Optimization of process parameters for ultrasonic-assisted laser welding of Ti6Al4V alloy and the impact of ultrasonic power on porosity, microstructure, and mechanical properties

doi:10.21203/rs.3.rs-4983650/v1

Download PDF

Research Article

Optimization of process parameters for ultrasonic-assisted laser welding of Ti6Al4V alloy and the impact of ultrasonic power on porosity, microstructure, and mechanical properties

https://doi.org/10.21203/rs.3.rs-4983650/v1

This work is licensed under a CC BY 4.0 License

You are reading this latest preprint version

In this study, the optimization of welding parameters, pore defects, microstructure and mechanical properties of Ti6Al4V alloy welded joints subjected to ultrasonic-assisted laser welding were studied. The optimal parameter combination and the most significant factor influencing the tensile strength of the joint were determined by three-factor and three-level orthogonal experiments with range and variance analysis methods. A control experiment was set up under the optimal parameters to reveal the reason of ultrasonic influence on joint strength from the pore defects and microstructure. The control test showed that the cavitation effect and stirring effect of ultrasonic could significantly improve the weld defects and microstructure. Compared with no ultrasonic, the porosity of the welded joint decreased from 3.06% to 0.08%, and the average grain size of prior-β and α' martensite were refined by 15.1% and 6.7% respectively under an ultrasonic power of 1000W. The ultrasonic vibration also contributed to the grain boundary transformation from low-angle to high-angle and significantly reduced the residual stress inside the joint. With the increase of ultrasonic power, the tensile strength increased by 4.37% and 8.38% respectively and the fracture mode changed from brittle-ductile mixed fracture to ductile fracture.

Ultrasonic-assisted laser welding

Ti6Al4V alloy

Porosity

Microstructure

Grain refinement

Mechanical properties

Ti-6Al-4V titanium alloys are widely used in the aviation, aerospace [1], medical [2], automotive, and other industries [3] due to their low density, high strength-to-weight ratio, exceptional tensile strength, excellent resistance to corrosion, outstanding biocompatibility, and ease of fabrication. Particularly, in deep space exploration, it exhibits significant advantages as a load-bearing structure of the detector [4]. Laser welding exhibits characteristics including a high level of adaptability, precision, efficacy, a narrow heat-affected zone, and minimal thermal distortion, which enable it to fulfill the processing requirements of complex structures and establish itself as an essential technique for joining titanium alloys [5, 6]. However, laser welding is a rapid melting and solidification process, resulting in significant temperature gradient and severe melt pool fluctuation in welded joint. The difference of temperature gradient will lead to an uneven weld microstructure, grain coarsening, grain deformation, and non-uniform grain deformation characteristics inside the welded joint [7–9]. Additionally, the unstable collapse of the molten pool and the involvement of protective gas during welding can lead to significant pore defects in the Ti6Al4V joint,which seriously deteriorate the welding quality of the welded joint [10, 11].

In order to improve the microstructure of the weld and reduce pore defects, some researchers have incorporated ultrasonic vibration into the laser welding process recently [12–15]. The utilization of ultrasonic-assisted welding technology in laser welding has attracted significant attention and practical application due to its cost-effectiveness and minimal energy consumption [16]. The application of ultrasonic in laser welding aims to improve the flow of the molten pool and enhance its solidification characteristics through ultrasonic cavitation, acoustic flow, and vibration effects. And the ultimate purpose is to reduce the porosity of the joint, refine the grain size and mitigate the segregation of microstructure so as to improve the overall performance of the welded joint. Wang et al. [17] studied the ultrasonic assisted laser welding of Inconel690 alloy and discovered that the cavitation effect of ultrasonic could effectively break dendrites, while the acoustic flow effect facilitated the transfer of disrupted dendrites to other regions of the molten pool, thereby creating numerous nucleation sites for equiaxed crystal formation and consequently promoting grain refinement. Liao et al. [18] incorporated ultrasonic vibration into underwater laser welding of SUS304 and observed that the use of ultrasonic vibration enhanced the stirring efficiency of the molten pool, suppressed the growth of columnar crystals, and increased the content of equiaxed crystals, leading to a significant reduction in texture strength. Song et al. [19] compared the microstructure and mechanical properties of T-ULW and F-ULW assisted welding methods, and found that the dendritic grains under F-ULW were finer and more evenly distributed, and the tensile strength of the joint was also higher. Zhou et al. [14] investigated the effect of ultrasonic vibration on the microstructure of C-276/304 heterogeneous metal, and found that the ultrasonic cavitation effect promoted the molten pool flow and enhanced the diffusion coefficient of the molten pool, resulting in more uniform element distribution and suppressed Mo segregation. Recently, Guo et al. [20] preliminarily tried to introduce ultrasonic vibration into the Ti6Al4V laser welding process and discovered that ultrasonic raised the overall temperature of the molten pool and reduced the temperature gradient, which facilitating the transformation of prior-β columnar crystals to equiaxed crystals. Additionally, ultrasonic has a significant effect on eliminating pore defects in laser welding [21]. Lei et al. [12, 22–24] conducted a study on ultrasonic-assisted laser welding of AZ31B magnesium alloy and observed that ultrasonic cavitation and sound flow effects enhanced the flow of the molten pool, facilitated bubble growth and release, reduced porosity from 4.3–0.9%, and increased tensile strength by approximately 7%. Cui et al. [22] explored the influence of ultrasonic assisted laser arc welding on the pores of high-nitrogen steel, and found that the cavitation effect of ultrasonic would produce a sound pressure gradient in the molten pool, which was conducive to the upward floating of bubbles, and reduced the formation of nitrogen element pore defects. Liu and Tan et al. [23, 24] integrated ultrasonic vibration into the laser-MIG welding process of aluminum alloy, and observed that the increase of bubble volume caused by ultrasonic cavitation enhanced the escape velocity of bubbles, resulting in a reduction in joint porosity. However, excessive ultrasonic intensity led to severe cavitation effects, generating an abundance of bubbles that were challenging to expel from the molten pool, ultimately deteriorating joint performance.

The aforementioned studies have demonstrated that ultrasonic assistance can effectively improve microstructure and reduce pore defects. However, numerous studies on ultrasonic-assisted laser welding are mainly focused on aluminum, magnesium, steel and other materials, with limited research on titanium alloys. The influence mechanism of ultrasonic vibration on pore defects in Ti6Al4V laser welding has not been reported, and the in-depth study on the microstructure is still limited. Besides, in order to optimize mechanical properties and obtain high quality welded joints, one of the critical goals of the paper is to identify the appropriate parameters of the ultrasonic-assisted laser welding process using orthogonal experiments. Compared with other experimental methods, orthogonal experiments are often more rapid and accurate in determining the best combination of process parameters, and it is also a key step to achieve high quality welding without increasing the cost. [25, 26]

Therefore, in this paper, the relationship between process parameters and tensile strength is established by orthogonal experiment. The optimum parameter combination was obtained and the influence of each factor on tensile strength was evaluated. In addition, the corresponding control experiments were carried out under the optimal combination, which further revealed the inhibition mechanism of ultrasonic on pore defects, and systematically studied the influence of ultrasonic assistance on the microstructure of Ti6Al4V such as grain boundary properties, dislocation density, grain size and texture strength. It is of great significance to expand the application of titanium alloy in the field of deep space exploration.

2.1 Materials

The experimental material used in this study was Ti6Al4V titanium alloy in the annealed state with the dimension of 100×100×2 mm³, and the chemical composition of the alloy was shown in Table 1.

Table 1

Chemical composition of the Ti6Al4V base metal (wt.%)
Materials	Elements (wt. %)
Ti6Al4V	Ti	Al	V	Mn	Fe	Ni	Cu	Zn
Ti6Al4V	Bal	6.01	4.02	0.320	0.003	0.001	0.006	0.65

2.2 Experiment

In this experiment, a fiber laser (RFL-C6000X) with a minimum spot diameter of 0.5 mm and a wavelength of 1060 nm was employed as welding device. The ultrasonic auxiliary system (JYD-2700S) employed in the experiment was composed of ultrasonic generator, horn, and transducer. The ultrasonic generator had a maximum amplitude of 20 µm, a frequency of 20 kHZ, and a power of 0 W to 2000 W. The ultrasonic energy was transmitted by direct contact between the horn and the base metal surface, the ultrasonic application angle was set to 45°. The intensity of the ultrasonic was controlled by controlling the output power of the ultrasonic during the experiment. The self-designed fixture was utilized to secure the horn and the base metal for ensuring optimal contact between them. The distance between the ultrasonic horn and the weld was maintained at 10 mm. Both the horn and the laser beam remain stationary during the welding process, while the mobile platform was controlled by the control system for movement. Before the welding experiment, the base metal was cleaned in absolute ethanol by ultrasonic for five minutes to remove the stains, and the oxidation layer on the surface of the titanium alloy was removed by laser cleaning machine. Argon gas was used as a protection gas in the welding process, and the protection gas flow rate was 15 mm³/min (as shown in Fig. 1).

2.3 Microstructure characterization and performance testing

After the welding experiment, the wire EDM cutter was employed to section metallographic, tensile, and pore characterization samples along the vertical weld direction. The position of the cutting sample was shown in Fig. 2a. The metallurgical samples were ground and polished using sandpaper, followed by etching for 42 seconds with Kroll reagent (HF: HNO₃: H₂O = 3:6:91). The cross-sectional morphology of the weld metallography was observed by optical microscope (OM, VHX-7000). The internal pore defects of the weld were detected by 3D industrial computed tomography (Easiy-TOM150), and the dimensions of the 3D scan was depicted in Fig. 2c. Prior to EBSD analysis, the metallographic sample was electrolytically polished with a polishing agent (10 ml perchloric acid + 30 ml n-butyl alcohol + 60 ml methyl alcohol). EBSD analysis was performed by using a EDAX-TSL type field emission electron scanning microscope with a detector (Nordly max3 EBSD). The detection area of EBSD was illustrated in Fig. 2b, with region 1 covering a detection range of 120×80 mm² and a scanning step size of 0.1µm, and region 2 covering a detection range of 500×400 mm² with a scanning step size of 0.6 µm. As depicted in Fig. 2d, the dimension of the tensile samples was determined in accordance with the international standard (ISO 6892: 1998). The tensile test was carried out using a stretching machine (UTM6104-20 kN) at 25°C at a rate of 1 mm/min. The tensile fracture morphology was observed using a scanning electron microscope (Carl Zeiss Evo 15).

2.4 Process parameters optimization

The welding parameters used in this study were given in Table 2, and the orthogonal experiment L9(3³) scheme was presented in Table 3.

Table 2

Welding parameters of the orthogonal experiment
	Laser power (W)	Welding speed (mm/s)	Ultrasonic power (W)
1	1200 (A₁)	15 (B₁)	0(C₁)
2	1300 (A₂)	20 (B₂)	500(C₂)
3	1400 (A₃)	25 (B₃)	1000(C₃)

Table 3

The orthogonal experimental layout scheme
	Laser power (W)	Welding speed (mm/s)	Ultrasonic power (W)
1	A₁	B₁	C₁
2	A₁	B₂	C₂
3	A₁	B₃	C₃
4	A₂	B₁	C₂
5	A₂	B₂	C₃
6	A₂	B₃	C₁
7	A₃	B₁	C₃
8	A₃	B₂	C₁
9	A₃	B₃	C₂

3.1 The orthogonal experiment

The range analysis was used to determine the extent of the influence of each factor on the experimental results by calculating the average value and range of the experimental results for each level factor, as presented in Table 5. Additionally, to reduce experimental errors and validate the effectiveness of the experiments, we conducted a statistical analysis of variance of the experimental results, as presented in Table 6 [27, 28]. The analysis of range and variance range were used to analyze the influence of factors on the tensile strength of ultrasonic assisted laser welding joints. The purpose was to assess the primary and secondary relationships, rank them and select the optimal combination of welding parameters. The results of the orthogonal experiment are given in Table 4.

Table 4

The results of the orthogonal experiment
	Laser power (W)	Welding speed (mm/s)	Ultrasonic power (W)	Tensile strength(MPa)
1	1200	15	0	862.6
2	1200	20	500	940.6
3	1200	25	1000	966.3
4	1300	15	500	969.2
5	1300	20	1000	997.2
6	1300	25	0	875.1
7	1400	15	1000	956.8
8	1400	20	0	874.2
9	1400	25	500	913.6

The results of the tensile strength range analysis are presented in Table 5. The range of ultrasonic power was 102.8, which was 3.18 times of welding power and 5.41 times of welding speed, indicating that the order of influence on the tensile strength of the joint was C (ultrasonic power) > A (laser power) > B (welding speed).

Table 5

The results of the tensile strength range analysis
	Laser power (A)	Welding speed (B)	Ultrasonic power (C)
K₁ total	2769.5	2788.6	2611.9
K₂ total	2841.5	2812	2823.4
K₃ total	2744.6	2755	2920.3
k₁ average	923.2	929.5	870.6
k₂ average	947.2	937.3	941.1
k₃ average	914.9	918.3	973.4
Range	32.3	19	102.8
Sequence	C > A > B
Optimized	A₂	B₂	C₃
Optimized combination: A₂B₂C₃

The results of tensile strength variance analysis were presented in Table 6. The P-value of factor C was 0.018, and the p-value was less than 0.05, indicating that the experimental results were statistically significant. Moreover, the p-value of C was smaller than the p-values of factors A and B. The results showed that ultrasonic power was a significant factor affecting laser welded joints, followed by laser power and welding speed. Therefore, the results of range analysis and variance analysis showed that the optimal combination of ultrasonic assisted welding parameters was laser power 1300 W, welding speed 20 mm/s, and ultrasonic power 1000 W.

Table 6

The results of the tensile strength variance analysis
Factor	SS	DOF	MS	F-value	P-value
A	1688.18	2	884.09	5.622	0.151
B	547.28	2	273.64	1.823	0.354
C	16581.38	2	8290.69	55.223	0.018
D(error)	300.26	2	150.13

The above results showed that the ultrasonic power has the greatest influence on the tensile properties of Ti6Al4V and the optimal parameter combination was Sample 5. In order to further analyze the reasons for the influence of ultrasonic assistance on the tensile strength of Ti6Al4V welded joints, we set the corresponding control groups as Sample 10 and Sample 11 under the optimal welding parameters. The specific experimental parameters are shown in Table 7.

Table 7

The experimental parameters of control group
	Laser power (W)	Welding speed (mm/s)	Ultrasonic power (W)
Sample 5	1300	20	1000
Sample 10	1300	20	500
Sample 11	1300	20	0

3.2 Weld section morphology

The macroscopic morphology of the weld section under different ultrasonic powers (Sample 5, 10, 11) was depicted in Fig. 3a, b and c. When the ultrasonic power was 0 W, obvious pore accumulation was observed in the bottom region of the FZ, whereas no pores were observed in the 500 W and 1000 W sections. Additionally, the prior-β columnar crystals were observed to grow from both sides to the weld center in the top region of FZ of different samples, which was typical epitaxial solidification feature [29]. And the prior-β columnar grain size was significantly reduced with 500 W and 1000 W ultrasonic treatment compared to without ultrasonic treatment.

3.3 Porosity

Porosity is the most critical factor affecting the tensile strength of joints. There are two main types of pores in the welding process, including the process porosity and the metallurgical porosity [10, 11]. The formation of metallurgical pores is primarily attributed to the entrapment of hydrogen or other gases within the weld pool during the welding process, leading to their inability to escape in a timely manner. These pores are characterized by small size and distribution at various locations within the weld. The formation of process porosity is attributed to the unstable collapse of the laser keyhole, which exhibits a large and irregular shape, primarily concentrated at the base of the weld. To quantitatively analyze the impact of ultrasonic-assisted laser welding on porosity, the 3D-CT tomography was employed to observe the size and quantity of pores in the joints. As shown in Fig. 4a, within the CT-3D field of view, a large number of pores were distributed throughout the weld when without ultrasonic power supplied. Among them, most of them were small pores with a diameter of 100 to 200µm, showing blue and green, and a few were large pores (more than 500 µm) with a diameter of 600 to 900 µm, showing orange. The statistical analysis results of pores were shown in Fig. 4d, e and f. With the increase of ultrasonic power, both the number of pores and the porosity gradually decreased. The formula for porosity calculation was as follows:

$$\:{\text{P}}_{\text{r}}=\frac{\sum\:\text{V}\text{r}}{\text{V}}\times\:100\%\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:（1）$$

In the laser welded sample without ultrasonic, the number of pores and porosity were 277 and 3.06%, respectively. Moreover, the number of macropores was 6. When the ultrasonic power was increased to 500W, the number of pores and porosity were reduced to 101 and 0.71%, respectively, and the number of macropores was reduced to 1. When the ultrasonic power was further increased to 1000W, the number of pores and porosity were reduced to 10 and 0.08%, respectively, and no macropores were present in the weld joint. The porosity of the joint was reduced by 97.4% compared with the weld without ultrasonic application.

The reduction of porosity was mainly attributed to the cavitation effect of ultrasonic. When the ultrasonic was applied to the molten pool, steady-state cavitation occurred under specific conditions. As the steady-state effect is generated, the interface between the melt was broken, thereby producing unstable cavitation bubbles. A surge was generated around the cavitation bubble during rupture, which accelerated the flow of the surrounding melt, drove the motion of bubbles in the melt, and promoted collision and merging of the bubbles. The merging of bubbles thus effectively increased the volume of bubbles in the molten pool. The increase in the volume of the bubble would increase its rising speed in the molten pool. The cavitation threshold induced by the cavitation effect could be expressed as follows [30]:

$$\:{\:P}_{T}={P}_{0}-{P}_{v}+\frac{2}{3\sqrt{3}}{\left[\frac{{\left(\frac{2\sigma\:}{{R}_{0}}\right)}^{3}}{{P}_{0}-{P}_{v}+\frac{2\sigma\:}{{R}_{0}}}\right]}^{\frac{1}{2}}\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(2\right)$$

where P₀ was 1 atm, which was equal to 0.1013 MPa [31]; P_v stood for the saturation vapor pressure, which was equal to 0 MPa[31]; σ was the surface tension coefficient, which was equal to 1.4 N/m [32]; R₀ was the radius of the cavitation nucleus, which was equal to 10^− 5 m [31]; The calculated result of formula (2) was 1.93×10⁵ MPa; When the intensity of the ultrasound exceeded a threshold, a cavitation effect occurred inside the molten pool. The intensity of the ultrasonic P_K in the molten pool could be expressed as follows [32]:

$$\:{P}_{K}=\:{\left[\frac{2P{C}_{l}{\rho\:}_{m}}{S}\right]}^{\frac{1}{2}}\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(3\right)\:\:$$

where P was the output power of the ultrasonic (0 W, 500 W and 1000 W); ρ_m was the density of melt, which was equal to 4208 kg/m³ [31]; C_l was the sound velocity of ultrasonic in titanium alloy melt, which was equal to 4407 m/s [32]; S was the area of horn end, which was equal to 7.854×10^− 5 m²; In this experiment, when the ultrasonic output power was P₁ = 500 W and P₂ = 1000 W, the corresponding homologous values were 1.537×10⁷ MPa and 2.250×10⁷ MPa, respectively. They were both above the threshold value P_T=1.93×10⁵ MPa.

Therefore, significant cavitation effects were observed in the molten pool under the ultrasonic power of 500 W and 1000 W. Whether these bubbles can transform into pore defects depends on their ability to rise out of the molten pool before the keyhole closes. According to the Stokes' formula[21], the bubble escape velocity V_e could be expressed as:

$$\:{V}_{e}=\frac{2\left({\rho\:}_{m}-{\rho\:}_{g}\right)g{r}^{2}}{9{\eta\:}_{m}}\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(4\right)\:$$

where g was the acceleration of gravity, which was equal to 9.8 N/kg; r was the pore radius; η_m was the liquid metal viscosity, which was equal to 0.005 gcm^− 1s^− 1 [32]; ρ_g was the bubble density, which was equal to 1.784 kg/m³ [33]. The solidification rate was assumed to be equivalent to a welding speed of 20 mm/s or higher [34]. The pore critical diameter calculated by Eq. (4) was 206.8 µm. The results indicate that when the diameter of bubbles within the molten pool exceeds 206.8 µm, they are able to escape before the keyhole closes, whereas pores with diameters below this critical threshold are unable to exit the molten pool and consequently lead to the formation of pores defect. Additionally, as shown in Fig. 4f, this conclusion was further supported by the observation that all remaining pores were smaller than 200 µm at the ultrasonic power of 1000 W.

3.4 Microstructure

3.4.1 Microstructure in the central of FZ

Figure 5 shows the microstructure of FZ region under different ultrasonic powers. It could be seen that the crisply martensite full the interior of the prior-β grain boundary. When cooling from the melting state to the solidification state, the liquid molten pool will be transformed into coarse prior-β grains. The prior-β grain boundaries of grains were shown in Fig. 5a.

According to the continuous cooling transition curve (as shown in Fig. 6), the high temperature β phase will transform into a structure related to its cooling rate. The cooling rate in the FZ region exceeds 410°C/s, resulting in the transformation of the high-temperature β phase into fine needle α' martensite [29, 35]. With the cooling of the FZ, the primary acicular α₁' martensitic phase grew at a 45° direction from the grain boundary in the prior-β grain and ceased its growth until reaching the other grain boundary. Then, as the molten pool continued to cool, a secondary precipitated α′ phase (α₂') was formed in a direction perpendicular to the α₁' martensite phase. The cross-growth of primary α₁' martensite and secondary α₂' martensite resulted in the formation of basket structures within the prior-β grains in the FZ region [36]. Figure 5b and c illustrated that, with the increase of ultrasonic power, the length of martensite α₁' was significantly reduced and the content of secondary precipitated phase α₂' was also increased, resulting in a more compact mesh basket structure.

3.4.2 Grain size

Grain size is one of the important factors affecting the tensile strength of joints. Due to the limitation of sampling range, the prior-β grain morphology was difficult to be observed completely,

Therefore, in order to ensure the rigor of scientific research, in this section, we expanded the sampling range (region 2 in Fig. 2b) and performed the parent grain crystallographic reconstruction (PGCR) for prior-β grains [37]. The Aztec-crystal software was employed to accurately assess the influence of ultrasonic treatment on the grain size of α' martensite and prior-β. As shown in Fig. 7a, e, i, it could be seen that there were large numbers of α' martensite with different orientations within the prior-β grains. However, the presence of a large number of α' martensite made it difficult to observe the grain boundary distribution and grain size of prior-β. Figure 7c, g and k show the reconstructed grain appearance, and no α’ martensite is observed at the reconstructed prior-β grain boundary. At the ultrasonic power of 0 W, it was observed that a large number of prior-β grains exhibited columnar crystal appearance. With the increase of the ultrasonic power, the number of prior-β columnar crystals was significantly decreased, while the number of small sized equiaxed crystals was increased. The statistical results of α' martensite are shown in Fig. 7b, f, and j. With the increase of the ultrasonic power, the average grain size of α' martensite was reduced by 10.4 µm to 10.0 µm and 9.7 µm respectively. Additionally, the proportion of small-sized grains (α' martensite size less than 9 µm) increased from 36.9–37.3% and 38.5% respectively. The degree of grain refinement for α' martensite reached 6.7%. The statistical results of prior-β shown in Fig. 7d, h, and l demonstrated that, with the increase of the ultrasonic power, the average grain size of prior-β was reduced by 83.6 µm to 73.6 µm and 71 µm respectively. Additionally, the proportion of small-sized grains (prior-β size less than 90 µm) increased from 62.54–67.34% and 70.14% respectively. The degree of grain refinement for prior-β reached 15.1%.

Ultrasonic vibration refines the grain size of prior-β through mechanical and cavitation effects. On the one hand, the application of ultrasonic to the molten pool increased the stirring frequency, which broke the epitaxial growth trend of prior-β columnar crystals. As a result, these coarse prior-β columnar crystals would subsequently be fragmented into numerous fine new dendrites. And these delicate dendrites offered a multitude of nucleation sites for the development of equiaxed crystals, thereby promoting grain refinement [19]. On the other hand, the ultrasonic cavitation effect generated a significant quantity of cavitation bubbles within the molten pool, the cavitation bubbles would expand, plug, and burst rapidly in the molten pool. Upon the collapse and closure of these bubbles, there would be instantaneous reduction in pressure surrounding the bubbles, leading to a rise in the melting point within the molten pool. It could be explained by the Clausius-Clapeyron equation [38]:

$$\:\frac{\varDelta\:T}{\varDelta\:P}={T}_{m}\frac{{V}_{1}-{V}_{2}\:}{\varDelta\:H}\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:（5）$$

where ΔT was the temperature change, T_m was the solidification temperature under atmospheric pressure, ΔP was the pressure change, ΔH was the latent heat of crystallization, V₁ was the liquid volume, and V₂ was the solid volume. The increase of melting point was equivalent to increasing the degree of undercooling in crystallization process, thereby increasing the nucleation rate of molten pool. The generation of a large number of new nuclei provides conditions for the subsequent growth of equiaxed crystals.

The reduction of α' martensite could be attributed to the following two aspects. On the one hand, the α' martensite nucleated and grew within the prior-β grain, thus the maximum length of the α' martensite was constrained by the prior-β grain boundary. The refined prior-β grain significantly reduced the overall size of the primary α₁' martensite. On the other hand, the cavitation effect of the ultrasonic vibration increased the cooling of the molten pool, which implied that the phase transformation (solidification) started at a temperature lower than the equilibrium phase transition temperature (994℃). The enhancement of undercooling facilitated the growth of the α₁' and α₂' martensite phase within the prior-β grain [39]. Compared with α₁' martensite, α₂' martensite exhibited a smaller phase size, thus the growth of large amounts of α₂' martensite perpendicular to α₁' martensite reduced the overall size of the α' martensite, which contributed to the formation of a compact basket structure. Hall-Patch provided the relationship between weld grain size and strength [40]:

$$\:{{\sigma\:}}_{1}={{\sigma\:}}_{0}-\:\text{k}{d}^{-1/2}\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:（6）$$

where $\:{{\sigma\:}}_{1}\:$was the strength of the weld, $\:{{\sigma\:}}_{0}$ was the material constant, k was the slope of Hall-Patch, and d represented the average grain size of the weld. Therefore, the joint strength of Ti6Al4V increased with the refinement of the average grain size of prior-β and α' martensite.

3.4.3 Grain boundary characteristics and dislocation density

The misorientation angle of grain boundaries reflects the atoms arrangement law, which greatly affects the microstructure characteristics and performances of materials [18]. Based on the orientation angle between adjacent grain boundaries, grain boundaries can be categorized into two types: high-angle grain boundaries (> 15°, HAGBs) and low-angle grain boundaries (2–15°, LAGBs). Figure 8a, b, and c show the grain boundaries distribution in the FZ region under different ultrasonic powers (region 1 in Fig. 2b), where the blue lines are LAGBs and the red lines are HAGBs. The results indicated that HAGBs were distributed across almost the entire region, with a limited LAGBs presented in the interior of α' martensite. Figure 8d, e and f illustrate the statistics charts of misorientation angle distributions. The results indicated that the grain boundary orientation exhibited multi-peak distribution characteristic under different ultrasonic powers, and mainly distributed around 2°, 10°, 60° and 90°. Furthermore, the majority of HAGBs were distributed around 60°. As the ultrasonic power increased, the proportion of LAGBs decreased from 10.8–8.95% and 7.03%, respectively, while the proportion of HAGBs increased from 89.2–91.05% and 92.97%, respectively. This indicated that ultrasonic assistance helped to promote the transition from LAGBs to HAGBs.

The phenomenon was attributed to the enhanced undercooling of the molten pool by ultrasonic cavitation effect. The decrease of the supercooling degree in the molten pool promoted the nucleation and growth of α₁' and α₂' martensite. A large number of α₂' martensite and α₁' martensite growing perpendicular to each other will form more HAGBs [41]. Compared with LAGBs, the HAGBs can impede the dislocation movement more effectively [42], and when the cracks expand to the HAGBs, it usually needs to consume a larger amount of energy, and thus plays a role in preventing crack expansion. Therefore, the sample of 1000 W had a stronger ability to retard brittle cracks, which significantly improved the joint strength.

During laser welding of Ti-6Al-4V alloys, the uneven of heat input inevitably results in an inhomogeneous distribution of residual stresses within the material, and this difference in distribution leads to inhomogeneity in grain deformation. As a result, many geometrically necessary dislocations (GNDs) are generated in the weld to maintain the coherence of grain deformation [43]. The distribution of GNDs in the weld can be reflected by the kernel average misorientation (KAM). Figure 9a, b and c show the KAM diagrams of 0 W, 500 W, 1000 W, where different colors reflect the distribution of GNDs within the material. The results indicated that at the ultrasonic power of 0W, the KAM diagram mainly showed green color with relatively high dislocation density. As shown in Fig. 9b and c, with the increase of ultrasonic power, the KAM map gradually transitioned from the original green to blue, and the average KAM value also decreased from 0.44° to 0.36° and 0.31° respectively. This indicated that the GNDs density of the weld was gradually reduced and the deformation of the weld grains were also reduced.

Ultrasonic assisted could reduce the dislocation density in the weld and improved the deformation resistance of the joint, which could be attributed to the following two aspects. From a microscopic point of view, residual stress caused the material lattice to be in a high-energy unstable state (dislocation state). When the ultrasonic wave was applied to the molten pool, the vibration effect it generated effectively increased the vibration frequency and amplitude of lattice atoms. This allowed the previously distorted atoms to return to their normal low-energy stable state after obtaining sufficient energy, thereby opening up dislocations and relaxing residual stresses [44]. From a macro point of view, the inhomogeneous of grain deformation is mainly affected by the uneven heat input. The stirring effect of ultrasonic waves in the molten pool promoted a more uniform temperature distribution, thus reducing the temperature gradient in the weld. Therefore, the residual stress was significantly reduced and the reduction in dislocation density significantly enhanced the resistance to grain deformation.

3.4.4 Texture strength

During the welding process, the uneven heat input leads to a significant temperature gradient inside the weld, which leads to the formation of coarse prior-β columnar grains in the weld pool. These prior-β columnar grains tend to have a specific grain orientation, which makes the joint strength of Ti6Al4V alloy significantly anisotropic in different loading directions[45]. Figure 10a, c and e show the polar figures of the α phase under different ultrasonic powers. It could be seen that the maximum texture strength of α phase all appeared on the crystal plane of {0001}, and with the increase of ultrasonic power, the maximum texture strength of α phase decreased from 9.48 to 6.71 and 6.34 respectively. The polar diagrams of reconstructed β phase are shown in Fig. 10b, d, f. It could be seen that the maximum texture strength of β phase under different ultrasonic power all appeared on the {100} crystal plane, and with the increase of ultrasonic power, the maximum texture strength of β phase decreased from 8.79 to 8.11 and 6.91, respectively. Previous studies have shown that the precipitation of α phase mainly distributed inside the prior-β grain boundary and followed the Burgers orientation relationship with the parent β phase, which makes the texture strength of the α phase consistent with that of the β phase [46]. The reduction of the texture strength of α and β phases by ultrasonic was mainly attributed to the refinement of prior-β grain. The stirring effect of ultrasonic broke the growth trend of prior-β columnar grains and promoted the formation of equiaxed grains, which reduced the anisotropy of the crystals. Therefore, the texture strength of α and β phases were both significantly reduced.

3.5 Mechanical properties

Figure 11 shows the tensile test results of the control sample and the base metal. The tensile strength of the base material was 1023.7 MPa and the tensile strength of the sample without ultrasonic was 920.1 MPa. When the ultrasonic power was gradually increased to 500 W and 1000 W, the tensile strength of the joint increased to 960.3 MPa and 997.2 MPa, respectively, representing a rise of 4.37% and 8.38%.

The joints under different ultrasonic power all failed in the FZ area. Figure 12a, d and g depict the macroscopic tensile fracture of the joint under different ultrasonic powers. It was evident that significant large-size pore defects were present on the fracture surface of the sample without ultrasonic treatment, but not on the fracture surface of the 500 W and 1000 W samples. Pore defects significantly deteriorate the mechanical properties of joints, as they lead to stress concentration during tensile loading, thereby accelerating the fracture of the weldment. Figure 12b and c depict the fracture surface (magnified in 500 times and 1000 times) under the ultrasonic power of 0 W. It was evident that a limited number of shallow dimples were distributed around the cleavage step, and the distinct characteristics of river patterns emerged on the cleavage step surface, indicating a mixed ductile and brittle fracture mode for the sample without ultrasonic treatment. Figure 12e and f show that, under the ultrasonic power of 500 W, the number of dimples distributed around the cleavage step increased, and the size of dimples also decreased significantly. As shown in Fig. 12h and i, under the ultrasonic power of 1000 W, a large number of dimples were observed in the fracture, and the number of dimples was larger and the size was also smaller, which was a typical ductile fracture feature. The large number of aggregated dimples significantly enhanced the strength of the joint.

The orthogonal experiment of ultrasonic-assisted laser welding of Ti6Al4V titanium alloy was performed, considering factors such as ultrasonic power, laser power, and welding speed. The range and variance methods were employed to analyze the influence of factors on the tensile strength of the joints. The control group experiment was set up to further analyze the reasons for the influence of ultrasonic assist Ti6Al4V joints. The main results are summarized as follows:

1 According to the orthogonal experimental results, the optimal parameters were ultrasonic power 1000 W, laser power 1300 W, and welding speed 20 mm/s. The most important factor affecting the tensile strength was ultrasonic power, followed by laser power and welding speed, and the maximum tensile strength of the joint was 997.2 MPa.

2 The ultrasonic cavitation effect significantly reduced the porosity of the joint. According to the calculation results, the cavitation effect was produced in the molten pool under the ultrasonic power of 500 W and 1000 W. The cavitation effect helped to increase the volume of the bubble, thus increasing the escape velocity of the bubble and reducing the porosity of the joint, compared with the sample without ultrasonic, the porosity of the joint decreased from 3.06–0.08%, and the porosity decreased by 97.4%.

3 The microstructure of the FZ region was composed of columnar prior-β grain boundary and acicular α' martensite. With the increase of ultrasonic power, the length of α₁' martensite decreased significantly and the content of α₂' martensite increased significantly. The mesh structure was more compact. The content of HAGBs increased from 89.2–92.97%, and the average value of KAM decreased from 0.44 to 0.41.

4 Ultrasonic could promote the transformation of prior-β columnar crystals into equiaxed crystals and refined the grain sizes of α' and prior-β. The growth trend of prior-β columnar crystals was broken by the stirring effect of ultrasonic on the melting pool, resulting in a reduction in the average grain size of prior-β from 88.6 µm to 71.0 µm and a decrease in the β phase on the {100} plane from 8.79 to 6.91. Due to the limitation of grain boundaries and the increase of undercooling, the average size of α' martensite decreased from 10.4 µm to 9.7 µm and the proportion of small grains (less than 20 µm) was increased from 36.9–38.5%. The maximum texture strength of the α' phase on the {0001} plane decreased from 9.48 to 6.34.

5 With the increase of ultrasonic power, the number of dimples gradually increased and the size gradually decreased. The joint fracture mode changed from ductile and brittle mixed fracture to ductile fracture mode. Compared with those without ultrasonic treatment, the tensile strength of the joint increased from 920.1 MPa to 997.2 MPa, which increased by 8.38%.

Author contribution

Yue Chen: revision, validation, methodology, writing original draft; Congyi Liang: methodology, data curation, writing; Haozhong Lin: data collection, Haoyuan Tan: validation; Longfei Zhang: revision; Wei Wei: revision; Zhou Jun: revision;

Funding

This work was supported by the Guangxi Science and Technology Base and Talent Special Project (grant no. GKAD21220038).

Ethical approval Not applicable.

Consent to participate Not applicable.

Consent to publish Not applicable.

Conflict of interest The authors declare no competing interests.

Z Wu, H Kou, N Chen, Z Xi, J Fan, B Tang, J Li (2022) Recent developments in cold dwell fatigue of titanium alloys for aero-engine applications: A review. J Mater Res Technol 20: 469-484. http://dx.doi.org/https://doi.org/10.1016/j.jmrt.2022.07.094
R Balasubramanian, R Nagumothu, E Parfenov, R Valiev (2021) Development of nanostructured titanium implants for biomedical implants–A short review. Materials Today: Proceedings 46: 1195-1200. http://dx.doi.org/https://doi.org/10.1016/j.matpr.2021.02.064
I Gurrappa (2003) Characterization of titanium alloy Ti-6Al-4V for chemical, marine and industrial applications. Mater Charact 51: 131-139. http://dx.doi.org/https://doi.org/10.1016/j.matchar.2003.10.006
D Kyratzis, H Collaboration (2023) Latest advancements of the HERD space mission. Nucl Instrum Methods Phys Res, Sect A: Accelerators, Spectrometers, Detectors and Associated Equipment 1048: 167970. http://dx.doi.org/https://doi.org/10.1016/j.nima.2022.167970
E Akman, A Demir, T Canel, T Sınmazçelik (2009) Laser welding of Ti6Al4V titanium alloys. J Mater Process Technol 209: 3705-3713. http://dx.doi.org/https://doi.org/10.1016/j.jmatprotec.2008.08.026
H Ertek Emre, Ş Arslan (2019) Effect of laser welding on microstructure and mechanical properties of biomedical Ti6Al4V. Applied Physics A 125: 755. http://dx.doi.org/https://doi.org/10.1007/s00339-019-3045-3
T Yang, J Liu, Y Zhuang, D Xu (2022) Micro-crystallographically unraveling strength-enhanced behaviors of the heavy-thick Ti-6Al-4V welded joint. Mater Charact 188: 111893. http://dx.doi.org/https://doi.org/10.1016/j.matchar.2022.111893
J Liu, T Yang, Y Zhuang, L Li, M Huang, X Su, S Dong (2024) Micro-crystallographically revealing the plasticity-evolved behaviors of thick Ti–6Al–4V laser-arc hybrid welded joint. J Mater Res Technol 28: 459-467. http://dx.doi.org/https://doi.org/10.1016/j.jmrt.2023.12.007
Y Fu, N Guo, B Zhu, X Shi, J Feng (2020) Microstructure and properties of underwater laser welding of TC4 titanium alloy. J Mater Process Technol 275: 116372. http://dx.doi.org/https://doi.org/10.1016/j.jmatprotec.2019.116372
X Kang, T Yan, L Wang, Q Gao, X Zhan (2022) Study on the distribution, element characteristics, and formation mechanism of porosity during laser welding for Ti-6Al-4V bottom-locking joint. Int J Adv Manuf Technol 123: 2691-2701. http://dx.doi.org/https://doi.org/10.1007/s00170-022-10302-3
X-L Gao, L-J Zhang, J Liu, J-X Zhang (2014) Porosity and microstructure in pulsed Nd: YAG laser welded Ti6Al4V sheet. J Mater Process Technol 214: 1316-1325. http://dx.doi.org/https://doi.org/10.1016/j.jmatprotec.2014.01.015
Z Lei, J Bi, P Li, T Guo, Y Zhao, D Zhang (2018) Analysis on welding characteristics of ultrasonic assisted laser welding of AZ31B magnesium alloy. Opt Laser Technol 105: 15-22. http://dx.doi.org/https://doi.org/10.1016/j.optlastec.2018.02.050
Z Lei, J Bi, P Li, Q Li, Y Chen, D Zhang (2018) Melt flow and grain refining in ultrasonic vibration assisted laser welding process of AZ31B magnesium alloy. Opt Laser Technol 108: 409-417. http://dx.doi.org/https://doi.org/10.1016/j.optlastec.2018.07.015
S Zhou, B Wang, D Wu, G Ma, G Yang, W Wei (2021) Follow-up ultrasonic vibration assisted laser welding dissimilar metals for nuclear reactor pump can end sealing. Nucl Mater Energy 27: 100975. http://dx.doi.org/https://doi.org/10.1016/j.nme.2021.100975
A Kolubaev, O Sizova, S Fortuna, A Vorontsov, A Ivanov, E Kolubaev (2020) Weld structure of low-carbon structural steel formed by ultrasonic-assisted laser welding. J Constr Steel Res 172: 106190. http://dx.doi.org/https://doi.org/10.1016/j.jcsr.2020.106190
S Kumar, C Wu, G Padhy, W Ding (2017) Application of ultrasonic vibrations in welding and metal processing: A status review. J Manuf Process 26: 295-322. http://dx.doi.org/https://doi.org/10.1016/j.jmapro.2017.02.027
D Wang, H Bo, H Meng, H Li, D Zhuang, N Xu, J Shen, X Li, Q Li, S Shao (2023) Ultrasonic vibration induced the improvement of mechanical properties and fracture behavior of Inconel 690 alloy joint during laser welding. J Mater Res Technol 24: 2064-2073. http://dx.doi.org/https://doi.org/10.1016/j.jmrt.2023.03.119
H Liao, Z Wang, B Zhang, P Chi, Y Wang, J Tian, X Wu, Q Zhang (2023) Microstructure and mechanical properties of SUS304 weldments manufactured by ultrasonic vibration assisted local dry underwater welding. J Mater Process Technol 322: 118183. http://dx.doi.org/https://doi.org/10.1016/j.jmatprotec.2023.118183
Z Liu, X Jin, J Zhang, Z Hao, J Li (2022) Microstructure evolution and mechanical properties of SUS301L stainless steel sheet welded joint in ultrasonic vibration assisted laser welding. Opt Laser Technol 153: 108193. http://dx.doi.org/https://doi.org/10.1016/j.optlastec.2022.108193
J Guo, J Wang, L Cheng, Y Duan, X Zhan (2024) Unravelling the mechanism of columnar-to-equiaxed transition and grain refinement in ultrasonic vibration assisted laser welding of Ti6Al4V titanium alloy. Ultrasonics 141: 107342. http://dx.doi.org/https://doi.org/10.1016/j.ultras.2024.107342
Q Li, G You, L Wang, S Zeng, F Yao, J Hou (2022) Coupled effect of ultrasonic vibration and post-weld remelting on the porosity and microstructure of die-cast magnesium alloy AZ91D. J Manuf Process 83: 742-755. http://dx.doi.org/https://doi.org/10.1016/j.jmapro.2022.09.033
B Cui, K Chen, Y Yang, Y Lv, F Zhang, S Liu (2024) Effect of ultrasonic vibration on the pores and properties of the laser-arc hybrid welding joint of high nitrogen steel. Mater Chem Phys 318: 129297. http://dx.doi.org/https://doi.org/10.1016/j.matchemphys.2024.129297
J Liu, H Zhu, Z Li, W Cui, Y Shi (2019) Effect of ultrasonic power on porosity, microstructure, mechanical properties of the aluminum alloy joint by ultrasonic assisted laser-MIG hybrid welding. Opt Laser Technol 119: 105619. http://dx.doi.org/https://doi.org/10.1016/j.optlastec.2019.105619
C Tan, B Xu, F Liu, Y Zhao, D Lin, L Wu, B Chen, X Song (2022) Effect of ultrasonic vibration on porosity suppression and columnar-to-equiaxed transition in laser-MIG hybrid welding of aluminum alloy. Int J Adv Manuf Technol 122: 2463-2474. http://dx.doi.org/https://doi.org/10.1007/s00170-022-10034-4
R Ranjan, SK Jha (2024) Optimization of welding parameters and microstructure analysis of low frequency vibration assisted SMAW butt welded joints. Int J Interact Des Manuf (IJIDeM) 18: 1687-1707. http://dx.doi.org/https://doi.org/10.1007/s12008-023-01562-8
C Kumar, M Das, C Paul, B Singh (2017) Experimental investigation and metallographic characterization of fiber laser beam welding of Ti-6Al-4V alloy using response surface method. Opt Lasers Eng 95: 52-68. http://dx.doi.org/https://doi.org/10.1016/j.optlaseng.2017.03.013
X Huang, L Zhang, J-M Zhang, C Chen, X Lu, L Sun (2024) Orthogonal design optimization for Cu/Sn58Bi-0.4 Mg/Cu solder joint strength in ultrasonic-assisted soldering. Mater Today Commun 38: 107669. http://dx.doi.org/https://doi.org/10.1016/j.mtcomm.2023.107669
G Li, Y Wang, Y Liang, P Gao, X Liu, W Xu, D Yang (2024) Microstructure and mechanical properties of laser welded Ti-6Al-4V (TC4) titanium alloy joints. Opt Laser Technol 170: 110320. http://dx.doi.org/https://doi.org/10.1016/j.optlastec.2023.110320
J Zhao, J Zhang, Q Gao, T Yan, X Zhan (2022) Research on microstructure evolution and tensile characteristics of Ti6Al4V double lap-fillet joints fabricated by dual laser-beam bilateral synchronous welding. Opt Laser Technol 145: 107472. http://dx.doi.org/https://doi.org/10.1016/j.optlastec.2021.107472
J Kang, X Zhang, S Wang, J Ma, T Huang (2015) The comparison of ultrasonic effects in different metal melts. Ultrasonics 57: 11-17. http://dx.doi.org/https://doi.org/10.1016/j.ultras.2014.10.004
D Yuan, S Shao, C Guo, F Jiang, J Wang (2021) Grain refining of Ti-6Al-4V alloy fabricated by laser and wire additive manufacturing assisted with ultrasonic vibration. Ultrason Sonochem 73: 105472. http://dx.doi.org/https://doi.org/10.1016/j.ultsonch.2021.105472
C Panwisawas, B Perumal, RM Ward, N Turner, RP Turner, JW Brooks, HC Basoalto (2017) Keyhole formation and thermal fluid flow-induced porosity during laser fusion welding in titanium alloys: Experimental and modelling. Acta Mater 126: 251-263. http://dx.doi.org/https://doi.org/10.1016/j.actamat.2016.12.062
J Kar, D Chakrabarti, SK Roy, GG Roy (2019) Beam oscillation, porosity formation and fatigue properties of electron beam welded Ti-6Al-4V alloy. J Mater Process Technol 266: 165-172. http://dx.doi.org/https://doi.org/10.1016/j.jmatprotec.2018.10.040
R Rai, G Roy, T DebRoy (2007) A computationally efficient model of convective heat transfer and solidification characteristics during keyhole mode laser welding. J Appl Phys 101. http://dx.doi.org/https://doi.org/10.1063/1.2537587
Y Liu, J Wang, T Yan, X Yin, X Zhan (2024) Effect of synchronous gas cooling method on microstructure and mechanical properties of laser welded joint for Ti6Al4V titanium alloy. Mater Today Commun 39: 108779. http://dx.doi.org/https://doi.org/10.1016/j.mtcomm.2024.108779
J Du, H Liu, F Wang, W Bao, H Li, T Liu (2023) Effects of heat input on microstructure evolution, mechanical and corrosion properties of TC4 alloy by keyhole TIG welding. J Mater Res Technol 27: 5266-5277. http://dx.doi.org/https://doi.org/10.1016/j.jmrt.2023.11.047
L Germain, N Gey, R Mercier, P Blaineau, M Humbert (2012) An advanced approach to reconstructing parent orientation maps in the case of approximate orientation relations: Application to steels. Acta Mater 60: 4551-4562. http://dx.doi.org/https://doi.org/10.1016/j.actamat.2012.04.034
T Yuan, S Kou, Z Luo (2016) Grain refining by ultrasonic stirring of the weld pool. Acta Mater 106: 144-154. http://dx.doi.org/https://doi.org/10.1016/j.actamat.2016.01.016
Z Li, W Zhao, H Zhang, G Xiao, K Yu (2023) Microstructure and corrosion resistance of fusion zone in Ti-6Al-4V alloy welded using pulsed-and continuous-wave lasers. Corros Sci 220: 111269. http://dx.doi.org/https://doi.org/10.1016/j.corsci.2023.111269
R Masumura, P Hazzledine, C Pande (1998) Yield stress of fine grained materials. Acta Materialia 46: 4527-4534. http://dx.doi.org/https://doi.org/10.1016/S1359-6454(98)00150-5
J Teng, P Jiang, Q Cong, X Cui, M Nie, X Li, Z Zhang (2024) A comparison on microstructure features, compression property and wear performance of TC4 and TC11 alloys fabricated by multi-wire arc additive manufacturing. J Mater Res Technol 29: 2175-2187. http://dx.doi.org/https://doi.org/10.1016/j.jmrt.2024.01.256
T Yan, Y Liu, J Wang, Y Zheng, X Zhan (2024) Bridging microstructure and crystallography with the crack propagation behavior in Ti-6Al-4V alloy fabricated via dual laser beam bilateral synchronous welding. J Mater Sci Technol 174: 1-14. http://dx.doi.org/https://doi.org/10.1016/j.jmst.2023.05.058
L Zhang, Z Wu, Y Li, Y Feng, F Zhao (2024) Study on microstructure, mechanical and corrosion behavior of Ti-6Al-4V titanium alloy by Keyhole gas tungsten arc welding. Mater Today Commun: 109530. http://dx.doi.org/https://doi.org/10.1016/j.mtcomm.2024.109530
Q Yang, Z Dong, R Kang, Z Wei (2024) Effect of ultrasonic vibration and deep cryogenic treatment on the residual stress, mechanical properties and microstructures of HR-2 austenitic stainless steel. Mater Sci Eng, A 900: 146502. http://dx.doi.org/https://doi.org/10.1016/j.msea.2024.146502
K Wang, R Bao, D Liu, C Yan (2019) Plastic anisotropy of laser melting deposited Ti–5Al–5Mo–5V–1Cr–1Fe titanium alloy. Mater Sci Eng, A 746: 276-289. http://dx.doi.org/https://doi.org/10.1016/j.msea.2019.01.031
M Qi, S Huang, Y Ma, SS Youssef, R Zhang, J Qiu, J Lei, R Yang (2021) Columnar to equiaxed transition during β heat treatment in a near β alloy by laser additive manufacture. J Mater Res Technol 13: 1159-1168. http://dx.doi.org/https://doi.org/10.1016/j.jmrt.2021.05.057

Download PDF

Reviewers agreed at journal
02 Sep, 2024
Reviewers invited by journal
02 Sep, 2024
Editor invited by journal
28 Aug, 2024
Editor assigned by journal
27 Aug, 2024
First submitted to journal
27 Aug, 2024

You are reading this latest preprint version

Optimization of process parameters for ultrasonic-assisted laser welding of Ti6Al4V alloy and the impact of ultrasonic power on porosity, microstructure, and mechanical properties

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental procedures

2.1 Materials

2.2 Experiment

2.3 Microstructure characterization and performance testing

2.4 Process parameters optimization

3. Results and discussion

3.1 The orthogonal experiment

3.2 Weld section morphology

3.3 Porosity

3.4 Microstructure

3.4.1 Microstructure in the central of FZ

3.4.2 Grain size

3.4.3 Grain boundary characteristics and dislocation density

3.4.4 Texture strength

3.5 Mechanical properties

4. Conclusion

Declarations

References

Status:

Version 1