**4. Heat treatment of laser processed titanium aluminide components**

The microstructural evolution of γ-TiAl clearly affects the end products mechanical properties as a result of processing and post-processing operations. Dependent on the characteristics anticipated for parts, several techniques could be utilized to fix microstructure. It has been reported that lamellar of coarse grains display comparatively superior fracture toughness, creep but lack ductility, notably at room temperature whilst duplex of fine grains connotes low fracture toughness, creep resistance nonetheless modest ductile property [27].

SLM of Ti-44.8Al-6Nb-1.0Mo-0.1B was investigated by Gussone et al. [62] and influence of heat-treatment during and after fabrication. The results showed that considerable changes stimulated by heating the unstable Ti-44.8Al-6Nb-1.0Mo-0.1B varies sharply. The initial heat treating of samples produces microstructures exemplified by increased orbicular γ and β/βο at the α2/γ boundaries colony.

Alloy Ti-47Al-2Nb-2Cr (at%) fabricated through selective electron beam melting (SEBM) by Guangyu et al. [92], examined the influence of post processing heat treatment on microstructure. Lamellae microstructure was revealed for the samples (**Figure 9**) because of SEBM cyclic heating. Hence, transformation of phases was clearly through β-phase. Though, various structures are gotten at different temperatures, oil quenching at 1250°C+ heating to 1200°C for 2 h produces a refined and homogeneous microstructure.

Klein et al. [93] investigated the microstructural formations of Ti-Al-Nb-Cr-Mo in understanding the requirements for controlling the microstructure formation and concomitant properties. Upon annealing, γ-platelets arise initially from the precipitating βo-phase. Cr and Mo are repelled throughout the γ-grain development.

The influence of annealing temperature was investigated by Ren et al. [19], on Ti-45Al-8.5Nb formation of controlled in lamellar structure. The regulated α<sup>2</sup> lamellae appears in alloys annealed at lesser than 650°C and vanishes beyond 700°C. Extensive hardening stimulates O phase on α<sup>2</sup> lamellae and the γ/(α<sup>2</sup> + O) been obtained attributed to refined α<sup>2</sup> lamellae. The schematic of the heat treatment procedure is depicted in **Figure 10** below.

studied. Small fraction of β<sup>o</sup> reduces the microhardness of the LMD alloy. Lamellar orderings are moderately greater in the LMD microstructure because annealing is achieved in the (γ + α) phase with noticeable coarsening of the non-melted γ

*Heat Treatment Pattern Adopted by Ren et al. 19 for the γ-Ti-45Al-8.5Nb alloy annealed at 900 oC for 2 hrs followed by air cooling with (a) annealing at 600 °C, 650 °C and 700 °C for 6 hrs each followed by air cooling (b) isothermal annealing at 600 °C for annealing time range from 0.5 to 500 hrs followed by air cooling.*

*Microstructures of heat-treated Ti-47Al-2Nb-2Cr alloy at (a) 1250°C for 2 h air cooled (b) 1300°C for 10 min air cooled (c) 1250°C for 10 min oil quenched (d) 1250°C for 10 min oil quenched followed heat*

*Laser Based Additive Manufacturing Technology for Fabrication of Titanium Aluminide-Based…*

Also, phase transformations were examined by Kastenhuber et al. [28] for βcontaining γ-TiAl based alloy in gas atomization of fast cooling. Initial variations of composition caused a primary β-phase, indicating to an uninterrupted escalation of the Al and lessening of Nb, Mo and Ti at the previous β-grain boundaries. Therefore, following L ! L + β reaction the solidification path continues unevenly,

lamellae.

**207**

**Figure 10.**

**Figure 9.**

*treating at 1200°C for 2 h air cooled [92].*

*DOI: http://dx.doi.org/10.5772/intechopen.85538*

Analysis of microstructure of high-Nb TiAl alloy carried out by Qiang et al. [94], for Ti-45Al-8.5Nb-0.2 W-0.2B-0.02Y (at.%) resulting from numerous cooling rate. The transformation path α ! α<sup>2</sup> + γ for lamellar microstructure is controlled chiefly by its rate of cooling. Fast rate of cooling causes the retention of α/α<sup>2</sup> phase of supersaturated microstructure. It was suggested that a rapid cooling rate and slow rate of cooling for α ! α<sup>2</sup> + γ and β ! α transformation respectively.

In the repair of defective or worn blade parts by Rittinghaus et al. [95], LMD of TNM™ β-containing alloy with composition of Ti-43.5Al-4Nb-1Mo-0.1B (at.%) was *Laser Based Additive Manufacturing Technology for Fabrication of Titanium Aluminide-Based… DOI: http://dx.doi.org/10.5772/intechopen.85538*

#### **Figure 9.**

alloying element restrains the oxygen dispersion making impact in the creation of

LENS was employed by Zhang and Bandyopadhyay [90], to deposit Ti6Al4V

and Al2O3 powders on Ti6Al4V substrate. For the pure Ti6Al4V, α-laths of Widmanstätten Ti was obtained while Ti6Al4V + Al2O3 parts revealed equiaxed grains with non-melted Al2O3. Microhardness results showed that Al2O3 section possessed the maximum hardness value followed by Ti6Al4V + Al2O3 sections. Direct laser cladding (DLC) of Ti45Al5Nb0.5Si had been studied by Majumdar

et al. [91], to determine the consequence of varying parameters on titanium aluminide. Dual phase α<sup>2</sup> + γ was revealed in DLC Ti45Al5Nb0.5Si alloy. The processing parameters had little effect on the microhardness of the clad. Rise in Si

**4. Heat treatment of laser processed titanium aluminide components**

The microstructural evolution of γ-TiAl clearly affects the end products mechanical properties as a result of processing and post-processing operations. Dependent on the characteristics anticipated for parts, several techniques could be utilized to fix microstructure. It has been reported that lamellar of coarse grains display comparatively superior fracture toughness, creep but lack ductility, notably at room temperature whilst duplex of fine grains connotes low fracture toughness,

SLM of Ti-44.8Al-6Nb-1.0Mo-0.1B was investigated by Gussone et al. [62] and influence of heat-treatment during and after fabrication. The results showed that considerable changes stimulated by heating the unstable Ti-44.8Al-6Nb-1.0Mo-0.1B varies sharply. The initial heat treating of samples produces microstructures exemplified by increased orbicular γ and β/βο at the α2/γ boundaries colony.

Alloy Ti-47Al-2Nb-2Cr (at%) fabricated through selective electron beam melting (SEBM) by Guangyu et al. [92], examined the influence of post processing heat treatment on microstructure. Lamellae microstructure was revealed for the samples (**Figure 9**) because of SEBM cyclic heating. Hence, transformation of phases was clearly through β-phase. Though, various structures are gotten at different temperatures, oil quenching at 1250°C+ heating to 1200°C for 2 h produces a refined and

Klein et al. [93] investigated the microstructural formations of Ti-Al-Nb-Cr-Mo in understanding the requirements for controlling the microstructure formation and concomitant properties. Upon annealing, γ-platelets arise initially from the precipitating βo-phase. Cr and Mo are repelled throughout the γ-grain development. The influence of annealing temperature was investigated by Ren et al. [19], on

Analysis of microstructure of high-Nb TiAl alloy carried out by Qiang et al. [94], for Ti-45Al-8.5Nb-0.2 W-0.2B-0.02Y (at.%) resulting from numerous cooling rate. The transformation path α ! α<sup>2</sup> + γ for lamellar microstructure is controlled chiefly by its rate of cooling. Fast rate of cooling causes the retention of α/α<sup>2</sup> phase of supersaturated microstructure. It was suggested that a rapid cooling rate and slow

In the repair of defective or worn blade parts by Rittinghaus et al. [95], LMD of TNM™ β-containing alloy with composition of Ti-43.5Al-4Nb-1Mo-0.1B (at.%) was

Ti-45Al-8.5Nb formation of controlled in lamellar structure. The regulated α<sup>2</sup> lamellae appears in alloys annealed at lesser than 650°C and vanishes beyond 700°C. Extensive hardening stimulates O phase on α<sup>2</sup> lamellae and the γ/(α<sup>2</sup> + O) been obtained attributed to refined α<sup>2</sup> lamellae. The schematic of the heat treatment

rate of cooling for α ! α<sup>2</sup> + γ and β ! α transformation respectively.

quantity improved the propensity to produce cracks.

creep resistance nonetheless modest ductile property [27].

more favored alumina films.

*Aerodynamics*

homogeneous microstructure.

**206**

procedure is depicted in **Figure 10** below.

*Microstructures of heat-treated Ti-47Al-2Nb-2Cr alloy at (a) 1250°C for 2 h air cooled (b) 1300°C for 10 min air cooled (c) 1250°C for 10 min oil quenched (d) 1250°C for 10 min oil quenched followed heat treating at 1200°C for 2 h air cooled [92].*

#### **Figure 10.**

*Heat Treatment Pattern Adopted by Ren et al. 19 for the γ-Ti-45Al-8.5Nb alloy annealed at 900 oC for 2 hrs followed by air cooling with (a) annealing at 600 °C, 650 °C and 700 °C for 6 hrs each followed by air cooling (b) isothermal annealing at 600 °C for annealing time range from 0.5 to 500 hrs followed by air cooling.*

studied. Small fraction of β<sup>o</sup> reduces the microhardness of the LMD alloy. Lamellar orderings are moderately greater in the LMD microstructure because annealing is achieved in the (γ + α) phase with noticeable coarsening of the non-melted γ lamellae.

Also, phase transformations were examined by Kastenhuber et al. [28] for βcontaining γ-TiAl based alloy in gas atomization of fast cooling. Initial variations of composition caused a primary β-phase, indicating to an uninterrupted escalation of the Al and lessening of Nb, Mo and Ti at the previous β-grain boundaries. Therefore, following L ! L + β reaction the solidification path continues unevenly,

subject to Al-content and quantities of Nb and/or Mo. At room temperature the γ-phase appears in three different morphologies.

duplex phase were observed. The duplex phase reveals greater room temperature

*Laser Based Additive Manufacturing Technology for Fabrication of Titanium Aluminide-Based…*

microhardness value rises as tensile strength increases for Ti-49Al (at.%)

The influence of cooling rate was investigated by Fan et al. [99], on mechanical and microstructural attributes of Ti-49Al alloy. Increased rate of cooling results in reduction of interlamellar and dendrite spacings. Whereas rise in cooling rate causes corresponding improvement in tensile strength and hardness values. Consequently,

According to Bünck et al. [58], recent manufacturing processes for aerospace parts are expected to be well developed and cost-effectively competitive methods. LAM parts have high surface roughness and structural inhomogeneity and usually necessitate supplementary treatments to diminish irregular microstructure. Inadequate ambient temperature ductile property of TiAl alloys deters processing. Even during forging using isothermal process, dies are preserved at considerably high temperatures. Practical issue still to be stopped is the reactivity of melting crucible with TiAl, besides the matters of cost [25]. Novel methods are required that permit the modification of satisfactory amount of ductility though preserving the exceptional strength and creep characteristics that aforementioned research works has unlocked. The solution to unravel this difficulty rests in essential consideration of fundamental relationship between the properties and microstructure mechanisms. Grain morphology control is a challenging issue for LAM of large metallic components. Numerous practices have extensively considered the additional characteristics of TiAl-based aerospace parts, comprising the practice of surface modeling,

Presently, LAM has the prospective to develop typical manufacture techniques, specifically in creating of small size of extremely complicated components [100]. Manufacturers have previously presented low-pressure turbine blades (LPTB) of TiAl alloy in aero engines and more interest is shown for the development of TiAl, demonstrating the distinctiveness of the alloy. This is with a view that engine efficiency would be substantially better-quality, indicating decreased fuel utilization, CO2 emission and noise. Consequently, increases stress is envisaged for TiAl-LPTBs and necessitates adjusting processing routes to produce functional materials. LAM is suggested to have benefits for aircraft engine components because it can easily translate designs to 3D component, produce customized parts and functional

designs with complex and intricate features [36]. It also has potential of manufacturing parts with no waste, lightweight and minimal lead time of

manufacturing. Hence, it has the ability to be exceptional scalable and manufacture

Based on the reviewed works, it can be generally deduced that LAM processing of titanium aluminide suffers several set-backs militating against its wider acceptance in aerospace application. Although, it has attractive features making it to be qualified as a candidate material for aerospace applications, the processing and post-processing problems still needs to be looked into. It is expected that combined processes of alloying and heat treatment (in-situ and ex-situ) could result in appreciable ductility while maintaining other mechanical properties. Consequently,

tensile property, whilst fully-lamellar had better strength.

directionally solidified alloy.

**5. Challenges and critical issues**

*DOI: http://dx.doi.org/10.5772/intechopen.85538*

application of coatings and heat-treatment.

final product with little or no post-processing.

**6. Summary and conclusions**

**209**

The consequence of heat treatments was studied by Li et al. [96], to evaluate grains features, transformation of phases and hardness of Ti-45Al-2Cr-5Nb (at.%) produced through SLM. The grains of heat-treated parts are overshadowed with HAGBs that rises as temperature of annealing increases. SLM-fabricated alloy has D019 structure of α<sup>2</sup> phase but the B2-phase is a bcc structure and γ-phase illustrate a L1o (bcc) structure.

The microstructural evolution and room temperature fracture toughness β-solidifying TiAl alloy studied by Chen et al. [30], for Ti-45Al-2Nb-1.5V-1Mo-0.3Y (at.%). Fine near-lamellar containing majorly of fine grains and combinations of γ + β phases with lamellar colony boundaries. The precipitation of fine β and γ grains is deemed an inherent toughness system due to α2/β and α2/γ boundaries creating limited displacement movement successfully.

The influence of heat treatment was studied by Tebaldo and Faga [97], on hardness of Ti48Al2Nb2Cr manufactured through EBM technology. It was noticed that only minor different in density occurred due heat treatment and micro porosity. Near-γ with equiaxed lamellar was obtained for samples not heat treated and fully lamellar achieved for heat treated samples. It was concluded that the developed alloy would be appropriate for manufacturing components in aircraft engines.

An incorporation of both HIP and heat treatment was developed by Chen et al. [13], to enhance mechanical properties of TiAl alloy. Corresponding increment in elongation and YS was noticed for both duplex and lamellar structures. Reduction in TiAl alloy mechanical properties was evident from the observed microcracks. However, the combined production method showed a microcrack free microstructure, thereby enhancing the properties. Illustration of the processing routes are shown in **Figure 11**.

Mechanical and microstructure governing techniques was investigated by Zhao et al. [98], for Ti-46Al-2Nb-2V-1Mo-Y (at%). The α + β + γ phase pre-treatment by annealing actually gave raise to the breakdown of lamellar α/γ structures. Fully-lamellar was attained with prolong annealing whereas for two-step process,

**Figure 11.** *The processing routes adopted by Chen et al. [13] (a) separately (b) combined processing.*

### *Laser Based Additive Manufacturing Technology for Fabrication of Titanium Aluminide-Based… DOI: http://dx.doi.org/10.5772/intechopen.85538*

duplex phase were observed. The duplex phase reveals greater room temperature tensile property, whilst fully-lamellar had better strength.

The influence of cooling rate was investigated by Fan et al. [99], on mechanical and microstructural attributes of Ti-49Al alloy. Increased rate of cooling results in reduction of interlamellar and dendrite spacings. Whereas rise in cooling rate causes corresponding improvement in tensile strength and hardness values. Consequently, microhardness value rises as tensile strength increases for Ti-49Al (at.%) directionally solidified alloy.
