Electron beam powder bed fusion (PBF-EB) is an additive manufacturing (AM) technology that enables the fabrication of metallic parts with arbitrary geometric complexity within a vacuum environment. Due to its ability to process materials at high temperatures (> 1000 °C), processing of crack and oxidation sensitive materials, as well as refractory alloys is possible. However, due to limited fundamental understanding of the intricate dynamics during powder consolidation and melt pool formation, the development of advanced processing strategies has mainly been limited to experimentally time-consuming parameter studies, as numerical models have mostly been unable to accurately predict processing conditions at the part or even layer scale. In this study, we perform high-speed in-situ X-ray imaging during multi-layer single track powder melting experiments on MiniMelt, a recently developed, custom-built PBF-EB machine for in-situ X-ray investigations. Our experiments reveal several key melt pool formation dynamics, some of which are being identified for the first time. They show how melt pool formation involves the coalescence of molten powder particles into larger droplets and how these droplets either fuse with the melt pool or solidify as balling particles. They also elucidate the origins of melt pool oscillations and spatter formation and demonstrate how the superposition of these mechanisms can lead to chaotic and escalating movement within the melt. We expect our results to improve and extend the phenomenological understanding of the powder consolidation mechanisms during PBF-EB and to aid in the development of new scanning strategies as well as the validation of numerical models.

In laser additive manufacturing (AM) of hot work tool steels, direct tempering (DT) of the tool from as-built (AB) condition without prior conventional austenitization and quenching results in enhanced tempering resistance. To date, intercellular retained austenite (RA) decomposition, leading to a shift in secondary hardening peak temperature, and finer martensite substructure are reported to be responsible for such a behavior. In this work, authors aimed at studying the strengthening contributions by performing isothermal tempering tests for long times (up to 40 hours) at elevated temperatures (up to 650 degrees C) on DT and quenched and tempered (QT) specimens. The thermal softening kinetics and the microstructural evolution were evaluated with the support of computational thermodynamics. The results suggest that the main contributor to enhanced temper resistance in DT condition is the larger fraction of thermally stable and extremely fine (similar to 20 nm) secondary (tempering) V(C,N) compared with QT. This could be explained by the reduction of available V and C in austenitized and quenched martensite for a later secondary V(C,N) precipitation during tempering, because of equilibrium precipitation of relatively large (up to 500 nm) vanadium-rich carbonitrides during the austenitization process. A complementary effect of the substructure refinement (i.e., martensite block width) in rapidly solidified highly supersaturated martensite was also quantified in terms of Hall-Petch strengthening mechanism. The significant effect of secondary V(C,N) was successfully validated by assessing a laser AM processed vanadium-free hot work tool steel in QT and DT condition, where no significant differences in strength and temper resistance between the two conditions were evident. [GRAPHICS] .

The formation of α-AlFeSi sludge in AlSi10Mg has been studied by computational thermodynamics based on the CALPHAD method. Both the amount of sludge and the sludge onset temperature T_Sludge have been investigated. We find that the amount of sludge increases linearly with the empirical sludge factor for the studied composition interval, which agrees well with published experimental data. We also find a notable difference between the total amount of α-AlFeSi phase and the amount of pre-eutectic sludge. The T_Sludge follows a similar linear relationship but only when there is no Cr present in the material. However, we propose a modified sludge factor like expression for the temperature onset with a significantly improved predictability compared to previous empirical expressions. With Cr present, a α-AlMSi phase, rich in Cr but poor in Mn, forms at a significantly higher temperature which leads to higher amounts of sludge suggesting Cr to be more detrimental for sludge formation than previously thought. Finally, we also suggest ranges of validity for the linearized sludge factor expression outside which full thermodynamic calculations accounting for all multicomponent effects must be used.

Electron beam-powder bed fusion (PBF-EB) is an additive manufacturing process that utilizes an electron beam as the heat source to enable material fusion. However, the use of a charge-carrying heat source can sometimes result in sudden powder explosions, usually referred to as "Smoke", which can lead to process instability or termination. This experimental study investigated the initiation and propagation of Smoke using in situ high-speed synchrotron radiography. The results reveal two key mechanisms for Smoke evolution. In the first step, the beam-powder bed interaction creates electrically isolated particles in the atmosphere. Subsequently, these isolated particles get charged either by direct irradiation by the beam or indirectly by back-scattered electrons. These particles are accelerated by electric repulsion, and new particles in the atmosphere are produced when they impinge on the powder bed. This is the onset of the avalanche process known as Smoke. Based on this understanding, the dependence of Smoke on process parameters such as beam returning time, beam diameter, etc., can be rationalized.

As a potent grain refiner for steel casting, TiN is now widely used to refine γ-austenite in steel additive manufacturing (AM). However, the refining mechanism of TiN during AM remains unclear despite intensive research in recent years. This work aims to boost our understanding on the mechanism of TiN in refining the γ-austenite in AM-fabricated 316 stainless steel and its corresponding effect on the mechanical behaviour. Experimental results show that addition of 1 wt.% TiN nanoparticles led to complete columnar-to-equiaxed transition and significant refinement of the austenite grains to ∼2 µm in the 316 steel. Thermodynamic and kinetic simulations confirmed that, despite the rapid AM solidification, δ-ferrite is the primary solid phase during AM of the 316 steel and γ-austenite forms through subsequent peritectic reaction or direct transformation from the δ-ferrite. This implies that the TiN nanoparticles actually refined the δ-ferrite through promoting its heterogenous nucleation, which in turn refined the γ-austenite. This assumption is verified by the high grain refining efficiency of TiN nanoparticles in an AM-fabricated Fe-4 wt.%Si δ-ferrite alloy, in which δ-ferrite forms directly from the melt and is retained at room temperature. The grain refinement is attributed to the good atomic matching between δ-ferrite and TiN. Grain refinement in the 316 steel through 1 wt.% TiN inoculation not only eliminated the property anisotropy but also led to a high strain-hardening rate upon plastic deformation and thereby a superior strength-ductility synergy with yield strength of 561 MPa, tensile strength of 860 MPa and elongation of 48%.

A high-energy white synchrotron X-ray beam enables penetration of relatively thick and highly absorbing samples. At the P61A White Beam Engineering Materials Science Beamline, operated by Helmholtz-Zentrum Hereon at the PETRA III ring of the Deutsches Elektronen-Synchrotron (DESY), a tailored X-ray radiography system has been developed to perform in-situ X-ray imaging experiments at high temporal resolution, taking advantage of the unprecedented X-ray beam flux delivered by ten successive damping wigglers. The imaging system is equipped with an ultrahigh-speed camera (Phantom v2640) enabling acquisition rates up to 25 kHz at maximal resolution and binned mode. The camera is coupled with optical magnification (5x, 10x) and focusing lenses to enable imaging with a pixel size of 1,35 micrometre. The scintillator screens are housed in a special nitrogen gas cooling environment to withstand the heat load induced by the beam, allowing spatial resolution to be optimized down to few micrometres. We present the current state of the system development, implementation and first results of in situ investigations, especially of the electron beam powder bed fusion (PBF-EB) process, where the details of the mechanism of crack and pore formation during processing of different powder materials, e.g. steels and Ni-based alloys, is not yet known.

An electron beam powder bed fusion (EPBF) printability study of a medium-C hot-work tool steel with focus on part density and surface roughness was performed using three different powder particle size distributions (PSDs) of 45–105 μm (typical for EPBF), 20–60 μm (typical for laser beam powder bed fusion), and a 50–50 wt.% mixture of these two powders. First, acceptable process parameter windows were generated based on as-printed density for each PSD. Full density parts (at least 99.5% dense according to NIST) were produced using the 20–60 μm PSD and the mix PSD. Fifteen different contouring strategies were also tested for potential improvement of the as-printed side surface roughnesses, which ranged from 23.3 to 25.7 μm among the three PSDs. Side surface roughness as low as 13.8 μm was attained by using contouring strategies employing two contouring lines, which were typically observed to be more effective than one-line strategies. Overall, the 20–60 μm PSD was determined to convey a better as-printed build quality over a wider range of parameters without sacrificing process productivity.

The development of a sample environment for in situ x-ray characterization during metal Electron Beam Powder Bed Fusion (PBF-EB), called MiniMelt, is presented. The design considerations, the features of the equipment, and its implementation at the synchrotron facility PETRA III at Deutsches Elektronen-Synchrotron, Hamburg, Germany, are described. The equipment is based on the commercially available Freemelt ONE PBF-EB system but has been customized with a unique process chamber to enable real-time synchrotron measurements during the additive manufacturing process. Furthermore, a new unconfined powder bed design to replicate the conditions of the full-scale PBF-EB process is introduced. The first radiography (15 kHz) and diffraction (1 kHz) measurements of PBF-EB with a hot-work tool steel and a Ni-base superalloy, as well as bulk metal melting with the CMSX-4 alloy, using the sample environment are presented. MiniMelt enables time-resolved investigations of the dynamic phenomena taking place during multi-layer PBF-EB, facilitating process understanding and development of advanced process strategies and materials for PBF-EB.<br />

During fusion-based metal additive manufacturing, there is an inherent directionality in heat transfer, which leads to columnar grain growth. This may result in cracking and anisotropic mechanical properties in many alloy systems. Therefore, it is important to study the conditions under which columnar-to-equiaxed transition in grain structure occurs. The grain morphology is determined by several factors such as process conditions, local alloy composition, and number density of nucleating sites. In the present work, a model for simulating columnar-to-equiaxed transition is formulated, considering nucleating site size distribution, rapid solidification and constitutional undercooling in multicomponent alloys. Furthermore, the model is coupled with multicomponent Calphad-based thermodynamic and diffusion mobility descriptions. It is demonstrated that including the above aspects is important in accurately predicting the columnar-to-equiaxed transition by comparing with experimental data for an additively manufactured TiB2-reinforced AlSi10Mg alloy. The framework developed in this work may be used to predict columnar-to-equiaxed transition in additively manufactured technical alloys consisting of multiple elements.