Melt-based Processes Research Articles

Understanding in-process responses in multi-layer friction stir additive manufacturing: Temperature, viscosity, tool torque, and mechanical properties

Most melt-based processes greatly inhibit additive manufacturing of high-strength aluminium alloys due to porosity, cracking, and distortion. Friction stir additive manufacturing (FSAM) greatly enhances the printability of such alloys by avoiding melting. However, the repeated heating and cooling during the multilayer fabrication severely degrades the structure and properties of the final build. The effects of thermal cycles, peak temperatures, and cooling rates that substantially degrade the properties are not well reported in the literature. The processing conditions that control the complex viscoplastic flow of the material and the in-process force responses on tool important in order to understand the influence of the possible defects in the final build need to be better reported. Therefore, a systematic numerical and experimental study is conducted to quantitatively understand the spatial and temporal evolution of the build properties, bead profiles, tool torque, and traverse force for the first time in the FSAM literature. The results, which have been rigorously tested and verified, show that the peak temperature, cooling rate, bead profile, tool torque and traverse force were more sensitive to the print height, followed by traverse speed and tool rotation speed. However, the degradation of mechanical properties was found to be least affected by the higher traverse speeds as a result of the lower peak temperatures and the duration of thermal exposure. The numerically computed results corroborated well with the corresponding experimentally measured results, and the results from the independent literature, further enhancing the reliability of our findings. Further, the direct correlation between process variables and the final build properties via in-process responses could substantially reduce the existing trial-and-error approaches in the manufacturing of aluminum alloy structures through the FSAM route.

Forefront Research of Foaming Strategies on Biodegradable Polymers and Their Composites by Thermal or Melt-Based Processing Technologies: Advances and Perspectives.

The last few decades have witnessed significant advances in the development of polymeric-based foam materials. These materials find several practical applications in our daily lives due to their characteristic properties such as low density, thermal insulation, and porosity, which are important in packaging, in building construction, and in biomedical applications, respectively. The first foams with practical applications used polymeric materials of petrochemical origin. However, due to growing environmental concerns, considerable efforts have been made to replace some of these materials with biodegradable polymers. Foam processing has evolved greatly in recent years due to improvements in existing techniques, such as the use of supercritical fluids in extrusion foaming and foam injection moulding, as well as the advent or adaptation of existing techniques to produce foams, as in the case of the combination between additive manufacturing and foam technology. The use of supercritical CO2 is especially advantageous in the production of porous structures for biomedical applications, as CO2 is chemically inert and non-toxic; in addition, it allows for an easy tailoring of the pore structure through processing conditions. Biodegradable polymeric materials, despite their enormous advantages over petroleum-based materials, present some difficulties regarding their potential use in foaming, such as poor melt strength, slow crystallization rate, poor processability, low service temperature, low toughness, and high brittleness, which limits their field of application. Several strategies were developed to improve the melt strength, including the change in monomer composition and the use of chemical modifiers and chain extenders to extend the chain length or create a branched molecular structure, to increase the molecular weight and the viscosity of the polymer. The use of additives or fillers is also commonly used, as fillers can improve crystallization kinetics by acting as crystal-nucleating agents. Alternatively, biodegradable polymers can be blended with other biodegradable polymers to combine certain properties and to counteract certain limitations. This work therefore aims to provide the latest advances regarding the foaming of biodegradable polymers. It covers the main foaming techniques and their advances and reviews the uses of biodegradable polymers in foaming, focusing on the chemical changes of polymers that improve their foaming ability. Finally, the challenges as well as the main opportunities presented reinforce the market potential of the biodegradable polymer foam materials.

Current status and future perspectives of EU ceramic breeder development

• EU reference tritium breeding pebbles are produced using a melt-based process. • Process scale-up is underway to ensure the supply for ITER. • Biphasic materials show enhanced mechanical strength and stability. • Design relevant data were summarised in a material property handbook. • Crucial neutron irradiation experiments will complement the material qualification Biphasic ceramic pebbles, consisting of Li 4 SiO 4 and Li 2 TiO 3 , are being developed as the EU reference tritium breeding material for ITER and DEMO. A modified melt-based process for the pebble fabrication was established and optimised with regard to the process stability and yield. In parallel, the properties of pebbles and pebble beds were evaluated in dedicated experiments. These include investigations into e.g. the long-term thermal stability of pebbles, the thermomechanical properties of pebble beds, as well as the compatibility between the ceramics and the structural material. The available data was recently summarised and issued as the Material Property Handbook on Advanced Ceramic Breeders. During the EU DEMO conceptual phase, two main issues are proposed to be pursued in view of ITER. Irradiation campaigns are planned to ensure that the advanced ceramic breeder material is adequately tested and an extensive process scale-up with an increased production rate is foreseen to secure the supply for the ITER test blanket modules.

Carbon nanotubes and their polymeric composites: the applications in tissue engineering

Carbon nanotubes (CNTs), with unique graphitic structure, superior mechanical, electrical, optical and biological properties, has attracted more and more interests in biomedical applications, including gene/drug delivery, bioimaging, biosensor and tissue engineering. In this review, we focus on the role of CNTs and their polymeric composites in tissue engineering applications, with emphasis on their usages in the nerve, cardiac and bone tissue regenerations. The intrinsic natures of CNTs including their physical and chemical properties are first introduced, explaining the structure effects on CNTs electrical conductivity and various functionalization of CNTs to improve their hydrophobic characteristics. Biosafety issues of CNTs are also discussed in detail including the potential reasons to induce the toxicity and their potential strategies to minimise the toxicity effects. Several processing strategies including solution-based processing, polymerization, melt-based processing and grafting methods are presented to show the 2D/3D construct formations using the polymeric composite containing CNTs. For the sake of improving mechanical, electrical and biological properties and minimising the potential toxicity effects, recent advances using polymer/CNT composite the tissue engineering applications are displayed and they are mainly used in the neural tissue (to improve electrical conductivity and biological properties), cardiac tissue (to improve electrical, elastic properties and biological properties) and bone tissue (to improve mechanical properties and biological properties). Current limitations of CNTs in the tissue engineering are discussed and the corresponded future prospective are also provided. Overall, this review indicates that CNTs are promising “next-generation” materials for future biomedical applications.

Study of lithium germanate additions to advanced ceramic breeder pebbles

1. Abstract Worldwide, lithium rich ceramic pebbles are being developed to be used as tritium breeders inside future fusion reactors. Upon irradiation, the pebbles will produce tritium which will be used alongside deuterium to fuel the fusion reaction. It is essential that these pebbles are of sufficient quality to withstand the various forces experienced within the wall of the reactor. Currently, a melt-based process is used at the Karlsruhe Institute of Technology for the production of pebbles composed of lithium orthosilicate with a secondary strengthening phase of lithium metatitanate. Recent studies have focused on incorporating lithium orthogermanate, which forms a solid solution with lithium orthosilicate. Various compositions were used to perform fundamental studies to determine phase transitions as well as melting points for processing. The melt-based KALOS process was then used to produce pebbles with selected compositions. Thereafter the pebbles underwent a series of characterisation tests including microstructure and surface analysis using REM as well as elemental mappings using EDX. XRD was used for determining the crystal structures present in various compositions and the unit cell volumes. The measurements showed that the unit cell volume of each structure linearly depends on the composition. Additionally, pebbles were held at reactor relevant temperatures to determine any changes to the mechanical strength. It was found that pebbles with 30 mol% lithium metatitanate and a lithium orthosilicate/orthogermanate ratio in the range of 1.5–4 showed an increased strength as well as good thermal stability and may have the potential to be used as tritium breeders.

Development of a Segmented Temperature Control for Targeted Solidification in Injection Molding

Abstract One of the largest challenges in production using melt-based processes is the manufacturing of precise parts. Especially due to high differences in temperature of the produced parts while processing, shrinkage cannot be avoided, since the processing material is typically heated up, molded and cooled down. In injection molding, the molten plastic is loaded by high variations in temperature and pressure while being processed. The gradients can lead to a significant change of the local specific volume, shrinkage potential and inner stresses, which can result in part warpage. In order to increase the precision of the manufactured parts, the shrinkage potential has to be homogenised to achieve an even shrinkage and therefore minimise part warpage. In this work, the approach for homogenisation of the shrinkage potential is a homogenisation of the specific volume and density respectively. Pursuing the goal of a homogeneous distribution of the specific volume leads to manipulation of the influencing factors namely temperature and pressure. The impact of temperature and pressure changes on the specific volume can further be quantitatively described by the material specific pvT-data. Based on geometric restrictions, a manipulation of the local pressure inside the cavity in most cases is impossible. However, on the other hand, a local control of the temperature is possible using highly dynamic tempering techniques. Based on this line of arguments, the paper describes the development of a highly segmented dynamic temperature control in injection molding to locally influence the mold and part temperature. Thereby, the specific volume of the part will be locally adjusted to reduce warpage as well as to compensate process variations occurring by changing material properties or varying ambient conditions. Due to the nature of thermal processes a special control strategy has to be developed to enable accurate temperature control, which is able to compensate slow thermal reaction in a highly dynamic process.

The flow properties and presence of crystals in drug-polymer mixtures: Rheological investigation combined with light microscopy

The presence of solid matter in polymer melts affects the rheological properties of a drug-polymer mixture, and thus the processability of these mixtures in melt-based processes. The particle morphological changes related to dissolution and crystal growth in the mixtures of paracetamol and ibuprofen with polyethylene oxide and methacrylate copolymer (Eudragit® E PO) were observed by polarized microscopy simultaneously while measuring their rheological properties within temperature ranges relevant for melt processes, such as hot melt extrusion and fused deposition modeling 3D printing. The dissolution of solid crystalline matter into the molten polymer and its effects on the rheological parameters showed that the plasticization effect of the drug was highly dependent on the temperature range, and at a temperature high enough, plasticization induced by the small-molecule drugs could enhance the flowability even at very high drug loads. Therefore, even supersaturated mixtures can be plasticized efficiently, enabling their melt processing, such as hot melt extrusion or 3D printing. The combination of rheometry and polarized light microscopy proved to be very useful for studying the link between morphological changes in the drug-polymer and the flow behavior of the drug-polymer mixtures at different temperature ranges and deformation modes.

Characterisation and radiolysis of modified lithium orthosilicate pebbles with noble metal impurities

Modified lithium orthosilicate (Li4SiO4) pebbles with additions of titanium dioxide (TiO2) are suggested as an alternative tritium breeding ceramic for the European solid breeder test blanket module. The noble metals – platinum (Pt), gold (Au) and rhodium (Rh), can be introduced into the modified Li4SiO4 pebbles during the melt-based process, due to the corrosion of Pt-Rh and Pt-Au alloy crucible components. In this study, the surface microstructure, chemical and phase composition of the modified Li4SiO4 pebbles with different contents of the noble metals was analysed. The influence of the noble metals on the radiolysis was evaluated after irradiation with accelerated electrons (E=5MeV), up to 12MGy absorbed dose at 300–345K in a dry argon atmosphere. Using electron spin resonance (ESR) spectroscopy, it was determined that the noble metals (up to 300ppm) do not significantly influence the formation and accumulation of radiation-induced defects (RD) in the modified Li4SiO4 pebbles.

The reprocessing of advanced mixed lithium orthosilicate/metatitanate tritium breeder pebbles

The recycling of tritium breeding materials will be necessary for any future use of nuclear fusion energy due to economical as well as ecological considerations. In the case of the solid breeder blanket concept, the ceramic pebble beds that are intended for the generation of tritium will eventually need to be restored due to depleted lithium levels as well as due to fractured pebbles, which will cause a deterioration of the pebble bed properties. It is proposed that the pebbles, which are fabricated using a melt-based process, are recycled using the same initial process, by replenishing the lithium levels and reforming the pebbles at the same time. To prove this recycling scheme, advanced ceramic pebbles were fabricated and then re-melted multiple times to prove that the reprocessing did not have any negative effect on the pebble properties and secondly, pebbles were produced with a simulated lithium burn-up and subsequently replenished by additions of LiOH to the melt. It was shown that the re-melting and lithium re-enrichment had no effect on the pebble properties, demonstrating that a melt-based process is suitable for recycling used breeder pebbles.

Continuous micro-/nano-fiber composites of polyamide 6/polyethylene oxide with tunable mechanical properties using a novel co-extrusion technique

Continuous fibrous composite tapes of polyamide 6 (PA6)/polyethylene oxide (PEO) containing over 16 thousand individual ribbon-like PA6 micro-/nano-fiber domains were produced using a novel co-extrusion and two-dimensional multiplication technique. This melt-based process is suitable for scaling-up, applicable to any melt-processable polymers, and highly adjustable in terms of material composition, tape geometry, and number/size of the fiber domains. The PA6 fibrous tapes were post-oriented to achieve superior mechanical properties that are comparable with commercial strong polymeric fibrous composite tapes. The high tunability of tape and fiber structure and properties makes the co-extruded PA6/PEO tapes a desirable candidate for various strong tape applications.

Effect of Molecular Weight on the Electrophoretic Deposition of Carbon Black Nanoparticles in Moderately Viscous Systems

Electrophoretic deposition from viscous media has the potential to produce in-mold assembly of nanoparticles onto three-dimensional parts in high-rate, polymer melt-based processes like injection molding. The effects of the media's molecular weight on deposition behavior were investigated using a model system of carbon black and polystyrene in tetrahydrofuran. Increases in molecular weight reduced the electrophoretic deposition of the carbon black particles due to increases in suspension viscosity and preferential adsorption of the longer polystyrene chains on the carbon black particles. At low deposition times (≤5 s), only carbon black deposited onto the electrodes, but the deposition decreased with increasing molecular weight and the resultant increases in suspension viscosity. For longer deposition times, polystyrene codeposited with the carbon black, with the amount of polystyrene increasing with molecular weight and decreasing with greater charge on the polystyrene molecules. This deposition behavior suggests that use of lower molecular polymers and control of electrical properties will permit electrophoretic deposition of nanoparticles from polymer melts for high-rate, one-step fabrication of nano-optical devices, biochemical sensors, and nanoelectronics.

Fabrication of modified lithium orthosilicate pebbles by addition of titania

Lithium orthosilicate pebbles are one of the ceramic tritium breeder materials destined for the European solid breeder test blanket modules of ITER, the large-scale scientific experiment intended to prove the viability of fusion as an energy source, presently under construction in Cadarache, France. While the current reference material is fabricated by melt-spraying with 2.5wt.% excess of silica, resulting in a two-phase material of lithium orthosilicate and metasilicate, a modified melt-based process was used to fabricate breeder pebbles with additions of titania in order to obtain pebbles with lithium metatitanate as a secondary phase. The fabricated two-phase pebbles exhibit a fine-grained microstructure and increased crush loads. The optimum titanate content has yet to be evaluated, nonetheless the pebbles may have the potential to combine the advantages of both lithium orthosilicate and metatitanate breeder ceramics.

Effect of Processing Parameters on the Electrophoretic Deposition of Carbon Black Nanoparticles in Moderately Viscous Systems

Polymer-melt-based manufacturing processes for nanostructures offer high-rate, environmentally friendly, and commercially viable alternatives to solution-based methods. In this work, electrophoresis of a model carbon black and polystyrene system with moderate viscosity was used to investigate the viability of adapting nanoassembly processes to the high viscosity environment of polymer melts. The presence of polystyrene did not prevent deposition of carbon black, but deposition rates decreased at shorter deposition times; deposition was not linear with increasing applied voltage; and greater solution concentrations reduced the critical voltages (i.e., the voltage at which the rate of deposition changed). X-ray photoelectron spectroscopy (XPS) results and comparison of experimental data with Hamaker's model showed that about 1.6% of the available polystyrene was initially deposited with the carbon black. At voltages above the critical voltage, the deposited mass was less than the Hamaker prediction, indicating the formation of electrically insulating layers on the electrodes. The overall behavior suggests that polymer melt-based processes could be employed for high-rate fabrication of nano-optical devices, biochemical sensors, and nanoelectronics.

Osteochondral Tissue Engineering Constructs with a Cartilage Part Made of Poly(L-lactic Acid) / Starch Blend and a Bioactive Poly(L-Lactic Acid) Composite Layer for Subchondral Bone

Articular cartilage has an inadequate natural rebuilding capacity. Tissue engineering has shown to have potential to provide an effective alternative to engineer the damaged cartilage. In this study, an integrated porous bi-layered scaffold was developed aiming to mimic the requirements of cartilage and underlying subchondral bone. The osteochondral approach explored in this work was to include a common polymeric component in both cartilage and bone components, which maximised the integration at the interface by mean of a melt-based processing route. A blend of starch and poly(Llactic acid),PLLA, was used in the cartilage side, which was found to possess an adequate water uptake capability. For the bone region, to induce bioactivity, PLLA had been reinforced with hydroxyapatite (HA) and bioactive glass (BG). The surfaces of the constructs were investigated as a function of soaking time in a simulated body (SBF) fluid using scanning electron microscopy (SEM) and FTIR. The SEM – FTIR indicated a bone-like apatite formation and the surface coverage by apatite layer increased with increasing soaking time, whereas the cartilage-layer did not exhibit the formation of any apatite like layer.

Alternative tissue engineering scaffolds based on starch: processing methodologies, morphology, degradation and mechanical properties

An ideal tissue engineering scaffold must be designed from a polymer with an adequate degradation rate. The processing technique must allow for the preparation of 3-D scaffolds with controlled porosity and adequate pore sizes, as well as tissue matching mechanical properties and an appropriate biological response. This communication revises recent work that has been developed in our laboratories with the aim of producing 3-D polymeric structures (from starch-based blends) with adequate properties to be used as scaffolds for bone tissue engineering applications. Several processing methodologies were originally developed and optimised. Some of these methodologies were based on conventional melt-based processing routes, such as extrusion using blowing agents (BA) and compression moulding (combined with particulate leaching). Other developed technologies included solvent casting and particle leaching and an innovative in situ polymerization method. By means of using the described methodologies, it is possible to tailor the properties of the different scaffolds, namely their degradation, morphology and mechanical properties, for several applications in tissue engineering. Furthermore, the processing methodologies (including the blowing agents used in the melt-based technologies) described above do not affect the biocompatible behaviour of starch-based polymers. Therefore, scaffolds obtained from these materials by means of using one of the described methodologies may constitute an important alternative to the materials currently used in tissue engineering.

A new approach based on injection moulding to produce biodegradable starch-based polymeric scaffolds: morphology, mechanical and degradation behaviour

One of the present challenges in polymer scaffold processing is the fabrication of three-dimensional (3D) architectures with an adequate mechanical performance to be used in the tissue engineering of hard tissues. This paper describes a preliminary study on the development of a new method to produce biodegradable scaffolds from a range of corn-starch-based polymers. In some cases, hydroxlapatite was also used as a reinforcement of the biodegradable polymers. The developed methodology consists of a standard conventional injection moulding process, on which a solid blowing agent based on carboxylic acids is used to generate the foaming of the bulk of the moulded part. The proposed route allows for the production of scaffolds with a compact skin and a porous core, with promising mechanical properties. By using the developed method it is possible to manufacture biodegradable polymer scaffolds in an easy (melt-based processing) and reproducible manner. The scaffolds can be moulded into complex shapes, and the blowing additives do not affect the non-cytotoxic behaviour of the starch-based materials. The materials produced using this method were evaluated with respect to the morphology of the porous structure, and the respective mechanical properties and degradation behaviour. It was demonstrated that it is possible to obtain, by a standard melt based processing route, 3D scaffolds with complex shapes that exhibit an appropriate morphology, without decreasing significantly the mechanical properties of the materials. It is believed that the optimisation of the proposed processing methodology may lead to the production of scaffolds that might be used on the regeneration of load-bearing tissues.

Superconducting cuprates prepared by the melt quench process and containing excess Y or additives

Bulk YBaCuO samples doped with excess Y or other additives (including rare earths and Nb) and prepared by means of the melt quench process exhibit high magnetization and diamagnetic susceptibility. These can be attributed to the formation of a microstructure which is highly aligned and has few cracks and voids. These characteristics are associated with the presence of a second phase. The importance of this study lies in its applicability to the preparation of polycrystalline superconducting oxides with very high J c values through careful control of their microstructure, in particular through the use of non-stoichiometric formulations and the introduction of additives in conjunction with melt-based processing methods.