Laser Assisted Machining of Inconel 718 Super Alloy

124 Pages·2009·18.46 MB·English

Laser Assisted Machining of
Inconel 718 Super Alloy

By

Salar Tavakoli Manshadi

Department of Mechanical Engineering
McGill University

Montréal, Québec
February 2009

A Thesis Submitted to the Faculty of Graduate Studies Research in Partial
Fulfillment of the Requirement for the Degree of
Master of Engineering

© Salar Tavakoli Manshadi
Abstract
This research work assesses the effect of Laser Assisted Machining (LAM) on the
machinability of Inconel 718 using a triple layer coated carbide and a sialon ceramic tool.
This study was motivated by issues related to poor machinability of IN718 under
conventional machining operations. In this work a focused Nd:YAG laser beam was used
as a localized heat source to thermally soften the workpiece prior to material removal.
Finishing operations were assumed throughout the experiments. Optimization screening
tests were performed over a wide range of cutting speeds (ranging from 100 to 500
m/min) and feeds (ranging from 0.125 to 0.5 mm/rev). Results showed a significant drop
in all three components of cutting force when thermal softening caused by the laser
power was in effect. These tests yielded the optimum cutting speed and feed to be 200
m/min and 0.25 mm/rev for the coated carbide and 300 m/min and 0.4 mm/rev for the
ceramic tool. Under these optimum conditions tool life tests were carried out. Drastic
increase in terms of the material removal rate (MRR) was demonstrated under LAM
conditions as compared to conventional machining. A nearly %300 increase in MRR was
established for the coated carbide tool while slightly reducing tool life, mainly because
the coatings offering thermal and wear protection could not withstand the high
temperatures associated with LAM. Nearly %800 increase in MRR for the ceramic tool
was achieved while improving tool life (about %50). In all cases, improvements in
surface finish and surface integrity were observed. The dominant mode of tool failure
was observed to be average flank wear for all tools tested. However, the coated carbide
tool exhibited signs of chipping and flaking in the coatings. The morphology of the chips
produced was analyzed and it was shown that temperature and increased chip thickness
were the main causes of transition from steady state to shear localized chip structure.
Shear localized or sawtooth chips tended to form at high temperatures when maximum
undeformed chip thickness reached 140 μm.

i
Résumé
Cette recherche évalue l'effet de l’usinage assisté par Laser (UAL) sur l’usinabilité
d'Inconel 718 en utilisant deux outils : Le premier est enrobé d’une triple couche de
carbure et le second est en céramique sialon. Cette étude a été motivée par la difficulté
d’usiner IN718 conventionnellement. Dans ce travail, un rayon laser Nd:YAG a été
utilisé comme une source de chaleur localisée pour adoucir thermiquement la pièce avant
l'usinage. Les expériences représentaient les opérations de finitions. Une optimisation a
été exécutée à travers une sélection unitaire pour une large gamme de vitesses de coupes
(aux limites de 100 à 500 m/min) et de vitesses d’avance (aux limites de 0.125 à 0.5
mm/rév). Les résultats ont manifesté une réduction significative dans toutes les trois
composantes de la force de coupe quand l'adoucissement thermique provoqué par le laser
était mis en effet. D’après les tests, les valeurs optimales de vitesse de coupe et d’avance
sont 200 m/min et 0.25 mm/rév pour l’outil avec la couche de carbure et 300 m/min et
0.4 mm/rév pour l’outil en céramique. Dans ces conditions optimales, des épreuves de
tenue d’outils ont été réalisées. Une augmentation du taux d’enlèvement de matière a été
démontrée lors de l’application de l’UAL en comparaison à l’usinage conventionnel. Une
augmentation dans le taux d’enlèvement de matière de 300% a été établie pour l’outil
enrobé de carbure avec une légère réduction en tenue d’outil. La raison de cette réduction
est le fait que ces couches qui offrent une protection thermique et une résistance d’usure
ne pouvaient pas résister aux températures élevées associées à l’UAL. Une augmentation
de 800% dans le taux d’enlèvement de matière a été accomplie pour l’outil en céramique
avec une amélioration de tenue d’outils d’environ 50%. Dans tous les cas, une
amélioration de l’intégrité de la surface à été observée. En générale, ça été remarqué que
le mode dominant d'échec des outils est l’usure frontale. Pourtant, l’outil avec une couche
de carbure a fait signe d’usure frontale et d’écaillage de la couche. La morphologie des
copeaux produits a été analysée; il a été montré que la température et l’augmentation de
l'épaisseur des copeaux étaient les causes principales de transition du régime permanent
au cisaillement localisé de la structure des copeaux. Le cisaillement localisé et les
copeaux en forme de dent de scie en tendance à se former à de hautes températures quand
l'épaisseur des copeaux non-déformée atteint une valeur maximale de 140 µm.
ii
Acknowledgements
First and foremost, I would like to express my sincere gratitude to my principal
supervisor Prof. Helmi Attia and co-supervisor Prof. Vincent Thomson for their
invaluable guidance in the accomplishment of this research. My extended gratefulness
goes to Mr. Raul Vargas for his critical contributions and constructive comments in the
development of this work. I would also like to thank the Natural Science and Engineering
Research Council of Canada (NSERC), the National Research Council of Canada (NRC)
and my principle supervisor for their financial support throughout the course of this
research.
All experiments performed in this work were carried out at the Aerospace Manufacturing
Technology Center (AMTC), National Research Council of Canada (NRC). I would like
to thank Prof. Attia for providing me with the opportunity to conduct my experiments at
the AMTC and to interact with leading researchers in the field.
Furthermore, I am grateful to Mr. Nicola De Palma for his technical help and support on
performing the machining tests at the AMTC. As well, my deep appreciation is extended
to all the members at AMTC, NRC who helped during the experimental phase, and to
Mrs. Helen Campbell at McGill University for her help with the SEM imagery.
Last but not least I would like to express my deepest gratitude to my parents for their
endless support and encouragement. Without you none of this would be possible.

iii
Contents
Abstract ............................................................................................................................... i 
Résumé ............................................................................................................................... ii 
Acknowledgements .......................................................................................................... iii 
Contents ............................................................................................................................ iv 
List of Figures .................................................................................................................. vii 
List of Tables .................................................................................................................. xiii 
Nomenclature ................................................................................................................. xiv 
Chapter 1: Introduction .............................................................................................. 1 
1.1  General ................................................................................................................. 1 
1.2  Terminal Thesis Objectives .................................................................................. 3 
1.3  Thesis Organization .............................................................................................. 3 
Chapter 2:  Literature Review ....................................................................................... 5 
2.1  Introduction .......................................................................................................... 5 
2.2  Superalloys ........................................................................................................... 5 
2.3  Inconel 718 ........................................................................................................... 6 
2.3.1  Microstructure ............................................................................................... 6 
2.3.2  Mechanical Properties ................................................................................... 8 
2.3.3  Machinability Issues ................................................................................... 10 
2.4  Machining of Inconel 718 .................................................................................. 11 
2.4.1  Cutting Forces ............................................................................................. 11 
2.4.2  Tool Wear ................................................................................................... 13 
2.4.3  Surface Integrity.......................................................................................... 14 
2.4.4  Chip Formation ........................................................................................... 15 
2.5  Thermally Enhanced Machining (TEM) ............................................................ 17 
iv
2.5.1  Plasma Enhanced Machining (PEM) .......................................................... 17 
2.6  Laser Assisted Machining (LAM) ...................................................................... 19 
2.6.1  LAM Principle of Operation ....................................................................... 19 
2.6.2  LAM of Ceramics ....................................................................................... 22 
2.6.3  LAM of Inconel 718 ................................................................................... 24 
2.7  Gap Analysis ...................................................................................................... 28 
2.8  Detailed Objectives of the Research Work ........................................................ 29 
Chapter 3:  Experimental Setup .................................................................................. 30 
3.1  Introduction ........................................................................................................ 30 
3.2  Workpiece Material ............................................................................................ 30 
3.3  Cutting Tool ....................................................................................................... 30 
3.4  Experimental Setup ............................................................................................ 32 
3.4.1  Machine Tool .............................................................................................. 32 
3.4.2  Laser ............................................................................................................ 32 
3.4.3  LAM Setup.................................................................................................. 34 
3.5  Cutting Forces .................................................................................................... 36 
3.6  Tool Wear ........................................................................................................... 37 
3.7  Surface Roughness Measurements ..................................................................... 38 
3.8  Workpiece Temperature Measurements ............................................................. 38 
3.8.1  IR Camera Calibration ................................................................................ 40 
3.9  Chip and Microstructure analysis ....................................................................... 42 
Chapter 4:  Effect of LAM on the Machinability of Inconel 718 .............................. 45 
4.1  Introduction ........................................................................................................ 45 
4.2  Experimental Results Obtained in Laser Heating Tests ..................................... 45 
4.2.1  Preliminary Heating Tests ........................................................................... 45 
v
4.2.2  Secondary Heating Test Results ................................................................. 47 
4.3  Test Results of LAM Optimization Tests .......................................................... 49 
4.3.1  Triple Layer Coated Carbide (KC8050) ..................................................... 49 
4.3.2  Sialon Ceramic (KY1540) .......................................................................... 63 
Chapter 5:  Tool Life, Surface Integrity and Chip Morphology Under Optimum
LAM Conditions....................................................................................... 74 
5.1  Introduction ........................................................................................................ 74 
5.2  Tool Life Analysis .............................................................................................. 74 
5.3  Tool Failure Modes ............................................................................................ 78 
5.3.1  Triple Layer Coated Carbide ...................................................................... 78 
5.3.2  Ceramic Tool .............................................................................................. 80 
5.4  Microstructure Analysis ..................................................................................... 82 
5.4.1  Bulk Material .............................................................................................. 83 
5.4.2  Machined Surfaces ...................................................................................... 84 
5.5  Productivity Analysis ......................................................................................... 89 
5.6  Chip Morphology ............................................................................................... 92 
5.6.1  Triple Layer Coated Carbide Tool .............................................................. 93 
5.6.2  Ceramic Tool .............................................................................................. 97 
Chapter 6:  Conclusion and Future Recommendations ........................................... 101 
6.1  Conclusions ...................................................................................................... 101 
6.2  Recommendations for Future Work ................................................................. 103 
References ...................................................................................................................... 104 

vi
List of Figures
Figure 2.1 SEM micrograph showing (a) δ -phase plates, (b) γ˝ discs and (c) γ´ spheroids
in IN718 treated isothermally at 850oC/24h. [11] ........................................... 7 
Figure 2.2 Yield strength of Inconel 718 vs. temperature. [14] .......................................... 9 
Figure 2.3 Optical micrographs of the Inconel 718 chip at 61 m/min. [37] ..................... 15 
Figure 2.4 Chip image for cutting speed of 75 m/min and feed of 0.125 mm/rev showing
signs of transition to shear localized chips. [12] ............................................ 16 
Figure 2.5 Resultant cutting force vs. surface temperature for various cutting speeds for
PEM. [14] ...................................................................................................... 18 
Figure 2.6 Surface roughness vs. surface temperature for various cutting speeds using
PEM. [14] ...................................................................................................... 18 
Figure 2.7 (a) LAM process overview. [4] (b) heat gradient created near laser beam. [2]
....................................................................................................................... 20 
Figure 2.8 LAM setup. (a) As demonstrated by Anderson [42], (b) LAM setup at the
NRC Aerospace Manufacturing Technology Center (Photo courtesy of NRC-
AMTC) .......................................................................................................... 21 
Figure 2.9 Variation of cutting force vs. T [47] ............................................................ 23 
mr
Figure 2.10 Results for LAM of PSZ material (Pfefferkorn) [41]. ................................... 24 
Figure 2.11 Reduction in cutting forces in LAM of Inconel 718 at different speeds.[40] 25 
Figure 2.12 (a) effect of different coating materials applied to Inconel 718 to increase
absorptivity. (b) Absorptivity of different laser wavelengths on Inconel 718.
[42] ................................................................................................................. 26 
Figure 2.13 Specific cutting energy vs. material removal temperature. [42] ................... 26 
Figure 2.14 Notch wear at different T .[42] ................................................................... 27 
mr
Figure 2.15 VB and VB vs. T .[42] .................................................................... 27 
max ave mr,ave
Figure 2.16 Tool cost comparison when machining 1m length of Inconel 718. [42] ....... 27 
Figure 3.1 Setup for laser heating experiments ................................................................ 33 
Figure 3.2 A spot burned by the laser viewed under magnification. Circle (a) shows the
true laser spot diameter for this test. .............................................................. 34 
Figure 3.3 Graph showing the results and the trend line obtained for the spot size tests. 34 
vii
Figure 3.4 (a) Schematic of laser assisted machining (b) Experimental setup used for the
laser assisted machining tests ........................................................................ 35 
Figure 3.5 Force directions during machining .................................................................. 36 
Figure 3.6 (a) Worn insert (KC8050), showing wear region (b) Flank wear criteria ....... 37 
Figure 3.7 Online roughness measurements; Portable Taylor Hobson Surtronic 3+ ....... 38 
Figure 3.8 IR camera image (viewing workpiece from the top) ....................................... 39 
Figure 3.9 Typical heating curve for thermocouple. The conditions for this test are 3000
W, 300 m/min, 0.3 mm/rev, 3 mm spot size. IR measurement point
temperature is calculated as 383 °C. .............................................................. 41 
Figure 3.10 Comparison of the temperature data obtained from the thermocouple and IR
camera during the heating tests at various cutting speeds. ε=0.09- 3,000 W-
2mm spot size- 0.3 mm/rev. .......................................................................... 42 
Figure 3.11 Prepared chip sample, mounted in bakelite powder ...................................... 43 
Figure.3.12 Etched and polished chip sample showing microhardness indentation.
(Magnification: 150X) ................................................................................... 44 
Figure 4.1 Typical heating curve for laser heating tests. Temperature vs. time; 500 W,
1179.9 rpm, 0.1 mm/rev ................................................................................ 46 
Figure 4.2 Workpiece showing surface damage along the heating length caused by the
laser. ............................................................................................................... 47 
Figure 4.3 Effect of increasing feed on the temperature of the workpiece during laser
heating at 3,000 W - 2mm spot size- 100 m/min. .......................................... 49 
Figure 4.4 Methodology for optimization tests ................................................................. 50 
Figure 4.5 Comparison of (a) cutting forces (b) feed forces (c) radial forces. Conventional
machining and LAM at 100 m/min and 0.125 mm/rev. ................................ 52 
Figure 4.6 Change in forces vs. cutting speed for conventional machining and LAM
(3,000 W) at feed= 0.125 mm/rev and DOC= 0.25 mm. ............................... 53 
Figure 4.7 Surface temperature measured with the IR camera vs. speed, for conventional
machining and LAM (3,000 W) at feed= 0.125 mm/rev and DOC= 0.25 mm.
....................................................................................................................... 53 
viii
Figure 4.8 Average wear measured on the flank face of cutting tool vs. speed. For
conventional machining and LAM (3,000 W) at feed= 0.125 mm/rev and
DOC= 0.25 mm. ............................................................................................ 54 
Figure 4.9 Tool wear on the flank face of the tool at 0.125 mm/rev and (a) 100 m/min,
conventional machining (b) 100 m/min, LAM (c) 150 m/min, LAM (d) 200
m/min, LAM (e) 250 m/min, LAM (magnification: 12X). Sliding distance=
30 mm. (Refer to Figure 3.6) ......................................................................... 56 
Figure 4.10 Surface roughness vs. speed, measured for conventional machining and LAM
(3,000 W) at feed= 0.125 mm/rev and DOC= 0.25 mm. ............................... 57 
Figure 4.11 Change in forces vs. feed under LAM (3,000 W). cutting speed= 200 m/min,
DOC= 0.25 mm. ............................................................................................ 58 
Figure 4.12 Surface temperature vs. feed, as measured by the IR camera for LAM (3,000
W) at cutting speed= 200 m/min and DOC= 0.25 mm. ................................. 59 
Figure 4.13 Average flank wear measured on the flank face vs. feed for LAM (3,000 W)
at cutting speed= 200 m/min and DOC= 0.25 mm. ....................................... 60 
Figure 4.14 Average wear on the flank face of the tool at LAM (3,000 W), 200 m/min
and (a) 0.125 mm/rev (b) 0.175 mm/rev (c) 0.2 mm/rev (d) 0.25 mm/rev (e)
0.3 mm/rev (f) 0.5 mm/rev (magnification: 12X). Sliding distance= 30mm.
(Refer to Figure 3.6) ...................................................................................... 61 
Figure 4.15 Average surface roughness vs. feed for LAM (3,000 W) at cutting speed=
200 m/min, DOC= 0.25 mm. ......................................................................... 62 
Figure 4.16 Cutting forces for the triple layer, coated carbide and the ceramic tool for
LAM (3,000 W), 200 m/min, 0.25 mm/rev. .................................................. 64 
Figure 4.17 Change in forces vs. cutting speed for conventional machining and LAM
(3,000 W), feed= 0.25 mm/rev and DOC= 0.25 mm. .................................... 65 
Figure 4.18 Surface temperatures measured with the IR camera vs. speed for
conventional machining and LAM (3,000 W), feed= 0.25 mm/rev and DOC=
0.25 mm. ........................................................................................................ 65 
Figure 4.19 Average flank wear of the cutting tool vs. speed for conventional machining
and LAM (3,000 W), feed= 0.25 mm/rev and DOC= 0.25 mm. ................... 66 
ix