Powder manipulation, microstructure, mechanical properties and bio-corrosion performance of titanium and titanium alloys additively manufactured by selective electron beam melting

摘要

Additive manufacturing (AM), defined as a process of joining materials to make parts from three dimensional (3D) model data, usually layer upon layer, as opposed to subtractive and formative manufacturing methodologies, has been recognized globally as a group of revolutionary near-net-shape or net-shape fabrication technologies. AM offers advantages of more freedom in design, lower buy-to-fly ratio and shorter lead times. Selective electron beam melting (SEBM) is a powder-bed-fusion-based AM process, developed by Arcam AB in Sweden in 2002, which offers high energy efficiency and power density, rapid scan speed and unique capability of manufacturing high reactive metals such as titanium (Ti) due to the vacuum build chamber involved. Although much research has been devoted to the SEBM of Ti alloys, particularly Ti-6Al-4V, current understanding of the mechanical performance of SEBM-fabricated Ti components is still limited in a number of aspects. This thesis aims at enhancing the current understanding of the AM process of Ti and Ti alloys by SEBM in the following four aspects. 1. Manipulation and characterization of a novel Ti powder precursor for SEBM applications A low-cost novel Ti powder precursor (sponge Ti particles) has been manipulated using a proprietary powder manipulation technology (PMT) in order to produce a low-cost, nearly spherical Ti powder for SEBM applications. Research has shown that the PMT is capable of producing more than 50 wt.% of nearly spherical Ti powder in the size range of 45–106 μm (usable for SEBM) and about 30 wt.% of less than 45 μm of nearly spherical powder (usable for AM by cold spray processes). PMT-processed Ti powder with a size range of 75–106 μm exhibited similar flowability and spreadability to those of recycled Arcam Ti-6Al-4V powder when assessed in an external Arcam powder bed evaluation system. Cubic samples were built with the PMT-processed Ti powder using an Arcam A1 SEBM system under different SEBM parameters. The resulting density, surface conditions and microstructures of the as-built samples were investigated. It was concluded that through appropriate modification of the SEBM parameters in conjunction with the use of suitable melt strategies, it is feasible to produce quality samples with the newly developed low-cost nearly spherical Ti powder. This research demonstrates the potential of developing low-cost feedstock powder for AM by SEBM. 2. Positional dependence of microstructure and tensile properties of a thick Ti-6Al-4V block additively manufactured by SEBM Limited information exists in the open literature about the microstructure and mechanical properties of SEBM-fabricated thick-section (≥25.4 mm) Ti-6Al-4V samples or parts, while thick sections are involved in many components for structural applications. A systematic study has been made of the positional dependence of the microstructure and tensile properties of a 34mm-thick Ti-6Al-4V block additively manufactured by SEBM. Marginally graded microstructures were observed along the build direction and from the side surface to the centre. Abnormally coarse α laths, thick and tortuous grain boundary α phase, and massive α phase transformation products were observed. To assess the tensile properties, a total of 27 tensile samples were prepared from nine different heights of the block sample, and all samples satisfied the minimum requirements for mill-annealed Ti-6Al-4V, irrespective of their positions in the thick block. This conclusion demonstrates the capabilities of SEBM in producing quality thick-section Ti-6Al-4V components. A range of other revealing observations were documented and discussed. 3. The influence of as-built surface conditions and hot isostatic pressing (HIP) on tensile and fatigue properties of SEBM Ti-6Al-4V Achieving a high surface finish is a major challenge for most current metal AM processes. A quantitative study has been made of the influence of as-built surface conditions on the tensile and fatigue properties of Ti-6Al-4V produced by SEBM as compared to acid-etched and machined conditions. The experimental results indicate that chemical etching can double tensile elongation and noticeably improve tensile strengths due to improved surface finish. However, the fatigue strength remained to be much inferior to that of mill-annealed Ti-6Al-4V due to residual surface defects. Consequently, it remains challenging to modify the as-built surfaces of SEBM-fabricated components for fatigue-critical structural applications, particularly for those components which contain deep and narrow internal channels and complex concave and convex surfaces. HIP was employed to enhance the fatigue properties of SEBM-fabricated Ti-6Al-4V. Samples with different surface conditions (as-built, etched, machined and insufficiently machined) were subjected to HIP and their fatigue properties were evaluated under uniaxial tensile loading conditions. Although HIP can effectively improve the fatigue performance by healing most internal defects (i.e. pores and lack-of-fusion defects), it was found that surface defects played a more decisive role than internal defects in determining the fatigue properties of HIP-processed SEBM Ti-6Al-4V samples. It is expected that the fatigue properties of additively manufactured Ti-6Al-4V will continue to be a subject of considerable research in the near future. 4. The electrochemical responses of Ti-6Al-4V alloy manufactured by seven different processes including SEBM in Hank’s solution at 37oC Considering the increasing applications of additively manufactured Ti-6Al-4V implants, an in-depth understanding of the bio-corrosion performance of additively manufactured Ti-6Al-4V is necessary. For this reasons, samples of Ti-6Al-4V were manufactured by seven different processes, including SEBM, SEBM + HIP, selective laser beam melting (SLM), SLM + HIP, casting, mill-annealing, and spark plasma sintering (SPS). A comparative study was then made of the corrosion characteristics of these samples via the potentiodynamic polarization tests in Hank’s solution at 37oC. The microstructural features of each group of samples were characterized prior to corrosion. The corroded surfaces including the corrosion products were analysed. Owing to different microstructures, Ti-6Al-4V samples manufactured by different processes showed distinctly different polarization responses. The order of corrosion resistance was found to be: SLM & mill-annealed & SLM + HIP & SEBM & SEBM + HIP Cast SPS. The point defect model was used to interpret the different corrosion responses. The findings of this research provide a different perspective for the selection of manufacturing process for Ti-6Al-4V for bone implant applications.