# Novel Composite Powders with Uniform TiB\({}_{2}\) Nano-Particle Distribution for 3D Printing
## 3 Results and Discussion  
As shown in Figure 2a, the largest Al-3.8 Cu-1.3 Mg composite powder particles are around 70 \(\upmu\)m. All the powders have a spherical morphology. Figure 2b shows the size distribution of the composite powders measured by laser diffraction and the average powder size is 3.68 \(\upmu\)m. The atomized composite powders exhibited a typical rapid solidification microstructure with a fine equiaxed grain structure, as shown in Figure 2(c1-f1). The grain size of the different powder fractions was measured from SEM images (more than 1000 grains were measured for each fraction), and the statistical results are illustrated in Figure 2(c2-f2). The equiaxed grain structure had a median grain size of 2.88 \(\upmu\)m in fraction A, and then decreased with the decrease of the powder size to 0.81 \(\upmu\)m in group D. Figure 2g shows the variation of the average grain size versus the powder size. The grain size decreased as the powder size decreased. [PERSON] et al. [10; 16] proved that the average grain size depends linearly on the powder size and is proportional to the cooling rate of the powders. The fine structure of the powders benefits from the high rate of solidification of the gas atomization process, in which the crystallization process has been suppressed due to the large under-cooling. TiB\({}_{2}\) particles distributed both inside the \(\upalpha\)-Al grains and along the grain boundaries, as indicated by the arrows in Figure 2(e1,f1). Figure 3a shows a synchrotron X-ray diffraction pattern of TiB\({}_{2}\)-reinforced Al-3.8 Cu-1.3 Mg composite powders. The diffraction peaks of Al, TiB\({}_{2}\) and Al\({}_{2}\)Cu were detected correspondingly, as shown in Figure 3a. From the diffraction pattern, the calculated value (7.1 wt. %) of the TiB\({}_{2}\) mass fraction was obtained through the reference intensity ratio (RIR) method [17], which is in agreement with the ICP result. Figure 3b shows the modified Williamson-Hall plot [18] obtained from the diffraction peaks of aluminum. The intercept of the plot indicates that the average grain size was 731.7 nm while the weighted average grain size calculated from the raw data of Figure 2b,g was 870 nm. The difference is due to the fact that synchrotron X-ray diffraction is more sensitive to low-angle grain boundaries,  
Figure 1: (a) Schematic drawing describing the principal of gas atomization technique and (b) set-up of the facility used in this work. while the SEM can only show relatively high-angle grain boundaries. It suggests small-angle grain boundaries exist in the composite powders, which was later observed in the EBSD. It is pointed out in [19] that along with the decreasing powder size, the rapid solidification microstructure can change from dendritic grains to cellular and even to equiax  
Figure 3: (**a**) Synchrotron X-ray diffraction patterns of the TiB\({}_{2}\)-reinforced Al-3.8 Cu–1.3 Mg composite powder; (**b**) Modified Williamson-Hall plot. Figure 2: SEM morphology (**a**) and powder size distribution (**b**) of the atomized TiB\({}_{2}\)-reinforced Al-3.8 Cu–1.3 Mg composite powder; SEM micrograph and corresponding grain size distribution of the atomized TiB\({}_{2}\)-reinforced Al-3.8 Cu–1.3 Mg composite powder: (**c1,c2**) group A: 63–75 \(\upmu\)m; (**d1,d2**) group B: 45–53 \(\upmu\)m; (**e1,e2**) group C: 10–26 \(\upmu\)m; and (**f1,f2**) group D: \(\leq\)10 \(\upmu\)m; (**g**) Variation of average grain size with powder size. study, [PERSON] et al. [14] investigated atomized Al-Cu-Mg (grade 2024) alloy powders, which exhibit a large dendrite structure. In the current study, the composite powders had a much finer equiaxed grain structure (Figure 2(c1-f1)). The modification of the grain structure was due to the effect of TiB\({}_{2}\) nanoparticles on the solidification process, since TiB\({}_{2}\) particles can significantly improve the crystal nucleation rate of \(\alpha\)-Al to refine the grains due to the interfacial effect reported in [5; 20].