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Article

Effect of In-Situ Pulse Current on Microstructure and Mechanical Properties of AISI 9310 Gear Steel by Laser Powder Directed Energy Deposition

by
Cenchao Xie
1,2,
Fei Yang
1,
Peng He
3,
Wenfa Liu
4,
Qiang Feng
5,
Liucheng Zhou
1,3,*,
Ping Liu
6 and
Xin Sun
1
1
National Key Lab of Aerospace Power System and Plasma Technology, Air Force Engineering University, Xi’an 710038, China
2
Army Aviation Military Representative Office in Tianjin, Tianjin 300000, China
3
School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an 710049, China
4
Army Aviation Military Representative Office in Chengdu, Chengdu 610043, China
5
State-Owned Jinjiang Machine Factory, Chengdu 610043, China
6
Institute of Aviation, Chongqing Jiaotong University, Chongqing 400074, China
*
Author to whom correspondence should be addressed.
Machines 2025, 13(4), 308; https://doi.org/10.3390/machines13040308
Submission received: 13 March 2025 / Revised: 2 April 2025 / Accepted: 7 April 2025 / Published: 10 April 2025
(This article belongs to the Section Advanced Manufacturing)
Browse Figures
Versions Notes

Abstract

AISI 9310 gear steel, renowned for its high hardenability, is widely employed in the manufacturing of aerospace gear components. Laser powder directed energy deposition (LP-DED) takes advantage of a laser heat source to melt metal powder, thus creating a molten pool and facilitating the quick achievement of material deposition and shaping. However, the issue of forming quality has been acting as a significant constraint on the development of LP-DED. To address this concern, the present research endeavors to enhance LP-DED by leveraging the assisted application of in situ pulsed current, with the aim of preparing high-quality deposited specimens. It has been observed that the pulsed current does not trigger any phase transformation within the deposition zone. Instead, the Joule heating effect brought about by the current serves as a catalyst for grain growth. Meanwhile, the electric-plastic effect of the pulsed current results in an elevation of plastic deformation. Moreover, it facilitates the transformation of dislocation defects from simple dislocation lines to intricate dislocation networks, consequently leading to a substantial increase in dislocation density. Furthermore, the contraction force induced by the current exerts a compressive influence on the molten pool, which in turn accelerates the discharge of gas.

1. Introduction

Gear components are prone to failure mechanisms such as wear and tooth root fracture [1], making gear repair and manufacturing crucial in aviation applications. AISI 9310 gear steel, a high-strength alloy characterized by exceptional hardenability, tensile strength, and toughness [2], presents significant manufacturing challenges. Conventional gear production involving rigorous cutting and heat treatment processes results in high costs, extended production cycles, and difficulties in broken tooth repair. Additive manufacturing technology offers a promising solution for direct gear fabrication, enabling cost reduction and enhanced production efficiency through integral molding [3].
Laser powder directed energy deposition (LP-DED) as a metal AM process utilizing high-energy laser beams to melt and solidify metal powder into designed geometries [4,5], demonstrates particular advantages for manufacturing complex gear components. While LP-DED eliminates the need for post-processing heat treatment, its deposition quality and mechanical properties remain inferior to conventionally manufactured gears due to limitations in process parameters including laser beam characteristics, powder feeding rate, material quality, and scanning speed [6].
To address these technical constraints, researchers have proposed various energy field-assisted enhancements for LP-DED [7], including ultrasonic [8], electromagnetic [9], and thermal field optimizations [10]. Zhai et al. [11] demonstrated that alternating current application during laser cladding of Ni-Cr-B-Si coatings significantly influenced microstructural characteristics, elemental distribution, crack formation, and hardness without altering phase composition. Xu et al. [12] successfully implemented pulsed current-assisted laser cladding with follow-up feed to produce crack-free Ni60A nickel-based alloy coatings on 42CrMo alloy, leveraging the crack tip flow and Joule heating effects. Wang et al. [13] investigated electric pulse effects on Ti-6Al-4V alloy, observing microstructural evolution from α’ martensite to basketweave α structure with increasing voltage. Lu et al. [14] reported that electromagnetic fields could effectively transform surface tensile stresses into compressive stresses in DMD parts while modifying molten pool cooling rates. Despite these advancements, limited research exists on LP-DED of gear steel, particularly regarding substrate–powder material compatibility in repair contexts. Furthermore, most energy field applications have focused on AC electric or magnetic fields, which introduce thermal effects that complicate the isolation of individual field influences on deposition quality.
Prior studies on energy field-assisted laser powder directed energy deposition (LP-DED) primarily focused on alternating current/magnetic fields and thermal optimization, with limited exploration of in situ pulsed current effects—particularly for high-strength AISI 9310 gear steel. Current research lacks systematic investigations into three critical aspects: (1) dislocation network modification, (2) defect closure mechanisms, and (3) crystallographic texture evolution during LP-DED. The interplay between Joule heating and non-thermal electroplastic effects remains unresolved. This study presents a controlled experimental investigation of AISI 9310 gear steel fabrication using in situ pulsed current field-assisted LP-DED. Comprehensive characterization of microstructural and mechanical properties was conducted, with analysis of microstructure formation mechanisms supported by solid phase transformation theory. This research establishes correlations between microstructural variations and mechanical property enhancements, validating the improvements achieved through pulsed current assistance. These findings advance the industrial application potential of LP-DED for high-quality AISI 9310 gear steel production. The aim of this study is to assess the influence of in situ pulsed current on the microstructure, defect formation, and mechanical properties of AISI 9310 gear steel fabricated via laser powder directed energy deposition (LP-DED), with a focus on elucidating the interplay between Joule heating, electroplasticity, and dislocation dynamics.

2. Materials and Methods

2.1. Material

The substrate utilized in this study is a circular AISI 9310 steel plate, possessing a thickness of 5 mm and a diameter of 10 cm. The heat treatment process is detailed as follows: Carburization was initiated at a temperature of 920 °C. During the carburization stage, the carbon potential was maintained at 1.3%, while in the diffusion period, it ranged from 1.1% to 1.2%. Subsequent to the carburization process, tempering was conducted at 680 °C for a duration of 3 h. Quenching was then carried out within a carbon-protected atmosphere at a quenching temperature of 870 °C. Subsequently, a box-type electric furnace was employed for high-temperature tempering at 680 °C for three hours, followed by furnace cooling. Subsequently, the specimen was heated in a salt bath at 800 °C for 15 min and then cooled in oil. Finally, tempering was performed at 200 °C for 3 h, and the specimen was naturally cooled in flowing air. The chemical composition of the substrate is presented in Table 1. The powder utilized for deposition is AISI 9310 gear steel spherical powder (as depicted in Figure 1a–c), and the particle size statistics are illustrated in Figure 1d. The main chemical element composition (by mass fraction) of the powder includes 3.81% carbon, 0.08% silicon, 1.24% chromium, 3.74% nickel, 0.63% manganese, and 90.5% iron. Insulation was applied using FR4 water glass fiber board.

2.2. LP-DED Method

The in situ pulsed current-assisted LP-DED process is carried out on the LP-DED equipment (model XKZZ-P1-00) manufactured by Xi’an Kongtian Electromechanical Intelligent Manufacturing Co., Ltd. (Xi’an, China). To optimize the pulse current parameters, a series of pre-experimental trials were systematically conducted. We explored a wide range of current values, frequencies, and duty cycles, with the aim of avoiding excessive Joule heating (which causes an unstable melt pool and disrupts deposition) while ensuring sufficient current density for electroplastic effects, vital for enhancing the deposited material’s mechanical properties. The power supply utilized for generating the pulse current is the KX-GF200A/80V model (Zhongshan Kexiong Power Technology Co., Ltd., Zhongshan City, China), which is capable of outputting pulse current at a voltage of 30 V. The frequency of the pulse current is 100 Hz, the duty cycle is 15%, and the current intensity is 40 A. A 15% duty cycle was chosen to balance thermal accumulation and mechanical effects. High duty cycles cause thermal accumulation, leading to grain coarsening. So, we minimize it. Meanwhile, we maximize mechanical effects for dislocation activation, which improves material plasticity and densification. These settings also align with prior studies on pulsed current-assisted laser cladding (e.g., Xu et al. [12] and Zhai et al. [11]), where similar parameters enhanced defect closure without phase changes. During the process, the laser power is set at 1200 W, while the scanning rate is configured to be 10 mm/s. The diameter of the laser beam is 0.7 mm. The powder is fed at a rate of 0.5 g/min within an argon environment. The lapping rate of the laser beam amounts to 50%. For the laser beam scanning path, the method of cross-deposition layer by layer is adopted, and a total of 4 layers are designed for the gyratory powder layup. Stable output of the pulse current is ensured throughout the LP-DED process. Additionally, insulation measures are implemented to separate the substrate from the fixture, thereby guaranteeing the stability of the pulse current loop within the deposited sample.

2.3. Characterization Methods

A block sample with dimensions of 10 mm × 10 mm × 1.3 mm was deposited onto an AISI 9310 steel base plate having a thickness of approximately 5 mm. The XOZ cross section was characterized through line cutting along the middle position of the Y-axis (as depicted in Figure 2b). For the observation of the samples, an optical microscope (OM, specifically ZEISS-Axiocam 208, Carl Zeiss Microscopy GmbH, Jena, Germany) was employed to examine the morphology as well as the distribution of pores. Field emission scanning electron microscopy (FESEM, Quanta 250FEG, Huntington Beach, CA, USA) was utilized to characterize the microscopic morphology of both powders and specimens. Concurrently, to analyze grain orientation, texture evolution, and local misorientation (critical for understanding pulsed current’s mechanical effects), the accompanying electron back scatter diffraction (EBSD), with an analytical step size of 0.225 μm, was used for the characterization of crystal features. Moreover, the associated energy dispersive spectrometer (EDS) component was applied for elemental analysis. The hardness in the depth direction of the specimen was measured using a hardness tester (HXD-1000TMC/LCD, Taiming Inc., Shanghai, China). A loading force of 1000 g and a loading time of 15 s were adopted during the measurement process. At the same horizontal position, three measurements were taken at intervals of 100 μm, and the average value was regarded as the final result. Phase analysis was conducted by means of an X-ray diffractometer (XRD, Ultima IV (Rigaku Corporation, Tokyo, Japan)) at a scanning rate of 5°/s. X-rays were generated using a Cu target for this purpose. Transmission electron microscopy (TEM) can offer nanoscale perspectives on dislocation networks and stacking faults. These features play a pivotal role in establishing the connection between microstructural alterations and improvements in mechanical properties. The microscopic dislocation defect features were observed through transmission electron microscopy (TEM, JEOL 2100f, JOEL Ltd., Tokyo, Japan). The TEM specimens were prepared via the ion thinning method.

3. Results

3.1. Morphology and Pore Characterization

In order to explore the impact of pulsed current on the microstructure of LP-DED, experiments for conventional LP-DED and pulsed current-assisted LP-DED control groups were established, respectively. LP-DED directly prints samples with a thickness of approximately 1.3 mm onto the substrate. From an overall perspective, after disregarding the minor disturbance of the cross-section current density caused by the difference in specimen height, the steady pulse current in a single direction after depositing four layers of powder exerts an isotropic effect on the deposition process. This is due to the scanning path being rotated 90° layer by layer. The as-received samples do not display significant distinguishing morphological features when compared to the pulsed current-assisted deposition samples. The first–second powder deposit layers are located near the substrate. Owing to the influence of high supercooling and recrystallization, the bottom region undergoes multiple alternating thermal cycles from the subsequent deposited layers during the deposition process. This leads to the formation of a large number of equiaxed fine crystals in the deposition region near the boundary of the melting pool [15] (as illustrated in Figure 3b). The third–fourth powder is deposited on the basis of the first–second deposition area. As a result, the accumulated heat that fails to be released in a timely manner reaches a relatively high level. Coarse parallel columnar crystals or dendritic morphology are generated and preferentially grow along the direction of the temperature gradient in accordance with the epitaxial growth theory. The reduced temperature gradient G in the top deposition region causes the heat dissipation to last for a longer period, and the solidification process in the melt pool becomes more relaxed. During this slow solidification process, the grains have more time to grow coarser [16]. Random nucleation (stray grains) is significantly suppressed due to the substantial reduction in constitutive supercooling in the solidification process of the high-temperature molten pool [17]. Since the heat diffusion coefficient of steel is much larger than that of air, the laser energy leads to severe heat accumulation in the top region, which is the primary reason for the generation of columnar grains. Consequently, in the deposition direction from the first to the fourth layer, a gradual transition from equiaxed grains and polygonal grains to columnar crystals occurs, and the grain growth also presents an obvious gradient. A duty cycle of 15% was selected after optimizing the pulse current parameters. Thus, the Joule heat induced by the pulse current was rather limited compared to the molten pool temperature [18], and the resistivity [11] of the molten pool did not change significantly. This is the crucial reason why the grain evolution trend of the as-received samples is essentially the same as that of the pulsed current-assisted deposition samples.
Porosity has a significant impact on the mechanical properties and printing quality of the deposited samples. Porosity measurements were conducted on cross-sectional samples (XZ plane) using ImageJ software (the version number: 1.8.0_345). Five regions (140 μm × 80 μm) spanning the entire deposition thickness (1.3 mm) were analyzed to account for volume porosity. The uniform pore distribution observed across these regions (Figure 4) suggests that surface and volume porosity are consistent, as gas entrapment occurs homogenously during LP-DED solidification [19]. However, future studies will employ X-ray computed tomography (XCT) for 3D volumetric porosity quantification. Each of these regions was positioned in the middle of the Z-axis direction of the deposited sample and was evenly spaced at intervals of 2000 μm along the X-axis direction. It was observed that all samples exhibited a uniform pore dispersion without any significant enrichment. Compared to the as-received sample, the average porosity of pulsed current-assisted samples decreased from 0.307% ± 0.02% to 0.215% ± 0.02%, with a statistically significant reduction in pore count (p < 0.05, Student’s t-test). This can be attributed to the fact that when the current passes through the deposited sample, the deformable liquid-phase melt pool is compressed by the contraction force of the carriers moving towards the central axis [20,21]. As a result, the gas within the pores experiences a squeezing action, which greatly accelerates the escape of the gas from the bottom of the melt pool. Figure 5 indicates that LP-DED primarily contains two forms of pores, namely circular and elongated. The failure of gas to escape from the molten pool in a timely manner during the deposition process is the main reason for the existence of most circular pores with pore sizes not exceeding 1 μm. A small number of elongated pores, which are often more than 4 μm in length, represent a serious defect caused by insufficient powder melting or gas retention in the melt pool during solidification [22,23]. The reduction in porosity brought about by the pulsed current can be ascribed to two aspects. Firstly, the electromagnetic force generated by the pulsed current stirs the molten pool, which in turn enhances the powder fluidity and creates a tendency for crack-type pores to close. Secondly, the gradient accumulation of Joule heat effectively prevents defects such as insufficient powder melting and poor powder bonding, thereby improving the consistency of the deposited samples. Consequently, the pulsed current-assisted LP-DED sample demonstrates better metallurgical bonding capabilities.

3.2. Phase Analysis by XRD

In order to circumvent interference arising from primary extinction and preferred orientation, Rietveld fitting was carried out on the X-ray diffractometer (XRD) results. This was conducted with the aim of enhancing the accuracy of the analysis regarding crystal behavior. The R-factors for both samples fall within the acceptable range, thereby demonstrating the high reliability of the Rietveld fit. The XRD pattern predominantly encompasses typical α-Fe diffraction peaks with crystal plane indices of 100, 200, and 211 (as depicted in Figure 6a,b). When compared with the standard card PDF#04-003-7116, it can be observed that the physical structure of the material belongs to the body-centered cubic (BCC) ferrite structure (Figure 6e). The process of pulsed current-assisted deposition does not give rise to any new structural phases. The statistical results of XRD are presented in Table 2. The average full width at half maximum (FWHM) of all diffraction peaks decreases from 0.187 to 0.143. The influence of the pulsed current led to a narrowing of the diffraction peaks; however, it did not bring about any alteration in the phase composition. In accordance with the Williamson–Hall formula [24], this might be the consequence of the pulsed current causing either a reduction in grain strain levels or an increase in grain size at the microscopic level.
β h k l cos θ h k l = k λ D W − H + 4 ε W − H sin θ h k l
where εW-H and DW-H are lattice strain and the size of the nanocrystals, respectively. k is the Scherrer constant (typically around 0.89). λ is the X-ray wavelength. βhkl is the FWHM converted to radians. θhkl is the Bragg angle. εW-H and DW-H can be calculated from the slope and Y-axis intercept of the βhkl cosθhkl vs. 4sinθhkl plots [25], respectively.
In the case of pulsed current-assisted deposition samples, when compared with the as-received samples, the 100 peaks, 200 peaks, and 211 peaks are shifted to the right by 0.132°, 0.205°, and 0.281°, respectively. It is noted that the high-angle diffraction peaks exhibit relatively large deviation angles. Moreover, the deviation of these three diffraction peaks with varying amplitudes rules out the possibility that the shift is due to instrument zero drift. The lattice constants corresponding to all diffraction peaks were calculated. Under the influence of the pulse current, the average lattice constant decreases from 6.2155 to 6.1599. According to the Bragg diffraction formula [26], a reduction in the lattice constant results in a shift of the diffraction angle towards a higher angle. Microscopic defects can lead to changes in the lattice constant, and it is thus speculated that the pulsed current induces a high density or high thermal stability type of dislocation within the deposition. The 220 diffraction peaks suggest the presence of a negligible amount of residual austenite, which is retained as a consequence of the laser heat quenching effect during the deposition process. However, due to the absence of the 200 diffraction peak representing austenite at the 50-degree diffraction angle, the quantitative evaluation of the residual amount of austenite lacks statistical significance.
The equilibrium phase diagram of AISI 9310 gear steel was simulated by employing the TCFe9 database within the Thermo-Calc software (the version number: 2021b. As depicted in Figure 6c). During the LP-DED process, as the temperature decreases, the liquid phase commences transforming into austenite starting from 1507.95 K. This transformation continues until all liquid phases are entirely converted into austenite at 1475.1 K. Subsequently, as the molten pool continues to cool, the conversion of austenite to ferrite takes place at 759.8 K, accompanied by the precipitation of a small amount of M23C6 and M7C3 carbides. At this stage, the ferrite phase constitutes the main component of the deposited sample. The continuous cooling transformation curves of supercooled austenite, calculated using the JMatPro software (the version number: V13.0), demonstrate that diverse cooling transformation products are formed at different cooling rates [27] (as illustrated in Figure 6d). When the cooling rate is 100 °C/s, the austenite will be completely transformed into the martensitic phase. As the cooling rate decreases, the precipitation of bainite, ferrite, and pearlite will occur in sequence. However, in the case of LP-DED, the cooling is carried out by room temperature at a typical cooling rate ranging from 1 to 10 °C/s. The characteristic cooling phase within this cooling rate range is the ferrite phase, which is in agreement with the results obtained from XRD analysis.

3.3. Microstructure Analysis by EBSD

Figure 7a,b presents the EBSD characterization of the grain distribution in the near-substrate region of both the as-received and the pulsed current-assisted deposition samples. In the as-received sample, the typical columnar morphology characteristic of LP-DED can be observed. The radial size of the columnar crystals ranges from 10 to 20 μm, and a substantial number of fine equiaxial crystals with other orientations are interspersed among them, which disrupts the original columnar crystal morphology. In contrast, the number of fine equiaxed crystals in the pulsed current-assisted deposition samples decreases significantly. This indicates that the degree of recrystallization has decreased. Statistics obtained from the AZtecCrystal software (the version number: 2.1.2) reveal that the degree of recrystallization is reduced from 1.004% to 0.504% after the application of the current. However, the effect of the current on grain size reduction [28] is not evident. Instead, the equiaxial crystals display an obvious growth tendency along the thermal gradient. The coupled thermal effects of the pulsed current and laser light jointly contribute to the growth of the grains. The pulsed current induces a significant reduction in the number of grains oriented parallel to the X-axis direction 101 of the specimen. This can be attributed to the Lorentzian force generated by the pulsed current, which guides the preferred solidification direction during the solidification process within the melt pool. The melt is pulled away from the molten pool in a specific direction by regular fluctuations, thereby resulting in a discernible change in orientation.
Both samples deposited via different methods demonstrated a relatively pronounced preferred orientation of 100//Y-axis. However, in the as-received sample, the preferred orientation deviated significantly and instead formed a preferred orientation of 114//Y-axis. In contrast, the pulsed current-assisted deposition sample exhibited an extremely intense 100//Y-axis selective orientation. Its peak intensity was nearly twice that of the as-received sample, and there was a 6.2° deviation from its pole position. It can be inferred from the reconstructed austenite grains that the stronger selective orientation is attributable to the grain size of the austenite. Specifically, the average austenite grain size of the pulsed current-assisted deposition sample is three times larger than that of the as-received sample. For BCC materials, the 100-crystal plane has the lowest surface energy. Moreover, the 100 directions also have the lowest resistance along the transverse direction. This is due to the fact that it has the smallest average number of colliding atoms per layer and the largest crystal plane spacing along the transverse direction. During the initial formation of the texture at high temperatures, numerous low-resistance nucleation sites are generated. The action of the pulsed current guides the direction of nucleation, causing the material to tend to change along the current direction during cooling and thus select a specific low-resistance orientation [29]. As a consequence, distinct texture features are manifested in the observed region.
Figure 8 presents the statistics regarding average grain diameter, local misorientation, misorientation angle, and grain orientation spread. In the original sample, the majority of grains are fine crystals with a size below 2 μm. The average grain size experiences an increase from 2.17 μm to 2.579 μm, and the proportion of grains smaller than 2 μm decreases notably. The thermal accumulation resulting from the pulse current leads to the prolongation of the holding time after deposition. This, in turn, supplies more energy for grain growth following nucleation and gives rise to an obvious coarsening trend of the recrystallized grains. The local misorientation serves as an indicator of the degree of uniformity of plastic deformation. The average local misorientation increases by 5.2%, rising from 1.16 to 1.22. This indicates that the degree of plastic deformation grows under the influence of the pulse current. Moreover, as observed in Figure 7c,d, the strain accumulation in the as-received sample is mainly concentrated at the grain boundary and other defect locations, while the plastic deformation within intact columnar crystals is relatively minor. The average grain orientation spread value decreases from 1.03 to 0.82. This implies a reduction in the degree of distortion within the grains and a slight decrease in the proportion of deformed grains. Consequently, the plastic deformation induced by the pulse current primarily develops with a high density at the grain boundary. Plastic deformation is the outcome of the movement and propagation of dislocations at the microscopic level. The increase in dislocation density is ascribed to the force effect of the pulse current rather than its thermal effect. Furthermore, the proportion of low-angle grain boundaries (where the orientation difference is less than 10°) increases from 0.42 to 0.52 after the application of the pulse current. Low-angle grain boundaries possess lower energy storage, and there is generally a smaller orientation difference between grains. Although the improvement in mechanical properties brought about by low-angle grain boundaries is to a limited extent, they are conducive to the movement and morphological evolution of dislocations. They also provide additional deformation mechanisms, such as the slippage or recombination of dislocations. The presence of a greater number of grain boundaries at low angles enables more plastic deformation to occur between these boundaries, thereby enhancing the overall geometric dislocation density of the specimen.

3.4. Dislocation Defects by TEM

In order to conduct a more in-depth analysis of the effect of in situ pulsed current on plastic deformation and microscopic defects within LP-DED samples, characteristic locations were selected for analysis using TEM, HRTEM, and SAED modes. Figure 9a presents the TEM observations of the as-received sample, while Figure 9b,c displays the TEM observation results at two typical locations of the pulsed current-assisted deposition samples. As revealed in Figure 9(a,a1), the dislocation density in the as-received samples is relatively low, and the dominant defect forms are discrete short dislocation lines and dislocation tangles. Notably, the size of these dislocation tangles is commonly smaller than 50 nm. The types and quantities of defects in the as-received sample are limited, which consequently results in a low impact of defects such as dislocations on the mechanical properties of the deposition sample. Calibration of the diffraction pattern in the SAED mode indicates that the physical phase of the pulsed current-assisted deposited samples remains α-Fe (as depicted in Figure 9(b2)), which is in line with the XRD results mentioned previously. However, it is worth noting that the diffraction pattern of the pulsed current-assisted deposition sample is more regular than that of the as-received samples. This finding serves as evidence that the pulsed current-assisted deposition sample exhibits a higher degree of crystallization.
The EBSD results suggest that a greater degree of plastic deformation occurs in the pulsed current-assisted deposition samples. In these samples, prior to solidification, the melt pool is squeezed by the Lorentz force. As a consequence, dislocations can diffuse over a larger extent and multiply. The pulsed current triggers a change in the form of dislocations, thereby creating favorable circumstances for the formation, deformation, and reconstruction of defects. Meanwhile, the thermal effects intensify the thermal gradient in the normal direction of the melt pool, which makes the Marangoni convection effect [30] more pronounced. During the convection process, the interweaving of dislocations forms a densely distributed network of dislocations (as shown in Figure 9(c1)). The dislocation entanglements are more severe, and their size is generally larger than 100 μm. The diffraction pattern in Figure 9(c2) reveals that there are a substantial number of stacking fault defects in the area with a high density of dislocations. It is hypothesized that the optimization of the pulse current parameters will continue to influence the dislocation enrichment state and distribution range. This, in turn, may lead to the formation of a greater variety of micro-defects that are beneficial to the overall mechanical properties of the material [31]. The regions of dislocation concentration in Figure 9(a2,b2,c2) were analyzed using the inverse fast Fourier transform image. The existence of dislocation defects can be verified by the blurred images of lattice fringes that cannot be restored [32]. In the as-received samples, the lattice fringes are relatively clearer. In contrast, the images corresponding to dislocation lines and dislocation nets in the pulsed current-assisted deposition samples are extensively blurred, which further confirms the higher dislocation density in these samples. Simultaneously, some of the lattice fringes appear bent to varying degrees. This phenomenon is attributed to the lattice distortion effect caused by defects occupying the lattice positions [33].

4. Discussion

4.1. Thermal Effects of Pulsed Current

The thermal influence of pulsed current in the LP-DED process is prominent. Prior research has suggested that the non-thermal electron wind effect on dislocations, although proposed [34,35], is negligible in magnitude [36], being far too small to instigate dislocation movement. Experimentally, our results diverge from the grain refinement reported by some scholars [28,37,38]. Instead, we observe grain coarsening, mainly ascribed to the thermal effect of the pulsed current. The frequency characteristics of the pulse current play a role here. The vibrational frequency of the force induced by the current field does not fully align with the intrinsic frequency of the unconsolidated tissue [39], resulting in a weak resonance effect. As a consequence, during LP-DED, the thermal effect dominates, causing accelerated grain growth.
Heat accumulation during epitaxial growth increases with the deposition height gradient. This gradually decelerates the nucleation rate, allowing grains more time to coarsen during solidification and evolve towards equiaxed crystals. Both equiaxed and columnar crystals tend to grow along the thermal gradient. As depicted in Figure 10, which shows the hardness in the cross-sectional direction along the X-axis and Y-axis for both the as-received and pulsed current-assisted deposition samples, in the 0–600 μm region near the surface, where numerous columnar crystals are present, the hardness is approximately 5% lower than in the region near the substrate with concentrated equiaxed fine crystals. The accelerated growth of columnar crystals, caused by the severe thermal accumulation of Joule heat from the pulsed current, is the main reason for this reduced hardness in the electrically assisted deposited samples. However, within the initial 1–2 layers of deposition, the heat accumulation from the pulse current is rapidly absorbed by the substrate.
In the molten pool, under severe temperature gradients, two types of convection occur simultaneously: natural convection driven by buoyancy/gravity and Marangoni convection. These convective forces drive the fluid to flow outward from the hottest center of the molten pool. As the melt-pool temperature rises, its viscosity decreases, prolonging the liquid-phase duration and enhancing its directional mobility [39]. The thermal effect of the pulsed current has a twofold impact. While it contributes to some negative outcomes like grain coarsening and reduced hardness in certain regions, it also has positive aspects such as facilitating the expulsion of gas and pore closure due to enhanced convection in the molten pool.

4.2. Mechanical and Microstructural Effects of Pulsed Current

Although the thermal effect is dominant, the pulsed current also exerts mechanical and microstructural effects. Pulsed current promotes cross-slip through electroplasticity [40] and modifies the local stress state at the microscopic level by altering the form of dislocations. This results in improvements in strength and ductility. Despite the absence of grain refinement, the pulsed current induces competitive local grain growth, which changes the texture preference.
During pulsed current deposition, the structural thermal stability of dislocation defects improves, and significant residual thermal stresses are not introduced. A small number of discrete dislocation lines in the samples recombine to form a high-density and concentrated dislocation network. This leads to the formation of numerous stacking faults and defect plugging. Additionally, the force-induced micro-defects, such as high-density and multi-type dislocations and macro-defects like pores, work together to cause a small-range increase in the sample’s hardness in the initial deposition layers. The more intense local convection, influenced by the pulsed current, further accelerates gas expulsion and pore closure, which is beneficial for the overall quality of the deposited material. It is crucial to note that these mechanical and microstructural effects, despite the overarching thermal influence, contribute to enhancing the integrated mechanical properties of the samples in the LP-DED process.

4.3. Potential Implications for Hydrogen Embrittlement and Wear Resistance

While hydrogen embrittlement (HE) was not directly measured in this study, the pulsed current’s influence on dislocation networks and defect structures may alter hydrogen trapping behavior. High-density dislocations and stacking faults (Figure 9c) could act as hydrogen traps, potentially mitigating HE susceptibility by reducing hydrogen diffusivity [41,42]. However, further studies are needed to quantify hydrogen interaction with pulsed current-modified microstructures, particularly for aerospace applications where environmental exposure is critical.
Although wear resistance is critical for gear applications, this study focused on establishing the foundational relationships between pulsed current, microstructure, and basic mechanical properties (e.g., hardness and dislocation density), the effect of pulsed current in the LP-DED process is shown in Figure 11. Prior studies [43,44,45,46] indicate that wear performance is strongly correlated with hardness, defect density (e.g., porosity), and grain structure. Our results demonstrate that pulsed current reduces porosity (0.307%→0.215%), increases hardness (Figure 10), and promotes dense dislocation networks (Figure 9c), all of which are expected to enhance wear resistance by reducing crack initiation sites and improving load-bearing capacity. Future work will explicitly measure wear rates (e.g., pin-on-disk tests) and analyze wear mechanisms (adhesive/abrasive) under simulated operational conditions to validate these hypotheses.

5. Conclusions

In this paper, a comprehensive investigation has been conducted regarding the effect of pulse current applied in situ on the microstructure, defects, and phase of AISI 9310 gear steel samples prepared by LP-DED. Through a series of analyses and verification, the role of pulse current in the deposition process has been systematically explored. The main conclusions are elaborated as follows:
(1) The introduction of pulse current plays a crucial role in effectively reducing the number and porosity of pores within LP-DED samples. Specifically, when the current passes through the deposited sample, the deformable liquid-phase melt pool experiences a squeezing effect exerted by the contraction force of the carriers moving towards the central axis. This squeezing action significantly accelerates the discharge of air trapped within the pores. Moreover, the accumulation of Joule heat resulting from the pulse current has a dual positive impact. On one hand, it increases the melting degree of the powder, ensuring that the powder is more thoroughly melted and integrated during the deposition process. On the other hand, it enhances the fluidity of the melt pool, enabling the molten material to flow more smoothly and uniformly. These two factors jointly contribute to promoting the closure of cracks and reducing the formation of pores, thereby improving the overall quality and integrity of the deposited samples.
(2) The thermal effect of the pulsed current has a significant influence on grain growth. In the case of pulsed current-assisted LP-DED samples, an observable increase in grain size is noted, while there is no alteration in the physical phase of the material. This indicates that the pulsed current primarily affects the growth kinetics of the grains without introducing new phases. Additionally, the application of pulsed current leads to an increase in the degree of plastic deformation within the deposition region. This is manifested through the formation of intense preferred orientation textures, particularly the 100∥Y-axis preferred orientation. Such a texture formation can be attributed to the combined action of various factors related to the pulsed current, including its influence on the nucleation and growth directions of grains during the solidification process within the melt pool.
(3) The pulsed current has a remarkable impact on the dislocation density within LP-DED samples, causing it to increase significantly. Through the mechanism of electroplasticity, the original single dislocation lines are induced to recombine and transform into a densely distributed dislocation network. Along with this, complex stacking fault defects are also generated. This alteration in the dislocation configuration not only changes the microscopic structure of the material but also has a profound effect on its mechanical properties. The force effect of the pulsed current, in particular, acts as a key factor in significantly improving the microstructure by refining and optimizing the distribution of dislocations. Consequently, this leads to enhancements in the mechanical properties of the LP-DED samples, such as improved hardness, strength, and ductility, making them more suitable for various engineering applications.
In summary, the in situ application of pulse current in the LP-DED process of AISI 9310 gear steel samples has a multi-faceted impact on their microstructure, defects, and mechanical properties. Understanding these effects provides valuable insights for further optimizing the deposition process and improving the performance of the fabricated components.

Author Contributions

Conceptualization, C.X. and L.Z.; methodology, C.X.; software, L.Z.; validation, F.Y., X.S. and Q.F.; formal analysis, P.H.; investigation, W.L.; resources, P.L.; data curation, X.S.; writing—original draft preparation, C.X.; writing—review and editing, L.Z.; visualization, F.Y.; supervision, L.Z.; project administration, X.S.; funding acquisition, F.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Natural Science Basic Research Program of Shaanxi (Grant No: 2023-JC-JQ-37), the Shaanxi province “Two-Chain” Fusion Photon Integration and Photon Manufacturing Key Project (Grant No: 2021LLRH-03), the National Key Research and Development Program of China (Grant No: 2022YFB4601700), and the National Youth Talent Program of China; National Natural Science Foundation Integration Project (Grant No: 9236030003).

Data Availability Statement

Data will be made available upon request.

Conflicts of Interest

Author Cenchao Xie was employed by the company Army Aviation Military Representative Office in Tianjin, Author Wenfa Liu was employed by the company Army Aviation Military Representative Office in Chengdu, Author Qiang Feng was employed by the company State-Owned Jinjiang Machine Factory. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
BCCbody-centered cubic
EBSDelectron back scatter diffraction
EDSenergy dispersive spectrometer
FWHMfull width at half maximum
LP-DEDlaser powder directed energy deposition
XRDX-ray diffractometer

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Figure 1. (a–c) SEM, particle size, EDS characterization of powder for deposition, (d) SEM observation of powder at different magnifications, (e–j) powder particle size test results. Powder chemical element composition EDS observation.
Figure 1. (a–c) SEM, particle size, EDS characterization of powder for deposition, (d) SEM observation of powder at different magnifications, (e–j) powder particle size test results. Powder chemical element composition EDS observation.
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Figure 2. (a) LP-DED equipment, (b) specimen after deposition, (c) in situ pulsed current field applied during deposition, (d) schematic diagram of pulsed current-assisted LP-DED.
Figure 2. (a) LP-DED equipment, (b) specimen after deposition, (c) in situ pulsed current field applied during deposition, (d) schematic diagram of pulsed current-assisted LP-DED.
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Figure 3. EBSD grain morphology of as-received sample. (a) Columnar crystal morphology of the near-top region (3–4th layers), (b) equiaxed crystal morphology of the near-substrate region (1–2nd layers).
Figure 3. EBSD grain morphology of as-received sample. (a) Columnar crystal morphology of the near-top region (3–4th layers), (b) equiaxed crystal morphology of the near-substrate region (1–2nd layers).
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Figure 4. Pore number and porosity results of the as received and pulsed current assisted deposition samples.
Figure 4. Pore number and porosity results of the as received and pulsed current assisted deposition samples.
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Figure 5. SEM observation of pores and morphology in the cross-section direction of the deposited samples. (a) Typical pore characteristics of as-received sample, (b) typical pore characteristics of pulsed current-assisted deposition sample, (c) morphology of pores and precipitates, (d) orientationally aligned columnar crystals common in deposition sample, (e) crack defects in the as-received sample, (f) crack defects in the pulsed current-assisted sample.
Figure 5. SEM observation of pores and morphology in the cross-section direction of the deposited samples. (a) Typical pore characteristics of as-received sample, (b) typical pore characteristics of pulsed current-assisted deposition sample, (c) morphology of pores and precipitates, (d) orientationally aligned columnar crystals common in deposition sample, (e) crack defects in the as-received sample, (f) crack defects in the pulsed current-assisted sample.
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Figure 6. (a) Rietveld fit mapping by XRD of the as-received sample, (b) Rietveld fit mapping by XRD of the pulsed current-assisted sample, (c) Thermo-Calc simulated equilibrium phase diagram of AISI 9310 gear steel, (d) continuous cooling transition curve of supercooled austenite calculated by JMatPro, (e) ball-and-stick structure diagram of α-Fe.
Figure 6. (a) Rietveld fit mapping by XRD of the as-received sample, (b) Rietveld fit mapping by XRD of the pulsed current-assisted sample, (c) Thermo-Calc simulated equilibrium phase diagram of AISI 9310 gear steel, (d) continuous cooling transition curve of supercooled austenite calculated by JMatPro, (e) ball-and-stick structure diagram of α-Fe.
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Figure 7. EBSD observation of deposited samples near the substrate. (a) IPF diagram of the as-received sample, (b) IPF diagram of the pulsed current-assisted sample, (c) KAM diagram of the as-received sample, (d) KAM diagram of the pulsed current-assisted sample, (e) pole and inverse pole diagram of the as-received sample, (f) pole and inverse pole diagram of the pulsed current-assisted sample.
Figure 7. EBSD observation of deposited samples near the substrate. (a) IPF diagram of the as-received sample, (b) IPF diagram of the pulsed current-assisted sample, (c) KAM diagram of the as-received sample, (d) KAM diagram of the pulsed current-assisted sample, (e) pole and inverse pole diagram of the as-received sample, (f) pole and inverse pole diagram of the pulsed current-assisted sample.
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Figure 8. (a) EBSD statistical results. Grain diameter, (b) local misorientation and misorientation angle, (c) the misorientation angle of experimental and gaussian fitting, (d) grain orientation spread.
Figure 8. (a) EBSD statistical results. Grain diameter, (b) local misorientation and misorientation angle, (c) the misorientation angle of experimental and gaussian fitting, (d) grain orientation spread.
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Figure 9. TEM observation of deposited samples near the substrate. (a) Microscopic defect characteristics of the as-received sample, (b,c) microscopic defect characteristics of pulsed current-assisted samples. In detail, (a1,b1,c1) are the feature amplification regions of (a,b,c), respectively. (a2,b2,c2) are the HRTEM results and SAED mode of (a1,b1,c1). (a3,b3,c3) are the lattice fringe image after the inverse FFT of (a2,b2,c2).
Figure 9. TEM observation of deposited samples near the substrate. (a) Microscopic defect characteristics of the as-received sample, (b,c) microscopic defect characteristics of pulsed current-assisted samples. In detail, (a1,b1,c1) are the feature amplification regions of (a,b,c), respectively. (a2,b2,c2) are the HRTEM results and SAED mode of (a1,b1,c1). (a3,b3,c3) are the lattice fringe image after the inverse FFT of (a2,b2,c2).
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Figure 10. Hardness in cross-section direction. (a) Testing along the X-axis, (b) testing along the Y-axis.
Figure 10. Hardness in cross-section direction. (a) Testing along the X-axis, (b) testing along the Y-axis.
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Figure 11. Schematic of pulsed current effects during LP-DED. The power deposition gradient direction (vertical arrow), Lorentz forces (blue arrows), pulsed current flow (red arrows).
Figure 11. Schematic of pulsed current effects during LP-DED. The power deposition gradient direction (vertical arrow), Lorentz forces (blue arrows), pulsed current flow (red arrows).
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Table 1. Chemical composition table of AISI 9310 steel substrate (wt%).
Table 1. Chemical composition table of AISI 9310 steel substrate (wt%).
CMnSiSCrNiMoCuFe
0.110.560.240.0041.323.190.120.13Bal
Table 2. XRD diffraction results after Rietveld fitting.
Table 2. XRD diffraction results after Rietveld fitting.
Sample TypehklFWHMPeak
Intensity		Crystal Lattice SpacingLattice
Constant		Mean Lattice Constant
As received1000.3861002.05142.05146.2155
2000.0729.51.43688.2576
2110.102151.17888.3374
Pulsed current assisted1000.1021002.02832.02836.1599
2000.10117.71.43728.2621
2110.22722.71.16838.1895
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MDPI and ACS Style

Xie, C.; Yang, F.; He, P.; Liu, W.; Feng, Q.; Zhou, L.; Liu, P.; Sun, X. Effect of In-Situ Pulse Current on Microstructure and Mechanical Properties of AISI 9310 Gear Steel by Laser Powder Directed Energy Deposition. Machines 2025, 13, 308. https://doi.org/10.3390/machines13040308

AMA Style

Xie C, Yang F, He P, Liu W, Feng Q, Zhou L, Liu P, Sun X. Effect of In-Situ Pulse Current on Microstructure and Mechanical Properties of AISI 9310 Gear Steel by Laser Powder Directed Energy Deposition. Machines. 2025; 13(4):308. https://doi.org/10.3390/machines13040308

Chicago/Turabian Style

Xie, Cenchao, Fei Yang, Peng He, Wenfa Liu, Qiang Feng, Liucheng Zhou, Ping Liu, and Xin Sun. 2025. "Effect of In-Situ Pulse Current on Microstructure and Mechanical Properties of AISI 9310 Gear Steel by Laser Powder Directed Energy Deposition" Machines 13, no. 4: 308. https://doi.org/10.3390/machines13040308

APA Style

Xie, C., Yang, F., He, P., Liu, W., Feng, Q., Zhou, L., Liu, P., & Sun, X. (2025). Effect of In-Situ Pulse Current on Microstructure and Mechanical Properties of AISI 9310 Gear Steel by Laser Powder Directed Energy Deposition. Machines, 13(4), 308. https://doi.org/10.3390/machines13040308

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