Development of Ti6Al4V and Pure Tungsten 3D printing process using DED

Abstract

This thesis presents experimental investigations of 3D printing Ti6Al4V and pure tungsten structures by Directed Energy Deposition (DED). Experiments were conducted using an in-house DED printing system. First, the effect of build direction, wall thickness and heat treatment on the tensile strength and elongation were investigated. The strength of as-built samples were high enough to meet the ASTM requirements. However, only the samples built in the transverse direction met the requirement of elongation. Heat treatment made the mechanical properties isotropic and enhanced the elongation up to ~10 %. As the thermal condition was different depending on the wall thickness, it affected mechanical properties, especially samples with short cooling time between subsequent line scans. Second, this work reports successful fabrication of pure tungsten structures by DED, revealing the required process conditions. The effect of laser power, scan speed, powder feed rate and carrier gas velocity on the stability and properties of the structures is first analyzed, based on which the proper process condition for effective 3D printing of tungsten parts is proposed. Analyses of the fabricated samples show that the density and the hardness can be as high as 18.9 g/cm3 (98.4 % of the theoretical value) and 3.9 GPa, respectively. The results indicate that the optimal condition for 3D printing of tungsten is 400 ~ 530 J/mm2 in terms of specific energy and that high-speed or high-mass injection of powder can induce waviness on the surface.