Miikka Karhu

Welding of thick austenitic stainless steel using Nd:yttrium-aluminum-garnet laser with filler wire and hybrid process

Autogenous laser welding has shown many advantages over traditional welding methods in numerous applications. However, there could be even more applications, but due to the power levels of present high power lasers, depth of penetration is limited. One way to overcome this limitation is to use multipass laser welding, in which a narrow gap and a filler wire are applied. By this process thick sections can be welded with a smaller heat input and therefore with smaller distortions, and the process seems to be very effective compared to “traditional” welding methods. Another way to increase penetration and fill the groove is by using the so-called hybrid process, in which laser and GMAW are combined. In this study thick section austenitic stainless steel is welded using a multipass technique with filler wire, and also by utilizing a hybrid process. For narrow gap conditions, groove angles of 8°, 10°, and 12°, are used with a partially grooved V joint. Parameters (e.g, filler wire feeding, placement of wire, a...

Optimization of parameters in hybrid welding of aluminum alloy

Numerous advantages of hybrid welding, in which laser beam and arc has combined, over autogenous laser welding has been reported. Especially in case of inaccurate joint preparation or fixturing of the plates to be welded because of the filler metal added to the process through MIG-welding. Also additional heat, coming from the arc to the process, enables higher welding speed and deeper penetration. Aluminum alloy (AlMg3) was used in the experiments. Welding was carried out by using the hybrid process (combination of Nd:YAG- and MIG-welding) in the flat position. The joint preparation was carried out as shear cut and different gap widths were used. Welding experiments were made systematically using a statistical experiment procedure called TAGUCHI-method. Parameters, for example alignment of point of arc and laser, varied in experiments. Also characteristic parameters of both welding methods were changed according to the experimental procedure. In this paper results of welding experiments are reported as well as parameters used. A phenomenona of the hybrid process with aluminum is discussed and also reasons for weld defects occurred are pointed out.

Solidification Cracking Studies in Multi Pass Laser Hybrid Welding of Thick Section Austenitic Stainless Steel

Although the austenitic stainless steel grades are commonly considered to be quite easily weldable, there are certain applications which make exception to the above statement. As an example, which has also attributed to this study, it could be mentioned a welded assembly which forms a very rigid structure and has welds with fully austenitic microstructure. It is generally known that hot cracking of austenitic stainless steel during welding is very much coupled to chemical composition and the strains formed during solidification stage of the weld. The level of strains is dependent on e.g. a groove design, used welding parameters and the rigidity of the structure to be welded. The object of this study was to evaluate the hot cracking susceptibility of specially customized ITER-grade heat AISI 316 L(N)-IG austenitic stainless steel when it is being narrow gap multi pass laser hybrid welded under very rigid conditions, similar to occur in very massive components assembly. During the study the self restrained-type test system was developed and further tested with multi pass welding trials. The test welds were carefully visually inspected and evaluated during the welding trials and further evaluated with macroscopic and microscopic examination from the etched weld cross-sections. The test system consists of a very rigid clamping system and specially designed 60 mm thick test piece configuration which was planned to be rigid as itself. The scope of the study did not include strain measurement during testing. However, results of welding tests showed that the developed test set-up can produce critical conditions to promote hot cracking in produced test welds. The test set-up is described and the results of hot cracking tests are reported. The effect of different important factors on the risk of hot cracking susceptibility, such as shape of weld cross-section, chemical composition of the used parent/filler material, dilution and prevailing microstructure of weld metal, is discussed as well.

Study of filler metal mixing and its implication on weld homogeneity of laser-hybrid and laser cold-wire welded thick austenitic stainless steel joints

An austenitic stainless steel AISI 316L grade base material with butt joint thickness of 15 mm and different root gap preparation was single pass welded using both laser-GMA hybrid and laser cold-wire feeding process. The aim was to be able to study the filler metal mixing in thick section laser welding and the effect of processing parameters on it. Particular interest was to examine how homogenous filler metal mixing is across the whole weld cross-section from the weld surface to the root portion. Avesta 2205 duplex stainless steel filler wire was used in order to produce an over-matched weld metal composition differing from the base metal. Chromium was used as a trace- element as its distribution and quantity within the weld cross-section was identified using SEM and EDS analysis. Moreover, micro- and macroscopic metallography was carried out to examine emerged weld microstructures and their features. In the technical paper the effect of influencing parameters on the amount and homogeneity of filler metal mixing such as root gap preparation, wire feeding orientation (leading vs. trailing) and welding process (hybrid vs. cold wire) are compared and discussed.An austenitic stainless steel AISI 316L grade base material with butt joint thickness of 15 mm and different root gap preparation was single pass welded using both laser-GMA hybrid and laser cold-wire feeding process. The aim was to be able to study the filler metal mixing in thick section laser welding and the effect of processing parameters on it. Particular interest was to examine how homogenous filler metal mixing is across the whole weld cross-section from the weld surface to the root portion. Avesta 2205 duplex stainless steel filler wire was used in order to produce an over-matched weld metal composition differing from the base metal. Chromium was used as a trace- element as its distribution and quantity within the weld cross-section was identified using SEM and EDS analysis. Moreover, micro- and macroscopic metallography was carried out to examine emerged weld microstructures and their features. In the technical paper the effect of influencing parameters on the amount and homogeneity of filler met...

Laser-GMA hybrid welding of direct quenched steel in lap joint configuration - A preliminary study

Welding experiments using laser-GMA hybrid welding of lap joint between 6.5 mm thick direct quenched Ramor 500 grade protection steel and 2 mm thick Optim 500 MC construction steel plates were carried out. The filler materials used were Union Thermanit X96 and Esab OK Autrod AISI 316LSi wire. Direct quenched Ramor 500 grade steel can be used e.g. as an armour plating applications because it possess excellent ballistic properties combined with high strength and hardness [1]. However, joining of above material by welding can be challenging because of local softening at heat affected zone regions. The aim was to evaluate feasibility of hybrid welding process in joining point of view and to evaluate base and filler material weldability concerning e.g. hardness distribution across the weld cross-section. Also dilution assessments were made using EDS measurements in order to reveal how base and filler material is distributed across the weld cross-section.Welding experiments using laser-GMA hybrid welding of lap joint between 6.5 mm thick direct quenched Ramor 500 grade protection steel and 2 mm thick Optim 500 MC construction steel plates were carried out. The filler materials used were Union Thermanit X96 and Esab OK Autrod AISI 316LSi wire. Direct quenched Ramor 500 grade steel can be used e.g. as an armour plating applications because it possess excellent ballistic properties combined with high strength and hardness [1]. However, joining of above material by welding can be challenging because of local softening at heat affected zone regions. The aim was to evaluate feasibility of hybrid welding process in joining point of view and to evaluate base and filler material weldability concerning e.g. hardness distribution across the weld cross-section. Also dilution assessments were made using EDS measurements in order to reveal how base and filler material is distributed across the weld cross-section.

Narrow gap hybrid welding of thick stainless steel

Hybrid welding, in which laser and some arc welding method work together, has been introduced to be effective joining method. Hot spot, made by laser to the joint, has been reported to stablize the arc and change the way of material transfer in arc. Hot spot is also guiding the arc towards to it. This phenomenon has been utilized in this study in order to get the process to be happened inside a very narrow groove. In this way, together with multipass technique, the thickness of the parts to be welded can be increased. Welding still happens with quite low total heat input and according to that, method can be used for joining thick sections with very low distortions. Also phenomena of laser keyhole welding still take place, so method is very effective to joining thick sections.In this study thick section austenitic stainless steel has been welded using multipass technique and hybrid process; Nd:YAG -laser+GMAW. Welding has been done using partially grooved V joint geometry and narrow gap with groove angle of 10 degrees. Parameters, for example in filler wire feeding and placement of the wire and the arc are varied. Results of experiments are presented and a phenomenon of the process is discussed.Hybrid welding, in which laser and some arc welding method work together, has been introduced to be effective joining method. Hot spot, made by laser to the joint, has been reported to stablize the arc and change the way of material transfer in arc. Hot spot is also guiding the arc towards to it. This phenomenon has been utilized in this study in order to get the process to be happened inside a very narrow groove. In this way, together with multipass technique, the thickness of the parts to be welded can be increased. Welding still happens with quite low total heat input and according to that, method can be used for joining thick sections with very low distortions. Also phenomena of laser keyhole welding still take place, so method is very effective to joining thick sections.In this study thick section austenitic stainless steel has been welded using multipass technique and hybrid process; Nd:YAG -laser+GMAW. Welding has been done using partially grooved V joint geometry and narrow gap with groove angle o...

Thick-section laser multi pass welding of austenitic stainless steel joints using defocusing technique

This study introduces an experimental work carried out in multi pass laser welding with cold filler wire and laser-arc hybrid welding of thick section austenitic stainless steel. As it has been demonstrated earlier, hybrid and cold wire welding with a keyhole-mode can offer very efficient way to produce multi pass welds in narrow gap thick section joints. However, when multi pass welding is applied to one pass per layer method without e.g. scanning or defocusing, the used groove width needs to be very narrow in order to ensure the proper melting of groove side walls and thus to avoid lack of fusion/cold-run defects. As a consequence of the narrow groove, particularly in thick section joints, the accessibility of an arc torch or a wire nozzle into the very bottom of a groove in root pass welding can be considerably restricted. In an alternative approach described in this paper, a power density of a laser beam spot was purposely dispersed by using a defocusing technique. In groove filling experiments, a power density of defocused laser beam was kept in the range, which led the welding process towards to conduction limited regime and thus enabled to achieve broader weld cross-sections. The object was to study the feasibility of defocusing as a way to fill and bridge wider groove geometries than what can be welded with focused keyhole-mode welding with filler addition. The paper covers the results of multi pass welding results of up to 60 mm thick joints with single side preparations. In order to reach final 60 mm joint thickness, it required the preliminary defocusing trials of bead-on plate samples, and tests of joint thicknesses of 10 mm and 20 mm with different groove openings.This study introduces an experimental work carried out in multi pass laser welding with cold filler wire and laser-arc hybrid welding of thick section austenitic stainless steel. As it has been demonstrated earlier, hybrid and cold wire welding with a keyhole-mode can offer very efficient way to produce multi pass welds in narrow gap thick section joints. However, when multi pass welding is applied to one pass per layer method without e.g. scanning or defocusing, the used groove width needs to be very narrow in order to ensure the proper melting of groove side walls and thus to avoid lack of fusion/cold-run defects. As a consequence of the narrow groove, particularly in thick section joints, the accessibility of an arc torch or a wire nozzle into the very bottom of a groove in root pass welding can be considerably restricted. In an alternative approach described in this paper, a power density of a laser beam spot was purposely dispersed by using a defocusing technique. In groove filling experiments, a pow...

Welding experiments using vacuum enviroment with Nd:YAG-Laser

It is known that with electron beam welding produced in vacuum environment, deep and narrow welds with superior quality can be achieved. In some studies, this has inspired to use vacuum also with laser in order to improve already good performance achieved in atmospheric pressure. It has been reported that the use of vacuum increases penetration, change profile of weld cross-section and produce less porosity.In this study bead-on-plate Nd:YAG-laser welding experiments were carried out in vacuum and in atmospheric pressure in order to clarify the effect of vacuum on penetration and weld bead geometry. Materials used in the experiments were austenitic stainless steels AISI 304L, AISI 316LN and basic mild steel S235. Evaluations of the welds were done by visually and macroscopically from the macro cross-sections. Also occurrence of weld defects was taken into account.It is known that with electron beam welding produced in vacuum environment, deep and narrow welds with superior quality can be achieved. In some studies, this has inspired to use vacuum also with laser in order to improve already good performance achieved in atmospheric pressure. It has been reported that the use of vacuum increases penetration, change profile of weld cross-section and produce less porosity.In this study bead-on-plate Nd:YAG-laser welding experiments were carried out in vacuum and in atmospheric pressure in order to clarify the effect of vacuum on penetration and weld bead geometry. Materials used in the experiments were austenitic stainless steels AISI 304L, AISI 316LN and basic mild steel S235. Evaluations of the welds were done by visually and macroscopically from the macro cross-sections. Also occurrence of weld defects was taken into account.