טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentZolotaryov Denis
SubjectPost-Processing of Additively Manufactured Ti-6Al-4V
Parts - Microstructure and Properties
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Emeritus Menachem Bamberger
Full Thesis textFull thesis text - English Version


Abstract

Titanium alloys are advanced structural materials that exhibit a number of unique properties, including high specific strength, high-fatigue performance and good fracture toughness. Ti-6Al-4V is a wildly used titanium alloy.

A relatively new production process called Additive Manufacturing (AM) has the potential to become a game-changer in the nearest future in the aerospace, automotive, microelectronics and other industries. Based on a layer-by-layer production process, this technology enables high flexibility in the production of components having complex geometry and superior properties, saving on energy costs, and quick prototyping capability.

In the last decade, Ti-6Al-4V has been a highly studied alloy for AM processes, from the point of view of printability, microstructure and mechanical properties. But there is still lack of knowledge about the influence of post-processing on the properties and microstructure of AM produced parts.

This study focuses on the effect of Laser Welding (LW) and Hot Isostatic Pressing (HIP) on the mechanical properties and microstructure of Ti-6Al-4V parts, produced by an Electron Beam Melting (EBM) system.

The effect of the Laser Welding parameters (laser beam power and welding speed) on the welding performance, was analyzed. A combination of high laser power (1.8 KW) and relatively high welding speed (10.5 - 12.0 mm/sec) was found suitable for full penetration of a sample of 2-mm thick and formation of a welded area free of pores or cracks.

According to the observed results, dramatic changes in the  microstructure were found in the HAZ and FZ with respect to the parent material, as evident by the  formation of martensitic α' plates in both the HAZ and FZ, which  leads to significant hardening of the material. The high microhardness observed in the HAZ and FZ is related to the formation of α′, whereas the low tensile strength of laser-welded samples is related to oxygen-based contaminations occurring in the seam line.

The influence of the HIP temperature regime on the mechanical properties was investigated. It was found that an increase in the HIP temperature resulted in reduction of the UTS the yield strength. For samples treated with HIP, both parameters, α-lath thickness and fraction of the β-phase, influence the tensile strength of Ti-6Al-4V - increase in the α-lath size and β fraction leads to  reduction in the yield and tensile strength of Ti-6Al-4V. The same correlation was found between the microhardness and the microstructural parameters (the α-lath thickness and fraction of the β-phase).

The lower the temperature of the HIP, the higher the average number of cycles to failure. The fatigue limit of EBM printed and HIPed (at 800⁰C) Ti-6Al-4V is around 670 MPa, which is fully comparable to mill annealed wrought Ti-6Al-4V. It was found that as the thickness of lamellae decreases, the fatigue strength increases. In addition, an increase in the content of the β phase related to an increase in the HIP results in a decrease in the fatigue resistance of the material.

It was observed that crack initiation during fatigue failure of HIPed material apparently occurs along the α/β interface for all specimens.