Effect of ultrasonic vibration characteristics on mechanical properties of stainless steel laser weld

2.1. Model establishment

The propagation rate of ultrasonic wave is greatly affected by temperature [7], and it can still maintain a stable wavelength in a closed environment. Therefore, the parametric model of ultrasonic in laser welding is established by finite element simulation method, and the variation law of characteristic frequency [8, 9] and sound pressure is solved and studied based on COMSOL. Due to the remarkable reflection of ultrasonic wave, the simulation and calculation are carried out in the way of sound vibration coupling. Its process and principle are shown in Fig. 1. Considering that the ultrasonic vibration can significantly affect the sound pressure, the scheme has high calculation accuracy.

Fig. 1Flow chart of coupling simulation

According to the physical properties of ultrasonic, under the condition of specific ultrasonic frequency, when the wavelength meets a certain size relationship with the propagation space, the standing wave mode [10] will appear. At this time, the characteristic frequency can be calculated accurately. Combined with the characteristics of 304 stainless steel laser welding process, the sound field simulation model is established, as shown in Fig. 2, including strong sound field area, sub strong sound field area and weak sound field area. In ICEM software, the model is meshed, the numerical calculation in the boundary layer is considered, and the perfect matching layer and absorption layer in the grid are distinguished to avoid the sound wave on the boundary being reflected and reduce the error interference. For the mesh generation of perfect matching layer, set the calculation width as 0.5-1 times of the wavelength, and define its equal fraction in the radial direction as 50. For the absorption layer, the minimum cell size is 0.12 mm according to the structural size of the laser beam emitting end, and the final meshing result is shown in Fig. 3.

Fig. 2Composition of ultrasonic sound field model

Fig. 3Meshing results

The analysis type of transient model is the display calculation based on convective wave equation, and the core algorithm is discontinuous Galerkin algorithm [11]. The algorithm has low requirements for grid distortion, and only two element grids are needed for calculation in a single wavelength. However, because laser welding is a typical high-temperature environment, it is difficult to achieve convergence under large temperature difference distribution. Therefore, in the preliminary calculation, it is necessary to verify the mesh density of the perfect matching layer. The characteristic frequency distribution of ultrasonic is essentially the instantaneous state of sound field, and the actual amplitude cannot be obtained. Only by determining the actual impedance law of ultrasonic, can the specific amplitude be solved. Different from the time domain analysis, the frequency domain analysis of ultrasonic field is the distribution law of stable sound field, which reflects the final response result, and the amplitude is the relative amplitude. In the calculation of frequency domain characteristics, the governing equation of sound pressure is:

where $p_t$ is the total sound pressure field, ${\rho }_0$ is density field, $q_d$ is the dipole source vector, c is ultrasonic velocity, $\omega$ is ultrasonic emission frequency, and $Q_m$ is the monopole source.

Sound velocity is one of the most typical characteristics of ultrasonic finite element model, and its boundary condition in the model is defined as:

where $n$ is the direction vector under the condition of constant background velocity and density, $c_0$ is the instantaneous ultrasonic velocity vector, $f$ is the characteristic frequency and $A_v$ is the amplitude.

The convergence diagram in the simulation process is shown in Fig. 4. It can be seen that the whole iterative calculation is very stable, the step interval difference is small, and the convergence of the model is very satisfactory.

Fig. 4Convergence diagram in simulation process

2.2. Characteristic frequency analysis

In COMSOL, the sound field state at different characteristic frequencies can be solved based on Helmholtz equation, and the relative amplitude can be normalized. According to the emission range of ultrasonic excitation frequency, the results of extracting the first four steps are shown in Fig. 5. It can be seen that at the first-order characteristic frequency, the maximum relative amplitude is close to the laser emission center, and the ultrasonic propagation is obviously restrained under the influence of the structure of the laser head. The standing wave energy generated by the second-order sound field is less, and the sound field separation in the weld area is large. Although the weld area is close to the sound source, the relative amplitude is small, and the traveling wave plays a leading role. The distribution of the third-order sound field is similar to that of the fourth-order sound field, which is dominated by the standing wave, and there is relatively large amplitude at the tip. It can be seen that when the ultrasonic wave works at the second-order characteristic frequency, it can achieve better weld excitation effect.

Fig. 5Relative amplitude distribution at different characteristic frequencies

a) 1st order frequency (14569 Hz)

b) 2nd order frequency (20145 Hz)

c) 3rd order frequency (22896 Hz)

d) 4th order frequency (23314 Hz)

2.3. Sound pressure distribution law

Because the ultrasonic emission model is a semi closed structure, the ultrasonic will be output after multiple reflections and superposition in the transmitting cavity of the horn. At this time, the output ultrasonic sound pressure will be distributed in a complex state, which has an important impact on the weld pool. Through the simulation analysis, the cloud diagram of sound pressure distribution under the first four characteristic frequencies can be obtained, as shown in Fig. 6.

Fig. 6Sound pressure distribution at different characteristic frequencies

a) 1st order frequency (14569 Hz)

b) 2nd order frequency (20145 Hz)

c) 3rd order frequency (22896 Hz)

d) 4th order frequency (23314 Hz)

It can be seen that since the sound pressure at the outlet of the ultrasonic transmitting cavity is determined by the initial pressure, reflection pressure and reverberation pressure, the closer to the central axis, the greater the value of sound pressure. The sound pressure is significantly affected by the excitation frequency, and the sound pressure distribution is similar to the relative amplitude distribution. Compared with other excitation frequencies, the sound pressure at the second-order characteristic frequency is more concentrated and effective.

3.1. Welding system design

In order to accurately test the dynamic characteristics of laser welding pool, the welding system is designed properly and innovatively in the paper, including laser welding system, data acquisition system and ultrasonic control system, as shown in Fig. 7. Through the adjustment of ultrasonic vibration parameters, the comparative results of flow response of 304 stainless steel pool can be obtained. The data acquisition system can realize the transmission of signals and instructions based on PCI data card, and feed back to the laser welding system with the application of LINKS - 11S01 linker to realize precise control of welding current, flow rate of shielding gas (Argon), and trajectory of laser welding gun. The working parameters of ultrasonic power supply and transducer can be effectively adjusted by ultrasonic control system, so as to realize the control of ultrasonic vibration frequency and amplitude. The ultrasonic excitation power supply with the maximum frequency of 500 kW is used for the ultrasonic excitation system. Through the ultrasonic laser welding system, the image acquisition of weld pool under different ultrasonic vibration frequencies [12], the real-time monitoring of weld pool flow rate, metal vapor pressure and other data can be tested simultaneously, which provides an important basis for the application efficiency of ultrasonic vibration in laser welding.

Fig. 7Composition of ultrasonic vibration laser welding system

3.2. Process parameter design

In order to improve the balance of energy and shielding gas flow, the inclination angle of laser welding gun is set to 10°, the included angle between welding wire and base metal is set to 15°, and the direction of argon flow rate is set to 45°. The motion control of the welding gun is realized based on the stepping motor, and except for the welding gun, other auxiliary mechanisms remain stationary in the working face. According to the welding procedure qualification standard, the base metal of stainless steel is processed into standard welding samples, as shown in Fig. 8. At the same time, the main parameters of its laser welding process are set as shown in Table 1.

3.3. Welding effect of ultrasonic vibration

According to the sound field simulation results of ultrasonic laser welding, the second-order characteristic frequency is more conducive to the improvement of energy efficiency. In order to better study the welding effect, the ultrasonic frequency is set to 20145 Hz.

Fig. 8Weld sample processing scheme

Through the image monitoring of 304 stainless steel weld during the welding process, the transient weld pool area is obtained, as shown in Fig. 9. It can be seen that although ultrasonic has little effect on the final weld appearance, it can effectively change the weld pool area under transient conditions [13]. The maximum weld pool area of conventional laser welding is 6.68 mm², and the fluctuation range is 18.3 %. Under the influence of ultrasonic vibration, the maximum molten pool area is reduced to 5.38 mm² and the fluctuation range is reduced to 3.9 %. It can be seen that ultrasonic vibration can effectively improve the stability of molten state of stainless steel weld pool, is conducive to full solidification and crystallization, and effectively reduce the probability of small pores and depressions.

Fig. 9Variation law of transient molten pool area

At different positions away from the center of the weld, the distribution laws of molten pool flow rate and metal vapor pressure at 1 s can be measured, as shown in Fig. 10 and Fig. 11 respectively. It can be seen that the flow velocity of 304 stainless steel weld pool in the weld center decreases significantly after ultrasonic wave is applied. When it exceeds ± 2 mm from the weld center, the ultrasonic wave has little effect on the flow velocity of the molten pool. The distribution of steam pressure is opposite to the flow velocity of molten pool. When it is far away from the weld center, ultrasonic vibration will significantly increase the steam pressure. With the decrease of the distance from the weld center, the effect of ultrasonic on metal vapor pressure is no longer obvious. It can be seen that ultrasonic vibration can enhance the curing effect of the center of the molten pool and improve the thermal stability of the central weld away from the heat source.

Fig. 10Distribution of maximum flow velocity

Fig. 11Distribution of metal vapor pressure

4.1. Metallographic structure analysis

In order to study the effect of ultrasonic frequency on the mechanical properties of weld, laser weld samples of stainless steel were prepared at the first-order (ultrasonic frequency 1), second-order (ultrasonic frequency 2) and third-order (ultrasonic frequency 3) characteristic frequencies respectively. The metallographic structure of the weld sample is shown in Fig. 12.

Fig. 12Metallographic structure of weld under different ultrasonic frequencies

a) No ultrasound

b) Ultrasonic frequency 1

c) Ultrasonic frequency 2

d) Ultrasonic frequency 3

It can be seen that ultrasonic has a significant effect on the microstructure morphology. The grain size under ultrasonic frequency 1 and ultrasonic frequency 2 is smaller than that without ultrasonic vibration. The grain size under ultrasonic frequency 3 is relatively coarse, and ferrite is evenly distributed around pearlite in the form of blade. The grain shape under the condition of ultrasonic frequency 2 is more similar to that without ultrasonic vibration. However, due to the small flow rate of the molten pool and the high pressure of the surrounding metal vapor, the possibility of splashing in the molten pool is greatly reduced, so the uniformity of the weld thickness direction is effectively improved.

In order to study the effect of ultrasonic on the wear resistance of 304 stainless steel weld, constant speed friction test was carried out on weld samples under different ultrasonic frequencies. The normal load of the friction pair is defined as a constant value of 150 N, and the relative angular velocity of the pin disc is set as 6 rad/s under the condition of dry friction. Finally, the wear morphology is obtained, as shown in Fig. 13. It can be seen that ultrasonic vibration can effectively improve the wear resistance of 304 stainless steel weld. Among them, the weld surface scratch depth under ultrasonic frequency 2 is the smallest, the furrow effect is weak, and there is no abrasive wear.

Fig. 13Metallographic structure of weld under different ultrasonic frequencies

a) No ultrasound

b) Ultrasonic frequency 1

c) Ultrasonic frequency 2

d) Ultrasonic frequency 3

4.2. Microhardness and corrosion resistance analysis

Hardness is one of the important indexes to measure the comprehensive mechanical properties of weld. Through the HVS-5Z microhardness tester, the hardness distribution under different ultrasonic frequencies can be obtained, as shown in Fig. 14. It can be seen that ultrasonic vibration can increase the microhardness value of stainless steel weld surface by more than 25 %. Under the action of limited ultrasonic excitation frequency, the hardness of weld increases with the increase of frequency. Generally, the increase of hardness will improve the strength and wear resistance of materials, and the probability of insufficient toughness will also increase. Therefore, the compromise value is preferred in engineering to meet good comprehensive properties.

According to these test results, it can be known that the second-order characteristic frequency of ultrasonic can effectively improve the mechanical properties of 304 stainless steel weld. In order to verify its influence on corrosion resistance, the three weld samples were placed in the acidic environment with constant temperature of room temperature and 100 ℃ respectively, and the change law of oxidation amount was tested, as shown in Fig. 15. It can be seen that ultrasonic vibration can significantly reduce the amount of oxidation, but has little effect on the stable corrosion time. After 40 hours, the oxidation amount of the weld under different test environments reached a stable state, which shows that the corrosion resistance of the weld is more affected by the physical properties of the material itself.

Fig. 14Microhardness distribution under different ultrasonic frequencies

Fig. 15Variation law of weld oxidation