Micro welding of stainless steel by pulsed Nd: YAG laser

Transcription

1 Micro welding of stainless steel by pulsed Nd: YAG laser Octavian Dontu *a, Stergios Ganatsios *b, Daniel Besnea *a, Paul Beca *c a POLITEHNICA University of Bucharest, Department of Mechanical Engineering and Mechatronics, Chair of Precision Engineering and Mechatronics; b President TEI KOZANI, University WEST Macedonia Grecia; c National Institute of Research and Development for Precision Mechanics, 6-8 Pantelimon Road, 2 nd District, , Bucharest, Romania ABSTRACT The paper presents the results of the experimental research regarding the microwelding of stainless steel by pulsed Nd :YAG laser (STARWELD PERFORMANCE SWP 6002 ROFIN). The analytical and experimental investigation on laser weldability of a stainless steel allowed to draw the following conclusions. The micro-welds produced by the optimized Nd: YAG laser parameters featured a narrow and regular weld metal zone devoid of any solidification and shrinkage defects. The as weld metal structure was made up of elongated ferrite graints with limited amount of austenite, mainly located at grain boundaries. 1. GENERAL CONSIDERATIONS Due to its advantages (strictly controlled temperature and heating fields, minimal thermal influence in the weld zone, high welding speed without the need of addition material, totally automated process), the use of lasers for the achievement of weld assemblies is continuously increasing, especially in the case of materials difficult to weld by classical methods, as stainless steels and aluminum alloys. Stainless steel parts can be weld using different methods, but it is important to choose the most appropriate one, in order to obtain best performance. It is to be noticed that, regardless the chosen method, the welding implies normally the heating till the melting of the materials to be weld, fact that can lead to structure modifications able to influence the properties of thermally affected materials. It is necessary to select the most appropriate laser generator for the chosen weld type and materials to be welded, in order to dispose of an emission power and a wavelength leading to higher absorption of the laser radiations in the welded material. Experimental research was performed using a YAG:Nd solid active medium laser equipment (adjustable maximum pulse power of kw) produced by ROFIN, model STARWELD PERFORMANCE SWP The equipment allowed the easy obtaining of high quality welding using different metals. Figure 1 presents the plant used in experiments.. Fig. 1 The SWP 6002 laser equipment (ROFIN). INDLAS 2007: Industrial Laser Applications, edited by Mircea Udrea Proc. of SPIE Vol. 7007, 70070O, (2008) X/08/$18 doi: / SPIE Digital Library -- Subscriber Archive Copy Proc. of SPIE Vol O-1

2 The plant allowed the manual or automated (CNC) achievement of a high variety of welded assemblies (the weld outline can materialize any curve) for a high range of materials. The experimental research intended to obtain laser welded assemblies for various stainless steels, the materials to be welded being positioned end-to-end (fig. 2), by superposing sheets (fig. 3) or at right angle (90 0, fig. 4). The rigorous choice of welding parameters, especially energy and duration of laser pulses, leaded to the generation of high thermal gradients in the parts to be weld without a significant loading of the material next to the weld. Fig. 2. End-to-end welding ' iii n-cy.y.. Fig. 3 Scheme of welding by superposition Fig.4 Angle welding Proc. of SPIE Vol O-2

3 Figure 4 presents a YAG:Nd laser weld assembly of two stainless steel sheets positioned end-to-end. The following parameters were used: - medium power: P med =51 W; - energy/pulse : E=8.6 J; - pulse duration t=8 ms; - shooting frequency: F r =6 Hz; The preparation of materials to be welded, their positioning and the direction of action of the laser beam are very important in the case of right-angle welding, as shown in fig. 5. Laser be am Fig. 5 Angle welding scheme (transversal section) The high thermal gradients produced by the laser in the weld area lead subsequently to the quick cooling of melted metals bath. The effect was the apparition of solidification structures presenting favorable metallographic components that increased the strength of the laser weld assembly [1]. Laser welding tests were performed for various qualities of stainless steel of type X Cr Y Ni Zmo. The composition indexes presented the following values: X = 25; 23; 22; 18 %, Y = 12; 8 ; 7 ; 5 ; 4 %, Z = 4; 3; 0 % and 301L. The following stainless steels were welded: 25Cr-7Ni-4Mo; 22Cr-5Ni-3Mo; 23Cr-4Ni; 25Cr-8Ni a.s.o. 2. RESULTS AND DISCUSSIONS Experimental research proved how to adjust work parameters (power, duration of action/ displacement speed, manner of focalization of laser beams) in correlation with the properties and features of materials to be welded, specifically the depth of metal melting without producing the vaporization of surface material [5]. Fig. 6 presents the optimal adjustment range of the laser power density and duration of action, in correlation to certain melting depths of materials to be welded. Proc. of SPIE Vol O-3

4 Melting depth (nun] Laser duration of action [s] Fig. 6 Optimal adjustment range of the laser power density and duration of action, in correlation to certain melting depths of materials to be welded From the experiments performed on stainless steels, the authors selected only the probes of interest for the field of mechatronics. Fig. 7 and 8 present the welds resulted from the assembly of two stainless steel sheets welded using a YAG : Nd laser. Fig. 7 Fig. 8 For the probe presented in fig. 8, it can be noticed that, due to the difference of thickness between the two materials, of 3 mm, respectively 0.5 mm, a YAG: Nd laser with pulse emission pw with τ = 28 ms and t p = 16.8 ms was used in order to compensate the asymmetry of heat dissipation in the two bands. The probes, made of stainless steel 18 Cr 8 Ni, having a high percentage of carbon (0.08 %), present the risk of higher sensitivity to intercristalline corrosion in the resulted weld, partially eliminated due to the pulse emission of laser pulses. The most frequent defects of the weld are presented in continuation, the causes of their apparition being specified as well. Proc. of SPIE Vol O-4

5 Some imperfections of the weld (welding by superposition of two 301L stainless steel sheets) caused by the lack of correlation among the welding speed, the emission frequency of laser pulses and the energy, as well as by the absence of protection gas, are presented in fig. 9. Fig. 9. Welding by superposition of two 301L stainless steel sheets Fig. 10. The presence of impurities in a weld obtained by end-to-end welding of two stainless steel plates (301L) The same causes lead to the defects that can be noticed in fig. 10 (stainless steel 301L). The measures to be taken to avoid such defects regard the correlation among the energy of laser pulses, their frequency and the displacement speed of probes in front of the focused laser beam. Proc. of SPIE Vol O-5

6 3. CONCLUSIONS The experiments performed on the SWP 6002 ROFIN plant confirmed the high versatility of the laser equipment, which allowed very accurate adjustments of the work parameters, as well as the preference for the automated manner of processing (connection of the CNC system) in order to obtain good quality welds, rigorously correlated with the frequency of laser pulses and the displacement speed of parts in front of the laser beam. All probes feature a very good quality of the weld and a reduced thermal influenced area compared to other weld procedures able to be used for this range of assemblies and materials to be welded. The hardness of laser welds was established using a Vickers micro-hardness testing machine with a load of 500 grams. The results are synthesized in fig a ISO IOU SO 0 ;ii ft_ -r, - Disthnce fio'n weld center linmi Fig. 11 It can be noticed that micro-hardness increased with about 100 units in the weld area, as effect of thermal modifications produced by the laser, which leaded also to changes of metallographic constituents, as proved by the research of the obtained metallographic structures. These results will be presented in another paper. REFERENCES 1. O. Dontu, a.s.o.., The laser welding of some stainless steels used in process plants in chemical industry (in Romanian), Romanian Review of Chemistry, No. 3, Cedric MAS, These de doctorat de l universite Paris, Modelisation phisique du procede de decoupe de metaux par laser, Carosena M., Analysis of stainless steel welded joints : a comparison between destructive and non-destructive techniques, Journal of Materials Processing Technology, Didier BOISSELIER, Olivier FRENEAUX, Jean-Paul GAUFILLET, Le soudage laser des faibles et moyennes epaisseurs, Publication CETIM, O. DONŢU, Laser processing technologies (in Romanian), Technical Publishing House, Bucharest, Zhang Li., Autogenous laser welding of stainless steel to free-cutting steel for the manufacture of hydraulic valves, Journal of Materials Processing Technology, Laser plant ROFIN model SWP 6002, manufactured by ROFIN. Proc. of SPIE Vol O-6