Laser welding of light alloys and superalloys

Laser welding of light alloys and superalloys

Robotics & Computer-Integrated Manufacturing, Vol. 4, No. 1/2, pp. 225-231, 1988 0736-5845/88 $3.00 + 0.00 Pergamon Press plc Printed in Great Brita...

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Robotics & Computer-Integrated Manufacturing, Vol. 4, No. 1/2, pp. 225-231, 1988

0736-5845/88 $3.00 + 0.00 Pergamon Press plc

Printed in Great Britain.

• Paper

LASER WELDING OF LIGHT ALLOYS AND SUPERALLOYS G. RICCIARDI, M. CANTELLO and F. G. MICHELETH Institute per le Ricerche di Tecnologia Meccanica e per l'Automazione, Torino, Italy The chemico-physical characteristics of aluminum cause considerable difficulty in the welding of this material. The high level of specific power and the restricted area involved put laser technology in the position of being most promising for resolving these difficulties. This paper discusses the welding of aluminum using laser beam technology and presents the most significant experimental results.

1. INTRODUCTION The physico-chemical characteristics of aluminum call for a prior evaluation of laser phenomenology when this material is to be processed by laser. To this end, the following presents some considerations which provide a summary of the field.

material, besides contributing to the melting of the surrounding material, exerts the necessary pressure to keep it melted. In steels the outflow of a certain amount of vaporized material takes place in a perfectly balanced way, without significantly disturbing the process. In the case of light alloys (for which there remains the need to operate by depth penetration) the vaporization of the material, the low temperature of transformation allied to the weaker molecular tension make it far more difficult to maintain the dynamic balance of the "key-hole" process. It is therefore necessary to operate within a range of specific power sufficient to vaporize the material without generating violent boiling which could cause excessive eruption of melted material.

Reflectance The surface of aluminum in its natural state at room temperature reflects almost all incident laser energy. The absorbed energy however, will gradually increase the surface temperature. As the surface temperature rises, so does the ability of the material to absorb energy. Thus there is a relation between temperature and absorption: the processes of melting and vaporization of the material stabilize at levels which continue until they cannot be determined in advance but are in any case higher than those desired. There is a need to maintain a constant absorption coefficient the value of which depends on the desired depth of penetration. This may be attained by reaching the pre-established level of absorbed energy immediately and then by maintaining it at a constant level, with an appropriate ratio between the work speed and the rate of heat diffusion in the material. A system based on this approach is described later.

Oxidization It is well known that the presence of oxides causes structural defects in the weld and which significantly reduces its mechanical properties. Such surface oxidization in aluminum demands that particular measures be adopted to eliminate oxides present before the welding process and to prevent their formation during welding.

Alloying elements For alloys, both the type and percentage of the elements present can have a significant bearing on the results of the welding. The most common elements are Mg Si Mn Zn and Cu. According to the percentage of each present, they can give rise to chill cracks, segregations, etc.

Penetration Laser penetration welding is based on the possibility (due to the high specific power) of vaporization of the material instantaneously, to form a so-called key-hole cavity. The expansion of the vaporized 225

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Fig. 1. Key-holeformationduring the laser weldingprocess. In welding using traditional methods, these defects are overcome by using filler metals of suitable composition. Laser welding is normally carried out without filler metals and the consequent defects, identified above, are not so easily eliminated. A measure to prevent the formation of "chill cracks" is that of operation at high speed, although this can induce the segregation phenomenon. It is possible to find a correct balance, but only through experimental tests.

Porosity due to hydrogen One of the most frequent causes of porosity in aluminum is the presence of hydrogen (dirt, oils, humidity) in the environment. The passage of the material from the solid state to the liquid state is a cause of the diffusion of gas in the metal. The passage from the liquid state to the solid state is so rapid that the gas does not have time to come out of the mold, and thus remains trapped in the form of bubbles which constitute the aforementioned porosity. Of paramount importance in this regard are preventive cleaning and effective covering with inert gas. 2. WORK PROGRAM AND EXPERIMENTAL TESTS

In this preliminary approach to laser welding of aluminum, the choice has been to operate on a single type of alloy considered to be weldable by traditional systems. The aluminum base alloy chosen for the tests was 6061, which has the following compposition: aluminum base, Si = 0.6%, Cu = 0.27%, Mg = 10% and Cr = 0.20%. The use of an alloy considered to be weldable using traditional systems, allowed for a valid comparison with results using

laser technology. With the intention of facing and overcoming the problem of surface reflectance, three conditions were considered: sandblasted surfaces, pickled surfaces and chamfered edges. Recovery of the reflected energy can be accomplished by inclining the incidence planes of the beam. To minimize the negative influence of surfacecontaminating agents, all grease was carefully removed from the samples using suitable solvents and the samples were deoxidized by mechanical brushing. In the subsequent welding phase three types of cover gas were tried (argon, helium, nitrogen), which are considered most effective for the protection of the melted bath. Finally, to consider the effect of depth on the delicate dynamic balance of the key-hole three levels of thickness were employed to study the possible limits of the process. The tests began by performing laser passes at full penetration. With this procedure, it was possible to evaluate immediately the effect of some boundary conditions. In particular, the factors under consideration in this phase were surface coating, cover gas and focal point position. The operating conditions which were determined from the penetration tests, were utilized in the subsequent welding tests. The presence of the chamfer, obtained by simple manual scraping of the edges, increased laser beam absorption considerably a significant increase in work speed for the same power supplied. Table 1 indicates optimal operating parameters obtained for the different test conditions, and with the following general characteristics: • laser source, Spectra-Physics 2.5 kW • focal length of the lens, 5"

Laser welding of alloys • G. RICCIARDIet al. Table 1. Laser parameters for different thicknesses Thickness (mm) 1.0

Alloys 6061

1.5 3.0

Power (kW)

Speed (m/min)

2.0 2.3 3.0

5.0 4.5 3.0

• cover gas, nitrogen • diameter of primary coaxial nozzle: 2.5 mm; flow rate Q = 80 1/min • secondary nozzle, Linde type; flow rate Q = 40 1/min;

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focal point position: for thickness of 1 m m and 1.5 mm, at 0.5 mm below the surface of the specimen; thicknesses of 3 mm, at 1 mm below the surface of specimen. The welding performed was examined as follows: E x a m i n a t i o n o f appearance. As illustrated by the photographs in Fig. 4, the seam is regular throughout its length, both in the front and the reverse; lack of material, due to chamfering, causes a slight grooving of the seam on the front; this can be ameliorated

Fig. 2. Spectra-Physics2.5 kW laser.

Fig. 3. Laser welding process on aluminum.

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Robotics & Computer-Integrated Manufacturing • Volume 4, Number 1/2, 1988

i!il

!i!Jil

~e

Front side (facing the Laser beam]

Reverse side Fig. 4. Alloy 6061 laser welding: width of seam.

by the presence of the copper support underneath the pieces, which prevents dripping.

be overcome by the use of filler metal of suitable quality and quantity.

X-Ray examination. X-Rays show the presence of porosity and some marginal irregularities in the seam; the porosities, even if not excessive, are evidence of imperfection in the procedure. A more accurate investigation of the nature of porosities will make it possible to find the necessary remedies to reduce their presence to standard requirements.

Tensile stresses (Fig. 6). In the tensile tests, all the specimens failed in the weld except for those of a thickness of 1 mm, in which the break took place in the base material. This is due to the fact that the 1 mm material has previously undergone annealing, and upon welding, it upon has induced into the area a hardened structure, which is mechanically more resistant. The ultimate tensile stresses, which have been measured and have give favorable results are those indicated in Table 2, In general the resultant ulti-

Macro- and micrographs. The macrographs of Fig. 5 show the geometry of the section in addition to porosity present in the weld: the defects found can

Laser welding of alloys • G. RICCIARDI et al.

Thickness: 3ram

Thickness: 1.5 mm

Thickness: I mm

Fig. 5. Macrographs of welding of AL 6061.

I.Omm

I.Smm Thickness

:3.0 mm

Fig. 6. Laser welding of 6061 alloy: tensile tests.

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Robotics & Computer-Integrated Manufacturing • Volume 4, Number 1/2, 1988 Table 2. Tensile tests for different thicknesses Alloy 6061

Base material

Thickness 1.0 (R)

Weld

90 N/mm 2

150 N/ram 2

1.5 mm TN TA*

300 N/mm 2 300 N/mm 2

250 N/mm 2 280 N/mm 2

3.0 mm TN TA*

300 N/mm 2 300 N/mm 2

250 N/mm 2 280 N/mm 2

*Sample which has undergone heat treatment TA before tensile tests: TN, hardening and natural ageing; TA, hardening and artificial ageing; R, annealing state.

mate tensile stress is on average around 10% less than the ultimate tensile stress of the base material. The break for thicknesses of 1.5 and 3.0 mm always occurred on one of the two border-lines between the melted area and the unaffected metal, and arose from the indentations alongside the seam. This means a smaller area of seam; consequently, the structure of the seam itself maintains a resistance comparable to that of the base metal despite the presence of pososities. 3. SUPERALLOY LASER WELDING 3.1. Introduction Superalloys are normally employed in the construction of parts which must resist corrosion and high operating temperatures. When the usual tech-

niques of welding are used on these materials the most frequent problem is the formation of segregations; these cause a deterioration in the mechanical, chemical and thermal resistance characteristics. The use of laser generating limited and controlled thermal energy, makes possible the welding of greater thickness with minimum extensions of the melted area and consequent reduction in the segregations. 3.2. Experimental welding tests Before performing welding, a series of complete penetration tests were carried out on different thicknesses using an A V C O laser of 15 kW (Fig. 7), with the aim of verifying how the operating parameters affect the morphology of the seams.

Fig. 7. AVCO 15 kW laser.

Laser welding of alloys • G. R1CCIARDIet al. Examining the sections of the seams obtained under different conditions has confirmed that the increase in speed (within the tested values and always allowing complete penetration) improves the ratio (penetration depth/average width of the weld), while maintaining the structural characteristics of the seam. The welding test employed a single thickness for each particular material; for IN 601, two levels of power and speed (Table 3) were used. Table 3. Laser parameters for different welding parameters of superalloy IN 600 10 mm IN 601 Thickness 4 mm Power S p e e d P o w e r S p e e d P o w e r Speed (kW) (m/min) ( k W ) (m/rain) ( k W ) (m/min) 8

1.3

11.5

5.9

15

7.0

Preliminary metallurgical and mechanical qualification analyses were carried out in conjunction with the Institute of Industrial Chemistry, University of Padova. By means of X-rays and' penetrating liquids it was possible to observe in IN 601 the absence of microcracks and the reduced dimensions of porosities. The IN 600 exhibited instead, the presence of interdendritic microcracks. For both materials the evaluation of hardness revealed the presence of an area outside the seam having a hardness superior both to the base material and to the welding area. The mechanical characterization was carried out with tensile tests; the break always occured in the base material with the following loads: 55 kg/mm 2 for Inconel and 58 kg/mm 2 for Inconel 601.

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4. CONCLUSIONS The welding of light alloys and superalloys with laser technology raises numerous doubts and considerations. Some of these issues were studied experimentally. The case where both oxidization and hydrogen contamination took place required particular measures of preventive cleaning, as well as adequate insulation during welding. Knowledge of the difficulties involved made it possible to conduct the tests using appropriate measures to minimize detrimental effects. The results, although incomplete, have been very encouraging and give reason to hope that in the future it will be possible to improve on the processing technology still further. REFERENCES 1. La Rocca, A.V.: Laser applications in machining and material processing. European Conference on Optical Systems and Applications. Brighton, U.K., April 1972. 2. Locke, E.V., Gnanamuthu, D., Hella, R.A.: High Power Lasers for Metalworking. Avco Everett Research Lab., 1974. 3. Micheletti, G. F.: Introduzione alle tecnologie del laser. Tec. J. RTM, No. 21, Nov. 1975. 4. Ready, J.F.: Industrial Applications of Lasers. New York, Academic Press 1978. 5. Masory, O., Koren, Y., Schachrai, A.: CNC systems for a laser beam cutter. CIRP Conference, Haifa, Israel, July 1978. 6. Schachray, A., Castellani Longo, M.: Application of high power lasers in manufacturing. Ann. CIRP 38 1979. 7. Micheletti, G.F.: Laser di potenza: panorama introduttivo. International Conference on Industrial Applications of Laser, Ivrea, 3-4 April, 1979. 8. Ricciardi, G., Cantello, M.: Welding and heat treatment of some difficult materials by laser beam. Ann. CIRP 32 1983.