Effect of Ultrasonic Assistance on Properties of Ultra-High-Strength Steel in Laser-Arc Hybrid Welding

Abstract

To address the challenge of achieving an optimal balance between strength and toughness in ultra-high-strength steel welds, this study investigates ultrasonic vibration-assisted laser-arc hybrid welding. The influence of ultrasonic vibrations, applied to the lower surface of laser-arc hybrid welding specimens at powers ranging from 60 W to 240 W, on various aspects of the weld, including macroscopic morphology, porosity, microstructure, and mechanical properties, was systematically examined. Experimental findings reveal that as ultrasonic power increases, weld porosity initially diminishes before rising again. Simultaneously, the fusion ratio of the weld gradually enhances, and the cross-sectional morphology of the weld transforms from a “goblet” shape to an “inverted triangle”, with the transition boundary between the arc zone and laser zone becoming less distinct. Furthermore, an increase in ultrasonic power leads to a gradual rise in the microhardness of the weld, and the mechanical properties of the weld joint exhibit an upward trend. Notably, at an ultrasonic power of 180 W, the weld attains a tensile strength of 1380 MPa and an impact toughness of 10.5 J, highlighting the potential of this technique in optimizing the welding characteristics of ultra-high-strength steel.

1. Introduction

The microstructure of ultra-high-strength steel (UHSS) is predominantly composed of martensite. Its high carbon content ensures that UHSS exhibits high strength and hardness, making it widely utilized in armored protection and transport equipment, including tanks, wheeled vehicles, and aircrafts [1,2,3]. As a fundamental protective material, UHSS is gradually being developed with a focus on achieving thinner, harder, and purer forms. During service, welded UHSS structures are subjected to high-velocity projectile penetration. Due to the martensitic microstructure, which lacks significant plastic deformation to absorb energy, these structures are prone to plug failure, fragmentation, and back-face spalling. These issues impose higher demands on the strength and impact toughness of UHSS and its welded joints [4,5,6].

Laser-arc hybrid welding technology has garnered significant attention from the welding industry due to its numerous advantages, such as deep penetration, excellent bridging ability across gaps, and high welding speeds. These features make it an ideal choice for various industrial applications. However, despite its benefits, the technology is not without its drawbacks. High porosity in the weld, large grain size, and stress concentration are some of the issues that can significantly compromise the strength and integrity of the welded joints [7,8,9,10,11].

To tackle these challenges and enhance the strength and durability of welded joints, experts have explored various external excitation techniques during hybrid welding. These approaches are aimed at controlling and optimizing welding conditions, leading to improved weld quality. By applying external energy in specific forms, a series of macro- and micro-scale changes are induced in the molten pool during welding [12]. These adjustments can profoundly affect the internal structure of the weld, leading to enhancements in its overall strength and durability, which results in more resilient and dependable bonds. Ultrasonic vibration-assisted welding technology can effectively reduce defects in welds and regulate the strength and toughness of weld joints, while also being relatively cost-effective [13,14]. As a result, ultrasonic vibration-assisted welding for ultra-high-strength steel has attracted significant attention from researchers, leading to a series of achievements. Li et al. [15,16] employed mechanical vibration to facilitate the welding process of high-strength steel, achieving successful welds devoid of defects. Their findings revealed that the application of mechanical vibration decreased the surface temperature gradient within the molten pool and consequently prolonged the solidification period. Kolubaev et al. [17] compared the laser welding of high-strength steel with and without ultrasonic assistance, observing that both the microhardness and tensile strength of the weld joints improved when ultrasonic vibration was applied. Ultrasonic vibration alters the solidification conditions of the melt, refining the grain structure. Liu Jia et al. [8] investigated the effects of different ultrasonic power levels on the microstructure and mechanical properties of high-nitrogen steel weld joints, showing that ultrasonic vibration reduces porosity in the weld, inhibits the continuous growth of dendrites, and changes the grain growth orientation. The refined weld microstructure significantly improves both tensile strength and impact toughness. Vinh Tran-The Chung et al. [18] studied the use of ultrasonic vibration in metal gas arc welding steel joints. The distance between the welding gun and the ultrasonic generator affects the shape of the welded joint. When the distance is 120 mm, the ultrasonic vibration has the most severe effect on the weld. Li et al. [19] welded Cf/Al using an ultrasonic brazing method and found that the shear strength of the joint could reach 23.5 MPa over 60 s under ultrasonic action, which was 77.5% of the base metal. Wang et al. [20] realized the real-time observation of ultrasonic cavitation bubbles during the solidification process of a Bi-Zn alloy under ultrasonic vibration by using the X-ray simultaneous radiation technique and found that the sound pressure in the melt determines the intensity of cavitation of the cavitation bubbles in the melt. Under the pressure amplitude of 14.5 MPa, the cavitation bubble implodes within one ultrasonic cycle after nucleation and growth, and the cavitation bubble shrinks in the pressure amplitude range of 0~0.33 MPa. Zhuang et al. [21] connected the substrate with the swinger by thread and then performed laser remelting on the surface of the substrate. The results of electron backscattering diffraction (EBSD) show that the grain size in the remelting zone is significantly refined, the temperature gradient of the melt is changed by the cavitation effect and the acoustic flow effect induced by ultrasonic vibration, and the coarse columnar crystals are transformed into fine equiaemic crystals, and the microhardness is increased after remelting. The surface friction coefficient is reduced from 1.3 to 0.4.

This study pioneers the application of ultrasonic vibration-assisted laser-arc hybrid welding (UV-LAHW) technology for joining ultra-high-strength steel (UHSS). The effect of ultrasonic power on the morphology, microstructure, and fracture mechanism of a laser-arc hybrid welded joint was obtained through experimental research. The laser-arc hybrid welding technology has a variety of heat source combinations, and the addition of ultrasound forms a complex welding pool environment influenced by many factors. The experimental study provides technical guidance for the ultrasonic-assisted laser-arc composite welding of ultra-high-strength steel.

2. Test and Methods

The material used in this experiment is a low-alloy ultra-high-strength steel (UHSS). The key mechanical properties of the steel are as follows: tensile strength ≥ 1600 MPa, yield strength ≥ 1200 MPa, elongation after fracture ≥ 8%, and impact energy absorption Akv ≥ 14 J (at −40 °C). The base metal’s microstructure consists of lath martensite, as shown in Figure 1a, with a microhardness ranging between 390 and 410 HV. In this study, an austenitic stainless steel welding wire, HCr20Ni10Mn7Mo, was employed, and its composition is listed in

Table 1. Primary composition of samples (wt%).

Chemical Element	C	Si	Mn	Cr	Ni	Mo	Fe
HSS	0.26~0.33	0.25~0.44	0.75~1.2	0.75~1.1	1.05~1.40	0.25~0.45	bal.
HCr20Ni10Mn7Mo	0.11	0.60	0.63	20.26	11.00	1.03	bal.

.

The weld porosity was detected by X-ray, and the flaw detection equipment was YXLON MG452 X of ECtron X-ray flaw detector (Comet Yxlon, Hamburg, Germany). The weld is white gray, and the pores in the weld area are black dots. Remove 100 mm at each end of the negative, and the arc starting and ending are not within the porosity statistical range. Select the middle length of 400 mm of the negative to calculate the porosity and use the software Image Pro Plus 6 to calculate the porosity of the flaw detection negative. According to ISO5871-2014 Welding—fusion welded joints in Steel, Nickel, Titanium and Their Alloys (Beam Welding Excluded)—Quality Levels for Imperfections [22], the weld width is marked by the red line segment in the figure, and the porosity is the ratio of the porosity area in the film to the projected area of the weld. The porosity P is defined as follows:

$$P={\displaystyle \frac{D_P}{D_W}}\times 100\%$$

where $D_P$ is the area occupied by the stomata and $D_W$ is the area of the weld.

During the welding process, a red laser pumped by an LD with a wavelength of 808 nm was used as the background light source, and the high-speed color camera recorded the image at a sampling frequency of 5000 frames per second. Due to the high brightness of the arc zone during the welding process, an interference filter with a wavelength of 808 nm was installed in front of the high-speed camera lens. Hannover arc quality analyzer with a sampling frequency of 5 kHz was used to record electrical signals during welding.

The experimental setup incorporates an HL4006D Nd solid-state laser and a YD-350AG2HGE MIG/MAG welding machine (Panasonic, Osaka, Japan), creating a laser-arc hybrid welding configuration. The welding process employs a design where the arc leads while the laser follows, with the laser directed vertically and the welding torch set at a 30° angle to the laser beam. Additional welding parameters can be found in

Table 2. Welding parameter.

Welding Parameter	Value
Welding speed (m/min)	0.8
Laser power (kW)	2.6
Welding current (A)	220
Arc voltage (V)	27
Distance between laser and wire (mm)	5
Defocusing value (mm)	−2
Ultrasonic power (W)	0 W, 60 W, 120 W, 180 W, 240 W

. Furthermore, an ultrasonic assistance system, consisting of an ultrasonic generator, amplitude transformer, transducer, and tool head, was integrated into the hybrid welding process. Various ultrasonic power settings were utilized during the ultrasonic-assisted hybrid welding tests on ultra-high-strength steel. The ultrasonic generator delivers a maximum output of 1000 W and operates at a frequency of 20 kHz. The test specimens were placed on a CNC-controlled platform, where movement and speed were precisely regulated by computer programming. Figure 1 presents the schematic (b) and setup diagram (c) of the ultrasonic vibration-assisted laser-arc hybrid welding system. The vibrating tool head was positioned beneath the workpiece, 5 mm behind the laser focal point, and aligned with the direction of the worktable’s motion.

Following the welding procedure, X-ray imaging was employed to examine the influence of ultrasonic vibration on porosity levels within the weld. The surface and grain characteristics of the weld were analyzed using a Leica DM 2700 M microscope (Leica Microsystems, Wetzlar, Germany) optical microscope from Germany. For a more detailed analysis of the weld’s microstructure and elemental composition, a JSM-6510F SEM equipped with an EDS system from JEOL, Tokyo, Japan was utilized. Tensile tests were conducted on the welded joints with an INSTRON 8850 universal testing machine (Instron, Norwood, MA, USA). The tensile specimens were fabricated according to the ISO 4136:2001 Destructive Tests on Welds in Metallic Materials—Transverse Tensile Test for welded joint testing [23]. To evaluate the hardness gradient across the weld, lateral microhardness measurements were taken from the center of the weld to the base metal using a MH-60 Vickers hardness tester. A load of 500 gf was applied during the microhardness test, with a loading duration of 10 s.

3. Result and Discussion

3.1. Effect of Ultrasonic Vibration on Macroscopic Morphology and Porosity Defects of Welds

The cross-sections and macroscopic morphologies of ultra-high-strength steel weld joints under different ultrasonic power conditions are shown in Figure 2. As seen in Figure 2a–e, when the ultrasonic power is low, there are no significant changes in the macroscopic appearance of the weld compared to the condition without ultrasonic vibration. The upper part of the weld still exhibits prominent arc welding characteristics, maintaining the “goblet” shape typical of hybrid welding. With an increase in ultrasonic power, the boundary between the arc-affected region and the laser-affected region becomes less pronounced. The weld cross-section gradually changes from a “goblet” shape to an “inverted triangle”. Additionally, the weld reinforcement (or excess height) decreases, dropping from 0.7 mm to 0.5 mm. This reduction in excess height helps to relieve stress concentrations in the welded structure, which can improve the overall performance of the weld [24].

As the ultrasonic power increases from 60 W to 240 W, certain changes occur on the weld surface. When the ultrasonic power reaches 240 W, the weld surface texture becomes disordered, and undercutting appears to some extent. The weld bead becomes irregular, and surface porosity begins to appear. Additionally, significant spatter is generated during the welding process, indicating that excessive ultrasonic power causes instability in the welding process. This instability results in poor surface quality and the formation of defects, suggesting that excessive ultrasonic power compromises weld quality.

Porosity is a prevalent but critical challenge in welding, as an increased pore density causes stress accumulation within the weld, weakening the overall integrity of the joint. In the absence of ultrasonic assistance, porosity related to random processes is observed in the weld. As the ultrasonic power increases up to 180 W, porosity gradually decreases, with the residual pores predominantly located at the weld’s initiation and termination points. This reduction in porosity is attributed to the influence of ultrasonic cavitation. After ultrasonic vibration is applied, bubbles in the molten pool require a longer time to collapse when in a stable cavitation state. Under the influence of acoustic pressure, these bubbles grow continuously. Simultaneously, under the negative pressure of the sound waves, the bubbles in the molten pool tend to migrate toward the cavitation bubbles, merging with them and increasing in size. When the bubble’s natural frequency aligns with the ultrasonic frequency, the oscillating bubbles cease to grow. At this point, the vibration in the molten pool accelerates the bubble growth, increasing their size and hastening their escape from the molten pool. This prevents bubbles from being trapped and forming pores in the solidified weld. When the ultrasonic power increases to 240 W, the weld porosity rate increases sharply, reaching 8.94%, as shown in Figure 2.

This occurs because excessive ultrasonic power transforms the stable cavitation effect into a transient cavitation effect, significantly shortening the bubble collapse time. As a result, the time available for bubbles to escape from the molten pool is reduced, and the bubbles are trapped, leading to the formation of porosity defects. At high ultrasonic power levels, the disturbance in the molten pool becomes excessive, and the curvature of the keyhole’s front wall is no longer able to maintain a dynamic equilibrium.

This increases the instability of the laser keyhole, exacerbating the formation of pores. The instability of the welding process was monitored using a Hanover analyzer and high-speed camera, as shown in Figure 3, further confirming the disruptive effects of excessive ultrasonic vibration on the molten pool and keyhole stability.

3.2. Effect of Ultrasonic Vibration on Microstructure of Ultra-High-Strength Steel

The weld microstructure of the hybrid welded joints under ultrasonic power ranging from 0 W to 240 W is shown in Figure 4. In the absence of ultrasonic power (0 W), the dendritic structure near the fusion line appears relatively large, growing outward from the fusion zone toward the weld center, as depicted in Figure 4a. After ultrasonic vibration is applied, the grain growth orientation improves significantly. As the ultrasonic power increases, Figure 4b–e shows a gradual shortening of the weld dendrites, with increasingly noticeable grain refinement. Additionally, the width of the columnar grain region near the fusion line decreases as ultrasonic power increases, and the distribution of equiaxed grains becomes more uniform.

Figure 5 illustrates the grain size distribution at the weld center for various ultrasonic power levels. As ultrasonic power rises, the columnar grain region shrinks, while the equiaxed grain region expands. The increase in ultrasonic power refines the grain structure by reducing the grain size. Ultrasonic waves cause bubbles in the molten steel to grow, contract, and collapse, with the bubble contraction velocity given by Equation [25]:

$$V=\sqrt{{\displaystyle \frac{2P_{\mathrm{L}}}{3{\rho }_{\mathrm{L}}}}\left({\displaystyle \frac{R_{\mathrm{m}}^3}{R^3}}-1\right)}$$

(1)

In Equation (1), Rm signifies the bubble’s largest radius, while PL indicates the static pressure within the molten material. As the bubble contracts to a radius of R, the highest pressure is observed at a point 1.587 R away from the bubble. This peak pressure can be computed using Rayleigh’s simplified model, as expressed below:

$$P_{\max }=P_{\mathrm{L}}{4}^{-{\displaystyle \frac{4}{3}}}{\left({\displaystyle \frac{R_{\mathrm{m}}}{R}}\right)}^3$$

(2)

As indicated by Equation (2), the local pressure surrounding the cavitation bubble can peak at thousands of atmospheres. This instantaneous pressure shift affects the melting point of the molten substance, as defined by the Clausius–Clapeyron relation [26]:

$${\displaystyle \frac{\mathsf{\Delta }T}{\mathsf{\Delta }P}}=T_{\mathrm{s}}{\displaystyle \frac{V_2-V_1}{\mathsf{\Delta }H}}$$

(3)

In Equation (3), ΔT refers to the change in temperature, Ts is the solidification temperature at ambient pressure, ΔP signifies the pressure variation, ΔH is the latent heat of fusion, V1 refers to the volume of the solid phase, and V2 is the liquid volume. Equation (3) shows that increasing pressure raises the solidification temperature of the melt and enhances the degree of supercooling during crystallization. As the cavitation bubble expands, it absorbs surrounding energy, causing localized undercooling. This increased undercooling raises the nucleation rate.

On the other hand, as ultrasound propagates through the molten pool, it intensifies flow within the pool due to cavitation-induced mechanical stirring. This induces changes in the orientation of grain growth. The collapse of cavitation bubbles generates shock waves that fragment the primary dendrites at the solid–liquid interface. The resulting dendrite fragments serve as nucleation sites for new grain formation. This interruption inhibits continuous dendritic growth, reducing their length. Additionally, the high viscosity of molten metal causes part of the ultrasonic energy to dissipate as it propagates, creating a sound pressure gradient within the pool. This creates irregular flow within the molten pool. Acoustic streaming mitigates the temperature gradient across the molten pool, limiting dendritic development, and at the same time, facilitates the flow of the molten substance. This enhanced flow facilitates the rapid dispersion of fragmented grains into the molten metal, promoting solute element diffusion and improving the segregation of alloying elements.

Figure 6 presents the inverse pole figure of the weld center microstructure, as identified by EBSD. The weld region’s microstructure primarily consists of lath martensite and elongated cellular austenite grains. In the absence of ultrasound, the phase difference within the cellular grains remains relatively small. However, when ultrasonic power is set to 180 W, the phase difference increases, indicating that grains on the opposite side of the cellular structure are less prone to slip under load, suggesting an enhanced resistance to plastic deformation and improved weld toughness. Figure 6a reveals subtle color differences within the columnar grains, indicating the presence of numerous subgrains within the larger columnar grains. This subgrain structure is a key factor contributing to the high strength of the ultra-high-strength steel weld.

The relative distribution probability of EBSD crystal phase differences in Figure 6 shows that variations in ultrasonic power alter grain boundary characteristics. Large-angle and small-angle grain boundary misorientations are distinguished by a boundary of 15° [26,27,28,29]. In the absence of ultrasound, the proportion of low-angle grain boundaries (with orientation differences less than 15°) is higher, and smaller angles between grains correlate with higher relative distribution probability, indicating adjacency to low-angle grain boundaries. As ultrasonic power increases, the ratio of low-angle grain boundaries decreases from 0.34 to 0.23, while the content of high-angle grain boundaries expands from 0.56 to 0.62. This indicates that ultrasonic-assisted welding improves the weld’s ability to resist crack propagation. Additionally, larger grain sizes extend the crack propagation path, significantly reducing the probability of crack formation and enhancing weld toughness. Moreover, the presence of large-angle grain boundaries after ultrasound application promotes greater grain mismatch, which helps prevent and pin dislocations or form a ’shielding wall’ for dislocation movement at grain boundaries. This method improves the strength and toughness of the weld.

3.3. Impact of Ultrasonic Vibration on the Mechanical Behavior of Welded Joints

3.3.1. Weld Microhardness Analysis

Figure 7 presents the microhardness distribution curves of ultra-high-strength steel weld joints under varying ultrasonic power levels. Measurements were taken 2 mm below the top surface of the base metal, reaching from the weld center to the base metal. The microhardness values of the weld range from 450 to 570 HV. As ultrasonic power increases, the microhardness of the weld rises correspondingly, with the effect becoming more pronounced at higher power levels. However, the maximum hardness in the heat-affected zone (HAZ) exhibits no significant variation.

3.3.2. The Impact of Ultrasonic Vibration on the Stretching Performance of Ultra-High-Strength Steel Joints

Figure 8 shows how different ultrasonic power settings impact the tensile properties of ultra-high-strength steel welds. Initially, as ultrasonic power increases, the tensile strength rises, peaking at 1380 MPa at 180 W, before dropping again at 240 W. This indicates that the ultrasonic power’s effect on the molten pool is power-dependent, influencing the weld’s microstructure and porosity, which subsequently alters its tensile behavior. The impact resistance of ultra-high-strength steel welds at −40 °C first increases and then decreases as ultrasonic power is applied from 0 W to 240 W. At 180 W, grain refinement results in the maximum impact resistance of 10.3 J. In contrast, at 240 W, the growth of porosity due to weld imperfections reduces the structural integrity, leading to the lowest impact resistance.

As shown in Figure 9, at 180 W, the weld exhibits significant grain refinement and fewer pores, leading to optimal tensile performance. Additionally, the fracture location shifts from the weld center to the heat-affected zone (HAZ). In contrast, Figure 10 shows that at 240 W, despite continued grain refinement, the number of pores increases significantly, resulting in a decline in the weld’s tensile performance being more pronounced at higher power levels. However, the maximum hardness in the HAZ exhibits no significant variation.

3.3.3. The Influence of Ultrasonic Vibration on the Tensile Behavior of Ultra-High-Strength Steel Welds

Figure 11 presents the SEM morphology of the low-temperature impact fracture sur-faces of the welds. In Figure 11a, at 0 W ultrasonic power, small pores appear on the fracture surface, with cracks forming around these pores. The impact fracture exhibits quasi-cleavage characteristics, characterized by localized crack formation and shortrange propagation, forming a riverlike pattern. Numerous short, curved tear ridges are observed on the fracture surface. Figure 11b shows the fracture morphology when the ultrasonic power is 120 w, which is similar to Figure 11c. Small dimples can be found in the fracture, and the area is relatively small. In Figure 11d, at 180 W ultrasonic power, the fracture surface exhibits distinct dimples, indicating ductile tearing. The dimples are uniformly distributed, with the surface covered in shallow dimples. The application of ultrasound induces vibrations in the molten pool, delaying the crystallization rate and breaking dendrites during grain formation, which results in finer grains. The increased grain boundary area and more tortuous grain boundaries enhance resistance to crack propagation. High-angle grain boundaries redirect the path of crack advancement, and as the crack crosses from one grain to another, finer grains result in higher overall impact toughness. In Figure 11e, it can be seen that with the increase in ultrasonic power, the number of dimples increases, and relatively large stomatal aggregation areas also exist. Thus, appropriate ultrasonic power improves the weld’s mechanical properties, enhancing its low-temperature impact toughness. Ultrasonic vibrations in the molten metal help inhibit and disperse impurities within pores, reducing the likelihood of defect formation. This ensures the formation of high-quality welds.

4. Conclusions

• As ultrasonic power increases, the weld penetration ratio rises, and the cross-sectional morphology transitions from a “goblet” shape to an “inverted triangle” shape. The distinction between the arc and laser regions becomes less pronounced. However, when ultrasonic power is excessively high, the weld surface becomes irregular and collapses, leading to a deterioration in weld formation quality.
• When the laser power is 2.6 kW, the welding current is 220 A, the welding speed is 0.8 m/min, and the ultrasonic power is 180 W, the maximum tensile strength of the welded joint is 1380 MPa, and the impact power is 10.3 J. The cavitation and acoustic streaming phenomena generated by ultrasound in the laser-arc hybrid welding process refine the grain structure, leading to finer columnar grains and reduced equiaxed grain size. The cavitation effect, induced by ultrasonic vibration, promotes bubble formation and growth, facilitating their upward movement in the molten pool and thereby inhibiting porosity formation. However, at 240 W ultrasonic power, transient cavitation occurs, generating short-lived bubbles. The elevated temperature, intense pressure, and powerful jets resulting from bubble collapse generate numerous tiny bubbles that fail to exit the molten pool promptly, causing porosity.
• As ultrasonic power rises from 0 W to 180 W, the structural integrity of the welded joints enhances. At 180 W ultrasonic power, the weld exhibits high strength and toughness, achieving a tensile strength of 1380 MPa and an impact toughness of 10.5 J.