Micro laser powder bed fusion (mu-LPBF) enables the fabrication of the components with higher resolution and accuracy compared with conventional laser powder bed fusion (c-LPBF). This study demonstrated the feasibility of fabricating materials with excellent mechanical properties due to the fine microstructures generated by mu-LPBF. The near fully dense austenite stainless steel 316L with high strength was successfully achieved. The tensile tests showed that the specimens had the yield strength ranging from 645 MPa to 690 MPa, ultimate tensile strength between 765 MPa and 795 MPa, and total elongation over 40%. The high strength without the sacrifice of the elongation was attributed to the fine cellular structures. Both cell size and cell wall thickness were critical factors for the yield strength. The thin wall contributed to the minor increase in the yield strength when compared with c-LPBF fabricated specimens in the literatures. The hatch spacing minorly affected the cellular structure but did affect the grain size and texture. Due to the similar characteristics of the cellular structure, the yield strength was not strongly affected by the hatch spacing. The critical stress for deformation twinning was determined by the texture varying as the hatch spacing, which was the reason for the lower elongation of the samples under the largest hatch spacing.

The mechanical properties of aerospace-grade Ti-6Al-4V parts with varying microstructures fabricated via an electron beam melting (EBM) additive manufacturing method have been investigated. We find that alpha lath width in microstructure is significant to the deformation mechanisms in EBM-built Ti-6Al-4V specimens. The deformation-induced nanoscale twinning is observed due to a high work hardening rate at room temperature with an average alpha lath width of -0.6 mu m, which results in high yield strength and ultimate tensile strength with extraordinary ductility. By contrast, the fine-microstructure specimens with an average alpha lath width of -0.2-0.3 mu m where a low work hardening rate occurs, exhibit only the dislocation plasticity. These findings provide an indepth understanding of the microstructure-dependent deformation mechanisms in additively manufactured Ti-6Al-4V. More importantly, this work sheds light on overcoming the strength-ductility trade-off in titanium alloy by additive manufacturing.

Electron beam melting (EBM),as an excellent Additive Manufacturing (AM) technol-ogy,enables the printing of Ti-6Al-4V alloy for a wide range of applications such as aerospace and biomechanical industries.It improves functionality and integrity of components and negates complexities in assembly processes.However,due to the poor surface and sub-surface integrity rep-resented by the rough surface finish and low dimensional accuracy,achieving a favorable surface condition is quite challenging.Therefore,post processing becomes essential for these electron beam melted (EBM-ed) Ti-6Al-4V alloys.Being the most common technique to improve such parts,milling of Ti-6Al-4V alloy is very challenging and resulting tool wear issues,due to its unique mate-rial properties.Thus,this paper presents a comprehensive study on the surface integrity of EBM-ed Ti-6Al-4V parts processed by precision grinding and electropolishing,aiming to qualitatively and quantitatively clarify the interrelation between process parameters and processed surface quality.The surface and subsurface characteristics such as profile accuracy,surface roughness,microstruc-ture,defective layer and residual stress before and after post processing were compared and evalu-ated.The results show that by precision grinding,the profile accuracy was improved from over 300 μm PV to 7 μm PV,while surface roughness (Ra) was reduced from 30 μm to about 2 μm.The layer with partially melt particles was removed,but introduced a deformed subsurface layer with more residual stress.Then by applying electropolishing,the residual stress was released and the deformed layer was removed.In addition,Ra was further reduced to 0.65 μm.The research can serve as a reference for the integration of post machining processes with AM.

Fiber reinforced sandwich structures are widely used as structural elements due to their lightweight and high load-bearing capacity. In this paper, effort is taken to fabricate fiber facesheet (0°/90° orientation) and corrugated cores (trapezoidal vertical pillared, core-1 and sinusoidal vertical pillared, core-2) of the sandwich structure using fused filament fabrication and Inkjet printing techniques, respectively. Firstly, the quasi-static indentation properties of the additively manufactured facesheet, cores, and sandwich structures were investigated. In addition, acoustic emission signals were monitored during quasi-static indentation testing and the resulting hits data were correlated with quasi-static indentation test results. The quasi-static indentation test results showed that sinusoidal sandwich structures have the highest load-bearing capacity of 1.79 kN until crack initiation and a total energy-absorption capacity of 20.05 J. Acoustic emission hits recorded in the range of 1–30, 30–100, and above 100 represented crack initiation, crack propagation, and specimen failure, respectively. Lastly, micro-computed tomography analysis was performed on the sandwich structures at intermediate indentation displacements, thus leading to the identification of hard to detect failure points and damage propagation. Fracture surface morphology of the sandwich structures showed that fiber pullout and core shear were the dominant damage mechanisms.

Electron beam melting (EBM) is a promising technology to manufacture various alloys with outstanding properties; however, the number of available alloys is limited. We propose in situ alloying to accelerate the development of advanced and novel alloys, based on thermophysical calculations and CALPHAD approach, during the EBM process. We demonstrate our concept through the design and fabrication of high entropy alloys (HEAs). Three CoCrFeNiMn-xTi (x=0.18, 0.50, 2.00, in molar %) HEAs are manufactured. EBM-built HEAs achieve a homogeneous distribution of elements while forming multiphase alloys resulted from the hot powder bed. The topological structures formed by secondary phases contribute to an increase in the hardness of EBM-built HEAs up to 900 HV1. Considering alloy design, a systematic analysis on CoCrFeNiMn-0.18Ti HEA elucidates the microstructural evolution in detail. These findings provide a deep understanding of in situ alloying and pave the way to develop new alloys specific to the EBM process.

A compatible artificial bone implant requires large pores for enhanced nutrients transports, small pores to allow cell seeding and bone-like mechanical properties to avoid stress shielding. Herein, we report novel improved gyroid lattices with millimetre-scaled gyroid wall spacings and micrometre-scaled additional pores on the walls. Designs are successfully fabricated by electron beam melting using Ti-6Al-4V to high part qualities while exhibiting bone-like mechanical properties with a range of Young's modulus of 8-15 GPa and strength of 150-250 MPa. The improved design also eliminates brittle failure by allowing the structure to deform more stably. (C) 2020 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

Tribological behaviors of 316SS/bronze (BJP316), 420SS/bronze (BJP420) and WC/bronze (BJPWC) composites fabricated using binder jetting were evaluated systematically by using a scratch tester and dry reciprocating ball-on-disc tribometer under various loads. The results showed that the BJP316, BJP420 and BJPWC are mainly subjected to delamination, abrasion and spalling damages, respectively. The scratch resistance mainly depends on the soft bronze infiltrant. The friction coefficients in dry sliding all decrease with increasing loads, whereas the wear rates present different trends with loads. All composites exhibit comparable wear resistance to traditionally fabricated counterparts. The findings will help to improve the tribological design of mechanical elements fabricated by binder jetting and lay the foundation for the applications in the tribological field.

Electron beam melted (EBM) Ti-6Al-4V functionally graded materials (FGM) with continuously graded densities are investigated for dimensional accuracy, compressive properties, fractography and build direction effect in comparison to uniform density counterparts of the same volume. It is found that FGMs exhibit progressive layer-by-layer deformation mode regardless of unit cell designs and build direction. This deformation behavior is highly favourable for uni-directional impact absorption applications compared to uniform density counterparts with random or diagonal failure. Overall, the EBM-built FGM exhibits superior energy absorption than counterparts of uniform density. Significant improvement in the quasi-elastic gradient and energy absorption is obtained by changing the build direction for specific designs. Compared with other FGM or uniform density lattice structures from the literature, the energy absorption of lattice structures with lower relative density could outperform those with higher relative density by changing the unit cell design or density profile.

Powder bed fusion additive manufacturing (AM) technology, such as electron beam melting (EBM) and selective laser melting, has attracted tremendous academic and industrial interests because of its capacity to fabricate components with greater complexity compared with traditional processes, without significantly increasing the cost. It provides significantly higher design freedom to the designers and can make the built components closer to the optimum design in theory when compared with traditional processes. However, the mechanical performance of the new design fabricated by AM has not been clarified yet. Here, we report the fabrication and tensile deformation behavior of the EBM-built lightweight car suspension double wishbone for both conventional and optimized designs. EBM process is an effective method to produce a highly-dense Ti-6Al-4V lightweight design component with good reproducibility and fine α/β duplex microstructure. A poor mechanical performance in the optimized design is observed, which results from the build thickness-dependent mechanical performance that is caused by both various microstructures and rough surfaces in the Ti-6Al-4V parts. Notably, the rough surface plays a dominant role in premature failure when the build thickness is below 2 mm. Based on these findings, the degraded mechanical performance in the optimized design is discussed. The experimental results and analyses provide a guideline for the design of lightweight structures that are mainly comprised of thin walls and/or struts.

Porous metallic components are known for their multifunctions in a variety of industries. However, the fabrication of porous metal parts via the conventional manufacturing processes, such as metal injection molding and metal sintering, is largely constrained due to their dependency on templates or supports. Additive manufacturing techniques have no such limitations. Binder jetting is a powder-bed-based additive manufacturing technique, and it has the inherent advantages in fabricating high porous components through the pores-by-process approach. In this work, the manufacturing process of porous CoCrFeMnNi high entropy alloy using binder jetting was investigated. Part porosities in the range of 35 %-40 % were achieved through binder jetting and subsequent sintering processes. X-ray computed tomography measurement indicated the morphology and size of the pores were uniformly distributed. The average compressive yield strength of the porous samples varied between 42-70 MPa, compressive elastic modulus varied between 3-12 GPa. Fracture surface investigations revealed transgranular quasi-cleavage fractures to be more dominant. The corrosion resistance of the porous CoCrFeMnNi was found to be comparable to the 316 L equivalent upon optimum sintering parameters. Overall, it is potential candidate for filtration applications.

Selective laser melting processes deposit and join metal powders to near net shape in a layer-by-layer manner. The process of melting and re-solidification of several layers of deposited material can result in geometric deviations, and the impact is particularly significant for sub-millimetre structures oriented at a wide range of overhang angles with respect to the building platform. This paper assesses and benchmarks the capabilities of a neural network-based geometric compensation approach for truss lattice structures with circular cross-sections. The neural network method is capable to generate free-form cross-sections with enhanced geometric freedom for compensation compared to more established analytical compensation approaches limited to predefined geometric shapes. For neural network training, lattice dome structures composed of trusses with different overhang angles were designed and printed by selective laser melting and measured via X-ray computed tomography, resulting in point cloud data sets containing more than 20,000 data points for each overhang angle. For experimental validation, neural network-compensated dome structures were benchmarked against dome structures with elliptical parameter compensation. Results show that the neural network compensated lattice trusses achieve higher printing dimensional accuracy compared to the uncompensated structures and those compensated based on elliptical parameter estimates.

The microstructure, mechanical properties and deformation mechanisms of the 304L stainless steel (SS) additively manufactured by selective laser melting (SLM) were systematically investigated. The SLM fabricated 304L SS contains two phases (face-centered-cubic γ-austenite and body-centered-cubic δ-ferrite) and exhibits a hierarchical microstructure with length scales spanning several orders of magnitude. The hierarchical microstructure includes the melt pools and slightly elongated columnar grains at the micron scale, cellular structures decorated with a high density of dislocations at the sub-micron scale and oxides at the nanoscale. Stacking faults formed due to the residual stress in addition to the low stacking fault energy of the 304L SS (19.2 mJ/m2) while massive annealing twins were generated arising from the combined effects of residual stress and intrinsic heat treatment. The as built 304L SS exhibits a significantly enhanced strength–ductility synergy compared to that of wrought and annealed counterparts. The enhanced yield strength stems from the hierarchically heterogeneous microstructure, while the outstanding tensile elongation is ascribed to the activation of multiple deformation mechanisms, involving the dislocation activities, the formation of stacking faults and mechanical twins, and the transformation-induced plasticity.

This paper proposes a novel flexible pressure sensor based on carbon black (CB), carboxy-methyl cellulose (CMC), and gelatin. CMC and gelatin are both food-grade materials that are degradable, non-toxic, and environmentally friendly. Freeze drying is performed to obtain a sensor porous honeycomb structure that achieves not only a wide monitoring range from 0 to 140% strain, but also high sensitivity (maximum gauge factor of 12.5, which is higher than ordinary conductive CB composite systems). After 3000 repeated presses, the sensor remains unchanged and retains its high sensitivity. This stable sensor response is promising for long-term practical applications. The proposed sensor is applied to a speech recognition system and can distinguish the different components of a sentence, thereby achieving accurate speech recognition. It is also capable of monitoring human body movements, including joint flexion and finger movement. Finally, a sensor integrated network is constructed for application to a human–machine interface. The proposed CB sponge pressure sensor has important applications for medical wearable devices and smart wear.

Additive manufacturing enables the fabrication of periodic ceramic lattices with controllable micro-architectures. Many studies reported their catastrophic brittle fracture behaviour. However, ceramic lattices may fail by a layer-by-layer pseudo-ductile fracture mode, by controlling micro-architectures and porosities. Moreover, their fracture behaviour can be optimised by introducing strut/wall thickness gradients. This paper investigates the fracture behaviour and the fracture mode transition of ceramic triply periodic minimal surface (TPMS) structures. Alumina TPMS structures with relative densities of 0.14-0.37 are fabricated by ceramic stereolithography. Quasi-static compression tests validate a transition density range for non-graded samples: low (<0.21) and moderate (>0.25) relative density samples show layer-by-layer pseudo-ductile and catastrophic brittle fracture modes, respectively. The pseudo-ductile failure mode increases the energy absorption performance, enabling load-bearing capacity for a compressive strain up to 50%. With appropriate thickness gradients, graded structures exhibit significant increase of energy absorption without a decrease of fracture strength compared to their non-graded counterparts.

316L austenitic stainless steel has a wide range of industrial applications. However, one of the major drawbacks is its low yield strength (170-300 MPa in annealed state). We report a method to strengthen 316L by adding 1 wt % and 3 wt% micron-sized TiC particles using low energy ball milling for the powder feedstock preparation followed by selective laser melting (SLM). The TiC particles were observed to be uniformly dispersed and well bonded to the 316L matrix after SLM. The 316L-TiC composites obtained were close to full density and the austenite grains were significantly refined with the addition of TiC particles. Tensile tests show that adding 1 wt % and 3 wt% TiC particles leads to a significantly increased yield strength (660 MPa and 832 MPa) and UTS (856 MPa and 1032 MPa) and maintains the good ductility (55% and 29% elongation). These findings offer a new perspective on the strengthening of 316L stainless steel

Nanoparticles reinforced steels have many advantaged mechanical properties.Additive manufacturing offers a new method for fabricating nanoparticles reinforced high performance metal components.In this work,we report the application of low energy ball milling in mixing nanoparticles and micron 316 L powder.With this method,0.3 and 1.0 wt％ Y2O3 nanoparticles can be uniformly distributed on the surface of 316 L powder with the parameters of ball-to-powder ratio at 1 : 1,speed at 90 rpm and 7 h of mixing.The matrix 316L powders remain spherical in shape after the mixing process.In the meantime,the effect of low energy ball milling and the addition of Y2O3 nanoparticles on the powder characteristics (flowability,apparent density and tap density) are also studied.Results show that the process of low energy ball milling itself can slightly decrease the flowability and apparent density of the 316 L powder.The addition of 0.3 and 1.0wt％ Y2O3 nanoparticles can also decrease the flowability,the tap density and the apparent density compared with the original 316 L powder.All of these changes result from the rough surface of the mixed powder produced by ball milling and the addition ofY2O3 nanoparticles.The powder's rough surface can increase the coefficient of friction of powders.The mixture of 316 L powder and Y2O3 nanoparticles can be successfully used for selective laser melting (SLM).The relative density of SLM 316 L-Y2O3 is measured at 99.5％.However,Y2O3 agglomerations were observed which is due to the poor wettability between 316 L and Y2O3.

To evaluate specimen size dependence on the tensile properties of additively manufactured (AM) components, various rectangular specimens, ranging from miniaturised to ASTM standard specimens, are machined from electron beam melted Ti-6Al-4V and used for the tensile testing. It is found that the elongation is strongly related to the sample size while the yield and ultimate tensile strengths exhibit an independent feature. Three major aspects, (i) presented location of lack of fusion, (ii) size and segregation of pores, and (iii) slimness ratio, have a synergic influence on the elongation of different specimen sizes with various cross-section area. Our findings suggest that microscale tests arise uncertainties in measurement, which must be considered in order to provide quantifiable levels of confidence when applying such tests to discriminate a material’s behaviour. The experimental results and analyses provide a guideline for the design and testing of non-standard specimens for AM components.

Due to rapid solidification of melted powders in metal additive manufacturing processes and high thermal gradients, large residual stresses are created in the build. This can lead to undesired distortions as well as crack initiation. The main aim of this work is to optimize the Additive Manufacturing (AM) process parameters by finite element modelling of the entire process to minimize the resulting residual stresses and distortions. We focus on two most important metal AM processes: (a) Laser Direct Energy Deposition (LDED) and (b) Selective Laser Melting (SLM). The ABAQUS AM module is employed to simulate both processes as it provides an automated interface allowing the user to define event data, such as element activation and heat input, as a function of both position and time to achieve process simulation of complex 3D parts. For the LDED processes, thin wall components are simulated, and residual stresses predictions are compared with both FIB-DIC and XRD measurement results at different scales. For the SLM process, overhanging structures with different support thicknesses are simulated and compared with experimental part distortion after support removal. It is shown that the support thickness together with selected process and material properties play a key role in resulting distortions.

A successful implant requires compatible mechanical properties. Herein, microlattice models with bone-like mechanical properties (compressive strengths of 169.5-250.9 MPa, Young's modulus of 14.7-25.3 GPa) and high porosity (up to similar to 60%) are designed and investigated through finite element modelling and electron beam additive manufacturing fabrication. Microlattices are designed to be hetero-porous, with large pores for nutrients flow and small pores for cell seeding. Size and boundary effects (3 x 3 x 6, 4 x 4x8 to 5 x 5x10) are studied of which revealed to affect the as-built mechanical properties via geometrical defects, generally deviating from that of modelling. The high proportion of increased nodals masses for fine features is found to greatly increase mechanical properties via the redistribution of stress concentrations. The effects of bounding edge additions to the lattices are also investigated, up to a 45% and 25% increase in strength and Young's modulus are respectively observed with a minimal increase in relative density of 4%, a large increase exceeding the modelled Gibson-Ashby trends. Mechanisms result from both the reduced fabrication defects and evenly distributed stress distributions are revealed by finite element analysis. The method allows effective enhancement of build qualities with retained pore sizes and porosities, which paves a new way for designing of microlattice with enhanced properties.

In this study, Sn-Ag-Cu solder alloys and Sn-Ag-Cu solder alloys reinforced with 0.1 wt% Ag-graphene nanosheets (Ag-GNSs) by mechanical mixing (H for abbreviation) and ball milling (Q for abbreviation), which were referred as SAC, H/0.1Ag-GNSs, and Q/0.1Ag-GNSs, respectively, were used to form solder joints. The creep behavior of the above solder joints was investigated by conducting nanoindentation tests. A method for calculating the strengthening stress generated by the load transfer and orientation of the Ag-graphene nanosheets was proposed. The method considers the geometry and grain data of the Ag-graphene nanosheets, which were obtained through scanning electron microscopy and electron back scattering diffraction. Considering other strengthening stresses generated by the dislocation strengthening, fine grain strengthening, Orowan strengthening for intennetathc compounds and metal matrix nanocomposites, and strengthening stress generated by Ag-graphene nanosheets, a modified constitutive model was proposed to investigate the constitutive behavior for creep performance of solder joints formed by SAC, H/0.1Ag-GNSs and Q/0.1Ag-GNSs. The results show good agreement with the experimental data.