Daniel M. Madyira

Effect of heat input on the electrical resistivity of dissimilar friction stir welded joints of aluminium and copper

This paper reports the effect of heat input on the resulting electrical resistivities of joints between aluminium and copper produced with the friction stir welding process. Welds were produced using three different shoulder diameter tools, viz: 15, 18 and 25 mm by varying the rotational speed between 600 and 1200 rpm and the traverse speed between 50 and 300 mm/min in order to vary the heat input to the welds. The microstructures of the joint interfaces were characterized, and the electrical resistivities measured. The resulting microstructural characterization revealed that metallurgical bonding was achieved at the joint interfaces of the welds produced. It was also observed that the electrical resistivity of the joint increased as the heat input to the welds increases.

High speed machining induced residual stresses in Grade 5 titanium alloy

The surface and near surface residual stress state induced by machining may have a significant effect on the structural integrity of some important mechanical components. This article describes an experimental investigation of this stress state during high speed machining of Grade 5 titanium alloy (Ti6Al4V). The size and depth of the stress field is evaluated by non-contact probing measurement techniques as a function of cutting speed (70–200 m/min) and depth of cut (0.25–1.0 mm). X-ray diffraction and synchrotron energy dispersive diffraction are used to evaluate the surface (X-ray diffraction) and sub-surface (energy dispersive diffraction) residual stress fields. The results indicate that both techniques may be successfully employed to evaluated machining induced residual stress fields and that the energy dispersive diffraction method is eminently suitable for probe depths up to 100 µm in Grade 5. The results clearly indicated that significant residual stresses are introduced during machining both at conventional and high cutting speeds. The stresses are largely compressive and aligned with the main cutting direction for both rough and finish cuts. The results also show that the residual stress level is a strong function of the cutting speed. The overall stress level effectively becomes more tensile but also decreases with increasing cutting speed.

Static Analysis of Advanced Composites for the Optimal Design of an Experimental Lightweight Solar Vehicle Suspension System

To create an energy efficient vehicle there are a number of aspects that need to be optimized, namely; the drive train of the vehicle and energy source, aerodynamics and weight. Focusing on weight reduction, while still maintaining the desired performance and structural strength, many manufacturers are turning to advanced composites due to their superior strength to weight characteristics. Solar car racing provides a research platform that drives this innovation through technology development and efficiency. A lightweight vehicle suspension system design is being presented, together with an introduction into future testing. A suspension system is made up of a number of critical components which are dynamically loaded during standard operation due to undulating forces imposed by the road surface. Unidirectional cross-wound carbon fiber tubing is used for suspension and steering arms. The tubing is interfaced with small steel inserts and pivoting arm tie rod ends. Concerns within the design are the adhesive bonding of the carbon tubing to the steel inserts, and what type of tensile loading the interface can withstand. Due to forces imposed on the system during cornering and shock loading the components are required to withstand a minimum of 1.2 times the weight of the overall vehicle, i.e. 258 kg. Tensile test results show that the mechanical properties of the adhesive joints rely somewhat on the surface characteristics and bond preparation. The target load of 258 kg was successfully obtained under static loading for two types of sample sets. The first based on the standard for describing the lap shear strength of adhesively bonded carbon fiber to aluminum, and the second based on the working component itself.Copyright

Energy content and combustion behaviour of loose biomass available in Limpopo

Solid biomass continues to be the primary energy source for a significant proportion of Sub-Saharan African society. It is estimated that 80% of energy for heating and cooking in this subcontinent is derived from round wood biomass resulting in estimated annual rate of deforestation of 0.7%. This is unsustainable. This is despite the existence of a substantial resource of loose biomass (forest and agricultural residues) that is produced and disposed of annually. However, one major challenge in harnessing loose biomass as a source of energy is low energy density and poor combustion behaviour. Biomass briquetting technologies can be deployed to improve energy density and combustion behaviour of loose biomass. This requires understanding of the energy content in locally available loose biomass sources. This paper investigates the calorific values (energy content) and combustion behaviour of loose biomass collected from a region in the Limpopo Province of South Africa. The aim of the investigation is to understand the energy value and hence viability of using such loose biomass with the overall goal of developing recipes for biomass briquetting in the region. Calorific values were measured for 12 samples of loose biomass and their combustion behaviour analysed. Certain loose biomass sources are then identified as potential briquetting candidates.

Cold storage for low-cost air-conditioning

Building heating, ventilation and air-conditioning (HVAC) consumes 50% of the building energy consumption. Such high costs are based on the conventional HVAC cycles for air cooling based on grid connected buildings. However, there are alternative passive and active cold storage systems that can be used to reduce building HVAC costs for on grid systems and provide a solution for off-grid HVAC. Such systems depend on harnessing the night time cold and using it for day time air cooling. Such systems can be driven by a solar photovoltaic powered fan. Although there is no mechanism for humidity control to attain recommended conditions (40–60% relative humidity and 21–26°C dry bulb temperature), temperatures can be attained to improve indoor thermal comfort. This paper reports tests conducted in Johannesburg that demonstrate the potential of such technology under sub-tropical conditions. Tests were conducted for a phase change material with a melting point of 25°C for varying air flow rates and air temperature. The PCM was encapsulated in aluminium casing providing good thermal contact between the PCM and air. Although the tests were conducted on a special test rig, results demonstrated potential to reduce air temperature by 3°C before supplying into the conditioned space.

Performance testing of a multi-layer biomass briquette stove

The production of greenhouse gases (GHG) has led to the Earths surface temperature increasing by an average of 0.6°C in the 20th century. This is partly due to over reliance on fossil fuels. Human activities are estimated to account for about 69% of GHG emissions. There is now an urgent need for alternative fuels, especially renewables such as biomass, to mitigate this. In most of Sub-Saharan Africa, cooking is conducted in open fires using fire wood. This practice has poor health implications due to gases emitted largely due to incomplete combustion. The consequence is reduced mortality especially for women and children due to respiratory related diseases such as chronic obstruction pulmonary disease (COPD). The main aim of this work is to report on tests conducted on a low-cost biomass briquette domestic stove that was developed to improved biomass briquette combustion. Thermal efficiency, rate of heat loss and combustion effectiveness of the design was analysed as key performance variables of the stove. The goal was to develop a safe and reliable low-cost biomass briquette stove that will be used for domestic cooking in communities with low income streams. Stove geometry and temperatures at various points were identified as major contributors to stove effectiveness. During the investigation, controlled cooking test (CCT) method was used. Results showed good stove performance for typical cooking times.

Loose biomass briquettes production process in Maphophe village of Limpopo province of South Africa

Deforestation has significantly affected availability of reliable low cost energy resources in low income rural households. As a result, off grid communities are becoming less and less reliant on round wood for energy. However, large amounts of agricultural and forestry residues that are produced annually are destroyed in perennial veld fires and some are deliberately destroyed to prepare for new crop fields. Accessing these loose agricultural and forest residues for energy use is one possible panacea. In the process, the challenges of deforestation can also be partly addressed. The main aim of this paper is to report on adopted methods to produce loose biomass briquettes using agricultural and forestry residues collected from the Maphophe village in Limpopo Province of South Africa. The goal was to avail workable processes for harnessing energy from loose biomass available in this locality. Production methods, energy content and combustion behaviour formed key performance variables. Two types of binders were tried, namely cow dung and cactus plant paste. A 30 tonne hydraulic press was using to compact the mixture of loose biomass and binder into briquettes. Out of 12 loose biomass samples collected, the best performing include ground nutshells, Mopani leaves, yellow thatching grass and sugarcane leaves. Performance was based on density, energy content and combustion behaviour. Cow dung was found to be the best binder and produced briquettes that offer a viable energy source for off grid communities.

Combustion behaviour of loose biomass briquettes resulting from agricultural and forestry residues

Reliance on round wood for heating and cooking has increased the percentage of deforestation to 0.7% per annum especially within sub-Saharan region where communities live off the electricity supply grid. The majority of these communities rely on agriculture and forestry activities for sustenance. Therefore, large amounts of agricultural and forestry residues are produced annually although they are considered to be of low energy content. This paper investigates the combustion behaviour of the loose biomass briquettes produced from residues collected from a rural community in the Limpopo Province of South Africa. These included mopane leaves, ground nut shells, sugar cane leaves, yellow thatching grass, cactus plant and cow dung among others. Biomass briquettes were produced from these residues for various proportions and compaction pressures (6, 12 and 19 MPa) with cow dung or cactus plant as a binder. The produced briquettes were tested for energy content and burnt in a biomass briquette stove while monitoring mass decay or burn rate and temperatures. The optimum briquettes were found to be of the cow dung binding compacted at a pressure of 19 MPa.

Design and sustainability of a biogas plant for domestic use

This paper presents a study on the design methodology of domestic mechanical anaerobic digesters. The energy crisis in Africa and especially in Southern Africa has raised concerns over the effects of depleting fossil fuel reserves. With recent improvement in biogas technology; development, implementation and commercialization of prefabricated plants for biogas production has led to the recognition of the benefits resulting from domestic bio digester plant use. Global efforts have been made in building and developing domestic biogas plants, but the lack of literature on design considerations for optimum development makes this challenging. Design parameters were studied through field surveys and literature reviewed on biogas technology. With relevant information, a domestic anaerobic digester was designed using a design process that consists of determining gas requirements, biomass selection and yield potentials, sizing the digester and design of additional components such as heating and mixing elements. Using a continuous digester system led to concluding that the organic loading rate, hydraulic retention time and type of waste to be processed play a major role in the design process of such systems.

Effects of forces on the welding tool during the dissimilar joining of aluminium and copper

In 1991, a game changing technique was introduced into the metal joining community. Patented in 1991 by The Welding Institute, the Friction Stir Welding (FSW) process has since transformed the way metal joining is being done. Since then, many industries have come to embrace the new joining technology due to its ease of operation and other numerous advantages it offers. Aside from the ease with which it is employed to join similar metals, FSW provides a way of joining dissimilar metals at the microstructural level without some of the problems that traditional metal joining technologies (especially fusion welding) encounter.