Pulsed laser polishing of steel surfaces

J.G.A.B. Simões, R. Riva, M.G. Destro,

M.S.F. Lima, A.L. Ribeiro

Instituto de Estudos Avançados

São José dos Campos – BR

Photonics Division

IX Brazilian MRS Meeting (Ouro Preto, 24-28 October 2010)2

Introduction/Objectives

• Laser polishing is quite a new technique developed in recent years, and appears as an attractive alternative to supplement the deficiency of those conventional abrasive methods since it is a non-contact process and would facilitate the automation of the polishing process of metalic parts� molds, dies, steel tools, medical implants.

• Laser polishing process is based on the melting and fast re-solidifying of a material microscopic layer of the same extent of the original surface roughness. Improvement on surface finishing is obtained when both the energy density and pulse duration of the laser beam are carefully controlled.

• In this work, we investigated the limits and optimal parameters for the laser polishing technique using a 532 nm pulsed Nd-YAG laser at a repetition rate of 5 kHz and pulse duration of 100 ~ 200 ns.

IX Brazilian MRS Meeting (Ouro Preto, 24-28 October 2010)3

Laser polishing concepts

T(z,t)Ts

z

z0(t)laser

T(z,t)Ts

z

T(z,t)Ts

z

z0(t)laserlaser

Diffusion length

pd TZ .2 α=

TV

TM

z

Laser-polishing: melting of a microscopic layer and a fast re-solidifying of the melted

material. The affected layer has to be deep enough to melt the roughness peaks, but it

must not be deeper than the valleys� the energy of the laser beam must be carefully

controlled to melt just a microscopic layer.

Limits on energy density:

t = Tp = pulse length

T0(Tp) = TV

pd

melt TZ

Z .2

α==

IX Brazilian MRS Meeting (Ouro Preto, 24-28 October 2010)4

Experimental Setup

Laser

4 X Telescope

Scan

head

F –Theta

objective

Z stage

Z

X

YScan head

(hurrySCAN, Scanlab)

• Aperture: 20 mm

• F-Theta: 250 mm

• Working surface: 100 x 100 mm2

• Resolution: 2 µm

• Max. speed: 1 m/s

Steel

sample

DPSS Nd-YAG Laser

(532 nm)(Corona, Coherent)

• Max. Ave. Power: 40 W

• Rep. rate: 1 – 20 kHz

• Beam diam.: 5 mm

• Pulse width: 80-200 ns

• Pulse energy: 7 mJ (5 kHz)

• M2 : 20

IX Brazilian MRS Meeting (Ouro Preto, 24-28 October 2010)5

Laser beam characterization

500− 400− 300− 200− 100− 0 100 200 300 400 5000

1

2

3

Radius (um)

Flu

ency

(J/

cm2

)

w− w

500− 400− 300− 200− 100− 0 100 200 300 400 5000

1

2

3

Radius (um)

Flu

ency

(J/

cm2

)

w− w

0

50

100

150

200

250

300

350

260 265 270 275 280 285 290 295

Distancia (mm)

Exp

M2=21

w0=182Bea

m r

ad

ius

(µµ µµ

m)

Lens distance (mm)

500− 400− 300− 200− 100− 0 100 200 300 400 5000

1

2

3

Radius (um)

Flu

enc

y (J

/cm

2)

w− w

500− 400− 300− 200− 100− 0 1002003004005000

1

2

3

Radius (um)

Flu

ency

(J/

cm2

)

w− w

500− 400− 300− 200− 100− 0 1002003004005000

1

2

3

Radius (um)

Flu

ency

(J/

cm2

)

w− w

100− 0 100 200 3000

0.2

0.4

0.6

0.8

1

Time (ns)

Am

pli

tud

e (A

.U.)

Laser pulse energy measured below

F-Theta lens using a power meter (Ophir)

Pulse energy

Average Power/ repetition rate

Laser pulse width measured with a

photodiode (MFOD 100)

Pulse width = 80 ns

Photodiode

Z stage

Fiber

(100 µm)

Laser beam

500 400 300 200 100 0 100 200 300 400 5000

0.5

1

1.5

2TEK0 013 -18 /10 /1 0- 17W -204 mm

posição (um)

Flu

enci

a (J

/cm

2)

w− w

Oscilloscope

Beam profiler

scan speed

Radius = speed x time

IX Brazilian MRS Meeting (Ouro Preto, 24-28 October 2010)6

Energy density limits for polishing

Applying only one pulse allows estimating of polishing process limits �

Melting (T ~ T melting)

Ablation/Vaporization ( T > T vapor)

500− 400− 300− 200− 100− 0 100 200 300 400 5000

1

2

3

4

TEK0032-18/10/10- 10W-192mm

posição (um)

Flu

enci

a (J

/cm

2)

Fmelt

141− 141

500− 400− 300− 200− 100− 0 100 200 300 400 5000

1

2

3

4

TEK0031-18/10/10- 15W-192mm

posição (um)

Flu

enci

a (J

/cm

2)

Fmelt

180− 180

Wmelt = 180 µm

Wmelt = 140 µm

Wvapor = 50 µm

Fvap

Fmelt ≈ 0.8 J/cm2

Fvapor ≥ 3 J/cm2

IX Brazilian MRS Meeting (Ouro Preto, 24-28 October 2010)7

Material characterization

0.620.74152

Rv

(µm)

Rp

(µm)

Ra

(nm)

Materials: Carbon Steel (AISI 1020) /

Stainless Steel (AISI 304)

Surface finishing: silicon carbide sandpaper (400 Gr)

on planar machined surfaces

0.300.3678

Rv

(µm)

Rp

(µm)

Ra

(nm)

0 1 2 31−

0.5−

0

0.5

1

X (mm)

Z (

um

)Ra−

Ra

0 200 400 600 800 1 103

×

0

0.01

0.02

lines/mm

Am

pli

tude

0 1 2 31−

0.5−

0

0.5

1

X (mm)

Z (

um

)

Ra−Ra

0 200 400 600 800 1 103

×

0

2 103−

×

4 103−

×

6 103−

×

8 103−

×

lines/mm

Am

pli

tud

e

1020

304Roughness profile

Frequency spectra

3D Surface profiler

(Taylor Hobson)

resol.: 0.8 nm

IX Brazilian MRS Meeting (Ouro Preto, 24-28 October 2010)8

Polishing procedures

∆h

∆w = pulse repetition rate/scan speed

Variable parameters:

�Laser Fluency (J/ cm2)

� Beam overlapping:

∆h: line spacing

∆w: beam radius spacingNumber of passing

Fixed parameters:

� Pulse repetition rate: 5 kHz

� Pulse width: 80 ns

� Beam radius: 300 µm� Beam profile:

500− 400− 300− 200− 100− 0 100 200 300 400 5000

1

2

3

Radius (um)

Flu

en

cy

(J/

cm

2)

w− w

IX Brazilian MRS Meeting (Ouro Preto, 24-28 October 2010)9

Results – Carbon Steel 1020

-1.27-0.70.42194200/100

5 pass

-0.4640.730.5521050/25

1 pass

-0.55-1.11.5268100/50

1´pass

-0.49-1.30.89231200/250

1 pass

-0.210.620.74152base

RskRv

(µm)

Rp

(µm)

Ra

(nm)

∆w/ ∆h

(µm)

0 1 2 31−

0.5−

0

0.5

1

X (mm)

Z (

um

)

Ra−

Ra

0 200 400 600 800 1 103

×

0

0.01

0.02

lines/mm

Am

pli

tud

e

0 1 2 31−

0.5−

0

0.5

1

X (mm)

Z (

um

)

Ra−

Ra

0 200 400 600 800 1 103

×

0

0.01

0.02

lines/mm

Am

pli

tud

e

0 1 2 31−

0.5−

0

0.5

1

X (mm)

Z (

um

)

Ra−

Ra

0 200 400 600 800 1 103

×

0

0.01

0.02

lines/mm

Am

pli

tud

e

0 1 2 31−

0.5−

0

0.5

1

X (mm)

Z (

um

)

Ra−

Ra

0 200 400 600 800 1 103

×

0

0.01

0.02

lines/mm

Am

pli

tud

e

0 1 2 31−

0.5−

0

0.5

1

X (mm)

Z (

um

)

Ra−

Ra

0 200 400 600 800 1 103

×

0

0.01

0.02

lines/mm

Am

pli

tud

e

Effect of beam overlapping � ∆h, ∆w and number of pass.

Conditions: Fma = 0.8 J/cm2, wlaser= 300 um, frp = 5 kHz

IX Brazilian MRS Meeting (Ouro Preto, 24-28 October 2010)10

Results – Carbon steel (1020)

0 1 2 31−

0.5−

0

0.5

1

X (mm)

Z (

um

)

Ra−

Ra

0 200 400 600 800 1 103

×

0

0.01

0.02

lines/mm

Am

pli

tud

e

-0.980.390.301441.3

-0.880.850.401361.8

-0.430.470.351141

0.510.920.891660.6

-0.210.620.74152w/laser

RskRv

(µm)

Rp

(µm)

Ra

(nm)

Fmax

(J/cm2)

0 1 2 31−

0.5−

0

0.5

1

X (mm)

Z (

um

)

Ra−

Ra

0 200 400 600 800 1 103

×

0

0.01

0.02

lines/mm

Am

pli

tud

e

0 1 2 31−

0.5−

0

0.5

1

X (mm)

Z (

um

)

Ra−Ra

0 200 400 600 800 1 103

×

0

0.01

0.02

lines/mm

Am

pli

tud

e

0 1 2 31−

0.5−

0

0.5

1

X (mm)

Z (

um

)

Ra−

Ra

0 200 400 600 800 1 103

×

0

0.01

0.02

lines/mm

Am

pli

tud

e

0 1 2 31−

0.5−

0

0.5

1

X (mm)

Z (

um

)

Ra−

Ra

0 200 400 600 800 1 103

×

0

0.01

0.02

lines/mm

Am

pli

tud

e

Effect of fluency � 5 passages.

Conditions: ∆h = 0.1 mm, ∆w = 0.1 mm wlaser= 300 um, frp = 5 kHz

IX Brazilian MRS Meeting (Ouro Preto, 24-28 October 2010)11

Steel 1020 – best results

Depth of melting layer for 80 ns pulses is not enough for the basis Ra�

� peak reduction is possible even without great improvement on Ra

� several passages present better results (homogeneous surface)

than only one passage and smaller beam/line spacing.

.

basis F = 1 J/cm2 F = 1.8 J/cm2

0.620.74152

Rv

(µm)

Rp

(µm)

Ra

(nm)

0.850.40136

Rv

(µm)

Rp

(µm)

Ra

(nm)

0.470.35114

Rv

(µm)

Rp

(µm)

Ra

(nm)

IX Brazilian MRS Meeting (Ouro Preto, 24-28 October 2010)12

Results – Stainless steel (304)

-0.520.120.165210 pass

-0.330.200.17465 pass

-0.190.150.25503´pass

-0.460.340.29501 pass

0.050.300.3678base

RskRv

(µm)

Rp

(µm)

Ra

(nm)

Number

pass

0 1 2 31−

0.5−

0

0.5

1

X (mm)

Z (

um

)

Ra−Ra

0 200 400 600 800 1 103

×

0

2 103−

×

4 103−

×

6 103−

×

8 103−

×

lines/mm

Am

pli

tud

e

0 1 2 31−

0.5−

0

0.5

1

X (mm)

Z (

um

)

Ra−Ra

0 200 400 600 800 1 103

×

0

2 103−

×

4 103−

×

6 103−

×

8 103−

×

lines/mm

Am

pli

tud

e

0 1 2 31−

0.5−

0

0.5

1

X (mm)

Z (

um

)

Ra− Ra

0 200 400 600 800 1 103

×

0

2 103−

×

4 103−

×

6 103−

×

8 103−

×

lines/mm

Am

pli

tud

e

0 1 2 31−

0.5−

0

0.5

1

X (mm)

Z (

um

)

Ra−Ra

0 200 400 600 800 1 103

×

0

2 103−

×

4 103−

×

6 103−

×

8 103−

×

lines/mm

Am

pli

tud

e

0 1 2 31−

0.5−

0

0.5

1

X (mm)

Z (

um

)

Ra− Ra

0 200 400 600 800 1 103

×

0

2 103−

×

4 103−

×

6 103−

×

8 103−

×

lines/mm

Am

pli

tud

e

Effect of beam overlapping � number of pass.

Conditions: Fma = 1 J/cm2, ∆h, ∆w = 0.1 mm wlaser= 300 um, frp = 5 kHz

IX Brazilian MRS Meeting (Ouro Preto, 24-28 October 2010)13

Results – Stainless steel

0.300.3678

Rv

(µm)

Rp

(µm)

Ra

(nm)

0.120.1656

Rv

(µm)

Rp

(µm)

Ra

(nm)

0.200.1746

Rv

(µm)

Rp

(µm)

Ra

(nm)

w/laser Laser5 pas

Laser

10 pas

With 10 passages �

Oxidation ???

�Alloy elements has

different vapor pressure

Best results � 5 pass �

40 % reduction of Ra

IX Brazilian MRS Meeting (Ouro Preto, 24-28 October 2010)14

Conclusions

�Ra is not a good parameter to qualify laser polishing process

� Spatial frequency analysis is better.

� Reduction of Rp was observed even without improvement on Ra.

�Laser polishing is only possible with a carefully choice of laser parameters,

� the beam parameters must be measured with confidence, specially the

beam profile.

�Future work: increase energy density;

investigate overlap (line spacing/beam separatiion)

and beam profile influences.

Thanks for your attention.

Laser polishing of steels using a high repetition

100 ns pulsed laser was demonstrated with

a reduction of 30-50% on initial Ra