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In laser welding the two regimes of heat-conduction welding and deep-penetration welding are distinguished. In order to characterize these two process regimes, the shown approach uses a sinusoidal laser power modulation together with timeresolved coaxial back-reflection measurement and high-speed imaging. Thereby the lower laser power levels lead t...
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... coaxial reflection measuring photodiode was synchronously triggered with the high-speed camera in order to obtain a time-resolved measurement. Figure 1 shows the detailed setup of the laser focussing unit and the coaxial back-reflection measurement. The laser radiation reflected and scattered back from the process was transmitted through a beam splitter, filtered to the laser wavelength of 1030 nm and reduced in intensity before being focussed onto the silicon photodiode with high photosensitivity at 1030 nm. ...
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... reveal that the formation and closure of the capillary has a significant influence characterizing the established process at high frequencies up to 1000 Hz. The high-speed videos -see figure 10 -show that in the case of 2.5 kW average laser power and a power modulation frequency of 1000 Hz no capillary is formed and the dominating welding process is heat-conduction welding. Whereas by increasing the average laser power up to 2.8 kW at the same frequency of 1000 Hz a deep capillary can continuously be observed [5]. ...
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... observations in the high-speed videos are underlined by the experimental process results, which show the corresponding weld cross-sections in figure 11 and figure 12. The weld with 2.5 kW average laser power shows an almost heat-conduction weld shape whereas the 2.8 kW weld has a typical deep-penetration shape. ...
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... figure 13 and figure 14, the 2.5 kW average laser power measurements are dominated by high reflections and therefore low absorption. On the opposite, figure 14 indicates almost no high reflections resulting in a high absorption. ...
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... figure 13 and figure 14, the 2.5 kW average laser power measurements are dominated by high reflections and therefore low absorption. On the opposite, figure 14 indicates almost no high reflections resulting in a high absorption. ...
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... figure 13 and figure 14, the 2.5 kW average laser power measurements are dominated by high reflections and therefore low absorption. On the opposite, figure 14 indicates almost no high reflections resulting in a high absorption. This confirms the observed phenomena in the high-speed videos, which showed that in case of 2.8 kW average laser power the capillary is kept continuously opened at 1000 Hz modulation frequency. ...
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... high power part of the modulation cycle widens the capillary and due to the still existing capillary the process is characterized by multiple reflections, enhanced incoupling efficiency. This results in typical deep-penetration welds, as can be seen on the right side of figure 12, and is confirmed by the slight increase of the 2.8 kW curve for frequencies up to 1000 Hz in figure 9. ...
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Citations
... In recent years, the improvement method based on laser power modulation has been concerned. Stritt et al. found that the keyhole formed and disappeared periodically in the process of laser power modulation power for magnesium alloy laser welding [13]. Zhang et al. conducted the laser welding for magnesium alloy with the sine modulation of laser power. ...
The laser welding was conducted for the magnesium alloy AZ31 to investigate the relationship between the power modulation and the keyhole behavior. The laser power was kept constant at 1.5 kW. The welding speed increased from 3.0 to 4.0 m/min with an interval of 0.2 m/min. With the increment of welding speed, the welding process changes from the high-absorptivity mode to the low-absorptivity mode gradually. The sinusoidal modulation of laser power was exerted in the welding processes with different welding speeds. The modulation frequency was from 50 to 200 Hz with an interval of 50 Hz. A method is proposed based on sequence images to extract the oscillation degree of the molten pool. After welding, the weld formation was analyzed in both cross sections and longitudinal sections. Experimental results show that the keyhole depth variation depends on both the laser power waveform and the intrinsic state of the keyhole. The occurrence probability of the keyhole deepening is low in the low-absorptivity mode for high-reflective metals. When the modulation frequency is close to the intrinsic frequency of the keyhole, the balance between the keyhole depth and the keyhole stability can be established in the medium-absorptivity mode.
... Power modulation has been introduced recently to improve the thermal efficiency and the stability of laser welding processes of highly reflective materials such as cooper. For examples, the sinusoidal power modulation was used by Stritt et al. [28] to reduce the power consumption by a transition of welding from thermal conduction to deep penetration; modulating the power improved the thermal efficiency of the laser welding process. The positive impact of power modulation on the stability of the copper laser welding process was confirmed by Heider et al. [29] that the modulated sinusoidal laser with a frequency of 400 Hz-600 Hz obtained the weld in a regular shape and with a reduced number of pores. ...
A new laser welding process with power modulation was proposed to promote the weld formation and improve the quality of welds over magnesium alloys. The impact of the parameters of power modulation on laser welding was investigated experimentally at the aspects of weld formation, keyhole entrance, molten pool flow, and microstructure and mechanical properties of welds. The experiments were performed for the butt joints on AZ31B magnesium alloy sheets with a thickness of 3 mm, and the results shown that the laser welding with a sinusoidal power modulation increased the melting volume and improve the efficiency of energy coupling in comparison with that with a constant power. Power modulation in laser welding could minimize even eliminate the underfill over the top surface of weld. The higher the frequency of power modulation was, the longer the columnar grains formed near the fusion line of weld. With an increase of the amplitude of power modulation, both of the cell grains and columnar grains were decreased in width; while the average size of equiaxed grains was increased. Power modulation in laser welding improved the tensile strength and the elongation of magnesium alloy at welds; when the power was modulated with the frequency of 200 Hz and the amplitude of 300 W, the maximum tensile strength was 237 MPa, and the ultimate elongation was 7.26% and the weld was broken by a mixed ductile–brittle fracture.
... Still, it is possible to expect that the temporally modulated behaviour will be maintained and that the material oscillatory behaviour will correspond to the input waveform frequency value. Analysing the scientific literature, it is possible to observe that the oscillatory behaviour generated by a temporally modulated beam has previously been reported by Stritt et al during the laser welding of Cu [48,49]. Considering the results by Temmler et al on IN718 with waveform modulation of the laser power, it may be expected that such a relationship may also be found truthful for Ni-based alloys [42]. ...
The Laser Powder Bed Fusion (LPBF) process has been historically operating with high brilliance fiber laser sources with Continuous Wave (CW) emission. Nonetheless, temporal waveform modulation of the laser emission power at high frequency levels can provide a means to enhance the deposition process by modifying the melt dynamics and solidification mechanisms. In order to disclose the effect of different waveform shapes and their parameters, an experimental study using an open LPBF system was conducted. This work configures as the second part of an investigation on this topic with the aim of validating the analytical model proposed in the first part of this work. The LPBF system developed enabled to program the power emission profiles during single track depositions. Four different waveform shapes were tested (namely Square Wave, Ramp Up, Ramp Down and Triangle Wave) at different levels of waveform amplitude (ΔP=200-400 W) and different frequencies (fw= 2 – 4 – 6 – 8 kHz) during the single track deposition of AISI316L. High speed imaging acquisitions allowed to disclose the melt dynamics and identify the melt oscillation frequency. Larger waveform amplitudes and waveforms with sudden variations of the emission power generated melt ejections and process instabilities. Stable conditions could be identified when employing Ramp Up and Triangle waveforms with ΔP=200. Melt surface oscillation frequency corresponded to the values imposed via the modulation of the laser emission power thus validating the analytical model of Part I which correlated the melt surface temperature to the recoil pressure induced over the molten pool. Optical microscopy images and metallographic cross-sections confirmed the high speed video observations. 3D reconstructions of the depositions via focus variation microscopy allowed to determine the build rates and roughness of the single tracks. Build rates obtained in stable deposition conditions with waveform modulation are analogous to values obtainable with CW emission and beneficial effects over the roughness were reported.
... It seems like the vertical oscillation seems to increase the laser absorptivity. Stritt et al. (2011) utilized laser power modulation to investigate the transition from heat-conduction to deep-penetration welding, it is found that the spot size was inversely proportional to the threshold of keyhole formation. Based on the research of Stritt et al. (2011), we can make the following hypothesis, which is well consistent with the phenomena in the experiment: The vertical oscillation of the laser beam has a similar effect to the laser spot modulation. ...
... Stritt et al. (2011) utilized laser power modulation to investigate the transition from heat-conduction to deep-penetration welding, it is found that the spot size was inversely proportional to the threshold of keyhole formation. Based on the research of Stritt et al. (2011), we can make the following hypothesis, which is well consistent with the phenomena in the experiment: The vertical oscillation of the laser beam has a similar effect to the laser spot modulation. While the laser spot is large, the dispersed energy is not enough to form a keyhole; while the laser spot is small, the concentrated energy makes the center of the molten pool begin to gasify and form a depression. ...
... And how to describe the threshold of keyhole mode in 3-d oscillation welding. Stritt et al. (2011) found that the keyhole formation threshold is higher than keyhole disappearance threshold. It means that when the energy input in the keyhole reaches the threshold, the keyhole will not disappear immediately, which is seems like the keyhole has a "delay". ...
Although 2-d (two-dimensional) beam oscillation can reduce the porosity defects during laser welding of aluminum alloys, it distributes the heat input, resulting in a higher power required to form a capillary. Therefore, this research draws attention to an alternative approach, which is considered to be beneficial to laser absorption in oscillation laser beam welding, 3-d (three-dimensional) oscillation of the laser beam. (3-d oscillation means that the laser spot moves on the two-dimensional plane and the defocus change simultaneously, causing the heat input to change) An experiment was designed where the variation of oscillation frequency leads to significant changes in capillary behavior. The result shows that the increase of vertical oscillation can promote the formation of a capillary. When the oscillation frequency reaches some special values, the capillary appears and disappears periodically, such as the vertical oscillating frequency is 5 Hz, rotation oscillating frequency is 200 Hz. Meanwhile, welding pores are generally distributed at the position where the capillary begins to disappear. The reason for promoting capillary formation during 3-d laser beam oscillation, and the change of normalized temperature, which is meaningful for understanding the welding mode transition, is illustrated by a simplified model. Compared with traditional linear welding, the experimental results show that it is possible to obtain lower porosity seam during 3-d laser beam oscillation welding in low feed rate, and the mechanism of stabilizing the molten pool is discussed qualitatively.
... The combination of spatial and temporal power modulation offers new possibilities to control the difference of the energy input by means of adapted laser power during the process [5,15,16]. A combination of spatial and temporal power modulation for processing aluminum and copper was demonstrated by Kraetzsch et al. with a 1D scanning system. ...
For joining metallic materials for battery applications such as copper and stainless steel, laser beam micro welding with beam sources in the near-infrared range has become established in recent years. In laser beam micro welding, spatial power modulation describes the superposition of the linear feed motion with an oscillating motion. This modulation method serves to widen the cross-section of the weld seam as well as to increase the process stability. Temporal power modulation refers to the controlled modulation of the laser power over time during the welding process. In this paper, the superposition of both temporal and spatial power modulation methods is presented, which enables a variable control of the weld penetration depth. Three weld geometries transverse to the feed direction are part of this investigation: the compensation of the weld penetration depth due to the asymmetric path movement during spatial power modulation only, a W-shaped weld profile, and a V-shaped. The weld geometries are investigated by the bed on plate weld tests with CuSn6. Furthermore, the use of combined power modulation for welding tests in butt joint configuration between CuSn6 and stainless steel 1.4301 with different material properties is investigated. The study shows the possibility of precise control of the welding depth by this methodology. Depending on the material combination, the desired regions with maximum and minimum welding depth can be achieved by the control of local and temporal power modulation on the material surface.
... This may cause problems with setting thresholds for weld quality classification and shows the challenges in finding principles for a universal monitoring system. Stritt et al. investigated reflected laser radiation to distinguish between heat-conduction and deeppenetration mode during disc laser welding with modulated laser power [5]. The authors also observed higher and more variable BR during the heat-conduction mode compared with deep-penetration mode and noticed that the BR is higher during the keyhole formation than the BR corresponding to keyhole closure. ...
... This was a simple verification that the coaxial camera imaging method does not give results that conflict with the other methods. Moreover, others [5] have shown, that capillary formation and closure is correlated with reflected laser radiation. There is the potential to monitor the keyhole presence during the welding process. ...
There are several approaches to weld quality monitoring during laser welding. Reflected laser radiation carries partial information about the welding process. Fibre lasers has usually a built-in diode to detect excessive back-reflected laser radiation to protect the laser source from damage. Reflected laser radiation measured in the laser source is compared with reflected laser radiation measured in the welding head. Moreover, coaxial high-speed imaging with a narrow bandpass filter on laser wavelength is used to visualize the reflected laser radiation. The advantage of this solution is that no additional illumination is needed and the reflected laser intensity and spatial distribution can be obtained from the image. Keyhole inlet dimensions are measured and related to the laser power. The transition between laser welding modes is studied.
... The formation and depth of the keyhole are determined by both energy density and the duration of laser beam irradiation of the metal [43]. At low frequencies (100 Hz), the instantaneous laser power varies gradually, and a large amount of energy is input in each positive half-cycle per unit time. ...
Modulated laser power hybrid welding (M-HW) of 4 mm-thick T2 pure copper was conducted to improve the welding stability. It was observed that the sinusoidal modulation of laser power in hybrid welding can reduce the incidence of weld defects such as spatters and pores, and decrease the transverse shrinkage of the weld seam. An observation of the evolution of the molten pool and keyhole using high-speed CCD photography reveals the main reason behind this to be the increase in the instantaneous maximum power because of power modulation, which increases the keyhole depth and frequency of formation of penetrated keyholes. The formation of a penetrated keyhole facilitates the escape of excess energy from the aperture at the bottom of the keyhole, thereby preventing the instability of the bottom of the keyhole caused by multiple reflections of laser beams thereat. This reduces the energy transfer within the surroundings of the molten pool, as well as the transverse shrinkage of the weld seam. The effects of the amplitude (modulation amplitude) and the frequency of the sinuous waveforms of laser power (modulation frequency) on the frequency of formation of penetrated keyholes were analysed.
... There is a large dispersion in drill-through times. This is not surprising since there are key differences between ablation of shallow pits and drilling of deep holes 34 . First, as the laser drills deeper into the channel, many effects come into play. ...
The results of detailed experiments and high fidelity modeling of melt pool dynamics, droplet ejections and hole drilling produced by periodic modulation of laser intensity are presented. Ultra-high speed imaging revealed that melt pool oscillations can drive large removal of material when excited at the natural oscillation frequency. The physics of capillary surface wave excitation is discussed and simulation is provided to elucidate the experimental results. The removal rates and drill through times as a function of driving frequency is investigated. The resonant removal mechanism is driven by both recoil momentum and thermocapillary force but the key observation is the latter effect does not require evaporation of material, which can significantly enhance the efficiency for laser drilling process. We compared the drilling of holes through a 2 mm-thick Al plate at modulation frequencies up to 20 kHz. At the optimal frequency of 8 kHz, near the resonant response of the melt pool, the drilling efficiency is greater than 10x with aspect ratio of 12:1, and without the collateral damage that is observed in unmodulated CW drilling.
... In other words, in laser welding of highreflectivity material, the response of the keyhole depth to transient changes of laser power showed certain lag effects. [33,34] also found this in the research on laser welding of aluminum alloy. They found that critical power for forming the keyhole was higher than that for collapsing the keyhole in laser welding of AlMgSi alloy, while the critical powers for forming and collapsing the keyhole were consistent in laser welding of St37 steel. ...
High-reflectivity of materials, such as magnesium, copper and aluminum, results in low thermal efficiency of their infrared laser welding processes. AZ31 magnesium alloy was selected to study the effects of power modulation on energy coupling efficiency in laser welding of highly-reflective materials. A model for the relationship between energy coupling efficiency and modulation parameters was obtained. The energy coupling efficiency in optimized modulated-power laser welding was about 1.58 times that in constant-power welding. The mechanism was explored by analyzing keyhole evolution and the resulted pressure distribution along keyhole wall during welding. The keyhole evolutions in laser continuous welding of common material (Q345 steel, reflectivity of 65%) and highly-reflective material (AZ31, reflectivity of 85%) were observed through high-speed imaging by utilizing a half sandwich method. The results indicated that the secret of improving energy coupling efficiency of laser welding process of highly-reflective materials through power modulation was the formation of a deep keyhole and its long life. When instantaneous power decreased from the peak, there was still enough recoil pressure at the bottom of keyhole to resist surface tension and hydrostatic pressure of liquid metal, which was the fundamental reason for the long time existence of keyhole with a large depth.
... Gao et al. [17] revealed that the porosity of welded joints of Ti 6 Al 4 V alloy can be inhibited by applying laser pulse modulation and adjusting the overlap factors of welding spots. Similarly, by carrying out the sinusoidal power-modulated laser experiment on two kinds of materials, Stritt et al. [18] found that highly reflective materials were associated with large improvement of the power coupling efficiency during the welding process using laser power modulation. Zhang et al. [19] effectively suppressed the generation of pores by employing laser power modulation in the welding process of magnesium (Mg) alloy. ...
Aiming to solve the serious porosity defects in laser welded girth joints of thin-walled tube and end plug made of nano-sized Ce 2 O 3 doped Mo alloy (NC-Mo), the influences of laser power modulation, multipass remelting and zirconium (Zr) addition on the number, size and distribution of porosity defects were experimentally studied. By utilizing X-ray computed tomography (XCT), scanning electron microscope (SEM), energy dispersive spectro-scopy (EDS) analysis, transmission electron microscope (TEM) and Raman spectrum (RS), the porosity feature of welded joints achieved under various conditions was analyzed. The results showed that welding cycles had a significant influence on the porosity ratio of fusion zone (FZ) while the amplitude and frequency of laser power waveform slightly influenced the porosity. When the welding cycles increased from 2 to 8, the porosity ratio of FZ decreased from about 1.00% to about 0.48% and the maximum and average pore diameters reduced by about 53% and 27%, respectively. Adding minor Zr in molten pool can further reduce the R P of FZ to about 0.35%. Through analysis, it can be seen that the pores in FZ can be divided into irregularly-shaped keyhole-induced pores and spherical metallurgy-induced pores. The latter was generated possibly because some impurity elements (including O and H) pre-existing in base metal (BM). Increasing welding cycles can promote the gas in molten pool to float and outflow, thus significantly decreasing the porosity. Moreover, Zr added in molten pool can be preferentially reacted with O to generate ZrO 2 , which can inhibit the precipitation of volatile MoO 2 to thus suppress the generation of metallurgy-induced pores.
