Microstructure and mechanical properties of high power diode laser (HPDL) treated cast aluminium alloys

The appliance and development of modern technologies in the areas of surface engineering can be extended by laser surface treatment, especially using high power diode laser (HPDL) for remelting, feeding and/or alloying. The purpose of this work was to determine technological and technical conditions for tungsten carbide (WC) ceramic powder feeding into the surface layer of the laser treated Al–Si–Cu cast aluminium alloys with high power diode laser, as well as to investigate the microstructure and ceramic powder particle distribution in the surface layer. Special attention was devoted to monitoring of the layer morphology of the investigated material and on the particle occurred. Light and scanning electron microscopy as well as X‐Ray diffraction were used to characterize the microstructure of the remelted zone. A wide range of laser powers was choose and implicated by different process speed rates. Also one powder in form of tungsten carbide was used for feeding with the middle particle size of 80 µm. As the main findings there was found that, the obtained surface layer is without cracks and defects as well as has a comparably higher hardness value compared to the non remelted material. The hardness value increases according to the laser power used so that the highest power applied gives the highest hardness value in the remelted layer. Also the distribution of the tungsten carbide particles is good, but there are still possibilities for further modelling. The major purpose of this work is to study the effect of a high power diode laser melting on the cast Al–Si–Cu alloys structure to provide application possibilities for automotive and aviation industry.     

60瓦半导体激光器光纤耦合器件

应用光纤列阵耦合方式，对大功率半导体激光器线列阵输出光束的快轴方向用一根柱透镜准直，准直后的光束耦合到光纤列阵中，实现出纤功率为60瓦的大功率半导体激光二极管线列阵光纤耦合器件，耦合效率大于80％，光纤的数值孔径NA为0．11。     

Laser Bending of Aluminum Foam Sandwich Panels

Aluminum foam sandwich panels were laser bent by means of a diode laser. Two panels different in thickness were used in bending tests; the effect of the main process parameters (laser power and scan velocity) on the bending efficiency was investigated as well as the contribution of the panel skin, and the protective gas. As a result, a very good formability was observed for the laser processed panels.     

Determining of laser surface treatment parameters used for light metal alloying with ceramic powders

There are presented in this paper the investigation results of microstructure as well hardness investigation of the surface layer of cast aluminium alloys in as‐cast state and after laser surface treatment using a high power diode laser (HPDL) with Al₂O₃ ceramic powder. The purpose of this work was to determine the proper laser treatment conditions for surface treatment of the investigated alloys and to describe the structural changes occurred in the surface layer after laser treatment. For investigation of the obtained structure there was used light as well scanning and transmission electron microscopy. The morphology and size of the ceramic powder particles as well the structure of the remelted aluminium surface layer was possible to determine. After the laser surface treatment carried out on the aluminium cast alloys there are visible structural changes concerning the microstructure as well as distribution and morphology of the fed particles occurred in the sample surface. Concerning the laser treatment conditions for surface hardening also the laser power and ceramic powder feed rate was studied. The structure of the surface laser tray changes in a way, that there are zones detected like the remelting zone and the heat influence zone. This investigation with appliance of a high power diode laser for Al alloys makes it possible to obtain or develop an interesting technology very attractive for different industry branches.     

Corrosion behaviour of laser surface melted magnesium alloy AZ91D

A high power diode laser (HPDL) was used to produce laser surface melting (LSM) treatments on the surface of the Mg alloy AZ91D. Different treatments with different microstructures were produced by varying the laser-beam power and laser-scanning speed. Corrosion evaluation, using hydrogen evolution and electrochemical measurements, led to a relationship between microstructure and corrosion. Most corrosion rates for LSM treated specimens were within the scatter of the as-received AZ91D, whereas some treatments gave higher corrosion rates and some of the samples had corrosion rates lower than the average of the corrosion rate for AZ91D. There were differences in corroded surface morphology. Nevertheless laser treatments introduced surface discontinuities, which masked the effect of the microstructure. Removing these surface defects decreased the corrosion rate for the laser-treated samples.     

Design and stability analysis of a CMOS feedback laser driver

A feedback laser driver circuit has been developed to control the average optical output power of a double heterostructure CW laser diode and also to operate with a wideband amplitude modulation. The average light power monitoring is realized by measuring the photoelectric current of a photodiode integrated inside the package of the laser diode. Safety features, including transient signal suppression, protect the laser against excessive light power. To obtain responses that are not out of the absolute maximum ratings of the laser diode light power, a study of the feedback loop stability is necessary. Two transconductance structures, using inverting and noninverting circuits are compared using the dominant pole compensation method. Then, the more stable driver circuit is analyzed and integrated using CMOS technology.     

近红外光纤激光与蓝光半导体激光焊接紫铜工艺对比研究

目的 提高紫铜激光焊接接头的力学性能,并分析激光工艺参数对焊缝外观及焊缝微观组织的影响规律。方法 分别对蓝光半导体激光与近红外光纤激光焊接紫铜的工艺参数进行优化设计,采用光学显微镜观察焊缝的外观形貌,采用拉力机测试焊缝的抗拉强度,采用金相显微镜观察和分析焊缝的微观组织。结果 当采用近红外光纤激光进行焊接时,功率为2 000 W,焊接速度为20 mm/s,焊缝抗拉强度为156 MPa。当采用蓝光半导体激光进行焊接时,功率为500 W,焊接速度为20 mm/s,焊缝抗拉强度为246 MPa,达到母材抗拉强度的80%。结论 由于铜对蓝光波长的吸收率较高,当采用蓝光半导体激光进行焊接时,热量输入较低,焊缝的变形相对较小,并且焊缝中心各个方向上的温度梯度相同,容易形成等轴晶,有利于力学性能的提高。     

Making fast cylindrical gradient-index lenses diffraction limited by using a wave-front-correction element

A wave-front-correction element (WFCE) is produced to make a cylindrical Ag-ion-exchanged gradient-index (GRIN) lens with a high numerical aperture (0.53) diffraction limited (wave-front error, 0.02lambda rms). The wave-front aberrations of the cylindrical GRIN lens are measured by a phase-shifting shearing interferometer, with a conventional microscope objective used as a compensation lens. The continuous surface relief of the WFCE is produced by a lithographic process. The wave-front-corrected GRIN lens is applied to collimate the strongly divergent light (57 degrees full diverging angle measured at 1/e(2) of maximum intensity) emitted by a high-power diode laser. The power irradiated into a full angle of 2 mrad can be enhanced by a factor of 1.8 with the WFCE.     

圆柱类金属构件表面裂纹的激光超声识别方法研究

为了利用激光超声技术有效地识别圆柱表面裂纹,提出利用圆柱表面波信号增强和小波包-奇异值分解(WPT-SVD)方法识别圆柱类金属构件表面裂纹的位置和深度。建立了圆柱的激光超声显式有限元模型,分析了圆柱表面裂纹对表面波的模式转化作用。利用圆柱表面裂纹在激发源位置附近时激光超声扫描信号增强的现象,识别圆柱表面裂纹的位置。在已识别圆柱表面裂纹位置的基础上,通过分析圆柱表面裂纹检测信号的时频特点,利用WPT-SVD提取圆柱表面信号的时频特征,定义参数k r表征裂纹深度的变化,识别圆柱表面裂纹深度。搭建了激光超声圆柱表面裂纹检测实验系统,开展了实验研究,实验结果表明所提出的圆柱表面信号增强和WPT-SVD方法可以识别出圆柱表面裂纹的位置和深度。     

Bending behavior of aluminum foam-filled double cylindrical tubes

Quasi-static experiments and numerical simulations are carried out to study three-point bending behavior of a new kind of structures, i.e., double cylindrical tubes filled with closed-cell aluminum foam. The deformation and failure mechanism of this new structure were observed and analyzed numerically using the finite element method. It is revealed that the stress distribution and fracture of the foam-filled double-tube structure are different from those of an empty tube and foam-filled single tube. Two cracks were found experimentally, and both experiments and numerical simulation show that cracks initiated in the aluminum foam. In comparison with empty and foam-filled single tubes, the load-carrying capacity of this new structure is much steadier, the bending resistance is enhanced, and the weight efficiency of energy absorption is higher. Parameters affecting the performance of the foam-filled double-tube structures are also studied.     

Mechanical characterization of glass/polyimide microjoints fabricated using cw fiber and diode lasers

This paper discusses the laser-irradiated microjoints between glass and polyimide for applications in neural implants. To facilitate bonding between them, a thin titanium film with a thickness of approximately 0.2 μm was deposited on glass wafers using the physical vapor deposition (PVD) process. Two sets of samples were fabricated where the bonds were created using diode and fiber lasers. The samples were subjected to tension using a microtester for bond strength measurements. The failure strengths of the bonds generated using fiber laser are quite consistent, while a wide variation of failure strengths are observed for the bonds generated with diode laser. Few untested samples were sectioned and the microstructures near the bond areas were studied using an optical microscope. The images revealed the presence of a sharp crack in the glass substrate near the bond generated with the diode laser. However, no such crack was observed in the samples made using fiber laser. To investigate the reasons behind such discrepancy in bond quality further, uncoupled three-dimensional finite element analyses (FEA) were conducted only for the samples created using diode laser. First, the transient heat diffusion-based FEA was conducted by using the laser power intensity distribution as a time dependent heat source. This model calculates the temperature distribution within the substrates as a function of time. Next, the structural model predicts the amount of residual stresses developed in the joint system as it is cooled down to room temperature. The out-of-plane normal component of residual stresses was within the failure strength range of glass that may have caused fracture initiation in the substrate.     

Efficient Continuous-Wave and Q-Switched Operation of a 946-nm Nd:YAG Laser Pumped by an Injection-Locked Broad-Area Diode Laser

The efficient, low-threshold operation of a 946-nm Nd:YAG laser pumped by an injection-locked broad-area diode laser is reported. The implications of pump-beam quality for efficient, low-threshold operation, particularly with intrinsically inefficient transitions, are discussed in the context of previously published models. Results are presented showing that the M(2) = 1.3 pump beam of the injection-locked diode laser enabled a cw slope efficiency of 48% and a threshold of 52 mW to be attained. When Q-switched, 335 mW of pump power gave 27-ns, 5.2-muJ pulses. These were frequency doubled to obtain 19-ns, 1-muJ pulses at 473 nm. These results represent significant improvements over similar systems pumped by free-running broad-area diode lasers or arrays.     

Shaping of the end of a multimode optical fiber for efficient coupling light from a laser diode

An entrance surface of a multimode optical fiber with the highest efficiency of coupling radiation from a laser diode is considered. The geometry of the surface is designed with the purpose of keeping a refracted ray inside the fiber with simultaneous minimization of the reflection coefficient in each point of the surface. As a good approximation to the ideal surface, the composite entrance surface of a cylinder-wedge shape is proposed. It has clearly improved the coupling efficiency as compared with that of the purely cylindrical surface. Analysis of the entrance surfaces is based on their transmission coefficients, depending on the incident ray direction.     

Amplitude-stabilized frequency-modulated laser diode and its interferometric sensing applications

A direct frequency-modulated (FM) laser diode light source without light power variation is developed. The amplitude variation of the FM laser diode is compensated by means of a feedback system with use of a superluminescent diode as an external light power controller. Output power greater than 1 mW is obtained at the modulation frequency to 5 kHz with a >10 stabilization factor. By use of the amplitude-stabilized FM laser diode, we measured subfringes with high accuracy in FM continuous wave interferometry, increased the dynamic range of the displacement measurement, and improved the stabilization factor in the laser diode feedback interferometer.     

Laser heat treatment of titanium GRADE 1 with iron‐nickel powder

Titanium alloys thanks to their low density and high mechanical properties are a group of materials that are being used willingly nowadays. A promising method of titanium heat treatment is laser alloying. Process parameters like laser beam power, its traverse speed, amount of alloying elements and shield gas, have influence on the resulting material. Different chemical composition and morphology can be achieved resulting in a change of properties on the surface of the material. The paper presents the investigation of titanium GRADE 1 processed with iron‐nickel powder using laser alloying. The treatment was performed using a high power diode laser. Different laser beam power values were used. Treatment resulted in obtaining good‐adhere, porous‐free, uniform composite material with no cracks. Formation of new phases and solid solution was the reason why it was possible to achieve an increase in microhardness on the surface. Light microscopy, scanning electron microscopy with energy dispersive X‐ray spectroscopy examination, microhardness results and X‐ray diffraction are presented within the paper.     

Frequency-shifted optical feedback in a pumping laser diode dynamically amplified by a microchip laser

Compared with conventional optical heterodyne detection, laser optical feedback imaging (LOFI) allows for a several orders of magnitude higher intensity modulation contrast. The maximum contrast amplification is typically 10(3) for a diode laser in the gigahertz range and 10(6) for a microchip laser in the megahertz range. To take advantage of the wavelength tunability of a laser diode and of the lower resonant detection frequency of a microchip laser, we used LOFI modulation induced by the frequency-shifted optical feedback in a laser diode as a modulated pumping power for a microchip laser for resonant dynamic amplification. In this way, we were able to transfer the optical feedback sensitivity of the laser diode to the megahertz range. Application to telemetry is also reported.     

High-resolution random-modulation cw lidar

A high-resolution random-modulation continuous wave lidar for surface detection using a semiconductor laser diode is presented. The laser diode is intensity modulated with the pseudorandom binary sequence. Its enhanced resolution is achieved via interpolation and a novel front-end analog technique, lowering the requirement of the analog-to-digital converter sampling rate and the associated circuitry. Its mathematical model is presented, including the derivation of the signal-to-noise ratio and the distance standard deviation. Analytical and experimental results demonstrate its capability to achieve distance accuracy of less than 2?cm within 2.6?ms acquisition time, over distances ranging from 1 to 12?m. The laser diode emits 1.4?mW of optical power at a wavelength of 635?nm.     

Single aspherical lens for deastigmatism, collimation, and circularization of a laser beam

An astigmatic and elliptically diverging laser beam generated by a gain-guided laser diode can be converted into a collimated and circular beam with a single specially designed lens. The designed lens also corrects for the astigmatism present in the laser diode. The input surface of the lens has a different concave and convex aspherical profile in the transverse and the longitudinal direction, respectively, for astigmatism correction. The output surface possesses a convex aspherical and a straight profile in the transverse and the longitudinal direction, respectively, for collimation and circularization of the final output laser beam.     

Application of holographic diffractive doublets as collimators for highly elliptical light beams edge emitted from diode lasers

Two schemes for collimation of diode laser light beams with high cross-sectional ellipticity by means of a doublet of holographic diffractive elements are proposed and designed; one of the schemes is realized and tested. In both the schemes the first element of the doublet collimates the beam in the plane of the longer axis of the ellipse, and the second element collimates it in the perpendicular plane. Each element simulates a cylindrical lens. The set-up with the focal line of the cylindrical beam oriented perpendicular to the meridional plane is realized experimentally. The elements are holographic surface-relief gratings recorded in photoresist. For recording, only homocentric diverging beams are used, which minimizes potential aberrations and optical dirt. The parameters of the elements are computed using four equations, including one equation for compensation of the aberration of the lowest order. The doublet is proposed for the He—Ne red wavelength. A collimated He—Ne laser beam is employed for quality testing of collimation in a reverse way, with this beam impinging upon the second element. Characteristics of an outgoing beam from the first element of the doublet are recorded with a charge-coupled device camera. Calculated spot diagrams are compared with cylindrical focal lines captured separately from both the elements.     

Excitation of capillary waves in strongly absorbing liquids by a modulated laser beam

Several mechanisms for the excitation of capillary waves and for the development of the average deformation of a liquid surface under the action of a modulated laser beam are considered. The amplitude of the capillary wave in a strongly absorbing solution of the dye LDS 751 in ethylene glycol is experimentally studied as a function of laser intensity. Consecutive changes in the predominant mechanism of the excitation with increasing laser intensity are observed and described. At low laser intensities the mechanism connected with the creation of a surface tension gradient prevails. This mechanism becomes nonlinear with increasing influence of the convective motion. In addition, pressure pulsations of the convective flow start to contribute significantly to the generation process. The resonances of capillary waves in a cylindrical container are also investigated and used for determining the surface tension and viscosity of the liquid.