It is difficult to collect the crack propagation signal under general continuous welding condition due to other signal interference of molten pool. In order to study the effect of residual stress on crack propagation, acoustic emission technology was successfully applied to monitor welding process according to the characteristics of pulsed laser welding. Crack free welding is achieved by reducing the pulse interval to limited the crack size of single pulse welding spot. The welding process was monitored synchronously by high speed photography and acoustic emission, the evidence of crack propagation after solidification of weld is successfully captured.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Laser welding has the advantages of high energy density and high efficiency. Solidification cracks are common defects in laser welding of aluminum alloys [1,2]. Compared with continuous lasers, pulsed lasers can not only inhibit the solidification cracks by designing ramp-down pulse shaping, but also reduce crack sensitivity by appropriately increasing the average laser power and pulse frequency [3–5].
At present, most researches on thermal cracking in the field of casting and welding focus on solidification process of the molten pool. In view of the limitations of the specific environment, it is challenging to accurately characterize the susceptibility to solidification cracks during welding process. Bakir et al.  used a special experimental device with a high-speed camera to measure the strain during welding. The critical strain required for the formation of stainless-steel solidification cracks is determined locally and globally. Similarly, the X-ray phase contrast method was employed to observe the internal characteristics of solidification and crack propagation directly in laser spot welding of aluminum alloys .
The welding residual stress (WRS) is a key part of welding research for metal components. However, there were few studies on how this part of the residual stress affects the propagation of solidification cracks. In the laser-arc hybrid welding of steel, the interaction of the phase change and strain hardening has a very significant effect on WRS . In the case of artificially adding constraints, the stronger constraint strength promotes the initiation of cracks. Under the condition of unilateral constraints, higher confinement strength is beneficial to promote crack growth. However, under the condition of bilateral constraints, it is beneficial to suppress crack growth . In the research of Lei et al. , the residual stress distribution of laser welded Al-Li alloy parts was studied by a combination of profile method and finite element simulation. The peak value of tensile stress appeared in the center of the weld, and decreased as the distance away from the weld center increasing. Thought combining the results of numerical simulation and high-speed photography, Vrancken et al.  determined the temperature range of tungsten microcracks appearing in additive manufacturing, and the direction of the principal stress determines the crack shape.
Similar to pore defects, it is difficult to observe the internal topography and propagation of the hot crack. Acoustic emission (AE) technology has applications in nondestructive of workpieces after welding. For example: location of cracks . Research has shown that AE is very sensitive to residual stress and can be used as a tool for monitoring the release of residual stress and qualitatively assessing residual stress in the future . The plasma generated recoil force on the molten pool during laser welding, and the energy generated by vibration process can be captured by AE equipment. In the pulsed laser welding process, Luo et al.  used AE technology to study the characteristics of the plasma plume and penetration of the weld. They also used the energy gradient and total energy of the AE signal to describe the impact energy and total impact energy, which was based on the AE detected transfer ratio from metal droplet to molten pool in the pulsed metal inert gas (P-MIG) welding of aluminum alloy . Combining with the finite element method, AE technology can detect the hydrogen-induced crack initiation behavior in high-strength steel, then by introducing the hardness and plastic strain to the hydrogen diffusion coefficient into the governing equation, the cracking criterion could be derived .
However, there is no report on the relationship between AE signal and crack propagation during laser welding up to now. The stress generated by the cracking of the molten pool could become the acoustic emission source. It is captured by the acoustic emission probe and converted into digital form, then processed by the amplifier, which displayed as AE waveform in the software finally. In this study, the AE technology is applied to the process monitoring of aluminum alloy pulsed laser welding to analysis the crack propagation. Combined with high-speed photography, the evidence of crack propagation after welding is captured by video and sound signal.
2. Materials and methods
In this experiment, pulsed laser was employed to weld 1 mm thick 5083 aluminum alloy sheet. Trupulse 556 laser equipment produced by TRUMPF was adopted. The laser wavelength and focal spot diameter are 1064 nm and 600 µm respectively, laser focus is TEM00 mode with gaussian energy distribution. The welding parameters are shown in Table 1, for the five sets of samples, the ratio of pulse frequency to welding speed is 3:1, which controls the distance between adjacent pulsed welding spot and ensures that the experimental results are not affected by the overlap rate. Using high-speed photography and acoustic emission to detect image and acoustic signals during pulsed laser welding, as shown in the Fig. 1.
3. Results and discussion
3.1 Solidification characteristics of spot welding
Figure 2 shows the top view of the weld morphology, there is no significant change in the weld width under different pulse intervals. The overlap rate of the weld joints is basically the same. As the pulse interval gradually decreases, the surface crack length gradually decreases. Figure 3 shows the weld microstructure of different section under different pulse intervals. The cross-section with pulse interval of 66.67ms has penetrating crack, while the pulse interval of 23.81ms has no crack. The decrease of pulse interval has no obvious effect on the solidification microstructure of aluminium alloy.
In the pulsed laser spot welding research of Jia et al. , the pulse shaping suppressed the occurrence of solidification cracks of the 5083 aluminum alloy. The application of trailing pulses not only reduced the solidification rate during the solidification of the molten pool, but also increased the temperature gradient and reduced the length of vulnerable zone. This study completely eliminate the crack from the perspective of the solidification process. In fact, if the previous pulsed laser welding spot already has thermal cracks. The crack of pulsed single welding spot determines the crack shape along the direction of the weld. The smaller crack radius and larger crack angle are favorable conditions for crack healing . In this research, high-speed photography was employed to monitor the welding process. At the same time, acoustic emission technology is used to collect the elastic wave of internal energy released during the welding process. In the continuous pulsed laser welding process, through high-speed photography of the welding process, it is found that even the smallest pulse interval (23.81ms) is much longer than the solidification time of a single pulse weld molten pool (3ms), as shown in Fig. 4(a). It could be inferred that if the cracks were only generated during the solidification process and do not propagate (residual stress effect) after solidification, the effect of the pulse interval on crack suppression cannot be explained.
In order to eliminate the interference of welding environment noise. Before welding, the experimental environment was tested for noise signals lasting two minutes. Since the amplitude of the environment noise value monitoring is 35.87dB, the threshold value is set of 40dB, as shown by the blue dotted line in Fig. 4(b). Therefore, after solidification, the AE signal obtained can only come from the workpiece. In the initial stage of welding, the laser acts on the surface of the aluminum alloy, causing the aluminum alloy to transform from solid to liquid, generating elastic waves with large instantaneous energy. As the welding process progresses, the stirring action of the laser on the molten pool causes it to oscillate, thereby maintaining a relatively strong sound signal. At the end of welding, there was a small fluctuation in the sound signal. It can be seen that when the state of the molten pool changes during the welding process, it will be reflected by the waveform of acoustic emission.
However, the focus of this research is on the strength of the elastic wave from the solidification process of the molten pool, and the internal energy release of the aluminum alloy after solidification. During the solidification of the molten pool, as the molten pool gradually stops oscillating, the intensity of the AE waveform gradually decreases. But as the solid-liquid interface advances, there is still an elastic wave of energy release in the molten pool. When the solidification is over, the strength of the AE waveform has a temporary increase. According to the physical characteristics of elastic wave propagation, there are two reasons: On the one hand, after the molten pool is completely solidified, the disappeared liquid metal reduces the elasticity wave absorption; on the other hand, the stress level at this time is at a higher intensity, which can cause stronger elastic waves. Combining the results of high-speed photography and acoustic emission, the welding process is divided into three stages: welding, solidification and crack propagation. In Fig. 4(b), when the solidification ended at 8.1ms, the cracks became more obvious with the passage of time. At the same time, in the crack propagation stage of the AE waveform, the sound signal also lasts after 30ms. Because of the signal threshold, the crack propagation time could be longer.
In the continuous pulsed laser welding process, the crack size of a single pulse welding spot plays a vital role in the crack propagation of the entire weld. Due to the remelting between the pulsed laser welding spot, when the crack size of the welding spot is small, the inheritance of the crack to the next can be avoided.
3.2 Solidification characteristics of continuous pulse welding
The welding process with the pulse interval of 66.67ms and 23.81ms was tested. AE test results are shown in Fig. 5. The AE waveform captured by the acoustic emission on the molten pool of pulsed laser welding can match the actual pulse shaping well. When the pulse interval is 66.67ms, there is a period of no AE signal between adjacent pulses; when the pulse interval is 23.81ms, the AE signal is continuous. According to Fig. 4, it can be inferred that in pulsed continuous laser welding, if the pulse interval is large, the stress of the weld will be fully released after the solidification process completed, which will cause the crack to propagate to a larger size. The next pulsed laser cannot remelt large-inch cracks on the welding spot, not only cannot eliminate the cracks, but also be an initiation region to propagate to the next welding spot. This is similar to the relation between liquation and solidification cracks, it proposed that the liquation cracks usually become initiation sites for solidification cracks . Therefore, before the next pulse, the crack size of the previous pulse weld spot will decrease with the decrease of the pulse interval. Because small size cracks are easier to be remelted, and the tendency of crack formation is also reduced.
The tensile fracture morphology of weld under different pulse intervals is shown in Fig. 6. Because solidification cracks formed under long pulse interval, two regions with different characteristics are formed in the tensile fracture. In Fig. 6, the area enclosed by the dotted line is the crystal crack area, outside the dotted line is the fracture area by tensile tests. When the pulse interval is reduced to 23.81ms, no crystal crack appears in the tensile fracture morphology, which means the solidification crack of aluminum alloy during pulsed laser welding is completely eliminated.
In summary, for aluminum alloy pulsed laser welding, by increasing the welding speed and pulse frequency, the time interval between pulses could be reduced from 66.67ms to 23.81ms, and the crack of weld could be successfully suppressed. In view of the particularity of solidification cracks in pulsed laser welding, the first combination of high-speed photography and acoustic emission has found that the solidification time of single pulse welding spot is about 3 ms, however the crack propagation caused by residual stress after solidification is at least 30 ms. Even if shortening pulse interval could not completely eliminate the cracks of a single welding spot, it effectively avoided the crack propagation and limited the crack size, which could be remelted to form a continuous pulsed laser weld without cracks.
National Natural Science Foundation of China (52075317, 51905333); Shanghai Sailing Program (19YF1418100); Royal Society through International Exchanges 2018 Cost Share (China) scheme (IEC\NSFC\181278); Shanghai Science and Technology Committee Innovation Grant (19511106400, 19511106402).
The authors declare no conflicts of interest.
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
1. X. Wang, F. Lu, H. P. Wang, Z. Qu, and L. Xia, “Micro-scale model based study of solidification cracking formation mechanism in Al fiber laser welds,” J. Mater. Process. Technol. 231, 18–26 (2016). [CrossRef]
2. A. C. Akué Asséko, B. Cosson, F. Schmidt, Y. Le Maoult, R. Gilblas, and E. Lafranche, “Laser transmission welding of composites - Part B: Experimental validation of numerical model,” Infrared Phys. Technol. 73, 304–311 (2015). [CrossRef]
3. Z. M. Beiranvand, F. M. Ghaini, H. N. Moosavy, M. Sheikhi, and M. J. Torkamany, “Solidification cracking susceptibility in pulsed laser welding of Al–Mg alloys,” Materialia 7(July), 100417 (2019). [CrossRef]
4. M. Rohde, C. Markert, and W. Pfleging, “Laser micro-welding of aluminum alloys: Experimental studies and numerical modeling,” Int J Adv Manuf Technol 50(1-4), 207–215 (2010). [CrossRef]
5. P. Von Witzendorff, S. Kaierle, O. Suttmann, and L. Overmeyer, “Using pulse shaping to control temporal strain development and solidification cracking in pulsed laser welding of 6082 aluminum alloys,” J. Mater. Process. Technol. 225, 162–169 (2015). [CrossRef]
6. N. Bakir, A. Gumenyuk, and M. Rethmeier, “Investigation of solidification cracking susceptibility during laser beam welding using an in-situ observation technique,” Sci. Technol. Weld. Joining 23(3), 234–240 (2018). [CrossRef]
7. M. Miyagi, Y. Kawahito, H. Wang, H. Kawakami, T. Shoubu, and M. Tsukamoto, “X-ray phase contrast observation of solidification and hot crack propagation in laser spot welding of aluminum alloy,” Opt. Express 26(18), 22626 (2018). [CrossRef]
8. L. Chen, G. Mi, X. Zhang, and C. Wang, “Numerical and experimental investigation on microstructure and residual stress of multi-pass hybrid laser-arc welded 316L steel,” Mater. Des. 168, 107653 (2019). [CrossRef]
9. X. Wang, F. Lu, H. P. Wang, H. Cui, X. Tang, and Y. Wu, “Mechanical constraint intensity effects on solidification cracking during laser welding of aluminum alloys,” J. Mater. Process. Technol. 218, 62–70 (2015). [CrossRef]
10. Z. Lei, J. Zou, D. Wang, Z. Guo, R. Bai, H. Jiang, and C. Yan, “Finite-element inverse analysis of residual stress for laser welding based on a contour method,” Opt. Laser Technol. 129(2020), 106289 (2020). [CrossRef]
11. B. Vrancken, R. K. Ganeriwala, and M. J. Matthews, “Analysis of laser-induced microcracking in tungsten under additive manufacturing conditions: Experiment and simulation,” Acta Mater. 194, 464–472 (2020). [CrossRef]
12. X. Liu, D. Xiao, Y. Shan, Q. Pan, T. He, and Y. Gao, “Solder joint failure localization of welded joint based on acoustic emission beamforming,” Ultrasonics 74, 221–232 (2017). [CrossRef]
13. M. Chai, M. Qin, Y. Zheng, X. Hou, Z. Zhang, G. Cheng, and Q. Duan, “Acoustic Emission Detection during Welding Residual Stresses Release in 2.25Cr1Mo0.25V Steel Welds,” Mater. Today: Proc. 5(5), 13759–13766 (2018). [CrossRef]
14. Y. Luo, L. Zhu, J. Han, X. Xie, R. Wan, and Y. Zhu, “Study on the acoustic emission effect of plasma plume in pulsed laser welding,” Mech. Syst. Signal Process. 124, 715–723 (2019). [CrossRef]
15. Y. Luo, Y. Zhu, X. Xie, and R. Wan, “Study on the transient impact energy of metal droplet transfer in P-MIG welding based on acoustic emission signals analysis,” Mater. Des. 90, 22–28 (2016). [CrossRef]
16. T. Shiraiwa, M. Kawate, F. Briffod, T. Kasuya, and M. Enoki, “Evaluation of hydrogen-induced cracking in high-strength steel welded joints by acoustic emission technique,” Mater. Des. 190, 108573 (2020). [CrossRef]
17. Z. Jia, P. Zhang, Z. Yu, H. Shi, H. Liu, D. Wu, X. Ye, F. Wang, and Y. Tian, “Effect of pulse shaping on solidification process and crack in 5083 aluminum alloy by pulsed laser welding,” Opt. Laser Technol. 134, 106608 (2021). [CrossRef]
18. P. von Witzendorff, S. Kaierle, O. Suttmann, and L. Overmeyer, “Monitoring of solidification crack propagation mechanism in pulsed laser welding of 6082 aluminum,” High-Power Laser Mater. Process. Lasers, Beam Deliv. Diagnostics, Appl. V 9741, 97410H (2016). [CrossRef]
19. F. M. Ghaini, M. Sheikhi, M. J. Torkamany, and J. Sabbaghzadeh, “The relation between liquation and solidification cracks in pulsed laser welding of 2024 aluminium alloy,” Mater. Sci. Eng. A 519(1-2), 167–171 (2009). [CrossRef]