# 3D Printing with Mixed Powders of Boron Carbide and Al Alloy
## 3 Results and Discussion  
Hardness, Young's modulus, SEM, and energy dispersive x-ray spectroscopy (EDX) were tested for a sample printed with 150 W power and 150 mm/s scan speed. Figure 5 shows the load versus displacement for a cube sample. Nanovea PB1000 Mechanical Tester is used for the hardness test. It appears that the hardness (110 MPa) and Young's modulus (11.7 GPa) of mixed B\({}_{4}\)C/Al are close to those of Al. This is mainly due to a large portion of Al powder in the sample. An important feature is that the synthesized part is less brittle and has better elastic property than B\({}_{4}\)C.  
Figure 6 shows the SEM and EDX results of a polished cubic sample with 99% density. EDX element analysis over an area of 1 mm x 1 mm indicates that Boron increases by 7 wt% and Al decreases by 30 wt% after the AM process. This has been verified with different testing areas and samples. One reasonable explanation is that a significant portion of Al was evaporated during the process due to its low melting temperature (660 \({}^{\circ}\)C) and boiling temperature (2740 \({}^{\circ}\)C). Its boiling temperature is close to the melting point of B\({}_{4}\)C (2763 \({}^{\circ}\)C). However, this will need to be further studied and confirmed in on-going research activities.  
Figure 4: Uniformity test for samples using different parameters. All samples on the top use scan speed 100 mm/s, whereas on the bottom uses scan speed 150 mm/s. From the bottom to the top rows, the hatching space is 0.13, 0.14, 0.15, 0.16, and 0.17 mm, respectively.  
Figure 5: Load curve.  
Figure 6: _Cont._Mapping of element test using EDX was done with the same testing area shown in Figure 6 to identify the mixing quality of B4C and Al. As shown in Figure 7, Boron element is well distributed in the Al structure. Again, it shows that Boron appears to have a larger percentage after the AM process. More interestingly, the X-ray powder diffraction data, shown in Figure 8, clearly demonstrates newly formed crystal phases of aluminum carbide (Al4C3) and aluminum diboride (AlB2) during the melting and solidification process for the mixed compound powders (i.e., 20 wt% B4C and 80 wt% AlSi10 Mg). This provides strong evidence that AM can not only melt powders to form 3D structures but also can carry on in situ synthesis to form new compounds to change their properties (such as mechanical, electrical, etc.) for specific applications. As a note, from the phase diagram of Al and B4C, aluminum carbide (Al4C3) forms at a temperature over 1400 \({}^{\circ}\)C and aluminum diboride (AlB2) at a temperature over 600 \({}^{\circ}\)C. Detailed study of AM synthesis is beyond the scope of this paper and will be present in a separate publication.  
Figure 6: SEM and EDX test results of a cubic sample with mixed B4C (20 wt%) and AlSi10 Mg (80 wt%).  
Figure 7: EDX mapping test of boron (left) and Al (right). Boron is well distributed in the Al structure.  
Two types of collimators were made with the same parameters for hardness test (150 W power and 150 mm/s scan speed). One is the honeycomb type (3D), of which geometrical parameters were taken from reference [2]; the results are shown in Figure 9. In control of the geometry and tolerance (10 \(\upmu\)m level), very consistent results have been achieved. Another type is the thin-wall collimator (2D). A small version of the optimized thin-wall type of collimator was printed and is given in Figure 10. It is under evaluation at Oak Ridge National Labs (ORNL). Part of the sample was cut and tested with EDX and XRD. It is consistent with the data from cubic samples (Figure 8) in terms of synthesis of Al\({}_{4}\)C\({}_{3}\) and AlB\({}_{2}\). Especially, we observed that Boron concentration has been increased significantly and Al has been decreased due to evaporation during the collimator printing.  
Figure 8: XRD test of mixed B\({}_{4}\)C and AlSi10 Mg sample.  
Figure 10: A small version of the optimized collimator design.  
Figure 9: 3D collimator structures (honeycomb).