Keyhole Modes Research Articles

Microstructural evolution in laser powder bed fusion of water-atomized high-carbon low-alloy steel: Analysis of melting mode effects

Microstructural evolution in laser powder bed fusion of water-atomized high-carbon low-alloy steel: Analysis of melting mode effects

Correlative spatter and vapour depression dynamics during laser powder bed fusion of an Al-Fe-Zr alloy

Spatter during laser powder bed fusion (LPBF) can induce surface defects, impacting the fatigue performance of the fabricated components. Here, we reveal and explain the links between vapour depression shape and spatter dynamics during LPBF of an Al-Fe-Zr aluminium alloy using high-speed synchrotron x-ray imaging. We quantify the number, trajectory angle, velocity, and kinetic energy of the spatter as a function of vapour depression zone/keyhole morphology under industry-relevant processing conditions. The depression zone/keyhole morphology was found to influence the spatter ejection angle in keyhole versus conduction melting modes: (i) the vapour-pressure driven plume in conduction mode with a quasi-semi-circular depression zone leads to backward spatter whereas; and (ii) the keyhole rear wall redirects the gas/vapour flow to cause vertical spatter ejection and rear rim droplet spatter. Increasing the opening of the keyhole or vapour depression zone can reduce entrainment of solid spatter. We discover a spatter-induced cavity mechanism in which small spatter particles are accelerated towards the powder bed after laser-spatter interaction, inducing powder denudation and cavities on the printed surface. By quantifying these laser-spatter interactions, we suggest a printing strategy for minimising defects and improving the surface quality of LPBF parts.

Single-track investigation of additively manufactured mold steel with larger layer thickness processing: Track morphology, melt pool characteristics and defects

Single-track investigation of additively manufactured mold steel with larger layer thickness processing: Track morphology, melt pool characteristics and defects

Comparison of melting efficiency between blue, green, and IR lasers in pure copper welding

Pure copper parts are commonly used in many industrial products because of their low thermal resistance and high electrical conductivity. However, connecting high-quality and high-efficiency copper materials remains a challenge. This is because pure copper has low absorption of near-infrared light, making it difficult to weld stably with a near-infrared laser. Visible light lasers should realize high-efficiency laser welding of pure copper. However, there are few reports comparing the laser wavelength dependence of welding efficiency for pure copper. In this study, bead-on-plate welding was performed on pure copper plates of 2 mm thickness using a 1.5 kW blue diode laser (445 nm), a 16 kW IR disk laser (1030 nm), and a 3 kW green disk laser (515 nm). Bead-on-plate welding of pure copper was performed in the thermal conduction mode or the keyhole mode by varying the laser spot diameter and power, and the amount of melting was measured from cross-sectional observations. As a result, compared to the IR disk laser, blue and green lasers showed higher melting efficiency in both the thermal conduction and keyhole modes, and the melting behavior was more stable. In thermal conduction mode welding, the melting efficiency was 0.2% with the IR disk laser and 0.7% with the blue diode laser. In keyhole mode welding, the melting efficiency with the blue diode laser or green disk laser was about 7%, which is equivalent to that with the IR disk laser with 2.5 times the output power.

Benchmark models for conduction and keyhole modes in laser-based powder bed fusion of Inconel 718

Benchmark models for conduction and keyhole modes in laser-based powder bed fusion of Inconel 718

Heat transfer and melt flow of keyhole, transition and conduction modes in laser beam oscillating welding

Melting modes have significant effects on the heat transfer and fluid flow in laser beam oscillating welding (LBOW), which in turn affects the welding quality and solidification microstructure. In this paper, a computational fluid dynamics (CFD) model for LBOW of 2219 aluminum alloy is developed to study the melting/solidification, fluid flow pattern, keyhole dynamics and energy absorption under different welding modes: keyhole, transition and conduction mode. A hybrid heat source model is developed to describe the influence of the vapor heat and multiple reflections of the laser rays on the keyhole surface. The predicted molten pool dimensions and weld profiles are in good agreement with the experimental results. By increasing the oscillating frequency, the line energy density significantly decreases due to a higher scanning speed that results in a shallower keyhole with wider opening. The laser rays are more likely to leak out after 2-3 reflections which reduces the energy absorption. Thus, the melting mode will transfer from keyhole to conduction mode. With further increase of the oscillating frequency, the heat transfer will be dominated by heat conduction when the energy density is below the threshold for keyhole formation. Our work lay the foundations for optimizing the LBOW process to obtain sound joints.

Melting modes of laser powder bed fusion (L-PBF) processed IN718 alloy: Prediction and experimental analysis

Melting modes of laser powder bed fusion (L-PBF) processed IN718 alloy: Prediction and experimental analysis

3D finite element modeling of selective laser melting for conduction, transition and keyhole modes

Selective Laser Melting (SLM) consists one of the most widely used 3D printing methods, capable of producing metallic parts of high density and quality. During SLM, complex physical mechanisms and phenomena are taking place, affecting the overall process, thus, extensive relevant research is conducted in the recent years. Along with the experimental research that is carried out, different modeling methodologies have been proposed to simulate the process and to gain a better understanding, exploiting the capability to predict and optimize the process' results. The current paper presents a modeling methodology to simulate the formation of the melting pool during a single track in SLM, for conduction, transition and keyhole modes depending on the utilized Volumetric Energy Density (VED). A heat transfer model has been combined with deformed geometry to simulate the material vaporization, depending on the VED, since the material vaporization mechanism can be considered as a major importance parameter in the keyhole formation. To retain the generality and the simplicity of the model, only one coefficient that needs an estimation was adopted, namely, the atom recombination factor βR, which is directly correlated with the VED, while other parameters were defined based on analytical solutions and basic physics models. The accuracy of the model was validated through a “blind” comparison with experimental results from literature in terms of melting pool depth and Heat Affected Zone (HAZ) depth and width, with the simulation result being in high agreement with experimental ones. Finally, conclusion regarding the material vaporization rate and the evolution of the process in a more microscopic level were also deduced.

Analysis of heat transfer and melt flow in conduction, transition, and keyhole modes for CW laser welding

Melting modes have a profound impact on heat transfer and fluid flow in laser material processing. In order to have a better understanding of the thermo-fluidic behavior of the melt pool for different laser melting modes, experiments were conducted to study temperature behavior, melt pool dynamics, and weld characteristics during the transition from conduction melting mode to keyhole melting mode. Infrared thermography was applied to analyze heat transfer for different melting modes. Melt pool dynamics captured by high-speed camera were used to interpret the driving force acting on the molten flow. In conduction mode, the temperature distribution in the weld pool is regular and smoother. Heat transfer in this mode is governed predominantly by heat conduction. In transition mode, the sideway of molten metal and fluid flow are improved by Marangoni convection, resulting in a wider melt pool. When the keyhole is developed, the keyhole mode is achieved and the recoil pressure generated by violent evaporation acts as the principal driving force to push molten metal flow backwards. An upward melt flow occurs along the rear of melt pool. The results also reveal that the surface ripple and grain growth direction is directly dependent on heat transfer and fluid flow in different melting modes.

Achieving ultrahigh-strength in beta-type titanium alloy by controlling the melt pool mode in selective laser melting

It is challenging to simultaneously achieve nearly full density and high strength in refractory alloys using selective laser melting (SLM). In this study, the achievement of ultrahigh-strength resulting from nearly full density has been reported in beta-type titanium alloy by controlling the melt pool mode in SLM. The melt pool mode was divided into the conduction and keyhole modes, which were determined from the macroscopic morphology of the melt pool in the SLMed Ti-34.2Nb-6.8Zr-4.9Ta-2.3Si (wt%) (TNZTS) alloy single tracks in combination with the keyhole threshold (P·V−0.5 = 251.3 W (m⋅s−1)−0.5) calculated theoretically. Compared with condition mode, the keyhole mode has higher porosity and inevitably causes poor mechanical property. Fortunately, by optimizing the SLM process parameters predicted via the keyhole threshold, an ultrahigh-strength and nearly full density (99.7%) TNZTS alloy with conduction mode was obtained by SLM. The alloy exhibited an ultrahigh compressive yield strength of 1286 MPa, which was higher than the majority of the beta-type titanium alloys reported so far. The microstructural analyses indicated that the ultrahigh-strength TNZTS alloy consisted of a thin shell-shaped (Ti, Nb, Zr)2Si (S2) phase (20–50 nm) around the columnar β-Ti grain boundaries together with an ultrafine dot shaped (Ti, Nb, Zr)5Si3 (S1) phase (50–300 nm) in the β-Ti matrix. The ultrahigh strength resulted from high-density dislocations and the effective dislocation blockage by the semi-coherent S1 and coherent S2 phases, thereby leading to the dislocation-strengthening and hardening effect. The strategy utilized in this study provides the fundamental guidelines for generating refractory metallic alloys with high density and excellent performance.

Numerical and analytical investigation on meltpool temperature of laser-based powder bed fusion of IN718

Prediction of meltpool features in Laser-Based Powder Bed Fusion (LB-PBF) is a complex non-linear multiple phase dynamic problem. In this investigation, numerical simulations and analytical models are offered to predict meltpool temperature and to provide a methodology to estimate melt track quality. By determining the meltpool temperature, different rheological phenomena including recoil pressure can be controlled. Recoil pressure is known to drive the keyhole and conduction modes in LB-PBF which is an important factor to qualify the melt track. A numerical simulation was carried out using Discrete Element Method (DEM) with a range of process parameters and absorptivity ratios; allowing observation of the variation of meltpool temperature and free surface morphology, as calculated by the volume-of-fluid (VOF) method. A spatially thermophysical-based analytical model is developed to estimate meltpool temperature, based on LB-PBF process parameters and thermophysical properties of the material. These results are compared with experimentally observed meltpool depth for IN718 specimens and found to have a good accuracy. The numerical and analytic results show good agreement in the conduction mode to estimate the meltpool temperature and related phenomena such as recoil pressure to control the melt track and layering quality. The analytical model does not accurately predict the keyhole mode which may be explained by evaporation of chemical elements in the examined material.

Differences in microstructure and nano-hardness of selective laser melted Inconel 718 single tracks under various melting modes of molten pool

Abstract Single tracks of Inconel 718 were manufactured by selective laser melting (SLM) under the same volumetric energy density (VED), but via various scanning speed and laser power, with the aim of discerning the influence of the melting mode of molten pools on their solidification behavior, microstructure and nano-hardness. For this purpose, the geometric appearance of each Inconel 718 single track was firstly investigated. The results indicate that the molten pool geometries are significantly different despite melting under the same VED condition; the typical melting modes of molten pools, including the keyhole and conduction modes, can be obtained. The solidified microstructures of both modes evolve from cellular at the bottom to cellular dendrites at the top; but the dendrites are more developed under the keyhole mode. In addition, the grains of both modes follow the epitaxial columnar crystal growth pattern along the boundary of molten pool. The preferential growth direction of grains under the conduction mode is closer to the direction, but the grains orientation of the keyhole mode tends to be and direction. Moreover, in comparison with the conduction mode, the average nano-hardness for different regions of molten pools corresponding to the keyhole mode is slightly higher.

Understanding Crack Formation Mechanisms of Ti–48Al–2Cr–2Nb Single Tracks During Laser Powder Bed Fusion

The crack formation mechanisms of Ti–48Al–2Cr–2Nb single tracks processed by laser powder bed fusion were extensively investigated in a wide range of laser powers and scan speeds. The crack patterns were categorized by their directionalities, which were parallel (longitudinal crack) and/or perpendicular (transverse crack) to the scan direction. For the representative process conditions of the keyhole, transition, and conduction modes, cracking behaviors were characterized by combining the fractography and the microstructural analysis. Further, thermal-mechanical finite element method simulations were performed to predict the distribution of temperatures and thermal stresses during the melt pool formation. On the basis of the combined results, the cracks formed in keyhole, transition, and conduction modes were clarified as a solidification crack and/or a thermal crack. In addition, the formation of these cracks was thoroughly understood in terms of thermal stresses and microstructural factors that affect the crack susceptibility. Finally, comprehensive mechanisms responsible for cracking of Ti–48Al–2Cr–2Nb single tracks under laser powder bed fusion were proposed for different process conditions (the keyhole, transition and conduction modes).

Microscale interaction between laser and metal powder in powder-bed additive manufacturing: Conduction mode versus keyhole mode

Abstract Metal additive manufacturing (AM) techniques, particularly laser powder-bed methods, have shown tremendous advantages for producing high-value, complex, and customized components. However, precisely controlling the microstructure and defects of the products in the AM process has been a long-standing issue. Here, we have developed a coupled thermal-mechanical-fluid model to reveal the microscale dynamic evolution of metal powders, particularly for Ti-6Al-4V, during laser irradiation. Using different laser powers, layer thicknesses, and hatch spacings, we have systematically compared powder evolutions in two typical processing modes – conduction mode and keyhole mode. We have revealed the heat and mass balance in these two typical printing modes for the first time. There is only one circular flow in the longitudinal section of the molten pool in the conduction model, while two circular flows present in the keyhole mode. Gravity drives the melted metal to fill the gaps between the powders and contributes to the formation of the molten pool. The simulation results demonstrate that a larger printable powder layer thickness is achieved in the keyhole mode than that in the conduction mode. The thermal distribution during the multiple-track melting in the conduction mode is more uniform than that in the keyhole mode, leading to more uniform resulting microstructure. This study presents opportunities to control the microstructure and defects at the microscale of the AM products by modulating processing parameters and switching between conduction and keyhole modes.

On thermal expansion behavior of invar alloy fabricated by modulated laser powder bed fusion

Abstract In the present research, structural analyses are conducted on Invar (64% Fe-36% Ni) samples manufactured by modulated laser powder bed fusion (LPBF) under different laser power regimes. Within the selected process window, the results obtained from X-ray computed tomography analysis indicate that the relative density of as-built samples increases with an increase in laser power. In addition, it is observed that irregular-shaped pores with the longest axis normal to the building direction are significantly reduced with increasing the laser power. Microstructural investigation suggests that with increasing the laser power, a transformation from conduction to transition and keyhole modes occurs. In X-ray diffraction patterns of the as-built samples, only fcc γ-phase is detected and no reflection of bcc α-phase is found. Texture measurements show that the intensity of 〈110〉 crystallographic orientation along the building direction of the as-built samples increases with increasing laser power. Coefficient of thermal expansion of the as-built samples is very low and comparable to that of conventionally manufactured Invar alloy. Furthermore, as-built samples fabricated at lowest (250 W) and highest (400 W) laser powers exhibit the lowest and highest thermal expansion displacement, respectively. Finally, the thermal expansion behavior and its correlation with structural integrity and texture are discussed.

The printability, microstructure, crystallographic features and microhardness of selective laser melted Inconel 718 thin wall

Abstract The thin plate is a promising light-weight structure to broaden applications of Inconel 718 superalloy in aerospace and transportation industries. In the present work, selective laser melting (SLM) technology was applied to manufacture a series of Inconel 718 thin walls (ultra-thin plates) to explore their printability, microstructures, crystallographic features and microhardness under keyhole and conduction modes. Results show that it is feasible to manufacture Inconel 718 thin walls with a thickness of ~0.2 mm by SLM. Keyhole mode is favorable for a better printability, finer dendrites, the precipitation of strengthening phases of γ′/γ″, a stronger 〈001〉 texture intensity and a higher microhardness in the center zone of the SLMed thin wall. Conduction mode contributes to the uniform microstructures and microhardness in the marginal and center zones of the thin wall. Also, the decrease of area fraction in strengthening phases and the increase of Laves phase result in a decreasing trend of microhardness along the deposition direction. Finally, microstructural formation and evolution mechanisms of SLMed thin walls under two modes are proposed based on the solidification conditions and vertical thermal cycles.

Effect of Melt-In and Key-Hole Modes on the Structure and Mechanical Properties of AISI 430 Steel Welded Using Plasma Transfer Arc Welding

In this study, AISI 430 stainless steel couple 10 mm thick were welded in the butt position by melt-in and key-hole plasma transfer arc welding (PTAW) processes without using any filler wire addition and pretreatment. Welded joints were manufactured choosing a constant nozzle diameter (2.4 mm), welding speed (0.01 m/min), shielding gas flow rate (25 L/min), two different plasma gas flow rates (0.8 and 1.2 L/min), three different welding currents in melt-in PTAW process (80, 85 and 90 A), and three current intensities of the key-hole PTAW process (120, 125, and 130 A). In order to determine the microstructural changes that occurred, the interface regions of the welded samples were examined by scanning electron microscopy (SEM), optic microscopy (OM), X-ray diffraction (XRD) after PTAW. Microhardness and V-notch impact tests were applied to determine the mechanical properties of the welded samples. In addition, fracture surface were examined by SEM after impact tests.

A Comparative Study of the Heat Input During Laser Welding of Aeronautical Aluminum Alloy AA6013-T4

The heat input is the amount of energy supplied per unit length of the welded workpiece. In this study, the effect of two different heat inputs in laser beam welding of a high strength aluminum alloy AA6013-T4 was evaluated from macrostructural and microstructural points of view. The experiments were performed using a continuous wave 2 kW Yb-fiber laser with 100 μm spot size on the upper surface of the workpiece. Keeping the heat input at a given level, 13 or 30 J/mm, the laser power was changed from 650 W to 2 kW and the welding speed from 33 to 150 mm/s. In the condition of higher heat input 30 J/mm it was possible to obtain both cutting and welding processes. For 13 J/mm, welding processes were obtained in conduction and keyhole modes. The equiaxed grain fraction changed with changing speed for the same heat input. The laser processing induced a decrease in the hardness of the weld bead of about 25% due to the solubilization of the precipitates. The estimated absorptivities of the laser beam in the liquid aluminum changed largely with experimental conditions, from 4.6% to 10.5%, being the most significant source of error in measuring the real amount of energy absorbed in the process. For the same heat input the macrostructure of the welded surfaces, i.e., humps and dropouts, changed as well. All these facts indicate that the heat input is not a convenient method to parameterize the laser beam welding parameters aiming the same weld features.

Prediction of Weld Joint Shape and Dimensions in Laser Welding Using a 3D Modeling and Experimental Validation

This paper presents an experimentally validated weld joint shape and dimensions predictive 3D modeling for low carbon galvanized steel in butt-joint configurations. The proposed modelling approach is based on metallurgical transformations using temperature dependent material properties and the enthalpy method. Conduction and keyhole modes welding are investigated using surface and volumetric heat sources, respectively. Transition between the heat sources is carried out according to the power density and interaction time. Simulations are carried out using 3D finite element model on commercial software. The simulation results of the weld shape and dimensions are validated using a structured experimental investigation based on Taguchi method. Experimental validation conducted on a 3 kW Nd: YAG laser source reveals that the modelling approach can provide not only a consistent and accurate prediction of the weld characteristics under variable welding parameters and conditions but also a comprehensive and quantitative analysis of process parameters effects. The results show great concordance between predicted and measured values for the weld joint shape and dimensions.

Control of aluminium laser welding conditions with the help of numerical modelling

Abstract Numerical modelling of laser welding in both conduction and keyhole modes is studied to highlight the influence of the main welding parameters on assembly performance. Numerical simulation enables the thermal history of assembly weld joints to be described. A hot cracking criterion is proposed. The model predicts that (1) a deviation in the localization of the shielding gas does not affect instability and that (2) fluctuations of the absorbed laser beam power generate cooling speed deviations. The results also demonstrate that laser welding in keyhole mode, when compared to laser welding in conduction mode, is more likely to induce higher cooling speeds, greater risks of hot cracking but better sensitivity to post weld age hardening heat treatment.