Advanced Materials Research Vol. 1019

Abstract: Two commercially pure (CP) titanium powders, with mean particle sizes of 78 and 32 μm, respectively, were used to manufacture titanium samples via the press-and-sinter powder metallurgy process. Compaction pressures ranging from 300 to 500 MPa were used to compact the powders into cylindrical and rectangular forms. The green samples were sintered for 2 hours under high vacuum, 10^-6 mbar, at 1100, 1200 and 1300 °C, respectively. Green density and green strength data were collected from the compacted samples, and sintered density, sintered strength and microstructure images were collected from the sintered samples. These data were used to characterise process models for the compressibility of the powders, and for the sinter densification, using the Master Sintering Curve (MSC) model. The results show that particle size influences the processing at both the compaction and sintering step. In modelling these two processes, separate MSC models must be characterised and each used individually to predict each one’s final sintered density. It is shown that if the densification parameter is used to characterise the sintering model, a unified Master Densification Curve (MDC) is found. The modified MDC model can be used to predict the final sintered density regardless of the initial green density or mean particle size of the powder.

Abstract: Titanium alloys have a number of features which make them attractive for use in aerospace, marine and chemical engineering, biological engineering, etc., due to their advantage of low density, high strength, and excellent corrosion resistance and biocompatibility. In this paper, Ti-6Al-4V (Ti-64) rods were prepared by vacuum sintering titanium alloy powder compacts at 1300°C for 2h and then hot extruding the as-sintered Ti-64 alloy billets at 1150°C in air. The microstructure and property changes, after vacuum sintering and hot extrusion, were investigated. The results showed clear evidence of porosity and a coarse lamellar microstructure in as-sintered Ti-64 alloy billets. Tensile testing of as-sintered material gave yield strength, ultimate tensile strength and ductility values of 850MPa, 985MPa and 2%, respectively. After extrusion at 1150°C, no obvious pores could be seen in the microstructure of as-extruded Ti-64 alloy rods and the lamellar microstructure was significantly refined, and the mechanical properties were significantly improved. The yield strength, ultimate tensile strength and the ductility reached 1130MPa, 1245MPa and 4.5%, respectively. Compared with the mechanical properties of Ti-64 alloy rod prepared by extruding a hot pressed Ti-64 alloy billet (1300°C for 5min, argon protective atmosphere) in air, the ductility of the Ti-64 alloy rod studied here is lower. The fracture characteristics of as-sintered and as-extruded Ti-64 alloy rods have also been investigated.

Abstract: The research focuses on the fatigue and fracture toughness of Ti-6Al-4V manufactured by Selective Laser Melting (slm) of fine powder. This manufacturing approach offers remarkable design flexibility as layers can be built up at approximately 50-100 micron layer increments. Consequently very complex shapes can be achieved and minimal machining is required. For very complex parts selective laser melting may be a viable alternative to conventional casting, forging and machining. The process does, however, result in significant residual stresses as well as significant surface roughness and some minor porosity, all of which can impair mechanical properties. The fatigue and fracture toughness behaviour of such slm Ti-6Al-4V alloys fabricated by laser melting can depend substantially on the interlayer bonding, as this method could lead to lines of weakness between melted layers or even very poorly melted and bonded layers. This project then endeavours to measure the fatigue behaviour in the form of a fracture mechanics based “Paris equation” and fracture toughness of selective laser melted material in two orientations, namely the fatigue crack grown (i) perpendicular and (ii) parallel to build direction. Such fatigue data would be invaluable in the assessment of fatigue lives of components made of such material.

Abstract: Direct Metal Laser Sintering (DMLS) is a layer-by-layer Additive Manufacturing (AM) process that creates physical metal parts from three dimensional Computer Aided Design (CAD) data. For DMLS to be generally accepted by industry as a manufacturing technology, high mechanical integrity of final components needs to be demonstrated. Mechanical properties of manufactured components are directly affected by the quality of each individual laser sintered track of each consecutive layer. In this study, the optimal ratio of laser power and scanning speed on single tracks is determined for Titanium-6Al-4V powder on an EOSINT M270 DMLS machine for a layer thickness that varies between 15 μm and 30 μm. Two different laser powers, namely 150 W and 170 W were considered. Scanning speeds varied between 600 mm/s to 2000 mm/s with 200 mm/s intervals. The most stable tracks resulted from high laser power, slow scanning speed and thin powder distribution. The empirical data were compared to a melt pool width prediction program, which was found to underestimate track width at all scanning speeds and re-melting depth at low scanning speeds. Further, it was found that decreased powder thickness can be used with an increased scanning speed and high laser power. This strategy may be used to increase surface quality. The penetration data during fusion of the tracks onto the building platform further validates the quality of each sintered track.

Abstract: The titanium alloy (Ti-6Al-4V) coated with commercially pure (CP) Ti particles was fabricated by the laser cladding process. Coating quality was investigated by scanning electron microscopy and X-ray diffraction, while wear resistance of the alloys was evaluated on a ball-on-flat dry sliding wear tester at room temperature under a load of 25 N, frequency of 5 Hz and stroke length of 2 mm using three counterface materials (302 SS, Brass and WC). Mass loss of the laser cladded samples as well as the wear rate of each counterface material was calculated. SEM images revealed growth of acicular widmanstätten α, also known as α′ martensite, embedded in a prior β matrix at the coating-substrate interface. Smearing of Ti from the cladded samples was observed on the worn surfaces of the counterface materials except for brass which showed high material removal.

Abstract: The aim of this work was to refine the as-cast Ti-6Al-4V grain size and its Widmanstätten morphology to optimise the mechanical performance of Ti-6Al-4V castings. Hydrogenation and deformation were used as variables in processing routes aimed at assessing the degree of refinement in the as-cast Ti-6Al-4V microstructure. Thermohydrogen processing (THP) refined the Widmanstätten morphology and not the prior beta grain boundary network. Therefore, the degree of refinement in THP processing is limited to morphology refinement within pre-existing prior beta grains. Deformation processing and recrystallisation is necessary to eliminate the indelible influence of the prior beta grain boundary network on the extent of THP refinement. In this context, a substantial degree of refinement is achieved from thermohydrogen and deformation processing (THDP). The as-cast Ti-6Al-4V grain boundary network was refined from an average diameter of 2000μm to 20μm. In addition, the Widmanstätten morphology was refined to submicron equiaxed alpha and beta grains.

Abstract: The hot deformation behavior of Ti-6Al-4V alloy with transitional microstructure over temperature 800°C~950°C and strain rate ranges of 0.001~10s^-1 has been studied by Gleeble-3500 hot working simulation testing machine. The flow softening of stress-strain curves is resulted from the spheroidization of transitional microstructure, dynamic recrystallization and superplasticity. Both temperature and strain rate are important factors affecting the deformation behavior. Flow instability induced by adiabatic shear bands occurs at 800-880°Cand 0.32-10 s^-1. With the increasing of strain rate and decreasing of temperature, the degree of strain localization increases. The optimum working region of Ti-6Al-4V alloy with a transitional microstructure is at 820-910°C and 0.001-0.1 s^-1.

Abstract: This paper reports on preliminary results obtained during exploratory research, investigating the feasibility of employing Friction Hydro Pillar Processing principles as an alternative technique for producing riveted joints in 3.17mm Ti-6Al-4V sheets. Influence of process parameters was investigated to understand material flow during the plasticisation phase. An understanding of joint performance from a macrostructural and hardness point of view was established and forms part of the data presented. Process energy was quantified in relation to rotational speed. The data presented will assist in guiding the overall research objective to determine the feasibility of Friction Hydro Pillar Riveting as a suitable alternative high integrity joining process for the Aerospace industry.

Abstract: This paper presents an investigation on the influence of varying heat input during friction stir welding of Ti6Al4V alloy, with respect to static and dynamic joint integrity. Weld heat input was controlled by varying the rotational-and tool travel speed. In the absences of large defects the welded samples failed predominately in the parent plate, while percentage elongation for all welds was lower than that of the original material. To quantify the influence of joint geometry on dynamic joint integrity, the test samples were categorised into “as-welded” and “polished” conditions for ease of comparison. Welds done at medium heat input exhibited improved fatigue strength in both conditions, while crack initiation sites for the as-welded condition was predominantly from tool shoulder marks whereas the polished sample initiation sites could be mainly linked to subsurface defects in the weld nugget. The relationship between welding tool geometry, weld defects and-process parameters is also discussed in an attempt to identify interrelationships that could be linked to joint integrity.

Abstract: Pure powders of titanium, aluminium, nickel and ruthenium were mechanically alloyed and melted in a button arc furnace under an argon atmosphere to produce two alloys of composition Ti-52.5Al-10.0Ni (at.%) and Ti-52.5Al-10.0Ni-0.2Ru (at.%). The alloys were then cut and metallographically prepared. Scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) were used to characterize the samples. Thermogravimetric analysis (TGA) was used to analyze the oxidation behavior from room temperature up to 1050°C. The alloys were also oxidized in air at 1050°C for 120 hours. The Ti-52.5Al-10.0Ni (at.%) alloy formed dendrites of γ-TiAl (55.6 at.% Al) surrounded by a eutectic of γ-TiAl + Al₃NiTi₂ (τ₃) phases. The Ti-52.5Al-10.0Ni-0.2Ru (at.%) alloy formed dendrites of γ-TiAl (53.6 at.% Al) surrounded by a eutectic of γ-TiAl + Al₃NiTi₂ (τ₃). The ruthenium was mostly in solid solution (0.3 at.%) in the Al₃NiTi₂ (τ₃) phase, although traces of it were present in the dendrites (0.1 at.% Ru). When oxidized in air from room temperature to 1050°C, the as-cast Ti-52.5Al-10.0Ni-0.2Ru (at.%) had a mass gain of 0.60% and the as-cast Ti-52.5Al-10.0Ni (at.%) had a mass gain of 0.97%. Isothermal oxidation of both alloys at 1050°C for 120 hours formed mixed metal oxides of TiO₂+Al₂O₃ on the surface.