Influence of heat treatments on microstructure of electron beam additive manufactured Ti-6Al-4V alloy

magistrsko delo

Damir Skuhala (Author), Tomaž Vuherer (Mentor), Fernando Gustavo Warchomicka (Co-mentor), Florian Pixner (Co-mentor)

Abstract

Additive manufacturing of metallic parts is increasing in popularity and starting to emerge as a new competitive manufacturing process. Printed structures from Ti-6Al-4V titanium alloy, produced by electron beam additive manufacturing (EBAM), possess columnar prior β grains and layer bands, alongside an ultrafine lamellar microstructure, which is prone to low ductility and thus requiring thermal post-processing. Several heat treatments were performed in α + β and β field, in one or multiple stages. The results showed that bi-lamellar microstructure can be obtained, and that selection of annealing temperature and cooling rate determines the morphology, thickness, and distribution of both primary and secondary α features. Mechanical properties were evaluated on three selected heat treatments. Annealing of the As-built condition was performed at 710°C (HT1) and 870°C (HT2), resulting in lamellar microstructure with basketweave morphology. In two-stage heat treatment (HT3), the temperature in the first stage has exceeded β transus, while in the second, annealing was performed again at 870°C. The microstructure was characterized as a mixture of lamellar and bi-lamellar with large α colonies inside the rearranged prior β grains. Air cooling was performed in all HT from the final annealing stage. Strength and hardness have decreased with increasingly coarser microstructural features, while fracture toughness was improved, except in HT1, where the decrease in the fracture toughness was mainly attributed to reduced intrinsic toughening. As-built and HT1 conditions were eﬀected by microstructural texture, causing inconsistent fracture morphology, reduced crack roughness and scattering in results. The influence of texture was decreased by coarser microstructure in HT2, while crack tortuosity was increased. Very unpredictable fracture behaviour was observed in HT3 due to large α colonies, as their orientation determines the areas of ductile or cleavage crack propagation.

Keywords

titanium alloys;Ti-6Al-4V;additive manufacturing;EBAM;heat treatments;microstructural optimization;mechanical properties;fracture toughness;

Data

Language:	English
Year of publishing:	2020
Typology:	2.09 - Master's Thesis
Organization:	UM FS - Faculty of Mechanical Engineering
Publisher:	[D. Skuhala]
UDC:	620.17/.18:669.295.055-15(043.2)
COBISS:	15353347
Views:	809
Downloads:	154
Average score:	0 (0 votes)
Metadata:

Other data

Secondary language:	Slovenian
Secondary title:	Vpliv toplotnih obdelav na mikrostrukturo aditivno navarjene titanove zlitine Ti-6Al-4V z uporabo elektronskega snopa
Secondary abstract:	Uporaba aditivnih tehnologij, s katerimi je mogoče izdelati kompleksne geometrije, postaja vedno bolj zanimiva alternativa klasičnim proizvodnim procesom. Strukture iz titanove zlitine Ti-6Al-4V so izdelane s tehnologijo taljenja dodajalne žice z elektronskim snopom. Strjevalne razmere med procesom navarjanja zaradi velikih ohlajevalnih hitrosti spodbujajo nastanek stebrastih (usmerjenih) β-zrn in toplotno vplivanih območij (meje med posameznimi varki), ki so vidna na makroskopski skali. Mikrostruktura je definirana kot zelo fino lamelarna, s prepleteno morfologijo. Izkazuje dobre trdnostne lastnosti, a je raztezek zelo omejen. Potrebna je naknadna toplotna obdelava, s katero optimiziramo mikrostrukturo. Toplotne obdelave so bile izvedene v območjih α+β in β, in sicer v eni ali večih stopnjah. Rezultati so pokazali, da s pravilno izbiro temperature in načinom ohlajanja lahko dosežemo bilamelarno mikrostrukturo z različnimi morfologijami, debelino in razporeditvijo tako primarne kot sekundarne α-faze. Tri toplotne obdelave so bile izbrane za določitev mehanskih lastnosti. Začetna mikrostruktura je bila žarjena pri 710 °C (TO1) in 870 °C (TO2). V obeh primerih je mikrostruktura lamelarna ter ohranja prepleteno morfologijo in stebrasta β-zrna. Tretja toplotna obdelava (TO3) je bila izvedena v dveh stopnjah. Temperatura v prvi stopnji preseže temperaturo alotropske modifikacije, v drugi stopnji je žarjenje ponovno potekalo na 870 °C. Mikrostruktura se spremeni v kombinacijo lamelarne in bilamelarne, z usmerjenimi α-kolonijami znotraj transformiranih β-zrn. Po toplotnih obdelavah postaja mikrostruktura vedno bolj groba, kar se odraža v zmanjšanih trdnostnih lastnostih in trdoti, poveča pa se lomna žilavost, razen pri TO1. Nižja žilavost je povezana predvsem z zmanjšanim deformacijskim utrjevanjem. Mikrostruktura pri začetnem in žarjenem stanju (TO1) je podvržena neenakomerni razporeditvi mikrostrukturnih značilnosti (velik vpliv mikrostrukturne teksture), kar se kaže v različni morfologiji preloma, raztrosu končnih rezultatov in obliki odpornostnih krivulj. Vpliv mikrostrukture teksture pri stanju TO2 je zaradi debelejših lamel manjši, fronta razpoke pa je bolj groba. Lomno obnašanje pri TO3 je nepredvidljivo zaradi velikih kolonij, ki so posledica neenakomerne razporeditve in velikosti β-zrn. Orientacija kolonij določa območja duktilnega ali krhkega napredovanja razpoke.
Secondary keywords:	titanove zlitine;zlitina Ti-6Al-4V;aditivne tehnologije (EBAM);toplotne obdelave;optimizacija mikrostrukture;mehanske lastnosti;lomna žilavost;magistrske naloge;
Type (COBISS):	Master's thesis/paper
Thesis comment:	Univ. v Mariboru, Fak. za strojništvo, Konstrukterstvo
Pages:	XII, 99 str.
ID:	11542126