Tribaloy T-400 is a cobalt based alloy with molybdenum additions, which has been developed for improved resistance to high temperature wear, galling and corrosion. Its hardness is provided by a hard intermetallic Laves phase, dispersed in a tough matrix of cobalt rich eutectic or solid solution. However, cracking limits its applications such as hard facing using laser surface cladding/coating. The primary aim of this work is accomplished by cladding crack free Tribaloy T-400 layer using a high power Nd:YAG laser. The optimal process conditions of cladding crack free Tribaloy T-400 coating on different steel substrates were obtained. The effects of iron dilution on the hardness of cladded Tribaloy T-400 coating are investigated. Dilution determined from clad geometry is verified from dilution calculated from an analysis of the composition of the clad. Microstructures of clad layers produced using optimal process parameters with and without preheating the substrate were analysed by Scanning Electron Microscopy (SEM). The chemical compositions of different phases present in the clad were analysed by Energy Dispersive X-ray Spectroscopy (EDS). Presence of phases with FCC and BCC structures and Laves phase (Co3.6Mo2Si0.4) in the clad were identified and analysed by X-ray Diffraction (XRD). The residual stresses in the clads were evaluated using hole drilling technique. The correlation between the process conditions and the resulting microstructures are discussed. Based on the results of this research, further scaling up to industrial application of laser cladding of Tribaloy T-400 is promising.

Ablation of bulk polycrystalline zinc in air is performed with single and multiple picosecond laser pulses at a wavelength of 1030 nm. The relationships between the characteristics of the ablated craters and the processing parameters are analyzed. Morphological changes of the ablated craters are characterized by means of scanning electron microscopy and confocal laser scanning microscopy. Chemical compositions of both the treated and untreated surfaces are quantified with X-ray photoelectron spectroscopy. A comparative analysis on the determination of the ablation threshold using three methods, based on ablated diameter, depth and volume is presented along with associated incubation coefficients. The single pulse ablation threshold value is found to equal 0.21 J/cm2. Using the calculated incubation coefficients, it is found that both the fluence threshold and energy penetration depth show lesser degree of incubation for multiple laser pulses.

A new approach is presented in this paper to link optical emission spectrum analysis to the quality of clad layers produced with laser metal deposition (LMD). A Nd:YAG laser (λ=1.064μm) was used to produce clad tracks with Metco 42C powder on AISI 4140 steel substrate. The laser power was ramped in steps of 220W from 800W to 2780W focusing the laser beam into a 4.8mm diameter spot. A constant cladding speed of 10mms−1 is used with a powder feeding rate of 0.36gs−1. The extent of metallic bonding is evaluated from dilution data obtained by measuring the melt depth in the substrate from cross sections of the clads. The spectra generated during the deposition process were collected with an optical spectrometer attached to the laser head. The discrete spectral lines from the collected spectra were identified from the NIST database as Fe I and Cr I lines. Two different spectral analysis methods were used, namely the calculation of the electron temperature Te and the intensity ratio IR. The electron temperature Te is computed from four discrete Cr I lines at 526.415, 529.827, 532.834, and 534.044nm wavelength. The intensity ratio IR is a measure of the absorption of a discrete Fe I line at 588.91nm. The analysis of the collected spectra indicates that onset and extent of metallic bonding can be detected with a spectrometer during the deposition process. A sudden decrease of the IR signal during the LMD process indicates the onset of metallic bonding. A further increase of the laser power mainly contributes to an increase in melt depth, leading to a higher dilution. In this region, the Te signal shows a better correlation with the dilution.

A 2D thermal model of laser cladding process based on mass and energy balance is built incorporating the powder efficiency and solved with the finite element software COMSOL MULTIPHYSICS® v4.4. Powder efficiency was used as one of the input parameters. Powder efficiency was determined with weight measurements before and after laser cladding on thin DIN 2393 steel plates. Powder efficiency was also calculated from the clad area measured with binary image processing technique applied on cross section micrograph. The powder efficiency obtained from these two methods is in good agreement. The changes in powder efficiency with cladding process conditions were analysed in detail. The effect of input energy on the powder efficiency and dilution were correlated. The powder efficiency increases with energy input to a maximum value, beyond which the increase is marginal. The dilution continues to increase within the tested effective energy levels. Two methods were used to validate predictions of the thermal model. The positions of the melt depth and depth of the HAZ are measured from optical micrographs and from the hardness profiles. The depths were computed by tracing the melting temperature and the Ac3 temperature of the substrate material against the isotherms generated in the numerical simulations. The agreements in general were good. Thermocouples were inserted in the substrate materials at different locations and depths to record the temperature changes during laser cladding of 11 overlapped clad tracks. Measured and simulated temperature cycles with time agree within 5% of error. The developed powder efficiency based model is able to predict accurately the clad geometry and thermal cycles during the laser cladding process.

Laser surface micro/nanopatterning by particle lens arrays is a well-known technique. Enhanced optical fields can be achieved on a substrate when a laser beam passes through a self-assembled monolayer of silica microspheres placed on the substrate. This enhanced optical field is responsible for ablative material removal from the substrate resulting in a patterned surface. Because of the laser ablation, the microspheres are often ejected from the substrate during laser irradiation. This is a major issue impeding this technique to be used for large area texturing. We explored the possibility to retain the spheres on the substrate surface during laser irradiation. A picosecond laser system (wavelength of 515 nm, pulse duration 6.7 ps, repetition rate 400 kHz) was employed to write patterns through the lens array on a silicon substrate. In this experimental study, the pulse energy was found to be a key factor to realize surface patterning and retain the spheres during the process. When the laser pulse energy is set within the process window, the microspheres stay on the substrate during and after laser irradiation. Periodic patterns of nanoholes can be textured on the substrate surface. The spacing between the nanoholes is determined by the diameter of the microspheres. The depth of the nanoholes varies, depending on the number of laser pulses applied and pulse energy. Large area texturing can be made using overlapping pulses obtained through laser beam scanning.

The effects of powder stream when using different feeding nozzles on the attenuation of the laser power were investigated theoretically and experimentally. The powder streams were recorded with a high speed camera. The average velocity of the particles was obtained using standard Particle Image Velocimetry (PIV) technique. These average velocities were computed at different powder feeding rates with an industrial ILT co-axial nozzle and an off-axial nozzle. Different carrier gas flow rates and pressures were used in the experiments. The recorded images were processed with ImageJ software to acquire the divergence of the powder streams. The attenuated laser power was computed with a simple model based on mass concentration. A power meter was used to measure the transmitted laser power for the experimental verification of the calculated attenuated laser power. A Nd:YAG laser beam of 4.8 mm spot size at focus was used at 1100 W. The calculated laser power transmitted through the powder stream, agrees with the experimental values within 5% of error margin. The results indicate that the attenuation of laser power is less with the off-axial nozzle as compared to co-axial nozzle under the same test conditions.Clad tracks were produced with two different types of feeding nozzles. The cross sections of clad tracks were prepared with standard metallographic procedures to investigate the geometry of the clad, dilution and the heat penetration. The relation between the geometry of a clad and corresponding transmitted laser power was investigated. The results indicate that there is a minimum threshold laser power that is required to produce clads with adequate metallic bonding.

Droplets are able to levitate when deposited over a hot surface exceeding a critical temperature. This is known as the Leidenfrost effect. This phenomenon occurs when the surface is heated above the so-called Leidenfrost point (LFP), above which the vapor film between the droplet and hot surface is able to levitate the droplet. Such a critical temperature depends on several factors. One of the most studied parameters has been the surface roughness. Almost all of the experimental studies in the literature have concluded that the LFP increases with the roughness. According to these results, it seems that the roughness is detrimental for the stability of the vapor film. In contrast with these results, we present here a micropatterned surface that significantly reduces the LFP. The temperature increase, relative to the boiling point, required to reach the LFP is 70% lower than that on the flat surface. The reasons for such an effect are qualitatively and quantitatively discussed with a simple semiempirical model. This result can be relevant to save energy in applications that take advantage of the Leidenfrost effect for drop control or drag reduction.

The developed feedback control system adjusts the laser power such that the width of the melt pool is kept at a user defined reference value. The system uses a CMOS camera and software algorithms to obtain the width of the melt pool. Using a discrete control filter, the required laser power to meet the reference melt pool width is determined. The control filter gain is designed such that the closed loop bandwidth of the cladding process is in the order of a few Hz. As a result, the closed loop system will effectively compensate for disturbances (such as heat sink changes and speed changes) within a second.Several experiments were performed to investigate the controller performance. The laser power was adjusted for the above effects. The resulting dilution and consequent hardness were constant over the entire clad layer.

Leidenfrost ratchets are structures with the ability of transporting liquid droplets when heated over the critical Leidenfrost temperature. Once this temperature is reached, the droplet levitates over the surface and moves in the direction marked by the slope of the ratchet at terminal velocities around 10 cm/s. Here we provide new experiments with micron-sized ratchets, which have been produced with picosecond pulse laser ablation. In the following work, we use a simple method to measure the thrust driving droplets of capillary size over the micro-ratchets. The mechanism responsible for the force acting on the drop on superheated ratchets has been recently under debate. We extend the recently proposed “viscous mechanism” proposed by Dupeux et al. [Europhys. Lett. 96, 58001 (2011)10.1209/0295-5075/96/58001] to capillary droplets and find good agreement with our measurements.

The combination of a dual-scale (nano and micro) roughness with an inherent low-surface energy coating material is an essential factor for the development of superhydrophobic surfaces. Ultrashort pulse laser (USPL) machining/structuring is a promising technique for obtaining the dual-scale roughness. Sheets of stainless steel (AISI 304 L SS) and Ti-6Al-4V alloys were laser-machined with ultraviolet laser pulses of 6.7 ps, with different numbers of pulses per irradiated area. The surface energy of the laser-machined samples was reduced via application of a layer of perfluorinated octyltrichlorosilane (FOTS). The influence of the number of pulses per irradiated area on the geometry of the nanostructure and the wetting properties of the laser-machined structures has been studied. The results show that with an increasing number of pulses per irradiated area, the nanoscale structures tend to become predominantly microscale. The top surface of the microscale structures is seen covered with nanoscale protrusions that are most pronounced in Ti-6Al-4V. The laser-machined Ti-6Al-4V surface attained superhydrophobicity, and the improvement in the contact angle was >27% when compared to that of a nontextured surface.

A model for the determination of the clad geometry and dilution in laser cladding is presented. The model uses a novel approach to determine the clad geometry. This involves a mathematical transformation of the balance equations describing the physical effects. The correlation between observable melt pool characteristics and dilution is investigated using this model. Simulations were performed for different combinations of cladding speed, laser power (distribution) and substrate temperature. Simulation results are compared to experimental results, showing a high degree of agreement. A high correlation between melt pool width and dilution was found, which was almost independent of the process settings and substrate temperature. This makes the width an excellent sensor for online dilution control.

Hydrophobic surfaces have attracted much attention due to their potential in microfluidics, lab on chip devices and as functional surfaces for the automotive and aerospace industry. The combination of a dual scale roughness with an inherent low-surface-energy coating material is the pre-requisite factor for the development of an artificial superhydrophobic surfaces. Ultra short pulse laser (USPL) machining/structuring is a promising technique to obtain the dual scale roughness. Moreover, ultra short laser pulses allow machining without or with limited thermal effects. Flat stainless steel (AISI 304L) were laser machined with ultraviolet laser pulses of 6.7ps, at different laser processing parameters. Next, the samples were coated with a monolayer of perfluorinated octyltrichlorosilane (FOTS) to get a superhydrophobic surface. The degree of hydrophobicity was accessed by static contact angle measurement. Laser patterned surface has longitudinal micro channels. Drag reduction in liquid flow can be obtained due to the shear free boundary condition at air-liquid menisci. The geometry of the patterns was analyzed with optical and scanning electron microscopy. Micro-Particle Image Velocimetry (μPIV) has been employed to measure and visualize the flow over such patterns

Laser welding has a large potential for the production of tailor welded blanks in the automotive industry, due to the low heat input and deep penetration. However, due to the small laser spot and melt pool, laser-based welding processes in general have a low tolerance for seam gaps. In this paper, five laser-based welding techniques are compared for their gap bridging capabilities: single-spot laser welding, twin-spot laser welding, single-spot laser welding with cold wire feeding, twin-spot laser welding with cold wire feeding and laser/GMA hybrid welding. Welding experiments were performed on 1.1- and 2.1-mm-thick AA5182 aluminium sheets. The resulting welds were evaluated using visual inspection, cross sectional analysis with optical microscopy, tensile tests and Erichsen Cupping tests. The results show that the use of a filler wire is indispensable to increase the gap tolerance. A proper alignment of this wire with the laser spot(s) is crucial. With the single spot laser welding with cold wire feeding, a gap up to 0.6 mm could be bridged as opposed to a maximum allowable gap width of 0.2 mm for single-spot laser welding without filler wire. For 2.1-mm-thick sheets, the laser/GMA hybrid welding process can bridge even gaps up to 1.0 mm. Most welds had a high tensile strength. However, during Erichsen Cupping tests, the deformation of the welds is significantly lower as compared to the base material.

Plastic incompatibility second-order stresses were determined for different orientations of a polycrystalline grain, using X-ray diffraction data and results of the self-consistent elasto-plastic model. The stresses in cold rolled ferritic steel were determined both in as-received and under tensile loaded conditions. It has been shown that the Reuss model and the self-consistent model applied to near surface volume provide the best approaches to determine diffraction elastic constants. For the first time, the elastic energy in an anisotropic material (arising from plastic incompatibilities between grains having various lattice orientations) has been determined. The second-order incompatibility stresses and stored elastic energy are presented in Euler space.

Residual stresses are an important parameter in the process optimization of laser cladding. These stresses are caused by high thermal gradients and different thermo-mechanical properties of the substrate and clad materials. An experimental setup was used to determine one dimensional average residual stresses based on deflection measurements. Overlapping clad tracks were produced on a bar. The stresses were calculated from the difference in the bar deflection after cladding and after removal of the clad layer. Experiments were performed using Stellite 12 and Inconel 625 as clad material. A carbon steel (S235JR) was used as substrate material. The powder feed rate was linearly increased with the cladding speed in all experiments, and the laser powder was adjusted in real-time by controlling the meltpool width. Residual stresses were found to increase with increasing cladding speed. The stresses observed in the Stellite 12 layers are larger than the stresses found in the Inconel 625 layers. The residual stresses on the clad layer surface were determined using X-Ray Diffraction (XRD) technique. The results from both techniques are compared.

A reliable maintenance and service of energy generation plants would be impossible without the professional designing, manufacturing and monitoring of welded joints. The lifetime assessment factors of welded components as implemented in the design codes must be updated accounting for the modern materials and the advanced steam parameters used in the piping construction [Weld Strength for high temperature components design and operation (WELDON). European Project No. GRD2-2000-30363, Gampe U, Seliger P, Creep crack growth testing of P91 and P22 pipe bends. Int J Pressure Vessels Piping 2001;78:859-64, INTEGRITY of repair welds in high temperature plants operating under steady and cyclic loading conditions. European Project No. G5RD-CT-1999-00118].Within the EU 5th Framework RTD project 'WELDON' tests at high temperatures are performed on component-like feature test specimens like welded pipes and large tensiles in addition to laboratory specimens to study the geometry and size effects on the damage evaluation methodology. The circumferential welds of the pipes are subjected simultaneously to internal pressure and an axial load. The large tensile specimens manufactured from the welded pipes are subjected to uniaxial loading. These components are made from steel grade P22 and P91 and are equipped with gages for on-line monitoring of temperatures, deformations and strains.Residual stresses are measured on these components in as welded and after post-weld heat treatment (PWHT) conditions and are monitored during creep testing with interruptions and after failure. X-ray and hole drilling techniques are employed in these measurements. These data will be used to validate the FE modelling of residual stresses and damage assessment.The results obtained from long time creep tests, metallographic investigations of damage development in the different zones of the weldment and residual stress measurements are presented in this paper. (c) 2006 Elsevier Ltd. All rights reserved.

For the production of steel to aluminium transition joints a laser joining process with an Nd:YAG laser source and a cold wire feeder has been developed. For a good joint strength it is critical to minimize the intermetallic phase buildup during the joining. With careful manipulation of the laser power and the spot position the steel is kept in a solid state and the aluminium is molten, thus ensuring an intermetallic layer thickness of less then 5 µm. To further optimize the joint strength the wetting of the steel by the aluminium has been maximized to increase the load bearing surface. Optimal wetting has been achieved with a 12% aluminium silicon filler wire and zinc coated steel, where the zinc significantly improves the wetting. Because of this the use of flux was not needed. The research has concentrated on the joining of sheet materials in a T configuration. Breaking strengths of up to 150 MPa have been realized for a material thickness of 3 mm and 140 MPa for 5 mm.For the production of steel to aluminium transition joints a laser joining process with an Nd:YAG laser source and a cold wire feeder has been developed. For a good joint strength it is critical to minimize the intermetallic phase buildup during the joining. With careful manipulation of the laser power and the spot position the steel is kept in a solid state and the aluminium is molten, thus ensuring an intermetallic layer thickness of less then 5 µm. To further optimize the joint strength the wetting of the steel by the aluminium has been maximized to increase the load bearing surface. Optimal wetting has been achieved with a 12% aluminium silicon filler wire and zinc coated steel, where the zinc significantly improves the wetting. Because of this the use of flux was not needed. The research has concentrated on the joining of sheet materials in a T configuration. Breaking strengths of up to 150 MPa have been realized for a material thickness of 3 mm and 140 MPa for 5 mm.

In this paper, the effects of residual stresses are considered in crack growth of cold bent and longitudinally cracked carbon–manganese tubes and pipes tested under pressure at 360°C, using the fracture mechanics parameters reference stress, σref, K and C∗. Residual stress measurements using the X-Ray diffraction technique and a successive layer removal method have been performed through the thickness of the pipe extrados. These data have been used in finite element analyses to model effects of the secondary stresses acting on the crack tip. Creep crack growth rates versus C∗ at 360°C, in cold bent tubes, were shown to be faster by a factor of ∼50 at constant C∗ compared to cracking in fracture mechanics CT test specimens. This difference is due to geometry as well as the method of C∗ analysis. In cold-bend tubes after 1000h thermal soaking at 650°C, there was an increase in incubation time and steady state cracking rate reduced by a factor of 10. Furthermore, it was found that residual stress measurements performed after a thermal treatment at 360°C for 50h indicated a reduction of 40–50%. However, the calculations of C∗ using a combination of primary plus secondary stresses showed only an increase by a factor of ∼2 which did not fully explain the difference in cracking rate in the tube specimens.

In life assessment the question of what is the effect of a pre-existing stress field on the behaviour of a crack at high temperatures needs to be addressed. This paper proposes a laboratory test method for inducing a superimposed residual stress in a C-ring specimen by plastic loading at room temperature. X-ray diffraction and neutron diffraction techniques are then compared in measuring the induced residual stresses across the crack plane of the C-ring specimen on three steels (P22, P91 and 316LN) taken from the 'HIDA' project [1]. The lack of standards and practices for both neutron and X-ray diffraction techniques makes any set of independent measurements difficult to validate. Therefore both analytical and numerical FE methods were used to apply, calculate and validate the residual stress profiles. Good agreement has been found between X-ray and neutron diffraction methods and the predicted FE results for all the three materials.