Abbott Vascular encountered strength and variability issues when attempting to resistively weld 304 Stainless Steel to equiatomic Nitinol. Initial observations suggested that passivation layer (Cr2O3, TiO2) formation affected the weld interface. One hundred 304 Stainless Steel/Nitinol pairs were allowed to oxidize in air at room temperature for allowed periods of time (.1, 1, 3, 5, 7, 12, 16, 24, 168, and 336 hours). Each pair was welded resistively with constant current. A Miyachi/Unitek Advanced Data Analysis Monitor (ADAM) recorded the peak resistance at the instance the weld was made. Resistances were compared to Instron 5900 tensile maximum break load (KgF). Use of optical microscopy and Scanning Electron Microscopy (SEM) revealed microstructural reduction of void size at the sample fracture surface (1-.5 µm). Literature suggested the existence of metastable precipitate forms at near equiatomic compositions within the theoretical temperature range (261.9-1425.2 0C). The Instron 5900 mechanically validated presence of precipitates, while Electron Dispersive X-Ray Spectroscopy (EDS) confirmed the existence compositionally. Literature confirms B19’ precipitates size increases with temperature. This suggests higher resistance samples will promote growth of precipitates due to increased heat input. Increased average particle size was observed with increased resistance (0-.3 µm). Crystal lattice inconsistencies between Nitinol parent phase (B2) and B19’ promote premature fracture due to increased misfit dislocation density. Therefore increased weld resistance promotes the growth of incoherent Ti3Ni4 precipitates which inhibit load bearing capabilities, causing premature failure.

An investigation was undertaken to better understand the corrosion behaviour of dissimilar welded Cu-lean AA7003 and AA7108 extrusions. The major variables under study were the heat-treated condition (as-welded T6 vs. as-welded T6+Paint Bake (PB)), extrusion alloy Cu composition (AA7003 vs. AA7108), weld filler composition (ER4043 vs. ER5356), weld joint geometry (lap-joint vs. T-joint), and weld direction with respect to extrusion direction (parallel (═) vs. perpendicular (┴)). The corrosion behaviour of the various weld configurations under investigation was observed using an ASTM standard practice for modified salt spray testing (ASTM G85-A2), a GM worldwide engineering standard for cyclic corrosion testing (GMW-14872), and potentiodynamic polarization measurements. The effect of exposure to GMW-14872 on the tensile-shear behaviour of the various weld configurations under study was also investigated using a custom tensile jig. Examination post exposure to ASMT G85-A2 revealed the presence of differing pitting corrosion morphologies between AA7003 and AA7108. Due to increased Cu-content, AA7003 displayed deep pitting corrosion which penetrated the entirety of the dynamically recrystallized top surface layer and reached the fine-grained interior. Shallow pitting of the recrystallized surface layer was observed on AA7108, with very few penetration sites that reached the underlying fine-grained interior. No difference in corrosion behaviour was observed between the heat affected zone (HAZ) and unaffected base alloy of welded AA7003 and AA7108, also consistent with potentiodynamic polarization results. However, the HAZ displayed dual corrosion bands separated by a thin band of unattacked alloy; a result of distinct local microstructural changes induced by thermal cycling from welding. Tensile-shear testing revealed four types of observed fracture modes: shear across the weld throat, fracture along the AA7xxx/ER5356 interface, fracture along the AA6063/ER5356 interface and fracture in the HAZ of AA7xxx. Little to no corrosion was observed on weld configurations exposed to GMW-14872, resulting in no differences in the tensile-shear behaviour of exposed and unexposed weld configurations. Large variations observed in the tensile-shear results were a result of numerous weld defects. / Thesis / Master of Applied Science (MASc)

Friction stir welding (FSWing) is a solid state, thermo-mechanical process that utilizes a non-consumable rotating weld tool to consolidate a weld joint. In the FSW process, the weld tool is responsible for generating both the heat required to soften the material and the forces necessary to deform and consolidate the former weld seam. Thus, weld tool geometry, material selection, and process parameters are important to the quality of the weld. To study the effects of the weld tool geometry on the resulting welds, a previous study was conducted using varying degree taper, microwave-sintered tungsten carbide (WC) weld tools to FSW Ti-6Al-4V. Fully consolidated welds were down selected for this study to evaluate the resulting mechanical properties and to document the microstructure. X-ray diffraction (XRD) was used to compare the parent material texture with that in the weld nugget. The purpose of this study is to quantify the temperatures obtained during FSWing by interpreting the resulting microstructure. This information is useful in process optimization as well as weld tool material selection.

No description available.

No description available.

Yes / The microstructure and molecular orientation distribution over both the length and the thickness of microinjection‐molded linear low‐density polyethylene with a weld line were characterized as a function of processing parameters using small‐angle X‐ray scattering and wide‐angle X‐ray diffraction techniques. The weld line was introduced via recombination of two separated melt streams with an angle of 180° to each other in injection molding. The lamellar structure was found to be related to the mold temperature strongly but the injection velocity and the melt temperature slightly. Furthermore, the distributions of molecular orientation at different molding conditions and different positions in the cross section of molded samples were derived from Hermans equation. The degree of orientation of polymeric chains and the thickness of oriented layers decrease considerably with an increase of both mold temperature and melt temperature, which could be explained by the stress relaxation of sheared chains and the reduced melt viscosity, respectively. The level of molecular orientation was found to be lowest in the weld line when varying injection velocity, mold temperature, and melt temperature, thus providing an effective means to identify the position of weld line induced by flow obstacles during injection‐molding process. / Jilin Scientific and Technological Development Program. Grant Number: 20180519001JH; National Key R&D Program of China. Grant Number: 2018YFB0704200; National Natural Science Foundation of China. Grant Numbers: 21674119, 21790342; Newton Advanced Fellowship of Royal Society. Grant Number: NA150222

There have been several design models for single plate shear connections in the past 20 years. The current design model states that the bolt shear rupture strength of a connection is a function of the number of bolts and the a-distance, which is the distance from the weld line to the bolt line. The evaluation of this design model demonstrates inconsistent predictions for the strength of the connection. The finite element program ABAQUS was used throughout the research to study single plate shear connections. Finite element analyses included model verification and investigations of parameters, including the effect of a-distance, plate thickness, plate material, and the position of a connection with respect to a beam neutral axis. In addition, double-column bolt connections were studied. The results show that bolt shear rupture strength of a connection is not a function of the a-distance. Plate materials and thicknesses that do not satisfy ductility criteria result in connections with significant horizontal forces at the bolts. This horizontal force reduces the shear strength of a bolt group and creates a moment that must be considered in design. The magnitude of the force depends on the location of the bolt with respect to the beam neutral axis. A new design model for single plate shear connections with bolts in a single column is proposed. It was found that in double-column bolt connections, force redistribution among the bolt columns occurs. Force redistribution does not occur when thick plates are used, resulting in bolts in the outer column (from the support) fracturing while bolts in the inner column resist much less force. Further study is needed for double-column configurations. The study of plate behavior shows that the shear stress distribution when a plate reaches the strain hardening stage is not constant throughout the cross section. A relationship for calculating plate shear yielding strength based on this shear distribution is proposed. / Ph. D.

Nowadays there is a strong demand for lighter vehicles in order to increase the pay load. Through this the specific fuel consumption is decreased, the amount of greenhouse gases is lowered and the transport economy improved. One possibility to optimize the weight is to make the components from high strength steels and join them by welding. Welding is the main joining method for fabrication of a large proportion of all engineering structures. Many components experience fatigue loading during all or part of their life time and welded connections are often the prime location of fatigue failure.Fatigue fracture in welded structures often initiates at the weld toe as aconsequence of large residual stresses and changes in geometry acting as stress concentrators. The objective of this research is to increase the understanding of the factors that control fatigue life in welded components made from very high strength steels with a yield strength of more than 800 MPa. In particular the influences of the local weld toe geometry (weld toe radius and angle) and residual stress on fatigue life have been studied. Residual stresses have been varied by welding with conventional as well as Low Transformation Temperature (LTT) filler materials. The three non-destructive techniques Weld Impression Analysis (WIA), Laser Scanning Profiling (LSP) and Structured Light Projection (SLP) have been applied to evaluate the weld toe geometry.Results suggest that all three methods could be used successfully to measure the weld toe radius and angle, but the obtained data are dependent on the evaluation procedure. WIA seems to be a suitable and economical choice when the aim is just finding the radius. However, SLP is a good method to fast obtain a threedimensional image of the weld profile, which also makes it more suitable for quality control in production. It was also found that the use of LTTconsumables increased fatigue life and that residual stress has a relatively larger influence than the weld toe geometry on fatigue strength of welded parts.

Welding, as one of the most effective joining methods for metals, has been extensively applied in engineering usage for a long time. When cracks occur in the vicinity of weldments, weld repairs are frequently considered for crack repair to extend service life. In order to evaluate to what extent the weld repair has improved the fatigue life of a cracked welded structure, it is necessary to be able to determine the residual life of the cracked welded joint, as well as the life of the weld repaired joint. Both these assessments require that the fatigue crack growth data be available. The determination of crack propagation rates of welded and weld repaired structures is thus of paramount importance to implement a damage tolerant approach to structural life extension. However, since most studies on welded joints so far have concentrated on fatigue life evaluation, at the present time only limited information is available on crack propagation rates in welded joints, and virtually none on fatigue behaviour and crack propagation in weld repaired joints. This thesis has focused on examination of fatigue and crack propagation behaviour in as welded and weld repaired aluminium alloy 5083, a weldable marine grade alloy extensively used in construction of high speed ferries and aerospace structures. Crack growth rates were measured during constant amplitude fatigue testing on unwelded, as-welded and weld repaired specimens of 5083-H321 aluminium alloy. A 3-D finite element analysis was conducted to determine the stress intensity factors for different lengths of crack taking into account the three-dimensional nature of the weld profile. The effects of crack closure due to weld residual stresses were evaluated by taking measurements of the crack opening displacements and utilised to determine the effective stress intensity factors for each condition. Metallurgical examinations and fractography of the fracture surface were conducted using an optical microscope and SEM. It was found that crack growth rates in welded plates are of the same order of magnitude as those of parent material when effective stress intensity factors were applied. However weld repaired plates exhibit higher crack growth rates compared to those of unwelded and once-only welded plates.

Selective corrosion of type 316L austenitic stainless steel welds during the production of organic acids resulted in losses in production due to unscheduled downtimes to perform repairs. Estimated corrosion rates of type 316L filler material welds were an order of magnitude higher than that of the base material. Alternative higher alloyed commercial filler materials were evaluated under actual production conditions. The evaluated filler materials were types 316L, 317L, 309L, 309MoL, 2205, 2507, 625, 825 and 904L. The effect of nitrogen on the corrosion properties of type 309L filler material was evaluated by manipulating the nitrogen concentration of the shielding gas during MIG welding. These changes in nitrogen concentration did not influence the corrosion resistance of the type 309L filler material. No correlation could be established between the corrosion rates, analysed chemical composition of the product and operating temperature during production. In almost all the cases where the chemical composition of the filler material was comparable with that of the base material the corrosion rates of the filler materials were higher than that base material. It might be expected that the ferrite phase with higher molybdenum and chromium should be more corrosion resistant while the austenite should be less resistant. This was, however, not the case with the corrosion of type 309L filler material. It would thus appear that in this case nickel enrichment of the austenite phase had a larger influence on the corrosion resistance of the austenite phase than the chromium and molybdenum had on the corrosion resistance of the ferrite phase. It appears that nickel and molybdenum had the largest contribution to the corrosion resistance of stainless steels welds under these operating conditions. It is, however, believed that a certain minimum concentration of chromium is also required to provide corrosion resistance to these alloys in hot organic acid environments. In contrast with the fact that a substantial alloying content is required to improve corrosion resistance of the filler material, the small difference in composition between ferrite and austenite phases, due to micro segregation, appeared to affect the corrosion resistance on micro scale. This is illustrated by the micrographs, which show corrosion to etch out the dendrite structure. Since the morphology of the austenite and ferrite phases is so similar, it could not always be conclusively established which one of the two phases corroded selectively. Analyses performed on the austenite and ferrite phases did not indicate a concentration difference within the phases itself. However, there were significant differences in the concentration of elements between the phases, with the austenite stabilising elements reporting to the austenite phase and the ferrite stabilizing elements reporting to the ferrite phase, in line with thermodynamic predictions. In the case of the filler materials following the austenite mode of solidification, no significant concentration differences were detected within the matrix. Although all highly alloyed high nickel alloyed filler materials (types 904L, 825 and 625) corroded at a lower rate than the type 316L base material, type 625 filler material was the filler material of choice due to the lack of any pitting of the weld. Pitting was detected in both the 825 and 904L filler materials. Galvanic corrosion was not noted at any of the weld/HAZ interfaces and in no case did the type 316L parent metal adjacent to the weld corrode preferentially to the material further away from the weld. Copyright / Dissertation (MEng)--University of Pretoria, 2010. / Materials Science and Metallurgical Engineering / unrestricted