Characteristics Of Molten Pool Research Articles

Manipulating molten pool in in-situ additive manufacturing of Ti-22Al-25 Nb through alternating dual-electron beams

Wired-based in-situ additive manufacturing (AM) technology couples in-situ metallurgy for material-synthesis and near net-shaping for part-fabrication. However, the manipulation and control of the in-situ metallurgy quality is one of the most critical challenges it faced now, as insufficient metallurgy not only causes inherent defects but also leads to fatal damages to the deposited component. Till now, there is almost no effective way to manipulate it. In this study, we proposed to use the alternating dual-electron beam to flexibly control the molten pool, thereby, achieving the purpose of manipulating the in-situ metallurgical process. Ti-22Al-25Nb (at.%) alloy was taken as the target alloy and fabricated in-situ with the raw materials of pre-alloyed TiNb wire and pure-Al wire. Results show that this technology not only exhibits powerful control ability to the molten pool characteristics but also brings additional in-situ co-frequency forced vibration behavior to the molten pool. The regulated elongated molten pool, as well as its vibration behavior further promotes composition homogenization, porosity reduction(densification as high as 99.999%), and grain refinement(about 32%). As a result, the tensile strength of the in-situ deposited components is as high as 993.6±32.4 MPa on average. In addition, employing an alternating heat source model, numerical simulation was also carried out for revealing the unique temperature evolution and molten pool flow mechanism of the alternating dual-beam AM technology. This work not only provides valuable guidance for the regulation of the in-situ metallurgy quality in the wired-based in-situ AM field but also supplies an unprecedented processing method for the Ti-22Al-25Nb alloy. • Ti-22Al-25Nb (at%) alloy was in-situ manufactured through twin-wire dual-electron beam AM technology. • The alternating dual-electron beam was employed to manipulate the in-situ metallurgical inside the molten pool. • The microstructure and mechanical properties of the dual-beam as-deposited sample were revealed. • The temperature evolution and molten pool flow mechanism during the alternating dual-beam AM process were studied. • The relationship between molten pool vibration behavior and grain refinement was explored.

Cracking mechanism and control of Hastelloy X prepared by laser powder bed fusion

In this study, orthogonal experiments were carried out under different processing parameters to tackle the cracking issue of Hastelloy X (HX) fabricated by laser powder bed fusion (LPBF). The metallurgical defects, molten pool characteristics, cracking and residual stresses under various parameters were studied. The results show that when the volume energy density (VED) is controlled within the range of 83.33–200.00 J/mm3, the as-built samples have a greater density and a smoother surface. Cracking propagation occurs mostly along high-angle grain boundaries, and Mo6C carbides develop at cracks and between dendrites, which are liquation cracking characteristics. Cracks can be effectively suppressed by altering the morphology of the molten pool and minimizing residual stress. The cracking density of as-built HX can be lowered by 93% after optimizing the process settings so that the produced samples with nearly full density and minimal surface roughness can be obtained.

2000W Blue laser directed energy deposition of AlSi7Mg: process parameters, molten pool characteristics, and appearance defects

ABSTRACT Aluminium alloys are difficult to be manufactured by directed energy deposition (DED) with the widely used infrared laser because of the low absorptivity. A blue laser with a 450 nm wavelength has a significantly higher laser absorptivity in processing aluminium. Here, we developed a 2000 W blue laser directed energy deposition (BL-DED) system for additive manufacturing. In this system, a coaxial camera, a paraxial high-speed camera, and a paraxial infrared camera were simultaneously adopted to analyze the formation and evolution of molten pool characteristics. The relationships among the molten pool characteristics, depositing states, and appearance defects were built. We successfully printed the curved thin wall with good appearance quality by the manual in-situ control of appearance defects. This work demonstrated the advantages of the blue laser in DED of aluminium alloy, which could melt the powders under lower energy input conditions and result in a more stable deposition process.

Investigation of IR pyrometer-captured thermal signatures and their role on microstructural evolution and properties of Inconel 625 tracks in DED-based additive manufacturing

In Laser Directed Energy Deposition (DED-L) process, based on the particular thermal cycle followed during the laser processing, a wide range of microstructural and dimensional outcomes are envisaged in the end product on account of the interaction of the material or the alloy with the laser. Various parameters can influence the thermal cycle and a direct correlation between the thermal characteristics with the mechanical/metallurgical properties would be helpful to control the process in a better way. Keeping in view of this, the present study is aimed at analyzing thermal signals received from infrared pyrometer and understanding the correlation between thermal signatures, phase prediction data using CALPHAD simulation, microstructure and deposition characteristics. The thermal signatures captured from infrared pyrometer revealed close interrelationship between the input process parameters and the molten pool characteristics, which have a huge influence on the solidification rate vis-à-vis microstructural properties. Total life of melt-pool showed an increase in magnitude from 137 ms to 264 ms with decrease in cooling rates from 3000 °C/s to 1750 °C/s. The grain size increased from 3.48 μm to 7.74 μm with increase in solidification shelf life from 32 ms to 110 ms. The higher solidification shelf life and lower cooling rates also resulted in the larger and the more prominent metal carbide precipitate and intermetallic formations as evident from XRD analysis. Microhardness of the deposits also increased with decrease in solidification shelf life and increase in cooling rates of molten pool. The dimensions of laser deposited tracks were also found to increase with increment in total molten pool lifetime both at increasing laser power and decreasing laser scan peed conditions.

Study of Microstructure and Properties of AZ91 Magnesium Alloy Welded Joint with Magnetic Field and TiO2 Activated Flux

In order to improve the weldability and bearing capacity of AZ91 magnesium alloy welded joints, magnetic field and active flux were added in the TIG welding process. In the welding process, the magnetic field and welding parameters were unchanged, and the coating amount of active flux was adjusted. The formability, mechanical properties, and microstructure of the welded joints under different coating amounts of activated flux were analyzed, and the crystallization nucleation characteristics of molten pool were discussed. The experimental results reveal that the combined effect of the magnetic field and activated flux has a significant effect on increasing the penetration and promoting mechanical properties. When the coating amount of activated flux is 3 mg/cm2, the highest penetration of the welded joint is obtained, which is 141% of that without activated flux. Meanwhile, the mechanical properties reach the maximum, which is a tensile strength of 292 MPa, elongation of 11.2%, and weld zone hardness of 75.6 HV (0.5 Kgf). The combined effect of TiO2 flux and magnetic field does not change the phase composition and the grain orientation of a weld metal but can affect the grain size. The average grain size of a weld metal under an activated flux coating amount of 3 mg/cm2 is 18.2% smaller than that under an activated flux coating amount of 1 mg/cm2.

Equivalent heat source approach in a quasi-steady-state laser-GMAW hybrid welding simulation

A quasi-steady-state laser-GMAW hybrid welding simulation method was developed. The comprehensive effects of buoyancy, thermal capillary force, electromagnetic force and arc shear force were considered in the model, thus the temperature and flow fields characteristics of molten pool were obtained. At the same time, the heat source motion was realized based on the deformed geometry (DG) approach. The model can be applied to laser welding processes with different joint configurations or moving paths.

Prediction of molten pool temperature and processing quality in laser metal deposition based on back propagation neural network algorithm

Stability of molten pool temperature directly affects the dimensional accuracy, metallurgical defects, solidification microstructure and mechanical properties of formed parts manufactured by laser metal deposition technology. Therefore, the optimization of the macroscopic morphology and microstructure of the formed parts through the stable control of the molten pool temperature has been intensively studied. In this study, three models, based on the back propagation neural network (BPNN), random forest (RF) algorithms and response surface methodology (RSM) approach, respectively, were used to establish prediction relationships between the deposition input parameters and processing status parameters, geometric morphology and mechanical property parameters. Then, the processing temperature and molten pool characteristics of thirty groups of 316L stainless steel single-track cladding layers were analyzed. The prediction results show that the average prediction error (APE) of the prediction of molten pool temperature, track width, track height and micro-hardness of the deposited layer based on BPNN model are 0.5%, 1.3%, 2.9% and 0.1%, respectively, which are better than the prediction results of RF and RSM models. Then, BPNN model was further used to predict the molten pool temperature and processing quality of the deposited layer under the combination of five new process parameters groups. Objective of this study was to lay the foundation for the subsequent design of the molten pool temperature control system to improve the morphology accuracy and mechanical properties of formed parts.

The investigation of molten pool dynamic behaviors during the “∞” shaped oscillating laser welding of aluminum alloy

To reduce the pore defect during the laser welding of aluminum alloy, a 3D numerical model is developed to study the dynamic behaviors characteristics of molten pool in the oscillating laser welding. The “∞” shaped oscillating laser welding process of 5052 aluminum alloy is calculated by the numerical model. The characteristics of morphology, temperature field, flow field of the molten pool are analyzed and discussed in details by the numerical calculation. The results show that the molten pool formed in the “∞” shaped oscillating laser welding is wide and shallow and the bottom part of the molten pool is flat. The complex fluid flowing characteristics induced by the formed vortexes are found in the molten pool. The influence of oscillating amplitude and frequency on “∞” shaped oscillating laser welding is also investigated. It is clearly seen that the maximum temperature and maximum flow velocity of the molten pool are decreased with the increase of oscillating amplitude or frequency and then the molten pool becomes more stable and shallower, which is benefit for the bubble escaping and weld porosity reduction. Therefore, the proposed numerical simulation method for the “∞” shaped oscillating laser welding is efficient for the weld porosity reduction and welding quality improvement in the laser welding of aluminum alloy.

Finite Element Analysis and Computational Fluid Dynamics Verification of Molten Pool Characteristics During Selective Laser Melting of Ti-6Al-4V Plates.

The finite element (FE) method is used to characterize the thermal gradient, solidification rate, and molten pool sizes of Ti-6Al-4V plates in the process of selective laser melting (SLM). The results are verified by using the computational fluid dynamics (CFD) simulation. The proposed FE model contains a series of toolpath information that is directly converted from a G-code file, including hatch spacing, laser power, layer thickness, dwell time, and scanning speed generated by using Slic3r software from a CAD file. A proposed multi-layer, multi-track FE model is used to investigate the influence of the laser power, scanning speed, and scanning path on the microstructure in the Ti-6Al-4V plate built via SLM. The processing window is also determined based on the proposed FE model. The FE results indicate that, with a decrease in the laser power and an increase in the scanning speed, the morphology of the crystal grains, showing fully columnar crystals, gradually deviates from the fully equiaxed region. The formed grains are dependent on the laser power, scanning speed, and deposition position, but they are not sensitive to the scanning path, and with the deposition from the bottom layer to the top layer, the size of the formed grains is gradually increasing, which shows a good agreement with the experimental results.

Selective laser melting of IN738 superalloy with a low Mn+Si content: Effect of energy input on characteristics of molten pool, metallurgical defects, microstructures and mechanical properties

IN738 superalloy with excellent high-temperature performance is of huge interest for hot-end component applications in aerospace, but its additive manufacturing via selective laser melting (SLM) is rather challenging due to the unsolved high crack susceptibility. Here, for the purpose of improving the formability of IN738 in SLM, Mn + Si content was then strictly controlled in raw powder. An orthogonal experiment was then carried out under different energy inputs to investigate the characteristics of the molten pool, metallurgical defects, microstructures, and mechanical properties of IN738 by SLM. The results showed that high laser power was more likely to shape large melting depth and depth-to-width ratio under the same linear energy density (LED), rather than scanning speed. Four morphologies of the molten pool were observed in the entire LED range. At lower LEDs, hot cracks were usually accompanied by irregular pores due to the stress concentration at the sharp corner of pores; at higher LEDs, cracks were located at the interface of grain boundaries with large misorientation angles. Energy dispersive spectrometry results revealed that the presence of Al-rich oxides and high Mn + Si content at the interface were the main reasons for cracking. The Scheil-Gulliver solidification modeling was used to examine the cracking susceptibility of Si and Mn elements. In addition, both dendrites and cellular dendrites were found in columnar grain structures. By reducing the Mn + Si content and optimizing process, crack-free and nearly fully dense samples of IN738 were obtained by SLM with excellent mechanical properties (ultimate tensile strengths = 1206 MPa, yield strength = 866 MPa and elongation = 16.5%), which were much higher than the mechanical properties of conventionally manufactured IN738.

Multiscale investigation of microstructure optimization in the arc additive manufacturing and arc welding by self-induced ultrasound

In the arc welding and arc additive manufacturing processes, utilizing self-induced ultrasound has attracted widespread attention for obtaining intensive arc plasma and optimizing metallurgical characteristics of molten pool, featuring with increased penetration, suppressed dendrites and refined grains in the deposited layer and weld joint of metal. However, the ultrasound emitting mechanism remains unclear and process optimization remains limited by incomplete understanding of ultrasound in the arc plasma and molten pool. In our work, Multiscale experiments and modeling (MHD and MHD waves modeling at meso scale, coupled phase-field and fluid flow modeling and fluid-structure modeling at micro scale) are proposed to reveal the physical essence of ultrasound generation, ultrasound induced streaming, suppressed dendrites growth and dendrites fragmentations.It demonstrates the effectiveness and efficiency of utilizing self-induced ultrasound in controlling microstructures of 316 stainless steel with a maximum of 24% decrease in grain size and transition from anisotropy orientation to isotropy orientation, and concrete directions of process optimization arc identified for facilitating the successful industrial use. We expect introducing self-induced ultrasound energy could be a more efficient and cost effective technology for controlling of microstructure in the arc metal processing related fields.

Molten pool characteristics of a nickel-titanium shape memory alloy for directed energy deposition

Fabrication of nickel-titanium shape memory alloy through additive manufacturing has attracted increasing interest due to its advantages of flexible manufacturing capability, low-cost customization, and minimal defects. The process parameters in directed energy deposition (DED) have a crucial impact on its molten pool characteristics (geometry, microstructure, etc.), thus influencing the final properties of shape memory effect and pseudoelasticity. In this paper, a three-dimensional numerical model considering heat transfer, phase change, and fluid flow has been developed to simulate the cladding geometry, melt pool depth, and deposition rate. The experimental and simulated results indicated that laser power plays a critical role in determining the melt pool width and deposition rate while scan speed and powder feed rate have less effect on cladding geometry and deposition rate. The fluid velocity has a huge influence on the distribution of elements in the molten pool. The temperature gradient G, solidification rate R, as well as shape factor G/R were calculated to illustrate the underlying mechanisms of grain structure evolution. The grain morphology distribution of cross-section from the experimental samples agreed well with the simulation results. The model reported in this paper is expected to shed light on the optimization of the deposition process and grain structure prediction.

A fast method for monitoring molten pool in infrared image streams using gravitational superpixels.

Additive manufacturing (AM) is one of the most trending areas in production that allows creating three-dimensional objects according to a predetermined design. AM finds its application in all kinds of niches, from medicine to the aerospace industry, although there are still several technological barriers that must be addressed. For example, monitoring techniques that guarantee decision-making to guarantee quality and repeatability of processes. An imaging-based methodology is presented to monitor and extract thermal and geometric characteristics of molten pool in real-time. A superpixel-based approach is proposed to reduce the dimensionality of the infrared images and facilitate the segmentation and tracking tasks. This algorithm is called gravitational superpixels. Using the color and temperature features, our algorithm groups the pixels. These superpixels have better adherence to the structures that form the images. Facilitating the segmentation tasks. Our algorithm is compared against superpixel-based and saliency-based already reported works. To validate the performance, infrared-image streams (LMD process) and standard datasets are using. The proposed algorithm has a molten pool segmentation uncertainty of $$0.1\; mm$$ . Reported results show that the performance of our proposal is applicable for tasks that require good precision when segmenting and fast runtime. It is important to highlight the relevance of this work for additive metal manufacturing processes.

Nonintrusive estimation of subsurface geometrical attributes of the melt pool through the sensing of surface oscillations in laser powder bed fusion

Molten pool geometry, whose surface parameters may be extrapolated through direct process observations, has been identified as a fundamental indicator of stability in laser powder bed fusion (LPBF). However, a parameter that cannot be directly measured on industrial systems by means of conventional sensing equipment is the molten pool depth. Indeed, methods based on x-ray imaging demonstrated in the literature have helped to better understand the process. However, retrofitting such solutions to industrial systems does not appear as a viable route currently. Within the present investigation, a nonintrusive sensing method for the indirect measurement of subsurface molten pool geometry based on the detection of surface oscillations is presented. The analysis of frames acquired using a high-speed camera and a secondary illumination light allows the identification of the crests of capillary waves through bright reflections on the surface of the molten pool. The characteristic oscillation frequency of the surface ripples may be correlated with the penetration depth or to other subsurface geometrical parameters. Proof of concept testing of the sensing principle was conducted on two different materials, namely, AISI316L and IN718, by means of single track LPBF depositions. Experiments were conducted at different levels of laser emission power to induce variations in molten pool characteristics. The process was observed by employing an off-axis illumination light and a high-speed camera, which allowed acquisitions with high spatial and temporal resolution. The acquired frames were postprocessed to extract the oscillation indicator, and analysis of the power spectral density of the signal allowed for the identification of the oscillation frequency. Results show that oscillation frequencies range from 3 to 5.5 kHz. Molten pool penetration depth and cross-sectional area could be correlated with the oscillation frequencies for the inline detection of these parameters during LPBF depositions. For both materials, higher oscillation frequencies corresponded to a shallower molten pool and a smaller mass of molten material. Moreover, different characteristic curves of oscillation frequency variations as a function of the melt pool cross-sectional area were determined for IN718 and AISI316L.

Investigation on coaxial visual characteristics of molten pool in laser-based directed energy deposition of AISI 316L steel

Abstract Laser-based directed energy deposition (LDED), an additive manufacturing technique, has received increasing attention from the industry and academia. Unstable reliability and poor repeatability in the production of metal parts are two challenges currently faced by LDED. In-situ monitoring and adaptive control technology based on visual characteristics serves as effective methods to ensure part quality and process stability. Although molten pools directly influence metallurgical behaviors in LDED, their visual characteristics still require further investigation. In this work, a coaxial visual image of a molten pool was captured during the LDED of 316 L material and divided into three regions. Then, the evolutionary behaviors of different regions during single-layer single-track processing, multi-layer one-track stacking, and one-layer multi-track overlapping were analyzed. Additionally, the metallurgical behaviors of the molten pool, including the formation mechanisms of slag characteristics, were explicated. Results showed that molten pool characteristics could be presented and described with visual images reasonably. These characteristics could indicate the state of melting, heat conduction and convection, and overlapping position. Meanwhile, coaxial visual characteristics were closely related to product qualities. This work could enhance the understanding of LDED process mechanisms and improve real-time monitoring capability. The proposed approach for the determination of molten pool characteristics is expected to be applicable to other materials.

TIG Stainless Steel Molten Pool Contour Detection and Weld Width Prediction Based on Res-Seg

As the basic visual morphological characteristics of molten pool, contour extraction plays an important role in on-line monitoring of welding quality. The limitations of traditional edge detection algorithms make deep learning play a more important role in the task of target segmentation. In this paper, a molten pool visual sensing system in a tungsten inert gas welding (TIG) process environment is established and the corresponding molten pool image data set is made. Based on a residual network, a multi-scale feature fusion semantic segmentation network Res-Seg is designed. In order to further improve the generalization ability of the network model, this paper uses deep convolutional generative adversarial networks (DCGAN) to supplement the molten pool data set, then performs color and morphological data enhancement before network training. By comparing with other traditional edge detection algorithms and semantic segmentation network, it is verified that the scheme has high accuracy and robustness in the actual welding environment. Moreover, a back propagation (BP) neural network is used to predict the weld width, and a fitting test is carried out for the pixel width of the molten pool and its corresponding actual weld width. The average testing error is less than 0.2 mm, which meets the welding accuracy requirements.

Study on keyhole coupling and melt flow dynamic behaviors simulation of 2219 aluminum alloy T-joint during the dual laser beam bilateral synchronous welding

In this research, the keyhole coupling and melt flow behaviors of 2219 aluminum alloy T-joint during dual laser beam bilateral synchronous welding (DLBSW) process are investigated by simulation, which involves fluid heat transfer and fluid flow. The calculated mesh model of the T-joint is established by simplifying the equal boss of the skin. The combined Gaussian laser heat source is adopted to simulate the laser energy loaded on the base metal during the welding process. The obtained simulation results are reliably matched with the experimental results. Subsequently, not only the coupling process of two keyholes and the evolution characteristics of the coupled keyhole are analyzed, but also the characteristics of molten pool and the flow behaviors of the liquid metal are studied in detail. The results demonstrate that the keyhole profile’s oscillations continuously exist, whether before or after the keyhole coupling. The flow direction of the liquid metal near the coupled keyhole wall includes the flow parallel to the keyhole wall and that around the keyhole wall. Besides, the maximum depth of the molten pool slightly lags behind the rear wall of the keyhole. On the coupled keyhole wall and molten pool surface, the melt flow velocities are significantly greater than that inside the molten pool. The vortexes are produced near the upper and lower regions of the coupled keyhole wall.

Real-time Process Monitoring of Laser Welding by Infrared Camera and Image Processing

Understanding and predicting relationships between laser welding process parameters, such as laser power and welding speed, and molten pool have been studied widely in order to critically control and improve laser welding. The laser welding processes are difficult to monitor in real time because of high temperature and rapid heating characteristics. In this study, infrared camera was set to collect data and provide real time monitoring system to determine the molten pool characteristics and weld quality. This study carried out a laser welding of SS400 low carbon steel and analyzed real-time image of the welding process to determine the average temperature of molten pool and calculate the size of molten pool. By varying the laser power and the welding speed, the infrared camera and imaging processing technique can monitor change of molten pool temperature in a range of 1000 C to 15000 C with about 1% temperature fluctuation. In addition, the size of molten pool can be calculated from the temperature profile of the welding zone. The calculated molten pool size was about 95% accurate compared to the measured size from microscope imaging.

Investigate on the porosity morphology and formation mechanism in laser-MIG hybrid welded joint for 5A06 aluminum alloy with Y-shaped groove

Abstract Laser-MIG hybrid welding is an advancing technology for the joining of aluminum alloy with the medium-thickness owing to the high efficiency and excellent gap bridging ability. In this paper, the laser-MIG hybrid welding experiment for 5A06 aluminum alloy with the thickness of 6.9 mm is carried out. The process porosities and metallurgical porosities in the weld cross-section and tensile fracture are investigated. Furthermore, it is novelty that the formation mechanism of process porosities and metallurgical porosities are comprehensively analyzed based on the porosity morphology after welding, as well as characteristics of molten pool and keyhole during laser-MIG hybrid welding. It is found that the formation of process porosity is owing to the fact that the tip of the keyhole is easily closed by the molten metal stream and separated to form a process bubble. The formation of the metallurgical porosity is attributed to the atomic hydrogen. The inner wall of the metallurgical porosity with the second phase is observed. It is inferred that the formation of second phase is accompanied by the atomic hydrogen. Besides, the area with dense metallurgical porosities and the aggregate metallurgical porosity are formed under the driven force of the liquid metal flow behavior. The formation process of aggregate metallurgical bubble goes through three states, separation state, contact state and aggregate state.

Observing molten pool surface oscillations during keyhole processing in laser powder bed fusion as a novel method to estimate the penetration depth

Various in-situ monitoring techniques have been developed for the detection of process drifts in the Laser Powder Bed Fusion (LPBF) process. Currently, optical emission monitoring can retrieve information regarding molten pool characteristics, such as temperature, width, length and area which provide substantial process signatures. Nonetheless, a fundamental indicator for the retrieval of a complete set of spatially distributed information is missing: the molten pool depth. Within the present investigation, a system for the estimation of the penetration depth based on the detection of molten pool surface oscillations is reported. Initially, the fundamentals of the monitoring technique are presented. The principle relies upon the observation of molten pool surface ripples through the measurement of probe light reflections in the melt area. Proof of concept testing of the sensing principle was conducted through an experimental investigation on a prototypal platform. A monitoring system (consisting of a high-speed camera and a secondary illumination light) was employed to view the process while realising both bead-on-plate material remelting and single track powder bed fusion depositions of AISI316L at different levels of laser emission power. Oscillation frequencies were extracted from the high-speed imaging acquisitions after image processing and signal analysis. The surface wave oscillations were measured to be in the range of 3.5–5.5 kHz in keyhole conditions. Metallographic cross-sections allowed to observe the effective molten pool penetration depth and cross-sectional area and were correlated to oscillation frequencies. Higher values of oscillation indicated shallower penetration and consequently a smaller mass of molten material.