Key Engineering Materials Vol. 964

Abstract: The effect of part geometry on premature thin wall part failure in laser powder bed fusion (LPBF) is investigated using FEM simulation. Two FEM models are used to simulate the residual stress and buckling modes. Two experimental parts with different lengths are used for model validations. A LPBF FEM model evaluates the residual stress associated with the two experimental parts. A parametric buckling model is developed to determine the eigenvalues for 100 different part geometries including different part lengths (20-60 mm), widths (0.5-2 mm), and heights (10-50 mm). The results show that thin wall parts are more susceptible to buckling mode 1 when part length is small and to a combination of mode 1 and 3 when part length increases. In both cases the threshold stress for buckling is mostly sensitive to part thickness and height.

Abstract: This investigation estimated porosity and dislocation density in austenitic stainless steel 316LSi thin walls fabricated by Cold Metal Transfer Wire and Arc Additive Manufacturing (CMT-WAAM). Porosity density was calculated using ImageJ software. MAUD software (Materials Analysis Using Diffraction) was used to analyze the microstructural parameters and dislocation density. The density of pores and microstructural parameters of 316LSi alloy exhibit typical values of AM conditions. The porosity values oscillate between 2.80 to 3.48 %. The obtained dislocation density values are 5.0 e+12, 4.3 e+12, and 3.2 e+12 for 2.4 e+12 m^-2 for 70, 80, 90, and 140 A current source, respectively. In 316LSi thin walls, the increases in the current input in CMT-WAAM are accompanied by the very lowest decrease in the dislocation density state.

Abstract: This study aims to investigate the influence of isothermal annealing on the residual stresses and fatigue properties of a 316L austenitic stainless steel, manufactured by the laser powder-bed fusion (LPBF), possessing a high density of 99.98%. Residual stresses were evaluated using the X-ray diffraction techniques. High-cycle fatigue tests were performed on cylindrical samples manufactured in both horizontal and vertical orientations, subjected to force-controlled axial fully reversed loading. Following fabrication, the samples underwent isothermal annealing in a furnace either at 600 °C for 120 minutes or at 900 °C for 30 minutes. Subsequently, the samples were machined to their final dimensions and electropolished to a mirror surface finish. Preliminary findings revealed that increasing the annealing temperature effectively reduced the surface residual stresses. However, this reduction did not lead to an improvement in the fatigue resistance of this nearly fully dense material in the high-cycle fatigue regime. Interestingly, the structure annealed at 600 °C exhibited a higher fatigue strength compared to the structure annealed at 900 °C, with no discernible difference between the printing directions. Fracture surfaces and microstructural features examined using light and electron microscopy revealed that cracking was primarily initiated at surface defects or slip bands. These results highlight the complex interplay between residual stresses, microstructure, strength, and fatigue behaviour of LPBF 316L austenitic stainless steel. Further analysis and investigations are required to fully understand the underlying mechanisms and develop strategies for enhancing the fatigue performance of additive manufactured components.

Abstract: While additive manufacturing of metals has been rapidly growing industry for the past decade, the quality and the fatigue properties of the materials are still not very well known. In this study, we focus on the laser powder bed fusion (PBF-LB) manufactured Ti6Al4V. The as built material was compared to the heat treated counterpart by microstructural analysis, and the mechanical properties, impact toughness and the fatigue strength were determined. Bending fatigue testing was conducted for both as built and polished material to reveal the effect of surface roughness. The results showed that the heat treatment and the resulting microstructural change is crucial for the material properties and the material showed very brittle behaviour without it. According to the results, the surface quality plays also an important role in the fatigue life of the material, especially if no heat treatment is used.

Abstract: An overview of the additive/subtractive hybrid manufacturing (ASHM) research on three heat resisting materials – 18Ni-300 maraging steel, 316L stainless steel, and Inconel 718 (hereinafter 18Ni-300, 316L and IN718) – is provided to bridge key knowledge gaps and establish the respective process-microstructure-property relationships. The results examine validating the final surface roughness properties in the as-built and machined conditions in terms of the linear and areal parameters. Microscopic observations are also detailed to identify the influence of dry machining intermittent passes and/or laser conditions on microstructural features, as well as the bulk density. Mechanical stability assessment involved hardness measurement and tensile testing to evaluate the mechanical response of the materials built by in-envelope ASHM.

Abstract: Solid-state additive manufacturing may solve critical issues typically arising during fusion-based additive manufacturing of high-strength aluminium alloys. In this work, the recently introduced Friction Screw Extrusion Additive Manufacturing (FSEAM) process was employed to manufacture wall-like rectangular builds of AA6060 T6 deposited with deposition speeds from 300 mm/min to 500 mm/min. All builds were manufactured at a tool rotation rate of 400 rpm with 1 mm layer thickness. The volumetric supply rates were adjusted to maintain constant build width. Solid builds were formed without major defects over the full range of deposition speeds. The process generated sufficient normal force and heat at all deposition speeds which resulted in manufacturing of defect free builds. The resulting average grain size was consistently below 5 micrometer throughout all builds independent of deposition speed or location through the height. Microhardness measurements revealed a decrease in hardness from a feedstock value of 80 HV to around 50 HV in all manufactured builds. Tensile tests in the building direction showed consistent results for all the samples as a result of defect-free parts, demonstrating a tensile strength of approximately 150 MPa, yield strength of 100 MPa, and uniform elongation of 12-15%. The fracture surfaces revealed large amounts of dimples at all deposition speeds in line with the high degree of plastic deformation preceding fracture observed from the tensile tests. The obtained results indicated that FSEAM is a promising process for solid-state additive manufacturing of aluminium alloys.

Abstract: Lattice structures and Topology Optimization are two of the main routes to design lightweight high resistance components. These design techniques often lead to complex geometries not obtainable by traditional manufacturing. In this work we show how Additive Manufacturing (AM) of metals can be a successful way to reach that result. At first, we studied Ti6Al4V samples produced by Electron Beam Melting (EBM) to determine the mechanical properties of the base material. Hot Isostatic Pressing (HIP) was performed on a part of these samples to understand the impact of this process on defects and material properties. The results we obtained showed that the properties of Ti6Al4V produced by EBM are comparable to the one of the conventionally produced one. Given these results we redesigned an automobile’s lower control arm to reduce its mass: considering both Topology Optimization (TO) and lattice structures. Ti6Al4V components with different lattice structures were successfully manufactured by EBM.

Abstract: Additive manufacturing (AM) techniques are based on the process of joining materials, layer upon layer, differently from subtractive manufacturing methods. The design for AM allows for the creation of complex shapes as well as for the improvement of the performance of critical components in several fields, spanning from aerospace and automotive to biomedical applications. On the other hand, unlike man-made high load-bearing capacity devices, which are usually dense solids, nature uses mesoscopic or microscopic cellular structures as a fundamental support for the design. The increasing applications of AM in industrial production have led to product reimagination from a novel standpoint, enabling the fabrication of advanced lattice structures using polymer-based materials. Over the past few years, many efforts have been made to develop strategies for finding the design which is best suited to the requirements. In the current research, specific design scenarios were explored, the aim being to develop novel lattice structures for energy absorption, using an AM technique (i.e., fused deposition modelling) and a modified Acrylonitrile Styrene Acrylate (ASA)-based material. The fabricated structures were preliminarily analysed by means of compression tests.

Abstract: With the aim of developing a high-strength aluminum alloy for laser powder bed fusion (L-PBF), an Al–10Si–4.5Mg alloy with the a-Al/Si/Mg₂Si three-phase microstructure was investigated. The Al–10Si–4.5Mg alloy processed by L-PBF exhibited a fine cellular microstructure including fine granular Mg₂Si phases, and therefore exhibited a higher hardness of 187 HV0.1 than those of the conventional Al–Si–Mg alloy. However, cracks were macroscopically propagated between the internal fabrication voids along the melt pool boundaries in the L-PBF processed samples, resulting in a limited relative density below 95.5%. The cracking could be attributed to the relatively coarse Mg₂Si particles decorated with the eutectic network. Although the improved strength suggests the advantage of strengthening by the Mg₂Si phase, further optimization of the processing conditions will be required to manufacture the intact L-PBF parts.