CMT+P welding is less susceptible to hot cracking than the MAG welding process due to use of low heat input properties. Solidification cracking was found in all weld specimens that had greater 0.39KJ/mm heat inputs. Cracking occurs because of the contraction stresses generates during cooling. Hydrogen cracking is found in HAZ with low heat input parameters, this type of cracking occurred because of very rapid cooling and therefore not enough time to allow the hydrogen to dissipate from the specimen. To eliminate this type of cracking the experiment could be repeated by adding heating during welding to control and reduce the cooling rate. Due to high carbon content in the steel and very fast cooling the microstructure of the weld is martensitic in the base metal as well as the HAZ. Microstructure in the weld and base metal is martensitic due to high carbon con-tent and rapid cooling. At low heat inputs dilution is less and therefore lower carbon content in weld pool. Better weld appearance and weld quality is achieved with CMT+P welding process than the conventional GMA welding processes because of the new wire movement technology during welding

The structural integrity of a welded structure depends mainly on the performance of the welded joints. Due to the welding process, the mechanical properties of the structure change and different regions are created in the weldment. The mechanical properties of welded joints change significantly around the heat affected zone (HAZ). So to predict stress distribution around the weld, these changes should be considered in the finite element model (FEM) of the welded structure. In this research, the changes of mechanical properties around the welded joint were experimentally tested and used to develop a FEM model of a welded joint which can predict the stress behavior around the weld. First, an experimental analysis was carried out on an ASTM standard arc welded joint of stainless steel specimen to observe the microstructural change in the HAZ. This enables to find out the HAZ width using an optical microscope. Moreover, a tensile testing was performed to investigate the change of Young’s modulus of the HAZ compared to the base metal (BM). Another experimental analysis was also performed on a real arc welded structure of the same material to observe its’ strain distribution around the HAZ. The HAZ width and Young’s modulus obtained from the experimental testing were then applied to generate the FEM model of an ASTM standard arc welded joint as well as a real arc welded structure of stainless steel. The finite element analysis (FEA) results of stress distribution around the weld joint in both cases show a good agreement with the experimental results. Therefore, the developed material property based FEM model can predict the stress behavior of similar type of structures with the same welding process on the same material studied in this research. / February 2017

Approved for public release, distribution is unlimited / Damping of welded structures is a subject of great interest and application for the navy as relates to ship shock survivability and acoustic transmission of ship noise. The purpose of this research is to study the effects of welding on damping. A generic model of a warship's hull structure was used to study damping effects. The model's natural frequencies and mode shapes were calculated using a finite element model prior to model testing. The frequency response and natural frequencies of the model were determined experimentally by exciting the model and measuring the response throughout the structure using Frequency Response Functions (FRF's). The results were compared with the finite element modeling. The damping ratio of the model in relation to position from excitation was calculated using the half-power point method and then a more detailed analysis of frequency dependent damping versus position was made using modal parameter extraction using the Complex Exponential Method. / Lieutenant, United States Navy

The feasibility of fully automatic GMAW processes may rely on the development of sophisticated equipment to emulate the manual welding torch oscillation pattern or on the development of high level methods of control to prevent the appearance of defects, especially the lack of sidewall fusion. An intermediate solution is to optimise the weaving parameters of a conventional pattern oscillator in such a way as to minimise the level of rejection. A prototype of a computerised system to work with Pulsed-GMAW equipment, in the vertical-up position, was proposed to produce a minimal level of rejection for welds in plates up to 25 mm thick. The system basically consists of optimised mode control algorithms, based on theoretical and experimental models of weld pool behaviour. Three tasks are performed by the system; the selection of parameters for an optimum working point, an off-line simulation of the operation and real-time error monitoring of the process. Statistical experimental modelling was applied in order to build most of the optimised models, because of the large number of variables to be treated and their complex inter-correlation. The welding variables were correlated with single responses. Partial and Correlation Analysis techniques were used to discover the relationship between the variables and the responses. Regression Analysis was then applied as a means of obtaining the 'weight' of the most significant variables. Finally, since some variables were found to be collinear, a corrective technique for biased variables was employed. Acceptance criteria for bead shapes were proposed and assessed. The effect of the oscillation parameters and other welding variables on the bead formation was analyzed and an operational 'envelope' for the parameters determined. A theoretical approach to predict the occurrence of poorly shaped beads, due to the lack of metal bridge between the joint walls, was successfully developed and applied in parallel with the statistical experimental methods. Equations for optimising the bead shape and for determining the operational envelope contours were subsequently generated and evaluated. An extension of the system to an actual adaptive control scheme was discussed and sensors and signals to be used were evaluated. Finally, a process instability phenomenon in long test plates was identified and investigated. This instability may prevent the use of GMA W in some conditions in the vertical-up position.

Argon TIG arc temperatures have been measured for a wide range of arc parameters using the 'Fowler-Milne' spectroscopic method. Prompted by widespread disagreement amongst temperatures published by previous groups, a detailed investigation has been carried out into those experimental and theoretical aspects of the measurement process that may have led to incorrect results. The tests have included the variation of experimental parameters, the choice of Abel inversion procedure, and the calculation of argon species number densities and partition functions. The existence of equilibrium within the TIG arc has been investigated by determining temperatures with a number of argon emission lines. Significant differences 'in derived values of the temperatures were observed within 1mm of the tungsten cathode tip, smaller variations were observed over the remainder of the arc. The results indicate that large scale departures from a Boltzmann distribution amongst argon atom excited states exists close to the tungsten tip, deviations decrease with increasing distance from the tip but do not disappear completely. lt is believed that the breakdown of equilibrium within the argon TIG arc may help to explain the disagreement in temperatures obtained by past groups that have used spectroscopic methods. Despite evidence for non-equilibrium within the arc. results from previous groups suggest that derived values of .the temperatures are substantially correct in the main body of the arc. The detailed temperature maps provide useful information on temperature changes with arc parameters for the purposes of modelling and understanding of the arc.

The main objective of the project was to conceptually design a friction stir welding machine for joining rails. The applicability of friction stir welding types and its application in rail joining was investigated. A number of machine concepts for joining rail using friction stir welding techniques were developed and a final workable concept was laid out. In addition, the existing methods and machines for joining rails were considered, including arc welding, exothermic welding, flash butt welding and manual joining (rails joined by means of splice plate). After comparing different methods of joining rails, an optimized method was selected. The capabilities of the new conceptual machine, such as its ability to accommodate various rail profiles, were demonstrated through designs and various calculations. The development cost analysis was performed and a comparison was made with the other three methods of joining rails. Consequently, it was concluded that friction stir welding concept could be applied in rail joining and the costs associated with it could be lowered

A survey has been made of the developments of welding processes and the literature concerned with the fatigue resistance of butt welded joints. Evident from the literature survey was the fact that a mass of experimental data was available on the fatigue properties of welds yet many fatigue failures were still being reported. It appears essential therefore that the design data derived from experimental results should be presented in a meaningful way to aid the safe design of welded connections. Fatigue tests were conducted on specimens cut from 1/2 in. thick plate of medium carbon steel, over a range of alternating and mean stress levels. The welds were produced using manual metal arc, gas metal arc and electron beam welding processes and manufactured in industry by accepted production methods. A non destructive examination together with static tests were carried out on both the parent material and the welded joints. The results obtained have enabled a comparison to be made between the data obtained from static, dynamic and non destructive tests. The analysis shows some evidence of correlation between the information derived from the static and non destructive tests with that of fatigue performance. A method of analysing fatigue data, based on the method of least squares, has been developed. This provides good fits to data when used in conjunction with modified versions by Goodman and Gerber of the basic Jefferson empirical fatigue equation. The fatigue curves generated by this method can be described by two equation constants which can subsequently be used to generate a family of fatigue curves over a range of mean loads. The method of analysis is also capable of calculating confidence limits on experimental data for both stress and fatigue life for the predicted fatigue curve.

Capacitor discharge welding (CDW) is a rapid solidification joining process under the influence of one-dimensional thermal gradients. Although CDW is useful for joining small parts and dissimilar metals, CD welded joints have a large variability in weld strength. CDW is not widely accepted because of a lack of automated process control. Studying the sources of variability in the CDW process can guide the automation of CDW. Therefore, the objectives of this study were to investigate sources of variability that affect weld strength and to generate a model to predict the weld strength variability in CDW. The source of variability was investigated by using screening experiments. Four different materials, stainless steel, Nitronic 50 Steel, copper, and low oxygen copper (C101), were selected to represent various levels of thermal conductivity and absorbed gas content. Thermal conductivity, percentage of gas content absorbed, diameter, and welding time were treated as the independent variables while the dependent variables were the standard deviation of CD weld strength as a percentage of base material strength and the mean of CD weld strength as a percentage of base material strength. A screening experiment and a statistical analysis of the data were used to develop a predictive model of the weld strength variability in CDW. Electron photomicrographs of weld fracture surfaces and dynamic current and resistance curves for each welding cycle were used to support conclusions from the statistical analysis. Conclusions of this study are that thermal conductivity and absorbed gas content do have a significant influence on weld strength variability in CDW. / Graduation date: 1999

Materials which can be changed in shape and size when exposed to a magnetic field are called magnetostrictive materials. The magnetostrictive material was developed for use in the many actuator industries. A single crystal of magnetostrictive material is used to maximize its magnetostrictive response. It can be produced by the crystal growth method. However, a single crystal from this method is limited in size and shape. The growth direction of a single crystal, sometimes, is different from its magnetostrictive axis. Capacitor discharge welding (CDW) is a high power density welding process at 10���-10��� Kelvin/second. The fusion zone of CDW is very small and the heat affected zone of CDW is rarely detectable. The amount of material affected by heat in the welding process is very small. The objective of this study is to use capacitor discharge welding (CDW) to join single crystal copper and determine the effect of CDW on the microstructure of the single crystal copper. To minimize the cost of using single crystal copper, low oxygen copper C101 (polycrystalline) is used as a replacement. By maximizing the weld strength, Response Surface Methodology (RSM) is used to find the optimum condition of single crystal copper. The single crystal copper was cut at different orientations and welded together with the one that had same orientation to compare the results. The metallography was used to study voids, fusion zone and dislocations of the single crystal copper after welding. After analyzing the effect of the CDW process on the single crystal, the effect of crystal orientation upon void formation was found statistically inconclusive. Small fusion zone as 0.09154 mm was found. No heat affected zone of single crystal was detected. Also, no microstructural damage was found along the fusion zone. Dislocation density before and after CDW were not different because there was no statistically significant difference between the etch pit density in the fusion zone and that outside the fusion zone. In summary, regardless of voids, CDW may be a useful method for welding single crystal metals. / Graduation date: 1999

Automation of Capacitor Discharge Welding (CDW) has not been investigated in industry due to the limited understanding of the process. The US Department of Energy's Albany Research Center (ALRC) has done an extensive study of the process and reported applications of welds in the manufacture of electro-mechanical products such as relays and circuit-breakers. The main purpose of this work was to investigate automation of the load/unload cycle of CDW. An automatic CDW machine was built with real-time quality monitoring capabilities. Tooling and fixtures were fabricated for joining silver to brass components used in a circuit breaker assembly. A machine controller was used to automatically sequence the process and the actuators. Computer-based instrumentation was developed for on-line data acquisition and quality monitoring of the weld. This demonstrational setup shows that the equipment can be compact and the process can be easily automated for high production rates. The automation architecture developed can be used as a general model for automating CDW operations in other environments. The automated CDW system was tested by welding silver contacts to brass brackets at different parameter settings. Metallographic examination of the resultant capacitor discharge (CD) welds (made by the automated CDW system) revealed the formation of continuous metallurgical bonds between silver and brass. A comparison of CD welds made by the manual and the automated system was performed to determine the effect of CDW automation on weld quality. The welds were also found to be superior to resistance welds. / Graduation date: 1997