Additive manufacturing letters最新文献

This letter explores the advantages of additively manufactured substrates with spatially varying electromagnetic properties. These engineered substrates will be constructed using space filling curves (SFC) of various orders. New advanced manufacturing systems such as the nScrypt 3Dn-300, have enabled the rapid fabrication of these SFC substrates. This letter will apply the engineered SFC substrate to the design and fabrication of metasurface antennas. By utilising a SFC to vary the local substrate permittivity, along with the printed conductive patch dimensions, the range of achievable surface impedances can be greatly expanded. This enlarged design space will be leveraged to yield increased gain for a given metasurface antenna size. Methods to characterise the substrate permittivity and conductive ink are discussed along with a complete description of the metasurface antenna design, fabrication and validation process.

Laser directed energy deposition (LDED) is a promising way for creating hard surfaces like ceramic-reinforced metal matrix composites (MMC), but it faces a significant challenge in identifying crack formation during the process. As an emerging solution, acoustic signal monitoring is easy to be integrated within the process, and significantly reduces the time needed to detect micro-cracks in as-built MMC surfaces. This study reports on cracking sounds produced while employing LDED with SiC particles on a stainless steel 316 L substrate, examining the sound characteristics across time and frequency domains. Different sound sources in LDED are analyzed in the frequency domain, specifying the suitable frequency range for crack monitoring. Interestingly, the in-process micro-cracking on the hard surfaces produces a distinct audible ‘ping’ sound typically ranging between 12000 and 16000 Hz. By recording this sound, an efficient approach is proposed to identify crack generation during the rapid cooling in the LDED process of hard materials.

In recent years, the topic of beam shaping for improved laser material processing has rapidly grown influencing metal laser powder bed fusion (PBF-LB/M). Given the need to reduce the cost and improve control of the PBF-LB/M process to make it more competitive with traditional manufacturing methods, increasing productivity of PBF-LB/M is critical. When research reports a new beam profile (e.g. ring profile) to improve productivity on a specific material, it is often generalised, and assumed to have the capability to improve productivity in PBF-LB/M across the board. In this work, we use both low-fidelity simulations and experimental work to investigate the difference between Gaussian and ring/spot beam profiles on metals with very different thermal properties (a stainless steel and an aluminium alloy). We show that the two materials have opposite responses to the change in beam profile (both in terms of melt pool dimensions and thermal gradients); further, the most beneficial intensity distribution is dependent on the energy input to the material. This exemplifies yet another way in which the PBF-LB/M process is non-linear and contradicts the idea that a ring/spot laser profile is beneficial for all laser processing technologies. This highlights the need for further research into the non-linear effect of varying intensity distributions on laser processing before the benefits of dynamic beam shaping can be truly realised.

An original container was designed to measure the ferritic content of powder batches by the Eddy Current (EC) technique. As opposed to the standard X-Ray Diffraction (XRD) or Electronic BackScatter Diffraction (EBSD) methods, the EC measurements can be implemented on powder batches of significant sizes, say a hundred grams or so. Using a methodology based on the multiple recycling of an initially ferrite-free virgin powder, it was shown that the EC signals are sensitive to the ferritic content of the recovered powder. On the other hand, in the frequency range scanned by the sensor, the EC signals are virtually independent on the oxygen concentration within the tested powder.

Laser Powder Bed Fusion/Metal (PBF-LB/M) is a promising technology for industrial applications, but challenges such as long process times remain. Innovations such as the shell-core approach aim to address this by creating a dense shell around a minimally exposed powder core, significantly reducing processing times, with full densification and property adjustments achieved by subsequent hot isostatic pressing (HIP). This study focuses on the fabrication of shell-core samples using a powder mixture of austenitic steel and Si₃N₄ to produce high nitrogen steel PBF-LB/M components, which are otherwise difficult to produce due to the limited nitrogen solubility in the melt. PBF-LB/M induces Si₃N₄ decomposition, resulting in Si and N loss through laser-powder interaction. Si₃N₄ particles in the still powdered regions serve as a source of N enrichment during HIP, circumventing the limitations of nitrogen solubility in the melt and exploiting the higher solubility in the solid. After HIP, energy dispersive spectrometry and electron backscatter diffraction reveal a fully austenitic matrix with Si diffusion seams mainly in non-laser-exposed areas. The Si₃N₄ dissolution during HIP contributes to an interstitial dissolved N content of about 0.189 mass%, which, together with the higher Si content, increases hardness. Wavelength dispersive spectrometry (WDS) and nanoindentation line scans show decreasing Si and N concentrations from core to shell, resulting in reduced (nano)hardness in the shell. This innovative approach demonstrates the potential to produce AM components with enhanced properties by overcoming the limitations of nitrogen solubility in the steel melt during PBF-LB/M.

Additive Friction Stir Deposition (AFSD) is an emerging solid-state metal additive manufacturing (AM) process that offers several key benefits, including high deposition rates and wrought-equivalent mechanical properties even in the as-deposited condition. The work presented is the first study to report on the development of microstructure and mechanical properties of AFSD-processed duplex stainless steel (DSS2507). The banded microstructure of the starting material was remarkably affected by AFSD processing; the austenite grains exhibited a refined and equiaxed morphology, while the ferrite grains appeared slightly larger and elongated. Microstructural observations revealed that the potential mechanism of microstructure evolution in austenite was discontinuous dynamic recrystallization (DDRX), while in ferrite, it was continuous dynamic recrystallization (CDRX). The occurrence of multiple thermal cycles during the AFSD process resulted in σ phase precipitation, which in turn led to considerable variation in mechanical properties with respect to the build direction. The top region of the as-built part with an insignificant σ phase fraction showed improved tensile strength and ductility combination compared to the as-received DSS2507 as well as other AM-processed DSS2507 alloys.

Meltpool dimensions play a pivotal role in defining the defects and microstructure state of Laser Powder Bed Fusion (LPBF) builds. Therefore, it is crucial to investigate variations in meltpool geometries under different process conditions. In this work, we fabricated single tracks of LPBF Hastelloy X (HX) alloy under 36 printing conditions and examined the corresponding cross-section meltpool dimensions at two locations across the build platform. This investigation demonstrates the impacts of laser power, scan speed, powder layer thickness, and printing locations on resultant meltpool dimensions. As expected, we observed that meltpool dimensions increase as laser power increases or scan speed decreases. It was also concluded that thicker powder layers lead to wider and shallower meltpools due to reduced laser energy penetration into the solid beneath the powder layer. Additionally, the meltpool dimensions show variations dependent on deposition locations due to the different levels of interaction of the laser and its induced vapor plume, resulting in shallower and wider meltpools. These findings provide a systematic understanding of meltpool dimension variations across various process conditions for LPBF HX alloy, which ultimately offer insights into the formation of defects and microstructure features.

This study investigates the fatigue behaviour of samples made by laser powder-bed fusion of Hastelloy X (LPBF-HX) with as-built and machined surface conditions at 700 °C under fully reversed strain-controlled cyclic loading. Samples with both surface conditions exhibited initially cyclic hardening followed by cyclic softening under large strain amplitude testing, where a slight continuous hardening was observed for tests with smaller strain amplitudes. The samples with machined surfaces showed longer endurance and higher stress ranges than those with as-built surfaces. Post-fatigue-test EBSD analysis showed the formation of the Goss texture and extensive local strain accumulation in the samples tested under high strain amplitude at 700 °C. Fractography investigations revealed that early crack initiation in the samples with as-built surfaces was from stress concentrations induced by valleys on the rough surface. No evidence of crack initiation induced by pre-existing defects was observed in the machined samples, and the excessive slip activity at the surface was found to be responsible for the crack initiation.

The evolution of heat exchangers (HXs) manufactured by additive manufacturing techniques is significantly needed. The depowdering solution is a necessity, especially if flow channels are incorporated into the design. In this study, a one-piece HX with multiple layers of internal channels (printed by binder jetting additive manufacturing) was completely depowdered through a developed approach. Each HX channel has a semi-elliptical geometry, four perpendicular turnings along the approximately 200-mm length, and an approximately 80-mm center segment that is inaccessible due to the turnings. To depowder this component, two approaches including the compressed air and the vortex motion were tested first. It was found that the compressed air or vortex motion alone could partially depowder the internal unbound powder of the printed heat exchanger. Consequently, for complete depowdering, a combined approach of the vortex motion and compressed air blowing with multiple cycles was developed and tested. A study of the effect of the vortex duration in each depowdering cycle was conducted, and results showed that an increase from five minutes to ten minutes resulted in a reduced number of stages for a complete depowdering.

In this study, we investigated the microstructural variation of Ti-6Al-4 V in inverted pyramid parts built using Laser Powder Bed Fusion (LPBF). Two parts were fabricated with and without ghost parts to study the effects of interlayer delay time on thermal history and microstructure. Finite Element Method (FEM) based process simulation was used to predict the thermal history and cooling rates during the LPBF process to understand the location-specific microstructure and mechanical properties variation. The thermal analysis findings revealed that the variations in the cooling rates and pre-deposition temperature were notably significant. Within the same part, the cooling rates exhibited significant variations, differing by up to three orders of magnitude in two scenarios: (1) within the same layer, influenced by the proximity to the edges, and (2) at different heights, attributable to the strongly varying cross-section. Comparing the two parts, the cooling rates of the part with ghost parts were approximately two orders of magnitude higher than in the part without the ghost parts. This significant difference can be attributed to the extended interlayer cooling time and lower pre-deposition temperature resulting from the presence of two ghost parts which introduced an effective delay time between laser scans. Experimental validation against microstructure images and hardness measurements showed similar trends with the predicted results. These findings provide valuable insights into controlling microstructure at specific locations during LPBF fabrication, which is essential for building complex geometries with controlled material properties.