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 6 manner as filament-based 3D print- ing of thermoplastics [18] . However, the modest availability of MG compositions with suitable thermal stability and vis- cosity in addition to the relative diffi- culty to source MG wire and rods have limited the maturation of FFF. PATH TOWARD TECHNOLOGY INFUSION Additive manufacturing of MGs has made substantial strides toward technology infusion and commercial- ization. The assortment of AM tech- nologies applied to MGs can produce parts that cannot be achieved in other materials. Although the AM process in- troduces porosity and contaminants into the resultant part, the wear prop- erties of the 3D printed MG is compa- rable to the traditionally cast MG [11] . Applications that require high hard- ness and wear resistance, such as gears and excavation tools for extraterrestrial robotic exploration, were the first pro- totyped with LPBF, which have shown promising results (Figs. 3a and b) [19] . Wear-resistant casings for watches and electronics have also been prototyped. Capitalizing on the elasticity of MGs, guitar bridges that are more efficient at transmitting vibrations were demon- strated (Fig. 3c). The bridge is designed such that the sounds mimic traditional materials used in guitar bridges, such as brass, but with improved perfor- mance and part longevity provided by Fig. 2 — Diagram of the temperature ranges of various MG AM technologies that are discussed in this article, overlaid with a differential scanning calorimeter measurement of an arbitrary MG to illustrate key thermal events. that will deteriorate the integrity of the component. Another related technology is la- ser directed energy deposition (LDED), where feedstock powder is fed through nozzles and then ejected into the melt pool or path of the laser to build the part. LDED of MGs is most common- ly used to fabricate MG composites for wear-resistant applications [13] . The reso- lution of LDED is worse than LPBF but it allows for intermixing of different pow- ders, supplied via different nozzles and hoppers, to tailor the desired composi- tion and even produce compositional gradients [14] . Microstructural gradients from amorphous to crystalline are also possible. LDED is also used to clad MG alloys to a crystalline substrate at de- sired locations. Thermal spray additive manufac- turing (TSAM) is another technology that uses powder as feedstock. Using a conventional high-velocity oxygen-fu- el (HVOF) gun, the powder is thermal sprayed at high temperature onto the substrate in layers. The morphology of each layer is controlled using stencils or masks [12,15] . TSAM produces parts with relatively high porosity but extremely high cooling rates, allowing for the pro- duction of MG compositions that are difficult to be made amorphous other- wise. The process also has a high-vol- ume throughput, allowing parts to be built quickly. Cold spray additive man- ufacturing (CSAM) is a complementary technology to TSAM where amorphous powders are ejected at high velocities but at temperatures below the glass transition [16] . Each layer adheres due to the mechanical deformation of the sprayed powder. Sheets and ribbons, generally fab- ricated through melt spinning, are an- other form factor of MGs that are readily available in varied chemistries. Laser foil printing (LFP) is a similar process to LPBF that uses sheets or foils as feed- stock instead of powder [10] . LFP pro- duces parts with lower oxygen content than powder-based processes due to the much lower exposed surface area in the sheet production process. How- ever, the technology is less mature than LPBF and LDED; most LFP components were fabricated on custom-built labora- tory-scale equipment. Ultrasonic additive manufactur- ing (UAM) also utilizes MG sheets as feedstock. In UAM, an ultrasonic horn is used to consolidate sheets of mate- rial at or near room temperature. The high frequency relative motion along the interface causes the materials to fuse without exposing them to elevated temperatures, avoiding issues like crys- tallization and high residual stresses [17] . Fused filament fabrication (FFF) takes advantage of the TPF process in MGs and the existing infrastructure be- hind AM of polymers. MG wires or rods could be extruded through a heated nozzle and then printed in the same