JMEPEG (2010) 19:1031 1036 ÓASM International
DOI: 10.1007/s11665-009-9567-7 1059-9495/$19.00
Investigations of White Layer Formation During
Machining of Powder Metallurgical Ni-Based
ME 16 Superalloy
S.C. Veldhuis, G.K. Dosbaeva, A. Elfizy, G.S. Fox-Rabinovich, and T. Wagg
(Submitted April 2, 2009; in revised form August 19, 2009)
Surface integrity of machined parts made from the advanced Ni-based superalloys is important for modern
manufacturing in the aerospace industry. Metallographic observations of the ME 16 alloy microstructure
were made using optical metallography and a high-resolution scanning electron microscope with energy
dispersive x-ray spectrometer (HR SEM/EDS). Tool life of cemented carbide inserts with TiAlN coating
during machining (finishing turning operation) of ME 16 superalloy has been studied and wear patterns of
the cutting tools were identified. Surface integrity of the machined part after completion of the turning
operation was investigated. The morphology of machined parts has been examined and cross-sections of the
machined surfaces have been analyzed. The formation of white layer on the surface of the machined part
was studied for varied machining conditions. It was found that a 2-4 lm thick white layer forms during
turning of the ME 16 superalloy. This layer was investigated using EDS and XRD. The studies show that
the white layer is an oxygen-containing layer with a high amount of aluminum, enriched by chromium and
tungsten. Under specific cutting conditions, the structure of white layer transforms into a c-alumina.
Formation of this thermal barrier ceramic white layer on the surface of the machined part negatively affects
its surface integrity and cutting tool life.
precipitations, powder metallurgy aerospace alloys can be
Keywords aerospace, machining, metallography, superalloys
designed or tailored to contain a higher alloy content than is
possible using casting techniques. This, in turn, should
contribute to the development of alloys with still greater
strength. Also, a more uniform structure compared to cast
materials gives a better, more homogeneous distribution of the
1. Introduction
strengthening phases throughout the powder compact and
should result in better properties (Ref 4, 5).
Fine grain ME 16 nickel base superalloy forgings are
Machining of advanced ME 16 alloy is a significant
produced by powder metallurgy processing (PM) for aero-
challenge. This is due to a more complex combination of
space applications (Ref 1). ME 16 has been recently developed
material properties, including lowering of thermal conductivity
for turbine and disk applications requiring strength and creep
that leads to elevating temperatures at the tool/chip interface
resistance at relatively high temperatures (600-800 °C), as well
during cutting, work hardening tendency during machining that
as well as resistance to fatigue crack initiation at the lower
becomes more severe with increased strengthening of this alloy,
temperatures (300-600 °C) (Ref 2). Generally, PM superal-
and intensive adhesion to the surface of the tooling under
loys having increased processability will meet increased
operation. Tool life can be significantly decreased (Ref 5, 6).
temperature capability while maintaining strength and lower
Generally, PM microstructure improvements such as absence of
density (Ref 3).
large carbide particles have been accompanied by decreased
Two major advantages accrue from making a powder
sensitivities to defects during machining (Ref 7, 8). On the
metallurgy alloy, due to the rapid solidification rates of
other hand, one of the key requirements for rotor blades and
atomized metal droplets. The grain size of the product is very
discs superalloys is fatigue strength (Ref 3). In order to
small, of the order of microns, much smaller than in typical cast
maintain fatigue strength, the most challenging aspects when
materials (Ref 3, 4). Because of lack of segregations or
machining these materials come from the workpiece surface
quality point of view. For instance, formation of the white layer
poses a significant potential danger to fatigue life (Ref 9). For
this material to be used in critical engine components, this issue
must be resolved first.
S.C. Veldhuis, G.K. Dosbaeva, G.S. Fox-Rabinovich, and T. Wagg,
To date there is not much information available on the
Department of Mechanical Engineering, McMaster University, 1280
machining of ME 16. This paper focuses on investigations of
Main St. W., Hamilton, ON L8S 4L7, Canada; and A. Elfizy,
structural characteristics of ME 16 alloy and surface integrity
Manufacturing Engineering Development, Pratt & Whitney Canada,
issues of the machined part, with an emphasis on the features of
1000 Marie-Victorin, Longueuil, QC J4G 1A1, Canada. Contact
e-mail: dosby@mcmaster.ca. the white layer formation.
Journal of Materials Engineering and Performance Volume 19(7) October 2010 1031
Table 1 ME 16 alloy elemental composition wt.% based
2. Experimental
on the quantitative EDS data
In this work, the structure and machinability of the powder
Elemental content
metallurgical nickel-based superalloy ME 16 has been studied
Al Ti Cr Co W Ta Mo Nb in detail. Intensive studies of the ME 16 alloy microstructure,
surface morphology of the machined part and the white layer
3.1 2.6 10.4 20.5 3.0 1.4 1.3 1.4
formation have been performed using various methods includ-
ing optical metallography, x-ray diffraction (XRD) and a high-
Balance is minor amount of nickel and alloying elements
resolution scanning electron microscope with energy dispersive
Fig. 1 Optical and SEM metallography of ME 16 alloy (workpiece material): (a) optical image, magnification 16009; (b) HR SEM image,
magnification 50009 and 200009
Fig. 2 SEM elemental map of ME 16 alloy. TaC and NbC formation
1032 Volume 19(7) October 2010 Journal of Materials Engineering and Performance
x-ray spectrometer (HR SEM/EDS). The machining experi-
3. Results and Discussion
ments were performed using a Boehringer VDF 180 turning
Centre. Tool life was studied under various cutting speed
3.1 Study of the Microstructure and Properties
conditions. The tool life was evaluated as a length of cut (m)
The general elemental composition of the ME 16 sample is
when flank wear of cutting insert reaches 300 lm. The
parameters of cutting used during turning experiments (finish- presented in Table 1. Figure 1 presents optical metallographic
images of the ME 16 alloy. There are evenly distributed fine-
ing operation) were the following: speed 30-65 m/min, depth of
grained carbides in the structure (Fig. 1a). These carbides have
cut 0.125 mm, feed 0.1225 mm/rev. Commercial cemented
low cohesion to the matrix (Fig. 1b) and were found to be
carbide WC-Co inserts (K-grade) with TiAlN PVD coating,
easily torn off during sample preparation (polishing and
commonly used for cutting of Inconel alloys, were employed in
etching). They have a complex composition. The matrix of
this work. All the cutting tests were performed under wet
machining conditions using Commonwealth water-based cool- ME 16 is extremely fine grained with an average grain size of 7
lm (fine grains of Ni-based c-phase, Fig. 1b). Figure 2 shows
ant CommCool Max.
the EDS elemental map for ME 16 alloy. The ME 16 alloy
Surface roughness measurements were carried out with a
contains fine (microns-sized) carbides, mainly (Ta, Ti, Nb) C,
surface roughness tester, Zygo New View 5000 interferometer
within a ductile (Co, Ni, Cr) matrix phase (Ref 10). The
optical profiling system, using evaluation and cut-off lengths of
hardness of ME 16 alloy was measured and compared to the
5 and 0.8 mm, respectively. The surface roughness was taken at
widely used Inconel 718 Ni-based superalloy. Hardness of both
four locations (90° apart) and repeated twice at each point on
alloys is similar: HRC 47-48 for the Inconel 718 and 46-47 for
the face of the machined surface and the average values were
the ME 16.
reported.
Fig. 3 Tool life and crater formation of CC inserts with TiAlN coating vs. cutting speed during machining of ME 16 alloy
Fig. 4 Surface finish of ME 16 at various cutting speeds: (a) 30 m/min, (b) 40 m/min, and (c) 50 m/min. Magnification 50009
Journal of Materials Engineering and Performance Volume 19(7) October 2010 1033
3.2 Tool Life and Wear Behavior Studies layer (Fig. 6). This layer has poor adhesion to the substrate. It is
almost flaked off in Fig. 5 and 6.
The tool life of cemented carbide inserts with TiAlN coating
XRD studies of the white layer formed on the surface of
versus cutting speed is presented in Fig. 3. Tool life notably
machined part indicate the formation of a c-alumina phase
decreased with increasing cutting speeds from 20 to 65 m/min.
(see corresponding spectrum in Fig. 7a) at cutting speed of
Wear patterns were studied for the coated cemented carbide
50 m/min. The c-alumina is a low-temperature modification of
inserts. Figure 3 presents SEM images of worn cemented
a-alumina (Ref 11, 12) and its formation indicates that the
carbide inserts. Cratering of the rake surface was observed to be
actual temperatures in the cutting zone are around 750-800 °C.
significant and increased rapidly with a rise in cutting speed.
However c-alumina has similar characteristics to the a-alumina
The cratering was found to be quite severe at 50 m/min and it
and has a similar effect on the surface integrity of the machined
was catastrophic at 65 m/min. This severe diffusive wear can
part. This undesirable phase cannot be detected by XRD at the
be caused by high temperatures at the rake surface, which is
lower cutting speeds of 40 m/min (Fig. 7b).
most likely due to the low thermal conductivity of the ME 16.
Alumina phase found in the white layer is ceramic and acts
as a thermal resistant layer that is formed in situ during cutting
3.3 Surface Integrity Studies
on the machined surface of the ME 16 alloy. A significant
The surface morphology of the machined part made of ME portion of the heat generated during cutting goes into the tool
16 alloy is presented in Fig. 4(a)-(c). No visible defects were instead of workpiece. These aspects may have encouraged
detected on the surface. Surface roughness data show that the intensive cratering on the rake surface of the cutting tool
average roughness Ra after turning experiments (finishing (Fig. 3). In addition, the ceramic layer is extremely brittle
operation) are almost similar: 2.535, 2.707, and 2.668 lm compared to the core. The machined part with this layer could
correspondingly. have reduced cycle fatigue strength due to high possibility of
However, metallographic sections showed the machining of crack formation within the white layer (Ref 13).
the ME 16 super alloy using cemented carbide inserts with The microhardness distribution for the machined surface of
TiAlN coating results in the formation of a white layer under ME 16 at the cutting speeds is presented in Fig. 8. The data
varying cutting conditions. At speeds of 30 and 40 m/min, the presented show that at cutting speeds above 40 m/min a
white layer is thick and noncontinuous (Fig. 5a, b) and its softening of a region close to the surface layer takes place. This
average thickness is 4 lm. At 50 m/min, the layer is continuous is related to the hardness of the workpiece material layer below
and its thickness is diminished down to 2 lm (Fig. 5c). The the superficial (2-4 lm thick), white layer (Fig. 5). White layer
EDS point analysis shows that the white layer is a metal- composed of ceramic alumina phase may prevent heat from
ceramic compound (Al-Cr-O) that forms on the surface during being evenly absorbed by the core of the machined part
machining (Fig. 5). EDS elemental map confirm data of point (Ref 14, 15). This could worsen cutting conditions at the higher
EDS analysis and also indicates aluminum and tungsten in this cutting speed.
Fig. 5 SEM images and EDS analyses of cross-sections of machined part made of ME 16 alloy, 50009. White layer, formed at cutting speed:
(a) 30 m/min, (b) 40 m/min, and (c) 50 m/min
1034 Volume 19(7) October 2010 Journal of Materials Engineering and Performance
Fig. 6 Elemental map of the white layer on the surface of machined part of ME 16 alloy. Cutting speed 40 m/min
18
18
17
17
16
16
Core material
Core material
15
15
14
14
13
13
12
12
Å‚-alumina phase
11
11 cannot be
Å‚-alumina phase,
detected by XRD
thin surface layer 10
10
White layer
White layer
9
9
8
8
7
7
30 40 30 40
(a) Diffraction angle, 2Åš, (degrees) (b) Diffraction angle, 2Åš, (degrees)
Fig. 7 XRD spectra of white layer and workpiece material for machined surface of ME 16 alloy. Cutting speed: (a) 50 m/min and
(b) 40 m/min
Journal of Materials Engineering and Performance Volume 19(7) October 2010 1035
arb. uints
arb. uints
X-ray intensity,
X-ray intensity,
Acknowledgment
This research was funded by Pratt & Whitney Canada.
References
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1036 Volume 19(7) October 2010 Journal of Materials Engineering and Performance
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