Additive manufacturing (AM) technology provides unique opportunities for producing net-shape geometries at the macroscale through microscale processing. This level of control presents inherent trade-offs necessitating the establishment of quality controls aimed at minimizing undesirable properties, such as porosity and residual stresses. Here, we perform a parametric study into the effects of laser scanning pattern, power, speed, and build direction in powder bed fusion AM on residual stress. In an effort to better understand the factors influencing macroscale residual stresses, a destructive surface residual stress measurement technique (digital image correlation in conjunction with build plate removal and sectioning) has been coupled with a nondestructive volumetric evaluation method (i.e., neutron diffraction). Good agreement between the two measurement techniques is observed. Furthermore, a reduction in residual stress is obtained by decreasing scan island size, increasing island to wall rotation to 45 deg, and increasing applied energy per unit length (laser power/speed). Neutron diffraction measurements reveal that, while in-plane residual stresses are affected by scan island rotation, axial residual stresses are unchanged. We attribute this in-plane behavior to misalignment between the greatest thermal stresses (scan direction) and largest part dimension.

Here we report the first example of a class of additively manufactured carbon fiber reinforced composite (AMCFRC) materials which have been achieved through the use of a latent thermal cured aromatic thermoset resin system, through an adaptation of direct ink writing (DIW) 3D-printing technology. We have developed a means of printing high performance thermoset carbon fiber composites, which allow the fiber component of a resin and carbon fiber fluid to be aligned in three dimensions via controlled micro-extrusion and subsequently cured into complex geometries. Characterization of our composite systems clearly show that we achieved a high order of fiber alignment within the composite microstructure, which in turn allows these materials to outperform equivalently filled randomly oriented carbon fiber and polymer composites. Furthermore, our AM carbon fiber composite systems exhibit highly orthotropic mechanical and electrical responses as a direct result of the alignment of carbon fiber bundles in the microscale which we predict will ultimately lead to the design of truly tailorable carbon fiber/polymer hybrid materials having locally programmable complex electrical, thermal and mechanical response.

Current additive manufacturing methods present the potential to construct net-shape structures with complicated architectures, thus eliminating the need for multi-step processing and fasteners/joints. Combined with these features is the ability to ascribe material properties at the sub-millimeter scale, inspiring multi-material, functionally graded designs. These features make additive manufacturing an attractive option for composite materials development. In an effort to extend this family of technologies beyond nano-and micro-composites, we explore the additive manufacture of multi-directional composite preforms. This exercise has served to highlight the aspects of additive manufacturing critical to composite and general materials processing, as well as to demonstrate the high fidelity between modeled and additively manufactured structures. Within the scope of composites development, we review the state-of-the-art and discuss challenges facing the broad adoption of additive manufacturing for directionally reinforced composites processing.

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The effect of tensile loading on the mechanical and electrical behaviour of carbon nanotube fibres spun continuously from an aerogel is discussed. While these fibres exhibit moderate strength (0.19 GPa) and elastic moduli (9.16 GPa), their piezoresistive behaviour, evaluated through active resistance measurements during tensile loading, demonstrates their potential for sensing applications in advanced composite materials. Insight into the failure behaviour of the aerogel-spun fibres is gained through microscopic examination of the failed fibres, as well as analysis of their electrical response. A new phenomenon for carbon nanotube fibres is reported-the aerogel-spun fibres are observed to undergo mild to severe kinking due to tensile failure. This kinking is attributed to compressive failure due to recoil from the fibre free ends. A simple analysis of experimental results yields a fibre compressive strength range of 172-177 MPa.