Study on the Microstructure and Properties of TC4 Alloy Based on Water-Jet-Guided Laser Technology

Abstract

Ti–6Al–4V (TC4) dual-phase titanium alloy is widely used in aerospace components owing to its excellent strength-to-weight ratio and high-temperature stability. However, conventional machining often generates a wide heat-affected zone (HAZ) and oxide or recast layers, which deteriorate the microstructure and reduce long-term reliability. In this study, the water-jet-guided laser (WJGL) process was applied to investigate how coupled laser–water interactions influence the groove morphology, elemental distribution, and crystallographic evolution of TC4 alloy. Under optimized parameters, the WJGL process reduced the HAZ width to less than 1 $\mathsf{\mu }$m, effectively removed the resolidified layer, and suppressed surface oxidation. SEM, EDS, and EBSD analyses confirmed that the $\alpha$ + $\beta$ dual-phase structure remained stable, with no significant phase transformation or grain coarsening. Compared with conventional laser cutting, WJGL achieved smoother surfaces, improved interfacial integrity, and reduced thermal damage. These findings highlight the potential of WJGL for precision machining of high-performance titanium alloys and provide theoretical and experimental support for enhancing the microstructural control and service reliability of aerospace TC4 components.

1. Introduction

Turbine blades, essential components of high-efficiency gas turbines, operate under extreme conditions exceeding 1200 °C and high mechanical stress [1,2]. Such harsh environments demand materials with high specific strength, oxidation resistance, and thermal stability. Among the candidate materials, TC4 alloy (Ti–6Al–4V) stands out due to its excellent balance of mechanical and chemical properties [3]. The $\alpha$-Ti phase, with a hexagonal close-packed (HCP) structure, provides superior strength and corrosion resistance, whereas the $\beta$-Ti phase, with a body-centered cubic (BCC) structure, enhances the alloy’s plasticity and machinability [4,5]. This $\alpha$ + $\beta$ dual-phase microstructure enables TC4 to maintain high strength, ductility, and oxidation resistance at elevated temperatures, making it an ideal material for turbine blades and compressor disks in aerospace engines.

However, the same characteristics that ensure its high-temperature performance—low thermal conductivity and strong chemical reactivity—also make TC4 extremely difficult to machine. During processing, excessive heat input can easily lead to the formation of large HAZ, recast layers, and microstructural degradation, ultimately reducing component reliability. Therefore, it is essential to develop machining technologies that can minimize thermal damage while maintaining the structural integrity of the alloy.

Despite its superior intrinsic properties, conventional machining methods such as Electrical Discharge Machining (EDM) and laser cutting still face significant challenges when applied to TC4 [6,7]. These techniques often introduce excessive heat, producing wide HAZs that cause surface deformation, cracking, and undesired microstructural transformations [8]. In particular, laser-based cutting frequently results in the formation of a thick recast layer, whose thickness and morphology greatly influence surface quality. The presence of this recast layer not only lowers the corrosion resistance of the alloy but also promotes grain coarsening due to repeated thermal cycling, thereby degrading the strength and toughness of the machined region [9,10]. Josef et al. used EDM with a high peak current (29 A) to modify the surface of TC4 alloy. SEM and EDX analyses revealed a martensitic surface layer, an HAZ, and carbon-rich residues. The EDM-treated samples showed poor fatigue performance, largely independent of the initial microstructure [11]. Yang et al. conducted an experimental and numerical study on the formation of the HAZ in TC4 alloy plates under laser heating. The parametric analysis showed that increasing the laser power enlarged the depth and width of the HAZ, while higher scanning speed or larger laser spot diameter reduced it [12]. Reck et al. investigated the HAZ and fatigue behavior of ($\alpha$ + $\beta$) Ti–6Al–4V–ELI alloy after laser cutting. A martensitic ${\alpha }^{\prime }$ phase formed in the HAZ, differing from the original equiaxed $\alpha$ + $\beta$ structure. Barrel grinding removed about 50% of the HAZ, while polishing completely eliminated it. Unoptimized laser cutting caused molten droplets and high roughness, reducing fatigue life, whereas optimized parameters effectively minimized the HAZ and improved fatigue performance [13]. Overall, although conventional methods are widely used for TC4 machining, thermally induced microstructural damage and recast layer formation remain major obstacles, especially under high-temperature or cyclic service conditions, severely limiting the long-term reliability of TC4 components. Thus, further optimization of machining techniques is crucial to improve both processing precision and material performance.

Recent technological progress has led to the development of several advanced cutting methods aimed at improving machining precision and reducing thermal damage. Among them, femtosecond laser cutting technology has attracted considerable attention for its ability to minimize the HAZ. The ultrashort pulse duration rapidly heats the material to a high temperature and induces instantaneous vaporization, enabling extremely precise cutting of microstructures that require high dimensional accuracy [14]. However, the high equipment cost and low processing speed of femtosecond lasers restrict their large-scale industrial applications [15].

To overcome these limitations, hybrid methods such as laser-assisted water-jet cutting have been proposed. This technique combines the localized heating capability of the laser with the cooling and cleaning effects of the water jet. During processing, the laser first melts the surface of the material, and the subsequent high-speed water jet rapidly cools the molten region and removes debris. This approach significantly reduces the overall heat input, thereby minimizing thermal damage and improving cutting efficiency and surface quality [16,17]. Nevertheless, the system’s high complexity and cost still limit its widespread adoption in industrial settings.

In contrast, conventional water-jet cutting operates as a purely non-thermal process, generating minimal heat and effectively avoiding thermal deformation and surface cracking common in laser or EDM machining [18]. It also helps maintain the stability of the original microstructure and prevents grain coarsening caused by repeated thermal cycles. However, its limited cutting precision and relatively low processing speed hinder its efficiency, especially when machining thick or hard materials [19].

In summary, although traditional and hybrid cutting techniques have improved the surface quality of TC4 alloys to some extent, they still face difficulties in maintaining microstructural stability and controlling thermal effects [20]. To further enhance machining precision and microstructural control, WJGL technology has emerged as a promising alternative [21]. By integrating the laser’s energy concentration with the water jet’s cooling and constraint effects, WJGL effectively minimizes the HAZ and suppresses grain coarsening induced by high temperatures. It also improves the surface flatness and reduces recast layer formation, thereby preserving the alloy’s superior mechanical and chemical properties [22,23]. Zhang et al. studied the jet stability in WJGL machining of hard and brittle materials. They found that high-pressure gradients at sharp corners caused laser divergence and reduced accuracy. Replacing sharp corners with low-curvature arcs stabilized the jet and improved surface quality sixfold [24]. Liao et al. examined the surface mechanism of WJGL cutting on Ni-based superalloy. They identified recast crystals and amorphous oxides from melt and plasma solidification and observed substrate twinning caused by plasma shockwaves [25]. Zhang et al. used WJGL to cut LTCC ceramics. Optimized parameters yielded smooth grooves with minimal debris and high straightness, outperforming conventional laser cutting in precision and efficiency [26].

Although previous studies have achieved notable progress in optimizing WJGL processing parameters and improving cutting precision, research on the mechanisms of defect formation, thermal effects, and microstructural evolution during laser–water interaction in titanium alloys remains limited. In particular, there is a lack of systematic investigation into how the water jet influences the $\alpha$ + $\beta$ dual-phase stability and HAZ characteristics of TC4 alloy.

Therefore, this study systematically investigates the microstructural features and defect evolution behavior of TC4 alloy after WJGL processing. Through comprehensive analyses of surface morphology, grain distribution, and elemental composition, the work elucidates the water-mediated mechanisms that preserve microstructural integrity and suppress HAZ expansion, providing theoretical insight and technical guidance for the precision WJGL machining of dual-phase titanium alloys.

2. Materials and Methods

2.1. Material

The material used in this experiment was a TC4 alloy sheet, which is widely employed in critical components such as aerospace engine turbine blades due to its excellent physical properties at high temperatures (Figure 1). As shown in the chemical composition of the TC4 alloy (

Table 1. Chemical composition of TC4 titanium alloy.

Element	Ti	Al	V	Fe	Si	C	N	O
Mass fraction (%)	Bal.	6.00	4.10	0.30	0.10	0.10	0.05	0.015

), vanadium enhances the alloy’s strength and high-temperature stability, whereas aluminum improves its oxidation resistance.

2.2. Experimental Details

The WJGL experimental platform used in this study employed a laser–water-jet composite energy conduction system, as shown in Figure 2. It mainly comprised a Nd:YAG laser, a high-pressure water jet module, a laser–water coupling system, an annular protective gas unit, and a three-dimensional motion stage. Filtered and deionized water was pressurized to 22.2 MPa, which was identified as the optimal condition to maintain stable jet flow, efficient cooling, and continuous laser–water coupling. Under this condition, the jet remains steady and continuous, ensuring a uniform laser–material interaction zone.

The focused laser beam enters the coupler and combines with the high-speed water stream (nozzle diameter: 70 $\mathsf{\mu }$m). Due to the refractive index difference between water and air, the beam propagates through the jet via total internal reflection, forming a “water-fiber” transmission mechanism without a focal length [27,28]. This mechanism enables simultaneous energy delivery, cooling, and debris removal, thereby minimizing heat input and suppressing the formation of the HAZ. An annular protective gas system surrounding the nozzle reduces air disturbance and oxygen interference, further stabilizing the jet [29]. Preliminary tests determined an effective power range of 5–35 W, within which stable machining and effective material removal were achieved.

The specimen was fixed on the platform using a clamping plate, while the three-dimensional motion stage enabled precise control of the machining path. The main parameters of the system are listed in

Table 2. WJGL equipment parameters.

Parameter	Value
Wavelength (nm)	532
Laser power (W)	0–40
Repetition rate (kHz)	10
Water pressure (MPa)	22.2
Feed rate (mm/s)	0–70
Nozzle inner diameter ($\mathsf{\mu }$m)	70
Spot diameter ($\mathsf{\mu }$m)	70
Flow rate of water (mL/s)	0.6–0.7
Fluence (J/${\mathrm{cm}}^2$)	0–2.1 × ${10}^4$
Intensity (W/${\mathrm{cm}}^2$)	0–3.9 × ${10}^6$

.

2.3. Microstructural Characterization

A series of characterization techniques were employed to analyze the surface and subsurface microstructures of the TC4 alloy after WJGL processing. The groove morphology was examined using metallographic microscopy, while scanning electron microscopy (SEM) was applied to analyze surface and cross-sectional features. Electron backscatter diffraction (EBSD) was used to assess grain structure and orientation, and energy-dispersive X-ray spectroscopy (EDS) was employed to evaluate elemental distribution and interfacial composition. All analyses were conducted under controlled laboratory conditions to ensure data accuracy and reproducibility.

3. Result and Discussion

3.1. Metallographic Analysis of Grooves

The groove morphology can directly reflect, to a certain extent, the effects of the water-guided laser during processing and its interaction with the material surface. The laser power used in this study ranged from 5 W to 35 W, which can be divided into a low-power range (5–15 W) and a high-power range (25–35 W). As can be seen from Figure 3a, when the laser power was 5 W, the groove wall was curved and non-vertical, there was incompletely removed molten material, the bottom of the groove was irregularly “V”-shaped, and there was no obvious HAZ at the edge of the groove. As the laser power increased to 15 W, as shown in Figure 3b, the groove wall tended to be vertical, and the bottom of the groove was gradually changed from an irregular “V” shape to a flat “—” shape, and cracks appeared at the edge of the groove as well as at the HAZ. The measurement results indicated that under that power condition, the width of the HAZ was approximately 2–4 $\mathsf{\mu }$m, suggesting limited thermal impact. Although the stability of the $\alpha$ and $\beta$ phases was largely preserved, partial melting and incomplete material removal still caused a slight reduction in machining accuracy.

Under higher-power conditions (25–35 W), as shown in Figure 3c,d, the groove walls became more vertical and well defined, while no significant HAZ was observed. This time, the width of the HAZ was less than 1 $\mathsf{\mu }$m and could almost be neglected. This indicates that the increased energy input under water-jet confinement did not induce excessive thermal damage but rather improved machining precision. According to the equation $I=P/A$ (with a spot diameter of 70 $\mathsf{\mu }$m, corresponding to an area of $A=3.85\times {10}^{-5}{\mathrm{cm}}^2$), the calculated laser intensity ranged from approximately $1.3\times {10}^5$ to $9.1\times {10}^5$ W/${\mathrm{cm}}^2$. Within that range, a moderate increase in laser power effectively improved the groove morphology and reduced the formation of resolidified material [30], indicating that controlled energy input is essential for achieving high precision and minimizing thermal effects during WJGL processing.

Small protrusions (tips) were observed at the bottom of each groove during the experiments, as consistently shown in all subfigures (Figure 3a–d). This phenomenon occurs because WJGL involves not only the thermal removal of material but also plasma explosion-assisted ablation [31]. During groove formation, the laser continuously ablates the material, releasing plasma that rapidly accumulates within the confined region. The sudden expansion and outward ejection of this plasma generate shock waves that contribute to the observed tip formation. The plasma explosion also influences the groove morphology, which is inevitable; however, its impact is much weaker than that of thermal removal. Nevertheless, the combined action of plasma-assisted ablation enhances the overall machining efficiency compared with conventional laser ablation.

3.2. Micro-Morphology Analysis of Water-Guided-Laser-Cut Grooves

As shown in Figure 4a, the cross-section of the slit after WJGL processing appears relatively flat. However, localized irregularities can be observed near the slit edge, and a higher-magnification image (Figure 4b) reveals distinct residues of the base material that were not completely removed in this region. The formation mechanism is closely related to the non-vertical groove walls and melt residues observed in the macroscopic morphology. When the laser power is low or the energy density is unevenly distributed, the local regions cannot fully absorb sufficient energy to reach complete melting. As a result, part of the molten material is not promptly expelled by the water jet and remains on the processed surface, leading to irregular “V”-shaped grooves and a noticeable reduction in machining accuracy. Such a decline in machining precision could compromise the dimensional tolerance and surface integrity required for aerospace components, potentially affecting their assembly accuracy and long-term operational reliability. As shown in Figure 4c,d, when the laser power increased to 15 W, the slit sidewalls of the TC4 duplex alloy exhibited good straightness and uniform morphology. No significant recast buildup or cracks were observed, indicating excellent processing quality and stable microstructure. These results suggest that an effective thermo-mechanical synergistic mechanism was established between the laser beam and the high-speed water jet under WJGL processing [32]. This synergy enhances the rapid discharge of molten material and suppresses its redeposition, resulting in a more uniform microstructural distribution and a highly controllable groove morphology with improved machining precision at the macroscopic scale. Such improvement in machining precision and microstructural uniformity is particularly valuable for aerospace applications, where even minor surface defects or geometric deviations can significantly affect aerodynamic performance and fatigue resistance. As shown in the magnified view of Figure 4d, the $\alpha$ and $\beta$ phases in the base material still coexist distinctly, with no evident change in their distribution or morphological boundaries. This indicates that the WJGL process did not induce any noticeable phase transformation. This microstructural stability can be attributed to two primary factors. First, the water jet serves as an efficient cooling medium that continuously removes heat during laser irradiation, thereby suppressing the temperature rise and minimizing the extent of the HAZ. This prevents the microstructure from entering the high-temperature $\beta$-phase region and undergoing drastic phase transformations. Second, the stable and moderate laser energy input avoids excessive heat accumulation, keeping the localized temperature below the critical $\beta$-phase transition threshold of approximately 882 °C [33]. In addition, TC4 is a thermally stable dual-phase titanium alloy that exhibits inherent resistance to structural degradation. The distributions of its $\alpha$ and $\beta$ phases possess a certain degree of thermal tolerance, and in the absence of severe thermal cycling, the microstructure remains stable with minimal phase boundary migration or reconstruction.

Compared with the ideal slit quality shown in Figure 4c,d, the regions illustrated in Figure 4e–h exhibit noticeable degradation in the microstructure and surface morphology as the laser power further increases. As shown in Figure 4e,f, the sidewalls of the slit exhibit a pronounced recast layer, and the surface morphology becomes uneven, displaying irregularly attached structures. Notably, the recast layer is not continuously or metallurgically bonded to the base material. A distinct interfacial gap can be observed, within which a weak bonding zone is formed during processing. The formation of this region is closely associated with heat transfer, phase transition behavior, interfacial tension, and the rapid solidification process. Under high laser energy, the local base material rapidly melts to form a molten pool, which is then rapidly solidified by the cooling effect of the water jet. Ideally, if the molten pool and substrate achieve sufficient wetting, diffusion, and crystallization [34,35], a continuous metallurgical bonding zone can be established. During this process, crystal growth [36] and elemental diffusion within the recast layer may lead to a loose microstructure, coarse grains, and even the formation of nonequilibrium phases. However, when the laser power is excessively high, the local temperature can far exceed the liquidus line, causing the molten metal in the pool to possess high kinetic energy and surface tension, which results in insufficient wetting of the substrate [37]. In addition, rapid cooling may prevent the molten metal from fully diffusing into the substrate, while the formation of a thin oxide film on the substrate surface can further hinder wetting. These factors collectively inhibit the solid–liquid metallurgical reaction, causing the resolidified recast layer to “float” on the substrate surface after solidification, leaving a distinct interfacial gap. Secondly, although the rapid cooling effect of the water jet significantly reduces the extent of the HAZ, it may also inhibit crystal growth and elemental diffusion within the recast layer, leading to a loose microstructure, coarse grains, and even the formation of nonequilibrium phases. The rapid solidification process results in an indistinct phase boundary, which further weakens the interfacial bonding force at the solid–liquid interface. Consequently, even when the recast layer is in physical contact with the substrate, it essentially behaves as a “weak bonding zone”. This interfacial gap is characterized not only by grayscale contrast and blurred microstructural boundaries but also by its potential to compromise the overall service performance of the material in engineering applications. This region may serve as a preferential pathway for corrosive media and a site for stress concentration, promoting interfacial delamination and the initiation of fatigue cracks. Under high-temperature, cyclic loading, or corrosive environments, such defects can significantly reduce the fatigue life of the material. Therefore, this weak interface must not be overlooked, especially when TC4 is used in high-reliability applications such as aerospace structural components and medical implants.

As shown in Figure 4g,h, the characteristics of the recast layer become more pronounced. Its thickness increases, the surface morphology becomes rougher, and localized bulges and melting traces appear. These phenomena can be attributed to the increase in energy density caused by the higher laser power. Excessive local melting expands the volume of the molten pool, while the instantaneous cooling capacity of the water jet cannot fully match the elevated thermal load. As a result, part of the molten material is not removed in time and resolidifies on the surface. At the same time, the rapid phase transition induced by the laser–water interaction generates intense thermal stresses [38]. These stresses promote the formation of cracks and pores within the recast layer and weaken its metallurgical bonding strength with the base material. This structural feature is particularly detrimental to dual-phase alloys such as TC4, whose superior performance relies on the uniform distribution and continuity of the $\alpha$ and $\beta$ phases. The formation of the recast layer disrupts this balance, leading to local heterogeneity in phase distribution and reduced overall microstructural integrity. Although TC4 exhibits excellent thermal stability up to 350 °C [39] and oxidation resistance up to 800–900 °C, its localized heat tolerance threshold (approximately 950–1000 °C) [40,41] may still be exceeded when the laser power is outside the optimal range (5–35 W), even with the aid of water jets, leading to localized microstructural destabilization [42]. Meanwhile, the recast layer is often accompanied by grain coarsening and blurred phase boundaries, which pose potential risks to the fatigue strength, corrosion resistance, and creep properties of the material. As shown in Figure 4e–h, excessive laser energy leads to a noticeable degradation in the microstructure and interfacial integrity of the cutting zone, particularly manifested by the formation of the recast layer and the appearance of interfacial gaps. These observations highlight the critical importance of precise parameter control during WJGL processing of TC4 alloys. These findings are of great significance for deepening the understanding of the material’s microscopic response mechanisms and for enhancing the microstructural continuity within the processed region.

3.3. Trench Edge Elemental Analysis

As shown in Figure 5a, the cross-sectional SEM image clearly reveals the microstructural distribution among the base material, transition zone, and recast layer. The left side corresponds to a dense and homogeneous base material region exhibiting excellent structural stability. Moving toward the right, a distinct transition zone (approximately 70–80 µm in width) appears between the base material and the recast layer. This zone displays a weakly bonded interface, microstructural variations, and partial interfacial discontinuities. The recast layer is located on the right side of the transition zone and exhibits a fine, continuous structure with a brightness contrast relative to the base material. The presence of a weakly bonded interface between the recast layer and the substrate indicates the existence of metallurgical bonding defects in this region, which can adversely affect the overall interfacial integrity of the material. Such weakly bonded zones may act as potential sites for stress concentration and crack initiation, reduce load transfer efficiency across the interface, and accelerate fatigue crack propagation under cyclic loading. In addition, these interfacial discontinuities can facilitate the ingress of oxygen or corrosive media, ultimately degrading the long-term mechanical reliability and corrosion resistance of aerospace components operating under complex thermal and mechanical environments. In Figure 5b–e, the elemental distributions of Ti, O, Al, and V across the cross-section are presented. As shown in Figure 5b, Ti exhibits the highest concentration in the base material region. Along the scanning line (“line data 1”), the Ti signal gradually decreases toward the recast layer, with a pronounced drop observed at approximately 70–80 $\mathsf{\mu }$m. This indicates that Ti migrates and becomes depleted near the interface as a result of localized melting and subsequent re-solidification. The O element (Figure 5c) is almost uniformly distributed within the base material. Once entering the weakly bonded region, its concentration rises sharply, and beyond 80 $\mathsf{\mu }$m in the recast layer, the oxygen content increases significantly. This enrichment reflects enhanced oxidation induced by localized melting and exposure to the ambient environment during WJGL processing. Even though a protective gas environment was employed during machining, indicating that transient oxidation reactions still occurred at high local temperatures where water and vapor interactions were present. In contrast, the Al (Figure 5d) and V (Figure 5e) signals also show a gradual decrease along the scan line toward the recast layer, indicating that both elements underwent partial depletion near the interface. However, compared with O and Ti, the degree of depletion for Al and V is relatively smaller, suggesting that their diffusion behavior is more limited under the thermal influence of WJGL. Such enhanced interfacial uniformity and compositional stability are highly beneficial for aerospace applications, as they improve the structural integrity and service reliability of critical components—particularly turbine blades and load-bearing frames that operate under high temperature, vibration, and oxidation-prone environments.

Figure 6 presents the line-scan profiles of O, Ti, Al, and V elements. Figure 7 shows the types and relative contents of elements in the recast layer, where oxygen (O) exhibits the highest peak, indicating that the recast layer is mainly composed of oxides, with a small number of carbides also present. The Ti curve remains smooth and stable within the 0–70 $\mathsf{\mu }$m range but drops sharply after entering the 70–80 $\mathsf{\mu }$m region. Beyond 80 $\mathsf{\mu }$m, the Ti signal in the recast layer decreases markedly. In contrast, the O curve rises rapidly at approximately 80 $\mathsf{\mu }$m, forming a distinct oxygen-rich zone. This sharp compositional transition at the interface highlights the localized oxidation effects induced during WJGL processing. The O curve rises sharply at approximately 80 $\mathsf{\mu }$m, forming a distinct oxygen-rich region that indicates a sudden compositional change at the interface caused by localized oxidation. This feature also correlates well with the weakened interfacial microstructural continuity observed in the SEM analysis.

The Al and V elements, which are the original constituents of the base material, can also be detected in the recast layer, indicating that these elements were partially retained during the melting and re-solidification process of WJGL machining. Taken together, the elemental distributions and line-scan profiles are consistent with the morphological characteristics shown in Figure 5a–e. They indicate that compositional redistribution and selective oxidation at the interface, caused by localized high temperatures and rapid cooling during WJGL cutting, are the fundamental origins of the weakly bonded zones. However, compared with conventional laser cutting, the oxide layer thickness and compositional fluctuation at the interface are more controllable in WJGL cutting due to the protective and cooling effects of the water film [43].

This mechanism effectively suppresses excessive oxidation and severe grain coarsening, resulting in a more continuous and compact recast layer with improved interfacial quality. Consequently, the process provides strong support for enhancing the overall machining quality and interfacial performance.

3.4. Effects of Water-Guided Laser Processing on Crystal Orientation and Morphology

Figure 8 shows the crystallographic features of the TC4 duplex alloy after WJGL processing in the slit region. As observed in the figure, the grain structure near the slit edge exhibits noticeable variations: several coarse grains are located adjacent to the slit, surrounded by numerous finer grains of smaller size, forming a typical mixed microstructure characterized by alternating coarse and fine grains. This phenomenon contrasts with the initial equiaxed lath-like grains in the unprocessed base material region on the left, which are uniformly oriented and arranged in an orderly manner. During WJGL processing, the high energy density of the laser beam generates an instantaneous high-temperature molten zone on the material surface. The water jet rapidly removes heat through its strong thermal conductivity, creating a rapid cooling environment. This coupling of “high temperature and rapid cooling” disrupts the initial thermal equilibrium, resulting in a steep thermal and stress gradient within the local region. Under these conditions, the base material experiences steep thermal and stress gradients. Grains in the HAZ near the slit may undergo partial remelting and re-solidification, leading to the disturbance, disintegration, and migration of the original grain boundaries. These processes promote recrystallization and result in the formation of high-density fine grains. The formation of coarse grains may be attributed to slightly higher local heat input and deeper melting induced by the laser, which accelerate atomic diffusion and promote rapid grain growth during subsequent solidification. In addition, the thermal input during WJGL processing and the impact of the water jet provide dynamic conditions that drive dislocation slip, twin formation, and grain boundary migration and reconstruction. The accumulation of in situ strain energy, together with high-temperature relaxation under short-duration heating, facilitates the transformation of subcrystalline structures and the rearrangement of the grain boundary network, ultimately promoting the formation of fine grains. This phenomenon is often reflected in the “dynamic recrystallization” (DRX) behavior in heat treatment [44,45], which also indirectly indicates that although WJGL processing is a non-traditional means of processing, its microstructural evolution of the material has a similar metallurgical process to control the ability.

On the other hand, TC4 alloy is a dual-phase titanium alloy mainly composed of $\alpha$-phase (hexagonal close-packed, HCP) and $\beta$-phase (body-centered cubic, BCC) at room temperature. Localized heating during WJGL processing may trigger the $\alpha \to \beta$ phase transformation and form metastable or unstable phases. Subsequent rapid cooling by the water jet prevents some high-temperature $\beta$ phases from fully transforming back to the equilibrium $\alpha$ phase, resulting in the formation of nonequilibrium residual structures. This metastable structure is often characterized by blurred grain orientations and diffuse boundaries in EBSD analyses. However, in this figure, no extensive phase transformation zone or continuous recast layer is observed, indicating that the WJGL process maintained good control over the thermal influence range. This observation is consistent with the results shown in Figure 3 and Figure 4.

At the service performance level, grain refinement is generally recognized as beneficial for enhancing the strength, hardness, and fatigue resistance of materials. In particular, under high-temperature operating conditions, finer grains contribute to improved thermal stability and creep resistance [46]. In contrast, the reduction in grain boundary density within coarse-grained regions offers certain advantages in terms of high-temperature creep and oxidation resistance, although it may lead to a slight decrease in strength [47]. Therefore, the mixed structure of coarse and fine grains observed in Figure 8 not only reflects the synergistic effect of the local thermal–stress field but may also serve as a complementary mechanism during actual service, providing a multiscale adjustment capacity for enhancing the overall performance of the material. Figure 8c shows that the crystallographic structure is stable and the internal organization is complete. However, near the edge of the slit, the grain color distribution becomes more scattered, exhibiting frequent color variations and signs of orientation disturbance. This indicates a diversification of the crystallographic texture, with locally interlaced grains appearing in multiple directions, in stark contrast to the stable and uniform fabric observed in the base material region. Such orientation perturbations originate from the localized melting behavior induced by the transient high heat input of the laser during the WJGL process. Subsequently, the strong cooling effect of the water jet promotes rapid solidification of the molten zone, leading to the formation of fine grains with discontinuous orientation and a typical disordered recrystallization structure. Further observations reveal that, in addition to a large number of newly formed fine grains at the edge of the slit, several larger equiaxed grains are also embedded among them, forming a mixed microstructure characterized by the interlacing of coarse and fine grains. This observation is highly consistent with the grain size distribution shown in Figure 8a,b, indicating that the local grain organization in the HAZ of the slit has undergone a complex thermally coupled evolution. On one hand, the formation of fine grains reflects the rapid solidification and recrystallization processes induced by the synergistic action of the WJGL. On the other hand, the appearance of coarse grains may result from localized grain growth caused by excessive heat input.

From the distribution of black and red–green grain boundaries in Figure 8c, it can be seen that the density of both high-angle and low-angle grain boundaries increases near the edge of the slit. This observation further confirms that the grains in this region undergo complex behaviors such as grain boundary migration, dislocation slip accumulation, and subgrain structural reconfiguration under the combined effects of thermal stress and strain fields.

The comprehensive BC and IPF diagrams in Figure 8c illustrate the microstructural evolution characteristics of the WJGL-processed region. It can be observed that although WJGL processing induces a certain degree of grain refinement and orientation disturbance near the edge of the slit, these changes are confined to a narrow HAZ. The overall crystallographic orientation continuity and grain boundary connectivity between the base material and the slit remain largely preserved, with no extensive phase transformation zones or recast structures observed. This “local perturbation–global stability” feature of the microstructure reflects the result of efficient heat control and strong cooling synergy during WJGL processing. Compared with conventional laser processing, which often causes deep and uncontrollable thermal damage, WJGL demonstrates a clear advantage in maintaining the integrity of the base material and suppressing the expansion of overheated regions.

The microstructural comparison between the unprocessed region and the WJGL-cut edge is shown in Figure 9. Figure 9a corresponds to the unprocessed base material, while Figure 9b shows the cut edge region after WJGL processing. The grain orientations in the inverse pole figure (IPF) maps of the two regions exhibit a high degree of spatial consistency and are both concentrated in specific crystallographic directions. This indicates that the overall crystallographic structure of the material remains largely intact, despite the combined effects of laser thermal input and water-jet cooling. It is worth noting that in Figure 9b, corresponding to the region near the slit edge, the color in the pole figure appears noticeably lighter than that of the base material. This reduction in pole intensity reflects a more dispersed grain orientation distribution and a weakening of the crystallographic texture strength in this region. This phenomenon indicates that although the main crystallographic orientation remains largely unchanged, certain local regions experience a degree of orientation relaxation and texture weakening. The mechanism behind this slight de-weaving effect can be attributed to two factors. First, during WJGL processing, the high-temperature molten zone formed by the laser beam on the material surface—despite being rapidly cooled by the water jet—may still trigger brief remelting and resolidification of some grains, thereby disturbing the original texture structure. Second, the orientation shifts induced by the coupled thermal–stress field should not be overlooked, particularly in the case of TC4 alloys, where the $\alpha$ and $\beta$ phases exhibit distinct responses to thermal gradients and residual stress. In such $\alpha$ + $\beta$ dual-phase materials, the differences in thermal conductivity and thermal expansion coefficient between the two phases easily lead to stress accumulation at the phase boundaries during processing, resulting in slight orientation deflection and local misorientation within the grains. This thermally induced texture perturbation behavior is highly consistent with the previously observed diversity in grain colors and scattered orientations in the IPF maps, providing additional evidence for the occurrence of local grain recrystallization. However, judging from the overall morphology of the IPF map, no new significant orientation clusters are formed in the processed region, and no randomization of the grain structure is observed. This indicates that the degree of crystallographic perturbation induced by WJGL processing remains effectively controlled within an acceptable range. In particular, compared with conventional laser processing—where the fabric is often severely disrupted—WJGL processing demonstrates a distinct advantage in maintaining both the continuity of the crystallographic texture and the stability of the microstructure. This “domain-limited control” of grain orientation perturbation at the microscopic scale not only demonstrates the capability of WJGL processing to precisely confine the HAZ but also provides a structural foundation for enhancing the long-term service stability of the processed components.

4. Conclusions

In this study, the effects of WJGL processing on the microstructural evolution of TC4 titanium alloy were systematically investigated. The results demonstrate that WJGL processing effectively suppresses the formation of the HAZ and recast layer, achieving precise control of thermal input and microstructural stability. Under optimized conditions, the HAZ width was limited to less than 1 $\mathsf{\mu }$m, confirming that the strong cooling effect of the water jet significantly reduces thermal accumulation and mitigates laser-induced damage. Elemental line-scan analysis revealed clear compositional redistribution at the interface, with Ti and O exhibiting pronounced gradients near 70–80 $\mathsf{\mu }$m, while Al and V showed only slight depletion, indicating limited diffusion and restrained oxidation under the confined thermal field of the WJGL. EBSD characterization further confirmed that although fine-grain formation and orientation perturbations occur locally near the slit edge, the overall crystallographic continuity and phase stability of the base material remain well preserved. These findings highlight the “local perturbation–global stability” feature of WJGL processing, where the synergy between laser heating and water-jet cooling ensures balanced energy input, prevents excessive grain growth, and enhances interfacial bonding quality. Overall, WJGL processing offers a highly controllable and thermally efficient approach for improving the machining quality and long-term structural reliability of TC4 titanium alloys, providing both theoretical and experimental support for its application in high-performance aerospace components.