# Effect of laser assistance machining on residual stress and fatigue strength for a bearing steel (100Cr6) and a titanium alloy (Ti 6Al 4V)

Guenael Germain, Franck Morel, Jean Lu Lebrun, Anne Morel, Bertrand Huneau

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## To cite this version:

Guenael Germain, Franck Morel, Jean Lu Lebrun, Anne Morel, Bertrand Huneau. Effect of laser assistance machining on residual stress and fatigue strength for a bearing steel (100Cr6) and a titanium alloy (Ti 6Al 4V). 7th European Conference on Residual Stresses (ERCS 7), 2006, Berlin, Germany. pp.569-574, 10.4028/www.scientific.net/MSF.524-525.569. hal-01008098

HAL Id: hal-01008098

https://hal.science/hal-01008098v1

Submitted on 16 Sep 2025

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Distributed under a Creative Commons Attribution 4.0 International License
Effect of laser assistance machining on residual stress and fatigue
strength for a bearing steel (100Cr6) and a titanium alloy (Ti 6Al 4V)

G. GERMAIN¹,ª, F. MOREL¹,ᵇ, J-L. LEBRUN¹,ᶜ, A. MOREL¹,ᵈ, B. HUNEAU²,ᵉ

¹ LPMI/EPPM-EA1467. Ecole Nationale Supérieure d'Arts et Métiers, CER d'Angers, 2, Bd du Ronceray - BP 93525 - 49035 Angers Cedex 01 France

² GeM, UMR CNRS 6183. Ecole Centrale de Nantes, 1, rue de le Noë – BP 92101 – 44321 Nantes Cedex 03

ª guenael.germain@angers ensam.fr , ᵇ franck.morel@angers ensam.fr ,

ᶜ lebrun@angers ensam.fr , ᵈ anne.morel@angers ensam.fr , ᶜ bertrand.humeau@ec-nantes.fr

**Keywords:** laser assisted machining, microstructure, residual stresses, fatigue

**Abstract.** The use of Laser Assisted Machining (LAM) can improve different aspects of the machinability of high strength materials. A study was undertaken to determine the optimum cutting parameters and to quantify their influence on fatigue strength according to the type of microstructure created. Two different materials were studied: a bearing steel (100Cr6 / AISI 52100) and an aeronautical titanium alloy (Ti6Al4V). In the bearing steel a significant increase of the fatigue resistance was observed due to the transformation of the surface layer into martensite. For the titanium alloy, a slight reduction in the fatigue strength was found as in this case the microstructure and residual stress state of the surface layer was less beneficial. The surface roughness has also been measured and no significant variation has been observed for different laser powers in each material.

**Presentation of Laser Assisted Machining (LAM)**

Laser Assisted Machining (LAM) can be considered as an alternative process for the machining of high strength material like ceramic [8], [6], [7], hard steel [3], graphite iron [9], titanium alloys [5], nickel alloys [1] and metal matrix composites [10]. It is important to be able to machine these materials because they have good mechanical characteristics, especially at high temperature, hence in terms of mechanical design these materials are being used more and more.

**The Principle of LAM**

LAM is a high temperature cutting process using a laser beam as the heat source (Fig. 1).

<figure><img src="image_3.png" /><figcaption>Fig. 1: LAM configuration during a turning operation (IREPA)</figcaption></figure>

The principle of the processes is to reduce the cutting force necessary to machine the material by increasing the temperature to the point where the strength of the material is reduced (Fig. 2). Indeed at high temperature, the specific cutting energy is weak, which improves workability. Fig. 2 [2] shows the characteristic evolution of hardness and yield strength of various materials with regard to the temperature. For most materials a drop in the yield strength or hardness occurs between 500 and 600°C. To be effective, the cutting tool must thus operate in the zone where the temperature remains higher than this value. However, the distance between the tool and the laser spot must be sufficient to prevent degradation of the tool due to high temperature.
<figure><img src="image_4.png" /><figcaption>Fig. 2: Sensitivity of yield strength and hardness to temperature for various materials [2]</figcaption></figure>

## Experimental procedure

A numerically controlled lathe (REALMECA RT-5) equipped with a 2.5 kW ROFIN YAG laser was used in this work. The laser nozzle can be controlled in three translations and two rotations. A numerical control system (NUM 1060) allows the control of the seven independent degrees of freedom. The high-power laser beam is delivered through a fibre optic cable to the lathe chamber, where it is focused on the workpiece surface. During machining, a gas under pressure (air) is forced through the laser nozzle to protect the focusing lens from being damaged by chips. To obtain the maximum power density, the focal plan of the laser is set to be coincident with the surface of the workpiece. In this case, the laser spot diameter is equal to 0.8 mm and the power density is approximately $2 \times 10^9$ W.m$^{-2}$. Three components of the cutting force are measured by a Kistler piezoelectric dynamometer. The output signals for the cutting force are amplified and collected by a data acquisition system with the LabView® software. A near infra-red camera is used for the acquisition of the thermal field and the measurement of heat flux in the tool will be soon possible.

The measurement of roughness is carried out using a Mitutoyo SJ400. Measurements of residual stresses and mean diffraction peak widths on the surface and in-depth were carried out on the machined components by using a portable X-ray diffractometer [4] from Proto-XRD. The residual stresses were determined in the axial $\sigma_{xx}$ and circumferential $\sigma_{yy}$ directions. Finally push-pull fatigue tests were carried out on hourglass shape specimens, using an Instron servo-hydraulic testing machine operating under load control with a load ratio of $R = -1$. Fatigue tests were carried out with specimens machined traditionally and using LAM. The fatigue failure surfaces of the broken specimens were observed using a Scanning Electronic Microscope.

## Results of LAM

Machining was carried out with a $Al_2O_3+TiN$ coated carbide insert tool (Sumitomo AC700G) with geometry CNMA 12 04 08 and using a holder PCLNL 20 20 K 12. The parameters of the selected process are cutting speed (Vc), feed rate (f), depth of cut (ap) and the laser power (P). The cutting force, the surface roughness (Ra) and the residual stresses were measured for each test. Two materials were used: a bearing steel 100Cr6 with 57 HRc hardness and, a titanium alloy Ti6Al4V. The steel (100Cr6) was quenched after an austenitization at 850°C. At this temperature, the carbides are not dissolved into the matrix.

## Evolution of the cutting force

For the two materials, the cutting force decreases with the power of the laser.
For the 100Cr6 heat treated steel, the cutting speed was kept constant at 240 m.min⁻¹. The distance between the tool and the laser beam was 5 mm. The average reduction of the cutting force for a 500 W laser power is 42% for ap = 0.1 mm and only 9% for ap = 0.4 mm (Fig. 3). The increase of the cutting force measured for a power of 1400 W and a depth of cut of 0.1 mm is due to a thermal degradation of the tool by laser beam reflection. In this configuration most of the laser beam is reflected onto the insert that is heated and wears quickly.

<figure><img src="image_5.png" /><figcaption>Fig. 3: Cutting Force (N) for 100Cr6 heat treated steel.</figcaption></figure>

For Ti6Al4V, the feed rate (f) was kept constant at 0.1 mm.tr⁻¹. The cutting force strongly decreases with the laser power (Fig. 4): an average reduction of 28% for the laser power of 500 W and 51% for 1000 W. The cutting force decreases also with the depth of cut (−34% for ap = 1 mm and −45% for ap = 0.5 mm) and with the cutting speed (−41% for Vc = 26 m.min⁻¹, −37% for Vc = 54 m.min⁻¹).

<figure><img src="image_6.png" /><figcaption>Fig. 4: Cutting force (N) for Ti6Al4V</figcaption></figure>

## Evolution of the roughness

The surface roughness remains stable regardless of the laser power used. For the 100Cr6, the roughness (Ra) with different laser power assistance and a depth of cut of 0.1 mm, lies between 0.6 and 0.75 µm for a feed rate of 0.1mm.tr⁻¹ and lies between 0.7 and 1.2 µm for a feed rate of 0.2 mm.tr⁻¹. For the Ti6Al4V, the surface roughness lies between 0.3 and 0.6 µm for the different conditions.

## Evolution of residual stresses

Measurements of residual stresses and mean diffraction peak widths on the surface and in-depth were carried out on the workpiece. The measurement error is approximately +/- 50 MPa. The residual stresses at the surface are influenced by the laser power. As the laser power increases, the compressive residual stresses decrease or tend towards tensile stresses. Fig. 6 illustrates the evolution of the residual stresses on the surface for Ti6Al4V. The same trend is observed for the 100Cr6. This increase is mainly due to the surface heating effects during laser assisted machining. The evolution with the laser
power of these stresses towards tension is faster for the lowest cutting speed (26 m.min⁻¹). For the highest cutting speed (54 m.min⁻¹) the surface temperature is lowered, leading to a smaller influence of the laser (Fig. 5). For cutting speeds higher than this, the heating effect of the laser becomes negligible as the heating time is too short. In this case, the residual stresses will tend towards the values measured in conventional machining (i.e. they increase with the cutting speed).

<figure><img src="image_7.png" /><figcaption>Fig. 5: Evolution of surface residual stresses with laser power on Ti6Al4V</figcaption></figure>

For the second material 100Cr6, the residual stresses were measured in-depth by successive electrochemical polishing. Fig. 6 shows the evolution of axial residual stresses as a function of the depth for traditional machining (without laser assistance) and with a 600W laser assistance, at a feed rate equal to 0.1 mm.tr⁻¹, a cutting speed of 240 m.mn⁻¹ and a depth of cut of 0.5 mm. The effect of the laser on the tangential residual stresses shows the same trend as that shown for the axial residual stress. The compressive residual stresses, induced by conventional machining, are strongly reduced by the laser assistance. Both the maximum compressive stress and the depth affected by residual stresses are decreased (Fig. 6). Measurements on the surface show that the surface residual stresses increase with the laser power, as was seen for Ti6Al4V.

<figure><img src="image_8.png" /><figcaption>Fig. 6: Evolution of axial residual stresses according to the depth (100Cr6)</figcaption></figure>

The diffraction peak width was also analysed. This value quantifies the level of work hardening experienced by the material during the machining process. A similar evolution to that seen in Fig. 6 for the residual stress was observed. The level of work hardening remains approximately the same with or without the laser but the depth of the hardened zone is much smaller with the laser assistance (Fig. 7).

<figure><img src="image_9.png" /><figcaption>Fig. 7: Evolution of diffraction peak width according to the depth (100Cr6)</figcaption></figure>
## Fatigue strength due to heat treatment during LAM

Push-pull fatigue tests were carried out on hourglass specimens whose gauge lengths were machined either with or without laser assistance. An Instron servohydraulic fatigue testing machine operating in load control was used. For each LAM condition, around fifteen specimens were tested using a staircase procedure. The mean fatigue limit was estimated for $10^6$ cycles.

LAM can also be used to perform certain heat treatments just after the cutting operation. The laser spot has a diameter (0.8 mm) which is much bigger than the chip width (approximately equal to the feed rate), hence if the position of the laser spot is shifted downstream so that not all of the heat affected material is removed by the cut, a superficial quench of the newly generated surface can be achieved.

The LAM parameters were determined in order to create a thin surface layer of material which was heat treated so that the microstructure of this layer was modified.

## Annealed 100Cr6 fatigue limit

In this study the 100Cr6 steel has an annealed microstructure. For annealed 100Cr6 traditionally machined, the average fatigue limit was determined to be 320 MPa.

In order to introduce a surface heat treatment using LAM, an optimal condition for the laser power and cutting parameters was used ($P=1000 \text{ W}, V_c = 52 \text{ m.min}^{-1}, f = 0.12 \text{ mm.rev}^{-1}$ and $ap = 0.2 \text{ mm}$). The resulting martensite layer had a depth of less than 100 µm and induces strong values of compressive residual stress. The corresponding fatigue limit was found to be approximately 400 MPa. The compressive surface residual stresses and the microstructure of the surface layer inhibit crack formation and propagation. This generally leads to crack initiation that takes place inside the material and not from the surface. The initiation usually occurs at an inclusion and result is a fish eye type rupture surface (Fig. 8).

<figure><img src="image_10.png" /><figcaption>Fig. 8: Fish eye on fatigue fracture surface for a specimen machined with laser assistance</figcaption></figure>

## Ti6Al4V fatigue limit

The fatigue tests conducted on titanium specimens obtained by conventional machining resulted in mean fatigue limit of 581 MPa and a standard deviation of 27 MPa.

The tests using specimens machined by LAM were carried out with a cutting speed of $20 \text{ m.min}^{-1}$, a depth of cut of 0.15 mm, a feed rate of $0.1 \text{ mm.rev}^{-1}$ and a laser power of 400 W. These parameters lead to a microstructure transformation characterized by a needle-shaped microstructure near the surface. The parameters were selected in order to avoid the formation of a dendritic layer with cracks, which would be catastrophic in terms of fatigue. The surface residual stresses were found to be approximately -35 MPa in the circumferential direction and -140 MPa in the tangential direction. These values are respectively -210 MPa and -390 MPa on the specimens machined without laser assistance. The fatigue limit is 550 MPa with a standard deviation of 23 MPa for the specimens machined by LAM. This slight reduction in the fatigue strength can be attributed to the new
microstructure created and/or to the increase in the residual stresses. The coupling between these two
parameters affecting the fatigue strength is important. The actual role of the microstructure compared
to that of the residual stresses is difficult to assess. A possible approach is to relieve the residual
stresses by an appropriate heat treatment and then quantify the sole effect of the microstructure. This
will be conducted in the future. Observation of the rupture surfaces show that in the case of
conventional machining, crack initiation occurs at the surface of the specimen. For LAM, crack
initiation generally occurs in the needle-shaped sub-surface layer.

General Conclusion

In the first part of this work, the influence of Laser Assisted Machining on cutting force, surface roughness and residual stresses were investigated using a quenched 100Cr6 bearing steel with a hardness of 57HRc and the titanium alloy Ti6Al4V. The LAM parameters used resulted in no microstructural change of the surface layer of the workpiece. It was found that with an increase in laser power: the surface roughness is not affected, the cutting force strongly decreases and, the residual stresses increase towards tensile or positive stresses.

The second part of this works was conduced on an annealed 100Cr6 steel and Ti6Al4V. The LAM
parameters were determined in order to create a thin surface layer of material which was heat treated
so that the microstructure of this layer was modified. For these two materials, the microstructures
created during LAM have different influences of the fatigue limit. For the steel an increase in the
fatigue limit (320 MPa to 400 MPa) was found and is due to the layer of martensite created on the
surface, which inhibits surface crack initiation. A slight reduction in the fatigue limit (580 MPa to
550 MPa) was observed for the titanium alloy and can be explained by the needle-shaped
microstructure created on the surface or/and by the increase in compressive residual stresses towards
tensile or positive values.

Acknowledgement

This study has been carried out in the framework of a *Contrat Plan Etat Région* (CPER) Pays de la Loire gathering ECN, ENSAM Angers and CETIM. We are indebted to Doctor D. Bellett to his encouraging comments.

**References**

[1] Anderson, Patwa, Shin, Laser-assisted machining of Inconel 718 with an economic analysis, International Journal of Machine Tools and Manufacture, In Press, Corrected Proof, Available online 18 January 2006,

[2] Ezugwu, Bonney, Yamane, An overview of the machinability of aeroengine alloys, Journal of Materials Processing Technology, Volume 134, Issue 2, 10 March 2003, p. 233-25

[3] Germain, Lebrun, Robert, Dal Santo, Poitou, Experimental and numerical approaches of Laser assisted turning, IJFP Vol. 8 Special Issue 2005, (2005) p. 347-361.

[4] Hauk, Structural and residual stress analysis by non-destructive methods, Elsevier, 1997

[5] Lesourd, Etude et modélisation des mécanismes de formation de bandes de cisaillement intense en coupe des métaux. Application au tournage assisté laser de l'alliage de titane TA6V, Thèse Ecole doctorale de Nantes ED 82-174 (1996).

[6] Malot, Usinage assisté par laser du bore, Thèse Université de Bourgogne TDIJON 2001/31 (2001)

[7] Melhaoui, Contribution à l'étude de l'usure d'outil de coupe en usinage assisté par laser et à l'usinabilité d'une céramique à base d'oxyde de zinc, Thèse Centrale Paris 1997-04 (1997)

[8] Rebro, Shin, Incropera, Design of operating conditions for crackfree laser-assisted machining of mullite, International Journal of Machine Tools and Manufacture, Volume 44, Issues 7-8, June 2004, p. 677-694

[9] Skvarenina, Shin, Laser-assisted machining of compacted graphite iron, International Journal of Machine Tools and Manufacture, Volume 46, Issue 1, January 2006, p. 7-17

[10] Wang, Yang, Wang, An investigation of laser-assisted machining of Al₂O₃ particule reinforced aluminium matrix composite, Materials Processing Technologie 129 (2002) p.268-272
# Effect of Laser Assistance Machining on Residual Stress and Fatigue Strength for a Bearing Steel
(100Cr6) and a Titanium Alloy (Ti 6Al 4V)

10.4028/www.scientific.net/MSF.524-525.569

## DOI References

[2] Ezugwu, Bonney, Yamane, An overview of the machinability of aeroengine alloys, Journal of Materials Processing Technology, Volume 134, Issue 2, 10 March 2003, p. 233-25
doi:10.1016/S0924-0136(02)01042-7

[8] Rebro, Shin, Incropera, Design of operating conditions for crackfree laser-assisted machining of mullite, International Journal of Machine Tools and Manufacture, Volume 44, Issues 7-8, June 2004, p. 677-694
doi:10.1016/j.ijmachtools.2004.02.011

[10] Wang, Yang, Wang, An investigation of laser-assisted machining of Al2O3 particule reinforced aluminium matrix composite, Materials Processing Technologie 129 (2002) p.268-272
doi:10.1016/S0924-0136(02)00616-7

[1] Anderson, Patwa, Shin, Laser-assisted machining of Inconel 718 with an economic analysis, International Journal of Machine Tools and Manufacture, In Press, Corrected Proof, Available online 18 January 2006,
doi:10.1016/j.ijmachtools.2005.11.005

[2] Ezugwu, Bonney, Yamane, An overview of the machinability of aeroengine alloys, Journal of Materials Processing Technology, Volume 134, Issue 2, 10 March 2003, p. 233-25
doi:10.1016/S0924-0136(02)01042-7

[8] Rebro, Shin, Incropera, Design of operating conditions for crackfree laser-assisted machining of mullite, International Journal of Machine Tools and Manufacture, Volume 44, Issues 7-8, June 2004, p. 677-694
doi:10.1016/j.ijmachtools.2004.02.011

[9] Skvarenina, Shin, Laser-assisted machining of compacted graphite iron, International Journal of Machine Tools and Manufacture, Volume 46, Issue 1, January 2006, p. 7-17
doi:10.1016/j.ijmachtools.2005.04.013

[10] Wang, Yang, Wang, An investigation of laser-assisted machining of Al2O3 particule reinforced aluminium matrix composite, Materials Processing Technologie 129 (2002) p.268-272
doi:10.1016/S0924-0136(02)00616-7