No description available.

No description available.

No description available.

No description available.

Solder joints in electronic packages provide mechanical, electrical and thermal connections. Hence, their reliability is also a major concern to the electronic packaging industry. Ball Grid Arrays (BGAs) are a very common type of surface mount technology for electronic packaging. This work primarily addresses the thermo-mechanical durability of BGAs and is applied to the exemplar alloys; traditional leaded solder and a popular lead-free solder. Isothermal mechanical fatigue tests were carried out on 4-ball test specimens of the lead-free (Sn-3.8Ag-0.7Cu) and leaded (Sn-37Pb) solder under load control at room temperature, 35°C and 75°C. As well as this, a set of combined thermal and mechanical cycling tests were carried out, again under load control with the thermal cycles either at a different frequency from the mechanical cycles (not-in-phase) or at the same frequency (both in phase and out-of-phase). The microstructural evaluation of both alloys was investigated by carrying out a series of simulated ageing tests, coupled with detailed metallurgical analysis and hardness testing. The results were treated to produce stress-life, cyclic behaviour and creep curves for each of the test conditions. Careful calibration allowed the effects of substrate and grips to be accounted for and so a set of strain-life curves to be produced. These results were compared with other results from the literature taking into account the observations on microstructure made in the ageing tests. It is generally concluded that the TMF performance is better for the Sn-Ag-Cu alloy than for the Sn-Pb alloy, when expressed as stress-life curves. There is also a significant effect on temperature and phase for each of the alloys, the Sn-Ag-Cu being less susceptible to these effects. When expressed as strain life, the effects of temperature, phase and alloy type are much diminished. Many of these conclusions coincided with only parts of the literature and reasons for the remaining differences are advanced.

No description available.

No description available.

No description available.

Friction Stir Welding (FSW) is a solid-state joining process that was invented in 1991; it is particularly useful for joints difficult to make using fusion techniques. Significant advances in FSW have been achieved in terms of process modelling since its inception. However, until now experimental work has remained the primary method of investigating tool wear in FSW. In this project, two main objectives were set; the first one was to produce a numerical approach that can be used as a useful tool to understand the effect that worn tool geometry has on the material flow and resultant weld quality. The second objective was to provide a modelling methodology for calculating tool wear in FSW based on a CFD model. Initially, in this study, a validated model of the FSW process was generated using the CFD software FLUENT, with this model then being used to assess in detail the differences in flow behaviour, mechanically affected zone (MAZ) size and strain rate distribution around the tool for both unworn and worn tool geometries. Later, a novel methodology for calculating tool wear in FSW is developed. Here a CFD model is used to predict the deformation of the highly viscous flow around the tool, with additional analysis linking this deformation to tool wear. A validation process was carried out in this study in order to obtain robust results when using this methodology. Once satisfied with the tool wear methodology results, a parametric study considering different tool designs, rotation speeds and traverse speeds was undertaken to predict the wear depth. In this study, three workpiece materials were used which were aluminium 6061, 7020 and AISI 304 stainless steel, while the materials used for the tools used were of H13 steel and tungsten-rhenium carbide (WRe-HfC) with different tool designs. The study shows that there are significant differences in the flow behaviour around and under the tool when the tool is worn and it shows that the proposed approach is able to predict tool wear associated with high viscous flow around the FSW tool. With a simple dome shaped tool, the results shows that the tool was worn radially and vertically and insignificant wear was predicted during welding near the pin tip. However, in other regions the wear increased as the weld distance increased. Additionally, from the parametric study that was undertaken for the two tool designs - a dome and a conical shape- the study has found that for both tool designs, wear depth increases with increasing tool rotation speed and traverse speed. It was also shown that, generally, the wear depth was higher for the conical tool design than the dome tool in the pin tip zone. The research concludes that a proposed methodology is able to calculate tool wear associated with high viscous flow around the FSW tool, which could be used as a method for calculating tool wear without the need for experimental trials. The CFD model has provided a good tool for prediction and assessment of the flow differences between un-worn and worn tools, which may be used to give an indication of the weld quality and of tool lifetime. Furthermore, from the results, it can be concluded that this approach is capable of predicting tool wear for different process parameters and tool designs and it is possible to obtain a low wear case by controlling the process parameters.

Dissimilar metal welds (DMWs) are widely used in steam vessels in thermal power stations to join low-temperature alloys, such as steels, to high temperature alloys, such as nickel-based alloys. This provides a cost-effective manufacturing solution. However, there is a history of DMWs failing due to creep in service environments. Many investigations have been performed on weld systems and failures in the traditional 2.25Cr-1Mo (P22) steels, but fewer have been performed on newer 9Cr-1Mo steels, such as P91 and P92. Failures involving these newer steels continue to occur, for reasons which are not thoroughly understood. The factors involved are believed to include system stresses, differences in thermal expansion and microstructural evolution due to interdiffusion across the weld interface. The overall aim of this research was to investigate the chemical and microstructural stability of a range of DMWs involving P91 and P92 steels. Much of the work is centred on a 3-bead manual metal arc weld of nickel alloy 625 onto a P92 substrate. This was subjected to tempering (760 °C for 2 hours) and furnace ageing (625 °C for 32 and 125 days). The as-welded and aged states were characterised by a range of techniques, including SEM, TEM, EDX and microhardness testing. Site-specific TEM samples were extracted from the weld interfaces using focussed ion beam (FIB) methods. EDX measurements show iron-enrichment in the weld metal (WM) up to 30 wt%, and a partially mixed zone (PMZ) up to 50 microns from the weld. TEM analysis of the as-welded state reveals the presence of a 1 – 2 micron wide band separating the two alloys, and of different crystallographic orientation to both. Following ageing, diffusion of carbon takes place from the P92 to the WM, leading to a carbon denuded zone (CDZ) in the former and an enriched zone (CEZ) in the latter. Precipitates in the CDZ (M23C6 and MX) dissolve to supply this diffusion, while Nb-rich MX phases have formed on grain boundaries in the CEZ. Additionally, carbides are found to form along the interface between the band and the P92. EDX measurements confirm that the aged WM is enriched in carbon near the interface. Microhardness measurements reveal slight softening of the P92 in response to ageing, and pronounced hardening of the WM. The thermodynamics of the alloys were modelled using the software Thermo-Calc, while diffusion across the interface was modelled using DICTRA. The findings support the trends of the experimental results in terms of diffusion behaviour and phase changes. An industrial case study, ex-service P91 – alloy 625 pressure vessel welds exposed to c. 565 °C for c. 40,000 hours, has also been undertaken. Creep failure occurred during service near the weld interface. SEM showed that the creep crack tip was advancing through the CDZ. Microstructural changes were similar to those in the P92 – alloy 625 system, only more pronounced; the CDZ was found to be almost entirely devoid of standard M23C6 and MX precipitates, having been replaced by a band of carbonitrides (either M23X6 or M6X) of unusual chemistry, parallel to the weld interface. A second industrial case study involved a weld between P91 and P92 steels using the P87 filler metal, recently developed by EPRI, which is designed to minimise interdiffusion. This system, in contrast to those involving alloy 625, shows evidence for only minimal interdiffusion after ageing at 649 °C for 131 days, with no CDZ being observed. These observations are supported by Thermo-Calc and DICTRA calculations. Therefore, systems of this type may be resistant to creep failure in long-term service.