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Curvy imagers that can adjust their shape are of use in imaging applications that require low optical aberration and tunable focusing power. Existing curvy imagers are either flexible but not compatible with tunable focal surfaces, or stretchable but with low resolution and pixel fill factors. Here, we show that curvy and shape-adaptive imagers with high pixel fill factors can be created by transferring an array of ultrathin silicon optoelectronic pixels with a kirigami design onto curvy surfaces using conformal additive stamp printing. An imager with a 32 × 32-pixel array exhibits a fill factor, before stretching, of 78% and can maintain its electrical performance under 30% biaxial strain. We also develop an adaptive imager that can achieve focused views of objects at different distances by combining a concave-shaped imager printed on a magnetic rubber composite with a tunable lens. Adaptive optical focus is achieved by tuning both the focal length of the lens and the curvature of the imager, allowing far and near objects to be imaged with low aberration. Curvy and shape-adaptive imagers with high pixel fill factors and tunable focusing power can be created by transferring an array of ultrathin silicon optoelectronic pixels with a kirigami design onto curvy surfaces using conformal additive stamp printing.

Successful regeneration of large segmental bone defects remains a major challenge in clinical orthopedics, thus it is of important significance to fabricate a suitable alternative material to stimulate bone regeneration. Due to their excellent biocompatibility, sufficient mechanical strength, and similar structure and composition of natural bone, the mineralized collagen scaffolds (MCSs) have been increasingly used as bone substitutes via tissue engineering approaches. Herein, we thoroughly summarize the state of the art of MCSs as tissue-engineered scaffolds for acceleration of bone repair, including their fabrication methods, critical factors for osteogenesis regulation, current opportunities and challenges in the future. First, the current fabrication methods for MCSs, mainly including direct mineral composite, in-situ mineralization and 3D printing techniques, have been proposed to improve their biomimetic physical structures in this review. Meanwhile, three aspects of physical (mechanics and morphology), biological (cells and growth factors) and chemical (composition and cross-linking) cues are described as the critical factors for regulating the osteogenic feature of MCSs. Finally, the opportunities and challenges associated with MCSs as bone tissue-engineered scaffolds are also discussed to point out the future directions for building the next generation of MCSs that should be endowed with satisfactorily mimetic structures and appropriately biological characters for bone regeneration., Graphical abstract Image 1, Highlights • The recent progresses for building MCSs are recommended in detail. • Various regulation approaches are presented for enhancing the osteogenesis of MCSs. • The opportunities for MCSs as bone tissue-engineered scaffolds are discussed.

In this work, 7075 Al alloys were ultrasonically soldered by pure Zn solder in air under different acoustic intensities. The dependence of the microstructure and mechanical properties of joints on acoustic intensity was revealed. Acoustic intensity is innovatively controlled by applying different lap widths without increasing the input ultrasonic power. The acoustic pressure inside the solder was simulated by finite element simulation, and results showed that the acoustic intensity increased by more than six times when the lap width decreased by 50 %. High acoustic intensity led to intense cavitation, thereby causing a pronounced erosion of substrate. The Al content inside the joint increased from 17.3%–24.5% when the clearance width decreased from 0.8 mm to 0.4 mm. The Clausius–Clapeyron equation helped explain that the melting point of Zn increased by approximately 73.8 °C under the high acoustic pressure caused by cavitation bubble collapse, which could cause the direct solidification of Zn and served as the main reason of grain refinement. Zn grains were refined from 6.9 μm to 4.2 μm when soldered at 0.8 mm and further to 3.5 μm when soldered at 0.4 mm because of the strong cavitation caused by high acoustic intensity. Acoustic intensity had a remarkable effect on the mechanical properties of joints. Joint shear strength and hardness increased from 117.3 MPa and 93.3 HV to 145.1 MPa and 109.4 HV, respectively, when the lap clearance width decreased from 0.8 to 0.4 mm. Particularly, the joint soldered at 0.4 mm failed through the substrate because of the high strength of the joint seam and the weakness of the substrate, which were explained through the Hall–Patch relationship, the Schmid factor, the dissolution of secondary phases, and the increase in high angle boundaries. This work provides a novel method by producing strong joints with fine grains without increasing input power.

Quantitative assessment of soft tissue elasticity is crucial to a broad range of applications, such as biomechanical modeling, physiological monitoring, and tissue diseases diagnosing. However, the modulus measurement of soft tissues, particularly in vivo, has proved challenging since the instrument has to reach the site of soft tissue and be able to measure in a very short time. Here, we present a simple method to measure the elastic modulus of soft tissues on site by exploiting buckling of a long slender bar to quantify the applied force and a spherical indentation to extract the elastic modulus. The method is realized by developing a portable pen-sized instrument (EPen: Elastic modulus pen). The measurement accuracies are verified by independent modulus measures using commercial nanoindenter. Quantitative measurements of the elastic modulus of mouse pancreas, healthy and cancerous, surgically exposed but attached to the body further confirm the potential clinical utility of the EPen.

The research on the ceramic/metal composites with interpenetrating continuous lamellar structure has attracted much interest in structural materials field. In this work, we proposed an innovative approach to fabricate co-continuous ZrB2–SiC/Zn–5Al composites by combining freeze casting with ultrasonic-assisted spontaneous infiltration. The ZrB2–SiC porous ceramic with orderly lamellar channels (30 vol%) was fabricated by freeze casting using ammonium polyacrylate as the dispersant. The effects of ultrasonication duration on the microstructure and mechanical properties of the composites were investigated in detail. Results indicated that the ultrasonication improved the wettability and facilitated the spontaneous infiltration. The composites obtained the excellent comprehensive properties when the duration was 300 s, which can be attributed to not only the sufficient infiltration of lamellar channels but also the longer infiltration distance inside the ceramic walls induced by the ultrasonic capillary and cavitation.

Two sound joints of Al–Cu bimetallic sheet with 6.5 mm in thickness were obtained in Cu (upper)–Al (lower) and Al (upper)–Cu (lower) welding configurations via the electron beam welding and their distinct coupling models were developed to well understand their flow behavior of the molten pool and microstructure evolution mechanisms during the electron beam welding. For the electron beam welding of the Cu–Al bimetallic sheet, liquid Al and Cu flowed into each other and fully combined in the fusion zone and the interface zone under high-temperature environment, forming large amount of intermetallic compounds. For the electron beam welding of the Al–Cu bimetallic sheet, liquid Al was floating on the Cu substrate, while liquid Cu was flowing at the bottom of the molten pool and mutually combined with liquid Al at the interface, forming a relatively small amount of intermetallic compounds. Moreover, a part of liquid Al flowed back to the seam root under the influence of the Marangoni effect. Therefore, the fusion zone and the interface zone with various phase compositions were formed in these two Al–Cu bimetallic joints and the mechanical properties of the corresponding joints were determined. Furthermore, the average tensile strengths of Cu–Al and Al–Cu bimetallic joints were 59 MPa and 67 MPa, respectively. The fracture locations of these two joints were both at the edge of the fusion zone along the Al2Cu intermetallic compound interlayer. Moreover, the fracture characteristics of these joints were mainly cleavage fracture.

In the modelling of thin-sheet piezoelectric actuators, the bonding condition between the actuator and the host structure can place a significant influence on the behaviour of the actuator. This paper provides a comprehensive theoretical study of the electromechanical behaviour of a thin-sheet piezoelectric actuator bonded to a host structure through a partially debonded adhesive layer under in-plane electric loading. The focus is on the coupled effect of the debonding and adhesive layer on the interfacial stress. The piezoelectric actuator is characterized by a general electro-elastic Euler–Bernoulli beam model featuring both axial and bending deformation. The theoretical solution of the problem is formulated by using singular integral equations, in terms of the interfacial shear and normal stresses, which are solved by using Chebyshev polynomials. The introduction of the adhesive layer in the model changes the singular behaviour of the problem. The current singular integral equations are shown to be effective in solving for both singular and non-singular stresses. Typical examples have been provided to show the effect of the geometry and the material property on the local stress field along the interface.

This study aimed to propose a novel method for ex-situ biogas upgrading by adding ammonium chloride to increase the concentration of ammonia nitrogen. The results showed that ammonia nitrogen had a significant effect on biogas upgrading. The maximum content of CH4 reached 94.1% when the concentration of ammonia nitrogen was 5500 mg/L. At the same time, CH4 yield of increased by 57.5%. High throughput sequencing results showed that the relative abundance of hydrogenotrophic methanogens reached 73.1%, while that of acetotrophic methanogens was only 1.3%, which greatly increased the content and yield of CH4. For the bacterial community, Clostridium was the dominant bacteria and the ammonia nitrogen concentration had little effect on it. These results demonstrate that upgrading biogas by increasing the concentration of ammonia nitrogen is feasible.

In this work, a novel friction stir diffusion bonding (FSDB) method was proposed to eliminate hook when joining thin sheets. Three tools, with several convex-vortex pins were innovatively designed to improve the diffusion bonding effect of the lap interface. Results showed that sound joints, excellently bonded at the lap interface and without hook, were obtained. The lap interfaces on the joints welded using the T-tool and F-tool almost remained intact, thereby indicating that they were formed by diffusion bonding. The material flow behavior was also simulated by FLUENT software and the results showed that the material flow along thickness was significantly enhanced by increasing the pin number. The joint welded by the T-tool showed shear fracture mode. Sound bonding was formed at the lap interface when using the F-tool and thus the joint showed tensile fracture.

AZ31B Mg alloys were joined with a pure Zn interlayer via ultrasonic-assisted transient liquid phase bonding. The formation and evolution of intermetallic compounds (IMCs) and the mechanical properties of the bonded joints were investigated. Results showed that joints could form within an extra short time of 0.1 s via eutectic reaction, eutectoid reaction and diffusion bonding. Mg-Zn eutectoid structures, IMCs of MgZn and MgZn2 formed inside the bonded joint. The width of the MgZn2 phase decreased with prolonged ultrasonication time, high ultrasonication power, and sonotrode pressure. The entire joint was characterized by eutectoid structures without MgZn2 at a high sonotrode pressure of 0.4 MPa. MgZn2 had a higher hardness and modulus than MgZn, Mg-Zn eutectoid structure, and Mg substrate. Joints fractured through the MgZn2 phase and eutectoid structure had shear strength values of 30 and 40 MPa, respectively.

This study was aimed to increase both the methane content and yield from rice straw by adding extra H2 into a two-stage anaerobic digestion system. The results showed that the efficiency of H2 utilization was 36.43%. Of the total amount of H2 used, 47.3% was converted to CH4, thereby increasing the CH4 content and yield by 45% and 101%, respectively. This conversion was performed at a H2/CO2 ratio of 4/1 and a recycling frequency of two times a day in an up-flow reactor (UR). Due to the different ways of CH4 production in continuously stirred tank reactor (CSTR) and UR, ethanol and acetate were produced as intermediaries, accounting for 65% and 87% of total intermediaries, respectively. Moreover, the pH values in CSTR and UR were 6.7 and 8.1, respectively. The results of the microbial community structures showed that the archaea genera of Methanosaeta and Methanobacterium were the most dominant in CSTR, whereas, Methanosaeta was the most dominant in UR. In terms of the bacterial community, Clostridia was the most abundant class in UR. Based on the results mentioned above, it is feasible to simultaneously improve both the CH4 content and yield from rice straw by adding extra H2 into the two-stage anaerobic digestion system.

This study investigates the optical polarization characteristics of low-earth-orbit (LEO) space targets at different operating attitudes for different surface materials by analyzing these characteristics using a microfacet analysis model. Subsequently, the polarization image of a typical LEO space target was simulated, and several laboratory and outfield polarization detection experiments were conducted. Simulated and experimental results validate the effectiveness of polarization in analyzing and evaluating the operating attitudes of the LEO space targets and in identifying their material properties. Results show that the polarization images can effectively determine the constituent structure of the LEO space targets.

Biomedical Ti-Fe-Zr-Y alloys were prepared by 3D printing on pure titanium substrate. The influences of Zr on mechanical, forming, and biological properties of the alloys were investigated in detail. The results showed that with increasing the Zr addition, the surface roughness, friction coefficient and worn volume decrease at first and then increase, the lowest values obtained at 5.86 at.% Zr addition. The ultimate compression stress and specific strength gradually decrease. The studied alloys have no cytotoxicity. They can promote the early adhesion and proliferation of cells. The eutectic alloy with 5.86 at.% Zr addition has the best ability of apatite deposition, it exhibits a better comprehensive performance among the studied alloys, which is superior to the Ti70.5Fe29.5 and Ti-6Al-4 V alloys.

Collagen is the critical organic component of bone matrix, which is the template for bone biomineralization. Phosphorylation modification of collagen plays an important role in the process of biomineralization in vivo, but its mechanism on in vitro biomimetic mineralization of bone matrix remains unclear at the molecular level. Sodium tripolyphosphate is used to phosphorylate collagen in this study and new phosphate groups appear on collagen fibrils after phosphorylation modification. The chelating amount of calcium is improved linearly with increasing the phosphorylation degree of collagen fibrils, which demonstrates that the introduced phosphate groups serve as new nucleation sites and participate in the formation of apatite minerals inside the collagen fibers. Stabilized nanosized amorphous calcium phosphate by polyacrylic acid can also permeate into collagen fibers and further transform into another layer of hydroxyapatite minerals. Both layers of apatite minerals eventually induce the formation of hierarchical intrafibrillar mineralization structure within the phosphorylated collagen fibers. The present research enriches the previous biomineralization mechanism of bone matrix, provides a facile strategy for biomimetic mineralization of collagen, and offers the basis for future investigation of the advanced bone substitute materials.

A Fourier telescope (FT) is an imaging system based on laser illumination and optical aperture synthesis and is suitable for imaging distant targets with high resolution through the atmosphere. The imaging time of a conventional FT is so long (about two hours) as to limit its practical applications. In order to shorten the imaging time of a Fourier telescope, we propose a new imaging system called the compressed sensing Fourier telescope (CS-FT). The image quality, the imaging time and the effects of noise of the new system are studied in detail. Based on the analysis, we find that by reasonably choosing the total sampling rate (TSR), the imaging time of the CS-FT is obviously shorter than that of the conventional FT while the image quality of the CS-FT is near that of the conventional FT. Worth noting is that with the smaller values of the low-frequency sampling rate (LSR) and the order of the sampling probability density function (OPDF), the CS-FT can achieve a better image quality. The reconstruction results of the field data show that the CS-FT can reduce the imaging time of a FT in an actual noise environment.

An ultrasonic-induced capillary action method was used to join porous ceramics for the first time. Results indicated that under ultrasonication, the solder infiltrated the porous ceramics and formed a soldered joint. The solder infiltration distance increased with prolonged ultrasonication time and reached 42 μm at an ultrasonication time of 120 s. Ceramic particles were eroded off and flowed into the joint seam because of the intense cavitation within narrow clearances. A fully penetrated layer formed near the joint seam at 120 s. This joint seam had high strength, so cracks propagated throughout the ceramic substrate during shear tests. Overall, our method was suitable for joining porous and microporous materials.

Engineering living systems is a rapidly emerging discipline where the functional biohybrid robotics (or “Bio-bots”) are built by integrating of living cells with engineered scaffolds. Inspired by embryonic heart, we presented earlier the first example of a biohybrid valveless pump-bot, an impedance pump, capable of transporting fluids powered by engineered living muscle tissues. The pump consists of a soft tube attached to rigid boundaries at the ends, and a muscle ring that squeezes the tube cyclically at an off-center location. Cyclic contraction results in a net flow through the tube. We observed that muscle force occasionally buckles the tube in a random fashion, i.e., similar muscles do not buckle the tube consistently. In order to explain this anomaly, here we develop an analytical model to predict the deformation and stability of circular elastic tubes subjected to a uniform squeezing force due to a muscle ring (like a taught rubber band). The prediction from the model is validated by comparing with experiments and finite element analysis. The nonlinear model reveals that the circular elastic tube cannot buckle irrespective of muscle force. Buckling state can be reached and sustained by bending and folding the tube before applying the muscle ring. This imperfection may appear during assembly of the pump or from nonuniform thickness of the muscle ring. This study provides design guides for developing advanced biohybrid impedance pumps for diverse applications.

Metamaterials are man-made materials engineered to possess certain desired and often counterintuitive properties. It is well-known that elastic metamaterials may exhibit unusual bulk elastic properties when subject to dynamic loads at certain frequencies, such as negative Poisson’s ratio, negative modulus, and/or negative mass density. This paper focuses on the modeling of a one-dimensional micropolar-type elastic metamaterial subject to transient dynamic loading. The metamaterial consists of unit cells that support both translational and rotational motion and can be modeled as a micropolar-type continuum. Interestingly, the equations governing the dynamics of the new micropolar-type continuum possess remarkable similitude with those governing the transient response of an elastic bar with elastic supports. The resulting governing equation is solved using the Fourier transform technique. The transient dynamic response of the metamaterial subject to an axial impact is then studied based on both the developed continuum model and the original discrete model. Finite element analysis of the equivalent bar model is also conducted. The results from the developed continuum model is compared with the discrete model and the finite element analysis to evaluate its suitability. This work presents a novel micropolar-type model for a specific metamaterial and investigates its transient response using both continuum modeling and discrete unit cell modeling. The results indicate that the micropolar-type model can accurately capture the transient behavior of the propagation of the strain pulse in the metamaterial

A nano-resolution PDMS sensor simultaneously measures cell force and matrix stiffening dynamics in 3D., Cells in vivo generate mechanical traction on the surrounding 3D extracellular matrix (ECM) and neighboring cells. Such traction and biochemical cues may remodel the matrix, e.g., increase stiffness, which, in turn, influences cell functions and forces. This dynamic reciprocity mediates development and tumorigenesis. Currently, there is no method available to directly quantify single-cell forces and matrix remodeling in 3D. Here, we introduce a method to fulfill this long-standing need. We developed a high-resolution microfabricated sensor that hosts a 3D cell-ECM tissue formed by self-assembly. This sensor measures cell forces and tissue stiffness and can apply mechanical stimulation to the tissue. We measured single and multicellular force dynamics of fibroblasts (3T3), human colon (FET) and lung (A549) cancer cells, and cancer-associated fibroblasts (CAF05) with 1-nN resolution. Single cells show notable force fluctuations in 3D. FET/CAF coculture system, mimicking cancer tumor microenvironment, increased tissue stiffness by three times within 24 hours.

Highlights • Thin liquid metal layer produces stronger cavitation with more uniform bubble size and distribution than bulk liquid. • Cavitation characteristics under different parameters are correlated with the acoustic pressure simulation results. • High acoustic pressure and strong cavitation can still be obtained at the weakly vibrated region. • High bubble growth speed of 16.8 m/s is obtained and evidenced by bubble dynamics calculation., Cavitation in thin layer of liquid metal has potential applications in chemical reaction, soldering, extraction, and therapeutic equipment. In this work, the cavitation characteristics and acoustic pressure of a thin liquid Ga–In alloy were studied by high speed photography, numerical simulation, and bubble dynamics calculation. A self-made ultrasonic system with a TC4 sonotrode, was operated at a frequency of 20 kHz and a max output power of 1000 W during the cavitation recording experiment. The pressure field characteristic inside the thin liquid layer and its influence on the intensity, types, dimensions, and life cycles of cavitation bubbles and on the cavitation evolution process against experimental parameters were systematically studied. The results showed that acoustic pressure inside the thin liquid layer presented alternating positive and negative characteristics within 1 acoustic period (T). Cavitation bubbles nucleated and grew during the negative-pressure stage and shrank and collapsed during the positive-pressure stage. A high bubble growth speed of 16.8 m/s was obtained and evidenced by bubble dynamics calculation. The maximum absolute pressure was obtained at the bottom of the thin liquid layer and resulted in the strongest cavitation. Cavitation was divided into violent and weak stages. The violent cavitation stage lasted several hundreds of acoustic periods and had higher bubble intensity than the weak cavitation stage. Cavitation cloud preferentially appeared during the violent cavitation stage and had a life of several acoustic periods. Tiny cavitation bubbles with life cycles shorter than 1 T dominated the cavitation field. High cavitation intensities were observed at high ultrasonication power and when Q235B alloy was used because such conditions lead to high amplitudes on the substrate and further high acoustic pressure inside the liquid.

Topological invariance has recently attracted rapidly growing attention, which can be characterized in a one-dimensional (1D) system by the Zak phase, a special kind of Berry phase. While acoustic systems are proven to be excellent platforms for exemplifying the diversity of topological properties, it remains challenging to downscale the device, since such interface states are usually attained at higher-frequency regimes. Here, we report both theoretically and experimentally the realization of interface states in the low-frequency band gap of a 1D chain of Helmholtz resonators. By numerically inspecting the Zak phase of the bulk band of the proposed system, we reveal the transition point of the band structure in a low-frequency range and elucidate the underlying mechanism of interface states at the lowest band gap in the spectrum. As a result, the lattice constant is reduced to the subwavelength scale, instead of being comparable to the wavelength. Our proposed mechanism is verified experimentally by measuring acoustic intensity for the interface state. The experimental results agree quite well with the theoretical predictions, showing the existence of interface states at the desired frequencies. We anticipate our mechanism to open up the possibility for the miniaturization and integration of acoustic functional devices and have far-reaching implications in diverse applications, such as enhanced sensing and biomedical imaging.

In this study, a new elastic metamaterial featuring multiple local resonators has been developed with negative effective mass and/or negative effective modulus. An illustrative example is presented to show the corresponding influence of the multiple local resonances. The current method opens a novel route to design elastic metamaterials by fully exploring the effect of multiple local resonances on the double negative behaviour featuring negative phase velocity, as well as the single negative behaviour featuring wave attenuation.

Significance Engineered living systems is a rapidly emerging field where functional biohybrid machines are built by integration of cells or tissues with engineered scaffolds for countless technological applications. Here, we present a biohybrid pumping machine or “pump-bot,” capable of generating unidirectional flow powered by engineered skeletal muscle tissue. It utilizes a bioinspired mechanism for valveless pumping and can achieve flow rates that are at least three orders of magnitude higher than previously reported biohybrid pumps. The proposed design offers several merits, including flexibility of material choices, ease of fabrication, robustness, and ability to operate without the need for valves, blades, or other moving parts. This biological pumping system can have broad utility and impact in tissue engineering, microfluidics, and biomedical devices., Pumps are critical life-sustaining components for all animals. At the earliest stages of life, the tubular embryonic heart works as a valveless pump capable of generating unidirectional blood flow. Inspired by this elementary pump, we developed an example of a biohybrid valveless pump-bot powered by engineered skeletal muscle. Our pump-bot consists of a soft hydrogel tube connected at both ends to a stiffer polydimethylsiloxane (PDMS) scaffold, creating an impedance mismatch. A contractile muscle ring wraps around the hydrogel tube at an off-center location, squeezing the tube with or without buckling it locally. Cyclic muscle contractions, spontaneous or electrically stimulated, further squeeze the tube, resulting in elastic waves that propagate along the soft tube and get reflected back at the soft/stiff tube boundaries. Asymmetric placement of muscle ring results in a time delay between the wave arrivals, thus establishing a net unidirectional fluid flow irrespective of whether the tube is buckled or not. Flow rates of up to 22.5 μL/min are achieved by the present pump-bot, which are at least three orders of magnitude higher than those from cardiomyocyte-powered valve pumps of similar size. Owning to its simple geometry, robustness, ease of fabrication, and high pumping performance, our pump-bot is particularly well-suited for a wide range of biomedical applications in microfluidics, drug delivery, biomedical devices, cardiovascular pumping system, and more.

An ultra-rapid transient liquid phase bonding method was successfully used to join AZ31B Mg alloys by using pure Zn interlayers in air. The bonding time was shortened to 1 s with ultrasonic assistance. Eutectic structures, MgZn, and MgZn2 formed at relatively low temperatures of 360 °C and 370 °C. Only eutectic structures were observed at a high temperature of 380 °C because the entire joint was filled with eutectic liquid phase at this moment. MgZn2 had higher hardness and modulus than those of MgZn. The shear strength of joint was inversely proportional to its width. The joint fractured through MgZn2 at 360 °C and 370 °C, and through eutectoid structures at 380 °C. Keywords: Ultrasonic, Transient liquid phase, Intermetallic compound, Shear strength, Diffusion

A new two-dimensional periodic elastic metamaterial system is proposed, which exhibits multiple local resonances and can generate negative effective parameters. In certain frequency ranges, negative bulk modulus, negative shear modulus and negative mass density can be achieved simultaneously. Analytical study of the new metamaterial system is performed based on a simplified model to study the effects of main material and geometric parameters. Numerical analysis is further conducted to simulate elastic wave propagation in the current metamaterial. The results show clearly the existence of negative dispersion. The association of negative mass and modulus with the mode of local resonance, the eigenstate, is evaluated. In the frequency range for negative parameters, negative phase velocities of the longitudinal and shear waves can be observed in the simulation. The developed metamaterial shows strong anisotropic properties. It can support both longitudinal and shear waves in general directions but in specific directions only longitudinal wave can propagate. In the latter case, it exhibits fluid-like properties. Typical examples of the current metamaterial under different loading conditions are presented to show both the modal response and the property of elastic wave propagation in the metamaterial.

In this work, pure Sn was used to wet Zr50.7Cu28Ni9Al12.3 bulk metallic glasses (BMGs) assisted by ultrasonic treatment. Without ultrasonic treatment, pure Sn showed a non-wetting condition to BMG. Ultrasonic vibration facilitated the wetting of Sn to BMG. Prior to ultrasonication for 30 s, only physical adsorption formed at the Sn/BMG interface. Increasing ultrasonic time led to the alteration of the bond at the Sn/BMG interface from point contact to local surface contact, and to diffusion layer. Two bonding modes of order–order and order–disorder were discovered at the Sn/BMG interface. Cu content was higher than the other elements near the bonding interface. Longer diffusion distances of Sn into the BMG were obtained at high ultrasonic power, high temperature and large dip depth.

In this work, an ultrasonic-assisted semi-solid brazing method was proposed to join Al/Mg. Oxide films on substrate surfaces can be successfully removed even when filler metal was at semi-solid state. A large quantity of intermetallic compounds (IMCs) formed at 460 °C because of large amount of Al and Mg dissolved into joint caused by strong cavitation. Decreasing the temperature to 430 °C could significantly reduce IMC. However, weak cavitation happened inside low liquid phase fraction at 420 °C, making it difficult to break IMCs and solid phases.

To prevent the formation of Al/Mg intermetallic compounds (IMCs) of Al3Mg2 and Al12Mg17, dissimilar Al/Mg were ultrasonic-assisted soldered using Sn-based filler metals. A new IMC of Mg2Sn formed in the soldered joints during this process and it was prone to crack at large thickness. The thickness of Mg2Sn was reduced to 22 μm at 285 °C when using Sn-3Cu as the filler metal. Cracks were still observed inside the blocky Mg2Sn. The thickness of Mg2Sn was significantly reduced when using Sn-9Zn as the filler metal. A 17 μm Mg2Sn layer without crack was obtained at a temperature of 200 °C, ultrasonic power of Mode I, and ultrasonic time of 2 s. The shear strengths of the joints using Sn-9Zn was much higher than those using Sn-3Cu because of the thinner Mg2Sn layer in the former joints. Sn whiskers were prevented by using Sn-9Zn. A cavitation model during ultrasonic assisted soldering was proposed.

An ultrasonic-assisted semi-solid brazing method was used to control Mg/Al intermetallic compounds (IMCs). Zn–20.95Al, which has a large semi-solid temperature range of 60°C, was chosen as ...

The formation of Al/Mg intermetallic compounds (IMCs) of Al3Mg2 and Al12Mg17 was successfully avoided through ultrasonic assisted transient liquid phase bonding Mg/Al using pure Sn interlayers. A new IMC of Mg2Sn formed in the joints. Different bonding parameters were used to control the formation of Mg2Sn. The optimum joint shear strength of 60.0 MPa was obtained at the temperature of 220 °C, ultrasonic power of Mode I and ultrasonic time of 4 s. The thickness of Mg2Sn should be depressed to less than 15% of the joint width. Joints were fractured through the Mg2Sn layer.

In this work, refill friction stir spot welding (RFSSW) was used to weld 2 mm-thick 5083-O alloy. The Box–Behnken experimental design was used to investigate the effect of welding parameters on the joint lap shear property. Results showed that a surface indentation of 0.3 mm effectively eliminated the welding defects. Microhardness of the stir zone (SZ) was higher than that of the base material (BM) and the hardness decreased with increasing the heat input during welding. The optimum failure load of 7.72 kN was obtained when using rotating speed of 2300 rpm, plunge depth of 2.4 mm and refilling time of 3.5 s. Three fracture modes were obtained during the lap shear test and all were affected by the hook defect.

Drilling and filling stages were successfully performed to produce a regular volume defect and then repair this fabricated defect. The microhardness distribution of the repaired joint was flat, and an excellent diffusion bonding occurred at the interface between the filler and the base material (BM) under the optimum combinations of a rotational velocity of 1600 rpm, dwell time of 20 s and plunge depth of 0.4 mm. A high-quality joint with the maximum tensile strength of 217 MPa was achieved. The fracture position of this joint was located at the thermo-mechanically affected zone rather than the interface between the filler and the BM of the joint with a low tensile strength.

In order to solve the problem of the short lifespan of the neural electrode caused by micro motion, this study designed a novel neural electrode based on lumped compliance compliant mechanism to control different modes of micro-motion in a more effective way. According to the mathematical modeling of the novel neural electrode, the equivalent bending stiffness and equivalent tensile (compression) stiffness were calculated. The results of the finite element analysis based on the mathematical modeling revealed that the novel neural electrode showed excellent micro-motion-attenuation capability. The static analysis results showed that the novel design dramatically reduced the maximum displacement of the brain in 51% and the maximum stress in 41% under longitudinal micro-motion environment. It also effectively reduced the 5.1% maximum stress while maintaining the maximum displacement under lateral micro-motion environment. The experimental results based on the tissue injury evaluation system also confirmed that the novel electrode is more effective in micro-motion attenuation than the reference one. In detail, the strain of the brain tissue caused by the implantation of the neural electrode was decreased by 1.26 to 27.84% at the insertion depth of 3 mm, and 0.522 to 17.24% at the insertion depth of 4.5 mm, which has convinced the effectiveness of the design. Graphical abstract The schematic of the novel neural electrode and evaluationsystem of tissue injury.

To obtain high joint strength, friction stir lap welding (FSLW) was used to join 6061-T6 aluminum alloy and Ti-6Al-4V alloy using low rotating speeds. The effect of tool rotating speed on the microstructure and lap shear failure loads of the joints was mainly discussed. Results showed that voids were easily observed at the lap interface when small penetration was used. The void was eliminated by decreasing the rotating speed. The hook acted as a “lock” during the lap shear test. At the inner side of hook, metallurgical bonding was formed due to the formation of intermetallic compounds. The joint failed through Al alloy and showed higher lap shear failure loads than other joints when the rotating speed of 300 rpm was used.

Continuous intermetallic compounds (IMCs) at the nugget zone (NZ) of dissimilar friction stir welded Al/Mg joint easily become crack initiation and propagation path, deteriorating mechanical properties. To reduce or eliminate the disadvantages induced by the continuous IMCs, ultrasonic-assisted friction stir welding (UaFSW) based on stationary shoulder system was employed to join 6061-T6 aluminum alloy and AZ31B magnesium alloy. Defect-free joint without the shoulder marks was obtained under the synergistic effect of the stationary shoulder and the ultrasonic. Vibration and acoustic streaming induced by the ultrasonic broke the continuous IMCs layer near the thermo-mechanically affected zone (TMAZ) of advancing side (AS) into pieces or particles. Facture path from the short Al/Mg interface near the TMAZ at the AS of the conventional joint was changed to the long Al/Mg interface at the retreating side (RS) of the UaFSW joint, improving tensile properties. Maximum values of tensile strength and elongation of the UaFSW joint were 152.4 MPa and 1.9%, which were 17 MPa and 0.8% higher than those of the conventional joint, respectively.

In order to enhance the weldable thickness and tensile properties of dissimilar ultra-thin Al/Mg alloy sheets, ultrasonic-assisted friction stir welding (UaFSW) was performed. The addition of ultra...

Using magnesium alloy as upper sheet, 3 mm-thick AZ31 magnesium alloy and 6061 aluminum alloy were joined using friction stir lap welding assisted by stationary shoulder. The effects of tool rotating speed on cross-sections, microstructure and mechanical properties of Mg/Al lap joints were mainly discussed. Results showed that stationary shoulder contributed to joint formation, by which stir zones (SZ) were characterized by big onion rings after welding. Because of the big forging force exerted by stationary shoulder, the upper region of hook was well bonded. SZ showed much higher hardness because of intermetallic compounds (IMCs). The bonding conditions at the base material (BM)/SZ interface at advancing side and the hook region played important roles on joint lap shear properties. The X-ray diffraction pattern analysis revealed that the main IMCs were Al3Mg2 and Al12Mg17.

This study focused on the effects of thread on hook and cold lap formation, lap shear property and impact toughness of alclad 2024-T4 friction stir lap welding (FSLW) joints. Except the traditional threaded pin tool (TR-tool), three new tools with different thread locations and orientations were designed. Results showed that thread significantly affected hook, cold lap morphologies and lap shear properties. The tool with tip-threaded pin (T-tool) fabricated joint with flat hook and cold lap, which resulted in shear fracture mode. The tools with bottom-threaded pin (B-tool) eliminated the hook. The tool with reverse-threaded pin (R-tool) widened the stir zone width. When using configuration A, the joints fabricated by the three new tools showed higher failure loads than the joint fabricated by the TR-tool. The joint using the T-tool owned the optimum impact toughness. This study demonstrated the significance of thread during FSLW and provided a reference to optimize tool geometry.

Refill friction stir spot welding (RFSSW) is a new spot welding technology, by which spot joint without keyhole can be obtained. In this work, RFSSW was used to join 2-mm-thick 2198-T8 aluminum alloy sheets and effects of the sleeve plunge depth on microstructure and lap shear properties of the joints were mainly discussed. Results showed that when using small plunge depths of 2.4 and 2.6 mm, joints showed good formation and no defects were observed. Incomplete refilling defect was observed with increasing plunge depth due to material loss during welding. Size of the grains at sleeve-affected zone (SAZ) is smaller than that at the pin-affected zone, and the size becomes bigger with increasing the plunge depth. More secondary phase particles can be observed at SAZ with increasing the sleeve plunge depth. The lap shear failure load firstly increased and then decreased with increasing the sleeve plunge depth. The maximum failure load of 9819 N was attained with plug fracture mode when using 2.6 mm. Fracture morphologies show ductile fracture mode.

Friction stir welding (FSW) was used to join Ti–6Al–4V alloy in air and under intense cooling conditions. The results show that the application of liquid nitrogen is beneficial in decreasing the pe...

Both simple interface joining and continuous intermetallic compounds (IMCs) layer formed at the interface of Al/Mg alloys during friction stir welding (FSW) deteriorates mechanical property. Based on stationary shoulder, ultrasonic was employed to assist FSW of 6061-T6 and AZ31B alloys to increase interface joining length and mixture degree of Al/Mg alloys at nugget zone (NZ). Sound joint with smooth surface is obtained, resulting from the combined effect of stationary shoulder and ultrasonic. Ultrasonic are beneficial to increasing material flow, interface joining length and mixture degree of Al/Mg alloys, improving mechanical interlocking. The tensile strength and elongation of joint with ultrasonic reach 120 MPa and 1.5%, which are higher than that without ultrasonic. Additionally, local fracture surface morphology consists of some dimples, presenting mixed fracture containing brittle and ductile fractures.

Defect-free joint of Ti–6Al–4V alloy was successfully friction stir welded below β-transus temperature and then tensile tests were performed. Microstructure, macrostructure, tensile properties and fracture position are mainly discussed in order to investigate how surface indentation and plate thickness influence the tensile property. Weld zone (WZ) attained below β-transus temperature that owns better tensile strength limit than base metal (BM). During the tensile test, the elongation is decided by whether BM yields. Compared with friction stir welding joint using 2.5 mm thick plate, it is very difficult for joint using 2 mm thick plates to get bigger elongation due to surface indentation. Due to the higher tensile property of the WZ, the joint without surface indentation fractures at BM, reaching the 58.46 % elongation of BM.

A three-dimensional finite volume model was established by ANSYS FLUENT software to simulate material flow behavior of the refill friction stir spot welding (RFSSW) process. The RFSSW experiment was performed to validate the rationality of the simulation results. Simulation results show that the maximum velocity appears at the sleeve outer wall. The velocity becomes smaller as the increase of the distance to the tool walls. The material flow behaviors are verified by the cross section and microstructure of the RFSSW joint. Low flow velocity in the joint center leads to alclad concentration, which easily results in shear fracture. The sleeve plunge depth must be bigger than the upper sheet thickness in order to obtain joint with large bonding area. Both increasing the rotational speed and refilling speed accelerate the material flow while increasing the rotational speed is a more effective method.

In this work, friction spot welding (FSpW) was used to weld dissimilar 6063/5083 aluminium alloys, and the effects of the sleeve plunging speed on joint microstructures and the lap shear properties were discussed. Results showed that the keyhole can be eliminated using FSpW. Bending hooks changed to a flat morphology when the plunging speed was increased. Excellent bonding was formed at the lap interface in the sleeve-affected zone. As the sleeve plunging speed was increased from 40 to 110 mm min−1, the lap shear failure loads for the joints first increased and then decreased. A maximum failure load of 8017 N was attained when using a sleeve plunging speed of 80 mm min−1. All joints fractured at the lap interface.

Periodic fluid shear stress (FSS) is one of the main mechanical microenvironments in mineralization of bone matrix. To elucidate the mechanism of periodic FSS in collagen mineralization, a mechanical loading induced mineralization system is developed and compared with traditional polyacrylic acid (PAA) induced mineralization. Fourier transform infrared (FTIR) spectroscopy, calcium-to-phosphorus molar ratio and transmission electron microscopy (TEM) demonstrate that both periodic FSS and PAA can control the size of amorphous calcium phosphate (ACP) to avoid aggregation and help the formation of intrafibrillar mineralization. Differently, periodic FSS under a proper cycle and range can accelerate the conversion of ACP to apatite crystals and alleviate the reduced transformation caused by PAA. Under the action of template analogues, periodic FSS can also promote the formation of highly oriented hierarchical intrafibrillar mineralized (HIM) collagen. These findings are helpful for understanding the mechanism of collagen mineralization in natural bone matrix and contribute to the design of novel bone substitute materials with hierarchical structures.

In this work, grain refinement of the aluminum soldered joint was obtained by applying cavitation within narrow channels and the possible grain refinement mechanism was proposed. Aluminum sheets with different channel widths were ultrasonically soldered by pure Sn in air. An ultrasonic system with a TC4 sonotrode, was operated at a frequency of 20 kHz and power of 1000 W during soldering. The effect of channel width on grain size, element distribution and strength of the soldered joint was studied. Results showed that the grain size decreased from 2.62 to 1.04 µm and the element ratio of Al in solder increased from 0.93 to 4.86% when the channel width decreased from 0.8 to 0.2 mm. Instant solidification of Sn grains was readily observed in the joint before cooling due to the large undercooling induced by the intensified cavitation inside the narrow channels. The random cavitation induced nucleation of Sn was believed to be mainly responsible for the grain refinement of the soldered joint. The shear strength of the joint increased from 29.5 to 48.8 MPa and the hardness increased from 16.5 to 25.2 HV due to the grain refinement of Sn and the presence of Al transferred from the substrates.

Using alclad 2024-T4 aluminum alloy as the research object, stationary shoulder technology was used in friction stir lap welding process to investigate its performance in this study. Joint features and mechanical properties of the lap joints were mainly investigated. Results show that lap joint with smooth surface, without shoulder marks and inner defects can be obtained using the stationary shoulder technology. With increasing the welding speed from 40 to 130 mm/min, effective sheet thickness (EST) at the advancing side (AS) shows rather stable values (from 1.17 to 1.31 mm), EST at the retreating side (RS) increases from 0.57 to 1.13 mm, and stir zone width decreases from 4.95 to 4.44 mm. Lap shear failure load of the SSFSLW joints firstly increases and then decreases with increasing the welding speed. Using 100 mm/min, the maximum failure loads of 15.85 and 9.01 kN were obtained when the RS and AS of the joint bear the main load during the lap shear test. Shear fracture mode and tensile fracture mode can be obtained during the lap shear test. All joints present ductile fracture mode.

To eliminate the tearing defect in friction stir welding (FSW) Ti-6Al-4V alloy joint, back heating assisted FSW (BHAFSW) method was put forward in this study. Using different back heating temperatures, joint formation and microstructure were mainly discussed. Results show that the temperature gradient along thickness plays a more important part than the welding peak temperature affecting the tearing defect. With increasing the back heating temperature, size of the tearing defect gradually decreases due to reduced temperature gradient. When using 480 °C, defect-free BHAFSW joint with fully lamellar structure along thickness and higher hardness can be obtained. Moreover, the BHAFSW can effectively reduce tool wear during welding.

For the purpose of reducing the hook defect in friction stir lap welding joint, Ti-6Al-4V alloy was lap welded by slightly penetrating into the lower sheet. Hook feature, microstructure and mechanical properties of the lap joints were mainly discussed. Results show that using slight penetration, plastic material mainly concentrates above the lap interface, which is beneficial to suppress the primary hook and broaden the stir zone (SZ) width. Simultaneously, a very small secondary hook is formed. The void-like defect, which is formed due to high peak temperature and big temperature gradient along thickness, can be eliminated by decreasing the rotating speed. Microstructures along thickness show much difference due to big temperature gradient. SZ hardness of both the upper and lower sheets is higher than the base material due to finer grains. Bigger lap shear failure load can be obtained using 150 rpm, which is 17.1 KN. Lap joints fracture along the secondary hook and present ductile fracture mode.

To widen the parameter range for friction stir welding (FSW) of Ti–6Al–4V alloy, back heating assisted FSW (BHAFSW) technology was proposed in the present study. Both the conventional FSW and BHAFSW processes were used to weld 2.5 mm thick Ti–6Al–4V alloy to investigate their performances. Results show that during conventional FSW, tearing defects always appear at the joint bottom and it can only be eliminated when using 100 rpm. The back heating method reduces the temperature gradient along thickness, which is beneficial to eliminate the tearing defect and therefore defect–free joints can be attained using wider parameter range. During BHAFSW process using 200 rpm, temperature of the whole stir zone (SZ) is above β–transus temperature and the SZ is characterized by lamellar α + β microstructures. Tool wear is reduced during the BHAFSW process. Maximum tensile strength of the BHAFSW joint reaches 958.45 MPa, which is 93.6% of the base material (BM). Keywords: Back heating assisted friction stir welding, Ti–6Al–4V alloy, Tearing defect, Microstructure, Tensile strength