Narrow Gap Laser Welding Research Articles

Welding Characteristics of Medium Titanium Plates with Autogenous Laser Welding and Narrow-Gap Laser Filling Welding Modes.

Titanium and titanium alloys with a medium thickness of 5 to 12 mm are widely used for ocean platforms, military equipment and in other fields because of their light weight, appropriate strength and corrosion resistance. In this study, autogenous laser welding and narrow-gap laser welding processes were researched and compared, and the welding characteristics, weld microstructure and joint strength were analyzed. The results showed that autogenous laser welding had higher efficiency, narrower weld width and higher microstructure uniformity. Autogenous laser welding can achieve the single pass weld penetration at laser keyhole mode. The weld width of narrow-gap laser welded joint was 12.5 mm, which was nearly three times than that of autogenous laser welding. The grain size of autogenous laser welding was obviously smaller and more uniform in depth than that of narrow-gap laser welding. In the weld zone, the coarse columnar α grains grew from the fusion line, while in the heat-affected zone, equiaxed α grains with needle and sawtooth α morphologies were presented. The microhardness of the heat-affected zone was higher than in the weld zone and the base metal due to the denser needle microstructure. The tensile samples all fractured at the base metal, indicating the welded joint strength efficiency was greater than 1.

Single-pass ultra-high-power laser welding of thick nickel-based GH3535 superalloy: Microstructure evolution and microhardness

In comparison to narrow-gap laser welding, the ultra-high-power laser welding (UHPLW) represents an efficient method for single-pass welding of 22 mm thick GH3535 superalloy. This paper explored the microstructure evolution and microhardness distribution along the direction of weld penetration. The results showed that the direction of grain growth changed from horizontal to an inclined downward direction towards the bottom of the joint along the penetration direction. Precipitates distributed in the interdendritic region were identified as M2C and M6C carbides. The rapid cooling rate in the lower zone (LZ) of the joint resulted in slight elemental microsegregation and a lower volume fraction of precipitates than in other zones. The smaller grain size and the higher volume fraction of the precipitates resulted in a higher microhardness in the middle zone (MZ) of the entire joint. The study results will facilitate the application of UHPLW for the efficient welding of GH3535 thick plates.

Study on molten pool flow and hot cracking of narrow gap laser welding of 316LN stainless steel for fusion reactor

In this paper, narrow-gap laser welding (NGLW) of 316LN stainless steel was studied. The results indicated that continuous laser welding with filler wire (CLWFW) and pulsed laser welding with filler wire (PLWFW) can obtain good weld formation under the transition of the wire bridge, and when the pulsed laser was a trapezoidal wave, the hot cracks of the filling layer can be greatly inhibited. The intermittent output of the pulsed laser reduced the temperature gradient of the molten pool and enhanced the morphology of the caudal region of the molten pool. Meanwhile, timely liquid backfilling effectively relieved the stress concentration of the weld, thus inhibiting the generation of hot cracks. Besides, the intermittent pulse energy input had a stirring effect on the molten pool, which disturbed the growth direction of the dendrite grain and refined the grain; consequently, the impurity elements were fully diffused, and the distribution of elements in the weld center area was more uniform.

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 on the suppression of solidification cracks in narrow gap laser welding of high-strength steel based on dual-beam gradient control weld solidification

Welding solidification cracks are considered as critical defects in narrow gap laser welding, and it is of utmost importance to develop reliable measures to suppress them. The fundamental approach to curbing solidification cracks lies in controlling the solidification behavior of the weld. A novel narrow gap laser welding technology is proposed to address this issue by controlling the solidification of the weld with double laser beam in this paper. The influence of process parameters on the suppression of welding solidification cracks is explored. Additionally, the mechanism behind the dual-beam laser's ability to suppress solidification cracks are comprehensively examined through weld formation, molten pool solidification behavior and microstructure evolution. The findings demonstrate that the energy matching of the dual-beam and the spacing between the beams are crucial factors that impact the effectiveness of welding crack suppression. When the beam spacing is greater than 6 mm, this method begins to work on suppressing welding solidification cracks. Three distinct effective modes of crack suppression are identified, namely molten pool expansion, gradient solidification, and weld remelting. Among these, the gradient solidification mode proves to be the most reliable method for suppressing welding solidification cracks, as it facilitates a transformation in the molten pool's solidification mode from "enclosed" solidification to "sequential" solidification. Compared to conventional single-beam Narrow Gap Laser Welding, this approach not only effectively suppresses welding solidification cracks but also improves welding efficiency by nearly doubling the filling speed.

Experimental analysis of the effect of ultrasonic vibration on porosity suppression in narrow gap laser welding

Abstract The welding process has a wide range of applications in aerospace, military manufacturing, machinery, and other fields, and with the continuous improvement of welding technology requirements, the inhibition of porosity in the welding process is also increasing. This study, through the narrow gap laser welding characteristics and porosity formation mechanism of the depth of analysis, for the existing narrow gap laser welding method exists in the side wall of the weld fusion is poor porosity, poor uniformity of organizational properties and other issues, proposed ultrasonic-based welding method to achieve effective inhibition of porosity. The experimental analysis of ultrasonic vibration on the formation of porosity is carried out based on narrow-gap laser welding. The peak sound pressure of narrow gap welding increases when the ultrasonic current amplitude changes from 24A to 32A. Under different ultrasonic vibration conditions, the number of welds without applied ultrasound was as high as 95, while the number of porosities gradually decreased to 48 with the increase of applied ultrasound amplitude from 12% to 37%. Furthermore, as the ultrasonic amplitude increased, the residual height decreased by 0.45 ml. In addition, at fixed ultrasonic amplitude, with the increase of laser power, the porosity increased from 0.89% to 2.21%, and the average diameter of the porosity increased from 0.29 mm to 0.43 mm. The porosity for the porosity diameter of less than 200 μm was reduced to 0.020%. The percentage of stomata with a diameter greater than or equal to 200μm increased to 2.098%. This study analyzes the inhibition effect of ultrasonic vibration on porosity to a certain extent. Moreover, the inhibition effect of ultrasonic vibration in narrow gap laser welding is significant and the smaller the laser power under the same amplitude, the better the inhibition effect, which provides a valuable reference for the research of the narrow gap laser welding process.

Penetration prediction of narrow-gap laser welding based on coaxial high dynamic range observation and machine learning

Narrow-gap laser welding is a novel joining process for thick-walled applications. However, it is challenging to obtain accurate root weld penetration as a crucial parameter for evaluating welding quality due to the limited spatial position. The images of the front-side weld pool are one of the most effective signals for reflecting the root weld penetration. Hence, this paper proposes using a high dynamic range (HDR) camera to capture the weld pool morphology under narrow-gap laser welding. The characteristics of weld pool under different root weld penetration states are obtained and analyzed through orthogonal experiments. Furthermore, the root weld penetration states prediction model is constructed based on a backpropagation (BP) neural network. The results show that the accuracy of this prediction model is approximately 94.74 %, so the proposed method can be further applied to establish real-time root weld penetration control system of narrow-gap laser welding process.

Numerical investigation of sidewall penetration in narrow gap oscillating laser welding process

The lack of sidewall fusion is one of the process challenges in narrow gap laser welding. To solve this problem, beam oscillation has been applied to narrow gap laser welding. Because the laser oscillation behavior had a great influence on the sidewall penetration, a 3D heat transfer model of narrow gap oscillating laser welding of high-strength steel was established for numerical simulation. The effect of oscillation parameters on sidewall penetration was investigated by quantitatively analyzing the sidewall penetration under different oscillation parameters. The results showed that the high-temperature region of the molten pool changed periodically with the laser oscillation. As the oscillation amplitude increased from 0 mm to 1.5 mm, the molten pool cross-section shape changed from tip shape to arc shape, the peak temperature of the sidewall increased from 1857℃ to 2115℃, and the sidewall penetration also increased from 0.82 mm to 0.96 mm. As the oscillation frequency increased from 20 Hz to 100 Hz, the peak temperature of the sidewall decreased from 2036℃ to 1936℃. However, more heat was accumulated in the high frequency mode, and the value of sidewall penetration increased from 0.9 mm to 0.94 mm.

Influence of High-Temperature Annealing on Microstructure and Properties of Welded Joints Using Narrow Gap Laser Welding of TC4 Titanium with Welding Wire

Influence of High-Temperature Annealing on Microstructure and Properties of Welded Joints Using Narrow Gap Laser Welding of TC4 Titanium with Welding Wire

Study on Porosity Defect Detection in Narrow Gap Laser Welding Based on Spectral Diagnosis.

As an advanced connection technology for large thick-walled components, narrow gap laser welding has the advantages of small heat input and high efficiency and quality. However, porosity defects are prone to occur inside the weld due to the complex welding environment. In this study, the influence of the process parameters and pollutants such as water and oil on the porosity defect were explored. The action mechanism of water on the electron temperature and spectral intensity of the laser-induced plasma was analyzed. The results showed that the spectral intensity during narrow gap laser welding was weaker than that of flat plate butt welding. Under the optimal welding process conditions, the electron temperature during narrow gap laser self-fusion welding was calculated as 7413.3 K by the Boltzmann plot method. The electron density was 5.6714 × 1015 cm-3, conforming to the thermodynamic equilibrium state. With six groups of self-fusion welding parameters, only sporadic porosity defects were observed according to the X-ray detection. When there was water on the base metal surface, a large number of dense pores were observed on the weld surface and in the weld through X-ray inspection. Compared with the spectral data obtained under the normal process, the relative light intensity of the spectrometer in the whole band was reduced. The electron temperature decreased to the range of 6900 to 7200 K, while the electron density increased. The spectrum variation during narrow gap laser wire filling welding was basically the same as that of laser self-fusion welding. The porosity defects caused by water and oil pollutants in the laser welding could be effectively identified based on the intensity of the Fe I spectral lines.

A coupled ray-tracing based CFD and cellular automaton model for predicting molten pool formation and microstructure evolution in narrow gap laser welding

Laser beam reflection plays a significant role in the temperature development history during various laser material processes. Different from other processes where laser beam reflection main happens inside the keyhole, in narrow gap laser welding (NGLW), the laser beam would interact with the sidewalls multiple times before existing the narrow groove, which would have a great impact on the defect formation as well as the grain structure development. However, the underlying mechanism of this process remains unknown in multi-pass NGLW. In this study, an integrated ray-tracing based CFD and cellular automaton (CA) modelling framework is established to investigate the impact of laser beam reflection on molten pool formation and microstructure evolution in multi-pass NGLW. From the CFD simulation, it is found that the overall laser beam absorption of NGLW is greater than laser metal deposition (LMD) while smaller than laser powder bed fusion (LPBF) process. Besides, the overall absorptivity in NGLW is increased 28% when considering the ray reflection during the first welding pass. The absorptivity would increase and then stabilise through the multi-pass welding process, which is attributed to the change in incident angle and number of reflections. Additionally, it is found that the undercut defect is more likely to occur with ray reflection due to the excessive temperature rise on the sidewalls, while lack of fusion is observed in the case without ray reflection. By incorporating the thermal history of each welding pass with CA, the impact of laser beam reflection on grain structure development is evaluated. Columnar grains would govern the microstructure evolution in the case with ray reflection, which grow epitaxially from the fusion line to the molten pool centre. Grain coarsening would occur around the lack of fusion region in the case without ray reflection. Moreover, the analysis on grain size distribution is conducted and a greater maximum grain size is obtained in the case without ray reflection as a result of lower cooling rate caused by lack of fusion. However, a higher grain size increase is obtained in the case with ray reflection during multiple welding passes, and larger grains are expected to form if more welding passes are to be conducted. The simulated molten pool morphology and grain structure are compared with the experimental observation, and a good agreement is achieved.

Dynamic Characteristics of Plasma in Ultrasonic-Assisted Narrow-Gap Laser Welding with Filler Wire.

Laser welding with filler wire was applied to Q345D in a narrow gap under ultrasonic assistance, and the dynamic characteristics of plasma were studied by high-speed imaging and spectral acquisition. The results showed that the plasma area decreased gradually with increasing distance between the ultrasonic loading position and welding seam. The electron density and temperature of the plasma with ultrasonic assistance were higher than those without ultrasound. The electron density was approximately 1016~1017 cm-3, and the plasma temperature was approximately 4000~6000 K. Ultrasonic assisted laser wire filling welding can bring about cavitation effect and significantly reduce the porosity problem.

Interlaminar Microstructure and Mechanical Properties of Narrow Gap Laser Welding of 40-mm-Thick Ti-6Al-4V Alloy.

Narrow gap laser welding (NGLW) is a common solution for the welding of thick structures. NGLW was carried out on narrow-gap butt joints of 40 mm-thick Ti-6Al-4V alloy plates with a U-shaped groove. The distribution characteristics of the interlaminar microstructure in different height ranges of the joint were investigated, and the evolution behavior and formation mechanism of the interlaminar microstructure of the joint were also revealed. This showed that a large amount of short needle martensite nucleated and grew up near the fusion line and the upper boundary of the remelting zone. The "softening" phenomenon occurred in all welds except the cover layer weld. The microstructure evolution and defect migration, induced by multiple welding thermal cycles in the upper weld forming process, were the main reasons for the "softening" of the lower weld. The tensile strength of each sample changed in the range of 920~990 MPa; the fracture mode of the sample belongs to a transgranular ductile fracture. In addition, compared with the upper part of the joint, the plasticity and toughness of the weld area in the lower part of the joint was improved.

Influence of the magnetic field on the melting and solidification behavior of narrow-gap laser welding with filler wire

This study analyzed the influence of magnetic induction intensity on the plasma electron temperature distribution and the molten pool flow behavior by high-speed imaging and spectral analysis. With the increase in magnetic induction intensity, the plasma high-temperature center region gradually increased, and the plasma electron temperature increased from the bottom of the groove to the edge of the groove. The magnetic field showed a stirring effect on the molten pool. Certain magnetic field intensities can make the molten pool more uniform and flow more stable. Compared with the penetration depth and penetration area size of the weld with or without a magnetic field, the penetration depth and penetration area of the weld under the magnetic field increased. A certain range of magnetic induction intensity can be coupled with other process parameters to achieve an ideal effect and form a weld.

Effects of sidewall grain growth on pore formation in narrow gap oscillating laser welding

How to suppress the pores is one of the most important challenges in narrow gap laser welding of aluminum alloys. In order to solve this question, narrow gap oscillating laser welding (O-NGLW) of dissimilar AA6061 and AA2024 aluminum alloys was carried out. With the increase of the laser oscillation frequency (f), the pores changed from the dense and linear chain at weld center to the scattered distribution segregating at AA2024 side, and disappeared when f ≥ 400 Hz. The normalized calculated stirring Reynolds number of O-NGLW was 1369 when suppressing the pores totally, 1.39–1.75 times that of the welding without narrow gap, indicating that a stronger stirring effect induced by laser oscillation was needed to capture the bubbles. The theoretical analysis suggested that this phenomenon was closely related to the effect of columnar grains growth on bubble interception (ECGGBI). Based on the interaction between ECGGBI and the stirring effect induced by laser oscillation, the pore formation was discussed, and the reason why stronger stirring effect was needed to suppress the pores was revealed.

Narrow-gap laser welding with beam wobbling and filler wire and microstructural performance of joints of thick TC4 titanium alloy plates

How to control poor fusion of sidewall and interlayer in narrow-gap laser welding with filler wire of thick plates is an unsolved problem. When the groove width changes in the direction of the plate thickness (for example, V-shaped narrow-gap grooves or preset inverse deformation to offset angular distortion caused by welding), incomplete fusion is more difficult to control, resulting in low yield. Laser welding with beam wobbling and filler wire (LWBWFW) was carried out on narrow-gap butt joints of thick TC4 titanium (Ti) alloy plates with V-shaped grooves. On this basis, the relationship between welding conditions and incomplete fusion under four widths of the groove bottom (4, 6, 8 and 10 mm) was studied. It is found that the key to eliminating incomplete fusion is shown as follows: on the one hand, the beam wobbling amplitude should be appropriate. On the other hand, the ratio of the welding line energy to the wire feed amount should be proper. The former mainly affects incomplete sidewall fusion, while the latter mainly influences incomplete interlayer fusion. Following the above principles, this research obtained well-formed joints of TC4 alloy with the thickness of 30 mm welded by LWBWFW (hereinafter referred to as LWBWFW joints) and the tensile strength of the joints can reach 99.6% that of base metal (BM).

Analysis of welding solidification crack in narrow gap laser welding of high-strength steel

Analysis of welding solidification crack in narrow gap laser welding of high-strength steel

Ultra-Narrow Gap Fiber Laser Conduction Welding Technology for 304 Stainless Steel Thick Plates and the Mechanical Properties of Welding Joints

The application of thick metal plates is increasing, and the welding problem is becoming more and more prominent. Narrow gap laser welding is one of the important methods, and it is also a research hotspot. The stainless steel thick plates were welded using the ultra-narrow gap fiber laser conduction welding with filler wire. Results show that the ranges of technological parameters for the achievement of the weld seam with no defects are smaller when the gap width is comparatively larger. Using the optimized technological parameters, the butt welding with no defects on the 3 mm gap between two 304 stainless steel plates with 60 mm thickness was achieved through the filling 20 times. This welding method of 304 metal with large thickness is rare in the literature. The tensile strength of the welding joint can be up to 87% of that of the base metal, and the micro-hardness and yield strength of the joint are comparable with those of the base metal.

Melting behavior in ultrasonic-assisted narrow-gap laser welding with filler wire

The performance of ultrasonic-assisted laser welding with filler wire was investigated using high-speed imaging and a mechanical analysis of the droplet. The present study demonstrated that the droplet transition was influenced by the distance between the ultrasonic-assisted position and the weld. Due to ultrasonic assistance and plasma and metal vapor injection, the droplet was blown away and not directly above the keyhole such that the shielding and the absorption of the laser were minimal. The weld pool received direct laser irradiation for a long period. The weld depth increased by 27.2%, while the width difference was not significant when the distance between the ultrasonic-assisted position and the weld center (DUL) was 10 mm. As the DUL increased to 20 mm, the weld was formed well, and the pores were significantly reduced. With increasing the DUL, the effect of ultrasound on the weld pool became weak. Also, the droplet transition did not significantly differ from that without ultrasound, when the distance was 30 mm.

Solidification cracking sensibility of narrow gap laser welding on ITER-grade austenitic stainless steel

This study aimed to investigate the solidification cracking in narrow gap laser welding (NGLW) of ITER grade austenitic stainless steel 316LN. The results show that NGLW of ITER grade 316LN presents high solidification cracking sensitivity. The cracking always occurred in the centerline of build-up weld with a certain depth, not in root and cosmetic layers. The crack mostly propagates on the surface of the weld and it could be be healed by the following layer welding. The healing efficiency depends on the overlapping zone between the two layers. Other various laser welding processes such as autogenous laser welding (ALW), double-sided NGLW, and narrow gap pulsed laser welding (NGPLW) were studied. These welding processes present better cracking resistance than that of NGLW because of the stress distribution, microstructural characteristic and constraint condition of welded joints.