Recent advances in additive manufacturing have significantly expanded the design and fabrication capabilities of carbon fiber-reinforced polymer (CFRP) structures, particularly in the context of electromagnetic microwave absorption (EMWA). This review provides a comprehensive overview of the current state of research on EMWA properties of additively manufactured CFRP structures, focusing on EMWA mechanisms, polymer material, and additively manufactured microwave absorbers. Key topics include the EMWA mechanisms inherent to various fiber-reinforced materials and the role of additive manufacturing processes in tailoring EMWA performance. Moreover, the review paper summarizes the electromagnetic characteristics of various fiber-reinforced materials and evaluates the microwave absorption performance of additively manufactured absorbers, highlighting the trade-offs between electromagnetic and load-bearing performance. Furthermore, challenges and future perspectives are discussed to provide new insights into enhancing EMWA and balancing EMWA with load-bearing capabilities. It explores new possibilities for next-generation advanced additively manufactured CFRP microwave absorbers that maintain excellent load-bearing properties.

The ability to manufacture complex designs from multiple materials has long been a key objective for applications operating in extreme environments. Multi-material (MM) additive manufacturing (MMAM) has significantly enhanced the functionality of additive manufacturing (AM) by enabling the integration of dissimilar alloys while leveraging the inherent advantages of AM, including design flexibility, reduced material waste, and rapid production, with the ability to tailor mechanical properties through spatial material distribution and local processing conditions. This process unlocks unprecedented opportunities across industries such as aerospace, automotive, biomedical, energy, and nuclear sectors. This article provides a comprehensive review of the state-of-the-art in MMAM, focusing on the manufacturing processes, molten pool formation, alloy compatibility, and bimetallic interface characteristics—including microstructural and mechanical properties—as well as modeling and simulation approaches for performance prediction and optimization, with developments tracked from 2013 to 2024. This review article predominantly focuses on: (i) MM-laser powder bed fusion, (ii) MM-directed energy deposition, and (iii) MM-wire-arc AM by detailing the mechanisms of molten pool formation at the interface and dissimilar alloy material compatibilities. Subsequently, the article provides an in-depth analysis of the meso- and micro-structural characteristics at the interface in bimetallic structures across widely employed MMAM alloys. The mechanics of MMs under various mechanical properties are presented, including microhardness/micro-indentation, tensile, flexural, compression, and fatigue strength, which are critical for MMAM applications in extreme conditions. In addition, current modeling and simulation approaches for MMAM are discussed with respect to the challenges and opportunities to increase MMAM adoption. The article concludes with a future roadmap for advancing MMAM by overcoming feedstock and build material cross-contamination, monitoring the in situ process, standardizing MM testing, and further developing thermo-mechanical modeling, specifically, for MMAM.

Graphical abstract

The structural materials, which exhibit high toughness and high strain energy absorption, can be used in impact-resistant applications such as bulletproof vests, automobiles, and aerospace. Numerous studies indicate that functional gradient materials, which contain non-uniform density, exhibit excellent performance in energy absorption. During the compression test, the struts of the gradient porosity materials collapsed layer by layer, and this phenomenon optimizes the energy absorption capability of the materials. Furthermore, the collapse region or direction can be predicted and controlled by the design of the gradient porosity materials. In addition to the above concepts, this research also improves its energy absorption capacity through the following two strategies: (1) Chamfering the node of the porous structure to avoid the stress concentration, and (2) optimizing the angle between the struts of the porous material to enhance the ductility of the material. To fabricate the complicated gradient porosity structure, the structural materials were printed using the selective laser melting process with Ti-6Al-4V ELI alloys. Through the experiments conducted in this study, the structural strength was enhanced by up to 28% through structure design, and the energy absorption was improved by 19% compared to the gyroid structure, which has been reported to exhibit good energy absorption capabilities.

In the growing additive manufacturing industry, there is increasing demand for improved as-built surface quality of parts fabricated by the powder bed fusion (PBF) process, particularly in the aerospace, medical, and tooling industrial sectors. The surface finish of PBF parts is often suboptimal due to the inherent layer-by-layer fabrication process. Depending on the material used, the average surface roughness (Ra) of PBF components typically ranges from 5 to 50 μm. To address this issue, various strategies have been investigated, including optimizing printing process parameters, refining support designs, and upgrading laser hardware. In this study, we investigated the machine factors on the as-built surface quality of parts in the PBF process. Fully dense as-built 1.2709 tool steel parts were produced with a relative density of 99.9% using platform pre-heating. Without heat treatment, the as-built part exhibited an ultimate tensile strength of 1,135 ± 75 MPa, yield strength of 915 ± 120 MPa, and an elongation of 12 ± 3%. Vickers hardness was measured at 339 ± 35. Surface measurements were performed on parts placed across the substrate plate, with the Ra of as-built vertical walls averaging 22.6 ± 11.9 mm. Results showed that the surface quality of as-built 1.2709 tool steel parts, with a layer thickness of 30 μm, was significantly affected by their distance from the inert gas outlet and the laser center. This study demonstrates that the as-built surface quality of PBF parts can be controlled through more effective build job preparation without changing key processing parameters.

Hot isostatic pressing (HIP) of Hastelloy X alloy is an essential heat treatment process in manufacturing hot-end components for aerospace engines. This study investigated the microstructure evolution and mechanical properties of laser powder bed fusion-manufactured Hastelloy X superalloy at room and high temperatures under various HIP treatments. The results showed that as the HIP temperature increased, the recrystallization degree increased, with the proportion of low-angle grain boundaries decreasing from 49.7% at HIP1100 to 0% at HIP1210. The carbides along the grain boundaries evolved from particle distribution at HIP1100 to chain-like distribution at HIP1180 and coarsened at HIP1210. In the room temperature tensile test, specimens treated at HIP1100 exhibited the highest tensile strength due to restrained dislocation slip, grain refinement strengthening, and carbide dispersion strengthening. In the high-temperature tensile test, significant carbide coarsening was induced at HIP1100, while minimal changes were observed at HIP1180 and HIP1210. As the HIP temperature increased, the tensile strength and elongation both improved due to the synergistic effect of the reduced number of grain boundaries and chain-like distribution of carbides. The cracks primarily propagated along the grain boundaries, with the HIP1210 specimen showing a better capacity for crack inhibition.