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Additive Manufacturing of Ti-Based Alloys
TiAl-based intermetallic alloys have come to the fore as the preferred alloys for high-temperature applications. Conventional methods (casting, forging, sheet forming, extrusion, etc.) have been applied to produce TiAl intermetallic alloys. However, the inherent limitations of conventional methods do not permit the production of the TiAl alloys with intricate geometries. Additive manufacturing technologies such as electron beam melting (EBM) and laser powder bed fusion (LPBF), have been used to produce TiAl alloys with complex geometries. EBM technology can produce crack-free TiAl components but lacks geometrical accuracy. LPBF technology has great geometrical precision that could be used to produce TiAl alloys with tailored complex geometries, but cannot produce crack-free TiAl components. To satisfy the current industrial requirement of producing
crack-free TiAl alloys with tailored geometries, the paper proposes a new heating model for the LPBF manufacturing process. The model could maintain even temperature between the solidified and subsequent layers, reducing temperature gradients (residual stress), which could eliminate crack formation. The new conceptualized model also opens a window for in-situ heat treatment of the built samples to obtain the desired TiAl (gama-phase) and Ti3Al (α2-phase) intermetallic phases for high-temperature operations. In situ heat treatment would also improve the homogeneity of the microstructure of LPBF manufactured samples.
2. TiAl Intermetallic Alloy Phases
The fully lamellar or nearly lamellar microstructure consists of the TiAl γ(α2-phase) and a small volume fraction of Ti3Al (α2-hase).
Near-gamma: the near-gamma alloy consists of a gamma (γ) grain microstructure with a moderate alpha grain.
Duplex: the duplex microstructure consists of gamma (γ) grains, B2 phase, and γ/B2 lamellar colonies.
3. Metal Additive Manufacturing
4. Electron Beam Melting of TiAl-Based Alloys
5. Laser Powder Bed Fusion of TiAl-Based Alloys
the smaller beam size reduces the burden of post-processing activities. As a result, it has been used extensively for manufacturing several biomedical and engineering components using titanium and steel-based alloys with great dimensional accuracy . The literature reveals that, compared to EBM manufacturing systems, LPBF technology has not been used
extensively to manufacture TiAl-based alloys . The production of TiAl-based intermetallic alloys via the LPBF manufacturing process only started about a decade ago . Unfortunately, the pioneers could not use the LPBF manufacturing process to produce crack-free TiAl samples . The high rate of heating and cooling (104–106 K/s)  during the LPBF manufacturing process, which results in the build-up of residual stress, was responsible for the inability to produce crack-free TiAl near-net components . A host of researchers tried to determine the optimum process parameters that could be used to produce crack-free TiAl near-net-shapes, but to no avail . These attempts
were based on the premise that there are more than 50 processing parameters  that influence melt pool geometry during the LPBF process and the appropriate combinations of these parameters might help to overcome the cracking effect.
Vilaro et al.  changed the solidification front to induce a smaller temperature gradient by a combination of the process parameters of slow scanning speed and wider beam diameter (scanning speed 0.02 m/s, laser beam diameter 380 m, laser power 60–250W). The thermal conditions (temperature gradient—G, cooling rate—T, solidification rate—Z) were optimized by changing the solidification behavior from dendritic, to cellular, up to planar front growth. Thus, an increase in the G/Z ratio progressively changes the solidification behavior, whereas the G/Z ratio determines the microstructure characteristics. A
careful combination of the process parameters could lead to the production of crack-free TiAl-based alloy components with intricate geometries by slowing down the cooling rate. The optimum process parameters cause the molten pool to take a longer time to solidify at a lower cooling rate. The effort of Vilaro et al.  was only able to reduce the cracking effect,
but could not suppress it completely. Vilaro et al.  reported that the cracks began from the interface of the solidified layers, and propagated along the building direction Z. The authors maintained the building substrate at 500 C (Figure 3) throughout the experiments to relieve the material of residual stress while the building process continued. This strategy could not prevent the cracking of the TiAl build parts because the LPBF manufacturing process works by lowering the base plate a distance equivalent to the powder layer thickness each time a layer is completed. Because a new layer of powder is delivered onto the powder bed and the manufacturing process continues, for a large multiple layer component, the effect of keeping only the base plate at 500 C becomes ineffective for preventing crack formation. In reality, the approach of holding the base plate at a high temperature has the tendency of inducing a high thermal gradient between the base plate section and the top section of the built component. The high temperature gradient introduces high residual stress, which could cause the sample to crack.
were manufactured at a scanning speed of 0.45 m/s, a laser power of 80W, and a powder layer thickness of 30 m. However, the intrinsic heat treatment could not eliminate the crack formation. It was reported that a tension crack evolved from the side of the specimen towards the middle, which implied that there a thermal gradient developed between the outer perimeter and the middle section of the sample. Hence, intrinsic heat treatment could not prevent the rapid heating and cooling process inherent in the LPBF manufacturing process. The intrinsic heating process led to the production of dissimilar microstructural
features in different areas of the samples. The researchers found that the microstructures of the samples were improved using hot isostatic pressing (HIP). HIP enabled the production of the (a2/g) lamellar microstructure. Loeber et al.  conducted a direct comparison between the EBM and LPBF manufacturing processes. These authors used the EBM and LPBF manufacturing process to produce Ti-48Al-2Cr-2Nb samples. Microstructural analysis revealed that the LPBF samples did not show any clear microstructure under the SEM-SE (secondary electron) contrast—thus no clear statement could be made of the microstructure of the as-built LPBF samples. A fine lamellar microstructure was observed after heat treating the samples at 1400 C for 2 h. The LPBF samples had elastic modulus values of 50 13 GPa, which were about one third lower than what is normally reported for TiAl alloys manufactured using the EBM and the conventional manufacturing methods [. The low elastic modulus of the LPBF samples was attributed to the presence of cracks during the LPBF manufacturing process. Obviously, the attempt by these authors to produce crack-free TiAl-based intermetallic alloys via the LPBF systems was not successful. Many preheating techniques were introduced (Figure 3) (scanning laser beam , IR heaters , fast and second defocused laser beams , baseplate induction circuits , and substrate resistive heating ) to overcome the cracking effect of TiAl components manufactured via LPBF. The preheating techniques were focused on heating the base plate to alleviate the cracking effect during the LPBF manufacturing process. However, as presented in Figure 2, the EBM preheating system is designed to keep the powder bed at a high temperature (about 1000 C)  throughout the manufacturing process prior to the melting of the powder feedstock by the lower power electron beams, as
opposed to the LPBF pre-heating system which is mainly focused on preheating of the base plate (Figure 3). The base plate normally acts as a heat sink, which could serve as a crack initiation point due to the high thermal gradients and thermal stresses at the bottom of the samples . As a result, many researchers and industry practitioners have focused on
preheating the base plate in an attempt to avoid the cracking effect. This analogy sounds very logical, because most of the cracking during the LPBF manufacturing process starts from the base plates (Figure 4). However, the building of large TiAl components requires keeping every layer of the powder bed at a high temperature to avoid the development of a high thermal gradient between the solidified layers and the new layers. Heating only the base plate provides a narrow window of producing only a few crack-free layers at the base plate level. As the building process continues and the manufactured samples increase in size, the effectiveness of heating only the base plate diminishes, and the samples begin to crack from the interface between the layers, the outer perimeter, and the middle section of the middle section of the samples, as previously reported by Vilaro et al.  and Gussone et al.  (Figure 4). It is obvious that the industrial panorama of producing TiAl-based intermetallic alloy components of intricate shapes with great geometrical accuracy is in high demand . Therefore, there is an urgent need for further research into the possibility of producing crack-free intricate geometries of TiAl-based alloy parts with great dimensional precision for high-temperature
6. The Proposed Next Generation LPBF Manufacturing Systems
even cooling during and after the building process, unlike the previous LPBF system which focused only on pre-heating the base plate. The layer-wise manufacturing strategy of the LPBF process ensures that the build platform is lowered one step according to the powder layer thickness after a layer is built. The current proposed system (Figure 5) would ensure that the temperature of the already built part on the built platform is at a temperature that would enable a slow cooling rate during and after the entire build process. Such an LPBF manufacturing process would not favor the development of a high thermal gradient. Since the temperature gradient between the bottom part of the sample and the upper parts is reduced, there is a greater possibility of producing crack-free TiAl samples with tailored geometrical configurations.
process, as already attempted by Caprio et al. . The cooling rate after the building process would also be controlled to prevent any crack formation due to rapid cooling. Such an approach would also open a window for the in situ heat treatment of the build samples to obtain the desired TiAl (g-phase) and Ti3Al (a2-phase) intermetallic phases for high temperature operations. In situ heat treatment would also prevent the formation of inhomogeneous microstructures, as it is well documented that the LPBF built parts normally present anisotropic microstructures.
geometry with dimensional accuracy for industrial applications is very near, as the LPBF technology is gradually attaining maturity.
||The infrastructure and knowledge base of producing TiAl-based intermetallic alloys via conventional methods is very mature, resulting in the mass production of simple shapes .|
|Powder bed fusion||EBM||
||Production of 3D components of less intricate shapes with the required TiAl (γ-phase) and Ti3Al (α2-phase) intermetallic phases for high temperature operations .|
|LPBF||Production of non-crack-free 3D components .||3D components of intricate geometries with high resolution and rigorous build accuracy .|
|The proposed LPBF manufacturing heating system||Figure 5||Optimum process parameters not yet determined.||Possible production of 3D components of intricate geometries according to the technical, functional, and geometrical dimensions of the required/intended applications.|
The entry is from 10.3390/ma14154317
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