Direct Laser Sintering Research Articles

Sintering of calcium phosphates with a femtosecond pulsed laser for hard tissue engineering

Direct laser sintering on hard tissues is likely to open new pathways for personalised medicine. To minimise irradiation damage of the surrounding soft tissues, lasers operating at wavelengths that are ‘safe’ for the tissues and biomaterials with improved optical properties are required. In this work laser sintering is demonstrated with the use of an ultrafast, femtosecond (100fs) pulsed laser operating at a wavelength of 1045nm and two existing calcium phosphate minerals (brushite and hydroxyapatite) which have been improved after doping with iron (10mol%). Femtosecond laser irradiation caused transformation of the Fe3+-doped brushite and Fe3+-doped HAp samples into β-calcium pyrophosphate and calcium-iron-phosphate, respectively, with simultaneous evidence for microstructural sintering and densification. After estimating the temperature profile at the surface of the samples we suggest that soft tissues over 500μm from the irradiated zone would be safe from thermal damage. This novel laser processing provides a means to control the phase constitution and the morphology of the finished surfaces. The porous structure of β-pyrophosphate might be suitable for applications in bone regeneration by supporting osteogenic cell activity while, the densified Fe3+-rich calcium-iron-phosphate may be promising for applications like dental enamel restoration.

Influence of the build orientation on the fatigue strength of EOS maraging steel produced by additive metal machine

AbstractThis paper presents a research dealing with the dependence of the fatigue strength of maraging steel parts, manufactured by direct selective laser sintering, on the production build orientation. Three sets of specimens have been manufactured according to the ISO 1143 Standard (2010) by EOSINT M280 additive manufacturing machine, with the following heat and mechanical treatments, in agreement with the recommendations by the material manufacturer and current literature. The expected outcomes are the Fatigue Limit values of the material and the maximum number of cycles observed at different stress levels for three different build orientations (three different angles, 0°, 45° and 90°, between the build direction and the longitudinal axis of the samples). The results have been processed and compared by statistical methods in order to determine the fatigue curves in the finite life domain and the fatigue limits, along with their confidence bands and intervals, and to investigate the significance of the build orientation factor.

Fabrication of high porosity Cu-based structure using direct laser sintering

Purpose – The purpose of this paper is to report the investigation on obtaining a high-porosity Cu-based structure which was sintered successfully using direct laser sintering and to reveal the mechanisms of obtaining high porosity and acceptable strength simultaneously as well as the effect of NaCl content on characteristics of the sintered samples. Design/methodology/approach – Experimental study has been performed. The powder mixture utilized in this experiment includes metal powder (Cu and Cu3P mixture) and porogen [sodium chloride (NaCl)]. Scanning electron microscope, energy-dispersive X-ray analysis and the Archimedes density measurement were used to detect the characteristics of the sintered sample and to reveal the mechanism of forming high porosity and strength structure. Findings – More than 57 per cent porosity can be achieved while the structural strength is acceptable, and the infiltrated experiment verifies the pores in the structure are inter-connected. During laser sintering, NaCl powder melts and brings a larger amount of liquid phase, inducing large rearrange force and fast rearrangement of Cu particles. Therefore, after NaCl is removed, the dense and strong longer-bar shape tracks with larger amount of pores in the microstructure can be obtained. The size of pores as well as the porosity of the sintered sample increase with the NaCl content in the powder mixture. Originality/value – The mechanism of obtaining high porosity and acceptable strength simultaneously was revealed. The effect of NaCl content on characteristics of the sintered samples was also disclosed.

Long Fatigue Crack Growth in Ti6Al4V Produced by Direct Metal Laser Sintering

Results of an experimental determination of the propagation of long fatigue cracks in Ti6Al4V alloy prepared by selective laser melting technology are presented. Direct metal laser sintering (DMLS) exploiting EOSINT M270 system was used for fabrication of compact tension specimens having different orientation in relation to the build direction.It has been found that there is no substantial influence of the build orientation and thus the microstructure exhibiting directionality on the growth of long fatigue cracks and on the crack propagation threshold, when optimal process parameters and subsequent stress relieving heat treatments at 380 °C for 8 hours are chosen.The results were compared with literature data on the growth of long cracks in Ti6Al4V alloy manufactured by DMLS technique and with data for conventionally produced forged material.The fatigue crack propagation path, fracture surfaces and fracture surface roughness were investigated. An analysis of the specific microstructure produced by the direct laser sintering was performed and discussed in terms of the mechanism of the propagation of long cracks.

Clinical acceptability of metal-ceramic fixed partial dental prosthesis fabricated with direct metal laser sintering technique-5 year follow-up.

Statement of Problem:In recent years, direct metal laser sintered (DMLS) metal-ceramic-based fixed partial denture prostheses have been used as an alternative to conventional metal-ceramic fixed partial denture prostheses. However, clinical studies for evaluating their long-term clinical survivability and acceptability are limited.Aims and Objective:The aim of this study was to assess the efficacy of metal-ceramic fixed dental prosthesis fabricated with DMLS technique, and its clinical acceptance on long-term clinical use.Materials and Methods:The study group consisted of 45 patients who were restored with posterior three-unit fixed partial denture prosthesis made using direct laser sintered metal-ceramic restorations. Patient recall and clinical examination of the restorations were done after 6months and every 12 months thereafter for the period of 60 months. Clinical examination for evaluation of longevity of restorations was done using modified Ryge criteria which included chipping of the veneered ceramic, connector failure occurring in the fixed partial denture prosthesis, discoloration at the marginal areas of the veneered ceramic, and marginal adaptation of the metal and ceramic of the fixed denture prosthesis. Periapical status was assessed using periodical radiographs during the study period. Survival analysis was made using the Kaplan–Meier method.Results:None of the patients had failure of the connector of the fixed partial denture prostheses during the study period. Two exhibited biological changes which included periapical changes and proximal caries adjacent to the abutments.Conclusion:DMLS metal-ceramic fixed partial denture prosthesis had a survival rate of 95.5% and yielded promising results during the 5-year clinical study.

Analysis of the Surface Integrity in Cryogenic Turning of Ti6Al4 V Produced by Direct Melting Laser Sintering

The Ti6Al4V is widely utilized in the biomedical field thanks to its high biocompatibility, however, due to its low machinability, is classified as a difficult-to-cut material. With the goal of improving the surface quality of biomedical components made of Ti6Al4V produced by the DMLS additive manufacturing technology and later on machined, Liquid Nitrogen was tested as a coolant in semi-finishing turning. The integrity of the machined surfaces is evaluated in terms of surface defects and topography as well as residual stresses. The obtained results showed that the cryogenic machining assured a lower amount of surface defects and higher values of the residual compressive stressed compared to dry cutting, but a general worsening of the surface topography was detected.

Review of selective laser melting: Materials and applications

Selective Laser Melting (SLM) is a particular rapid prototyping, 3D printing, or Additive Manufacturing (AM) technique designed to use high power-density laser to melt and fuse metallic powders. A component is built by selectively melting and fusing powders within and between layers. The SLM technique is also commonly known as direct selective laser sintering, LaserCusing, and direct metal laser sintering, and this technique has been proven to produce near net-shape parts up to 99.9% relative density. This enables the process to build near full density functional parts and has viable economic benefits. Recent developments of fibre optics and high-power laser have also enabled SLM to process different metallic materials, such as copper, aluminium, and tungsten. Similarly, this has also opened up research opportunities in SLM of ceramic and composite materials. The review presents the SLM process and some of the common physical phenomena associated with this AM technology. It then focuses on the following areas: (a) applications of SLM materials and (b) mechanical properties of SLM parts achieved in research publications. The review is not meant to put a ceiling on the capabilities of the SLM process but to enable readers to have an overview on the material properties achieved by the SLM process so far. Trends in research of SLM are also elaborated in the last section.

Finite Element Simulation of Semi-finishing Turning of Electron Beam Melted Ti6Al4V Under Dry and Cryogenic Cooling

In the last few years, important step forwards have been made on Finite Element Simulation of machining operations. Wrought Ti6Al4 V alloy has been deeply investigated both numerically and experimentally due to its wide application in the industry. Recently, Additive Manufacturing technologies as the Electron Beam Melting and the Direct Melting Laser Sintering are more and more employed in the production of biomedical and aeronautical components made of Ti6Al4 V alloy. Fine acicular microstructures are generated by the application of additive manufacturing technologies, affecting the mechanical properties and the machinability. By the consequence, this peculiarity has to be considered in modelling the material behaviour. In this work, a numerical analysis of cylindrical external turning on Electron Beam Melted (EBM) Ti6Al4 V alloy is presented. A Johnson-Cook constitutive equation was implemented as a flow stress model and adapted with respect to the wrought Ti6Al4 V alloy. The model was calibrated and validated through the cutting forces and temperatures measurements acquired under dry and cryogenic lubricating conditions.

Fabrication of water-cooled laser silicon mirror by direct laser sintering.

Coppery heat sink with micro pores and Sub-millimeter channel has been fabricated by direct laser sintering on the back of the silicon mirror. To verify the heat dissipation capability of the fabricated heat sink, a Twyman-Green interferometer was employed to measure the thermal deformation of the silicon mirror radiated by a high power laser. It is shown that the thermal deformation of the mirror increases with the irradiating time and laser intensity. The heat balance can be achieved after several seconds of laser irradiation even when the net absorbed laser power density is up to 5.3 × 10(5) W/m(2). The time for reaching the heat balance also increases with the laser intensity. The maximum thermal deformation of the mirror is 0.65 µm if the net absorbed laser power density is 5.3 × 10(5) W/m(2).

Fabrication of Cu Heat Sink on Silicon Substrate Using Direct Laser Sintering

Silicon is applied widely because of its good electrical properties, thermal conductivity and optical processing. It is necessary to fabricate a heat sink on silicon substrate to improve the heat dissipation ability for modern industrial application. There are many traditional methods of processing heat sink on silicon substrate. However, it is hardly to meet the requirements of today’s technology for the disadvantages such as residual stress exist, processing shape limited and inefficiency. The investigation on fabricating heat sink silicon substrate by direct later sintering was conducted in this study. By sintering a transition layer, coppery heat sink with channel width of several hundred micrometers and circinate shape has been fabricated by direct laser sintering. Furthermore, the bonding mechanism and the influence of the powder components on the interface morphology and structure have been investigated.

Direct Selective Laser Sintering/Melting of High Density Alumina Powder Layers at Elevated Temperatures

Abstract Direct selective laser sintering (SLS) or selective laser melting (SLM) are additive manufacturing techniques that can be used to produce three-dimensional ceramic parts directly, without the need for a sacrificial binder. In this paper, a low laser energy density is applied to SLS/SLM high density powder layers of sub-micrometer alumina at elevated temperatures (up to 800 C ). In order to achieve this, a furnace was designed and built into a commercial SLS machine. This furnace was able to produce a homogeneously heated cylindrical zone with a height of 60 mm and a diameter of 32 mm. After optimizing the layer deposition and laser scanning parameters, two ceramic parts with a density up to 85% and grain sizes as low as 5 μ m were successfully produced.

Fabrication and Characterization of Porous 45S5 Glass Scaffolds via Direct Selective Laser Sintering

A porous 45S5 bioactive glass-ceramic scaffold has been designed and fabricated via direct selective laser sintering (SLS). The scaffolds sintered over a range of laser powers from 6.0 to 30.0 W were characterized using scanning electron microscope (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). Their mechanical properties were tested with Vickers hardness tester. The testing results showed that the 45S5 glass in an initial amorphous state was transformed into the favorable crystallization phase Na2Ca2Si3O9 by inhibiting other phase transformation due to the rapid heating and cooling of laser. The 45S5 particles began to soften and fuse together with the increased laser power during the sintering process. At a proper range of laser power (around 15.0 W), the sample became denser and had higher degree of crystallinity with superior fracture toughness. Further increasing the laser power, the 45S5 glass powders were melted, and there are the undesired holes or even sinking emerged on the surface of the sample. So, the scaffold with the superior fracture toughness and degree of crystallinity has been obtained under the laser power of 15.0 W.

Rapid prototyping of porcelain products by layer‐wise slurry deposition (LSD) and direct laser sintering

PurposeThe purpose of this paper is to study the rapid prototyping of porcelain products by using layer‐wise slurry deposition (LSD), in order to reduce the time to market of new or customized porcelain products or artworks.Design/methodology/approachThe properties such as phase composition, microstructure, shrinkage, density, and mechanical strength, of laser sintered (LS) and biscuit fired (BF) samples, before and after post sintering in a furnace, were studied and compared with each other.FindingsThe laser sintered sample was comparable with the biscuit fired sample in porosity, but had just half the mechanical strength of the latter due to the layer‐wise fabrication process. The feasibility of rapid prototyping of porcelain products was validated by the successful fabrication of some porcelain samples, which showed that the relatively low mechanical strength of the laser sintered sample was still high enough for the following handling processes, such as surface glazing and glost firing.Originality/valueThe paper demonstrates the possibility of rapid prototyping of porcelain components and the models produced by using LSD process.

Laser sintering of blended Al‐Si powders

PurposeThe purpose of this paper is to study the effects of particle size distribution, component ratio, particle packing arrangement, and chemical constitution on the laser sintering behaviour of blended hypoeutectic Al‐Si powders.Design/methodology/approachA range of bimodal and trimodal powder blends were created through mixing Al‐12Si and pure aluminium powder. The powder blends were then processed using selective laser sintering to investigate the effect of alloy composition, powder particle size and bed density on densification and microstructural evolution.FindingsFor all of the powder blends the sintered density increases with the specific laser energy input until a saturation level is reached. Beyond this saturation level no further increase in sintered density is obtained for an increase in specific laser energy input. However, the peak density achieved for a given blend varied significantly with the chemical constitution of the alloy, peaking at approximately 9 wt% Si. The tap density of the raw powder mixture (assumed to be representative of bed density) was also a significant factor.Originality/valueThis is the first study to consider the usefulness of silicon as an alloying element in aluminium alloys to be processed by selective laser sintering. In addition the paper outlines the key factors in optimising processing parameters and powder properties in order to attain sound sinterability for direct laser sintered parts.

Defectoscopy of direct laser sintered metals by low transmission ultrasonic frequencies

This paper focuses on the improvement of ultrasonic defectoscopy used for machine elements produced by direct laser metal sintering. The direct laser metal sintering process introduces the mixed metal powder and performs its subsequent laser consolidation in a single production step. Mechanical elements manufactured by laser sintering often contain many hollow cells due to weight reduction. The popular pulse echo defectoscopy method employing very high frequencies of several GHz is not successful on these samples. The aim of this paper is to present quadraphonic transmission ultrasound defectoscopy which uses low range frequencies of few tens of kHz. Therefore, the advantage of this method is that it enables defectoscopy for honeycombed materials manufactured by direct laser sintering. This paper presents the results of testing performed on AlSi12 sample.

S041015 レーザ積層造形による工具鋼粉末の積層造形

In this study, the fabrication conditions of tool steel powder by direct selective laser sintering were investigated. The effects of powder size and laser scanning conditions on the state of laser-sintered body were examined to fabricate a sound laser-sintered body of tool steel. The optimum laser power, scan speed and scan pitch were examined by experiments. As a result, it is difficult to fabricate a sound laser sintered body by using SKD11 powder of 10 μm. It was, however, found that the continuously smooth single-scan track can be obtained at wider range between laser power and scan speed by using SKD11 powder below 45 μm. The laser-sintered body could be fabricated at a laser power of 10 W, a scan speed of 10 mm/s and a scan pitch of 0.08 mm by using SKD11 powder below 45 μm.

Evolution of Direct Selective Laser Sintering of Metals

Direct Metal Selective Laser Sintering (DMSLS) is a layer-by-layer additive process for metal powders, which allows quick production of complex geometry parts. The aim of this study is to analyse the improvement of DMSLS with “EOSINT M270”, the new laser sintering machine developed by EOS. Tests were made on sintered parts of Direct Metal 20 (DM20), a bronze based powder with a mean grain dimension of 20 μm. Different properties and accuracy were evaluated for samples manufactured with three different exposure strategies. Besides mechanical properties, the manufacturing process was also examined in order to evaluate its characteristics. The quality of laser sintered parts is too affected by operator experience and skill. Furthermore, critical phases are not automatic and this causes an extension of time required for the production. Due to these limitations, DMSLS can be used for Rapid Manufacturing, but it is especially suitable to few sample series.

Characterization of Porosity in a Laser Sintered MMCp Using X-Ray Synchrotron Phase Contrast Microtomography

Direct Laser Sintering (DSL), a technology enabling the production of dense metal components directly from 3D CAD data, was used for the first time to produce a Metal Matrix Composite (MMCp) based on Al-Si-Cu alloy in view of its application in different fields, in particular for aeronautics. The porosity of the material obtained so was investigated by using optical and electron microscopy and, in particular, X-ray computed microtomography techniques. DSL is a unique technique to produce complex components in an economical way while computed microtomography is a unique technique to evaluate the porosity and pore and cracks distribution in a not destructive way. A near homogeneous distribution of the porosity and pore sizes was observed both comparing different regions of the same specimen and also by comparing different samples obtained by using the same DLS production method. A quantitative analysis of the damage in the composite is also reported.

Microstructures and properties of direct laser sintered tungsten carbide (WC) particle reinforced Cu matrix composites with RE–Si–Fe addition: A comparative study

The poor wettability between ceramics and metals is a main obstacle in obtaining high-performance metal-matrix composites (MMCs) parts using direct metal laser sintering (DMLS). Rare earth (RE) elements, due to their unique physical and chemical properties, have high potential for improving laser processability of MMCs. In this work, a comparative study was performed to investigate the influence of RE–Si–Fe addition on microstructural features and mechanical properties of DMLS processed tungsten carbide (WC) particle reinforced Cu MMCs parts. It showed that by adding 3 wt% RE–Si–Fe, the WC reinforcing particles were refined, the particle dispersion state was homogenized, and the particle/matrix interfacial compatibility was enhanced. The RE–Si–Fe-containing WC/Cu MMCs parts possessed significantly elevated mechanical properties, i.e., densification level of 95.7%, microhardness of 417.6 HV, fracture strength of 201.8 MPa, and friction coefficient of 0.8. The metallurgical functions of the RE–Si–Fe additive for the improvement of DMLS quality of MMCs parts were discussed.

Balling phenomena in direct laser sintering of stainless steel powder: Metallurgical mechanisms and control methods

Balling effect, as an unfavorable defect associated with direct metal laser sintering (DMLS), is a complex physical metallurgical process. In this work, two kinds of balling phenomena during DMLS of 316L stainless steel powder were investigated and the metallurgical mechanisms of balling were elucidated. It was found that using a low laser power gave rise to the first kind of balling characterized by highly coarsened balls possessing an interrupted dendritic structure in the surface layer of balls. A limited amount of liquid formation and a low undercooling degree of the melt due to a low laser input was responsible for its initiation. The second kind of balling featured by a large amount of micrometer-scaled (∼10 μm) balls on laser sintered surface occurred at a high scan speed. Its formation was ascribed to laser-induced melt splashes caused by a high capillary instability of the melt. Feasible control methods were proposed to alleviate balling phenomena. It showed that increasing the volumetric density of energy input, which was realized by increasing laser power, lowering scan speed, or decreasing powder layer thickness, decreased the tendency of balling. The addition of a trace amount of deoxidant (H 3BO 3 and KBF 4) in the powder yielded a smooth laser sintered surface free of balling.