Molten Pool Size Research Articles

Development of sensing and control system for robotized laser-based direct metal addition system

Laser-based direct metal addition (LBDMA) is a promising directed energy deposition technology that is well suited for the production of complex metal structures, low-volume manufacturing, and high-value component repair or modification. LBDMA is finding wide application in the automotive, biomedical, and aerospace industries. However, the process reliability and the repeatability of finished components are still problems. This work offers a solution by developing a sensing and control system for the robotically controlled 8-axis LBDMA system developed at the Research Center for Advanced Manufacturing of Southern Methodist University, Dallas, TX. The developed system consists of sensing and control units for the powder flow rate and the molten pool size. An optoelectronic sensor was developed to sense the powder flow rate. It is a main component in an on-line control system of powder flow rate in a LBDMA system. An infrared imaging setup was installed on the laser head to monitor the top full-field view of the molten pool. A simple proportional integral derivative (PID) controller, combined with feed-forward compensation was used to build a closed-loop control system for achieving a uniform molten pool size. Two L-shaped single-bead walls were built with and without closed-loop control, respectively. A good performance on achieving uniform geometry by closed-loop control of the molten pool size was approved.

Microstructure and mechanical properties of Ti-6Al-4V components fabricated by laser micro cladding deposition

Laser micro cladding deposition manufacturing (LμCDM) is a newly developed rapid manufacturing method for metals. The LμCDM technology adopts a novel powder feeding method based on alternating friction and inertia force, and this powder feeding method can effectively improve the accuracy and orientation of the powder injection, resulting in a smaller molten pool size and a higher cooling rate of liquid metal. Therefore, the components fabricated by LμCDM could obtain the finer microstructures and the improved mechanical properties. It is found that the components fabricated by LμCDM are fully dense free of cracks or pores and exhibit columnar prior β grains with a finer acicular α′ phase microstructure. The microhardness (HV0.2) of the thin-wall component is HV 400–HV 500 in the majority part of the cross section and can reach about HV 850 in the top region. The ultimate tensile strength (UTS) and elongation show insignificant dependence on the testing directions of the tensile specimens. The UTS is between 1,002 and 1,100 MPa, and the elongation is between 10.0 % and 14.7 %.

Numerical simulation of AZ31B magnesium alloy in DE-GMAW welding process

The prerequisite to predict the welding temperature field accurately is to select the thermal efficiency η correctly. In this paper, the AZ31B magnesium alloy was used to study the thermal efficiency η of the bypass arc in double-electrode gas metal arc welding (DE-GMAW) process. The experimen- tal molten pool images were captured using a charge-coupled device (CCD) camera. Then, the molten pool size was extracted using an image-processing algorithm. A finite element model capturing the features of DE-GMAW welding was established by using ANSYS software. The simulated values of the molten pool size were obtained according to the numerical simulation results of the tem- perature field in DE-GMAW welding. By comparing the simulated molten pool sizes with the experimentally mea- sured values, it was found that the best match is obtained when η=0.28∼0.32. In addition, the simulation results show that when only the bypass current decreases, the weld width and the weld penetration depth, as well as the heat input of the base metal, increase. These results are in good accordance with the experimental observations.

Comparison of weld characteristics between longitudinal seam and circumferential butt weld joints of cylindrical components

The control of weld induced residual stresses and distortions are very important in longitudinal seam (L-Seam) and circumferential (C-Seam) butt joints of cylindrical components. In this study, a three-dimensional, sequentially coupled thermo-metallurgical-mechanical analysis was performed for L-Seam and C-Seam butt weld joints of AISI 304 cylindrical components to evaluate weld characteristics during gas tungsten arc welding (GTAW) process. Initially, weld experiments were carried out to obtain the heat input and the macrographs which were further validated with the predicted weld molten pool sizes during heat source fitting (HSF) analysis. The Goldak's double ellipsoid heat source function obtained in HSF analysis was employed in the thermal and mechanical analysis. The results reveal that there are significant differences in residual stress distributions between L-Seam and C-Seam butt joints. Full scale shop floor welding experiments were performed to verify the effectiveness of the proposed numerical models and these are in good agreement with the experimental measurements.

Three-Dimensional Temperature Gradient Mechanism in Selective Laser Melting of Ti-6Al-4V

Selective laser melting (SLM) is widely used in making three-dimensional functional parts layer by layer. Temperature magnitude and history during SLM directly determine the molten pool dimensions and surface integrity. However, due to the transient nature and small size of the molten pool, the temperature gradient and the molten pool size are challenging to measure and control. A three-dimensional finite element (FE) simulation model has been developed to simulate multilayer deposition of Ti-6Al-4 V in SLM. A physics-based layer buildup approach coupled with a surface moving heat flux was incorporated into the modeling process. The melting pool shape and dimensions were predicted and experimentally validated. Temperature gradient and thermal history in the multilayer buildup process was also obtained. Furthermore, the influences of process parameters and materials on the melting process were evaluated.

Parameter determination of double-ellipsoidal heat source model and its application in the multi-pass welding process

The parameters of the heat source model have significant influence on the temperature field and sequentially affect the residual stress field. In this paper, a neural-network programme based on the Levenberg–Marquardt algorithm is developed to predict the parameters of Goldak's double-ellipsoidal heat source model. The analytical solution of the heat conduction equation based on the double-ellipsoidal heat source is obtained by integrating a series of instant point heat sources over the volume of the ellipsoidal heat source. The transient temperature distribution and the sizes of the molten pool are obtained under various welding processes by using the analytical method. Then, a neural-network programme is employed to train and predict the heat source parameters. These results of temperature and molten pool size obtained by the numerical simulation with the predicted heat source parameters are calibrated by the published experimental results. The numerical results show a good agreement with the experimental measurements. Finally, the developed Levenberg–Marquardt neural network is employed to predict the heat source parameters in the multi-pass welding process in the laboratory. By comparing the finite element (FE) numerical results with experimental results, the heat source parameters have been successfully identified in the multi-pass welding process.

Growth Characteristics of TiCxNy Ceramics under Laser Melting Conditions

By laser melting technique, TiCxNy ceramics are prepared under the laser powers of 300, 400, 500, 600, and 700 W, respectively. The size of molten pool decreases with the increase of scanning speed from 1 to 4 mm/s at a certain power. TiCxNy microstructure continuously changes from the top to bottom of molten pool. Based on the XRD spectrum, the composition of ceramic can be confirmed as Ti(O0.19C0.53N0.32), C0.7N0.3Ti, C2MoTi, and Ti8C5. Based on the analysis of the growth process, the growth conditions can be confirmed. The maximum depth of molten pool obtained at the laser energy density of 500 W/cm2. The more deviation from this value, the more shallow the pool. The corrosion experiment result shows that TiCxNy ceramic has good corrosion resistance to various corrosive agents except HNO3.

Characteristics of weld pool behavior in laser welding with various power inputs

This paper investigates the numerical simulations of multi-kilowatt disk laser and fiber laser welding, ranging from 6 to 18 kW to study the behavior of molten pool in 20-mm-thick steel plate by using Volume-Of-Fluid (VOF) method and several mathematical models like Gaussian heat source, recoil pressure, Marangoni flow, buoyancy force, and additional shear stress and heat source due to the metallic vapor. Vortex flow pattern is observed for higher laser power except for 6-kW case, and the higher the laser power, the bigger the vortex flow pattern. Welding speed has an influence on molten pool in terms of depth of penetration and size of molten pool, but overall shape of molten pool remains the same. The reasons for the vortex flow pattern in high-power laser welding are the absorption of more energy at the bottom of keyhole, which promotes more liquid metal at the bottom, while for lower power with lower speed, the melt formation is more uniform in the thickness direction and most of the molten metal in the lower part of keyhole reaches the top of molten pool, and consequently, no vortex flow pattern is observed in the keyhole bottom.

Numerical Simulation of Transport Phenomena for a Double-Layer Laser Powder Deposition of Single-Crystal Superalloy

A turbine blade made of single-crystal superalloys has been commonly used in gas turbine and aero engines. As an effective repair technology, laser powder deposition has been implemented to restore the worn turbine blade tips with a near-net shape capability and highly controllable solidified microstructure. Successful blade repair technology for single-crystal alloys requires a continuous epitaxial grain growth in the same direction of the crystalline orientation of the substrate material to the newly deposited layers. This work presents a three-dimensional numerical model to simulate the transport phenomena for a multilayer coaxial laser powder deposition process. Nickel-based single-crystal superalloy Rene N5 powder is deposited on a directional solidified substrate made of nickel-based directional-solidified alloy GTD 111 to verify the simulation results. The effects of processing parameters including laser power, scanning speed, and powder feeding rate on the resultant temperature field, fluid velocity field, molten pool geometric sizes, and the successive layer remelting ratios are studied. Numerical simulation results show that the maximum temperature of molten pool increases over layers due to the reduced heat dissipation capacity of the deposited geometry, which results in an increased molten pool size and fluid flow velocity at the successive deposited layer. The deposited bead geometry agrees well between the simulation and the experimental results. A large part of the first deposition layer, up to 85 pct of bead height, can be remelted during the deposition of the second layer. The increase of scanning speed decreases the ratio of G/V (temperature gradient/solidification velocity), leading to an increased height ratio of the misoriented grain near the top surface of the previous deposited layer. It is shown that the processing parameters used in the simulation and experiment can produce a remelting ratio R larger than the misoriented grain height ratio S, which enables remelting of all the misoriented grains and guarantees a continuous growth of the substrate directional-solidified crystalline orientation during the multilayer deposition of single-crystal alloys.

Selective laser melting of Ti6Al4V alloy for biomedical applications: Temperature monitoring and microstructural evolution

• Temperature measurements of molten pool were done using CCD camera. • Temperature of molten pool versus scanning speed and laser power was determined. • Microstructures and microhardness of SLM samples were analyzed. • Influence of heat treatment on microstructure were discussed and presented. Selective laser melting (SLM) is a kind of additive manufacturing where parts are made directly from 3D CAD data layer-by-layer from powder material. SLM products are used in various industries including aerospace, automotive, electronic, chemical, biomedical and other high-tech areas. The properties of the parts produced by SLM depend strongly on the material nature, characteristics of each single track and each single layer, as well as the strength of the connections between them. Studying the temperature distribution during SLM is important because temperature gradient and heat transfer determine the microstructure and finally mechanical properties of the SLM part. In this study a CCD camera was applied for determination of the surface temperature distribution and the molten pool size of Ti6Al4V alloy. The investigation of the microstructure evolution after different heat treatments was carried out to determine the microstructure in terms of applicability for the biomedical industry.

Thermo-mechanical Modeling of Laser Deposited Bimetallic Components

A thermo-mechanical three-dimensional finite element model is developed for use in determining the temperature history and residual stress in a Cu-H13 thin wall plate deposited by means of the laser engineered net shaping process. The same model is also applied to an H13-H13 sample in order to compare the results. The input laser power is adjusted for each layer and three different scanning speeds so as to maintain a steady molten pool size and a predefined pool depth of one and a half layers in thickness. It is observed that for a constant scanning speed, the required laser power decreases with the addition of more layers, and with an increase in the scanning speed the laser power needs to be increased. During the deposition of the first two layers, the required input laser power is significantly higher for the H13-H13 sample than for the Cu-H13 one because of the high heat conduction rate through the Cu substrate. The z-component (growth direction) of the residual stresses is found to be dominant over the other components and is compressive near the center of the wall and tensile at the free edges, which is consistent with the experimental results presented in the literature. The residual stress levels near the free edges are found to be higher in the Cu-H13 sample than in the H13-H13 sample. In these regions, the unidirectional scanning strategy results in a higher stress accumulation than the alternative scanning strategy.

Evolution of laser-fired aluminum-silicon contact geometry in photovoltaic devices

The evolution of temperature and velocity fields during laser processing of solar cells to produce an ohmic contact between an aluminum thin film and a silicon wafer is studied using a transient numerical heat transfer and liquid metal flow model. Since small changes in pulse duration, power, and power density can result in significant damage to the substrate and, in extreme cases, expulsion of droplets from the molten zone, the selection of optimal laser processing parameters is critical. The model considers the unusually large heat of fusion of the Al-Si alloy formed during processing and the large composition-dependent two phase region. The calculated size and shape of the fusion zone were in good agreement with the corresponding experimental data, indicating the validity of the model and providing a basis for using the model to develop a better understanding of the laser-assisted fabrication of contacts for solar cell devices. The transient changes in the composition of the Al-Si molten region are found to have a major impact on the heat transfer during the formation of the contact. Consideration of the time-dependent concentration of Al in the molten region is also essential to achieve good agreement between the experimental and computed molten pool sizes. Process maps showing peak temperatures and the depth and width of the molten pool are presented in order to assist users in the selection of safe process parameters for the rapid fabrication of these silicon-based photovoltaic devices.

Numerical simulation of temperature distribution and TiC growth kinetics for high power laser clad TiC/NiCrBSiC composite coatings

A three dimensional model was proposed to simulate high power laser clad TiC/NiCrBSiC composite coatings on Ti6Al4V alloys. The temperature distribution, temperature curves on different nodes, three dimensional shape and size of TiC melting region, molten pool and heat affected zone (HAZ) of the substrate were obtained. To have a clear physical insight into the phase transformation and microstructure evolution in the coatings during laser cladding process, a theoretical kinetic analysis was performed to elucidate the nucleation, growth velocity, and size of TiC particles on the basis of simulated temperature curves of the molten pool. A good quality TiC/NiCrBSiC composite coating with low dilution rate and excellent metallurgical bond was fabricated under optimal processing parameters using powder mixture of TiC and NiCrBSiC as clad material and cuboid of Ti6Al4V alloys as substrate. To validate the reliability of the proposed model, the theoretical results were compared with the microstructure of the coatings. It shows that these theoretical results are in excellent agreement with the experiment cases.

Study of improvement of gap tolerance in laser MIG arc hybrid welding of aluminium alloy

Hybrid welding combining YAG laser or fibre laser with MIG arc was carried out on aluminium alloy, and the effects of set-up conditions of a laser beam and a wire, and the laser beam parameters on wire melting phenomena and gap tolerance in butt joint were investigated. It was found that, in order to obtain a deeper penetration, the laser beam should be separated from the wire with a distance at which there was no direct interaction between a laser beam and a droplet during its transfer. On the contrary, in order to get a wider gap tolerance for a butt joint, for example, it was better for the laser beam to be set to cross with a wire over 2 mm so that the laser beam could directly irradiate on the wire surface to melt it. As a result, the arc current could be decreased so efficiently that the molten pool size formed by MIG is decreased and the gap tolerance increased. In the case of the utilization of a fibre laser, it was found that the wire melting phenomena were affected by the laser beam parameters such as the beam diameter and the defocused conditions when welding was not done at a focal position. It was clarified that even at the same laser beam diameter on the work surface, the wire melting phenomena could be different under the different defocused conditions. In the defocused conditions where the focal position was over the work surface, the molten droplet on the wire tip evaporated more easily than the case where the focal position was within the work surface. From the viewpoint of laser absorption by the wire or the molten droplet, according to an arc phenomenon approach, it was found that about 10% of laser energy was absorbed during hybrid welding when the laser beam was directly irradiated on the wire surface.

316L Stainless Steel with Gradient Porosity Fabricated by Selective Laser Melting

To fabricate 316L stainless steel part with a pore gradient structure, the method using selective laser melting (SLM) technique is exploited. Scan tracks feature, densification, and tensile property of SLM-produced samples prepared via different scan speeds were investigated. The results show that the porosity is strongly influenced by scan speed. On this basis, a gradient changed scan speed was applied in every SLM layer for the purpose of producing a gradient porosity metal. The results indicate that the structure exhibits a gradually increased porosity and a reduced molten pool size along the gradient direction of scan speed variation. The forming mechanisms for the gradient porosity were also addressed.

Experimental and Numerical Study of the LENS Rapid Fabrication Process

Several aspects of the thermal behavior of deposited stainless steel 410 (SS410) during the laser engineered net shaping (LENS™) process were investigated experimentally and numerically. Thermal images in the molten pool and surrounding area were recorded using a two-wavelength imaging pyrometer system, and analyzed using THERMAVIZ™ software to obtain the temperature distribution. The molten pool size, temperature gradient, and cooling rate were obtained from the recorded history of temperature profiles. The dynamic shape of the molten pool, including the pool size in both travel direction and depth direction was investigated, and the effect of different process parameters was illustrated. The thermal experiments were performed in a LENS™ 850 machine with a 3 kW IPG Photonics laser for different process parameters. A three-dimensional finite element model was developed to calculate the temperature distribution in the LENS™ process as a function of time and process parameters. The modeling results showed good agreement with the experimental data.

In-process monitoring and feedback control for stable production of full-penetration weld in continuous wave fibre laser welding

Laser micro-welding has been applied for device sealing in electronics and automobile industries. Welding of corners in goods and products is a problem owing to easier formation of a weld with burn-through, shallow penetration or a non-bonded part when a drastic change in the welding speed or laser power occurs. This research was therefore undertaken with the objective of obtaining a fundamental knowledge of in-process monitoring and feedback control for the stable production of a full-penetration weld with a constant bead width on the bottom surface irrespective of the changes in the laser power and the welding speed. Variation in weld penetration geometry was investigated by rapid deceleration and acceleration in the welding speed during lap welding of pure titanium thin sheets with a continuous wave (CW) single-mode fibre laser beam. The rapid deceleration in the welding speed led to a considerable change in the full-penetration weld geometry or a partially penetrated weld (if the power was accordingly reduced), resulting in the difficulty in the stable production of a full-penetration weld bead. The heat radiation intensity measured from the laser-irradiated area was useful as an in-process monitoring signal for detecting the molten pool size on the laser-irradiated surface. However, the utilization of monitoring of heat radiation was difficult for predicting the weld bead width on the bottom surface due to the formation of partial penetration or the change in the penetration shape. The laser power was controlled at a 4 ms interval according to the heat radiation signal in order to adjust the weld bead width on the laser-irradiated surface to the target weld penetration geometry affected by thermal storage. Consequently, the feedback-controlled laser power produced a stable full-penetration weld with the designed bead width on the bottom surface irrespective of the rapid deceleration of the welding speed and the corresponding decrease in laser power. Furthermore, the developed feedback control algorithm was effective in rapid acceleration of the welding speed. From these results, it was confirmed that the consideration of the feedback control algorithm including a thermophysical property such as thermal storage was essential for the suppression of the effect due to rapid deceleration and acceleration of the welding speed and the laser power in lap seam micro-welding with a CW fibre laser.

Optimization of the LENS ® process for steady molten pool size

A three-dimensional finite element model was developed and applied to analyze the temperature and phase evolution in deposited stainless steel 410 (SS410) during the Laser Engineered Net Shaping (LENS ®, 1 1 LENS ® is a registered trademark of Sandia Corporation. ) rapid fabrication process. The effect of solid phase transformations is taken into account by using temperature and phase dependent material properties and the continuous cooling transformation (CCT) diagram. The laser beam is modeled as a Gaussian distribution of heat flux from a moving heat source with conical shape. The laser power and translational speed during deposition of a single-wall plate are optimized in order to maintain a steady molten pool size. It is found that, after an initial transient due to the cold substrate, the dependency of laser power with layer number is approximately linear for all travel speeds analyzed. The temperature distribution and cooling rate surrounding the molten pool are predicted and compared with experiments. Based upon the predicted thermal cycles and cooling rate, the phase transformations and their effects on the hardness of the part are discussed.

裏面加熱源を用いた温度場制御による角変形低減効果に関する検討

Back heating method is proposed for angular distortion decreasing technique. Back heating is expected for angular distortion decrease to the following two effects; that is, bending in the opposite direction and homogenization of temperature distribution through thickness direction. Best condition of temperature distribution control for angular distortion decrease has been evaluated with numerical simulation. Back heating source is appropriately arranged to welding heat source, then, angular distortion is decreased more effective than the above-mentioned effects, and perfectly decreased by back heating of ten percent to welding heat input. In back heating, angular distortion can be decreased by the change in generation behavior of inherent strain according to nonconventional temperature history. Angular distortion decreasing technique is effective in any condition concerning heat input parameter and the best condition concerning multiple-heat-source arrangement is decided to be dependent on the size of mechanical molten pool.

In-process Monitoring and Adaptive Control for Laser Spot and Seam Welding of Pure Titanium

In-process monitoring and adaptive control is increasingly expected for the decisive detection of laser welding defects and for the stable formation of sound welds since laser welding has been applied in several industries. The objective of this research is to obtain a fundamental knowledge of in-process monitoring and availability of adaptive control for laser spot and seam welding of pure titanium with a pulsed Nd:YAG laser beam. The reflected light and heat radiation from the laser-irradiated area were monitored in synchronism with 20,000 F/s high-speed video observation of a molten pool. With respect to in-process monitoring, it was found that the reflected light was sensitive to the molten area formation and the heat radiation had clear correlation to the molten pool sizes for respective laser powers. Moreover, the heat radiation intesity had a quick response to the rapid change of laser peak power, and then the correlation between the heat radiation and the molten pool size was maitained in the designed laser power. As for adaptive control, spatters and pores were drastically reduced by controlling laser peak power and pulse duration according to the obtained features of the heat radiation-monitoring signal.