Molten Track Research Articles

Investigations on the influence of adapted metal-based alloys on the process of laser beam melting

Additive manufacturing technologies are characterized by complex process interrelations. Consequently, specifically adapted alloys are required to enable a robust building process. In particular, laser beam melting (LBM) is increasingly used for the fabrication of sophisticated functional parts for various applications in numerous industrial sectors, such as automotive and aerospace. However, process stability and repeatability are major challenges for industrializing LBM. This paper presents a comprehensive investigation of the influence of AlSi10Mg additives in a 316L stainless steel powder during LBM. A two-stage experimental approach was applied, during which the temperature field around the molten track and the number of spatters during the LBM process were determined by means of high-speed thermographic imaging. Furthermore, the microstructure of the additively manufactured specimens, the modified 316L stainless steel powder, and the respective raw materials was characterized by scanning electron microscopy. The experimental study described in this paper aimed to obtain correlations between the additive content (input), the temperature field of the molten track, and the microstructure (outputs). It was found that the cooling rate decreases with a higher amount of AlSi10Mg in the powder. Furthermore, the microstructure analysis demonstrated an increasing formation of the body-centered cubic phase with a higher fraction of AlSi10Mg. The conclusion is that additives in the powder considerably affect important key characteristics of the LBM process.

Effect of Line Energy Density and Wall Thickness on the Top Surface Quality of AlSi10Mg Sample Fabricated via Selective Laser Melting

The effect of line energy density (LED) and wall thickness on the top surface quality of AlSi10Mg sample fabricated by selective laser melting (SLM) has been investigated in this paper. It is observed that the surface morphology has great evolution which the molten track turns to regular and then to irregular with the increase in LED. The line roughness (Ra) and peak height (Rz) decrease first and then increase as the LED increases. The optimal LED for low Ra/Rz increases from 0.1389 J/mm to 0.30 J/mm with the wall thickness increases from 0.1 mm to 2 mm. This work can provide a guide of manufacturing complexy geometrical thin-walled parts with a fine top surface quality.

Multi-physics modelling of molten pool development and track formation in multi-track, multi-layer and multi-material selective laser melting

Selective laser melting (SLM) is a promising powder-based additive manufacturing technology due to its capability to fabricate metallic components with complex geometries. While most previous investigations focus on printing with a single material, recent industry-orientated studies indicate the need for multi-material SLM in several high-value manufacturing sectors including medical devices, aerospace and automotive industries. However, understanding the underlying physics in multi-material SLM remains challenging due to the difficulties of experimental observation. In this paper, an integrated modelling framework for multi-track, multi-layer and multi-material SLM is developed to advance the in-depth understanding of this process. The main novelty is in modelling the molten pool evolvement and track morphology of multiple materials deposited on the same and across different layers. Discrete element method (DEM) is employed to reproduce the powder deposition process of multiple materials in different deposition patterns, with particle size distribution imported from a particle size analyser. Various phenomena including balling effect, keyhole depression, and lack of fusion between layers are investigated with different laser energy inputs. As a result of the different thermal properties, several process parameters including energy density and hatch spacing are optimised for different powder materials to obtain a continuous track profile and improved scanning efficiency. The interface between two layers of different materials is visualised by simulation; it was found that the phase migration at the interface is related to the convection flow inside the molten pool, which contributes to the mixing of the two materials and elemental diffusion. This study significantly contributes to the challenging area of multi-material additive manufacturing by providing a greater in-depth understanding of the SLM process from multi-material powder deposition to laser interaction with powders across multiple scanning tracks and different building layers than can be achieved by experimentation alone.

General Investigations on Manufacturing Quality of Permalloy via Selective Laser Melting for 3D Printing of Customized Magnetic Shields

Selective laser melting (SLM)-assisted 3D printing was employed to directly fabricate Ni-Fe-based permalloy magnetic shields of fiber optic gyroscopes in spacecraft. Process optimization was conducted for the quantitative analyses of the influences of laser processing parameters on the bulk density and surface roughness of the as-printed parts. It contributed to the determination of the suitable SLM process conditions to improve the as-printed quality and further explore the formation mechanisms of Ni-Fe-Mo soft magnets. The SLM-generated microstructures with hierarchical characteristics, involving molten pools or tracks, austenite grains and cellular substructures, were dissected before and after the recrystallization annealing treatment. The soft magnetic properties of as-printed permalloy Ni-15Fe-5Mo with and in the absence of the annealing were comparatively evaluated, proving to be comparable to those of the conventionally processed permalloy parts. The lightweight magnetic shields were 3D-printed, showing the feasibility and applicability of SLM.

A research on the surface morphology, microstructure evolution and wear property of selective laser melting Al2O3/P20 composites

The Al2O3/P20 composites were successfully manufactured by selective laser melting (SLM) from powder mixture of Al2O3 and P20 mold steel with different Al2O3 contents (0.5 wt%, 1 wt%, 2 wt%) in this work. The surface morphology analysis indicated that Al2O3 particles were partially or fully melted for agglomerates during the SLM process with segregating between contiguous molten tracks. The model of Al2O3 diffusing during the SLM process was built for illustrating the generated mechanism of Al2O3 agglomerates. Moreover, the densification behavior of specimens became poor (the relative density declined from 99.3% to 94.2%) as the Al2O3 contents increased. The microstructure within samples had some obvious variations including coarsening martensite microstructure, tempering microstructure transformation, reverted austensite and precipitation of fine carbide particles. The result of Rockwll hardness test manifested that 1 wt% obtained the maximum value of 57.6 HRC, while 2 wt% had the minimum value of 45.9 HRC. Tribological test revealed that coefficient of friction (COF) and wear rate significantly declined when the Al2O3 contents were 1 wt% and 2 wt%, which emphasized the superior wear resistance property.

Impact of oxygen content in powders on the morphology of the laser molten tracks in preparation for additive manufacturing of silicon

Powder additive pulsed Nd:YAG laser (wavelength 1064 nm), with the aim of preparing AM of such material. The influence of oxygen content in the initial silicon powders on the continuous tracks morphology was investigated. It was found that the initial oxygen content of the processed powders must be lower than 0.1 wt% to produce a smooth silicon melted track. This result is explained as due to the formation of gaseous manufacturing (AM) of materials is affected by the oxygen present in the raw material, especially if they possess a native oxide. The latter influences the properties of the laser molten track. We carried out a study on the interactions between silicon fine powder (<3 μm) pellets and a silicon monoxide (SiO) by reaction between silicon and its native oxide during the process.

Selective laser melting of AISI 304 stainless steel composites reinforced by Al2O3 and eutectic mixture of Al2O3–ZrO2 powders

The effect of Al2O3 and eutectic mixture of Al2O3–ZrO2 reinforcements on the densification, microstructural, and mechanical behaviors of selective laser melted 304 stainless steel composites have been investigated in this study. An optimum volume content was identified for the eutectic mixture of Al2O3–ZrO2 reinforcement, while the samples reinforced by Al2O3 particles exhibited a detrimental trend with increasing Al2O3 content. The addition of ZrO2 particles led to the formation of regular and even scanning tracks, whereas swelling of the molten tracks was observed due to the melting-point depression in addition to the low viscous melt pools and greater fluidity. Moreover, process-induced cracks and voids were observed in the samples reinforced by Al2O3, while the use of the eutectic mixture of Al2O3–ZrO2 was observed to suppress the cracks, leading to a dense structure. The microhardness of the fabricated samples reinforced with the eutectic mixture of Al2O3–ZrO2 particles was observed to be considerably higher than that of those reinforced with Al2O3 composites due to the homogeneous dispersion of the eutectic structures. The results of this study confirm that the use of a eutectic mixture of Al2O3–ZrO2 particles suppresses process-induced cracks and significantly enhances the reinforcing effect on 304 stainless steel composites.

Evolution of molten pool during selective laser melting of Ti–6Al–4V

The characteristics of molten pools provide valuable insight into the complexity of the metal additive manufacturing process, which has a significant influence on the quality of the parts built using this process. In our work, we develop a three-dimensional multiphysics finite element model of a selective laser melting (SLM) process on a Ti–6Al–4V alloy. The dynamic characteristics of the molten pool are studied by multiphysics simulation with consideration of phase transitions, recoil pressure, surface tension, and Marangoni effect. The results show the time-evolution of temperature distribution, flow field, and surface morphology of a single track during the SLM process. The recoil pressure caused by evaporation plays a significant role in molten pool dynamics and induces a depression at the head of the molten pool. As a result of the backward Marangoni flow, the material is shifted to the tail region and a vortex is generated. In addition, a protrusion is presented at the middle and start points of the scanned track, while a depression is formed at both sides and at the terminal point. The simulation and the experimental results on the surface morphology of the molten track during the SLM process are in good agreement. Furthermore, it is found that metal evaporation may take place not only on the surface of the molten pool but also inside it, due to the drop in pressure. This is a significant contributor to the formation of porosity in SLM parts.

Fiber laser surface remelting of a nickel-based superalloy by an integrated rectangular laser spot

In some specific industrial applications employing nickel-based superalloy components, a surface enhancement of product tribological properties is required. In this work, the effects of a Laser Surface Remelting on a nickel-based superalloy was investigated. An Ytterbium fiber laser, with maximum power of 4 kW, and a focusing segmented mirror, which produces an integrated laser beam (in a knife blade or rectangular spot) as well as a laser intensity homogenization, were used. Different values of laser power and scanning speed were tested. Remelted tracks, having a width of 8 mm, were characterized in terms of surface visual inspection, morphology, metallographic examination as well as longitudinal and transverse micro-hardness. Moreover, a comparison was performed between properties of the remelted material and the base alloy. The particular optical setup allowed the width of the molten tracks remained almost constant also varying the process parameters. The quality of the treated surface, for a given power, improved with increasing scanning speed, even if the depth of the molten area reduced. Micro-hardness increased of about 14% because of the formation of very fine columnar-dendritic grains, due to the rapid remelting and resolidification. Process efficiency was also evaluated by two different parameters: the analysis of both allowed to state that, keeping the laser power constant, the efficiency of the process increased almost linearly with the speed.

Cracking Behavior in Additively Manufactured Pure Tungsten

In this study, near fully dense (96.5%) pure tungsten bulks were additively manufactured and the cracking behavior was investigated. A crack network with a spacing of ~ 100 μm was observed in the fabricated bulks. It was observed that the laser scanning strategy, which could tailor the microstructure, affected the crack distribution pattern in fabricated tungsten. The calculated surface temperature difference (7300 K) was much higher than the cracking criterion (800 K) of tungsten, indicating that cracking is almost inevitable in laser additive manufacturing of tungsten. It could be concluded that crack network formed because the cracks emerged in every laser molten track and then interconnected in the layer-by-layer building process.

Microstructure prediction of selective laser melting AlSi10Mg using finite element analysis

Selective Laser Melting (SLM) technology is a complex process controlled by mass and heat transfer. It is very important to understand the microstructure of single track. This work predicted the single track's microstructure of SLMed AlSi10Mg by calculating the thermal variables and using columnar to equiaxed transition (CET) criterion. Firstly, a high accurate transient three-dimensional finite element model was established with anisotropic thermal conductivities verified by both molten pool dimensions and track surface feature. Secondly, the finite element model was used to accurately predict the thermal variables, such as temperature gradient, cooling rate and solidification rate of the molten pool. Thirdly, a columnar to equiaxed transition criterion was established by connecting the predicted thermal variables and experimental microstructure, the experimental results verified that it can predict the microstructure of SLMed AlSi10Mg very well.

Fundamental analysis of the influence of powder characteristics in Selective Laser Melting of molybdenum based on a multi-physical simulation model

Selective Laser Melting (SLM) offers great potential for the processing of molybdenum as it allows the production of parts that can hardly be fabricated in a classical way. The workpiece is built up by melting up powder layer by layer. Besides the applied laser parameters, the characteristics of the powder and the deposited powder layer have a crucial influence on the dynamics of the process and the final processing result. In order to get a fundamental understanding of these influences, numerical simulations are a versatile tool as they allow a detailed look on isolated parameters. In this contribution a coupled thermo-fluid dynamical simulation model for SLM is applied for a fundamental analysis of the influence of powder characteristics in SLM of molybdenum. The results allow a deeper insight into the influences of powder particle size and powder particle arrangement on the SLM process and the formation of the molten track.

Dense Pure Tungsten Fabricated by Selective Laser Melting

Additive manufacturing using tungsten, a brittle material, is difficult because of its high melting point, thermal conductivity, and oxidation tendency. In this study, pure tungsten parts with densities of up to 18.53 g/cm3 (i.e., 96.0% of the theoretical density) were fabricated by selective laser melting. In order to minimize balling effects, the raw polyhedral tungsten powders underwent a spheroidization process before laser consolidation. Compared with polyhedral powders, the spherical powders showed increased laser absorptivity and packing density, which helped in the formation of a continuous molten track and promoted densification.

Elongation of metallic nanoparticles at the interface of silicon dioxide and silicon nitride

In the process of shape transformation of metal nanoparticles (MNPs) embedded in dielectrics resulting from swift heavy-ion irradiation, key parameters such as the energy deposition threshold, the necessity of a molten track and the NP size have been identified for amorphous silicon dioxide. The extension of such parameters to other dielectrics is yet unclear. We present experimental evidence of the shape transformation of nearly spherical NPs located at the interface of amorphous silicon nitride and silicon dioxide thin layers upon irradiation with 185MeV Au ions at fluences of 0.3 and 1×1014cm−2. After irradiation the ∼16–18nm diameter Au and Ag NPs transformed into continuous nano-rods exhibiting a high aspect ratio with a clear preference of elongation into the silicon dioxide layer. The results are discussed in the context of Thermal Spike calculations, which indicate that the track formation timescales may have an important influence on the NP elongation process.

Determination of True Temperature in Selective Laser Melting of Metal Powder Using Infrared Camera

The objective of this study is to restore the true temperature in during the process of Selective Laser Melting (SLM) of metal powder (in the present case INOX 316L), that is the actual problem in laser assisted additive technologies. To meet this objective, at the first stage, the temperature was measured on the surface of metal substrate INOX 304L without a powder layer. Based on the results of studies the method of temperature measurementof the molten pool surface on metallic plates and during melting of metal powder layer, that were exposed to laser radiation, using an infrared camera (IR), in the present case FLIR Phoenix RDAS TM, was proposed. To restore the true temperature based on the brightness temperature values measured by IR camera, the results of temperature measurements were compared with the width of the molten track on the surface of the plate in the absence of powder. In case of SLM, the results of IR camera measurements were compared with the width of a welded track (bead). The true temperature profiles and temperature gradients values were determined along the axis of the laser beam for melting of plates without powder. In the case of powder melting, the developed method allows to determine the influence of several key SLM parameters on the molten pool shape.

Synthesis of embedded Si nano-particles using swift heavy ions and its optical properties

ABSTRACTEmbedded Si nano-particles of average size around 5nm were synthesized in an amorphous Si matrix by two stage ion implantation processes. It has been observed that amorphous Si (a-Si) layers were recrystallized using 50 MeV Au ions with enhanced regrowth rate with activation energy in the range of 0.29 eV. During the crystallization process Si nanocrystals were formed in the a-Si layers due to sudden quenching of the molten tracks created by MeV Au ions. The recrystalizations were confirmed by Rutherford backscattering spectrometry-Channeling (RBSC) technique. The structural modification and nanocluster creation that emerged during recrystallization process was observed in high-resolution transmission electron microscopy and photoluminescence (PL) spectroscopy. The PL emission was observed over a broad band of 2.8 – 3.4 eV and centered at 3.25 eV. The Si nano-crystal formation can be explain by a mechanism combining the melting within the ion tracks by thermal spike process and the subsequent recrystallization nucleated from the crystalline sides at the interface.

Experimental Investigation into the Single-Track of Selective Laser Melting of IN625

Selective laser melting(SLM) is driven by the need to fabricate functional metallic parts and tools with near shape and density. The method of process to fabricate a metal part will save materials, time and energy compared to the traditional manufacturing methods. Unlike the selective laser sintering (SLS), the metal powder particles are molten by the laser beam during the process of selective laser melting. In this paper, IN625 powders were adopted to investigate the characters of single molten track. The factors that affect the surface quality and relative density are the process parameters such as the laser energy, scan speed and so on. They were studied to find out the correlation between the parameters and formation of single-track. It has been found that Optimal ratio between laser power and scanning speed (P/v) is 1-1.5 for IN625 SLM. P/v is the linear energy density. It also has been found that the width and height of single-track can be calculated when the linear energy density is given. In this study the laser power, scan spacing and the hatch spacing which affect the surface quality and the relative density of the metallic parts were optimized.

Selective laser melting W–10 wt.% Cu composite powders

Tungsten–Copper (W–Cu) alloys are promising materials for electrical and thermal applications. However, its forming method still remains limited in conventional powder metallurgy technique which is not suitable for manufacturing parts with intricate shapes. In this work, selective laser melting technology was introduced for fabricating W–10 wt.% Cu alloys parts. The morphological feature of a single molten track was analyzed. The results show that liquid phase sintering with complete melting of the binder (Cu), but nonmelting of the structural metal (W), acts as the main mechanism in this process. The melting conditions of single layers in different processing parameters were investigated. The results show that a moderate melting zone can be acquired from an established process map. Moreover, investigations on multilayers forming show that the final density increases with the decrease of scan speed until it reaches a plateau due to the insufficient rearrangement in liquid phase sintering and the balling effect.

Liquid-like phase formation in Gd2Zr2O7 by extremely ionizing irradiation

Isometric Gd2Zr2O7 with the ordered, pyrochlore structure has an extremely high resistance to radiation-induced amorphization. Ion-beam irradiations at keV to GeV energies result in a disordered, defect-fluorite structure that remains crystalline to very large fluences. However, we report liquid-like phase formation of droplet-like surface hillocks and quenched molten tracks in Gd2Zr2O7. The extremely high energy density of 12-MeV C60 clusters creates tracks with substantial volumes of amorphous material, accompanied by the formation of nanocrystals of the disordered, defect-fluorite structure. This is the first evidence of irradiation-induced amorphization of Gd2Zr2O7. In contrast, irradiation of Gd2Zr2O7 pyrochlore with swift heavy ions of U resulted in an order-disorder transformation to defect-fluorite without any evidence of amorphization. Thermal-spike calculations highlight the dominance of the effect of deposited energy density, controlled by the projectile velocity, as compared with the energy loss.

Atom probe tomography of swift ion irradiated multilayers

Nanometer scale layered systems are well suited to investigate atomic transport processes induced by high-energy electronic excitations in materials, through the characterization of the interface transformation. In this study, we used the atom probe technique to determine the distribution of the different elements in a (amorphous- Fe 2 Tb 5 nm / hcp - Co 3 nm ) 20 multilayer before and after irradiation with Pb ions in the electronic stopping power regime. Atom probe tomography is based on reconstruction of a small volume of a sharp tip evaporated by field effect. It has unique capabilities to characterize internal interfaces and layer chemistry with sub-nanometer scale resolution in three dimensions. Depth composition profiles and 3D element mapping have been determined, evidencing for asymmetric interfaces in the as-deposited sample, and very efficient Fe–Co intermixing after irradiation at the fluence 7 × 10 12 ion cm - 2 . Estimation of effective atomic diffusion coefficients after irradiation suggests that mixing results from interdiffusion in a molten track across the interface in agreement with the thermal spike model.