The major classes of additive manufacturing (AM).
Titanium aluminides has the potential of replacing nickel-based superalloys in the aerospace industries because its density is almost half that of nickel-based alloys. Nevertheless, the room temperature properties (ductility) have made the wider application of this class of intermetallic alloy far from being realized. This has led to various research been carried out in adjusting the production processing and/or material through alloying, heat treatment, ingot metallurgy, powder metallurgy and most recently additive manufacturing processing. One of the additive manufacturing processing of titanium aluminide is laser engineered net shaping (LENS). It is used to produce components from powders by melting and forming on a substrate based on a computer-aided design (CAD) to shape the components. This contribution will focus on the laser processing of titanium aluminides components for aerospace applications. Also, the challenges confronting this processing techniques as well as suggested finding to solve the problems would be outlined. The objective of this work is to present an insight into how titanium aluminides components have been developed by researchers with emphasis on aerospace applications.
- additive manufacturing
- titanium aluminide
- laser additive manufacturing (LAM)
- aerospace components
Aerospace engine components are exposed to high temperatures as a result of enormous heat generation in the engine and formation of large temperature rise across the structural parts. These components are generally expected to possess a high strength-to-weight ratio in order to give high thrust, improved fuel efficiency at minimal fuel consumption while increasing speed of flight. Aircraft engines are usually fabricated by titanium alloys, nickel-based super alloys, superior steels or ultrahigh-temperature ceramics. Moreover, an important engineering material for manufacturing of gas turbine and aircraft engine components are alloys of titanium (Ti) because of good properties like corrosion resistance, excellent specific strength and most especially their low density . Also, due to the biocompatibility of these alloys, they have been applied in biomedical industries as implants [2, 3, 4] and prosthetic beak . The poor wear performance and low hardness of titanium alloys constrained their adoption in fabricating components to be subjected in contact forces of friction and wear [3, 6]. Likewise, titanium alloys have not been impressive for load-bearing components (LBC) in the aerospace industry because of uneven stress distribution on parts .
Beta titanium (β-Ti) alloys is reported to have been used to fabricate landing gears in Boeing aircraft from the Ti-5Al-5Mo-5V-3Cr because of better strength-toughness mishmash in β-Ti alloys compared to α + β and α alloys . Titanium intermetallic alloys have great potentials in aerospace, automotive and power plants especially titanium aluminides (TiAl) because of their excellent creep and light-weight in relation to nickel-based superalloys . The most feasible TiAl alloys with engineering interest are those of 44–48 at.% Al content because of the eutectoid transformation reaction occurring during solidification and cooling process . Figure 1 displays the phase diagram of Ti-Al between 0 and 60 at. % aluminum. Titanium matrix composites (TMCs) reinforced with ceramics has increased due to improvement in heavy load-bearing, wear and friction performance in service due to material efficiency with lower cost of parts fabrication.
A possible replacement of Ni-based superalloys is gamma-titanium aluminide (γ-TiAl) considered in manufacturing aero-engine and automobile engine parts because of their high strength, high stiffness, light weight good oxidation and corrosion resistance [11, 12, 13, 14, 15, 16, 17, 18, 19, 20]. The choice of γ-TiAl is hinged on the fact that its density is almost half of Ni-based superalloy and high temperature creep properties [21, 22] with application temperature up to 750°C . These has made γ-TiAl to be favorably accepted in aerospace industry. However, aero engine component like turbocharger operate at temperatures between 700 and 950°C which presents a concern of oxidation reaction apart from its room temperature ductility . γ-TiAl of 48 at.% Al generally comprise of a γ-phase and α2-phase with L1o and DO19 structure respectively (major: γ-TiAl, minor: α2-Ti3Al) typically referred to as γ-TiAl [10, 24]. This γ-TiAl-based alloy are continuously been improved upon in the manufacturing nozzles, low pressure turbine blade (LPTB) and turbochargers in aerospace engines .
General Electric has applied the Ti48Al2Cr2Nb alloy called the GE4822 to manufacture the Boeing-787 Dreamliner’s engine known as GEnx-1B that reduces both fuel consumption and CO2 emission by 15% [11, 21, 26]. Due to the GE4822 alloys high-temperature capability, there has been works on improving the ductile property of γ-TiAl which have been used for low-pressure turbine (LPT) blades [11, 16, 18, 27] and to expand the temperature range 800–850°C . High Nb-containing TiAl known as TNM have been produced with higher yield strength, oxidation resistance and better creep than conventional TiAls [15, 19, 23, 26, 28, 29].
However, the extremely poor tensile ductility (<1% elongation) of TiAl-based alloys at room temperature, poor fracture toughness and high-temperature poor workability makes it difficult to fabricate and strongly limits their applications in the industry [10, 13, 15, 16, 18, 30]. This has made conventional processing techniques very problematic in manufacturing TiAl-based components. Hence, a lot of interest in improving the ambient temperature characteristics and oxidation resistance at elevated temperature, methods such as alloying and post-processing heat treatment has been suggested to help advance its properties.
Apart from the potentially improved geometrical complexity, additive manufacturing (AM) permits manufacturing of innovative materials with functionally graded capabilities during fabrication . The applicability of TiAl alloys is becoming more progressive due to the growing knowledge of the connections concerning mechanical properties and microstructural evolution as well as modifications in processing operations . However, even with AM technologies issues of the mass production quantities is still not making the technique cost-effective .
Currently, AM has gained acceptance for manufacturing customized products with complex geometrical freedom, more homogeneous microstructure and functionally enhanced parts gradually replacing traditional processes [14, 28, 32]. Due to this property, it is possible to manufacture components and finished parts which could not be implemented with conventional manufacturing technologies or only at great expense. AM routes is poised to produced γ-TiAl parts for aerospace applications.
Laser additive manufacturing (LAM) is a class of AM process that makes use of high-powered laser beam in melting and fabricating metal powders into three-dimensional (3D) components from a predesigned computer-aided design (CAD) model. One of the highly desirable LAM techniques is laser engineered net shaping (LENS®) regarded for producing ceramics, composites and functionally graded materials (FGMs) which are difficult to produce [33, 34].
The objective of this work is to give an overview of LAM of TiAl alloys and composites while presenting a review of crucial matters, investigation tendencies, fabrication and progress achieve on γ-TiAl based materials for aerospace. A succinct overview of TiAl was first presented followed by AM of titanium alloys particularly those applied in aerospace industries. The paper subsequently highlights a comprehensive appraisal of latest studies on γ-TiAl laser processing and post-processing effects on microstructural evolution and mechanical properties. Prospects, challenges and suggested findings were discussed at the end of this work.
2. Additive manufacturing (AM)
AM is a material joining process from a three-dimensional (3D) CAD model data to build an object, in typically layer by layer [32, 35]. AM has been known by names which includes freeform fabrication (FFF), additive layer manufacturing (ALM), rapid prototyping (RP) and 3D-printing (3DP). AM have been widely adopted in industries especially the aerospace and medical field with recent application in radio-frequency (RF) for microwave and millimeter wave devices . The distinguishing feature of this technology is ability to manufacture objects through layer upon layer building  and consistent component functions requiring little or no additional process .
The two key classes of AM technologies are direct energy deposition (DED) and powder bed fusion (PBF) as depicted in Table 1 . The DED methods offers prospects in engineering applications for producing functional parts with intricate geometries from metallic powders . The powder particles used are preferably spherical with the DED being more sensitive in size distribution and wires are adopted as precursor for certain DED processes for higher production rate . The process melts the material feedstock (powder or wire) using a high-powered laser beam. The most commonly used lasers are CO2 and ND: YAG which has been used for fabricating complex and customized parts, prototyping of metallic parts and repairing/cladding of components . This method is ideal to directly make components with limited tooling but the produced parts exhibits relatively low fracture toughness and prone to defects like cracks and pores due to fast heating and cooling rates involved [39, 40].
In the PBF techniques, a heating source (laser or electron beam) scans metallic powders on a preplaced build platform to melt the powders in a predefined path. This is controlled based on CAD model to build the object in a layer by layer tool path. It has the benefits of reduced lead time and material waste  and control of melt pool dynamics and stability .
There have been remarkable improvements in AM primarily by the Ti alloys production for aerospace components. Though, PBF is competent to make near-net shaped objects and DED also can be applied to refurbish/repair parts and modify features apart from fabricating 3D objects. The high-temperature gradient due to focused instant heating and cooling is the main difficulty encountered by these techniques.
The majority of investigations on AM materials demonstrates that their mechanical properties of better than the traditionally manufactured titanium alloys. The principal application of additive manufactured Ti-alloy is the aerospace industry followed by the medical industry while its gradually been accepted in automotive industry. AM presents encouraging platform to produce highly effective Ti-alloy component with intricate geometry [43, 44]. Whereas, these alloys are continuously been studied, the problem encountered is due to their additional processing requirements.
Laser-based AM, however, achieves the built parts defined in a CAD model by melting and solidifying powder on a layer by layer basis . This method is known to have been adopted in aerospace to manufacture and create components of graded material using Ti-based alloys. The LAM technologies produce fully dense parts from wire or powder feedstock [46, 47]. It has the merit of cost and time saving in comparison to traditional methods of casting and forging. A pertinent problem encountered in LAM when processing titanium matrix composites form various literatures is the unevenness of particle distribution. Also, the appearance of non-melted particles leading to reduced plasticity.
2.1 Additive manufacturing of titanium alloys
One of the titanium alloys that are ideal for aerospace applications is the β-Ti alloy because it has a combined property of high strength and lightweight . Titanium alloys have proven to be extremely attractive and important materials in aerospace applications because of excellent combinations of outstanding corrosion and mechanical properties [39, 47, 49, 50]. Recently, there has been increased interests in the development of additively manufactured titanium (and their alloys) parts in industries. However, the widespread applications are mitigated by expensive processing involved compared to some material of similar function .
A lot of studies have shown that Ti-alloys produced by AM possess considerable better mechanical properties than the conventional materials. Also, recent development has led to increased acceptance of the technologies, contributing new potential concerning functional incorporation and lightweight components.
Saboori et al.  presented a review of AM on titanium components by direct laser deposition (DLD) analyzing the significance prospects of DED process. The connection among titanium alloys microstructural characteristics and process parameters were outlined. It was understood that DED processed parts undergo complex thermal history severely induced due to process uncertainties and process parameters. Even with attempts in optimizing process parameters to curtail flaws, enhance mechanical properties and microstructure of the component, there still existed the challenge of striking a balance. However, titanium parts fabricated by DED still had increased strength and poor ductile properties. It was stated that this alloy suffer anisotropy in the tensile characteristics because components fabricated horizontally demonstrate increased ductile strength compared with those build vertically.
In the work of Hu et al. , titanium matrix composites (TMC) has been fabricated by LENS using a 3D quasi continuous network (3DQCN) and TiB as reinforcement. The author examined the influence of energy input and reinforcement on parts quality and wear resistance. Superior wear performance was recorded as a result of TiB addition as a reinforcement. In a related work by Hu et al. , the author examined the influence of laser power and TiB reinforcement on mechanical properties. It was shown that laser power greatly affects the resulting microstructures but the strengthening of TiB increases the ultimate compressive strength (UCS) and microhardness of TiB-TMCs. Table 2 shows influence of laser power in varying microhardness of LENS processed TiB-TMCs.
|Classes||Technology||Source of energy||Material used|
|Powder bed fusion (PBF)||Electron beam melting (EBM)||Electron beam, laser beam||Metal, ceramics and polymers|
|Selective laser melting (SLM)|
|Direct metal laser sintering (DMLS)|
|Selective laser sintering (SLS)|
|Directed energy deposition (DED)||Laser engineered net shaping (LENS)||Laser beam||Metals (powder and wire)|
|Electronic beam welding (EBW)|
Also, the mechanical properties and defects (porosity) of TiB-TMCs parts fabricated using an innovative ultrasonic vibration-assisted (UV-A) LENS have been investigated by Ning, Hu and Cong . The procedure adopted exhibited remarkable influences on porosity, pore size, TiB whisker size and distribution, QCN microstructural grain size and microhardness. Consequently, fine microstructures were obtained due to refinements of both TiB whiskers and QCN microstructural grains UV-A LENS providing larger area of grain boundaries to hinder the dislocation motion, thus improving the microhardness of fabricated parts.
A research was conducted by Grove et al.  to investigate and compare the surface integrity and machinability of Ti-5553 alloy with composition Ti-5Al-5V-5Mo-3Cr produced by casting, SLM and SLM + heat treatment (in-situ). Higher tool wear was noticed in the in-situ heat treated alloy while the tool wear for the other two does not seem to differ. Therefore, it proves titanium alloys’ machinability is extremely affected further processing methods.
According to Uhlmann et al. , the aviation industry requires a drastic reduction in NOx and CO2 emission in the coming years ahead. The authors’ study was focused on highly effective and efficient titanium alloys with optimizing process parameters for structures manufactured by the SLM machine. The post-processing of TiAl6V4 parts fabricated by SLM display vast possibility of enhancing surface quality. Examination of computed tomography and microstructure confirms that porosity of titanium alloy parts can be minimized through HIP-process.
Silica (SiCO2) coatings have been created on commercially-pure titanium (Cp-Ti) by Heer and Bandyopadhyay  using LENS with subsequent stress relieve heat treatments and post-deposition laser passes. Ti5Si3 phase was formed leading to high hardness of ∼1500 HV in the coatings and wear rate was mostly reduced in comparison to Cp-Ti irrespective of heat treatment as well as rise in laser pass commonly reduce wear rates. The microstructures revealed dendrite shaped and morphology of columnar deposit around primarily matrix of α-titanium.
A process of reactive deposition AM was adopted by Traxel and Bandyopadhyay  for maintaining Ti-Zr-BN composite parts using commercially pure titanium (Cp-Ti) with addition of Zr and BN. This was aimed at improving elevated temperature and wear resistance abilities if TMC using LENS. The as-fabricated BN-containing components shows TiB2, TiN and TiB as reinforcement with SEM image of CpTi-BN and CpTi-Zr-BN presented in Figure 2 below. Zr-addition displays a combination of composites of composites and alloys leading to high hardness and ultimate compressive strength (UCS) with enhanced wear resistance. This method is expected to be used in creating novel coatings and structures from vast powder feedstocks to develop the bulk and surface characteristics of titanium related metals.
Effects of thermal history and build direction were studied by Mantri and Banerjee  on β-Ti alloys containing Mo-12 wt% and V-20 wt%, produced through LENS technique. A specific single β-phase is identified in 20 wt% V while the dual-phase microstructure in Ti-12 wt% Mo system revealed α-phase in β-grain morphology. This was accredited to the build directions that had different thermal cycles at various portion of the builds.
In the work of Li et al. , LAM was used to fabricate Ti-6Al-2V-1.5Mo-0.5Zr-0.3Si alloy in order to examine tensile strength and microhardness of the alloy. Mixed columnar β-grains with no equiaxed microstructure was observed for as-deposited alloy. The addition of Zr, Mo and Si causes increase in microhardness value because of hardening effect of their solid solution. Also noticed, was the rise in ductility in comparison to Ti-6Al-4V.
Song et al.  reported the titanium alloy to fabricate a prosthetic beak as shown in ( Figure 3 ) for a bird (Grus japonensis) using SLM. A model was first developed for the titanium alloy customized beak and was successfully completed to save the endangered bird.
A comparative study of SLM and electron beam melting (EBM) carried out by Zhao et al.  to fabricate Ti-6Al-4V parts. Both components largely contained α + β and α′ phase, respectively. SLM samples had higher tensile strength but lower ductility than EBM fabricated parts. Even though porosity was observed in both cases it was much higher in SLM parts. While generally, samples fabricated in the vertical direction had increased ultimate tensile strength, yield strength, and improve ductility than those in the horizontal orientation.
Direct metal deposition (DMD) has been used by Pouzet et al.  to build samples with varying volume fractions of TiB + TiC. The Coaxial deposition process fabricated TMC from powder mixture of T-6Al-4V + B4C. Pure and consistent Ti-6Al-4V-TiB microstructure was noticed and boron isolation during solidification of β-Ti resulted in grain refinements. At low B4C, TiC was not observed which was ascribed to high solubility of C in α-Ti at high solidification rate leading to decreased tensile strength at elevated and room temperature.
2.2 Laser additive manufacturing (LAM) of titanium aluminides
TiAl are known to be favorable for high-temperature component because of their high strength-to-weight ratio, high melting point, attractive creep and superior modulus of elasticity . Recently, TiAl-based alloys have been effectively applied in aircraft engine productions with increase in demand for these alloys expected in the near future .
Microstructure and phase composition are immensely determining the properties of intermetallic TiAl-based alloys. Elements like Mo, V, Zr, Mn, Nb, Cr, Ni and Fe are known to be employed in developing enhanced mechanical properties for TiAl [58, 59]. The site effects for most of the elements remains unchanged with the different alloy compositions. However, Cr, Mn and V site substitution vary subject to Ti:Al ratio . It is commonly accepted that a microstructure of equiaxed fine grains are desirable to enhance strength and ductility whereas coarse microstructure of fully lamellar is better for high creep.
The lack of ductility of TiAl at room temperature makes it problematic to machine and fabricate components by traditional methods. AM is very promising to incorporating design freedoms and processing when building components . LENS and selective laser melting/sintering (SLM/SLS) are primarily known as LAM techniques. Various AM technologies are called by their trade names from the manufacturers or company of establishments . A laser engineered net shaping (LENS) Optomec 850R machine for AM system is shown in Figure 4 . This technique is suitable for manufacturing and realization of complex geometries with reduced lead time and material loss . The process uses a focused laser beam for melting and fusion producing 3D parts from feedstock powder. Apart from building components ( Figure 5 ), it refurbishes components through a process called laser cladding. This is believed to be the state-of-the-art technique for aero engine parts repairing .
For aerospace components materials, when compared with conventionally applied materials like Ni-based superalloys, TiAl is far harder and offers close to 50% weight savings. This results in decrease cost of turbine operations and emission levels due to highly efficient fuel usage . Advances in recent development techniques has led to collaborations among manufacturers. This is made possible to fabricate turbine blades using electron beam melting (EBM) from pre-alloyed Ti4822 powder, presently employed in GEnx engines [60, 64].
For aerospace components materials, when compared with conventionally applied materials like Ni-based superalloys, TiAl far harder and offers close to 50% weight savings. This results in decrease cost of turbine operations and emission levels due to highly efficient fuel usage . Advances in recent development techniques has led to collaborations among manufacturers. This is made possible to fabricate turbine blades using electron beam melting (EBM) from pre-alloyed Ti4822 powder, presently employed in GEnx engines [60, 64].
DED was adopted by Hoosain et al.  to create clads of γ-TiAl using in-situ elemental Ti and Al powder. The purpose of the study was to optimize formation of γ-phase during laser processing through mass flow rates. Depending on flow rate of Al and heat treatment both duplex and lamellar phases are observed. Also, Al content largely determines the microhardness and twinning existence. In another work, Hoosain et al.  investigated the consequences of Al flow rate on microstructure of in-situ alloy composition. The γ-TiAl was compared with Ti4822, a duplex phase microstructure was observed as Al feed rate increase. Fine lamellar grains were not generated for heat treatment temperature at 1200°C because it was below the temperature of α-transus. Figure 6 presents the microstructure of commercial GE alloy Ti-48Al-2Cr-2Nb alloys.
In the feasibility study carried out by Dilip et al. , Ti-6Al-4V and Al powder was employed to samples of TiAl using binder jetting AM and reactive sintering. Initially, Al3Ti formation was obtained in the liquid phase due to Ti64 and Al reaction. Afterwards, TiAl was formed has diffusion of Al continues with sintering time. Other intermetallic together with TiAl phases were obtained in the final microstructure.
Todai et al.  presented a study to manipulate Ti4822 alloy microstructure fabricated through EBM process. Duplex-like and γ-bands of layered microstructure were formed by recurring heat treatment of melt pool area. The angle of building substantially makes the tensile elongation and yield strength (YS) to vary. The XRD patterns of the EBM built Ti-48Al-2Cr-2Nb alloy in different directions are displayed in Figure 7 . The anisotropy of YS reduces as temperature rises. The heat treatment at 700°C gave the highest YS which show decrease in value as temperature rises. EBM was also adopted by Murr et al.  in fabricating TiAl from precursor powders to investigate to hardness and microstructure produced. Microstructure of α2-Ti3Al was noticed for powder precursor whereas lamellar colony at γ/α2 with equiaxed γ-TiAl was obtained for EBM samples. It was reported that the EBM-fabricated TiAl had microhardness of ∼4.1GPa which makes the YS far exceed that of EBM-fabricated Ti64.
Selective electron beam melting (SEBM) was adopted by Chen et al. , to investigate the microstructural variations of TiAl alloy samples and their impact on microhardness. A near lamellar structure was observed TiAl alloys. During the process, three phase transformation of α2 + γ → B2, α2 → B2, α2 → γ was noticed. An increase in hardness was ascribed to rapidly raising quantity of lamellar colonies α2/γ has the built height increases.
The structure of TiAl alloys was examined by Li et al.  for detailed characterization of SLM processed Ti-45Al-2Cr-5Nb alloy. Anisotropy of microstructure were obtained has the sides contained columnar grains while the top showed equiaxed microstructure. The processed sample was overshadowed with high angle grain boundaries (HAGBs) which is favored by the energy density. Also, evenly dispersed minute quantities of B2 and γ phases (nano meters range) were found in α2 matrix γ and B2 phase reduces with raising energy density whereas α2 content increases.
Sharman, Hughes and Ridgway  stated that previous works on LAM had concluded that it was merely impossible to manufacture component of TiAl that are crack free without additional heating to influence the solidification process. It was demonstrated that by defocusing the laser across build surface heats the particles ahead of entering melting zone. Hence, decrease rate of cooling lower than the critical level that permits crack formation.
Oxide dispersion strengthen (ODS) has been used by Kenel et al.  to fabricate Ti-45Al-3Nb- < 0.2Y2O3 at.% and studied the microstructure. The traditional microstructure of α2 + γ and near-lamellar were observed. The ODS-TiAl alloy displays greater hardness than ones without the reinforcement. β-solidifying TiAl was studied by Gussone et al.  fabricated by SLM of γ-TiAl pre-alloyed powder. Under cooling of β occurred due to tremendous Al loss leading to β/B2 microstructure. During SLM and hot isostatic pressing (HIP), the raise in content of oxygen make the sample less ductile.
Seifi et al.  conducted and experiment to evaluate the mechanical properties and microstructure of EBM γ-TiAl with HIP. The two conditions presented microstructure of heterogenous interchanging γ-grains of coarse and fine bands. It is worthy to note that the EBM samples revealed microcracks at secluded sections all over the builds. The consequence of beam current on microstructure and grain boundary orientation had been studied by Yue et al.  for Ti-47Al-2Cr-2Nb manufactured by selective electron beam melting (SEBM). Increases beam current resulted in resulted in microstructural transformation to near-γ structure to fine duplex phase and B2-phase volume fraction steadily rises. Also, HAGB increases progressively with beam current while the low angle grain boundary (LAGB) reduces.
In establishing a relationship among scan speed, microstructure and mechanical properties, Li et al.  fabricated Ti-45Al-2Cr-2Nb parts using SLM. The grain size reduces with laser scan while crystallographic topography basically remains unchanged. The phases of B2 and γ rises with increased scan speed whereas α2-phase is lowered. It was also reported that nanohardness and compressive strength rises with improved scan speed for the SLM TiAl alloy.
Chen, Yue and Wang  studied SEBM sample using various scan speed to produce Ti-47Al-2Cr-2Nb alloys. Microstructural changed occurred to duplex from near-γ structure as the scan speed rises. However, both α2 and B2 phases are homogenously dispersed in the matrix of γ. Ultimate compressive strength (UTS) of SEBM-produced TiAl alloy generally increased with the scanning speed.
Statistical examination was used by Shi et al.  to optimize process parameter of SLM Ti-47Al-2Cr-2Nb on Ti-6Al-4V substrate. It was understood that unsuitable amalgamation of process parameters would lead to instability of melt pool, cracks balling and other defects. To optimize the process, Al loss need to be taken into account. The ideal process parameter obtained was able to build TiAl components with 97.34–98.95% densities.
3. Development of titanium aluminide composites for aerospace applications
LENS technique uses high-powered laser to deposit materials and it has been significant in repairing and building of metallic components . Nevertheless, the qualities and mechanical properties of built components are greatly affected by imperfections like porosity, crack cavitation and inhomogeneous microstructure. Composite are of very high demand in aerospace industry because of requests for property combination like high stiffness, fracture toughness, strength, oxidation resistance and lightweight. Moreover, the impetus for composites in aerospace is the ever-increasing demand, thus substitute for metal alloys as weight reduction, cost-saving and fuel efficiency . The aviation industry needs components made with materials with high thermal and mechanical properties at reduced cost [75, 76]. Presently, AM of TiAl is developing, but cracks do emanate due to the integrally brittle nature of the material .
According to Kablov et al. , the qualitative improvement of the technical and performance indicators of aviation gas turbine engines (AGTEs) and power plants largely depends on the implementation of new materials with a previously unachieved combination of properties. It was established that alloying intermetallic titanium ortho alloy with gadolinium in the amount of 0.1 at.% results to the rise in strength at room temperatures owing to the realization of the mechanism of dispersion strengthening with finely dispersed gadolinium oxides .
A crucible-less technique was adopted by Kartavykh et al.  to cast TiAl alloy with composition with composition of Ti-44Al-5Nb-3Cr-1.5Zr (at.%) and high-gradient float zone (FZ) re-solidification. Dual phase of γ + α2 lamellar with small quantities of B2 + γ was observed for the FZ processed alloy. This alloy had high tensile strength which is promoted by Cr accumulation in B2 phase.
A third-generation Ti-43.5Al-4Nb-1Mo-0.1B (TNM) fabricated with casting+HIP was studied by Dahar, Tamirisakandala and Lewandowski  to determine the influence of thermo mechanical treated specimen and applied force fracture behavior. Noteworthy alterations of phases were noticed due to the processing.
A coating of TiAl resistant to oxidation was developed by Sienkiewicz et al. , for thermal shield structures of material. A coating of titanium-aluminum-silicon system of varying chemical configurations was produced through warm spraying. Silicide of Ti5(Si,Al)3 was observed for two kind of morphologies. Generally, the coating shows attractive cyclic and isothermal resistance.
A complete processing chain develop by Juechter et al. , to demonstrated that Ti-45Al-4Nb-C processed by selective electron beam melting (SEBM) could be applied in manufacturing turbocharger wheels. This was made possible by enhanced optimization scan approach coupled with heat treatment.
The microstructures and mechanical properties of Ti6Al4V and Ti45Al7Nb powder mixtures in ratio 1:1 was studied by Wenjun, Chao and Feng  using Electron beam selective melting (EBSM) to build samples resulting in Ti22Al3.5Nb2V alloy. Two distinct portions of microstructure were noticed, a dual (α2 + β) phase and martensite portion. The dual phase region had lower microhardness value compared to the other portions.
The effects of carbon (C) addition was studied by Klein et al. , on γ-TiAl based alloys. Generally, C induces more α2-phase formation and reduces βo-phase. However, substantial hardening of γ and α2 phases occurred with rise in C concentration.
Laser cladding TiN/Ti3Al intermetallic composite coatings have been successfully used to increase the elevated temperature resistance to oxidation and mechanical properties of Ti6Al4V alloy . The clads were largely constituted by phases of α-Ti, Ti3Al and TiN. The intermetallic of TiN/Ti3Al are high oxidation resistant with microhardness far above the value of Ti6Al4V.
An examination of the connection between boron (B) content and microstructural characteristics of TiAl/B (Ti-46.5Al-2.5Cr-2Nb-0.5Y/B) composites produced by SLM was carried out . The grain size reduces as B increases but the HAGBs also rises with B. Owing to grain refinement strengthening system, the highest value of 1610.53 MPa and 5.17% was recorded for compressive strength and strain respectively.
The effects of Nb, Ta and Zr had been investigated by Bresler et al. , for Ti-44Al-5X (X = Nb, Ta, Zr) to examine creep properties ( Figure 8 ). All alloying elements mostly improve creep performance substantially and reduce interspacing of α2/γ while enhancing stability of microstructure.
TiAl-based alloys of two molybdenum (Mo) content was fabricated by Jiang et al.  through vacuum levitation melting (VLM). Mo addition is understood to refine lamellar size colonies. It also encourages twining capabilities and reduces cracks in β phase.
Laser metal deposition (LMD) was adopted by Carrullo, Falcón and Borrás , to analyzed films produced on Ti48Al2Cr2Nb coating during oxidation through optimization of parameters. The coatings demonstrated high oxidation resistance at 800°C. The layers of oxides formed showed complex microstructure and development of consecutive layers on coating.
Ambient temperature mechanical characteristics were evaluated by Dai et al. , for coatings of Ti-Al-Si on the substrate of Ti6Al4V through laser surface alloying. Thermal expansion between coating and substrate resulted in crack formation and propagation. The coating basically consists of Ti5Si3 reinforcement in Ti-Al-phase. Si content increase causes resultant rise in brittle Ti5Si3 which had increased hardness than Ti-Al. The process of high-temperature oxidation resistance of Si in Ti-Al-Si system coatings involves increased refining of oxide grains and boosting the creation of Al2O3.
An investigated of microstructure carried out by Maliutina et al. , on laser cladding of TA6Zr4DE using Ti48Al2Cr2Nb powders. The coatings had γ-TiAl and α2-Ti3Al phases of fully lamellar microstructure. The occurrence of niobium as an alloying element restrains the oxygen dispersion making impact in the creation of more favored alumina films.
LENS was employed by Zhang and Bandyopadhyay , 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. , to determine the consequence of varying parameters on titanium aluminide. Dual phase α2 + γ was revealed in DLC Ti45Al5Nb0.5Si alloy. The processing parameters had little effect on the microhardness of the clad. Rise in Si quantity improved the propensity to produce cracks.
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 .
SLM of Ti-44.8Al-6Nb-1.0Mo-0.1B was investigated by Gussone et al.  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. , 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.  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. , on Ti-45Al-8.5Nb formation of controlled in lamellar structure. The regulated α2 lamellae appears in alloys annealed at lesser than 650°C and vanishes beyond 700°C. Extensive hardening stimulates O phase on α2 lamellae and the γ/(α2 + O) been obtained attributed to refined α2 lamellae. The schematic of the heat treatment procedure is depicted in Figure 10 below.
Analysis of microstructure of high-Nb TiAl alloy carried out by Qiang et al. , for Ti-45Al-8.5Nb-0.2 W-0.2B-0.02Y (at.%) resulting from numerous cooling rate. The transformation path α → α2 + γ for lamellar microstructure is controlled chiefly by its rate of cooling. Fast rate of cooling causes the retention of α/α2 phase of supersaturated microstructure. It was suggested that a rapid cooling rate and slow rate of cooling for α → α2 + γ and β → α transformation respectively.
In the repair of defective or worn blade parts by Rittinghaus et al. , LMD of TNM™ β-containing alloy with composition of Ti-43.5Al-4Nb-1Mo-0.1B (at.%) was studied. Small fraction of βo 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.  for β-containing 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.
The consequence of heat treatments was studied by Li et al. , 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 α2 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. , 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 , 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. , 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. , 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, 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. , 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.
5. Challenges and critical issues
According to Bünck et al. , 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 . 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, application of coatings and heat-treatment.
Presently, LAM has the prospective to develop typical manufacture techniques, specifically in creating of small size of extremely complicated components . 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 . 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 final product with little or no post-processing.
6. Summary and conclusions
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, researchers would be able to develop a mechanism of fabricating TiAl with better room and elevated temperature properties.
The authors would like to acknowledge the financial support of African Laser Centre-National Laser Centre; Council of Scientific and Industrial Research (ALC-NLC; CSIR), Project Number LHIP500 Task ALC S100.