Microstructure, Variant Selection, and Mechanical Properties of Laser-Welded Ti-4Al-2V Joints

Abstract

Laser welding of the near α-phase titanium alloy Ti-4Al-2V, used for complex components in the nuclear industry, has been rarely reported. In this study, butt weld joints made of Ti-4Al-2V alloy plates under different parameters, including the laser power, the welding speed, and the defocus distance, were manufactured and analyzed. The results showed that adjusting the combination of 4.2 kW of laser power, a 20 mm/s welding speed, and a −2 mm defocus distance could achieve a penetration depth exceeding 6 mm. Porosity defects were prone to forming in the middle and bottom parts of the fusion zone, due to rapid cooling. The microstructure of the fusion zone was mainly needle-like α martensite, which precipitated in the form of specific clusters. The interior of a cluster was composed of three types of variants with <11−20>/60° phase interfaces to achieve the lower boundary’s energy. Affected by the microstructure and welding defects, the strength of the weld joint was basically similar under different welding conditions, namely about 720 MPa, slightly higher than that of the base metal, while the rupture elongation at breaking decreased by more than 50%. The micro-Vickers hardness of the weld joints was about 50–60 HV higher than that of the base metal, while the impact toughness was about 40 KJ, almost half that of the base metal. This research lays a solid foundation for the engineering application of laser welding of Ti-4Al-2V alloys.

1. Introduction

Titanium alloys have aroused great interest due to their high melting point, high strength, and low density, and are widely used in various fields such as aerospace, biomedical, and nuclear energy [1,2]. Numerous studies have shown that most titanium alloys exhibit good applicability and excellent welding performance when used to assemble complex structural components [3,4]. Titanium welding structures with excellent performance can be achieved through different welding methods [4,5,6].

Laser welding has been the most common and widely used welding method for titanium alloys. The main characteristics of laser-welded titanium alloys are as follows:

(1) The welding process has a significant impact on the microstructure and mechanical properties of the weld joint. Affected by parameters such as the laser power and welding speed, the evolution of the microstructure mainly includes changes in the microstructure and the grains’ morphology [5,7]. Different microstructures in the fusion zone (FZ), the heat-affected zone (HAZ), and the base metal (BM) cause different deformation mechanisms, resulting in a diversity of mechanical properties. Generally, the tensile strength of FZ is higher than that of the BM, but its ductility is lower [8,9].

(2) During the rapid cooling processes of laser welding, titanium alloys exhibit significant variant selection. Numerous researchers have systematically analyzed the selection patterns of variants in different titanium alloys. Lu et al. found that the variants of Ti-4Al-2V prepared by laser metal deposition (LMD) mostly precipitated in the form of Type I triple-alpha-variant clusters, which possesses lower boundary energy [10], similar to the results for the additively manufactured Ti-6Al-4V, according to Stephenson et al. [11]. However, Farabi et al. characterized two different types of variant clusters in pure commercial titanium [12]. In addition, Shi et al. assessed different variant selection rules of the grain boundary α in a Ti5533 alloy, and found that variant selection was not only governed by the empirical rules, such as Burgers orientation relationship, but was also influenced by the primary β grain boundaries [13].

(3) Laser welding is more likely to cause porosity defects in the weld joint, thereby reducing the mechanical properties. Since the 1990s, researchers have conducted a series of related works through experiments and theoretical analyses [14,15,16]. Zhao et al. conducted in-situ research on the keyhole behavior in a titanium alloy during laser irradiation using high-energy X-rays, and basically elucidated the mechanism of the formation of pore defects [17]. They established the relationships among the materials, the process of formation, and the formation of pore defect.

Ti-4Al-2V is a near α-phase titanium alloy with excellent properties, including welding performance. The current research focused mainly on its microstructure’s composition and its mechanical properties. Li et al. [18]. first conducted a preliminary study on the phase composition of Ti-4Al-2V alloys. Subsequently, Liu et al. [19], Omidbakhsh et al. [20], and Rodchenkov et al. [21] investigated the irradiation behavior, oxygen behavior, and superplastic deformation performance of Ti-4Al-2V, respectively. In recent years, the influence of the processes of formation, such as additive manufacturing and welding, on the microstructure and mechanical properties of Ti-4Al-2V has been studied. Lu et al. analyzed the features of α-phase precipitation and the phase composition of Ti-4Al-2V during laser additive manufacturing [10]. Our team preliminarily analyzed the microstructure and properties of tungsten inert gas (TIG)-welded Ti-4Al-2V [22]. Wang et al. investigated the performance of different forms of dissimilar Ti-4Al-2V/Ti-6Al-4V welded joints [23]. Moreover, the influence of the welded pores on the fatigue behavior of Ti-4Al-2V at different temperatures was investigated by Liu et al. [24]. However, experimental research on laser-welded joints, which have been implemented for industrial applications of Ti-4Al-2V, especially the microstructure and mechanical properties, is still lacking.

This study systematically analyzed the microstructure and mechanical properties of a laser-welded Ti-4Al-2V alloy for the first time. Nine sets of butt weld joints with different levels of laser power, welding speeds, and defocus distances were prepared. The microstructure and variant selection were characterized using optical microscopy (OM), scanning electron microscopy (SEM), electron back-scatter diffraction (EBSD), and transmission electron microscopy (TEM). The influence of the laser power and the welding speed on the formation of internal porosity defects in a Ti-4Al-2V alloy was analyzed. Their mechanical properties at room temperature were tested. Then the mechanism of defect formation and variant selection were analyzed, as well as the relationships among the laser welding parameters, the welding defects, the microstructure, and the mechanical properties. Finally, a systematic understanding of the microstructure and properties of laser-welded joints of the Ti-4Al-2V alloy was obtained.

2. Materials and Methods

2.1. Welding Material and Method

The chemical composition of the near-α Ti-4Al-2V plates, as shown in

Table 1. Chemical composition of the BM, wt. %.

Ti	Al	V	C	O	N	Zr	Fe	Si	Co
Bal.	4.90	2.30	0.01	0.09	0.01	<0.10	<0.05	<0.01	<0.01

, was measured by a physicochemical phase analysis method. Laser welding was conducted by an HWF-160 laser welding system, and Figure 1 shows the schematic diagram of the laser welding process on a Ti-4Al-2V plate with the dimensions of 150 mm × 100 mm × 6 mm. The welding parameters, including different levels of laser power, welding speeds, and defocus distances, are shown in

Table 2. Laser welding parameters of the weld joint.

	Laser Power, kW	Welding Speed, mm/s	Defocus Distance, mm
Joint (1)	2.2	20	−2
Joint (2)	3.2	20	−2
Joint (3)	4.2	20	−2
Joint (4)	4.7	20	−2
Joint (5)	5.2	20	−2
Joint (6)	4.7	15	−2
Joint (7)	4.7	25	−2
Joint (8)	4.7	20	0
Joint (9)	4.7	20	2

. The dimensions of the joined Ti-4Al-2V plate were 150 mm × 200 mm × 6 mm. During the welding process, above 99.99% purity argon was used as the shielding gas on the front and back of the weld joints to prevent oxidation.

2.2. Mechanical Property Tests

After welding, the micro-Vickers hardness, tensile properties, and Charpy impact toughness of the weld joints were investigated at room temperature. Figure 2a shows the locations on the laser-welded plates where the specimens for the mechanical property tests were cut from. The micro-Vickers hardness test covered the BM, HAZ, and FZ of the weld joint, as shown in Figure 2b, and was conducted by the FALCON511 automatic Vickers hardness testing machine using a 0.05 kg indenter for 15 s based on the Chinese standard GB/T4340.1-2009, which is similar to ASTM E92, with a 1 mm step. The gauge section of the tensile specimen was located in the FZ. The room-temperature tensile properties were tested according to GB/T 228-2015, which is similar to ASTM E08, on the plate specimen, as shown in Figure 2c, using a CMT4105 universal electronic testing machine. The V-notch of the Charpy impact toughness specimen was located in the middle of the FZ. The Charpy impact toughness test was carried out on a JB-300b pendulum impact testing machine with a 5 mm thick V-notch specimen according to GB/T 229-2007, which is similar to ASTM E23. A schematic diagram of the impact specimen is shown in Figure 2d.

2.3. Characterization of the Microstructure

The microstructure of the weld joints was characterized with a Leica DMI 8C optical microscope and a JSM-IT500 scanning electron microscope equipped with electron back-scattered diffraction (EBSD). The OM, SEM, and EBSD specimens were mechanically polished with 2000# SiC paper, then polished by electrolysis with a solution of methanol, 2-butanoxyethanol, and perchloric acid at a DC voltage of 2 V at about −30 °C. After polishing, the sample could be directly used for EBSD characterization. The OM and SEM samples were etched for 15 s at room temperature in a solution of HF:HNO₃:H₂O = 1:4:15 (volume). The fracture morphology of the samples used for the tensile and Charpy impact toughness tests was also characterized by SEM. A transmission electron microscopy (TEM) investigation was conducted for the FZ of Joint (6), and TEM specimens were prepared via twin-jet electrochemical polishing in a solution containing 10% perchloric acid and 90% methanol at 18 V and −35 °C.

3. Results

3.1. Microstructure

The macroscopic morphology of the cross-section of the weld produced by varying laser irradiation parameters on I-type butt joints made of the Ti-4Al-2V alloy, with a thickness of 6 mm, is depicted in Figure 3. Through a comparison of the welds’ macroscopic morphology, the following conclusions can be drawn. (1) As the laser power increased, the weld’s penetration depth increased. When the defocus distance and the welding speed were fixed at −2 mm and 20 mm/s, respectively, as shown in Figure 3a–e, a laser power exceeding 4.2 kW could ensure a penetration depth in the weld of more than 6 mm. (2) When the laser power and the defocus distance were fixed at 4.7 kW and −2 mm, respectively, all the welding speeds could ensure sufficient penetration, as shown in Figure 3d,f,g, while the width of the FZ increased with a decrease in the welding speed, i.e., an increase in the heat input. (3) When the laser power and welding speed were fixed at 4.7 kW and 20 mm/s, respectively, and the defocus distance was 0 or −2 mm, the weld’s penetration depth was guaranteed to exceed 6 mm. When the defocus distance was 2 mm, the penetration depth was only 5.5 mm. Similarly, the width of the FZ and the penetration depth increased with a decrease in the defocus distance, i.e., an increase in the heat input. In summary, the macroscopic morphology of the weld joints changed with the welding parameters, which is consistent with conventional understanding. In the nine weld joints in Figure 3, the FZ, HAZ, and BM can be clearly defined on the basis of the different microstructures, but the characteristics of the FZ and HAZ obtained under different parameters were roughly the same. In addition, porosity defects appeared in multiple welds, often in welds with deeper penetration, which were related to the behavior of keyholes and are discussed in detail in Section 4.1.

Since there was no significant difference in the microstructures of the FZ and HAZ of the weld joints under the nine conditions shown in Figure 3, the enlarged FZ and HAZ microstructure of the BM, Joints (4)–(6) were analyzed further, as shown in Figure 4. As shown in Figure 4a, equiaxed grains with a size of approximately 30 μm can be observed in the BM, which formed after rolling and annealing. Under the three conditions shown in Figure 4b1 (4.7 kW, 20 mm/s, and −2 mm; Joint (4)), Figure 4c1 (5.2 kW, 20 mm/s, and −2 mm; Joint (5)), and Figure 4d1 (4.7 kW, 15 mm/s, and −2 mm; Joint (6)), two types of boundaries can be observed inside the FZ, namely the grain boundaries of the primary β-phase and the boundaries of the α-phase, as shown by the arrow in Figure 4b1. Within the FZ, the needle-like phase, which is almost divided into specific clusters by the primary β grains’ boundary and the phase boundaries, was the main constituent phase. The needle-like phase was detected by TEM, as shown in Figure 5. It was suggested that the needle-like phase should be the α-phase. It is evident that some α-phases penetrate the original β grains, which was different from the previously characterized α martensite of the same titanium alloys [10,25].

Figure 4b2,c2,d2 show the HAZs of Joints (4)–(6). A clear gradient microstructure from the equiaxed grains on the side near the BM to needle like phases on the side near the FZ can be observed. This is because the shorter the distance to the FZ, the more heat is received and the higher the cooling rate, resulting in larger β grains and needle-like α-phases. Nevertheless, in a comparison of the three welding conditions, there was no significant difference in the microstructure of the HAZ.

3.2. Texture

The {0001}, {11−20}, and {10−10} pole figures of the BM, HAZ, and FZ under different welding conditions were investigated to analyze the textural evolution during laser welding. The pole figure of the BM in Figure 6a shows that the Ti-4Al-2V plate formed by rolling and annealing mainly contained a {0001} fiber texture, with the maximum intensity being about 9. The texture of the HAZ was more complex. From the pole figure in Figure 6b, it can be seen that the {0001} <11−20> plate’s texture showed the highest intensity, about 8, and the intensities of other texture types were less than 3. There was no significant difference in the type of texture and the intensity of the HAZ under different welding conditions. Figure 6c–e show the pole figures of the FZ for Joints (4)–(6). The types of microstructural texture formed under the three conditions were similar, including {0001} <10−10>, {11−20} <10−10>, and {11−20} <0001> plate textures. However, the maximum texture intensity of Joint (4) was only 16, while those of Joints (5) and (6) were about 30, which was influenced by the heat input of welding (Joints (5), (6) > Joint (4)).

3.3. Variant Selection

The variant selection of the weld joints with different welding conditions was investigated by EBSD. The inverse pole figures (IPFs) of the center part in the FZ under the three conditions, as shown in Figure 4, together with that of the reconstructed parent β-phase are shown in Figure 7. Compared with the morphology of the variants, they were mainly needle-like structures, indicating that under the three conditions above, the variants tended to precipitate in the form of clusters, and meaning that specific types of variants among the 12 variants appeared in a certain region, as shown by the black circles in Figure 7a1. In a variant cluster, there was a type of variant that precipitated in parallel with the others, and its length and content were significantly greater than others in the unified cluster. Massive transformation of the α variant can be detected near the original β grains’ boundaries, as shown by the black arrow in Figure 7a1. Similar phenomena can also be observed in Joints (5) (Figure 7b1,b2), and (6) (Figure 7c1,c2).

3.4. Mechanical Properties

Table 3. Tensile properties and Charpy impact toughness of the BM and laser-welded joints at room temperature.

	Yield Strength, MPa	Tensile Strength, MPa	Rupture Elongation, %	Charpy Impact Toughness, J
BM	638	697	23	80.7 ± 3.2
Joint (4)	625 ± 7	729 ± 4	11.2 ± 1.2	45.4 ± 9.7
Joint (5)	615 ± 3	722 ± 9	7.2 ± 0.1	48.1 ± 5.3
Joint (6)	601 ± 9	720 ± 3	7.7 ± 0.2	36.6 ± 2.0

shows the room-temperature tensile properties of Joints (4)–(6). The yield strength and tensile strength of Joint (4) were 625 MPa and 729 MPa, respectively. Those of Joints (5) and (6) were slightly lower than those of Joint (4), about 615 MPa and 722 MPa for Joint (5), and 601 MPa and 720 MPa for Joint (6), respectively. Moreover, the rupture elongations of Joints (5) and (6) were both only about 7%, which was much lower than that of Joint (4) (~11%). It should be noted that the yield strength, tensile strength, and rupture elongation of the BM were 638 MPa, 697 MPa and 23%, respectively.

Figure 8a–c show the fracture morphology of the tensile specimens. The fracture morphologies of the three joints were mainly composed of dimples, and there was no obvious difference in the dimples’ size and distribution among the different conditions. Therefore, it can be considered that the lower elongation of Joints (5) and (6) may be caused by their internal defects. In the following discussions, the formation of porosities during laser welding and the impact of the microstructures on the tensile properties are analyzed.

The Charpy impact toughness results showed that the impact absorption energies of Joints (4)–(6) were 45.4 ± 9.7J, 48.1 ± 5.3 J and 36.6 ± 2.0 J, respectively, and were much lower than that of the BM, which was 80.7 ± 3.2 J. The fracture morphology of the specimens for the Charpy impact toughness test (Figure 8d–f) also showed that there was no obvious difference in the three weld joints. Through a comparison of the tensile and impact (shear) fractures under the same welding conditions, shown in Figure 8, it was seen that the dimples in the shear fractures were generally smoother and shallower, while those of the tensile fractures were bigger and deeper. The difference was mainly caused by the different fracturing processes of different tests. Specifically, the process of impact fracturing occurs instantaneously, while tension fractures occur relatively slowly at a certain rate.

Figure 9 shows the micro-Vickers hardness distribution of Joints (4)–(6). The hardness of the FZ at the three joints were higher than those of the BM. Moreover, the hardness of Joints (4) and (6) was slightly higher than that of Joint (5). Specifically, the microhardness of the FZ is about 50–60 Hz higher than that of the BM.

4. Discussion

4.1. The Weld Pool and the Porosity of the Laser-Welded Ti-4Al-2V Alloy Plate

Auwal et al. compared different welding methods for titanium alloys. Laser welding showed a high energy density, concentrated heating, a fast welding speed, and small amounts of welding deformation [4], which led to different structures of the weld pool and the microstructure of the weld joint from other welding methods. Cho et al. found that during the process of laser welding, the metal on the surface of the weld pool would volatilize and sag downward under the action of additional stress, and finally form a small pit on the surface [15]. In the results of this study, as shown in Figure 3, there was a depression of about 0.2 mm on the surface of the weld joints, which was caused by vaporization and volatilization under high temperatures.

Matsunawa et al. [26], Lee et al. [14], Cho et al. [15], and Fabbro et al. [27] reported that during the heating process, the laser could be directly injected into the pit’s bottom, forming a slender “keyhole”. When the recoil pressure of the metal vapor is balanced by the surface tension and the gravity of the liquid metal, the keyhole will not go further. When the spot density is very high, the keyhole will run through the whole thickness of the plate to form a deep penetration weld. The keyhole moves along the direction of welding with the beam relative to the workpiece. The metal melts in front of the keyhole, flows around the keyhole to the rear, and re-solidifies to form a weld. In this study, multiple processes led to the laser penetrating the workpiece to be welded, so the solidification behavior of the weld was the same as that of a keyhole weld.

In addition, researchers have found that the in the process of laser welding, spherical porosity defects with sizes ranging from tens to hundreds of microns can easily appear inside of the weld joint [28,29,30,31]. In the early stages, limited by the methods of characterization, researchers, such as Panwisawas et al. [32], have mostly used numerical simulation and theoretical analyses, supplemented by X-ray, SEM, and CT methods, to characterize the joints’ structure after welding, trying to explore the formation and disappearance of keyholes and the mechanism of internal porosity in the welding process. It was suggested that for the plate to be welded with a certain thickness, when the laser power was in the appropriate range, the keyhole was stable, the shielding gas was inside the keyhole, and the metal vapor that formed in the front of the keyhole could escape in time, thus forming a sound weld joint. However, when the laser power was too high or the welding speed was too low, the stability of keyhole would decrease [16,31]. At this time, in the welding process, the keyhole moves forward, the rear wall of the keyhole collapses, and the gas (shielding gas and metal vapor) that has not yet escaped will be sealed in the weld joint, eventually forming a porosity [33].

With the development of characterization technology, Zhao et al. investigated the keyhole’s behavior in-situ using high-energy X-rays during the process of the laser’s action on a titanium alloy [17]. It was observed that the process of forming internal porosity during laser welding was mainly determined by the laser power and welding speed, which was confirmed by previous theoretical derivations.

In this study, no internal porosity defects were observed in Joint (1)–(4) (Figure 3a–d). Nevertheless, when the energy density of welding increased, caused by the laser power increasing or the welding speed decreasing, evident internal porosity defects appeared, as shown in Figure 3e–g. The results showed that for a 6 mm thick Ti-4Al-2V alloy plate, a 4.2 kW laser power combined with a 20 mm/s laser speed could avoid internal defects. Further increases in the laser power or decreases in the welding speed will lead to the keyhole’s instability and internal porosity defects.

4.2. Characteristics of the Formation of Variant Clusters

When the transformation from the β-phase to the α-phase occurs in titanium, 12 different oriented variants of the α-phase may be formed, following Burgers orientation relationship, as shown in

Table 4. The 12 α-variants that can form in a single prior β grain in Ti [34].

Variant	Burgers Orientation Relationship (BOR)	Rotation Angle/Axis from V1
V1	(1−10)β//(0001)α; [111]β//[11-20]α	/
V2	(10−1)β//(0001)α; [111]β//[11-20]α	60°/[1 1 −2 0]
V3	(01−1)β//(0001)α; [111]β//[11-20]α	60°/[1 1 −2 0]
V4	(110)β//(0001)α; [-111]β//[11-20]α	90°/[1 −2.38 1.38 0]
V5	(101)β//(0001)α; [-111]β//[11-20]α	63.26°/[−10 5 5 -3]
V6	(01−1)β//(0001)α; [-111]β//[11-20]α	60.83°/[−1.377 −1 2.377 0.359]
V7	(110)β//(0001)α; [1-11]β//[11-20]α	90°/[1 −2.38 1.38 0]
V8	(10−1)β//(0001)α; [1-11]β//[11-20]α	60.83°/[−1.377 −1 2.377 0.359]
V9	(011)β//(0001)α; [1-11]β//[11-20]α	63.26°/[-10 5 5 -3]
V10	(1−10)β//(0001)α; [11-1]β//[11-20]α	10.53°/[0 0 0 1]
V11	(101)β//(0001)α; [11-1]β//[11-20]α	60.83°/[−1.377 −1 2.377 0.359]
V12	(011)β//(0001)α; [11-1]β//[11-20]α	60.83°/[−1.377 −1 2.377 0.359]

[34]. The 12 α-variants usually do not precipitate uniformly, as the theory suggests, but rather result in the selection of specific variants. The common forms of variant selection include the following: (1) some variants have significantly higher precipitation rates than others, resulting in an uneven proportion of variants precipitating; (2) some specific combinations of variants exhibit clustered precipitation, with a very high volume fraction in a certain region. The most common types of clusters are V1/V2/V3, V4/V5/V6, V7/V8/V9, and V10/V11/V12, which are called Type I clusters, in which each of the two neighboring α-variants are separated by a [11-20]/60° α/α boundary. This type of cluster is often characterized in additive manufactured Ti-4Al-2V, commercially pure Ti, and Ti-6Al-4V alloys [10,11,12,35]. In addition, the other four types of clusters are V1/V5/V9, V2/V4/V12, V3/V7/V11, and V6/V8/V10, named the Type II clusters, in which a [1055-3]/63.26° α/α boundary exists between each of the two neighboring α-variants. This type of cluster is relatively more difficult to form, but it has also been characterized in pure commercial titanium and Ti-6Al-4V [11,12].

As shown in Figure 7, there are obvious characteristics of the variant cluster in the FZ of the Ti-4Al-2V alloy’s weld formed by laser welding. To characterize the type of variant clusters, further analysis of the microstructure (Figure 7a) showed that Type I clusters dominated in this FZ. Figure 10a shows that the proportion of <11−20>/60° interfaces between the two variants accounted for 64.9% of the total length of the phase interface, far exceeding the 18.2% of randomly precipitating variants. Figure 10b shows three types of variants with a phase disorientation of <11−20>/60° in α-cluster A. The {0001} pole figure is shown in Figure 10c, where the poles of the three types of variants intersect at the same point, satisfying the theoretical characteristics of a Type I cluster. In this region, the area of the three variants above accounted for approximately 70%. Further analysis of the microstructure, shown in Figure 7b,c revealed similar results. In summary, there was a significant variant selection phenomenon in the FZ of Ti-4Al-2V laser welds, and the variants tended to precipitate in the form of Type I clusters. The kernel average misorientation (KAM) maps of the microstructures shown in Figure 7a are shown in Figure 11. It can be seen that the area with the <11−20>/60° interface were more concentrated and had a higher KAM value, as shown by the white arrow in the figure, indicating that the variants precipitated in the form of Type I clusters can reduce interface’s energy.

4.3. Relationship among the Laser’s Parameters, Macro- and Microstructures, and Mechanical Properties

It has been suggested that the mechanical properties of weld joints are closely related to the microstructure, the weld defects, and the residual stress, which are all determined by the welding parameters [2]. In this study, under different welding conditions, the microstructures of the FZ and HAZ were basically the same. In the FZ, the α-phase precipitated, with a needle-like morphology, predominantly in the form of Type I clusters. It can be inferred that under different welding conditions, the influence of the microstructure on the mechanical properties is not significant, which is also explains that the Vickers hardnesses of the FZ were equivalent under the three welding conditions in Figure 9. Moreover, the formation of the needle-like α phase in the FZ made the tensile strength of laser-welded joints reach 720–729 MPa, which was slightly higher than 695 MPa in the BM, and the rupture elongation was reduced by half. Meanwhile, due to the differences in the needle-like α-phase and the annealed α-phase, the microhardness of the FZ was about 50–60 HV higher than that of the BM, and the Charpy impact toughness was only half that of the BM. In addition, the formation of internal porosity defects in the weld joints further reduced its tensile elongation. As a result, the rupture elongation of Joints (5) and (6) was only about 7%, which is much lower than 11% in Joint (4). In the HAZ, the microhardness was about 20–30 HV higher than that of the BM, which was also caused by the formation of a new α-phase during the αβα phase transformation during the cooling of the joints.

In summary, for laser welding of a Ti-4Al-2V alloy, if better mechanical properties are desired, it is necessary to focus on welding defects such as porosity, and the formation of the weld. Correspondingly, the cooling rate of the weld joints, including the FZ and HAZ, did not change significantly under different conditions, resulting in a similar phase transformation process in the weld joint. Therefore, the influence of welding parameters on the microstructure is not remarkable and does not significantly affect the mechanical properties.

5. Conclusions

In this study, laser welding with different parameters was used to join a 6 mm thick plate made of a Ti-4Al-2V alloy, and the microstructure, variant selection, and mechanical properties of the weld joints were characterized and analyzed. The main conclusions are as follows.

• A welding speed of 20 mm/s, a defocus distance of −2 mm, and a welding power of 4.7 kW could ensure penetration of the weld. On this basis, increasing the defocus distance to 2 mm or reducing the welding power to 3.2 kW prevented the weld from fully penetrating, but adjusting the welding speed to 25 mm/s could also ensure deep penetration. There were porosity defects inside the weld bead under multiple conditions, especially in the middle and lower parts. There was no significant difference in the microstructure of the FZ among different welding conditions, and all were needle-shaped α martensite.
• The α-variants of the Ti-4Al-2V alloy welds formed by laser welding showed significant selective precipitation characteristics, mainly characterized in the form of Type I clusters, and the formation of <11−20>/60 ° interfaces between the variants within the clusters reduce the interface’s energy. Meanwhile, in order to balance the overall energy of the weld joint, a larger proportion of<11−20>/60 ° interfaces inside the cluster existed, with higher KAM values inside the variant.
• Under different welding conditions, the yield strength and tensile strength of the weld seam were basically similar. The yield strength of the weld joints was slightly lower than that of the BM, while the tensile strength was slightly higher than that of the BM. In addition, when the laser power increased from 4.7 kW to 5.2 kW and the welding speed decreased from 20 mm/s to 15 mm/s, the elongation after fracture decreased by about 40%, which was significantly lower than that of the BM. The micro-Vickers hardness of laser-welded joints was significantly higher than that of the BM, while the impact toughness was much lower.