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Materials Science in Additive Manufacturing LPBF of Ti-Al-graded multi-materials

consistent with the density data for Ti6Al4V/AlMgScZr- (Figure 11D-F) revealed that cracks occurred at the
graded multi-material. As interface cracks reduced, both interface between the graded layer and Ti6Al4V.
the ultimate compressive strength and strain of the samples Figure 12 summarizes the microhardness and
improved, indicating that the compression performance compressive strength of Ti alloys, Al alloys, and
of the samples primarily depended on the quality of the multi-material parts consisting of Ti alloys or Al
formation.
alloys. Ti alloys exhibit higher microhardness values
The fracture mechanism of the Ti6Al4V/AlMgScZr- (approximately 380 HV) and compressive strengths
graded multi-material was further elucidated through ranging from 1109 MPa to 1393.8 MPa, 41,42 whereas Al
additional analyses conducted on the fracture alloys demonstrate lower microhardness values (≤200 HV)
morphologies of the corresponding samples. Figure 11 and compressive strengths ranging from 211 – 621 MPa. 43,44
presents the morphologies of the fractures on both sides The significant discrepancy in microhardness between
(AlMgScZr and Ti6Al4V) of the samples at different laser these two materials results in a performance mismatch at
scanning speeds. At a laser scanning speed of 2600 mm/s, the interface, which is a key factor contributing to interface
distinct cleavage steps and cleavage facets were observed susceptibility to crack. In this study, the microhardness
on the AlMgScZr side (Figure 11A), indicative of a brittle value of TiAl fell between that of Ti6Al4V and AlMgScZr,
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fracture mechanism. The excessive energy input resulted and the interface microhardness exhibited a graded
in the formation of porosities and cracks on the fracture decrease along the building direction, promoting favorable
surface, which could serve as the initiation points for metallurgical bonding at the interface. This microhardness
fracture and lead to reduced compressive strength. With profile significantly improved the interfacial bonding
an increase in scanning speed to 2800 mm/s, the fracture strength, resulting in an enhanced compressive strength
exhibited features of cleavage steps, cleavage planes, and of up to 1531 MPa. In addition, a gradual transition of
a few dimples (Figure 11B), suggesting a combination of microhardness was observed at the interface. According
ductile and brittle behavior in the fracture mechanism. to Zhang and Bandyopadhyay, who fabricated Ti6Al4V/
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Further increase in scanning speed to 3000 mm/s resulted Al12Si multi-material parts using LDED technology, the
in the presence of unmelted Ti6Al4V powders on the interface obtained Ti Al IMCs with a high microhardness
fracture surface, with cracks originating and propagating value, resulting in a relatively lower compressive strength
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from these unmelted powders. This observation indicates of 507.8 MPa. In addition, researchers have attempted
that unmelted powders in the graded layer induced the to improve the mechanical properties of multi-material
formation of cracks under higher laser scanning speeds
of 3000 mm/s during the compression process, thereby parts by constructing multi-material systems, including
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weakening the mechanical properties of the samples Ti6Al4V/W7Ni3Fe, Ti6Al4V/SS316, and Al12Si/
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(Figure 11C). Analysis of the distribution of Ti and Al SS316. Although most of these systems achieved high
elements on the fracture surface of the Ti6Al4V side microhardness in multi-material parts, they often exhibited
low compressive strength due to the formation of cracks
and brittle IMCs at the interface. It is also worth mentioning
that due to the presence of cracks commonly found at
interfaces, Ti6Al4V/AlMgScZr-graded multi-material
samples may not meet the tensile requirements. Therefore,
further process optimization and investigation are required
in further studies. Tensile and fatigue tests should be
conducted to determine whether these functionally graded
materials meet the functional requirements.
This study demonstrated that LPBF-processed Ti6Al4V/
AlMgScZr-graded multi-material parts enable effective
control of density behavior, interfacial metallurgical
reactions, and mechanical properties. These desired
characteristics are achieved through a graded interface that
connects the two different materials with a composition-
graded layer. A well metallurgically bonded-graded multi-
material interface was obtained by controlling the laser
Figure 12. The hardness and compressive stress of Ti alloys, Al alloys, and scanning speed of the graded layer, thereby improving
their multi-materials. 40-48 the overall mechanical properties of the multi-material

Volume 3 Issue 2 (2024) 11 doi: 10.36922/msam.3088