EFFICIENT PROCESSING OF HARD, BRITTLE MATERIALS · 2018-04-26 · When hard and brittle ceramic...
Transcript of EFFICIENT PROCESSING OF HARD, BRITTLE MATERIALS · 2018-04-26 · When hard and brittle ceramic...
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Successfully competing in a
global market requires a
combination of having a range
of unique advantages and ways
of standing out from the crowd.
Precision manufacturing offers
this opportunity, but at the same
time it poses challenges in terms
of machinery, control and tooling.
Six domains were identified in
which a company can make the
difference.
EFFICIENT PROCESSING OF HARD, BRITTLE MATERIALS
SIX DOMAINS PROVIDING OPPORTUNITIES TO EXCEL
Hard materials such as ceramics and cemented carbide are
very useful in numerous components that are exposed to high
mechanical, chemical and/or thermal stresses. However,
these materials are not used as often as they could be as they are
not only hard but also brittle. That means that they can break
unpredictably. They are therefore slow and expensive to machine.
But there are innovative solutions in the pipeline. Scientists are
actively looking for ways to process brittle materials quickly and
cost-effectively. Such as milling with very high precision, thanks to
diamond-coated cutting equipment. Or by removing material with
electrical discharges or vibrations.
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MARKET NEEDA higher wear resistance, a fatigue strength, a longer lifetime and a
light weight are sought-after properties when examining hard mate-
rials, such as cemented carbide and ceramics, in product design. Al-
though these materials would be ideally suited for this purpose, they
are not used because of the difficulties in machining, which would
create a high-cost, slow production process. To establish break-
through cost-effective machining, technologies for both small and
large series need to be introduced.
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POTENTIAL & OPPORTUNITYThe challenges when machining carbides and ceramics are, on the
one hand, ensuring the required level of precision and, on the oth-
er, keeping down machining costs. When hard and brittle ceramic
material gets chipped, this results in unpredictable dimensions, and
a high level of tool wear and a low material removal rate add sub-
stantially to the price tag.
However, the high levels of hardness and melting temperatures of
hard and brittle materials make them ideal for a wide range of appli-
cations. In the automotive industry, high-quality technical ceramics
are used so as to consistently comply with requirements that gener-
ally cannot be met by metal- or plastic-based materials. The l-sensor
with doped ZrO2 as an electric conductor is a perfect example of
this. Power stations use ceramic components in any machinery that
is subject to high levels of mechanical, chemical and thermal stress.
In the plastic industry too, components that face high levels of stress
are made of high-quality technical ceramics. In other sectors as well,
these materials can be found being used for various applications:
mechatronics and semiconductors (bearings, precision compo-
nents), the pump industry (valves, bearings, plungers), the food in-
dustry (valves, cutters), the chemicals industry (nozzles), the medical
industry (hip balls, teeth, knives), aerospace (valves, sensors) and
offshore navigation (guidance, bearings).
Grinding is still the standard technology used for machining of hard
and hardened materials. For ceramic components, an NNS will be
produced in soft state (green part), followed by a sintering process
which will lead to the shrinkage – to an unpredictable extent – of
the dimensions of the relevant part, thus making a finishing grinding
operation inevitable. For hard materials that are electrically conduc-
tive, EDM technology is also frequently used. However, nowadays
new technologies are also available for machining free-form shapes
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out of hard, brittle materials. The challenge is to incorporate these
technologies into the production chain and achieving a cost-effec-
tive solution.
RESEARCH RESULTSThe paragraphs that follow will discuss various existing and prom-
ising prospective alternatives to grinding and EDM for machining
cemented carbides and ceramics.
High-precision milling
High-precision milling allows machining of cemented carbide parts
to take place without there being the drawbacks of both grinding
and/or EDM. It thus improves the level of geometrical freedom (in
the form of ensuring, within feasible limits, high surface quality and
short lead times as a result of direct milling). To enable milling, a
very hard diamond coating is required at the cutting edge (HV10 >
9,000 kg/mm2), combined with new milling strategies. Other consid-
erations that need to be taken into account when milling cemented
carbide are a high level of machine stability; avoiding spindle ex-
pansion; minimising tool overhang; cooling due to compressed air;
and high spindle speeds (30,000 rpm for a 2-mm diameter tool).
However technological feasible it is though, the economics of mill-
ing cemented carbides mean that it not suited to all applications:
tool cost (approx.. €250/tool diameter of 2 mm), dimensional limits
(only small sizes) and a limited Material Removal Rate (MRR) of 2.2
mm3/min).
High-precision (micro-)milling of ceramics is even more difficult. Ex-
periments with a 1-mm end mill with different hard coatings and CBN
showed that a nanograin diamond coating is most appropriate. With
cutting parameters (ap = 4 µm, fz = 3 µm and vc = 120 m/min), a
roughness of less than 60 nm and an MRR of 0. 912 mm³/min over a
distance of 2 m are achieved. Delamination of the coating limits tool
performance. The hard carbon coating in the tool breaks up above
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66 mm. The micrograin diamond coating has an intermediate per-
formance, which can be explained by the larger size of the diamond
crystals. With CBN tools and cutting parameters (ap = 10 µm, fz =
2-10 µm and vc = 60 m/min), a roughness of less than 150 nm and
a maximum cumulative milling length of 341 mm was achieved. This
process is mainly restricted in practice to finishing operations and
micro applications.
Electrical discharge machining (EDM)
Electrical discharge machining is a machining technique in which
material is removed by controlled electrical discharges (sparks) be-
tween an electrode (a wire or die in most cases) and a workpiece,
both of which are submerged in a dielectric fluid. The EDM process
imposes one big limitation on the workpiece material: it has to be
sufficiently conductive. By adding a secondary conductive phase
(e.g. WC, TiB2, NbC and TiN) to a non-conductive matrix (ZrO2,
Al2O3, Si3N4), EDM can be enabled by creating a percolating con-
ductive network inside the composite material. This network can
form between 30 and 40 vol.% of conductive secondary phase and
is dependent on the grain size, with smaller grains giving better re-
sults. As well as allowing EDM, the creation of a composite material
has a positive effect on the mechanical properties, such as strength
and fracture toughness.
Different coatings for milling ceramics
Micrograin diamond (DM) Nanograin diamond (DN) Hard Carbon (HC)
Coating material CVD diamond crystalline CVD diamond nanocrystalline Ta-C (>80% sp³)
Coating technology CVD CVD PVD-Arc
Coating morphology Crystals of 1-2 µm Crystals of 30-40 nm -
Hardness (HV 0.05) 10.000 10.000 7.000
Max. service temp. (°C) 600 600 500
Coating thickness 6-10 µm 6-12 µm 0,3-1,2 µm
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Unlike metals, in which melting and evaporation are the main material
removal mechanisms (MRMs), in ceramics and ceramic composites,
other phenomena can play an important role in determining the ma-
terial removal rate, electrode wear and surface quality. Grains can be
dislodged during EDM due to differences in thermal expansion co-
efficients between the two phases, a process called thermal shock.
This in general leads to a high level of surface roughness and a low
level of tensile strength. Another similar MRM is spalling, in which a
newly formed recast layer (a layer of molten material) breaks loose
during the cooling process following a spark due to thermal stresses.
In general this also leads to higher levels of surface roughness. A
number of materials also undergo chemical reactions at the elevat-
ed temperatures occurring during EDM. Si3N4, a popular technical
ceramic, does not melt but decomposes above 1,800°C into Si metal
and N2 gas. The main MRM in WC is oxidation, leaving a nanometric
WO3 layer behind which can be easily removed by warm water.
In general these MRMs occur simultaneously, but most of the time
one is dominant. Thermal conductivity is the main material property
behind the material removal rate (MRR) when melting and evapora-
tion are the dominant MRMs, due to the fact that a lower thermal con-
ductivity ensures that the heat is concentrated at the surface during
EDM. However, fracture toughness and grain boundary strength be-
come important when thermal shock and spalling occur, and grain
size appears to exert a strong influence on the oxidation behaviour.
Therefore, when machining ceramics by means of EDM, knowledge
of the interactions between material properties and machining be-
haviour is crucial when trying to achieve optimal MRRs and surface
quality.
Electro-chemical machining (ECM)
ECM is similar to EDM, but there are a few major differences: no
melting or evaporation takes place; no stress deformations are intro-
duced, material removal occurs by means of dissolution of the anode
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(workpiece) (MRR = 1.5 cm³/min), an electrolyte is used to transport
ions between the cathode and the anode and often the electrode
vibrates to improve evacuation of the electrolyte.
An electrically conductive part can be machined regardless of its
hardness down to a very low surface roughness. Due to the very
narrow gap between the cathode and the anode, parts can be ma-
chined with very high precision and very low Ra values (approx. 20
nm).
Vibration-assisted machining
In vibration-assisted machining, a vibration is added to the cutting
tool. This can be a turning tool, a drilling tool or a grinding/milling
tool. Typically, the amplitude ranges between 1 and 40 µm, and the
frequency can be anything up to 80 kHz. At frequencies between
18 and 25 kHz, the process is known as ultra-assisted machining.
The vibration significantly increases the ability to machine hard and
brittle materials (SiC, Al2O3, ZrO2, etc.). The vibrating movement has
the advantages of lower cutting forces which allow higher material
removal rates, and the machining of small details. The vibration also
results in better cooling conditions, and the different wear mecha-
nisms have a tool sharpening/dressing effect.
Electrolytic in-process dressing (ELID) grinding
ELID grinding enables the use of metal-bonded grinding wheels
which are very durable and difficult to deal with in comparison with
resinous and vitrified wheels. Furthermore, with ELID grinding, it
is possible to obtain both good geometrical accuracy and a very
smooth surface on a workpiece (because of very small abrasives of
1 µm).
Several types of cemented carbide (WC-Co) and ceramics (ZrO2,
Si3N4, etc.) have been ground with ELID to a surface roughness Ra
of a few nanometres. This results in mirror-like surfaces of very pol-
ished quality. You will find below two images of ground workpieces:
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the WC-Co piece on the left measures 40 x 50 mm, has a surface
roughness Ra of 0.007 µm (Rz of 0.063 µm) and a straightness error
of 1 µm over both its width and its length. On the right-hand side is a
piece of ZrO2 of 50 mm in length and 20 mm in width which has a
roughness Ra of 0.006 µm (Rz of 0.050 µm) and a straightness error
of 0.6 µm over its length.
Laser-assisted turning
In laser-assisted turning, a laser beam locally heats up the work-
piece material just before cutting. Locally heating this material im-
proves the machinability of high-strength materials like ceramics.
Besides better machinability, laser-assisted machining holds out
several benefits: higher cutting volumes and longer tool life (MRR of
up to 10 mm3/s), a shorter manufacturing time and lower costs, the
elimination of cooling lubricants (dry machining), geometrical flexi-
bility, affordable manufacturing of complex components made from
technical ceramics, and a highly reproducible manufacturing quality
due to a very good level of control of the laser source.
ELID ground WC-Co ELID ground ZrO2
Laser-assisted turning on Monforts RNC LaserTurn
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A comparison between turning and laser assisted turning of ZrO2
Turning Laser assisted turning
Parameters
vc [m/min] 30 60
F [mm] 0.01 0.01
Ap [mm] 0.17 0.42
Laser power [W] 0 2000
Material removal rate [mm3/s] 0.86 4.31
Results
Ra 0.08 0.10
Rz 0.60 0.67
Tool wear VBmax [µm] 40 46
An example providing an overview of the techniques above
Material E [GPa] Hv [kg/mm³]KIC 10kg
[MPam0.5]K [W/m°K] ρ [10-5Ωm]
ZrO2 - TiN 280 1350 9.7 6.41 2.94
Techno-logy
ELID-grind-ing
Turning
Laser
assisted
turning
UAG
Die-
sinking
EDM
Micro
EDM
Micro
millingMilling
Material removal rate [mm3/s]
1.18 0.86 4.31 1.66 0.15 0.0002 0.0152 2.2
Surface roughness (Ra) [µm]
0.020 0.08 0.10 0.2 0.65 0.5 0.03 0,06
Shape
flexibilityLow Medium Medium High High Medium High High
Milling
DIe sinking EDM
UAG
ELID-Grinding
Micro-EDM
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Laser ablation
Laser ablation is a mass removal technique involving coupling laser
energy to a target material. It is already widely applied in metal-cut-
ting processes. However, for ceramics, due to their extremely high
melting and boiling points and high thermal conductivities, the effi-
ciency of conventional lasers is limited. The development of fem-
tosecond lasers holds out new opportunities in terms of the laser
machining of ceramics with an extreme high melting point. Femto-
second lasers (10-15 s) allow for a non-thermal ablation regime (τpulse
< Tthermal) yielding a reduced dependency on the thermal properties
of the target material. Instead of melting/evaporation, the material
removal mechanism is adsorption/excitation of the target material at
a rate faster than heat is conducted inside the material. In addition
to enabling ceramics to be machined, the non-thermal machining
mechanism of femtosecond lasers considerably increases the sur-
face quality and precision of laser-machined parts.
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INDUSTRIAL EXAMPLETo demonstrate the milling of carbides, a hexalobular-shaped punch
die has been produced (see the image below). The pocket, which
has a diameter of 9 mm and a depth of 6 mm, is machined in ce-
mented carbide WC/Co 90/10 to a precision level of 0.02 mm. Two
diamond-coated micro ball-end mills were used, one for roughing
and one for finishing, both of them 1 mm in diameter. The rough-
ing operation took about 156 minutes, and the finishing oper-
ation 44 minutes at 20,000 rpm and a feed rate of 200 mm/min.
The cutting parameters can be set even higher, since while there
was tool wear, this was at a rather low level, but in this case the limits
of the machinery were reached.
The table above shows that it is possible to machine the punch die
but the economics of the process need to be looked at. Tool costs,
at about €250/tool, are rather high. Alternative technologies, like
wire EDM, can combine multiple parts in one operation, if the shape
of the product allows this. However, as the number of applications
increases, surface tool costs will inevitably go down and the milling
of carbides will become competitive for a wide range of such appli-
cations.
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Carbide hexalobular-shaped punch
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SEIZING THE OPPORTUNITYMachining hard and brittle materials is no longer the exclusive pre-
serve of grinding. When looking to increase cost-effectiveness and
geometrical flexibility, a variety of novel technologies are available.
However, it is important to set up the correct combination of tech-
nologies to achieve a cost-effective production chain. The key here
is to remove as much material as possible as fast as possible in the
roughing stage and so reach an optimal starting point for the more
expensive finishing operation. For each product the make-up of this
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EXPERTISE AND FACILITIES AT YOUR DISPOSALThe Precision Machining Lab at Sirris:
• the Fehlmann Versa 825 five-axis high-precision milling centre;
• the high-precision Erowa clamping system;
• the Mitutoyo Apex-S 3D coordinate measuring machine;
• a laser texturing machine for surface functionalization
• an acclimatised chamber.
Various specifications:
• milling of precision components to an accuracy of 3 μm;
• machine travel range: X: 820 mm; Y: 700 mm; Z: 450 mm;
• spindle: 20,000 rpm, 24 kW and 120 Nm at 50-1,920 rpm;
• clamping with micrometric repeatability;
• CNC-controlled (scanning) measurements from CAD;
• measurement accuracy of 1.7 μm + 0.3 L/100 μm (L in mm).
The precision machining lab, its infrastructure and engineers,
are at your service to:
• realise your prototype precision components for new applications;
• become conversant with precision machining before investing
yourself;
• provide you with support with regard to the machinability and
cost-effective manufacturing of precision components.
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THE AUTHORSSirris is the collective centre for the Belgian technology industry. The
Advanced Manufacturing Department boasts more than 60 years of
experience in the field of machining technology. Sirris was the first or-
ganisation in Belgium to introduce NC programming, damped-boring
bars, tool management, high-speed milling, five-axis simultaneous mill-
ing, hard turning and laser ablation. Over the last four years the focus
has been on achieving micrometric precision levels on five-axis milling
machines that, while high-end, is within the reach of SMEs. Working
with industry, our applied research has led to game-changing results.
Peter ten Haaf
Program Manager - Precision Manufacturing
As responsible for the Precision Manufacturing department Peter
defines the research strategy and supports industry in detecting their
own opportunities.
Olivier Malek
Expert Machining Advanced Materials and Surface Functionality
Olivier is responsible for research and industrial projects on high
precision machining. His interests lay in non-traditional machining
technologies and advanced materials in particular.
Krist Mielnik
Expert High-precision Milling
Krist focuses on the finishing process optimisation of the gear
prototype, realignment problems and precision finishing of additive
manufactured parts and methods to evaluate and improve machine
precision.
Tom Jacobs
Expert Machining Advanced Materials and Monitoring Solutions
As a senior engineer, Tom is helping companies with research on
methods to control precision during production with the help of
sensors and real-time data.
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PARTNERSThe research descripted within this publication was a collaboration
between
This publication has been made within the framework of “VIS” and
supported by Agentschap voor Innovatie door Wetenschap en
Technologie (IWT).
DIAMANT BUILDINGBoulevard A. Reyerslaan 80 B–1030 Brussel +32 2 706 79 44 www.sirris.be [email protected] blog.sirris.be
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