Keyhole Mode Research Articles

High Reflectivity and Thermal Conductivity Ag–Cu Multi-Material Structures Fabricated via Laser Powder Bed Fusion: Formation Mechanisms, Interfacial Characteristics, and Molten Pool Behavior

Ag and Cu have different advantages and are widely used in key fields due to their typical highly electrical and thermal conductive (HETC) properties. Laser powder bed fusion (LPBF), an innovative technology for manufacturing metallic multi-material components with high accuracy, has expanded the application of Ag-Cu in emerging high-tech fields. In this study, the multi-material sandwich structures of Ag7.5Cu/Cu10Sn/Ag7.5Cu were printed using LPBF, and the formation mechanism, interface characteristics, and molten pool behavior of the Ag7.5Cu/Cu10Sn (A/C) and Cu10Sn/Ag7.5Cu (C/A) interfaces were studied to reveal the influence of different building strategies. At the A/C interface, pre-printed Ag7.5Cu promoted Marangoni turbulence at a relatively low energy density (EA/C = 125 J/mm3). Due to the recoil pressure, the molten pool at the A/C interface transformed from a stable keyhole mode to an unstable keyhole mode. These phenomena promoted the extensive migration of elements, forming a wider diffusion zone and reduced thermal cracking. At the C/A interface, the molten pool was rationed from the conduction mode with more pores to the transition mode with fewer defects due to the high energy density (EC/A = 187.5 J/mm3). This work offers a theoretical reference for the fabrication of HETC multi-material structures via LPBF under similar conditions.

Comparison of melt evolution and flow mechanisms of Inconel 718, Ti6Al4V, 304 stainless steel, and AlSi10Mg manufactured by laser powder bed fusion, structures, and properties after heat treatments

Process parameters and material type are the key influencing factors in laser powder bed fusion (LPBF) manufacturing. In this study, a three-dimensional high-fidelity discrete heat-flow-coupled model was developed to study the surface morphology, melt evolution, and defect formation mechanisms of four types of alloys formed using LPBF under different laser processes at the powder scale to optimize the process parameters. Four alloys were manufactured by LPBF with optimal process parameters. Multiple characterizations were adopted to disclose the effect of heat-treatment on the microstructure and mechanical properties of the as-built parts of four materials. During LPBF forming, the melts developed from continuous and semicylindrical to discontinuous and “medicine spoon”-shaped at low and high linear energy densities (LEDs), respectively. The Ti6Al4V powder bed exhibited a long trailing melt pool and acted deeper (i.e., keyhole mode). The AlSi10Mg powder bed exhibited a smaller melt pool and did not exhibit trailing (i.e., heat-transfer mode). For Inconel 718 alloy and AlSi10Mg alloy, the improvement of mechanical properties is due to the precipitation of nanoparticle phase. The equilibrium of α+β phase results in the ideal strength and plasticity of Ti6Al4V alloy. The gradual disappearance of the cellular substructure reduces the strength of 304 stainless steel, but increases the plasticity. Process parameters correlate simulations with experiments and establish a "process-structure-properties" system. This study provides insights into melt evolution and flow behavior and is a reference for selecting and optimizing materials, laser-process parameters, and heat treatment regimes in practical engineering.

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.

Keyhole and Conduction Mode of Welding AISI 304HCu: A Comparison

Keyhole and Conduction Mode of Welding AISI 304HCu: A Comparison

A thermo-mechanical phase-field fracture model: Application to hot cracking simulations in additive manufacturing

Thermal fracture is prevalent in many engineering problems and is one of the most devastating defects in metal additive manufacturing. Due to the interactive underlying physics involved, the computational simulation of such a process is challenging. In this work, we propose a thermo-mechanical phase-field fracture model, which is based on a thermodynamically consistent derivation. The influence of different coupling terms such as damage-informed thermomechanics and heat conduction and temperature-dependent fracture properties, as well as different phase-field fracture formulations, are discussed. The model is numerically implemented with the finite element method. Finally, the model is applied to simulate the hot cracking in additive manufacturing. Thereby not only the thermal strain but also the solidification shrinkage is considered. As for the thermal profile, both analytical temperature solution and numerical thermal field around the melting pool are taken into account. Based on the latter approach, the influence of different process parameters is further studied. The study reveals that the solidification shrinkage strain takes a dominant role in the formation of the circumferential crack, while the temperature gradient is mostly responsible for the central crack. Process parameter study demonstrates further that a higher laser power and slower scanning speed are favorable for keyhole mode hot cracking while a lower laser power and quicker scanning speed tend to form the conduction mode cracking. The numerical predictions of the hot cracking patterns are in good agreement with similar experimental observations, showing the capability of the model for further studies.

Emissivity Prediction for an IR Camera During Laser Welding of Aluminum

Laser processing is becoming increasingly important in industrial applications. The success of the process relies on two fundamental parameters: the surface temperature of the medium and the thickness of the hardened layer. One of the most important factors during a laser process is certainly the temperature, which presents high temperature gradients. The speed at which a material undergoes a phase transition, the chemical reactions that take place during processing and the properties of the material are all dependent on temperature changes. Consequently, the measure of temperature is a demanding undertaking. This study proposes to measure temperature for the duration of laser welding with the infrared camera (IR) Optris PI. To restore the real temperature based on the brightness temperature values measured by the IR camera is needed to evaluate the emissivity to be attributed to the IR camera. For this purpose, firstly, the isotherms consistent with the melting point of aluminum (785 K) were assessed and then compared with the temperature distribution gauged in the zone of irradiation of the laser. Such data were then compared with the thickness of the melted zone. The use of the melting point isotherm allowed the calculation of the value of emissivity and the restoration of the temperature. Thermography software data acquisition wrongly presupposes the emissivity value does not change. This generates incorrect thermographic data. The surface emissivity normally hinges on temperature. Therefore, the values on which the literature relies may not work for materials of interest in the conditions of the process. This is particularly the case, where welding is carried out in keyhole mode (Tmax = Tvap). However, the physical phenomena involved, including evaporation and plasma plume formation, high spatial and temporal temperature gradients, and non-equilibrium phase transformations, influence the optical conditions of the brightness of the emission of light from the molten pool, making, De Facto, the emissivity value not constant. Thus, what we propose here is a methodological procedure that allows the measurement of the effective emissivity of the surface, at the same time taking into consideration the consequence of physical phenomena and the conditions of the surface. Two procedures (Standard and Simplified) capable of providing the correct emissivity value in relation to the working parameters have been proposed. The results showed that the procedures are correct, fast, and easy to use.

Mitigating keyhole pore formation by nanoparticles during laser powder bed fusion additive manufacturing

Keyhole pore formation is one of the most detrimental subsurface defects in the laser metal additive manufacturing process. However, effective ways to mitigate keyhole pore formation beyond tuning laser processing conditions during keyhole mode laser melting are still lacking. Here we report a novel approach to mitigate keyhole pore formation during laser powder bed fusion (LPBF) process by using stable nanoparticles. The critical keyhole depth for keyhole pore generation (i.e., the largest keyhole depth without keyhole pore formation) during LPBF of Al6061 increases from 246 µm to 454 µm (85% increase) after adding TiC nanoparticles. In-depth x-ray imaging studies and thermo-fluid dynamics simulation enable us to identify that two mechanisms work together to mitigate keyhole pore generation: (1) adding nanoparticles prevents the keyhole from collapsing by increasing the liquid viscosity to impede the protrusion development; (2) adding nanoparticles slows down the keyhole pore movement by increasing the liquid viscosity, resulting in the recapturing of the pore by the keyhole. We further demonstrate that adding TiC nanoparticles can also eliminate the keyhole fluctuation induced keyhole pore during LPBF of Al6061. Our research provides a potential way to mitigate keyhole pore formation for defect lean metal additive manufacturing.

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

The present study explores a combination of numerical and simulation approaches to generate a process map for identifying the regimes of conduction and keyhole modes of melting and verify the same with experimental data. Finite Element based simulation studies were conducted to determine the regions of conduction mode, keyhole mode, and transition by varying the laser power and speed. Single tracks and density cubes were processed based on the simulation results to confirm the melting modes and study its effect on microstructure and hardness. Increase in volumetric energy density (VED) causes a shift in microstructure of single tracks, from a mix of short columnar and equiaxed grains in conduction mode to long columnar grains in keyhole mode, with an overall increase in the grain size. The melt pool depth to width ratio also increases with VED. The VED based criteria alone cannot determine melting modes as the single-track samples at 81 J/mm3 exhibited both conduction mode (at 250 W) and keyhole mode (at 350 W). Almost all the printed cubes showed high density (>99.9%) irrespective of melting mode. Similar to single track the average grain size of bulk samples increased with increase in VED. The bulk samples were subjected to three different heat treatments (Homogenisation, Solution treatment and Direct Double Aging) to study their effect on the microstructures and mechanical properties. Homogenisation resulted in near identical equiaxed microstructure irrespective of processing parameters. The highest hardness of about 470 HV was observed for the direct double aged samples.

Multiphase and multi-physical simulation of open keyhole plasma arc welding

Plasma arc welding (PAW) is an important technique in the manufacturing industry. To improve the welding efficiency, the special keyhole mode has attracted considerable attention because it has the potential to achieve full penetration without a groove. However, rigorous process conditions are required to maintain a stable keyhole mode. For better and more widespread application in industry, a multiphase and multi-physical model is established to reveal the complicated electro-magneto-thermo-hydro-mechanical phenomena and gas-liquid-solid interactions in the keyhole PAW process. Arc backward reflection and deviation of the heating center occur because of the relative movement in the system, and molten liquid metal is pushed backward and upward to form a salient metal layer at the rear. The maximum height reaches 1.45 mm. Outflow phenomenon occurs through the open keyhole, and the outflow temperature and velocity are approximately 6000 K and 60 m/s by the calculation, respectively. The predicted weld pool and keyhole basically coincide with measured results. This study provides a comprehensive understanding of the multiphase and multi-physical interactions in the PAW process, which may help improve the associated techniques.

Investigation of the transient coupling between the dynamic laser beam absorptance and the melt pool - vapor depression morphology in laser powder bed fusion process

A predictive powder-bed model has been developed using open-source environments to investigate the coupling between the optical and the thermo-hydrodynamical phenomena for the laser powder bed fusion process. Firstly, discrete element modeling has been carried out to predict the configuration of powder particles over a substrate. Then, thermo-fluidic simulations for both the stationary and the moving laser irradiation have been carried out to investigate the melt pool dynamics, the vaporization induced keyhole depression instabilities, and the multiple reflections of the laser beam in the powder bed and the keyhole. The local absorptance and the reflectance of the laser beam at the static curvature of the solid powder particles and the dynamic interface of the vaporization-induced keyhole depression have been incorporated using a parallelized ray-tracing algorithm. The influences of laser power, scanning speed, beam spot diameter, and powder layer thickness on the melting mode and the transient beam absorption have been investigated. During the initial phase of melting in the powder bed, it was found that the laser absorption is very high (up to 73%) due to multiple reflections induced beam trapping. The absorption behavior for the conduction mode of melting was found to be dependent on the scanning speed of the laser beam. For the keyhole mode of melting, the temporal variation of the laser beam absorption shows a highly non-linear fluctuating behavior that is directly related to the vapor depression morphology of the melt pool. The numerically predicted attributes such as the initial laser beam absorptance, the transient melt pool-vapor depression morphology for the stationary laser irradiation, and the melt pool dimensions for the single-track deposition matches well with the findings of the published powder calorimetric measurement, published high-speed transmission X-ray imaging and the measured single-track deposited using the in-house laser powder bed fusion setup, respectively.

Physics-based analytical modeling of keyhole mode in laser powder bed fusion

Physics-based analytical modeling of keyhole mode in laser powder bed fusion

The influence of beam focus during laser powder bed fusion of a high reflectivity aluminium alloy — AlSi10Mg

LPBF of Al alloys is associated with numerous challenges when compared to other commonly used alloys due to their higher reflectivity and thermal conductivity. In this work, processing diagrams, temperature prediction models, XCT, and metallography are used for establishing criteria in process parameter optimization of high reflectivity Al alloys based on AlSi$_{10}$Mg response in using 57 different process parameter combinations - 21 using a focused Gaussian laser beam and 36 using divergent beams. For LPBF systems with focused beam diameters <100 {\mu}m, divergent beams obtained by defocusing to a position above the LPBF build plate primarily lead to conduction mode melt pools, while a focused beam leads to transition and keyhole mode melt pools. Conduction mode melting helps in avoiding keyhole mode defects, resulting in parts with densities >99.98%. Additionally, an analytical model-guided selection of laser power and velocity settings for a focused beam help in stabilizing melt pool and spatter dynamics in the transition melting mode thereby enabling a potential to obtain density values close to conduction mode densities (~99.98%). A dimensionless keyhole number (Ke) was derived in this work to identify distinct regions of conduction (Ke of 0-12), transition (Ke of 12-20), and keyhole (Ke > 20) mode melting during LPBF of AlSi$_{10}$. A melt pool aspect ratio (ratio of melt pool depth to width) of ~0.4 is observed to be the threshold between conduction and transition/keyhole mode melt pools for AlSi$_{10}$, different from the conventionally assumed 0.5. Lastly, inferred laser absorptivity values (from experimental melt pools) of transition/keyhole mode melt pools are observed to be >40% higher when compared to conduction mode melt pools. This work demonstrates a dimensionless-process map method to obtain near fully dense parts that can be generalized for LPBF of high reflectivity alloys.

In-situ alloyed CoCrFeMnNi high entropy alloy: Microstructural development in laser powder bed fusion

• Grain development can be inherited from single tracks to multiple layers. • Modelling methods show dependence on different meltpool modes. • The homogenisation of in-situ alloyed Mn is mainly attributed to remelting. In-situ alloying has the potential to combine the compositional flexibility of high entropy alloys (HEAs) and the advanced forming capability of laser powder bed fusion (LPBF). This study fundamentally investigated the elemental homogenisation and grain development in the in-situ alloying process of CoCrFeMnNi HEA, by analysing the basic units, i.e., tracks and layers, and introducing Mn as an alloying element to the base CoCrFeNi HEA. Different modelling methods were employed to predict meltpool dimensions, and the results indicated the dependence of the modelling on practical meltpool modes. Delimitation of elemental distribution was found in keyhole meltpools since an intensive flow was generated due to recoil pressure. The homogeneity of in-situ alloyed Mn in single tracks was insufficient whether operated in conduction mode or keyhole mode, which required remelting from adjacent tracks and following layers to promote homogenisation significantly. The preferred orientation in single tracks along scanning directions changed from <001> to <101> as the scanning speed increased, although the cross-sections were similar in size with identical linear energy density. Such preference can be inherited during the printing process and lead to different textures in three-layer samples. It was also observed that applying hatch spacing smaller than a half meltpool width could coarsen the grains in a layer. The results from this study provide structure-parameter correlations for future microstructural tailoring and manipulation.

Three-Dimensional Numerical Simulation of Grain Growth during Selective Laser Melting of 316L Stainless Steel.

The grain structure of the selective laser melting additive manufactured parts has been shown to be heterogeneous and spatially non-uniform compared to the traditional manufacturing process. However, the complex formation mechanism of these unique grain structures is hard to reveal using the experimental method alone. In this study, we presented a high-fidelity 3D numerical model to address the grain growth mechanisms during the selective laser melting of 316 stainless steel, including two heating modes, i.e., conduction mode and keyhole mode melting. In the numerical model, the powder-scale thermo-fluid dynamics are simulated using the finite volume method with the volume of fluid method. At the same time, the grain structure evolution is sequentially predicted by the cellular automaton method with the predicted temperature field and the as-melted powder bed configuration as input. The simulation results agree well with the experimental data available in the literature. The influence of the process parameters and the keyhole and keyhole-induced void on grain structure formation are addressed in detail. The findings of this study are helpful to the optimization of process parameters for tailoring the microstructure of fabricated parts with expected mechanical properties.

Keyhole-induced Porosity in Laser Manufacturing Processes: Formation Mechanism and Dependence on Scan Speed

Laser welding and laser-based powder-bed fusion additive manufacturing in the deep penetration (keyhole) mode are promising technologies for the synthesis of metal components. The significant potential of these technologies remains latent because of structural defects (porosity), which significantly degrade the structural integrity and performance of the end products. Practical strategies for reducing those defects are addressed through fundamental understanding of their formation. In this study, pore formation of hydrodynamic origin is investigated, including the dynamics and mechanisms of the formation based on the above mentioned technologies. The pore volume and frequency of pore appearance, depending on the amplitude and frequency of capillary vibrations, are considered. Physical analysis is performed to obtain the scanning velocity values for the maximum and zero amplitudes and the frequency of capillary waves. A comparison between calculated curves and experimental data confirms both the capillary origin of the pores and the estimated scanning speeds at which the parameters of the pores exhibit their maximum values or vanish. The results obtained may facilitate in the selection of the optimal scanning speed when designing a pore-free technology.

Laser additive manufacturing of a 316L/CuSn10 multimaterial coaxial nozzle to alleviate spattering adhesion and burning effect in directed energy deposition

Multimaterial additive manufacturing is aimed at the integration of functionality and mechanical performance of dissimilar materials for various applications. In this work, a 316L/CuSn10 multimaterial coaxial nozzle was designed and additively manufactured by laser powder bed fusion (LPBF) for alleviating spattering adhesion and burning effect in directed energy deposition. The applicability of the 316L/CuSn10 nozzle was then validated in the laser-based directed energy deposition (L-DED) process. The interfacial diffusion zone of the 316L/CuSn10 structure was measured with a length of ~400 μm, indicating a well bonding between 316L and CuSn10. A high laser power (P = 230 W) and a low scanning speed (v = 300 mm/s) adopted for printing CuSn10 on 316L could lead to “keyhole mode”, resulting in a large penetration depth of the pre-solidified layer and wide diffusion zone at the interface. The spattering behavior of the 316L/CuSn10 nozzle applied in the L-DED process showed that the heterogeneity of dissimilar materials could reduce the adverse effect of the adhesion of spattering droplets. High laser reflection and high thermal conductivity of CuSn10 could relieve the burning effect of the reflected laser beam on the tip of the 316L/CuSn10 nozzle. Therefore, the utilization of CuSn10 on the nozzle tip could improve its service life. These findings may serve as a new way for low-cost and rapid mass production of the coaxial nozzles that can be applied in the L-DED technique.

Mathematical model to study the keyhole formation in pulsed Nd:YAG laser welding of SS 316L material and its experimental verification

A mathematical model to study keyhole formation and its propagation in the material is developed for laser welding performed in an open atmosphere. The present model overcomes the limitations of existing models in assuming sonic vapor jet velocity to calculate vaporization-induced recoil pressure responsible for keyhole formation. In the present model, the exact value of vapor jet velocity is calculated using gas dynamics equations. The minimum threshold value of absorbed laser beam intensity required to perform keyhole welding irrespective of laser pulse duration for laser beam radius of 0.6 mm has been found to be 0.8 × 105 W/cm2 and is in good agreement with the experimental value. In between conduction mode welding and keyhole mode welding, a transition mode exists where a keyhole mechanism develops itself and melt displacement is not considerable in this zone. Weld penetration occurs mainly through heat diffusion in this transition mode. The predicted values for keyhole penetration velocity are also in good agreement with the experimental values. At a longer pulse duration, the model over-predicts the keyhole penetration velocity as compared to the experimental value due to nonconsideration of vapor plasma absorption of the laser beam.

Selective laser melted AZ91D magnesium alloy with superior balance of strength and ductility

In the context of global carbon neutrality, the application of lightweight magnesium alloys is becoming increasingly attractive. In this study, selective laser melting (SLM) was employed to achieve nearly full dense and crack-free AZ91D components with fine equiaxed grain structure. The formation mechanism of typical pore defects (gas pore, lack-of-fusion pore and keyhole pore) and melting modes (keyhole mode and conduction mode) were systematically studied by varying the laser power and scanning speed. The morphology and volume fraction of the pores under different processing conditions were characterized. A criterion based on the depth-to-width ratio of the melt pool was established to identify different melting modes. The strength and ductility (tensile strength up to 340 MPa and uniform elongation of 8.9%) of the as deposited AZ91D are far superior to those of the casting components and are comparable to those of its wrought counterparts. The superior balance of strength and ductility of SLMed AZ91D, as well as the negligible anisotropic properties are mainly ascribed to the extremely fine equiaxed grain structure (with average grain size of ∼1.2 µm), as well as the discontinuous distribution of β-Al12Mg17 phases. It thus provides an alternative way to fabricate high-strength magnesium alloys with complex geometry.

Solidification cracking of a nickel alloy during high-power keyhole mode laser welding

Nickel alloy Inconel 740H, a candidate material for use in ultra-supercritical power plants, is susceptible to solidification cracking during high power deep penetration laser welding. Here we examine how cracking is affected by welding variables and determine the locations where the cracks occur experimentally and theoretically. We use a solidification cracking model to calculate the effects of welding variables on cracking and the locations where the cracks form during high power laser keyhole mode welding of IN 740H. The parameters needed for the cracking model are obtained from a well-tested numerical heat transfer and fluid flow model for keyhole-mode welding. Model predictions of cracking and their locations for different welding conditions are verified by experiments. • Solidification cracking of nickel alloys during laser welding is poorly understood. • We propose a novel methodology to understand why, when, and where cracking occurs. • A heat and fluid flow and solidification model, and experimental data are used. • Model predictions of cracking locations and conditions are verified by experiments. • The work can be used to prevent weld failures in thick section laser keyhole welds.

Laser welding Al–Si coated hot stamping steel in conduction mode: weld formation and Al-rich microstructure

The non-uniform mixture microstructure is always revealed in laser welds of Al–Si coated hot stamping steel. It is very necessary to understand aluminum-rich segregation on phase morphology evolution. At same time the uniform and high aluminum welds were tried to obtain by replacing keyhole mode with conduction mode. The uniform fusion zones contained 3–6 wt.% Al, higher than normal keyhole welds. The weld microstructure was mixed of delta ferrite, lath martensite and few discussed needle phases with different morphology. Lower Al region revealed high fraction of island martensite, when aluminum content w(Al) > 5 wt.%, the needle phase was precipitated from the matrix of delta ferrite. High resolution transmission electron microscopy confirmed it long periodical stacking ordered structure in body-cubic crystal matrix.