Abstract
M.Ing. (Mechanical Engineering)
Titanium and its alloys have the potential to serve as a strategic economic
driver of the South African economy. The manufacture and use of high
strength, lightweight materials such as titanium alloys have become of great
importance in the aerospace and biomedical industries over the past few
decades. The manufacturing costs of titanium alloy components however, are
considered high due to the poor machinability of the material. Furthermore, as
with all metals during machining, surface residual stresses are induced into the
material. These are of particular interest in the aerospace industry as they can
be either detrimental or beneficial to the performance and fatigue life of
materials. The aim of this investigation is therefore to examine the effect that
machining parameters have on the magnitude, sign and distribution of residual
stresses induced in Grade 4 and 5 titanium alloys during high performance
machining (turning).
The effect of these machining parameters is investigated by residual stress
measurements conducted with X-ray diffraction and grain structure analysis of
the machined surfaces by optical microscopy. Results show that cutting speed
and depth of cut have a significant effect on the residual stresses. At low
cutting speeds, the surface residual stresses are largely compressive, becoming
more tensile with an increase in cutting speed. An increase in depth of cut also
introduces more compressive residual stresses into the material. The
microstructural analysis of the alloys shows that grain deformation decreases
with an increase in cutting speed and cutting depth.