Page 52 - MSAM-1-2

Materials Science in Additive Manufacturing Process study of DED steel matrix composites

content in the MMC samples may be significantly lower weight fraction of 6 wt.%. This indicates that the powder
than 2 wt.%. This may account for the negligible differences clouds formed during the DED process can be the TiB
2
in the results obtained for 316L/TiB MMC as compared to particles that escaped during the powder deposition.
2
pure 316L stainless steel.
Even though TiB particles were observed to be
2
3.2. Density adhering onto the 316L stainless steel particle surfaces, the
tumble mixing process did not induce sufficient adhesion
The mean and S/N ratio of the measured density is which caused the TiB particles to separate from the 316L
tabulated in Table 5, with the calculated weight fraction of stainless steel particles during powder deposition. The
2
TiB in the DED samples. The average weight fraction of powder deposition step involves forces induced by the
2
TiB is 2 wt.% which is significantly lower than the added
2 gas flow to expulse the powder from the nozzle. Tumble
mixing is not suitable for mixing nanoparticle composites
A B for DED as the process involves injecting the mixed powder
into a fast-moving gas stream. Larger TiB particles can
2
also be used as they are less likely to escape due to higher
gravitational force acting on them. Ball milling can be used
[18]
to strengthen the adhesion during the mixing . During
ball milling, mechanical alloying is achieved due to the
repeated deformation, fracturing, and cold welding of
the powder particles [18,26] . At the start, the reinforcement
Figure 6. Energy dispersive X-ray spectroscopy locations for (A) equiaxed particles facture due to their brittleness before sticking onto
and (B) columnar grains. Location 1 is within the matrix, location 2 is at the matrix powder in which cold welding predominates due
the sub-grain boundary, and location 3 is at the TiB particle. to plastic deformation. During the deformation and cold
2
welding of the matrix powder particles, the reinforcement
Table 4. Composition for equiaxed and columnar grains particles then dispersed inside the matrix. Fracture then
takes over due to the matrix hardening. Finally, a dynamic
Element Equiaxed (wt.%) Columnar (wt.%) balance between cold welding and fracture ensures the
Matrix Boundary TiB Matrix Boundary TiB
[27]
2 2 absence of agglomeration . In addition, the collisions can
S 1.06 - 1.6 0.48 - - cause the break-up of agglomerated particles . However,
[18]
Cr 16.48 22.27 19.18 16.26 21.86 22.75 it is of interest to note that using larger particles or ball
Mn 1.38 1.83 - 1.33 1.8 2.6 milling result in a morphological change to the powder,
Fe 67.69 54.09 66.34 69.81 62.03 46.25 which, in turn, may affect the powder flowability. Hence,
Ni 10.13 6.38 9.03 10.66 9.3 3.86 a careful control of the powder preparation parameters
is needed. With controlled parameters used during the
Ti - - 3.85 - - 5.57
Mo - 10.69 - - 5.01 4.4 process, the centrifugal effect of ball milling results in
uniform dispersion of the reinforcement particles in
Others 3.26 4.74 - 1.46 - 14.57 the powder mixture with minimum change in powder
Total 100 100 100 100 100 100 morphology. It has been found that mechanical alloying

Table 5. Mean, S/N ratio of sample density, and calculated weight fraction of TiB
2
Taguchi number Laser power (W) Scanning speed (mm/min) Hopper speed (rpm) Mean density (g/cm ) S/N f
3
1 1000 200 200 7.891±0.004 17.943 1.8%
2 1000 400 300 7.879±0.011 17.929 2.0%
3 1000 600 400 7.874±0.016 17.924 2.1%
4 1200 200 300 7.887±0.002 17.938 1.9%
5 1200 400 400 7.876±0.002 17.926 2.0%
6 1200 600 200 7.855±0.012 17.903 2.4%
7 1400 200 400 7.876±0.003 17.927 2.0%
8 1400 400 200 7.883±0.006 17.934 1.9%
9 1400 600 300 7.862±0.008 17.910 2.3%

Volume 1 Issue 2 (2022) 6 http://doi.org/10.18063/msam.v1i2.13