Page 56 - MSAM-1-4

Materials Science in Additive Manufacturing LPBF of Mg and its bio-applications

A B 2.2.2. Scanning speed
The appropriate scanning rate can increase the heating
time of metal powder. The fabrication of compact
component can be achieved with low scanning speed,
as it allows longer contact time between powder and
laser, thereby increasing the rate of energy transfer to
the powder bed. With the scanning speed increasing,
Figure 3. Morphology of (A) the pre-atomized Mg-15Gd-0.4Zr (wt%) less energy is transferred to powder, which always results
alloy powder and (B) the gas-atomized Al powder. in incomplete melting of the particles. Shuai et al. [41]
reported the influence of SLM processing parameters on
metal powders in the market. The efficient preparation of the corrosion performance of ZK60 alloy. At an energy
high-quality spherical metal powders is vital to promote density of 600 J/mm , the final as-built part with a
3
AM technology upgrades and industrial applications. relative density of 97.3% was obtained. Spierings et al. [42]
Therefore, the existing powder preparation methods analyzed the impact of different laser scanning speeds
need to be improved. For the gas atomization method, on the static mechanical properties of SLM-treated
optimizing the structure of the atomizer and improving the scandium (Sc) and zirconium (Zr)-modified Al-Mg
kinetic efficiency of the atomized gas are the most effective alloys. The results showed that the scanning speed could
means to improve the quality and production efficiency affect the hardness and mechanical performance of the
of metal powders. For the PRE atomization method, to alloy, while yield strength was barely affected by the laser
efficiently prepare high-quality spherical powders, the scanning speed.
rotation speed of the electrode bar is increased, while the
reasonable plasma heat source and power are also fed. 2.2.3. Laser energy density
2.2. Parameters optimization Although the variety of laser powers or scanning speeds
can significantly influence the forming quality and
The performance of SLM prepared product strongly performance of AM-processed part, it is difficult to
depends on the processing parameters, such as laser power, describe their effects individually. An empirical formula
scanning speed, hatch spacing, and layer thickness. The for evaluating the input energy of laser additive using laser
previous studies have shown that the process parameters are energy density is given below:
[43]
considered appropriate when the following requirements
are fulfilled in scanning tracks and layers : (π) The E = P
[38]
scanning tracks should be continuity, (θ) each layer has to V V ..HD
be high enough to build up the part cumulatively, (ρ) every
3
layer should be high enough, and (σ) the connection angle Where, E represents energy density (J/mm ), P is the
v
between two adjacent layers should be close to 90°. laser power (W), V is the scanning speed (mm/s), H is the
hatch spacing (mm), and D is the layer thickness (mm).
2.2.1. Laser power After forming, the forming quality of samples is measured
High laser power can improve the wettability of the molten by density and surface roughness. In general, the surface
pool and provide more power for powder consolidation. Ji of the sample with high density is relatively flat and has a
[39]
et al. reported an analytical model to predict the grain low surface roughness, as shown in Figure 4. In the case
size of the part after LPBF process. Results show that the of low laser energy density, the temperature in the molten
maximum surface temperature of the component is not pool is also relatively low so that the metal powder cannot
influenced by the scanning speed. In addition, the average be fully melted, and noticeable unmelted powder particles
grain size reduced with the increasing of laser power. He et and a large number of pores can be observed between
al. prepared AZ61 alloy with a relative density of 98%. The the adjacent molten layers, which result in low density.
[40]
forming zone of AZ61 with input laser power from 60 W to When the laser energy density is too high, the temperature
90 W was determined. The results show that the increase in in the molten pool easily exceeds the boiling point of
laser power helps improve the densification of the material magnesium powder because the melting and boiling
and the formation of equiaxed grains, thus improving the point of magnesium powder is relatively close, resulting
resistance to degradation and microhardness. However, in a large amount of powder evaporation, which leads to
too high laser power can lead to grain coarsening and a the formation of local vapor pressure, powder splashing
decrease of Al solid solution in the Mg matrix, resulting in phenomenon, and formation of a large number of pores, as
increased mass loss and decreased microhardness. displayed in Figure 5.

Volume 1 Issue 4 (2022) 5 https://doi.org/10.18063/msam.v1i4.24