2.4.1 Particle Size Analysis
The particle size distribution for all powders used and produced in this study was Recrystallization annealing Beta annealing
analyzed by a Malvern Mastersizer 2000 laser-scattering instrument. For the analysis, a few grams of a powder sample were dispersed in water to reach the detection level for determining the particle size distribution. The maximum diameter of powder particles which can be detected is 2 mm.
2.4.2 Oxygen Analysis
As-forged samples was cut into 1mm×1mm×1mm cubes, and then quantitative chemical analysis (oxygen, nitrogen and hydrogen) was done at Durkee Testing Laboratories, Inc. USA, using a LECO combustion method.
2.3.3 Thermal Analysis
Differential thermal analysis (DTA) was carried out on blended and milled powders using a TA Instrument SDT 2960. The powders were heated up to 1300
o
C in an alumina crucible at a heating rate of 10 oC/min, with flowing argon as the protective atmosphere.
2.3.4 Density Measurement
The density of green and as-sintered powder compacts were calculated in Equation 2.1 by dividing their mass with their corresponding volumes, measured by a Nextengine 3D scanner shown in Figure 2.11.
Density =VolumeMass (2.1) To evaluate the pososity eliminated by sintering, a densification parameter is calculated from the sintered density and green density of a powder compact as follows:
Figure 2.11: Nextengine 3-D scanner.
2.4.5 Optical Microscopy
An examination of the powder and the as-forged and heat treated parts by optical microscopy was carried out using an Olympus BX60 optical microscope equipped with a digital camera. For metallographic examination of the powders, samples were prepared by mounting the powders in an epoxy resin at room temperature. For examining consolidated parts, samples for metallography were cut from the as-forged and heat treated parts by electric discharge machining (EDM) wire cutting. Both mounted and bulk samples were ground step by step using 120, 320, 600, 1200, 2000 and 4000 grit SiC papers to produce flat surfaces. After grinding, the flat surfaces were polished to a “mirror” finish using an alumina dispersion with a particle size of 0.3 μm. The polished samples were used for X-ray diffraction (XRD) analysis, optical microscopy (OM), porosity distribution measurement and scanning electron microscopy (SEM).
To observe the microstructure of the samples, their polished surfaces were etched using Kroll’s solution, consisting of 2 ml HF, 6 ml HNO3 and 92 ml distilled water.
Images of the microstructures were captured using digital cameras attached to the optical microscope and scanning electron microscope.
2.4.6 Porosity Distribution Measurement
camera attached to an optical microscope. Two-dimensional porosity distributions, perpendicular to and along the forging direction, were determined by analyzing these digital OM images using IQ image analysis software. The OM images were taken at equal intervals of distance from the edge to the center of the parts along the two directions mentioned above.
2.4.7 X-ray Diffraction
X-ray diffraction (XRD) analysis was carried out using a Philips X’ pert system diffractometer with Cu Kα radiation source and an incidence beam at a scan rate of 2o/min. The working conditions of the X-ray tube were a voltage of 20 kV and current of 40mA. The scan range of 2θ was from 20 o
C to 90 oC, and the results were matched with standard X-ray diffraction powder pattern from PDF cards of Ti, AlV and Ti-6Al-4V.
2.4.8 Scanning Electron Microscopy
A Hitachi S4700 scanning electron microscope (SEM) was used to observe the microstructures and morphology of specimens, and the elemental content and distribution of specimens were measured by an energy dispersive X-ray spectrometer (EDS) attached to the scanning electron microscope. Resin mounted specimens were coated by a thin layer of carbon before examination, but it was not necessary to coat the bulk samples since they are electrically conductive.
2.4.9 Transmission Electron Microscopy
The microstructure of as-forged parts was examined using a CM30 Philips transmission electron microscope (TEM). A double tilt holder was used to hold the specimens. To prepare specimens for TEM, slices with a thickness of around 0.5 mm were cut from as-forged parts using an EDM wire cutter, and then the slices were ground to reduce their thickness to about 50 μm by 120, 320, 600, 1200 and 2000 grit SiC abrasive papers. Then several disks with a diameter of 3
mm were punched from each slice, and finally each disk was further thinned by electrical jet polishing to produce a hole. The conditions used for jet polishing were a voltage of 10 mV a current of 10 mA and a temperature ranging from -40
o
C to -30 oC. The jet polishing solution was composed of 60vol.% methanoll, 35vol.% Butanol and 5vol.% perchloric acid.
2.4.10 Tensile Testing
The shape and dimensions of specimens used for tensile testing are shown in Figure 2.12. Tensile testing was done using an Instron 33R4204 universal testing machine with a load cell of 5 kN at room temperature. An extensometer with a gauge length of 10 mm was used to record the strain during tensile testing. A strain rate of 8.3×10-5 s-1 was used for all of the tensile tests. For the as-forged parts, tensile test specimens were cut one by one in a direction perpendicular to the forging direction using an EDM wire cutter, as shown in Figure 2.13. Specimens from as-sintered parts, were cut along the powder pressing direction. The rough surfaces of the cut specimens were ground by 120, 320, 600, 1200 and 2000 grit SiC papers to remove the effect of surface roughness on tensile properties.
Figure 2.12: Shape (a) and dimensions (b) of the tensile test specimens.
Figure 2.13: A schematic diagram showing the orientation used for cutting tensile test specimens from an as-forged rocker arm.