Non-equiatomic W10Mo27Cr21Ti22Al20 high-entropy alloy produced by mechanical alloying and spark plasma sintering: Phase evolution and mechanical properties

In this work, non-equiatomic W10Mo27Cr21Ti22Al20 refractory high-entropy alloy (RHEA) was produced using mechanical alloying followed by spark plasma sintering. The phase formation, microstructure, and compressive mechanical properties of the alloy were studied. During mechanical alloying, a Body-centered cubic (BCC) solid solution phase with a particle size of less than 1 µm was obtained after 18 h ball milling. The microstructure of the sintered sample exhibits three distinct phases consisting of two solid solution phases BCC1 and BCC2 as well as fine TiCxOy precipitates distributed in them. The volume fractions of each phase were about 79%, 8%, and 13%, respectively. The sintered W10Mo27Cr21Ti22Al20 showed yield strengths of 2465, 1506, 405, and 290 MPa at room temperature 600, 1000, and 1200°C, respectively, which are about twice that of the same refractory high-entropy alloy produced by vacuum arc melting. At 1000 and 1200°C, the strength after yielding gradually increased to 970 and 718 MPa at a compressive strain of 60%. The studied refractory high-entropy alloy can have good potential in high-temperature applications due to its high specific strength at elevated temperatures compared to conventional Ni-based superalloys and most as-reported refractory high-entropy alloys.

Mechanical characterization and creep strengthening of AZ91 magnesium alloy by addition of yttrium oxide nanoparticles

In the current research, the authors have attempted to improve the mechanical properties and creep behavior of the magnesium alloy Mg–9Al–1Zn (AZ91) in three different stress levels. To this end, the present study investigated experimentally the addition effects of different values of yttrium oxide nanoparticles to the AZ91. In this regard, weight percentages of 0.5%, 1%, 1.5%, and 2% nanoparticles were added to the material using the vortex casting method. Then, various test specimens were fabricated based on the ASTM standards by utilizing a Computer Numerical Control lathe machine. Different experiments were performed, and the results of different groups were compared with each other. The results revealed that the addition of yttrium oxide (Y2O3) nanoparticles increases the strength of AZ91 magnesium alloy until the nanoparticles do not clump in the microstructure. In other words, the tensile strength of the nanocomposite increased by adding nanoparticles up to 1.5%, but by adding 2% of nanoparticles, we found that the tensile strength is lower than that of pure magnesium. Moreover, one of the most important achievements of this study is that if the nanoparticles do not clump in the material microstructure, the addition of Y2O3 increases the rate of stable creep (the secondary creep stage). Also, the experimental results indicated that the highest stable creep rate is related to the nanocomposite with 1.5% yttrium oxide nanoparticles. Furthermore, the maximum hardness of the material was obtained in the same case.

Thermo-mechanical modelling of the Friction Stir Spot Welding process: Effect of the friction models on the heat generation mechanisms

Friction Stir Spot Welding involves complex physical phenomena, which are very difficult to probe experimentally. In this regard, the numerical simulation may play a key role to gain insight into this complex thermo-mechanical process. It is often used to mimic specific experimental conditions to forecast outputs that may be substantial to analyse and elucidate the mechanisms behind the Friction Stir Spot Welding process. This welding technique uses frictional heat generated by a rotating tool to join materials. The heat generation mechanisms are governed by a combination of sliding and sticking contact conditions. In the numerical simulation, these contact conditions are thoroughly dependent on the used friction model. Hence, a successful prediction of the process relies on the appropriate selection of the contact model and parameters. This work aims to identify the pros and cons of different friction models in modelling combined sliding-sticking conditions. A three-dimensional coupled thermo-mechanical FE model, based on a Coupled Eulerian-Lagrangian formulation, was developed. Different friction models are adopted to simulate the Friction Stir Spot Welding of the AA6082-T6 aluminium alloy. For these friction models, the temperature evolution, the heat generation, and the plastic deformation were analysed and compared with experimental results. It was realized that numerical analysis of Friction Stir Spot Welding can be effective and reliable as long as the interfacial friction characteristics are properly modelled. This approach may be used to guide the contact modelling strategy for the simulation of the Friction Stir Spot Welding process and its derivatives.

Increasing flexibility of self-piercing riveting by reducing tool–geometry combinations using cluster analysis in the application of multi-material design

A frequently used mechanical joining process that enables the joining of dissimilar materials is self-piercing riveting. Nevertheless, the increasing number of materials as well as material–thickness combinations leads to the need for a large number of rivet–die combinations as the rigid tool systems are not able to react to changing boundary conditions. Therefore, tool changes or system conversions are needed, resulting in longer process times and inflexibility of the joining processes. In this investigation, the flexibility of the self-piercing riveting process by reducing the required tool–geometry combinations is examined. For this purpose, various joints consisting of similar as well as dissimilar materials with different material thickness are sampled and analysed. Subsequently, a cluster algorithm is used to reduce the number of rivet–die combinations required. Finally, the effect of the changed tool geometries on both the joint formation and the joint load-bearing capacity is investigated. The investigation showed that a reduction by 55% of the required rivet–die combinations was possible. In particular, the rivet length influences the joint formation and the joint load-bearing capacity. An exclusive change of the die (e.g. die depth or die diameter) did not show a significant influence on these parameters.

Joining sheets made from dissimilar materials by hole hemming

This research work presents a new joining process based on the hemming process for attaching sheets made from dissimilar materials with very different mechanical properties. The process is termed ‘hole hemming’ and consists in producing a mechanical interlock between pre-drilled holes which can be made anywhere on the sheets. The process is carried out in a two-stage operation including flanging the hole of an outer sheet and bending the flange over the hole of an inner sheet. First, the joining stages and the required tools are designed. Then, the joining of DP780 steel and AA6061-T6 aluminium alloy sheets, which are applied to manufacture lightweight structures in the automotive industries, is investigated using finite element analysis. Results show that the hole hemming process is able to successfully join these materials without fracture. The hole-hemmed joint withstood the maximum forces of 2.5 and 0.5 kN in single-lap shear and peel tests, respectively, and failed with hole bearing mode which is known as a gradual failure mode. The results demonstrate the applicability of the hole hemming process for joining dissimilar materials.

Compression and morphological properties of a bio-based polyurethane foam with aluminum hydroxide

Compression and morphological evaluation of a new bio-based polyurethane foam (PUF) with aluminum hydroxide (ATH) added as flame retardant were carried out. The PUF was obtained from a blend of vegetable oils. Compression behavior of the polyurethane with different mass fractions of flame retardant (ATH) was investigated according to ASTM D1621–16. The ATH addition highly increased the compression yield strength of the specimens, going from 0.85 MPa (no ATH) to 2.34 MPa ( + 50%wt ATH). The compression yield strain did not show a noteworthy difference up to 40% ATH, presenting a significant decrement in the PUF + 50%ATH. The compression elasticity modulus increased from 15.40 MPa (no ATH) up to 139.77 MPa ( + 50%wt ATH). SEM images were used in order to evaluate the morphological structure of the foam. Regarding the cell sizes, there was no pattern observed, therefore, the cell sizes were adopted as random. The shapes of the cells were detected as elliptical in two different directions in the same cross-sectional area. The digital image correlation (DIC) technique showed higher strain values where the transverse ellipsoid-shaped cells were located, therefore, the load-oriented ellipsoids presented higher stiffness. Thus, the results for PUF with addition of ATH show that the bio-based material presented an important improvement in the compression properties, which allows this material to become more useful for different applications, such as furniture, building and automobile industries, as well as sandwich structures.

Textile-reinforced mortars; an experimental comparative study of tensile strength improvement methods

In this article, an experimental study is conducted to compare eight improvement methods for the tensile strength of textile-reinforced mortars (TRM). 12 series of samples with different modification methods are compared to determine the most effective factors on crack initiation force and tensile strength of TRM. Eight modification methods are categorized under three main groups of mortar modification, fabric modification, and fabric-mortar interface modification. TRM's first crack force and ultimate force are considered as indices of method performance. One-way ANOVA and factorial analysis were also conducted to statically determine the most significant methods for improving TRM tensile behavior. The results showed that the modification of mortar by short fiber is the most effective method for the enhancement of TRM's first crack force. Also, the methods which led to the transfer of failure mode from mortar to fabrics were the most effective methods on TRM ultimate force improvement. The result showed that coating fabrics with epoxy affects TRM tensile strength more than all other methods. Extra enhancement of TRM ultimate force is achieved by adding silica fume to epoxy before coating the fabrics and spreading the sand and short fibers on impregnated fabrics.

Dynamic response analysis of Euler–Bernoulli beam on spatially random transversely isotropic viscoelastic soil

A novel dynamic soil-structure interaction model is developed for analysis for Euler–Bernoulli beam rests on a spatially random transversely isotropic viscoelastic foundation subjected to moving and oscillating loads. The dynamic equilibrium equation of beam-soil system is established using the extended Hamilton's principle, and the corresponding partial differential equations describing the displacement of beam and soil and boundary conditions are further obtained by the variational principles. These partial differential equations are discretized in spatial and time domains and solved by the finite difference (FD) method. After the differential equations of beam and soil are discretized in the spatial domain, the implicit iterative scheme is used to solve the equations in the time domain. The solving result shows the FD method is effective and convenient for solving the differential equations of beam-soil system. The spring foundation model adopted the modified Vlasov model, which is a two-parameter model considering the compression and shear of soil. The advantage of the present foundation model is avoided estimating input parameters of the modified Vlasov model using prior knowledge. The present solution is verified by publishing solution and equivalent three-dimensional FE analysis. The present model produced an accurate, faster, and effective displacement response. A few examples are carried out to analyze the parameter variation influence for beam on spatially random transversely isotropic viscoelastic soil under moving loads.

Dislocation re-emission induced staged work hardening in graphene-nanotwin reinforced Cu: A molecular dynamics simulation study

Graphene and nanotwins are two effective reinforced microstructural features to achieve improved mechanical properties of metallic composites, while the two features are generally applied separately. In this study, graphene/nano-twinned Cu nanocomposites models with different arrangement of the graphene and twin boundaries were designed by using molecular dynamics (MD) simulations, and the dislocation processes and the interactions between dislocation and graphene/twin were simulated and investigated. The simulation results indicated the arrangement of graphene and nanotwin affects the work hardening behaviors in the graphene/nano-twinned Cu composites, i.e., two staged work hardening behavior corresponded to cyclic process of dislocation hindrance-absorption-reemission in the model with relatively small twin spacing and twin-graphene spacing, while the work hardening dominated by dislocation intersection and multiplication occurred in the model with large twin-spacing. The simulation provided herein demonstrated that the special arrangement of graphene and nanotwins led a way to tailoring the mechanical properties of metallic composites with various work hardening behaviors. Graphical abstract Highlights 1. Dislocation reactions between twins and graphene were simulated and analyzed. 2. Twin-graphene distance and the twin distance play key roles in the reaction. 3. The mechanism corresponding to work hardening changes in the limited two distances.

Collapse analysis of Al 6061 alloy conical shells with circular cutouts under axial loading: experiment and simulation

The current research is conducted to investigate the experimental and numerical study of crushing behavior and buckling modes of thin-walled truncated conical shells with or without cutouts and discontinuities under axial loading. In this regard, Instron 8802 servohydraulic machine is used to perform the experiments. Additionally, the buckling modes, derived from the axial collapse phenomenon, are simulated with Finite Element (FE) software. The force-displacement diagrams extracted numerically are compared with experimental results. Various factors, including maximum force, energy absorption, specific energy, and failure modes of each case, are also discussed. The results indicate that the increasing cutout cause a decrease in the maximum force and energy absorption. Moreover, with cutouts reduction, the failure modes of the samples changed from the diamond asymmetric mode and single-lobe mode to multi-lobes, and with removing cutouts, the failure mode is observed to be completely symmetric.