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Materials Science in Additive Manufacturing Crack-free AA7075 with Zr modification via LPBF

1. Introduction escape as well. Laser re-melting between layers promotes
existing pores in the previous layer to coalesce and grow at
Laser powder bed fusion (LPBF), as an important laser- the expense of others. They also found that the subsequent
based additive manufacturing (AM) technique for metal laser re-melting caused by the next cycles helps to disperse
components, is considered one of the most innovative large pores into smaller ones, rather than eliminating them
technologies in Industry 4.0 [1-4] . It provides an opportunity due to the relationship of gas solubility and temperature.
to manufacture components with complex geometries in However, the whole re-melting of laser tracks helps to
the aerospace, automotive, and medical industries [5,6] . LPBF degas the melt track because it allows the entrapped gas
enables successive layer assembly with a laser beam as the at the bottom of the molten pool to float up and escape .
[16]
heat source. The localized heat input in micron length
and time scale induces rapid melting and consolidation Cracking is another typical defect in LPBF-
through the formation of a molten metal pool [7,8] . The manufactured parts [17,18] . Solidification cracking is the most
numerous thermal cycles during processing thereby lead common in LPBF and is mainly related to the materials
to unique microstructural features, which are in favor compositions [19,20] although elaborate control of processing
[21]
of high mechanical strength. However, there are several may only work to some extent . It occurs in the mushy
concerns for the widespread application of the LPBF zone of an alloy at the final stage of solidification. At
technique, among which defects are important obstacles. this stage, the liquid films between adjacent dendrites
Porosity and cracks, as the most common defects, have can be easily torn due to the solidification shrinkage and
been investigated for decades to reveal the mechanisms of thermal stresses upon cooling. In addition, adjacent arms
defect formation and migration methodology [9,10] . coalesce and form a solid skeleton, resulting in poor liquid
feeding . Cavity forms in the weak intergranular regions
[22]
Porosity formation in LPBF largely depends on the and propagate through interdendritic colonies and even
thermal history of the molten pool [11,12] . Lack of fusion layers. Therefore, most conventional alloys cannot be
defects are easily formed if the thermal input is too low to readily additively manufactured. Appropriate alloys must
fuse the powder to the substrate or the underlying layer. be carefully selected or modified so as to increase the
Conversely, if the thermal input is too high, the molten printability of LPBF. High-strength 7xxx series aluminum
metal evaporates vigorously, and the intense vapor recoil alloys (AA) are typical types that are unfriendly to the
pressure will suppress the melt surface, creating large LPBF process due to their high laser reflectivity, easy
keyhole-induced porosity at the bottom of the molten reactivity with oxygen, and especially, crack sensitivity [23,24] .
pool. Even when the thermal input is intermediate, there Crack elimination is of the first priority for the qualified
are possibilities for the formation of metallurgical pores, performance of the LPBF-manufactured high strength AA
originated from entrapped gases, such as shielding gas, parts. Recently, a growing number of researchers began
vapor, hydrogen, and gases entrapped during powder focusing on this topic [25-29] . Otani et al. found that silicon
[30]
atomization process. These are usually small spherical additions into AA7075 help to yield better process ability
pores that are driven by Marangoni flow and fail to escape by suppressing the voids and cracking. However, high
from the molten pools. Migration of porosity relies on silicon content would induce a tradeoff between strength
[13]
optimizing the processing parameters. Tang et al. and ductility and must be carefully adjusted. Modification
proposed a prediction model for lack of fusion porosity with with transition elements, such as Sc and/or Zr, has shed
the fundamental parameters, including melt-pool cross- new light on achieving crack-free aluminum components
sectional dimensions, hatch spacing and layer thickness, by LPBF. These elements offer the possibilities for a strong
and investigated their influence on the defect. Plessis et grain refinement due to the precipitation of coherent L1
2
[14]
al. elaborated how different processing parameters structured Al Sc, Al Zr, and Al (Sc,Zr) and have proven
3
3
3
result in different pores formation mechanisms. The three- to be effective in crack reduction through equiaxed
dimensional morphologies of lack of fusion porosity, microstructure in recent works . Pre-alloyed Sc- and
[31]
metallurgical pore, and keyhole mode pore have been Zr- modified AA were found to enable high proportion of
revealed as irregular, near-spherical, and rounded, but not fine equiaxed grains and therefore lead to high mechanical
[15]
spherical. Leung et al. uncovered mechanisms of pore strength [32-34] . However, the application of Sc has been
migration by Marangoni-driven flow, pore dissolution, and limited due to the inherent corrosion sensitivity and laser
dispersion by laser re-melting through in situ and operando incompatibility issues . Zr, a less expensive alternative,
[35]
high-speed synchrotron X-ray imaging. Marangoni-driven has therefore found more applications in the refinement
flow causes the melt with a high surface tension to fold of grains in both traditional and LPBF processing. Er has
over the surface of the melting track and entrain an argon attracted a growing interest as a modification element for
bubble, and allows the pores inside the molten pool to the high-strength AA manufactured through LPBF .
[36]
Volume 1 Issue 1 (2022) 2 https://doi.org/10.18063/msam.v1i1.4