October_2021_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 2 1 1 5 (a) (b) (c) (d) (e) (f) I n recent years, additive manufactur- ing (AM), or 3D printing, has been at the forefront in the world of mate- rials science and engineering. AM is a part fabrication methodology by which the part is produced by adding materi- al layer-by-layer until the final designed geometry is achieved [1,2] . Numerous AM technologies have since been devel- oped that use feedstock material of var- ied shape and chemistry. Industry and academia are diligently working to in- crease the variety of materials that can be fabricated using AM. Metallic glasses (MGs) are alloys that do not have a crystal structure. This is generally achieved by rapidly quench- ing the alloy melt such that the atoms do not have sufficient time to crystal- lize, hence bypassing the crystallization process. Though, in theory, any metal couldbemade amorphous if thequench rate is sufficiently fast, most MG compo- sitions are designed with intrinsically slow crystallization behavior such that amorphous parts could be made us- ing experimentally achievable cooling rates. This amorphous microstructure is the reason MGs have properties that are different from conventional alloys. For example, most MGs possess high elas- ticity, strength, hardness, and corrosion resistance, which have inspired using MGs in high-performance applications like spacecraft hardware, biomedical technologies based on different feed- stock shapes and temperature rang- es (Fig. 2). Feedstock material in both amorphous and crystalline forms could be transformed through AM to produce fully amorphous parts or metal matrix composites, which is controlled by ad- justing the build parameters. Approximately 70% of the scien- tific literature covering 3D printing of MGs describes laser powder bed fusion (LPBF), where a laser (ormultiple lasers) is selectively exposed onto a powder bed to consolidate regions of interest [6] . Then a new layer of powder is applied on top and the process repeats until the final geometry is achieved. LPBF produces highly accurate parts using powder as feedstock, which could be sourced with relative ease and is avail- able in multiple chemistries. LPBF has enabled bulk MG parts produced out of any alloy system with no thickness limitation. However, the limitations present in LPBF of conventional met- als are also present when printing MGs such as porosity, high residual stress- es, introduction of oxygen and other defects during powder production and the printing process, scalability of part sizes, and overall cost [11,12] . Additional- ly, with printing MGs, the temperature range within the heat affected zone of subsequent layers must be controlled to not induce unwanted crystallization implants, and sports equipment [3,4] . Due to the fast cooling rate require- ments, there are strict dimensional lim- its in casting or melt spinning MGs, only allowing for relatively thin and simple geometries to ensure sufficient cooling throughout the part to avoid crystalli- zation. This hinders the ability for MGs to be made into net shape, hence re- ducing their versatility. Attempts have been made to circumvent this limita- tion, most notably through thermo- plastic forming (TPF) [5] . However, the tooling, temperature control, and limit- ed compatible alloy compositions have prevented the proliferation of the TPF of MGs. Another method to fabricate MG without the dimensional restraints due to the requisite cooling rates is through AM. Most AM processes employ high heating and cooling rates in the se- quential layering process and therefore it is a superb technology to fabricate MG components [6-8] . CURRENT AM TECHNOLOGIES Though MGs were first report- ed in the 1960s [9] , only recently has the global supply chain matured suf- ficiently to source MGs in a variety of chemistries and geometries as shown in Fig. 1 [10] . These form factors include powders, pellets, foils, sheets, plates, wires, and cylinders. This has allowed the development of multiple MG AM Fig. 1 — Photographs of metallic glasses in different forms: (a) wires, (b) pellets, (c) rods and cylinders, (d) powder, (e) cast plate, and (f) sheets and foils. Images reproduced fromBordeenithikasem [10] .