Papers by Author: Robert G. O'Donnell

Abstract: The addition of a constriction in the melt flow path of high pressure die castings is discussed in terms of its influence on modifications to mechanical properties. It is shown through experimentation that the ultimate tensile strength and elongation to fracture of as-cast tensile specimens increased when the melt flowed through a constricted path. It is proposed that defect-forming inclusions were disintegrated more efficiently in the constricted runner through increased strain rates and turbulent dissipation rates. Increased turbulence is also presumed to be the cause for the greater dispersion of defects. The suggestions are supported with calculations aided by computational fluid dynamics simulations.

Abstract: The application of moderate cooling rates to metal alloys of certain composition can generate metals that exhibit an amorphous microstructure on a bulk scale. This phenomenon is related to the avoidance of the nucleation of the competing crystalline phases associated with the alloy during solidification. This work describes the devitrification behaviour of the bulk glass forming Mg60Cu29Gd11 system through the use of a number of analytical techniques including DSC, laser confocal microscopy, SEM and XRD. Attention is drawn to the correlation between the more common analytical techniques and the observation of phase transformations on the surface of the metal, evident using a laser scanning confocal microscope fitted with a heating stage.

Abstract: Friction stir processing (FSP) combines frictional heating and severe plastic deformation to produce microstructural modification, either locally targeted at the near-surface regions or through the bulk, of metallic components fabricated by conventional processing routes. In this paper, we highlight the capabilities of this process by applying it to a high-pressure die cast Al-Si-Mg-(Cu) alloy and examining the resulting microstructure and mechanical properties.

Abstract: Melt flow and solidification within a die casting cavity is a complex process dependent in part on melt pressure (with or without intensification), melt velocity, melt flow path, thermal gradients within the die, die lubrication and melt viscosity. Casting defects such as short shots, cold shuts and shrinkage porosity can readily occur if casting conditions are not optimised. Shrinkage porosity in particular is difficult to eradicate from castings that comprise thick sections, since these sections will usually solidify late in the casting cycle and may be starved of melt supply during the critical solidification (and contraction) stage. The current work seeks to elucidate the influence of the melt shearing on the die casting process and demonstrates that the modifications made to the melt through introduction of a local constriction in the melt path can generate improvements in casting microstructure and reduce shrinkage porosity.

Abstract: ATM high pressure die casting technology (ATM) is a variant of the traditional high pressure die casting (HPDC) process and is distinguishable by its characteristic lean runners that increase process yields. Reduced raw material consumption helps ATM leave a smaller footprint on the environment by lowering greenhouse gas (GHG) emissions during primary processing of the alloys and in their melting and handling in the foundry. Further avenues for reducing GHG emissions are raised by the use of ATM technology which improves the integrity of castings - facilitating the adoption of lighter weight components in automobiles. In the present paper, reductions in GHG emissions achieved by ATM are illustrated with the aid of a commercial case study; potential mass reduction opportunities for the automotive sector are explored with the aid of finite element analysis.

Abstract: Conventionally produced high pressure die-cast (HPDC) components are not considered to be heat treatable because gases entrapped during the die-casting process expand during solution treatment causing unacceptable surface blistering. Components may also become dimensionally unstable. Both these effects prevent the heat treatment of die-castings as these phenomena are detrimental to the visual appearance, mechanical properties and utilisation of the component. Recent work has revealed a process window in which HPDC aluminium alloys that are capable of responding to age hardening may be successfully heat treated without encountering these problems. As a result, improvements of greater than 100% in the tensile properties are possible, when compared with the as-cast condition. The new heat treatment schedules are described for HPDC parts of different size and shape, the role of chemistry on ageing is discussed and microstructural development during heat treatment examined†.