Shu Beng Tor

Metallic powder-bed based 3D printing of cellular scaffolds for orthopaedic implants: A state-of-the-art review on manufacturing, topological design, mechanical properties and biocompatibility

Metallic cellular scaffold is one of the best choices for orthopaedic implants as a replacement of human body parts, which could improve life quality and increase longevity for the people needed. Unlike conventional methods of making cellular scaffolds, three-dimensional (3D) printing or additive manufacturing opens up new possibilities to fabricate those customisable intricate designs with highly interconnected pores. In the past decade, metallic powder-bed based 3D printing methods emerged and the techniques are becoming increasingly mature recently, where selective laser melting (SLM) and selective electron beam melting (SEBM) are the two representatives. Due to the advantages of good dimensional accuracy, high build resolution, clean build environment, saving materials, high customisability, etc., SLM and SEBM show huge potential in direct customisable manufacturing of metallic cellular scaffolds for orthopaedic implants. Ti-6Al-4V to date is still considered to be the optimal materials for producing orthopaedic implants due to its best combination of biocompatibility, corrosion resistance and mechanical properties. This paper presents a state-of-the-art overview mainly on manufacturing, topological design, mechanical properties and biocompatibility of cellular Ti-6Al-4V scaffolds via SLM and SEBM methods. Current manufacturing limitations, topological shortcomings, uncertainty of biocompatible test were sufficiently discussed herein. Future perspectives and recommendations were given at the end.

Fabrication and microstructural characterisation of additive manufactured Ti-6Al-4V parts by electron beam melting

An industrial impeller has been successfully fabricated by using Ti-6Al-4V extra low interstitial (ELI) powder via an Arcam A2XX electron beam melting (EBM®) machine. With the introduction of the A2XX, Arcam allowed the production of complex large-scale parts for industrial applications. However, the implementation of as-built metallic additive manufacturing (AM) parts in the industry is still far from straightforward and requires a sound understanding of the processing and the material properties of as-built part. This study examines the EBM thermal process influencing the microstructure evolution of Ti-6Al-4V by the EBM process. Ti-6Al-4V blocks with varying thicknesses were fabricated to simulate the different thickness of complex parts. The microstructure and Vickers hardness were evaluated. This paper mainly contributes to the EBM manufacturing of the industrial impeller and the build thickness dependence of microstructure and mechanical properties in EBM additive manufactured Ti-6Al-4V.

Low temperature and deformation-free bonding of PMMA microfluidic devices with stable hydrophilicity via oxygen plasma treatment and PVA coating

In the present study, a low temperature and deformation-free bonding method to seal hydrophilicity-stable microchannels in polymethyl methacrylate (PMMA) microfluidic devices was proposed. Oxygen plasma pre-/post-treatments and polyvinyl alcohol (PVA) coating were performed on the PMMA microfluidic substrates before thermal bonding. Different surface treatments were characterized using a confocal profilometer, tensiometer, attenuated total reflection Fourier-transform infrared spectrometer (ATR-FTIR), differential scanning calorimeter (DSC), and scanning electron microscope (SEM). The effects of surface modification on bond strength and microchannel integrity were studied. Oxygen plasma treatment prior to PVA coating was found to be significant in improving the coating uniformity and enhancing the adhesion between the PVA layer and the PMMA base. The PMMA microchannel substrate could be bonded to a homogeneous PMMA cover plate at about 70 °C (30 °C lower than the PMMA glass transition temperature, Tg) with the coated PVA layer serving as a medium material which had high wettability, high surface energy and low Tg. With oxygen plasma post-treatment of the coated PVA layer, the bond strength was improved and comparable to that obtained by pure thermal bonding at a temperature near the Tg of PMMA. Due to the low bonding temperature, the microchannel integrity was well retained with negligible deformation. The hydrophilicity stability of different surface treatments was evaluated and compared under both dry and wet storage conditions for a month. The results suggested that the PVA-coated PMMA substrates with oxygen plasma post-treatment present the highest hydrophilicity and lowest hydrophobic recovery. As a demonstration, monodispersed oil-in-water (O/W) droplets with volumes of sub-10 nL were successfully and reliably generated in the hydrophilic microfluidic devices fabricated with the proposed bonding method.

Investigation on processing of ASTM A131 Eh36 high tensile strength steel using selective laser melting

ABSTRACT In this paper, selective laser melting (SLM) technique was used to investigate the processing of EH36 high tensile strength steel commonly used in the shipbuilding applications. EH36 powder was produced according to ASTM A131 standards using gas atomisation process. SLM process parameters, including scanning speed and hatch spacing, were investigated to produce test specimens with high density. Parts were successfully built using SLM without cracks. Density tests were performed according to ASTM B962 standards. Light optical microscopy and scanning electron microscopy showed slight porosities and martensitic microstructure respectively. The study concluded that EH36 parts could be produced using SLM and this provided foundation work for the technical feasibility of fabricating high tensile strength steel components for the shipbuilding industry.

Geometry dependence of microstructure and microhardness for selective electron beam-melted Ti–6Al–4V parts

ABSTRACT In an additive-manufactured metallic part, distinct and different microstructure and mechanical properties may exist in different areas due to differences in shape and location. Two parts, one with straight-finned structure and the other with curve-finned structure, were fabricated by the selective electron beam melting method using pre-alloyed Ti–6Al–4V ELI powder. Microstructural characterisation of these two parts that have varying fin thickness and shape was carried out to investigate the synthetical influence of 2D planar build geometry and in-fill hatching strategy on selective electron beam melting. It was found that the β interspacing is larger in the curve-finned structure, leading to a lower microhardness as compared to the straight-finned structure. It suggests a slower cooling rate in the curve-finned structure due to the differences in build geometry and in-fill hatching strategy.

Effects of Injection Molding Parameters on the Production of Microstructures by Micropowder Injection Molding

ABSTRACT Micropowder injection molding (μPIM) is a potential low-cost process for the mass production of metal or ceramic microstructures. In order to obtain good molded microstructures and to avoid molding defects, it is important to select suitable injection molding parameters. In this paper, the selection of injection molding conditions for the production of 316L stainless steel microstructures by μPIM is presented. Silicon mold inserts with 24 × 24 microcavities were injection molded on a conventional injection molding machine. The dimensions of each microcavity were Φ 100 μ m × depth 200 μm, giving an aspect ratio of 2. The distance between each microcavity was 200 μm. Five sets of experiments were conducted by varying one injection molding parameter at a time. The parameters included injection pressure, holding pressure, holding time, mold temperature, and melt temperature. Higher injection pressure and holding pressure were required during the injection molding process due to the small dimensions of the microcavities and the large number of microcavities (576 microcavities). High mold temperature was required for complete filling of the microcavities. Molded microstructures without visual defects were obtained using appropriate injection molding parameters. Catalytic debinding and sintering of the 316L stainless steel microstructures were successfully conducted.

Anisotropic Mechanical Properties in a Big-Sized Ti-6Al-4V Plate Fabricated by Electron Beam Melting

In this study, in order to realize the application of the electron beam melting (EBM) technology for the printing of large components, the microstructure and mechanical properties of a big-sized Ti-6Al-4V plate (6 mm×180 mm×372 mm) additively manufactured by EBM were investigated. The paper focused on the graded microstructure and anisotropic mechanical properties by using x-ray diffraction, optical microscope, scanning electron microscope, microhardness and tensile test. A gradual change in microstructure with an increase in build height was observed. The formation of a graded microstructure was observed and discussed based on the thermal history experienced during printing. The mechanical properties were influenced accordingly by the graded microstructure. Moreover, the specimens which were printed parallel and perpendicular to the printing directions exhibited high elongation of ~18% and ~14%, respectively. The anisotropy in ductility was also observed and discussed according to the columnar prior β structure and grain boundary α phases present.

Simultaneously enhanced strength and ductility for 3D-printed stainless steel 316L by selective laser melting

Laser-based powder-bed fusion additive manufacturing or three-dimensional printing technology has gained tremendous attention due to its controllable, digital, and automated manufacturing process, which can afford a refined microstructure and superior strength. However, it is a major challenge to additively manufacture metal parts with satisfactory ductility and toughness. Here we report a novel selective laser melting process to simultaneously enhance the strength and ductility of stainless steel 316L by in-process engineering its microstructure into a <011> crystallographic texture. We find that the tensile strength and ductility of SLM-built stainless steel 316L samples could be enhanced by ~16% and ~40% respectively, with the engineered <011> textured microstructure compared to the common <001> textured microstructure. This is because the favorable nano-twinning mechanism was significantly more activated in the <011> textured stainless steel 316L samples during plastic deformation. In addition, kinetic simulations were performed to unveil the relationship between the melt pool geometry and crystallographic texture. The new additive manufacturing strategy of engineering the crystallographic texture can be applied to other metals and alloys with twinning-induced plasticity. This work paves the way to additively manufacture metal parts with high strength and high ductility.3D printing: adding strength and ductility to stainless steelA steel alloy with both high tensile strength and ductility has been three-dimensional (3D) printed by researchers in Singapore. Additive manufacturing builds 3D objects by adding materials layer by layer, a relatively simple process for plastics. However, this manufacturing process is much more difficult for metals, which are susceptible to defects and internal pores. This is particularly problematic when the final product needs excellent mechanical properties, such as hardness or strength. Xipeng Tan and co-workers from Nanyang Technological University used a specific laser scanning strategy to melt metallic powders and form a stainless steel alloy with a zig-zag crystallographic microstructure. The tensile strength and ductility of their stainless steel samples were increased by approximately 16% and 40%, respectively, compared to an alloy with the typical microstructure.A creative approach to substantially enhance both the strength and ductility of SLM-printed metal parts was successfully demonstrated on the ubiquitous marine-grade stainless steel 316L. The new discovery improves the strength and ductility of stainless steel parts by ~16% and 40% compared with the typical 3D printing process and conventional manufacturing methods. Control of the crystallographic texture is key for this breakthrough, which was achieved by tailoring the geometrical features of the melt pool involved in the laser-based 3D printing process. The desired <011> crystallographic texture favors the activation of the nano-twinning mechanism, which simultaneously enhances the strength and ductility.

Application of Electron Beam Melting (EBM) in Additive Manufacturing of an Impeller

An industrial impeller has been successfully fabricated by using Ti-6Al-4V ELI powder via a newly installed Arcam A2XX electron beam melting (EBM) machine. EBM was found to be preferable to build the circular or complex-shaped parts with thin walls. Several problems that frequently take place during the EBM fabrication are proposed based on the practical experience. It is found that metallization peeling-off, warpage, “swelling” and arc trips are the main reasons resulting in the failures of building jobs. It is suggested to pay close attention to the following aspects: a thorough cleanliness of the entire EBM system before start, a reasonable placement of builds on the start plate, an optimized design on geometries, etc., for a successfully built part.

Morphological Box Classification Framework for supporting 3D scanner selection

ABSTRACT 3-Dimensional (3D) scanning systems are becoming more common in the industry nowadays, for inspection and reverse engineering (RE) purposes. Although technical specifications are provided with commercially available scanners, a question could be raised pertaining to the degree of sufficiency of the technical specifications typically provided, with regard to specific application needs such as the scanning of challenging objects. These challenging objects present a less than ideal working condition for some 3D scanners, and the specified accuracy cannot be achieved. This effect varies across different types of 3D scanning technology. A more intuitive specification with regard to the time taken and ease of use will be beneficial to the user, but often not available. Hence, this paper proposes a Morphological Box Classification Framework based on the functional decomposition of the non-contact 3D scanning technology, in order to help users better understand and compare 3D scanners efficiently, and choose a scanner for their application that is able to perform within their desired accuracy, time taken, and ease of use. A case study of 3D scanners evaluation using the proposed framework for a RE application is conducted, and results presented.