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A comprehensive understanding of cracking mechanisms and the prevention of interfacial microcrack formation are imperative for additive manufacturing of high-performance multi-material heterostructures. This study systematically investigated 316L/CuSn10 heterostructures and identified solidification cracking and solid-state cracking as the predominant mechanisms. Solidification cracking is closely linked to the copper content within the mixing zone, particularly evident at 10% copper content, which heightens sensitivity to solidification cracking due to the widening of intergranular spacing and the elongation of the liquid film channel. Solid-state cracks tend to initiate from pre-existing solidification cracks, propagate along high-angle grain boundaries (HAGBs), particularly within a specific misorientation angle range of 20°–50°, terminating eventually at low-angle grain boundaries (LAGBs). This is mainly controlled by the distribution of dislocations at crack tips, which are dispersed within the grains at LAGBs, and the resulting back stress contributes to crack termination. These findings contribute valuable insights into the cracking mechanisms in heterostructures and offer guidance for the fabrication of crack-free steel-copper components.

Porous NiTi lattice structures are widely used in the manufacture of crucial components owing to their excellent shape memory effect, superelasticity, and high damping capacities. However, the specific strength and lightweight characteristics of porous NiTi lattice structures fabricated by conventional technologies are limited by unpredictability. In this work, three types of porous NiTi structures based on triply periodic minimal surface (TPMS) – Diamond, Gyroid, and Primitive – were designed and manufactured by the laser powder bed fusion (LPBF) additive manufacturing process. This work demonstrates LPBF is a feasible and efficient approach to fabricate highly accurate porous NiTi TPMS structures. Moreover, the influence of each of these structures on the mechanical and shape memory properties was investigated. Among the three structures, Gyroid had the smallest volume fraction deviation. Furthermore, the Diamond structure had the largest compressive modulus (782.82 MPa) and ultimate yield strength (163.14 MPa). The Gyroid and Primitive structures exhibit excellent elastic recovery deriving from high values of compressive modulus (662.44 MPa, and 703.29 MPa), and can maintain reliable structural robustness. The Primitive structure exhibited the lowest mechanical properties (37.80 MPa). During the cyclic compression test, Gyroid and Primitive show a smaller unrecovered strain than Diamond. Primitive shows the largest recovered strain during the heating process (6.98%). The higher mechanical flexibility of Primitive structure endows this structure with higher recovery ratio. During the direct compression test, the residual strain exhibits a positive correlation with the loading strain. All three structures exhibit good deformation recovery capability with a strain of 4%. At a strain of 12%, recovered strain during heating became the dominant factor in the recovery of the TPMS structure. Overall, porous NiTi TPMS structures are capable of reversible compressibility composed of rapid elastic recovery and controllable shape memory recovery. The unique performance of porous NiTi TPMS structure fabricated by LPBF renders it a highly efficiency energy-absorbing structure.

Although self-healing elastomers have been developed in a great breakthrough, it is still a challenge to develop one kind of material that can respond to the fracture instantly even though this characteristic plays an essential role in emergency circumstances. Herein, we adopt free radical polymerization to construct one polymer network equipped with two weak interactions (dipole–dipole interaction and hydrogen bonding). The elastomer we synthesized has a high self-healing efficiency (100%) and a very short healing time (3 min) in an air atmosphere, and it can also self-heal in seawater, showing an ideal healing efficiency of >80%. Additionally, on account of its high elongation (>1000%) and antifatigue capacity (no rupture after loading–unloading 2000 times), the elastomer can be utilized in a wide range of applications, including e-skin and soft robot fields.

The porous metallic biomaterials have attracted significant attention for implants because their lower young's modulus matches the human bones, which can eliminate the stress shielding effect and facilitate the growth of bone tissue cells. The porous metallic biomaterials fabricated by selective laser melting (SLM) have broad prospects, but the surface of the SLM-built porous structure has been severely adhered with unmelted powders, which affects the forming accuracy and surface quality. The porous metallic biomaterials face the corrosion problem of complex body fluid environments during service, so their corrosion resistance in the human body is extremely important. The surface quality will affect the corrosion resistance of the porous metallic biomaterials. Therefore, it is necessary to study the effect of post-treatment on the corrosion resistance of SLMed samples. In this work, the mechanical response and the electrochemical corrosion behavior in simulated body fluid of diamond and pentamode metamaterials Ti–6Al–4V alloy fabricated by SLM before and after sandblasting were studied. After sandblasting, the mechanical properties of the two porous metallic biomaterials were slightly improved, and the self-corrosion potential and pitting potential were more negative; meanwhile, the self-corrosion current density and passive current density increased, indicating that its corrosion performance decreased, and the passive film stability of sandblasted samples got worse.

• This paper proposes a novel lattice-surfaced PEEK cage to provide tailored mechanical performance, better stress absorption and deformation resistance. • The compression modulus and elastic limit can be tailored by adjusting the lattice-surfaced area without sacrificing the energy absorption efficiency. • Multiple point-plane stress transfer mechanism is found for lattice-surfaced PEEK cage, which plays an important role in stress absorption and deformation resistance. • The high-strength PEEK shows a characteristic radial morphology and a more ordered double-stranded orthorhombic structure. Porous structure design on the contact surface is crucial to promote the osseointegration of the intervertebral cage while preventing subsidence and displacement. However, the stress response will undergo significant changes for the current random porous cages, which can directly affect the mechanical properties and long-term usability. Here, this paper proposed a newly designed polyetheretherketone (PEEK) cage with the triply periodic minimal surface (TPMS)-structured lattice surfaces to provide tailored 3D microporosity and studied the mechanical performance, stress/strain responses, and microstructure changes in depth. The lattice-surfaced PEEK cage mainly exhibits a multiple-point-plane stress transfer mechanism. The compression modulus and elastic limit can be adjusted by controlling the area of the Diamond TPMS surface while the energy absorption efficiency remains stable. The microstructure of high-strength PEEK is featured by the radial pattern morphology. Meanwhile, the double-stranded orthorhombic phase is more ordered, and the benzene plane subunit and lattice volume become more expanded. [Display omitted] [ABSTRACT FROM AUTHOR]

A room-temperature self-healing silicone elastomer was prepared based on the synergistic effect of multiple H-bonding and dynamic covalent bond. The multiple bonds constructed by inserting thiourea into the polyurea network can inhibit the crystallization of hard urea H-bonds segments and activate the diffusion movement of polymer chains. Dynamic imine endows materials with a strong connection for the fracture interface by imine metathesis. The effect of thiourea on urea H-bonds was confirmed by the Fourier transform infrared spectrum, which showed obvious changes of H-bond density according to peak revolution of C═O and N–H. Differential scanning calorimetry demonstrated the transition from the crystalline to amorphous state after the introduction of thiourea. Tensile tests and scratch-healing tests showed that this design method can improve the self-healing property without sacrificing the mechanical strength. Finally, the optimized self-healing process was analyzed from the perspectives of the contact process, the interpenetration diffusion of the polymer chain, and rebuilding of crosslinking points between the two interfaces, which would build an avenue for constructing a fast, self-healing, and tough material.

Selective laser melting (SLM) is an additive manufacturing technology that has an extensively applied foreground and practical value in many fields. Despite its powerful manufacturing ability, defects are prone to occur and therefore a more reliable and repeatable manufacture process is in high demand. During the SLM process, the melt pool signature is the key to understanding the dynamic process status, with which it is possible to predict process failure and give guidance to real-time feedback control. In this paper, a novel method to capture melt pool signature using a U-Net-based convolutional neural network is described. A lightweight architecture was used to reduce the inference time, and an improved loss function with penalty maps was applied to better remove interferences. The model performance was evaluated by comparing both the processing time and accuracy with two conventional image segmentation algorithms, including the threshold segmentation method and the active contour method. Mean intersection over union (MIoU) was chosen as the segmentation metric. Unlike traditional algorithms, U-Net successfully eliminated the interferences, and reached the highest MIoU (0.9806) at a relatively low computational cost of 37 ms on average. The collected information from the melt pool area in various scenarios was analyzed, and its potential to indicate the problem of melt pool overheating was investigated.

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Additive manufacturing (AM) technology enables the creation of a wide variety of assemblies and complex shapes from three-dimensional model data in a bottom-up, layer-by-layer manner. Therefore, AM has revolutionized the modern manufacturing industry, attracting increasing interest from both academic and industrial fields. The Rapid Manufacturing Center (RMC) of the School of Materials Science and Engineering at the Huazhong University of Science and Technology (HUST), one of the earliest and most powerful AM research teams in China, has been engaged in AM research since 1991. Aiming to address the “stuck neck” problems of specific high-strength products for AM, the RMC has conducted full-chain research in the aspects of special materials, processes, equipment, and applications for AM. Moreover, it has formed a multi-disciplinary research team over the past three decades. Relevant research achievements in the AM field include winning five national awards, more than ten first prizes, and more than ten second prizes at the provincial and ministerial levels. The RMC was complimented as “the world's most influential organization in the laser AM field in 2018” by Virtual and Physical Prototyping (an international authoritative magazine of AM). Moreover, their industrialization achievements were evaluated as “having affected countries such as Singapore, South Korea, and the United States” by an international authoritative Wohlers Report on AM. In this study, we first summarize the representative research achievements of the RMC in the AM field. These include the preparation and processing technology of high-performance polymeric, metallic, and ceramic materials for AM; advanced processing technology and software/equipment for AM; and typical AM-fabricated products and their applications. Further, we discuss the latest research achievements in cutting-edge 4D printing in terms of feedstock selection, printing processes, induction strategies, and potential applications. Finally, we provide insights into the future directions of AM technology development: (ⅰ) Evolving from three-dimensional printing to multi-dimensional printing, (ⅱ) transitioning from plane slicing to curved surface slicing to woven slicing, (ⅲ) enhancing efficient formation from dot-line-sheet-volume printing, (ⅳ) shifting from single material to multi-materials AM, (ⅴ) advancing from the multiscale direction of macroscopic–mesoscopic–microscopic structures, (ⅵ) integrating material preparation with forming integration, (ⅶ) expanding from small batch to large batch.

Laser-based powder bed fusion (L-PBF) is an additive manufacturing (AM) technology that uses a high-energy–density laser to scan through a powder bed and completely melts the metal powder. The environment inside the printer chamber, including the flow field of the shielding gas and the spatter particles induced by laser–powder interactions, is essential for product quality. For the first time, this work built a full-size model of printer chamber, and numerically investigated the interaction between the shielding argon flow and the laser induced spatter particles with considering laser temperature. A full-size geometric model of a commercial L-PBF printer with a Gaussian heat source was constructed, as well as a movable particle-release source model for particle injection. The distribution of the argon flow field, the temperature field, and the trajectory and deposition of spatter particles, particularly above the workbench during the L-PBF process were quantitatively analyzed. The results show that the gas flow within 30 mm above the workbench is uniform, and in the upper region of the printer chamber, the flow field is disorderly. The laser can only induce high temperature and upward gas flow in a small region close to the workbench (the height less than 1.6 mm), and the laser induced velocity disturbance in rest regions of the L-PBF printer is negligible. Particles injected towards the outlet (ID4) are mostly blown into the outlet, and in the other four injection directions, more than 90% of spatter particles are deposited inside the printer chamber, especially more than 50% deposited on the workbench. Increasing the laser power (from 100 to 200 W) has little effect on particle deposition on the workbench. Results will be helpful for improving the L-PBF product quality.

High relative density (>98%) Al-15Si alloy samples were fabricated by selective laser melting (SLM) with laser re-melting process. The micro-hardness and wear resistance of Al-15Si alloy samples were investigated in this study. The addition of 5 wt% TiC improved the micro-hardness of SLM-produced Al-15Si alloy, which had a remarkable micro-hardness of 172.8 Hv. After the heat treatment (annealing or quenching), the hardness of all tested samples decreased significantly, while the Al-15Si/TiC composites with the laser re-melting kept the higher hardness values (145 Hv). This sample presented the lower coefficient of friction (0.42) and wear rate (2.75 × 10-5mm3N-1m-1) than other conditions. Based on the results, the addition of 5 wt% TiC with laser re-melting in SLM process, together with annealing treatment afterwards, was a feasible approach to obtain a high micro-hardness and good wear resistance of Al-15Si alloys.

Bimetallic structures combining steel and copper offer the advantage of merging robust mechanical strength of steel with exceptional thermal conductivity of copper, making them valuable in applications such as cooling channels, heat-exchange equipment, and aviation industries. Nonetheless, creating a high-strength bimetallic interface proves challenging due to the inherent disparities in their physical and chemical properties. In this investigation, we present a novel approach to tailor the microstructure of laser powder bed fused steel-copper bimetallic structures, aiming to enhance their interface strength significantly. The resulting steel-copper bimetallic structure exhibited remarkable interface strength, boasting an ultimate tensile strength of 619 MPa. This enhanced strength can be attributed to the presence of a heterogeneous interlayer structure proximate to the interface. This heterogeneity manifests as a gradient microstructure consisting of two columnar crystal outer layers and an equiaxed crystal interlayer. This microstructural diversity engenders a substantial back stress during plastic deformation, thus yielding a synergistic strengthening effect. Furthermore, these heterogeneous structures exert control over the fracture behavior of the samples, shifting it from cleavage fracture to ductile fracture. Consequently, this study introduces an innovative approach for optimizing and fine-tuning the microstructure of interfaces comprising multi-materials.

A comparative study on the influence of different manufacturing methods (selective laser melting and hot rolling) on the microstructure, mechanical and thermal behaviours of tungsten (W) was presented for the first time. The results indicated that the selective laser melting (SLM) W exhibited a finer grain sizes, a lower strength ductility, hardness and thermal conductivity compared to hot-rolled W. The main reason for this result was that the laser underwent rapid heating and cooling when it was used to melt W powder with high energy density, resulting in large internal stress in the sample after manufacturing. Subsequently, the internal stress was released, leading to the generation of micro-cracks at the grain boundaries, thereby affecting the performance of SLM W samples. In addition, the higher fraction of high-angle grain boundaries (HAGBs) of SLM W was found to be the key factor for intrinsic brittleness. Because the HAGBs are the preferred crack paths, which could promote crack propagation and decrease fracture energy.

Alumina-based ceramic molds are extensively applied in investment casting processes of hollow turbine blade due to their outstanding high-temperature chemical stability and creep resistance. Herein, laser powder bed fusion (L-PBF) method was utilized to fabricate an alumina-based ceramic mold with integral core and shell (CMCS). The influences of the component contents in the composite powder on fluidity and packing density were comprehensivley studied, and the optimized formulation for the composite powder was consequently determined as 88 wt% mixed spherical alumina (coarse: fine = 9:1), 12 wt% epoxy resin E12, and 0.1 wt% fumed silica. The vacuum infiltration process was adopted to improve mechanical properties of alumina ceramic after the sintering, where the solid loading of ceramic slurry was investigated. The CMCS was divided into two parts for fabrication, i.e., the core&shell and the base with a spiral grain selector, which could facilitate convenient of cleaning residual powder due to enlarged openings. After the first infiltration and pre-sintering, the separated two parts were bonded by a ceramic binder and then underwent second infiltration and final-sintering to obtain a complete CMCS. The alumina-based ceramic fabricated through L-PBF and post-treatment possessed low sintering shrinkage (1.51–2.03%), adequate apparent porosity (30.82 ± 0.01%), and room-temperature strength (13.03 ± 2.90 MPa for unbonded specimen, 9.72 ± 1.34 MPa for bonded specimen). Strikingly, the subsequent casting experiment has proved that the method proposed in this paper is promising to fabricate superior alumina-based CMCSs.

This research investigated the influence of laser energy density on the phase transformation characteristics and structure, together with the mechanical property of NiTi shape memory alloys fabricated by laser powder bed fusion (LPBF). Results showed that randomly oriented needle-shaped martensite was distributed in the B2 austenite matrix in samples with the higher energy density. In contrast, typical columnar grains were observed throughout multiple melt pools, showing obvious epitaxial growth characteristics in the samples with lower energy density. Besides, the martensite transformation temperature varied approximately linearly with the increasing laser energy density, showing a positive correlation. It can be explained that more intense evaporation of Ni element in the melt pool under higher energy density leads to a reduction in the Ni content of the matrix and contributes to a reduction in the lattice distortion of the B2 matrix. Finally, the samples with the energy density of 138.89 J/mm3showed optimal tensile and compression strength of 648 and 2970 MPa, respectively. This work provides a novel approach to optimize the performance of LPBF NiTi sample by tailoring the phase composition and structure through the laser energy density.

The self-healing property of biological tissues gives humans inspiration to endow artificial synthetic materials with self-healing functions. Herein, we report a thermoplastic elastomer that can be self-repaired in a short time, which uses dual dynamic bonds (hydrogen bonds and disulfide bonds) coupled to realize its self-healing function through the rapid conversion of dynamic hydrogen bonds between the hard and soft segments, as well as the dynamic bonding and dissociation of disulfide bonds. Furthermore, the maximum tensile strength and strain of the elastomer can reach 17.4 MPa and 3780%, respectively, and the maximum self-healing rate is close to 100% (temperature: 90 °C, time: 5 h). The arrays’ arrangement and reversibility of hydrogen bonds endow the elastomer with high toughness and self-healing properties and thus solve the problem that ordinary polymers cannot self-heal.

Graded lattice structures (GLSs) have drawn much attention in engineering and biological areas due to the enhanced mechanical properties and energy absorption capacity that benefit from the graded design. This study aims to investigate the effect of gradient on fatigue behaviors of lattice structures under the cyclic loading lateral to the gradient direction. The Gyroid-type triply periodic minimal surface was utilized to design GLS with continuously varied volume fraction (VF) from 10% to 40%. Uniaxial compressive testing and compression-compression fatigue testing were conducted on the Ti-6Al-4V Gyroid lattice structures with both uniform and graded VFs fabricated by laser powder bed fusion. Three typical fracture modes and mixed fracture modes were all observed on the fracture surfaces of struts after fatigue testing. The fatigue life of Gyroid GLSs is 1.21–1.67 times that of uniform counterparts with an identical overall VF. The enhancement mechanism on fatigue properties coming from the gradient design is elaborated through the finite element analysis and experimental characterization. The lower level of tensile stress, bigger macro-area and micro-plastic zone on struts of the main loading-bearing constituent of Gyroid GLS relieve the stress concentration around the crack tip, thus, lower the crack propagation rate and provide stronger fatigue crack resistance. Furthermore, the struts in the main loading-bearing constituent are more prone to stretching, therefore, provide long-term carrying capacity even after the occurrence of penetrating cracks. Experiment results show that the carrying capacity can be still provided by these struts even with penetrating cracks.

Recent reports on the selective laser melting (SLM) process under a vacuum or low ambient pressure have shown fewer defects and better surface quality of the as-printed products. Although the physical process of SLM in a vacuum has been investigated by high-speed imaging, the underlying mechanisms governing the heat transfer and molten flow are still not well understood. Herein, we first developed a mesoscopic model of SLM under variable ambient pressure based on our recent laser-welding studies. We simulated the transport phenomena of SLM 316L stainless steel powders under atmospheric and 100 Pa ambient pressure. For typical process parameters (laser power: 200 W; scanning speed: 2 m∙s−1; powder diameter: 27 μm), the average surface temperature of the cavity approached 2800 K under atmospheric pressure, while it came close to 2300 K under 100 Pa pressure. More vigorous fluid flow (average speed: 4 m∙s−1) was observed under 100 Pa ambient pressure, because the pressure difference between the evaporation-induced surface pressure and the ambient pressure was relatively larger and drives the flow under lower pressure. It was also shown that there are periodical ripple flows (period: 14 μs) affecting the surface roughness of the as-printed track. Moreover, the molten flow was shown to be laminar because the Reynolds number is less than 400 and is far below the critical value of turbulence; thus, the viscous dissipation is significant. It was demonstrated that under a vacuum or lower ambient pressure, the ripple flow can be dissipated more easily by the viscous effect because the trajectory length of the ripple is longer; thus, the surface quality of the tracks is improved. To summarize, our model elucidates the physical mechanisms of the interesting transport phenomena that have been observed in independent experimental studies of the SLM process under variable ambient pressure, which could be a powerful tool for optimizing the SLM process in the future.

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