metal AM components.
1.2 Powder Bed Fusion AM
Powder bed fusion (PBF) refers to a class of AM processes. The most common machine tool for metal additive manufacturing is the PBF systems. This type of machine operates by selectively fusing powder following a layer‐by‐layer deposition principle. The powder is contained in a silo and fed to a re‐coater to evenly distributed powder between consolidation operations. The tool distributing the powder can be in the form of a scraper, a roller, or a blade that is designed to both fluidize and distribute the powder. As this tool has a major influence on the distribution of the layer, the powder and the tool have to be carefully matched to the application. Two kinds of PBF technologies are based on the laser (L‐PBF) and the electron‐beam (EB‐PBF) as the power source [21]. The L‐PBF process is more versatile since it can operate under a neutral gas atmosphere while EB‐PBF requires a vacuum. However, electron beams are faster and can be controlled efficiently through magnetic mirrors and hence offer certain advantages. The high‐density parts and small voxel size distinguish powder bed fusion from other metal AM processes for manufacturing functional components. In the past 20 years, PBF technologies (especially L‐PBF) have seen a rapid increase across various sectors, such as aerospace, biomedical, tooling, and energy sectors [3]. Several review articles discuss PBF process–structure–property relations and establish applications for Type‐I functional materials [22, 23]. A much smaller portion of the literature is focused on more complex functionally gradient metal PBF. Design of functionally gradient additive manufacturing (FGAM) components encompasses distributed site‐specific properties with gradual transitions in geometry, chemical compositions, constituents, or microstructures. A lack of guidelines on the selection and distribution of materials has hindered the development of FGAM, thereby limiting the microstructural design and arrangement of transition phases. Although some commercial software packages exist for FGM's and multi‐materials 3D printing, they are still far removed from real industrial applications and the ever‐increasing demands of novel functionally graded components. FGAM of PBF is a highly underexplored field for metals, especially in the material gradients domain. An overview of the literature on metal PBF functional components is presented in the following sections according to geometrical and material gradients.
1.2.1 Geometric Gradients in PBF
Porous metals in various forms, including foams and lattices, have been investigated for many years for a wide variety of applications [24, 25]. These range from lightweight structures to functional applications such as heat exchangers or electrodes. Cellular solids have been investigated to determine relationships between their geometry and their physical properties (thermo‐mechanical, acoustic, impact‐toughness, etc.). The motivation for much of this research is high‐performance applications; for example, in simultaneous structural light‐weighting and sound dampening in motor vehicles/aircraft, thus providing high functionality with minimized resource consumption. Recent advances in lattice design have enabled the creation of structures with spatially varying solid volume fraction; these are termed as functionally graded lattices. Gibson‐Ashby models have developed a theoretical framework for cellular lattice materials that are well suited for generating topology optimized designs. Powder bed fusion systems are uniquely suitable to manufacture the highly complex lattice structures with struts as small as 100 μm.
Maskery et al. [26] investigated the mechanical properties of graded density Al‐Si10‐Mg lattice structures manufactured by L‐PBF for structural applications and found significant improvement in compressive and impact strength. Choy, Sun, Leong, and Wei [27] showed similar improvements in mechanical properties for Ti‐6Al‐4V. Li, Hassanin, H., Attallah, Adkins, and Essa [28], and Tan et al. [29] investigated TiNi Shape Memory Alloys (SMA) fabricated by L‐PBF. The process conditions that lead to the microstructural characteristics play a major role in the SMA alloy properties. L‐PBF of TiNi lattices can enable tailored properties, in addition to the functionality associated with the shape‐memory effect. Li et al. [28] used a topology optimized design to manufacture a negative Poissons ratio metamaterial.
Functionally graded metal lattice structures with tailored mechanical response are most commonly used in biomedical applications like implants. The human bone is a functionally graded anisotropic structure, and hence any biocompatible implants need to have similar mechanical performance. A large surface area also helps toward osseointegration and thus leads to fewer implant rejections. A large body of research thus focuses on scaffolds of biocompatible materials like CoCrMo, Titanium alloys, and, more recently, biodegradable alloys of Mg, Zn, and Fe [30]. Hazlehurst, Wang, and Stanford [31] investigated L‐PBF of CoCrMo FGMs to imitate bone for a hip implant that is half as light, and twice as flexible compared to a solid metal structure. Ataee, Li, Fraser, Song, and Wen [32] researched high porosity (82–85%) Ti64 scaffolds with varying unit cell dimensions by EB‐PBF and found that the ratio of elastic modulus anisotropy in orthogonal directions was comparable to those of a human trabecular bone. Yan, Hao, Hussein, and Young [33] and Yu, Sun, and Bai [34] focused on increasing the surface area by using Triply Periodic Minimal Surfaces (TPMS) in their lattice designs. Zhang, Fang, Leeflang, Zadpoor, and Zhou [35] took an alternate approach through a stepwise topological design based on diamond unit cells to mimic the structure of the femoral diaphysis through L‐PBF of Ti‐6Al‐4V.
1.2.2 Material Gradients in PBF
The application of multi‐materials in a component expands the already large design space provided by additive manufacturing processes. Directed energy deposition processes (DED) are typically associated with multi‐material additive manufacturing in metals due to their intrinsic system‐related flexibility of changing to different feedstocks during operation, as further discussed in Section 1.3. However, PBF‐based additive manufacturing processes exhibit specific advantages over DED. A prominent example is the possibility to generate more intricate geometries and achieve smaller feature sizes. At the same time, most commercially available PBF systems are limited to a single powder feedstock restricting the deployment of different materials during the build process. This section deals with Type‐II, III material gradients in PBF [36].
The most common kind of multi‐material PBF components are Type‐II involving the use of an existing prefabricated structure of material A and adding a second material B. While the process itself doesn't change, the multi‐material components often reduce the manufacturing time thus combining the best of conventional and additive manufacturing. Another Type‐II multi‐material methodology involves premixed powder compositions that offer significant improvements in desired properties. The review paper by Bandyopadhyay and Heer [20] showcases several examples of Type‐II PBF multi‐material. More recently, Chen et al. [37] investigated L‐PBF of steel–bronze multi‐material components and found brittle cracks at the interface. Hinojos et al. [38] used EB‐PBF to make IN718/SS316 and SS316/IN718 components and found that the high temperatures in EB‐PBF make it less susceptible to solidification cracking. AlMangour, Grzesiak, and Yang [39], Han et al. ([40]), Kun, Beibei, Wenheng, and Cailin ([41]), and Xia et al. [42] worked with premixed titanium composite powders to improve the wear resistance and hardness of L‐PBF Ti parts. Recent works successfully demonstrated microstructural control during PBF to make Type‐III components. Niendorf et al. [43] used process control to modify the microstructure of SS316 in L‐PBF and Koptyug et al. [44] used a similar mechanism in EB‐PBF of SS316. Biondani, Bissacco, Mohanty, Tang, and Hansen [45] showcased a hybrid process chain to make Type‐II multi‐material L‐PBF components with a mirror‐like finish for molding applications.
Anstaett, Seidel, and Reinhart [46] from Fraunhofer, IGCV showcased a multi‐material FGAM part of Copper–Chrome–Zirconia and Tool Steel 1.2790. New systems were used for handling the selective layout of powder within the individual recoated layer for powder‐bed systems [47]. Demir and Previtali [48] introduced a prototype L‐PBF system with two powder feeders in combination with piezoelectric transducers
