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Du, Xi (2024) Performance of Ti-6Al-4V by laser powder bed fusion: powder feedstock and heat treatment considerations. PhD thesis, University of Nottingham.

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Abstract

Laser powder bed fusion (LPBF) has received significant attention in the past decade due to its capacity to produce complex shaped 3D metallic parts from digital models. Ti and Ti alloys exhibit high potential for medical and aerospace applications due to their low density, high specific strength, good corrosion properties, and good biocompatibility. Among these Ti alloys, Ti-6Al-4V (α+β type) is the most common titanium alloy for various applications. The fabrication of Ti-6Al-4V by LPBF has received extensive attention in recent years. However, the performance of Ti-6Al-4V by LPBF with powder feedstock and heat treatment considerations is not fully understood. This thesis aims at understanding and optimising the performance of Ti-6Al-4V by LPBF considering the role of powder feedstock (including particle size distribution (PSD) and chemical composition) and heat treatment.

First, the PSD changes of reused powder were examined by laser diffraction and scanning electron microscope (SEM) image method. PSD coarsening of reused powder was observed. The PSD coarsening mechanisms were systematically investigated by analysing various types of powder collected in different locations of the build chamber. PSD of various collected powders and interactions between laser and powder material were analysed. There are three main reasons for coarsening mechanisms, namely, layer thickness, spatter formation, and sintered powder after laser scanning borders. Besides, the effect of PSD coarsening of reused powder on the porosity and surface quality of as-built parts was investigated. After that, three different PSD powder feedstocks were obtained via the sieving method with a 38 µm sieve mesh. The effect of different PSD powder feedstocks on in-process powders, single-track formability, and the performance of Ti-6Al-4V by LPBF was systematically studied. A link can be established between different PSD powder feedstocks, in-process powders, and the performance of Ti-6Al-4V. It is suggested that fine powder feedstock (PSD range ~15-38 µm) presents better surface quality and relative density. Significant differences in the microstructure and mechanical performance of Ti-6Al-4V by different PSD powder feedstocks cannot be found.

As-built Ti-6Al-4V by LPBF typically shows columnar prior-β grain boundaries with fine martensite α′ inside, resulting in high strength (tensile strength more than 1200 MPa) but poor ductility (elongation less than 10%). Due to the limitations of PSD on tailoring the microstructure and mechanical performance of Ti-6Al-4V, chemical composition modification and heat treatment were considered to tune the microstructure and achieve balanced strength-ductility. In-situ chemical composition modification of Ti-6Al-4V via CP Ti additions was employed in this thesis. After diluting Ti-6Al-4V with different CP Ti additions, different nominal Ti-4.5Al-3V (with 25 wt.% CP Ti, called PT25), Ti-3Al-2V (with 50 wt.% CP Ti, called PT50) alloys can be obtained. Microstructure, mechanical properties, and related mechanisms of Ti-6Al-4V, nominal Ti-4.5Al-3V, and nominal Ti-3Al-2V were studied. It is found that the martensite α′ size is increased, and the crystal lattice parameters c/a ratio is decreased when CP Ti addition increases. A trend of decreasing strength and increasing ductility can be observed with increasing addition of CP Ti. PT50 presents a well-balanced strength (tensile strength ~1000 MPa) and ductility (~13%) in the as-built state without post heat treatment. The decreased c/a ratio promoting the dislocation slip may mainly make contribute to the increased ductility.

Since balanced strength and ductility can also be achieved by common heat treatment, heat treatment was employed in the thesis to compare with the chemical composition tailoring approach. Microstructure and mechanical properties of Ti-6Al-4V at stress relief temperature (730 ℃, called HT730) and annealing temperature (900 ℃, called HT900) were investigated. The main work focused on the heat treatment of Ti-6Al-4V in this thesis was to investigate the deformation behaviour of heat treated Ti-6Al-4V after tensile testing using electron backscatter diffraction (EBSD). Deformation twins can be observed in heat treated Ti-6Al-4V, especially in HT900. Deformation twinning behaviour of heat treated Ti-6Al-4V was further analysed, including twin variants and the Schmid factor. More deformation twins, more twin variants, and more twin-twin intersections can be found in HT900 compared to HT730. Besides dislocation slip, twinning-induced plasticity (TWIP) may also contribute to increased ductility after heat treatment, in contrast to chemical composition modification. The differences in microstructure and mechanical properties mechanisms for chemical composition modification and heat treatment were discussed.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Clare, Adam Simonelli, Marco Murray, James
Keywords:	Laser powder bed fusion; Ti alloys; Powder feedstock; Particle size distribution; Heat treatment; Deformation behaviour; Deformation twins
Subjects:	T Technology > TN Mining engineering. Metallurgy
Faculties/Schools:	UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering
Item ID:	77430
Depositing User:	Du, Xi
Date Deposited:	26 Apr 2024 08:11
Last Modified:	26 Apr 2024 08:11
URI:	https://eprints.nottingham.ac.uk/id/eprint/77430

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