The ability to monitor and control the depth of a laser weld in real-time is critical in many laser welding applications. Consequently, we have investigated the use of an optical method to sense weld depth. Welds were generated on kovar samples, using a pulsed Nd:YAG laser. The sensing method uses digital high-speed photography to measure the velocity of the plume of vaporized metal atoms ejected from the metal surface. An energy balance equation is then used to relate the plume velocity to the size of the weld. Numerical solution of the energy balance equation yielded values for weld depth that were within 8% of the actual measured values.
In order to investigate the progression of tensile strength at the joints of laser beam welded sheet metal, the results of an optical measurement procedure applied during a tension test are being utilized. With the aid of a video camera and an interactive interpretation of the diagrams, the tensile strength of the samples equipped with a measuring grid may be observed during the tension test. This measuring device allows for the investigation of its viscoelasticity and eliminates the local tensile strength in the area of the joint including the heat affected zone. It is our intention to identify the properties of the sheet metal joints, which may serve as an aid for optimizing future production processes and characteristic features.
Laser welding has become a standard manufacturing technique, particularly in industries where weld quality and performance are critical, such as the aerospace, nuclear, medical devices and automotive sectors. In many laser welding applications, flaws are not acceptable in the final product, so every weld must be inspected. Post-process inspection is time-consuming and, if a systematic problem develops, many flawed parts could be produced before the problem is identified and corrected. The preferred approach is therefore to perform in-process inspection as the weld is produced. This paper describes a weld process inspection system based on a compact, computer controlled optical spectrometer, which observes the laser welding plume in real time. From the plume spectrum, one is able to determine the temperature of the weld site and the elements present in the fusion zone. A sudden change in weld temperature may indicate a weld flaw, either from a loss of laser energy coupling and therefore a loss of fusion, or from excess energy input and burn-through. An indication of the elements present in the fusion zone can be used for seam tracking or penetration monitoring when dissimilar materials are being joined in the butt or lap configurations, respectively.
While coating automotive body steels with zinc has a beneficial effect on their corrosion properties it can give significant problems when individual panels are subsequently welded to form the body shell. This work shows that a pulsed Nd:YAG laser can be used as a precleaning tool to locally remove the zinc and then as a spot welding device to join 2 sheets in a lap weld configuration. Laser spot welding can produce joints having adequate strength at a range of conditions but the localized nature of the heat input can produce damage on the top surface, this is often not acceptable in structures such as an automotive body shell. In this work, conditions have been identified which are capable of precleaning the area to be welded and able to produce welds having adequate strength with cosmetically acceptable surfaces.
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.
The year 1997 will become known as the 'year of commercial excimer lasers.' The number of installations on manufacturing and medical floors will be much higher than ever before. Besides the medical uses the most important applications are DUV lithography, TFT annealing for flat panel displays and microdrilling for nozzle arrays, especially for ink jet printer heads. Recently two very important breakthroughs were achieved. The first is in regard to the performance of the excimer laser itself. The second is a much more efficient way of using the UV photons. These are the main reasons for the actual success. Technology development by laser manufacturers has resulted in remarkable improvements of component lifetime, reliability, cost of ownership and ease of use. With gas and optics lifetimes in excess of 108 pulses, laser tube exchange intervals longer than 5 multiplied by 109 pulses and integration of internal halogen generators a quasi sealed-off excimer laser with hands-free operation is accomplished. The new generation NovaLineTM combines the experience of more than 4000 installed excimer lasers with a completely new laser engineering design. Progress in advanced UV optics will be demonstrated with two examples. In commercial production of very precise nozzle arrays, high-end optics allow the drilling of all nozzles simultaneously with an optical distortion of less than 0.5 micrometer over a full 0 18 mm processing field. For scanning applications as AMLCD annealing, the rectangular laser beam can be optically transformed into a very homogeneous line, up to 300 mm long and 0.5 mm wide.
Recent improvements in 'surface engineering' have helped to increase one-sun silicon solar cell efficiencies to more than 24% for float-zone grown single-crystal silicon. Texturing of the cell surface, to enhance the light coupling into cell, constitutes a significant part of this dramatic progress. Most single-crystal silicon substrates with a (100) surface orientation can be textured with relative ease using a selective or anisotropic chemical etching method. Other silicon materials, like ribbon-grown, (111) dendritic web and polycrystalline substrates do not lend themselves to chemical material removal without elaborate micro- lithographic masking method. This paper investigates the feasibility of using excimer micromachining as an alternative method of texturing silicon solar cells in general. Experiments are conducted with (111) float-zone and dendritic web-grown substrates. Using a 'diamond' patterned mask and a Kr2 excimer laser, contiguous arrays of V- shaped micro-grooves are formed on each substrate. The resulting surface texture is examined by surface profilometry and the results are correlated to the original surface micro characteristics of the samples. Sample carrier lifetimes and solar reflectances are measured prior to- and after the laser processing. The results verify the technical feasibility of excimer micro machining of (111) float zone and dendritic web single crystal substrates.
Over a period of many years a wealth of information has been accumulated on processing parameters for laser micromachining of different materials. This information includes threshold fluences, etch rates versus fluence curves, taper effects, debris minimization and assist gas effects to name a few. It is the intent of the authors to compile this empirical information into an easily accessible catalog to be used as a reference guide for those interested in laser micromachining. Information will be included on material type, lasers used in machining (carbon-dioxide, solid state, excimer), laser processing parameters (wavelength, fluence, pulse width, cutting speed) and other material related information (thickness, feature sizes, etc.).
MCC has developed a rapid prototyping technology for manufacturing high density interconnections for multichip modules (MCMs). By using laser direct writing to customize a high density pre-manufactured programmable substrate, the time required to make an MCM is reduced from several months to weeks. No photomasks are required, so the non-recurring engineering costs that are typical of full custom designs are significantly reduced or eliminated. The substrates are manufactured by commercial manufacturers using their standard fabrication processes, so the technology offers high performance, reliability, and robustness. The substrates are laser customized with subtractive and additive metallization processes. The subtractive processes are used to divide wiring segments. The additive processes are used to link the segments into the design-specific nets. A rerouting process is used to enable flip chip connections to area array and perimeter pad ICs so that prototype flip chip MCM work can be done immediately with pre-existing IC perimeter pad layouts. The rerouting process uses photoablation of vias in polyimide and laser lithography to define an interconnect metal between substrate wiring and the flip chip landpads. The laser prototyping process was used to make a signal integrity multichip module that has 1296 flip chip connections and 192 I/Os and designed to operate at over 400 Mb/s between ICs.
This paper is devoted to the arrangement of trajectory of the laser beam by scanner and plotter systems. The problem appears only when the trajectory changes from one direction to the other. The motion along direction 1 to point A and along direction 2 after point B can be provided by the same speed V1 equals V2 equals V*. But between the points A and B it is necessary to make some manipulations to provide the same quality of the irradiated zone -- the width d, the accuracy (Delta) (maximum deviation from the desired trajectory), momentary values of energy intensity q, W/cm2, etc. The general way to hold the constant value of q, which defines the temperature of irradiated zone for laminated objects manufacturing (LOM) and selective laser sintering (SLS) and photons density for stereolithography (SL), is to control speed of motion of scanners and plotters. We discuss an approach for optimization of control algorithms for both types of delivery systems.
The laser engineered net shaping (LENSTM) process, currently under development, has demonstrated the capability to produce near-net shape, fully dense metallic parts with reasonably complex geometrical features directly from a CAD solid model. Results to date show that excellent mechanical properties can be achieved in alloys such as 316 stainless steel and Inconel 625. In fact, due to the highly localized nature of the laser heating, a fine grain structure will occur resulting in a significant increase in yield strength at no expense of ductility. The current approach lends itself to produce components with a dimensional accuracy of plus or minus .002 inches in the deposition plane and plus or minus .0.015 inches in the growth direction. These results suggest that this process will provide a viable mens for direct fabrication of metallic hardware directly from the CAD solid model.
New approach to laser engraving of half tone images has been proposed and tested. Combining two basic approaches to laser engraving -- single pulse mask imaging and raster element construction by pack of laser pulses -- the new system constructs every individual raster element by imaging on the workpiece surface a dynamic mask of controlled size. The dynamic mask shape corresponds to the required raster element shape. This approach offers several important advantages over the conventional ones: (1) analog control of the mask shape provides gray level continuum, thus ensuring the image quality, unattainable by other means; (2) raster element marking by single laser pulse provides very good marking rate. It takes only one scan of the writing laser head to mark raster line. Much more powerful laser pulses can be used to engrave complete raster element by single pulse instead of its point-by-point construction by consecutive laser pulses; (3) the influence of laser beam quality parameters, such as beam divergence, and power instabilities on the gray level has been greatly reduced because raster element shape primarily depends on the mask shape and not on the power level and beam divergence. Dynamic mask system can be used both with cw and pulsed laser. Gray scale tones can be reproduced by the linear raster line width in the first case. Advantages of the new device have been demonstrated by engravings on stone, wood, etc. made with 50 W carbon-dioxide laser.
A comparative study of material processing with Q-switched Nd:YAG lasers at the fundamental (1064 nm) and the second harmonic wavelength (532 nm) is presented. Two different laser-systems are used: (1) A flash-lamp pumped, phase- conjugated Nd:YAG oscillator-amplifier-system with average output powers of up to 45 W at both wavelength at a repetition rate of 100 Hz; and (2) A cw-pumped, periodically Q-switched dual-rod Nd:YAG laser with an average power of 50 W at 1064 nm and up to 18 W at 532 nm. The lasers are used for cutting of copper (Cu) with a thickness between 0.1 and 0.5 mm and percussion drilling of copper and alumina (Al) with a thickness of up to 1 mm. The material-processing efficiency is compared for nearly identical laser-parameters at both wavelengths. With the flash-lamp-pumped system cutting of Cu is possible with typically 30% higher speed at 532 nm compared to 1064 nm for average powers up to 15 W. At higher average powers the cutting speed starts to saturate at nearly the same level for both wavelength caused by plasma effects on the surface. With the cw-pumped system percussion drilling in Cu and Al is compared at different wavelengths. While drilling of copper is significantly more efficient for thicknesses above 0.5 mm using the green laser beam, the efficiency for drilling of alumina was slightly higher in the infrared.
A frequency doubled Nd:YLF laser is investigated as a potential source for photoablation of polyimide films. The hole diameter as a function of incident laser power is studied in a 25 micron thick free standing polyimide film. The photoetched vias are inspected by optical microscopy and show a relatively clean surface surrounding the entrance diameter. The microscopy studies also reveal that the vias exhibit a shape which has three characteristic diameters. The entrance and exit diameters are of similar dimension and define a melt zone region. A uniform circular waist diameter, which is smaller than entrance and exit diameters, can only be obtained with 5 to 10 laser pulses and pulse energies above 4 mJ.
As part of a larger program investigating the machining of advanced composite materials using both traditional and laser techniques, a series of three polymeric matrix composites (PMCs) have been subjected to lasing trials using a pulsed Nd-YAG laser. This paper presents an overview of this work and comments on the application of YAG laser processing to PMC materials in general based on the examination of three specific composite systems.
An inherent characteristic of laser machined features is taper. For a particular laser the degree of taper is largely dependent on two things: the type of material being machined and the laser energy per unit area or fluence at the processing surface. The latter of these is dependent on different parameters for the three lasers mentioned above. In this paper we review the particular laser parameters that control taper and what degree of taper can be expected for certain materials. More often than not taper is a nondesirable characteristic and is, in many cases, the limiting factor regarding the maximum thickness of material that a laser is practically capable of drilling or cutting through. We therefore discuss methods of minimizing taper and its effects. Under some circumstances taper can be used to one's advantage to achieve a desired feature size if one has control over the taper angle.
The paper presents the results of the study of silicon and copper surface destruction under pulsed and pulse-periodic action of YAG:Nd laser radiation. The anomalies have been discovered in the probe beam of He-Ne laser scattered from the silicon surface under its irradiation with a series of YAG:Nd laser pulses at I less than Imelt. (where Imelt. is the surface melting threshold). These anomalies are supposedly related to formation of surface layer saturated with defects.
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.
We report the first demonstration of high resolution mm dynamic range measurements using sinusoidal phase modulation interferometry. Our novel method has increased the range by over 3 orders of magnitude, while maintaining ultra-high accuracy. Simple commercial laser diodes were used and a new algorithm allowed for the first ultra-high accuracy and high dynamic range in real time for a slightly vibrating environment.
A new type of two-wavelength interferometer for the measurement of absolute distance is described. Since the wavelength of the laser diode is temporally multiplexed, only one laser diode light-source is required, thus eliminating the necessity of aligning two independent optical axes. Absolute distance is calculated from the difference of the phases in the interference signals generated by each wavelength. The self-correlation function is applicable in our system for detecting this phase- difference between two kinds of interference signals, thereby, the signal processing is very simple and there is a possibility that the real-time distance measurement becomes possible in our new method.
Carbon dimer formation during ablation plume expansion and subsequent interaction with a substrate is recognized to affect substantially the process of pulsed laser deposition (PLD) of thin films. In this work we report formation of C2 molecules during laser ablation from graphite targets under conditions typical for PLD. Two types of lasers were used for ablation -- 5 microsecond carbon-dioxide laser operated at 1 J and 10 ns Nd-YAG laser operated at 100 mJ in the second harmonic. We present results of time-resolved (50 ns) emission spectroscopy, plasma imaging and time-of-flight techniques. C2 molecules formation has been studied in a variety of experimental conditions, including ablation plume interaction with target surface, plume interaction with the surface of a substrate and two-plasmas interaction. Particularly no molecules formation was observed at maximum energy and there was an optimum laser pulse energy for dimers formation for short pulses of Nd:YAG laser. The plumes in both cases had shell like structure with high ions inside and neutrals located in the outer layers. Molecules formation occurred to be more efficient in the outer layers of the plume in the case of carbon-dioxide laser.
Measurement is an integral part of optics manufacture, where grinding and polishing steps are linked iteratively with the testing steps. While numerous test methods have been developed, many of these are prone to vibration effects that limit their application to in-situ monitoring, or have other limitations. We have applied Shack-Hartmann wavefront sensors to the problems of optics measurement. We have developed an instrument that allows testing in common configurations, and also provides new ways to test optics. The current device is extremely sensitive. We have demonstrated this device for testing various optical elements including lenses, mirrors and laser rods.
The use of lasers in industrial and medical procedures continues to increase. A fundamental question in many laser- material interactions is how is the incident laser power transferred to the target material, and how is the power distributed among the phases (solid, liquid, vapor) of the material. This paper describes the results of a fundamental calorimetry experiment to determine the fraction of incident carbon-dioxide laser energy which is used to vaporize water from a target volume, and the fraction of power used to simply heat the remaining liquid. The experiment was performed over a range of incident laser powers from 60 to 300 W. Over most of the range of incident power, the fraction used to vaporize water is 30 to 35 percent. This fraction increases at the lowest powers.
Studies of laser pulsed deposition of thin films show that introduction of an obstacle like a grid screen into ablation plume substantially affects dynamics of plasma expansion and results in dramatic changes of plasma parameters. Two regimes of plasma-grid interaction have been studied in detail -- regime of the free-molecular flow and the throttling. The grid screen may break plasma plume into several interacting with each other flows. Some features of plasma-grid interaction (like transformation of velocity distribution function and variation of plasma temperature and density) can be attributed to the interaction of plasma flow with the generated shock wave. Different dimensional effects controlling plasma-grid interaction have been considered. We report the effect of velocity distribution transformation on the stage of interaction between multiple plasma flows behind the screen. Two modes of such transformation have been observed: linear growth of density in the resulting flow, and formation of the additional slow peak in the velocity distribution. We associate these two modes of transformation with two different regimes of plasmas interaction -- with regime of lateral loss reduction, and regime of expansion in the background gas.
Frequency selected pulsed CO laser was used for surface heat treatment of polymeric materials. Nylon and poly(ethyleneterephthalate) samples having strong absorption bands near wavelength of approximately 6 micrometers were processed by the laser radiation of different temporal, spectral and energy density characteristics by using different geometry and methods of irradiation. Different types of microstructure were formed on the surfaces of the samples. Experimental conditions corresponding to each type of microstructure were defined. Visual (macro) changes of polymeric material properties (if any) and their correlation with formed microstructures were analyzed.
Initial results from studies of the effect of high power 1064 nm light from a Q-switched Nd:YAG laser on Scottish granites, commonly used in buildings and monuments, are described. Pink granites, were shown to undergo bleaching at energy densities close to those necessary for successful removal of black soiling. Element maps produced by energy dispersive x-ray analysis showed no change in stone composition before and after laser exposure. Further work is currently being carried out in order to obtain an understanding of the bleaching mechanism and damage thresholds, as it is necessary to determine damage thresholds for laser cleaning in order to protect vulnerable stone minerals.
The precision laser machining (PLM) consortium is developing advanced lasers for machining and investigating process improvements. Recent cutting experiments completed by Boeing at TRW on aluminum alloys included pulse repetition rates up to 220 kHz from a diode-pumped neodymium YAG laser. Overall, cutting speed increased with power over the range tested and results were of a quality and speed to offer promise for manufacturing at the average power levels tested.
Industrial application of a high power laser usually requires an instrumentation for directing and aiming of the laser beam. Optical-mechanical beam control and imaging system is described. This system allows the operator to arbitrarily change the point of impingement of the laser beam along the treated object at any time during the technological process. Herewith the system provided the opportunity of continuous, precise monitoring of the invisible (infrared) laser focal spot position on the image of the treated object. Both visual and TV options are presented. This system is especially useful for industrial application in the following circumstances and their combinations: the object cannot be moved arbitrarily against the laser beam; the object is at the substantial distance from the laser; the visible pilot laser beam cannot be used for aiming.