Influence of process conditions in laser-assisted low-pressure cold spraying

A laser-assisted low-pressure cold spraying (LALPCS) is a one-step coating process in which the laser irradiation interacts simultaneously with the spraying spot on the substrate or deposited coating surface in order to improve coating properties. It is expected that the LALPCS could be an effective method to improve a low-pressure cold sprayed coating deposition efficiency and denseness. The purpose of the additional energy from the laser beam is to create denser and more adherent coatings, enhance deposition efficiency and increase the variety of coating materials.In this study copper and nickel powders with additions of alumina powder were laser-assisted low-pressure cold sprayed on carbon steel. Coatings were sprayed using air as process gas. A 6 kW continuous wave high power diode laser and a low-pressure cold spraying unit were used in the experiments. The influence of laser energy on coating microstructure, density and deposition efficiency was studied. The coatings were characterized by optical microscopy and SEM. The coating denseness was tested with open cell potential measurements. Results showed that laser irradiation improved the coating denseness and also enhanced deposition efficiency.     

纳秒高能密度粒子束与瞬态作用过程研究

多极板赝火花 (Pseudospark)放电室 ,在外加直流电压下 ,能产生自收缩纳秒强脉冲电子束和多种离子束。电子束瞬间功率密度可达 10 10 W /cm2 。与高功率脉冲激光相似 ,该高能束流与物质相互作用将产生瞬态高温等离子体 ,并伴随高温、高压和烧蚀过程。本文将介绍用脉冲粒子束进行金属表面改性的试验方法 ,以及材料表面微区快速熔凝等一系列非平衡的物理、化学变化 ,阐述材料表面轰击前后显微结构、性能等分析测试结果 ,讨论了纳秒脉冲粒子束与固体介质间的瞬态作用过程。试验结果表明 ,纳秒脉冲电子束和离子束以其特有的作用过程 ,在材料表面改性、表面涂敷以及低维材料制备等领域中 ,有着诱人的应用前景     

Diamond synthesis at low temperatures

Diamond growth at room temperature [i.e., on unheated monocrystalline silicon (111) substrates], has been demonstrated by the laser irradiation of CO/H₂ gas mixtures at low pressures. The CO is photodissociated by a multiphoton process using 193 nm radiation from an excimer laser. This process, referred to as laser chemical vapor deposition (LCVD), is distinct from the laser-excited CVD (LECVD) process described in an earlier publication. Although no quantitative measurements have yet been made on the gas-phase species compositions in LCVD, the growth of diamond particles at room temperature marks a beginning to enabling diamond deposition on materials with low melting temperatures.     

WC nano-particle surface injection via laser shock peening onto 5A06 aluminum alloy

A novel technique, named nano-particle surface injection via laser shock peening-NPSI/LSP, for surface modification of light alloys is reported. The WC nano-particles were implanted into a 5A06 aluminum alloy surface under a very high pressure (up to Giga or even tens of Giga Pascal), produced by a Q-switched pulsed Nd:Glass laser system. The results confirmed that the surface performance of the 5A06 aluminum alloy have been dramatically improved by the NPSI/LSP process based on an integrated strengthening mechanism of laser shock peening, nano-particles and nano-particle intensified shock peening.     

飞秒激光加工参数对RB-SiC表面形貌的影响

目的 揭示飞秒激光加工参数对反应烧结碳化硅(Reaction-Bounded Silicon Carbide,RB-SiC)表面形貌的影响规律。方法 通过改变激光能量密度和有效脉冲数,研究RB-SiC表面烧蚀槽的形貌变化规律,确定飞秒激光加工RB-SiC的去除机理。采用扫描电镜、共聚焦显微镜、X射线能谱仪和拉曼光谱仪分析RB-SiC烧蚀前后的表面形貌演变行为。结果 激光能量密度在0.62~10.48 J/cm²时,Si富集区域形成凹陷结构,SiC颗粒区域形成周期性结构(Laser-Induced Periodic Surface Structures,LIPSS),周期约为970 nm。随着激光能量密度的增加,凹陷结构扩大加深,表面球形纳米颗粒增多,烧蚀槽宽度呈对数增长。有效脉冲数在69~ 1 379,Si富集区域的去除量高于SiC颗粒区域的去除量。随着有效脉冲数增加,烧蚀槽深度显著加深,凹陷结构扩展成深坑结构,飞溅至烧蚀槽外侧的纳米颗粒聚集成团簇物,由Si、SiC和非晶态SiO2构成的沉积物在烧蚀槽边缘形成堆积层。结论 降低激光能量密度能够减少RB-SiC表面凹陷和纳米颗粒,有助于提升烧蚀形貌的一致性。增加有效脉冲数会促进烧蚀槽底部深坑结构的产生,进而扩大Si与SiC去除量之间的差异。     

Laser-enhanced electrodeposition of nickel coatings

Optimal conditions for a laser-enhanced electrodeposition of nickel coatings have been specified. Comparative results of structure and mechanical properties of electrically deposited nickel coatings produced by a laser-enhanced galvanostatic method and without laser emission were analysed. It was found that laser medium wave IR radiation has a local thermal effect on the process of electrodeposition and stimulates formation of a nickel coating structure which is of a more equilibrium state, less defective and has reduced internal stress in the area of local laser heating. Studies of plating rate showed a 1.4 times increase for nickel coatings in conditions of laser-enhanced electrodeposition due to reduction of common cathode overpotentional and increase of cathode metal efficiency, that involves generation of a more coarse microcrystal structure. Results of studies of the laser radiation effect on structure and properties of nickel coatings show potential for increased adhesion of electrodeposited coatings with a copper substrate.     

Laser ablation of titanium alloy under a thin and flowing water layer

Underwater laser ablation has become an alternative machining process that is able to reduce the thermal damage in work materials caused by lasers. However, the disturbance of water to the laser beam is a crucial concern for the ablation performance in water and cut surface quality obtained. In this study, a new laser ablation technique has been proposed, in which a waterjet was applied to impinge the top workpiece surface in order to form a thin and flowing water layer. With the assist of such water layer during the laser ablation, the redeposition and heat-affected zone can be minimized. Titanium alloy (Ti–6Al–4V) selected as a work sample was grooved by using a nanosecond-pulse laser under different machining conditions. The cut geometry and heat-affected zone were observed and analyzed to justify the process performance. The metallurgical change and cracks that occurred on and underneath the groove surface were also investigated in this study. The experimental results revealed that a clean cut with less thermal damage can be obtained when the workpiece was ablated by a laser under the flowing water layer. In addition, a narrower and deeper groove can be fabricated when a higher waterjet flow rate was applied. The laser ablation under the flowing water layer developed in this study could be a potential method for machining titanium alloy or even other thermal-sensitive materials.     

表面反射层在激光防护上的作用

为了研究激光辐照时反射层对基材的防护作用,采用铝箔作为高反射层进行激光辐照对比试验.结果表明:铝箔高反射层的存在降低了激光辐照对基材的热烧蚀和热影响,这种防护方式对基材抗激光防护效果显著.     

Densification,microstructural evolutions of 90W-7Ni-3Fe tungsten heavy alloys during laser melting deposition process

Thin walled 90W-7Ni-3Fe tungsten heavy alloys (WHAs) have been prepared by a laser melting deposition (LMD) additive manufacturing technique using different input laser volume energy densities. Detailed investigations on densification and microstructural evolutions of the LMD process have been carried out. The result shows that the sample density increases with increasing input energy density by elimination of “lack of fusion” defects. However, some gas delivered with powders gets trapped in the molten pool due to the fast cooling rate and complex shapes of W particles, resulting in a prevalence of small round pore defects even under high input energy densities. Near full density can be obtained when the energy density reaches 74 J/mm³. The LMD WHAs have two phase microstructures consisting of polygonal or dendritic W particles embedded in a nickel‑iron matrix, which has large sized columnar grains due to epitaxial growth. The polygonal particles are partially dissolved/melted W powder particles. The dendritic particles are newly formed grains due to the reprecipitation of supersaturated W in the nickel‑iron matrix during solidification. The proportion of dendritic particles increases with the increasing input energy density. A microstructure evolution process adapted from traditional liquid phase sintering process containing three stages of rearrangement, solution-reprecipitation, and solid state is proposed for the LMD process.     

Optimisation of a paint coating ablation process by CO2 TEA laser: Thermal field modelling and real-time monitoring of the process

The ablation of coatings (paint, varnish, oxides, etc.) using a pulsed laser beam is a process that has a number of major benefits compared with conventional cleaning techniques (mechanical and chemical), the main advantage being the possibility of removing the surface layer without any risk of damaging the substrate. Another benefit, and certainly not the least, is the non-use of toxic solvents or chemical products that are difficult to eliminate and, in any case, will be banned from use in the near future. However, it must be stressed that, notwithstanding the above benefits, laser ablation of painted surfaces can only give good quality results if the process is properly controlled. The procedure discussed in this paper has two aims: the first involves establishing a thermal model of the interaction, applied to the ablation of paint from metal surfaces by infra-red laser. The second involves proposing an optimised paint removal process validated by experimental measurements. This second objective will be based on an analysis method of the acoustic signal transmitted by the ablation plasma beam.     

Laser-assisted synthesis of carbon nanofibers: From arrays to thin films and coatings

Carbon nanofiber assemblies in the form of non-aligned films, arrays of vertically aligned nanofibers, aligned nanofiber mats and composite coatings were produced by laser-assisted catalytic chemical vapor deposition. A visible argon ion laser was used to thermally decompose pure ethylene over alumina supported nickel catalysts. Straight, vermicular, beaded, branched and coiled individual nanofibers were observed. The effects of the laser irradiation time on individual nanofiber characteristics, thus on overall nanofiber assembly characteristics were investigated. The arrays, nanostructured films and coatings were examined by scanning electron microscopy. The individual nanofibers were examined by transmission electron microscopy. Nanofiber texture and nanotexture were assessed by lattice fringe analysis of high resolution transmission electron microscopy images. The observed variation in the interfringe distance along the nanofiber wall suggests a pulsed growth mode. This growth mode and the nanofiber shaping mechanism are discussed. Recommendations on how to control nanofiber characteristics such as shape and internal structure are provided.     

Micro/nanometric investigations of the effects of laser ablation in the generation of micro-tools from solid CVD diamond structures

Using pulsed laser ablation, a novel method has been successfully developed for the generation of micro-features in diamond thus enabling the production of precision abrasive like micro-cutting elements into solid diamond micro-grinding tools. This paper examines the effects of the laser ablation on the generation of abrasive micro-features in diamond by using electron energy loss spectroscopy (EELS) and transmission electron microscopy (TEM). Site specific specimen preparation by a focus ion beam (FIB) material removal process has been made possible by the application of platinum and tungsten protective layers, providing the necessary specimen surface stability during the ion beam milling and lift-out operations. The prepared thin section orientated orthogonally to the cutting edge of a laser generated abrasive feature, has allowed the identification of the graphite and diamond allotropes across the ablated interface using EELS. A sequence of TEM images has revealed the extent of surface graphitisation and the high integrity of the diamond substructure. TEM examinations at nanometric resolutions have allowed the visualisation of the diamond and graphite allotropes at molecular level while depicting the abrupt graphite to diamond transition resulting in a highly defined boundary, thereby endorsing the capabilities of the laser ablation process to generate cutting edges with minimal structural damage.     

Al2O3陶瓷表面化学镀镍工艺及其低温连接

进行了Al2O3陶瓷表面的化学镀镍处理,对Al2O3陶瓷表面前处理工艺各个工序进行研究,制定了前处理流程.研究了硫酸镍浓度、次亚磷酸钠浓度、施镀温度、施镀时间等对镀层沉积速率的影响,对镀层进行了金相分析,在此基础上得到化学镀镍的优化工艺.进行了镀镍后Al2O3陶瓷低温钎焊工艺试验,研究了不同化学镀工艺条件对接头组织及力学的影响.结果表明,在试验条件下,硫酸镍、次亚磷酸钠浓度等化学镀工艺,焊接温度、焊接时间及压力等焊接工艺对接头组织及力学性能均有重要的影响.     

飞秒激光烧蚀铝片的热弛豫效应模拟与实验研究

研究了在不同能量密度的单脉冲飞秒激光下烧蚀铝片的过程。通过分析基于双温模型有限差分法所模拟出的数据图,并结合飞秒激光烧蚀实验的结果,从而研究不同激光能量密度与铝片烧蚀之间的联系。研究表明：飞秒激光能量密度与飞秒激光烧蚀的关联,而实验所测的结果进一步表明了提升飞秒激光能量密度对加工铝材料的加工效率以及加工质量的影响。通过研究和实验结果能够得出随着飞秒激光能量密度的增加,飞秒激光烧蚀期间铝材料的热弛豫过程将加长,烧蚀强度也将有所增加,同时铝材料加工后得出形貌质量也将有所提高,这个结论对于飞秒激光烧蚀金属材料的研究具有一定的意义。     

Deuterium trapping in irradiated nickel

The effect of cold plastic deformation in nickel on the deuterium segregation induced by ion irradiation has been studied by the nuclear-reaction method. The irradiation and discrete measurements have been performed at room temperature and coordinated in time due to the use of a beam of accelerated deuterons with an energy of 700 keV in the reaction D (d,p) T. An analysis of the experimental dose dependences of the accumulation of implanted deuterium and of the results of calculation of the process of deuterium implantation using an SRIM program made it possible to reveal the basic laws of the deuterium segregation. It has been established that deuterium trapping in the irradiated volume occurs into initial traps (deformation-induced vacancies) and radiation-induced traps (vacancies and their clusters). The dislocations at room temperature are not efficient traps of deuterium. Dose ranges of deuterium trapping by different-type traps have been revealed, and a model of the formation and evolution of the deuterium segregates in nickel has been put forward.     

“in situ” XPS studies of laser induced surface cleaning and nitridation of Ti

A titanium foil has been subjected to laser irradiation “in situ” in a pre-chamber of an X-ray photoemission spectrometer under different atmospheres (vacuum, Ar, O₂, air, N₂ and H₂). As a result of these treatments, a high amount of the carbon contamination layer was removed and other changes in composition were induced. Nitridation was achieved by laser irradiation under nitrogen. The most effective treatment protocol included an initial cleaning procedure induced by irradiation in vacuum, followed by a second irradiation process performed under nitrogen. Partial nitridation is also observed when irradiating under synthetic air. Lateral and depth analysis of the nitrogen concentration around the laser spot has been also carried out. It is found that the outermost layers present a similar concentration of nitrogen. In addition, the measured nitrogen profile indicates that the amount of nitrogen within the laser spot region is relatively lower than within the immediately surrounding area. Almost no nitrogen remains in the spot area after sputtering for 30 min. A model is proposed to account for the observed titanium surface nitridation processes.     

Nanocrystalline Nickel Deposition on the Titanium Alloys Using High Solution Flow Velocity

An innovative process has been developed for electroplating of nickel on titanium surface using fast solution flow technique. Nickel was directly deposited on a titanium alloy without using any pre-treatment process. Level of adhesion was determined using quantitative peel test and characterization of the deposition was performed by scanning electron microscopy. Results showed that the rate of nickel deposition at 60 °C was higher than that of the rate of nickel deposition at 40 °C. Moreover, Watts solution provided higher rate of nickel deposition compared to the sulfate-based nickel solution. The rate of deposition increased with increasing the solution flow velocity from 1.5 to 3 m/s and raising current density from 0.4 × 10⁴ to 1.6 × 10⁴ A/m² for both solution baths. Adhesion test indicated good level of adhesion between the deposited nickel and titanium surface. The bonding toughness increased to 4 J/m² for 1.2 × 10⁴ A/m² as a result of higher deposition rate. However, the mechanism responsible for the coating process was discussed in detail.     

Deposition Mechanism and Microstructure of Laser-Assisted Cold-Sprayed (LACS) Al-12 wt.%Si Coatings: Effects of Laser Power

Surface treatment is one of the most costly processes for treating metallic components against corrosion. Laser-assisted cold spray (LACS) has an opportunity to decrease those costs particularly in transportation systems, chemical industries, and renewable energy systems. This article highlights some of those potential applications. In the LACS process, a laser beam irradiates the substrate and the particles, thereby softening both of them. Consequently, the particles deform upon impact at the substrate and build up a coating. To circumvent the processing problems associated with cold-spray (CS) deposition of low-temperature, corrosion-resistant Al-12 wt.%Si coatings, a preliminary investigation detailing the effect of laser power on its LACS deposition mechanism and microstructural properties is presented. The deposition efficiency, the microstructure, and the microhardness of the LACS-deposited coatings produced by a 4.4-kW Nd:YAG laser system were evaluated. The outcome of this study shows that pore- and crack-free Al-12 wt.%Si coatings were deposited via softening by laser irradiation and adiabatic shearing phenomena at an optimum laser power of 2.5 kW.     

Crystallographic texture in pulsed laser deposited hydroxyapatite bioceramic coatings

The orientation texture of pulsed laser deposited hydroxyapatite coatings was studied by X-ray diffraction techniques. Increasing the laser energy density of the KrF excimer laser used in the deposition process from 5 to 7 J/cm(2) increases the tendency for the c-axes of the hydroxyapatite grains to be aligned perpendicular to the substrate. This preferred orientation is most pronounced when the incidence direction of the plume is normal to the substrate. Orientation texture of the hydroxyapatite grains in the coatings is associated with the highly directional and energetic nature of the ablation plume. Anisotropic stresses, transport of hydroxyl groups and dehydroxylation effects during deposition all seem to play important roles in the texture development.     

激光照射原位生成SiC晶须的晶须形态

利用冷等静压法将掺有粘结剂的SiC纳米颗粒压制成块体并经大功率CO2激光器照射,在激光照射区域获得SiC晶须,利用扫描电镜对晶须进行观察,分析照射层不同区域和不同气氛环境下晶须形态的变化。结果表明:不同气氛环境下晶须形态有显著区别,分别表现为放射状团簇特征、螺旋状团簇特征和密排短棒状团簇特征;同一照射层内,不同区域晶须形态也不一致,沿光束入射方向,可见以较大晶粒为核心的团簇状小长径比晶须和数量丰富、形态良好的大长径比晶须;不同的气氛组分和光强梯度分布是造成晶须形态不同的主要原因。