Page 83 - MSAM-2-4

Materials Science in Additive Manufacturing Laser DED-produced Ti-6Mn-4Mo alloy

by dashed lines. The existence of long grains along the elements in certain regions can be explained by the
build direction is supported by the similar observations in unmelted or partially melted powder particles. This is
the literature, such as DED-produced Ti-6Al-4V by Tan especially relevant for Mo, which is hard to melt due to its
et al. , and DED-produced Ti-Mo alloy by Kang et al. very high melting temperature. In addition, the deposited
[37]
[33]
Some partially melted powder particles can be found in Ti-6Mn-4Mo samples are overall free of apparent cracks
the bulk of material, as shown in Figure 6. As seen from and only a few micropores can be found in the cross-
the figure, as-built alloy is composed of predominantly section. This, further, confirms the soundness of the
β phase grains. However, α + β lamellar microstructure selected DED process parameters.
is also present mostly near the bottom of melt pools After the heat treatment, the microstructure undergoes
(Figure 6C). This observation agrees with the XRD results, a noticeable change (Figure 7). A dual-phase α + β lamellar
which show that β phase is the dominant phase while a microstructure was formed within the primary phase
small amount of α phase is also present. Alshammari grains. A similar microstructure was observed in Ti-5.8wt%
et al. indicated that the number of α + β lamellar Mn produced by laser DED . In such a microstructure,
[40]
[35]
microstructure present in Ti-Mn alloy depends on the darker lamella is an HCP α-Ti while brighter lamella is
amount of Mn. As Mn increases from 1 to 5%, the amount BCC β-Ti (Figure 7D). This observation is consistent with
of β phase becomes more abundant. At 10% Mn, the XRD results, which show that heat-treated Ti-6Mn-4Mo
microstructure is entirely composed of β phase rather than consists mainly of α phase. As beta alloy with metastable
α + β lamellar microstructure. An increase in the volume β phase undergoes heat treatment, α phase starts to
fracture of β phase along with an increase of Mn was also precipitate from the β matrix. Such transformation from
reported for Ti-Mn produced by laser DED . From these β to α + β during aging is a slow process, during which
[35]
observations, it can be assumed that as-built Ti-6Mn-4Mo α plates grow in β matrix through a diffusion-controlled
consists of mostly β phase, but a small amount of α phase mechanism.
is also present.

Several elongated patches of about 100 μm long can 3.3. Elemental composition analysis
be visible at the bottom of melt pools (Figure 6B). The Figure 8 shows the EDS mapping results for a region
microstructure within these patches is different from the of 550 × 550 μm on the cross-section of as-built
matrix. It consists of a long and fine-needle-like structure. Ti-6Mn-4Mo. It is evident that Ti and Mn elements
The presence of such patches is a sign of inhomogeneous are distributed homogeneously. Although there is a
distribution of alloying elements. Depletion of alloying distinct segregation of Mn from melt pool boundaries

A B A B

C D C D

Figure 6. Scanning electron microscopy micrographs of the Figure 7. Scanning electron microscopy micrographs of the
microstructure of as-built Ti-6Mn-4Mo alloy. (A) Observation at ×65 microstructure of directed energy deposition-produced Ti-6Mn-4Mo
magnification. Arrows point at the partially melted Mo powder particles, after heat treatment. (A) Observation at 65× magnification. Black box
and the black box represents the close-up view area for the next higher represents the close-up view area for the next higher magnification
magnification level; (B) observation at ×120 magnification. The black box level; (B) observation at 120× magnification. The black box represents
represents the close-up view area for the next higher magnification level; the close-up view area for the next higher magnification level; (C)
(C) observation at ×500 magnification. Black box represents the close-up observation at 500× magnification. The black box represents the close-up
view area for the next higher magnification level; and (D) observation at view area for the next higher magnification level; (D) observation at
×2000 magnification. 2000× magnification.

Volume 2 Issue 4 (2023) 6 https://doi.org/10.36922/msam.2180