High-power Fiber Laser Welding Research Articles

Active control effect of shielding gas flow on high-power fiber laser welding plume

Plume are common physical phenomena in fiber laser keyhole welding and have serious negative effects on the welding process. Based on this, this paper explores the regulation law of conventional shielding gas flow on plume. The results show that the shielding gas has a very significant effect on the suppression of the slender part of the plume, and the greater the gas flow rate, the better the plume removal effect. The addition of the shielding gas makes the welding process more stable, the molten pool flows stably, and the frequency of spatter eruption is reduced. Under the experimental conditions, the optimal shielding gas flow rate is around 15 l/min, and the penetration depth and width are increased by about 10% and decreased by about 22%, respectively, compared with that without adding the shielding gas. Based on the gas flow simulation, the gas flow pressure (about 132 Pa) generated by an appropriate amount of shielding gas (about 15 l/min) can press the liquid column and spatter near the keyhole mouth into the molten pool to avoid the spatter eruption. Excessive shielding gas flow will interfere with the flow of the molten pool excessively, and the weld surface will show a serious undercut phenomenon.

Research on fiber laser welding formation, microstructure, and mechanical properties of 7.5 mm 304 stainless steel

304 austenitic stainless steel with a thickness of 7.5 mm was welded by high-power fiber laser welding. The effects of main welding parameters, including laser power, welding speed, and defocusing amount, on weld formation were studied in detail. The welding parameters were optimized according to the weld formation. On this basis, the microstructure and mechanical properties of the welded joints were further analyzed. The results showed that depression and hump defects easily occurred once the joints were penetrated. The defects could be effectively avoided when two specific parameters were adopted: namely, high-power laser welding with high speed at the focal position or low speed at the far defocusing position; both could reduce the impact of metal vapor in the keyhole on the molten pool, and meet the requirements of penetration and stability to obtain well-formed welds. High-speed welding obtained nail-shaped weld, and Peanut-shaped weld was obtained by low-speed welding. The microstructure of the welds was mainly composed of massive austenite and a small amount of ferrite, indicating that the solidification abided by the F-A mode according to the pseudo-binary phase diagram. Due to the smaller grain size, the hardness of the Nail-shaped weld (238.8HV0.3) was higher than that of the Peanut-shaped weld (223.4HV0.3). The tensile strength of the Nail-shaped weld joint and Peanut-shaped weld joint were 677 MPa and 689 MPa, which reached more than 95% of the base metal.

Formation, microstructure and mechanical properties of double-sided fiber laser welded ultra-high strength steel

The ultra-high strength steel with a thickness of 20 mm was welded by double-sided high-power fiber laser welding. The welding parameters were optimized according to the behavior of molten metal and formation of the weld. Necking, spatters and uneven formation occurred when the welding parameters were improper. Microstructure of the weld at most region was made up of martensite, while austenite could be found at the final solidified position where the solidification rate was relatively slow. The thermal effects of the second welding process resulted in the austenization of original martensite and then transformed to ferrite and carbide within the interlaminar weld. Hardness of the weld was much higher than that of base metal owing to the formation of martensite. Hardness decreased at the austenite zone and the interlaminar zone made up of massive ferrite. Solid solution of C element within the martensite improved the tensile strength but weakened the toughness of the weld, leading to the formation of cleavage planes at fracture surfaces of the top and the mid welds. Large angle between the grain growth direction and force loading direction was benefit to improve the tensile properties.

High-Power Fiber Laser Welding of High-Strength AA7075-T6 Aluminum Alloy Welds for Mechanical Properties Research.

The results disclosed that both the microstructure and mechanical properties of AA7075-T6 laser welds are considerably influenced by the heat input. In comparison with high heat input (arc welding), a smaller weld fusion zone with a finer dendrite arm spacing, limited loss of alloying elements, less intergranular segregation, and reduced residual tensile stress was obtained using low heat input. This resulted in a lower tendency of porosity and hot cracking, which improved the welded metal’s soundness. Subsequently, higher hardness as well as higher tensile strength for the welded joint was obtained with lower heat input. A welded joint with better mechanical properties and less mechanical discrepancy is important for better productivity. The implemented high-power fiber laser has enabled the production of a low heat input welded joint using a high welding speed, which is of considerable importance for minimizing not only the fusion zone size but also the deterioration of its properties. In other words, high-power fiber laser welding is a viable solution for recovering the mechanical properties of the high-strength AA 7075-T6 welds. These results are encouraging to build upon for further improvement of the mechanical properties to be comparable with the base metal.

A New Method to Assess Fiber Laser Welding Quality of Stainless Steel 304 Based on Machine Vision and Hidden Markov Models

High-power fiber laser welding has a broad range of applications in industrial processing and modern intelligent manufacturing, but how to assess the quality of laser welding has always been a concern. In the welding process, the geometric feature of the keyhole generated by the high temperature of laser can directly reflect quality of the laser welding. By using machine vision to acquire images in real time and analyze related features, the quality of laser welding can be evaluated, which reduce the cost and time in later inspection. Nevertheless, due to the dynamic complexity of the keyhole as well as the blurring of the keyhole and full penetration hole boundary caused by metal vapor and spatter, identification of multiple type defects through machine vision is still a problem that requires a solution. This paper presents a novel laser welding defect classification method using keyhole boundary-based feature based on machine vision and Hidden Markov process. After effective segmenting the collected welding image, the shapes of the keyhole as well as the full penetration hole were automatically extracted using the characteristics of gray projection distribution and the Poisson extinction method. Subsequently, a pre-trained Hidden Markov Model was employed to establish the connection between the keyhole's geometry and the welding quality defects. Experiments indicate that our theory can efficiently and accurately extract the key geometric shapes. Welding experimental data involving stainless steel 304 verifies the feasibility of the classification theory, which can monitor welding quality and reveal potential porosity and penetration defects.

高功率光纤激光深熔焊接小孔的形貌特征

高功率光纤激光深熔焊接小孔的形貌特征

High Power Fiber Laser Welding with Filler Material

AbstractIndustry today is continually challenged to improve quality, reliability, reduced cycle times, and improve the overall manufacturing efficiency. This is driving manufacturers across various market sectors to consider fiber laser welding and the possibility to automate their welding processes to improve consistency and part quality. Another driver is the changing workforce dynamics. It is hard to find or train skilled workers who can consistently and reliably weld aluminum, titanium, and high‐strength nickel alloys. This is particularly true when welding with filler material where the base materials are subject to porosity and cracking during the welding process.

Influence of carbon dioxide shielding gas on microstructure and corrosion property of welds

AbstractThe microstructures and corrosion properties of weld joints are studied during 10 kW high power fiber laser welding of 304 stainless steel with shielding gases of 100 % argon, 80 % argon containing 20 % carbon dioxide and 100 % carbon dioxide, respectively. As the content of carbon dioxide in argon shielding gas increases from 0 % to 20 % then to 100 % during 10 kW high power fibre laser welding, the interdendritic δ‐ferrite in weld joints changes from lathy, short strip and trivial structures to thick‐long strip with short secondary dendrite arms then to skeletal‐network structure. Oxide inclusions, observed in 100 % carbon dioxide shielded weld, promote the formation of the skeletal‐network structure of δ‐ferrite and the homogenization of chromium and nickel in weld joint. The 100 % argon shielded weld joint has a highest initial corrosion current density, the 80 % argon containing 20 % carbon dioxide shielded weld has a lowest pitting potential, while the 100 % carbon dioxide shielded weld has a highest initial corrosion potential. The microstructures and element distribution play important role in determining the corrosion properties of the weld joints.

Dynamic keyhole behavior and keyhole instability in high power fiber laser welding of stainless steel

A three-dimensional numerical model, considering the real-time multiple reflections of a laser beam, adiabatic bubble model and shear stress, was developed to study the dynamic keyhole behavior and keyhole instability in fiber laser welding of stainless steel. The inner dynamic keyhole behavior and weld defect formation were directly observed in a high resolution assisted by transparent glass. The numerical and experimental results showed that the keyhole width reached the quasi-steady state earlier than the keyhole depth did during fiber laser welding of stainless steel. Due to the large recoil pressure at rear keyhole wall caused by the irradiation of laser energy reflected by the bulge at the front keyhole wall, the rear keyhole wall was severely deformed at keyhole bottom and keyhole middle. The rear keyhole wall was collapsed due to the high surface tension pressure and hydrostatic pressure. The whole keyhole collapse was attributable to the capillary instability of the keyhole associated with large depth/width ratio and the strong flow of the bulges at the keyhole wall. When the laser power was increased, the keyhole depth/width ratio was increased, so the keyhole was more capillary instable. The average inclined angle of the front keyhole wall was decreased.

Ultra high power (100 kW) fiber laser welding of steel.

A 100kW fiber laser was first used to weld steel. Speeds at the range between 0.3 and 5.0m/min were tested, and the maximum weld bead depth of 70mm was achieved by single pass welding. Solidification cracking and porosity occurred when the welding speed was lower than 0.5m/min, while undercut appeared when the welding speed was higher than 3.0m/min. Both the ratio of depth to width and the cross section area of the weld bead had a positively linear relationship with the welding speed. A high speed camera was used to observe the characteristics of the keyhole and molten pool. The average number of spatters increased with the welding speed, while the keyhole diameter and the length of the molten pool in front of the keyhole decreased with the welding speed. This Letter validates the application potential of a 100kW ultra high power fiber laser in manufacturing, e.g., welding, cutting, and additive manufacturing.

Mechanisms for Improvement of Weld Appearance in Autogenous Fiber Laser Welding of Thick Stainless Steels

High-power fiber laser welding is an efficient and effective way to produce heavy section structures. However, there is a significant challenge in producing the welds with free of imperfections such as nail-head-shaped welds, spatters, and root sagging. This is partially due to a lack of understanding of the welding mechanism of high-power fiber laser. In this paper, we were especially interested in the mechanism to improve the appearance of welds, and we focused on the autogenous laser welding on thick stainless steel plates by a 10 kW fiber laser. To look into the relations of process parameters and the quality of welds, a high-speed imaging system was applied to observe the molten pool flow and vapor plume during the welding process. The appearances of welds subjected to different welding conditions were analyzed. The results showed that (1) nail-head-shaped welds were suppressed by using a gas jet during laser welding process. (2) In the forward welding, a gentle upwelling molten metal flow on the rear keyhole wall, a deeper weld pool and a weaker vapor plume resulted in no spatter. (3) The gravity affected the formation of underfills and root sagging significantly during autogenous laser welding of thick plates. (4) When the workpiece was placed vertically in the transverse position, the welding process was stable without an aggregation of molten melt at the back surface. Moreover, the mechanisms of forming root sagging and humps were different at the top surface.

Influence of CO2 Shielding Gas on High Power Fiber Laser Welding Performance

The weldabilities were investigated during 10 kW high-power fiber laser welding of 304 stainless steel with the shielding gases of 100% Ar, 80% Ar + 20% CO2 and 100% CO2, respectively. As the proportion of CO2 in shielding gas increased from 0% to 20% then to 100%, the molten pool became unstable and the optional parameter range for obtaining a good weld appearance became narrow. The defocused distance was more negative during the CO2 shielded welding, where the weld joint without apparent defect, the maximum penetration, and a necking of weld width were formed. Porosity has been suppressed and eliminated in the CO2 shielded weld joint. The highest microhardness was obtained from the Ar + CO2 shielded weld joint, because the denser δ-ferrite appeared in the Ar + CO2 shielded weld joint. The microhardness of CO2 shielded weld joints was relative low because of the oxidation of the elements of C, Si, Mn, and Cr, which reduces the solid solution hardening tendency during the welding process.

Study on the role of recondensation flux in high power laser welding by computational fluid dynamics simulations

Partial penetration welding with fiber laser on 20 mm thick plates was carried out in horizontal position to study the role of secondary heating in modeling of high power fiber laser welding. Experiments were carried out using 18.8 kW laser with 1.5 m/min welding speed at Ar assist gas flow rates of 0, 17, 29, and 40 l/min, all four cases show similar bead shape with bright emission of vapor plume. Numerical simulations were performed using volume of fluid method by considering three different models as models A–C. Model A considers only Fresnel reflection inside the keyhole using real time tracking of free surface. Model B considers vapor recondensation flux inside keyhole along with model A. Finally, model C is used, which considers vapor plume heating at 4100 K temperature along with models A B. Secondary heating by recondensation and vapor plume is vital in modeling of high power fiber laser welding; especially, the upper part of the bead is more influenced due to secondary heating. Tungsten particles are also used to visualize the flow pattern of melt pool.

High Power Fiber Laser Welding of Single Sided T-Joint on Shipbuilding Steel with Different Processing Setups

Laser welding of thick plates in production environments is one of the main applications of high power lasers; however, the process has certain limitations. The small spot size of the focused beam produces welds with high depth-to-width aspect ratio but at times fails to provide sufficient reinforcement in certain applications because of poor gap bridging ability. The results of welding shipbuilding steel AH36 with thickness of 8 mm as a single-sided T-joint using a 10 kW fiber laser are presented and discussed in this research paper. Three optical setups with process fibers of 200 µm, 300 µm and 600 µm core diameters were used to study the possibilities and limitations set by the beam delivery system. The main parameters studied were beam inclination angle, beam offset from the joint plane and focal point position. Full penetration joints were produced and the geometry of the welds was examined. It was found that process fibers with smaller core diameter produce deeper penetration but suffer from sensitivity to beam positioning deviation. Larger fibers are less sensitive and produce wider welds but have, in turn, lower penetration at equivalent power levels.

Interaction between the laser beam and keyhole wall during high power fiber laser keyhole welding

The crucial factor of laser welding is the laser energy conversion. For a better understanding of the process, the interaction process between the laser beam and keyhole wall was investigated by observing the keyhole wall evaporation during high-power fiber laser welding. The results show that the evaporation vapor, induced by the laser beam, discretely distributed on the keyhole wall. A tiny 'hollow' zone was observed at the spot center-action region on the FKW. The evaporation vapor induced by the spot center moved downward along the front keyhole wall (FKW) with a period of about 0.3~0.75 ms, which indicates that the keyhole formation is reminiscent of a periodical laser drilling process on the FKW. The evaporation vapor on the keyhole wall suggest the assumption that the laser energy coupling mode in the keyhole was multiple-reflection, and the keyhole depth was mainly determined by the drilling behavior induced by the first absorption on the FKW.

Online detection method of weld penetration based on molten pool morphology and metallic vapor radiation for fiber laser welding

A novel detection method of penetration status was presented for a high-power fiber laser welding. The metallic vapor and molten pool was recorded by a high-speed camera during welding process. The radiation intensity of metallic vapor, as well as the morphology of molten pool end, was calculated by image processing algorithm as image features. Four image features, the radiation intensity of metallic vapor (RIMV), the area of molten pool end (AMPE), the rear angle of molten pool end (RAMPE), and the aspect ratio of molten pool end (ARMPE), were extracted. The mean value, relative range, variation coefficient, and frequency ratio were computed for the four features to obtain the 16 characteristic parameters. Aiming at penetration status, the characteristic parameters were reorganized to form two complex indicators by the principal component analysis. Experimental results showed that the detection method was potential for online detection on the penetration status in a high-power laser welding process.

Space-dependent characterization of laser-induced plasma plume during fiber laser welding

The role of a plasma plume in high power fiber laser welding is of considerable interest due to its influence on the energy transfer mechanism. In this study, the space-dependent plasma characteristics including spectrum intensity, plasma temperature and electron density were investigated using optical emission spectroscopy technique. The plasma temperature was calculated using the Boltzmann plot of atomic iron lines, whereas the electron density was determined from the Stark broadening of the Fe I line at 381.584 nm. Quantitative analysis of plasma characteristics with respect to the laser radiation was performed. The results show that the plasma radiation increases as the laser power increases during the partial penetration mode, and then decreases sharply after the initiation of full penetration. Both the plasma temperature and electron density increase with the increase of laser power until they reach steady state values after full penetration. Moreover, the hottest core of the plasma shifts toward the surface of the workpiece as the penetration depth increases, whereas the electron density is more evenly distributed above the surface of the workpiece. The results also indicate that the absorption and scattering of nanoparticles in the plasma plume is the main mechanism for laser power attenuation.

Effects of laser focusing properties on weldability in high-power fiber laser welding of thick high-strength steel plate

The effect of various high-power laser-welding parameters on obtaining deep penetration welds without weld defects has been investigated. However, there are no studies on the effect of laser focusing. In this study, high-power fiber laser welding of a 12-mm-thick high-strength steel plate was performed by using two optics systems with different power density distributions and focus depths (2 or 4 mm) to investigate the effects of laser focusing properties on weldability. Full penetration welds without weld defects were obtained with the 4 mm focus depth optics system at low welding speeds of 25–50 mm/s. High-speed video and X-ray transmission images showed that the behavior of the molten pool on the top surface during laser welding was more stable for the 4 mm system than for the 2 mm system. The keyhole was stable with no large fluctuations, and no bubbles were formed in the keyhole. This result was attributed to keeping the power density within 50–120 kW/mm2 to maintain a stable keyhole shape during the welding. Consequently, it was concluded that the laser-focusing properties during laser welding of a thick steel plate affected the weldability strongly, and the optics system with long focus depth was particularly useful in producing sound welds.

Laser-induced infrared characteristic analysis for evaluating joint deviation during austenitic stainless steel laser welding

During high-power fiber laser welding, molten pool thermal radiation contains plenty of information which is related to the welding quality. In order to study the relationship between the molten pool characteristics and the welding stability during high-power (10 kW) fiber laser butt joint welding of type 304 austenitic stainless steel plates, the laser beam spot was controlled to move along the weld joint center, parallel to the weld joint with deviation, and cross the weld joint with two different slopes. Under these four different experimental conditions, the infrared radiation characteristics of a molten pool infrared image in laser welding process was investigated based on welding thermal radiation and heat conduction mechanism. An infrared-sensitive high-speed camera was used to capture the molten pool dynamic infrared images during laser welding. The image processing and pattern recognition algorithm were applied to analyze the molten pool characteristics. The molten pool width, keyhole area, keyhole swing, keyhole sharp degree, and position coordinates were defined as the characteristic parameters of a molten pool to analyze the welding stability. It was found that the joint deviation would affect the welding stability which could be reflected by the welding thermal balance, temperature distribution of a molten pool, and the shape of the corresponding infrared thermal image. Welding experimental results showed that the stability of high-power fiber laser welding process could be estimated by the proposed infrared characteristic parameters.

Effect of plume on weld penetration during high-power fiber laser welding

To clarify the effect of the laser-induced plume on weld penetration during high-power fiber laser welding, this paper investigates the change in weld penetration, plume behavior and temperature, particles present in the plume, and the attenuation of the horizontal probe laser. Results show that the weld penetration had an improvement of about 20% as the plume was blown away by using a supersonic cross jet. The plume temperature at a height of 5 mm was approximately 5078 K. The weld penetration, plume temperature, and plume attenuation to a probe laser decreased as plume height increased. A plume height of approximately 80 mm resulted in a shape similar to that of a focused laser beam; a cone of white smoke composed of numerous particles appeared around the plume. Calculated and measured results indicate that the interaction of laser particles in the plume accounts for the primary impact on laser-plume interaction. It is found that, if the plume height increases, the weld penetration decreases correspondingly due to the increasing attenuation of the high-power fiber laser beam by the particles in the plume.