A tube rolling process throughout which the so called "polygonisation" appears has been analyzed first experimentally and second numerically. Twelve stand stretch reducing mills were chosen and a series of experiments was carried out. Following those results, some laboratory tests were carried out using a draw bench. The results show that eccentricity appears when there is a change in the mean wall thickness. When "polygonisation" appears the middle part of the tube between the flange and groove sides becomes thickner than the other parts of the tube. The rigid-plastic finite element method was adopted, to analyze the process numerically. Some new techniques have been developed, e.g. a scheme for dealing with free surfaces and a method for dealing with three dimensional friction. After checking the validity of the computer programs, both two dimensional and three dimensional analyses have been carried out. It has been demonstrated that the two dimensional analysis is not entirely acceptable. As a result of the three dimensional analysis, some new information has been obtained, e.g. the three dimensional metal flow in the roll bite and the effect of friction on the eccentricity and on the front and back tensions. The final results indicate that eccentricity can be analyzed by the present computer program, therefore "polygonisation" can be analyzed and understood. The basic phenomenon of "polygonisation" has been understood to some extent.

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No description available.

With the widespread use of zinc coated steels in the manufacture of high volume spot welded assemblies, such as the automotive body-in-white, there is a need to address the inherent difficulties in welding this particular product. The presence of the zinc coating increases the rate of degradation of the welding electrodes, and so there is a need for frequent electrode maintenance to combat the deterioration in weld quality associated with electrode wear. This results in short production runs and reduced productivity. Pre-programmed current stepping of the welding current may be used to compensate for the reduction in weld size with electrode wear, and so extend electrode life. However, this open-loop technique is difficult to optimise, particularly when welding zinc coated steel. In order to develop a feedback control system for current stepping, it is necessary to relate the weld diameter to some measurable parameter, in order to perform continuous monitoring of the weld quality. In view of the difficulty of deriving a suitable mathematical description of the physical process, on which to base a control algorithm, alternative techniques for spot weld quality monitoring and control have been examined. A neural network based model of the spot welding process has been produced, to predict weld quality from the measured electrical data. Guidelines have been developed for selecting the ideal network parameters for maximising the prediction performance over the life of the welding electrodes. In order to overcome the difficulties observed in optimising the pre-programmed current stepping control system, the feasibility of using a rule based fuzzy logic controller has been assessed. Rules were defined for determining the required step change in welding current to maintain weld quality, given the diameter of the previous weld and the estimated electrode tip diameter.

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Several AA-5083 welds were fabricated using the mechanical MIG welding process with different welding parameters and two different filler wires. The investigation focuses on the effect of welding variables such as current, welding speed, and heat input (linear energy) on mechanical properties and weld metal porosity in AA 5083 weldments. Image analyses was used to determine the percentage of weld porosity. Results shows that the weld porosity observed in all welds varies with welding parameters employed. The percentage of porosity decreases with increasing welding current range from 250 to 280 A, as it does as the welding speed increases from 8.33 to 11.24 mm/s. However, porosity increased with increasing heat input from 596 to 703 J/mm. Moreover, the percentage of porosity in welds with the ER5087 filler wire was considerably lower than that of welds with the ER55565A filler wire. All weldments had the UTS, the yield strength and elongation in the range of weld mechanical properties standard according to EN and ASTM Standard. The porosity appeared to have a much reduced effect on the strength of weldments. Analysis of the hardness profiles across the weld interface was employed for both welds using ER5556A and ER 5087 filler wire. The hardness values vary with welding parameters employed. The trend shows that increasing arc power input (Q=EI) by increasing welding current, produces increased erratic hardness distribution. Some welds with a high arc power input (Q) exhibit very low hardness (down to 30% of hardness of base metal) in a very narrow area, less than 1 mm width immediately adjacent to the weld interface. The fracture location for weldments with bead-on plate varied between the fusion line and the base metal. However, the fracture location for weldments with bead-off plate was generally at the weld zone. SEM photographs shows that the base metal and the HAZ fracture are ductile whilst the weld metal and the fusion line fracture are the mixed rupture between ductile and brittle. The optimum mechanised MIG welding parameters having a low net heat input and a low porosity for 5 mm thick AA5083 butt-welds using ER5556A filler wire comprised a welding current of 270 amperes, a welding speed of 10.5 mm/s and a net heat input 603 J/mm. For weldments using ER5087 filler wire, the optimum welding parameters consisted of a welding current of 271 amperes, a welding speed of 10.7 mm/s and a net heat input 609 J/mm.

The weld inspection system developed in this work is based on the principle of optical profiling. Optical profiling is a method for visualising the surface profile of an object while it is being viewed by an imaging source which is perpendicular to its surface. The signal to be viewed by the imaging source is provided by a line of laser light. The laser light is angled relative to the weld surface it is incident upon. The distortion in the laser line gives an indication of weld surface quality. A traditional imaging source for such systems has been the CCD camera. This has been replaced with a novel CMOS camera. The two main benefits a CMOS camera has over standard CCD cameras are (1) Random Pixel Access (2) Logarithmic Response to Light. Taking advantage of the random pixel access capabilities of the CMOS camera procedures were developed to extract the information relevant to weld analysis from minimal sections of the full 2-D image. Using the grey scale signals (logarithmic scale) from just a few single rows or lines within the 2-D image of the projected laser line, it was possible to (a) determine weld edges using wavelet analysis (b) achieve weld surface classification using multivariate analysis. These procedures have been implemented in laboratory set ups, but were also tested successfully online during laser welding.

It has been found that growth of the electrode tip is the dominant electrode degradation process controlling electrode life. Three distinct mechanisms then contribute to overall growth of the electrode tip. The relative contribution of these mechanisms is dependent on coating type and welding process details. In general, electrode tip growth rate is governed by the current and force density at the electrode contact face, but is independent of electrode material. Alloying between the electrode contact face and the coating is considered to be a secondary factor influencing electrode life. Techniques have been developed for measuring the change in resistance during weld formation at the electrode/sheet and sheet/sheet interfaces. The electrode/sheet interfacial resistance has been found to be significantly greater than that measured at the sheet/sheet interface. It has been established that the electrode degradation processes affect these resistance values. In particular, the resistance across the welding electrodes is inversely related to electrode tip contact area. Improvements in electrode life are thus possible through the use of current stepping techniques. In the present investigation, a number of criteria for establishing optimum rates of current increase have been evaluated. The most significant improvements in life were obtained using a rate of current increase based on the rate of electrode tip growth. An iterative technique has been derived for establishing an optimum non-linear current stepping programme. The implications of the results to manufacturing procedures have been considered, including the recommendation of preferred electrode geometry, practices for electrode dressing and current stepping. These practical considerations should lead to a more efficient mode of operation in modern manufacturing plants operating at high production rates.

The finite element simulation of filling thin section cavities has been studied in the present research, with an emphasis on the casting application. This thesis describes the development of appropriate governing equations for filling thin section cavities and finite element analysing software used to simulate some selected filling examples as case studies. As the transverse velocity profile across a thin section can be approximated between the parabolic and plain distributions, the flow throughout the cavity can be represented by in-plane velocity components and simulated using a quasi three-dimensional formulation. By applying an appropriate velocity profile and accounting for any thickness variation, the governing filling formulation was derived using the conservation laws of mass, momentum and free surface, which produced the thickness-integrated continuity, Navier-Stokes and pseudo-concentration equations, respectively. Appropriate transformations were incorporated into the thickness-integrated Navier-Stokes equations to accommodate the three-dimensional form of the cavity. Since surface tension also affects the flow in thinner sections, a study on the inclusion of this topic was also included. The finite element flow formulation was constructed by discretising the governing equations spatially and temporally using the conventional Galerkin method and the implicit backward difference method, respectively, and was solved via a mixed formulation. This was found to be the best approach for the cavity having thickness changes. For free surface tracking, the explicit Taylor-Galerkin method was used to discretise the pseudo-concentration equation since it gave more accurate results. This filling simulation model was applied to three case studies, where the numerical results were compared with the experimental data for the benchmark gravity sand casting as gathered by other researchers, a cylindrical shaped cavity in the laboratory and a high pressure die casting in the foundry. Good agreement was obtained in each case.

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