Title

Manufacturing and properties of titanium-based materials produced by selective laser melting

Date of Award

2015

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Engineering

Faculty

Faculty of Health, Engineering and Science

First Advisor

Associate Professor Lai-Chang Zhang

Second Advisor

Associate Professor Mariana Calin

Third Advisor

Professor Jürgen Eckert

Abstract

This PhD study firstly presents the results of using selective laser melting (SLM) to produce commercially pure titanium (CP-Ti) parts. Accurate manipulation of SLM parameters is applied to produce nearly fully dense (>99.5%) CP-Ti parts without any posttreatments. Compared with output from traditional technologies, the microhardness, compressive and tensile strengths of SLM-processed CP-Ti parts have been improved to 261 Hv, 1136 MPa and 757 MPa, respectively, due to the formation of refined martensitic α΄ grains during SLM. Optimizing manufacturing parameters could enhance the strength and hardness of CP-Ti while maintaining its ductility. Fractography study of the tensilefailed specimens showed that incompletely melted particles and porosities caused early fracture in porous samples. Mixture of dimples and minor quasi-cleavage facets covered most fracture surface in fully dense samples.

Secondly, optimized tuning of the SLM manufacturing parameters achieved almost fully dense in-situ Ti-TiB composites from optimally milled Ti-TiB2 powder. X-ray and electron diffraction patterns as well as microstructural investigations indicated a chemical reaction during SLM in which irregular-shape titanium diboride (TiB2) particles react with pure Ti to form needle-shape TiB particles. Transmission electron microscopy (TEM) investigations revealed that Ti grains are refined significantly due to the existence of B. The microhardness, yield strength and compressive strength of the SLM-produced Ti-TiB composites increased to 402 Hv, 1103 MPa and 1421 MPa, respectively, compared to those of SLM-produced CP-Ti parts. These improvements are mainly due to strengthening and hardening effects induced by TiB particles and refinement of Ti grains. Fractography analyses showed that a mixture of splitting/shearing and smooth/rough zones covers the fracture surfaces of failed composite samples after compression testing.

Thirdly, SLM, powder metallurgy (PM) and casting technologies were applied to produce Ti-TiB composite parts from Ti-TiB2 powder. The ultimate compressive strength of SLM-processed and cast samples were 1421 MPa and 1434 MPa respectively, whereas the ultimate strengths of PM-processed 25%, 29% and 36% porous samples were 510 MPa, 414 MPa and 310 MPa respectively. Young’s modulus for porous composite samples were 70 GPa, 45 GPa and 23 GPa, for 25%, 29% and 36% porosity levels respectively which are lower than those of SLM-processed (145 GPa) and cast (142 GPa) samples. Fracture analysis of the SLM-processed and cast samples showed similar features but failure of porous samples occurred due to porosities and weak bonds among particles.

Fourthly, CP-Ti and Ti-TiB composite specimens with different porosity levels were obtained by SLM. Young’s moduli and yielding strengths of tested porous samples decreased with increasing of porosity and are observed in the range of 120-963 MPa and 12-68 GPa respectively for CP-Ti and 234‒767 MPa and 25‒84 GPa respectively for Ti- TiB. Elastic moduli of both 37% porous CP-Ti and Ti-TiB samples are close to that of human bone, leading to reduce stiffness mismatch between implant and bone.

This research demonstrates that SLM is a promising method for fabricating titanium-based samples with superior mechanical properties (i.e. tensile, compression and hardness) to those produced by casting and powder metallurgy techniques, thus improving the reliability of the SLM process for biomedical applications.

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