As we weld with a small focus distance in the small area, the shielding gaz has new functions : heat removing, optics shielding and liquid metal shielding. Therefore, the gaz usually used in laser welding (He, N2, Ar or mixture) was compared with and without a small percentage of oxygen.The benefit influence of a small percentage of oxygen has been demonstrated for the three aimed points but also an important and benefit effect on the weldshape has been shown, ie with the same welding speed value and the same welding energy value, the weld penetration can be increased from 0,5 mm up to 2 mm, only by cptimization of the gaz composition with a small amount of oxygen). 190 / SPIE Vol. 1502 Industrial and Scientific Uses of High-Power Lasers (1991) 0-8194-0611-2/91 /$4.00 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/23/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

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This work presents the results of investigations in case of laser cladding with a pulsed Nd:YAG laser. We discuss about the experimental parameters in view to realize single clad and particulary the laser data (energy, frequency, pulse duration). The main results are the clad formation beyond an energy threshold and beyond a threshold pulse dwation, the clad behaviour is the same as a clad obtained with a cw laser. Thanks to a cross-section modeling, we compare calculations with experiments and we conclude that the interaction efficiency depends widely on powder spray characteristics.

A comparison of TIG welding and laser welding of INCOLOY 800 is presented under two different aspects: weld pool sensitivity to minor elements and modelling of stresses and temperatures distribution. For hG welding, welding parameters have to be adjusted when changing ofINCOLOY 800 master melt. It appeared that laser welding did not depend on the material chemistry and gave a good reproducibility of the results, whatever is the master melt, with the same parameters. Thermal modelling was developed for a tube-tubesheet configuration. TIG modelling showed a stress localisation for several chemistries, which indicated a risk of cracking at the root of the weld. The modelling showed how it is related both to the temperature of the base material close to the molten pool, and the duration of the process. For the INCOLOY 800, longer the temperature is in the 800-1000°C range, higher the risk of cracking exists. For laser welding, the risk is much lower, because the process is incomparably faster. . INTRODUCTIONIt was observed that for TIG welding, the shape of the welds were not reproducible when changing the master melt and keeping the same process parameters. This behaviour was related to slight differences in the material chemistry, particularly to minor elements percentage. The fact that parameters have to be adjusted with chemical variations of INCOLOY 800, was a major problem from an industrial point of view. It is to be known that this welding process can be used as a repair process in running plants, and that the chemistry ofthe tubes to weld is not often available.The study consisted in a double approach of the problem: first, work on an other technique such as laser welding, to solve reproducibility problems, and second, try to understand why, in a tubetubesheet configuration, INCOLOY 800 is so sensitive to hot cracking. INCOLOY 800 WELDABILITY WITH LASER PRQCESSThe welding process was developed for tube welding configuration. The tubes were 25mm diameter, and 2.6mm thick. INCOLOY 800 is an austenitic steel type (Z5 NCTA 33.21).The TIG study was limited to 8 master melts having a S% and (Ti + Al)% composition included respectively between 0.002% and 0.008% and between 0.46 and 0.68%. The chemistry is given in Annex 1. On the 8 macrographies resulting of TIG welding, the 4 most representative are presented in Annex 2: it is shown how different can be the weld shape with the various kinds of tubes. O-8194-1508-1/941$6.OO SPIE Vol. 2207 / 439 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/22/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

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