New technology will soon enable the high-speed precision machining of CMCs at a significantly reduced cost.
Figure 1. Test article design.
matrix composites (CMCs) are becoming widely used in aerospace applications
(especially in rocket motors and turbine engine hot exhaust areas) as a method
to reduce weight while increasing performance and efficiency. Continuous-fiber
CMCs typically consist of a silicon carbide (SiC) ceramic matrix and SiC
ceramic fibers, which together create a SiC/SiC composite. These composites can
withstand extreme temperatures, making them ideal for propulsion system
CMCs are replacing the nickel, chromium and titanium alloys historically used
in high-temperature applications, resulting in reduced structure weight (as
much as 50% for certain applications). Although significant weight savings and
performance benefits can be achieved through the introduction of CMCs into
spacecraft and aerospace components, some drawbacks need to be considered and
addressed to fully realize the potential of this family of materials.
The machining and grinding of ceramic matrix composites, particularly SiC CMCs,
are difficult and costly processes. CMC machining costs can represent up to 75%
of the total production cost for a particular component.
carbide CMCs are abrasive in nature, causing traditional cutting tools to
suffer from short operating lives. The geometry and construction material of
traditional cutting tools have been major factors in their inability to
increase material removal rates and thereby decrease manufacturing costs. New
tool materials and/or manufacturing methods must be developed for the machining
of CMCs to reduce manufacturing costs and make them a more viable material
option for spacecraft and aircraft components.
A variety of materials and coatings, including polycrystalline diamond (PCD),
have been applied to the machining of CMCs. It has been demonstrated that a
small increase in material removal rate is possible, though not enough to
drastically reduce manufacturing costs. Different
machining coolants have also been used in place of traditional glycol-based
coolants to dissipate the large amount of heat generated during the machining
of these materials. Coolants like alcohol have reduced the heat at the
tool/substrate interface and produced a high-precision surface on the final
part, but this does not solve the problem of high manufacturing costs.
Grinding techniques have also been employed in the fabrication of CMC
components, and they typically produce good surface quality. However, low
material removal efficiencies and high grinding wheel wear result in high
machining and recurring tooling costs. Also, standard grinding techniques are
not applicable to surfaces with complex geometries.
recently, no practical method to economically machine ceramics and CMCs has
existed with the high precision required for many components. Laser-assisted
machining (LAM) was developed in an effort to reduce the high manufacturing
costs typically associated with ceramics while maintaining the base structure
of the material. In laser-assisted machining, a laser is used as an intense
heat source to change the deformation behavior of the ceramic substrate from brittle to
ductile just prior to material removal with a cutting tool, without melting or
To maintain the integrity
of the material, the power output of the laser must be carefully controlled so
as to heat only the substrate to the depth of cut of the machining process.
Cutting tools, such as PCD-coated tools, are typically used in conjunction with
the LAM process. This helps to extend the tool life far beyond that of standard
machining practices in ceramics.
When compared to traditional
machining techniques for ceramics, LAM has been shown to achieve material
removal rates nearly 20 times that of ultrasonic machining and over 100 times
that of grinding. Typically, ceramics are machined using a series of expensive
grinding machines, which can be replaced by a single (and potentially
lower-cost) LAM system.
Developing a New System
Flight Sciences (AFS) and Kansas State University (KSU) have teamed up to
develop multi-axis LAM capability for SiC CMC materials.* The program is
sponsored through the West Virginia High Technology Consortium (WVHTC)
Foundation’s Innovative Research initiative, which seeks to stimulate
technological innovation in the private sector and strengthen the role of small
businesses and universities in conducting research that supports NASA’s latest
critical technology needs.
The technology to be developed during this program will enable the high-speed
precision machining of CMCs at a significantly reduced cost, making production
of current and new CMC structures more cost-effective and viable. The overall
goals of this program are to demonstrate the technical feasibility of using LAM
for the fabrication of CMC components, perform limited micro-structure analysis
and physical property testing on LAM-processed articles, and demonstrate the
cost/time savings that can be achieved with the developed technology. The CMC
material used during this program was SiC fiber in a SiC matrix.
In order to determine the LAM operational parameters for the CMC material,
predictive numerical models were developed using a finite element analysis
(FEA) model approach. The heat transfer and thermal stress distribution in the
material was modeled and verified using actual experimental data collected
during the fabrication of a test article. The FEA models were used to predict
the laser power, beam diameter and preheating time, as well as the tool path
The team also developed a distinct element method (DEM) model to simulate the
material removal process using LAM techniques. The development of the models
assists with the future development of LAM technology by providing a means to
rapidly predict operational parameters in a production environment.
A 3 x 5 in. key characteristic test article for fabrication using LAM
techniques was developed to simulate features typically found in space
propulsion and turbine engine components. Key features included leading and
trailing edge geometries, 0.093-0.248 in. diameter holes and a 3-axis slot (see
Figure 1). The test article design represents features currently found in
propulsion structures and features that are not possible to install using
current techniques, such as 3-axis slot milling and counterbores.
The holes, with their associated counterbores, represent the range of hole
sizes of interest to the program. The article also features a full radius edge,
as well as a radius and chamfer combination edge, to represent the leading and
trailing edges and throat geometries found in propulsion structures. The
modeled article relies on 0.090-in. material that is face-milled to a final
thickness of 0.085 in. by removing 0.0025 in. per side.
retaining ownership of all key intellectual property, Aurora is open to collaborations and
licensing options for its new technology.
tool materials were selected to support operational sequences and to function
properly with laser heating requirements. The operational sequences for
fabrication of the article were determined in an effort to minimize the
equipment and setup time/costs associated with fabrication. In any
manufacturing operation, non-recurring costs like setup, equipment and tooling
are a major factor in selecting the fabrication method, and often result in
costs higher than the performance of the machining operation. Working to reduce
non-recurring costs was a key aspect during all steps of the program to reduce
fabrication costs associated with laser-assisted machining technology.
The selected cutting tools included a custom-designed polycrystalline cubic
boron nitride (PCBN) insert cutter for face milling and diamond-coated fluted
cutters. The use of an insert cutter instead of a traditional fluted cutter
reduces non-recurring tooling costs associated with manufacture by enabling the
replacement of just the insert and not the entire tool. With coated fluted
cutting tools, once the cutting surfaces wear and the cutting edge is
diminished, regrinding the surface is impossible without removing the coating.
The selection of a diamond application method for the fluted tools was a
critical aspect to the final tool selection. There are many commercially
available PCD tool coatings that utilize various application methods. The most
common method of application is to resin bond or “glue” the diamond particles
to the outside of the cutting tool. Another method is to grow the crystals on
the tool surface. Both of these methods produce acceptable coatings for
standard applications, but can only withstand temperatures of up to
Due to the high tool path temperature on the CMC surface as a result of
laser heating, both the resin bond and PCD growth application methods would
fail during use. A novel metal bond coating method was developed that is
capable of withstanding temperatures in excess of 2400°F.
Because of the thermal conductivity of the material, the area to be machined
must be relatively small in order to facilitate laser heating of the CMC
substrate during hole installation. When using helix drills, the tool must
remove the entire area of the hole, which in a LAM application would require
that the entire area to be removed be heated. This technique would be
applicable to small hole sizes (<0.015 in.) but is not possible with larger
An alternate method is to step up the drill sizes to the final desired hole
size, but this would significantly increase manufacturing costs. Therefore, a
hole installation technique was developed that only requires a small area be
heated in order to produce the final hole size in a single operation, thereby reducing
operational run time.
2. Experimental 3-axis LAM setup at Kansas State University.
Testing and Results
determine an initial assessment of the developed LAM operational parameters and
machining methodology, trials in homogenous silicon nitride
) were completed using a
3-axis LAM system currently in place at Kansas State
University (see Figure 2).
Figure 3 illustrates the face milling, groove and chamfer installations that
have been completed on the Si3
3. LAM-fabricated features, including (a) face milling, (b) groove installation
and (c) chamfer.
fabrication, the tool cutting path temperature was monitored using a laser
pyrometer, and cutting forces were measured using a dynamometer attached to the
machine center bed. The collected data were compared to the predicted results
determined using the developed FEA models. The results of these trials
indicated that the parameters selected work with large material specimens. It
was also determined that the tooling materials selected for use with the LAM
process are capable of withstanding the intense heat generated without
degradation to the coating or surface.
The results of this trial have indicated the need for the development of an
improved heating method for the fabrication of large components. The program
team is currently revising the heating strategy for LAM prior to the
fabrication of the test article, and this should be completed in early 2008.
For more information, contact: Aurora Flight
Sciences, 9950 Wakeman Dr.,
Manassas, VA 20110; (703) 396-6304; e-mail firstname.lastname@example.org; or visit www.aurora.aero.com. Kansas State University,
IMSE Department, 212 Durland Hall, Manhattan, KS 66506; (785) 532-3731; e-mail email@example.com; or visit www.ksu.edu.