Revealing the microstructural evolution of electron beam powder bed fusion and hot isostatic pressing Ti-6Al-4V in-situ shelling samples using X-ray computed tomography

Electron beam powder bed fusion/hot isostatic pressing (E-PBF/HIP), also known as in-situ shelling, is an emerging technology that produces components by only forming their shells whilst retaining sintered powder at the core, and then using HIP to consolidate the entire structure. E-PBF/HIP can boost additive manufacturing productivity, however, the fundamental understanding of the process-microstructure-property correlations remains unclear. Here, we systematically investigate the microstructural evolution of E-PBF/HIP Ti-6Al-4V parts as a function of hatch melting parameters. All HIPped samples achieve full densification, however, their microstructures are significantly different from one another. Using X-ray computed tomography (XCT) and optical microscopy, our results show that the HIPped Ti-6Al-4V microstructure can be controlled by varying the porosity, P (%), pore surface areas and morphology in the as-built parts with a single set of HIP parameters. The HIPped microstructures still exhibit the as-built columnar grains when the as-built porosity, P 5 % with a highly dense pore network. This work suggests two main drivers for the grain morphology transitions during HIP: (1) a dramatic increase in pore volume increases the localised applied pressure up to 4 times at the core region of the sample and (2) minimise lack-of-fusion pores with high surface energies, promoting dynamic recrystallisation. This study provides a fundamental insight into the E-PBF/HIP technology, showing the feasibility to tailor microstructural properties of E-PBF built parts whilst boosting additive manufacturing productivity

Auxetic response of additive manufactured cubic chiral lattices at large plastic strains

Auxetic lattices exhibit a negative Poisson’s ratio and excellent energy absorption capability. Here, we investigate the compressive performance of auxetic cubic chiral structures. By utilising finite element analysis (FEA) verified by interrupted mechanical testing and x-ray computed tomography, the auxeticity and failure mechanisms at the large strain deformation have been evaluated. The FEA results show that the initial elastic–plastic response agrees with the prediction of the classic scaling laws of bending-dominated lattices. At increasing plastic deformation, the energy absorption and auxeticity are dependent on relative density, i.e., the slenderness ratio, of the constitutive struts. In the plastic regime, the auxeticity decreases with relative density. Ductile fracture precedes densification in relative densities above 1.2%, thus dictating a new scaling law for the variation of the maximum energy absorbed with density. The numerical model predicts the scaling of mechanical properties, fracture strains, and energy absorption of the constitutive unit cell and finite-sized specimens in the relative density ranging from 0.3% to 6.5%. However, to accurately model the failure mechanism, geometrical imperfections should be included. The scaling laws derived from this work may aid the design of next generation auxetic lattices with tailored mechanical properties

Keyhole fluctuation and pore formation mechanisms during laser powder bed fusion additive manufacturing

Keyhole porosity is a key concern in laser powder-bed fusion (LPBF), potentially impacting component fatigue life. However, some keyhole porosity formation mechanisms, e.g., keyhole fluctuation, collapse and bubble growth and shrinkage, remain unclear. Using synchrotron X-ray imaging we reveal keyhole and bubble behaviour, quantifying their formation dynamics. The findings support the hypotheses that: (i) keyhole porosity can initiate not only in unstable, but also in the transition keyhole regimes created by high laser power-velocity conditions, causing fast radial keyhole fluctuations (2.5–10 kHz); (ii) transition regime collapse tends to occur part way up the rear-wall; and (iii) immediately after keyhole collapse, bubbles undergo rapid growth due to pressure equilibration, then shrink due to metal-vapour condensation. Concurrent with condensation, hydrogen diffusion into the bubble slows the shrinkage and stabilises the bubble size. The keyhole fluctuation and bubble evolution mechanisms revealed here may guide the development of control systems for minimising porosity

In situ X-ray imaging of hot cracking and porosity during LPBF of Al-2139 with TiB2 additions and varied process parameters

Laser powder bed fusion (LPBF) additive manufacturing of 2XXX series Al alloys could be used for low volume specialist aerospace components, however, such alloys exhibit hot cracking susceptibility that can lead to component failure. In this study, we show two approaches to suppress the formation of hot cracks by controlling solidification behaviour using: (1) TiB2 additions; and (2) optimisation of LPBF process parameters. Using high-speed synchrotron X-ray radiography, we monitored LPBF of Al-2139 in situ, with and without TiB2 under a range of process conditions. In situ X-ray radiography results captured the crack growth over 1.0 ms at a rate of ca. 110 mm s−1, as well as pore evolution, wetting behaviour and build height. High-resolution synchrotron X-ray computed tomography (sCT) was used to measure the volume fraction of defects, e.g. hydrogen pores and microcracks, in the as-built LPBF samples. Our results show adding TiB2 in Al-2139 reduces the volume of cracks by up to 79 % under a volume energy density of 1000 to 5000 J mm−3, as well as reducing the average length, breadth, and surface area of cracks

Thermoelectric magnetohydrodynamic control of melt pool flow during laser directed energy deposition additive manufacturing

Melt flow is critical to build quality during additive manufacturing (AM). When an external magnetic field is applied, it causes forces that alter the flow through the thermoelectric magnetohydrodynamic (TEMHD) effect, potentially altering the final microstructure. However, the extent of TEMHD forces and their underlying mechanisms, remain unclear. We trace the flow of tungsten particles using in situ high-speed synchrotron X-ray radiography and ex situ tomography to reveal the structure of TEMHD-induced flow during directed energy deposition AM (DED-AM). When no magnetic field is imposed, Marangoni convection dominates the flow, leading to a relatively even particle distribution. With a magnetic field parallel to the scan direction, TEMHD flow is induced, circulating in the cross-sectional plane, causing particle segregation to the bottom and side of the pool. Further, a downward magnetic field causes horizontal circulation, segregating particles to the other side. Our results demonstrate that TEMHD can disrupt melt pool flow during DED-AM

In situ correlative observation of humping-induced cracking in directed energy deposition of nickel-based superalloys

Directed energy deposition (DED) is a promising additive manufacturing technique for repair; however, DED is prone to surface waviness (humping) in thin-walled sections, which increases residual stresses and crack susceptibility, and lowers fatigue performance. Currently, the crack formation mechanism in DED is not well understood due to a lack of operando monitoring methods with high spatiotemporal resolution. Here, we use inline coherent imaging (ICI) to optically monitor surface topology and detect cracking in situ, coupled with synchrotron X-ray imaging for observing sub-surface crack healing and growth. For the first time, ICI was aligned off-axis (24° relative to laser), enabling integration into a DED machine with no alterations to the laser delivery optics. We achieved accurate registration laterally (0.93), directly tracking surface roughness and waviness. We intentionally seed humping into thin-wall builds of nickel super-alloy CM247LC, locally inducing cracking in surface valleys. Crack openings as small as 7 µm were observed in situ using ICI, including sub-surface signal. By quantifying both humping and cracking, we demonstrate that ICI is a viable tool for in situ crack detection

3D Correlative Imaging of Lithium Ion Concentration in a Vertically Oriented Electrode Microstructure with a Density Gradient

The performance of Li+ ion batteries (LIBs) is hindered by steep Li+ ion concentration gradients in the electrodes. Although thick electrodes (≥300 µm) have the potential for reducing the proportion of inactive components inside LIBs and increasing battery energy density, the Li+ ion concentration gradient problem is exacerbated. Most understanding of Li+ ion diffusion in the electrodes is based on computational modeling because of the low atomic number (Z) of Li. There are few experimental methods to visualize Li+ ion concentration distribution of the electrode within a battery of typical configurations, for example, coin cells with stainless steel casing. Here, for the first time, an interrupted in situ correlative imaging technique is developed, combining novel, full-field X-ray Compton scattering imaging with X-ray computed tomography that allows 3D pixel-by-pixel mapping of both Li+ stoichiometry and electrode microstructure of a LiNi0.8 Mn0.1 Co0.1 O2 cathode to correlate the chemical and physical properties of the electrode inside a working coin cell battery. An electrode microstructure containing vertically oriented pore arrays and a density gradient is fabricated. It is shown how the designed electrode microstructure improves Li+ ion diffusivity, homogenizes Li+ ion concentration through the ultra-thick electrode (1 mm), and improves utilization of electrode active materials

Oxidation induced mechanisms during directed energy deposition additive manufactured titanium alloy builds

To prevent oxygen contamination, additive manufacturing (AM) techniques normally operate in an inert gas chamber (GC). An alternative method, useful for large builds and components repair, is the application of localised shielding gas (LSG). The effect of oxygen contamination on Ti6242 during directed energy deposition (DED) AM using an inert GC compared to LSG was investigated by in situ synchrotron x-ray experiments. When processing in LSG mode, the amount of oxygen absorbed from the atmosphere was sufficient to reverse the Marangoni flow leading to an alteration of the molten pool geometry and strongly influencing defect formation. Microstructural analysis reveals that, at high oxygen levels, the commonly developed α' martensitic microstructure was completely suppressed, forming precipitation of a tetra modal microstructure of α phase consisting of globular, primary and secondary lamellae (in colonies) and basketweave structure. These results help elucidate the influence of oxygen contamination in additively manufactured Ti alloys, potentially enabling improved industrial practices for AM of titanium alloy

X-ray imaging of powder consolidation during laser additive manufacturing

The laser-matter interaction and powder consolidation in laser additive manufacturing (LAM) occur on very short time scales (10-6 - 10-3 s); and they have proven difficult to characterise. A better understanding of the underlying mechanisms during LAM is crucial for prediction and optimisation of part properties. This thesis highlights the development and applications of a LAM process replicator (LAMPR), combined with in operando high-speed synchrotron X-ray imaging and image analysis to study these mechanisms. Using this setup, the sequential powder consolidation phenomena were revealed in LAM of stainless steel (SS316L), a Fe-Ni alloy (Invar36) and bioactive glass (13-93). The consolidation mechanisms of alloy powders are driven by molten pool wetting and vapour-driven powder entrainment. The principal consolidation mechanism of 13-93 bioactive glass is driven by viscous flow. The evolution of porosity and spatter were revealed during LAM of virgin and oxidised Invar 36 powders under different build conditions. The oxide films altered the Marangoni convection from centrifugal to centripetal, restricted the melt flow (and gas transport) in the melt track and promoted pore growth. Several new pore mechanisms were uncovered, including pore migration, dissolution, dispersion, and bursting. The laser-induced gas/vapour jet promoted the formation of melt tracks and denuded zones while ejecting spatter at velocities up to 1 m/s along the argon gas flow and laser scanning directions. In addition, a new spatter mechanism has been discovered; spatter can be formed by laser-driven gas expansion. Laser re-melting of large pre-existing pores can result in two extreme outcomes: (1) pore healing by Marangoni flow and (2) formation of droplet spatter and open pore. These results have clarified the physics behind previous hypotheses and proposed new mechanisms in LAM, which are critical for the development of LAM and simulation models of the process