S0440105 レーザービーム積層造形により作製したIN 718造形材の組織と強度特性([S044]次世代3Dプリンティング,機械材料・材料加工部門)

Effects of building direction and heat treatment on the local mechanical properties of direct metal laser sintered 15-5 PH stainless steel

In this work, we numerically investigate the effect of building direction on the solidification behaviour and microstructure evolution of direct metal laser sintered (DMLS) AlSi10Mg. The building direction, as previously proved in experimental studies, can influence the solidification behavior and promote morphological transitions in cellular dendritic microstructures such as columnar-to-equiaxed transition (CET). We develop a thermal model to systemically address the impact of laser processing conditions and building direction on time-dependent solidification parameters including pulling velocity, thermal gradients, and cooling rates of the molten pool during DMLS of AlSi10Mg alloy. We then study the microstructure evolution of DLMS-AlSi10Mg for horizontal and vertical building directions. The present model includes heterogenous nucleation on inoculant particles that triggers the CET, and its results are consistent with the predictions of a previously developed model. In addition, findings are compared with experimental observations to ensure the accuracy of obtained numerical results.

The complex surface morphology and multiscale surface features inherent with additively manufactured (AM) components contributes to the overall flow characteristics and heat transfer of cooling passages. As the AM process and cooling data in literature continues to evolve, so does the need for more accurate heat transfer and pressure loss correlations for AM cooling schemes. This study improves the predictability of pressure loss and heat transfer for AM cooling passages by fabricating a range of coupons and investigating samples in literature. Twenty-seven test coupons were manufactured using direct metal laser sintering in an assortment of build directions and build locations that produced a variety of surface morphologies. Nondestructive evaluation, computed tomography scanning, was used to quantify the surface morphology as well as capture the as-built geometric dimensions of the cooling schemes. Friction factor and bulk Nusselt number of the coupons were measured using an experimental rig. Pressure loss and heat transfer correlations in literature were compared with the experimental results from the current coupons and datasets from literature. Arithmetic mean roughness correlations in literature struggled to predict the cooling performance of AM channels since the bulk roughness statistic did not capture the overall form of the surface morphology. A combination of root mean square roughness and skewness of the roughness was able to best predict pressure loss and heat transfer for the present samples and those in literature while being independent of build location, build direction, material, machine, and laser parameters. The maximum absolute error was 25% and average absolute error was 12% for the friction factor correlation. The maximum absolute error was 39% and average absolute error was 8% for the Nusselt Number correlation.

Effect of volume energy density on selective laser melting NiTi shape memory alloys: microstructural evolution, mechanical and functional properties

Selective laser sintering (SLS) is an efficient process in additive manufacturing that enables rapid part production from computer-based designs. However, SLS is limited by its notable lack of in-situ process monitoring when compared to other manufacturing processes. We report the incorporation of optical coherence tomography into an SLS system in detail and demonstrate access to surface and sub-surface features. Video frame rate cross-sectional imaging reveals areas of sintering uniformity and areas of excessive heat error with high temporal resolution. We propose a set of image processing techniques for SLS process monitoring with OCT and report the limitations and obstacles for further OCT integration with SLS systems.

Growth of long fatigue cracks in Ti6Al4V alloy manufactured by direct metal laser sintering (DMLS) was investigated. Two DMLS systems, EOSINT M270 and EOSINT M290, with different process parameters were used for production of CT specimens having three different orientations of crack propagation with respect to the DMLS build direction. The as-built specimens were stress relieved at 740 °C. The fatigue crack growth curve and the threshold values of the stress intensity factor for crack propagation were experimentally determined. It has been found that the chosen DMLS processing parameters and the used stress relieving procedure results in material exhibiting isotropic crack growth behavior, i.e. the crack growth was found to be independent of the DMLS build direction. The fatigue crack growth rates and the threshold values for the crack growth were compared with published results characterizing the as-built material and material after different post processing heat treatments.

The paper describes experiments on selective laser sintering (SLS) of a high-temperature ceramic material – silicon carbide powder F320 – using the MeltMaster3D-160 SLS unit equipped with a fiber ytterbium laser with a peak power of 200 W. We investigated the sintering mechanism and the impact of technological parameters on the microstructure, phase composition, and density of the resulting 3D cubic samples. The technological properties of the initial powder were also investigated, including morphology, granulometric composition, bulk density, and flow rate. The powder morphology mainly consists of acicular particles with an aspect ratio of 1:5. Granulometric analysis revealed an average particle size of 48 μm. Measurements indicated that the bulk density reached 1.11 ± 0.01 g/cm3, approximately 36.6 % of the theoretical density value. The average time of powder outflow from the Hall funnel was 21.0 ± 0.1 s, with 2–3 hits on the funnel during the measurement process. Experimental cubic samples of 10×10 mm were manufactured using 75 technological modes. Silicon carbide powder particles sinter due to the thermal effect of laser radiation and the release of SiC microparticles on the surface of the powder particles, with silicon (average size less than 1 μm) prevailing in the composition, followed by mutual bonding of neighboring powder particles in the sintering region. X-ray phase analysis demonstrated that due to the laser radiation, the resulting 3D samples contain the following phases: SiC (6H), Si, and C. It was revealed that a scanning step larger than the actual spot diameter (spot diameter + thermal influence zone), 60–70 μm in size, causes the formation of unsintered areas between sintering tracks. The key parameters affecting the density index of the obtained samples are layer height, energy density, and scanning step. The best density index for the obtained samples is 86.7 % relative to the absolute density of the material (3.21 g/cm3). Further research will be devoted to the development of techniques for post-processing the resulting porous samples-blanks to obtain a density close to 100 %.

Objectives: To compare two of the most popular rapid prototyping processes of Stereolithography (SLA) and Selective Laser Sintering (SLS) from a product-based perspective, to help customers analyze them and make a choice between the two. Methods: This paper includes the qualitative testing of identical specimens created by SLA and SLS. The specimens are evaluated on the parameters of dimensional accuracy, tensile strength, water absorption, surface roughness, density, Vickers hardness and microscopic defect structure. The outcome of this study aims at helping people to understand SLS and SLA better in terms of the products they create so that it becomes easier for users to make a choice between the two. It also aims at highlighting the above mentioned statistical information about SLA and SLS so that they may be improvised and enhanced in the future. Findings: Based on the tests conducted, it was confirmed that the SLA specimens were better than the SLS specimens in the tensile strength, water absorption, surface roughness and density tests. The SLS specimens outperformed the SLA specimens in the dimensional accuracy and Vickers hardness tests. Thus it was concluded that the SLA specimens exhibited better mechanical and physical characteristics than the SLS specimens.

Direct metal laser sintering is an additive manufacturing process which is capable of fabricating three-dimensional components using a laser energy source and metal powder particles. Despite the numerous benefits offered by this technology, the process maturity is low with respect to traditional subtractive manufacturing methods. Relationships between key processing parameters and final part properties are generally lacking and require further development. In this study, residual stresses were evaluated as a function of key process variables. The variables evaluated included laser scan strategy and build plate preheat temperature. Residual stresses were measured experimentally via neutron diffraction and computationally via finite element analysis. Good agreement was shown between the experimental and computational results. Results showed variations in the residual stress profile as a function of laser scan strategy. Compressive stresses were dominant along the build height (z) direction, and tensile stresses were dominant in the x and y directions. Build plate preheating was shown to be an effective method for alleviating residual stress due to the reduction in thermal gradient.

Microheater array powder sintering: A novel additive manufacturing process

A Round Robin study for selective laser sintering of polymers: Back tracing of the pore morphology to the process parameters

Microstructural evolution and anisotropic regulation in tensile property of cold metal transfer additive manufactured Ti6Al4V alloys via ultrasonic impact treatment

Improvement of densification and microstructure of ASTM A131 EH36 steel samples additively manufactured via selective laser melting with varying laser scanning speed and hatch spacing

The aim of this work was to investigate the microstructure and mechanical properties of samples produced by direct metal laser sintering (DMLS) with varied laser beam speed before and after heat treatment. Optical analysis of as-built samples revealed microstructure built of martensite needles and columnar grains, growing epitaxially towards the built direction. External and internal pores, un-melted or semi-melted powder particles and inclusions in the examined samples were also observed. The strength and Young’s modulus of the DMLS samples before heat treatment was higher than for cast and forged samples; however, the elongation at break for vertical and horizontal orientation was lower than required for biomedical implants. After heat treatment, the hardness of the samples decreased, which is associated with the disappearance of boundary effect and martensite decomposition to lamellar mixture of α and β, and the anisotropic behaviour of the material also disappears. Ultimate tensile strength (UTS) and yield strength(YS) also decreased, while elongation increased. Tensile properties were sensitive to the build orientation, which indicates that DMLS generates anisotropy of material as a result of layered production and elongated β prior grains. It was noticed that inappropriate selection of parameters did not allow properties corresponding to the standards to be obtained due to the high porosity and defects of the microstructure caused by insufficient energy density.

Selective Laser Sintering (SLS) and Selective Laser Melting (SLM) are parent Additive Manufacturing processes using a laser to solidify metallic, ceramic, polymer or composite powders. During the process, the object is built layer by layer. A laser source is responsible for the consolidation, by local heating. The light is deviated by a scanning head according to the instructions of an STL file. Then partial fusion of the particles takes place, followed by a solidification of the liquid created. Kinetics of these steps are very high and play an important role in the final microstructure (rearrangement of the particles, pore creation, residual stresses). In the case of polymers and composites the technique is now well understood and widely used, but for metals and ceramics it suffers from a lack of precision, surface roughness and poor mechanical properties. The goal of this Ph.D. work is to understand the effect of the thermal gradients on the consolidation process, using different laser parameters (power, pulse frequency, scan speed,…). In SLS, microscopic thermal gradients arise, due to the fact that the laser is pulsed and that only the exterior of the powder particles is molten. A thermal model to describe the interaction between a laser beam and a (spherical) grain is proposed. This model allows for the incorporation of the latent heat of fusion and for a realistic surrounding. The absorbed laser intensity is modeled by means of the Mie theory. Experiments where two particles of powder are isolated and illuminated by the laser are carried out in order to measure the interparticular necks and the volume of liquid formed for different repetition rates. The thermal model leads to good predictions of the particles final sintering state. Fluid flow models are investigated in order to determine the dynamic of the molten liquid. The main issue is to explain the capillary flow mechanisms leading from the molten material to the neck formation. Two models are derived to simulate the fluid flow between the particles. The first one simulates a capillary flow between two parallel plates. The second one deals with energetic considerations arising from Frenkel's principle. In both cases, the final output is the liquid life-time necessary to get the interparticular neck lengths experimentally observed. We point out that the two models predict the same liquid lifetime, although they are derived with different hypotheses. In SLM, continuous lasers are used and one has to deal with macroscopic thermal gradients, since the particles are completely molten. A very high laser power is used and the negative thermal effects (like thermal stresses or balling effects) are important. A solution to control them is to adjust the scanning strategy. Four scanning strategies are investigated for material with different thermal conductivities. The three-dimensional model used to describe the laser-matter interactions and the temperature evolution of the scanned powder bed allows for a finite latent heat (Stefan-problem) and for conductivity modifications due to the consolidation. We show that this finite element thermal model can be efficiently used to anticipate most of the problems (like cracks or balling) arising in practice. The benefit of avoiding thermal gradients is shown, in particular by EBSD analysis. Finally, applications of the SLS/SLM technique on different pieces built during this work are shown. A new way to build support structure is also proposed.