State of the Art
Machining operations like turning, milling and drilling are common
manufacturing processes in mechanical engineering, especially if
complex geometries have to be produced. Nowadays, high-speed cutting
(HSC) of metallic materials gets more important in industry to
reduce manufacturing times and production costs. Hence, several research
activities have been started to investigate high-speed machining
processes beginning in the 1980s. As a result, the cutting speed during
machining of magnesium and aluminium alloys has been raised to about 6000 m/min.
Comparing these cutting speeds to the state of the art for machining beta-titanium
(120m/min) and nickel-base superalloys and cobalt-base alloys (60m/min)
clearly indicates that the problem of the poor machinability of these
high-performance materials has not been solved.
Chip Formation Mechanism
Figure 1: Principle of a milling process (left). Two-dimensional view of a segmented chip (right).
From all aspects, the chip-formation mechanism is the key factor influencing the
machinability of a material and only a thorough understanding of this mechanism
can lead to the optimisation of the cutting process. The chip-formation process can be described
as follows: At the beginning of the cutting action, the material is dammed in front of the tool
and the plastic deformation is concentrated in a narrow zone, the primary shear zone
(point 1 in fig. 1), leading from the tool tip to the upper surface of the workpiece.
Most of the energy used for the plastic deformation is transformed into heat in
the primary shear zone. The rate of the heat dissipation into the surrounding
material strongly depends on the material properties (e.g. heat conductivity
and the flow curve) and the cutting parameters (e.g. the cutting speed,
cutting depth and feed rate). In case the heat can dissipate quickly, the material
is deformed homogeneously leading to the formation of a continuous chip with
constant chip’s thickness. The cutting force remains almost constant. On the
other hand, if the heat cannot dissipate quickly into the material surrounding
the primary shear zone, the material softens and the deformation therefore localises.
In the end, the material is deformed in a narrow zone of a few microns (the so-called
adiabatic shear band) which leads to the formation of segmented chips,
(fig. 1, right). Additionally, the possibility of shear crack development during
the shear deformation followed by rewelding of the surfaces is discussed especially
for the nickel-base superalloy Inconel IN718 at the moment. The chip is afterwards
guided along the rake face of the tool, the so-called secondary shear zone leading
to an increase in temperature in the contact area (point 2 in fig. 1).
Independent of the mechanisms leading to segmented chips, the chip segmentation will
be reflected on the finished surface of the workpiece showing a wavy structure
at the micro scale leading to higher surface roughness. The strain in the shear band
can easily exceed 1000% at strain rates up to 10 7
/s and the local temperature in
the shear bands can even reach the melting point of the material. During the formation
of segmented chips, the cutting forces are fluctuating.
Materials investigated in the MAMINA project
Three of high-performance metallic materials of industrial relevance have been selected
for the investigations within the MAMINA project:
The beta-titanium alloys (e.g. Ti 15V 3Al 3Cr 3Sn) are the only ones
being able to be precipitate strengthened. Due to the unique combination
of low density and high strength, beta-titanium alloys are widely used
in aerospace engineering, e.g. for landing gears. In addition, extensive
research is carried out to introduce the beta-titanium alloys into medical
applications, e.g. for tools, products for osteosynthesis and implants.
Machining of beta-titanium alloys, however, involves high production costs.
This difficulty arises from the physical, chemical and mechanical properties
of beta-titanium: The low Young’s modulus (the beta titanium alloys
exhibit the lowest Young’s modulus of all titanium alloys) in
combination with the high strength of beta-titanium cause chatter and
tolerance problems. Additionally, during the cutting action fresh
metallic titanium surfaces are produced. Due to the high chemical
reactivity of titanium, chemical reactions occur between the tool
and the workpiece leading to rapid destruction of the cutting tool.
Finally, due to formation of very long chips, the cutting operations
cannot be automated as the machining process must be interrupted as
often as it is necessary to remove the chips from the process zone
by an operator.
Inconel IN706 is a typical representative of the whole group of
nickel-iron-based superalloys. IN706 is an alloy based on Inconel IN718,
which is normally used in stationary turbines, e.g. for the production
of turbine blades for the final stages of compressors and turbine disks
where the manufacturing requires complex machining operations. The
machinability is generally poor as the high strength of IN706
(even at elevated temperatures up to 650°C) results in rapid tool wear.
The relatively low thermal conductivity of the alloy leads to heat
concentration in the process zone. Therefore, poor surface integrity
and even the ignition of the workpiece are possible during machining of
Cobalt-based corrosion-, temperature- and wear-resistant alloys, e.g.
the alloy X40, are classified as moderate to difficult to machine as
they are resistant to metal removal because of their high shear strengths.
These alloys are used in turbine and aircraft engine components as cheaper
alternative to the nickel-based superalloys, because of easy manufacturing
(air casting instead of vacuum casting). They exhibit excellent thermal
shock resistance which makes them a good choice e.g. for vane applications.
Cobalt-base alloys can only be processed using conventional production
methods at low cutting speeds. During machining at higher cutting speeds,
these alloys rapidly harden and due to the cutting-induced heat generation
the rake face welds to the tool resulting in built-up edge formation.