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CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx

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Surface integrity of nickel-based alloys subjected to severe plasticdeformation by abusive drilling

Rachid M’Saoubi (1) a,*, Dragos Axinte (1) b, Christopher Herbert b,c, Mark Hardy c, Paul Salmon b

a Seco Tools AB, Fagersta, Swedenb Rolls-Royce University Technology Centre (UTC) in Manufacturing, Faculty of Engineering, University of Nottingham, UKc Rolls-Royce plc, Derby, UK

1. Introduction

It has been well-documented the importance of surfaceintegrity upon performance of machined components [1]. Espe-cially, the safety critical (e.g. aerospace) components that work atcyclic high mechanical loads and elevated temperatures aresubject of strict surface integrity checks. Of particular interestare Ti and Ni-based superalloys used for aeroengine componentsthat work in the areas characterised by high level of mechanicaland thermal loads. Thus, industrial surface quality standardsspecify that severe plastic deformations and/or thermally abusedlayers are not accepted on the part surfaces. Studies on residualstresses, hardness and metallurgical characteristics of Ni-basedsuperalloys of drilled surfaces [2–5] were reported. As such, inabusive machining conditions (e.g. excessive tool wear in absenceof coolant) so called ‘‘white layers’’, with distinct metallurgicalcharacteristics (e.g. ultra-fine grain size, increased hardness), incombination with severe plastic deformation (SPD) underneathcan appear [5]. Thus, with a great variety of Ni-based superalloys,there is key interest to understand the nature/level of metallurgicalchanges occurring after abusive machining.

In this respect, the paper investigates the sensitivity of four Ni-based superalloys used for aeroengine applications when SPDlayers are generated and aims to identify the causes of theirmechanical/microstructural differences in the context of abusive

damages and evaluate the capabilities of the analysed Ni-basuperalloys to withstand abusive cutting conditions.

2. Experimental approach and methodology

SPD layers were generated on a set of four Ni-based superalby drilling (TiN/TiAlN coated carbide Ø6.0 mm drill) using

same set of abusive (not employed in real industrial applicatioextreme conditions (cutting speed, vc = 35 m/min, feed rf = 0.12 mm/rev; no coolant; flank wear, VB = 0.3 mm) on a 5-machining centre equipped with a 4-axis (Fx,y,z; Mz) dynamomeAlloy 718, Waspaloy, Alloy 720Li and coarse grain (CG) RR1displaying different thermal, metallurgical and mechanical prerties (Table 1) were utilised for testing.

Waspaloy is a wrought alloy that can be used in disc rotor

casing applications at temperatures above 650 8C; as with allbase superalloys it contains a face-centred-cubic austenitic

A R T I C L E I N F O

Keywords:

Surface integrity

Drilling

Ni alloy

A B S T R A C T

Surface integrity of nickel-based superalloys after machining operations has become a topic of m

interest in the aerospace sector. In the present work, the characteristics of nickel-based alloys (Alloy

Waspaloy, Alloy 720Li, and RR1000) subjected to abusive drilling conditions have been investigated u

experimental methods such as FEG-SEM, EBSD, XRD, TEM and nano-indentation. The results indicated

presence of nano-sized grains typical of severe plastic deformation in the machined surface while

presence of plastic slip bands was observed in the sub-surface layers. Correlations between the ther

mechanical properties of the nickel-based alloys and the severe plastic deformation features of

machined surfaces are presented.

� 2014 C

Table 1High temperature mechanical properties.

Alloy 718 Waspaloy Alloy 720Li RR10

UTS650 8C 1195 1130 1420 1448

Rp 0.2650 8C 980 809 1060 1034

UTS650 8C – ultimate tensile strength at 650 8C (MPa).

Rp 0.2650 8C – yield strength at 0.2% strain and 650 8C (MPa).

Contents lists available at ScienceDirect

CIRP Annals - Manufacturing Technology

journal homepage: http: / /ees.elsevier.com/cirp/default .asp

(g0)e, isera-ipleipi-

drilling operations. This refers to correlation of scanning electronmicroscopy (SEM), transmission electron microscopy (TEM),electron back scattered diffraction (EBSD), X-ray diffractionsupported by dynamometry that enabled the understanding ofcauses/phenomena leading to different magnitudes of SPD surface

othigh* Corresponding author.

Please cite this article in press as: M’Saoubi R, et al. Surface integritabusive drilling. CIRP Annals - Manufacturing Technology (2014),

http://dx.doi.org/10.1016/j.cirp.2014.03.067

0007-8506/� 2014 CIRP.

matrix. Waspaloy is hardened mainly by gamma prime

submicron precipitates. Alloy 718, here considered as referenca cheaper alternative to Waspaloy and is restricted to temptures <600 8C; it is a Ni–Fe based wrought alloy with the princstrengthening phase being the gamma double prime (g00) prectate. Alloy 720Li (5–20 mm fine grain size), available in bpowder metallurgy and wrought forms, strengthened by a h

y of nickel-based alloys subjected to severe plastic deformation byhttp://dx.doi.org/10.1016/j.cirp.2014.03.067

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hase, can be used up to 700 8C. RR1000 is a powder metallurgye superalloy that can operate at around 725 8C; having alarly high g0 fraction as Alloy 720Li, it was reported in bothse and fine grain sizes. However, in this study, the grain size

ed as follows: Alloy 720Li < CG RR1000 < Alloy < Waspaloy. Note that Alloy 718 utilised here is, unlike ther materials, a reference commercial off-the-shelf grade that islikely to satisfy disc quality metallurgical conditions.he subsurface characteristics of the abused sample holes werestigated in axial and hoop directions (Fig. 1a) with particularrest on the latter that corresponds to the cutting directiong which the plastic deformation mainly occurs.

BSD measurements were performed on colloidal silica-shed hoop cross sections using a FEGSEM LEO ULTRA 55 at aking distance of 15 mm and accelerating voltage of 20 kV.

nel 5 EBSD software from HKL Technology/Oxford Instru-ts was used to acquire and process the EBSD data. Dataction was performed at 133 Hz on a scan area ofmm � 150 mm from the edge of the drilled hole surface. A

size of 0.1 mm was used during the EBSD scans, resulting incquisition of 150,000 data points resulting in 3.25 h scan time.

amples of the machined surface for TEM investigation wereared by the Focused Ion Beam (FIB) lift-out method in a FEI

nta 200 3D FIB system (Fig. 1b). The samples were furtherined in a JEM-2000FxII TEM (at 200 kV) to investigate the

cture of the SPD layers. The machining induced surfaceual stresses were determined by X-ray diffraction and sin2 c

hod on a Bruker D8 Advance diffractometer using Cu Ka

ation and the analysis of (3 1 1) and (4 2 0) diffraction peaks.he nanohardness profiles of the machined subsurface layerse determined by nano-indentation on a CSM Nanohardnessr using a load of 5 mN and penetration depth of 300 nm.

icrostructure and deformation characteristics of theurface layers

Scanning electron microscopy (SEM) analysis

ig. 2 shows SEM-back scattered electron mode (BSE) picturese subsurface of the four machined Ni-based alloys specimensn in the hoop plane. Two distinct regions can be observed: (i)rted grains containing plastic slip bands and elongated

ipitates typical of the subsurface layer; (ii) SPD ‘‘white’’ layere very near free-surface.

Fig. 3 depicts a closer view of region (i) and (ii) using SEM-secondary electron imaging mode (SEI) revealing varying thick-nesses of the SPD white layers: �2 mm (Alloy 718); �3 mm(Waspaloy); �4 mm (Alloy 720Li); �10 mm (RR1000). It can alsobe observed that the SPD layer in the Waspaloy and Alloy 718appears as very uniform bands while Alloy 720Li and RR1000, withtheir deeper SPD layers, present delaminations/cracks that occur atthe free-surface as well as at the interface between this layer and thebulk material. These delaminations are likely to be caused by highmechanical loads occurring at elevated temperature gradients inheating/cooling the workpiece surface that result in increasedhardness and tensile residual stresses; all samples displayed thethinnest SPD layers at the drill entrance and thickest around 1 mmabove the drill exit; this could be caused by the heat built-up(leading to easier plastic deformation) when the tool advances intothe workpiece until a point (e.g. close to exit) when the free-surfaceprovides increased cooling.

3.2. Electron back scattered diffraction (EBSD) analysis

EBSD IPF and local misorientation component maps (Fig. 4) onthe hoop cross sections display small orientation changes on themap, highlighting regions of higher deformation caused by abusivedrilling. As expected, regions of increasing strain are located nearthe machined surface (green-yellow-red colour) and region of lowstrain near the bulk (blue colour).

In order to visualise the extent of plastic deformation in thesubsurface, the average intragrain misorientation (AMIS) parame-

Fig. 3. SEM-SEI close view of the SPD layers produced by abusive drilling of Alloy

718 (a), Waspaloy (b), Alloy 720Li (c) and RR1000 (d).

Fig. 4. EBSD inverse pole figure (IPF) and local misorientation maps of deformed

areas in the near machined surface, Alloy 718 (a, e), Waspaloy (b, f), Alloy 720Li (c, g)

and RR1000 (d, h).

. Schematics of sectioned machined hole (a) and FIB lift out in the hoop

tion for TEM analysis (b).

. SEM-BSE views of the machined layers produced by abusive drilling: Alloy

a), Waspaloy (b), Alloy 720Li (c) and RR1000 (d).

ase cite this article in press as: M’Saoubi R, et al. Surface integritusive drilling. CIRP Annals - Manufacturing Technology (2014),

ter was determined according to the method described in previousinvestigations [6,7]. In Fig. 5 the AMIS profiles extending from themachined surface down into the bulk are presented for theanalysed Ni-based superalloys.

A decrease of the AMIS value is observed when moving awayfrom the machined surface down into the bulk material and thus,the extent of plastic deformation in the subsurface can therefore beestimated: �40 mm (Alloy 718); �50 mm (Waspaloy); �60 mm(Alloy 720Li); �120 mm (RR1000). Further EBSD analysis of theSPD ‘‘white layer’’ in RR1000 was enabled due to its largerthickness when compared to the other Ni based alloys. The results

y of nickel-based alloys subjected to severe plastic deformation byhttp://dx.doi.org/10.1016/j.cirp.2014.03.067

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R. M’Saoubi et al. / CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx 3

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CIRP-1178; No. of Pages 4

are shown in Fig. 6 where the IPF and pole figure maps of the SPD‘‘white’’ layer are displayed. A clear evidence of a fine equiaxedgrain structure with a mean EBSD grain size estimated to �105 nmis noticeable in the SPD layer. The corresponding pole figure mapsshow the presence of a retained shear-induced deformationtexture, typical of face-centred cubic material. The latter correlatesvery well with previous XRD pole figure measurement obtained onmachined Inconel 718 [7].

3.3. Transmission electron microscopy (TEM)

A further examination of the SPD layers in the TEM (Fig. 7)reveals the existence of a zone of equiaxed ultra-fine grained (UFG)microstructure indicative of grain refinement that most likelyoccured by dynamic recrystallization (DRX).

The average grain size values estimated in the SPD layers arthe range of �20–100 nm (Table 2). ‘‘Coarser’’ nano-sized grainsobserved in RR1000 and Alloy 720Li materials when compareWaspaloy and Alloy 718, the latter displaying only evidencepartially recrystallized grains. The selected area diffraction (SApatterns taken from the SPD layers in the four Ni-based alloyscomposed of more or less well-defined rings which also is indicaof the presence of UFG microstructure. The broken rings in the SApatterns taken from the SPD layers in Alloy 718 and Waspasuggest the presence of more elongated UFG band microstrucwith a preferred crystallographic orientation. On the other hand,presence of more continuous rings in the SAED patterns of SPD layin Alloy 720Li and RR1000 are indicative of an increase in grain

further to the SPD process. The latter is also consistent withresults obtained during the EBSD analysis of the RR1000 SPD la

3.4. Mechanical characteristics of the SPD layers

To close the loop in understanding the results of the previinvestigations, the nanohardness distribution across the deptSPD layer and the residual stresses upon the surface have bevaluated; note that the first measurements started about 1.5

under the free surface to avoid the ‘‘softening’’ edge effect. FrFig. 8 it can be noted that Alloy 718 and Waspaloy show lvariations on these distributions, while a distinct zone of increananohardness can be observed for Alloy 720Li and, especiaRR1000. This is somehow expected when correlating

nanoharndess distribution under the machined surface with

depths of the work-hardened plastically deformed (Fig. 5) and

‘‘white’’ (Fig. 3) layers.

When considering the residuals stresses, in axial direc

Fig. 6. Detailed EBSD IPF and pole figure maps of SPD ‘‘white’’ layer in RR1000

showing a fine grain structure and shear-induced deformation texture.

Fig. 5. AMIS profile obtained from EBSD for the four Ni-based superalloys subjected

to abusive drilling.

Table 2Summary of mechanical properties of SPD layers: Nanohardness, grain size (

and near-surface residual stress.

Material Alloy 718 Waspaloy Alloy 720Li RR1000

Nanohardness

[GPa]

6.95 � 0.2 6.96 � 0.1 7.82 � 0.3 8.74 �

Grain size

[nm]

�20–30 �30–40 �50–70 �80–100

Axial stress

[MPa]

135 � 52 �36 � 46 �80 � 80 251 � 74

Hoop stress

[MPa]

797 � 283 522 � 127 1564 � 319 1256 � 132

Fig. 8. Nanohardness profiles under the abused drilled surfaces for the four Ni-b

superalloys.

tiontedote

pth,riedsed.loysheir718faceFig. 7. TEM pictures of the near surface SPD layers Alloy 718 (a), Waspaloy (b), Alloy

720Li (c) and RR1000 (d).

Please cite this article in press as: M’Saoubi R, et al. Surface integritabusive drilling. CIRP Annals - Manufacturing Technology (2014),

negligible tensile values were found while in the cutting direc(hoop) where intense thermal and mechanical loads are expecin abusive drilling, highly tensile levels have been observed. Nthat using Cu Ka radiation source, the information decalculated for 90% contribution to the scattered X-rays vabetween 3.3 and 5.5 mm depending on the diffraction angle uTherefore the high tensile residual stresses evaluated for the al(Alloy 720Li and RR1000) are more characteristic from trespective SPD ‘‘white’’ layers while those evaluated for Alloy

and Waspaloy also include the contribution from their subsurwork-hardened layers.

y of nickel-based alloys subjected to severe plastic deformation byhttp://dx.doi.org/10.1016/j.cirp.2014.03.067

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iscussion of the characteristic differences of the abusedhined layer

o support the understanding of the material response toive cutting parameters, and hence the formation of SPD layers,

level of cutting torque (Mz) was correlated to the severity, i.e.h, of the plastic deformations (see Figs. 5, 8): decreasing from

000 (Mz = 3.8 � 0.2 N m; Dx = �10 mm), Alloy 720Li= 2.2 � 0.10 N m; Dx = �4 mm), Waspaloy (Mz = 2.1 � 0.09 N m;

�3 mm) to Alloy 718 (Mz = 1.1 � 0.05 N m; Dx = �2 mm).R1000 and Alloy 720Li showed larger SPD layers, larger AMISes and higher nanohardness compared to the other two alloys.e alloys also showed highly tensile residual stresses in the SPDr. These characteristics are due to the higher UTS at highperature and reduced thermal conductivity resulting in highering temperatures and steeper thermal gradients duringhining. Delaminations and cracks were only observed in000 and Alloy 720Li showing that higher thermal andhanical stresses were present here.lthough the cutting temperatures were not measured in thisy, previous investigations [8,9] reported values of 500–650 8Cng drilling of Alloy 718 under similar cutting speeds but in thence of tool wear. It is therefore reasonable to assume thater temperatures would be expected in these tests due to theence of severe flank wear resulting from abusive drillingitions. Preliminary 3D FEM simulation of temperatureibution in drilling of Alloy 718 with a worn tool (VB = 0.3 mm)

performed with commercial implicit FE code Deform-3DTM.results showed a maximum temperature reaching up to 700 8Ce tool/workpiece interface (Fig. 9). Considering the differencesechanical properties (Table 1) and resulting mechanical loadseen the different Ni alloys, the following temperatures are

y to be expected, �725 8C (Waspaloy), �750 8C (Alloy 720Li)�775 8C (RR1000).

ased on these temperatures values, the Zener–Hollomon) parameter [10] can be determined for the different Ni alloys,

e � expQ

R � T

� �(1)

re e is the average strain rate estimated from FEM � 105 s�1), Q is the activation energy for the respective Ni

[11], R is the gas constant (8.3144621 J K�1 mol�1) and T is thermation temperature.urther, the average grain size in the SPD layer can be estimatedg a relation previously derived for Alloy 718 in the hot working

5. Conclusions

With the development of advanced aerospace materials there hasbeen identified the need not only to evaluate the capabilities of these,e.g. Ni-based superalloys, to work under high temperature and stressconditions, but to understand their sensitivities to abusive machin-ing operations; this will give an indication on the surface integritydamages that could occur if process malfunctions (e.g. lack ofcoolant) occur. In this respect, the paper reports on an in-depthmetallurgical and mechanical analysis of the surface integrityresulting from abusive drilling operation on a set of four Ni-basedsuperalloys, i.e. Alloy 718, Waspaloy, Alloy 720Li and RR1000. It wasfound that although the alloys, i.e. Alloy 720Li and RR 1000, arereported with better mechanical performances at higher operatingtemperatures, they seem to be more sensitive (i.e. thicker severeplastic deformation layers, higher tensile residual stresses) toabusive drilling conditions. As a general conclusion, it could becommented that while more mechanically performant alloys aredeveloped for higher temperatures, these combinations of propertiesare likely to lead to the decrease of their machinability and thereforeto reduced tolerance to intense heat and mechanical loads that couldbe developed when inadequate machining conditions occur.

Acknowledgements

This work was partially carried out under the sponsorship ofPhD CASE award (University of Nottingham and Rolls-Royce)EPSRC–UK (EP/H501460/1). The authors would like to thank M.Schwind and M. Fallqvist at Seco Tools AB for helping withacquiring the EBSD and nanohardness data, respectively. M.Abouridouane (RWTH-WZL Aachen) is also greatly acknowledgedfor performing the preliminary 3D FEM simulations.

References

[1] Jawahir IS, Brinksmeier E, M’Saoubi R, Aspinwall DK, Outeiro JC, Meyer D,Umbrello D, Jayal AD (2011) Surface Integrity in Material Removal Processes:Recent Advances. Annals of the CIRP 60(2):603–626.

[2] Kwong J, Axinte DA, Withers PJ (2009) The Sensitivity of Ni-Based Superalloy toHole Making Operations: Influence of Process Parameters on Subsurface Damageand Residual Stress. Journal of Materials Processing Technology 209:3968–3977.

[3] Kwong J, Axinte DA, Withers PJ, Hardy MC (2009) Minor Cutting Edge–Workpiece Interactions in Drilling of an Advanced Nickel-Based Superalloy.International Journal of Machine Tools and Manufacture 49(7–8):645–658.

[4] Soo SL, Hood R, Aspinwall DK, Voice WE, Sage C (2011) Machinability and SurfaceIntegrity of RR1000 Nickel Based Superalloy. Annals of the CIRP 60(1):89–92.

[5] Herbert C, Axinte DA, Brown P, Hardy MC (2012) Investigation into theCharacteristics of White Layers Produced in a Nickel-Based Super Alloy fromDrilling Operations. Machining Science and Technology 16(1):40–52.

[6] Brewer LN, Othon MA, Gao Y, Hazel BT, Buttrill WH, Zhong Z (2006) Compari-son of Diffraction Methods for Measurement of Surface Damage in Superalloys.Journal of Materials Research 21:1775–1781.

[7] M’Saoubi R, Larsson T, Outeiro J, Guo Y, Suslov S, Saldana S, Chandrasekar S(2012) Surface Integrity Analysis of Machined Inconel 718 Over MultipleLength Scales. Annals of the CIRP 61(1):99–102.

Fig. 9. FE 3D temperature predictions in drilling of Alloy718.

Table 3Zener–Hollomon parameter and estimated grain size (d) in the SPD layers for the

different Ni alloys.

Material Alloy 718 Waspaloy Alloy 720Li RR1000

T [8C] 700 725 750 775

Q [kJ mol�1] 403 385 467 402

Z [s�1] 8.62 � 1026 2.83 � 1025 1.40 � 1029 2.17 � 1027

Grain size,

d (nm)

30 47 52 90

rmation range [12] and used as a first approximation for theased alloys of our study:

Þ ¼ �0:027 � lnðZÞ � 7254

Tþ 8:49 (2)

he values obtained for the Z–H parameter and grain size arerted in Table 3 for the different Ni alloys. It can be seen that the

n sizes predicted by Eq. (2) appear to be consistent with thervations made by TEM.

ase cite this article in press as: M’Saoubi R, et al. Surface integritusive drilling. CIRP Annals - Manufacturing Technology (2014),

[8] Okada M, Ueda T, Hosokawa A, Tanaka R (2012) Drilling of Difficult-to-CutMaterials by Using Indexable Insert Drill with Non-Axisymmetrical Geometry.Transactions of the Japan Society of Mechanical Engineers Series C78(785):252–261.

[9] Beno T, Hulling U (2012) Measurement of Cutting Edge Temperature inDrilling. Procedia CIRP 3:531–536.

[10] Zener C, Hollomon JH (1944) Effect of Strain Rate Upon Plastic Flow of Steel.Journal of Applied Physics 15:22–33.

[11] Bi Z, Zhang M, Dong J, Luo K, Wang J (2010) A New Prediction Model of SteadyState Stress Based on the Influence of the Chemical Composition for Nickel-Base Superalloys. Materials Science and Engineering A 527:4373–4382.

[12] Medeiros SC, Prasad YVRK, Frazier WG, Srinivasan R (2000) Modeling GrainSize During Hot Deformation of IN 718. Scripta Materialia 42:17–23.

y of nickel-based alloys subjected to severe plastic deformation byhttp://dx.doi.org/10.1016/j.cirp.2014.03.067