Multi-pass Laser Welding Research Articles

Narrow gap multi-pass laser welding with 52M/308L double wires filler addition for SA508/316L dissimilar metals

Recently, considerable effort has been directed at the development of laser welding with wire filler addition as an alternative manufacture technique for the substantial productivity gains it would offer to the nuclear pressure vessels fabrication. However, single wire filler such as 52M and 308 L addition is difficult to take into account many properties such as the strength, toughness and resistance to stress corrosion cracking at the same time. In this work we evaluate the feasibility of applying 52 M/308 L dissimilar double wires filler addition for the multi-pass narrow gap laser welding (NGLW) of SA508/316 L dissimilar metals. The objectives of this paper is to regulate the microstructure of the welded joint by adjusting the ratio of the 52 M/308 L double wires filler. The results showed that welded joints without crack and lack-of-fusion were acquired. An excellent comprehensive property welded joint with relative high hardness and low residual stresses is acquired when the 52 M/308 L double wires proportion was 75 %:25 %. The results also indicated that a certain proportion of 308 L stainless steel wire filler addition was conducive to reducing the microhardness of the welded joint. However, when that ratio of the 308 L wire was up to 75 %, harmful element S was obtained in the precipitation zone, which might reduce the mechanical properties of the welded joints.

Study of multi-pass laser welding deformation of bipolar plates by experiments and simulations

Study of multi-pass laser welding deformation of bipolar plates by experiments and simulations

Numerical and experimental study of residual stress in multi-pass laser welded 5A06 alloy ultra-thick plate

The laser welding technology of ultra-thick plates has been widely applied in many crucial engineering structures, where its safety and stability still suffer from the residual stress brought by the welding temperature. In this work, experimental and numerical studies are carried out to reveal the evolution and distribution of the residual stress. A 5A06 aluminum alloy plate with 30 mm thickness is used to conduct the laser welding experiment, and the short-wavelength X-ray diffractometer (SWXRD) is employed to measure the residual stress. Furthermore, a multi-pass laser welding numerical model is developed to predict the residual stress during the welding process. The simulation results of the weld pool geometry and residual stress agree well with the experimental data, which gives a good validation of the numerical model. Based on the proposed numerical model, the influence of the welding parameters, including inter-pass cooling time and welding rate are also investigated. It is discovered that the reduction of the inter-pass cooling time results in weakness effect of the temperature gradient, which hinders the generation of plastic deformation and leads to a lower residual stress value. Meanwhile, the simulation results also show that the increase in welding rate promotes the generation of thermal stress, which brings a higher residual stress value appeared in the structure.

Microstructures and performances of the butt joint of TA1/Q235B bimetallic sheet with addition of a Mo interlayer by using narrow gap laser welding with filler wire

By taking molybdenum (Mo) having favorable metallurgical compatibility with both titanium and iron as the interlayer material, the butt joint of TA1/Q235B layered bimetallic sheet with thickness of 6 mm was prepared by utilizing narrow gap multi-pass laser welding with low heat input. Mo generally can favorably isolate Ti from Fe, meanwhile the transition zone of Mo and steel adjacent to Mo interlayer was softened according to the results of microhardness test. The results of face bending testing showed that the softened zone was beneficial to the mechanical properties of the welded joint, which could restrict the crack propagation along the interface of Mo/steel in fusion zone so that fracture appeared in the BM side. The microstructures of the softened zone exhibited a low dislocation density observed by a transmission electron microscope (TEM), which may be the main reason leading to a low microhardness. In additon, Mo interlayer was confined in a small zone, which only accounted for about 6.5% of the total area of cross section of the fusion zone while a small amount of FeTi and λ brittle intermetallic compounds (IMCs) still appeared around the Mo/steel interface in fusion zone. The tensile strength of the welded joint was 467 MPa, which took up about 93% that of base metal. The results of this paper will help guide endeavours for the fabrication of pipes and containers made of Ti/steel bimetallic sheets.

Multi-Pass Laser and Hybrid Laser-Arc Narrow-Gap Welding of Steel Butt Joints

Multi-pass laser and laser-arc narrow-gap welding make it possible to obtain high-quality butt joints of large-thickness pipe steels with hardness up to 2600...2800 MPa and toughness close to the base metal toughness at a temperature of 20°C. When multi-pass welding methods are used, the subsequent pass makes heat treatment of the previous ones, and reduces the rate of their cooling yet. This allows excluding heat treatment of the joint after welding, which significantly reduces the time and costs for manufacturing large-sized structures.

The interface microstructure, mechanical properties and corrosion resistance of dissimilar joints during multipass laser welding for nuclear power plants

This study presents the interface microstructure, mechanical properties and corrosion resistance of dissimilar joints between Inconel 52M overlays and 316L stainless steel during multipass laser welding for nuclear power plants. The results indicate that the microstructure at the interface beside 316L stainless steel consists of cellular with the width of 30–40 μm, which also exhibits numerous Cr and Mo-rich precipitates like flocculent structure and in chains along grain boundaries as a mixed chemical solution for etching. Many dendritic structure with local melting characteristics and Nb-rich precipitates are exhibited at the interface beside Inconel 52M overlays. Such Nb-rich precipitates at the interface beside Inconel 52M overlays deteriorate the tensile strength and toughness of dissimilar joints at room temperature. The tensile strength of 316L stainless steel at 350 °C significantly decreases with the result that dissimilar joints are fractured in 316L stainless steel. The correlation between corrosion behavior and microstructure of weld metals is also discussed. The difference in high corrosion potential between Nb-rich precipitates and the matrix could result in establishing effective galvanic couples, and thus accelerating the corrosion of weld metals.

An evaluation of multipass narrow gap laser welding as a candidate process for the manufacture of nuclear pressure vessels

Nuclear pressure vessels are currently fabricated using arc welding processes. Recently, considerable effort has been directed at the development of electron beam welding as an alternative fabrication technique owing to the substantial productivity gains it would offer. However, little attention has been directed at laser-based techniques. In this work we evaluate the potential for applying multipass narrow-gap laser welding (NGLW) to the fabrication of nuclear pressure vessels, based on the characterization of a 30 mm thick weld in SA508 steel. Although still a multipass process, the number of passes is reduced in comparison to an arc weld of the same thickness, and the deposition of successive passes provides a degree of tempering to previously deposited weld metal in a way that the electron beam welding process does not. Principal engineering challenges for the implementation of multipass NGLW include the achievement of appropriate joint fit-up, and the shielding of a molten pool at the base of a deep and narrow weld groove.

The Segregation and Liquation Crackings in the HAZ of Multipass Laser-Welded Joints for Nuclear Power Plants

The segregation and liquation crackings in the heat-affected zone (HAZ) beside Inconel 52M overlays of multipass laser-welded joints are investigated for nuclear power plants. The results indicate that Nb-rich precipitates are distributed in chains and some aggregate together as coarse particles in the local regions at the interface. With increasing heat input, the transition width becomes widened and the fraction of the precipitates at the interface significantly increases. Closely associated to Nb segregation, liquation crackings occur along columnar dendrites in the HAZ beside Inconel 52M overlays. Due to no deformation coordination of the columnar dendrites with Nb-rich segregates in Inconel 52M overlays, liquation crackings occur in thermal cycle of multipass laser welding. The enrichment of Nb element in the interdendritic regions increases favorable factors of liquation crackings at grain boundaries.

The defects and microstructure in the fusion zone of multipass laser welded joints with Inconel 52M filler wire for nuclear power plants

The defects and microstructure in the fusion zone of multipass laser welded joints with Inconel 52M filler wire are investigated for nuclear power plants. Experimental results indicate that the incomplete fusion forms as the deposited metals do not completely cover the groove during multipass laser welding. The dendritic morphologies are observed on the inner surface of the porosity in the fusion zone. Many small cellular are found in the zones near the fusion boundary. With solidification preceding, cellular gradually turn into columnar dendrites and symmetrical columnar dendrites are exhibited in the weld center of the fusion zone. The fine equiaxed grains form and columnar dendrites disappear in the remelted zone of two passes. The dendrite arm spacing in the fusion zone becomes widened with increasing welding heat input. Nb-rich carbides/carbonitrides are preferentially precipitated in the fusion zone of multipass laser welded joints. In respect to high cooling rate during multipass laser welding, element segregation could be insufficient to achieve the component of Laves phase.

Defects inhibition and process optimization for thick plates laser welding with filler wire

One of the challenges in laser welding of thick plates is the formation of lack of fusion defects. In this paper, it is found that the formation of such defects has relations with the surface profile of the weld joint. Experiment results indicate that the weld joints can form perfect fusion when the surface of solidified filling metals is concave. Then the author studied the relations between input welding parameters and single bead geometry, which can affect the surface shape of solidified metals, filling passes and defects formations during the multi-pass laser welding process. Further investigations and analysis show that P/Vf (the radio of laser power to wire feed rate) and Vf/Vw (the radio of wire feed rate to welding speed) can be used to quantify the input variables and impede the formation of lack of fusion defects. Verification results show that the optimized parameters for perfect fusion are: P/Vf >1.5 and Vf/Vw<6.

Microstructural characteristics of the laser welded joint of ITER correction coil sub case

The ITER correction coil (CC) case reinforces the winding packs against the electromagnetic loads, minimizes stresses and deformations to the winding pack. The cases are made of high strength and high toughness austenitic stainless steel (316LN) hot rolled heavy plate and have a thickness of 20mm. Considering the small cross-section and large dimensions of the case, deformation of the case when welding becomes a challenge in the case manufacturing. Therefore, laser welding was developed as the main welding technology for manufacturing. In this paper, multi-pass laser welding technology is used, the laser weldability of a 20mm thick 316LN austenitic stainless steel plate is studied and the microstructure of the welded joint is analyzed. The welding experiment used an YLS-6000 fiber laser (IPG) and weld filler of 316LMn to match the base metal was used. The result shows that the welded joint has no obvious surface and internal defects based on the optimized welding parameters. The weld joint have a fine austenite microstructure and display columnar dendrites and cellular grains with strong directional characteristics. No apparent heat affected zone is observed and approximately 2μm an austenite microstructure of the fusion line is clearly presented.

Defocusing Techniques for Multi-pass Laser Welding of Austenitic Stainless Steel

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 of up to 60mm thick joints with single side preparations.

Multipass Narrow Gap of Heavy Gauge Steel with Filler Wire

This article describes method of heavy gauge welding using laser radiation as beam source of energy. The article contains the results of single-pass laser-arc welding and multipass laser welding with filler wire; highlight benefits and drawbacks of each welding method. The results obtained were compared with the traditional methods of welding of the same thickness.

The Laser Welding with Hot Wire of 316LN Thick Plate Applied on ITER Correction Coil Case

ITER correction coil (CC) cases have characteristics of small cross section, large dimensions, and complex structure. The cases are made of heavy thick (20 mm), high strength and high toughness austenitic stainless steel 316LN. The multi-pass laser welding with hot wire technology is used for the case closure welding, due to its low heat input and deformation. In order to evaluate the reliability of this welding technology, 20 mm welding samples with the same groove structure and welding depth as the cases were welded. High purity argon was used as the shielding gas to prevent oxidation because of the narrowness and depth of the weld. In this paper investigation of, microstructure characteristics and mechanical properties of welded joints using optimized welding parameters are presented. The results show that the base metal, fusion metal, and heat affected zone (HAZ) are all have fully austenitic microstructure, and that the grain size of fusion metal was finer than that of the base metal. The welding resulted in an increase of hardness in the fusion metal and HAZ. It was confirmed that the tensile strength of fusion metal was higher than that of base metal and the impact toughness value is higher than industry standard requirement. Thus, this welding process was determined to be reliable for manufacture of the ITER CC cases manufacture.

Applying statistical models optimize the process of multi-pass narrow-gap laser welding with filler wire

Laser welding of thick plates has been used in automobile and shipping industry owing to its focusing heat input, little welding deformation, and high productivity. Using narrow-gap technique can decrease the filling volumes of wire and increase the welding efficiency. However, its process is more complicated since it introduces filler wire to narrow-gap weld configurations. Usually the welding path, planning mainly depends on man’s experiences. Human factors have big influences on the quality of the joint. The aim of this paper is to study the interactions between welding parameters and the geometry of single bead using statistical methods. Then, the founded models were employed to plan the welding path, the number of filling passes, and layers in the multi-pass narrow nap joining. These results will provide supports for automatic multi-pass laser welding. The welding system includes a 10-kW Nd:YAG laser system, a KUKA robot, a Fronius wire feeder, and a 20-mm-thick high-strength ship steel with narrow gap. Laser power, welding speed, and wire feed rate are input variables. The responses include the transversal area of the added metals, bead width, bead height, and the ratio of bead width to height. Verification experiments indicate that all these models can forecast the responses within the factor domain. Transversal section micrographs of the joints show that this method can get welding joint with less defects.

Thick-section Laser and Hybrid Welding of Austenitic Stainless Steels

Austenitic stainless steels are generally known to have very good laser weldability, when ordinary grades of sheets are concerned. But it is not necessarily the case, if special grades of fully austenitic structures with e.g. high molybdenum, or thick-section are used. It is also known that hot cracking susceptibility is strictly controlled by composition and welding parameters. If solidification is primary ferritic, hot cracking resistance is dramatically increased. It is also well known that laser welding needs a careful control of weld edge preparation and air gap between the edges. The dependence on edge quality can be decreased by using filler metal, either cold wire, hot wire or hybrid laser-arc welding. An additional role is high molybdenum contents where micro segregation can cause low local contents in weld which can decrease the corrosion properties, if filler metal is not used. Another feature in laser welding is its incomplete mixing, especially in thick section applications. It causes inhomogeneity, which can make uneven microstructure, as well as uneven mechanical and corrosion properties In this presentation the features of laser welding of thick section austenitic stainless steels are highlighted. Thick section (up to 60 mm) can be made by multi-pass laser or laser hybrid welding. In addition to using filler metal, it requires careful joint figure planning, laser head planning, weld parameter planning, weld filler metal selection, non-destructive and destructive testing and metallography to guarantee high-quality welds in practice. In addition some tests with micro segregation is presented. Also some examples of incomplete mixing is presented.

Multi-pass laser welding of thick plate with filler wire by using a narrow gap joint configuration

A 17 mm low-carbon steel plate is welded by a fiber laser with a narrow gap joint configuration and multi-pass technique. A high-speed camera is used to real-time monitor the welding process, and the effects of groove size and the side shielding gas on the weld quality are studied. Experimental results showed that a concave shape in the bottom of weld can be formed when use a general shielding gas nozzle with the 8.0 mm external diameter. Through a special design of the shielding gas nozzle with the 2.0 mm external diameter which can deep into the interior of groove, the pressure from shielding gas can balance the surface tension from the liquid in the top of groove, so the shielding effect can be dramatically improved and the concave shape in the bottom of weld can be eliminated. When the filler wire and laser beam can smoothly enter the groove, using a relatively small groove size not only reduce the consumption of filler wire but also reduce the deflection of filler wire in the gap that can improve the fusion of groove.

Understanding the process parameter interactions in multiple-pass ultra-narrow-gap laser welding of thick-section stainless steels

In laser welding, typical welding penetration depths are in the order of 1–2 mm/kW laser power. The multipass laser welding technique, based on the narrow-gap approach, is an emerging welding technology that can be applied to thick-section welds by using relatively low laser power, but the process is more complicated since it is necessary to introduce filler wire to narrow-gap weld configurations. The aim of this work was to understand significant process parameters and their interactions in order to control the weld quality in ultra-narrow-gap (1.5 mm gap width) laser welding of AISI grade 316L stainless steel. A 1-kW IPG single-mode fiber laser was used for welding plates that were 5 to 20 mm in thickness using the multiple-pass narrow-gap approach. Design of experiments and statistical modelling techniques were employed to understand and optimise the processing parameters. The effects of laser power, wire feed rate, and welding speed on the weld homogeneity, integrity, bead shape, gap bridgability and surface oxidation were studied. The results were evaluated under different optimising constraints. The results show that the models developed in this work can effectively predict the responses within the factors domain.

厚板不锈钢激光填丝多道焊的接头组织分析

厚板不锈钢激光填丝多道焊的接头组织分析

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, and arc) are varied. Results of the experiments are presented and phenomena of the process, as well as reasons for weld defects, are pointed out.