Material Form Research Articles

Tough and Moldable Sustainable Cellulose-Based Structural Materials via Multiscale Interface Engineering.

All-natural materials derived from cellulose nanofibers (CNFs) are expected to be used to replace engineering plastics and have attracted much attention. However, the lack of crack extension resistance and 3D formability of nanofiber-based structural materials hinders their practical applications. Here, a multiscale interface engineering strategy is reported to construct high-performance cellulose-based materials. The sisal microfibers are surface treated to expose abundant active CNFs with positive charges, thereby enhancing their interfacial combination with the negatively charged CNFs. The robust multiscale dual network enables easy molding of multiscale cellulose-based structural materials into complex 3D special-shaped structures, resulting in nearly twofold and fivefold improvements in toughness and impact resistance compared with those of CNFs-based materials. Moreover, this multiscale interface engineering strategy endows cellulose-based structural materials with better comprehensive performance than petrochemical-based plastics and broadens cellulose's potential for lightweight applications as structural materials with lower environmental effects.

Study on the Flow Behavior of 5052 Aluminum Alloy over a Wide Strain-Rate Range with a Constitutive Model Based on the Arrhenius Model Extension

The formability at room temperature and low speed limits the application of aluminum alloy, while high strain rates positively improve the formability of materials. The constitutive behaviors of materials under high strain rates or impact loadings are significantly different from those under quasi-static conditions, while few constitutive models consider the effect of the mobile dislocation and forest dislocation evolution on the dynamic strain aging (DSA) over a wide strain-rate range. The 5052 aluminum alloy, of which the primary source of strain-hardening is dislocation–dislocation interaction, is widely used in manufacturing automotive covering parts and is considered one of the most promising alloys. Therefore, this study conducts uniaxial tensile tests on AA5052-O under conditions of temperatures ranging from 293 K to 473 K and strain rates ranging from 0.001 s−1 to 3000 s−1, and compares the stress–strain relationships of AA5052-O under different conditions to illustrate the constitutive relationship affected by the dislocation evolution over a wide strain-rate range. The Arrhenius model based on the thermal activation mechanism is modified and extended by considering the effects of dynamic strain aging (DSA), drag stress, and the evolution of mobile dislocation and forest dislocation. Thus, a new physics-based constitutive model for AA5052-O is proposed, which can well reflect the change in strain-rate sensitivity with the strain rate increasing. The mobile dislocation density and total dislocation density are predicted with a modified Kubin–Estrin (KE) model, and the influences of variable mobile dislocation on DSA and dislocation drag are discussed as well. In order to verify the reliability of the new constitutive model, the dislocation densities of the specimens before and after deformation are obtained with TEM and XRD, which are in good agreement with the predicted values. This study also compares the newly proposed model with classic constitutive models using multiple statistical evaluation methods, which shows that the new physics-based constitutive model has not only more clear physical meanings for its parameters but also has a higher prediction accuracy.

Contactless single point incremental forming: experimental and numerical simulation

The demand for small-batch manufacturing processes has increased considerably in recent years due to the need for personalized and customized products. Single point incremental forming (SPIF) has emerged as a time-efficient approach that offers increased material formability when compared to conventional sheet metal forming techniques. However, the complexity of SPIF requires a complete understanding of the material deformation mechanism. In this study, a non-conventional contactless tool in the form of hot compressed air is employed to form a polycarbonate sheet. The influence of the contactless tool on the shaping process is modeled and analyzed with a finite element modelling (FEM). Two different models were developed and coupled to estimate the resulting shape of the sheet. A CFD model was created to obtain pressure and temperature values of the air impacting the sheet, while a transient structural model was employed to study the deformation of the sheet. The research provides a working model that is able to predict the performance of this contactless incremental forming process of polymers with high accuracy. The comprehensive FE model developed in this work is able to forecast the final part geometries and dimensions in addition to the normal strain progression. It also revealed that the primary modes of deformation in SPIF were stretching, thinning, and bending. The model was validated by experimental results, and the predicted sheet deformation was compared to the one generated experimentally, and the results obtained were in good agreement.

Enhancing forming accuracy in aluminum alloy variable-diameter tubes through dual-coil controllable electromagnetic forming

High-accuracy forming of aluminum alloy variable-diameter tubes imposes strict requirements on force field design. Although electromagnetic forming has shown unique advantages in enhancing the formability of aluminum alloy materials, its shortcomings lie in the single and difficult-to-adjust force field, which limits its application in variable-diameter tubes. To address this challenge, a novel dual-coil electromagnetic forming approach with good controllability of force field is proposed. Numerical simulations and experiments are both conducted to showcase the superiority of this approach in forming variable-diameter aluminum alloy tubes. The results demonstrate that the forming accuracy of AA6061-O variable-diameter tubes has significantly improved compared to a traditional single-coil system, in which the maximum die-fitting gap is reduced from approximately 2 mm to less than 0.22 mm. Furthermore, it is identified the force field produced by the single-coil system causes a local cavity gap between the tube and the die, which adversely affects the forming accuracy. In contrast, the improved force field produced by the dual-coil system facilitates bottom-to-top deformation of the tube, thereby avoiding undesirable cavity gaps and achieving high forming accuracy. Importantly, the dual-coil forming system offers significant controllability of the Lorentz force field by adjusting the coil discharge, making it suitable for variable-diameter tubes of different sizes without changing the coil structure. This work represents a substantial advancement in controllable tube forming, offering a new approach to force field design and holding promise for extensive applications in the forming of variable-diameter tubes.

Unsupervised Deep Learning for Advanced Forming Limit Analysis in Sheet Metal: A Tensile Test-Based Approach.

An accurate description of the formability and failure behavior of sheet metal materials is essential for an optimal forming process design. In this respect, the forming limit curve (FLC) based on the Nakajima test, which is determined in accordance with DIN EN ISO 12004-2, is a wide-spread procedure for evaluating the formability of sheet metal materials. Thereby the FLC is affected by influences originating from intrinsic factors of the Nakajima test-setup, such as friction, which leads to deviations from the linear strain path, biaxial prestress and bending superposition. These disadvantages can be circumvented by an alternative test combination of uniaxial tensile test and hydraulic bulge test. In addition, the forming limit capacity of many lightweight materials is underestimated using the cross-section method according to DIN EN ISO 12004-2, due to the material-dependent occurrence of multiple strain maxima during forming or sudden cracking without prior necking. In this regard, machine learning approaches have a high potential for a more accurate determination of the forming limit curve due to the inclusion of other parameters influencing formability. This work presents a machine learning approach focused on uniaxial tensile tests to define the forming limit of lightweight materials and high-strength steels. The transferability of an existing weakly supervised convolutional neural network (CNN) approach was examined, originally designed for Nakajima tests, to uniaxial tensile tests. Additionally, a stereo camera-based method for this purpose was developed. In our evaluation, we train and test materials, including AA6016, DX54D, and DP800, through iterative data composition, using cross-validation. In the context of our stereo camera-based approach, strains for different materials and thicknesses were predicted. In this cases, our method successfully predicted the major strains with close agreement to ISO standards. For DX54D, with a thickness of 0.8 mm, the prediction was 0.659 (compared to ISO's 0.664). Similarly, for DX54D, 2.0 mm thickness, the predicted major strain was 0.780 (compared to ISO 0.705), and for AA6016, at 1.0 mm thickness, a major strain of 0.314 (in line with ISO 0.309) was estimated. However, for DP800 with a thickness of 1.0 mm, the prediction yielded a major strain of 0.478 (as compared to ISO 0.289), indicating a divergence from the ISO standard in this particular case. These results in general, generated with the CNN stereo camera-based approach, underline the quantitative alignment of the approach with the cross-section method.

Laser surface texturing of dies in strip drawing of DP600 steel sheet

ABSTRACT Friction at the blank-die contact influences material flow, formability, and product surface quality in processes like deep drawing. Surface modification of the dies by laser texturing is a potential technique to alter the tribological characteristics at the interface during sliding of the sheet over the die surface in lubricated condition. In this work, the effect of laser surface texturing of dies has been investigated under high normal loads during strip drawing of dual phase (DP600) steel. The effect of texture parameters and process parameters on the interface friction between the strip and the dies has been studied. It has been found that the friction coefficient reduced, and the surface finish improved significantly with the textured dies when compared to the conventional die. This study emphasises the importance of surface modification of the dies by laser texturing for its potential application in sheet metal forming processes.

Hot-Air Contactless Single-Point Incremental Forming

Single-point incremental forming (SPIF) has emerged as a time-efficient approach that offers increased material formability compared to conventional sheet-metal forming techniques. However, the physical interaction between the forming tool and the sheet poses challenges, such as tool wear and formability limits. This study introduces a novel sheet-forming technique called contactless single-point incremental forming (CSPIF), which uses hot compressed air as a deformation tool, eliminating the requirement for physical interaction between the sheet and a rigid forming tool. In this study, a polycarbonate sheet was chosen as the case-study material and subjected to the developed CSPIF. The experiments were carried out at an air temperature of 160 °C, air pressure of 1 bar, a nozzle speed of 750 mm/min, and a step-down thickness of 0.75 mm. A Schlieren setup and a thermal camera were used to visualize the motion of the compressed hot air as it traveled from the nozzle to the sheet. The results showed that the CSPIF technique allowed for the precise shaping of the polycarbonate sheet with minimal springback. However, minor deviations from the designed profile were observed, primarily at the starting point of the nozzle, which can be attributed to the bending effects of the sample. In addition, the occurrence of sheet thinning and material buildup on the deformed workpiece was also observed. The average surface roughness (Ra) of the deformed workpiece was measured to be 0.2871 microns.

Effect of laser power and heat treatment on microstructures and tensile properties of LPBF fabricated Hastelloy X superalloy

Laser powder bed fusion (LPBF) technic is suitable for manufacturing metal parts with complex shapes and internal channels. The energy input and post heat treatment have great influence on the formability, microstructure and properties of the metallic materials. In this work, Hastelloy X superalloy is fabricated by LPBF with a variation of laser power, and then post-treated by hot isostatic pressing and solution treatment. Influences of laser power and heat treatment on microstructure features of as-built sample are evaluated. The correlation among laser power, microstructure features and tensile properties of heat-treated samples are revealed. The results reveal that average grain size, aspect ratio, texture intensity increases while the proportion of high-angle grain boundaries decreases with the increase of laser power. Precipitates of Cr-rich carbides along grain boundary are observed after post heat treatment. Post-treatment promotes the twinned recrystallization, grain coarsening and texture degradation. Yield strength increases while tensile strength decreases with the increase in laser power. The higher vertical yield strength of sample with laser power of 320 W could be attributed to its larger aspect ratio and more twin boundaries. 280 W is the preferred laser power to obtain the best overall tensile properties. This study is helpful for the exploration of optimal LPBF process and post heat treatment, by which Ni-based superalloys with a good balance between strength and ductility can be produced.

Determining the hot forming limits of titanium alloy sheet under different strain paths by constant equivalent strain rate hot gas bulging tests

This study aims to determine the hot forming limit diagram (FLD) of titanium alloy sheet using constant equivalent strain rate hot gas bulging tests. A novel method was proposed to calculate pressure-time curves under different strain paths, ensuring that the equivalent strain rate during hot deformation remains constant. The key of this method is the development of a VUMAT material subroutine that accurately reflects anisotropic hardening and r-value evolution, and the pressure loading curves under different strain paths were determined by coupling with the VUAMP amplitude subroutine embedded with a PID control algorithm. Based on dies with different cavity structures and unbonded double-layer samples, the FLDs of TA32 titanium alloy sheet under linear and nonlinear strain paths were obtained by carrying out hot gas bulging tests at 800 ℃ with a strain rate of 0.001 s-1. The results demonstrate that the proposed approach is applicable for determining the pressure-time curves under various complex strain paths, and it is experimentally verified that the equivalent strain rate of the sample apex is maintained near the target value during the bulging process. Furthermore, the significant effects of anisotropy and strain path change on the forming limits were quantitatively analyzed. The developed method is not only beneficial for testing the hot biaxial mechanical properties of materials, but also useful for controlling the strain rate during the hot forming of complex thin-walled components to improve the formability of materials.

Stable strain hardening behavior of Ti-4Al-5Mo-5V-5Cr-1Nb alloy during plastic deformation

Strain hardening is crucial to the formability and reliability of structural materials, however, such capacity will be exhausted as strain increases. In current work, a stable strain hardening character is found in the compression of Ti-4Al-5Mo-5V-5Cr-1Nb alloy up to 0.4 strain. Microstructural characterization on deformed samples reveals the development of multiscale α′′ lath frames. The stable strain hardening behavior is understood via unveiling the roles of α′′ lath during deformation. It is noted that these α′′ laths impede dislocations and reduce their mean free path as strain increases. Meanwhile, a high density of dislocations can be observed within these nanoscale α′′ laths, indicating they are also involved into deformation. Such a strain accommodation mechanism greatly relieves the stress concentrated at β/α′′ interfaces enabling the resistance to crack initialization.

Glass and Liquid Chemistry of Coordination Polymers and MOFs

Abstract The chemistry of coordination polymers and metal-organic frameworks (MOFs) has progressed largely with the development of crystallography. The study of “disordered systems” such as the liquid and glassy states of coordination polymers and MOFs has been developed in recent years, and various methods have been recognised to produce glasses from crystals. The structures of the glass and liquid phases are studied using synchrotron radiation X-rays and spectroscopy. Many of them have the coordination bond-based network structure found in the crystal structures. This suggests that the structural design principles of coordination chemistry can be applied to glasses and liquids of coordination polymers and MOFs. The inherent properties of the disordered system, such as wide compositions and high internal degrees of freedom, high material formability and softness, contribute to the coupled physicochemical properties.

Effects of thermoforming operation and tooling on the thermoformability of plastic‐coated fibre‐based materials

AbstractAdvances in the three‐dimensional (3D) forming of fibre‐based materials require the formulation of more formable materials and the development of process lines, machinery, and tools. Using a thermoforming process to convert fibre‐based materials into 3D forms is an emerging area of research which requires further investigation into the practicality of the process line and tooling in forming such materials. Accordingly, this study evaluated the impact of the thermoforming process operation and tooling on the thermoformability of plastic‐coated paperboards. The main objective was to provide design recommendations for the future development of thermoforming lines, followed by guidelines for tooling design to improve the performance of materials utilising the currently available machinery. This study examined the thermoforming behaviour of two different plastic‐coated paperboards in vacuum and pressure thermoforming by investigating their maximum acquired depth, shape accuracy, and damage mechanisms. The research findings, based on the depth and linear elongation achieved, indicate that the inferior performance of plastic‐coated paperboards in thermoforming cannot be wholly attributed to restrictions in the three‐dimensional formability of materials; the inability of the current process lines to utilise the maximum potential of materials can also lead to their inferior performance. Notably, the method of pressure supply and cooling of materials requires adjustment of these materials. From a tooling perspective, owing to the spring‐back effects, the enlargement of the mould dimensions should be considered during the design stage. Additionally, based on potential opportunities with the current unmodified machinery and materials, products in the size of standard food trays have a higher likelihood of being optimised with tooling design than smaller sized shapes, which still require additional developments in materials. Moreover, designing moulds without draft angles can reduce the risk of rupture owing to the prevention of localised stress formation in materials.

Improving Material Formability and Tribological Conditions through Dual-Pressure Tube Hydroforming

Dual-pressure tube hydroforming (THF) is a tube-forming process that involves applying fluid pressure to a tube’s inner and outer surfaces to achieve deformation. This study investigates the effect of dual-pressure loading paths on material formability and tribological conditions. Specifically, pear-shaped and triangular cross-sectional parts were formed using dual-pressure modes where fluid pressure on the inside of the tubular blank was alternated with pressure on the outside surface of the tubular blank, causing the tube to expand/stretch and contract. During expansion, the tube conformed to the die’s cavity, while during contraction, the contact area between the die and the workpiece reduced, leading to decreased friction stress at the tube–die interface. Additionally, the reversal of pressure loadings caused the tubular blank to buckle, altering the stress state and potentially increasing local shear stress, improving material formability. Dual-pressure THF has demonstrated that the pressure loading paths chosen can substantially influence material formability. Comparing the geometries of parts formed by dual-pressure THF and conventional THF shows a significant increase in the protrusion height of both the pear-shaped and triangular specimens using dual-pressure THF.

Development of Forming Limit Curve for Glare in a semi-cured state using Hydro-bulging test method

Forming limit curve (FLC) is a valuable tool for predicting the formability of materials. Considering the need to develop FLC for fiber metal laminates (FMLs), Glare in a 2/1 sequence was prepared for the development of FLC. The prepreg used was glass fiber stacked in between the aluminum sheets in a semi-cured state, which allows for enhanced strain percentage and helps predict the maximum forming limit of Glare. The experimental data was accumulated by comprehensively testing parameters like pressure rate, temperature, and blank holding pressure. The results show that at higher temperatures and reduced pressure rate and blank holding pressure, the formability of Glare can be enhanced. This study helps predict the safe zone when forming Glare and manufacturing small intricate parts of Glare material in engineering applications.

Application of Digital Image Processing Techniques to Detect Through-Thickness Crack in Hole Expansion Test

Advanced high-strength steels (AHSS) have become increasingly popular in the automotive industry due to their high yield and ultimate tensile strengths, enabling the production of lighter car body structures while meeting safety standards. However, they have some setbacks compared to conventional steels, such as edge cracking through sheet thickness caused by forming components with shear-cut edges. When characterizing the formability of sheet metal materials, the hole expansion test is an industry-standard method used to evaluate the stretch-flangeability of their edges. However, accurately visualizing the first cracking is usually tricky and may be subjective, often leading to inconsistent results and low reproducibility with some impact of the operator on both direct and post-processing measurements. To address these issues, a novel digital image processing method is presented to reduce operator reliance and enhance the accuracy and efficiency of the hole expansion test results. By leveraging advanced image processing algorithms, the proposed approach detects the appearance of the first edge cracks, enabling a more precise determination of the hole expansion ratio (HER). Furthermore, it provides valuable insights into the evolution of the hole diameter, allowing for a comprehensive understanding of the material behavior during the test. The proposed method was evaluated for different materials, and the corresponding HER values were compared with the traditional method.

Tough and Strong All-Biomass Plastics from Agricultural and Forest Wastes via Constructing an Aggregate of Hydrogen-Bonding Networks

The development of all-biomass materials to replace conventional plastics has been gradually becoming a focus. However, all-biomass plastics, especially those fabricated from agricultural and forestry wastes, have the obstacles of poor formability and/or low toughness. Herein, we demonstrated a facile, efficient, and easy-to-scale method to significantly improve the formability and toughness of biomass materials via constructing an aggregate of hydrogen-bonding networks, where the relatively weak hydrogen bonding could be sacrificed during stretching. After a continuous preparation process that combined a paper-making process with an in situ welding process, the regenerated cellulose material with a layered microstructure was spontaneously formed. The interlayer hydrogen-bonding interactions could dissipate energy during stretching. As a result, the cellulose plastics were tough and strong. The tensile strength, strain, and toughness reached 154.9 MPa, 57.7%, and 81.76 MJ/m3, respectively, which were markedly higher than those of previous cellulose-based materials. The corresponding cellulose hydrogel exhibited an excellent strength of 9.5 MPa and a high strain of 171.4% also. During this scalable process, a 1-ethyl-3-methylimidazolium acetate (EmimAc) aqueous solution worked as a dispersant and a solvent, and a high solid content of cellulose/EmimAc (20 wt %) was used. Based on such an effective method, various agricultural and forestry wastes, including corn straw, wheat straw, grass, and wood powder, could be directly processed into high-tough all-biomass films, indicating a huge potential in ecofriendly materials, environmental protection, and bioresource utilization.

Characterization of the Fracture Forming Limits by Radial Extrusion

This paper introduces a new formability test based on double-action radial extrusion to characterize material formability in the three-dimensional to plane-stress material flow transitions that are found in bulk metal-formed parts. The presentation draws from a multidirectional tool, which was designed to convert the vertical press stroke into horizontal movement of the compression punches towards each other, aspects of experimental strain determination, fractography, and finite element analysis. Results show that three-dimensional to plane-stress material flow transitions at the radially extruded flanges lead to different modes of fracture (by tension and by shear) that may or may not be preceded by necking, such as in sheet metal forming. The new formability test also reveals adequate characteristics to characterize the failure limits of very ductile wrought and additively manufactured metallic materials, which cannot be easily determined by conventional upset compression tests, and to facilitate the identification of the instant of cracking and of the corresponding fracture strains by combination of the force vs. time evolutions with the in-plane strains obtained from digital image correlation.

Local and global forming potential of present dual-phase steel developments

Modern car bodies are constantly relying higher proportions of high and ultra-high strength steels. In the past, the increase in material strengths led to a conflict regarding the complexity of component geometries. Limits of material formability are reached and compromises had to be made in part design. By integrating application-oriented problems into the material development, these limits are expanded with respect to the given requirements. Within the present study, for several dual-phase steels with a tensile strength ≥800 MPa, practical laboratory tests are presented and their benefits in press shop application. In particular influencing factors on stretch flange formability are presented and discussed for example cutting parameters, anisotropy, time between cutting and forming or the orthogonal strain gradient.

Stretching and Forming Limit Curve of Steel–Glass Fibre Reinforced and Non-Reinforced Polyamide–Steel Sandwich Materials

This paper focuses on investigating the forming behaviour of sandwich materials composed of steel sheets and glass fibre-reinforced polyamide 6 (GF-PA6), i.e., thermoplastic-based fibre metal laminates (FML). Stretching and forming limit curve (FLC) determination of FML with different cover/core layer thickness ratios at various forming temperatures, i.e., at room temperature (RT), 200 and 235 °C, are the main approaches for characterizing their formability. In addition, the formability of mono-materials and non-reinforced sandwich materials is investigated as a reference. For a successful test and reliable results, several technical issues are considered, such as the suitable lubrication configuration and digital image correlation at elevated forming temperatures. The results revealed that the formability of non-reinforced sandwich materials with different core layer thicknesses exhibited compared formability to their monolithic steel sheet and no remarkable improvement in their formability with increasing the temperature up to 200 °C. Conversely, the formability of FML shows significant improvement (approx. 300%) with increasing temperature with a forming depth of about 33 mm at 235 °C compared to only 12 mm at RT.

Modeling of constitutive behavior and ductile fracture of AA5A06 aluminum alloy sheet in warm hydroforming process

In order to predict the formability of lightweight materials, the flow stress and fracture behavior of AA5A06 aluminum alloy which is very sensitive to temperature and strain-rate were investigated in this study. First, a modified Misiolek constitutive equation with a work-soften term is proposed to calculate the flow stress of the sheet metal. To predict ductile fracture in sheet forming operations, Cockcroft-Latham damage criterion is applied here and ductile fracture threshold under each condition is estimated using Response Surface Method. Finally, warm hydroforming deep drawing tests were conducted at various conditions and the experiment results are compared with the numerical simulation result to evaluate the new model’s feasibility and accuracy in forecasting the mechanical response of the material in warm forming conditions. Results show that the proposed model has a good accuracy compared with the traditional power law models and the FEM simulation conforms well to the tests.