Microstructure and Mechanical Properties of Ti35421 Alloy: A Comparison Between Laser Directed Energy Deposition (L-DED) and Rolling

Abstract

In this study, the newly developed Ti35421 (Ti3Al5Mo4Cr2Zr1Fe wt.%) alloy was prepared by laser directed energy deposition (L-DED) because it contains several major elements that can refine grains, which is expected to enable the transformation from columnar to equiaxed grains. The results show that the L-DED Ti35421 alloy is predominantly composed of equiaxed grains and features various α-phase morphologies, including grain boundary α, lath α, and acicular α′ structures. These microstructural features are attributed to the rapid cooling conditions during processing. Such a microstructure enhances the alloy’s tensile strength (1446 MPa) while leading to limited ductility (1.7%). Following the solution and aging treatment, the grain boundary α phase undergoes coarsening, while the matrix β phase transforms into numerous fine lamellar α phases. This leads to a reduction in strength but an improvement in ductility. Therefore, the optimal heat treatment process for the L-DED Ti35421 alloy is determined to be a two-stage procedure: first, heating at 780 °C for 2 h followed by air cooling, and subsequently heating at 575 °C for 8 h with air cooling. Under this treatment, the alloy exhibits excellent mechanical properties, including a tensile strength of 1196 MPa, a yield strength of 1162 MPa, an elongation of 6.8%, and a reduction in area of 16.7%. Since there are no continuous grain boundaries in α, the rolled Ti35421 alloy exhibits better ductility than the L-DED Ti35421 alloy. This article is a revised and expanded version of a poster presentation entitled “Microstructure and mechanical properties of Ti-3Al-5Mo-4Cr-2Zr-1Fe alloy fabricated by laser deposition manufacturing”, which was accepted and presented at the 15th World Conference on Titanium (Ti-2023), Edinburgh, UK, 12–16 June 2023.

1. Introduction

Additive manufacturing (AM) includes binder jetting, material jetting, material extrusion, powder bed fusion (PBF), laser directed energy deposition (L-DED), etc., [1,2]. Among them, the rapid manufacturing of large-scale metal components primarily depends on the L-DED process [3,4]. The L-DED process often deposits material layer-by-layer using a high-energy laser and multiple powder nozzles. Melting and solidification are often accompanied by large temperature gradients and high cooling rates. The whole process goes through multiple cycles, resulting in multiple dynamic changes in microstructure [5].

Generally, the temperature gradient (G) and solidification rate (R) can be used to evaluate whether equiaxed grains can be formed [6]. The G value is at a high level during the L-DED process. The R value is mainly related to the laser scanning speed and has a small range of variation, leading to the formation of columnar grains and making it difficult to control. The characteristics of columnar grains are closely related to the heat source and process parameters: the width of columnar grains in SLM samples is usually below 500 μm [7,8]; the width of columnar grains in L-DED samples is mostly above 1 mm [9]; the columnar grains in WAAM are the coarsest and often run through the entire sedimentary layer [5,10].

It is challenging to avoid the formation of columnar grains by adjusting the process parameters of the L-DED process. Therefore, promoting nucleation kinetics and increasing the number of nucleation sites have been extensively studied [11,12]. Hu et al. [13] selected TiB as the reinforcing component to mitigate the anisotropic characteristics inherent in the solidification process, resulting in the formation of an ultrafine, three-dimensional interconnected microstructure. This microstructural evolution improves the fracture toughness of titanium alloys and enables a uniform, isotropic enhancement in overall material properties. Zhang et al. [14] developed Ti-Cu alloys via laser directed energy deposition (L-DED), which promoted the in situ formation of Ti₂Cu particles that acted as heterogeneous nucleation sites. Consequently, a microstructural transition from columnar to equiaxed grains was achieved, leading to a significant improvement in the mechanical properties of the titanium alloy. Simonelli et al. [15] studied the effect of Fe element on the microstructure of Ti-6Al-4V alloy during the SLM process and found that the addition of 3wt.% Fe can significantly refine the primary β grains. Zhang et al. [16] studied the effect of Fe element on the microstructure and mechanical properties of L-DED Ti-6AI-4V alloy and found that Fe element caused the formation of $\omega$-phase and TiFe precipitation and promoted the CET transformation. Guo et al. [17,18] investigated the effectiveness of Fe micro-alloying in modifying the microstructure of L-DED porous titanium and found that increasing Fe content induces progressive microstructural refinement, transitioning from coarse columnar grains to fully equiaxed β grains. The above results show that the addition of Fe element can refine primary β grains of additive manufactured titanium alloys. Brooke et al. [12] produced Ti-Fe, Ti-Cu, Ti-Cu-Fe, and Ti-Mo alloys produced via direct energy deposition and verified that supercooling parameter is the most reliable parameter to guide the selection of alloying elements for additively manufactured alloys. McDonald et al. [19] investigated the influence of Fe and Cr on the grain size of titanium alloys and observed that both elements tend to segregate during solidification, leading to a refinement of the prior-β grains.

Adding grain-refining elements has emerged as a primary approach for eliminating columnar grains in titanium alloys. In this research, a newly designed near-β titanium alloy, Ti35421 (Ti3Al5Mo4Cr2Zr1Fe, wt.%), was produced using the laser direct energy deposition (L-DED) process. The composition of Ti35421 was developed by substituting 1 wt.% Fe with 5 wt.% V in the Ti-B19 alloy, while maintaining the same molybdenum equivalent [20]. The Ti35421 alloy incorporates Cr and Fe, elements known for their grain-refining capabilities, and is anticipated to generate a significant amount of equiaxed grains during the L-DED manufacturing process. Therefore, as a potential titanium alloy, the as-fabricated microstructure of the L-DED sample is worth studying. Meanwhile, the heat treatment and mechanical properties of the L-DED and rolled Ti35421 alloys are optimized based on the microstructural investigation. Finally, the L-DED Ti35421 alloy and the rolled Ti35421 alloy are compared, and further optimization suggestions for the L-DED Ti35421 alloy are provided.

2. Materials and Methods

2.1. Material Preparation

Ti35421 alloy ingot is made of high-purity sponge titanium, Al-80Mo alloy, Al-85V alloy, high-purity aluminum wire, high-purity chromium, high-purity iron, and high-purity zirconium through a three-time vacuum consumable arc melting process. After melting, the ingot is forged at 1050 °C and rolled at 900 °C. The microstructure of the as-rolled Ti35421 alloy is shown in Figure 1. The chemical composition of the ingot was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) (Jena, Thuringia, Germany), and the results are listed in

Table 1. Chemical composition (wt.%) of rolled Ti35421 alloy.

Ti	Al	Mo	Cr	Zr	Fe	O	N	H	C
Bal.	3.29	5.26	3.82	1.98	1.34	0.045	0.004	0.001	0.008

.

The Ti35421 powder was manufactured from the rolled bar using Plasma Rotating Electrode Processing (PREP) (Shangji, Nanjing, China), and the powder with a particle size between 53 and 150 μm was sieved for L-DED process. The powder morphology used for L-DED is shown in Figure 2. The powder properties of the Ti35421 powder are listed in

Table 2. Powder properties of Ti35421 alloy.

Chemical Composition (wt.%)
Ti	Al	Mo	Cr	Zr	Fe	O	N	H	C
Bal.	3.24	5.23	3.85	1.96	1.29	0.063	0.012	0.002	0.007
Particle size			Flowability		Apparent density			Particle shape
D10/μm	D50/μm	D90/μm	s/50 g		g/cm3			%
97.3	130.5	165.6	22.5		2.76			0.97

.

2.2. Laser Directed Energy Deposition

The Ti35421 specimen was produced utilizing a laser direct energy deposition (L-DED, RayCham, Nanjing, China) system equipped with a 3000 W YLS Yb fiber laser (YLS-3000) (IPG, Newton, MA, USA), which provided a laser spot diameter of 3 mm. The processing conditions were configured as follows: a laser power of 1600 W, a laser speed of 600 mm/min, a hatch distance of 1.6 mm, and a layer height of 0.5 mm. A bidirectional zigzag scanning pattern was employed for consecutive layers, with each subsequent layer rotated 90° relative to the prior one. The fabrication process was carried out in an argon environment maintaining an oxygen level below 200 ppm. Schematic diagram of L-DED process is shown in Figure 3.

2.3. Heat Treatment

Place samples of 10 mm $\times$ 10 mm $\times$ 10 mm at 790~820 °C for 30min and then cool them with water. The content of phase $\alpha$ is measured by metallography method. When the content of phase $\alpha$ is completely transformed into phase β the critical temperature is the β transition temperature. The as-fabricated L-DED Ti35421 alloy and rolled Ti35421 alloy were subject to heat treatment in two stages. In the initial stage, the samples were maintained at a temperature of 30–40 °C below the β phase transition point for 2 h, followed by water quenching to preserve a sufficient amount of β phase. In the subsequent stage, the samples underwent aging treatment at various temperatures for 8 h and were then air-cooled. This process aimed to generate a secondary α phase in order to enhance the mechanical properties.

2.4. Microstructure Characterization

The samples intended for microstructural analysis were polished using SiC paper followed by colloidal silica (OP-S) and subsequently etched with Kroll’s reagent for 10 s. The initial microstructure of the L-DED sample was examined using optical microscopy (OM) (Zeiss, Oberkochen, Germany) and transmission electron microscopy (TEM) (FEI, Hillsboro, OR, USA). To assess the β phase transformation temperature, the microstructures of both the L-DED and rolled samples after solution treatment were analyzed via OM (Zeiss, Oberkochen, Germany). Additionally, the microstructures of solution-treated and aged L-DED and rolled samples were investigated using scanning electron microscopy (SEM, FEI, Hillsboro, OR, USA) in order to measure the phase volume fraction and average phase size.

2.5. Mechanical Properties Testing

The tensile test specimens, which had a gauge length of 25 mm and a dumbbell-like geometry, were extracted along the OZ direction from both the L-DED and rolled samples in accordance with the ISO 6892-1:2009 standard [21]. Tensile testing was conducted using a servo-hydraulic testing machine (MTS, Eden Prairie, MN, USA) at ambient temperature. The tests were carried out at an extension rate of 0.375 mm/min, and each test was repeated three times to ensure data consistency and reliability.

2.6. Computational Methods

A three-dimensional thermal simulation model of L-DED was established using COMSOL Multiphysics 5.5 (COMSOL, Stockholm, Sweden), which considered the heat conduction, heat convection, heat radiation, and phase transition. In order to reduce the amount of calculation, three deposition layers were simulated, and each layer contained two tracks. To diminish the calculation difficulty, the effect of the metal flow behavior was not considered in the model. A moving Gauss body heat source was used to simulate the energy transfer of laser beam in this paper. The initial temperature of the computing environment was set to 25 °C.

3. Results and Discussion

3.1. As-Fabricated Microstructure

As shown in Figure 4, obvious molten pool boundaries can be seen in the building direction (YZ plane or XZ plane), the bottom of molten pool is of cellular structure, and the top of molten pool is of equiaxed grain structure. The cellular crystals are located within the equiaxed grains, with each equiaxed grain containing multiple cellular crystals. These cellular crystals grow perpendicular to the molten pool boundaries, as their growth direction is opposite to the direction of heat flow. The thickness of the deposition layer was measured according to the boundaries of the molten pool and it was found that the distance between two adjacent parallel boundaries (two layers) is about 1100 μm, that is, each layer is about 0.55 mm, which was close to the set layer thickness of 0.50 mm. The length and width of grains were measured, and the results are shown in Figure 5. The average grain width is about 106 μm and the average grain length is about 214 μm. These results indicate that a significant quantity of equiaxed grains is formed in the L-DED Ti35421 alloy during the L-DED process.

The microstructural characteristics and phase composition of the as-fabricated L-DED Ti35421 alloy were analyzed using TEM along with corresponding diffraction patterns, as illustrated in Figure 6 and Figure 7. The alloy primarily comprises a β matrix, acicular α′, and lath α phases. The acicular α′ phase is predominantly found within the grains, and its formation is attributed to the extremely rapid cooling rate during processing [22,23]. The lath α phase is observed both within the grain interiors and at the grain boundaries, with continuous grain boundary α also being present. During the rapid solidification process, elements such as Mo, Fe, and Cr are unable to diffuse out from the β phase, leading to the formation of acicular α′, and consequently, these elements are significantly less concentrated in the lath α regions [24]. The chemical composition of region A in Figure 6, which includes the β matrix and martensitic α′, is determined to be Ti5.14Al6.06Mo4.48Cr3.09Zr0.92Fe (wt.%), whereas the composition of region B in Figure 7, corresponding to the lath α phase, is Ti6.12Al3.50Mo2.42Cr2.65Zr0.32Fe (wt.%).

3.2. Heat-Treated Microstructure

Due to variations in elemental composition, different batches of titanium alloys may exhibit varying phase transition temperatures. The β transition temperature was determined using the metallographic method, as illustrated in Figure 8. It is evident that when the solution temperature reaches 800 °C, the boundaries of the molten pool remain clearly visible, with a significant amount of α phase present near these boundaries. At 810 °C, the molten pool boundaries are nearly indistinguishable, and some grain boundaries begin to fade. Upon increasing the solution temperature to 820 °C, the molten pool boundaries vanish entirely, and the α phase is almost completely absent. Based on these observations, it can be concluded that the transition temperature lies between 810 °C and 820 °C.

Figure 9 displays the microstructural characteristics of the L-DED Ti35421 alloy following solution-aging treatment, while

Table 3. Microstructure features of α phases of L-DED Ti35421 alloy after aging for 8 h.

Aging Temperature /°C	Lath α	Lamellar α
	Volume Fraction /vol.%	Volume Fraction /vol.%	Average Length /μm	Average Width /μm
515	14.1 ± 1.5	43.3 ± 2.7	0.43 ± 0.04	3.75 ± 0.62
535	13.5 ± 0.6	37.0 ± 2.8	0.51 ± 0.05	3.84 ± 0.02
555	14.1 ± 1.3	35.7 ± 1.3	0.67 ± 0.04	4.64 ± 1.15
575	13.8 ± 3.2	33.1 ± 2.1	0.82 ± 0.02	6.30 ± 0.35

provides data on the volume fraction and size of the α phase. According to prior studies, phase transformation from β to ω_iso occurs within the temperature range of 209–375 °C. As the temperature increases to between 375 and 475 °C, the ω_iso phase gradually dissolves. During heating from 475 to 590 °C, the α phase becomes more concentrated, and with further temperature elevation, the α phase progressively transforms back into the β phase [25,26]. The lath-shaped α phase primarily forms during the solid solution heat treatment process. Notably, a coarse and continuous grain boundary α phase is clearly visible. The lamellar α phase is predominantly formed during the aging treatment, and its volume fraction decreases with increasing aging temperature. However, the dimensions of the lamellar α phase consistently increase from 515 °C to 575 °C, regardless of whether width or length is considered.

Figure 10 displays the microstructural characteristics of the rolled Ti35421 alloy following solution-aging treatment, while

Table 4. Microstructure features of α phases of rolled Ti35421 alloy after aging for 8 h.

Temperature /°C	Lath α	Lamellar α
	Volume Fraction /vol.%	Volume Fraction /vol.%	Average Length /μm	Average Width /μm
525	13.3 ± 1.3	46.2 ± 4.1	0.47 ± 0.02	3.47 ± 0.69
535	12.2 ± 1.9	36.3 ± 1.9	0.63 ± 0.01	5.58 ± 0.63
545	14.6 ± 1.3	31.5 ± 5.8	0.84 ± 0.04	6.64 ± 0.39
555	12.4 ± 1.0	31.0 ± 4.1	0.84 ± 0.05	6.84 ± 0.47

provides the corresponding volume fraction and size measurements of the α phase. After rolling, the grains were broken, and there is no obvious grain boundary after heat treatment. Compared with L-DED Ti35421 alloy, the lath α of rolled Ti35421 alloy is short and wide, but the volume fraction is about the same (12.4 vol.%~13.3 vol.%). The growth rule of lamellar α with aging temperature is consistent with that of L-DED Ti35421 alloy, but it is more sensitive to temperature. When the aging temperature increases from 515 °C to 575 °C ($∆$ = 65 °C), the volume fraction of lamellar α in L-DED Ti35421 alloy decreases from 43.3 vol.% to 33.1 vol.% ($∆$ = 10.2 vol.%). In contrast, when the aging temperature increases from 525 °C to 555 °C ($∆$ = 30 °C), the volume fraction of lamellar α in L-DED Ti35421 alloy decreases from 46.2 vol.% to 31.0 vol.% ($∆$ = 15.2 vol.%). The size comparison of lamellar α is also similar.

3.3. Mechanical Properties

Figure 11 shows the engineering stress–strain curves of Ti35421 alloy under different heat treatment conditions and different manufacturing conditions. As-fabricated L-DED Ti35421 alloy has the strongest strength and poor ductility. Following solution treatment (780 °C for 2 h with water cooling), there is a notable reduction in strength accompanied by a considerable increase in ductility. After aging, the strength increases in varying degrees, while the ductility decreases in varying degrees. Specifically, within the set temperature range, the higher the aging temperature, the smaller the strength increase and the smaller the ductility decrease. The tensile strength, yield strength, elongation, and reduction in area of the L-DED Ti35421 alloy following 8 h of aging treatment can reach values within the ranges of 1196 to 1329 MPa, 1162 to 1279 MPa, 1.4% to 6.8%, and 4.9% to 16.7%, respectively. And the data are summarized in

Table 5. Mechanical properties of L-DED Ti35421 alloy after aging for 8 h.

Aging Temperature /°C	Strength		Ductility
	Tensile Strength ${\mathit{\sigma }}_{\mathit{s}}$/MPa	Yield Strength ${\mathit{\sigma }}_{\mathit{y}}$/MPa	Elongation $\mathit{\delta }$/%	Reduction in Area $\mathit{\phi }$/%
As-fabricated	1446 ± 46	1377 ± 52	1.7 ± 0.3	/
515	1329 ± 10	1279 ± 16	1.4 ± 1.1	4.9 ± 1.1
535	1273 ± 5	1226 ± 9	1.8 ± 0.6	5.7 ± 1.6
555	1253 ± 12	1216 ± 6	2.0 ± 0.4	6.7 ± 1.6
575	1196 ± 7	1162 ± 8	6.8 ± 1.6	16.7 ± 1.9

. Those of rolled Ti35421 alloy after aging for 8 h can be achieved in ranges of 1128~1268 MPa, 1088~1175 MPa, 1.2~12.5% and 3.5~31.6%, respectively. And the data are summarized in

Table 6. Mechanical properties of rolled Ti35421 alloy after aging for 8 h.

Aging Temperature /°C	Strength		Ductility
	Tensile Strength ${\mathit{\sigma }}_{\mathit{s}}$/MPa	Yield Strength ${\mathit{\sigma }}_{\mathit{y}}$/MPa	Elongation $\mathit{\delta }$/%	Reduction in Area $\mathit{\phi }$/%
515	1145 ± 7	--	1.2 ± 0.1	3.5 ± 1.0
525	1268 ± 8	1175 ± 7	8.5 ± 1.2	21.8 ± 3.3
535	1197 ± 9	1135 ± 9	10.7 ± 1.5	25.9 ± 1.9
545	1174 ± 7	1113 ± 6	10.6 ± 0.8	22.1 ± 3.2
555	1128 ± 6	1088 ± 8	12.5 ± 1.6	31.6 ± 4.1

. The static tensile properties at room temperature are important indexes to evaluate metal materials. It can be seen that the rolled Ti35421 alloy has better ductility than and equivalent strength to L-DED Ti35421 alloy. For example, when the aging temperature is 535 °C, the tensile strength of L-DED Ti35421 alloy is 1273 MPa, which is equivalent to the strength of rolled Ti35421 alloy with aging temperature of 525 °C (1268 MPa), but the elongation of the former is only 1.8%, which is obviously smaller than that of the latter (8.5%).

In addition, the rolled Ti35421 alloy treated under the condition of 780 °C/2 h/WQ + 515 °C/8 h/AC exhibits completely brittle behavior. This is primarily due to the fact that at the aging temperature of 515 °C, the volume fraction of lamellar α reaches its maximum. Cracks are difficult to propagate through a microstructure containing a large amount of lamellar α, leading to a significant decrease in ductility.

Figure 12 illustrates that the alloy experiences repeated cycles of swift melting and solidification throughout the deposition process, a result of the distinct heating and cooling techniques employed. The process is characterized by extremely high cooling rates and thermal gradients. Additionally, the insufficient constitutional undercooling at the liquid–solid interface within the melt pool significantly reduces the rate of spontaneous nucleation during solidification, making it highly prone to the formation of columnar grains that extend along the building direction [27,28]. Fe and Cr are eutectoid β stable elements, and both elements segregate upon solidification, which leads to high constitutional undercooling and promotes more equiaxed grain growth [19]. The cooling rates at the bottom and top of the molten pool have little difference during solidification (3500 °C/s~4000 °C/s) according to the simulation, but the temperature gradient at the bottom (>7 $\times$ 10⁵ °C/m) is obviously greater than that at the top (4 $\times$ 10⁵ °C/m~5 $\times$ 10⁵ °C/m). With the decrease in temperature gradient, the solidification interface will change from planar to cellular, and then to dendritic. The low ratio of temperature gradient to cooling rate (G/R) may be the main reason for the appearance of cellular crystals at the bottom of the molten pool. Cellular crystal only appears when the alloy is directionally solidified (G > 0), which is a grain form with the growth direction opposite to the heat flow direction, and its growth condition is close to the constitutional undercooling limit. Dendrites grow at the top of the molten pool and finally contact each other. Dendrites are usually not seen in titanium alloys, but the dendrite contact point (grain boundary) can be seen [6].

In the Ti35421 alloy, the volume fraction and dimensions of the lamellar α phase are primarily determined by the free energy difference between the α phase and the metastable β phase. Following solution treatment, a metastable β matrix is generated, which subsequently evolves into lamellar α during aging. Within a certain temperature range, lowering the aging temperature increases the free energy difference between the two phases, thus increasing the driving force for phase transformation [25,26]. Figure 13 shows the relationship between aging temperature and mechanical properties. The tensile strength and yield strength of the L-DED Ti35421 alloy decrease monotonously with the increase in aging temperature. The ductility of L-DED Ti35421 alloy increases slightly with the increase in aging temperature, and increases significantly at 575 °C. It is noteworthy that the rolled Ti35421 alloy shows obvious brittle fracture after aging at 515 °C, resulting in its mechanical strength being significantly lower than that of other aging temperatures. Except for 515 °C, the tensile strength and yield strength of the rolled Ti35421 alloy decrease with the increase in aging temperature. The elongation after fracture and reduction in area are positively correlated, which increase with the increased aging temperature, but those of the rolled sample aged at 545 °C decrease slightly.

Since the main α phase is affected by the solid solution temperature and maintains a relatively stable state within the temperature range of 515 °C to 575 °C, the mechanical behavior of the Ti35421 alloy is primarily governed by the lamellar α phase. Figure 14 and Figure 15 depict the relationship between lamellar $\alpha$ characteristics and mechanical properties. The smaller the size of lamellar $\alpha$, the better the strength, but the lower the ductility. When the content (size) of lamellar $\alpha$ is the same, L-DED Ti35421 alloys tend to have higher strength and lower ductility than rolled Ti35421 alloys, which is related to the coarse grains and continuous grain boundaries of L-DED titanium alloys. What is more, the mechanical properties of rolled Ti35421 alloys show a step increase or decrease with the increase in the content (size) of lamellar $\alpha$, which indicates that rolled Ti35421 alloys are more sensitive to the change in the content (size) of lamellar $\alpha$ than L-DED Ti35421 alloy.

Figure 16 presents a comparison of the combined mechanical properties among the L-DED Ti35421 alloy, rolled Ti35421 alloy, and several other commonly used near-β titanium alloys [29,30,31]. In this plot, the horizontal axis represents yield strength, while the vertical axis indicates elongation after fracture. All data points fall within the region bounded by two parallel dashed lines. The mechanical properties demonstrate that Ti35421 alloy has a wide performance window by adjusting the manufacturing process and heat treatment process. The L-DED Ti35421 alloy without heat treatment has the highest strength, but its ductility is unacceptable. After undergoing solution treatment and aging treatment, the material exhibits a notable reduction in strength, while ductility improves to a lesser extent. When the aging temperature is raised to 575 °C, the alloy achieves satisfactory overall mechanical performance. Consequently, the optimal heat treatment procedure for the L-DED Ti35421 alloy is determined to be 780 °C for 2 h with water cooling (WC), followed by aging at 575 °C for 8 h with air cooling (AC). Under this treatment, the alloy demonstrates excellent comprehensive mechanical properties, with a yield strength of 1162 MPa and an elongation after fracture of 6.8%.

The rolled Ti35421 alloy only treated by solid solution has poor comprehensive properties (the points are close to the lower dotted line in Figure 16) and has improved comprehensive properties after solution and aging treatment (the points are close to the upper dotted line). Thus, the optimal heat treatment process for the rolled Ti35421 alloy is determined to be 780 °C for 2 h with water cooling, followed by 525 °C to 555 °C for 8 h with air cooling. Under this condition, the alloy achieves excellent comprehensive mechanical properties, with a yield strength ranging from 1088 to 1175 MPa and an elongation after fracture of 8.5% to 12.5%. It is evident that, at comparable yield strength levels, the ductility of the L-DED Ti35421 alloy is inferior to that of the rolled Ti35421 alloy. In other words, the rolled Ti35421 alloy exhibits superior mechanical properties compared to the L-DED Ti35421 alloy. Nevertheless, the L-DED Ti35421 alloy demonstrates an exceptional strength ceiling that cannot be achieved by the rolled Ti35421 alloy.

As observed in Figure 12, the molten pool experiences an extremely rapid cooling rate, preventing the timely diffusion of solutes. This intense cooling promotes the formation of a significant amount of acicular α′ in the L-DED Ti35421 alloy. Furthermore, the following thermal cycles contribute to the development of lath-shaped α structures, which are illustrated in Figure 6 and Figure 7. The formation of lamellar $\alpha$ needs a long time of heat preservation, while the addition process is mainly instantaneous heating. According to Figure 12, in the process of L-DED process, the sample is immediately heated, then rapidly cooled, and kept between 200 and 400 °C for a long time, which cannot meet the formation conditions of lamellar $\alpha$. The acicular ${\alpha }^{\prime }$ could improve the strength significantly, which may be the main reason for the high strength of as-fabricated L-DED Ti35421 alloy. Coarse grains, acicular ${\alpha }^{\prime }$, and continuous grain boundary $\alpha$ result in difficult crack propagation, as shown in Figure 17a. As a result, as-fabricated L-DED alloy shows high strength and low ductility. In practical applications, near-$\beta$ titanium alloys are generally subject to solution treatment for a short time and then aged for a long time to meet the engineering requirements. The solution temperature is selected around the $\beta$ transition temperature of the near-$\beta$ titanium alloy. Although the solution temperature range available for selection is relatively small, it strongly affects the microstructure and mechanical properties of the alloy. Solution treatment could fully dissolve various elements and phases in the matrix, eliminate the stress generated during hot working, and form uniform supersaturated solid solution, which is ready for aging treatment [32,33]. L-DED Ti35421 alloy after solution treatment is mainly composed of $\beta$ matrix and a small amount of lath $\alpha$, resulting in slow crack propagation (the strength is low and the ductility is good), as shown in Figure 17b. After solution and aging treatment, a large amount of lamellar $\alpha$ is formed in the alloy, and the lath $\alpha$ and grain boundary $\alpha$ become thicker, resulting in the difficulty of crack propagation (the strength increases significantly and the ductility decreases significantly), as shown in Figure 17c. However, the contribution of lamellar $\alpha$ to strength is less than that of acicular ${\alpha }^{\prime }$, which leads to the strength of solution- and aging-treated Ti35421 alloy being lower than the as-fabricated L-DED Ti35421 alloy. The solution and aged rolled Ti35421 alloy has no grain boundary $\alpha$, and the crack propagation is not hindered by the grain boundaries. What is more, there are a small amount of lath $\alpha$ and a large amount of lamellar $\alpha$ in the solution and aged rolled Ti35421 alloy, which could improve the strength, as shown in Figure 17d. Therefore, the solution and aged rolled Ti35421 alloy has better comprehensive mechanical properties than solution and aged L-DED Ti35421 alloy, as shown in Figure 16.

4. Conclusions

In this study, a potential near-β titanium alloy, Ti35421 (Ti3Al5Mo4Cr2Zr1Fe, wt.%), was fabricated by L-DED process; the microstructure of L-DED samples was studied in detail; and the heat treatment and corresponding comprehensive mechanical properties were optimized and discussed. Meanwhile, L-DED Ti35421 alloy and rolled Ti35421 alloy were compared. The conclusions are as follows:

(1) The microstructure of the as-fabricated L-DED Ti35421 alloy is characterized by clearly defined pool boundaries, with cellular crystalline formations appearing near these regions. Most of the grains display an equiaxed morphology. The alloy comprises a β phase matrix, along with continuous grain boundary α phase, small amounts of lath-like α phase, and a substantial volume of acicular α’ phase. This particular arrangement of microstructural elements is responsible for the material’s high tensile strength (1446 MPa) and limited ductility (1.7%).

(2) After undergoing solution and aging treatments, the grain boundary α phase in the L-DED Ti35421 alloy gradually coarsens, and the β phase within the matrix evolves into a multitude of fine lamellar α structures. With the increase in aging temperature, the strength of the material steadily decreases, while its ductility shows a progressive enhancement.

(3) The optimal heat treatment regimen for the L-DED Ti35421 alloy was found to be a two-step process: first, heating at 780 °C for 2 h followed by air cooling, then heating again at 575 °C for 8 h with subsequent air cooling. Under this treatment, the alloy exhibits its highest overall mechanical properties, including a tensile strength of 1196 MPa, a yield strength of 1162 MPa, an elongation of 6.8%, and a 16.7% reduction in area.

(4) The optimal heat treatment procedure for the rolled Ti35421 alloy has been identified as a two-step process: first, heating at 780 °C for 2 h followed by air cooling (AC), and subsequently heating within the range of 525–555 °C for 8 h, also ending with air cooling. Under these conditions, the alloy exhibits outstanding mechanical properties, such as a tensile strength between 1128 and 1268 MPa, yield strength ranging from 1088 to 1175 MPa, an elongation of 8.5–12.5%, and a reduction in area of 16.7%. In comparison to the L-DED Ti35421 alloy, the rolled variant displays enhanced overall performance, which can be attributed to the lack of continuous grain boundary α phase.

Although Ti35421 alloy obtains the microstructure of all equiaxed grains in the L-DED process, the heat-treated comprehensive mechanical properties were worse than that of rolled Ti35421 alloy, which may be the reason for coarse grains and continuous grain boundary $\alpha$. It is anticipated that the mechanical properties of the L-DED Ti35421 alloy can be further enhanced and brought up to the level of those of rolled Ti35421 alloy through the exploration of additional grain refinement and breaking techniques, such as the incorporation of the B element or the application of online rolling. This will be the primary focus of future research.