Keng H. Leong

High power Nd:YAG and CO2 laser welding of magnesium

The laser beam weldability of AZ31B magnesium alloy was examined with high power pulsed Nd:YAG and continuous wave (cw) CO2 lasers. The low solid absorptivity, liquid viscosity, and liquid surface tension of magnesium make it more difficult to weld than steel. Welding parameters necessary to obtain sound autogenous welds were determined for both pulsed Nd:YAG and cw CO2 lasers. The weldability of magnesium was significantly better with the Nd:YAG laser. This observation was attributed to the higher absorption of the Nd:YAG beam, which in turn reduced the threshold irradiance required for welding and produced a more stable weldpool. The signal from an infrared weld monitor was correlated with both the penetration depth and the quantity of black powder on the workpiece after welding. The fine black powder was shown to result from evaporation of magnesium and zinc. The Nd:YAG welds were only slightly softer than the base material and had very narrow heat-affected zones.

Inversion of light-scattering measurements for particle size and optical constants: experimental study.

Measurements of the light scattered by a sample contain information regarding the physical properties of the sample. Laser light-scattering measurements can be made unobtrusively in environments that are inaccessible to other types of measurements, so laser light-scattering techniques have become an important tool in aerosol research. The primary difficulty associated with using light-scattering techniques to determine aerosol particle properties is inverting the measurements or extracting the desired information from the measurements. In this study we use a 15-channel polar nephelometer to measure the light-scattering patterns of monodisperse polystyrene spheres. The light-scattering measurements are inverted, and the particle size distribution function and optical properties of the particles are retrieved. The results of these inversions are presented after a brief description of the calibration and operation of the nephelometer.

Laser welding of aluminum alloys

Recent interest in reducing the weight of automobiles to increase fuel milage has focused attention on the use of aluminum and associated joining technologies. Laser beam welding is one of the more promising methods for high speed welding of aluminum. Consequently, substantial effort has been expended in attempting to develop a robust laser beam welding process. Early results have not been very consistent in the process requirements but more definitive data has been produced recently. This paper reviews the process parameters needed to obtain consistent laser welds on 5000 series aluminum alloys and discusses the research necessary to make laser processing of aluminum a reality for automotive applications.

Real-time monitoring of laser beam welding using infrared weld emissions

A nonintrusive, solid-state device has been developed to monitor in real time the infrared emissions during laser welding. The weld monitor output is an analog signal (100–1000 mV) that depends on the beam power and weld characteristics. The dc level of this signal is related to weld penetration, while ac portions of the output can be correlated with surface irregularities and part misalignment or contamination. Changes in dc behavior are also noted for both full and deep penetration welds. Full penetration welds are signified by an abrupt reduction in the weld monitor output. Bead-on-plate welds were made on steel, aluminum, and magnesium with both a cw CO2 laser and a pulsed Nd:YAG laser to explore the relationships between the weld characteristics and the weld monitor output.

Selecting a high-power fiber-optic laser beam delivery system

Multimode optical fibers enable efficient flexible laser beam delivery but at a loss in the quality of the delivered beam. The fiber-optic beam delivery system effectiveness is strengthened by the optimal selection of its components not only for minimizing beam quality degradation but also for robustness. Smaller fibers tend to produce less degradation to beam quality but the minimum usable fiber size is limited by the quality of the laser beam, focusing optic, and the numerical aperture of the fiber. Selection of appropriate fiber type is important because the characteristics of the output beam enhance or degrade the utility of the fiber-optic-delivered beam for different applications. The other components of the beam delivery system also impact performance. High-power handling requires high- quality fiber end-face finish and special connectors that can withstand back-reflections. Recent developments in optical materials allow manufacture of very low aberration optics useful for short focal length lenses in output optics. Harnessing the power of a high- power laser requires that knowledgeable and prudent choices be made when selecting the laser and its beam delivery system. We will focus on issues relevant to understanding and specifying a fiber-optic beam delivery system and provide guidelines for specifying a system. Data obtained with high power Nd:YAG lasers will be used as examples.

Laser surface alloying of silicon into aluminum casting alloys.

Aluminum alloys that are easily castable tend to have lower silicon content, and hence, lower wear resistance. The use of laser surface alloying to improve the surface wear resistance of 319 and 320 aluminum alloys was examined. A silicon layer was painted onto the surface to be treated. A high power pulsed Nd:yttrium–aluminum–garnet laser with fiber-optic beam delivery was used to carry out the laser surface treatment to enhance the silicon content and produce a very fine silicon-rich phase. One advantage of using a pulsed laser beam to carry out the surface alloying is it provides a vigorous turbulence in the molten pool and enhances the dissolution of the fine silicon into the molten bath and its dispersal in a short process time. Process parameters were varied to obtain smooth single treated tracks and minimize the surface roughness from overlap of the treated tracks. Our goal is to take advantage of the vigorous turbulence characteristic of the pulsed beam to obtain desired microstructure of laser-al...

Designing a fiber optic beam delivery system

One of the advantages offered by visible and NIR lasers over carbon-dioxide lasers is that they can be delivered through optical fibers. Fiber-optic beam delivery is ideal when the beam must be delivered along a complex path or processing requires complicated manipulation of the beam delivery optics. Harnessing the power of a high-power laser requires that knowledgeable and prudent choices be made when selecting the laser and its beam delivery system. The purpose of this paper is to discuss a variety of issues important when designing a beam delivery system; data obtained with high power Nd:YAG lasers will be used as illustrative examples. (1) Multimode optical fibers are used for high-power applications. The fiber imposes, to varying degrees, a structure on the beam that is different from the laser output. Fibers degrade the beam quality, although the degree of degradation is dependent on the fiber length, diameter and type. Smaller fibers tend to produce less degradation to beam quality, but the minimum usable fiber size is limited by the quality of the laser beam, focusing optic and the numerical aperture of the fiber. (2) The performance of the beam delivery system is ultimately determined by the quality of the optics. There for, well- corrected optics are required to realize the best possible performance. Tests with both homogeneous and GRADIUMTM lenses provide insights into evaluating the benefits offered by improvements in the output optics from gradient-index, aspheric and multi-element lens systems. Additionally, these tests illustrate the origins of variable focused spot size and position with increasing laser power. (3) The physical hardware used in the beam delivery system should have several characteristics which enhance its functionality and ease of use, in addition to facilitating the use of advanced diagnostics and monitoring techniques.

Threshold laser beam irradiances for melting and welding

A model based on the conservation of energy for a moving heat source incident on a flat plate is used to predict the threshold laser beam irradiance required to initiate melting on a metal surface. With the use of a non‐dimensional variable and its solution, the threshold irradiance can be predicted with the use of a simple equation. This equation is a function of the absorptivity of the surface, thermal conductivity of the metal, temperature increase for melting, diameter of the laser beam at the surface and the non‐dimensional variable value for the ratio of the thermal diffusivity of the metal to the product of the beam diameter and velocity used. Laser beam irradiances exceeding these predicted values are required for welding. The model predictions are validated with experimental results of beam irradiances required for welding different metals.

Understanding high‐power fiber‐optic laser beam delivery

Fiber‐optic beam delivery is commonly used in industrial laser systems. This article examines the conditions for the optimal propagation of high power beams through optical fibers. Beam quality effects by step and gradient index fibers of different lengths are considered. The differences between the diverging beam from a fiber and the beam at focus and on the fiber face are illustrated. Estimates are provided of the worst‐case beam quality to be expected from fibers. Guidelines are also provided for the selection of beam delivery components based on the limitations of the optical system and the tasks to be performed.

Design and Test of a Polar Nephelometer

This paper describes the design, calibration, and testing of a prototype polar nephelometer that uses a diode laser as the light source. The instrument continuously extracts a stream of aerosol into a sensing head. The scattering of an ensembles of aerosol particles illuminated by a laser beam is measured simultaneously at 15 polar angles from 23° to 128°. Absolute calibration of the nephelometer is obtained using Freon-12 as the scattering medium and the geometry of the sensing head. The prototype has been used successfully to retrieve particle size and refractive index from a monodisperse nonabsorbing aerosol and determine the angular scattering response of atmospheric aerosols with scattering coefficients to 10−6 m−1. Problems and limitations encountered with the prototype in long term unattended operation are discussed.