Gerald Albert Knorovsky

Effect of surface tension on melt pool dynamics during laser pulse interaction

A previously developed model for simulation of recoil-pressure induced melt displacement during laser pulse interaction has been upgraded to include the restraining effect of surface tension. The results of numerical simulations of melt displacement/ejection during laser welding and drilling using this enhanced model are presented. In particular, the dependences of the threshold pulse energies for melt displacement and melt ejection as functions of laser pulse duration, beam radius and beam intensity distribution are computed and analysed.

On the possibility of microwelding with laser beams

We define microwelds as having fusion zone dimensions of <100 μm. At such small dimensions the required laser irradiance to produce melting is at or above conventional estimates of the irradiance required to produce drilling. The question thus arises if such small microwelds can be made via laser processes. A theoretical criterion defining the threshold pulse energy and beam intensity required for melt displacement (necessary for penetration-mode welding or drilling) is proposed. The results of numerical simulation present dependences of the threshold pulse energy and beam intensity as functions of laser pulse duration and beam radius. Experimental verification of the proposed criterion is described and a comparison of theoretical predictions and measurements is presented.

Microjoining with a scanning electron microscope

Abstract In the present work the authors describe the adaptation of a standard SEM into a flexible microjoining tool. The system incorporates exceptional control of energy input and its location, environmental cleanliness, part manipulation and especially, part imaging. Beam energetics, modelling of thermal flow in a simple geometry, significant effects of surface energy on molten pools and beam size characterisation are treated. Examples of small to micro fusion welds and molten zones produced in a variety of materials (Ni, tool steel, Tophet C, Si) and sizes are given. Future directions are also suggested.

Surface alloy depletion and martensite formation during glass to metal joining of austenitic stainless steels

Abstract Preoxidised and glass to metal (GtM) sealed austenitic stainless steels displayed a ferritic (bcc) layer near the metal/oxide interface, as determined by electron backscatter diffraction and X-ray diffraction. Through electron probe microanalysis, it was determined that this layer was depleted of alloying elements due to the oxidation and sealing processes. Characterisation of the layer morphology suggested that it formed through the martensite transformation mechanism. Thermochemical modelling with ThermoCalc also supported a martensitic transformation as opposed to diffusional ferrite formation. The composition gradient through the layer was correlated to the Eichelman and Hull empirical relationship for martensite start (Ms) temperatures. Because of Cr, Mn and Si depletion during preoxidation and glass sealing, Ms temperatures near ambient are possible in this surface region. The martensite layer was non-uniform, however, with laths extending deeper into the alloy due to stabilised growth in the material above its Ms temperature. This behaviour was characterised by image analysis techniques and discussed in terms of martensite stability and microstructural effects. Possible negative aspects of bcc phase formation on GtM seal properties are discussed, and analyses of alternative alloys 21-6-9 (tradename Nitronic 40; Armco Holding Corp., West Chester, OH, USA) and 22-13-5 (Nitronic 50) showed reduction or elimination of martensite after GtM joining.

Selection of parameters for μE-beam welding

Abstract Electron beam welding is a well known process used where high precision, high reliability welds are needed, often in exotic materials. Recently, it has been proposed to apply the electron beam produced in a standard scanning electron microscope (SEM), with reversible modifications to increase beam current, for microscale welding. In addition to providing the clean environment associated with the column vacuum, the SEM in imaging mode provides exceptional capabilities in visualising extremely small parts. Furthermore, the standard stage and beam motion controls offer the possibility of flexible programming of beam path with relatively minor software additions. In order to better evaluate the requirements for and effects of μE-beam welding (μEBW) on typical microtechnologically important materials, a clear understanding of the characteristics of the SEMs beam and its interaction with possible target materials is needed. The penetration ability of electrons depends strongly upon their accelerating voltage and the target they are being directed at. Hence, in some circumstances the beam may interact as a surface heat source, while in others it may act as a volume heat source, with important consequences on weld schedule development for the parts and geometry being welded. In this work, the authors explore some of the factors involved and propose simple models for the electron beam heat source which depend on the parameters being used.

CO2 laser welding fused silica.

The feasibility of laser welding of fused silica (aka quartz) has been demonstrated recently by others[1,2]. An application requiring hermetic sealing of a thin, pressure-bearing quartz diaphragm to a thicker frame led us to explore this technique. We found that laser welding techniques normally used for metallic parts caused scorching and uneven melting. Contrary to “standard” practices (near-focus, high travel speed, high power density), successful welds in fused silica required a broad heat source applied over a large area under a slow rotation to gradually heat the glass through the annealing, softening and finally working temperatures. Furthermore, good mechanical contact between the parts to be joined played an even more important role in this process than in typical metallic joints.Temperature profiles are measured and discussed as related to laser power, rpm’s, number of passes, joint geometry and beam orientation; and compared to an FEM thermal model.We will discuss the effects of laser power, travel speed, number of passes, joint geometry and part thicknesses on achieving hermeticity and cosmetically-acceptable joints. Finally, we will examine the differences between standard and antithetical laser welding techniques used.The feasibility of laser welding of fused silica (aka quartz) has been demonstrated recently by others[1,2]. An application requiring hermetic sealing of a thin, pressure-bearing quartz diaphragm to a thicker frame led us to explore this technique. We found that laser welding techniques normally used for metallic parts caused scorching and uneven melting. Contrary to “standard” practices (near-focus, high travel speed, high power density), successful welds in fused silica required a broad heat source applied over a large area under a slow rotation to gradually heat the glass through the annealing, softening and finally working temperatures. Furthermore, good mechanical contact between the parts to be joined played an even more important role in this process than in typical metallic joints.Temperature profiles are measured and discussed as related to laser power, rpm’s, number of passes, joint geometry and beam orientation; and compared to an FEM thermal model.We will discuss the effects of laser power, tr...

Unified characterization of surfaces and gases in MEMS devices

Chemical and physical materials-aging processes can significantly degrade the long-term performance reliability of dormant microsystems. This degradation results from materials interactions with the evolving microenvironment by changing both bulk and interfacial properties (e.g., mechanical and fatigue strength, interfacial friction and stiction, electrical resistance). Eventually, device function is clearly threatened and as such, these aging processes are considered to have the potential for high (negative) consequences. Sandia National Laboratories is developing analytical characterization methodologies for identifying the chemical constituents of packaged microsystem environments, and test structures for proving these analytical techniques. To accomplish this, we are developing a MEMS test device containing structures expected to exhibit dormancy/analytical challenges, extending the range of detection for a series of analytical techniques, merging data from these separate techniques for greater information return, and developing methods for characterizing the internal atmosphere/gases. Surface analyses and data extraction have been demonstrated on surfaces of various geometries with different SAMS coatings, and gas analyses on devices with internal free volumes of 3 microliters have also been demonstrated.

Recoil force measurements during pulsed ND:YAG laser spot welds (1008).

Recently, the evaporative recoil pressure effect induced by high intensity laser irradiation on molten zone motion in welds has become increasingly appreciated. Theory indicates that so-called conduction mode welds are in fact rarely encountered. Given that shapes and sizes of fusion zones are so dependent upon recoil force, the ability to model fusion zone behavior requires correct implementation of the physics involved, particularly as size scales decrease and surface energy effects increase in relative magnitude. Our presentation discusses validation experiments supporting such model development. Two techniques are discussed, a calibration method using sensitive piezoelectric force gauges, and a more general tool using a microphonic method. Each technique has advantages and disadvantages, which will be discussed. For example, while the piezo force gauge technique is readily understandable, it requires a very lightweight sample in order to avoid smearing of the force signal. However, when the sample size becomes very small, other phenomena begin to affect the gauge, giving apparently negative force measurements! The microphonic technique can be applied to actual welds, but needs careful consideration as well to eliminate comb-filtering, echoes and sample ringing. Measurements on 304L will be presented and discussed relative to contemporary theories.Recently, the evaporative recoil pressure effect induced by high intensity laser irradiation on molten zone motion in welds has become increasingly appreciated. Theory indicates that so-called conduction mode welds are in fact rarely encountered. Given that shapes and sizes of fusion zones are so dependent upon recoil force, the ability to model fusion zone behavior requires correct implementation of the physics involved, particularly as size scales decrease and surface energy effects increase in relative magnitude. Our presentation discusses validation experiments supporting such model development. Two techniques are discussed, a calibration method using sensitive piezoelectric force gauges, and a more general tool using a microphonic method. Each technique has advantages and disadvantages, which will be discussed. For example, while the piezo force gauge technique is readily understandable, it requires a very lightweight sample in order to avoid smearing of the force signal. However, when the sample siz...

Application of melt ejection criterion in simulation of micromachining with laser

The theoretical criterion defining the threshold pulse energy and beam intensity required for melt ejection is proposed. The results of numerical simulation present dependencies of the threshold pulse energy and beam intensity as functions of laser pulse duration and beam radius. The experimental verification of proposed criterion is described and the comparison of theoretical predictions and measurements is presented. The criterion is applied for simulation of laser drilling metal foil with thickness in the range 25 μm - 125 μm using laser beam with 12 μm beam radius and pulse durations 10 ns and 100 ns. The computational results are used to interpret the results of experimental study of laser drilling of 125 μm aluminum foil using a single mode beam of a XeCl laser performed at the Nederlands Centrum voor Laser Research (NCLR) and the University of Twente. Additional results on Nd:YAG spot welds in pure Ni are also presented.

Shear Testing of Laser Spotwelds.

A shear test was used to investigate the effect of shielding gas (Argon, Nitrogen and air) on the mechanical properties of laser spot welds in Fe-28Ni-17Co alloy (Kovar). The load vs. displacement curves obtained, while superficially resembling those of a standard tensile test, were quite non-reproducible, and obscured the differences due to process conditions. Fractographic examination of the samples and analysis of the testing conditions led to significant conclusions about how to correctly interpret the shear test results, which in turn enabled a determination of the real effects of the change in shielding gas. Several different types of fracture morphology were noted, depending upon how the fracture surface developed relative to the original weld. This resulted in the disparate nature of the load-displacement curves. The results of the shear testing, fractography and metallography will be used to support interpretation of the differences found with respect to porosity formation, strength and work hardening behavior.A shear test was used to investigate the effect of shielding gas (Argon, Nitrogen and air) on the mechanical properties of laser spot welds in Fe-28Ni-17Co alloy (Kovar). The load vs. displacement curves obtained, while superficially resembling those of a standard tensile test, were quite non-reproducible, and obscured the differences due to process conditions. Fractographic examination of the samples and analysis of the testing conditions led to significant conclusions about how to correctly interpret the shear test results, which in turn enabled a determination of the real effects of the change in shielding gas. Several different types of fracture morphology were noted, depending upon how the fracture surface developed relative to the original weld. This resulted in the disparate nature of the load-displacement curves. The results of the shear testing, fractography and metallography will be used to support interpretation of the differences found with respect to porosity formation, strength and work hard...