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The objectives of this paper are to measure the absorption rate based on water calorimetric method for both full and partial penetration welding and to evaluate the energy loss by spatter and metallic plume in 6 kW laser welding of 304 austenitic stainless steel according to the X-ray observed keyhole characteristic and loss of specimen weight. In partial penetration welds of 9 mm thickness, the absorption rate decreased from 73% to 62% with the welding speed from 50 to 150 mm/s due to the decrease in multiple reflections of laser beam inside the keyhole. In full penetration welds of 3 mm thickness, the absorption rate increased from 42% to 61% with the welding speed owing to the decrease in the amount of beam escaped from the keyhole inlet on the bottom surface of the sheet. At 50 mm/s in full penetration welds, fluctuation in keyhole diameter appeared, and the minimum absorption rate might come near 0 when the keyhole diameter was 3 times of the normal value. [ABSTRACT FROM AUTHOR]

TIG welded joint had higher strength than laser welded joint, while magnetic properties of the TIG welded laminations degraded much more seriously. Residual stress distribution was measured to reveal the relationship between stress and magnetization properties. And eddy current simulation was conducted to analyze the effect of welding process on the eddy current losses of the welded laminations. This manuscript could act as a guide for choosing the proper welding methods to join the laminated electrical steels. [ABSTRACT FROM AUTHOR]

Weldability of the high-grade electrical steel B27ahv1500 has been researched. Weld bead characteristics, torsion strength and magnetic properties of the welded laminations were measured and analyzed. Torsion strength of the welded laminations was linearly related to the weld bead area. Iron loss of the laminations welded at the speed of 10 mm/s had a percentage of increase of 24 compared to that of the laminations without welding when the magnetic flux density was 1.0 T. Lower boundary of the welding speed can be determined by the magnetic properties requirement and upper boundary of the welding speed can be determined by the torsion strength requirement. [ABSTRACT FROM AUTHOR]

Torsion strength of the laser-welded stator was modeled in this study to reduce the number of destructive experiments. A semi-empirical mathematical model was developed to estimate the feature dimensions of the weld bead at various welding speeds and an approximate equation to calculate the weld bead area was then derived. A torsion strength estimation model was built to describe the quantitative relationship between the torsion strength and the weld bead area. The feature dimensions, area of the weld bead and the torsion strength of the laser-welded stator were measured in experiments to validate the models. Modeling results fit well with the experimental results. The required weld bead area for the stator with various specifications could be derived from this model. [ABSTRACT FROM AUTHOR]

The keyhole phenomenon is uncovered to exist in the printable range of laser powder bed fusion process. The pores due to unstable keyhole collapse can be divided into bottom-pore (B-pore) and rear-pore (R-pore) based on the formation mechanisms. Till now, how to distinguish keyhole stability and predict pore formation remains unclear. Here, a dimensionless number (R kl) is determined to establish a relationship between the keyhole morphology and pore formation. When R kl is below 0.8, B-pore occurs at the keyhole bottom due to the bridge formed between the front and rear keyhole walls. With the increase of R kl and the standard deviation, pore formation mechanisms gradually change to R-pore in which a bubble pinches off at the rear wall. The cases with no pore formation corresponding to stable keyhole. These precise thresholds can be further specified with more in situ imaging data in the future. The fluid dynamics under the two pore formation mechanisms are further explored via a multiphysics simulation model. R-pore is smaller and has large probability to escape due to the smaller escaping distance and thicker liquid film. In addition, there is a tendency from B-pore to R-pore under higher ambient pressure, providing a solution for pore regulating in laser powder bed fusion. Based on the analysis results, it is expected to guide the laser-material interaction process to fabricate high performance parts by the keyhole morphology, especially under the high energy input to improve the efficiency, such as high-energy printing and welding. [Display omitted] • A dimensionless number is determined to identify the distribution of pore-formation mechanisms in the power-velocity map. • B-pore is formed when R kl is below 0.8, occurring in the region with high power and low velocity. • R-pore requires both large R kl and standard deviation, generated under the conditions with high power and medium velocity. • The clockwise flow and an anticlockwise flow collision accounts for the R-pore. • Higher ambient pressure induces a tendency from B-pore to R-pore, providing a strategy for pore regulating in LPBF. [ABSTRACT FROM AUTHOR]

Laser Directed Energy Deposition (DED) is an important process for additive manufacturing because of the advantage of high manufacturing speed. 450 nm blue laser has a higher absorption rate than traditional infrared laser for highly reflective materials such as Al and Cu. However, it is still unclear how the blue laser affects the particles and the molten pool flow in the DED process. In this work, we first designed two models to reveal the mechanism of the molten pool behaviors in the blue laser DED process: the transient model and the steady deposition model. The models are validated by in-situ and ex-situ experiments respectively. It is found that the three stages can be concluded for a particle impacting the molten pool: particle impacts, surface oscillates and the molten pool recovers. The particles whose diameter is less than 100 µm are necessary for steady deposition. The influence of process parameters on DED is investigated, and a suitable process parameters window is found. we also verified the superiority of the blue laser for DED of highly reflective materials through the simulation models. The current study provides new insights into the mechanism from the interaction in the molten pool scale to single-track forming in the blue laser DED, and can provide theoretical guidance on the application of blue laser DED of highly reflective materials. [Display omitted] • A 3D multi-physics field model was first developed to visualize the molten pool dynamics in the blue laser Directed Energy Deposition (DED) process. • We found that insufficient melt pool depth and unmelted powder agglomeration were the main causes of porosity in blue laser DED. • We verified the superiority of the blue laser for DED of highly reflective materials through the simulation models. [ABSTRACT FROM AUTHOR]

A single track, as the basic unit of laser directed energy deposition (L-DED) process, plays a significant role in the dimensional accuracy and mechanical performances of the ultimate products. However, there is almost no systematic investigation on the formation process and three-dimensional characteristics of the internal pore defects. Here, we used a high-speed camera, laser scanning confocal microscope (LSCM), and synchrotron radiation X-ray computed tomography (SR-CT) to study single tracks of AlSi10Mg alloy fabricated by blue laser directed energy deposition (B L -DED). A comprehensive investigation is conducted on the impact of processing parameters on the sizes, shapes, and formation mechanism of pore defects. Three types of pore defects are examined in single tracks: Type I lack of fusion, Type II spherical gas pores and Type III large irregular pores. Besides, large irregular pores are the transition between other two types. In particular, the results of SR-CT show that porosity decreases gradually with the increment of laser power and scanning speed. Therefore, high laser power accompanying with fast scanning speed will reduce the porosity. The lowest porosity of 0.074% is achieved under the power at 1600 W with scanning speed at 1080 mm/min, which has an obvious improvement over the current infrared L -DED. In addition, the mapping relationship among laser power, scanning speed and pore defects is established, which will provide a fundamental understanding of the origin of the defect and strategies for controlling the defect in L -DED towards high-quality printing. [Display omitted] • The porosity decreases with the increase of laser power and scanning speed by blue laser directed energy deposition. • Three types of defects are observed: Type I lack of fusion, Type II spherical gas pores, Type III large irregular pores. • The lowest porosity is 0.074% under the power of 1600 W and scanning speed of 1080 mm/min. • The mapping relationship among laser power, scanning speed and types of pore defects is established. [ABSTRACT FROM AUTHOR]

Laminated electrical steels are the key material structure of the stator and rotator of a motor, where high-quality welding is crucial for improving the performance of the electric machine. However, it is challenging to achieve a desired balance among the magnetic properties, mechanical properties, and welding efficiency for the laminated electrical steels at present. This work first adopted a 450 nm flat-top blue laser with a maximum power of 2000 W, which has a higher primary absorption rate to Fe compared to that of the conventionally used 980–1080 nm infrared laser, to join the laminated electrical steels. The dynamic process, weld bead characteristics, mechanical properties, and magnetic properties of the blue laser welding of laminated electrical steels were investigated. The results showed that a flat-top and large-spot blue laser could make the molten pool have a self-stabilizing ability to keep the continuity when encountering the gap and unsmooth surface. Moreover, a further improved self-stabilizing ability was observed for a small welding speed. The largest depth and width of the blue laser weld bead in this work were 2.30 and 0.57 mm, respectively, with a shear strength of up to 1700 N, which were achieved at the laser power of 1500 W and welding speed of 10 mm/s. In addition, the work also validated that the blue laser could significantly reduce the splashing in welding of electrical steel laminations and improve the welding efficiency, which provides a promising solution for high-performance electrical machine manufacturing. [Display omitted] [ABSTRACT FROM AUTHOR]

A method of laser remelting with an ultra-thin additional element coating on the surface was put forward to improve both the hardenability of mild steel and the hardening efficiency. Both the 100 % C and the 50 % C-50 % Cr (in mass) coatings were used. The additional surface coating increased the fusion efficiency of the laser irradiation because the carbon particles on the surface had a higher laser absorption rate than the mild steel. With the introduction of both surface coatings, the hardness of the melted zone was significantly improved, and the maximum hardness reached the hardness of the high carbon steel. The molten flows drove the diffusion of the elements, which contributed to the increase of hardness in the melted zone. This manuscript provides a potential method for improving the hardness of the surface layer by introducing new elements (C, Cr) in laser remelting, especially for the material with low hardenability. [ABSTRACT FROM AUTHOR]