Spectroscopic comparison of aluminium welding plasmas produced by high power CO and CO2 lasers

Optics

&

Laser Technology,

Vol. 28, No. 5, pp. 405-407,

Copyright Printed

0

in Great

1996 Elsevier Britain.

All rights reserved

0030-3992/96

ELSEVIER ADVANCED

1996

Science Ltd $15.00+0.00

0030-3992(95)00114-x

TECHNOLOGY

Technical

note

Spectroscopic comparison welding plasmas produced power CO and CO2 lasers M. SCHELLHORN,

of aluminium by high

A. EICHHORN

Welding tests on the aluminium alloy AlMgSil (6082) by the use of a high power CO laser with good beam quality show higher penetration depths and better weld seam quality compared with the results obtained with a commercial industrial CO2 laser. Spectroscopy of the laser-induced welding plasma shows a strong decrease of the intensities of AI lines and no appearance of AI(III) lines in CO laser aluminium welding compared with CO2 laser welding at the same process parameters. This is a consequence of the shorter 5 to 5.6 pm wavelength of the CO laser leading to reduced beam-plasma interaction. Copyright @ 1996 Elsevier Science Ltd. KEYWORDS:

lasers (CO, CO*), aluminium

laser welding,

Introduction

plasma spectroscopy

interaction due to the shorter of the CO laser.

Laser welding of aluminium is a major research subject in industry. It is well known that the processing range in CO2 laser welding of aluminium alloys is small’.2. The maximum usable process intensity is limited by the onset of a statistically appearing shielding plasma leading to weld defects, such as porosities and crater formation. Since the plasma absorption coefficient for the laser radiation is proportional to the square of the wavelength, the use of an Nd : YAG laser seems to be appropriate but it suffers from an inherent bad beam quality. The CO laser with its wavelength around 5 urn offers an attractive compromise between the attributes of CO2 and Nd : YAG lasers, particularly through its potential for very high power and efficiency’, possibly high power transmission through chalcogenide glass fibres4 and improved processing5.

Aluminium

welding

wavelength

(5 to 5.6 urn)

experiments

Welding experiments on AlMgSil targets (20 mm in width, 55 mm in length and a depth of 6 mm) have been carried out by the use of a laser welding head which is formed by a plane and a 90” off-axis parabolic mirror with 125 mm focal length. This welding head has been used for both CO and CO2 lasers. In the case of the CO laser we also used a CaF2 lens with a focal length of 200 mm. Helium is used as a process gas at a flow rate of 30 1 min’. A copper tube of 4 mm inner diameter produces a tangential process gas flow opposite to the weld direction. Figure 1 shows the penetration depth as

1

Recently, we developed a gas-dynamically cooled CO laser6 with an unstable resonator with a magnification of A4 = 2. The laser beam is 1.3 times diffraction limited at a power level up to 4.5 kW. Aluminium welding tests show higher penetration depths and better weld seam quality compared with the results obtained with a commercial industrial CO;! laser. Spectroscopic results of the laser-induced welding plasma presented in this note show a strong decrease of the intensities of Al(H) lines and no appearance of Al(II1) lines, as in the case of COz laser aluminium welding. This shows that the temperature and the electron density must be smaller, which can be explained by a reduced beam-plasma

CO laser f=200 mm p-pol. 0 - Nd-YAG laser 0.6 mm fibre CO, laser f=125 mm p-pol.

0 ‘0:

working gas: helium

L z f’P material: AlMgSil

0 0

I 2

I 4

I 6

I 8

10

Weld speed (mlmin) The authors are in the Deutsch-Franz6sisches ISL, F-68301, Saint Louis, France. Received Revised 15 November 1995.

Fig. 1 Welding depths in AIMgSil function of feed velocity at a power CO, COP and Nd : YAG lasers

Forschungsinstitut 29 August 1995.

405

(6082) aluminium alloy as a level of 2.2 kW obtained with

406

Spectroscopic

comparison

of aluminium

welding plasmas: M. Schellhorn and A. Eichhorn spectrograph by means of fibre optics with a core diameter of 200 urn. The centre of the interaction zone at the target surface was imaged by a 1 : 1 optic to the entrance of the optical fibre at an angle of 30” with respect to the target surface.

5500

5550

5600

5650

Wavelength

5700

5750

5700

5750

(r$)

1

-I 5500

5550

5600

5650

Wavelength

(A)

Fig. 2 Spectra of AlMgSil (6082) welding plasmas at a power level of 2.5 kW and a feed velocity of 6 m min-’ in the case of (a) CO* and (b) CO laser welding

a function of weld speed at a power level of 2.2 kW for CO and CO2 lasers. For comparison there is also shown the penetration depth obtained with an Nd : YAG laser system taken from the literature7. The welding depths are greater with the CO laser. This is a consequence of the smaller focal spot size of the CO laser (150 and 240 urn diameter when using f= 125 and ,f= 200 mm optics, respectively) compared with the CO* laser (290 urn) and the Nd : YAG laser (300 urn). Thus, the intensity in the focus is higher with the CO laser. The most important result is the good quality of the CO laser weld seams characterized by a homogeneous and smooth surfaces9, especially at high feed velocities. The surface quality of all the CO2 laser aluminium weld seams is not good because of melt ejection and crater formation resulting from a statistically appearing shielding plasma. Owing to the shorter wavelength of the CO laser the beam-plasma interaction is drastically reduced and thus smooth weld seams are obtained. Thus, the process parameter range is significantly extended, which is important for industrial laser applications. To prove this assertion, spectroscopic investigations of the laser-induced aluminium plasma have been carried out in CO and COZ laser welding. Spectroscopy

of aluminium

laser

Spectra of aluminium laser welding plasma are taken at a power level of 2.5 kW and a feed velocity of 6 m min-’ for CO2 and CO laser welding of AlMgSil by the use off= 125 mm optics. The peak intensity with the COz laser is measured with a beam diagnostic system (Prometec) to be 4.8 x lo6 W cmp2. In the case of the CO laser the peak intensity is higher; that is, of the order of lo7 W cm-*. Because of the damage of the needle used by the beam diagnostic system, it was not possible to measure the intensity directly. Figures 2(a) and (b) show time-integrated spectra of the plasma in CO1 and CO laser welding, respectively. In the CO2 laser plasma strong Al(I1) lines and also Al(II1) lines are observed. However, in the CO laser plasma the intensity of the Al(I1) line is drastically reduced and there is no appearance of Al(II1) lines. The electron density and temperature in plasmas can be estimated by the Starkbroadening method and by the relative line intensities of subsequent ionization stages of the same element described in detail by Griem”, assuming local thermodynamic equilibrium. In the case of the CO2 laser spectrum we obtain an electron density of 4 x lOI cmm3 by using the calculated line broadening parameters” of the Al(I1) line at 5593.23 A. With this value of the electron density we estimate from the line ra$o method of the Al(II1) lines (5696.47 A and 5722.65 A) and the Al(I1) line at 5593.23 A a plasma electron temperature T,-‘/’ of 1.5 eV ( _ 17 500 K). The relative density of aluminium atoms and ion species as a function of temperature is shown in Fig. 3 by solving the Saha equation. In this temperature range the Al(I1) and Al(II1) are the dominant ion species, which is consistent with the experimental observation. The different behaviour of the ionized aluminium lines in the CO laser induced welding plasma (no Al(II1) lines, Al(I1) line intensity strongly reduced) shows that the temperature and the electron density must be smaller, as in the case of CO2 laser aluminium welding. Since the plasma absorption coefficient is approximately proportional to T,- 3/2, the square of the electron density

plasmas

Time-integrated spectra were recorded by means of a grating spectrograph (SPEX 500) with a 600 grooves mm-’ grating and a modified CCD cameralo. The spectra were taken with a total accumulation time of about 1 s, thus smoothing short fluctuations of the line intensities. The apparatus width is about two pixels corresponding to 1 A. The light was fed into the

5000

10000

15000

20000

Temperature

25000

30000

(K)

Fig. 3 Relative density of electrons, atoms and ion species as a function of temperature in an aluminium plasma calculated by solving the Saha equation

Spectroscopic

comparison

of aluminium

welding plasmas: M. Schellhorn

and the square of the laser wavelengthI the beamplasma interaction by the effect of inverse bremsstrahlung is reduced in CO laser welding. This is the reason for the better weld seam quality obtained with the CO laser. Owing to the simple and compact design of the CO laser head and the better aluminium welding results it is an attractive candidate for an industrial high power laser system. Summary In summary, welding tests on the aluminium alloy AlMgSil (6082) by the use of a high power CO laser with good beam quality, show higher penetration and better weld seam quality compared with the results obtained with a commercial industrial CO2 laser. Spectroscopy of the CO and CO;! laser induced aluminium welding plasma has been carried out. In the case of the CO* laser, an electron density (-4 x 1016cmP3) and a temperature of w 1.5 eV of the laser-induced welding plasma are estimated from the Stark-broadening of an Al(I1) line and the intensity ratio of Al(I1) and Al(II1) lines. In the case of CO laser aluminium welding the intensities of Al(I1) lines are strongly reduced and there is no appearance of Al(II1) lines. From this fact we conclude that the temperature and the electron density and thus the plasma absorption coefficient must be smaller, as in the case of COz laser

and A. Eichhorn

407

aluminium welding. This is the reason for the better weld seam quality obtained with the CO laser. References Shen, G., Roth, G., Maisenhiilder, F. Luser und Optoelekrronik, 25 (1993) 96 Schiifer, R., Behler, K., Beyer, E. Pror. of the 9th In! Congress. LASER 8Y, Springer Verlag, Berlin (1989) 544 Dymshits, B.M., Ivanov, G.V., Mescherskiy, A.N., Kovsh, LB. SPIE Proc, 2206 (I 994) IO9 Sato, S., Igarashi, K., Taniwaki, M., Tanimoto, K., Kikuchi, Y. Appl Phvs Lert, 62 (1993) 669 Maisenhiilder, F. Proceedings of the lnternu~ional Conference of Laser Advanced Material Processing (LAMP), Vol. I, Nagaoka, Japan (High Temperature Society of Japan, Osaka, 1992). 43 Schellhorn, M., Btilow, H.v. Opr Left, 20 (I 995) 1380 Brite-Euram II BE 7997. Welding of advanced materials with high power cw and pulser-sustained Nd-YAG lasers Schellhorn, M. Mecanique et Optique, Societe Fran$aise des Mechaniciens, 39-41 rue Louis-Blanc, F-92400 Courbevoie (1995) 355 Schellhorn, M. Application of a high power CO laser in aluminium welding, 5th Int Conf Industrial Lasers and Applications ILLA’95, 24-26 June, 1995, Moscow Region, Shatura, SPIE Proc, 2713 (1995) 287 Eichhorn, A., Werner, U., Mach, H., Masur, H. ISL-Report 120/94, B.P. 34, 68301 Saint Louis Cedex, France (1994) Griem, H.R. Plasma Spectroscopy, McGraw-Hill, New York (1964) Mulser, P., Sigel, R., Witkowski, S. Physics Reports, Section C qf Physics Letters, 6 (1973) I87

Optics & Laser Technology