Showing papers in "Advanced Engineering Materials in 2019"

Selective Laser Melting to Manufacture “In Situ” Metal Matrix Composites: A Review

Abstract: After a brief introduction on selective laser melting (SLM) and ex situ manufacture of metal matrix composites (MMCs), this paper reviews the capacities and benefits of SLM to activate and control ...

Polymer‐Derived Ultra‐High Temperature Ceramics (UHTCs) and Related Materials

Abstract: Ultra-high temperature ceramics (UHTCs) represent an emerging class of materials capable of providing mechanical stability and heat dissipation upon operation in extreme environments, e.g., extreme heat fluxes, chemically reactive plasma conditions. In the last few decades, remarkable research efforts and progress were done concerning the physical properties of UHTCs as well as their processing. Moreover, there are vivid research activities related to developing synthetic access pathways to UHTCs and related materials with high purity, tunable composition, nano-scaled morphology, or improved sinterability. Among them, synthesis methods considering preceramic polymers as suitable precursors to UHTCs have received increased attention in the last few years. As these synthesis techniques allow the processing of UHTCs from the liquid phase, they are highly interesting, e.g., for the fabrication of ultra-high temperature ceramic composites (UHT CMCs), additive manufacturing of UHTCs, etc. In the present review, UHTCs are in particular discussed within the context of their physical properties as well as energetics. Moreover, various synthesis methods using preceramic polymers to access UHTCs and related materials (i.e., (nano)composites thereof with silica former phases) are summarized and critically evaluated.

Processing Magnesium and Its Alloys by High-Pressure Torsion: An Overview

Abstract: Magnesium and its alloys have attracted significant attention in recent years because they display high strength-to-density ratios, they are biodegradable and they provide a potential for hydrogen storage. Many investigations have examined the effect of high-pressure torsion processing on the microstructures and properties of these materials so that numerous reports are now available. This overview provides a summary of the observations reported to date on the structure and mechanical property evolution including the nature of grain refinement, the grain boundary misorientation distributions, texture evolution and the minimum grain size. For convenience, the mechanical properties are separated into hardness, tensile behavior and superplastic properties. It is shown that the mechanism of grain refinement differs from other metallic materials processed by severe plastic deformation but high strength may be achieved in magnesium alloys and exceptional ductility in pure magnesium. Hydrogen storage and corrosion behavior are also examined together with a discussion of recent attempts to produce magnesiumbased nanocomposites through processing by high-pressure torsion.

Sandwich Structures with Prismatic and Foam Cores: A Review

Abstract: This article provides a comprehensive review of recent research efforts on the development and characterization of sandwich structures with corrugated, honeycomb, and foam cores. The topics discussed in this review article include aspects of core-face bonding and reinforcement, enhancement of core mechanical properties and panel performance (including the role of structural hierarchy), and multifunctional advantages offered by different core constructions. In addition, the review discusses potential applications, including in morphing wing design, impact resistance, and ultralightweight applications. Future research directions are discussed.

Embedding Tracking Codes in Additive Manufactured Parts for Product Authentication

Abstract: Additive manufacturing (AM) process chain relies heavily on cloud resources and software programs. Cybersecurity has become a major concern for such resources. AM produces physical components, which can be compromised for quality by many other means and can be reverse engineered for unauthorized reproduction. This work is focused on taking advantage of layer-by-layer manufacturing process of AM to embed codes inside the components and reading them using image acquisition methods. The example of a widely used QR code format is used, but the same scheme can be used for other formats or alphanumeric strings. The code is segmented in a large number of parts for obfuscation. The results show that segmentation and embedding the code in numerous layers help in eliminating the effect of embedded features on the mechanical properties of the part. Such embedded codes can be used for parts produced by fused filament fabrication, inkjet printing, and selective laser sintering technologies for product authentication and identification of counterfeits. Post processing methods such as heat treatments and hot isostatic pressing may remove or distort these codes; therefore, analysis of AM method and threat level is required to determine if the proposed strategy can be useful for a particular product.

Investigation of Lightweight and Flexible Carbon Nanofiber/Poly Dimethylsiloxane Nanocomposite Sponge for Piezoresistive Sensor Application

Abstract: In this paper, highly flexible and compressible piezoresistive nanocomposites consisting of carbon nanofibers (CNFs) and polydimethylsiloxane (PDMS) are developed for sensing applications. Porous PDMS with a porosity of 74.7% is manufactured using sugar templates, and CNFs are infiltrated into the porous structures to tailor the material’s electrical resistivity. The highly flexible pressure sensors show piezoresistive characteristics up to 70% compressive strains due to the conductive CNF network and the porous microstructures of the fabricated nanocomposites. The reorganization of CNF conductive network is characterized using in-situ micromechanical tests within a scanning electron microscope, validating the piezoresistive sensing mechanism. The sensing performance under various maximum applied strains, elevated load rates, and long-term repeatability is characterized. The developed porous nanocomposite sponge is employed as a tactile wearable sensor.

Laser‐Induced Keyhole Defect Dynamics during Metal Additive Manufacturing

Abstract: Laser powder bed fusion (LPBF) metal additive manufacturing provides distinct advantages for aerospace and biomedical applications. However, widespread industrial adoption is limited by a lack of confidence in part properties driven by an incomplete understanding of how unique process parameters relate to defect formation and ultimately mechanical properties. To address that gap, high‐speed X‐ray imaging is used to probe subsurface melt pool dynamics and void‐formation mechanisms inaccessible to other monitoring approaches. This technique directly observes the depth and dynamic behavior of the vapor depression, also known as the keyhole depression, which is formed by recoil pressure from laser‐driven metal vaporization. Also, vapor bubble formation and motion due to melt pool currents is observed, including instances of bubbles splitting before solidification into clusters of smaller voids while the material rapidly cools. Other phenomena include bubbles being formed from and then recaptured by the vapor depression, leaving no voids in the final part. Such events complicate attempts to identify defect formation using surface‐sensitive process‐monitoring tools. Finally, once the void defects form, they cannot be repaired by simple laser scans, without introducing new defects, thus emphasizing the importance of understanding processing parameters to develop robust defect‐mitigation strategies based on experimentally validated models.

The Effect of High‐Pressure Torsion on Microstructure, Hardness and Corrosion Behavior for Pure Magnesium and Different Magnesium Alloys

Abstract: Severe plastic deformation by high pressure torsion (HPT) is used to process and refine the grain structure of commercial purity magnesium and AZ31, AZ91, and ZK60 magnesium alloys. Transmission electron microscopy shows that the microstructure of pure magnesium is characterized by a bi-modal grain size distribution with grains in the range of a few microns and ultrafine grains after HPT, whereas the magnesium alloys display a homogeneous ultrafine grain structure after processing. X ray diffraction analysis reveals that the AZ91 alloy displays the largest lattice microstrain and this alloy also exhibits the highest hardness after processing. The processed AZ31 and the ZK60 alloys show similar microstructures and maximum values of hardness. Contrary to earlier reports of significant improvements in the corrosion resistance of magnesium alloys in biological environments, the present results show that processing by HPT has no significant effect on the corrosion behavior of magnesium alloys in a 3.5% NaCl solution. By contrast, pure magnesium exhibits an increased corrosion resistance after HPT.