# 3D Printing with Mixed Powders of Boron Carbide and Al Alloy
## 2 Materials and Methods  
Boron carbide (melting temperature 2763 \({}^{\circ}\)C, thermal conductivity 29 W/m.K, density 2.52 g/cm\({}^{3}\), and thermal expansion coefficient \(3\times 10^{-6}\)K) is a very common material used for radial collimator, because it is relatively cheap, and safe in terms of handling. However, its brittleness makes it hard to use in fabrication of thin-wall radial collimators with 3D printing technology. It is found that by adding Al (melting temperature 660 \({}^{\circ}\)C, thermal conductivity 237 W/m.K, density 2.70 g/cm\({}^{3}\), and thermal expansion coefficient \(23\times 10^{-6}\)K) to B4C, the strength and thermal handling capability can be improved significantly and the brittleness can be reduced without increasing in weight. In this paper, B4C powder (American Elements, Los Angeles, CA) and Al alloy (AlSi10 Mg) powder (CNPC, China) are used to make radial collimator parts. These powders are readily available standard commercial products. The mixing ratio of 20% B4C and 80% AlSi10 Mg is selected to make thin walls matching the performance of neutron absorption (B [10] concentration) by using the conventional method, whose Myler septa are coated with Boron [2].  
Microscopic images and size characterization are given in Figures 1 and 2 for B4C powder and AlSi10 Mg powder, respectively. The size of the B4C particles is generally in the range of 10 to 45 \(\upmu\)m. However, the shapes are irregular, which is not ideal for AM process. In reality, we have no other choice since this is the best quality we could find from all suppliers. AlSi10 Mg particle size is finer than B4C, since 80% of the particles are measured within 10 to 30 \(\upmu\)m. Moreover, it has spherical shape, which has better flowability. Practically, different powder sizes can help make dense components [10].  
Figure 1: Characterization of boron carbide (B4C) powder.  
Figure 2: Characterization of Al alloy (AlSi10 Mg) powder.  
80 wt% of AlSi10 Mg alloy powder was mixed with 20 wt% B4C powder before the AM process, by using a jar mill machine to run 24 h. A series of cubic samples were printed to measure the density and surface roughness. Powder bed fusion (PBF) system made by PolarOnyx was used to do the AM process. This system was modified to fit for low-mass ceramic 3D printing and filled with Argon gas. The process parameters are listed as follows: laser average power: 100, 150, and 200 W on targets; laser type: fiber laser; laser operation mode: pulse repetition rate of 200 MHz, pulse width of 1 ns; laser wavelength: 1070 nm; scan speed: 100 mm/s to 500 mm/s; patterning type: lines; hatching order: bidirectional; hatching space: 50 to 150 \(\upmu\)m; and powder layer thickness: 50 \(\upmu\)m. Fiber laser can provide stable operation and reasonable absorption, which is essential to the AM process.  
All of the measurement data on roughness and density are summarized in Figure 3. It is noted here that some of the data are not complete due to the fact that those samples are damaged during polishing process. However, the rest of the data are sufficient enough to make a conclusion on process optimization. Over 99% relative density (image-j is used) and acceptable roughness (<50 \(\upmu\)m, top surface) were obtained when the average power on target was greater than 150 W. Further study on uniformity was carried out by fixing 150 W average power and scan speed of 100 and 150 mm/s. The hatching space varies from 130 to 150 \(\upmu\)m (or 0.13 mm to 0.15 mm) at a step of 10 \(\upmu\)m. Figure 4 shows that all of the samples have good uniformity of relative density (<2% variation, from 97-99%).  
Figure 3: Top surface roughness (left column) and density measurement (right column) of mixed B4C (20 wt%) and AlSi10 Mg (80 wt%).