Research progress on selective laser melting (SLM) of bulk metallic glasses (BMGs): a review

Amorphous alloys possess much higher strength and hardness, but relatively lower Young’s modulus, compared with traditional crystalline materials [121]. Among all the BMG systems, Fe-based BMGs are a special family with the higher strength and hardness value. The elastic modulus is similar to that of traditional stainless steels but the strength and hardness are even 3–4 times higher than those of super high-strength steels [64]. Fe-based bulk metallic glass is the most widely used in industrial systems. In 2013, Pauly was the first to report that the Fe-based bulk metallic glass was formed by SLM technology, and the 3D scaffold structure was successfully manufactured [69]. Additive manufacturing of amorphous alloy powder by SLM requires the regulation of laser power, scanning speed, scanning distance, and powder thickness. These parameters affect two important standards of amorphous 3D printing production: formation and crystallization. Figure 1 is the cooling curve of Fe-based amorphous additive manufacturing demonstrated by Pauly. The cooling rate R_SLM of Fe-based amorphous in SLM process is much higher than its critical crystallization cooling rate R_C of Fe-based BMGs.

Although a higher cooling rate can inhibit crystallization, it is inevitable that high-speed cooling rate will lead to the instability of the molten pool, resulting in bad molding in SLM. Thus, inhibition of crystallization and the perfect formation tend to be two conflicting requirements in the process of amorphous alloy manufacturing. Many experimental studies show that the amorphous structures with good molding and high amorphous ratio can be obtained within a specific range of parameters for the BMGs studied.

A large number of experimental studies by Jung have shown that the laser scanning rate to form completed amorphous structure should not be higher than 2500 mm/s [70]. When the laser scanning rate was too high, the powder particles cannot completely melt, resulting that many unmelted spherical particles can be observed in the internal metallograph. When the scanning rate was 1500mm/s, the structure with a density of more than 99% can be obtained which is shown in Fig. 2 a. X-ray diffraction (XRD) test showed that the amorphous samples with few nanocrystals could be obtained. Wang set a record to make the size of 45 mm × 20 mm block with Fe₅₅Cr₂₅Mo₁₆B₂C₂ bulk metallic glass, and amorphous samples with different parameters were also carried out [75]. The as-printed microstructure stays in a fully amorphous state, and good relative density has been realized. When the laser power was 100 W and the scanning rate was 300 mm/s (at the red box in Fig. 2b), almost completely dense and completely amorphous samples are obtained. Compared with the research of Jung, the parameters such as power and scanning rate vary greatly. Considering the difference of powder and device, difference in parameters is acceptable. In the SLM process, the general energy density is determined by the formula E=P/(V×t×h), P is the laser power, v is the laser scanning speed, t is the powder layer thickness, and h is the scanning spacing distance. On the other hand, as the energy density increases and the heat input increases, the formation capacity of the printed sample will undoubtedly increase. The parameters are different according to the powder composition, particle size, sphericity, and even the difference of equipment. According to most studies, the reasonable energy density is in the range of 30–100 J/mm³ [50, 79, 80, 83‐86]. Hofmann studied the effect of processing on the microstructure and properties of an FeCrMoBC glass-forming alloy through a variety of manufacturing parameters by SLM [74]. In this study, energy densities ranging from 20 to 300 J/mm³ were used and high energy density resulted in overmelted parts that would crack during solidification and low energy density resulted in loosely sintered parts. A single composition, fully amorphous powder feedstock, can be used to create an assortment of phases and microstructures in a single print: from fully amorphous to dendrite-reinforced metal matrix composite to fully crystalline.

Thus, the GFA of BMG powders are significantly lower compared to high-purity cast material [123]. SLM process parameters need to be carefully balanced to avoid crystallization while securing sufficient densification of the fabricated BMGs [97]. Wang pointed out that although the temperature in the heat-affected region can be higher than Tg, the corresponding time window/life of the high-temperature period is quite short, about 0.8 ms [75]. Crystallization is theoretically possible during this time, but in practice, an incubation period of about 1.87ms is required. In this case, nucleation cannot be completed, and the amorphous structure still existed due to kinetic factors. It was clarified that the lifetime of the molten pool is at the millisecond scale, corresponding to a rapid cooling rate of ∼1.38 × 106 K/s. During printing, other locations of the sample experience either low temperatures (< Tg) or very short exposure time to high heat input, therefore avoiding crystallization due to thermodynamic and/or kinetic reasons and ensuring an amorphous structure across the whole as-printed sample. Mahbooba used electron back scatter diffraction (EBSD) method (shown in Fig. 3) to detect smaller grains in microstructures because EBSD has a higher resolution than XRD. EBSD analysis showed that there were clusters of nanoparticles with low concentration [57]. Temperature, contamination, and mechanical stress are the main causes driving the nucleation of metallic glass [124]. Prolonged thermal exposure and thermal cycling during SLM may be the cause of nanocrystalline nucleation in his work. EBSD analysis showed that the nano-grains existed only in localized clusters in the whole cylinder. However, grain nucleation due to hot annealing usually shows uniform distribution [125].

In addition, although the heat flow in AM is directional, the nanoparticles are relatively equiaxial. Based on the localized and equiaxial nanocrystals, thermal annealing is unlikely to provide energy for grain nucleation. Alternatively, nucleation may be the result of oxygen contamination and mechanical stress. Oxygen can promote the formation of metastable quasicrystal which has lower activation energy than the competitive primary phase [126, 127]. However, information about the role of oxygen in Fe-BMGs is limited. Li reported that the GFA in FeMoCSiBP BMG was improved when oxygen addition was 0.02 to 0.15% [64]. It has been speculated that the critical level of oxygen lowers the liquidus temperature and inhibits the precipitation of primary phase. FeNiPC BMG has been reported to have similar results, with oxygen up to 0.0252% improving thermal stability without causing crystallization [128]. Interestingly, the composition of the alloy affects the oxygen content; in addition to the process elements, Zhang observed the top surface morphology of SLM Fe-based amorphous on stainless steel and found that some light gray sediments formed and tend to be distributed along the pool in the powder bed [87]. It was suggested that yttrium first reacts with oxygen during the melting of the amorphous alloy in the SLM process. Yttrium has a strong deoxidation capacity and yttrium oxide was easily formed, so the low gravity oxide will be pushed away and distributed along the molten pool. It was found that oxygen could be extracted with the aid of element Y to produce low-density oxides that floated onto the surface of metal parts. The conclusion is that adding appropriate elements can realize the purification of impurities in the SLM process and use in metal materials. Based on the localized and equiaxial nature of the nanograins in the SLM of FeCrMoCB alloy, it is unlikely that thermal annealing during DMLS processing provided the energy for grain nucleation [57]. A low concentration of nano-grains was exposed within the BMG microstructure.

Compared with iron-based amorphous, Zr-based amorphous alloys have better glass forming ability [129]. Therefore, although SLM technology was first used in the manufacture of iron-based amorphous, the research on Zr-based amorphous SLM is more extensive, according to the cooling conditions of Zr-based amorphous, as shown in Fig. 5. Because of the stability of zirconium atoms, the cooling conditions for the formation of Zr-based amorphous structure are less severe than that of Fe-based amorphous structure. On the one hand, this shows that the Zr-based amorphous alloy is not easy to crystallize; on the other hand, this reduces the difficulty of manufacturing. According to many studies [93, 97], the reasonable energy density range of SLM manufacturing Zr-based amorphous alloy is from 10 to 50 J/mm³, in which, when the energy density is less than 20 J/mm³, it is easier to form a completely amorphous structure [104].

Many studies have focused on improving crystallization and forming in addition to the process parameters. Wei [91] analyzed the residual stress of Zr-based amorphous alloy in the process of selective laser melting: (1) experimental and finite element simulation results show that, compared with X and Y strategies, the residual stress of XY cross-scanning strategy is relatively low, and the residual stress increases with the increase of bar thickness. On the other hand, residual stress can be adjusted by annealing or preheating the substrate. (2) Theoretical analysis shows that the XY cross-scanning method to reduce the thermal gradient and preheating the matrix can improve the uniform shrinkage during rapid cooling, thus reducing the residual stress. Multiple scans resulted in a more homogenous distribution of the constituent elements within the melt pool and therefore increased the amount of the amorphous phase. Single scan is likely to induce an unevenly distributed force which would cause elemental segregation to occur within the melt pool, which results in a very inhomogeneous and random distribution of the constituent elements. Li [104] used the one-way single scanning, two-way single scanning, and two-way double scanning, as shown in Fig. 6, and found that single scanning is easy to cause segregation of elements, while molten pool flow can exist for a long time so as to improve the element distribution by multiple scanning, which makes it more even to get more amorphous structure. In contrast to single scan, multiple scans can result in an averaging of the melt flow and therefore a more homogenous distribution of the elements [104]. The method of using multi-scanning to increase the amorphous rate is applicable to both iron-based and zirconium-based amorphous materials. However, whether the method of high energy density is beneficial to increase the amorphous rate remains to be verified.

Ouyang [93, 94] studied the heat treatment in Zr-based amorphous alloy SLM process. It was found that the toughness of the sample was strengthened, and the macroscopic performance was reduced after heat treatment. Due to the huge thermal stress during heat treatment, cracks are easy to form between the molten pool and the heat-affected area, which should be improved by controlling the reasonable cooling rate. In addition, structural heterogeneity was found between the molten pool and the heat-affected region, as shown in Fig. 7. It tends to be completely amorphous, and there are some crystalline phases in the heat-affected region, which is also proved by the differential scanning calorimetry (DSC) curve. Other peaks appear after the wider crystal peaks, while the different diffraction ring diameters of the amorphous phase indicate that the two amorphous phases are also different.

According to the high-resolution TEM observation of the microstructure after annealing, the microstructure in the molten pool is the clumps of very fine nanocrystals, and the mixture of fine nanocrystals and coarse crystals (the growth of the original nanocrystals) is in the heat-affected zone, which also proves the structural heterogeneity between the molten pool and the heat-affected zone. In fact, there is no difference in the phase transition temperature of alloys produced by different synthesis methods. The difference in phase transition temperature may be caused by the difference in nanocrystal and oxygen content. It was confirmed that the SLM sample had more supercooled liquid regions (T_x-T_g) and the transition time was very short during the isothermal heat treatment, and the oxygen content in the SLM sample was high. It was found that the nanocrystallized particles were rich in aluminum and oxygen, as shown in Fig. 8. It was found that different metastable crystal phases appeared in the initial crystal phases of SLM and castings. This difference is caused by different oxygen levels. But with the disappearance of metastable state and the formation of thermodynamically stable phases CuZr₂ and Al₃Zr₄ at high temperature, the final crystallization stage is similar. The results show that reheating of the consolidated material and higher oxygen content lead to the formation of nanocrystals in the SLM component. This is because the density produced by SLM is lower than that of as-cast [131]. Ouyang studied two representative BMG alloys with different GFA, Zr₅₅Cu₃₀Ni₅Al₁₀ (named as Zr55 with high GFA), and Zr_60.14Cu_22.31Fe_4.85Al_9.7Ag₃ (named as ZrAg with low GFA) metallic glass by SLM [132]. The results show that the 3D-printed BMG with low GFA always has higher amorphous phase content than the BMG with high GFA. ZrAg BMG followed the mode of primary-type crystallization with multiple phase formation, leading to a low crystal growth rate, while Zr55 BMG follows the mode of polymorphous-type crystallization, which is characterized with formation of a simple crystalline phase, thus leading to a high crystal growth rate.

Zhang [92] studied the crystallization behavior of SLM Zr55 alloys samples with the powder under different annealing conditions. The XRD patterns and DSC curves of different Zr55 powders are shown in Fig. 9 a and b. As shown in Fig. 9 a, for the original powder and the powder after 600 K annealing treatment, a superposition of a typical broad halo peak and a specific set of weak peaks (mixing of amorphous matrix and Al₅Ni₃Zr₂ phase) can be observed. When the annealing temperature is 800k, severe crystallization will occur; in addition to the Al₅Ni₃Zr phase, NiZr₂, Al₂Zr₃, and Cu₁₀Zr₇ phases are generated. During annealing at 1000K, the main crystallization peaks correspond to the CuZr₂ phase and Al₂Zr₃ phase. This indicates that during the high temperature annealing process, the metastable phases of NiZr₂ and Cu₁₀Zr₇ gradually decomposed and disappeared. They were replaced by the stable phases of CuZr₂ and Al₂Zr₃. Figure 9 c and d are the XRD diffraction patterns and DSC curves of SLM samples with different powder compositions. The X-ray diffraction patterns of all deposits are mainly composed of broad halo peaks and crystal diffraction peaks of different intensities shown in Fig. 9 c. Compared with the unannealed powder, the peak relative intensity of the ZrCu and CuZr₂ crystals in the deposit prepared by the annealed powder decreased significantly, indicating that the volume fraction of the crystals decreased accordingly. The remelting zone (upper left corner or upper right corner of Fig. 10) shows a featureless structure, which can remain amorphous. It should be noted that the majority of deposits and dendrites are made of unannealed powder (Fig. 10), except for the size of the annealed powder (Fig. 10f, h), which indicates that more serious crystallization from unannealed powder. The influence of three different shielding gases (N₂, Ar, and Ar₉₈H₂) on the SLM process and the resulting structural, mechanical, and thermophysical properties of Zr_59.3Cu_28.8Al_10.4Nb_1.5 were investigated by Wegner [98]. Applying high-purity Ar as shielding gas led to fully amorphous processing with a relative density of 99.8%. The usage of N₂-shielding gas led to a reduction of the processable parameter range accompanied by severe cracking and crystallization. Despite a similar impurity as the applied N₂, the introduced Ar₉₈H₂ as shielding gas led to comparable results as Ar. Introducing hydrogen as a reducing element, therefore, appears as a promising approach to reduce the influence of oxygen impurities in the introduced gases. In the recent study carried out by Wegner, the influence of different oxygen contents, partial crystallinity, and the usage of flow-aid in the feedstock of the commercially available gas atomized Zr-based BMG-forming alloy AMZ4 during SLM was investigated [133]. It should be emphasized that the laser-induced melting in the SLM process is sufficient to eliminate the thermal history and preexisting crystals in the powder feedstock. Therefore, the originally introduced method of widening the grain size distribution for fluidity can be used as a promising technique to improve machinability. However, increasing the size distribution range is at the expense of the attainable surface quality, so further optimization is needed. The detrimental effect of oxygen on the GFA is reflected by a reduction of the applicable parameter range for amorphous processing. A detailed study on Zr_59.3Cu_28.8Al_10.4Nb_1.5 was carried out by Sohrabi [101]. Nanocrystals were detected in the heat-affected zones (HAZs) close to the melt-pool boundary. The incubation time at temperatures close to the melting point is very short, but that cooling was not fast enough to avoid any crystallization. Meticulous characterization via synchrotron experiments and electron microscopy was required to observe the nanocrystals, which may not be detectable via laboratory XRD. A benchmark was printed and showed excellent geometrical accuracy with deviations of only 5–40 μm after sandblasting. The various results show that SLM is a great choice for the fabrication of complex BMG parts, with no required post-processing steps except for surface finishing. Ericsson fitted the scattering data to a spherical particle model and it revealed a higher number density of particles and smaller average particle size in the SLM processed Zr_59.3Cu_28.8Al_10.4Nb_1.5 material [134]. The high density of particles is due to the increased oxygen content of the material during SLM processing, which reduces the energy barrier of nucleation.

Gao [55] studied the SLM Ti/Zr-based bulk metallic glass matrix composites (BMGCs) as (Ti_0.65Zr_0.35)₉₀Cu₁₀ alloy. The main processing parameters were a laser power of 200 W, layer thickness of 30 μm, and scan speed of 500 mm/s. The point distance and hatch length were 105 and 40 μm, respectively. Gao [114] continued to study the SLM Ti/Zr-based BMGCs with (Ti_0.65Zr_0.35)_100−xCu_x (x = 5, 10, 15 at.%) alloys systematically. The parameters used in SLM process are basically the same as those used in previous work. With the change of Cu content, the microstructure and phase composition of the samples prepared by SLM changed obviously. Gao’s [116] research showed that SLM can successfully produce Cu₄₆Zr₄₇Al₆Co₁ metallic glass composite materials without cracks. The layer thickness was 50 μm. The power and diameter of the laser were 200 W and 75 μm, respectively. The hatch spacing between scanning lines was 60 μm and 85 μm, respectively. Compared to previous work [55], the sample sizes of Cu₄₆Zr₄₇Al₆Co₁ metallic glass can be obtained are larger. Deng [113] studied the biocompatible glass as Ti₄₇Cu₃₈Zr_7.5Fe_2.5Sn₂Si₁Ag₂ alloy by SLM. It is worth noting that although the cooling rate should be similar to gas atomization, the alloy will vitrify during the SLM process. A complete glass structure can be obtained by SLM and it eliminated the crystalline phases in the powder by remelting the powder in the SLM process. For additively manufacturing Ti-based BMGs, it could be of high interest for a wide variety of applications in biomedical materials. Recently, a monolithic bulk metallic glass and glass matrix composites with a relative density above 98 % were produced by processing Cu₄₆Zr₄₆Al₈ (at.%) via SLM by Deng [118]. For laser powers ranging from 90 to 110 W at a scanning velocity around 1000 mm s⁻¹, a relative density above 98% can be achieved and the maximum relative density measured was 98.6%. The 3D-printed BMGC components had large dimensions with complex geometries, demonstrating the good manufacturability of the chosen Zr₅₀Cu₅₀ system by SLM [117]. Cu₅₀Zr₄₃Al₇ alloy specimens were manufactured by the SLM method from corresponding gas-atomized amorphous powders, and the laser energy density was in the range of 20.8–31.7 J/mm³ [115]. The size of the SLM-fabricated cylinder samples was 4 mm in diameter and 8 mm in height. For the first time, a high-density amorphous and crack-free bulk metallic glass (BMG) based on a precious metal (Pd₄₃Cu₂₇Ni₁₀P₂₀) was produced via SLM by Sohrabi [119]. The samples produced with a power of 40 W and 60 W have more regular shapes. As power increases, the surface becomes curved and irregular, and thus reduces the accuracy of the part.

Figure 15 shows the microstructure evolution of the SLM with Ti/Zr-based BMGCs [55]. There are amorphous phase, β phase, and a small fraction of the (Ti, Zr)₂Cu phase in the alloy. For the SLM of T (Ti_0.65Zr_0.35)_100−xCu_x (x = 5, 10, 15 at.%) BMG alloys, with the increase of copper content, the average volume fraction and width of the β phase decreased, and the morphology of β phase changes from spherical to continuous slender sphere and network [114]. With the increase of copper content, the decrease of the average volume fraction and size of the β phase is not conducive to hindering the rapid propagation of the shear band, and the plasticity of the alloy decreases rapidly. Figure 15 a shows the pseudo binary phase diagram of (Ti_0.65Zr_0.35)_100−xCu_x. Cu₄₆Zr₄₇Al₆Co₁ metallic glass composite materials without cracks can be prepared by SLM, and the CuZr phase is deposited in a glassy matrix and some micrometer-sized crystalline phases already exit in the overlapping area for the latter laser scanning [116]. The fracture strength of the SLM samples is lower than that by conventional copper mold cast method [146]. In this research, when the stress is parallel to the printed layer, some special deformations may occur in the elastic region according to the stress-strain curve. The abnormal stress-strain curve may be related to the internal stress release between the layers. This can be attributed to the thermal stress accumulates layer by layer in SLM process [43]. When SLM is performed with a lower energy input, B2 CuZr nanocrystals (30–100 nm in diameter) are uniformly dispersed in the metallic glass matrix (Cu₄₆Zr₄₆Al₈) [118]. These B2 CuZr nanocrystals became the nucleation centroid of the large cubic phase with stable oxygen content during the remelting step. The presence of these nanocrystals increases structural heterogeneity, which can be indirectly revealed by microhardness and nanoindentation measurements. Zhang applied the SLM technique to fabricate a Zr₅₀Cu₅₀ bulk metallic glass composite reinforced with in situ–generated B2 austenite in HAZs, where the B2 phase could transform to B19′ martensite during deformation and enhance the ductility due to the transformation-induced plasticity (TRIP) effect [117]. The fraction of the amorphous phase in the 3D-printed samples was 64 wt% and the phases such as B2-ZrCu, B19′-ZrCu, and Cu₅Zr intermetallics can be found.

Some cracks were found in (Ti_0.65Zr_0.35)₉₀Cu₁₀ and (Ti_0.65Zr_0.35)₈₅Cu₁₅ BMGs prepared by SLM, but no crack was found in the (Ti_0.65Zr_0.35)₉₅Cu₅ BMG [114]. However, when Cu content was 5, the amorphous content in the sample is very low. The relative densities of the specimens (Archimedean) range from 98.5 to 98.7% and small pores with a typical diameter of 10–30 μm can be found in the SLM of Ti₄₇Cu₃₈Zr_7.5Fe_2.5Sn₂Si₁Ag₂ BMGs [113]. The pores were uniformly distributed throughout the sample. It is the presence of these holes that reduces the density of the sample. The pore sizes range from 10 to 90 μm and most pores (above 50%) have an equivalent diameter between 15 and 30 μm when energy input was 14.3 J/mm⁻³ for SLM of Cu₄₆Zr₄₆Al₈ [118]. The pore sizes of the specimens produced with energy input of 17.4 J/mm⁻³ (relative density about 98%) mainly range between 20 and 35 μm. Lu [115] found that many nanoscale crystal phases precipitated from the glass matrix during the SLM process. Due to the high viscosity of BMG melt, the prepared BMG sample without big external cracks has many micro-scale pores at the boundary of solidification pool, which may be the main reason for the low relative density and high porosity of Cu₅₀Zr₄₃Al₇ BMG prepared by SLM. A porosity of approximately 0.76% with pores dispersed randomly in the printed sample [117]. The size of the major pores is 5–30 μm with a mean size of 15 μm, similar to the results observed previously for other 3D-printed Zr-based BMGs [107].

Many results have revealed that partial crystallization occurred at heat-affected zones (HAZ) around the molten pools in BMGs by SLM. Obviously, it is important to deeply understanding the thermal cycle in HAZ. FEM simulations were performed by Ouyang [88] and Xing [91] using a moving heat source which obeys Gaussian distribution. In the SLM process, the maximum temperature in the center of the molten pool can reach 2200°C, which is much higher than the melting point of 892°C of the alloy system (Zr₅₅Cu₃₀Ni₅Al₁₀). Note that the critical cooling rate required to form the amorphous phase in the Zr₅₅Cu₃₀Ni₅Al₁₀ system is about 10 °C/s [151], so the high cooling rate can ensure the formation of a complete amorphous structure in the molten pool during the SLM process. However, although the cooling rate in the HAZ is also high enough, the occurrence of crystallization is not determined by the cooling rate but by the elevated temperature. If the temperature in the HAZ is higher than the glass transition temperature (T_g), it is difficult to prevent crystallization. In addition, the reheating of the heated area only lasts about 5 ms, which results in the formation of nanocrystals in the amorphous matrix due to lack of growth time, which explains why HAZ usually has a composite structure of amorphous and nanocrystalline. The Xing’s model analysis shows that reducing the thermal gradient and preheating the substrate through the XY cross-scan strategy can enhance the uniform shrinkage during the rapid cooling process, thereby reducing the residual stress [72]. In the current 3D-printed Fe_43.7Co_7.3Cr_14.7Mo_12.6C_15.5B_4.3Y_1.9 BMG components, if the size of the micro-pore reaches 30 μm or more, micro-cracks will occur in the alloy which can be seen in Fig. 16 a–d. Xing also used a computational fluid dynamics (CFD) model to verify that the various defects formed in the 3D-printed Zr-based BMG are related to the melt flow behaviors in the molten pools under different energy densities [108]. The present work provides in-depth understandings of defect formation in the SLM process and provides methods for eliminating these defects in order to enhance the mechanical performance of 3D-printed BMGs. Ouyang [94] also introduced a model to calculate the stress field during the post heat treatment process of SLM Zr₅₅Cu₃₀Ni₅Al₁₀ BMG with the purpose of revealing the mechanism of microcracks. The thermal stress field during the heat treatment of 3D-printed metallic glass is obtained, as shown in Fig. 16 e. Obviously, there will be a huge stress concentration around the heat-affected zone, and the maximum tensile stress is higher than the tensile strength of Zr₅₅Cu₃₀Ni₅Al₁₀ BMG (1.53 GPa [152]), which proves the cause of the microcracks.

Lindwall [150] presented three models which was refer to computational welding mechanics (CWM) and compared regarding predicted temperature history and computational time. The CWM model can be used in combination with other models to obtain microstructure evolution and other characteristics, so that the microstructure can be predicted [153]. But because the simulation of the welding process is very complicated, its calculation model is also too complicated. When applying the CWM model to SLM process of Zr_59.3Cu_28.8Al_10.4Nb_1.5 BMG, it is necessary to improve the computational efficiency of the model. Afazov [154] combined the re-division of the layer group and the analysis of the temperature field to improve the calculation speed. This method was introduced into the modeling of SLM by Lindwall. After the simplified model, the calculation can be completed in 0.2% of the original calculation time, but the thermal simulation of the SLM can still be ensured.

The tribological properties of SLM-BMGs were studied, and it was found that the friction coefficient showed a similar trend to the friction coefficient of as-cast samples [102]. The coefficient of friction and the surfaces of the SLM samples at different stages of wear testing are illustrated in Fig. 23. The coefficient of friction (Fig. 23a, b) shows a similar trend like the as-cast specimen. In addition, when the initial steady-state ends, the wear pattern changes after 35 min. But compared with as-cast glass, the friction coefficient fluctuated more obviously after 80 min, and the maximum value was significantly higher than as-cast glass. As long as the wear mode is pure wear (within the first 60 min), the wear surface of the SLM sample cannot be distinguished from the as-cast sample. In the later period of wear, more plastic deformation materials with smaller area were found. The study found that in the later stage of wear, the wear form of metallic glass changed to a “brittle-tough” mixed phase. It is worth noting that the friction coefficient scattering of SLM samples is more obvious than that of as-cast materials. This may be related to high porosity at small scales or inherent structural heterogeneity. Zhang observed many furrows parallel to the sliding direction in the friction and wear experiment, and found some flaky wear debris around the wear track [107]. EDX analysis shows that in addition to metal elements such as Zr, Cu, and Al (base metals in 3D-printed BMG), wear debris also contains rich oxygen, indicating that complex metal oxide SBF was formed during the oxidation process in the wear test. These patterns illustrate the mechanism of simultaneous abrasive wear and oxidative wear during 3D printing BMG wear. In general, the resulting oxide film is easily broken due to brittleness, which can further promote the accumulation of wear debris and plows. In the experiment of strengthening 316L composite material with SLM-BMG, Zhang found that the COF curve of the composite material tends to be stable, while the 316L COF curve has been fluctuating throughout the wear test [87]. The average value of composite COF is 0.49, which is 21% lower than 316L (0.62). Compared with 316L, the wear quality loss of the composite material is also reduced by about 20%, indicating that the amorphous alloy strengthening can significantly improve the wear resistance of the 316L matrix. The effect of normal load and sliding speed on the microstructure, tribological properties, and wear mechanisms of ex situ additively manufactured FeCrMoCB/Cu BMG was systematically investigated [81]. The unique amorphous-crystalline microstructure gave rise to a well wear resistance. The friction coefficient fluctuated due to the different shear stress on the amorphous matrix and crystal phase at the initial stage of friction and wear.

Conventional properties such as magnetism [64, 192] and corrosion resistance [193‐195] have been extensively studied in as-cast amorphous. Jung [70] found that the geometrical structure and microstructure of the magnetic material have a certain influence on the shape hysteresis of M-H. As mentioned above, micropores and cracks were often observed in the SLM samples. These structural defects will generate a large demagnetizing field and inhibit the movement of domain walls. Therefore, the micropores and cracks formed in the SLM sample delayed the magnetic response to the applied magnetic field. Similarly, the thermal history of the material is the main factor affecting the external magnetism. The high cooling rate and residual stress caused by the directional solidification process introduce stress-induced magnetic anisotropy into the material. A double-scan strategy has been employed with the aim of amorphous phase fraction enhancement as well as densification [50]. The results shown that double scanning effectively fills voids generated due to partial melting of powders, and leads to relative density of up to 96% and corresponding saturated magnetization of 1.22 T. More importantly, the double-scan strategy stimulates reflow in the melt pool, securing compositional uniformity, and eventually enhances amorphous phase fraction to a value as high as 47%. Consequently, coercivity has been significantly decreased, which in turn reduces core loss and increases permeability. In Fig. 24(a, b), M_s and coercivity as a function of E_d. M_s has a linear dependence on the density, and maximum M_s of 1.22 T was obtained with double scanning.

Deng [102] believed that the polarization curve of the sample prepared by SLM is generally similar to that of the as-cast sample, as shown in Fig. 25. However, the susceptibility to pitting corrosion of BMG prepared by SLM is slightly reduced, and the surface repair ability is slightly improved. The main reason is that the common structural defects in the as-cast samples are more harmful to the corrosion stability than the pore defects in the SLM samples. The former is characterized by a clear chemical discontinuity with the vitreous matrix phase boundary, while the latter is a geometric discontinuity, leading to low chloride ion erosion driving, leading to the formation and diffusion of pits. Luo [109] compares amorphous and polycrystalline components of the same component. Large metallic glasses with amorphous structures usually exhibit good corrosion resistance. Two orders of magnitude are higher than polycrystalline alloys of the same composition, which can be explained by the absence of structural defects, such as dislocations or grain boundaries [196]. A variety of corrosion-resistant zirconium-based BMGs have been developed, and corrosion resistance has been shown to depend on the above two factors. Therefore, the corrosion rate of SLM samples increases with the increase of exposure time, which is related to the decrease of amorphous content caused by laser thermal effect. In order to further explore the SLM process, amorphous has a positive effect on corrosion resistance. By measuring the potential polarization curves of as-cast 316L, SLMed 316L, and SLMed composite, the corrosion behavior of these three materials were studied [87]. The pitting potential (E_pit) values of the two samples are shown in Fig. 25. The E_pit value of the SLMed 316L is 0.64 v, which is about 100 mV higher than that of the as-cast 316L sample (0.55v). In addition, the E_pit value of the composite sample is 0.75 V, which is about 200 mV higher than the as-cast 316L sample. Therefore, the amorphous alloy 316L manufactured with SLM has the highest corrosion resistance.

BMGs with advanced catalytic properties have attracted increasing attention in the field of catalysis due to their disordered atomic packing structure (compared with the well-arranged structure in crystal alloy) [68, 197]. The attractive catalytic power of BMGs stems from the low thermal activation barrier [198]. Liang [75] made porous Fe-based BMG matrix composites with complex diamond-shaped dodecahedron microstructure. The iron-based metallic glass composite porous material produced by SLM has been used for catalytic activation in Fenton-like processes and sulfate-based reactions. The results show that up to 45 times the reusability can be achieved in the sulfate-based reaction without any obvious efficiency degradation, which is the highest reusability of the catalyst under high efficiency so far. Studies believe that the excellent catalytic reusability comes from the extremely low surface attenuation in the sulfate radical reaction. In addition, structural analysis shows that α-Fe nanocrystals can trigger more electron transfer, but too much α-Fe will cause the catalytic effect of the metal glass matrix composite to be inhibited. Catalytic performance of SLM-produced Fe-based MG matrix composite in the Fenton-like process is shown in Fig. 26. Yang [89, 90] found SLM 3D printing may be a promising way to produce BMG with complex geometry and enhanced catalytic properties. In order to make use of the complex geometry of BMGs in 3D printing, the printed Zr₅₅Cu₃₀Ni₅Al₁₀ BMG specimens were treated with chemical dealloying to generate the nano-pore structure of Cu on the surface of BMG. It was found that the nanostructured structure showed outstanding catalytic performance for MO degradation [89]. In addition, lattice structure has better catalytic performance than hollow structure and cubic structure, as shown in Fig. 27. The resulting 3D-printed nanoporous Cu (3D NP-Cu) has a specific surface area that is 660 times that of conventional 2D belts or powder catalysts. The 3D NP-Cu catalyst shows high degradation efficiency for azo dyes, and its kinetic reaction constant is 0.147 min⁻¹, which is 14 times that of commercial Cu2 + catalysts and 4 times that of CuO powder. What is impressive is that the 3D-printed NP-Cu exhibits quite good reusability and excellent versatility [90]. These two works of Yang may stimulate new 3D printing micro/nano-structured metal catalysts to treat wastewater as a new academic hotspot. These present studies also demonstrate that the SLM 3D printing technique could be a promising approach for manufacturing BMGs with complex geometries and enhanced catalytic properties. In addition, Yang has developed a 3D porous MG/Cu catalyst structure prepared by SLM, which exhibits extraordinary catalytic efficiency in the degradation of RhB, and its normalized rate constant is about 620 times higher than that of commercially available nano-zero-valent iron. This performance parameter exceeds the Fenton type catalyst that has been reported the most so far. Surprisingly, these catalysts have excellent reusability and can be used more than 100 times (the highest record so far) without a significant drop in efficiency [82].

Schroers [199] reported that BMGs exhibited better cell growth and adhesion support than the crystals. But the technology has never been used because BMGs have been stuck with a problem that is hard to process. However, with the development of SLM technology, it is possible to apply BMGs to biological materials. Zhang [107] made a comprehensive study of the relevant properties in the application of SLM-Zr BMG biological materials. The SLMed BMGs in SBF solution also showed excellent anti-biological corrosion performance and the results can be seen in Fig. 28. The metal ions released after soaking in SBF for 30 days were lower than the safety limit for biological implantation applications. In vitro cell culture assessment showed that 3D-printed BMG has good biocompatibility and cell proliferation support, which is also comparable to the commercial Ti6Al4V alloy. In addition, the 3D-printed BMG with Ag also showed outstanding antibacterial ability against Escherichia coli. Deng [113] applied Ti-based BMGs without the toxic element Be by SLM to be potential biological materials and also obtained positive results. The SLM samples fracture at lower stress (1690 ± 50 MPa) without any significant plastic deformation.