Electron Beam Melting (EBM) is a relatively new Powder Bed Fusion (PBF) Additive Manufacturing (AM) process. Unlike a very similar laser PBF process, the EBM process occurs in an Ultra-High Vacuum (UHV) and high temperature (~700C) chamber, reducing residual stress and providing superior protection against oxidation. This makes EMB ideal for processing oxygen sensitive materials like Ti-6Al-4V, whose high strength-to-weight ratio, corrosion resistance, and high temperature performance have drawn the interest of aerospace and other high-performance manufacturing. Due to the nature of these industries, fatigue life is of particular interest. However, the relationship between EBM processing and fatigue life is not well studied and is thus the focus of this dissertation.First, a L16 Taguchi Design of Experiments (DOE) was constructed to investigate the effects of Focus Offset, Line Offset, Speed Function, Hot Isostatic Pressing (HIP) treatment, and surface roughness on the Very High Cycle (VHC) fatigue life of Ti-6Al-4V. Two HIP treatments were 800°C and 200 MPa for 2 hours and 1100°C and 100 MPa for 2 hours with 2.5°C/min quench. Half of the samples were tested in the as-machined condition with an average roughness, Ra, of 0.2 μm and the other half were further polished using Magnetic Assisted Finishing (MAF) to Ra = 0.1 μm. An ultrasonic fatigue testing machine was used to test fatigue life at 500 and 550 MPa loads, with a load ratio of R = -1. Nearly 225 samples were tested with 7 repeats per load condition. Fatigue results indicated that none of the machine parameters and surface roughness had a statistically significant correlation with fatigue life. However, a statistically significant correlation between HIP treatment and fatigue life was found. The 800°C samples performed as well as, if not superior, to conventional Ti64 with the average fatigue lives of 8.08E+07 and 3.28E+06 cycles for 500 and 550 MPa, respectively. In contrast, the 1100°C samples had significantly lower fatigue performance with the average fatigue lives of 7.21E+05 and 1.38E+05 cycles for 500 and 550 MPa, respectively. Microstructure and fractography investigations suggest that the poor performance of 1100°C samples can be attributed to coarsening of the prior beta (β) grains during the super-transus HIP treatment leading to the formation of large colonies of similarly orientated alpha (α) grains, allowing for easier dislocation movement across aligned preferential slip directions. This study concluded that the most important factor controlling fatigue life of EBMed Ti-6Al-4V is post HIP/heat treatment and that fine-tuning of print settings beyond those required to prevent obvious porosity and swelling defects will not have significant effects on the fatigue life of HIPed Ti-6Al-4V.
Read
