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Applied Surface Science 253 (2007) 3208–3214

Study of magnesium and aluminum alloys absorption coefficient

during Nd:YAG laser interaction

Nicolas Pierron *, Pierre Sallamand, Simone Matteı

Laboratoire Laser et Traitement des Materiaux, Universite de Bourgogne, EA 2976, FR, CNRS-2604, 71200 Le Creusot, France

Received 4 May 2006; received in revised form 16 June 2006; accepted 3 July 2006

Available online 26 September 2006

Abstract

In laser processes, the absorption factor of laser Nd:YAG by metals plays a very important role. In order to model laser welding, we need to

know its evolution during the process. The theoretical calculation does not enable the prediction of the absorption factor in the case of a keyhole

mode. It is difficult to predict the effect of plasma and recoil pressure on the shape of the keyhole. In this paper, an integrating sphere is used to

determine the absorption factor during the laser process, which is carried out on two types of magnesium alloys (WE43 and RZ5) and an aluminum

alloy. We obtain the evolution in time of the absorption factor according to different steps of the evolution of the keyhole.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Laser; Nd:YAG; Absorption; Surface; Aluminum alloys; Magnesium alloys; Integrating sphere

1. Introduction

The automotive industry wants to reduce energy consump-

tion and vehicle emissions. Reducing vehicle weight is one

means of reaching this goal. The use of magnesium is a solution

but this choice requires surface treatments to improve corrosion

resistance and finding ways of assembling these alloys. Laser

applications play an important role in these industrial

processes. The main problem is the material behavior during

such treatments. In fact, the interaction with laser is very

complex. This type of material reflects a large part of the

energy. However, using Nd:YAG laser (l = 1.06 mm), it is

possible to limit the loss of energy by reflection, because metals

are more absorbent for lower wavelengths. In laser processes,

the absorption factor is a very important parameter. The results

are strongly dependent on its evolution during the interaction. It

is impossible to accurately predict the values of this coefficient

because it depends on a lot of factors, including the surface

roughness, the presence of oxide layers, the angle of incidence

of the laser beam and the temperature. In 2004, Ignat et al. [1]

presented some results of variations of the absorption with

incident energy in the case of magnesium alloys: WE43 and

* Corresponding author. Tel.: +33 385731056; fax: +33 385731120.

E-mail address: n.pierron@iutlecreusot.u-bourgogne.fr (N. Pierron).

0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2006.07.035

RZ5 using an isothermal differential micro-calorimeter

(SETARAM C80). In fact, he considers that the absorbed

energy is transformed into calorific energy within the material

[2]. As a consequence, a mean value of the absorption

coefficient is obtained. However, this value cannot be used in a

numerical model for example. Indeed, information on each

moment of the interaction is needed.

In this paper, we present a method based on the measurement

of reflected energy during the interaction with the material.

In a first part, we compare Ignat’s results with our

measurements. We use two types of coated substrates for

RZ5: HAE treatment (anodizing process) and a conversion

process (mordancage process). The aim of this part is to

validate the experimental method. We use laser parameters,

which only enable having a molten pool without the formation

of a keyhole. These parameters are used to realize surface laser

treatments.

In a second part, we present results for magnesium and

aluminum alloys when a keyhole appears. We obtain an

evolution in time of the absorption factor. This information can

be introduced in a numerical model. We observe different

stages during the interaction, for example the solid–liquid

transition and the beginning of the formation of the keyhole.

Many explanations are presented in order to understand the

interaction with the material. We compare these hypotheses

with metallographic analysis.

N. Pierron et al. / Applied Surface Science 253 (2007) 3208–3214 3209

Fig. 2. Reflection inside the integrating sphere (ORIEL).

Table 1

Nd:YAG laser characteristics

Mean power (W) 400

Peak power (W) 500–6000

Pulse duration (ms) 0.3–20

Fiber diameter (mm) 400

Focal (mm) 200

Fig. 3. Electric signals from photodiode.

2. Experimental details

2.1. Equipment

An ORIEL integrating sphere (Fig. 1) is used to measure the

reflected energy during a laser pulse. The diameter of the sphere

is 250 mm.

There is a homogenization of light by multiple reflections.

The homogenization time is 10�9 s. A photodiode, a 1.06 mm

interferential filter and a numerical oscilloscope are used. The

electric signal delivered by the photodiode is proportional to

the reflected energy. The response time of the photodiode is

10�6 s. The sample is in the center of the sphere. The

reflectivity inside the sphere reaches 97% because of a BaSO4

treatment (Fig. 2).

A Trumpf pulsed Nd:YAG laser is used (HL 304P). Its

characteristics are reported in Table 1.

2.2. Principle of measurement

First of all, we make a reference shot IM using a mirror of

which the reflectivity rate RM (at wavelength of 1.06 mm) is

known. Then, we make a laser pulse on the sample and, thus, we

obtain a sample signal IS. Both signals represent the reflected

energy measured by the photodiode.

From the reflected energy we determine the equivalent

absorptivity by formula (1):

A ¼ 100� RMIS

IM

(1)

For example Figs. 3 and 4, respectively, show the electric

signals from photodiode and the evolution of the equivalent

absorptivity over time. A sample of magnesium alloy is

used. The peak power is 1000 W and the pulse duration is

15 ms.

The determination of equivalent absorptivity is identical

everywhere in the paper. We are now going to compare our

measurements with Ignat’s results [1] in order to validate the

method.

Fig. 1. Integrating sphere.

N. Pierron et al. / Applied Surface Science 253 (2007) 3208–32143210

Fig. 4. Equivalent absorption.

Table 3

Laser parameters for RZ5 HAE

Power (W)

1000 500 500 500 500

Pulse duration (ms) 15 20 16 8 4

Energy (J) 15 10 8 4 2

Table 4

Laser parameters for WE43 untreated

Power (W)

2000 2000 2000

Pulse duration (ms) 15 20 16

Energy (J) 15 10 8

3. Validation of the method

In this part, we study the experimental laser absorption of

magnesium alloy RZ5 HAE and WE43 untreated (Table 2).

The laser parameters for magnesium alloys are not sufficient

to reach metal evaporation, so only the liquid phase is

present.

3.1. Experimental details

The laser parameters for measurements are presented in

Tables 3 and 4. The coating of substrate is an anodising process

for RZ5 HAE. The beam diameter is 2 mm at the sample

surface.

The reflectivity rate of the mirror is 80% at 1.06 mm. The

angle of incidence of the beam to the normal of the sample

surface is 08. To compare our measurements with Ignat’s

results, we integrate the equivalent absorptivity at the time of

interaction to obtain a mean value of absorptivity. This

calculation is realized for each pair of parameters.

3.1.1. Spectral analysis and statistics uncertainty

Initially, the electric signals from photodiode are noisy. In

order to verify if the noise is due to electronic or physical

phenomena, a Fourier transformation of the signal is carried out.

Table 2

Substrate additional element content (RZ5 HAE)

WE43 (wt.%)

Y 4.11

Nd 2.28

Dy 0.27

Gd 0.19

Yb 0.09

Zr 0.45

RZ5 (wt.%)

Zn 4.37

La 0.32

Ce 0.78

We take the electric signals from Fig. 3. The spectral

analysis shows that the noise is essentially due to the electronic

equipment. No particular frequency is present (Fig. 5). The

continuous component of the signal is eliminated.

The approximation of white noise is considered. We can

calculate the statistical uncertainty on absorption factor sm(A)

[3,4] from Eq. (1):

smðAÞ ¼ ð100� AÞ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi�sIM

IM

�2

þ�

sIS

IS

�2s

(2)

This formula is used in Fig. 6 to print the error bar with a 95%

confidence interval. sIMand sIS

are noise amplitude average.

The aspect of the mirror noise amplitude is the same as in Fig. 5.

The noises IM and IS are signal amplitude averages. A is the

value of absorption factor from formula (1).

3.1.2. Results and discussions

We present the results from the integrating sphere and the

micro-calorimeter for each case. The order of statistical

uncertainty is 7%.

Fig. 5. Noise amplitude after laser pulse on the sample.

N. Pierron et al. / Applied Surface Science 253 (2007) 3208–3214 3211

Fig. 6. Equivalent absorption for RZ5 HAE.

Table 5

Laser parameters for WE43 Brut

Power (W)

500 750 1000 1250 1500 1750 2000 2500

Pulse duration (ms) 8 8 8 8 8 8 8 8

Energy (J) 4 6 8 10 12 14 16 20

3.1.2.1. RZ5 HAE. The values measured by the integrating

sphere are slightly inferior. However, the shape of the curve is

similar (Fig. 6). The metal reflectance changes with surface

conditions (temperature, surface roughness). It is important to

know that the HAE treatment is not easy to reproduce. The

composition and the thickness of the HAE protective layer can

change according to chemical bath composition. For this

reason, it’s very difficult to obtain the same surface if the

samples are not extracted from the same sheet. The samples

used for the measurement do not have the same surface

conditions. The interaction between the laser beam and the

substrate changes according to the sample. The advantage of an

integrating sphere is that the data acquisition is quick and we

obtain the evolution of data over time.

3.1.2.2. WE43 Brut. For these measurements we use high

power (2000 W) (Fig. 7). For example with 700 W, the

absorbed energy is not sufficient and the reflectivity is higher.

The mirror and sample signals are practically at the same level.

Fig. 7. Equivalent absorption for WE43 Brut (2000 W).

The absorption calculation is difficult because of partial

superposition of signals and noise.

3.1.3. Partial conclusion

We can say that the integrating sphere gives correct results.

We obtain approximately the same absorption values from the

integrating sphere and the micro-calorimeter. Results seem to

be better at high energy. At low energy, it is difficult to

differentiate sample signal and mirror signal. This technique is

therefore validated for measuring the absorption factor. The

acquisition is very fast unlike with the micro-calorimeter. The

precision of the measurement is good according to uncertainty.

We can think that for a keyhole mode, where energy is

important, this method gives good results.

4. Study at high power

4.1. Magnesium alloy WE43

In this part, the laser beam is focused on the surface piece

with a diameter of 400 mm. The angle of incidence of laser

beam is 458 to limit the projection on optics instruments. A

mirror with a reflectivity rate of 100% is used. The laser

parameters are presented in Table 5.

We determine the equivalent absorption for different peaks

of power. We can see the differences according to the evolution

of the keyhole (Fig. 8).

The initial value of the absorption factor depends on the

surface roughness which can act as a light-trap [5]. The

decrease is due to the melting of surface roughness. There is a

Fig. 8. Equivalent absorption for different peaks of power.

N. Pierron et al. / Applied Surface Science 253 (2007) 3208–32143212

Fig. 9. Zoom from Fig.8.

Table 7

Laser parameters for aluminum alloy 1050

Power (W)

1000 1250 1500 1750 2000 2250 2500 2750 3000 3250

Pulse duration

(ms)

8 8 8 8 8 8 8 8 8 8

Energy (ms) 8 10 12 14 16 18 20 22 24 26

Fig. 10. Equivalent absorption for different peaks of power.

stage of a few microseconds that corresponds to the solid-liquid

transition (Fig. 9). When the temperature is high enough to

vaporize the material, the recoil pressure generates a depression

in the liquid and finally a cavity inside the molten pool. The

absorptivity increases because of Fresnel reflections in the

keyhole [6–8]. When the keyhole is stabilized (2500 and

2000 W), the value is maximal (90%) (Fig. 8). For lower peaks

of power (1000 and 1500 W), the keyhole has not reached his

final depth at 8 ms. The evolution of the keyhole is faster for

high power and the recoil pressure is higher.

4.2. Aluminum alloys 1050

The aluminum alloy 1050 is used (Table 6). The peaks of

power are higher to reach the vaporization of the material

(Table 7). Its thermal conductivity and its reflectivity are more

significant. The experimental conditions are the same.

The evolution of the absorption factor is different. The

explanation for initial value is the same as that for the

magnesium alloy. The beginning of the formation of the

keyhole appears later. In the case of magnesium alloy, the

keyhole formation begins at 0.5 ms. For aluminum alloy, it

begins at 1 ms. The keyhole is stabilized at 3250 W (2000 W

for magnesium alloy). The instabilities at the end of interaction

are due to the opening-closing of the keyhole [8]. The mean

value is 70%. For 2000 W there is only a liquid phase. For each

curve, the absorption factor during the liquid phase is around

20% (Fig. 10).

Table 6

Chemical composition (composition limits) for aluminum alloy 1050

Al 99.5 max

Si 0.25 max

Fe 0.4 max

Cu 0.05 max

Mn 0.05 max

Mg 0.05 max

V 0.05 max

Other (each) 0.03 max

If we take the curve for 3250 W, we can see the different

steps of the laser–material interaction (Fig. 11). The same

explanations can be used for magnesium alloy.

Initially the value is around 30%. The duration of the

transition solid–liquid is 466 ms with an absorption factor of

21%. In Fig. 11, we see the melted zone after an interaction time

of 690 ms corresponding to the end of the liquid phase

(Fig. 12a). The different evolutions of the absorption factor are

significant according to statistical uncertainty.

Fig. 11. Equivalent absorption for 3250 W.

N. Pierron et al. / Applied Surface Science 253 (2007) 3208–3214 3213

Fig. 12. (a–c) Mel

At two milliseconds (part 2) the Fresnel reflections inside the

cavity, caused by recoil pressure, increase the absorptivity and

also the depth of the keyhole (Fig. 12b).

The absorption factor increases to reach 70% (part 3) and the

keyhole is stabilized (Fig. 12c).

5. Theoretical considerations

It is possible to determine the theoretical absorption factor

according to the surface temperature of the material, the

thermal conductivity and the change in optical properties

during the interaction laser–material. From the Drude model

[9], we can determine the complex permittivity ec of metal [5],

as well as the refractive and absorption indexes for the radiation

in the metal (n and k).

For a perpendicular incidence, the polarisation of radiation is

not important. But for an oblique incidence, the polarisation

plays a role. In laser welding we consider that the polarisation is

circular. We take the average between the absorption at the

metal surface of linearly polarised radiation directed parallel

(index pa) and perpendicular (index pe) to the surface. The

relations are given by (for n2 + k2� 1) [5,12]:

Apa ¼ 1� ðn2 þ k2Þ cos2 u � 2n cos u þ 1

ðn2 þ k2Þ cos2 u þ 2n cos u þ 1(3)

Fig. 13. Theoretical absorption factor at 458.

Ape ¼ 1� ðn2 þ k2Þ � 2n cos u þ cos2 u

ðn2 þ k2Þ þ 2n cos u þ cos2 u(4)

Ac ¼1

2ðApa þ ApeÞ (5)

where u is the angle of incidence of laser beam to the normal of

the sample surface.

The variation of surface temperature has a great influence

on the values of the absorption factor. The collision frequency

of free electrons increases with temperature. The Debye

model considers the free electrons like a Fermi gas. We can

deduce a relation between the collision frequency and the

temperature:

G ¼e0w2

pL

KiT s (6)

The indexes i correspond to solid or liquid phases of metal.

We only change the value of the thermal conductivity according

to temperature [10,11]. We use this expression to calculate the

complex permittivity. We can deduce the absorptivity of metal

according to surface temperature, metal phase and the angle of

incidence of the laser (Fig. 13).

The initial value of the absorption factor is lower than the

measurement made with the integrating sphere (�30% for

aluminium). In fact the theory is for ideal surfaces without

impurities and roughness. The surface conditions play an

important role for the result. The absorptivity is defined by an

intrinsic absorptivity Ai which depends on optical properties

and an external absorptivity Aext. We can write Aext = Ar-

o + Aim + Aox [5] in which we have, respectively, the

absorptivity due to surface roughness, impurities and oxide

layer. At high temperatures, the difference can be explained by

the presence of the keyhole which increases the value of

absorption factor.

6. Conclusions and perspectives

In the first part, we have proved that the experiment is

reliable as we find the same results as Ignat, apart from small

differences due to the type of experiment. Furthermore, it is

very difficult to obtain the same coating on different samples.

The surface conditions are not identical and the interaction

laser–material is modified. The integrating sphere gives good

results at high power.

ted zone 2 ms.

N. Pierron et al. / Applied Surface Science 253 (2007) 3208–32143214

With laser parameters which allow the formation of a

keyhole, we observe the same evolution for magnesium alloys

and aluminium alloys. In the case of magnesium alloys, the

absorption factor is generally higher. The phenomena during

laser interaction are faster. Moreover at the beginning of

interaction there is always a maximum value and then a

decrease due to the presence of surface roughness.

We reach an absorption factor of 90% for magnesium alloys

and 70% for aluminium alloys. Here, we obtain a temporal

evolution. Because of the theoretical calculation, we can see the

important effect of surface defaults on initial values of

absorptivity. The absorption factor can be modified by

impurities, oxides layer and roughness [5]. Impurities can

produce peaks of absorption.

In future experiments we will study the influence of this

roughness on interaction. In fact we want to modify the surface

conditions by laser surface texturing. The aim is to see if the

initial value of absorptivity is modified and, if so, to what

extent.

In all experiments we can see the solid–liquid transition

and the beginning of the formation of the keyhole. The

absorption factor increases because of multiple reflexions in

the cavity formed by recoil pressure. At present, a calculation

of Fresnel reflexions using the shape of the melted zone, in

presence of a keyhole, is being realized. The aim is to know if

the maximum values found with the sphere (70 and 90%) are

close to reality. In this calculation, we take into account the

value of absorption factor according to the angle of incidence

and the surface temperature as well as the evolution of

thermal conductivity.

Acknowledgements

I would like to thank the CEA, Valduc, France for having put

the integrating sphere at my disposal.

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