Publications

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A method has been developed to temporally characterize the power and energy absorbed in laser beam spot welding (LBSW). As a spot weld is created, the absorption of laser power changes as the surface of the weld pool changes from initial melting through the development of the keyhole. By relating the instantaneous delivered power and pulse energy to the scattered power during welding, a time-resolved description of the power and energy absorption can be obtained. The method uses two goldplated integrating spheres containing Nd:YAG notch-filtered photodiodes to capture and detect the scattered laser light. Under various welding parameters (pulse energy, duration, and shape), the level of scattered light changes with the y condition of the weld pool. For high depth-to-width aspect ratio keyhole mode welds, power transfer efficiency (or instantaneous energy transfer) ranges from -40 to 80% depending on the state of the weld pool. In contrast, low aspect ratio conduction mode welds maintain less than 50% transfer efficiency throughout the welding process. Overall energy transfer efficiencies measured by this method show good agreement with calorimetric (Refs. 1, 2) and thermal expansion measurements (Ref. 4). Time-resolved energy absorption was also evaluated for square and constant ramp down (CRD) pulse shapes. Through characterization of keyhole formation and transfer efficiency in relation to welding parameters, the laser welding process can be optimized, and insight into keyhole phenomena necessary for developing and improving modeling capabilities can be obtained.

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Surface scabbling of concrete by laser processing has been demonstrated in the literature for large-area problems ({approx}50 mm wide x 10 deep) using physically large, high-power consumption, multi-kW CW laser systems. With large spot diameters ({approx}50 mm) and low power densities ({approx} 300 W/cm{sup 2}), large volume thermal stresses are induced which promote concrete cracking. This process is highly power-density and heat-input (J/m) dependent. Too high power densities cause melting and generate potentially toxic fumes by vaporizing the cement matrix material. New applications require concrete removal with more portable, lower power equipment, and low particulate and fume generation. Recent results investigating the process for small-area ({approx} 2 x 2 mm) removal are examined and discussed. Tests performed were limited to < 700W output power. Ablation via thermal cracking was observed at larger spot sizes but as the spot size approached 10 mm (with constant power density) ablation ceased and melting predominated. Scaling effects involving temperature gradients through the ITZ (Interfacial Transition Zone), the probability of including an ITZ in the beam path at decreasing spot sizes, and the gradient effects on bulk properties between rock and sand zones will be presented and discussed.

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Weld porosity is being investigated for long-pulse spot welds produced by high power continuous output lasers. Short-pulse spot welds (made with a pulsed laser system) are also being studied but to a much small extent. Given that weld area of a spot weld is commensurate with weld strength, the loss of weld area due to an undefined or unexpected pore results in undefined or unexpected loss in strength. For this reason, a better understanding of spot weld porosity is sought. Long-pulse spot welds are defined and limited by the slow shutter speed of most high output power continuous lasers. Continuous lasers typically ramp up to a simmer power before reaching the high power needed to produce the desired weld. A post-pulse ramp down time is usually present as well. The result is a pulse length tenths of a second long as oppose to the typical millisecond regime of the short-pulse pulsed laser. This study will employ a Lumonics JK802 Nd:YAG laser with Super Modulation pulse shaping capability and a Lasag SLS C16 40 W pulsed Nd:YAG laser. Pulse shaping will include square wave modulation of various peak powers for long-pulse welds and square (or top hat) and constant ramp down pulses for short-pulse welds. Characterization of weld porosity will be performed for both pulse welding methods.

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Nd:YAG laser joining is a high energy density (HED) process that can produce high-speed, low-heat input welds with a high depth-to-width aspect ratio. This is optimized by formation of a ''keyhole'' in the weld pool resulting from high vapor pressures associated with laser interaction with the metallic substrate. It is generally accepted that pores form in HED welds due to the instability and frequent collapse of the keyhole. In order to maintain an open keyhole, weld pool forces must be balanced such that vapor pressure and weld pool inertia forces are in equilibrium. Travel speed and laser beam power largely control the way these forces are balanced, as well as welding mode (Continuous Wave or Square Wave) and shielding gas type. A study into the phenomenon of weld pool porosity in 304L stainless steel was conducted to better understand and predict how welding parameters impact the weld pool dynamics that lead to pore formation. This work is intended to aid in development and verification of a finite element computer model of weld pool fluid flow dynamics being developed in parallel efforts and assist in weld development activities for the W76 and future RRW programs.

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In an on going study of gap bridging for thin plate Nd:YAG laser lap welds, empirical data, high speed imaging, and computer modeling were utilized to better understand surface physics attributed to the formation and solidification of a weld pool. Experimental data indicates better gap bridging can be achieved through optimized laser parameters such as pulse length, duration, and energy. Long pulse durations at low energies generating low peak powers were found to create the highest percent of gap bridging ability. At constant peak power, gap-bridging ability was further improved by using a smaller spot diameter resulting in higher irradiances. Hence, welding in focus is preferable for bridging gaps. Gas shielding was also found to greatly impact gap-bridging ability. Gapped lap welds that could not be bridged with UHP Argon gas shielding, were easily bridged when left unshielded and exposed to only air. Incident weld angle and joint offset were also investigated for their ability to improve gap bridging. Optical filters and brightlight surface illumination enabled high-speed imaging to capture the fluid dynamics of a forming and solidifying weld pool. The effects of various laser parameters and the weld pool's interaction with the laser beam could also be observed utilizing the high-speed imaging. The work described is used to develop and validate a computer model with improved weld pool physics. Finite element models have been used to derive insight into the physics of gap bridging. The dynamics of the fluid motion within the weld pool in conjunction with the free surface physics have been the primary focus of the modeling efforts. Surface tension has been found to be a more significant factor in determining final weld pool shape than expected.

Gap bridging of thin plate pulsed Nd:YAG lap welds is optimized by focused welding at low peak powers without gas shielding. High speed images reveal effects of varying welding parameters and weld pool and laser beam interactions. Improved bridging with out gas shielding is attributed to changes in Marangoni convective flow. Development and verification of finite element models for weld pool physics is being conducted. Copyright © 2006 ASM International®.

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A straightforward and accurate method for measuring the laser beam diameter at focus is desired in order to develop fundamental understanding and for routine process control. These measurements are useful for laser materials processing by assuring laser performance consistency at the workpiece. By employing multiple-shot exposures on Kapton™ film, an unambiguous and precise measurement of the focused Nd:YAG laser beam diameter for spot welding lasers was obtained. A comparison of focused beam measurements produced with the Prometec laserscope and an ISO variable aperture method found that these two methods, which both measure the 86% energy contour, do closely agree. In contrast, Kapton film was found to measure the 99% beam energy contour and to diverge from measurements made with the other two methods. The divergence between Kapton and the other two methods was shown to be due to changes in the laser irradiance distribution that do not affect the location of the 99% energy contour. Since the 86% beam diameter was seen to not always be representative of the true beam diameter, the 99% Kapton film diameter can provide a more representative measurement of the focused laser for in-situ process control.