Abstract
M.Ing.
Grade 4 titanium is a commercially pure grade titanium alloy extensively used in various industries
including the chemical industry and more recently in the biomedical industry. Grade 4 has found a
niche as a biomedical material for production of components such as orthopaedic and dental implants.
Its physical properties such as high corrosion resistance, low thermal conductivity and high strength
make it suitable for these applications. These properties also make it hard-to-machine similar to the
other grades of titanium alloys and other metals such as nickel based alloys.
During machining of titanium, elevated temperatures are generated at the tool-workpiece interface
due to its low thermal conductivity. Its high strength is also maintained at these high temperatures.
These tend to impair the cutting tool affecting its machinability. Various investigations on other
grades of titanium and other hard-to-machine materials have shown that machining at high cutting
speeds may improve certain aspects of their machinability. High speed machining (HSM) is used to
improve productivity in the machining process and to therefore lower manufacturing costs. HSM
may, however, change the surface integrity of the machined material.
Surface integrity refers to the properties of the surface and sub-surface of a machined component
which may be quite different from the substrate. The properties of the surface and sub-surface of a
component may have a marked effect on the functional behaviour of a machined component. Fatigue
life and wear are examples of properties that may be significantly influenced by a change in the
surface integrity. Surface integrity may include the topography, the metallurgy and various other
mechanical properties. It is evaluated by examination of surface integrity indicators. In this
investigation the three main surface integrity indicators are examined. These are surface roughness,
sub-surface hardness and residual stress. White layer thickness and chip morphology were also
observed as results of the machining process used.
The effect of HSM on the surface integrity of grade 4 is largely unknown. This investigation therefore
aims to address this limitation by conducting an experimental investigation on the effect of HSM on
selected surface integrity indicators for grade 4.
Two forged bars of grade 4 alloy were machined using a CNC lathe at two depths of cut, 0.2mm and
1mm. Each bar was machined at varying cutting speeds ranging from 70m/min to 290m/min at
intervals of approximately 20m/min. Machined samples were prepared from these cutting speeds and
depths of cut. The three surface integrity indicators were then evaluated with respect to the cutting
speed and depth of cut (DoC).
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Results show that a combination of intermediate cutting speeds and low DoC may have desirable
effects on the surface integrity of grade 4. Highest compressive stresses were obtained when
machining with these conditions. High compressive stresses are favourable in cases where the fatigue
life of a material is an important factor in the functionality of a component. Subsurface hardening was
noticed at 0.2mm DoC, with no subsurface softening at all cutting speeds. Surface hardness higher
than the bulk hardness tends to improve the wear resistance of the machined material. Though surface
roughness values for all depths of cut were below the standard fine finish of 1.6μm, roughness values
of samples machined at 0.2mm DoC continued to decrease with increase in cutting speed. Low
surface roughness values may also influence the improvement of fatigue life of the machined
components. These machining conditions, (intermediate cutting speeds and low DoC), seem to have
promoted mechanically dominated deformation during machining rather than thermal dominated
deformation. Thermal dominated deformation was prominent on titanium machined at DoC of 1mm.