The development of high strength steel enables constructions to be made with less material than with previous used steel grades. The possibility of creating lightweight constructions with the same or even better mechanical properties is of high focus in the steel industry today. This can reduce productional costs as well as transportation related costs and have a less negative environmental impact. As steels are developed to be more durable, the processing of these steels grow more challenging and the use of conventional welding methods such as MIG/MAG-welding prove difficult (impossible for some steel grades) to obtain acceptable quality with single pass welds. The interest and use of laser-arc hybrid welding has increased during the last decade for its high welding speed and deep weld penetration. There are however still areas of the process that are not well documented. In this master thesis project, joining of laser cut high strength steel sheets with laser-arc hybrid welding has been tested to evaluate weld quality and process stability. The tests was performed on threesteel grades with increasing strength (S960QL, S1100QL and S1300QL). Steel sheets with different pre-welding preparations has been laser-arc hybrid welded and a documentation displaying resulting differences in weld geometry, microstructure, process stability and hardness has been created. For example, samples welded withthe mill scale still present was found not to have negative impact in terms of weld geometry and hardness when using laser-arc hybrid welding. Cooling rates was collected to aid the evaluation of microstructural composition in the welded area. The results of the work in this thesis project can be of aid in making the joining process of high strength steels more efficient.

Orthotropic steel deck has been widely used over the decades especially on long-span bridges due to its light weight and fast construction. However fatigue cracking problems on the welds have been observed in many countries. Rib-to-deck welds need special care since they are directly under wheel loads, which cause large local stress variations and stress reversals. Currently the only requirement by AASHTO bridge code is that the rib-to-deck welds need to be fabricated as one-sided partial penetration welds with minimum penetration of 80% into the rib wall thickness. However considering the thin rib plate thickness, it is very difficult to achieve this penetration without a "melt-through" or "blow-through" defect. Large cost has been caused for the repair. However recent research has found that the fatigue performance of the rib-to-deck weld is not directly related to its penetration. Other factors contribute to the fatigue performance as well. Therefore, alternative requirements which are more cost-effective and rational are desired. The objective of this research is to provide recommendations to the design and fabrication of rib-to-deck welds by investigating their fatigue performance with different weld dimensions, penetrations, and welding processes. Fatigue tests were performed to 95 full-scale single-rib deck segments in 8 specimen series fabricated with different welding processes and root gap openness. Specimens were tested under cyclic loads till failure. Three failure modes were observed on both weld toes and the weld root. Test results showed that the fatigue performance was more affected by other factors such as failure mode, R-ratio and root gap openness, rather than the weld penetration. The failure cycles were recorded for the following S-N curve analysis. Finite element analysis was performed to determine the stress state on the fatigue cracking locations. Special considerations were made for the application of hot-spot stress methodology, which post-processes the FEA results to calculate the stress values at cracking locations with the structural configuration taken into account. The hot-spot stress range values were derived and adjusted accounting for the fabrication and test error. Hot-spot S-N curves were established for each specimen series. Statistical analyses were performed to study in depth the effect of weld dimensions and test scenarios. Multiple linear regression (MLR) was performed to investigate the effects of different weld dimensions; and multi-way analysis of covariance (Multi-way ANCOVA) for the effects of specimen series, failure mode, R-ratio and weld root gap. It was found that the weld toe size was more relevant to the fatigue performance, other than the weld penetration. The failure mode and R-ratio were very influential on the fatigue performance. Recommendations to the weld geometry were proposed based on the MLR model fitting. S-N data were re-categorized based on ANCOVA results and the lower-bound S-N curve was established. AASHTO C curve was recommended for the deck design. / Master of Science

<p>Duplex stainless steels are a very attractive alternative to austenitic grades due to their higher strength and good corrosion performance. The austenitic grades can often be welded autogenously, while the duplex grades normally require addition of filler metal. This is to counteract segregation of important alloying elements and to give sufficient austenite formation to prevent precipitation of chromium nitrides that could have a negative effect on impact toughness and pitting resistance. The corrosion performance of the recently-developed lean duplex stainless steel LDX 2101 is higher than that of 304 and can reach the level of 316. This thesis summarises pitting resistance tests performed on laser and gas tungsten arc (GTA) welded LDX 2101. It is shown here that this material can be autogenously welded, but additions of filler metal, nitrogen in the shielding gas and use of hybrid methods increases the austenite formation and the pitting resistance by further suppressing formation of chromium nitride precipitates in the weld metal. If the weld metal austenite formation is sufficient, the chromium nitride precipitates in the heat-affected zone (HAZ) could cause local pitting, however, this was not seen in this work. Instead, pitting occurred 1–3 mm from the fusion line, in the parent metal rather than in the high temperature HAZ (HTHAZ). This is suggested here to be controlled by the heat tint, and the effect of residual weld oxides on the pitting resistance is studied. The composition and the thickness of weld oxide formed on LDX 2101 and 2304 were determined using X-ray photoelectron spectroscopy (XPS). The heat tint on these lean duplex grades proved to contain significantly more manganese than what has been reported for standard austenitic stainless steels in the 300 series. A new approach on heat tint formation is consequently presented. Evaporation of material from the weld metal and subsequent deposition on the weld oxide are suggested to contribute to weld oxide formation. This is supported by element loss in LDX 2101 weld metal, and nitrogen additions to the GTA shielding gas further increase the evaporation.</p><p> </p>

This study addresses the fatigue life of the head–to–skirt welds of tall distillation columns. Fatigue tests were done on two types of weld geometries which approximate the head–toskirt configurations. From the fatigue tests it was determined that the fatigue life of the experimental samples can be substantially improved by applying weld build–up between the head and the skirt. The expected fatigue life of the test samples was determined by way of calculation employing the so called Nominal–Stress–Approach, the Effective–Notch–Stress–Approach and the Stress–Life–Approach. For both the Nominal–Stress–Approach and the Effective–Notch–Stress–Approach the predicted fatigue life was found to be overly conservative compared to the experimental results. The Stress–Life–Approach predicted the fatigue life to within a factor of 1.3 for both the geometries under investigation when displacements due to welding are taken into account. If displacements due to welding is omitted this factor is increased, for the geometry without weld build–up, to 2. For the geometry with weld build–up the factor remains 1.3. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2011.

This study addresses the fatigue life of the head–to–skirt welds of tall distillation columns. Fatigue tests were done on two types of weld geometries which approximate the head–toskirt configurations. From the fatigue tests it was determined that the fatigue life of the experimental samples can be substantially improved by applying weld build–up between the head and the skirt. The expected fatigue life of the test samples was determined by way of calculation employing the so called Nominal–Stress–Approach, the Effective–Notch–Stress–Approach and the Stress–Life–Approach. For both the Nominal–Stress–Approach and the Effective–Notch–Stress–Approach the predicted fatigue life was found to be overly conservative compared to the experimental results. The Stress–Life–Approach predicted the fatigue life to within a factor of 1.3 for both the geometries under investigation when displacements due to welding are taken into account. If displacements due to welding is omitted this factor is increased, for the geometry without weld build–up, to 2. For the geometry with weld build–up the factor remains 1.3. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2011.

Duplex stainless steels are a very attractive alternative to austenitic grades due to their higher strength and good corrosion performance. The austenitic grades can often be welded autogenously, while the duplex grades normally require addition of filler metal. This is to counteract segregation of important alloying elements and to give sufficient austenite formation to prevent precipitation of chromium nitrides that could have a negative effect on impact toughness and pitting resistance. The corrosion performance of the recently-developed lean duplex stainless steel LDX 2101 is higher than that of 304 and can reach the level of 316. This thesis summarises pitting resistance tests performed on laser and gas tungsten arc (GTA) welded LDX 2101. It is shown here that this material can be autogenously welded, but additions of filler metal, nitrogen in the shielding gas and use of hybrid methods increases the austenite formation and the pitting resistance by further suppressing formation of chromium nitride precipitates in the weld metal. If the weld metal austenite formation is sufficient, the chromium nitride precipitates in the heat-affected zone (HAZ) could cause local pitting, however, this was not seen in this work. Instead, pitting occurred 1–3 mm from the fusion line, in the parent metal rather than in the high temperature HAZ (HTHAZ). This is suggested here to be controlled by the heat tint, and the effect of residual weld oxides on the pitting resistance is studied. The composition and the thickness of weld oxide formed on LDX 2101 and 2304 were determined using X-ray photoelectron spectroscopy (XPS). The heat tint on these lean duplex grades proved to contain significantly more manganese than what has been reported for standard austenitic stainless steels in the 300 series. A new approach on heat tint formation is consequently presented. Evaporation of material from the weld metal and subsequent deposition on the weld oxide are suggested to contribute to weld oxide formation. This is supported by element loss in LDX 2101 weld metal, and nitrogen additions to the GTA shielding gas further increase the evaporation. / QC 20101126

With the growth in pipeline installations all over the world, there is a great demand for highly productive and robust welding systems. Mechanised pipe welding has been developed over the last 50 years and the present focus is towards development of automated pipeline welding systems. Pipeline welding automation is aimed at reducing costs and improving the installation quality. To attain fully automated pipe welding systems there is a need to rely on sensors and controls systems to mimic human like capabilities, such as visual inspection, in real time. The key aim of this work is to develop and evaluate methods of automatic assessment of weld bead shape and quality during narrow gap GMAW of transmission pipelines. This implies that the measured bead profile will be assessed to determine whether the bead shape will cause defects when the subsequent pass is deposited. Different approaches have been used to conquer the challenge that is emulating human reasoning, all with different objectives in mind. In spite of extensive literature research performed, very little information was found concerning the real time determination and assessment of bead shape quality and none of it was reported to be applied successfully to the pipeline industry. Despite the continuous development of laboratory laser vision systems commercial ones have been on the market for decades, some specifically developed for the welding application. Laser vision sensor systems provide surface profile information, and are the only sensors which can satisfactorily measure bead profile on a narrow groove. In order to be able to use them to automatically assess weld bead shape and quality, a deep understanding of their characteristics and limitations needs to be achieved. Once that knowledge was attained it was then applied to determine the best sensor configuration for this purpose. After that the development of human like judgment algorithms were developed to accomplish the aim that was set. Empirical rules were obtained from an experienced welder regarding the acceptability of bead shapes and were then applied in the developed system with good results. To scientifically evaluate and determine the rules to use in this system, further experiments would be required. The output of the system developed showed very accurate, reliable and consistent results that were true to the external measurements and comparisons performed. The developed system has numerous applications in the pipeline industry and it could easily be implemented on commercial systems.

In this thesis, new methods for shear strength are proposed and backed up through extensive experimentation, ABAQUS models and data analysis of Titanium welds of three different alloys. The results are compared with those obtained by using the procedure outlined by AWS B4 for calculating Shear Strength in the transverse and longitudinal directions; this equation is widely used by the American Welding Society (AWS) and all those in search of more efficient designs involving welding. It is a well-documented issue that the equation provided by AWS yields a large discrepancy between the values for shear strength of longitudinal and transverse welds.

Student Number : 9811923K - PhD thesis - School of Mechanical Engineering - Faculty of Engineering and the Built Environment / Methods are presented for obtaining models used for predicting welded sample resistance and effective weld current (RMS) for desired weld diameter (weld quality) in the resistance spot welding process. These models were used to design predictive controllers for the welding process. A suitable process model forms an important step in the development and design of process controllers for achieving good weld quality with good reproducibility. Effective current, dynamic resistance and applied electrode force are identified as important input parameters necessary to predict the output weld diameter. These input parameters are used for the process model and design of a predictive controller. A three parameter empirical model with dependent and independent variables was used for curve fitting the nonlinear halfwave dynamic resistance. The estimates of the parameters were used to develop charts for determining overall resistance of samples for any desired weld diameter. Estimating resistance for samples welded in the machines from which dataset obtained were used to plot the chart yielded accurate results. However using these charts to estimate sample resistance for new and unknown machines yielded high estimation error. To improve the prediction accuracy the same set of data generated from the model were used to train four different neural network types. These were the Generalised Feed Forward (GFF) neural network, Multilayer Perceptron (MLP) network, Radial Basis Function (RBF) and Recurrent neural network (RNN). Of the four network types trained, the MLP had the least mean square error for training and cross validation of 0.00037 and 0.00039 respectively with linear correlation coefficient in testing of 0.999 and maximum estimation error range from 0.1% to 3%. A prediction accuracy of about 97% to 99.9%. This model was selected for the design and implementation of the controller for predicting overall sample resistance. Using this predicted overall sample resistance, and applied electrode force, a second model was developed for predicting required effective weld current for any desired weld diameter. The prediction accuracy of this model was in the range of 94% to 99%. The neural network predictive controller was designed using the MLP neural network models. The controller outputs effective current for any desired weld diameter and is observed to track the desired output accurately with same prediction accuracy of the model used which was about 94% to 99%. The controller works by utilizing the neural network output embedded in Microsoft Excel as a digital link library and is able to generate outputs for given inputs on activating the process by the push of a command button.

The demand for high energy battery assemblies is growing in sectors such as transportation. Alongwith it is the need for reliable, efficient and cost-effective ways to electrically connect the batteries toensure their performance. Battery cells are most often put into modules or packs when produced forelectrically driven vehicles. The variable of greatest influence when welding battery packs is thecontact resistance between the cell and the connection tab. It is crucial to minimize this variable asmuch as possible to prevent energy loss in the form of heat generation. The purpose of this project is to conduct a comparative literature study of different weldingtechniques for welding batteries. The compared techniques are resistance spot welding, laser beamwelding and ultrasonic welding. The performance was evaluated in terms of numerous factors such asproduction cost, degree of automation and weld quality. All three methods are tried and proven to function in the production of battery applications. Eachmethod has separate strengths and limitations which makes them complement each other. Thus, it isimportant to look at several factors when deciding which welding technique is the most suitable forthe desired application. The scale of production, economical aspects as well as battery cell geometrywere concluded to be the most important in making this decision.