Scanning galvanometer is a technology that can accurately control the laser beam. It has the advantages of high processing accuracy and good reliability. It is widely used in the field of advanced laser forming. Porosity and cracks are common defects in laser advanced forming processes such as laser welding and laser cladding. A large number of studies have shown that the above defects can be effectively controlled by oscillating the molten pool by means of ultrasound or pulsed laser. Compared with the above methods, the scanning galvanometer can not only stir and oscillate the molten pool by swinging the laser beam, but also control the motion trajectory and temperature gradient distribution of the molten pool, thereby effectively changing the solidification speed of the molten pool, refining the grains, and suppressing pores. gaps and cracks, which can effectively improve product quality. In recent years, scanning galvanometers have begun to be studied and applied in the fields of laser welding and laser cladding. This paper summarizes and summarizes the research on scanning galvanometers in laser welding and laser cladding advanced forming, and looks forward to the future development trend of scanning galvanometers in the fields of laser welding and laser cladding advanced forming. (Scanning galvanometer; Laser welding; Laser cladding; Overview)

The galvanometer scanning system is mainly composed of an optical focusing lens, a galvanometer scanning head, an information acquisition card, and a host computer control system. It is an optical scanning device based on the principle of a permanent magnet motor [1, 2]
. It mainly changes the direction and deflection angle of the laser by controlling the galvanometer so that the laser beam can reach the required working area. It has the characteristics of low step response time, high positioning accuracy, economic efficiency and large scanning range. [3]
. It has been widely used in laser processing, laser communication, laser radar, biological imaging, laser projection and other fields [4-6]
.
The traditional laser processing technology mainly fixes the laser head. The direction of the laser beam remains unchanged during the processing. The laser head is driven by a mechanical device to scan the workpiece back and forth. The energy utilization rate of the laser is low. After adding a scanning galvanometer to the laser processing technology, the deflection angle and scanning trajectory of the laser can be flexibly adjusted, which can significantly improve the effect of laser processing. The main scanning trajectories are sinusoidal, circular, “one” shape, and “∞” shape [7, 8]
. The scanning galvanometer laser can scan the molten pool back and forth, which increases the stirring effect of the laser on the molten pool and can greatly reduce defects such as pores during welding. It is currently mainly used in laser welding [9, 10]. Since laser oscillation can destroy the growth of dendrites in the microstructure, increase the number of nuclei, produce finer grains, and effectively improve the performance of the product, it has also begun to be used in additive manufacturing technologies such as laser cladding [11, 12]. The first part of this paper mainly introduces the research progress of scanning galvanometers in laser welding, the second part introduces the research progress in laser cladding, and the third part summarizes and prospects the development trend of scanning galvanometers in laser welding and laser cladding. The purpose of this paper is to summarize and summarize the research on scanning galvanometers in the fields of laser welding and laser cladding, in order to improve the quality of laser processed products. 1 Research on scanning galvanometers in laser welding Compared with traditional laser welding technology, scanning galvanometer laser welding technology has the advantages of wide scanning range, high flexibility, small heat input, and small welding thermal deformation [13, 14]. In addition, due to its multi-point welding characteristics, it can effectively increase the effective area of ​​the welding area, reduce welding defects, and improve process stability [15, 16]
.Studies have shown that in the welding process, the combination of high solidification rate and low temperature gradient is conducive to the formation of equiaxed grains, while the combination of low solidification rate and high temperature gradient is prone to the formation of cellular or columnar dendrites [17, 18]. The scanning galvanometer laser beam controls the peak temperature and solidification rate of the molten pool during the swinging process, which is conducive to the formation of equiaxed grains, refines the grains in the fusion zone and reduces the sensitivity of thermal cracks [19-21], which can effectively reduce welding defects in the laser welding process of structural steel, aluminum alloy, magnesium alloy, titanium alloy, copper alloy and their heterogeneous alloys.

1.1 The influence of scanning trajectory on welding characteristics
Scanning galvanometer laser has a variety of scanning trajectories. Different scanning trajectories will produce different stirring modes for the molten pool, which will change the flow mode of the molten pool, thereby changing the solidification time of the molten pool, increasing the time for bubbles to escape, and thus having a certain influence on the weld morphology and the suppression of defects such as pores.
Wu[24] et al. studied the influence of linear scanning trajectory and circular scanning trajectory on the pores in the weld area of ​​AA5182-O aluminum alloy. The pores in the weld area under the circular scanning trajectory were significantly reduced, and the tensile strength was significantly enhanced. Fetzer[25] et al. used three scanning trajectories, longitudinal, transverse, and circular, to study the weld morphology of AlMgSi Laser welding of aluminum alloy can produce welds without pores under circular trajectory. This is because under circular scanning trajectory, the laser interacts with existing bubbles repeatedly, which is conducive to the escape of bubbles. Cheng[26] et al. studied the effects of transverse, longitudinal, circular and non-scanning methods on the morphology, microstructure and mechanical properties of ultra-high strength steel (D406A) welded joints. The deepest penetration, finest grains and largest equiaxed grain area can be obtained under circular scanning trajectory, and the joint performance is the best. Chen[27] et al. found that the molten pool is unstable and spatter is easy to generate during circular scanning welding. They used “∞” scanning trajectory to conduct welding experiments on 5052 aluminum alloy, and the welds formed were of high quality and low porosity. Ke[28] and Wang[29] et al. studied the effects of circular and “∞” scanning trajectories on 5A06 The influence of porosity on aluminum alloy welded joints. Compared with the circular trajectory, the use of the “∞” scanning trajectory can significantly reduce the porosity of the weld. Wang [30] et al. used the “8” scanning trajectory to conduct welding experiments on TC31 high-temperature titanium alloy. The weld had good mechanical properties and could effectively inhibit the generation of pores. Gerhards [31] et al. used the “8” scanning trajectory to perform laser welding on 22MnB5. The results showed that the tensile strength and elongation of the weld joint were significantly improved. The influence of different scanning trajectories on the molten pool, weld joint morphology, microstructure and microhardness are shown in Figure 1.
In summary, compared with ordinary laser welding technology, different scanning trajectories of the galvanometer laser have different stirring effects on the molten pool, which has a greater impact on the performance of the weld joint. The circular and “∞”-shaped scanning trajectories can produce weld joints with better performance and effectively reduce the porosity of the weld. Among them, the “∞”-shaped scanning trajectory performs better. This may be because the “∞”-shaped trajectory can have a better stirring effect on the molten pool. In the future, we should increase the research on other scanning trajectories and study their effects on the morphology and performance of the weld joint.

1.2 Effect of oscillation frequency on welding characteristics
The generation of pores in laser welding is closely related to the solidification rate of the molten pool. The oscillation frequency of the galvanometer laser can control the stirring rate of the molten pool, thereby playing an important role in the solidification rate of the molten pool, and thus has a greater impact on the formation and suppression of pores.
Hao et al. [32] studied the effects of different oscillation frequencies on AISI304 The influence of oscillation frequency on the welding morphology of stainless steel is that with the increase of oscillation frequency, the penetration depth becomes shallower, the molten width becomes wider, and the weld cross section becomes more uniform. Cheng et al. [33] conducted laser butt welding experiments on Ti4Al6V and 6061Al dissimilar metals. The stability of the keyhole gradually decreased with the increase of oscillation frequency. At the appropriate oscillation frequency, the tensile strength of the weld joint was significantly improved. Hu et al. [34] found that for GH909 low expansion high temperature alloy, high oscillation frequency is more conducive to the formation of good performance joints. The improvement of joint mechanical properties is mainly due to the fact that the oscillation frequency can cause solid solution strengthening and precipitation strengthening. Zhang et al. [35] studied the effect of different oscillation frequencies on the suppression of pores in laser welding of Al-6Mg alloy. Pores are more likely to be generated below the keyhole. This is because the bubbles are far away from the keyhole and are not easy to escape. When the oscillation frequency increases, the pore suppression effect becomes better. Gong Jianfeng et al. [36] conducted a laser welding experiment on 5A06 Laser welding experiments were conducted on aluminum alloys, and the flow behavior of the molten pool was analyzed. Increasing the oscillation frequency can inhibit the flow of the molten pool, which is beneficial to reducing the porosity. Miyagi et al. [37] used different oscillation frequencies to conduct welding experiments on pure copper (C1100). The phenomena such as molten pool splashing and surface pores and the weld depth decreased with the increase of the oscillation frequency. Li et al. [38] conducted laser welding experiments on 5083 aluminum alloy thin plates. When the oscillation frequency was greater than 200 Hz, the weld porosity decreased, and the grains in the fusion zone were refined. Compared with non-oscillating laser welding, the microhardness of the weld zone and the tensile strength of the weld joint were significantly improved. Liu et al. [39] conducted laser welding experiments on 7075 aluminum alloy. The beam oscillation can transform the irregular oscillating fluid into regular oscillation. When the oscillation frequency of the laser beam reaches a suitable level, a stable molten pool can be obtained, and the pore generation can be inhibited. Hao [40] and Gao et al. [41] studied the effect of different oscillation frequencies on laser welding of AZ31B and AZ31 magnesium alloys. When the oscillation frequency is appropriately increased, a more uniform temperature gradient can be obtained, which is conducive to the generation of finer equiaxed grains. Li et al. [42] conducted laser welding experiments on 304 stainless steel. The oscillation frequency has a great influence on the solidification rate of the molten pool and can effectively refine the grains. Yan et al. [43] studied the effect of different oscillation frequencies on the temperature field and residual stress distribution of laser welded 316N stainless steel. With the increase of oscillation frequency, the weld temperature, residual stress and deformation will decrease. The effect of oscillation frequency of galvanometer laser welding on weld morphology, molten pool and mechanical properties is shown in Figure 2. As shown.
In summary, the oscillation frequency has a great influence on the keyhole of the laser welding weld, and at a relatively high oscillation frequency, it is easier to produce a laser welding joint without porosity defects, but when the oscillation frequency is too high, it will lead to an increase in porosity and cause molten pool splashing, which shows that there is a suitable threshold range for the oscillation frequency of the scanning galvanometer laser welding

1.3 Effect of oscillation amplitude on welding characteristics
Another important parameter in scanning galvanometer laser welding is the oscillation amplitude. The effect of the oscillation amplitude is mainly reflected in the stirring area of ​​the molten pool and the temperature gradient of the molten pool. The effects of the oscillation amplitude and the oscillation frequency on the weld morphology are very similar. Both have a greater impact on the weld width and depth, but a smaller impact on the residual height.
Haeusler et al. [44] conducted galvanometer laser welding experiments on copper. At higher oscillation amplitudes, the overlap degree and maximum penetration depth of the weld will decrease. Compared with traditional laser welding technology, the weld geometry can be adjusted without increasing the depth of penetration. Zhang et al. [45] studied the effects of different oscillation amplitudes on the heat transfer and melt flow of the molten pool. With the increase of oscillation amplitude, the temperature gradient of the molten pool will drop rapidly, and the melt flow rate will increase sharply. In addition, the molten pool vortex generated by non-scanning galvanometer laser welding mainly moves up and down in the vertical direction, while the molten pool vortex generated by the scanning galvanometer laser moves on the horizontal plane, which has a certain impact on the movement state of the keyhole and inhibits the formation of pores. Cheng et al. [46] studied the effect of different oscillation amplitudes on the microstructure and mechanical properties of 40Cr steel/45 steel fillet weld joints. The melt width gradually increased with the increase of laser oscillation amplitude, and the melt depth gradually decreased with the increase of laser oscillation amplitude. A smaller laser oscillation amplitude is beneficial to improving the quality of welded joints. Yamazaki [47] and Shiganov [48] conducted laser welding experiments on different types of steels and found that when the oscillation amplitude increased, the melt width would also increase, but the penetration depth of the weld joint would decrease. Ai et al. [49] used “∞” scanning galvanometer laser welding to conduct laser welding experiments on 5052 aluminum alloy. With the increase of oscillation amplitude, the maximum temperature and maximum flow rate of the molten pool decreased, and the molten pool became more stable. Ren et al. [50] used scanning galvanometer laser and arc composite technology to prepare Al-Mg on the surface of AZ80 magnesium alloy. Alloy coating, the stirring effect of the laser beam can effectively inhibit the segregation of the alloy, but if the oscillation amplitude is too large, the hardness and wear resistance of the coating will be reduced. The effect of the oscillation amplitude on the morphology and width of the microstructure of the heat-affected zone, the melt width, the melt depth, the mechanical properties, the weld temperature distribution and the eddy current velocity of the molten pool are shown in Figure 3. In summary, the change of the oscillation amplitude will cause the change of the laser welding melt width and melt depth. This is mainly because the oscillation amplitude will change the repeated action time of the laser on the molten pool, thereby affecting the temperature gradient of the molten pool. The larger the oscillation amplitude, the larger the action area of ​​the laser on the molten pool, the longer the repeated action time on the same area, the smaller the unit heat input, the lower the molten pool temperature and the melt depth of the weld joint, but the melt width will increase. 2 Research on Scanning Galvanometer in Laser Cladding
Laser cladding is a technology for depositing a protective layer on a substrate. It mainly uses a laser beam to heat metal powder or wire to prepare a coating with high hardness, wear resistance, corrosion resistance and oxidation resistance on the surface of the substrate [51-54]. Compared with traditional surface modification technology, it has the advantages of smaller heat-affected zone, lower dilution, good metallurgical bonding with the substrate, etc., which can achieve local or large-area repair of parts [55-57]. It has been widely used in metallurgical industry, shipbuilding industry, aerospace, automobile industry and biomedicine [58-60]. However, during the laser cladding process, the rapid cooling and solidification of the molten pool can easily lead to uneven mixing of the substrate and the alloy powder, and the bubbles released when the powder melts are not easy to escape, resulting in defects such as pores and cracks [61, 62]. In order to improve the shortcomings of laser cladding, some scholars have integrated the scanning galvanometer system into laser cladding.
2.1 Effect of adding galvanometer on laser cladding layer
Since the laser used in laser cladding emits a Gaussian laser beam, the laser energy is easily over-concentrated, resulting in a large difference in the temperature gradient in the molten pool. The large temperature gradient is not conducive to the growth of equiaxed crystals, resulting in more columnar crystal areas inside the cladding layer, which seriously affects the mechanical properties of the cladding layer. By adding a scanning galvanometer, the direction of the laser can be changed to produce different oscillation patterns, which can have different stirring effects on the molten pool and affect the temperature gradient distribution in the molten pool, destroying the growth of columnar crystals, thereby improving the performance of the cladding layer.
Klocke et al. [63] first added a sinusoidal scanning galvanometer system to laser cladding, which greatly improved the cladding efficiency. The high-frequency scanning of the laser beam can obtain a uniform and variable temperature distribution, and the stability of the cladding process is improved. With the development of scanning galvanometer technology, other scanning galvanometer systems such as circular galvanometers have been used for laser cladding. Xia et al. [64] used a circular scanning galvanometer system to prepare a WC-reinforced GH3536 nickel-based high-temperature alloy coating. The micro-dendrites were destroyed, the number of nuclei increased, and due to the different laser action time, the oscillation The temperature gradient in the middle is relatively low, while the temperature gradient at the edge is higher. An equiaxed crystal zone is formed in the middle of the cladding track, and a columnar crystal zone is formed at the edge of the laser spot. Li et al. [65] studied the effect of the circular scanning galvanometer system on the microstructure, mechanical and electrochemical corrosion properties of laser cladding chromium-nickel-iron alloy. Compared with the cladding without oscillation, the pores of the cladding layer under the circular track were significantly suppressed, and the growth of columnar crystals was destroyed due to the laser oscillation applied by the circular scanning track. There were more equiaxed crystals in the cladding layer, and the mechanical properties were better, but the corrosion resistance was poor due to the increase in grain boundaries. Lei et al. [66] used the same method to prepare NiCu cladding layer and WC/NiCu composite cladding layer, and compared them with the cladding layer prepared without oscillation. Under the circular scanning trajectory, the oscillating laser will destroy the growth of columnar crystals, resulting in the formation of more nucleation points, promoting the growth of equiaxed crystals, and making the grains inside the cladding layer finer, thereby improving the hardness and wear resistance of the cladding layer. However, due to the increase in grains, the cladding layer is more susceptible to grain boundary corrosion, so the corrosion resistance of the cladding layer under the circular trajectory is reduced. Dai et al. [67] deposited Ti6Al4V cladding layers on the Ti6Al4V surface using circular, sinusoidal, “∞” and “8” shapes, respectively. The results showed that the cladding layer prepared under the infinite laser trajectory had the best tensile strength and elongation. Compared with the circular and sinusoidal scanning trajectories, the flow of the molten pool under the infinite trajectory is more complex, and the distance required to complete the oscillation cycle is longer, which can effectively reduce the maximum temperature and temperature gradient of the molten pool, and impose a new flow direction, increase the nucleation points, and reduce the grain size. Compared with the “8” shape, the “∞” laser oscillation direction is more similar to the scanning direction of the cladding head, which leads to repeated rupture of the molten pool and is more conducive to the formation of equiaxed grains. The influence of the scanning galvanometer on the movement of pores in the molten pool and grain growth is shown in Figure 4. In summary, compared with the cladding layer without oscillation, the addition of the scanning galvanometer can stir the molten pool when the laser swings, reduce the temperature gradient, and destroy the growth of columnar crystals, so the mechanical properties of the cladding layer can be significantly improved. However, the current research mainly focuses on the influence of simple scanning trajectories such as sinusoidal and circular on the performance of the cladding layer, while the molten pool flow, microstructure evolution law and performance of the cladding layer under other more complex scanning trajectories need further in-depth research and development. 2.2 Effect of different oscillation frequencies and oscillation amplitudes on cladding layers

As important parameters in the scanning galvanometer system, oscillation frequency and oscillation amplitude can increase the laser scanning speed, thereby improving the oscillation effect, and further reduce the temperature gradient in the molten pool, promote the transformation of columnar crystals to equiaxed crystals, refine the grains, and improve the performance of the cladding layer.

Pekkarinen et al. [68-70] found that the oscillation amplitude and oscillation frequency have a great influence on the basic size and dilution rate of the cladding layer.

Cen et al. [11] studied the effect of different oscillation frequencies on the laser cladding structure and solidification cracks of Al-Cu-Mg alloy. Increasing the oscillation frequency can effectively reduce the solidification cracks of the cladding layer. With the increase of the oscillation frequency, the average hardness value of the cladding layer increases and the cracks decrease, but it does not affect the elemental composition of the microstructure of the cladding layer. Dai et al. [71] deposited Ti6Al4V cladding layers on Ti6Al4V surfaces using a wire feeding method using circular, sinusoidal, “∞”, and “8” shapes. The results showed that with the increase of oscillation frequency and oscillation amplitude, metallurgical defects such as porosity, organization, and columnar grain size tended to decrease, and the β grains could be effectively refined by stirring the molten pool. The increase in laser oscillation frequency has an inhibitory effect on the formation of defects under circular oscillation, but the inhibition of defects under sinusoidal, “∞”, and “8” shapes is not obvious. Compared with the oscillation frequency, the oscillation amplitude has a more obvious inhibitory effect on metallurgical defects. This is because the oscillation amplitude has a greater influence on the molten pool width, molten pool depth, and molten pool area. With the increase of oscillation frequency, the molten pool width and molten pool area increase, and the molten pool depth decreases. The unit energy area of ​​the laser acting on the substrate will decrease, avoiding excessive evaporation of local materials, thereby maintaining the stability of the molten pool. Fang et al. [72] studied the effects of different oscillation frequencies and oscillation amplitudes on the morphology of the Ti6Al4V cladding layer under a sinusoidal trajectory. The increase in oscillation frequency and oscillation amplitude can effectively reduce the internal defects of the cladding layer, and increasing the oscillation amplitude has a more obvious effect on the suppression of defects in the cladding layer. Xia et al. [73] studied the effects of different oscillation frequencies and oscillation amplitudes on the macroscopic morphology, microstructure and mechanical properties of GH3536 alloy under a circular trajectory. Compared with a non-oscillating laser beam, under a circular trajectory, the relative energy flux distribution of the laser is high at the edge and low at the center. Increasing the oscillation frequency and amplitude can significantly reduce the energy flux difference and help to generate a smaller temperature gradient. The lower the oscillation frequency, the more uneven the energy flux distribution in the molten pool; the larger the amplitude, the lower the energy flux in the molten pool. Therefore, if the frequency is too low and the amplitude is too large, it may cause the cladding layer to be discontinuous. In addition, the oscillation frequency and oscillation amplitude have a great influence on the dilution rate of the cladding layer. As the oscillation frequency increases, the dilution rate of the cladding layer will decrease. This is because increasing the oscillation frequency will increase the speed of the laser operation. The higher speed will reduce the energy absorbed by the substrate, thereby reducing the dilution rate. As the oscillation amplitude increases, the speed of the laser operation will also increase, but because the heat flux acting on the molten pool decreases with the increase of the amplitude, when the oscillation amplitude is too high, it cannot provide enough energy to melt the metal powder, so most of the laser energy is directly absorbed by the substrate instead of the metal powder, resulting in the dilution rate of the cladding layer will first decrease and then increase. At the same time, similar to the research of other scholars, the circular trajectory can destroy the growth of columnar crystals, and with the increase of oscillation frequency and oscillation amplitude, the laser will stir the molten pool at a higher speed, thereby driving the melt flow and promoting the transformation of long columnar dendrites into short columnar dendrites and equiaxed crystals, thereby refining the grains and increasing the hardness of the cladding layer. More equiaxed crystals can prevent crack propagation during the tensile process, which is beneficial to improve the toughness of the cladding layer. The effects of different oscillation frequencies and oscillation amplitudes on the average grain size, average microhardness and grain growth are shown in Figure 5. In summary, increasing the oscillation frequency and oscillation amplitude can suppress the metallurgical defects of the cladding layer, and the suppression effect of the oscillation amplitude is more obvious. However, when the oscillation amplitude is too large, the dilution rate of the cladding layer will be too large, thus affecting the performance of the cladding layer.

3 Summary and Outlook
Compared with traditional laser welding and laser cladding technology, scanning galvanometer laser welding and cladding technology has higher flexibility, larger working range and higher work efficiency. The scanning galvanometer laser system can not only stir the molten pool during the processing, thereby reducing metallurgical defects such as pores, cracks and uneven melting, but also increase the nucleation points, refine the grains, obtain equiaxed crystal structure, and thus improve the mechanical properties of the product. Although scanning galvanometer laser has many advantages, there are still relatively few studies on laser welding and laser cladding. The author believes that the future development may have the following points:
(1) The scanning trajectories currently studied are relatively simple, and the quality of products produced by different scanning trajectories is different. More should be revealed about the flow, organizational evolution and performance enhancement mechanism of the laser welding and laser cladding molten pool under complex scanning trajectories. (2) The scanning galvanometer focusing lens currently used in large quantities is the f-θ focusing lens. The focusing effect of the f-θ lens edge is relatively poor. This scanning system is not very suitable for large-area laser scanning. In the future, more improved scanning galvanometer systems should be used to further improve efficiency and product performance. (3) Most researchers have studied the scanning trajectory, oscillation frequency, and oscillation amplitude under a fixed laser power and scanning speed. Only a few scholars have studied the synergistic effect of multiple parameters on laser processing results under different laser powers, defocus amounts, scanning speeds, spot diameters, etc. In the future, the research on the mixed effect of laser process parameters and scanning galvanometer parameters should be strengthened. (4) When studying the scanning trajectory, oscillation frequency, and oscillation amplitude, many scholars chose the single-factor method to control the variables and change one parameter at a time. This requires a large number of experiments, which is easy to waste time and increase experimental costs. In addition, the process parameters are rarely optimized. The parameters used may not be the optimal parameters. In the future, optimization methods such as response surface method, Taguchi method, and machine learning should be introduced to optimize the parameters.