Heat Treatment of In Situ Laser-Fabricated Titanium Aluminide

Shaik Hoosain,Christopher Freemantle,Monnamme Tlotleng,Sisa Pityana

doi:10.3390/met8090655

Abstract

Direct energy deposition (DED) via laser processing, operated under standard conditions with a localised shielding gas, is a potential method for the manufacture of the γ-TiAl alloy. The freedom of operation, which includes the production of components via in situ melting of elemental powders, makes this method economically attractive. The goal of this study was to optimise the mass flow rates that lead to gamma phase formation during laser in situ melting of Ti and Al. A 3 kW Nd:YAG laser was used to melt Ti and Al elemental powders. Single clads were produced on Ti6Al4V substrates under localised argon shielding. The samples were heat-treated to promote microstructural homogenization and to provide thermal stress relief, after which they were characterized. Lamellar and duplex microstructures were obtained; depending on the Al feed rate and heat treatment temperatures. The Vickers microhardness was found to be predominantly dependent on Al content and the amount of twinning present. X-ray diffraction detected a proportional increase in the intensity of the γ phase peak with an increase in Al content, while α2 peaks were dissolved and the twin γ-Ti3Al5 peaks diminished slightly. An alloy produced in this work achieved the target microstructure and properties associated with superior ductility and tensile strength in these materials, indicating that the technology has future potential in the production of Ti-Al materials for applications such as structural components or thermal barrier coatings.

Highlights

Titanium aluminide (Ti-Al) alloys have a number of interesting properties such as low density, high specific strength, substantial mechanical strength at high temperatures, and good creep resistance [1,2]
Heat treatment was performed in accordance with the binary phase diagram and was dependent on the Al content and the target phase to be achieved during heating and cooling
Additive manufacturing of Ti-Al is usually conducted in an inert enclosure using electron beam melting and selective laser melting processes, which can be restrictive and costly

Summary

Introduction

Titanium aluminide (Ti-Al) alloys have a number of interesting properties such as low density, high specific strength, substantial mechanical strength at high temperatures, and good creep resistance [1,2]. Various applications have been investigated with a view to replacing the heavier nickel-based superalloys for the generation of aircraft and automotive engine components [1,2,3,4,5,6]. From an engineering point of view, the most interesting and viable alloys are within the 44–48 at. This is due to the eutectoid reaction that takes place during cooling and solidification. The properties of these alloys are controlled by their chemistry and the resulting microstructure.

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