Volume 27, Issue 3, August 2015
Index of content:
Effect of process parameters on the formation of laser-induced nanoparticles during material processing with continuous solid-state lasers27(2015); http://dx.doi.org/10.2351/1.4916081View Description Hide Description
During laser material processing with high laser beam intensities, a laser-induced vapor formation can occur. Due to the shockwave behavior of the vapor plume and the associated rapid cooling, a significant particle formation can be initiated by nucleation. The laser radiation interacts with the particles which can result in a dynamic change of the intensity distribution on the surface. Especially in the field of laser remote processing, the attenuation of laser radiation by nanoparticles can influence the process stability and reduce the processing quality. The presented work is focused on the particle formation at a height of 10 mm above the material surface during the laser welding of stainless steel with a fiber laser. The laser beam intensity on the surface was varied between 1.3 and 5.1 MW/cm2. Transmission electron microscopy images of the nanoparticles and high speed images of the vapor propagation in the ambient atmosphere were analyzed. The attenuation of a probe beam in the vapor plume was evaluated in dependence on the wavelength. The results indicate a linear connection between the laser beam power and the particle formation rate.
Morphology based statistical analysis of nanosecond pulsed laser texturing of the multicrystalline silicon27(2015); http://dx.doi.org/10.2351/1.4917043View Description Hide Description
In this paper, we investigate the surface morphology of the textured multicrystalline silicon with nanosecond (ns) Nd: YVO4 laser (wavelength of 1064 nm, repetition rate of 10 KHz, and pulse duration of 14 ns). Various surface topographies have been achieved with different laser as well as irradiation parameters. The textured area average roughness and depth have been statistically analyzed through ANOVA test, which could evaluate the significance and effectiveness of the adopted design of experiment. This research work is based on three control factors: Laser fluence, laser pulse overlap percentage, and number of irradiations. The statistical assessments were conducted based on roughness and depth values measured by optical interferometry. The effect of roughness and depth on solar weighted reflectance (SWR) was analyzed and significant reduction in SWR with increase in Ra was observed. In addition, time and energy consumption, which are highly significant in the industrial applications, have been investigated.
27(2015); http://dx.doi.org/10.2351/1.4916532View Description Hide Description
Mass production of carbon fiber-reinforced plastic parts has lately started in the automotive industry. Due to no abrasive wear in combination with a high degree of automation and ability for 3D processing, laser remote cutting is a suitable method for machining purposes in this context. In the automotive environment, solid-state lasers are favored because optical waveguides may then be used. In turn, the low absorption of the radiation of such lasers in the matrix material presents a drawback in terms of comparatively large heat affected zones (HAZ) and flaking at the cutting kerf. This paper deals with the question, if a laser absorbing additive can be used to enhance the absorption within the matrix material, while the optical properties in the visible spectrum are kept. For this purpose, an additive known from laser transmission welding has been added to the matrix material. Cutting experiments have been carried out while varying concentration of the additive. The investigations show that a significant reduction of the mean HAZ of 25% and the standard deviation (1 σ) of 56% can be achieved by adding 4% w/w of the additive to the resin. In addition to that, the flaking behavior can be avoided. Compared to adding soot particles, the optical properties of the laminate do not change in the visible spectrum, leaving the fiber textile visible.
27(2015); http://dx.doi.org/10.2351/1.4916979View Description Hide Description
As semiconductor based devices are manufactured on ever thinner silicon substrates, the required associated die break strength has to increase commensurately to maintain pick yields. In this study, the influence of laser processing parameters on the die break strength in laser dicing of silicon oxide-coated silicon wafers and silicon-based memory devices is investigated experimentally using ultraviolet lasers spanning a wide range of pulse width, from 400 fs to 150 ns. It is found that the net fluence, an accumulated pulse energy per surface area, is a meaningful process metric for damage induced by heat-affect zone to compare lasers processes with a large variety of pulse widths, laser scan speed, average powers, and repetition rates. Optimized process conditions for both nanosecond and femtosecond pulse widths are identified for achieving the highest die break strength in the target devices. The dependence of heat-affected zone on pulse width and net fluence during nanosecond laser processing is further demonstrated using multiphysical simulations. Simulations suggest that the thickest heat-affected zone section during laser scribing is typically located at the boundary of the laser incident surface. Simulation results also show that for a given repetition rate the heat-affected zone becomes larger as the net fluence increases due to smaller interpulse separation, consistent with the experimental observation.
27(2015); http://dx.doi.org/10.2351/1.4917346View Description Hide Description
Laser treatment of steel surfaces in air using continuous-wave radiation emitted by a fiber laser at 1.07 μm is investigated using a spectroscopic method that monitors the presence of FeO molecular fluorescence. For all conditions tested, the irradiance levels remained below 106 W/cm2 thus inhibiting the formation of plasma. In this paper, we demonstrate that FeO emissions are related to laser-induced steel vaporization and can be used to monitor the performances of the laser system for drilling and cutting applications. The heated Fe atoms oxidize rapidly forming solid and liquid FeO at the interface with the oxygen-filled atmosphere. As the formation of FeO is exothermic and that the presence of the oxide further increases laser absorption, the laser-induced oxide is rapidly vaporized and ejected off the surface, leaving an empty hole. The presence of FeO molecules can be monitored via the characteristic fluorescence emitted from the well-known orange system which is excited by the treating of laser itself. Excellent quantitative agreement was found between the FeO signal strength and the volume of material ablated by the laser beam in a drilling configuration allowing real-time monitoring of the interaction for process optimization.
27(2015); http://dx.doi.org/10.2351/1.4918975View Description Hide Description
This work presents an experimental-numerical methodology aimed at deriving the absorptance of both a low output power CO2 laser beam and a Yb fiber laser beam applied to an AISI 304 stainless steel cold-rolled sheet under two different conditions of the sample surface: without and with a spray graphite coating. The absorptance values were obtained by minimizing the error between temperature evolution measurements at various locations of the irradiated sheet and the corresponding finite element predictions. The values obtained for the CO2 laser were a = 0.20 for the stainless steel and a = 0.41 for the graphite coating. For the Yb fiber laser, the values were a = 0.40 for the stainless steel and a = 0.64 for the graphite coating. Degradation of the graphite layer was observed when the resistance temperature of the graphite coating was exceeded.