compared to Inconel 718 [70].
The review articles by Bandyopadhyay and Heer [20] list out several applications and research studies conducted on multi‐material AM components. Hence, in Table 1.1, the focus was on more recently published work and classifying them according to Type II–IV material gradients. The rapid increase in research on FGAM's and better DED process control, coupled with new software modules for design and simulation of FGAM components, indicates the potential for growth in this section. The ultimate goal is to have integrated materials design freedom at the voxel‐scale and the process‐control necessary to manufacture the designed FGAM's. Out of all the metal AM processes, DED is the closest toward achieving complete material design freedom.
Table 1.1 An overview of recently representative work on different types of materials gradients in DED.
| Materials | Type‐II | Function | References |
|---|---|---|---|
| 316/IN625 |
|
Optimized to avoid cracks | [55] |
| IN625/Cu | Bimetallic structure for thermal applications | [56] | |
| IN718/SS316L | Change in hardness and mechanical strength with no cracking | [57] | |
| TA15/IN718 with Cu/Nb interlayers | Interlayers allow the manufacture of traditionally unweldable alloys | [58] | |
| Type‐III | Continuous gradients | ||
| IN625/NiCrAlY |
|
Functionally graded interface with few/no cracks | [59] |
| 2.25Cr‐1Mo‐Steel/Alloy 800H | Graded Ferritic to Austenitic steels with limited Carbon diffusion | [60] | |
| Bulk metallic glasses Zr50 alloy (BMG) | Large gradient BMG structures that are otherwise difficult to produce | [61] | |
| Ti/Ti‐6Al‐4V | Compositionally graded with phase and microstructural control | [62] | |
| Ti‐6Al‐4V/Invar | Continuously graded samples with no cracks compared to the discrete interface | [63] | |
| Type‐IV | Composites + Continuous gradients | ||
| Ti‐B4C composite |
|
TiB precipitate control leads to MMC with equiaxed, fine α grains | [64] |
| Cp Ti‐NbC composite | Metal‐ceramic composites for high impact, high toughness applications (armor) | [65] | |
| Ti‐Al2O MMC w/ nanopowders | Fine α2 lamellar microstructure within a Ti matrix. High hardness was achieved | [66] | |
| TiB‐Ti composites | 3D quasi‐continuous precipitate network with high strength | [67] | |
| Hybrid Ti‐6Al‐4V wire + WC powder | High hardness and wear‐resistant composite | ([68]) |
1.4 Solid‐State Additive Manufacturing
Solid‐state AM is a class of processes that use friction and diffusion‐based mechanisms to produce mechanical bonding without melting. Solid‐state processes like ultrasonic welding and friction‐stir welding have established applications [71, 72]. They can also be converted into AM processes by integrating the weld head onto a three or five‐axis robot coupled with a CNC mill to make a hybrid AM system. Three such processes are Ultrasonic Additive Manufacturing (UAM), Friction Stir Additive Manufacturing [73], and Cold Spray Additive Manufacturing [74]. FAM and CSAM are relatively new technologies with few research papers in literature exploring hybrid functional components. Yin, Yan, et al. [75] showcased the use of cold spray as a hybrid AM process applied to L‐PBF components. UAM, on the other hand, is a well‐established and often ignored AM technology that can make unique functional parts. The rest of this section is hence dedicated to functional components made by UAM.
Ultrasonic Additive Manufacturing (UAM) is a solid‐state joining manufacturing process that is commonly used in conjunction with a CNC mill to make functional metal components [76]. UAM offers advantages in material properties as compared to traditional metal joining and forming technologies. It uses normal force coupled with low frequency mechanical ultrasonic vibrations to create a solid‐state weld between a thin foil and an existing substrate. Ultrasonic welding is an established industrial bonding process for plastics and soft metals. UAM is a hybrid AM process which is essentially layer‐by‐layer ultrasonic welding combined with CNC machining after each layer, hence providing freeform fabrication capability [77]. Given the right bonding parameters, completely solid‐state metallurgically bonded welds can be fabricated. The quality of UAM components depends on several process parameters, including normal force, vibration amplitude, and speed of bonding along with geometrical and environmental factors. For many years, the material systems available to be bonded by the UAM process were limited to softer Aluminum alloys (Al 3003) due to the high energy requirement for other engineering materials. The Fabrisonic UAM systems overcome this hurdle by using a high‐power transducer and load cell, which makes it feasible to build components from Copper, Nickel, and Iron‐based alloy systems.
UAM involves high‐speed relative motion between foils to be joined. Several researchers have studied the mechanism of bonding in ultrasonic metal welding [89]. The bonding process is a function of the three input parameters; the force applied, the velocity of bonding, and the vibration amplitude. It is also dependent on surface roughness, temperature, base plate characteristics, part geometry, and build height, among other factors. The bonding process can be separated into (a) volumetric bonding and (b) surface bonding effects. Volumetric bonding effects include elastic and plastic deformation enhanced through reduced yield stress due to acoustic and
