Blisk Production of the Future - MTU Aero Engines

ISABE-2009-1169
Blisk Production of the Future
Technological and Logistical Aspects of Future-Oriented Construction and Manufacturing
Processes of Integrally Bladed Rotors
Abstract
This paper describes the state-of-the-art in the
manufacturing technology for integrally bladed
compressor and turbine rotors (blisks) in today’s
engine construction and addresses the
technological and logistical challenges to be
resolved to ensure flexible and future-oriented
blisk manufacture.
Abbreviations
ECM Electrochemical Machining
PECM Precise Electrochem. Machining
HPC High Performance Cutting
Blisk Bladed integrated Disk
IBR Integrated Bladed Rotor
LFW Linear Friction Welding
IHFP Inductive High Frequency Pressure-Welding
LPC Low pressure compressor
AM Adaptive Milling
CMM Coordinate measuring
machine
2. Introduction
Integrally bladed rotors (blisks) are increasingly
being used in modern engine construction.
Meanwhile, advanced compressors for both
commercial and military engines frequently
come as all-blisk designs. Essential benefits
over conventional rotor designs where the
blades are fitted in slots in the disk rim include
weight reductions up to 20 percent and significant
efficiency improvements.
The next step of integration which promises
further weight savings — welding of individual
blisks to form multi-stage blisk drums — has
already been put to practice and is being further
developed (see figure1: blisk drum). The
same applies to the hollow fan blade technology
which is used to achieve further weight
reductions. Furthermore, investigations are
presently being carried out into the feasibility of
manufacturing dual-material blisks and into the
use of blisks in highly stressed turbine stages.
With respect to manufacturing technology this
means that a wide range of materials, sizes
and geometries has to be investigated.
Martin Bußmann, Dr. Erwin Bayer
MTU Aero Engines
Figure 1: Blisk drum.
A company that intends to manufacture the
entire spectrum of blisks in a cost-effective
manner, must focus on some core manufacturing
techniques for airfoils and be able to combine
these techniques to best effect. In the
process, additional requirements in terms of
manufacturing logistics and facility planning
strategy must be met.
And last but not least, the blisk design poses
new challenges for repair technology. The
established shop repair process chains which
base on the repair of the detail parts, blades
and disk, must be modified or newly developed
from scratch.
Figure 2 below is an overview of the various
blisk designs which differ in size and configuration.

1.2 Range
EJ 200
LPC
Ti64
LFW + AM
TP400 Drum
IPC
Ti64
EBW / IFW
HPM
Figure 2: Overview of the blisk types manufactured by MTU Aero Engines.
3. From the strategic approach to the present-day
situation at MTU Aero Engines
A strategic approach towards market-driven
blisk manufacture was defined by MTU Aero
Engines back in 2004.
To be able to manufacture various blisk types
to satisfy the demand on the commercial and
military markets long-term, several core manufacturing
technologies for the production of the
flow duct profile must be developed that can be
used in any combination, depending on the
type of blisk involved.
Machining resistance (material)
ECM ECM / / PECM
PECM
HPC
Current current
blisk Blisk spectrum
spectrum
HPC
LFW / IHFP
Blade surface area / airfoil length (geometry)
Exc. : Applications of airfoiling processes
as referred to two parameters
Figure 3: Core airfoiling processes for blisk
PW6000
HPC
Ti6242
HPM + IFW
EJ 200 Drum
HPC
Ti6242
HPM + IFW
NGPF
HPC
U720;IN718
(PECM)
Focusing on these core technologies makes
sense because the production of the flow duct
accounts for the major part of the manufacturing
costs. In addition, the use of suitable core
technologies may help reduce raw material
consumption and hence costs.
These core technologies are then supplemented
by hub machining and surface treatment
processes to obtain an optimum process
chain tailored to suit the respective type of
blisk.
If these manufacturing technologies are then
used in conjunction with advanced design and
construction methods a component can be
manufactured that is optimized in terms of
function, quality and costs.
Based on this approach, a number of development
projects were launched which focused
on:
- design to cost
- new or further developed joining, milling
and ECM/PECM techniques
- shot peening techniques
- new quality assurance methods
- new repair strategies

The Tool Box Approach in Process Chain Generation
Peripheral Peripheral
processes processes
Niblisk
Tiblisk
Pre-airfoiling processes
Figure 4: Tool box approach
3.1 Design to cost
Airfoiling
Rough-machining Finishing
ECM HPC LFW /
IHFP
ECM /
PECM
HPC
LFW /
IHFP +
AM AM
X X
X
X X
X X
: : : : : :
X
X X
X X
: : : : : :
The primary objective of design-to-cost measures
in blisk production is to reduce the manufacturing
costs by bringing the requirements for
airfoil and annulus surfaces as well as leading
/ trailing edge geometries as closely as possible
in line with the fluid mechanical requirements.
In the process, the friction losses of
processed surfaces (milled, shot peened,
ground) and/or the geometry variations of the
edges must be accounted for in the aerodynamic
design as accurately as possible.
TT
Surface- -
treatment
CG
:
X
X
:
Grinding,
etc.
X
:
X
:
Peripheral
processes
-
Post-airfoiling processes / QM
For the annulus surfaces, proof could be furnished
that milling marks that extend in the
direction of the flow have no adverse effects
whatsoever, as long as their depth does not
exceed a certain threshold value. This significantly
reduces the finish-machining effort.
The results of investigations into possible leading
edge geometries on compressor blades
have also revealed technical potential for expanding
the acceptance standards.
basic profile 1/4 T stub 1/3 T stub 1/8 T concave 1/8 T convex 1/6 T concave
Figure 4: Edge geometries investigated.
∆ω
mittel
∆ω
∆ω
∆ω
0,00%
1/4 1/4 stumpf stub 1/3 1/3 stumpf stub 1/8 1/8 concave konkav 1/8 1/8 convex konvex 1/6 1/6 concave konkav
1-2 Grafiken
3.2 Manufacturing technologies
Edge Kantenform geometries
3.2.1 Joining
Figure 5: Losses ∆ω of the deviating edge profiles investigated as compared with a basic profile
with constant edge radius.

Linear friction welding (LFW) has proven to be
a suitable method to produce the highly
stressed joint between the blades and the disk
and is now used in production. Further development
efforts in this field are mainly targeted
at the machine technology to achieve higher
flexibility with respect to the various component
geometries to be joined.
Since the LFW technique calls for highly engineered
machinery and presupposes certain
geometric conditions regarding the components
to be produced, the next generation of
blisk joining techniques is presently being developed:
the inductive high-frequency pressure
(IHFP) welding process.
Figure 6: Detail parts and
blisk joined by LFW
Here, the energy required to heat the materials
to be joined is produced by high-frequency
alternating electromagnetic fields so that the
complicated machinery needed in the LFW
process to produce the mechanical vibrations
and high joining forces is no longer required.
Another benefit of the technique is that the
component proper is subjected to lower
stresses during welding.
1
4
3
2
1= HF generator
2= Welding equipment
3= Hydraulic system
4= CNC control unit
Figures 7 IHFP welding machine
1
Figure 8: section through a IHFP welded joint.
Figure 9: Simulation of the heating process
during IHFP welding
Presently, the technology is being matured for
common joints on titanium materials and crosssections
from the fan area to the low-pressure
compressor. In the process, a capable simulation
of coil / alternating field is used which is
particularly suited for the component geometry
and significantly reduces parameter testing.
3.2.2 Milling
At MTU Aero Engines, milling is the most
commonly used technique to produce blisk
airfoils. In this field, a holistic approach to the
milling process—machine, setup, milling strategy,
tooling, tool holders, coolant-lubricant,
etc.—helped achieve considerable savings. A
significant contribution came from the MTUpatented
circular stagger milling process.
While with conventional milling the angle of
contact may be up to 180°, this angle is limited
to 30° to 60° with circular milling. This reduces
the cutting forces and increases the material
removal rate by means larger cutting depths
and higher cutting speeds.

angle of contact
angle of contact
Figure 10: Angle of contact with circular
and conventional milling
Figure 11: Milling on a 5-axis machine
Essential prerequisites for high-performance
machining using the circular milling approach
are the selection of the most suitable cutter
material and the definition of an optimum cutter
geometry. Also, the life of the rough-milling
tools can be more than doubled with an appropriate
edge preparation.
Rounding of the cutting edge markedly reduces
the risk of micro-chipping, and the cutting
edge normally exhibits a uniform rubbing
wear.
Figure 12: Sharp-edged and rounded edge of
a rough-milling cutter for blisk machining
These highly dynamic milling processes are
performed using special 5-axis milling machines.
The necessary acceleration can be
achieved by means of direct axis drives.
3.2.3 ECM / PECM
So far, the advantage of electrochemical machining
processes used in the production of
airfoil geometries, i.e. minimum tool wear, has
been offset by the drawback of inaccurate
contours particularly in the edge and radius
areas and a relatively long iteration process.
These disadvantages have now been overcome
by a further development of the process,
the so-called precise electrochemical machining
(PECM) technique. A reliable and precise
control of this process, which involves very
small gaps between component and vibrating
electrode, has now been made possible by
further developments in semiconductor technology
and control systems (precise pulsing
and response times in the microsecond range
at high machining currents).
The results of trials have shown that, in the
case of materials which are difficult to machine
(nickel-base alloys, nickel PM material), PECM
is distinctly superior to milling in terms of machining
speed, tool wear, surface finish, and
process stability of the geometric features. The
technology has been matured for production
use by MTU Aero Engines and first development
hardware is being machined on prototype
facilities.
Continuous
feed
Part
Part
´Tool
Tool
(-)
(+)
ECM for rough-machining
(electrochemical machining)
• gap > 1 mm
• lower accuracy
• high removal rates
(-)
(+)
Electrolyte
Pulsed
feed
PECM for finishing
(precise electrochemical machining)
•extremely narrow, controlled gap
< 0.1 mm
• pulsed current
• increased accuracy of forming
• lower removal rates
Figure 13: ECM / PECM operating principle

ECM premachining
PECM finishing
Figure 14: Blisk manufacture by a combination
of ECM premachining and PECM finishing –
airfoil geometries and surfaces.
3.2.4 Shot peening
Most of Blisk blades are subject to controlled
shot peening to increase their fatigue strength.
Alongside conventional shot peening MTU
Aero Engines uses ultra-sound assisted shot
peening which offers the advantage of minimum
component distortion, since both airfoil
sides are normally compacted simultaneously.
schematic
Figure 15: Test setup for US shot peening
Part
Peening Chamber
Sonotrode
Piezo Shaker
Controller
+ Amplifier
SONATS
Figure 16: Principe of US shot peening
Conventional shot peening
Intensity: 0.2 A, coverage: 125 %
US shot peening
Intensity: 0.2 A, coverage 150 %
250µm
Figure 17: Comparison of the surface texture
achieved by a) conventional and b) US shot
peening
Since US shot peening does not increase the
roughness of the airfoil surfaces, in particular
in the leading and trailing edge areas, postpeening
finishing processes that had to be
performed after conventional shot peening are
no longer required.

3.2.5 Quality assurance
In this area, optical measuring techniques hold
promise of time savings and more reliable
quality statements as compared with conventional
measurements using coordinate measuring
machines (CMM).
In contrast to conventional assembly of rotors,
the manufacture of blisks is subject to particularly
stringent requirements in terms of airfoil
location, radius and airfoil profile tolerances.
Depending on the stability of the manufacturing
process used, therefore, extensive dimensional
inspections must be performed in the
course of in-process and final inspection.
Compared with the tactile systems used on
CMMs today’s advanced optical systems offer
great potential of reducing the inspection effort.
They need far less time to provide complete
models of the components which can then be
verified by means of computation routines, for
example to assess manufacturing deviations in
boundary cases.
In cooperation with the measuring system
manufacturers, MTU Aero Engines has succeeded
in improving the accuracy of the systems
also for dimensional inspection of leading
and trailing edges, for example.
Figure 18: Dimensional inspection of a blisk
Figure 19 : Leading and trailing edges inspected
using optical systems
3.2.6 Repair
Due to the integral construction of blisks a
variety of repair processes needs to be modified
or newly developed. Repairs that go beyond
the scope of blending, a standard repair
method also for individual blades, involve
processes, such as buildup welding or patching
that call for perfectly matched process
steps, particularly when recoating or subsequent
heat treatments are required.
Repair processes
Figure 20 : Repair categories A, B, C, D
Of the repair categories illustrated above, the
replacement of complete airfoils poses the
most exacting requirements, as this repair
involves the use of cutting and joining processes
in the vicinity of critical component
zones. Category A to C repairs are already
used at MTU AERO ENGINES, category D is
being developed.
4. Future developments
4.1 New designs
The blisk design having become state-of-theart
in the entire compressor area, including the
high-temperature-resistant high-pressure compressor
stages, MTU Aero Engines is now
working on the development of fabricated,
high-stressed low-pressure turbine blisks.
Unlike with the smaller, integrally cast variants
used, for example, in auxiliary power units
(APUs), the special challenge to be overcome
here in manufacturing is the material mix of
forged disks and high-temperature cast blades
and even single-crystal materials.
Figure 21: Design example of a fabricated
turbine blisk

4.2 Materials
New engine concepts with enhanced efficiencies
call for the use of high-strength and hightemperature
alloys. In the low-pressure compressor,
the simple titanium alloy Ti6V4 is increasingly
being replaced by the alloys Ti6242
or Ti6246, and in the high-pressure compressor,
the modified alloy DA718 with approx. 7 %
more strength is being used instead of the
standard alloy IN718. The last compressor
stages upstream of the combustion chamber
which are subject to higher thermal stresses
will be made from PM nickel alloys. With these
materials, electrochemical machining processes
are particularly cost-effective.
4.3 Surfaces
To protect blisk blades from erosion by sand
and dust, MTU Aero Engines has developed
new protective coatings which have meanwhile
become the standard coatings for airlines operating
in erosive environments. In the coating
area, too, poor accessibility of the blisk calls for
special processes.
Figure 22: Blisk airfoils with anti-erosion coating
With aerodynamically optimized compressors,
highest surface finish requirements have to be
satisfied especially in the high-pressure compressor
area. To achieve the surface finishes
needed blisks with their special geometrical
constraints have to be processed using other
and, above all, more cost-effective techniques
than those used on individual blades. In this
field, automated electrochemical process sequences
and other processes are being tested
at present.
4.4 Quality assurance
Blisks are inspected using the standard inspection
methods developed for conventional
disks, such as fluorescent penetrant inspection,
ultrasonic inspection and segregation
etching. For the inspection of blanks, phased
array technologies are presently being further
developed and adapted to suit the particular
requirements. Such technologies with their
high resolution permit production inspection to
be performed quickly and effectively.
4.5 Design
Highly advanced 3D design tools are used in
the design of the blisk blades. Present-day
airfoils are highly twisted, the edges frequently
having the form of an “S”. The blade tips are
hard-faced to protect them against rubbing
wear. These features result in new challenges
for manufacturing technology.
Figure 23 : 3D CAD model of a high-pressure
compressor blade
4.6 Industrial production concepts
The manufacturing concept presented here
bases on the following prerequisites:
- Tool box concept
The cost-effective and high-quality manufacture
of a wide spectrum of different blisk types
is possible only by the combination of various
core manufacturing techniques for airfoiling.
- High investments into the special machinery
required are necessary to put the tool box concept
in place. Therefore, the manufacturing
concept must ensure optimum utilization of the
special machines.
- Owing to the integral construction and the
criticality of the component and the associated
interdependence of important quality characteristics
maximum control over an essential
part of the manufacturing process chain must
be ensured.

- Long response times regarding the current
quality situation of individual manufacturing
processes must be avoided. A prompt quality
control of the processes with effective possibilities
to interfere with the processes is a must.
Against this background two basic manufacturing
approaches are conceivable:
I) Central concept
Processes that do not form part of the core
manufacturing techniques are used for the
entire parts spectrum and are designed to
ensure adequate flexibility of machines and
fixtures. Whether and when a new core manufacturing
technique is included in the tool box
depends on the parts volume to be processed.
As long as this volume is below a cost-effective
threshold value the parts must be processed
using other methods.
II) Decentral concept
The parts volume for an optimum combination
of core manufacturing techniques must be
such that a complete autonomous process
chain is justifiable from the economic point of
view.
pre-stage
parts
Figure 24: Central Concept
p re-stag e
p arts
p re-stag e
p arts
.
.
.
Figure 25: Decentral concept
pre-airfoiling
processes
p re-airfoiling
p ro cesses I
p re-airfoiling
p ro cesses II
.
.
.
rough airfoiling
5. Summary
In advanced engine concepts, blisk construction
makes an important contribution towards
optimizing efficiencies and reducing weight.
Therefore, blisks will find increasing use, also
in the turbine area.
To be able to manufacture these critical and
highly stressed engine components in a costeffective
manner, a variety of sophisticated
manufacturing and quality techniques is
needed. These have then to be combined into
process chains optimally designed to suit the
various types of blisks to ensure best results in
terms of costs and quality.
finish airfoiling
milling PECM
welding
X X
milling X X
ECM X X
airfo ilin g
p ro cess I
airfo ilin g
p ro cess II
.
.
.
post-airfoiling
processes
p o st-airfo ilin g
p ro cesses I
p o st-airfo ilin g
p ro cesses II
.
.
.
quality
inspection
q u ality
in sp ectio n
q u ality
in sp ectio n
.
.
.