|M.Sc Student||Malka Markovitz Dennis David|
|Subject||Heat Treatments and their Influence on Mechanical Properties|
and Microstructure of 3D Printed Ti-6Al-4V
|Department||Department of Materials Science and Engineering||Supervisors||Professor Emeritus Menachem Bamberger|
|Dr. Shaul Avraham|
|Full Thesis text|
The investigation of the microstructure and mechanical properties (tensile strength, ductility and hardness) of Ti-6Al-4V alloy manufactured by Selective Laser Melting (SLM) and their evolution during different Heat Treatments (HT) is presented in this work. SLM-produced samples are composed of a strained and brittle Martensite phase, thus, supplementary post-manufacturing HT is necessary in order to optimize the mechanical properties. The manufactured samples were subjected to various HT carried out in Argon and in air, by employing two different approaches:
1. Two-stage HT: in the first stage the samples undergo a preliminary β-phase solution treatment followed by Water Quenching (WQ). Then a second stage is applied, including annealing treatments at various temperatures within the αβ range, followed by Furnace Cooling (FC).
2. One-stage HT: annealing within the αβ range, followed by FC.
Both HT approaches induced partial decomposition of the strained and brittle Martensite phase during annealing at low temperatures (650°C-750°C), resulting in decreased strength and hardness along with increased ductility, altogether with the formation of a bi-phasic lamellar αβ microstructure for higher annealing temperatures (800°C-950°C), showing reduction of strength and hardness, yet with an increase in ductility. Regardless of the obtained microstructure, higher annealing temperatures led to a coarser microstructure. Nevertheless, in contrast to the SLM-fabricated parent material which was composed of a fine microstructure, induced by high Cooling Rates (CR) applied during the manufacturing process, and retained during annealing, leading to improved mechanical properties (2nd HT approach), the solution treatment applied in the 1st HT approach dissolved the unique fine-microstructure obtained in the SLM-manufacturing process, which was not formed again upon quenching, due to the substantially slower CR developed during water quenching, leading to reduced hardness and tensile strength due to the coarser microstructure. HT carried in air according to the 1st approach resulted in a substantial hardness increase as against the Argon-treated counterparts, as a result of the dissolution of O, N and C within the sample, inducing an interstitial strengthening effect which also lead to embrittlement.
In light of the findings of this study, it is recommended to implement a 2-hour annealing treatment at 800°C in Argon, followed by FC without a preceding β-phase solution treatment. This annealing treatment is sufficient to induce decomposition of the brittle Martensite phase, resulting in the formation of lamellar αβ, hence it increases the ductility, yet maintains its fine microstructure, that in turn, leads to high strength and hardness.