Microstructures and surface roughness of EBM produced Ti-6Al-4V

The work presented in this dissertation is concerned with the microstructures and surface roughness of test slabs of Ti-6Al-4V produced by one of powder based Additive Manufacturing (AM) technique namely Electron Beam melting (EBM). The effects of process parameters of a EBM system and geometry factors of a EBM build such as slabs’ thickness and height etc. on the microstructure and the surface roughness of the EBM produced Ti-6Al-4V have being investigated. The processing parameters of the EBM system involved in the present work include beam current, scan speed, offset focus and scanning length etc. In this study three different batches of samples were prepared. Microstructures of EBM built Ti-6Al-4V were studied using Optical Microscopy (OM), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and X-ray diffraction (XRD). Confocal microscopy and image analysis codes ImageJ’s routine SurfCharJ were used to quantify the surface roughness of the test slabs. The microstructures of EBM built Ti-6Al-4V in general consist of columnar grains of prior β phase. Inside the columnar grains there is a typical (α+β) microstructure of titanium alloys containing Widmanstätten α platelets and rod-shaped β phase. Grain boundary α layer has also been observed on the grain boundaries of prior b columnar grains. By using TEM, the β phase in EBM built Ti-6Al-4V has been identified as a rod-like structure located on the grain boundaries of the fine a grains and often grows up along the build direction. The size of the β rods is about 200nm and the distance between the β rods is ranged between 0.5 -2μm. Chemical compositions for different phases have been measured by TEM/EDX and volume fraction of the β phase in the EBM Ti-6Al-4V has been determined to be 2.7%. The phase transformation sequence in EBM built Ti-6Al-4V has been discussed according to processing history and microstructures observed. It has been observed that, the size, number and geometry of prior β columnar grain depend on the sample thickness and other process parameters setting. The diameter of columnar grain varies between 2-70μm. The increase in size, number and regularity of columnar grains has been 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 platelets increases with increase in diameter of prior β columnar grain. The β phase rods are unaffected by sample thickness and process parameter settings. The sample thickness and beam energy density has a strong effect on the surface roughness of the test slabs. The value of surface roughness coefficient Ra for different test slabs varies between 1-20μm. It is observed that the surface roughness increases of the test slab increases with increase in thickness of the slab and beam energy density. The possible reasons for these variations in microstructures and surface roughness have been discussed