Powder bed metal additive manufacturing (AM) utilizes a high-energy heat source scanning at the surface of a powder layer in a pre-defined area to be melted and solidified to fabricate parts layer by layer. It is known that powder bed metal AM is primarily a thermal process and further, heat conduction is the dominant heat transfer mode in the process. Hence, understanding the powder bed thermal conductivity is crucial to process temperature predictions, because powder thermal conductivity could be substantially different from its solid counterpart. On the other hand, measuring the powder thermal conductivity is a challenging task. The objective of this study is to investigate the powder thermal conductivity using a method that combines a thermal diffusivity measurement technique and a numerical heat transfer model. In the experimental aspect, disk-shaped samples, with powder inside, made by a laser powder bed fusion (LPBF) system, are measured using a laser flash system to obtain the thermal diffusivity and the normalized temperature history during testing. In parallel, a finite element model is developed to simulate the transient heat transfer of the laser flash process. The numerical model was first validated using reference material testing. Then, the model is extended to incorporate powder enclosed in an LPBF sample with thermal properties to be determined using an inverse method to approximate the simulation results to the thermal data from the experiments. In order to include the powder particles’ contribution in the measurement, an improved model geometry, which improves the contact condition between powder particles and the sample solid shell, has been tested. A multi-point optimization inverse heat transfer method is used to calculate the powder thermal conductivity. From this study, the thermal conductivity of a nickel alloy 625 powder in powder bed conditions is estimated to be 1.01 W/m·K at 500 °C.

A new approach to manufacturing biocomposite sandwich structures is introduced. Materials used in the biocomposite are natural textile reinforcement, mycelium-bound agricultural waste as core, and bioresin. This paper focuses on three specific steps of the seven-step manufacturing process: filling pre-stamped textile shells with core mixture; allowing the core material to grow thereby binding reinforcement particles and textile skins into a unitized preform; and oven drying said preform to drive off moisture and inactivate the mycelium. Specific process details highlighted include design and thermoforming of growth trays, tray sterilization, filling trays with mycelium-inoculated substrates filling and allowing growth to occur, and finally conduction and convection drying/inactivation of the grown parts. To study the new material’s stiffness using different materials and under different processing conditions, specimen dimensions were based on ASTM D7250 and C393 standards. All dried samples were tested in flexure by three-point bending method to determine the stiffness and strength of the resin-less preforms and to identify optimal material combinations.

Self-folding structures such as origami-based structures have been studied by artists, mathematicians and engineers. New applications are emerging in fields such as biomedical and electronics. Self-folding structures can potentially extend three-dimensional (3D) printing into four-dimensional (4D) printing by intelligently designed material distribution in order to achieve controlled shape deformation over time. Traditionally, self-folding structures are folded along pre-defined hinges such that neighboring facets can transform their shapes. In this study, we present a new design and fabrication approach of self-folding structures with no foldable hinges. A significant benefit is its capability in fabricating shapes with smooth curved surfaces. Our self-folding method is based on a thermally responsive mechanism, where a thermal responsive film is used as the active material while another polymer material coated on the film is used as the constraining material. When the structure is heated, the two sides of the film will shrink differently due to the constraining material. Consequently the structure will fold. By changing the constraining patterns and coated material properties, the film can be self-folded into different shapes. Three types of folding features are presented in our study. Based on them, an unfolding algorithm is presented for a given shell structure with curved surfaces. Theoretical analysis and experimental tests are presented to demonstrate the capability of the self-folding method. Its limitation and future work are also discussed.

Today most industrial sectors are faced with several challenges such as the reduction of CO 2 emission, the shortage of resources and the increasing individualization of products. This situation particularly appears in the automotive industry. To reduce CO 2 emission of cars there is no alternative to decreasing weight. Advanced lightweight design implies new materials, innovative component design and finally new production methods. Based on this not only the CO 2 emission during the operation phase becomes a focus, also the manufacturing process becomes more and more important. Following upcoming requests for lightweight materials, new design principles and energy and resource efficient production processes, conventional forming technologies are reaching their limits. By including temperature as an active temperature parameter into the production process, advanced final component properties are possible, forming limits can be extended and process chains can be shortened. This is valid particularly for hydroforming. Advantages and disadvantages of this technology are well known, and today there are a few typical automotive components in series production. Compared to a blank half shell design of car components, hydroformed profiles allow a flangeless design, the reduction of individual parts in a component and an excellent degree of material utilization. To implement high temperatures into this technology, alternative heat-resistant forming media are mandatory. The substitution of water by nitrogen increases the thermal process limits up to 1,000°C, but this also requires new systems engineering. This paper gives an overview on the development of hot gas forming technology and illustrates prospects and limits by means of automotive-related parts for metallic lightweight materials such as ultra-high strength and stainless steel, Aluminum or Magnesium. Beside the determination of temperature-related material characteristics, the research focused on process and tool design based on thermo-mechanically coupled FE simulation. The consecutive manufacturing of prototyping parts allows the validation of simulation results and assesses the prospects to shorten existing process chains in the future.

In this paper, the key assumptions in the M-K and effective stress ratio models are investigated for AISI 1018 steel specimens with a thickness of 0.78 mm using experimental and numerical data from Marciniak tests. The experimental procedure included Digital Imaging Correlation (DIC) to measure the major and minor in-plane strains. Strain components were obtained at points inside (i.e., the defect region) and adjacent (i.e., the safe regions) to the high strain concentrations for four different strain paths. In the numerical analysis, FEA simulations with Marc Mentat were performed with shell elements to investigate the four specimen geometries. The key assumptions of interest are the incremental major strain ratio from M-K model and the critical stress concentration factor from effective stress ratio model. Thus, the mechanics- and material-based failure phenomena in these two analytical models are examined in this paper to provide insight into the material behavior at failure.

This paper develops a three dimensional (3-D) finite element modeling (FEM) to predict the workpiece thermal distortion in minimum quantity lubrication (MQL) deep-hole drilling. Drilling-induced heat fluxes on the drilled hole bottom surface (HBS) and hole wall surface (HWS) are first determined by the inverse heat transfer method. The proposed 3-D heat carrier model consisting of shell elements to carry the HWS heat flux and solid elements to carry the HBS heat flux conducts the heat to the workpiece to mimic drilling process. A coupled thermal-elastic analysis is used to calculate the workpiece thermal distortion at each time step based on the temperature distribution. The heat carrier model is validated by comparing the temperature profiles at selected points with those from an existing 2-D axisymmetric advection model. The capability for modeling distortion in the case of drilling multiple deep-holes is also demonstrated.

Strain-based forming limit diagrams (FLDs) are the traditional tool used to characterize the formability of materials for sheet metal forming processes. However, this failure criterion exhibits a significant strain path dependence. Alternatively, stress-based forming limit diagrams (FLDs) have been proposed and shown to be less sensitive to the deformation path. The stress-based failure criterion can be conveniently implemented in numerical simulations. However, for reliable numerical modeling, the sensitivity of the models to the selection of discretization parameters, in particular, the element type must be assessed. In this paper, Marciniak tests have been numerically simulated to investigate failure prediction using three different element types (shell, solid and solid-shell). Seven different specimen geometries were modeled in order to vary the loading paths. The results show that despite differences in stress calculation assumptions, shell, solid and solid-shell elements do not provide differences in failure prediction when a stress-based failure criterion is used.

An attempt has been made to synthesize SrFeO 3-δ powder by sol-gel process involving oxalate formation, its digestion for 4h, drying at 150°C for 24h, and decomposition at 800°C for 10h. The resulting powder is shown to a) exhibit a single phase with a perovskite-type cubic structure and lattice parameter a = 3.862±0.002A˚, b) contain irregular shape particles, and c) display optical absorption peaks corresponding to charge transfer from oxygen to iron (3.73 and 3.41eV), t 2g to e g transition of Fe 3+ (1.57eV), and crystal field (3d-3d) charge transfer of Fe 3+ (1.25eV). Impedance over a wide frequency range of 20Hz-2MHz at 118–318K has contributions from two parallel ‘RC’ circuits belonging to bulk and grain boundaries with the later displaying significant space charge polarization. The relaxation time of polarization follows an Arrhenius behaviour (τ = τ o exp[E a /k B T]) with τ o as ∼10 −8 s and activation energy E a as ∼50meV. Further, the sample having magnetic character with transition temperature as 853K, coercivity (H c ) = 3748Oe and magnetization 0.09 μ B per iron atom (at 17kOe). The zero field cooled and field cooled magnetization versus temperature data in conjunction with constricted hysteresis loops near the origin suggest core-shell morphology for the particles, core being antiferromagnetic with net uncompensated moment and shell conforming to disordered disposition of spins.

This paper proposes a novel thin shell curved surface forming method by dieless single point forming technology embodying the thought of the parallel running for both design and manufacture. And a sheet metal superplastic forming prototype system with a heterogeneous master-slave manipulator by force-reflecting control has been developed. The system mainly consists of mechanical transmission components, servo system, online optical scaler for the measurement and the path planning of the forming tool. The final ideal forming experiment has physically testified the feasibility of the mutual forming method for the shell sample of a panda model. This paper brings up the tentative idea of real object modification with the use of heptic force-feedback.

Double-layered microcapsules, which usually consist of a core (polymeric) matrix surrounded by a (polymeric) shell, have been used in many industrial and scientific applications such as microencapsulation of drugs and living cells. Compound concentric nozzle-based jetting has been favored due to its efficiency and precise control of the core-shell compound structure. Thus far, little is known about the underlying formation mechanism of double-layered microcapsules in compound nozzle jetting. This study aims to understand the formability of double-layered microcapsules in compound nozzle jetting by coupling a theoretical analysis with numerical simulations. A linear temporal instability analysis is used to define the perturbation growth rate and its ratio as a function of the wavenumber; and a computational fluid dynamics (CFD) method is implemented to model the microcapsule formation process in order to determine the good microcapsule forming range based on the growth ratio curve. Using a pseudo-bisection method, the lower and upper bounds of the good formability range have been determined for a given materials-nozzle system. The proposed formability prediction methodology has been implemented and validated in modeling a water-poly (lactide-co-glycolide) (PLGA) – air compound jetting system with satisfactory prediction results.

Microchannel arrays are being developed across a wide spectrum of applications including blood processing, fuel cells and thermal management among many others. Typical arrays are based on a flat laminated architecture produced in a vacuum hot press (VHP) through uniaxial bonding pressure at elevated temperatures. Due to the cylindrical nature of certain microchannel applications, such as chemical processing within chemical facilities and heat rejection within Stirling cycle engines, the packaging of microchannel devices within cylindrical geometries would greatly simplify interconnect. This paper describes the development of a novel technique for axisymmetric diffusion bonding pressure application with the use of a VHP capable of producing cylindrical laminated devices. Uniaxial bonding pressure is converted to axisymmetric pressure through the use of conical wedges. Pressure magnitude and distribution are controlled by the geometry of the pressure delivery platens, the design of which was aided by finite element analysis. Cylindrical test articles were produced by patterning thin metal foils by laser machining a single roll of metal shim stock, which was then coiled within an outer tube shell and axisymmetrically bonded with the VHP to produce a cylindrical laminated workpiece. Preliminary bonding results show quality bond lines with minimal void fractions. This new architecture is shown to be feasible for producing cylindrical laminated devices by microlamination using traditional VHP equipment.

Continuous three roller bending process is widely used in practice to bend the plates into cylinders. Bending load for plate material under bending is affected by plate thickness, width and shell diameter combinations. Maximum top roller load is encountered during the edge pre-bending stage as top roller is set at an offset distance from its mid position. Shell diameter, thickness and material for cylindrical structural element to be produced are fixed by design. Width of the plate for roller bending decides number of cylindrical segments required to achieve the designed shell length. Maximum pre-bending width depends on maximum top roller load imparting capacity. Looking to the above considerations, maximum width which can be pre-bend at limiting top roller load (for designed shell diameter, thickness and material combinations) specifies the capacity. Presented work aims at developing the mathematical model of top roller load for pre-bending. Top roller offset for pre-bending were calculated based on practical top roller pre-bending load data, for different grades of C-Mn steel plates (as per ASME sec II part-A). Based on these top roller offsets, finite element analysis (FEA) of pre-bending stage were performed using Hyperform LS-DYNA. Effect of co-efficient of friction at roller plate interfaces was analyzed. FE simulation of pre-bending of cladded plate (54 mm thick C-Mn steel plate of material grade SA-387Gr11Cl2 having 3 mm thick layer of stain less steel material grade SS-308) was performed. FEA load results were found in good agreement with the practical load results and can be used for capacity assessment and analysis of roller bending machines.

This work examines the release of a model water-soluble compound from electrospun polymer nanofiber assemblies. Such release attracts attention in relation with biomedical applications, such as controlled drug delivery. It is also important for stem cell attachment and differentiation on biocompatible electrospun nanofiber scaffolds containing growth factors, which have been encapsulated by means of electrospinning. Typically, the release mechanism has been attributed to solid-state diffusion of the encapsulated compound from the fibers into the surrounding aqueous bath. Under this assumption, a 100% release of the encapsulated compound is expected in a certain (long) time. The present work focuses on certain cases where complete release does not happen, which suggests that solid-state diffusion may not be the primary mechanism at play. We show that in such cases the release rate can be explained by desorption of the embedded compound from nanopores in the fibers, or from the outer surface of the fiber in contact with the water bath. After release, the water-soluble compound rapidly diffuses in water, whereas a release rate is determined by the limiting desorption stage. A model system of Rhodamine 610 fluorescent dye embedded in electrospun monolithic Poly(methylmethacrylate) PMMA or Poly(caprolactone) PCL nanofibers, or in nanofibers electrospun from PMMA/PCL blends, or in core/shell PMMA/PCL nanofibers is studied. Both the experimental results and theory point at the above-mentioned desorption-related mechanism and the predicted characteristic time, release rate, and effective diffusion coefficient agree fairly well with the experimental data. A practically important outcome of this surface release mechanism is that only the compound on the fiber and pore surfaces can be released, whereas the material encapsulated in the bulk cannot be freed within the time scales characteristic of the present experiments (days to months). Consequently, in such cases complete release is impossible. We also demonstrate how the release rate can be manipulated by the polymer content and molecular weight affecting nanoporosity and the desorption enthalpy, as well as by the nanofiber structure (monolithic fibers, fibers from polymer blends and core-shell fibers). In particular, it is shown that by manipulating the above parameters, release times from tens of hours to months can be attained.

Hemming is a manufacturing process of folding a panel onto itself or another sheet. Quality of hemming is characterized by geometry and formability. This paper presents a response surface study of 3D curved-surface-curved-edge hemming of an aluminum alloy, AA6111-T4, using finite element analysis. Solid elements and explicit FE solver are used for simulations of flanging, pre- and final hemming, and shell elements with implicit solver are deployed for springback prediction. A novel procedure called “solid to shell mapping” is developed to bridge the solid elements with the shell elements. Verified to be accurate and efficient, the model is utilized in a Central Composite Design to quantitatively explore the relationships between certain key process variables and the hem dimensional quality and formability. The most significant variables are identified as (i) pre-hemming angle on roll-in/roll-out; (ii) nominal surface curvature on sheet springback; (iii) initial sheet strain and flanging die radius on the maximum hemline surface strain of the produced hem. These results provide insights for process parameter selections in designing and optimizing 3D hems under material formability constraints.