Laser continuous/dispersed quenching techniques were used for surface hardening Cr8 cold work die steel, and its metallographic microstructures and mechanical properties are investigated in this work. Typical mixed structure which composed of carbides in different morphologies and acicular martensite can be obtained by laser quenching. The excellent wear resistance and surface hardness of Cr8 die steel are attributed to the synergistic effect of solid solution strengthening, grain refinement and high dislocation density. Meanwhile, the phase change hardening layer structure and residual tensile stress induced by full area coverage scanning (LCQ) are the main factors to deteriorate the impact toughness and bending performance after laser quenching. The soft substrate-hardened unit structure with laterally spaced distribution (LDQ) obtained by graphical scanning reduces the heat accumulation effect on the surface, which reduces concentration of deformation near the hardened layer. The bending property and fracture mechanism are studied. Compared with the forging die material treated by LCQ, the impact energy and bending resistances of forging die materials treated by LDQ are enhanced significantly. Consequently, Cr8 die steel takes into account the same wear resistance, and scanning strategy should be controlled to retain more interval distribution hardening units in layer structure for superior impact toughness and bending properties.
This paper aims at reducing iron loss of oriented silicon steel and improving its electromagnetic properties. The laser ablation was carried out with different scribe spacing by 532nm picosecond laser. The characteristics of ablation morphology and ablation mechanism were analyzed and discussed using 3D confocal microscope, SEM and EDS. Key magnetic parameters of oriented silicon steel such as relative permeability, dynamic hysteresis loop, and iron loss were measured using iron loss meter. The results show a regular and defect-free ablation morphology obtained by 532nm ultrafast laser, which has the characteristics of force effect damage. The performance improvement effect of silicon steel with the scribe spacing 3.5mm has been shown to be the best. The P1.5/50 and P1.7/50 decreased by 11.5% and 13.8% respectively, the peak of relative permeability increased by 13.6%, and the coercivity and loop area decreased by 12.3% and 11.9% respectively. The improvement of magnetic properties of silicon steel mainly depends on the domain structure and residual stress distribution.
The accumulation effects in high-reflectivity (HR) HfO2/SiO2 coatings under laser irradiation at different laser wavelengths are investigated respectively. It was found that the multi-shot laser induced damage thresholds (LIDTs) were always lower than that under single-shot at 1064 nm and 532 nm because of "fatigue effect". And the evolution of LIDTs versus shot number, namely the optic lifetime was acquired. The previous analysis verified the accumulation damage mechanism was mainly due to the influence of newly created defects, namely, the laser-induced defects or intrinsic defects with irreversible changes under multiple pulse irradiations. Thus, a correlative theory model based on critical conduction band electron density is constructed to elucidate the experimental phenomena in nanosecond at different wavelengths with a 5Hz repetition rate. In particular, shallow trap (defect state) of varying absorption crosssection with irradiated shot numbers is used to simulate the material modification process under multiple pulse irradiations. It’s found that the absorption cross-section of the defect state at 532 nm is about one order of magnitude higher than that at 1064 nm, and defects at 532 nm need much less shot numbers to reach saturation with a higher growth factor.
In view of the fact that the weak laser damage resistance of HfO2 / SiO2 coatings at 355 nm hinders the observation of the fatigue effect, nanosecond single and multiple pulse laser damage studies on Al2O3 / SiO2 high-reflective coatings were performed at 355 nm. Relative to that at the long wavelength, the fatigue effect at 355 nm is very weak and complicated. The damage probability curves and the evolution of the laser-induced damage threshold under multiple irradiations reveal that the fatigue effect is affected by both laser fluence and shot number. As the laser fluence or number of shots increases, the fatigue effect becomes more apparent. The damage morphologies induced by single and multiple irradiations both manifest as micrometer-scale pits without plasma scalding around, with the characteristics of a high defect density and high absorption coefficient. In particular, the accumulation damage mechanism at 355 nm may be reflected not only in the newly created defects but also in the modification of the coating material around the damage precursors. Thus, the coatings at 355 nm “seem to” have no damage growth threshold, no matter what the laser fluence is; once damage occurs, the damage site will grow sharply under subsequent pulses finally resulting in catastrophic damage.
Laser processes of crystalline silicon solar cells become increasingly attractive, because they are fast, accurate and contact-free. Nanosecond and picosecond laser ablations with wavelength of 532nm were performed on the anti-reflection layers deposited on silicon. The laser ablated grooves of AR coatings on monocrystalline and polycrystalline silicon were both characterized to verify the influence on the underlying silicon. And the threshold fluences were specified by contrasting with the corresponding performances under certain laser pulse duration. More importantly, the groove edges were analyzed to further expose the laser ablation mechanism under different laser pulse durations.
To research the formation and variation principle of the weld seam and molten pool for aluminum alloy high power fiber
laser welding, the welding experiments for 5052 aluminum alloy were carried out. The influences of laser power,
scanning velocity and protection gas on the welding process were systematically researched. The results show that with
the increase of power and scanning velocity, the depth to width ratio first increases and then decreases. The ratio reaches
the maximum value at 2.6 KW and 30 mm/s, respectively. When the power located at 2.6 KW to 2.8 KW or the velocity
located at 25 mm/s to 30 mm/s, stable deep penetration welding can be obtained. The weld seam shows relative flat
appearance and the molten pool presents typical “T shape” topography. Moreover, the protection gas also influences the
appearance of the weld seam. Using the independently designed fixture, the quality of the weld seam can be well
improved.
The lifetime of optics in high power laser system is typically limited by both laser-initiated damage and the subsequent
growth of laser-initiated damage sites. The single- and multiple-shot irradiations for HfO2 /SiO2 high-reflective coatings,
deposited from hafnia and silica at 1064nm in nanosecond were investigated. It was found that when shot number
increased, the possibility of damage growth increased as well. The relationship between field distribution and damage
morphologies and inner structures was discussed to reveal mechanism of damage initiation. Additionally, the damage
morphologies under different laser fluence and shot numbers were characterized to discuss the damage growth
mechanism upon subsequent pulses. The tested results illustrated the absorbers which induced damages were random
distribution, and the second highest peak of field intensity at the fourth interface was high enough to induce the
micron-sized damage pits. It was found that defect density had a significant impact on the damage site whether growing
or not upon subsequent laser pulses. Additionally, the growth resulted in delamination, and in turn delamination
accelerated damage growth, finally the catastrophic damage happened.
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