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In laser powder bed fusion, unconsolidated metal powder on the build plate tends to comprise a proportion of oxidised powder after repeated use. This is generally caused by the generation of oxidised spatter particles during processing which can contaminate the powder bed, and be incorporated into future builds, ultimately undermin- ing part integrity. Oxidised metal powder often results in porosity, poor layer-layer bonding and detrimental oxides in the printed part. This work uses a new chemical etching approach to remove oxides from the surface of oxidised stainless steel spatter powder. It was shown that a ten-fold reduction in oxide area coverage on spatter powder was possible through submersion in a solution of heated Ralph’s etchant for one hour. Oxide removal is thought to occur mostly via dissolution of the metal surrounding and underneath oxide islands on the powder, allowing more aggressive oxide removal. LPBF processing was performed using spatter, etched and virgin powder sieved to an identical powder size range. Etched spatter showed a reduced oxide slag layer on track surfaces com- pared to spatter. In addition, incorporation of powder into tracks appeared improved after chemical etching of the powder. This work demonstrates that chemical etching has the potential to be used to increase the re-usability and lifetime of spatter or heavily used powder from widely used and corrosion resistant stainless steel powder., Additive Manufacturing Letters, 3, ISSN:2772-3690

A common challenge in direct energy deposition (DED) is eliminating the anisotropy in mechanical performance associated with microstructure and the formation of coarse columnar grains. In this work, a heterogeneous nucleation mechanism was introduced into the melt pool, and, from this mechanism, an almost fully equiaxed grain morphology was obtained in the DED of Ti-6Al-3Mo. Three types of grain morphologies in DED Ti-6Al-3Mo, including full columnar grains, near-equiaxed grains and almost fully equiaxed grains were obtained from premixed and satellite powder blends from Ti, 6 wt.% Al and 3 wt.% Mo, respectively. Combined with the analysis of the interactions between powder particles and the melt pool in DED, the formation mechanism of the equiaxed grains caused by the incomplete melting of high melting point Mo particles was revealed. As the prior-β grains transformed from coarse columnar grains to fine-equiaxed grains, the strong fiber texture along the deposition direction was weakened, while the size of the α-laths in the prior-β grains slightly decreased, and the selection of α-variants was weakened. Due to the transformation of the prior-β grains from coarse columnar grains to fine-equiaxed grains, the tensile strength of the deposited samples increased from 982 MPa to 1082 MPa, while the yield strength increased from 840 MPa to 922 MPa, and the elongation of the as-deposited alloy also increased from 9.0 % to 9.8 %, which confirmed that the presence of fine-equiaxed grains is beneficial to the strength and plasticity of the DED alloy. This work further demonstrates the role that satelliting powders can play in terms of enhancing the columnar to equiaxed transition (CET) behavior associated with DED.

Icing hazards often pose operational and safety challenges. Various surface and coating approaches have been attempted for ice mitigation purposes, but the durability remains an outstanding issue. Inspired by traditional powder metallurgy and slippery icephobic surfaces, a new strategy for slippery liquid-impregnated porous metallic structure (LIPMS) with gradient porosity was proposed in this study for ice mitigation, by directly impregnating selected liquids into sintered porous copper components with hydrophobic/oleophilic guarding consideration. The results indicated that robust LIPMS with desired porosity were obtained, significantly delaying the icing of surface water droplets, providing good frost resistance in a high humid environment, and demonstrating ultra-low ice adhesion strength (less than 1 kPa). It was confirmed that the hydrophobic/oleophilic guarding design significantly improved icephobic durability, attributing to its role in repelling external water and maintaining internal slippery liquid. To offer a comprehensive understanding of icephobic mechanisms and the potentials of LIPMSs, a concept of ice initiation position was also proposed and theoretically discussed. The pivotal factor of icephobic LIPMS is to maintain the icing initiation position at the unfrozen liquid–liquid interface or inside the homogeneous liquid, thus inhibiting icing and facilitating the ice removal.

The concept of employing air volumes trapped inside polymer shells to make a lens for ultrasound focusing in water is investigated. The proposed lenses use evenly-spaced concentric rings, each having an air-filled polymer shell construction, defining concentric water-filled channels. Numerical simulations and experiments have shown that a plane wave can be focused, and that the amplification can be boosted by Fabry-Pérot resonances within the water channels with an appropriate choice of the lens thickness. The effect of the polymer shell thickness and the depth of the channels is discussed, as these factors can affect the geometry and hence the frequency of operation. The result was a lens with a Full Width at Half Maximum value of 0.65 of a wavelength at the focus. Results obtained on a metal-based counterpart are also shown for comparison. An advantage of this polymeric design is that it is easily constructed via additive manufacturing. This study shows that trapped-air lenses made of polymer are suitable for ultrasound focusing in water near 500 kHz. [Abstract copyright: Copyright © 2022. Published by Elsevier B.V.]

The geometrical design freedoms associated with additive manufacturing techniques are currently well exploited and finding commercial application. The capability of layer-based processes to allow modification of composition and microstructure in process to achieve functional grading is currently a growing topic. In this work, a method is demonstrated for varying layer thickness within single components that allows part sections to be interlaced for the purpose of locally manipulating material and structural properties. Demonstrator geometries are explored here which exhibit the interfaces within specimens constituted of both 30 µm and 150 µm. Accordingly, a new design freedom for laser powder bed fusion is created.

Curved toolpaths challenge accurate electrochemical jet processing due to resulting distortion of the cut profile. A more elaborate path planning approach is required to mitigate these negative effects. A 3-dimensional process model is presented here, to predict the resulting surface geometry. This is applied to simulate machining of curved channels and calculate optimised feed rates for locations of the toolpath with impaired geometry. Through the presented predictive planning methodology, achieved depths of curved sections of the machined channels were within 10.5% of the depth for straight sections, compared to up to 62.2% increase when feed rate compensation was not applied.

Funding for this work was provided through the UK Engineering and Physical Sciences Research Council (EPSRC), Grant Numbers EP/N034163/1, EP/N034201/1 and EP/N034813/1. Acoustic metamaterials constructed from conventional base materials can exhibit exotic phenomena not commonly found in nature, achieved by combining geometrical and resonance effects. However, the use of polymer-based metamaterials that could operate in water is difficult, due to the low acoustic impedance mismatch between water and polymers. Here we introduce the concept of “trapped air” metamaterial, fabricated via vat photopolymerization, which makes ultrasonic sub-wavelength imaging in water using polymeric metamaterials highly effective. This concept is demonstrated for a holey-structured acoustic metamaterial in water at 200–300 kHz, via both finite element modelling and experimental measurements, but it can be extended to other types of metamaterials. The new approach, which outperforms the usual designs of these structures, indicates a way forward for exploiting additive-manufacturing for realising polymer-based acoustic metamaterials in water at ultrasonic frequencies. Publisher PDF

Additively manufactured (AM) components possess microstructures different from conventional forms in the same material and as a consequence, the mechanical properties and machinability of components produced using these technologies differ. Also, the microstructures from these processes are often anisotropic and as such different regions possess varying properties and hence varying response during post process machining. An adaptive control (AC) machining system for addressing this variation in microstructures during post process machining is developed here and its use demonstrated in this paper. The adaptive control system is used in milling and drilling trials for machining components manufactured by Directed Energy Deposition (DED). Surface roughness and residual stress analysis in the ‘as built’ state and after post process machining with the prevailing cutting forces are also presented here. Microstructures of the deposited material and the subsurface effects at the machined region due to the action of the cutting tool is also analysed. Milling trials are investigated from the standpoint of the use of adaptive machining for the removal of the outermost layer of DED produced Ti6Al4V and in the machining of bulk regions after preliminary machining of the undulating layer. Drilling experiments are carried out for investigating the effects of the machining process on the resulting surface condition, comparing the process with and without adaptive control. The effects of variation in cutting forces during drilling through the different material systems comprising of the DED region and the wrought substrate is also discussed. The effect of periodic changes in cutting forces during machining due to the variation in microstructure along the build direction is analysed and discussed. The use of adaptive control machining for post processing is shown to improve surface finish as it reduces the initiation and generation of chatter marks on the machined surface resulting from material anisotropicity.

Self-lubricating aluminium-based composites reinforced with solid lubricants promise to meet the demand for lightweight materials in green tribological applications. The design advantages granted by additive manufacturing (AM) processes coupled with their capacity for in-situ production of composite materials are yet to be exploited in the realm of Al-transition metal dichalcogenides composites. In this work, laser powder bed fusion (LPBF) was deployed for the in-situ fabrication of Al-WS2 composites for the first time, elucidating the process-structure–property relationships in comparison to reference spark plasma sintering (SPS) samples. The WS2 response to the respective fabrication technique was also firstly investigated through a holistic characterisation. The formation of new phases (W for LPBF, Al5W and Al12W for SPS) provided the potential for microstructural tailoring for optimal tribological performance. For tribological properties, LPBF Al-WS2 exhibited a coefficient of friction (COF) 0.55 ± 0.01 and specific wear rate 3.4 ± 0.3 × 10−3 mm3/N∙m, slightly better than the SPS counterpart (COF 0.57 ± 0.02, specific wear rate 3.6 ± 0.3 × 10−3 mm3/N∙m). Furthermore, a novel methodology for studying the evolution of worn surfaces is proposed and validated, by which a tribo-layer formed at lower friction cycles was observed for the LPBF samples, meaning that AM will also be advantageous for the performance aspect of self-lubricating materials.

The combination of ultrasound and microbubbles allows treatment of indications that would be impossible or too risk adverse with conventional surgery. During treatment, subharmonic and ultraharmonic components that can only be generated from microbubbles are of great interest for intraoperative monitoring. However, the microbubble emissions are several orders of magnitude lower in power compared to that of the fundamental frequency component from the ultrasound applicator, resulting in a low signal‐to‐noise ratio (SNR) for monitoring. A 3D acoustic metamaterial (AMM) immersed in water is proposed for suppressing unwanted ultrasound waves, which allows the improved sensitivity for detecting weak microbubble emissions. Numerically, the importance of shear waves on the AMM transfer properties is highlighted, though only longitudinal ultrasound waves are transmitted through water. Experimentally, the design is implemented in titanium using additive manufacturing, with an attenuation level of 40 dB at the fundamental frequency. Consequently, the application of the AMM efficiently improves the SNR for subharmonic and ultraharmonic microbubble emissions by 11.8 and 11.9 dB, respectively. The subharmonic components originally overwhelmed by noise are recovered. This is the first time that AMMs have been applied to passive acoustic monitoring and this work stands to improve treatment outcomes from cavitation‐mediated focused ultrasound therapy.

The integrity of additively manufactured components is limited by the number, size, type and location of defects encapsulated in the build. Our ability to manufacture fatigue resistant components by the powder bed fusion process is still nascent as a result. The location of defects within a build volume is known to be of significance but efforts are yet to achieve superior manufacturing strategies resulting in tolerable fatigue performance. In this work the role of laser scan strategies is investigated in determining fatigue performance of printed components. Fractography and X-ray computed tomography data are presented to support this.

Laser Powder Bed fusion is capable of rapid production of parts, from conception, compared with traditional manufacturing methods. This said, the time taken to fabricate a single part can still be significant – typically many hours. Processing thicker layers, and hence fewer total layers, in the Laser Powder Bed Fusion process, is an effective way to reduce build times. However, mechanical performance can suffer as a result of this strategy. This study proposes and demonstrates a method to enable the interlacing of multiple layer thicknesses within one part, allowing for finer layers within regions where they are specifically required, whilst maintaining overall component integrity for specific load cases. Thicker layers are used within regions with lower property requirements in order to optimise an overall part for improved production rate. The design of interfaces between two disparate layer thickness regions could also be tailored for control of material properties and such will be investigated in an independent study. Ti6Al4V LPBF samples are fabricated, characterised by way of tensile testing, porosity analysis and microstructural analysis. The study demonstrate parts can be additively built using multiple layer thickness regions with consistent ultimate tensile strength (1110 - 1135 MPa) and varying penalties to ductility, depending on layer thickness and interface design (elongation to failure reductions up to 40% in the most extreme case).

Remarkable interfacial behaviors are observed in nature. Our efforts, directed toward replicating the structures, chemistries, and therefore functional properties of natural nonwetting surfaces, are competing with the result of billions of years of natural selection. The application of man-made surfaces is challenged by their poor longevity in aggressive environmental or applied service conditions. This study reports on a new approach for the creation of multiscale hierarchical surface patterns in metals, which exploits thermodynamic phenomena in advanced manufacturing processes. While hydrophobic coatings can be produced with relative ease by electrodeposition, these fractal-type structures tend to have poor structural integrity and hence are not durable. In this method, "seed surfaces" are directly written onto substrates by selective electrodeposition, after which they are irradiated by a large-area, pulsed electron beam to invoke a beading phenomenon, which is studied here. The length scale of these beads is shown to depend upon the melt time of the liquid metal. The created surfaces are shown to yield high water contact angles (145°) without subsequent chemical modification, and high adhesion properties reminiscent of the "rose petal" hydrophobic effect. The size and morphology and hence the hydrophobic effect of the surface beads generated are correlated with the thickness of the electrodeposited coating and hence the melt lifetime upon electron irradiation. This new rapid approach for tunable hydrophobic surface creation has applications for developing precision hydrophobic patterns and is insensitive to surface complexity.

In this study, we describe a new methodology to create complex coatings with functionally graded compositions, using electrochemical jet processing (EJP). Cu/Zn alloy coatings were electrodeposited, in a layer-by-layer manner, onto steel substrates through resin masks created with a desktop 3D printer. By adapting the electrolyte composition through the jet by drawing from multiple reservoirs, the ratio of cations (Cu2+:Zn2+) in the electrolyte supplied directly to the substrate surface at any point in the processing cycle can be manipulated. This allows for control over electrodeposit composition ratios through a given coating, while the mask allows for microfabrication at higher aspect ratios than achievable by electrochemical jet processing alone. After processing, masks were removed from the substrates and samples were subsequently characterised to verify changes in composition and deposition properties.

Material development for powder bed fusion is critical to enhance the utility of the process. Establishing process parameters for new materials limits the rate at which process performance can be appraised. Adapted laser calorimetry is a useful technique for rapid material screening. Here, multi-body feedstocks which have the potential to improve laser coupling and mechanical properties of structures (Al + multi-walled carbon nanotubes) are investigated to demonstrate the flexibility of laser calorimetry for understanding consolidation and heat transfer phenomena in powder bed fusion. It is shown that the modifications to powder surface condition/chemistry improve consolidation behaviour in this case.

In prior work, the authors demonstrated a method for the creation of composite coatings containing TiC particles in a pure Al matrix by cold spray via a satelliting technique. Here, the authors investigate the effects of using the ‘Satelliting’ technique on the deposition and characteristics of A6061-TiC coatings fabricated by cold-spray. The results showed clear improvements in the deposition efficiency and porosity of the satellited powder obtained by using the satellited feedstock. Annealing heat-treatment was then applied for further enhancement of the coating's density and wear properties. The heat-treated coating exhibited a ~70% reduction in interlamellar porosity. Dry sliding wear testing using a steel counter body at 5 N showed that using a satellited feedstock with TiC yielded a 55% and 20% reduction in coating specific wear rate in comparison to coatings made using pure 6061 and blended 6061-TiC, respectively. This was further reduced by ~70% and 40% in comparison to the 6061 and blended 6061-TiC coating after heat-treatment with good repeatability.

Improvement to the integrity of laser powder bed fusion (PBF-LB) processed Nickel-base superalloys is crucial for their wider adoption in industrial aerospace applications. The nature of PBF-LB, essentially an extended powder micro-welding process, allows opportunity for material contamination and oxidisation. This study reports how oxide formation occurs in Hastelloy X corresponding to ASTM composition, and in particular, in the material ejected from the process zone during fabrication (spatter). It is shown that oxidation occurs despite processing in an Ar atmosphere with

Thin-wall structures of Ti-6Al-4V were fabricated by low-power pulsed laser directed energy deposition. During deposition, consistent with prior reports, columnar grains were observed which grew from the bottom toward the top of melt pool tail. This resulted in a microstructure mainly composed of long and thin prior epitaxial β columnar grains (average width ≈200 μm). A periodic pattern in epitaxial growth of grains was observed, which was shown to depend upon laser traverse direction. Utilizing this, a novel means was proposed to determine accurately the fusion boundary of each deposited layer by inspection of the periodic wave patterns. As a result it was applied to investigate the influence of thermal cycling on microstructure evolution. Results showed that acicular martensite, α' phase, and a small amount of Widmanstatten, α laths, gradually converted to elongated acicular α and a large fraction of Widmanstatten α laths under layer-wise thermal cycling. Tensile tests showed that the yield strength, ultimate tensile strength and elongation of Ti-6Al-4V thin wall in the build direction were 9.1%, 17.3% and 42% higher respectively than those typically observed in forged solids of the same alloy. It also showed the yield strength and ultimate tensile strength of the transverse tensile samples both were ˜13.3% higher than those from the build direction due to the strengthening effect of a large number of vertical β grain boundaries, but the elongation was 69.7% lower than that of the build direction due to the uneven grain deformation of β grains.

Environmental legislation continues to drive optimisation of internal combustion engines in the automotive sector in an effort to reduce harmful emissions and promote fuel efficiency. Significant frictional losses occur at the ring pack, prompting research on reducing sliding friction at the ring – liner interface through the use of surface texturing. This study assesses the influence of contact area on the friction and wear behaviour of light-weight monolithic hyper-eutectic Al-Si alloy with a textured surface generated by electrochemical jet texturing. Flat rectangular samples were prepared from a cast cylinder liner via electro-discharge machining and flat lapped to 6 µm diamond finish prior to immersion in a 1 mol solution of NaOH for 120 s to simulate the mechanical honing process. Surfaces were electrochemically jet textured using a 2.3 mol solution of NaCl and current density of 220 A/cm2. An offset array of 1.3 × 0.3 mm rectangular features at 1.2 mm spacing was created with an average depth of 39 µm. Lubricated reciprocating sliding was carried out at a stroke length of 25 mm in a bath of PAO (4 cSt) at 100 °C against a 6 mm diameter, 13 mm long 52100 steel cylinder with three different contact areas corresponding to initial contact pressures of 157, 12 and 4 MPa. Sliding frequencies were varied between 1 and 15 Hz at a load of 50 N in order to establish the influence of both the contact pressure and velocity on the friction behaviour as a function of the lubrication regime. Textured features were observed to reduce the average coefficient of friction by up to 37% via a lubricant reservoir mechanism if the counter-surface contact width was greater than the feature dimension. Increases in contact pressure prevented a micro-EHL effect due to adhesive wear rather than pressurization of textured features.

Electrochemical jet processing encompasses a group of non-contact and ‘tool-less’ technologies, relying on localised electrolyte jets to affect changes to the workpiece in a site-specific manner. This is achieved without thermally or mechanically modifying the underlying material giving rise to a unique class of manufacturing methods. Jet techniques have been applied to remove and deposit material selectively, for example to machine microscale pits and grooves, to process larger surface areas, and to selectively coat materials through a variety of accretion phenomena. The jet itself also has the potential to serve as analysis/metrology tool. The potential to unify a broad range of site-specific manufacturing methods under one platform presents a unique opportunity to enable bespoke programmable surface geometries, finishes and compositions, guided by design and independent of material precondition. This review seeks to enrich the literature by drawing together these interdisciplinary research avenues into a single extensive but critical literature survey, incorporating the process fundamentals and theory, recent developments, and applications of jet processing methods, including hybrid jet processes. Finally, this review attempts to provide new insight and propose the direction of future research with the view to enhancing the areas in which electrochemical jet processes can add value on the factory floor and become widely applied industrial practice. ispartof: International Journal Of Machine Tools & Manufacture vol:180 issue:September 2022 status: Published online

Metamaterials exhibiting Fabry–Pérot resonances are shown to achieve ultrasonic imaging of a sub-wavelength aperture in water immersion across a broad bandwidth. Holey-structured metamaterials of different thickness were additively manufactured using a tungsten substrate and selective laser melting, tungsten being chosen so as to create a significant acoustic impedance mismatch with water. Both broadband metamaterial behavior and sub-wavelength imaging in water are demonstrated experimentally and validated with finite element simulations over the 200–300 kHz range.

Components manufactured by laser powder bed fusion (LPBF) are limited by their performance for use in critical applications. LPBF materials have microstructural defects, such as suboptimal grain size and morphology, and macroscale anomalies, such as lack of fusion. This results in LPBF components performing below their wrought counterparts for various mechanical properties, such as creep which has seldom been researched. To understand the creep behaviour of LPBF alloy 718, parts were fabricated using different scanning strategies and build orientations and creep tested at 650 °C under a 600 MPa load. Heat treatment increased the creep life by a factor of 5, confirming its necessity. The build orientation and stress state were shown to be determining factors in the creep failure mechanisms. The Meander scanning strategy resulted in a 58% increase in creep life compared to the Stripe strategy, due to the detrimental effects of the numerous laser overlapping regions in the Stripe strategy. For a given strategy, a 24% increase in creep life compared to wrought alloy 718 was observed, indicating that LPBF has the potential to surpass wrought material properties. As a result of this work, it is possible to propose build strategies for high temperature creep applications.

Fabry-Pérot ultrasonic metamaterials have been additively manufactured using laser powder bed fusion to contain subwavelength holes with a high aspect-ratio of width to depth. Such metamaterials require the acoustic impedance mismatch between the structure and the immersion medium to be large. It is shown for the first time that metallic structures fulfil this criterion for applications in water over the 200–800 kHz frequency range. It is also demonstrated that laser powder bed fusion is a flexible fabrication method for the ceration of structures with different thicknesses, hole geometry and tapered openings, allowing the acoustic properties to be modified. It was confirmed via both finite element simulation and practical measurements that these structures supported Fabry-Pérot resonances, needed for metamaterial operation, at ultrasonic frequencies in water. It was also demonstrated the the additively-manufactured structures detected the presence of a sub-wavelength slit aperture in water. Publisher PDF

The demonstration of salt dissolution incorporated within laser powder-bed fusion fabrication processes has allowed the creation of complex porous structures without the need for sophisticated design algorithms. This serves to simplify the process, for porous structure creation in powder-bed fabrication techniques, creating a new opportunity for the realisation of optimised structures. A new methodology is presented here in which modulation of the energy density while using a single feedstock material enables three-dimensional control of porosity, ranging from 20 % to 49 %. Through structured experimentation, the response of the material to varying the process parameters in selective laser melting is evaluated and nested structures of distinct densities and morphologies are created. Correlation of the process parameters with modulus and ultimate compressive stress are established. A simple-assembly algorithm was used to generate complex parts consisting of locally assigned porosities having characteristic properties.

In this work we employ additive manufacturing to print a circular array of micropillars on an aluminium slab turning its top surface into a graded index metasurface for surface acoustic waves (SAW). The graded metasurface reproduces a Luneburg lens capable of focusing plane SAWs to a point. The graded index profile is obtained by exploiting the dispersion properties of the metasurface arising from the well-known resonant coupling between the micropillars (0.5 mm diameter and variable length ∼3 mm) and the surface waves propagating in the substrate. From the analytical formulation of the metasurface’s dispersion curves, a slow phase velocity mode is shown to arise from the hybridisation of the surface wave with the pillar resonance. This is used to compute the radial height profile corresponding to the refractive index given by Luneburg’s equation. An initial validation of the lens design, achieved through ray theory, shows that ray trajectories have a strong frequency dependence, meaning that the lens will only work on a narrow band. An ultrasonic experiment at 500 kHz where plane SAWs are generated with a piezoelectric transducer and a laser scanner measures the out of plane displacement on the metasurface, validates the actual lens performance and the manufacturing technique. Finally, comparison between the ray analysis and experimental results offers insight into the behaviour of this type of metasurface especially in the proximity of the acoustic bandgaps and highlights the possibility for acoustic shielding., Ultrasonics, 111

This work demonstrates for the first time an augmentation to electrolyte jet processing which allows plasma electrolytic oxidation to be undertaken. Using a sequential process it is possible to produce micro features (≈500 μm width) with a resolution limited only by nozzle geometry but with significant enhancement to substrate surface properties. Here the technique is demonstrated and the resulting modification is compared on the microscale to properties of conventionally produced plasma electrolytic oxide coatings. The technique is further demonstrated through the creation of an exemplar titanium component which features localised surface oxidation treated mating features.

Laser Powder Bed Fusion (LPBF) is a commercially available technology to industry for a selected range of materials only. Opportune to the next stage of LPBF will be the development of multi-material capabilities alongside methods for rapid formulation of new materials. To this end the present work compares two viable material preparation techniques, namely ball milling and satelliting of powder feedstock, in which a binary material mixture is prepared. For both techniques, material handling, flowability and particle size distribution of the mixture are observed experimentally. Finally, for the first time initial experimental results with LPBF processing of a Metal Matrix Composite (MMC) composed of a magnesium (matrix) and silicon carbide (reinforcement) mixture are presented.

Metallurgical and materials transactions / A 51(5), 2444-2459 (2020). doi:10.1007/s11661-020-05692-6, There remains a significant challenge in adapting alloys for metal-based additive manufacturing (AM). Adjusting alloy composition to suit the process, particularly under regimes close to industrial practice, is therefore a potential solution. With the aim of designing new Ti-based alloys of superior mechanical properties for use in laser powder-bed fusion, this research investigates the influence of Fe on the microstructural development of Ti-6Al-4V. The operating mechanisms that govern the relationship between the alloy composition (and Fe in particular) and the grain size are explored using EBSD, TEM, and in situ high-energy synchrotron X-ray diffraction. It was found that Fe additions up to 3 wt pct lead to a progressive refinement of the microstructure. By exploiting the cooling rates of AM and suitable amount of Fe additions, it was possible to obtain microstructures that can be optimized by heat treatment without obvious precipitation of detrimental brittle phases. The resulting microstructure consists of a desirable and well-studied fully laminar $α$ + $β$ structure in refined prior-$β$ grains., Published by Springer, Boston

In laser powder bed fusion (LPBF), recovered unfused powder from the powder bed often degrades upon sequential processing through mechanisms like thermal oxidation and particle satelliting from ejected weld spatters and particle-laser interactions. Given the sensitivity of LPBF performance and build quality to powder properties, spent powder is generally discarded after a few build cycles, especially for materials that are sensitive towards surface oxidation. This increases feedstock material costs, as well as costs associated with machine downtime during powder replacement. Here, a new method to chemically reprocess spent LPBF metal powder is demonstrated under ambient conditions, using a heavily oxidised Cu powder feedstock recovered from prior LPBF processing as a model material. This is compared to an equivalent virgin Cu powder. The near-surface powder chemistry has been analysed, and it is shown that surface oxide layers present on spent Cu powder can be effectively reset after rapid reprocessing (from 5 to 20 min). Diffuse reflectance changes on etching, reducing for gas-atomised virgin Cu powder due to the formation of anisotropic etch facets, and increasing for heavily oxidised spent Cu as the highly absorptive oxide layers are removed. The mechanism of powder degradation for moisture sensitive materials like Cu has been correlated to the degradation of LPBF deposits, which manifests as widespread and extensive porosity. This extensive porosity is largely eliminated after reprocessing the spent Cu powder. Chemically etched spent powder is therefore demonstrated as a practical feedstock in LPBF in which track density produced is comparable to virgin powder., Additive Manufacturing, 52, ISSN:2214-8604

In this study, the creep performance of laser powder bed fusion manufactured Inconel 718 specimens is studied in detail and compared with conventional hot-rolled specimens alongside as-built then heat-treated and as-built then hot-isostatic pressed specimens. Hot-rolled specimens showed the best creep resistance, while the hot-isostatic pressed specimens yielded the worst performance, inferior to the as-built condition. Creep testing of all samples showed increased secondary creep rate was consistently correlated with a reduced life. Fractography revealed intergranular fracture was the primary failure mode for all as-built samples. Preferential intergranular precipitation in the case of the hot-isostatic pressed specimens during hot-isostatic pressing extensive intergranular cracking as the primary failure mechanism. Heat-treated specimens possessed only sparse intergranular precipitates, thereby explaining an improved creep lifetime. The hot-rolled specimens, having smallest grain size, showed the least extensive cracking, particularly in locations of finest grains, explaining avoidance of intergranular fracture as a key creep mechanism, thereby explaining the ductile creep fracture surfaces in the case of the hot-rolled samples.

Additive manufacturing (AM) technologies offer new processing routes for functionally graded materials. At present, parts built using these processes often require additional processing as a result of the characteristic surface finish limitations synonymous with AM processes. A difficulty thus arises in the post processing of these components as volumes within the part have differing material properties by definition and will therefore exhibit variable machinability. In this study, machining of functionally graded Ti6Al4V/ WC components consisting of a metal matrix composite (MMC) region and a single alloy region produced via direct energy deposition using commercially available tooling is explored. The influence of post processing on surface integrity is investigated and reported. The effect of material variation on cutting forces and tool response along the component is also analysed and reported. Cutting forces within the MMC region are found to increase by as much as 40% which has been subsequently related to the periodic changes in microstructure generated by the layer by layer build strategy. Tool wear mechanisms are investigated and the influence of material pull out on surface integrity of both MMC and single material regions is explored. This study provides an insight into how the layer building strategies, particularly with multiple materials and the resulting variation in microstructure influences the machining of resulting components.

The process of electrical discharge coating (EDC) may be used to deposit hard materials on conformal substrate surfaces. Next generation EDM'd components may exploit attachment phenomena to enhance recast layer properties, to avoid the need for recast layer removal. Here, a ceramic based composite layer was developed without cracking and porosity for the first time, using sequential coating using sacrificial TiC and Si electrodes. Attenuation of the discharge process by gap widening using Si debris in the gap explained improved layer properties. Composite coatings combining WC and TiC were also demonstrated, with good elemental intermixing. Attachment level was correlated strongly with melting point, with high melting point materials resisting ejection due to more rapid solidification. Nanoindentation showed the TiC and WC/TiC layers possessed the highest mean hardness values, approximately double that of the Cu based machined layer which itself yielded a much higher hardness of 11.0 GPa compared to 1.9 GPa of the substrate. Keywords: Electrical discharge coating, Nanohardness, EDM, Cracking, Porosity, TiC

Ribbed holes can serve to increase the efficiency of the heat exchangers and improve the performance of industrial equipment. Increasing demand for small ribbed holes is a key driver in manufacturing technology research. Electrochemical machining has been shown to be a promising method for this. In this paper, an intermittent low-frequency vibration tool is used to demonstrate significant improvement to the geometry of internal ribbed holes. The process stability, material removal rates, and uniformity of features along the flow direction are improved. Firstly, a 3D model of the flow field within the interelectrode gap was developed to calculate and governing flow regime in the interelectrode gap. The simulation demonstrated that the fluid velocity fluctuates periodically and this enhances electrolyte flushing in the interelectrode gap during the machining process. Then, experimental tests for the manufacture of spiral ribs on small holes (Ø1.5 mm and 20–40 mm depth) are also demonstrated with accompanying variation of the tool vibration amplitude and frequency, respectively. Results show that groove depth was most greatly influenced by the vibration amplitude and that better uniformity could be obtained at higher vibration frequencies. The groove depth increased by 15% over nonvibrating control tests with enhanced uniformity.

A novel type of metallic feedstock material for powder-bed additive manufacturing (AM) processes is proposed that enables the manufacture of cellular structures without the time consuming and computationally intensive step of digitally representing the internal geometry of a part. The feedstock is a blend of metal and salt particles and, following Selective Laser Melting (SLM) processing, the salt is dissolved to leave a metallic, cellular structure. The conditions for successfully processing the feedstock are first demonstrated, followed by an investigation into how the feedstock composition can be used to control the relative density of the cellular material. Mechanical testing reveals that the strength and stiffness of the cellular structures can be tuned through control of feedstock composition, and hence, relative density. This presents a significant enhancement to the state-of-the-art for materials preparation for AM since, for the first time, cellular structures can be created with specific properties without explicitly defining or analysing the unit cell geometry. Keywords: Additive manufacturing, Selective laser melting, Cellular structures, Porous structures

Additive manufacturing lends itself well to the manufacture of aerospace parts due to the high complexity and small volume of many components found in modern aero engines. By exploiting additive manufacturing design freedoms, enhanced part functionality can be achieved and lead time can be reduced. However, the integrity of these parts is a primary concern which often cannot be guaranteed with current generation additive manufacturing methods and materials. Studies on the performance of additively manufactured parts under service conditions are therefore required. In this study, laser powder bed fusion is used to produce specimens for creep testing. To allow this a novel specimen design, i.e. Two Bar Specimen, was applied for creep testing. The performance of these specimens, in the as-build condition, is showed to be largely poor because of surface integrity defects and unfavourable microstructure formation. These are clearly highlighted and explored. Further specimens, subjected to heat treatments, have also been tested. These showed a marked improvement of the microstructure. The lifetime of the heat-treated sample prepared with milling + wire electrical discharge machining was enhanced by as much as four times compared to the as-build specimens. However, this lifetime performance remains 33% below that of samples machined from the equivalent wrought material. This work then proposes manufacturing strategies to significantly enhance the performance of Inconel 718 when processed via laser powder bed fusion and post-heat-treatments.

The tribological behaviours of TiC-based cermet coatings, prepared by electrical discharge coating (EDC) using a semi-sintered TiC tool electrode, have been investigated. The as-deposited coatings exhibited complex microstructures, comprising TiC grains within an Fe matrix, on both high speed steel (HSS) and 304 stainless steel (304-SS) substrates. The wear resistance of TiC/Fe cermet coatings, on both substrate types, increased dramatically (one and two orders of magnitude improvement in specific wear rate), compared to as-polished substrates. Further, EDC cermet coatings on HSS were typically 2–4 times more wear resistant, depending on loading, than those deposited on 304-SS, with wear performance reflecting the composite nature of the coating coupled with the mechanical properties of the substrate. Laser surface treatments used to improve surface integrity of the as-deposited coatings, through elimination of cracks and porosity characteristic of ED coating, acted to increase wear rates for all samples, with the exception of coatings on HSS under conditions of high loading. The general increase of wear rate was attributed to a significant reduction in the proportion of TiC within the ED coatings, after laser treatment, combined with an increase in grain size; whilst improvements to the wear performance of laser treated, cermet coated HSS, under high loading, was attributed to the avoidance of an abrasive wear mechanism.

Direct Metal Deposition (DMD) is becoming increasingly attention-attracting technology for new component manufacture and repair. However, fundamental process understanding is not yet comprehensive. Without considering the realistic spatial distribution of powder flow, some important process conditions have been neglected in layered deposition, such as the initial stand-off distance of nozzle, scanning direction (orientation of nozzle), change in stand-off distance through process and stability of multi-layer deposition. This then limits the efficacy of deposition strategies which serves to limit the industrial uses of DMD. In this paper, a realistic model was built for the simulation of multi-layer deposition, using real spatial powder flow concentration. Then, the influences of the orientation of nozzle, the stand-off distance of nozzle and single-step rise on geometric characteristics are investigated. It is showed that the stand-off distance of nozzle significantly affects the geometric characteristics of the deposited layer thickness, while the influence of the orientation of four-tip nozzle on deposition can largely be neglected. Furthermore, the stability of multi-layer deposition was discussed, and the steady condition was obtained by analyzing relation among single-step rise, maximum deposited layer thickness and stand-off distance of nozzle. This also allows a deposition strategy to be optimized for the purpose of manufacturing given procedure. The approach taken here is also verified by experiments with the strategies proposed by simulation.

Deployment of Al components produced by Additive Manufacturing (AM) is inhibited by the low strength of the casting Al alloys currently being repurposed for use with this process. As a result, the use of AM to produce Al matrix Composites (AMCs) which have superior properties to their castable counterparts is an emerging area of research. In this paper, micro TiB2 particles were decorated to the surface of larger AlSi10Mg powder particles to create feedstocks through the ‘satelliting’ method. TiB2/AlSi10Mg composites were then successfully fabricated by Direct Metal Deposition (DMD) with a uniform distribution of the reinforcing TiB2 particles observed. Results show that macro agglomerations of TiB2 particles decrease significantly with decreasing traverse speed and TiB2 content. Porosity is also shown to decrease markedly with increasing traverse speed for this material feedstock. The hardness of deposits is shown to increase with addition of TiB2 content, which implies improvement of material strength.

The high optical reflectance of Cu at near-infrared wavelengths narrows the process window to fabricate Cu parts by laser powder bed fusion (LPBF). Coating powders with optically absorptive materials has been investigated to improve processability and enhance part properties. However, given the intense heat localization and thin coating layers relative to the powder, the mechanisms of thin film coating interaction in LPBF remain unclear, despite recent work showing the importance of the near-track environment in deposition behavior. In this study, optically absorptive Zn-coated Cu powders were prepared by physical vapor deposition and characterized. Single LPBF tracks were fabricated to elucidate material incorporation phenomena influenced by the volatile Zn coating. It is shown that Zn-coated powder enhances accretion at fastest effective scan speed tested (100 mm/s), where mean track volumes are increased from 0.72 ± 0.05 mm3 (as-received) to 0.91 ± 0.01 mm3 (Zn-coated). This has been correlated to the stronger vapor jet from the volatile Zn-coating, which denudes the surrounding powder bed. This exhausts the powder bed at slower effective scan speeds, causing instability and balling when compared to the as-received powder. It is shown that Zn is localized at the track surface and is undetectable in the track bulk, indicating Zn vaporization on interaction with the incident beam. Zn present mainly occurs through secondary deposition mechanisms like spatter and condensation, rather than in-process alloying. Coating powder feedstocks for use in LPBF therefore affects composition, laser beam absorptivity, and the near-track vapor environment that is known to influence material incorporation behavior.