In situ radiographic and ex situ tomographic analysis of pore interactions during multilayer builds in laser powder bed fusion

Porosity and high surface roughness can be detrimental to the mechanical performance of laser powder bed fusion (LPBF) additive manufactured components, potentially resulting in reduced component life. However, the link between powder layer thickness on pore formation and surface undulations in the LPBF parts remains unclear. In this paper, the influence of processing parameters on Ti-6Al-4 V additive manufactured thin-wall components are investigated for multilayer builds, using a custom-built process replicator and in situ high-speed synchrotron X-ray imaging. In addition to the formation of initial keyhole pores, the results reveal three pore phenomena in multilayer builds resulting from keyhole melting: (i) healing of the previous layers' pores via liquid filling during remelting; (ii) insufficient laser penetration depth to remelt and heal pores; and (iii) pores formed by keyholing which merge with existing pores, increasing the pore size. The results also show that the variation of powder layer thickness influences which pore formation mechanisms take place in multilayer builds. High-resolution microcomputed tomography images reveal that clusters of pores form at the ends of tracks, and variations in the layer thickness and melt flow cause irregular remelting and track height undulations. Extreme variations in height were found to lead to lack of fusion pores in the trough regions. It is hypothesised that the end of track pores were augmented by soluble gas which is partitioned into the melt pool and swept to track ends, supersaturating during end of track solidification and diffusing into pores increasing their size

Characterisation and correlation of areal surface texture with processing parameters and porosity of High Speed Sintered parts

High Speed Sintering is an advanced powder bed fusion polymer Additive Manufacturing technique aimed at economical production of end-use parts in series manufacture. Surface finish is thus of high importance to end users. This study investigates the surface topography of High Speed Sintered parts produced using a range of different energy-related process parameters including sinter speed, lamp power and ink grey level. Areal surface texture was measured using Focus Variation microscopy and the sample porosity was systematically examined by the X-ray Computed Tomography technique. Surface topography was further characterised by Scanning Electron Microscopy, following which the samples were subject to tensile testing. Results showed that areal surface texture is strongly correlated with porosity, which can be further linked with mechanical properties. Certain texture parameters i.e. arithmetic mean height Sa, root-mean-square Sq and maximum valley depth Sv were identified as good indicators that can be used to compare porosity and/or mechanical properties between different samples, as well as distinguish up-, down-skins and side surfaces. Sa, Sq and Sv for up- and down-skins were found to correlate with the above energy-related process parameters. It was also revealed that skewness Ssk and kurtosis Sku are related to sphere-like protrusions, sub-surface porosity and re-entrant features. Energy input is the fundamental reason that causes varying porosity levels and consequently different surface topographies and mechanical properties, with a 10.07 μm and a 30.21 % difference in Sa and porosity, respectively, between the ‘low’ and ‘high’ energy input

Effects of TiC content on microstructure and mechanical properties of nickel-based hastelloy X nanocomposites manufactured by selective laser melting

The nickel-based Hastelloy X (HX) superalloy is widely applied in the aerospace industry because of its exceptional oxidation resistance and various beneficial properties at high temperatures. HX-based nanocomposites manufactured by additive-manufacturing processes based on powder-bed fusion, such as selective laser melting (SLM), are expected to further enhance the material's mechanical and thermophysical performance. This paper systematically studies the effects of TiC nanoparticle content on the microstructure and tensile performance of SLM-fabricated HX nanocomposites. The results reveal that the microcracking that formed in pure HX was successfully eliminated in the fabricated nanocomposites when 1 wt% and 3 wt% TiC nanoparticles were introduced. The fabricated HX-3 wt.% (HX-3) TiC nanocomposite showed several TiC clusters and a much higher pore-volume percentage (0.15%) compared to the HX-1 wt.% (HX-1) TiC nanocomposite, in which this percentage was determined to be 0.026%. Compared to SLM-fabricated pure HX alloy, the HX-1 nanocomposite exhibited over 19% and 10% improvements in ultimate tensile strength and elongation to failure, respectively. A further increase in TiC content to 3 wt% was not found to further enhance the tensile strength but did result in a 10% loss in elongation to failure in HX-3 nanocomposite. These findings offer a promising pathway to employ SLM to manufacture both high-strength and high-ductility materials through the careful selection of nanoparticle materials and their content

A pragmatic continuum level model for the prediction of the onset of keyholing in laser powder bed fusion

Laser powder bed fusion (L-PBF) is a complex process involving a range of multi-scale and multi-physical phenomena. There has been much research involved in creating numerical models of this process using both high and low fidelity modelling approaches where various approximations are made. Generally, to model single lines within the process to predict melt pool geometry and mode, high fidelity computationally intensive models are used which, for industrial purposes, may not be suitable. The model proposed in this work uses a pragmatic continuum level methodology with an ablation limiting approach at the mesoscale coupled with measured thermophysical properties. This model is compared with single line experiments over a range of input parameters using a modulated yttrium fibre laser with varying power and line speeds for a fixed powder layer thickness. A good trend is found between the predicted and measured width and depth of the tracks for 316L stainless steel where the transition into keyhole mode welds was predicted within 13% of experiments. The work presented highlights that pragmatic reduced physics-based modelling can accurately capture weld geometry which could be applied to more practical based uses in the L-PBF process

Process-structure-properties relationships in laser powder bed fusion additive manufacturing

Laser powder bed fusion (L-PBF) Additive Manufacturing (AM) has exponentially grown in the last decades and is now being used for the production of both aerospace components and biomedical components, showing great potential to become a robust technology in the near future. Because of the plethora of process parameters and variables in L-PBF, the industry has focused on optimizing the required properties (e.g. part density, yield strength, UTS) by means of brute force (i.e. conducting expensive and time-consuming experimental campaigns). As a consequence, a deep theoretical understanding of process-structure-properties relationships of LPBF-AM is still missing. In this work we investigate how these process parameters (controllable) and variables (uncontrollable), along with the physical properties of the printed alloy, determine the complex physical phenomena that dominate LPBF. Among the many findings, we demonstrate the limited validity of a physical parameter (energy density) that was previously widely used by the AM community, finding its applicability to be limited to very narrow process windows. Thanks to in-situ high-speed imaging capabilities, we offer new insight into the complex interaction between gas-entrained powder particles and the liquid metal pool. We also conduct a dedicated analysis of the solidification behavior during L-PBF AM, finding that thermal gradient (G) and solidification rate (R) are strongly dependent on AM process parameters, thus offering practical advice to control the resulting microstructure. Finally, a Design of Experiments (DoE) study is carried out to elucidate the relationships between selected process parameters and the resulting stress distribution produced in cantilever beam-shaped samples. These results suggest that - depending on sample geometry - the amount of distortion may be more closely linked to the stress gradient developed across the beam’s thickness rather than to the average magnitude of the Von Mises residual stress

Process-structure-properties relationships in laser powder bed fusion additive manufacturing

Laser powder bed fusion (L-PBF) Additive Manufacturing (AM) has exponentially grown in the last decades and is now being used for the production of both aerospace components and biomedical components, showing great potential to become a robust technology in the near future. Because of the plethora of process parameters and variables in L-PBF, the industry has focused on optimizing the required properties (e.g. part density, yield strength, UTS) by means of brute force (i.e. conducting expensive and time-consuming experimental campaigns). As a consequence, a deep theoretical understanding of process-structure-properties relationships of LPBF-AM is still missing. In this work we investigate how these process parameters (controllable) and variables (uncontrollable), along with the physical properties of the printed alloy, determine the complex physical phenomena that dominate LPBF. Among the many findings, we demonstrate the limited validity of a physical parameter (energy density) that was previously widely used by the AM community, finding its applicability to be limited to very narrow process windows. Thanks to in-situ high-speed imaging capabilities, we offer new insight into the complex interaction between gas-entrained powder particles and the liquid metal pool. We also conduct a dedicated analysis of the solidification behavior during L-PBF AM, finding that thermal gradient (G) and solidification rate (R) are strongly dependent on AM process parameters, thus offering practical advice to control the resulting microstructure. Finally, a Design of Experiments (DoE) study is carried out to elucidate the relationships between selected process parameters and the resulting stress distribution produced in cantilever beam-shaped samples. These results suggest that - depending on sample geometry - the amount of distortion may be more closely linked to the stress gradient developed across the beam’s thickness rather than to the average magnitude of the Von Mises residual stress