Advanced Materials Research Vol. 1019

Abstract: The interaction between the Ti6Al4V alloy and the mould materials was investigated. The alpha-case was characterized by Vickers hardness tester, optical and scanning electron microscopy equipped with electron dispersive X-ray spectrometry (EDX). X-ray diffraction (XRD) analysis was performed on as cast and on YFSZ or YZ-Blended face-coats. From the experimental results, a distinct alpha-case formation was revealed. The YFSZ led to a thicker and harder alpha-case than the YZ-Blended face-coat. The EDX revealed the presence of Zr and Si elements in both alpha-cases. Therefore, from experimental results and thermodynamic calculations, pure ZrO₂ and SiO₂ may react with Ti.

Abstract: Titanium alloys offer excellent corrosion resistance, good strength to weight ratio, is nonmagnetic and biocompatible. This allows them to be used in demanding applications and specialised industries ranging from aviation to medicine. However, at high temperatures the metal is chemically reactive which requires several manufacturing processes such as melting and welding to be performed either in vacuum or inert gas atmosphere. Some processes such as rolling are impractical to be performed in inert gas atmosphere. Titanium alloys, notably Ti-6Al-4V, develops a hard oxide layer on the outer surface during hot processing, such as rolling, in atmospheric conditions. This oxide layer, commonly referred to as the alpha case, is both hard and brittle. The increased Young's modulus of the alpha case creating an outer layer with increased stiffness where maximum stresses occur, results in localized micro failure. The micro failures in this layer serve as a fatigue crack initiation zone, compromising the integrity of the component and causing it to fail. Traditionally alpha case is removed by means of chemical milling in hot acid baths using aggressive acids such as Hydrofluoric acid. The facilities needed for chemical milling require high capital costs as well as stringent and costly safety requirements. Lastly, the disposal of used acids is proving to become increasingly difficult due to the strict South African environmental laws. Removal of this layer by means of light pass machining has therefore become more desirable, however at present it remains economically infeasible. This study presents an overview of the materials background, alpha case formation and related machining considerations. Experiments that investigate alpha case properties are included.

Abstract:

Titanium and its alloys have been experiencing extensive development over the past few decades. They have found wide applications in the aerospace, biomedical and automotive industries owing to their good strength-to-weight ratio and high corrosion resistance. Machining performance is often limited by chatter vibrations at the tool-workpiece interface. Chatter is an abnormal tool behaviour which is one of the most critical problems in the machining process and must be avoided to improve the dimensional accuracy and surface quality of the finished product. This research aims at investigating chatter trends in the end milling process and to identify machine parameters that have effects on chatter during machining. The machine parameters investigated include axial feed rate, spindle revolute speed and depth of cut. In this research, experimental data was collected using sensors to analyze the existence of chatter vibrations on each processing condition. This research showed that the combination of the machine parameters, feed rate and spindle speed within certain proportions has an influence on machine vibrations during end milling and if not managed properly, may lead to chatter.

Abstract: The effect of thermal processing (TMP) parameters on grain growth in a low C - Mn steel and a C - Mn steel microalloyed with Nb, Ti and V were compared as part of a wider study on grain growth in microalloyed steels. The grain growth rate was found to be low at low temperatures and short soaking times but increases significantly with both increasing temperature and time. The activation energy Q, the grain growth equation constants n and A were found to be higher in the microalloyed steel than the plain C-Mn steel. A constitutive model for predicting austenite grain growth in the low C-Mn steel and the microalloyed steel has been developed. The predictive potential of the model is in good agreement with the experimental data.

Abstract: For an effective thermomechanical process control, knowledge of the start and finish temperatures of the austenite transformation is critical. Continuous Cooling uniaxial-Tension (CCT*) is a useful way to measure these values. Therefore, the dynamic transformation of austenite to ferrite (γ → α) was investigated in C-Mn steels to understand the hot ductility behaviour of these steels after varying the cooling paths i.e. simulating “hard” and “soft” cooling rates of the strand during continuous casting. Results show that hard cooling into the dual phase (γ + α) region significantly increases the dynamic transformation temperatures due to a higher driving force owing to double transformation and precipitation of AlN during the process. A comparison of dynamic versus static transformation (Ar₃ and Ar₁) temperatures and equilibrium transformation temperatures gives a better understanding of the contribution of strain to transformation during casting or hot deformation.

Abstract: The influence of the strain sequence during slab hot rolling (also known as “roughing”) on the evolution of austenite in plain carbon, C-Mn-V and C-Mn-Nb-Ti-V steels was investigated. Reheating and roughing simulations were conducted in a Bähr deformation dilatometer using a constant austenitising temperature, constant soaking time and various heating rates and roughing strain sequences. Stress analysis was used to quantify the austenite softening behaviour and the prior austenite grain size was measured from quenched specimens. The austenite grains of the plain carbon steel were coarser than those of both microalloyed steels, with the C-Mn-Nb-Ti-V grade being the finest due to effective pinning of the grain boundaries. Pass strains greater than 0.2 were sufficient for initiation of dynamic recrystallisation (DRX) for the C-Mn and C-Mn-V steels and led to uniform austenite microstructure with austenite grain sizes less than 40µm after the roughing stage.

Abstract: Vanadium carbide is less dense, harder and tougher than tungsten carbide, and nickel is more corrosion resistant that cobalt. Replacing the binder with Ni should improve the corrosion resistance. Since there are eutectics with a wide range of compositions in the C-Ni-V system, this system has potential for wear resistant alloys with ~VC as the discrete hard phase and the nickel solid solution as the binder. The aim of this work was to ascertain if eutectic microstructures with vanadium carbides could be beneficial to wear resistance. Phase studies confirmed the wide range of the eutectic alloys, with VC + (Ni), VC + σ′, VC + (V) and graphite + (Ni) and different morphologies. Preliminary wear tests showed results comparable to WC-Co alloys.

Abstract: Cobalt-based Fischer-Tropsch catalysts typically contain noble metals as reduction promoters to enhance the amount of the catalytically active metal in these catalysts after activation. The noble metal is typically co-impregnated with cobalt, which does not necessarily ensure the optimum contact between the noble metal and cobalt. The noble metal can be selectively deposited on the precursor of the catalytically active metal for the Fischer-Tropsch synthesis, if strong electrostatic adsorption (SEA) is the dominant mechanism. The point of zero charge of silica (Davisil 646) (PZC = 2-3) and that of Co₃O₄ (PZC = 9-10) differs significantly. This results in a pH region where it is theoretically possible to selectively deposit the noble metal onto Co₃O₄ using anionic exchange (e.g. using PtCl₆^2- ). The effect of pH on the uptake of these metal anions was investigated and found that adsorption is favored at low pHs. The reduction characteristics of Co₃O₄/SiO_2, promoted by SEA, were investigated by temperature programmed reduction (TPR) and thermal gravimetric analysis (TGA). The peak maxima for reduction temperature for both steps of the reduction of Co₃O₄ to CoO and of CoO to Co₃O₄ decrease in the promoted catalyst, the activation energy for the first step decreased from 84±11kJ/mol to 50kJ/mol upon promotion. The degree of reduction was also noted to increase from 48% in unpromoted Co/SiO₂ to 56% and 84% in two different Pt-Co/SiO₂ systems.

Abstract: Noble metals, such as platinum or gold, may promote the reduction of cobalt in supported cobalt catalysts either by direct contact or indirectly via hydrogen spill-over. The synthesis go gold nano-particles using THPC in the presence of a calcined Co/TiO₂-catalyst precursor results in a broadening of the gold particle size distribution possibly due to the association of gold with Co₃O₄. This was not observed for platinum and platinum-gold alloys synthesized in the same manner. Platinum-gold alloys are less effective than platinum as a reduction promoter, possibly due to the presence of gold on the surface of the alloy particles.

Abstract: The thermal properties of amorphous Ti-50at.%Pt alloy produced by mechanical alloying were studied by differential scanning calorimetry (DSC). The powders, sampled at various time periods during the milling process, displayed non-reversible exothermic reactions in the temperature range of 641-668 °C, and in the higher temperature range of 839-970 °C. The reaction peak temperatures and heat effects vary with milling time. The reactions at the lower temperature range were determined by X-ray diffraction analysis of the powder, heated beyond the onset temperature of the exothermic peaks, to be crystallisation into B19 TiPt. The reactions at higher temperatures are suspected twinning transformation of the B19 grains to form martensite. This requires confirmation by analysis with a transmission electron microscope, planned as part of future work.