Effect of helium as process gas on laser powder bed fusion of Ti-6Al-4V studied with operando diffraction and radiography

The utilisation of helium as process gas in laser powder bed fusion (LPBF) limits the generation of Ti-6Al-4V hot and incandescent spatters and enhances their cooling rate. In the present study, operando X-ray diffraction using synchrotron X-rays permits to verify that the cooling rates experienced by the deposited material are not significantly affected by the process gas unlike spatters. Topography measurements of the top printed surface reveal lower roughness of He-produced samples, attributed to the previously observed reduction of spatters with He and thus a reduction of redepositions on the powder bed and printed surfaces. Operando radiography provides with insights on the spatter formation mechanisms namely particle entrainment, agglomeration, melting and spheroidization

Effects of Additive Manufacturing Techniques on the Magnetocaloric Properties and Chemical Stability of LaFexCoySi13-x-y Alloys

Additive manufacturing (AM) is an emerging process to fabricate net shape, intricate, engineering components with minimal material waste; however, traditionally it has been largely applied to structural materials. AM of functional materials, such as magnetic materials, has received much less attention and the field is still in its infancy. To date, AM of magnetocaloric regenerators for magnetic refrigeration (an energy-efficient alternative to the conventional vapor-compression cooling technology), remains a challenge. There are several magnetic refrigerator device designs in existence today that are predicted to be highly energy-efficient, on condition that suitable working materials can be developed. This challenge in manufacturing magnetocaloric devices is unresolved, mainly due to issues related to shaping the mostly brittle magnetocaloric alloys into thin-walled channeled regenerator structures to facilitate efficient heat transfer between the solid refrigerant and the heat exchange fluid in an active magnetic regenerator (AMR) cooling device. To address this challenge, we explore the possibility of using extrusion-based additive manufacturing (AM) for 3D printing magnetocaloric structures in this work. Nominal compositions of LaFexCoySi13-x-y alloys were used for this investigation. The effects of extrusion printing on the composition were evaluated by microstructural, crystal structure, and magnetic characteristics probing. Chemical stability of precursor powders was assessed by simulating partial in-operando conditions of an Active Magnetic Regenerator (AMR) setup where heat transfer fluid (DI water) was circulated through the magnetocaloric structure with the aid of a circulating rig. 3D printed parts were immersed in a beaker setup with room temperature tap water (300ml) placed on a magnetic stirrer to simulate flow. Results were presented as comparisons of precursor powders and 3D printed scaffold in terms of composition as well as magnetic properties. X-Ray Diffractometry (XRD) data showed no changes in the composition of the 3D printed samples with similar amounts LaFeCoSi and α-Fe phases present in the structure. Immersed samples of precursor powders showed introduction of Fe3O4 oxide phases where higher compositions of oxide were seen for samples of longer immersion. Magnetometry data showed degradation of magnetocaloric response in polymer blended 3D printed structures with a ΔSmag decrease of 35% and lowered saturation magnetization (Ms). Water immersed precursor powders showed gradual degradation of ΔSmag­ for longer immersion times as well as lowered Ms with no changes in the curie temperature (Tc) among all the samples. Broadly speaking, this work demonstrated the printability of the magnetocaloric material into a functional regenerator type structure and the poor chemical stability of LaFexCoySi13-x-y alloys

Residual stresses and porosity in Ti-6Al-4V produced by laser powder bed fusion as a function of process atmosphere and component design

The influence of the process gas, laser scan speed, and sample thickness on the build-up of residual stresses and porosity in Ti-6Al-4V produced by laser powder bed fusion was studied. Pure argon and helium, as well as a mixture of those (30% helium), were employed to establish process atmospheres with a low residual oxygen content of 100 ppm O-2. The results highlight that the subsurface residual stresses measured by X-ray diffraction were significantly lower in the thin samples (220 MPa) than in the cuboid samples (645 MPa). This difference was attributed to the shorter laser vector length, resulting in heat accumulation and thus in-situ stress relief. The addition of helium to the process gas did not introduce additional subsurface residual stresses in the simple geometries, even for the increased scanning speed. Finally, larger deflection was found in the cantilever built under helium (after removal from the baseplate), than in those produced under argon and an argon-helium mixture. This result demonstrates that complex designs involving large scanned areas could be subjected to higher residual stress when manufactured under helium due to the gas\u27s high thermal conductivity, heat capacity, and thermal diffusivity

Tailored process gases for laser powder bed fusion

Metal laser powder bed fusion (L-PBF) allows for production of complex components using the energy from a laser to locally melt micron-sized powder following a layer-wise approach. Considerable scientific efforts are focused on addressing the influence of the process parameters on the melting stability and the control of material properties, while developing necessary monitoring and characterization tools. The importance of the process atmosphere has largely been undermined in favour of first order parameters connected to the laser scanning. The role of the atmosphere has been limited to the reduction of the operating residual oxygen level down to typically 1000 ppm. This thesis focuses on providing knowledge on the influence of the process atmosphere on the laser – metal powder interaction during L-PBF and the resulting properties of the built material in terms of generated defects, microstructure and mechanical properties. Different purities and compositions of generated atmospheres have been investigated to manufacture the most used materials in the field, namely 316L stainless steel, Alloy 718 and Ti-6Al-4V. The scope of process gases was extended from the traditionally employed argon to also include nitrogen, helium and mixtures of argon and helium. Purities from the typical 1000 ppm O2 threshold down to a few ppm were achieved using external monitoring of the atmosphere on both industrial- and laboratory-scale production machines.The investigated materials displayed different sensitivities to the atmosphere composition. 316L stainless steel had limited differences in terms of composition and strength when processed with high purity argon or nitrogen. Only processing with a built-in nitrogen generator, with which the process starts as soon as 10000 ppm residual O2 is reached, led to the increased oxidation of spatter particles and the appearance of large lack-of-fusion defects. A reduction in residual oxygen down to few ppm allowed to significantly hinder the development of thick Cr- and Al-rich particulate oxides on the surface of Alloy 718 spatter particles exposed to the L-PBF environment. In addition, Ti-6Al-4V had the highest sensitivity to the presence of impurities with significant oxygen and nitrogen pick-ups leading to embrittlement. This could be partially mitigated by limiting heat accumulation with longer interlayer time at the expense of productivity or by decreasing the oxygen level in the build chamber to below 100 ppm. Finally, helium was introduced as a new process gas that allowed to reduce the generation of spatter particles, favouring a stable melt pool, without significantly disrupting the residual stress state of the built part, which is critical for the productivity of L-PBF

In situ electrochemical grazing incidence small angle X-ray scattering: From the design of an electrochemical cell to an exemplary study of fuel cell catalyst degradation

Nowadays, electrochemistry has a considerable technological impact, involving fuel cells, super capacitors and batteries. These devices are based on complex architectures, which complicates monitoring their evolution in situ under operating conditions to reveal the reasons for reduced lifetime and performances. Here, we present a design of a multipurpose electrochemical cell for grazing incidence small and wide angle X-ray scattering (GISAXS and GIWAXS) where the environment for operating conditions can be recreated. We focus on proton exchange membrane fuel cells (PEMFCs) which operational conditions are simulated by means of potentiodynamic-based accelerated stress tests, applied to a thin film of Pt nanoparticles representing a model system of a benchmark catalyst. Two different upper potentials are used to mimic fuel cell operating conditions: at 1.0 V RHE the catalyst film preserves its initial morphology, while at 1.5 V RHE (simulating fuel cell start-up/shut-down cycles) significant coarsening has been observed. The initial dimension of the Pt particles of 4.0 nm increases to 8.7 nm due to the predominant process of coalescence and final Ostwald ripening. In parallel, the distance between the particles increases, the catalyst film (9 nm thick) becomes thinner at first and exhibit a higher roughness at the end