Fatigue Performance of Additive Manufactured Ti6Al4V in Aerospace Applications

Abstract

Additive Manufacturing (AM) for metals includes is a group of production methodst hat use a layer-by-layer approach to directly manufacture final parts. In recent years, the production rate and material quality of additive manufactured materials have improved rapidly which has gained increased interest from the industry to use AM not only for prototyping, but for serial production. AM offers a greater design freedom, compared to conventional production methods, which allows for parts with new innovative design. This is very attractive to the aerospace industry, in which parts could be designed to have reduced weight and improved performance contributing to reduced fuel consumption, increased payload and extended flight range. There are, however, challenges yet to solve before the potential of AM could be fully utilized in aerospace applications. One of the major challenges is how to deal with the poor fatigue behaviour of AM material with rough as-built surface. The aim of this thesis is to increase the knowledge of how AM can be used for high performance industrial parts by investigating the fatigue behaviour of the titanium alloy Ti6Al4V produced with different AM processes. Foremost, the intention is to improve the understanding of how rough as-built AM surfaces in combination with AM built geometrical notches affects the fatigue properties.This was done by performing constant amplitude fatigue testing to compare different combinations of AM material produced by Electron Beam Melting(EBM) and Laser Sintering (LS) with machined or rough as-built surfaces with or without geometrical notches and Hot Isostatic Pressing (HIP) treatment. Furthermore, the material response can be different between constant amplitude and variable amplitude fatigue loading due to effects of overloads and local plastic deformations. The results from constant amplitude testing were used to predict the fatigue life for variable amplitude loading by cumulative damage approach and these predictions were then verified by experimental variable amplitude testing. The constant amplitude fatigue strength of material with rough as-built surfaces was found to be 65-75 % lower, compared to conventional wrought bar, in which HIP treatments had neglectable influence on the fatigue strength. Furthermore, the fatigue life predictions with cumulative damage calculations showed good agreement with the experimental results which indicates that a cumulative damage approach can be used, at least for a tensile dominated load sequences, to predict the fatigue behaviour of additive manufactured Ti6Al4V

Effect of Post-Processing Treatment on Fatigue Performance of Ti6Al4V Alloy Manufactured by Laser Powder Bed Fusion

“…Roughness has been verified to be the most crucial parameter in fatigue, more than the presence of defects, residual stresses or microstructure. According to [16], HIPed samples did not show significant improvements in fatigue life with rough as-built surfaces. Fatigue behavior was dominated by the rough surface rather than by internal defects of the material.…”

“…Machining is one of the most common treatments for surface modification of AM parts and leads to the highest fatigue strength if combined with HIP treatment [17]. As explained by Kahlin [16], the fracture mechanism is changed in machined samples with respect to as-built rough ones. Cracks initiate in the interior or subsurface of machined samples, whereas cracks occur on the surface of as-built parts.…”

3D-printing for Aerospace : Fatigue Behaviour of Additively Manufactured Titanium

“…This thesis builds upon the previous work of the licentiate thesis Fatigue Performance of Additive Manufactured Ti6Al4V in Aerospace Applications [3], which was presented in June 2017 and hence the text in chapter 3 in this thesis is taken nearly directly from [3].…”

A Comprehensive Review of Additive Manufacturing (3D Printing): Processes, Applications and Future Potential

“…The surface microstructure has an influence on the mechanical properties such as tensile strength, yield strength, fatigue strength, compressive properties, crack extension, shear resistance, harness and ultimate strength of the AM production parts. The effect of microstructure on the ultimate performance, durability and reliability of AM production parts was investigated by several researchers (Kahlin, 2017), (Chan, 2015). Calignano et al investigated the influence of process parameters (power, hatching distance and scan speed) on Aluminium alloy (AlSi10Mg) surface roughness using Direct Metal Laser Sintering (DMLS).…”