Li, Zhang, and Chueh [49] demonstrated, for the first time, full spatial control in all directions in L‐PBF using a custom‐made system with multiple powder feeders and a point‐by‐point micro‐vacuum selective material‐removing system. Figure 1.3 shows the schematic of an Aurora Labs system with custom‐built software for multi‐material processing at DTU [36]. The powder feeder systems are accurate within a 1% error by weight of powder dispensed per layer. This allows precise control of the mixing percentage within each layer. An example is presented with a graded Type‐III steel sample with a hard phase (SS440C) and a soft phase (SS316L) alternating sinusoidally. Preliminary microstructure and microhardness tests reveal that the component is functionally graded as designed. One of the primary concerns of multi‐material PBF is the issue of material waste, due to difficulties in recycling the powder. Aerosint solves this issue through selective powder deposition. At DTU, the issue is addressed by means of tracking software, which automatically adjusts the ratio of the powder materials based on the history of the mixed powder and desired final composition per layer. Bodner et al. [50] used a unique PBF system with integrated liquid dispersed metal powder dispensation. The setup was used to additively manufacture a multilayered structure based on alternating Inconel 625 alloy (IN625) and 316L stainless steel (316L) layers on a 316L base plate. New methodologies for applying multi‐material powder in PBF are necessary for the wider adoption of FGAM components for critical functional applications.
Figure 1.3 Working schematic of the L‐PBF equipment, and functionally graded steel composite components built at The Technical University of Denmark (DTU).
1.3 Direct Material Deposition
1.3.1 Powder and Wire Feedstock for Near‐Net‐Shape AM
Direct Material Deposition (DMD) can be divided into two classes of AM processes based on the feedstock being powder or wire. The class of powder feedstock systems is called Directed Energy Deposition (DED), and with wire, the systems are classified underwire arc additive manufacturing (WAAM) [51]. In both cases, the deposition head consists of a power source (Laser/Wire arc) and a material deposition module to continuously add new material into the process. Often the deposition head is mounted on a five‐axis robot, and typically, there is an option to mix multiple materials in‐situ. The material flexibility in DMD processes makes them the most suitable for manufacturing functionally graded AM components. Theoretically, any additive process that allows adjustment of feedstock material mid‐build is viable. However, directed energy deposition (DMD) processes lend themselves best to functional grading due to the comparatively small feedstock losses as opposed to multi‐material PBF, since powder blends cannot be easily separated and recycled for reuse. Figure 1.4 shows a schematic of the DED process, wherein the deposition head is mounted on a five‐axis robot. Up to six materials can be used simultaneously in the setup. Laser deposited materials experience a complex thermal history marked by rapid solidification, high cooling rates, steep thermal gradients, and cyclic reheating and cooling from multiple laser passes. This can lead to the formation of nonequilibrium microstructures and significant variations in microstructure from layer to layer, as well as within individual layers. Hence, an online process‐control is achieved with the help of an in‐axis thermal camera that ensures similar thermal gradients in each new layer.
Hybrid Additive Manufacturing (HAM) combines the cost‐saving additive process with the dimensional accuracy of subtractive CNC machining [53]. When a metallic part is additively manufactured by direct material deposition, HAM allows the part to be machined after deposition occurs in the same system. This is largely different compared to the steps taken for conventional CNC techniques, where a block of a single material is placed into a CNC machine and is subjected to extensive subtractive processes to create a final part. A large amount of material waste is generated from the excess material during milling, which translates directly to manufacturing cost and environmental impact. With HAM, however, the initial block material is no longer present because the operator is building the part near net shape by direct deposition, requiring only surface finishing where necessary and creating little material waste. The presence of the deposition head on a robot‐arm leads to a straightforward integration with CNC to convert a typical DMD process into HAM. Several FGM applications are found for DED as compared to WAAM because the near‐net‐shape resolution of DED is small enough to have a much wider range of complex functional applications in the as‐built condition. Further, DED is better at repairing complex components due to the coaxial powder feeding mechanism and faster cooling rate, which allows for material deposition against gravity. The further sections thus focus on DED toward making functionally graded materials.
Figure 1.4 Schematic of powder feedstock based Direct Energy Deposition (DED) system installed at Force technology [52].
Photo credit: Venkata K Nadimpalli.
1.3.2 Functional Material Gradients in DED
A promising application of functionally gradient AM is the replacement of problematic dissimilar metal welds with additively manufactured graded transition joints. In the nuclear and aerospace industries, failures related to joints, and interfaces between dissimilar metals are frequent [54]. Typical joining processes produce sharp gradients in composition and properties that ultimately promote failure mechanisms in service. Furthermore, designs are limited by the need to assemble many individual parts composed of different materials in order to meet various services and cost requirements. Hence, for the case of low‐volume, high‐complexity parts, incorporating functional grading with additive manufacturing presents an attractive alternative. A major advantage of using gradient AM techniques is the ability to fabricate custom compositions, using phase diagrams as a map between desired compositions. In traditional alloy development, multicomponent alloys are made one point at a time until the phase diagram is understood. The gradients in AM processes allow for many compositions along a line to be fabricated in the same part, thus accelerating alloy development. For each gradient alloy, a defined gradient path can be selected for the AM building process, thus unlocking the possibility to reach the desired phase while avoiding the detrimental intermetallics. Table 1.1 below gives an overview of recent research on functional DED components.
Parts can be designed with gradient material properties and can be made by changing powder input. By doing this, the material behavior could be designed across the part by utilizing the changing material/mechanical properties of the material. In an example from Gu, Meiners, Wissenbach, Poprawe [23], a Ni‐Cr part was made with a designed negative coefficient of thermal expansion. Ideally, this process could be used to make structures that are piezoelectric and even have a negative Poissons ratio or make a ductile metal with a negative thermal expansion. Compositional gradation also increases the overall properties and integrity of the part since weld‐seam stress concentrations weakening joints are reduced. Hofmann et al. [69] performed finite elemental analysis (FEA) and have shown that a gradient transition in an automobile valve stem from steel to Inconel has approximately 10 times less stress concentration at the transitioning zone compared to a traditionally welded joint at the same operating temperatures. Another FGM example is that of LENS‐deposited Inconel transitioning to a copper alloy for increased thermal conductivity behaviors in high temperature heat exchangers.
