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.

The study aims to develop an infrared temperature-sensing system by applying to thermal radiation theory. The system consists of an optic unit, a photodetector, and an electronic unit. This system detects thermal radiation at 1310 nm wavelength, the temperature range of the system is 600~4000¢J, rise time 2£gs, spatial resolution 400£gm. The calibration was performed in the temperature low at 1200¢J by using a K-type thermocouple that can gain between temperature and output voltage relations, but beyond the temperature 1200¢J applying to Planck¡¦s law as calculate to predict. In the calibrated temperature range, the measurement error is ¡Ó80¢J for the low temperatures and ¡Ó20¢J for the high temperatures. The system was used to measure temperature variation during Nd:YAG pulse laser welding process. Experiments ware performed with stainless steel plates as specimen radiation by a laser pulse of 7ms duration time and various energy in the rang of 1245~5313mJ. The experimental results show the feasibility of the infrared temperature-sensing system in application of Nd:YAG pulse laser welding process.

Laser Transmission Welding (LTW) involves localized heating at the interface of two pieces of plastic (a laser transparent plastic and laser absorbing plastic) to be joined. It produces strong, hermetically sealed welds with minimal thermal and mechanical stress, no particulates and very little flash. An ideal transparent polymer for LTW must have: a low laser absorbance to avoid energy loss, a low level of laser scattering so it can provide a maximum energy flux at the weld interface and also have a high resistance to thermal degradation. The objective of the project was to analyze the effect of blend ratios of polybutylene terephthalate and polyethylene terephthalate (PBT/PET) on these laser welding characteristics. The blends were manufactured by DSM (Netherlands). They were characterized using Differential Scanning Calorimetry (DSC) and Thermal Gravimetry Analysis (TGA). The latter technique was used to estimate the order (n), activation energy (ΔH) and frequency factor (A’) of the degradation reaction of the polymer blends. The normalized power profile of the laser after passing through the transparent polymer was measured using a novel non-contact technique and modeled using a semi-empirical model developed by Dr.Chen. Adding more PET ratio to the blend, did not change beam profile of the transmitted beam significantly. Laser welding experiments were conducted in which joints were made while varying laser power and scanning speed. Measuring the weld strength and width showed that the blends containing PET have higher strength in comparison to pure PBT. The temperature-time profile at the interface during welding was predicted using a commercial FEM code. This information was combined with the degradation rate data to estimate the relative amount of degraded material at the weld interface. It showed that increasing the ratio of PET in the blend makes it more resistant against thermal degradation which can be one of the reasons the PET containing blends reach higher weld strengths when compared to pure PBT. / Thesis (Master, Chemical Engineering) -- Queen's University, 2010-08-31 10:03:42.167

Thesis (Ph. D.)--Worcester Polytechnic Institute. / Keywords: Optoelectronic holography; Microwelding; Microelectronic packaging; Microdrilling; Laser micromachining; Computational modeling. Includes bibliographical references (p. 204-212).

No description available.

Made available in DSpace on 2014-10-09T12:54:49Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:07:36Z (GMT). No. of bitstreams: 0 / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energéticas e Nucleares - IPEN/CNEN-SP / FAPESP:06/52778-1

Made available in DSpace on 2014-10-09T12:54:49Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:07:36Z (GMT). No. of bitstreams: 0 / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / O titânio é um material leve e resistente que possui aplicações em várias áreas, das quais podemos destacar a área médica, a aeronáutica e a nuclear. Porém, devido à sua alta reatividade a altas temperaturas com o oxigênio e outros elementos químicos presentes no ar, a soldagem deste material pode ser muito difícil. O uso de fontes de energia de alta intensidade como o laser, produz uma menor zona afetada pelo calor, diminuindo a área oxidada. Porém, mesmo assim há a necessidade do uso de uma atmosfera de proteção neutra, formada por hélio, argônio ou misturas destes gases. Esta atmosfera interage com o processo através de mudanças na formação de plasma, modificando as características da solda, como a largura do cordão e a penetração, podendo também provocar o aparecimento de porosidades nestes cordões. Neste trabalho foi verificada a influência do uso de argônio, hélio e misturas destes gases na soldagem com laser pulsado da liga Ti6Al4V. Também foi feito um estudo verificando a necessidade do uso de diferentes fluxos e dispositivos de proteção na soldagem. Foi verificado que as características físicas e mecânicas do cordão d solda não são modificadas significativamente pelos gases, e que apesar de haver um aumento na dureza pela falta de uma proteção de raiz, esta também não causa efeitos negativos na resistência da solda. Na soldagem do mesmo material com laser contínuo foram estudadas as influências dos parâmetros de soldagem, comparando-os com simulações matemáticas. Os resultados mostraram que a simulação pode ser utilizada para prever a largura do cordão de solda e das zonas afetadas pelo calor e oxidadas. / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energéticas e Nucleares - IPEN/CNEN-SP / FAPESP:06/52778-1

Thick-section austenitic stainless steels have widespread industrial applications, especially in nuclear power plants. The joining methods used in the nuclear industry are primarily based on arc welding processes. However, it has recently been shown that the Narrow Gap Laser Welding (NGLW) technique can be used to join materials with thicknesses that are well beyond the capabilities of single pass autogenous laser welding. The heat input for NGLW is much lower than that of arc welding, as are the expected levels of residual stress and distortion. The multi-pass laser welding technique, based on the narrow gap approach, is an emerging welding technology which can be applied to thick-section welds using a relatively low-power laser, but the process is more complicated than autogenous laser welding, since it is necessary to introduce filler wire to narrow gap weld configurations. Despite this complexity, the technique is very promising for improving the penetration capabilities of the laser welding process. However a limited amount of research has been conducted on the development of the NGLW technique; the control and optimization of weld bead quality inside the narrow gap is still an area of weakness. The research described in this thesis involves investigations on NGLW of AISI grade 316L austenitic stainless steel, and the performance of the resulting welds. Design-of-experiments and statistical modelling techniques were employed to understand and optimize the welding process. A statistical model was used in order to understand the significant process parameters and their interactions, allowing improved control of the weld quality in ultra-narrow gap (1.5 mm gap width) welds. The results show a significant improvement in weld quality can be achieved through the use of statistical modelling and multi-variable optimisation. The microstructure characteristics and mechanical properties (e.g. tensile strengths, fatigue, bending strength and fracture toughness) of the NGLW samples were examined and compared with those of other welding techniques - autogenous laser welding and gas-tungsten arc welding (GTAW). The work shows that NGLW of 316L steel sheets up to 20 mm thickness have generally better or comparable mechanical properties than those of GTAW but with much higher welding productivity. The results of detailed investigations of the 2D residual stress distributions, material distortions, and plastic strain characteristics of the NGLW technique are described. The contour method was employed for residual stress evaluation of the NGLW technique, and the results were validated using X-Ray and neutron diffraction measurements. The results were compared with those obtained with GTAW. The results suggest that the longitudinal tensile residual stresses in NGLW joints are 30-40% lower than those for GTAW joints. The influence of the laser power and number of passes for the NGLW technique, on the developed residual stress and plastic strain has been investigated, and the influence of welding strategy and the use of restraint during welding were also investigated. To understand the thermal history in NGLW and its effect on residual stress, finite element analysis was carried out using ABAQUS to numerically model the behaviour of residual stress across the multipass NGLW weld joints. The model has been validated with the experiments using temperature measurements and in terms of residual stresses the model is compared with neutron diffraction and the contour method. There is a very good correlation between the model and experimental results. The influence of both the laser power and welding speed on the induced residual stress during the NGLW process was also investigated using the model. The aqueous, pitting and stress corrosion cracking behaviour of the NGLW joints were investigated, and the results compared to those for GTAW joints under the same conditions. The results show that NGLW joints have better resistance to pitting corrosion than the GTA welds. Preliminary results also suggest that NGLW has better resistance to stress corrosion cracking.

No description available.