The present work researches influence of energy focusing effect to the fusion zone defects in the electron beam welding, which include fusion zone shape, welding depth, welding width, rippling and spiking. And to verify the model of welding defects base on theory and experiment. A spike is a sudden increase in penetration beyond what might be called the average penetration line. Many spikes have voids and cold shuts in their lower portions, because molten metal does not fill in completely, producing a condition similar to a cold shut in a casting. Ripples on a workpiece surface are generally associated with segregation, porosity and other microstructure defects. These become the objectives in this project.

In this study, the capacitance displacement system (CDS) is employed to measure the post-weld-shift (PWS) induced by laser welding in butterfly type laser module package. The advantage of CDS is able to simultaneously and immediately measure the direction and the magnitude of PWS. Furthermore, with the aid of notch clip, the PWS can efficiently and quantitatively be compensated by laser hammering technique to regain the coupling power. Reduction of the PWS is an important issue in developing low-cost and high-performance laser modules. The package yield of laser modules can be imp roved due to the real-time measurement and quantitative compensation. In comparison with the high-magnification camera with image capturing system (HMCICS) having 0.7£gm resolution, the capacitance sensor achieves 25.4nm and 0.1£gm in its resolution and accuracy, respectively. Besides, during the package procedure, the real-time displacement detection can be used to adjust the package parameters. As a result, the PWS is reduced that contributes to less coupling power loss. After welding, the result reveals that the PWS was measured as X=0.15£gm and Y=-4.58£gm, while the coupling power is 43.19%.

Using ultrasonic guided waves improve a problem of time-consuming and laborious with conventional ultrasonic method involving point¡Vby-point inspection. In order to measure of hundreds meters of pipeline in oil and chemical industries, guided waves inspection technique is used and developed widely. For fast and long range inspection, a pulse-echo system is used to excite Lamb waves propagating along the pipe. The collection and analysis of the returning echoes indicate the present of corrosion. But if the pipe features, like bends or supports, are corrosive, the signal of corrosions is often covered with echoes of pipe features. Then it causes the inspection more difficult. In this study, the pipe feature which discussed is the welded support. The focus of the advanced analysis, Discrete Wavelet Transform, of the echoes reflected from the welded support on a 3 inch pipe by experimental and finite element method, so as to improve the ability of corrosion inspection on pipe features. To study the feasibility of the improvement in the effect of coherent noise for guided wave inspection by Wavelet analysis. Results show that original signals can not be differentiated by comparing with signals of normal support, general corrosive support, and support with notch. But after processing these three signals by Wavelet analysis, the situation of the 3 type supports from the signals can be differentiated. The simulated results of two different models and five exciting frequencies show the similar trend to the experimental results after Wavelet transform processing. It is success on separating the signals of normal support and corrosive support with Wavelet Analysis, and this method of this study is useful to improve the effect of coherency noise for guided wave inspection.

Tailor welded blanks (TWBs) are used in the automotive industries as a method to meet economic, environmental and governmental demands. Conventionally, TWBs incorporated mild and low strength steels such as interstitial free and draw quality steels because of their excellent formability traits. However, due to their low strength they are unsuitable for energy absorption applications; thus, the interest of incorporating advanced high strength steels (AHSS) into the TWBs. Dual phase (DP) steel is a type of AHSS that is of interest because of its combination of high strength and good formability that is comparable to high strength low alloy (HSLA) steels. However, welding DP steel causes softening in the heat affected zone (HAZ), which leads to premature failure and reduces formability. The aim of this thesis was to study the effect of weld design on the formability of TWBs with DP steels and with HSLA steel. This thesis is divided into three parts; the first part examines TWBs with different weld line positions, weld line orientations and strain paths. The second part investigates bead-on plate curvilinear blanks and its effect on formability of the blanks. The last part examines the effects of multiple welds on the formability of TWBs.

Existing steel bridges are subjected to both increasing traffic loads and natural aging, both are capable of causing severe durability problems. Dependable rehabilitation methods are attracting attention as the promising methods to enhance structural durability and/or structural performance. One possible rehabilitation method, for improving fatigue performance, is the use of residual stress-based post-weld treatments such as peening. A number of studies has been performed and it has been proven that residual stress-based treatments are an effective way of increasing the fatigue lives of newly built steel bridges, and even enhancing the fatigue performance of existing structures. Provisions have been developed to ensure the proper execution of peening and several codes have considered its beneficial effect in the fatigue design of welded structures. Various analytical approaches are used to predict the fatigue performance of welded structures and the beneficial effects of residual stress-based post-weld treatments. In most codes and recommendations, variations of the “S-N curve” approach are employed. Linear elastic fracture mechanics (LEFM) and strain-based fracture mechanics (SBFM) are widely accepted approaches for making more precise predictions of the treatment benefit. Cohesive zone fatigue models are also recently introduced for predicting fatigue crack growth in as-received and peened welds. Despite all the research conducted, there are still two main unanswered questions related to the application of peening treatments. First, it is claimed that peening can be more effective for civil structures where a considerable portion of the total applied stress is due to permanent loads and thus peening is applied under load. However, most of research done so far has studied effects peening prior to the introduction of the structural self weight. Secondly, considering the nature of these treatments, some concerns have been raised regarding their effectiveness under actual in-service loading conditions, as most of the reported test-based studies only demonstrated the fatigue performance improvement under constant amplitude tension-only loading conditions. The current study was undertaken to examine the fatigue performance of welds peened-under load and to determine the effectiveness of peening for improving the fatigue performance of welds subjected to realistic in-service loading conditions. Moreover, a previously developed strain-based fracture mechanics (SBFM) model for predicting fatigue performance of welded details under different loading and treatment conditions, and a previously developed damage-based cohesive zone model for steel specimens were evaluated and calibrated. Fatigue tests were conducted on welded steel specimens, simulating different loading and peening conditions. Dye penetrant was used to stain cracked specimens upon detection of cracks and a crack front marking loading scheme was used to study the crack front shape. The alternating current potential drop (ACPD) method was used for continuous crack growth monitoring for both as-welded and peened specimens under different loading schemes. It was observed that cracks propagated at different rates in specimens treated under load than in the normally peened and as-welded specimens. Material tests were also conducted to determine the mechanical properties of the steel base metal. Secondary effects of peening were investigated by microhardness measurements and weld toe measurements. A number of typical weld toe defects was also detected. Residual stress measurements showed a uniformly distributed tensile residual stress near the surface of the untreated specimen. Needle peening the specimen resulted in a significant change in the residual stress distribution through the specimen thickness. In all cases, peening resulted in a significant increase in the fatigue life. However, greater fatigue life improvements were observed in lower stress ranges. Of the specimens tested under constant amplitude loading, those peened under load experienced the largest fatigue lives. For the variable amplitude loading tests, the untreated specimens had mean fatigue lives slightly less than observed in the constant amplitude tests. A previously developed strain-based fracture mechanics (SBFM) model was used to estimate analytically the effectiveness of peening applied to welded details. The model was able to predict the fatigue lives for both the as-welded and peened specimens for all loading conditions. It correctly estimated the additional benefit of peening when applied under a relatively small prestress level. The model predictions were used to estimate the additional benefit of peening under load. A previously developed cohesive zone model was introduced and applied to predict fatigue crack growth in a weld detail under cyclic loading. Fatigue tests were simulated using the finite element program ABAQUS. The material parameters α and β were chosen by iteration. Other fatigue tests were simulated and the model correctly predicted the effects of varying the applied stress range, R ratio, and residual stress level on the fatigue behaviour.

This study was undertaken with the objective of determining the effect of gap distance on the weld bead geometry and the mechanical properties of the weldments. Low-alloyed and low carbon steel plates were welded under different conditions where each weldment had different gap distance and weld bead grooves. The influences of welding parameters namely, welding speed, current and voltage on the weld bead were examined in terms of weld bead penetration and heat affected zone and weld metal zone hardness variations.

Keyhole laser welding is a joining technology characterised by the high focussed power density applied to the workpiece, facilitating deep penetration at high processing speeds. High aspect-ratio welds produced using this process invariably have narrow heat-affected-zones and minimal thermal distortion compared with traditional arc welding processes. Furthermore, the ability to process out of vacuum and the easy robotic manipulation of fibre optically delivered 1μm wavelength laser beams, allow keyhole laser welding to process geometrically complex components. The widespread uptake of keyhole laser welding for the production of titanium alloy components in the aerospace industry has been limited by the stringent weld quality requirements. Producing welds with levels of subsurface weld metal porosity content meeting the required weld quality criteria has been the primary obstacle. Here, three techniques for controlling the levels of weld metal porosity when welding titanium alloys with Nd:YAG rod lasers have been developed. Characterisation of the welding processes using high speed photography and optical spectroscopy, have allowed an original scientific understanding of the effects these methods have on the keyhole, melt pool and vapour plume behaviour. Combining this with a thorough assessment of the weld qualities produced, has enabled the effects of these process behaviours on the formation of weld metal porosity to be determined. It was found that with the correct process parameters a directed gas jet and a dual focus laser welding condition can both be used to reduce the occurrence of keyhole collapse during Nd:YAG laser welding. The directed gas jet prevents the formation of a beam attenuating vapour plume and interacts with the molten metal to produce a stable welding condition, whereas the dual focus laser welding condition reduces fluctuations in the process due to an enlarged keyhole. When applied, both techniques reduced the occurrence of porosity in the weld metal of full penetration butt welds produced in titanium alloys. A modulated Nd:YAG laser output, with the correct waveform and modulation frequency, also reduced the occurrence of porosity in the weld metal compared with welds produced with a continuous-wave output. This was a result of an oscillating wave being set-up in the melt pool which manipulated the keyhole geometry and prevented instabilities in the process being established. In addition, the potential for welding titanium alloys to the required weld quality criteria with state-of-the-art Yb-fibre lasers has been assessed. It was found that the high power densities of suitably focussed laser beams with excellent beam quality, were capable of producing low-porosity full penetration butt welds in titanium alloys without the techniques required for laser beams with a lower beam quality. These new techniques for keyhole laser welding of titanium alloys will encourage the uptake of keyhole laser welding for producing near-net-shape high-performance aerospace components. The advantages offered by this joining technology include high productivity, low heat input and easy robotic automation.

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An acousto-ultrasonic approach is used to explore the damage development in tensile shear spot welds during fatigue loading. There is reasonable data to support the hypothesis that a decrease in an AU signal is indicative of the presence of an internal crack and could be used for monitoring and evaluation purposes. / Master of Science