Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references (leaves 198-212). Available also in a digital version from Dissertation Abstracts.

Contour laser transmission welding (LTW) is a technology that has potential for joining large and complicated thermoplastic parts. Thermal expansion is the primary driving force to bridge potential gaps at the weld. A comprehensive investigation into gap bridging was performed using experimental studies, finite element (FE) thermal-mechanical coupled modeling, and analytical analysis of the contour welding process for polycarbonate (PC), polyamide 6 (PA6) , and glass fibres reinforced polyamide 6 (PA6GF). The effects of material properties (carbon black level, glass fibres and crystallinity), process parameters (laser scan power, scan speed) and weld gap thickness on weld shear strength were assessed. The experimental study indicated that low concentration of laser absorbing pigment accompanied with high power laser scan improves gap bridging. Damage on the top surface of the laser-transparent part limited the allowable laser power that could be delivered onto the weld interface. Maximum gaps of 0.2, 0.4 and 0.25 mm were bridged in the experiment for the three types of polymers respectively. The thermal behavior of polymers in contour LTW was analyzed by the 3-D quasi-static thermal FE models. Thermal expansion into the gap was simulated by the simplified 2-D transient, thermal-mechanical coupled FE models. An analytical model describing laser beam transmission and absorption in light-scattering polymers was developed and applied in the FE simulation for PA6 and PA6GF. FE simulated results agree well with the experiment in contour welding with gap of PC and PA6. The optimum material and process parameters have been searched in the model to maximize gap bridging for PC. An analytical model has been developed to predict the temperature rise and the thermal expansion in high speed contour welding of amorphous polymers. The model indicates that the maximum temperature at weld increases linearly with the laser line energy and the laser absorption coefficient. Thermal expansion and hence gap bridging increases with laser line energy. Lower laser absorption coefficient allows higher laser scan energy to be delivered onto the weld interface so helps bridge larger gap. The predicted thermal expansions by the model agree well with the measured maximum gaps bridged for polycarbonate. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2009-09-24 22:24:11.734

Thesis (PhDApSc(MineralsandMaterials)--University of South Australia, 2004.

Automotive and aerospace industries consume a significant amount of Al alloys in structures and framing. There is, however, a significant challenge to join the alloy components by laser welding. A key problem is the presence of large amount of porosity in the welds. This research work aimed to understand factors affecting porosity formation in laser welding of AA6014 Al alloy and identification and verification of a suitable method for the porosity reduction and elimination. AC-170PX (AA6014) Al alloy was welded, for the first time, using a 5 kW disk laser in two different configurations: fillet edge and flange couch joints using a number of different filler wires. The experimental results showed that laser power (2-5 kW) and welding speed (20-50 mm/s) had a significant influence on porosity generation. Also, the introduction of a 0.2 mm gap between the sheets significantly reduced porosity for the fillet edge joint while it had a marginal effect for the flange couch joint. The effect of the chemical composition of the filler wire on the AA6014 weld quality was also evaluated for the first time by using different filler wires (AA3004, AA4043, AA4047 and AA5083) over a range of laser powers and welding speeds. The increase in Mg and Mn content in the filler wire's composition was found to reduce porosity in comparison with high silicon content filler wires. Nanosecond pulsed Nd:YAG laser cleaning was investigated as a surface preparation method for laser welding for AA6061, and its effect on porosity at various welding parameters was examined. The effect of laser cleaning on porosity reduction during laser welding using a filler wire has not been reported before. The surface characteristics before and after laser cleaning were analysed. The results showed that laser cleaning played an essential role in significantly reducing porosity in both the fillet edge and flange couch joints at different levels of power and laser welding speed. The developed surface preparation technology as a method for porosity reduction in laser welding has been successfully implemented in one of the largest UK/international car manufacturers. To study the laser cleaning process, a novel Smoothed particle hydrodynamics (SPH) meshless model has been implemented using a new 3-D multi-phase transient model. For the first time, a study was conducted to validate the temperature field distribution predicted in SPH method under nanosecond pulsed laser heating. The need for special surface treatment of the kernel truncation was also investigated. The proposed model accurately predicted the laser ablation depth and the crater shape and was validated using a significant number of experimental and numerical data reported in the literature. Moreover, a primitive laser welding model has been created to predict the material flow inside the welding pool. The research work has resulted in four publications in peer-reviewed journals. The research highlighted that future work should include the development of a more advanced SPH model for the prediction of porosity in laser welding and to fully describe the relationship between laser cleaning and porosity reduction in laser welding.

No description available.

Laser welding was investigated as a potential joining technology for alumina ceramic substrates. The objective of this study was to develop a method to preheat the ceramic using a single defocused laser beam prior to welding. Engineering ceramics are employed in a variety of systems and environments due to their unique properties. Joining technologies must be developed to facilitate the manufacture of complex or large ceramic components. Laser welding is advantageous as it forms joints rapidly, and does not introduce intermediate materials to form the bond, which can have deleterious effects. The Laser Machining System (LMS) at Queen’s University was adapted for this study. A defocused far-infrared (FIR) laser beam was positioned to overlay a focused near-infrared (NIR) laser beam; the defocused FIR beam preheated the ceramic substrate and the focused NIR beam formed the weld. A finite element model was developed in COMSOL MultiPhysics to simulate the preheating processes and to develop a preheating protocol. The protocol was implemented using the FIR beam and adjusted to achieve preheating temperatures of 1450, 1525, and 1600degC. Welds were performed on 1 mm thick alumina plates using the preheating protocols and NIR beam powers of 25, 50, and 75 W. Weld speed was held constant throughout the study at 0.5 mm/s. The preheating protocols were successful at achieving near-constant preheating temperatures, with standard deviations below 32 degrees. Partially penetrating welds were formed with the NIR beam at 25 W, and fully penetrating welds at 50 and 75 W. Large pores were present in the 25 W and 50 W welds. Minimal porosity was observed in the welds formed at 75 W. All of the welded plates experienced a transverse fracture that extended perpendicular to weld, and a longitudinal fracture extending parallel to the weld. This study shows that a fixed defocused laser beam can successfully preheat alumina substrates to the high temperatures required for welding; however, non-homogenous cooling results in fracture. Increasing the preheating beam diameter or introducing an auxiliary means to provide a controlled cool-down cycle may mitigate these effects. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2013-04-29 17:59:57.43

<p>This study is concerned with investigating the effects of warm laser shock peening (wLSP) on the enhancement of mechanical performance of laser welded joints. A 3-D finite element model is presented which predicts the surface indentation geometry and in-depth compressive residual stresses generated by wLSP. To define the LSP pressure on the surface of the material, a 1-D confined plasma model is implemented to predict plasma pressure generated by laser-coating interaction in an oil confinement regime. Residual stresses predicted by the finite element model for wLSP reveal higher magnitude and depth of compressive residual stresses than room temperature laser shock peening. A novel dual laser wLSP experimental setup is developed for simultaneous heating of the sample, to a prescribed temperature, and to perform wLSP. The heating laser power is tuned to achieve a predefined temperature in the material through predictive analysis with a 3-D transient laser heating model.</p><p>Laser welded joints of AA6061-T6 and TZM alloy in bead-on-plate (BOP) and overlap configurations, created by laser welding with a high power fiber laser, were post processed with wLSP. To evaluate the strength of the welded joints pre- and post-processing, tensile testing and tensile-shear testing were carried out. To understand the failure modes in tensile-shear testing of the samples, a 3-D finite element model of the welded joint was developed with weld regions’ material strength properties defined through microhardness testing. The stress concentration regions predicted by the finite element model clearly explain the failure regions in the experimental tensile testing analysis. The tensile tests and tensile-shear tests carried out on wLSP processed AA6061-T6 samples demonstrate an enhancement in the joint strength by about 20% and ductility improvement of about 33% over as-welded samples. The BOP welds of TZM alloy processed with wLSP demonstrated an enhancement in strength by about 30% and lap welds demonstrated an increase in joint strength by 22%.<br></p><p></p>

Thesis (Ph. D.)--Oregon Graduate Center, 1982.

Limiting dome height (LDH) tests were used to evaluate the formability of both base metal and laser butt welded blanks of AHSS (including High strength low alloy (HSLA), Dual phase (DP) steels of different grades). Mechanical properties of the base metal and welded blanks were assessed by uniaxial tensile and biaxial LDH tests, and related to measured microhardness distributions across the welds. The formability ratio of laser welded dual phase sheet steels generally decreases with increased base metal strength. A significant decrease of LDH was observed in the higher strength DP steel welded specimens due to the formation of a softened zone in the Heat Affected Zone(HAZ). Softened zone characteristics were correlated to the LDH. Larger softened zones led to a larger reduction in the LDH. HAZ softening has been shown to be a function of the base metal martensite content and the weld heat input. Formability also decreased with increased weld heat input. Both in experiment and numerical simulations strain is localized in the softened HAZ in the uniaxial tensile testing, indicating that strain localization decreases tensile strength and elongation of laser welds in DP980.

Due to finite fossil fuel resources and the impact on our environment of burning fossil fuels, the automotive industry has been investigating ways to reduce the overall weight of automotive vehicles. This has led to increased interest in ways that light weight alloys such as magnesium can be used in fabrication of automotive parts and manufacturing processes such as welding that would enable increased use of magnesium. The objectives of this project were to characterize and determine the weldability of 2 mm thick AZ31B-H24 magnesium alloy by three different laser beam welding processes: a 4 kW Nuvonyx ISL-4000L high power diode laser, a 5 kW Trump TLC-1005 CO2 laser, and a 10 kW YLR-10000-WC fibre laser. The diode laser operated with a 0.9 by 12 mm spot size and with a maximum power density of 37 MW/m2. Due to its low power density, the diode laser was restricted to conduction-mode welding which produced wide fusion zones. The AZ31B magnesium laser welds exhibited a number of defects including hydrogen porosity, solidification cracking, liquation cracking, high vaporization rates, molten expulsions, and poor weld bead quality due to low surface tension. It was found that the majority of these defects could be controlled through the proper use of clamping and shielding of the weld pool and joint preparation and surface cleaning prior to welding. The as-received base material was delivered with a dark grey hydrated oxide layer. This surface condition was found to increase the overall diode laser beam absorption but was detrimental to the welding process when disrupted. When incorporated into the weld pool, the oxide created weak facets where solidification cracks would initiate or acted to localize strain during tensile testing. Proper joint preparation was required to produce a high quality diode laser weld: machining of the joint interface to remove interfacial gaps, chemical cleaning with acetone and ethanol to remove residual oils or grease, and stainless steel wire brushing to remove the oxide. Diode laser welds made using 3 kW power and 0.75 m/min welding speed achieved approximately 60% of the base metal’s ultimate strength and less than 15% of the base metal ductility. The reduced strength and ductility were attributed primarily to the weld defects which acted as strain localizers during plastic deformation and the lack of strain hardening in the weld metal. Both the CO2 and fibre lasers beams had focal spot sizes of 300 μm diameters and maximum power densities of 70 and 140 GW/m2, respectively. At these power densities, the CO2 and fibre lasers operated in keyhole-mode and produced welds which had narrower columnated fusion zones. The CO2 laser keyhole-mode welds exhibited keyhole instability and bulk material loss through vaporization that resulted in macro-porosity, under-fill, and generally poor weld bead quality. Welds produced using 5 kW power and 8 m/min welding speeds achieved approximately 70% of the base metal’s ultimate strength. The highest quality fibre laser welds were produced at 2 kW power and 100 mm/s welding speeds. These defect-free welds achieved transverse tensile strengths that were 86% of the base metal’s ultimate strength. The 14% loss of strength was attributed to the difference in temper of the base metal and the weld metal. The base material was received in a half-hard H24 temper and the as solidified weld metal is naturally in the softer F temper. This also resulted in a corresponding 15% reduction in hardness. Failure always occurred in the softened fusion zones of the welded samples where the measured hardness was reduced to an average 60 VHN25 from the base metal’s 75 HVN25. The fibre laser weld samples also experienced the greatest extension of any of the tested welds with a cross head displacement of 30% of the base metal. The extreme reduction in overall cross head displacement was attributed to the lower strength of the fusion zone. This led to strain localization in the transverse tensile specimens and premature failure that occurred prior to plastic deformation of the surrounding base material. Proper joint preparation was found to be critical when laser welding AZ31B magnesium sheet. Machined interfaces were required to minimize the gap and degreasing and stainless steel wire brushing were required for removal of the pre-existing hydrated oxide in order to produce sound laser welds. Helium shielding gas was found to improve the weld bead surface quality compared to argon. The keyhole-mode welds produced with the CO2 and fibre lasers were superior compared to the conduction-mode welds produced with the diode laser. This was due to the narrower fusion zone and reduced bulk material loss. Of the three laser welding processes examined in this study, the fibre laser produced the highest quality, strongest, and most ductile welds when analyzed in transverse tensile testing. However, direct comparisons between the CO2 and fibre laser welds could not be made because they were made using different joint preparations and welding conditions.