Residual Stress Measurements and Structural Integrity Implications for Selective Laser Melted Ti-6al-4v

Abstract

Selective laser melting (SLM) of Ti-6Al-4V has significant potential in the aerospace and biotechnology industries. SLM employs a focused laser beam to melt successive layers of metallic powder into complex components. This process can result in the generation of high thermally-induced residual stresses. These residual stresses, together with micro-flaws/ pores from the inherent fabrication process, may lead to premature fatigue crack initiation and propagation at relatively low cyclic stresses. The hole-drilling strain gauge method was used to evaluate residual stresses within SLM Ti-6Al-4V specimens, with the intention of understanding the associated mechanisms for the successful application of SLM Ti-6Al-4V in industry. OPSOMMING'Selective laser melting' (SLM) van Ti-6Al-4V het aansienlike potentiaal in die lugvaart en biotegnologiese bedrywe. SLM maak gebruik van 'n gekonsentreerde laser straal om agtereenvolgende lae metaal poeier te smelt en sodoende intrieke metal komponente te vorm. Gedurende SLM kan hoë resterende spanning veroorsaak word as gevolg van hoë temperature. Hierdie spanning, asook mikrodefekte wat gedurende die vormingsproses ontstaan, mag lei tot voortydige materiaalverswakking deur spleet vorming en verspreiding gedurende betreklik lae sikliese spannings. Resterende spanning in Ti-6Al-4V eksemplare is bereken deur 'n boor-versonke rekstrokie metode om sodoende die meganismes verantwoordelik vir die suksessvolle toepassing van SLM in die Ti-6Al-4V bedryf te bepaal.

Heat Treatment of Ti-6al-4v Produced by Lasercusing

“…High hardness values were observed for the as-built specimens, and are characteristic of the martensitic microstructure [12]. The course microstructure of the samples exposed to treatment A resulted in the lowest hardness values, possibly indicating a low strength-high ductility material property combination.…”

“…The heat treatments were conducted in a vacuum furnace in order to prevent oxidation of Ti-6Al-4V above 427˚C [10]. A standard mill anneal is shown in Figure 1a for the purpose of comparison [12].…”

“…One concern is the introduction of residual stresses due to the localised thermal input from the laser. These stresses, which can exceed the yield strength of the material, result from the temperature gradient mechanism in which previously solidified layers hinder the thermal expansion of the newly-formed areas [12]. Residual stresses have been attributed to the degradation of mechanical properties, part distortion, and micro-crack formation [12,14,15].…”

“…These stresses, which can exceed the yield strength of the material, result from the temperature gradient mechanism in which previously solidified layers hinder the thermal expansion of the newly-formed areas [12]. Residual stresses have been attributed to the degradation of mechanical properties, part distortion, and micro-crack formation [12,14,15]. Furthermore, 100 per cent density is often difficult to achieve by SLM methods due to the absence of mechanical pressure (as in conventional procedures) and the difficulty of laying an even powder layer [14].…”

“…The good biocompatibility properties of Ti-6Al-4V, combined with the geometrical flexibility of LaserCUSING, makes the alloy an attractive option for creating customised medical implants [11]. The ability of SLM techniques to fabricate intricate cooling channels also extends the material's scope of applications to aero-engine turbine blades [12].…”