Advanced Materials Research Vol. 1019

Abstract: The main goal of investigations on semi-solid metal processing is to produce complex parts with enhanced mechanical properties. Flow behavior and die filling in semi-solid processing is an intrest area of researchers. A comprehensive understanding of the behavior of the under investigation metallic slurry is the design key of a successful manufacturing process. In this way the material’s transient response is very important for understanding of the rheological properties and also further commercial development of the process. The material used in this research is an Al-Si alloy which is widely used in automotive and aerospace applications. The experiments were conducted using a Searl rheometer which is specially designed for high temperature rheological measurements. For rheological tests of semi-solid alloy in different solid fractions, the isothermal holding and shear rate step change experiments were examined. Using practical rheological results, a single phase thixotropic model has been derived and adopted with measurements. The investigated alloy behavior was obviously thixotropic which is quantified in a theoretic model. Also instantaneous rheopectic behavior was observed for this alloy. The state of the microstructure has been applied through a structural parameter. The presented model for A356 alloy fits well with the experiments.

Abstract: NASA developed aluminium alloy 2139 for use in airframe structural applications as the requirement for higher strength, corrosion resistant, aluminium alloys is increasing, however the need to weld ‘unweldable alloys’ is also becoming apparent. 2139 is seen to be in this class. Semi-solid rheocast aluminium plates (Al-Cu-Mg-Ag) were cast in high pressure die casts. In total 27 welds were fabricated in experimentation, 3 on each plate to avoid distortion by using a continuous wave Nd:YAG. The weldability of this Al-Cu alloy was assessed by optical microscopy to indicate the quality and geometry of the passes. Comparisons were made with As-Cast (F) tempers. Laser power parameters ranged from 3 kW to 4 kW and weld speeds from 4 m/min to 6 m/min. Welds were allowed to cool naturally. The results show that by varying speed and power, when combined with heat treatments, produce diverse results for this unweldable alloy.

Abstract: In order to fabricate high quality aluminium products, it is first essential to produce high quality billets/slabs. One of the key objectives associated with casting processes is to be able to control the as-cast structure. A novel direct chill (DC) casting process, the melt conditioned direct chill (MC-DC) casting process, has been developed for production of high quality aluminium billets. In the MC-DC casting process, a high shear device is submerged in the sump of the DC mould to provide intensive melt shearing, which in turn, disperses potential nucleation particles, creates a macroscopic melt flow to uniformly distribute the dispersed particles, and maintains a uniform temperature and chemical composition throughout the melt in the sump. The effect of intensive shearing on the complex microstructure evolution observed after MC-DC is explained on the basis of nucleation and growth behavior. Complete suppression of typical columnar grain growth and significant equiaxed grain refinement is observed. The solidification mechanisms responsible for the significant grain refinement by intensive shearing and the morphological evolution of Mg₂Si and Fe–containing intermetallic phases are discussed.

Abstract: In this study, the energy absorption capacity and crush strength of cylindrical thin-walled structures is investigated using nonlinear Finite Elements code LS-DYNA. For the thin-walled structure, Aluminum A6063 is used and its behaviour is modeled using power-law equation. In order to better investigate the performance of tubes, the simulation was also carried out on structures with other types of cross-sections such as triangle, square, rectangle, and hexagonal, and their results, namely, energy absorption, crush strength, peak load, and the displacement at the end of tubes was compared to each other. It was seen that the circular cross-section has the highest energy absorption capacity and crush strength, while they are the lowest for the triangular cross-section. It was concluded that increasing the number of sides increases the energy absorption capacity and the crush strength. On the other hand, by comparing the results between the square and rectangular cross-sections, it can be found out that eliminating the symmetry of the cross-section decreases the energy absorption capacity and the crush strength. The crush behaviour of the structure was also studied by changing the mass and the velocity of the striker, simultaneously while its total kinetic energy is kept constant. It was seen that the energy absorption of the structure is more sensitive to the striker velocity than its mass.

Abstract: In this study tensile deformation mechanisms of aluminium alloy 5083 were investigated under observations made from SEM equipped with a tensile stage. Observations during tensile testing revealed a sequence of surface deformation events. These included micro-cracking of large intermetallic particles, decohesion of small intermetallic particles from the matrix producing microvoids and slip bands distribution. The fracture surface was characterised with closely spaced dimples, typical for aluminium alloys.

Abstract: In industrial applications tight control during weld set-up equates to increased manufacturing cost. Solid state welding processes, particularly friction sir welding, do not generally make use of filler metals and hence a weld gap will have an influence on joint quality. In the current study the influence of a weld gap of 20% of the plate thickness (i.e. 1mm) in friction stir welded (FSW) joints was compared dynamically to welds made without a gap. These results were benchmarked against samples joined through MIG welding, a widely used method to join aluminium plates in industry. Tests revealed that the introduction of a 1mm gap caused a steeper slope of the stress-life curve of the friction stir welded joints when compared to flaw free welds made without a weld gap. Crack initiation in the 1mm gap FSW samples occurred at the advancing side weld edge where the tool shoulder interacts with the material, while the flaw free ‘zero gap’ welds failed in the parent material from the marks induced during oxide removal. Intermittent root flaws were, however, present in the welds made without a weld gap which decreased the life of the samples as the area of the flaw was increased. MIG welded samples always failed from the weld toe and the fatigue life of these samples were considerably lower than that of the 1mm gap friction stir welded samples. In the friction stir welded samples containing root flaws, a flaw across the entire sample width was required to reduce the life of the friction stir welds to that reached by the MIG welded samples.

Abstract: This research presents the potential for improved joint integrity of friction stir welding by controlling the plasticisation temperature in the weld nugget. During a typical FSW, temperature fluctuates with position along the length of the weld. Working from a basis that for all material and tool geometries, there is an Optimal Plasticisation Temperature (OPT), this paper provides a strategy for maintaining this optimal weld temperature by adjusting selected weld input parameters ensuring consistent joint quality, irrespective of component geometry or clamp configuration. This proposed methodology can also be used to determine the OPT for different FSW tool geometries and material combinations. Advanced monitoring and control strategies are essential to ensure that FSW can be made a more robust industrial process that can keep pace with the modern demand for more consistent production and reliability of welded structures. The potential lies in the possibility for an operator to now select an OPT point for a specific approved welding program and allow the welding platform to maintain the OPT via closed-loop temperature control which adjusts tool rotation and or tool traverse speed. This paper further reports on the potential of integration of a closed-loop temperature control algorithm for FSW. The system measures the temperature inside the FSW tool using thermocouple sensors (creating the feedback signal). The controller then applies a PID algorithm which in turn drives the spindle speed (and if necessary, tool traverse speed) in order to change the energy input rate to the weld for controlling plasticisation temperature.

Abstract: Studies show that, for viable product realisation and maintenance, a spectrum of novel processing technologies and materials to improve performance and reduce costs and environmental impact must constantly be addressed. One of these technologies, namely the cold spray process has enabled a broad range of coatings and applications, including many that have not been previously possible or commercially practical, hence its potential for new light metal applications. Therefore, the purpose of this paper is to highlight some advantages of this technology and explore how the cold spray process could be used to repair light metal unserviceable components and develop new light metals including light metal porous structures.

Abstract: South African Foundries have promptly been met with challenges from international competition due in part to trade “liberalisation” of South African foreign trade. To objectively evaluate the challenge, data from selected local foundries were analysed together with and compared to international data from selected countries from industry and specific foundry macro level. The review seeks to evaluate the impact of resource use efficiency and resource specific cost on the local foundry and compare the challenges and opportunities available specifically to the local foundry industry. A simplified financial model was used to output an income statement type result containing the major cost elements, to arrive at levels of profitability, which was used to evaluate and compare the competitiveness of the local to international foundry industry. From these results, some macro suggestions for the local foundry industry to achieve sustainable cost competiveness improvements become apparent. Index Terms: South African Foundries, Sand, Investment, Die, Gravity, High Pressure, Benchmarking, Resource efficiency, Resource usage, Resource costs, International comparison.

Abstract: High Quality Cost Effective Die Castings rely to a large extent on successful, effective and practical die design. A review is carried out of some runner and gate designs which resulted in successful high quality castings. Some High Pressure and Gravity cast casting are evaluated. Simulation results for evaluating the impact on the flow during filling due to Runner Geometries, Gate Geometries and positions as well as venting, overflows or risers are discussed. Thermal intervention through the use of internal thermal channels are also evaluated through the use of simulation results. The paper centres on a discussion of analysis of simulation results, predicting defects which impact on some aspects of surface finish and porosity. A case study is presented showing the value of following and taking heed of lessons learnt from simulation results, to carry out die designs with reduced reliance on simulation. Index Terms: South African Foundries, High Pressure Die Casting, Computerised Fluid Dynamics Simulation, Die Design, Defect Reduction.