The sensitisation of a 12% chromium ferritic stainless steel, conforming to EN 1.4003 and available commercially in South Africa under the trade name of 3CR12, was investigated during the course of this project. 3CR12 was designed to pass through the (<font face="symbol">a</font>+<font face="symbol">g</font>) phase field on cooling, with the austenite transforming to martensite on subsequent cooling to room temperature. The aim of this investigation was to verify that 3CR12 can sensitise during continuous cooling after welding, provided that low heat input levels are used. Two grades of 3CR12 with slightly different chemistries, designated 41220 (A) and 41311 (B), were evaluated. Grade 41220 has a higher austenite potential than grade 41311. 3CR12 plate was joined autogenously to AISI 316L by means of a series of square butt welds. Gas tungsten arc welding with argon shielding was used, and the heat input was varied from approximately 30 J/mm to 450 J/mm, in conjunction with welding speeds ranging from 2.36 mm/s to 33.3 mm/s. Rosenthal’s heat flow equations were used to calculate the cooling rate from 1500ºC to 800ºC for each experimental weld, and to illustrate the influence of the effective heat input and welding speed on the martensite content of the high temperature heat-affected zone. An increase in welding speed reduces the heat input and increases the cooling rate after welding. At lower heat input levels (less than approximately 100 J/mm), austenite nucleation was found to be suppressed by the rapid cooling rates, and a continuous network of ferrite-ferrite grain boundaries formed in the high temperature heat-affected zone. Higher heat inputs resulted in slower cooling with more martensite in the high temperature heat-affected zone after cooling. At heat input levels above approximately 250 J/mm, enough martensite formed during cooling to eliminate a continuous network of ferrite-ferrite grain boundaries in the high temperature heat-affected zone. Sensitisation was evaluated using an electrolytic oxalic acid etch (ASTM 763-99, Practice W), and a potentiostatic etch in 0.5M H2SO4. During the potentiostatic etch test, the potential was maintained at 0 VSCE to reveal the presence of any chromium depleted zones. Both grades of 3CR12 were found to be sensitised when a continuous network of ferrite-ferrite grain boundaries was present in the high temperature heat-affected zone (i.e. after welding at low heat input levels). When the heat input during welding was high enough to ensure the presence of martensite on the majority of the heat-affected zone grain boundaries, thereby effectively eliminating continuous ferrite-ferrite grain boundary networks, the welds were not in the sensitised condition. The austenite that forms during cooling acts as a carbon sink, absorbing any excess carbon. This prevents supersaturation of the ferrite and subsequent carbide precipitation that can lead to chromium depletion and sensitisation. Due to its higher austenite potential, grade 41311 can be welded at lower heat input levels and with faster cooling rates than grade 41220 without inducing continuous carbide precipitation and sensitisation. In order to prevent sensitisation, a fusion-line cooling rate of 80ºC/s should not be exceeded in 3 mm 3CR12 plate during welding. / Dissertation (MSc(Applied Science))--University of Pretoria, 2007. / Materials Science and Metallurgical Engineering / unrestricted

Creep degradation of steam carrying vessels in the power generation industry is a concern that needs to be constantly monitored. The Weldcore® process has been earmarked as a potential method of creep sampling which will allow for thick-walled sections to be analysed. A component of the process involves plugging the resultant hole after removing a creep sample using a novel welding technique called friction hydro pillar processing. At the commencement of this study, insufficient data was available to warrant safe industrial application of the process. This research was conducted to evaluate the performance of 10CrMo910 friction hydro pillar process welds. The effects of downward force, stud taper angle, hole taper angle and hole base diameter on process response, defect population, static properties and dynamic performance were evaluated. The variation of downward force showed that higher forces produce significantly smaller defects and higher fatigue life. The occurrence of defects was linked to process parameters and geometry thereby identifying the correct parameters for safe use in the power generation industry. Flash formation was identified as an early indicator of weld defects and can assist with quality control in industrial applications. Methods of standardising the plunge depth and forge force were developed to identify the correct magnitudes for different geometries, without the need for testing. Defects were shown to populate specific regions of the weld and produce variations in fatigue life. Crack initiation sites were detected which will aid in identifying areas of focus in further research and development. Temperature measurements were linked to the occurrence of defects and crack initiation sites and have been identified as a method of identifying defective welds. The effects of process parameters and stud and hole taper angles on energy inputs and near interface temperatures were statistically evaluated. Downward force was shown to have the largest effect on energy input rates, total energy input and temperatures at the 11.5mm and 20.5mm positions. Smaller hole and stud taper angles produced lower energy inputs and were identified as more energy efficient than the larger taper angles. A regression model was also developed to predict the fatigue life of welds and can assist with critical process related decision making. A range of hole base diameters were identified which produced welds with low defect populations and fatigue performance similar to that of the parent plate. Larger hole base diameters were shown to produce significant defects along the hole bottom fillet, in the weld nugget and along the bond line. Temperature measurements of the larger diameter welds showed a delay in response and are attributed to a delayed contact of plasticised stud material with the sidewall. Welds with hole base diameters larger than 11mm produced unrepeatable and defective welds, and also required higher energy inputs making smaller diameters more desirable. Analysis of all welds in this study revealed that clearance and interfacial pressures characterise the quality of friction hydro pillar process welds, therefore models were developed to aid in critical decision making with respect to downward force and geometry. This study has successfully evaluated the effects of process parameters and geometry on the properties of friction hydro pillar process welds and thereby has increased understanding of the process.

A multi-input multi-output system to implement on-line process monitoring and intelligent control of complex curvature friction stir welding was proposed. An extra rotation axis was added to the existing three translation axes to perform friction stir welding of complex curvature other than straight welding line. A clamping system was designed for locating and holding the workpieces to bear the large force involved in the process between the welding tool and workpieces. Process parameters (feed rate, spindle speed, tilt angle and plunge depth), and process conditions (parent material and curvature), were used as factors for the orthogonal array experiments to collect sensor data of force, torque and tool temperature using multiple sensors and telemetry system. Using statistic analysis of the experimental data, sensitive signal features were selected to train the feed-forward neural networks, which were used for mapping the relationships between process parameters, process conditions and sensor data. A fuzzy controller with initial input/output membership functions and fuzzy rules generated on-line from the trained neural network was applied to perceive process condition changes and make adjustment of process parameters to maintain tool/workpiece contact and energy input. Input/output scaling factors of the fuzzy controller were tuned on-line to improve output response to the amount and trend of control variable deviation from the reference value. Simulation results showed that the presented neuro-fuzzy control scheme has adaptability to process conditions such as parent material and curvature changes, and that the control variables were well regulated. The presented neuro-fuzzy control scheme can be also expected to be applied in other multi-input multi-output machining processes.

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Existing methods of joining automotive aluminium alloys are either expensive (Self Pierce Rivets) or di cult to implement (Resistance Spot Welding). Ultrasonic spot welding (USW) is a new alternative method using ~2% of the energy of resistance spot welding. USW is a solid state welding process that combines vibration and pressure at the interface of a joint to produce a weld. Much of the existing research focuses testing under laboratory conditions, using simple coupon sample geometry, and has proven to be an extremely robust process. This thesis shows a detailed investigation into the implementation of USW on automotive body panels, in collaboration with Jaguar Land Rover. Weld performance, bonding mechanisms and temperature gradients found in AA5754 align well with other research conducted using 6XXX series aluminium alloys. A laboratory trial was completed to verify all joints could be achieved on a Jaguar XJ dash panel, followed by installation of a USW machine in a production cell. A detailed statistical analysis was performed on strength and sticking data gathered from 60 Jaguar XJ dash panels that were welded in the trial. Results showed difficulty to apply USW in certain areas of the panel, although previous trials had suggested it was possible. A collaboration with Ford Motor Company allowed research to be conducted at the Ford Research and Innovation Center. Experiments were designed to discover which elements of the USW equipment had the most profound effect on weld strength, and a full factorial Design of Experiments was produced to and the most effective method of reducing variation in weld strength. Results showed that the vibrational response of complex geometry parts makes USW very difficult to predict, making it difficult to successfully implement in the automotive industry.

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In the field of welding metallurgy, much attention is given to the effects which the welding arc or flame will have on the structure and properties of the metals being joined. The engineer is interested in the intensity and extent of physical changes brought about by the unavoidable "heat treatment" which accompanies the execution of a weld.

Since Bernandos of Russia in 1887 applied an electric arc to weld metals, welding has become an important method of fabrication in industry. The ever increasing use of welding on a still widening variety of materials demands m ore and m ore understanding of the basic mechanisms of welding.

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