresses, and in particular the
residual stresses provided by the model were compared with the ones experimentally measured
using the hole drilling technique. Good agreement, in terms of stress range, was recorded both for
the maximum and the minimum stress.