Date of Award

January 2018

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

First Advisor

Meysam Haghshenas

Abstract

Ti-6Al-4V alloy has been favored by the transportation applications in the automotive and aerospace industries due to its good combination of excellent physical and mechanical properties. Ti alloys are naturally suited to additive manufacturing (AM) method, a layer wise manufacturing technique, since conventional manufacturing method of Ti alloys are quiet challenging. However, cooling rate and thermal processing history of AM Ti-6Al-4V alloy are quite different in comparison to conventionally fabricated Ti-6Al-4V alloy which leads to undesirable microstructures in the AM Ti-6Al-4V alloy with respect to large columnar prior β grains being found to grow potentially across the entire height from bottom layer to top layer. Therefore, it is required to assess the microstructure-process-structure-property-performance relationship of the additive manufactured Ti-6Al-4V alloy to assess whether it could meet the demands of engineering design considerations.

The samples studied in this research were prepared using laser powder bed fusion (L-PBF) method, a well-developed AM process to print Ti-6Al-4V alloy in different scan direction and scan size. Instrumented indentation testing technique, a robust, reliable, convenient, and non-destructive characterization method to study small-scale mechanical properties in metals and alloys at ambient and elevated temperatures, was used to assess ambient-temperature indentation creep of AM Ti-6Al-4V alloy. To examine depth-sensing indentation creep behavior of Ti-6Al-4V alloy at ambient temperature, a dual-stage scheme (loading followed by a constant load-holding and unloading) at different peak loads of 250 mN, 350 mN, and 450 mN with holding time of 400 s was performed. Creep parameters i.e. creep rate, creep stress exponent, and indentation size effect were analyzed and compared with conventional findings, according to the Oliver and Pharr method, at different additive manufacturing scan directions and scan sizes.

The effect of post heat treatment (i.e. aging and solutionizing with different cooling rates) on the microstructure and micromechanical properties of a Ti-6Al-4V alloy processed by laser powder bed fusion (L-PBF) technique is studied. Heat treatment cycles employed in this study include solutionizing at 950 °C (for 1 h) followed by three different cooling rates (water quench, air cooling, and furnace cooling). A separate set of samples were also used toward artificial aging (solutionizing followed by water quenching and artificial aging). To assess small-scale properties of as-printed/ heat treated materials, instrumented nanoindentation testing technique as a robust, convenient, and non-destructive approach is employed. The martensitic α and ά in as -printed Ti-6Al-4V alloy grows in lamellar structure in epitaxial way upon various heat treatments below β- transus temperature. With the relatively steep cooling rate, the β phase recrystallization transforms into a compact secondary basket-weave α phase since the primary α-phase develops and connects each other with different orientations.

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Microstructural quantitative analyses (i.e. optical microscopy and scanning electron microscopy) were performed as well to assess processing parameter-microstructure-property correlations in the additively manufacture Ti-6Al-4V alloy. These studies were done in parallel to the two main tasks of this project to be able to elaborate the mechanical measurements with microstructural evidences. Also, the obtained results were compared against traditionally processed Ti-6Al-4V.

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