Laser Powder Bed Fusion Technology Research Articles

Micro-scale residual stress and deformation analysis in bimetal bronze-stainless steel samples produced by laser powder bed fusion technology

Additive manufacturing technologies attract significant interest as a route to fabricate structural elements with complex net shapes. 3D printing may offer the option of combining dissimilar materials within one build, like dissimilar metals in the case of laser powder bed fusion. The strength of bimetallic interfaces formed by immiscible metals such as stainless steel and copper alloys depends on the mechanism of bonding, the presence and nature of defects, and residual stresses, placing the focus of interest on the structure–mechanical performance relationship in the vicinity of the bimetallic interface. In this article, we report an investigation of laser powder bed fusion fabrication of thin plate bimetallic bronze-stainless steel samples. Sample characterization was performed using several techniques including surface topography and fractography, energy dispersive X-ray spectroscopy, electron backscatter diffraction, mechanical testing with digital image correlation mapping of strains during in situ tension inside scanning electron microscope. Spatially resolved evaluation of the local residual stress was carried out using the Korsunsky Focused Ion Beam micro-ring-core drilling method (FIB-DIC). Pure bronze and stainless steel samples fabricated separately possessed good strengths (ultimate tensile strengths of 150 MPa and 850 MPa, respectively) and significant amounts of ductility (local strains to failure of 0.8% and 1.2%, respectively), in spite of the presence of a relatively coarse and porous structure. Although the values for the bimetallic sample were somewhat reduced, they also demonstrated good mechanical properties, with the ultimate tensile strength of 105 MPa and 0.7% ductility. The fracture of bimetal takes place within the bronze at a distance of about 150 μm from the interface. The present report discusses the insights obtained, and charts the pathways towards further improvement of bimetal joint strengths through simple modifications of the laser powder bed fusion printing procedures.

3D printed metal-insulator layered structure

Additively manufactured metal–insulator layered structures are facing a contradictory demand. Laser sources with different wavelength are required to melt the two materials. In the present study we attempt to additively manufacture metal–insulator composite using a fiber laser. Three different procedures are presented. Firstly, production of a core–shell soft magnetic composite by laser-powder bed fusion technology from a unique amorphous alloy. Lastly the preparation of Fe-Si and Fe-TiN layered structure is discussed. Further improvement of Fe-TiN can be achieved by oxidation heat-treatment. Microstructure, porosity and complex permeability spectra were studied.

The Study on Resolution Factors of LPBF Technology for Manufacturing Superelastic NiTi Endodontic Files

Laser Powder Bed Fusion (LPBF) technology is a new trend in manufacturing complex geometric structures from metals. This technology allows producing topologically optimized parts for aerospace, medical and industrial sectors where a high performance-to-weight ratio is required. Commonly the feature size for such applications is higher than 300–400 microns. However, for several possible applications of LPBF technology, for example, microfluidic devices, stents for coronary vessels, porous filters, dentistry, etc., a significant increase in the resolution is required. This work is aimed to study the resolution factors of LPBF technology for the manufacturing of superelastic instruments for endodontic treatment, namely Self-Adjusting Files (SAF). Samples of thin walls with different incline angles and SAF samples were manufactured from Nickel-Titanium pre-alloyed powder with a 15–45 μm fraction. The printing procedure was done using an LPBF set-up equipped with a conventional ytterbium fiber laser with a nominal laser spot diameter of 55 microns. The results reveal physical, apparatus, and software factors limiting the resolution of the LPBF technology. Additionally, XRD and DSC tests were done to study the effect of single track based scanning mode manufacturing on the phase composition and phase transformation temperatures. Found combination of optimal process parameters including laser power of 100 W, scanning speed of 850 mm/s, and layer thickness of 20 μm was suitable for manufacturing SAF files with the required resolution. The results will be helpful for the production of NiTi micro objects based on periodic structures both by the LPBF and μLPBF methods.

Effect of Preheating on the Residual Stress and Material Properties of Inconel 939 Processed by Laser Powder Bed Fusion.

One of the main limitations of laser powder bed fusion technology is the residual stress (RS) introduced into the material by the local heating of the laser beam. RS restricts the processability of some materials and causes shape distortions in the process. Powder bed preheating is a commonly used technique for RS mitigation. Therefore, the objective of this study was to investigate the effect of powder bed preheating in the range of room temperature to 400 °C on RS, macrostructure, microstructure, mechanical properties, and properties of the unfused powder of the nickel-based superalloy Inconel 939. The effect of base plate preheating on RS was determined by an indirect method using deformation of the bridge-shaped specimens. Inconel 939 behaved differently than titanium and aluminum alloys when preheated at high temperatures. Preheating at high temperatures resulted in higher RS, higher 0.2% proof stress and ultimate strength, lower elongation at brake, and higher material hardness. The increased RSs and the change in mechanical properties are attributed to changes in the microstructure. Preheating resulted in a larger melt pool, increased the width of columnar grains, and led to evolution of the carbide phase. The most significant microstructure change was in the increase of the size and occurrence of the carbide phase when higher preheating was applied. Furthermore, it was detected that the evolution of the carbide phase strongly corresponds to the build time when high-temperature preheating is applied. Rapid oxidation of the unfused powder was not detected by EDX or XRD analyses.

L-PBF for the production of metallic phononic crystal: design and functional characterization

Vibration abatement often requires the adoption of peculiar solutions and/or foundations. This paper presents an innovative solution to this problem, consisting in a phononic meta-material realized via Laser Powder Bed Fusion (L-PBF) capable to prevent the propagation of vibrations within specific frequency ranges. The integration of this meta-material within existing supporting structures can, therefore, greatly reduce the needing of foundations capable to stop vibrations. After a description of the design procedure of the meta-material that shows how to satisfy the constraints imposed by L-PBF technology, the manufactured sample is described and analyzed to predict its band-gaps. Finally, the theoretical results are compared with experimental measurements. These results show a good agreement between expected and actual meta-material behavior.

Comparative evaluation of printability and compression properties of poly-ether-ether-ketone triply periodic minimal surface scaffolds fabricated by laser powder bed fusion

Porous poly-ether-ether-ketone (PEEK) scaffolds have a good application prospect in orthopedics because of their excellent biocompatibility, chemical stability, and permeability. However, the porous PEEK scaffolds fabricated by conventional manufacturing methods are restricted to the uncontrollable and weak mechanical properties. In this work, porous PEEK scaffolds with highly-tunable elastic modulus are designed using typical triply periodic minimal surface (TPMS) structures, Gyroid, Diamond, I-WP, and then printed by laser powder bed fusion (LPBF). The effects of unit cell type and volume fraction on the manufacturing deviation, stress distribution, and mechanical properties of LPBF-printed PEEK scaffolds are investigated. The results illustrate that the Gyroid scaffolds present the lowest volume fraction deviation, followed by the I-WP scaffolds. By adjusting the volume fraction, the elastic modulus of the TPMS scaffolds is in the range of 42.17–512.12 MPa, which is comparable with those of human trabecular bones. Using the finite element (FE) simulation method, the experimental mechanical properties of the TPMS scaffolds can be predicted. In summary, the Gyroid structure is the optimal choice for the design and LPBF fabrication of porous PEEK scaffolds due to the best manufacturing fidelity and relatively good mechanical properties. • PEEK TPMS scaffolds were successfully fabricated by LPBF technology. • A FE method was proposed and it could accurately predict the mechanical responses of the PEEK TPMS scaffolds. • The elastic moduli of PEEK TPMS scaffolds can match those of human trabecular bones.

Investigating the Melt-Pool Temperature Evolution in Laser-Powder Bed Fusion by Means of Infra-Red Light: A Review

Recent decades seen the success of Additive Manufacturing (AM) in many industrial applications including aerospace, biomedical, automotive, and tooling. In the manufacturing of metallic parts, AM technology has the ability to produce parts with complex geometries which are difficult or impossible to produce using the conventional fabrication methods, such as machining and casting. Another benefit of AM is the employment of metal and metal alloys which are difficult to machine. Alloys such as titanium, nickel-titanium, and stainless steel have a wide range of applications particularly in the aerospace and biomedical industry. Selective Laser Melting (SLM), also known as Laser Powder Bed Fusion (L-PBF) is a type of AM technology used for the 3D printing of metal and alloy parts. The major drawback in L-PBF technology is the anisotropic properties of the produced parts. From L-PBF, these anisotropies exist due to instant melting and re-solidification of the metal powder, the ultra-high cooling rates and the variant temperature levels across the build layers and within the single layer itself. This article explores the essential role of the melt-pool temperature and temperature gradients that occur during the L-PBF process and their effects on the additively manufactured part’s properties.

Characterizing the strength of support-model connection in the case of laser powder bed fusion technology by torsion test

Additive manufacturing of metal parts was investigated in this research work, which connects to the models' supports. The connection between the support and the model has a high impact on the mechanical fixturing, the heat conduction, the removability of the support and the success of printing. In this research work, some support and tooth parameters such as the top length, the hatch distance, and Z offset were investigated to create a model-support connection strength map. This knowledge makes it possible to select the most appropriate connection type between the support and the model by the requirements specified in different cases. The strength of the support-model connection was characterized by the shearing properties using the torsion test. The results are illustrated and compared by different support parameters and materials. The 316L stainless steel and Ti6Al4V alloy were investigated. Different failure modes were determined depending on the chosen parameters.

Revealing dynamic processes in laser powder bed fusion with in situ X-ray diffraction at PETRA III.

The high flux combined with the high energy of the monochromatic synchrotron radiation available at modern synchrotron facilities offers vast possibilities for fundamental research on metal processing technologies. Especially in the case of laser powder bed fusion (LPBF), an additive manufacturing technology for the manufacturing of complex-shaped metallic parts, in situ methods are necessary to understand the highly dynamic thermal, mechanical, and metallurgical processes involved in the creation of the parts. At PETRA III, Deutsches Elektronen-Synchrotron, a customized LPBF system featuring all essential functions of an industrial LPBF system, is used for in situ x-ray diffraction research. Three use cases with different experimental setups and research questions are presented to demonstrate research opportunities. First, the influence of substrate pre-heating and a complex scan pattern on the strain and internal stress progression during the manufacturing of Inconel 625 parts is investigated. Second, a study on the nickel-base superalloy CMSX-4 reveals the formation and dissolution of γ' precipitates depending on the scan pattern in different part locations. Third, phase transitions during melting and solidification of an intermetallic γ-TiAl based alloy are examined, and the advantages of using thin platelet-shaped specimens to resolve the phase components are discussed. The presented cases give an overview of in situ x-ray diffraction experiments at PETRA III for research on the LPBF technology and provide information on specific experimental procedures.

Experimental Validation and Evaluation of the Bending Properties of Additively Manufactured Metallic Cellular Scaffold Structures for Bone Tissue Engineering.

The availability of additive manufacturing enables the fabrication of cellular bone tissue engineering scaffolds with a wide range of structural and architectural possibilities. The purpose of bone tissue engineering scaffolds is to repair critical size bone defects due to extreme traumas, tumors, or infections. This research study presented the experimental validation and evaluation of the bending properties of optimized bone scaffolds with an elastic modulus that is equivalent to the young’s modulus of the cortical bone. The specimens were manufactured using laser powder bed fusion technology. The morphological properties of the manufactured specimens were evaluated using both dry weighing and Archimedes techniques, and minor variations in the relative densities were observed in comparison with the computer-aided design files. The bending modulus of the cubic and diagonal scaffolds were experimentally investigated using a three-point bending test, and the results were found to agree with the numerical findings. A higher bending modulus was observed in the diagonal scaffold design. The diagonal scaffold was substantially tougher, with considerably higher energy absorption before fracture. The shear modulus of the diagonal scaffold was observed to be significantly higher than the cubic scaffold. Due to bending, the pores at the top side of the diagonal scaffold were heavily compressed compared to the cubic scaffold due to the extensive plastic deformation occurring in diagonal scaffolds and the rapid fracture of struts in the tension side of the cubic scaffold. The failure in struts in tension showed signs of ductility as necking was observed in fractured struts. Moreover, the fractured surface was observed to be rough and dull as opposed to being smooth and bright like in brittle fractures. Dimple fracture was observed using scanning electron microscopy as a result of microvoids emerging in places of high localized plastic deformation. Finally, a comparison of the mechanical properties of the studied BTE scaffolds with the cortical bone properties under longitudinal and transverse loading was investigated. In conclusion, we showed the capabilities of finite element analysis and additive manufacturing in designing and manufacturing promising scaffold designs that can replace bone segments in the human body.

A segmented heat source for efficiently calculating the residual stresses in laser powder bed fusion process

To improve the efficiency and accuracy of the numerical simulation for the laser powder bed fusion (LPBF) process, an optimized segmented heat source (OSHS) model is proposed in this work. The results show that the geometrical size and temperature distribution contour of the calculated cross-sectional molten pool are almost the same as those calculated by the moving heat source (MHS). Under the same mesh scheme, the calculation efficiency of the OSHS model improves about 9 times to that of the traditional MHS model. Moreover, the OSHS model can achieve stable calculations with the medium-size elements grid, which can reduce the required calculation time to one-third of the fine mesh scheme and achieve almost the same thermal-mechanical calculation result. This model was successfully applied on the simulation of the LPBF single layer multi-tracks thermal-mechanical numerical simulation. To verify the calculated results, the sample of the same size was prepared using 316L stainless steel by LPBF technology. The surface residual stress components of the as-built sample were measured by X-Ray Diffraction (XRD). The calculated residual stress curves were in good agreement with the measured results. The simulation and experimental results indicated that the primary surface residual stress component was the tensile stress along the laser scanning direction, which was about 280 MPa. The shear stress was concentrated at the corners of the as-built sample. Therefore, the OSHS model proposed in this work greatly improved the calculation efficiency and obtained very accurate results for the thermal-mechanical simulation of the LPBF process.

Effect of Laser Scanning Speed on the Microstructure and Mechanical Properties of Laser-Powder-Bed-Fused K418 Nickel-Based Alloy.

Laser powder bed fusion (LPBF) is a powder-bed-based metal additive manufacturing process with multiple influencing parameters as well as multi-physics interaction. The laser scanning speed, which is one of the essential process parameters of the LPBF process, determines the microstructure and properties of the components by adjusting the instantaneous energy input of the molten pool. This work presents a comprehensive investigation of the effects of the laser scanning speed on the densification behavior, phase evolution, microstructure development, microhardness, and tensile properties of K418 alloy prepared by laser powder bed fusion. When the scanning speed is 800 mm/s, the microstructure of the material is dominated by cellular dendrite crystals, with coarse grains and some cracks in the melting tracks. When the scanning speed is increased to 1200 mm/s, a portion of the material undergoes a cellular dendrite–columnar crystal transition, the preferred orientation of the grains is primarily (001), and internal defects are significantly reduced. When the scanning speed is further increased to 1600 mm/s, columnar crystals become the main constituent grains, and the content of high-angle grain boundaries (HAGBs) within the microstructure increases, refining the grain size. However, the scanning speed is too fast, resulting in defects such as unmelted powder, and lowering the relative density. The experimental results show that by optimizing the laser scanning speed, the microhardness of the LPBF-ed K418 parts can be improved to 362.89 ± 5.01 HV, the tensile strength can be elevated to 1244.35 ± 99.12 MPa, and the elongation can be enhanced to 12.53 ± 1.79%. These findings could help determine the best scanning speed for producing K418 components with satisfactory microstructure and tensile properties via LPBF. In addition, since the LPBF process is largely not constrained and limited by the complexity of the geometric shape of the part, it is expected to manufacture sophisticated and complex structures with hollow, porous, mesh, thin-walled, special-shaped inner flow channels and other structures through the topology optimization design. However, due to the relatively narrow LPBF process window, this study will benefit from LPBF in producing a lightweight, complex, and low-cost K418 product, greatly improving its performance, and promoting the use of LPBF technology in the preparation of nickel-based superalloys.

Micro wire and arc additive manufacturing (µ-WAAM)

• A novel variant of WAAM is developed. • Increased accuracy, while maintaining high depositions rates. • µ-WAAM can overcome some limitations of powder-bed fusion additive manufacturing processes. In this work we explore the wire and arc additive manufacturing (WAAM) process scale limits by using a wire diameter of 250 µm and about 2 mm stickout. This WAAM variant, named µ-WAAM, aims at competing with laser powder bed fusion technology, by enabling the fabrication of smaller parts with significantly higher deposition rates. The main issues of descaling the WAAM process are discussed and an acceptable parameter window to fabricate thin walls is presented. Several depositions were successfully performed with ASTMA 228 steel using a wire feed speed ranging from 75 to 90 mm/s, travel speed from 7 to 10 mm/s, a current intensity of 16 A RMS and power of ≈ 35 W RMS.

Acoustic properties of low-carbon 2% Mn-doped steel manufactured by laser powder bed fusion technology

The results of studies of acoustic anisotropy and anisotropy in terms of Young's and shear moduli, Poisson's ratio, and transverse strain of low-carbon 2% Mn-doped steel manufactured by laser powder bed fusion technology are presented in this article. The results on non-uniformity of anisotropy over the surface area of disc-shaped samples are also presented. The electromagnetic-acoustic method is used to input and receive longitudinal and transverse waves of various polarizations. The dependences of the tH-wave velocity and Poisson's ratio on the modes of heat treatment of the samples are obtained. The angular indicatrices of the change in the velocity of the transverse tH-wave depending on the direction of polarization and the indicatrix of Poisson's ratio are presented, as well as the calculated values of the weighted average Poisson's ratio over the plane of the samples.

Towards customized heat treatments and mechanical properties in the LPBF-processed Ti-6Al-2Sn-4Zr-6Mo alloy

The Ti-6Al-2Sn-4Zr-6Mo alloy was recently successfully processed using the laser powder bed fusion technology. This material is characterized by a very low strength in the as-built state, due to the presence of α” martensite. In this work, the transformations taking place at different temperatures in this alloy were investigated with the aim of achieving the necessary knowledge to customize the mechanical properties via heat treatments. Martensite decomposition, residual stress relaxation and the β-transus temperature were identified at progressively increasing temperatures. Then, different annealing treatments, corresponding to the temperatures identified, were conducted. For each condition, dissimilar α + β microstructures were obtained, varying in terms of morphology (lamellar, bi-lamellar) and size of the α laths (0.5–2.2 µm). The corresponding tensile properties were also significantly different. In particular, outstanding strength values were achieved when the residual stress was retained. For higher temperatures, a slight drop in strength was coupled with an outstanding improvement in terms of ductility. Overall, the most balanced combination of tensile properties was obtained for an annealing temperature of 875 °C. In this condition, the bi-lamellar microstructure grants a reduction in the slip length due to the presence of secondary α laths.

Hybrid surface characterisation of intra thin-walled Ti6Al4V surfaces produced by laser powder bed fusion technology

The proposed work investigates the hybrid surface characterisation of intra thin-walled Ti6Al4V surfaces fabricated using laser powder bed fusion technology. The thin-walled samples were characterised using scanning electron microscopy and Opto-digital microscopy techniques. The fractal dimensional analysis was performed using ImageJ software integrated with an open-source MultiFrac plug-in. The surface microscopy analysis revealed satellites powder particles, partially melted powder particles, spherical balling, and pores on the thin-walled surface. The fractal dimension establishes a correlation between the surface roughness values. The surface areal surface parameters analysis suggested variation along the build direction of the thin-walled Ti6Al4V sample. The development of sharp peaks and thus higher Ra, Sku and Ssk values were found along the build direction of the intra thin-walled samples. Therefore, the combination of areal surface topography analysis and fractal dimension approach can be a promising methodology towards surface characterisation of additively manufactured complex thin-walled surfaces.

Micro-Scale Residual Stress and Deformation Analysis in Bimetal Bronze-Stainless Steel Samples Produced by Laser Powder Bed Fusion Technology

Micro-Scale Residual Stress and Deformation Analysis in Bimetal Bronze-Stainless Steel Samples Produced by Laser Powder Bed Fusion Technology

Novel design for an additively manufactured nozzle to produce tubular scaffolds via fused filament fabrication

Additive Manufacturing of scaffolds with controlled porosity has demonstrated its value in many fields such as tissue regeneration and filtration applications. However, there is a scarcity of reports investigating the manufacturing of scaffolds with hollow fibers. Mainly, this was due to the lack of proper tools (extrusion nozzles) that enable the production of such structures. In this study, a methodology for designing and manufacturing a nozzle that can be fitted onto a fused filament fabrication 3D printer is proposed. Titanium laser powder bed fusion technology, coupled with chemical polishing, was used to produce the complex design of the nozzle with minimal post processing steps. Through this approach, we were able to produce a nozzle with the required internal structure. Numerical simulations and experimental analyses were conducted to better understand the melt flow behavior within the nozzle and the extrudate swell. The designed nozzle was capable of extruding tubular fibers with an outer and inner diameter of 1.3 mm and 0.5 mm, respectively. This, for example, enables the fabrication of tissue regeneration scaffolds with hollow fibers. This proposed method for designing and manufacturing can be used to produce nozzles capable of 3D printing scaffolds that best suit the intended application.

Effect of high-temperature preheating on pure copper thick-walled samples processed by laser powder bed fusion

The Laser Powder Bed Fusion (LPBF) also called Selective Laser Melting technology uses for the layer-based part fabrication laser beam as the main energy source for melting a powdered material. The processing of pure copper by LPBF technology is a challenge due to its high laser beam reflectivity, thermal conductivity and affinity to oxygen. Thus, for reaching homogeneous sample in an acceptable quality a high power Nd: YAG fibre laser sources are used. In this study, the effect of Nd: YAG fibre laser with a maximum power of 400 W and 400 °C high-temperature base plate preheating as another heat source on the relative density of pure copper thin-walled and thick-walled samples was investigated. Moreover, the effect of layer thickness, laser speed, laser velocity, hatch distance, sample width, scanning and remelting strategy was studied. Further on, the effect of powder bed preheating on the copper powder melting, laser beam reflectivity, wetting and solidification conditions and powder and part oxidation was discussed. Using the statistical methods for experimental planning the behaviour of each observed process parameter was revealed and the proper combination of process parameters was stated. It was found that for reaching relative density over 99% the process parameters should be set from observed range as following: layer thickness 0.03 mm, laser power 400 W, laser velocity 505 mm∙s−1, hatch distance 0.06 mm and powder bed preheating of 400 °C. Despite the high value of relative density, the issues connected with layer and track bonding attributed to thin oxide layers were not successfully eliminated.

Parameter Optimization for Printing Ti6Al4V-Alloy Patient-Customized Orthopaedic Implants by Laser Powder Bed Fusion Using Physio-mechanical Properties and Biological Evaluations.

A class of additive manufacturing technologies called Laser powder bed fusion (LPBF), which allows fabricating metallic components with complex geometries in near-net-shape, can be employed for fabricating patient-customized orthopaedic implants. Selection and optimization of the LPBF process parameters are critical to achieving the required biomechanical properties and fabricability of such implants. The process parameters of direct metal laser sintering, the most widely used LPBF process, were optimized for fabricating Ti6Al4V ELI orthopaedic implants, based on ASTM and ASM standards. The parameters included Laser power, Laser velocity and hatch distance, which were varied using Taguchi approach. A multi-criteria decision-making technique (TOPSIS) was employed to optimize the process parameters considering yield and ultimate tensile strength, percentage elongation, part density, volumetric energy density and printing time. In-vitro cytotoxicity and in-vivo muscle implantation were performed on the optimized samples for determining the suitability of the parameters for biomedical applications. A combination of medium laser power, higher laser velocity, and lower hatch distance with values 200W, 2200mm/s and 0.08mm, respectively, was found to be suitable for producing implants. Based on the type of LPBF technology in use, an implant manufacturer can select the initial set of parameters using a similar approach and improve them further based on experimental results. The optimized parameters were found to be suitable for developing orthopaedic implants, in terms of physical, mechanical and biological criteria. The methods and results presented in work are expected to assist the implant manufacturers in meeting the expected user requirements and quality standards.