Molten Pool Area Research Articles

Experimental and Numerical Investigation of the Use of Ultrasonic Waves to Assist Laser Welding.

This study evaluates the enhancement of laser welding using ultrasonic waves aimed at reorganising the intermetallic position in such a fashion that leads to increased mechanical properties of welds in battery pack assemblies for electric vehicles. The experiment employed 20 kHz and 40 kHz High-Power Ultrasound Transducers (HPUTs) in both contact and contactless modes. A simplified experimental configuration is suggested to represent conditions similar to those found in electric vehicle battery pack assemblies. Measurements of vibration transmission to aluminium alloy 1050 plates revealed more than a 1000-fold increase in acceleration amplitude in contact mode compared to contactless mode. The 20 kHz transducer in contactless mode demonstrated superior performance, showing a 10% increase in load and 27% increase in extension compared to welding without ultrasonic assistance. On the other hand, the 40 kHz transducer, while still improved over non-ultrasonic methods, showed less pronounced benefits. This suggests that lower-frequency ultrasonic assistance (20 kHz) is more effective in this specific context. The study explores ultrasonic assistance in laser welding copper (Cu101) to aluminium alloy 1050 using 20 kHz and 40 kHz HPUTs, showing that both transducers enhance microstructural integrity by reducing copper homogenisation into aluminium, with the 20 kHz frequency proving more effective in this context. A numerical simulation was conducted to evaluate the transmission of pressure into the molten pool of the weld, correlated with the vibration results obtained from the 20 kHz transducer. The numerical simulation confirms that no cavitation is initiated in the molten pool area, and all improvements are solely due to the ultrasonic waves.

Application of ultrasonic vibration in magnesium alloy lap welding

In order to enhance the performance of magnesium alloy and galvanized steel welds, ultrasonic vibration was applied to the laser welding process, and the effect of ultrasonic on mechanical properties was verified through experimental methods. The laser welding system for the lap weld seam was designed, and the ultrasonic vibration module and image detection module were added, which can obtain the influence of ultrasonic vibration on the molten pool area. Under the conditions of ultrasonic vibration power of 1000 W and 0 W, the characteristics of the weld pool area, metallographic structure, tensile strength, fracture morphology, hardness, residual stress, wear resistance, and corrosion resistance of the weld specimen were compared. The image data acquisition structure indicated that ultrasonic vibration can effectively reduce the ineffective area of the molten pool and make the energy in the molten pool more concentrated. Under the influence of ultrasonic vibration, the maximum molten pool area decreased to 5.38 mm2, with a variation range of 3.9 %, and the proportion of pores was greatly reduced. Research found that ultrasonic vibration can significantly improve the microstructure characteristics of the fusion welding zone, with an average grain size reduced to 23 μm. The reduction of grain size and refinement of microstructure were beneficial to the improvement of mechanical properties of magnesium alloy joints, with a yield strength increase of 6.5 %. Ultrasonic vibration had little effect on the hardness of the heat affected zone, it can increase the average hardness of the weld zone by more than 5 % and reduce the maximum residual stress by more than 50 %. Under different pressure and friction speed conditions, the maximum wear amount can be reduced by more than 25 %. At the same time, the resistance to oxidation corrosion and electrochemical corrosion also can be improved to a certain extent.

Drop and hump behaviors in robotic arc-directed energy deposition with vertical position

Arc-directed energy deposition (Arc-DED) with the vertical position offers significant potential for in-situ fabrication of overhang structures. However, the vertical position application is seriously hindered by drop and hump defects since Arc-DED possesses serious heat accumulation resulting from multi-layer depositions. The novelty of this study is to reveal the formation mechanisms of drop and hump defects in vertical Arc-DED of low-carbon steel walls. Force models of the molten pool in vertical-down and vertical-up Arc-DED are established. Two indicators, i.e., the molten pool area and the molten pool dimensional ratio, are proposed to evaluate the probability of drop and hump occurrence, respectively. Furthermore, how the wire feed speed (WFS), travel speed (TS), and the constant ratio of WFS to TS affect the molten pool behavior is explored and discussed. In the vertical-down deposition, decreasing the TS or increasing the WFS will increase the possibility of drop defects occurring when the molten pool area exceeds 45 mm². In the vertical-up deposition, increasing the TS or the WFS will increase the possibility of hump defects occurring when the molten pool dimensional ratio exceeds 4.1. Maintaining the WFS/TS constant, the probabilities of hump and drop defects increase as the WFS and the corresponding TS increase. This study offers a better understanding of the formation mechanisms underlying the drop and hump phenomena and lays a solid foundation for the practical application of vertical Arc-DED to fabricate overhang features.

Study on the influence of processing technology on the properties of 2024-T3 aluminum alloy

2024-T3 aluminum alloy is widely recognized for its low density and excellent mechanical properties. However, it has some shortcomings at high temperatures. Using Abaqus simulation technology, the surface heat source during the VIES (Variable Input Energy Source) treatment process was simulated, with the energy originating from the discharge channel. Through orthogonal experimental methods, it was indicated that the distribution and depth of the molten pool varied under different process parameters. Meanwhile, electric spark surface strengthening mainly affected the surface of the workpiece, with minimal impact on the internal structure. When the impact frequency is constant, increasing the input current enlarges the maximum heat flux density (Qmax) at the center, leading to an expansion of the molten pool area. Conversely, when the input current is constant, increasing the impact frequency shortens the contact time between the electric spark and the plate, resulting in reduced heat input from the electric spark. Consequently, the surface molten pool area decreases.

Molten pool image processing and quality monitoring of laser cladding process based on coaxial vision

Aiming at the problems of the traditional detection method of cladding quality in laser cladding process, which relies on experience, has low accuracy and long recognition time, a method based on the image area of molten pool is proposed to detect cladding quality. Firstly, the median filter algorithm with dynamic adjustment of filter window is used to reduce noise and the iterative threshold segmentation algorithm is used to separate the molten pool from the boundary, then the edge of the molten pool is extracted by Canny algorithm which can adaptively change the high and low thresholds and finally the area value of the molten pool is obtained according to the correspondence between the pixel value and the actual value. The relationship between three key factors (nozzle angle, scanning speed and laser power) and molten pool area is analyzed to achieve favorable cladding quality. The results show that the influence of nozzle angle on the molten pool area is anisotropic. And the molten pool area decreases monotonically with the increase of scanning speed, while the higher velocity will destroy the formation of the molten pool. In addition, with the increase of laser power, the expansion of molten pool area is intensified. At the same time, the analysis of the change of molten pool area over time can quickly and accurately detect the three defects (protrusion, depression, and slag inclusion) in the process of cladding.

Experimental and numerical study on welding temperature field by double-sided submerged arc welding for orthotropic steel deck

Orthotropic steel deck (OSD) is susceptible to fatigue damage when subjected to repeated traffic loads, and the welding residual stress (WRS) is an important factor contributing to fatigue damage in OSD. WRS is formed by the shrinkage of the welds due to the inhomogeneous temperature field during the welds fabrication process, and thus the study of the welding temperature field is of significance for accurately predicting the WRS. As an emerging welding technology, the double-sided submerged arc welding (DSSAW) technology can generate higher welding temperature and consequently higher welding penetration rate on U rib and deck for welding thicker base metal. However, the DSSAW technology generates more complex welding temperature field compared to the traditional arc welding technology, which has not been evaluated so far. In this study, the finite element model (FEM) of OSD with U rib-to-deck (RTD) welded joints is established to simulate the welding temperature field generated by DSSAW. Moreover, the temperature field in OSD by DSSAW is measured by using the infrared thermal imager (ITI) and the industrial temperature measurement gun (TMG) to verify the FEM simulated temperature results. It is found that large surface temperature gradient and large temperature gradient along the thickness direction would be formed at the weld toe of U rib and deck, which is generally the location where the maximum WRS is distributed and where the fatigue cracks are susceptible in OSD. Compared to the inside welding, the outside welding may generate higher welding temperature, higher temperature gradient, and larger molten pool area, resulting in larger WRS in the OSD. The findings in this study can help to an in-depth understanding of the welding temperature field caused by the new DSSAW technology and support further studies on WRS.

Coaxial and synchronous monitoring of molten pool height, area, and temperature in laser metal deposition

Laser Metal Deposition is an additive manufacturing technology that enables to realize of one-of-a-kind large components with a free-form geometry. However, the advantages of laser metal deposition from the economic and sustainability points of view can be attenuated by the presence of defects in the realized components. The process should be real-time monitored to immediately identify the onset of defects or process drifts that may undermine the outcome of the build, also enabling the possibility to immediately act against them with feedback control. Many of the proposed solutions in monitoring the Laser Metal Deposition process exploit the use of only one sensor or more but decoupled ones, which does not provide a comprehensive overview of the process. In this work, a coaxial multi-sensor monitoring system is proposed and exploited to capture the variation of three different process signatures. A custom laser triangulator, a near-infrared camera and a ratio pyrometer for simultaneous monitoring of the molten pool height, molten pool area, and molten pool temperature, respectively, are coaxially integrated. Thanks to the possibility to extract concurrent (in spatial and time domains) signals directly from the workpiece area and in real-time, a comprehensive status of the deposition can be deduced. The influence of the process parameters on the sensed outputs is investigated, and the relationships between the process signatures can be researched. While the molten pool area and its temperature are mostly affected by the laser power, the scan speed is more impactful on the molten pool height. A good linear correlation between the molten pool temperature and the area is assessed, while no correlation with the molten pool height is found, making it almost independently manipulatable.

Analytical Simulation of the Microbubble Collapsing in a Welding Fusion Pool.

This paper explains the use of remote ultrasound vibration at the optimum position and frequencies to vibrate plates under welding, with the aim of initiating cavitation in the molten pool area. It has been shown in the literature that ultrasound cavitation changes microstructure morphology and refines the grain of the weld. In practice, the plates are excited through narrow-band high-power ultrasound transducers (HPUTs). Therefore, a theoretical investigation is carried out to identify the plate-mode shapes due to the ultrasound vibration aligned with the frequency bandwidth of HPUTs available in the marketplace. The effect of exciting the plate at different locations and frequencies is studied to find the optimum position and frequencies to achieve the maximum pressure at the area of the fusion zone. It was shown that applying the excitation from the side of the plate produces an order of 103 higher vibration displacement amplitude, compared with excitation from the corner. The forced vibration of cavitation and bursting time are studied to identify vibration amplitude and the time required to generate and implode cavities, hence specifying the vibration-assisted welding time. Thus, the proposed computational platform enables efficient multiparametric analysis of cavitation, initiated by remote ultrasound excitation, in the molten pool under welding.

Numerical simulation and experimental investigation of the effect of three-layer annular coaxial shroud on gas-powder flow in laser cladding

Laser cladding is a notable metal additive manufacturing (AM). The outstanding benefit of laser cladding is that the cladding process is more flexible and it can be completed in an open environment. However, oxidation phenomenon of active metals such as titanium alloys will unavoidably clad in the open environment. To solve this problem, a three-layer annular coaxial shroud (TACS) has been designed using computational approach and evaluated by experimental data. A four-stream nozzle of gas-powder computational fluid dynamics (CFD) model was established by employing Euler-Lagrange framework to analyze the powder feeding process. Simulation results show that when the velocities of inner-layer gas of TACS and carrier gas are equal, the powder stream exhibits the best concentration within molten pool area due to the formation of the stable laminar powder stream. When the flared angle of outer-layer gas equals 45°, the vortex toroidal flow moves away from the molten pool and the argon fills in the entire cladding region to form a high-quality barrier. The optimized parameters of TACS have been applied to the practical coaxial laser cladding of Ti-6Al-4V (TC4). A single-track cladding sample with lower height to width ratio and lower wetting angle can be observed due to the flattening of the shielding gas. The oxidation is greatly reduced.

Effect of ultrasonic vibration characteristics on mechanical properties of stainless steel laser weld

The laser welding efficiency of stainless steel is high, but the welding quality is still insufficient. In order to improve the welding performance of stainless steel, a controllable ultrasonic laser welding system was designed in the paper. Through numerical simulation and experimental test, the influences of ultrasonic vibration on the mechanical properties of weld were obtained. For the simulation study, the sound vibration coupling scheme was applied to the welding model. The closed environment model was established, and the characteristic frequency and sound pressure distribution of ultrasonic were simulated and analyzed by COMSOL software. In the aspect of performance test, through the real-time monitoring of the welding system, the effect of ultrasonic on the molten pool area, molten pool flow rate and metal vapor pressure is obtained. Under different ultrasonic excitation frequencies, the changes of weld microstructure, wear morphology, hardness and oxidation were studied. The results show that ultrasonic vibration can significantly improve the wear resistance, hardness and corrosion resistance of stainless steel welds. The ultrasonic energy efficiency at the second-order characteristic frequency is higher, the flow rate in the molten pool is low, and the joint quality is better.

Online monitoring of an additive manufacturing environment using a time-of-flight mass spectrometer

Laser-based Directed Energy Deposition (L-DED) is a promising additive manufacturing technology, which has broad application prospects and practical value in many fields. The nondestructive testing technology of L-DED work piece quality monitoring requires high accuracy and capability for real-time process. This work developed an online monitoring system based on a miniature electron impact ion source time-of-flight mass spectrometer (EI-TOF) and a high-speed camera, which monitored the atmosphere above the molten pool area. The number of spatters was measured using a high-speed camera. The composition of the samples and spatters was measured by using element analyzer, inductively coupled plasma mass spectrometry, and electron spectroscopy. The variation in the atmosphere was measured using EI-TOF, and the results confirmed that the O2, N2 and H2O content decreased during the L-DED process. At the laser power of 400 W, the oxygen consumptions were 92.5% and 86.4% compared with those at the laser power of 500 W and 600 W, respectively. During the L-DED process, the number of spatters decreased when the laser power was increased from 400 W to 600 W. And at the laser power of 600 W, the average number of spatters was reduced to 59.3% compared with the laser power of 400 W. It can be found that dust contained a large amount of virgin powder and little spatters. It also can be found that at the laser power of 400 W, 500 W and 600 W, the amount of oxygen in the dust was 0.544%, 0.242% and 0.159%, respectively. The relative content of oxygen in the cross section of the seam was 35.58%, 43.79% and 44.30%, respectively. The deviation of the monitoring results (H2O, N2, O2) are significant at the same power when different materials was used as the substrate.

Characteristics of droplet transfer, molten pool formation, and weld bead formation of oscillating laser hot-wire tungsten inert gas hybrid welding

Oscillating laser hot-wire tungsten inert gas (TIG) hybrid welding of 10 mm thick 316LN austenitic stainless steel in the vertical position was performed. The characteristics of droplet transfer, molten pool formation, and forming mechanism of different oscillating parameters were investigated by the experimental method. A high-speed camera was employed to observe the arc, droplet transfer, and molten pool formation. Results showed that the larger molten pool area, more intense fluid flow, and better weld bead formation were achieved when the oscillating laser joined to TIG as compared to TIG or normal laser-TIG hybrid welding. Due to the dynamic wire feed, the form of droplet transfer was mainly contact transfer, and the frequency of droplet transfer was equal to the frequency of wire feed and was independent of the parameters of the heat source. The electrical signals in the TIG and oscillating laser-TIG weld processes were collected, respectively, and the results showed that the change of conductivity might be the reason why the laser can compress the arc. The addition of laser increased the arc voltage and active power and improved the efficiency of hybrid welding.

Comparative investigation on single laser beam and dual laser beam for lap welding of aluminum alloy

Aluminum alloy is considered to be one of the ideal materials for light-weighting on aerospace and new energy automotive. Laser welding without a filler wire is an efficient way for weight reduction. The common defects in laser welding of aluminum alloys are porosities and cracks, resulting in poor mechanical performances and short service life of joints. In order to investigate the methods to suppress defects, 1 mm thick automotive aluminum alloy AA6014 was performed for single and dual-beam laser lap welding in this study. The effects of two welding processes on microstructure, cracks, and mechanical properties were investigated. The results showed that the welding adaptability of dual laser beam lap welding was much better than single laser beam lap welding. Well-formed weld could be obtained in a suitable line energy range by both welding processes. The dual beam with an energy ratio of 1:1 was at an angle along the welding direction, which could increase the molten pool area and stir the weld pool more sufficiently. Thus, dual laser beam lap welding could significantly increase the welding speed and alleviate collapse caused by single laser beam welding. The gaps of lap welding tended to be crack sources and generate cracks, while dual-beam laser welding was advantageous for reducing such cracks. This research is of great significance to improve the laser welding quality of all-aluminum automotive bodies, increase production efficiency, and further reduce the weight of the automotive.

Numerical simulation of bubble formation behavior in oxygen coal combustion melting and separating furnace

AbstractIn order to improve the mass transfer efficiency of the high‐temperature molten pool area in the oxygen coal combustion melting and separating furnace, water model experiment and VOF model were used to study the mechanism of bubble formation in the area, investigate the changing rules of oxygen lance parameters on the shape and frequency of bubbles. The results show that the basic law of bubble generation is the transition from lateral injection to longitudinal growth, and the bubble generation frequency is 7.5 Hz under standard conditions. Increasing the oxygen lance mass flow will reduce the generation frequency, but the local bubble distribution is more compact. Increasing the oxygen lance inclination angle increase the generation frequency, but it will cause local gas phase accumulation, if too close to the bottom of the furnace. Small oxygen lances diameter is beneficial to increase the generation frequency and enhance mass transfer efficiency. Increasing the oxygen lance immersion depth is conducive to increasing the generation frequency, the gas‐liquid interface and improving molten and separation efficiency. On the other hand, it will weaken the effect of “adherence to the wall” and reduce the erosion of furnace lining by inhibiting the flow of bubbles and slag.

Modeling three-dimensional rough surface and simulation of temperature and flow field in laser transmission welding

• A numerical model for rough surface is established by applying the anisotropic 3D surface reconstruction method. • A 3D FEM model considering rough surface is developed to simulate and analyze the temperature field, molten pool geometry and flow fields in the LTW process. • The temperature of the contact surface between the transparent and absorbent layer is discontinuous, which is consistent with the incomplete contact under real welding conditions. The traditional Laser Transmission Welding (LTW) simulation method assumes that the welding contact interface is smooth. However, the actual welding interfaces exhibit a large number of rough surfaces. The mismatch may decrease the accuracy of the traditional finite element model (FEM). Thus, this work aims to establish a three-dimensional (3D) FEM model considering the rough surface. Numerical model for 3D rough surface of welding specimen is established. The distribution of molten pool area, temperature and flow field coupling are discussed. Then, a series of experiments were set up to obtain the weld width to verify the model. The results show that the temperature of the contact surface between the transparent part and the absorbent layer is discontinuous, which is consistent with the incomplete contact of the contact surface under real welding conditions. The error of weld width values between the simulated and experiment is smaller, which also confirm that the established model is more precise.

Real-time monitoring of immobilized oxide defects on bead surface in welding of high-strength galvanized steel

Welding is widely used for joining steel components in the automobile industry. With the use of high-quality materials, welding defects began to become increasingly complex. Immobilized oxide is one of the recent complex defects. Immobilized oxide defects generated in gas metal arc welding (GMAW) of high-strength steel and high-strength galvanized steel result in unpainted part on the surface of welding beads. Thus, it is necessary to be eliminated by using grinding and peening. However, it is difficult to find where oxides generate and how oxides generate in order to eliminate it. Few studies have explored the mechanism of immobilized oxide formation in other fields so far. In this study, the generation patterns of oxide defects were confirmed. Further, an oxide defect monitoring system for GMAW was developed by using an infrared camera and image processing equipment. The chemical properties of the immobilized oxide were verified by performing energy dispersive spectroscopy (EDS). The oxide appeared to be brighter in the infrared image, and then, image processing was conducted to determine the boundary of oxide defects. The processed images were laid over the original images, and the oxide defect area was displayed on the monitoring panel. Patterns of oxide defect generation were confirmed when it was combined with the following molten pool area moving in the opposite direction of the welding direction in our results. The developed monitoring system can be easily applied to the automatic welding process in the automobile industry.

Quality detection of laser additive manufacturing process based on coaxial vision monitoring

PurposeThis paper aims to explore the influences of different process parameters, including laser power, scanning speed, defocusing distance and scanning mode, on the shape features of molten pool and, based on the obtained relationship, realize the diagnosis of forming defects during the process.Design/methodology/approachMolten pool was captured on-line based on a coaxial CCD camera mounted on the welding head, then image processing algorithms were developed to obtain melt pool features that could reflect the forming status, and it suggested that the molten pool area was the most sensitive characteristic. The influence of the processing parameters such as laser power, traverse speed, powder feed rate, defocusing distance and the melt pool area was studied, and then the melt pool area was used as the characteristic to detect the forming defects during the cladding and additive manufacturing process.FindingsThe influences of different process parameters on molten pool area were explored. Based on the relationship, different types of defects were accurately detected through analyzing the relationship between the molten pool area and time.Originality/valueThe findings would be helpful for the quality control of laser additive manufacturing.

Research on Quality Control of Arc Welding Robot Based on Molten Pool Contour extraction

In order to overcome the influence of various uncertainties on the welding quality in robot welding process, this paper proposes a multi-information fusion detection method. At first, in order to obtain the geometrical characteristics of the molten pool image, the adaptive Wiener filtering algorithm was used to denoise the image, and the Otsu threshold method was used to binarize the image. Then the binarized image was co rroded, swelled, removed and small area operation. The complete clear edge information of the weld pool is extracted; then the simple and effective geometric feature parameters are used to describe the shape of the weld pool. According to the characteristics of the arc welding temperature field, the method of radiation temperature measurement and the wavelength selection principle are discussed. From the arc temperature, wavelength and the angle of the thermometer with respect to the molten pool area, the interference of the arc at the temperature measurement point in the near-melting pool area was analyzed; It is proved quantitatively that the radiation temperature measurement method is feasible under the condition of small arc current.

Complementary use of pulsed and continuous wave emission modes to stabilize melt pool geometry in laser powder bed fusion

The most common defects in laser powder bed fusion (LPBF) namely porosity, geometrical errors, roughness and thermal deformations are principally linked with the energy input to the process. In common practice, a single set of process parameters is used to produce a whole component independently from the dimensions of the actual scan path within a given layer. However, melt pool stability is highly dependent on the scanned geometry. A possible strategy to maintain a constant melt pool is the mixed use of pulsed wave (PW) and continuous wave (CW) emission regimes. Accordingly, this work investigates the complementary use of continuous and modulated emission at fixed energy density on large and thin sections respectively. The proposed approach is tested on AISI 316L stainless steel and melt pool observations are conducted employing a coaxial monitoring system built for purpose. Temporally resolved measurements of intensity and geometrical properties of the melt pool were extracted as well as a three-dimensional spatial mapping of the molten pool area. The results demonstrate that moving from CW to a PW regime at the transition zones to thinner sections is effective in maintaining a constant melt pool size thus avoiding heat build-up and part extrusion from the powder bed.

Simulation of molten pool dynamics and stability analysis in laser buttonhole welding

Molten pool dynamics in laser buttonhole welding with wire feeding and laser beam oscillation were studied using computational fluid dynamics (CFD). The simulation results, such as temperature fields and molten pool velocities, were used to analyze the characteristics of molten pool dynamics. The dynamics in the rear part of the molten pool area were significantly reduced in comparison to those in the front when a buttonhole (a cavity in the molten pool) was present.