Laser Beam Oscillation Research Articles

Study of mode transition in three-dimensional laser beam oscillating welding of aluminum alloy

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.

Laser welding of 16MnCr5 butt welds with gap: resulting weld quality and fatigue strength assessment

For laser welding, gaps should normally be avoided to ensure stable weld processes and high weld quality. Nonetheless, sometimes, gaps are resulting from non-optimal weld preparation in industrial applications. Within this investigation, the effects of a defocussed beam and laser beam oscillation on gap bridging abilities at reduced ambient pressure were investigated. For reference purpose, conventional laser welding with zero gap at ambient pressure was performed, too. The resulting weld geometry was investigated and correlated to gap bridging strategies and weld quality groups according to ISO 13919–1. The welds were characterized regarding their hardness and weld microstructure. Residual stress was determined by means of X-ray diffraction, and tensile tests as well as fatigue tests were conducted. The fatigue tests were evaluated by the nominal stress approach, the critical distance approach, and the stress averaging approach and correlated to weld quality measures. Resulting from this, fatigue resistance of laser welded butt welds with gap can be estimated by the FAT160 design S–N curve. The stress evaluation parameters for the determination of keff-values (ρ* = 0.4 mm, a = 0.1 mm) were confirmed.

Enhancing cutting performance with an oscillating laser beam

AbstractFiber lasers feature higher energy conversion efficiency than CO2 lasers, which enables a simple structure and energy‐saving design. Consequently, in the laser cutting processes for metal sheets, operators have been shifting usage from CO2 lasers to fiber lasers at an accelerated pace because of their reduced maintenance, improved metal absorption rate due to their wavelength and other advantages. In actual practice, fiber lasers are superior to CO2 lasers in high‐speed cutting processes for thin metal sheets. However, as the thickness of the metal sheet increases, problems arise where fiber lasers have inferior performance to CO2 lasers in terms of dross adhesion, cutting surface quality, and other factors.

Effects of laser beam oscillation welding parameters on Al-Si coated 22MnB5 weld joint properties

Al-Si coated 22MnB5 steel is one of the primary materials to fulfill vehicle lightweight and improve its fuel efficiency. In this study, laser beam oscillation welding approach was introduced to weld the Al-Si coated 22MnB5 steel in a butt joint configuration. To improve the joint strength, the process parameters were optimized through orthogonal experiments. Furthermore, the tensile tests were applied to evaluate the tensile strength of the welded joint and the base metal. The microstructure and fracture morphology of the welded joint and the base metal were observed and compared by scanning electron microscopy. Experimental results showed that the laser power had the largest effect on the tensile strength of the welded joint, followed by the oscillation frequency, the welding speed, and the oscillation amplitude. In addition, the optimized values of the laser power, the welding speed, the oscillation frequency and the oscillation amplitude to obtain the best performance of the welded joint were 3200 W, 3.1 m/min, 240 Hz, and 0.6 mm, respectively. Under this set of parameters, the tensile strength and elongation of the welded joint were 1510.59 MPa and 2.56% still lower than the ultimate strength of 1603.20 MPa and the elongation of 6.90% of the base metal, respectively. The formation of α-ferrite in the fusion zone promoted the initiation and propagation of the cracks under the tensile loading, which was the main reason for the degraded mechanical properties.

Optimization of beam oscillation on macro/micro formation and mechanical property in low-expansion superalloy laser-welded joint

Low-expansion superalloys have been widely used in aerospace, ships, nuclear power fields due to the high strength and low expansion coefficient. Generally, laser beam oscillation was adopted to solve the defects including poor bead formation, inner pores, and solidification cracks in the weldment of GH909 low-expansion superalloys medium-thick plates. Therefore, the molten pool flow, joint formation, microstructure, and mechanical properties were systematically studied in this paper. Results showed that beam oscillation effectively stabilized the molten pool and keyhole inside, thus forming a well-formed joint without obvious pores. In tensile tests, the strength and elongation of the joint at 300 Hz were measured as 859.5 MPa and 38.6 %, significantly higher than those of laser joint (731.1 MPa and 17.9 %) with plenty of pores and cracks. The average microhardness of the fusion zone (FZ) using laser oscillating welding (LOW) was about 16 HV larger than that of single laser welding (SLW). In FZ, γ/Laves eutectic precipitated phase was distributed discretely without obvious solidification cracks. There existed coherent and incoherent interfaces between eutectic γ and Laves phases. Moreover, coherent interfaces were detected to account for 73.9 %, presenting a major fraction. The improvement of the mechanical properties of LOW joint was mainly caused by solid solution strengthening and precipitation strengthening. This research has a guiding reference for well-formed and high-quality low-expansion superalloy welding and relevant industrial application.

Towards robust dynamic beam shaping for laser cutting applications

Dynamic beam shaping brings a large potential for the laser cutting process by increasing the cutting speed and improving the cut edge quality. However, laser beam oscillation is almost exclusively realized with 2-mirror galvanometer scanners that limit the industrial implementation of the technology. Within this work, a new concept platform based on a single-mirror scanning system with a 4 kW fiber laser is tested for cutting of 12 mm thick plates in different materials. Technology knowledge transfer from existing datasets with common 2-mirror scanners has been used to speed up the optimization of the novel system. The proposed solution is able to substitute conventional deflection mirrors and can be easily integrated into the optical beam path of existing industrial machines. This paper introduces the advanced laser cutting process possibilities and experimentally demonstrates the dynamic beam shaping performance compared to the static beam reference system.

Influence of laser beam oscillation on the cutting front geometry investigated by high-speed 3D-measurements

3D measurements of the cutting fronts during laser fusion cutting were performed with and without beam oscillation. The used 3D measurement equipment offers up to 75.000 frames per second and a local resolution less 50 µm. This allows high-speed recording of dynamic changes at the cutting front geometry, followed by a three-dimensional reconstruction. Out of an extensive study was picked out a parameter set for 8 kW laser power and a feed rate of 4.5 m/min cutting 10 mm stainless steel (AISI 304), compared to a cut with additional longitudinal beam oscillation by a frequency of 600 Hz and an amplitude of 0.15 mm. The large bright spots of common high speed videos can be interpreted as melt accumulations, pushed down by the gas pressure. They leave behind cold spots on the cutting front, which are very steep or even have an undercut. The influence of beam oscillation on the cutting front is also clearly visible. In the places where the laser beam is not currently located, the melt and the resulting structures appear to solidify. As a result, melt is not continuously ejected, but rather melt bursts are created with the oscillation frequency.

Influence of high frequency wobbling parameters on the weld quality of aluminium LBW

Currently available commercial remote laser beam welding (LBW) scanners allow a fast laser beam manipulation up to 1 KHz. In this work, wobbling (fast laser beam oscillation) was applied to weld aluminium alloy (5754) sheets of 1.6 mm thickness that are subjected to stamping processes later on. Influence of process parameters (laser power and speed, incidence angle and wobbling frequency and amplitude) in weld quality of T joint and edge lap joint configurations was studied. In T joint configuration, the use of 4047 wire filler metal of 1.2 mm of diameter was beneficial to avoid excessive undercuts and provide good penetration, weld shape and dimensions. Metallographic characterisation confirmed the absence of other defects such as lack of fusion, pores or cracks. In addition, wobbling led to optimal results without removing the protective coating of Ti/Zr from the sheets. Finally, mechanical characterisation for post-welding stamping suitability assessment of the welds was performed using shear and bending tests.

The effect of beam oscillation on laser welding of AA2219-T87 under subatmospheric pressure

Aluminum alloy AA2219 is widely used in the aerospace industry due to its high specific strength, and corrosion resistance, especially for the production of propellant tanks of space rockets. With the rapid development of laser sources, the welding of metal materials changes gradually from arc welding to laser welding, but pore defect drags the application of laser welding of aluminum alloy. To solve this bottleneck problem, this paper investigates the effect of laser beam oscillation on welding of AA2219 under subatmospheric pressure (LOWSP). The experimental results showed that LOWSP has significantly improved the surface morphology, penetration depth, microstructure,and mechanical properties of the weld seam. The EBSD testing analysis shows that the low-angle grain boundaries (LAGBs) take up the largest proportion in the microstructure with the increased proportion of equiaxed grains, and the grain size is further refined in LOWSP. When welded at the optimal laser beam oscillation frequency of 80 Hz under the pressure of 102 Pa, the pore was almost inhibited and the tensile strength of the weld seam reached 388.62 MPa, up to 83.16% of the base metal (the current level of the industry is generally between 300 ∼ 360 MPa). It is believed that the reduced boiling point of the aluminum alloy under subatmospheric pressure accelerated the molten metal flow rate, the stirring effect of laser beam oscillation on weld microstructure within molten pool significantly eliminate porosity, therefore the LOWSP process generated the refined grain size and the improved mechanical properties of the weld seam.

Effects of sinusoidal oscillating laser beam on weld formation, melt flow and grain structure during aluminum alloys lap welding

A numerical model of 1.5 mm 6061/5182 aluminum alloys thin sheets lap joints under laser sinusoidal oscillation (sine) welding and laser welding (SLW) weld was developed to simulate temperature distribution and melt flow. Unlike the common energy distribution of SLW, the sinusoidal oscillation of laser beam greatly homogenized the energy distribution and reduced the energy peak. The energy peaks were located at both sides of the sine weld, resulting in the tooth-shaped sectional formation. This paper illustrated the effect of the temperature gradient (G) and solidification rate (R) on the solidification microstructure by simulation. Results indicated that the center of the sine weld had a wider area with low G/R, promoting the formation of a wider equiaxed grain zone, and the columnar grains were slenderer because of greater GR. The porosity-free and non-penetration welds were obtained by the laser sinusoidal oscillation. The reasons were that the molten pool volume was enlarged, the volume proportion of keyhole was reduced and the turbulence in the molten pool was gentled, which was observed by the high-speed imaging and simulation results of melt flow. The tensile test of both welds showed a tensile fracture form along the fusion line, and the tensile strength of sine weld was significantly better than that of the SLW weld. This was because that the wider equiaxed grain area reduced the tendency of cracks and the finer grain size close to the fracture location. Defect-free and excellent welds are of great significance to the new energy vehicles industry.

Effect of focal position offset on joint integrity of AA1050 battery busbar assembly during remote laser welding

This paper aims at studying the impact of focal position offset on joint integrity of AA1050 battery busbar during remote laser welding. Welding experiments were conducted at different focal position offsets in the AA1050 L-joint with the integration of laser beam oscillation. The impact of focal position offset was evaluated by multiple joint indicators including weld geometry, thermal profile, weld porosity and mechanical strength. Results showed that defocusing of the laser beam tends to be favourable for the reduction of peak temperatures and weld porosity. However, a defocusing more than 3 mm can lead to 60% drop in bonding integrity and 40% loss of joint strength. A tolerance window of [-2 1] mm was determined for the focal position offset, corresponding to 70% and 30% of the Rayleigh length. This result is a useful reference to inform precise focal position control with respect to the accumulated variations of part tolerance, part-to-part gap and part positioning errors. This study also indicated that the thermal measurement can be a potential approach for in-process monitoring of joint integrity. Indeed, the peak temperature near the weld zone showed good linear correlation with off-line detectable joint indicators including weld depth, weld width at interface, weld tensile strength and weld porosity.

Effects of beam oscillation modes on microstructure and mechanical properties of laser welded 2060 Al-Li alloy joints

In the present work, laser welding of 2-mm-thick 2060 Al-Li alloy was performed under different beam oscillation modes. Weld seam morphology, microstructure, mechanical properties were compared. The results indicated that joints welded by oscillated laser beam possessed improved collapse, refined grains, higher microhardness and tensile strength. In fusion zone, nondendritic equiaxed grains, parallel dendrite grains and cellular dendrite grains were formed in all welded joints. After applying laser beam oscillation, the collapse decreased from 0.54 mm to 0.1 mm. The maximum tensile strength of welded joint was obtained in the case of ‘8′ shaped laser beam oscillation reaching 338.15 MPa, which was improved by 68% compared with that of joint without oscillation (200.58 MPa). In this case, grain characters and molten pool fluid were studied to reveal the enhancement mechanism of beam oscillation. Enhanced fluid velocity produced with ‘8′ shaped laser oscillation refined grains and more high angle grain boundaries (HAGBs) were formed. The grain size was reduced from 105.27 μm to 89.67 μm in cellular dendrite zone (CDZ). The percentage of HAGBs increased from 65.2% to 75.2%. The refined grains resulted in larger and more uniform microhardness value and better mechanical behavior. Shallower, crystal-sugar shaped dimples were formed in fracture surface obtained by pure laser welding without oscillation while deeper and bigger dimple, including equiaxed dimple and conical dimple were generated in the fracture surfaces of oscillated laser welded joints.

Laser beam oscillation welded SiCp/2xx Al alloy: Microstructure, phase interface and mechanical properties

SiCp/Al matrix composites (SiCp/AMCs), as light and high strength structural alloys, have been widely used in aerospace structural parts, space station solar cell structural framework and other fields. To develop its potential as a structural alloy, SiCp/2 xxxx Al matrix composite plate was selected to investigate its weldability. In this work, the 4 mm thick SiCp/2xx Al joints were prepared via three types laser beam oscillating welding. The formability, microstructure, phase interface and mechanical properties were studied in detail. Results showed that the laser oscillation path significantly affects the formability and tensile properties of the SiCp/2xx Al joint. The tensile strength of the SiCp/2xx Al joint prepared via Clock Wise (CW) oscillation mode reached 192 MPa. The unmelted SiC particles and Al matrix produced a mild ~4 nm Al4C3 transition layer, which had a coherent interface and enhanced the load transfer mechanism. In addition, a formula to quantitatively predict the yield strength of SiCp/Al welded joints was optimized, and the load transfer enhancement mechanism of welded joints was calculated as 14 MPa. It is found that after rapid non-equilibrium solidification, three closely bonded interfaces were characterized, i.e., (200)Al ‖ (104)Al4C3, (103)SiC ‖ (104)Al4C3 and (200)Al ‖ (−511)AlCu. This work provided a process window for laser oscillation mode welding of SiCp/2xx Al matrix composites plate, and also provided a strategy for in-situ construction of transition layer to improve the load transfer mechanism of SiCp/2xx Al welded joint.

Improvement of Surface Quality and Process Area Rate in Selective Laser Melting by Beam Oscillation Scan Technique

Selective laser melting (SLM) has potential benefits for additive manufacturing of complex and precise metal parts with required mechanical and material properties. However, the improvement of surface quality and the minimization of fabrication time still remark a challenge for efficient manufacturing by SLM. Among the process parameters, the scan strategy has a significant influence on process stability and quality of the final manufactured three-dimensional products. This paper demonstrates the investigation of an alternative scan strategy to improve the surface quality with a reduction of fabrication time and laser energy input. Hereby, the scan strategy is performed by a conventional linear and oscillating laser beam movement in a self-developed SLM process chamber. Comparative analysis of the manufactured specimens regarding surface quality and density was performed. The investigations in oscillating laser beam movement show a decrease in the average variation of surface profile up to 50% relative to the experimented conventional linear scan technique. Due to an increased melt pool during the oscillation beam movement, the hatch distance was increased. As a result, the applied laser energy input was reduced by more than 28%, and the area rate was raised up to 50% compared to the conventional linear laser beam movement.

Laser beam oscillation welding of aluminum alloy using the spatially modulated beam by diffractive optical element (DOE)

In laser welding, the spatial beam intensity profile of the laser beam is an important parameter as it has a considerable effect on the temperature distribution and cooling rate of the melt pool, which determine the weld properties. In this study, a laser beam with a spatially modulated intensity profile by a diffractive optical element (DOE) was applied to the laser beam oscillation welding of an aluminum alloy (AA5052), and the weld characteristics of modulated beam welding (MBW) were analyzed in relation to those of Gaussian beam welding (GBW). According to the result of experiment verifying full penetration condition, the MBW generally required a slower maximum beam speed for full penetration than the GBW due to laser energy spreading by the core and ring beam constitution. On the other hand, this dispersed laser energy of the modulated beam was more effective in formation of a stable melt pool, which in turn reduced weld defects (bumpy bead surface, pores, and cracks) compared with the GBW. It also played a role in improving the asymmetric morphology of the weld by generating a relatively even temperature distribution of the melt pool. From an electron back scatter diffraction (EBSD) analysis, it was found that columnar grains were mostly distributed at the edge area of the weld, and equiaxed grains were largely distributed at the center area of the weld. The asymmetric feature in the microstructure was also found because of the asymmetric beam oscillation pattern and the resultant cooling rate difference. It was found that the area solidified at the lower cooling rate exhibited relatively large columnar grains. For the MBW, the asymmetric feature in the proportion of the low and high angle grain boundaries within the weld zone was diminished, which resulted from the relatively uniform temperature distribution by the dispersed laser energy. Due to the slow cooling rate, the MBW sample could have larger grains than GBW; this, however, slightly reduced the tensile strength of the joint welded by the modulated beam. The highest tensile strength obtained from the GBW and the MBW sample was equivalent to 88 % and 83.4 % of the base material’s tensile strength, respectively. It was found that the hardness value and its distribution across the weld zone was similar for the GBW and MBW samples. It was also found that the weld zone has a lower hardness value than base material owing to the increase of grain size through recrystallization.

Numerical study of keyhole-induced porosity suppression mechanism in laser welding with beam oscillation

The suppression mechanism of keyhole-induced porosity of oscillating laser beam welding (OLBW) was investigated via numerical simulation with keyhole dynamics and melt flow. The established model was verified by the corresponding experimental results. Results show that the weld porosity of 304 stainless steel can be controlled within 0.6% in OLBW and the large keyhole and well-balanced energy of the melt pool are contributing to porosity reduction. Also, two kinds of porosity suppression mechanisms in OLBW were clarified: first, the stirring effect of the oscillating beam keeps the bubble migration speed is superior in the competition with the solidification rate; second, bubbles can be absorbed timely by the dynamic keyhole when the moving speed of the keyhole reaches 504.6 mm s−1.

Improving the interfacial bonding strength of dissimilar PA66 plastic and 304 stainless steel by oscillating laser beam

Achieving high interfacial bonding strength between plastics and metals is one of the key challenges of structural lightweight. Oscillating laser beam was employed to improving the strength in this paper. Based on orthogonal test and variance analysis, the effects of oscillating radius, frequency and defocus distance on morphological characteristics, interfacial microstructure and mechanical properties of lap welds were discussed. It was found that the weld interface at the PA66 side could be divided into three zones, which were completely melted zone (CMZ), partially melted zone (PMZ) and soften zone (SZ). The binding force dominated by COCr chemical bond was the main force of interfacial joining. The PMZ was the most cohesive zone because its relative content of COCr chemical bond was twice higher than that of CMZ. Its area ratio to the whole weld interface dominated the shearing strength, and was in direct proportion to it. The results showed that increasing the oscillating frequency was beneficial to increase the area ratio of PMZ, and it had most significant effect on the shearing strength, whose contribution percentage was 63.07%. The shearing strength increased from 2.9 MPa to 7.75 MPa when the area ratio of the PMZ increased from 35.9% to 52%.

Study of the influence of the oscillation frequency on the surface hardening process of the 42CrMo4 alloy using an oscillating laser beam

A study of the surface hardening process of the 42CrMo4 steel alloy by means of an oscillating laser beam has been carried out. For this purpose, a fiber coupled high power diode laser source and a galvanometric mirror scanner have been used. The study is focused on determining the influence of the oscillation frequency parameter on the temperature, geometry, microstructure, microhardness and stress state of the treated region, as well as its relationship with the key parameters of the laser hardening process. The results confirm that the surface hardening process of the 42CrMo4 alloy by means of an oscillating laser beam is an alternative technique to the conventional laser surface hardening methods based on fixed optical guidance systems. On the other hand, the selected frequency of oscillation has a significant influence on the treated region, mainly on the temperature, and, consequently, on the geometry, microstructure and stress state of the same. The selection of a high oscillation frequency, able to maintain the material temperature above that of austenization and below that of melting, is decisive to ensure the stability and uniformity of the treatment.

Closed-loop controlled compensation of thermal lensing in high-power thin-disk lasers using spherically deformable mirrors

We present the closed-loop controlled compensation of the thermally induced lens in a high-power thin-disk oscillator to obtain fundamental-mode operation over a wide range of pump powers. The radius of the oscillating beam was measured in real time at the position of the thin-disk crystal. This signal was used by the control system to adapt the curvature of a spherically deformable mirror to maintain a constant radius of the oscillating laser beam.

The hot cracking susceptibility subjected the laser beam oscillation welding on 6XXX aluminum alloy with a partial penetration joint

A laser beam oscillation method using Galvano mirrors, which allows wide weld beads and controls thermal stress distribution, was suggested to suppress the formation of solidification cracks in laser welds. In order to understand the solidification cracking behavior in relation to the bead shape, laser beam oscillation welding was performed under various oscillation widths and frequency conditions. To evaluate the effect of the oscillation parameter on solidification cracking susceptibility, a regression analysis based on the shape of the bead was performed. Stress distribution generated during the laser beam oscillation welding process was also analyzed using finite element modeling simulation. From the results, it was demonstrated that a high shrinkage stress field at the bottom of the partial penetrated bead suppresses the solidification cracking.