Laser-based Powder Bed Fusion Research Articles

Laser-based Powder Bed Fusion of dispersion strengthened CoCrNi by ex-situ addition of TiN

In this study, metal matrix composites (MMC) of a CoCrNi medium entropy alloy (MEA) reinforced with titanium nitride (TiN) nano-sized particles were designed for production by laser-based powder bed fusion (LB-PBF). Two different additions, 0.5 wt% and 5 wt% of TiN nano- sized particles, were mixed with CoCrNi using tumbling and ball milling, respectively. The results suggest that although the powder morphology is not significantly affected by the TiN addition, the processability of CoCrNi-5 wt% TiN is worse than that of CoCrNi and CoCrNi-0.5 wt% TiN. The hardness of CoCrNi, originally 278 HV, is enhanced by addition of 0.5% and 5% TiN to 309 HV and 357 HV respectively. This hardening is mainly due to the presence of reinforcement particles in the matrix. The work presented here shows promising processability of MMC based on MEAs by LB-PBF, as well as the beneficial effect of the reinforcement phase on the mechanical properties of MEAs.

Support Structure Impact in Laser-Based Powder Bed Fusion of AlSi10Mg

Laser-Based Powder Bed Fusion (L-PBF) is one of the most established additive manufacturing methods used to build metallic components. During L-PBF, cross-sections of the components are melted in a powder bed by a laser. To connect the workpieces to the building platform, to dissipate the heat induced by the laser and to reduce characteristic distortion, support structures are applied. So far, different support structures were mostly compared with each other with regard to the resulting process results, such as microstructure, residual stresses and distortion. In this study, however, the influence of the area supported by the support structures is systematically investigated. For two different supports (columns and meanders), the percentage of the supported area is varied between 0 and 70% and the influences on the microhardness profiles, melt path geometries and distortion are analyzed. For instance, by increasing the supported area, it has been possible to reduce distortion by up to approximately 30%.

Surface layer porosity and surface quality in dependency of contour scanning strategy of Ti6Al4V parts

Laser based powder bed fusion of metals (PBF-LB/M) grants the potential for the combination of modern design approaches like topology optimization with the production of near-net shape parts suitable for high performance application. But there is not only the need for high relative density of the parts but also a low surface layer porosity to achieve mechanical properties comparable or better than forging or casting. The surface layer porosity depends among other factors on scanning strategy, precisely the contour parameters, filler parameters and hatch offset. This work investigates the influence of the named parameters and energy input on the correlation of surface porosity and surface roughness of Ti6Al4V specimens. To correlate surface roughness and surface layer porosity with the used laser parameters micro computed tomography (µ-CT) measurements are used.

Influence of beam shape on spatter formation during PBF-LB/M of Ti6Al4V and tungsten powder

Laser based powder bed fusion of metals (PBF-LB/M) is an advanced manufacturing technology with high design freedom and a broad range of processable materials. A special advantage is the possibility of using local in-situ alloying. This in-situ modification needs local powder deposition and therefore an adjusted scanning strategy which decreases spatter formation and spatter range to impede feedstock contamination. Beam shaping is a valid option to reduce spatter formation by creating a less dynamic melt pool and vapor stream. This works aim is to investigate the influence of a ring-spot profile on the amount of generated spatters in comparison to a Gaussian beam by scanning over tungsten and Ti6Al4V powder. The amount of tungsten particles found in the molten material around the process zone via cross sections functions as indicator and shows the reduced amount of particles leaving the process zone using the beam shaped profile in comparison to a Gaussian beam.

Grain Refinement of Stainless Steel by Strontium Oxide Heterogeneous Nucleation Site Particles During Laser-Based Powder Bed Fusion

Grain Refinement of Stainless Steel by Strontium Oxide Heterogeneous Nucleation Site Particles During Laser-Based Powder Bed Fusion

Influence of preheating temperatures on material properties of PBF-LB manufactured hot-work tool steel X37CrMoV5-1

Processing of high-carbon tool steels with laser-based powder bed fusion (PBF-LB) can lead to thermal and stress-induced cracks. Defect-free processing is often realized by using base plate preheating. The temperature level during the building process is influenced by laser energy input and the surrounding conditions. Due to the layer-by-layer structure, heat treatment effects occur along the building height of the specimen, affecting the material properties. In-situ heat treatment can be used to reduce post-processing. A tempering process is required for preheating temperatures up to 500°C, which can be omitted at higher preheating temperatures.The hot-work tool steel X37CrMoV5-1 is processed at preheating temperatures between 200°C and 800°C. Base plate and top-level sample surface temperature are monitored during the building and cooling. The influence of heat treatment effects is discussed based on the microstructure, retained austenite and hardness along the building height.

Simulation of the temperature gradient in laser-based powder bed fusion using machine learning

Laser-based powder bed fusion of metals (PBF-LB/M) is an additive manufacturing (AM) process for producing metal components. In current works, the simulation of the temperature gradient (TG) of PBF-LB/M is done by solving partial differential equations (PDEs) that describe the heat transfer between a laser beam and a powder bed. However, solving PDEs is time-consuming, especially when the number of mesh elements is large. Therefore, we have used machine learning to realize TG simulations, which is faster than solving PDEs. We have considered TG simulation as a problem of semantic segmentation and have studied five state-of-the-art machine learning models.

Combined Factors for Enhanced High-Temperature Strength of Al-Mn-Cr Heat-Resistant Alloy Fabricated Using Laser-Based Powder Bed Fusion

Combined Factors for Enhanced High-Temperature Strength of Al-Mn-Cr Heat-Resistant Alloy Fabricated Using Laser-Based Powder Bed Fusion

Design of additively manufacturable injection molds with conformal cooling

Additive manufacturing enables the production of intricate geometries including internal structures. This design freedom can be used advantageously to enhance heat transfer in injection molds by means of conformal cooling. The main goal is to reduce cycle times and to improve part quality through uniform cooling of the plastic products. This paper presents cooling design concepts for mold inserts. Their underlying approaches differ with respect to the shape and the cross-sectional geometries of cooling channels. Distinct inserts are additively manufactured by laser-based powder bed fusion (PBF-LB) of AISI 420 stainless steel. Experiments are carried out on a custom thermal test bench. Infrared thermography is used to examine the surface temperature, showing a reduction in cooling time by up to 41 % compared to conventional concepts. Additionally, the coolant flow is measured. The evaluation of the cooling characteristics reveal a critical trade-off between cycle time and uniformity of the surface temperature.

High-Temperature Behavior of the Heat-Treated and Overaged Alsi10mg Alloy Produced by Laser-Based Powder Bed Fusion and Comparison with Conventional Alsimg Casting Alloys

High-Temperature Behavior of the Heat-Treated and Overaged Alsi10mg Alloy Produced by Laser-Based Powder Bed Fusion and Comparison with Conventional Alsimg Casting Alloys

Flexible and highly dynamic beam shaping for Laser-Based Powder Bed Fusion of metals

In state-of-the-art laser-based powder bed fusion processes, a thin layer of metal powder is typically melted by means of a small, Gaussian-shaped laser spot, limiting the stability and productivity of the process. The application of alternative beam shapes is a current research topic to stabilize, accelerate and extend laser-based powder bed fusion of metals, e.g. by tailoring the microstructure. Static beam shapes and sizes limit the theoretically possible process and geometry freedom. To overcome those limitations, an approach for flexible and highly dynamic beam shaping is presented. The experimental setup is based on two perpendicularly oriented acousto-optic deflectors. A synchronized superposition of the ultrafast deflections (switching rates above 100 kHz) is used to generate sequentially compound quasi-static beam shapes. This quasi-static beam shaping is combined with a state-of-the-art beam deflection system and proved to be a viable solution for improved processing. Specimens with a relative density of more than 99.5 % could be manufactured by applying three selected beam shapes. The proof of concept demonstrates the potential of the setup for flexible, fundamental research on the influence of alternative beam profiles in laser-based powder bed fusion of metals.

Qualification of uniform fatigue damage tolerance law for additively manufactured and cast Al-Si alloys

The demand on lightweight structures and materials are steadily growing in automotive and aerospace industries. Hypo-eutectic Al-Si alloys are promising candidates due to its high specific material strength and corrosion resistance. Near net shape manufacturing by additive manufacturing (AM) or castings allow a high resource efficiency. The complex geometries lead to a wide range of structural features due to process-induced variations in local cooling rate. Therefore, the relationships between structural characteristics (e.g. dendrites, grain, eutectic, porosity, lack of fusion) and fatigue behavior have to be understood to enable a lightweight and reliable design of Al-Si alloys. In this study, the effect of process and process-induced structural characteristics on stress-strain behavior (quasi-static, cyclic) and high cycle fatigue behavior were characterized. The specimens were processed by laser-based powder bed fusion (PBF-LB) and sand casting (SC) with variation in porosity and dendrite arm spacing. Hereby, the age-hardenable Al-Si alloys AlSi10Mg (AM, SC) and AlSi7Mg (SC) were investigated. AM batches were tested in as-built condition. All casting batches got a HIP densification and T6 heat treatment. The fatigue results were analyzed based on nominal stresses according to Woehler (S-N curve) and local stress intensity based on linear-elastic fracture mechanics (stress intensity factor) as well as elastic-plastic fracture mechanics (J integral). J integral based Shiozawa diagrams allowed a sufficient and uniform fatigue damage tolerance (FDT) assessment for all AM and SC alloys. Hereby, the FDT of Al-Si alloys were dominated by defects (size, position) and cyclic stress-strain behavior (yield strength, cyclic hardening).

Grain Refinement of Stainless Steel by Strontium Oxide Heterogeneous Nucleation Site Particles During Laser-Based Powder Bed Fusion

Grain Refinement of Stainless Steel by Strontium Oxide Heterogeneous Nucleation Site Particles During Laser-Based Powder Bed Fusion

Influence of the effective thermal conductivity of the powder bed on the properties of overhanging areas in Laser-based Powder Bed Fusion of Ti-6Al-4V

In this paper the influence of a variation of the effective thermal conductivity of the powder bed on the properties and manufacturability of overhanging component areas in laser-based powder bed fusion (PBF-LB/M) of Ti-6Al-4V is investigated. The variation of the effective thermal conductivity of the powder bed is realized by contactless support structures under the over-hanging test specimens. The downskin angle between the overhanging test specimens and the base plate as well as the distance between the overhanging area and the test specimens are varied in a full factorial experimental design. The test specimens are examined with regard to the properties of relative density, surface roughness and geometric accuracy. In addition, a key figure is introduced which makes the results comparable with those obtained with other system configurations and parameters such as laser spot diameter, hatch distance and layer thickness.

In-situ integration of piezoelectric stacks with laser-based powder bed fusion

Powder bed fusion is a layer-wise manufacturing technique and emerging technology capable of manufacturing highly customized and complex shapes. Due to the layerwise process characteristic, powder bed fusion has high potential to enable access to each cross section during the manufacturing process. Thus, the integration of 3-dimensional objects can be performed at locations where access is not possible after part production anymore. Thermal shrinkage can be exploited to apply pre-stress on these components. Especially piezoelectric stacks need to be pre-stressed and can be used for a wide range of applications, which is the reason the integration of these types of sensors and actuators is investigated. Piezoelectric stacks need to be pre-stressed, which can be done in-situ during manufacturing eliminating additional pre-stress elements. This study contributes to the understanding of the pre-stress mechanism with in-situ measurements. Furthermore, the tolerances of prefabricated top caps needed for this manufacturing technique is investigated.

Targeted vibration excitation for increasing the powder deposition efficiency in electrophotographic powder application for Laser-based Powder Bed Fusion of polymers

High efficiency in terms of powder consumption and the ability to generate multi-material parts using additive manufacturing are key features of the electrophotographic powder application for laser-based powder bed fusion of polymers (PBF-LB/P). The reliable deposition of powder particles onto the build platform is one decisive process step. The aim is to achieve a high degree of coverage and, at the same time, maintain an accurate contour of the deposited powder layer.In addition to the use of a suitable electrical transfer field, which has already been investigated, targeted vibration excitation offers enormous potential for improvement. For this purpose, a mechanical simulation model is set up in this study, which is subsequently validated experimentally. By using the knowledge gained from the simulation, for example with regard to suitable vibration modes, the powder deposition has been significantly improved.

Additive manufacturing of open porous functional structures: roadmap from manufacturing to the application

Additive Manufacturing has proven to be a valuable complement to conventional means of manufacturing. As it is a data-driven process, its flexible customization of part shapes and manufacturing parameters plays a crucial role for this development. This study investigates the novel possibilities enabled by laser-based powder bed fusion for the development of functionality driven design. As an example, open porosities are manufactured, structures known to be advantageous to flow and heat transfer applications. Samples are investigated by laboratory tests and 3D imaging to gain knowledge about application-related properties. The results contribute to correlating manufacturing parameters to resulting properties and thus to design future applications.

Influence of flow aid additives on optical properties of polyamide for Laser-Based Powder Bed Fusion

Aim of this paper is to investigate the influence of flow aid additives on beam matter interaction of polyamide for laser-based powder bed fusion of polymers (PBF-LB/P). The optical energy input determines the laser beam powder interaction, quantified by absorption within the optical properties of a material. Additives like fumed silica for increasing powder flowability are important for using PBF-LB/P materials. The aid of those additives and restrictions in mechanical properties of built components using high portions of additives are already investigated, but not the change in relevant optical properties. Dry blends of polyamide powder with different weight percentage of additives, such as fumed silica, alumina and titania are investigated. A measurement system with a double-integrating-sphere setup and CO2 laser is used to characterize the beam matter interaction as the absorptance of optical energy in the powder layer. Based on this, the change in absorptance due to the influence of flow aid additives is investigated in order to be able to estimate possible effects on the processability.

Increasing the precision of Laser-Based Powder Bed Fusion by process combination with in situ laser ablation

Laser-based powder bed fusion of metals is limited in terms of achievable accuracy, surface quality, and minimal structure size due to its inherent melting process. The limits are primarily determined by the focal diameter of the laser beam and the grain size of the metal powder. Small structures like deep and narrow slits with a width below 100 µm, are still a major challenge especially for high aspect ratios. To overcome this limitation a quasi-simultaneous manufacturing process combining additive and subtractive laser processes with continuous wave and ultrafast lasers was developed. The additive manufactured components can be precisely machined by means of ultrafast laser ablation after each added layer, achieving detailed structures with feature sizes below 50 µm. In this work, the latest results of the production of narrow slits with a depth of several millimeters in components made of pure iron will be shown.

Design and Characterization of a Cobalt-Free Stainless Maraging Steel for Laser-Based Powder Bed Fusion

Design and Characterization of a Cobalt-Free Stainless Maraging Steel for Laser-Based Powder Bed Fusion