High Strength Research Articles

Study on cementitious grout for scour protection of offshore wind turbine foundations

To study scour-resistant applicability, 6–10% mass ratio of polyurethane (PU) was added to sulphoaluminate cement (SAC) and ordinary Portland cement (OPC) grouts to generate OPCPU and SACPU. Sodium silicate (SS) with slurry volume ratio of 1:1–5:1 was added to SAC and OPC grouts to generate OPCSS and SACSS. The water-cement ratio (w/c) of grout was 0.8–1.5. Fresh-state properties and mechanical properties were investigated. Hydrated minerals and microstructures were analyzed, scouring tests were conducted considering key formulations. The results indicated that SACSS and OPCSS have a shorter setting time and higher strength, which makes them suitable as scour resistance materials. At the w/c of 1.0, OPCSS with a volume ratio of 3:1 and SACPU with 6% PU were selected for scour resistance tests. It showed that maximum scour depths and sediment transports were 7.0–12.2 and 1.1–1.2% as those without reinforcing conditions. The critical shear stress on the seabed under reinforcement is ∼103 times greater than the bed shear stress. This inhibits the occurrence of scouring. The study evaluated the applicability of cementitious reinforcement for scour resistance. This study analyzed material and mechanical properties, hydrated minerals, and microstructures, and conducted scour tests to optimize grouting material ratios for seabed scour protection, providing references for soil reinforcement and grouting protection.

The Learning Curves of Adelaide- and Gan-Modified Lim-Tsai Flexor Tendon Repair Techniques

Surgical performance that improves with experience is often depicted as representing a "learning curve." Although numerous studies examine the tensile properties of various flexor tendon repairs, few compare the associated learning curves. This study aims to address this gap by comparing the learning curves of Adelaide- and Gan-modified Lim-Tsai repairs. Emphasizing the difference in learning curves is crucial because it highlights the tension between achieving biomechanically superior repairs, which may be challenging to many surgeons, and opting for possibly incrementally less strong but more feasible techniques. We organized a workshop attended by 20 medical students whose experience in surgery was limited to a few suturing exercises. Each participant repaired five porcine tendons in situ either with Adelaide- or Gan-modified Lim-Tsai, followed by a peripheral suture. We tested all tendons with linear static testing to measure ultimate and yield loads. In addition, repair times were recorded for each repair. We used a linear mixed model to compare learning between the techniques. Ultimate loads increased with experience and were higher in Adelaide technique during the first two repairs, compared with Gan-modified Lim-Tsai (80 N vs 63 N and 79 N vs 66 N, respectively). Yield loads also increased with experience but did not differ between the repair techniques at any time point. Mean repair times decreased from 44 to 28 minutes and from 46 to 25 minutes with Adelaide- and Gan-modified Lim-Tsai repairs, respectively. The Adelaide core suture had a higher initial ultimate load capacity despite fewer suture strands, possibly indicating better tension consistency. The ultimate load of the Gan-modified Lim-Tsai repair increased between the first and fifth repair, and repeats were needed to achieve comparable results with the Adelaide repair. The results of this study suggest that both repair methods are suitable for novice surgeons, but Adelaide tends to result in higher strength from the first repair. Generalizability to other repairs should be made with caution.

Interfacial binding and heterogeneous nucleation mechanisms of HfC/HfO2 in HfO2/DZ125 composites: Insights from first-principles calculations and experiments

In this paper, the interfacial bonding strengths of the precipitated phase (carbides) and addition phase (HfO2) in the DZ125 nickel-based superalloy, as well as the nucleation potential of carbides with HfO2 as a heterogeneous substrate, were calculated by first-principles calculations. Results show that the mismatches of HfC(011)/HfO2(011) and HfC(111)/HfO2(011) satisfy the semi-coherent relationship.Adhesion work (Wad) and interfacial energy were computed for six interfacial structures with varying terminations and stacking sequences. Model 3 in HfC(111)/HfO2(011) exhibited the highest Wad (6.67 J/m2), with an interfacial spacing of 0.95 Å and interfacial energy ranging from −3.2–1.81 J/m2, indicating the strongest interfacial bonding strength. Electronic structure analysis confirmed that the strong bonding in Model 3 is due to the formation of a robust Hf-C covalent bond at the interface. Uniaxial tensile tests revealed that Model 3 has a broad strain range and high tensile strength, with the Hf-C bond maintaining its integrity without fracture. Model 3, the most stable structure, supports the adhesion and growth of HfC on HfO2. The experiment confirms that HfO2 can serve as a heterogeneous nucleation site for HfC and contribute to the refinement of HfC grains.

High mechanical strength, flame retardant, and waterproof silanized cellulose nanofiber composite foam for thermal insulation

High mechanical strength, flame retardant, and waterproof silanized cellulose nanofiber composite foam for thermal insulation

Sensory Discrimination Tests for Low- and High-Strength Alcohol

Research is limited on consumers’ ability to detect perceptible sensory differences between low- and high-strength alcoholic beverages. This study, therefore, conducted three pilot experiments using ISO sensory analysis methods to assess accuracy for evaluating beverages of different strengths. Participants were food production professionals trained in sensory analysis. Experiment 1 used a wide-range discrimination test to estimate low- to high-strength beverages (0–60% alcohol by volume (ABV) in 10% intervals). Experiment 2 included a narrower range of intermediate to high strengths (25–45% ABV in 5% intervals). Experiment 3 used 3-alternative forced choice tests (ISO 13301) to discriminate between beverages of varying strengths. Experiment 1 (n = 16) indicated that estimation ability was dependent upon the beverages’ ABV; as ABV increased, estimation significantly decreased (p < 0.005). These findings were not replicated in Experiment 2 (n = 13). In Experiment 3 (n = 17), a significant perceptible difference between high- and low-strength samples was observed in two of nine conditions (35% vs. 31% ABV (p = 0.009); 41% vs. 37% ABV (p = 0.037)). While people can detect large differences in beverage ABVs, they may have a moderate to poor ability to discriminate between beverages of similar strengths. These findings provide support for public health interventions that promote lower-strength alcoholic beverages.

Effect of Bamboo Nodes on the Mechanical Properties of Phyllostachys iridescens

Bamboo is a significant natural resource, recognized for its rapid growth, lightweight composition, high strength, and excellent mechanical properties, making it increasingly valuable in the furniture and construction industries. A critical structural aspect of bamboo is its nodes, yet there has been limited research on their impact on bamboo’s mechanical properties. This study investigates the mechanical properties of round bamboo tubes in three different states: internodes (S1), nodes with diaphragm removed (S2), and nodes with diaphragm (S3). The results show that the mechanical properties of S1 are a compressive strength (CS) of 29.72 MPa, a shear strength parallel to grain (SSp) of 11.82 MPa, a radial stiffness (Sr) of 155.59 MPa, an impact toughness (IT) of 20.74 kJ/m2, a modulus of rupture (MOR) of 16.45 MPa, a modulus of elasticity (MOE) of 408.53 MPa, a tensile modulus of rupture parallel to grain (MORT) of 189.62 MPa, and a tensile modulus of elasticity parallel to grain (MOET) of 431.05 MPa. Compared with S1, these above parameters change by CS +11%, SSp 6%, Sr +100%, IT −29%, MOR +5%, MOE +63%, MORT −29%, and MOET −58% in S2 and CS +10%, SSp 28%, Sr +250%, IT −31%, MOR +28%, MOE +92%, MORT −25%, and MOET −42% in S3. It demonstrates that the bamboo diaphragm and nodes significantly influence the mechanical properties of bamboo; they have a significant positive effect on the bending properties across the transverse grain, radial ring stiffness, and shear properties along the grain, but negatively impact the tensile properties along the grain.

Finite element study on the mechanism of the influence of pretension force and block strength on the shear resistance of the anchor cable with C-shaped tube

To prevent the early breakage of anchor cables under shear loads in support engineering, a combined structure of Anchor Cable with C-shaped Tube (ACC) has been proposed. The shear resistance enhancement mechanism of this structure and the mechanisms of various influencing factors have yet to be fully revealed. A refined nonlinear finite element model of ACC was original established using ABAQUS software, taking into account the actual structure of the steel strands and the interactions, such as contact and failure between the various components. Various anchor cable pretension forces and block strengths were set to investigate their effects on the shear mechanical response of ACC. The results successfully demonstrated a high correlation between peak shear load and pretension force. The results demonstrate that an increase in pretension force reduces the ACC’s peak shear load and break displacement. Additionally, the structure exhibited higher flexural stiffness, the block strength was mobilized earlier, and the block failed locally more quickly. Under high pretension forces, the system exhibited higher shear stiffness in the early stages of shearing due to the influence of the axial force component. With low pretension forces, the ACC exhibited a larger break displacement due to the minor tensile deformation at the shear plane position for the same shear displacement. At low pretension forces, the structure’s bending angle increased more rapidly during the middle and later stages of shearing, accompanied by a larger break displacement. Both of these factors led to a greater bending angle at the shear plane position at the point of failure. The results reveal the characteristic of the peak shear load initially increasing and then decreasing with the increase in test block strength, along with its underlying mechanism. As the block strength increased, the bending angle of the structure at the shear plane position increased more rapidly, resulting in higher shear stiffness. With high block strength, the combination of smaller break displacement and greater shear stiffness led to an initial increase followed by a decrease in peak shear load. A comprehensive RSSB (Relative Stiffness between Structure and Test Block) that considers both structural and test block stiffness was proposed. The deformation pattern of the structure was controlled by the RSSB. The higher the RSSB, the wider the plastic hinge extension range for the same shear displacement, the smaller the bending angle at the shear plane position, and the smaller the maximum curvature of the structure. The contact force of the C-shaped tube generally exhibited a “single peak” distribution. As the shear displacement increased, the peak position of the contact force moved away from the shear plane, and the maximum contact force increased rapidly and remained relatively stable. At the end of the shearing process, the contact force of the C-shaped tube exhibited a “double peak” distribution.

Fresh and Hardened Properties of Alkali-Activated POFA-GGBFS Pastes Cured in Ambient Temperature – An Initial Mix Design

Weak soils are characterised by their high compressibility and low shear strength, which requires stabilisation to improve their compaction and strength characteristics before being used for construction projects. Recent trends in soil stabilisation show an increased interest in the application of waste-derived geopolymers as low-carbon materials to address the carbon emission problem related to cement production. This paper presents the preliminary findings of using Palm Oil Fuel Ash (POFA) and Ground Granulated Blast Furnace Slag (GGBFS) binary blends as geopolymer precursor materials. These binary waste material blends were activated using alkaline solutions, namely Sodium Hydroxide (NH) and Sodium Silicate (NS), to synthesise geopolymers at ambient temperatures. Three main factors were considered for the optimisation of the geopolymer mix design, as recommended by past research: alkali equivalent (8-11.5%), activator modulus (0-0.7), and slag replacement (fixed at 30%). Alkali equivalent and activator modulus parameters were varied and tested to establish its initial and final setting times, workability and strength properties, while the slag replacement percentage was set at 30%. This study aimed to determine the most influential parameters to produce the geopolymer material with the highest compressive strength and satisfactory setting times and workability. Single-source alkali activators (NH only) used on POFA-GGBFS produced specimens with smaller flow diameters, longer initial and final setting times, and lower compressive strength than specimens activated with NH and NS. Geopolymer specimens synthesised with NH and NS produce stiffer compounds possessing higher compressive strength values and explosive-type failure modes due to their higher silica content. At ambient temperatures, TM-Mix 2 (7% alkali equivalent and 0.7 activator modulus) produced the highest compressive strength value of 67.3 MPa at 28 days curing, flow diameter of 227 mm, with the initial and final setting time of 25 minutes and 45 minutes, respectively. Consequently, the results show that the preliminary POFA-GGBFS geopolymer mix design could produce sustainably sourced geopolymer compounds with adequate strength and acceptable setting time and workability. This study is part of a current investigation to further optimise the POFA-GGBFS mix design for the purpose of weak soil stabilisation.

Elasticity and plasticity of PLA, PETG, ABS polymers for printing automotive parts

The objective of this paper is to explore the elasticity and plasticity properties of PLA, PETG and ABS polymers in the context of 3D printing of automotive parts. The present research is qualitative, descriptive, focusing on the theoretical analysis of the mechanical properties of PLA, PETG and ABS polymers, which is based on the collection of information from different authors. An analysis of the properties of these polymers is very significant for their application in the automotive industry, allowing a correct selection of materials. The research found very important data, highlighting that PLA, with its high stiffness and low plasticity, is ideal for prototypes and low-stress parts, while PETG offers a balance between flexibility and strength, suitable for functional components that require durability. ABS, known for its high impact strength and ductility, is recommended for applications that demand shock absorption and durability under demanding conditions. Selecting the right material is crucial to optimize the performance and longevity of 3D printed automotive parts, contributing to the advancement of additive manufacturing technology in the industry.

Effects of microstructure and phosphorus segregation on tensile properties of friction stir welded high phosphorus weathering steel

Defect-free sound joints of high-phosphorus (0.094 wt%) weathering steel (SPA-H) were successfully fabricated using friction stir welding (FSW). This study examines the microstructural evolution and mechanical properties of joints produced at different FSW parameters. Tensile tests of the joints revealed that fractures occurred in the base metal (BM) region, indicating almost 100 % welding efficiency. Furthermore, the stir zones (SZ) demonstrated a marked improvement in tensile properties. Particularly, at the rotation rate of 80 rpm and axial load of 45 kN condition (below A1), the microstructure featured ultra-fine ferrite and cementite, resulting in high hardness (270 HV) and tensile strength (704 MPa) in steel with just 0.08 wt% C, while maintaining nearly 80 % of the total elongation of BM. However, the SZ at 80 rpm exhibited an unusual decrease in local elongation despite the fine ferrite grain size and cementite presence. Electron probe microanalysis (EPMA) and nano-hardness revealed pronounced phosphorus segregation in the ferrite region and significant localized hardness disparities induced by the segregation behavior. The strain concentration at the interfaces between these regions during the tensile process leads to crack initiation and rapid propagation. The negative factor caused by the phosphorus segregation accelerates the failure of the specimen during the necking stage and ultimately shows decreased local elongation.

Refinement of microstructure and improvement of mechanical properties of directed energy deposited titanium alloys by adding Fe

Elemental alloying is a validated strategy for enhancing the mechanical properties of directed energy deposited titanium alloys. However, it is still a challenge to obtain titanium alloys with both high strength and high ductility. In this work, to balance strength and ductility, the alloying element Fe was in-situ added to the TC4 melt pool during the directed energy deposited process and the effects of its content on the microstructure and mechanical performance of the TC4 alloy deposits were investigated. The results showed that the Fe additive ranging from 0.5 to 2.0wt.% were completely solid-solved in the retained β phase of TC4 alloys. Moreover, the primary β columnar grains were transformed into fully equiaxed grains. Meanwhile, the presence of Fe refined the α-laths, increased the proportion of the retained β phase, and the continuous grain boundary α (GB-α) became discontinuous. The tensile strength of the deposited alloys was continually improved with increasing Fe content, while the elongation had a peak value. When the Fe content was 1.0wt.%, the deposited alloy exhibited the best comprehensive performance, with an ultimate tensile strength of 1071MPa and an elongation of 9.3%. The enhancement of tensile strength was attributed to the solid solution strengthening and the refinement of α-laths and primary β grains. The increased proportion of retained β phase and the formation of discontinuous GB-α were beneficial for the alloy ductility. However, increased α/β interfaces inhibited dislocation movements, resulting in a decrease in ductility. The development of this alloy is important for aerospace and other fields that require high strength, high ductility and lightweight.

Effect of heat treatment on microstructure and mechanical anisotropy of direct energy deposited Ti-6.0Al-2.5Nb-2.2Zr-1.2Mo alloy

Ti-6.0Al-2.5Nb-2.2Zr-1.2Mo alloy (Ti-6321) exhibits excellent corrosion resistance and impact toughness, which can be used as a deep-sea pressure-resistant hull. However, the alloy tends to have high strength but low plasticity, poor impact toughness, and anisotropic mechanical properties by fabricating by direct energy deposition (DED) technology. In order to improve the impact toughness of DEDed Ti-6321 alloy while reducing its anisotropy of mechanical properties, this study manipulates the microstructure and mechanical properties of the alloy through heat treatment. The research finds that with the increase of the single-stage annealing temperature within the (α+β) two-phase region, the width of the α phase increases, the Ultimate Tensile Strength (UTS) remains essentially unchanged, the Yield Strength (YS) decreases, and the impact absorption energy significantly improves. For double annealing heat treatment at two-phase region, temperature increase at the first-stage annealing enlarges the width and decreases the aspect ratio of the primary α phase. Leading to the decrease of the UTS & YS and increase of the impact absorption energy, with a conversely tendency at 1000°C. The increase in the second-stage annealing temperature raises the size and volume fraction of the secondary α phase, causing the alloy's UTS and YS to initially rise then fall, with a slight increase in impact absorption energy. The anisotropy in the mechanical properties of DEDed Ti-6321 alloy mainly addressed from the different distribution of α phase orientations at different loading directions, and can be eliminated through sufficient β single-phase zone annealing.

Study on the mechanism of improvement of mechanical properties of Al-Mg-Si aluminum alloys by Cu and Ce

First, the stability and tensile properties of the Al/Mg2Si interface doped with alloying elements in the Al-Mg-Si system of aluminum alloys are calculated from first principles. Then, the two experimental alloys with different components were heat treated and the properties were characterized. Calculations show that when the interface is doped with Cu, the tensile stress that the interface can withstand increases to 3.13 GPa; when the interface is doped with Cu and Ce, the tensile strain that the interface can withstand increases to 20% and the tensile stress that the interface can withstand increases to 4.17 GPa; when the interface is doped with Cu and Sc, the tensile strain that the interface can withstand increases to more than 25% and the tensile stress that the interface can withstand increases to 5.08 GPa or more. Characterization of the organization and properties of the experimental alloys showed that Ce not only increased the peak solid solution temperature of the alloys, but also increased the tensile and yield strengths of the alloys in the extruded state by 16% and 13.74%, respectively. During hot extrusion, Ce was able to promote the precipitation of precipitated phases. During aging, the addition of Ce promoted the aging precipitation of the alloy, which not only increased the number of precipitated phases in the alloy, but also refined the precipitated phases and enhanced the aging strengthening effect of the alloy. This study provides technical guidance for the development of new aluminum alloy materials with high strength and low cost.

Effect of laser remelting on microstructure evolution and high-temperature fretting wear resistance in a laser-cladding self-lubricating composite coating on titanium alloy substrate

TC21 titanium alloy possesses the advantage including high strength, low density, and great resistance to damage. Combining it with surface modification technology, it is expected to improve the performance of aviation structural components. This work applied laser remelting technology to realize surface modification, and the hardness and fretting wear properties of the coating were investigated. This research has found that laser remelting technology can effectively refine grain size and improve the performance of coatings. By exploring different laser remelting powers (600 W, 900 W, 1200 W) and the different diameters of laser beam (3 mm, 6 mm), the optimal laser process was obtained with laser power and spot diameter of 600 W and 6 mm, respectively. The average grain size is 2.1 μm. The laser remelting coating has an average microhardness about 980 HV0.2, and it is 2.5 times and 1.3 times higher than the substrate and laser cladding hardness, respectively. A minimum coefficient of friction (COF) about 0.5 was obtained by the laser remelting coating, and the wear rate reaches the lowest of 1.87 × 10−6 mm3/(N·m) at fretting temperature with 500 °C. The abrasive wear and adhesive wear were the main wear mechanisms, and accompanied by oxidative wear. This study provides important reference for improving the performance of high-strength titanium alloy to obtain low defect, high-performance coatings with stable quality.

Microscopic impacts of ultrasonic vibration on interfacial strength of SiCp/Al composites

SiCp/Al composites are used in aerospace and deep-space exploration equipment because of their extremely high strength and thrust-to-weight ratios; however, the differences in the properties of the reinforcement and matrix materials in this type of composites have restricted their applications. The ultrahigh-frequency vibration characteristics of ultrasonic vibration processing technology can effectively solve the above bottlenecks, but the effect of high-frequency vibration on the interfacial properties of SiCp/Al composites is still unclear. The effects of ultrasonic vibration on the interface strength of composites were analyzed from a microscale perspective by means of single particle push-out Molecular Dynamics (MD) simulations and tests under different conditions. The results show that the interface strength is negatively correlated with particle size but positively correlated with ultrasonic amplitude, with a maximum increase of about 51% relative to no ultrasound. Brittle-plastic transition was observed on the surface of particles with high interface strength, and lateral microcracks due to stress concentration were present on the surface of particles with low interface strength. Higher strains and grain refinement were obtained for larger amplitudes, and stacking faults and tangle dislocations appeared on the side of the interface layer close to the Al matrix. The results provide potential insights to improve the micromechanical and mechanical properties of SiCp/Al composites, enhance the longevity of the materials, and realize the sustainable use of resources by expanding the efficient, precise, and clean machining of such materials.

Achieving high strength and ductility of Al-Zn-Mg-Cu alloys via laser shock peening and spray forming

In order to further enhance the strength and ductility of ultra-high strength aluminum alloys, the laser shock peening technology was applied to ultra-high strength Al-Zn-Mg-Cu alloy. The ultimate tensile strength, elongation and hardness can reach to 751 MPa, 11 % and 208.3 HV by combining spray forming, secondary extrusion, solid solution, retrogression and reaging as well as laser shock peening. The high strength and hardness of the alloys is mainly attributed to the fine-grained layer on surface as well as new grain boundaries, dislocation cells and high-density dislocations introduced by laser shock peening, uniform nano-sized strengthening phases with high density precipitated during heat treatments. The excellent ductility of the alloys is mainly ascribed to multiple structures including fine-grained layer on surface and slip lines inside different grains introduced by laser shock peening, smaller size of fibrous grains and Al7Cu2Fe phase produced by secondary extrusion and spray forming. The aging treatment after laser shock peening can lead to the annihilation of high-density dislocations as well as significantly promote the formation of stable and coarse η phase, which can greatly reduce the strength of the studied alloys.

Design of non-heat-treatable Al-Fe-Ni alloys with high electrical conductivity and high strength via CALPHAD approach

Electrical conductivity and strength are two critical properties concerned for rotors materials in electric vehicles. However, the two properties are often mutually exclusive in cast Al alloys. The present work proposes a novel non-heat-treatable Al-Fe-Ni alloy by considering the intermetallic compound’s content and morphology. With the modification of Ni and Fe content, the solidification path of designed alloy were calculated to predict the phase composition, which would greatly impact their performance. The as-cast Al-2Fe-1Ni (wt.%) alloy exhibits comprehensive properties with an ultimate tensile strength of 131.2±2.5MPa, an elongation of 20.2±3.6% and an electrical conductivity up to 52.0±0.1%IACS. The eutectic microstructure of Al13(Fe,Ni)4+α-Al in Al-2Fe-1Ni alloy accounts for over 90%, much larger than the eutectic content in Al-0.4Fe-3.4Ni and Al-1Fe-1Ni alloys. The fine eutectics in Al-2Fe-1Ni alloy improve the second-phase strengthening effects and plastic deformation capability. In addition, the fibrous rod-like intermetallic reduces the electron scattering during the propagation process and leaves more space for free electron movement. The dominant second phase transforms from Al3Ni phase to Al9FeNi phase, further enhancing the thermal stability of the alloy. After aging treatment at 180 °C for 100h, it shows no noticeable change in electrical conductivity and mechanical properties. It is hoped that this work can provide a new design approach for high-strength and high-conductivity Al alloys.

Dynamic compressive behavior of functionally graded triply periodic minimal surface meta-structures

Triply periodic minimal surface (TPMS) lattice structures have gained considerable attention because of their light weight, high strength, and excellent energy absorption capabilities. However, the effect of the amplitude that can control the topological morphology of a TPMS on the dynamic properties of the TPMS structure is not yet fully understood, as previous studies have focused on the relative density and size as well as their quasi-static mechanical properties. In this study, three types of uniform sheet-based TPMS structures with different amplitudes and three types of functionally graded sheet-based TPMS structures were proposed. Experiments and numerical simulations were conducted under quasi-static and dynamic loading conditions. Six types of TPMS lattice structures made of 316L stainless steel were manufactured via powder bed fusion. Quasi-static compression tests were performed at a strain rate of 0.001 s⁻¹. The experimental results indicate that increasing the amplitude can increase the elastic modulus, plateau stress, and energy absorption capacity of a structure. Moreover, the functional gradient amplitude structure has a higher energy absorption capacity, and the structures with line and log gradient strategies improved by 17.38% and 35.43%, respectively, compared to the uniform structure with an amplitude of 1. Additionally, an idealized rigid-linear plastic hardening (R-LPH) model was proposed to predict the mechanical response of the structures. The finite element method (FEM) was used to construct dynamic compression numerical models, and their validity was verified through split Hopkinson pressure bar (SHPB) tests at a strain rate of 695 s⁻¹. The mechanical response, deformation modes, and stress enhancement effects of the structures under dynamic compression were systematically studied. The results show that the mechanical performance and energy absorption capacity of the structures under dynamic impact loading increase with increasing strain rate. The critical velocity for the transition from the quasi-static mode to the impact mode increases with amplitude. For strain rates below 6000 s⁻¹, the strain rate effect is the main factor influencing the dynamic stress enhancement. As the strain rate continues to increase, the dynamic stress enhancement results from the combined effects of inertia and strain rate, with inertia effects gradually becoming the dominant factor. This study shows that functional gradient TPMS meta-structures have excellent mechanical and energy absorbing properties under quasi-static compression and dynamic compression, with potential applications in passive safety protection.

Development of a 7000 series aluminium alloy suitable for laser-based Additive Manufacturing

High strength aluminium alloys are of significant interest for laser-based Additive Manufacturing as they can provide a high specific strength but have limited assembly possibilities due to poor weldability and the presence of large intermetallic particles challenging the conventional manufacturing route. Increasing the geometrical freedom by enabling their use with laser-based Additive Manufacturing would create new possibilities. This study investigates the possibilities of using an aluminium 7000 series alloy with Laser Powder Bed Fusion (L-PBF) and secondly for comparison with Direct Energy Deposition (DED). The chemical composition targets 5.1 – 6.1 wt% Zn and 2.1 – 3.9 wt% Mg. The level of Zn and Mg in the powder was increased to 10 wt% Zn and 3 wt% Mg to compensate for the expected evaporation during the laser-based AM process and guarantee a sufficient Zn and Mg content for precipitation strengthening. Solidification cracking in the initial alloy composition was resolved by the addition of 1 wt% Zr and 1 wt% V. For L-PBF, a study of the impact of the laser spot size showed a larger spot size is beneficial to improve the process stability. A study of the capability of preheating and of slowing down the process showed both further improve the process stability as well which is required to manufacture successfully larger parts to characterise the quasi-static tensile properties. For optimal process parameters for a fast process, meaning a scan speed of 700 mm/s, a yield stress of 425 ± 7 MPa and an ultimate tensile strength of 442 ± 5 MPa was obtained after a homogenisation and ageing heat treatment. In the as built state, the strength was significantly lower. For optimal process parameters for a slow process, meaning a scan speed of 250 mm/s, a yield stress of 398 ± 11 MPa and an ultimate tensile strength of 440 ± 16 MPa were obtained in as built state. For DED the mechanical properties of the alloy showed to be significantly dependent on process parameters. An excessive energy input results in more Zn and Mg evaporation and decreased cooling rates. Such parameters also impact the process stability of DED throughout the build cycle. Limiting the energy input resulted in dense samples and stable build cycles which after a heat treatment to peak ageing reached a yield stress of around 400 MPa and an ultimate tensile stress of around 440 MPa in the X- and Z-direction.

A comparison of wire-feed additive manufacturing and hybrid additive/subtractive manufacturing of high nitrogen steel

High nitrogen austenitic stainless steel, referred to as high nitrogen steel (HNS), has been widely studied and applied because of its excellent properties such as high strength, good plasticity and toughness. However, its high nitrogen content makes the deposition metal face problems such as nitrogen porosity, nitrogen escape and loss when it is processed by additive manufacturing (AM), which has a negative impact on the mechanical properties. Two thin-walled specimens without obvious defects were obtained by wire-feed AM and hybrid additive/subtractive manufacturing (HASM) using the self-made HNS wire with 0.7 wt.% N, respectively. The porosity, microstructures and tensile properties of thin-walled specimens were compared. The porosity of the two specimens was tested by an industrial computed tomography (CT) system. The defect volume ratio and the diameter of the largest pore of HASM specimen were reduced about 54% and by 46.6% than those of wire-feed AM, respectively. The morphology of ferrite at different height of both specimens was different including skeletal, lathy and granular. The tensile strength of HASM specimen was better than that of wire-feed AM, which is mainly attributed to the decrease of porosity and grain size.