Materials Science Forum Vols. 618-619

Abstract: This paper reports the preliminary results of an investigation on the synthesis of a Ti-Mg alloy powder through mechanochemical processing of TiO2 and Mg powders. TiO2 was mixed with elemental Mg according to a nominal stoichiometric composition with 15% excess Mg. The powder mixture was mechanically milled in a Simoloyer high energy ball mill for 5 different durations. Contamination was minimised by processing under a high purity argon atmosphere. Changes in phase composition were studied by XRD techniques. TiO2 was reduced, as shown by the formation of MgO. The extent of the reduction, as indicated by XRD peaks’ intensities, increased with milling time. XRD spectra of powders milled for 24 hours revealed virtual disappearance of TiO2 peaks and there was no evidence of elemental Ti. The lattice parameter of the resulting Ti metal was larger than that of elemental Ti. This implies that the Ti was alloyed with free Mg to produce Ti-Mg alloy powder. The lattice parameter increased with increasing milling time.

Abstract: The performance of extruded AZ31, AZ61 and AM-EX1 tubes was examined in three-point bending. Different extrusion temperatures were used to investigate the effect of grain size on the load-carrying capacity, energy absorption and fracture propensity of the tubes. Results showed that while the peak load increased with a smaller average recrystallised grain size, the retention of large elongated un-recrystallised grains in the microstructure reduced the load. The presence of the large elongated grains also appeared detrimental to the ability of the tube to deform before fracture.

Abstract: Tensile and compressive tests of a large AZ31 extrusion were performed along extrusion and long transverse directions over a range of strain rate (0.00075 s-1 to 9 s-1) and temperature (100°C to -143°C). The effects of strain rate, temperature, sample orientation and load direction (tension or compression) on mechanical properties are reported and discussed. The yield strength of tensile samples along the extrusion direction can be described by a constitutive equation using the conventional rate parameter for thermally activated deformation.

Abstract: Hot tearing, or hot cracking, is one of the most severe solidification defects commonly encountered during casting. It is such a complicated phenomenon that a full understanding is still not yet achieved, though it has been extensively investigated for decades. Most contributions are still based on qualitative characterisations. The purpose of this work is to develop a method that can quantitatively evaluate and investigate hot tearing behaviour. The principle is based on contraction stress/force measurements. The measured contraction force has been proven to be able to evaluate the hot tearing susceptibility as a more straightforward and quantitative index. By analyzing the contraction force curve, information can be obtained for both the initiation and the propagation of the hot tear. With this method, the influence of mould temperature and Al content on hot tearing behaviour of Mg-Al binary alloys has been investigated. The contraction force curves also indicate that the liquid refilling plays an important role during the hot crack propagation. With a lower cooling rate and higher onset temperature of hot tear, the remaining liquid is more favourable to refill the initiated hot crack, and consequently interrupts the propagation of cracks or possibly completely heals the cracks.

Abstract: In recent years, the pursuit of higher strength metals and alloys has led researchers to nanometer scale grain refinement. New nanocrystalline engineering techniques have successfully increased properties for a wide range of materials. Here we report a nanocrystalline 7075 alloy processed by high-pressure torsion that exhibits ultra-high strength and features a hierarchical solute architecture. The new hierarchy of solute architecture was discovered through high-resolution characterisation using novel techniques we have developed in atom probe tomography. These new techniques – nanotexture and fine scale solute cluster measurements, are the focus of this paper. Our results indicate that nanometer-scale engineering of solid solutions could offer a pathway towards a new generation of super-strong alloys that hold promise for creating entirely new regimes of property-performance space.

Abstract: The objective of this research is to develop an existing crucible furnace fueled by diesel fuel into an energy saving LPG fueled furnace. The Non- ferrous melting furnace will be used to melt Aluminium and Copper alloys. The results of the research are as follows: 1. The furnace and equipment improvements outlined in this research will save an average fuel cost of 26.90 percent when using diesel oil. 2. When comparing the use of LPG fuel with diesel oil to melt aluminium alloy, LPG use results in an average cost saving of 43.18 percent as compared to diesel, while copper alloy melting results in an average cost saving of 50.83 percent. The world energy crisis has impacted economic growth in the developing countries mainly due to the high costs of importing energy such as crude oil from middle eastern countries where the high price of the oil has lead to a higher cost of products. Thus manufacturers must consider this when embarking of significant capital expenditure. It is believed that alternative energy is the way to relieve the energy crisis situation. Using suitable energy is one way to save on production costs. Foundries which produce non - ferrous metals such as aluminium and copper Alloy still use oil crucible furnaces as the source of heat energy. If they change the energy source from oil to be liquefied petroleum gases (LPG) the melting cost will be decreased. This research was the experimentally conducted by melting non - ferrous metals within a crucible furnace using either LPG or diesel oil and then comparing the consumption levels. Before the experiment we modified an existing crucible furnace which used diesel oil to be the experiment furnace. However for reference data this researcher took the Capacity table of crucible furnaces from Pyro - Industrial Systems Company U.S.A.

Abstract: A multilayered sheet composite of commercial purity Al and Al-0.3%Sc alloys was produced by accumulative roll bonding. The final sheet material consisted of 64 ultra fine grained layers, each of ~7.8mm in thickness. The as-deformed material was annealed at temperatures ranging from 250 to 350°C to study the changes in microstructure and their associated influence on mechanical properties. The as-deformed structures largely comprised of high angle grain boundaries in the Al layers and low angle grain boundaries in the Al(Sc) layers. During annealing, the structures in the Al(Sc) layers remained unaltered, whereas the Al layers recrystallized rapidly to the full layer thickness. The mechanical properties of the Al-Al(Sc) composite were measured and found to be unique in strength and ductility with annealing temperature having a significant influence on these properties.

Abstract: Understanding and prediction of the mechanical properties of aluminium alloys are of great importance with respect to e.g. strength requirements and forming operations. In the 7xxx alloying system several mechanisms influence the hardening behaviour of the alloys, e.g. particle size and distribution, dislocation density, and alloying elements in solid solution. This work is an experimental study of work- and age-hardening considering a commercial AA7108 alloy in the as-cast and homogenized condition. Tensile specimens have been exposed to a solution heat treatment and a two-step age-hardening treatment with varying time at the final temperature. The tensile data for the different tempers have been evaluated in elucidation of already existing models based on the one-parameter framework by Kocks, Mecking, and Estrin. The particle size has been further investigated in the transmission electron microscope for one under- and one over-aged condition and the influence of particles on work-hardening behavior has been discussed.

Abstract: A textured microstructure was produced in the TiAl turbocharger blades and platelike TiAl samples in the process of conventional casting, in which the lamellar orientation is parallel to the plate surfaces. A microstructure design specific to the turbocharger blades was therefore proposed based on the preponderance in tensile properties, creep rupture life and creep resistance along the preferential lamellar orientation. The mechanical capacity of the TiAl turbochargers has been validated through endurance tests.

Abstract: Fatigue crack growth of long and small cracks was investigated for various Al-Si-Mg and Al-Mg cast alloys. Low residual stress was ensured during processing to concentrate on microstructural effects on crack growth. Compact tension and single edge tension specimens were fatigue crack growth tested at room temperature and stress ratio, R = 0.1. Microstructure related mechanisms were used to explain the near-threshold behaviour and crack growth response in Regions II and III for each material considering relevant microstructural features such as SDAS, grain size, and volume fraction and morphology of eutectic Si. Threshold behaviour of long cracks is attributed to closure-dependent mechanisms. In Regions II and III, the changes in crack growth mechanisms were explained through correlations between the extent of the plastic zone ahead of the crack tip and material-specific microstructural damage. Threshold behaviour of small cracks is explained through closure-independent mechanisms, specifically through the barrier effects of controlling microstructural characteristics specific to each material. Recommendations for integrating materials knowledge in structural design for fatigue performance are given.