The effect of sample dimensions and process parameters (beam current, scan speed, offset focus and scan length) of electron beam melting (EBM) system on microstructure of the EBM built Ti-6Al-4V alloy was investigated. The microstructure of EBM built Ti-6Al-4V alloy consists of columnar grains of prior β phase. Inside the columnar grain, typical (α+β) structures, namely Widmanstätten α platelets and rod-like β phase formed on the interfaces of the fine α grains, are observed. Grain boundary α layer forms along grain boundary of prior β columnar grain. With the increase of thickness of the test slab, beam energy density and scanning length, the prior β columnar grain grows along the build direction and diameter of which increases. The columnar grain diameter also decreases with the increase in height of the test slab. With increasing the thickness of the test slab and beam energy density, α platelets get coarser. 

本文研究了试件尺寸和工艺参数（电子束强度，扫描速度，焦点偏移量和扫描长度）对EBM加工的Ti-6Al-4V微观结构的影响。通常，可以观察到EBM加工的Ti-6Al-4V的微观结构是由原始β相的柱状晶粒组成。在柱状晶粒内部观察到典型的（α+β）结构，即魏氏体α片和在细小的α晶粒的界面上形成的杆状β相。还发现沿原始β柱状晶粒的晶界形成α层晶界。随着试件厚度，电子束能量密度和扫描长度的增加，先前的β柱状晶粒的直径增大，并且生长的方向与加工方向一致。同时，柱状晶粒直径也随着高度的增加而减小。随着厚度和电子束能量密度的增加，α片会变得更粗大。

There is an internal, self-balanced stress known as residual stress (RS) that exists in all alloy systems without any external applied forces. Depending on the compressive or tensile nature and magnitude of the RS, it significantly affects the mechanical properties of the materials. Therefore, it is crucial to know the nature and magnitude of RS in material for safe and economical operation. In this work, we used unique, multipurpose, high energy (50-150 KeV) beamline I12-JEEP (Joint Engineering, Environment and Processing) at Diamond Light Source in UK with Energy Dispersive X-Ray Diffraction (EDXRD) setup to map up the RS states in one of the most popular titanium alloys with a code name Ti6Al4V. This type of titanium alloy is widely used in biomedical and aerospace industry because of their excellent combination of a high strength/weight ratio and good corrosion resistance. The Ti6Al4V which we were investigated are produced using electron beam melting (EBM) technique as a function of EBM processing parameters. In addition to relation between RS and processing parameters of EBM, the hydride formation versus processing parameters and as well as the effect of residual stress to the hydride precipitation in EBM built Ti6Al4V were investigated. To find out the effect of EBM processing parameters to the residual stress development, various samples produced with different beam size, scanning speed and different building thickness were investigated. From each type of the sample four specimens were prepared and three of them loaded with hydrogen in different concentrations, i.e. the hydrogen concentration of the various samples are 262, 772, 951 and 1410 ppm. Other than these alloy samples we also measured clean and hydrided original powder samples which are used for make these solid samples in our studies. After data collection, the whole pattern fitting method Rietveld and Pawely were performed with structure analysis software package Topas-Academic and GSAS. .

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.

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 β phase, that formed during the initial solidification, possesses a column shaped morphology with growing direction parallel to built direction. Typical (α+β) structures namely Widmanstätten α platelets with rod-like β phase formed on the interfaces of the fine α grains, have been observed in the columnar prior β grains. Grain boundary α phase was found to be formed around the boundaries of the columnar prior β grains. Different phases present in the parts, especially the BCC β phases have been characterized. The TEM/EDX results indicate very high V composition in the β phase. Results of TEM/SAED and XRD also revealed that a superlattice structure could be present in the β phase. Phase transformation sequence is discussed according to the processing history and the microstructures observed.

The work presented in this dissertation is concerned with the microstructures andsurface roughness of test slabs of Ti-6Al-4V produced by one of powder based AdditiveManufacturing (AM) technique namely Electron Beam melting (EBM). The effects of processparameters of a EBM system and geometry factors of a EBM build such as slabs’ thickness andheight etc. on the microstructure and the surface roughness of the EBM produced Ti-6Al-4Vhave being investigated. The processing parameters of the EBM system involved in the presentwork include beam current, scan speed, offset focus and scanning length etc. In this study threedifferent batches of samples were prepared. Microstructures of EBM built Ti-6Al-4V werestudied using Optical Microscopy (OM), Scanning Electron Microscopy (SEM), TransmissionElectron Microscopy (TEM), and X-ray diffraction (XRD). Confocal microscopy and imageanalysis codes ImageJ’s routine SurfCharJ were used to quantify the surface roughness of the testslabs. The microstructures of EBM built Ti-6Al-4V in general consist of columnar grains of priorβ phase. Inside the columnar grains there is a typical (α+β) microstructure of titanium alloyscontaining Widmanstätten α platelets and rod-shaped β phase. Grain boundary α layer has alsobeen observed on the grain boundaries of prior b columnar grains. By using TEM, the β phase inEBM built Ti-6Al-4V has been identified as a rod-like structure located on the grain boundariesof the fine a grains and often grows up along the build direction. The size of the β rods is about200nm and the distance between the β rods is ranged between 0.5 -2μm. Chemical compositionsfor different phases have been measured by TEM/EDX and volume fraction of the β phase in theEBM Ti-6Al-4V has been determined to be 2.7%. The phase transformation sequence in EBMbuilt Ti-6Al-4V has been discussed according to processing history and microstructuresobserved. It has been observed that, the size, number and geometry of prior β columnar graindepend on the sample thickness and other process parameters setting. The diameter of columnargrain varies between 2-70μm. The increase in size, number and regularity of columnar grains hasbeen observed with increase in sample thickness, beam energy density and scanning length.While with increase in height of the build it decreases. The length and smoothness of α plateletsincreases with increase in diameter of prior β columnar grain. The β phase rods are unaffected bysample thickness and process parameter settings. The sample thickness and beam energy densityhas a strong effect on the surface roughness of the test slabs. The value of surface roughnesscoefficient Ra for different test slabs varies between 1-20μm. It is observed that the surfaceroughness increases of the test slab increases with increase in thickness of the slab and beamenergy density. The possible reasons for these variations in microstructures and surfaceroughness have been discussed.

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.

Ti-6Al-4V alloy is an attractive biomaterial. The current work evaluates the microstructures of the solid and net-shape Ti-6Al-4V alloy produced by Electron Beam Melting (EBM) system using SEM/EDX and optical microscope. The microstructures are influenced by the cooling rate, processing parameters of the EBM system and re-heating of the existing layer during the melting of subsequent layers. Layer structure and columnar grains have been observed, with growing direction parallel to the built direction. The interior of these grains consists of alternating α / β phases. The β phase in the colonies resembles rod shape embedded in the α platelet. Along the grain boundaries more or less continuous α layers were observed. In comparison to solid samples uneven surfaces and pores were seen in the net shape structure. Microhardness evaluation of the EBM produced alloys was also carried out and compared with conventionally produced alloys.