Papers by Author: Rian J. Dippenaar

Abstract: This study considers the weldability of Ti-6Al-4V alloys formed by various powder consolidation methods. Samples were prepared from commercially sourced pre-alloyed Ti-6Al-4V powder using both conventional press-and-sinter (PS) and the new novel resistance-heated pressing (RHP) methods. Fusion welding was executed by the gas tungsten arc (GTA) process with arc stability assessed in-situ by observations of the arc as well as monitoring of transient arc voltage. Results indicated equivalent arc stability between samples of RHP and commercially sourced wrought material while samples formed by PS showed high instability in arc initiation, attributed to high levels of porosity. Post weld analysis of mechanical hardness in powder based samples revealed no significant deviation in weld metal properties from welds conducted on commercially sourced wrought material. In all cases weld microstructures typical of Ti-6Al-4V alloys were observed with significant grain growth in the fusion and heat affected zones. Samples prepared by PS methods showed internal porosity due to gas evolution upon solidification, which may again be attributed to the highly porous initial microstructure.

Abstract: Intermetallic TiAl alloys are a class of innovative high-temperature materials which are developed to replace the substantially denser Ni-base alloys in low-pressure turbine blades of jet engines. By streamlining the production process of these parts, a substantial decrease in production costs can be achieved. To this end, a profound knowledge of the microstructural processes occurring during hot deformation is a prerequisite. To investigate the microstructural development during forming operations, cast and extruded as well as only cast specimens were hot-deformed and the microstructural development investigated in-situ by means of a novel diffraction method. This powder diffraction method utilizes the behavior of individual reflection spots on the Debye-Scherrer rings for deriving the materials response to the deformation imposed. It was found that the behavior of the two specimens is rather similar, although the starting microstructures show pronounced differences.

Abstract: The automotive industry is increasingly utilizing advanced high-strength steels, primarily to reduce the mass of motor vehicles. However, many of these steels fall within the peritectic composition range, which are notoriously difficult to cast by continuous casting techniques. Against this background, a brief review is given of our current understanding of the peritectic reaction as such and the subsequent peritectic phase transformations.

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.