Evaluation of microstructural development in electron beam melted Ti-6Al-4V

In the current work an investigation of the microstructures of EBM built Ti-6Al-4V test bars has been performed using OM, SEM, TEM and XRD. It has been found that the prior beta phase, that formed during the initial solidification, possesses a column shaped morphology with growing direction parallel to built direction. Typical (alpha+beta) structures namely Widmanstatten alpha platelets with rod-like beta phase formed on the interfaces of the fine alpha grains, have been observed in the columnar prior beta grains. Grain boundary alpha phase was found to be formed around the boundaries of the columnar prior beta grains. Different phases present in the parts, especially the BCC beta phases have been characterized. The TEWEDX results indicate very high V composition in the beta phase. Results of TEWSAED and XRD also revealed that a superlattice structure could be present in the beta phase. Phase transformation sequence is discussed according to the processing history and the microstructures observed

Near-Edge X-ray Absorption Spectra of Carbon-Nitride Molecules and Solids

Near-edge x-ray absorption fine structure spectra have been calculated for different carbon-nitrogen molecules, clusters and solids. The compounds investigated are used to model the chemical bonding in carbon nitride thin films. The molecular and cluster spectra are calculated employing the static exchange ab initio technique, while the solid state calculations are performed with the density functional full potential augmented plane wave method

3D printed Ti6Al4V implant surface promotes bone maturation and retains a higher density of less aged osteocytes at the bone-implant interface.

For load-bearing orthopaedic applications, metal implants having an interconnected pore structure exhibit the potential to facilitate bone ingrowth and the possibility for reducing the stiffness mismatch between the implant and bone, thus eliminating stress-shielding effects. 3D printed solid and macro-porous Ti6Al4V implants were evaluated after six-months healing in adult sheep femora. The ultrastructural composition of the bone-implant interface was investigated using Raman spectroscopy and electron microscopy, in a correlative manner. The mineral crystallinity and the mineral-to-matrix ratios of the interfacial tissue and the native bone were found to be similar. However, lower Ca/P ratios, lower carbonate content, but higher proline, phenylalanine and tyrosine levels indicated that the interfacial tissue remained less mature. Bone healing was more advanced at the porous implant surface (vs. the solid implant surface) based on the interfacial tissue ν1 CO3(2-)/ν2 PO4(3-) ratio, phenylalanine and tyrosine levels approaching those of the native bone. The mechanosensing infrastructure in bone, the osteocyte lacuno-canalicular network, retained ∼40% more canaliculi per osteocyte lacuna, i.e., a 'less aged' morphology at the interface. The osteocyte density per mineralised surface area was ∼36-71% higher at the interface after extended healing periods

Assessing the validity of theoretical results

We present recent theoretical results for the V3 and Au4 clusters. Calculations of the V3 doublet system indicate that the 6-311+G(d) basis set is sufficiently flexible to provide reliable minimum energy structures and vibrational frequencies, that these structures and frequencies are insensitive to spin contamination of the wave function when the BPW91 functional is used, and that changing to the B3LYP functional may result in very different structures and frequencies. A computationally less expensive scalar relativistic treatment of Au4 clusters gives structural properties that are in good agreement with those obtained using a four-component method

Effect of EBM processing parameters on surface roughness and microstructure of Ti-6Al-4V

Titanium and its alloys especially Ti-6Al-4V is an attractive biomaterial due to their excellent biocompatibility. Electron Beam Melting (EBM) is one of the Solid Free Form Fabrication (SFFF) methods to build 3D solid and near-net shape objects for medical implants and aerospace industry. EBM system utilizes a high energy electron beam to selectively melt a powder layer according to CAD file in a vacuum chamber. EBM system can manufacture complex 3D geometries requiring no or very little machining before use. The EBM systems are energy and material efficient. The microstructures and surface properties of objects produced by EBM can be influenced by the setting of different processing parameters in the EBM system. In this study solid slabs of Ti-6Al-4V, approximately 5 x 5 cm with various thicknesses were produced with different sets of processing parameters such as beam current, offset focus, scan speed and scan direction. The effects of these parameters on surface roughness, surface morphology and microstructure of slabs have been evaluated by using confocal microscopy, SEM /EDX and optical microscopy. The samples for optical microscopy and SEM were prepared by using standard metallographic methods. Microstructures of Ti-6Al-4V alloy produced by EBM usually consist of columnar grains as shown in Figure 1. These grains always grow parallel to build direction. Layers of different contrasts were observed in the samples where the layer interface is perpendicular to the build direction as shown in figure 2. The growth of columnar grains and appearance of layers with different contrasts were observed irrespective of the parameter values. These two observed phenomena can be attributed to the partial reheating / re-melting of the solidified layer by the electron beam during the melting of subsequent layer. The diameter of individual grain and density of grains are not uniform and usually decreases with increase in build height. Upon cooling from the β-transus temperature, more or less continuous α-layers were found to form along the prior β grain boundaries. In the EBM produced Ti-6Al-4V alloy the β -phase was found to be in rod-like geometry, with a size of 0.05-0.1μm in diameter, imbedded in the α-plates. It was observed that the high value of off set focus can cause porosity in the sample. For example the resultant porosity could be up to 11%. On the other hand where the value of offset focus is relatively small no such phenomenon was observed. Reconstruction of 3D surface topography and roughness coefficient (Ra) were computed by using images taken from confocal microscope and novel computer program “COMSTAT’’ [A.Heydorn et al (2000)]. Figure 3 shows a 3D reconstructed surface of the EBM produced sample. The Ra is computed by using the equation below: Where Lfi is the thickness of ith point, Lf is the mean thickness and N is the number of measurements. It has been observed that the value of Ra is processing parameters dependent. A sample with bigger thickness or higher current values tends to have relatively higher values of Ra. The scan speed and scan direction can also influence the surface morphology and microstructures of the EBM produced alloys

Effect of process parameters settings and thickness on surface roughness of EBM produced Ti-6Al-4V

Purpose – Ti-6Al-4V is one of the most attractive materials being used in aerospace, automotive and medical implant industries. Electron beam melting (EBM) is one of the direct digital manufacturing methods to produce complex geometries of fully dense and near net shape parts. The EBM system provides an opportunity to built metallic objects with different processing parameter settings like beam current, scan speed, probe size on powder, etc. The purpose of this paper is to determine and understand the effect of part's thickness and variation in process parameter settings of the EBM system on surface roughness/topography of EBM fabricated Ti-6Al-4V metallic parts. Design/methodology/approach – A mathematical model based upon response surface methodology (RSM) is developed to study the variation of surface roughness with changing process parameter settings. Surface roughness of the test slabs produced with different parameter settings and thickness has been studied under confocal microscope. Response surface methodology was used to develop a multiple regression model to correlate the effect of variation in EBM process parameters settings and thickness of parts on surface roughness of EBM produced Ti-6Al-4V. Findings – It has been observed that every part produced by EBM system has detectable surface roughness. The surface roughness parameter Ra varies between 1-20 µm for different samples depending upon the process parameter setting and thickness. The Ra value increases with increasing sample thickness and beam current, and decreases with increase in offset focus and scan speed. Originality/value – Surface roughness is related to wear and friction property of the material and hence is related to the life time and performance of the part. Surface roughness is an important property of any material to be considered as biomaterial. The surface roughness of the material depends upon the manufacturing method and environment and hence it is controllable either during fabrication or by post processing. From the 1st order regression model developed in this study, it is also evident that sample thickness, scan speed and beam current have relatively more effect on roughness value then the offset focus. With the model obtained equation, a designer can subsequently select the best combination of sample thickness and process parameter values to achieve desired surface roughness