Page 39 - MSAM-3-2

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

surface (Figure 2C-1) under different scanning speeds are A B
presented in Figure 5. The examination of the cross-section
sample revealed the presence of three phases: α-Al, α-Ti,
and TiAl . In addition, other phases composed of elements
3
such as Mg, V, Sc, and Zr may exist in the microstructure
but cannot be detected by XRD due to their low volume
fraction or small size. Meanwhile, α-Ti and TiAl were
3
detected at the graded layer surface. Based on the Ti-Al C
phase diagram displayed in Figure 5A, several IMCs,
including Ti Al, TiAl, TiAl , TiAl , and Ti Al were expected
2
2
3
3
5,
to form during the metallurgical reaction between Ti and
Al. The Gibbs free energy for the formation of the Ti-Al
29
IMC was calculated using the formula proposed by Kattner
et al. The formation of TiAl and Ti Al entails a series
30
2
2
5
of reactions with TiAl as the initial phase, which were
not thermodynamically considered, indicating that TiAl D
3
has the lowest free energy of formation among phases.
Consequently, as the laser irradiated, the graded powder
underwent melting and reacted to form TiAl through the
3
Ti+3Al→TiAl reaction. When the scanning speed of the
31
3
graded layer was increased from 2400 mm/s to 2800 mm/s,
the intensities of the Ti and TiAl diffraction peaks changed
3
from 6307 and 10992 to 8655 and 10211, respectively. As Figure 6. The scanning electron microscopic images of the cross-section
the scanning speed further increased to 3000 mm/s, the microstructure evolution at the interface of Ti6Al4V/AlMgScZr-graded
intensity of the Ti peak surpassed that of the TiAl peak as multi-material parts. (A) Low-magnification image of the interface.
3
the dominant phase. This phenomenon can be attributed (B) The top region of the interface. (C) The middle region of the
to the decreased interaction time between the laser and interface. (D) The bottom region of the interface. Scale bars: (A) 50 μm,
the powder as well as the lower melting pool temperature. magnification ×400; (B-D) 10 μm, magnification ×8000.
As a result, the Ti6Al4V powders incompletely melted and
retained in the molten pool, leading to a reduced formation between Ti6Al4V and AlMgScZr, and the lower laser
of TiAl . Therefore, for samples processed with lower absorptivity of mixed powders caused by highly reflective
3
scanning speeds, due to the formation of a greater number AlMgScZr powders. The insufficient absorption of laser
of brittle IMCs at the interface, there is a poorer resistance energy of the mixed powder bed led to a lower temperature
to crack propagation, resulting in larger areas of cracks. of the molten pool and the inadequate melting of Ti6Al4V
In contrast, the sample scanned at 2800 mm/s exhibited powders. 32,33 To further characterize the microstructure
fewer brittle phases at the interface, hence demonstrating a and chemical composition, the cross-section SEM images
stronger resistance to crack propagation and a reduction in of the sample were amplified, as shown in Figure 6B-D. In
crack area. However, when the scanning speed increased to Figure 6D, rod-like structures are observed at the bottom
3000 mm/s, despite the reduced quantity of brittle phases region of the graded layer, where the Al/Ti atomic ratio
formed, the lower thermal input led to the occurrence is approximately 2.48. Based on the results of the XRD
of defects, such as unmelted powders at the interface, pattern and the Ti-Al phase diagram, it can be inferred
resulting in the formation of cracks under thermal stress. that these rod-like structures are TiAl . The rod-like TiAl
3
3
Therefore, at a scanning speed of 2800 mm/s, the sample precipitated along the interface between Ti6Al4V and the
exhibited the optimal behavior of densification. graded layer and grew in the direction of the temperature
gradient. In addition, several finer dendrites precipitated
3.3. Microstructure evolution of Ti6Al4V/AlMgScZr- above the rod-like TiAl3. In Figure 6C, corresponding to
graded multi-material parts the middle region of the interface, a significant number of
Figure 6 illustrates the cross-section microstructure dendritic precipitates is presented. The atomic percentages
evolution of Ti6Al4V/AlMgScZr-graded multi-material of Al and Ti were 76.36% and 23.64%, respectively, with an
parts at the interface. In Figure A, several unmelted atomic ratio of 3.2. It could be inferred that these dendritic
Ti6Al4V powder particles are visible at the interface, structures were also composed of TiAl3. Figure 6B depicts
attributed to the significant difference in melting point TiAl fine dendrites with Al and Ti atomic percentages of
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Volume 3 Issue 2 (2024) 7 doi: 10.36922/msam.3088