dc.description.abstract |
Titanium
alloys are used extensively in the aerospace industry due to their
high specific strength and excellent corrosion resistance. However, their poor
thermal
conductivity and high chemical reactivity with tool materials make the
machining difficult, especially when grinding. During grinding, the excessive
heat generated at the wheel-work piece interface may result in poor surface
finisil,1_, a transformed surface layer, excessive plastic deformation, thermallyinduced
residual tensile stress, burn and micro-cracking on the ground
surface. The
poor surface integrity and metallurgical changes of the surface
and sub-surface
may impair the surface sensitive properties such as fatigue
life
during service.
ln order to overcome the thermal
problems when grinding titanium alloys, a
new
cooling strategy, cryogenic grinding, was studied which involves the
supply of liquid nitrogen into the grinding zone using a nozzle jet system. lt
was found that
cryo-cooling with conventional grinding wheel decreased
surface
roughness values, burn and plastic deformation of Ti-6Al-4V alloys
and
produced better a cutting mechanism than when using water-based
coolant, especially at higher depths of cut. However, the thermal problems
still introduced
high residual tensile stress which degraded the fatigue life of
ground specimens. Another approach to reducing the grinding temperature
was to use a
superabrasive wheel (diamond), because of its superior thermal
conductivity, the integrity of the ground surface was improved and the fatigue
life
properties of the specimen were maintained at higher values than for
conventional
grinding wheels.
An ultra stiff machine tool, Tetraform C, was also used to grind Ti-6Al-4V
alloys and under selected conditions the ground surface reached a good
surface finish and the
fatigue properties were also retained at lower depth of
cut. An ELID
system on the Tetraform C was also investigated. It produced a
rougher surface finish in this study. However, it is believed that reducing the
wheel
loading problem when grinding titanium alloys may contribute to
maintaining the fatigue properties. |
en_UK |