Mechanical Properties of Ti6Al4V Fabricated by Laser Powder Bed Fusion: A Review Focused on the Processing and Microstructural Parameters Influence on the Final Properties

: Ti6Al4V alloy is an ideal lightweight structural metal for a huge variety of engineering applications due to its distinguishing combination of high speciﬁc mechanical properties, excellent corrosion resistance and biocompatibility. In this review, the mechanical properties of selective laser-melted Ti6Al4V parts are addressed in detail, as well as the main processing and microstructural parameters that inﬂuence the ﬁnal properties. Fundamental knowledge is provided by linking the microstructural features and the ﬁnal mechanical properties of Ti6Al4V parts, including tensile strength, tensile strain, fatigue resistance, hardness and wear performance. A comparison between Laser Powder Bed Fusion and conventional processing routes is also addressed. The presence of defects in as-built Ti6Al4V parts and their inﬂuences on the mechanical performance are also critically discussed. The results available in the literature show that typical Laser Powder Bed–Fused Ti6Al4V tensile properties (>900 MPa yield strength and >1000 MPa tensile strength) are adequate when considering the minimum values of the standards for implants and for aerospace applications (e.g., ASTM F136–13; ASTM F1108–14; AMS4930; AMS6932).


Introduction
In recent years, Additive Manufacturing (AM) has faced a tremendous process of maturation due to the great development of AM techniques and materials.AM has become one of the important topics of research, and from the industrial point of view, AM has been attracting the attention of manufacturing industries worldwide.The event "3D printing to the factory floor" is real, and it is occurring very quickly.Lux Research, Boston, Massachusetts, USA, a provider of tech-enabled research and innovation advisory services, has released a new report, "Will 3D Printing Replace Conventional Manufacturing?" which anticipates that the Additive Manufacturing market will reach USD 51 billion within a decade [1].AM fabrication processes offer significant benefits for widespread industries, positively influencing society, the economy and the environment, increasing production sustainability.Some key benefits are pointed out in the following.
By using AM techniques, a physical part is directly and easily obtained from CAD data [2][3][4].
When using AM fabrication methods, tremendous product customization is possible without the extra costs usually associated with extra tools, dies, molds and others, which are higher in conventional processing routes such as casting or machining [5][6][7].
AM is triggering a revolutionary strategy in terms of engineering design, and AM techniques give clients, producers and users the possibility of designing with an exclusive focus on the component function rather than manufacturing process difficulties [8,9].Components with complex internal features would be almost impossible or unsustainable to produce using conventional manufacturing techniques [10][11][12][13][14]. AM opens new prospects to develop/fabricate a mechanical part regardless of its geometrical complexity [8,9].AM's high design freedom and flexibility enable the production of lightweight components able to retain structural strength, despite exhibiting a significant reduction in weight.Several cellular or lattice structures have been proposed as advantageous solutions for new and disruptive biomedical solutions such as hip implants [15][16][17][18][19].
When using AM techniques, the material waste is almost zero, and no toxic chemicals are directly used, in contrast, for instance, to conventional machining [20,21].
AM has the capability of producing near-net-shape components with high geometrical and dimensional accuracy with the ability to combine several machining steps into a single production step, which dramatically reduces the fabrication time [6,13].
According to ISO 17296-2:2015 [22], AM techniques can be grouped into the following seven categories: VAT Photopolymerization, Material Jetting, Binder Jetting, Sheet Lamination, Material Extrusion, Directed Energy Deposition and Powder Bed Fusion.The present review is focused on the Laser Powder Bed Fusion (LPBF) technique, which is a Powder Bed Fusion technique.In LPBF, a laser beam is used as an energy source to melt powder beds [2,3].CAD data are incorporated into the software of the equipment and then sliced into many layers.After ensuring an adequate operating atmosphere (typically argon), the first layer of powder is deposited over the build platform.Afterwards, the laser scans the pattern instructions for the respective layer, promoting the melting and fusion of powder.Subsequently, a new layer of powder is deposited over the previous one, melting the new powder and also promoting fusion to the previous layer.In this context, the huge potential of Laser Powder Bed Fusion has been used to design, develop and study new solutions for a wide range of applications.LPBF's high design freedom offers great potential for lightweight design and can bring to life completely new component designs by incorporating extraordinary tools, such as Topology Optimization, for achieving lightweight and high-performance components [23][24][25].The production of Ti6Al4V components by Laser Powder Bed Fusion has been one of the important topics in research targeting custom-fitting solutions with high added value for aerospace, aeronautics and biomedical industries [26,27].The present review aims to address the physicomechanical properties of LPBFed Ti6Al4V-based components in detail and analyze the main processing and microstructural parameters that influence the final properties.

Ti6Al4V Alloy
At room temperature, pure titanium displays a hexagonal close-packed (HCP) structure, named the α phase.At the β transus temperature (approximately 885 • C), this structure is transformed into a body-centered cubic (BCC) structure called the β phase [28,29].Commonly, five classes are discerned: α alloys, near-α, α-β, near-β and β alloys.Ti6Al4V alloy is considered an α-β alloy that contains 6 wt % aluminum as the α stabilizer and 4 wt % vanadium as the β stabilizer [30,31].For Ti6Al4V, the β transus temperature is approximately 995 • C under equilibrium conditions, so above this temperature, Ti6Al4V becomes 100% β phase [28,29].The final microstructure is highly correlated to the cooling rate that occurs from above the β transus temperature [32].Slow to intermediate cooling rates lead to the nucleation and growth processes of α-lamellae (α phase) to form α-Widmanstätten laths within the β matrix [33].Such microstructures are commonly observed in wrought and cast components.As the cooling rate increases, the length and thickness of α-lamellae decrease, leading to enhanced mechanical strength [28].Furthermore, when the cooling rate is sufficiently fast, the β phase undergoes a diffusionless transformation to the martensitic α phase [32].This transformation is an aspect of interest, because it increases the strength and hardness of this alloy, although reducing ductility.Figure 1 shows typical SEM images of cold rolling, hot rolling and Laser Powder Bed Fusion [3,[34][35][36].Figure 1c shows a finer microstructure of Ti6Al4V fabricated by LPBF.It exhibits the presence of the α martensitic phase with a needle-like feature as a consequence of the extremely high cooling rate [3,37].Figure 2 shows typical TEM images of Ti6Al4V obtained by Electron Beam Melting (EBM) and Laser Powder Bed Fusion.
of α-lamellae decrease, leading to enhanced mechanical strength [28].Furthermore, when the cooling rate is sufficiently fast, the β phase undergoes a diffusionless transformation to the martensitic α′ phase [32].This transformation is an aspect of interest, because it increases the strength and hardness of this alloy, although reducing ductility.Figure 1 shows typical SEM images of cold rolling, hot rolling and Laser Powder Bed Fusion [3,[34][35][36].Figure 1c shows a finer microstructure of Ti6Al4V fabricated by LPBF.It exhibits the presence of the α′ martensitic phase with a needle-like feature as a consequence of the extremely high cooling rate [3,37].Figure 2 shows typical TEM images of Ti6Al4V obtained by Electron Beam Melting (EBM) and Laser Powder Bed Fusion.The TEM images given in Figure 2 show differences in the final microstructural features of Ti6Al4V produced by EBM and LPBF.In the EBM specimens, both body-centered cubic (bcc) and hexagonal close-packed (hcp) diffraction spots are observed, which confirms the existence of both the α and β phases.Additionally, it has to be mentioned that the thickness of the β phase is much thinner (~0.30μm) than that of the α phase.Further, the volume fraction of β is quite low.On the other hand, in the Laser Powder Bed Fusion specimens, only the typical hcp diffraction spots are present, and the structure is fine α′ martensite [38], while in EBM specimens, both BCC (β phase) and HCP (α phase) phases are present, and these differences are explained by the higher cooling rate in the LPBF of α-lamellae decrease, leading to enhanced mechanical strength [28].Furthermore, when the cooling rate is sufficiently fast, the β phase undergoes a diffusionless transformation to the martensitic α′ phase [32].This transformation is an aspect of interest, because it increases the strength and hardness of this alloy, although reducing ductility.Figure 1 shows typical SEM images of cold rolling, hot rolling and Laser Powder Bed Fusion [3,[34][35][36].Figure 1c shows a finer microstructure of Ti6Al4V fabricated by LPBF.It exhibits the presence of the α′ martensitic phase with a needle-like feature as a consequence of the extremely high cooling rate [3,37].Figure 2 shows typical TEM images of Ti6Al4V obtained by Electron Beam Melting (EBM) and Laser Powder Bed Fusion.The TEM images given in Figure 2 show differences in the final microstructural features of Ti6Al4V produced by EBM and LPBF.In the EBM specimens, both body-centered cubic (bcc) and hexagonal close-packed (hcp) diffraction spots are observed, which confirms the existence of both the α and β phases.Additionally, it has to be mentioned that the thickness of the β phase is much thinner (~0.30μm) than that of the α phase.Further, the volume fraction of β is quite low.On the other hand, in the Laser Powder Bed Fusion specimens, only the typical hcp diffraction spots are present, and the structure is fine α′ martensite [38], while in EBM specimens, both BCC (β phase) and HCP (α phase) phases are present, and these differences are explained by the higher cooling rate in the LPBF The TEM images given in Figure 2 show differences in the final microstructural features of Ti6Al4V produced by EBM and LPBF.In the EBM specimens, both body-centered cubic (bcc) and hexagonal close-packed (hcp) diffraction spots are observed, which confirms the existence of both the α and β phases.Additionally, it has to be mentioned that the thickness of the β phase is much thinner (~0.30µm) than that of the α phase.Further, the volume fraction of β is quite low.On the other hand, in the Laser Powder Bed Fusion specimens, only the typical hcp diffraction spots are present, and the structure is fine α martensite [38], while in EBM specimens, both BCC (β phase) and HCP (α phase) phases are present, and these differences are explained by the higher cooling rate in the LPBF technique with respect to EBM.In general, lower substrate temperatures (≈200 • C) and an argon atmosphere are used for LPBF of Ti6Al4V-based components.These processing parameters lead to an increased convective cooling rate compared to the higher substrate temperatures (≈450 • C) and vacuum atmosphere used in EBM [31,39].The influence of each microstructural feature on the strength, ductility, hardness, fracture toughness, fatigue properties, wear resistance and corrosion behavior is meticulously addressed in the pertinent literature.The high strength [40], low weight ratio [41] and superior corrosion resistance of Ti6Al4V alloy make it suitable for a broad range of high-added-value products, from transportation and automotive industries [33] to chemical plants, oil and gas extraction, aerospace, medicine and aeronautics [31,32,[42][43][44].Table 1 lists the relevant physicomechanical properties of Ti6Al4V alloy and compares them with the properties of widely used materials in biomedical applications of cortical bone (i.e., CoCrMo alloys, 316 L stainless steel, and A357 aluminum alloy).The attractive mechanical (i.e., lower Young s modulus than cobalt alloys and stainless steels) and physical (i.e., low weight) properties of Ti6Al4V alloy, as well as its advantageous tribological (i.e., high corrosion resistance) and biological (i.e., excellent soft and hard tissue biocompatibility) performance, make this alloy very appealing for biomedical products such as orthopedic and dental implants [43,[49][50][51][52].With respect to aeronautics products such as engines (discs, blades and cooler parts), airframes, skins, flaps and slat tracks of wings and engine mountings, Ti6Al4V alloy is an appropriate choice, as it allows substantial weight savings and volume reduction compared to commonly used steels and aluminum.Additionally, this alloy presents good compatibility with composite materials allied to a high fatigue resistance and high-temperature mechanical properties [27,45].

Properties and Performance of Ti6Al4V Manufactured by LPBF
Laser Powder Bed Fusion provides very different mechanical properties from those of casting or wrought Ti6Al4V [9,53].As highlighted in Figure 3, the inherent specificities of the LPBF process as a layer-by-layer building strategy, the powder feedstock, melting phenomena and thermal gradients will define the final properties, as they will dictate the microstructural features (e.g., grain size, crystal growth direction, residual porosity and defects, among others) [27,32,54,55].
Ti6Al4V suitability for a wide range of applications, particularly in industries such as aerospace and medical devices [39, [56][57][58][59], is grounded in its material properties, especially when the strength-to-weight ratio and wear performance are key aspects.In this sense, this section is devoted to tensile properties and hardness (Section 3.1), fatigue behavior (Section 3.2) and wear performance (Section 3.3).
Table 2 provides a general outline of the latest studies on Ti6Al4V parts produced by LPBF, aiming to provide information regarding LPBF equipment specifications (and equipment manufacturing company) and the relevant properties being experimentally assessed.This table intends to help designers and manufacturers to quickly select the most effective approach for fabricating Ti6Al4V parts using a given type of equipment.Ti6Al4V suitability for a wide range of applications, particularly in industries su as aerospace and medical devices [39, [56][57][58][59], is grounded in its material properties, esp cially when the strength-to-weight ratio and wear performance are key aspects.In t sense, this section is devoted to tensile properties and hardness (Section 3.1), fatigue b havior (Section 3.2) and wear performance (Section 3.3).
Table 2 provides a general outline of the latest studies on Ti6Al4V parts produced LPBF, aiming to provide information regarding LPBF equipment specifications (a equipment manufacturing company) and the relevant properties being experimentally sessed.This table intends to help designers and manufacturers to quickly select the m effective approach for fabricating Ti6Al4V parts using a given type of equipment.

Tensile Properties
The consolidation of the metal powder by LPBF is achieved by the temperature effect (laser as the energy source for melting), gravity and capillary forces [55,99].Due to the large number of influential processing parameters on layer-by-layer AM processes, the production of high-quality Ti6Al4V parts having high densification requires complete control of the process [3,56,100].Most of the published studies on Ti6Al4V manufactured by LPBF report near full density and superior strength to that obtained when using conventional processing routes (such as cast or wrought) [66].Nevertheless, it should be highlighted that a proper assessment of the most suitable processing parameters is very relevant [34,101,102] for each piece of Laser Powder Bed Fusion equipment used in the fabrications, as the density of energy is not a sufficient parameter to optimize the processing parameters, as shown in Figure 4.In fact, several studies have been devoted to the assessment of the effects of LPBF processing parameters on several physical and mechanical properties, correlating them with the microstructure, defect generation, etc. [3,31,55,69,103].A summary of the tensile properties of Ti6Al4V parts produced by LPBF is found in Table 3.
cations, as the density of energy is not a sufficient parameter to optimize the processing parameters, as shown in Figure 4.In fact, several studies have been devoted to the assess ment of the effects of LPBF processing parameters on several physical and mechanica properties, correlating them with the microstructure, defect generation, etc [3,31,55,69,103].A summary of the tensile properties of Ti6Al4V parts produced by LPBF is found in Table 3.   Considering the standards for Ti6Al4V alloy for surgical implants and for aerospace applications (e.g., ASTM F136-13; ASTM F1108-14; AMS4930; AMS6932), the minimum values for tensile properties can be defined as follows: yield strength of 758 MPa, tensile strength of 827 MPa and tensile strain of 8%.LPBF Ti6Al4V displays superior yield and tensile strength to those of cast or wrought alloy, mainly due to microstructural features such as grain refinement [66].On the contrary, when regarding tensile strain, Ti6Al4V fabricated by LPBF exhibits lower ductility when compared to cast or wrought [53].As depicted in Table 3, several studies on LPBF Ti6Al4V parts show yield and tensile strengths that are significantly higher compared to the ASTM specification [33,41,53,72,106]; however, when regarding tensile strain, most of the studies report values lower than the minimum required (10%).This aspect is mainly the reason why heat treatments are usually performed on Ti6Al4V alloy fabricated by LPBF.
The Ti6Al4V alloy microstructure is dependent on the thermal history occurring during the fabrication but can be defined by post-processing heat treatments.Table 4 and Figure 5 show that different heat treatments (selected temperatures and cooling rates) induce substantial differences in the microstructural features of LPBF Ti6Al4V alloy in comparison to the as-built LPBF alloy and the ensuing mechanical properties [55,58,80,106].
The mechanical properties of Ti6Al4V are dictated by its microstructure, particularly by the constituent phases (α , α and β) and grain size [54, 109,110].Typically, Ti6Al4V alloy exhibits a microstructure that ranges from lamellar to globular [67, 77,111].While the first is usually desirable for enhanced fracture toughness, globular microstructure displays, on average, superior tensile strength and ductility [21].A typical equiaxed microstructure with a globular α phase in an α + β matrix is observed in wrought Ti6Al4V alloy [29].This microstructure typically leads to tensile strengths ranging from 897 to 984 MPa and tensile strains ranging from 10 to 19% [53,55,112,113].1a) As-built 736 1051 11.9 α acicular, column width < 0.5 µm (Figure 1b)   The mechanical properties of Ti6Al4V are dictated by its microstructure, particularly by the constituent phases (α′, α and β) and grain size [54, 109,110].Typically, Ti6Al4V alloy exhibits a microstructure that ranges from lamellar to globular [67, 77,111].While the first is usually desirable for enhanced fracture toughness, globular microstructure displays, on average, superior tensile strength and ductility [21].A typical equiaxed microstructure with a globular α phase in an α + β matrix is observed in wrought Ti6Al4V alloy [29].This microstructure typically leads to tensile strengths ranging from 897 to 984 MPa and tensile strains ranging from 10 to 19% [53,55,112,113].
During LPBF fabrication, this alloy undergoes extremely high cooling rates (10 3 -10 8 K/s), resulting in an acicular martensite phase known as the α′ phase [54, 114,115].Consequently, the as-built LPBF Ti6Al4V microstructure evidences a needle-like morphology, as shown in Figures 1c and 2a [55,77,80,106].Several heat treatments have been reported in the literature, mostly performed to increase the low ductility displayed by LPBF Ti6Al4V by promoting significant microstructural changes.When observing Figure 5, it is possible to conclude several aspects.In this heat treatment, the α′ martensite can decompose into an α + β phase, along with the formation of GB-α, and the thickness of the α lath after sub-transus heat treatment (HT) is primarily dependent on the maximum HT temperature and the cooling rate.In addition, the morphology of GB-α is mainly dependent on the HT temperature, and the GB-α exhibits a discontinuous morphology when HT temperatures increase to nearly the β transus (950 °C).It can be highlighted that for heat treatments below the β transus (≈950 °C) [5], β transformation occurs, with the cooling rate dictating the final phases, and even when the same phase is attained, their shape and size (e.g., α plate width) are also ruled by the cooling velocity [54, 58,106,116].When performing a heat treatment below the β transus temperature, a coarsening of the acicular martensite occurs, with larger lamellae when compared to the as-built alloy.Figure 5A,B evidence this phenomenon for heat treatments performed at 750 and 850 °C, respectively.Figure 5C shows that when performing a heat treatment of 950 °C, an elongated primary α grain in a β matrix is observed.Considering heat treatments above the β transus, again, the cooling rate defines the final microstructure: typically, furnace cooling creates a lamellar α + β structure, air cooling leads to an α-Widmanstatten structure, and water quenching (>410 °C/s) leads to an α′ martensite structure or fine, fine α lath [116].As an example, During LPBF fabrication, this alloy undergoes extremely high cooling rates (10 3 -10 8 K/s), resulting in an acicular martensite phase known as the α phase [54, 114,115].Consequently, the as-built LPBF Ti6Al4V microstructure evidences a needle-like morphology, as shown in Figures 1c and 2a [55,77,80,106].Several heat treatments have been reported in the literature, mostly performed to increase the low ductility displayed by LPBF Ti6Al4V by promoting significant microstructural changes.When observing Figure 5, it is possible to conclude several aspects.In this heat treatment, the α martensite can decompose into an α + β phase, along with the formation of GB-α, and the thickness of the α lath after sub-transus heat treatment (HT) is primarily dependent on the maximum HT temperature and the cooling rate.In addition, the morphology of GB-α is mainly dependent on the HT temperature, and the GB-α exhibits a discontinuous morphology when HT temperatures increase to nearly the β transus (950 • C).It can be highlighted that for heat treatments below the β transus (≈950 • C) [5], β transformation occurs, with the cooling rate dictating the final phases, and even when the same phase is attained, their shape and size (e.g., α plate width) are also ruled by the cooling velocity [54, 58,106,116].When performing a heat treatment below the β transus temperature, a coarsening of the acicular martensite occurs, with larger lamellae when compared to the as-built alloy.Figure 5A,B evidence this phenomenon for heat treatments performed at 750 and 850 • C, respectively.Figure 5C shows that when performing a heat treatment of 950 • C, an elongated primary α grain in a β matrix is observed.Considering heat treatments above the β transus, again, the cooling rate defines the final microstructure: typically, furnace cooling creates a lamellar α + β structure, air cooling leads to an α-Widmanstatten structure, and water quenching (>410 • C/s) leads to an α martensite structure or fine, fine α lath [116].As an example, Figure 5C shows a microstructure with α laths in a β matrix, which demonstrates that when performing a heat treatment above the β transus (950 • C) followed by air cooling, α acicular is no longer present.For this same example, Table 4 shows that this change in microstructure led to a decrease in the tensile strength (from 1191 to 984 MPa), while tensile strain was enhanced from 5.4 to 14.2%.

Fatigue Behavior
The fatigue performance is the response of a material to repeated cyclic loads and/or strains, and the fatigue strength dictates whether a certain material under continuous cyclic stresses is capable of ensuring a long operating life [117].Components under cyclic loading are rather common in a variety of structures and equipment, and for that reason, fatigue failure is one of the most common failure modes [27].The fatigue performance is crucial for load-bearing medical implants and for aerospace components [41,118].
The assessment of the fatigue performance usually comprises three different approaches: stress-based (S-N), strain-based (ε-N) or fracture mechanisms, in which N represents the number of cycles before failure.As seen in Figure 6, the evolution of a fatigue crack progresses through three main regions, starting with crack initiation, followed by crack propagation and, lastly, the fracture [119].Crack initiation is difficult to detect and sensitive to the size of the microstructural grain (REF).In fact, finer grains lead to the closer spacing of grain boundaries, which the crack has to break through, delaying crack initiation.On the other hand, crack propagation (Region II in Figure 6) is not influenced by the microstructure, being better described by a power law such as Paris's law [119].
Figure 5C shows a microstructure with α laths in a β matrix, which demonstrates that when performing a heat treatment above the β transus (950 °C) followed by air cooling, α′ acicular is no longer present.For this same example, Table 4 shows that this change in microstructure led to a decrease in the tensile strength (from 1191 to 984 MPa), while tensile strain was enhanced from 5.4 to 14.2%.

Fatigue Behavior
The fatigue performance is the response of a material to repeated cyclic loads and/or strains, and the fatigue strength dictates whether a certain material under continuous cyclic stresses is capable of ensuring a long operating life [117].Components under cyclic loading are rather common in a variety of structures and equipment, and for that reason, fatigue failure is one of the most common failure modes [27].The fatigue performance is crucial for load-bearing medical implants and for aerospace components [41,118].
The assessment of the fatigue performance usually comprises three different approaches: stress-based (S-N), strain-based (ε-N) or fracture mechanisms, in which N represents the number of cycles before failure.As seen in Figure 6, the evolution of a fatigue crack progresses through three main regions, starting with crack initiation, followed by crack propagation and, lastly, the fracture [119].Crack initiation is difficult to detect and sensitive to the size of the microstructural grain (REF).In fact, finer grains lead to the closer spacing of grain boundaries, which the crack has to break through, delaying crack initiation.On the other hand, crack propagation (Region II in Figure 6) is not influenced by the microstructure, being better described by a power law such as Paris's law [119].The crack propagation threshold, ΔKth (MPa√m), the Paris slope, the fracture toughness (Kc (MPa√m) and the number of cycles before failure can be assumed as the main parameters to assess the fatigue behavior of a material.Table 5 summarizes this fatigue data for as-built and post-treated Ti6Al4V alloy fabricated by LPBF.The crack propagation threshold, ∆K th (MPa √ m), the Paris slope, the fracture toughness (K c (MPa √ m) and the number of cycles before failure can be assumed as the main parameters to assess the fatigue behavior of a material.Table 5 summarizes this fatigue data for as-built and post-treated Ti6Al4V alloy fabricated by LPBF.
LPBF processing parameters are one crucial aspect that dictates the fatigue performance of Ti6Al4V alloy produced by this technology [69,84,121].When considering the role of processing parameters on the quality of Ti6Al4V parts fabricated by LPBF, two types of defects are commonly described.The first is the incomplete powder melting or improper fusion between successive tracks or layers caused by insufficient energy input.The second occurs with the entrapment of gases due to excessive energy.Regarding the influence of LPBF processing parameters on the fatigue performance of Ti6Al4V, Gong et al. [69] studied five processing conditions, varying the scan speed (see Figure 7) to obtain five different energy densities (from 27 to 100 J/mm 3 ).The results showed that when using 42 and 74 J/mm 3 , comparable fatigue life limits (≈350 MPa for 10 7 cycles) were obtained, with the highest being among the tested conditions.Specimens fabricated using 100 J/mm 3 exhibited pores with larger sizes and numbers (higher porosity) than those in the previously mentioned conditions, consequently displaying a lower fatigue life (300 MPa).Finally, for the lowest energy densities (27 and 32 J/mm 3 ), the presence of lack-of-fusion defects seriously compromised the fatigue performance (fatigue limit of 100 MPa).As-built The microstructure of Ti6Al4V alloy is another aspect when thinking about performance, and several studies have reported the influence of thermal and therm chanical treatments on the fatigue behavior of this alloy [21,56,85].As an example The microstructure of Ti6Al4V alloy is another aspect when thinking about fatigue performance, and several studies have reported the influence of thermal and thermomechanical treatments on the fatigue behavior of this alloy [21,56,85].As an example, Leuders et al. [56] showed that heat treatments have a strong influence on the fatigue behavior of LPBF Ti6Al4V parts.In their study, while the fatigue life for as-built Ti6Al4V was 27,000 cycles, after performing heat treatments at 800 • C (below β transus) or 1050 • C (above β transus), this number was respectively increased to 93,000 or 290,000 cycles (for a stress amplitude of 600 MPa).After performing a thermo-mechanical treatment (HIP), none of the specimens fractured before 2 × 10 6 cycles, showing the ability of this thermomechanical treatment to improve the fatigue life of LPBF Ti6Al4V through a reduction in detrimental internal defects.
S. Leuders et al. [56] showed that as-built Ti6Al4V has a crack growth behavior at a lower crack growth rate, which is similar in both directions.As detected for the fatigue life, a significant enhancement of the crack propagation threshold (∆K th ) is observed when performing thermal or thermo-mechanical treatments on as-built Ti6Al4V, as proven by the evident shift of the curves to the right.This trend was verified in specimens tested perpendicular or parallel to the build direction.
Another work from Riemer et al. [85] confirmed the effect of thermal and thermomechanical treatments on the fatigue behavior of as-built LPBF Ti6Al4V.As seen in Figure 8, the results are aligned with those from Leuders et al. [56], with the as-built LPBF Ti6Al4V showing low and insufficient fatigue performance.Moreover, heat treatments at 800 • C, 1050 • C and HIP also lead to a significant improvement, with higher crack propagation threshold values (3.9, 3.6 and 4.2 MPa √ m, respectively) when compared to the as-built condition (1.4 MPa √ m) [85] (see Figure 8).The surface condition is a key aspect that has a huge impact on the fatigue perfo mance of Ti6Al4V alloy produced by LPBF [21,69,74].Being a powder-bed fusion tech nique, LPBF displays an inherent surface condition for Ti6Al4V.As shown in Figure LPBF induces the presence of two types of roughness, a primary roughness resulting from the solidification of the melt pool and a second one due to partially melted powder part cles (Ra ≈ 13 μm) [74].The surface condition is a key aspect that has a huge impact on the fatigue performance of Ti6Al4V alloy produced by LPBF [21,69,74].Being a powder-bed fusion technique, LPBF displays an inherent surface condition for Ti6Al4V.As shown in Figure 9, LPBF induces the presence of two types of roughness, a primary roughness resulting from the solidification of the melt pool and a second one due to partially melted powder particles (R a ≈ 13 µm) [74].
The surface condition is a key aspect that has a huge impact on the fatig mance of Ti6Al4V alloy produced by LPBF [21,69,74].Being a powder-bed f nique, LPBF displays an inherent surface condition for Ti6Al4V.As shown i LPBF induces the presence of two types of roughness, a primary roughness resu the solidification of the melt pool and a second one due to partially melted pow cles (Ra ≈ 13 μm) [74].On this subject, Greitemeier et al. [21] reported the fatigue performan Ti6Al4V specimens, as-built and milled (see Figure 10), also addressing the in thermal and thermo-mechanical treatments on the fatigue behavior.From thei is possible to conclude that, independently of the heat treatment, the fatigue lif LPBF specimens is superior to that of as-built specimens.As reported [21,74,111,122], Greitemeier et al.'s [21] study highlights the detrimental effec inherent surface condition (see Figure 11) on the fatigue performance of this a comparing as-built and milled specimens subjected to thermal or thermo-m treatments, significant differences in fatigue life stress limits were found.Fo specimens, an improvement in the fatigue stress limit of 2.4 times was obse On this subject, Greitemeier et al. [21] reported the fatigue performance of LPBF Ti6Al4V specimens, as-built and milled (see Figure 10), also addressing the influence of thermal and thermo-mechanical treatments on the fatigue behavior.From their results, it is possible to conclude that, independently of the heat treatment, the fatigue life of milled LPBF specimens is superior to that of as-built specimens.As reported elsewhere [21,74,111,122], Greitemeier et al.'s [21] study highlights the detrimental effect of LPBF's inherent surface condition (see Figure 11) on the fatigue performance of this alloy.When comparing as-built and milled specimens subjected to thermal or thermo-mechanical treatments, significant differences in fatigue life stress limits were found.For annealed specimens, an improvement in the fatigue stress limit of 2.4 times was observed when changing the surface condition from as-built to milled.Similarly, for HIP specimens, an enhancement of 3.4 times was obtained by performing milling.
The high impact of the surface condition on the fatigue behavior of LPBF Ti6Al4V was also reported by Gong et al. [69].These authors compared LPBF parts fabricated using very dissimilar processing energy densities (from 32 to 74 J/mm 3 ), and although leading to significantly different fatigue limits, similar crack initiations were found, all at surface or sub-surface defects, as shown in Figure 11.
In sum, the fatigue properties of Ti6Al4V alloy produced by Laser Powder Bed Fusion are dictated by three main aspects: (1) The densification level of the produced parts, which is defined by the processing parameters used in the fabrication.When defects such as pores and lack of fusion are present in higher amounts (porosity higher than 5%), the fatigue performance tends to be poor [4,69].In this scenario, cracks can initiate either in the bulk or at the surface due to these defects [84].(2) The microstructural features are another important aspect because by performing thermal and thermo-mechanical post-treatments, it is possible to substantially improve the fatigue performance of this alloy by altering its microstructure.Hot Isostatic Pressing (a thermo-mechanical treatment) proves to be the most effective post-treatment to increase the fatigue performance of LPBF Ti6Al4V [4,21,84] (3) The surface condition has a crucial impact on the fatigue performance of this alloy, and regarding LPBF, the natural surface condition was found to be extremely detrimental, even when performing post-treatments on LPBF as-built parts (see Figure 10).In this sense, machining LPBF as-built parts seem to be an effective way to enhance the fatigue performance of Ti6Al4V parts manufactured by this technology.
changing the surface condition from as-built to milled.Similarly, for HIP specimens, an enhancement of 3.4 times was obtained by performing milling.The high impact of the surface condition on the fatigue behavior of LPBF Ti6Al4V was also reported by Gong et al. [69].These authors compared LPBF parts fabricated using very dissimilar processing energy densities (from 32 to 74 J/mm 3 ), and although leading to significantly different fatigue limits, similar crack initiations were found, all at surface or sub-surface defects, as shown in Figure 11.In sum, the fatigue properties of Ti6Al4V alloy produced by Laser Powder Bed Fusion are dictated by three main aspects: (1) The densification level of the produced parts, which is defined by the processing parameters used in the fabrication.When defects such as pores and lack of fusion are present in higher amounts (porosity higher than 5%), the fatigue performance tends

Hardness and Wear Performance
The data found in the literature for the hardness of Ti6Al4V alloy produced by LPBF show higher values when compared to those of wrought alloy (314 HV) [3,55,123,124].This fact is explained by the inherent microstructure that LPBF induces in Ti6Al4V due to the typical high cooling rate (10 3 -10 8 K/s).As-built LPBF Ti6Al4V alloy exhibits an acicular microstructure (α phase), which is a harder phase when compared to the α and β phases present on the wrought alloy [125,126].
As previously mentioned for strength, heat treatments also influence the hardness of this alloy due to changes in the microstructural phases.Depending on the heat treatment temperature and cooling rate, it is possible to change the full acicular microstructure (α ) of as-built LPBF Ti6Al4V to a coarser acicular microstructure; to an acicular α mixed with α + β microstructure; or even to an α + β microstructure (absent α phase).These differences in the microstructure, as reported by Kasperovich et al. [55], result in higher hardness for the as-built condition (360 HV) when compared to the alloy after heat treatments performed at 700 • C (351 HV) and 900 • C (324 HV).
Typically, LPBF Ti6Al4V in the as-built condition exhibits higher tensile strength, lower ductility and higher hardness, while after performing post-treatments (thermal), lower tensile strength, higher ductility and lower hardness are generally attained (see Tables 3 and 5).Table 6 presents an overview of the hardness values found in the literature for LPBF Ti6Al4V, either as-built or after thermal treatments [3,55,107,123].As-built 380-420 α acicular Bartolomeu et al. [3] As-built 389 α acicular Li et al. [120] As-built ≈400 -Amaya-Vazquez et al. [127] As built 440 α acicular Song et al. [124] As-built 450 -Vilaro et al. [58] As-built 354 α acicular 730 The wear performance of a material is usually correlated to its hardness; i.e., higher hardness leads to a higher wear performance [29].To the authors' knowledge, there are a very limited number of studies on the wear performance of Ti6Al4V alloy or pure titanium produced by LPBF [110,128,129].Kumar and Kruth [129] showed that LPBF Ti6Al4V exhibited the lowest fretting wear performance when compared to stainless steels, tool steel and a cobalt-chrome alloy, but the comparison was made between LPBF Ti6Al4V and this same alloy fabricated by different routes.Gu et al. [110] studied the wear behavior of commercially pure titanium fabricated by LPBF using four different processing parameters.Their results show an inversely proportional relationship between the nanohardness and the wear rate for all processing conditions.The processing parameters that led to the highest hardness value (≈4 GPa) were the same that were used for fabricating the material that exhibited the highest wear performance (lowest wear rate ≈ 7 × 10 −4 mm 3 /N•m).These authors also reported that the wear performance of this material produced by LPBF was higher than that of the reference material fabricated by powder metallurgy due to the presence of a refined α acicular microstructure [110].
Bartolomeu et al. [2] studied the wear performance of Ti6Al4V produced by LPBF, comparing it with hot-pressed and cast Ti6Al4V. Figure 12 shows the overall results of this study showing the interrelation between the microstructure, the hardness and the wear rate for the three different processing routes investigated.Figure 12 shows that the highest wear performance (lowest wear rate ≈ 6.5 × 10 −4 mm 3 /N•m) was observed for the LPBF alloy, which displayed the highest hardness (≈388 HV).On the other hand, the lowest wear performance (highest wear rate ≈ 8.3 × 10 −4 mm 3 /N•m) was detected for cast Ti6Al4V, which had the lowest hardness (≈342 HV).Considering that these three materials (cast, HP and LPBF) were almost fully densified, these authors stated that the wear performance is explained by the microstructural features and the resulting hardness [125,126].

Conclusions
This review covered 132 studies relevant for a critical understanding of the mechanical properties of Ti6Al4V parts fabricated by Laser Powder Bed Fusion.This review was focused on the processing and microstructural parameters' influences on this alloy.Most of the published studies on Ti6Al4V manufactured by LPBF report near full density and superior strength to that obtained when using conventional processing routes (such as cast or wrought).When regarding tensile testing, the available results available show that typical Laser Powder Bed-Fused Ti6Al4V tensile properties (>900 MPa yield strength and >1000 MPa tensile strength) are adequate when considering the minimum values of the standards for implants and for aerospace applications (e.g., ASTM F136-13; ASTM F1108-14; AMS4930; AMS6932).It is important to highlight that heat treatments are an excellent method to obtain the best compromise between strength and ductility for a given application.For instance, for surgical implant applications, the best compromise between strength and ductility can be obtained by performing a heat treatment at 850° for 2 h, followed by furnace cooling.The fatigue properties of Ti6Al4V alloy produced by Laser Powder Bed Fusion are dictated by three main aspects.The first is the densification level of the produced parts, which is defined by the processing parameters used in the fabrication.When defects such as pores and lack of fusion are present in high amounts, the fatigue performance tends to be poor, and cracks can initiate either in the bulk or at the surface due to these defects.The second aspect is the microstructural features, as by performing thermal and thermo-mechanical post-treatments, it is possible to substantially

Conclusions
This review covered 132 studies relevant for a critical understanding of the mechanical properties of Ti6Al4V parts fabricated by Laser Powder Bed Fusion.This review was focused on the processing and microstructural parameters' influences on this alloy.Most of the published studies on Ti6Al4V manufactured by LPBF report near full density and superior strength to that obtained when using conventional processing routes (such as cast or wrought).When regarding tensile testing, the available results available show that typical Laser Powder Bed-Fused Ti6Al4V tensile properties (>900 MPa yield strength and >1000 MPa tensile strength) are adequate when considering the minimum values of the standards for implants and for aerospace applications (e.g., ASTM F136-13; ASTM F1108-14; AMS4930; AMS6932).It is important to highlight that heat treatments are an excellent method to obtain the best compromise between strength and ductility for a given application.For instance, for surgical implant applications, the best compromise between strength and ductility can be obtained by performing a heat treatment at 850 • for 2 h, followed by furnace cooling.The fatigue properties of Ti6Al4V alloy produced by Laser Powder Bed Fusion are dictated by three main aspects.The first is the densification level of the produced parts, which is defined by the processing parameters used in the fabrication.
When defects such as pores and lack of fusion are present in high amounts, the fatigue performance tends to be poor, and cracks can initiate either in the bulk or at the surface due to these defects.The second aspect is the microstructural features, as by performing thermal and thermo-mechanical post-treatments, it is possible to substantially improve the fatigue performance, and HIP seems to be the most effective post-treatment to increase the fatigue performance of this alloy.The surface condition has a crucial impact on the fatigue performance of this alloy, and regarding LPBF, the natural surface condition was found to be extremely detrimental, and the machining of LPBF as-built parts seems to be an effective way to enhance the fatigue performance of Ti6Al4V parts manufactured by this technology.Typically, LPBF Ti6Al4V in the as-built condition exhibits higher tensile strength, lower ductility and higher hardness, while after performing post-treatments (thermal), lower tensile strength, higher ductility and lower hardness are generally attained.

Figure 3 .
Figure 3. Schematic representation of Laser Powder Bed Fusion technology.

Figure 3 .
Figure 3. Schematic representation of Laser Powder Bed Fusion technology.

Figure 5 .
Figure 5. Microstructures of Ti6Al4V parts produced by LPBF with different heat treatments: (a,b) heat stage followed by water quenching; (c,d) heat stage followed by air cooling; (e,f) heat stage followed by furnace cooling (adapted with permission from [5].Copyright 2021 Emerald Publishing Limited).

Figure 5 .
Figure 5. Microstructures of Ti6Al4V parts produced by LPBF with different heat treatments: (a,b) heat stage followed by water quenching; (c,d) heat stage followed by air cooling; (e,f) heat stage followed by furnace cooling (adapted with permission from [5].Copyright 2021 Emerald Publishing Limited).

Metals 2022 ,Figure 8 .
Figure 8. Fatigue crack growth data of Ti6Al4V by LPBF for different material conditions (buildin direction is normal to crack growth direction (reproduced with permission from [85].Copyrigh 2016 Andre Riemer).

Figure 8 .
Figure 8. Fatigue crack growth data of Ti6Al4V by LPBF for different material conditions (building direction is normal to crack growth direction (reproduced with permission from [85].Copyright 2016 Andre Riemer).

Figure 9 .
Figure 9.Typical surface roughness of Ti6Al4V produced by Laser Powder Bed Fusion

Figure 9 .
Figure 9.Typical surface roughness of Ti6Al4V produced by Laser Powder Bed Fusion.

Table 2 .
Summary of studies addressing Ti6Al4V parts produced by LPBF.

Table 2 .
Summary of studies addressing Ti6Al4V parts produced by LPBF.

Table 3 .
Tensile properties of LPBF Ti6Al4V, indicating the testing direction with respect to the building direction.

Table 5 .
High cycle fatigue properties of LPBF Ti6Al4V, indicating the testing direction with respect to the building direction (for a stress ratio of 0.1).

Table 6 .
Summary of LPBF Ti6Al4V hardness data extracted from literature.