O. Carvalho

Tribological behavior of Ti6Al4V cellular structures produced by Selective Laser Melting

Additive manufacturing (AM) technologies enable the fabrication of innovative structures with complex geometries not easily manufactured by traditional processes. Regarding metallic cellular structures with tailored/customized mechanical and wear performance aiming to biomedical applications, Selective Laser Melting (SLM) is a remarkable solution for their production. Focusing on prosthesis and implants, in addition to a suitable Youngs modulus it is important to assess the friction response and wear resistance of these cellular structures in a natural environment. In this sense, five cellular Ti6Al4V structures with different open-cell sizes (100-500µm) were designed and produced by SLM. These structures were tribologicaly tested against alumina using a reciprocating sliding ball-on-plate tribometer. Samples were submerged in Phosphate Buffered Saline (PBS) fluid at 37°C, in order to mimic in some extent the human body environment. The results showed that friction and wear performance of Ti6Al4V cellular structures is influenced by the structure open-cell size. The higher wear resistance was obtained for structures with 100µm designed open-cell size due to the higher apparent area of contact to support tribological loading.

Improvement on Sliding Wear Behavior of Al/Cast Iron Tribopair by CNT's Reinforcement of an Al Alloy

The results presented in this work show the wear characterization of Al-Si matrix composites reinforced by multiwall carbon nanotubes (MWCNTs) under dry reciprocating sliding conditions against a grey cast iron (GCI) The wear resistance is investigated as a function of the carbon nanotube (CNT) content that varied from 2 to 6 wt%. The results demonstrated that the CNT content plays a relevant role in the wear behavior by substantially reducing the wear loss of Al-Si CNT composites. Further, it reduces the wear loss of the grey cast iron counterface. A physical model able to explain the improved behavior in both mating materials is depicted from experimental results.

Effect of sintering stage in NiTi short-fibre-reinforced aluminium–silicon composites interface properties

This study is concerned with the influence of sintering stage duration on the interface load transfer capacity between NiTi fibres and AlSi matrix alloy and thus in the final composite strength. Short-fibre-NiTi/AlSi composites were produced by a pressure-assisted sintering process, under vacuum, for sintering stages of 20, 25 and 60 min. All the obtained composites, with different sintering stages, have shown the occurrence of load transfer from the matrix to fibres, indicating the development of an effective interface between fibre and matrix. Performed shear tests have shown a decaying of shear strength with increasing sintering stage, in the selected range of sintering times, with the lowest sintering time leading to higher properties. It was also concluded that with increasing sintering stage, the interface volume around the fibres increases and the load transfer capacity decreases.

Carbon nanotube dispersion in aluminum matrix composites—Quantification and influence on strength

ABSTRACT Carbon nanotube (CNT) incorporation in metal matrix composites is hampered by the development of attractive van der Waals forces, which lead to the formation of CNT agglomerates. Therefore, a detailed quantification and characterization of CNT agglomerates and consequently CNT dispersion can enlighten their influence in composites strength. This work presents a comprehensive study, quantifying agglomerates by size, for three different composites (2, 4, and 6 CNT vol%), made with an aluminum-silicon matrix by a powder metallurgy route. From the obtained experimental data, an empirical model that attempts to predict the tensile strength of these composites from CNT agglomerates analysis is presented.

Wear behavior of Ti6Al4V biomedical alloys processed by selective laser melting, hot pressing and conventional casting

Abstract The aim of this work was to study the influence of the processing route on the microstructural constituents, hardness and tribological (wear and friction) behavior of Ti6Al4V biomedical alloy. In this sense, three different processing routes were studied: conventional casting, hot pressing and selective laser melting. A comprehensive metallurgical, mechanical and tribological characterization was performed by X-ray diffraction analysis, Vickers hardness tests and reciprocating ball-on-plate wear tests of Ti6Al4V/Al 2 O 3 sliding pairs. The results showed a great influence of the processing route on the microstructural constituents and consequent differences on hardness and wear performance. The highest hardness and wear resistance were obtained for Ti6Al4V alloy produced by selective laser melting, due to a markedly different cooling rate that leads to significantly different microstructure when compared to hot pressing and casting. This study assesses and confirms that selective laser melting is potential to produce customized Ti6Al4V implants with improved wear performance.

Laser surface structuring of Ti6Al4V substrates for adhesion enhancement in Ti6Al4V-PEEK joints

PEEK is a promising polymer possessing high mechanical strength and biocompatibility and therefore it can be associated to titanium for biomedical applications. This study aimed at producing Ti6Al4V-PEEK joints with enhanced adhesion through laser-structuring Ti6Al4V treatments. Ti6Al4V cylindrical substrates were prepared by two types of surface treatments: alumina blasting and laser structuring. The holes number and size in laser-structured surfaces was varied. PEEK was then hot pressed against the metallic substrate to completely filling the surface cavities. The adhesion of the PEEK/Ti6Al4V joint was assessed by a shear bond strength test. Fracture surfaces and interfaces were investigated by SEM/EDS. Significant differences were found in the shear bond strength between alumina blasted and laser-structured samples. Bond strength improvement (exceeding 300%) was registered for the laser-structured specimens relative to grit-blasted ones. The laser-structuring technique showed to be very promising in the production of specifically designed surfaces for high strength and mechanically stable Ti6Al4V/PEEK joints.

Mechanisms governing the tensile, fatigue, and wear behavior of carbon nanotube reinforced aluminum alloy

ABSTRACT This work is concerned with understanding the influence of reinforcement mechanisms of carbon nanotubes (CNTs) on mechanical, wear, and fatigue tests on an Aluminium-Silicon (AlSi) alloy. The reinforcement mechanism is presented through the observation of fracture morphology of the different tests. Results of mechanical properties, fatigue life performance, and wear loss is presented and discussed. It is shown that the CNTs reinforcement effect is active simultaneously in all previous properties and the reinforcement physical mechanism seems to be essentially due to a reinforcement effect of the interface that seems to be similar in all mentioned mechanical solicitations.

Properties assessment of nickel particulate-reinforced aluminum composites produced by hot pressing

Nickel particulate-reinforced aluminum-silicon composites, with 5, 12.5 and 20 wt%Nickel were produced by a hot pressing route. Microstructural characterization showed a uniform distribution of the Nickel particulates in the aluminum-silicon matrix. Ultimate tensile strength and hardness of the composites were found higher than for the aluminum-silicon alloy, while ductility suffered a decrease. Fracture surface analysis showed evidences of load transfer from the matrix to the reinforcement indicating the development of an effective interfacial bonding between Nickel particulates and aluminum-silicon matrix. Energy dispersive spectrometer and X-ray diffraction analyses performed in the particle/matrix interface revealed that it was formed by Al3Ni intermetallic. It was found that the amount of Al3Ni intermetallic has a paramount influence in the composites properties.

Evaluation of CNT Dispersion Methodology Effect on Mechanical Properties of an AlSi Composite

The aim of this paper was to evaluate the effect of different dispersion methodologies on mechanical properties of the aluminum-silicon (AlSi) composites reinforced by multi-walled carbon nanotubes (MWCNTs) coated with Ni. Different mixing procedures of MWCNTs with AlSi powder were tested, and AlSi-CNT composites were produced by hot pressing—powder metallurgy technique. The shear tests were performed to get the mechanical properties. Scanning electron microscopy with x-ray energy dispersive spectroscopy analysis and thermal analysis was used to investigate the microstructure of AlSi-CNT composites, interface reactions, and fracture morphology after shear tests. The experimental results proved that an improvement of dispersion of CNTs was achieved by using a combination of different mixing processes.

Effect of Zirconia and Alumina Fillers on the Microstructure and Mechanical Strength of Dental Glass Ionomer Cements

Background: Glass-ionomer cements perform a protective effect on the dentin-pulp complex considering the F ions release and chemical bonding to the dental structures. On the other hand, those materials have poor physic-mechanical properties in comparison with the restorative resin composite. The main aim of this work was to evaluate the influence of zirconia and/or alumina fillers on the microstructure and strength of a resin modified glass-ionomer cement after thermal cycling. Methods: An in vitro experimental study was carried out on 9 groups (n = 10) of cylindrical samples (6 x 4 mm) made from resin modified glass-ionomer (Vitremer, 3M, USA) with different contents of alumina and/or zirconia fillers. A nano-hybrid resin composite was tested as a control group. Samples were mechanically characterized by axial compressive tests and electron scanning microscopy (SEM) coupled to energy dispersive X-ray spectrophotometry (EDS), before and after thermal cycling. Thermal cycling procedures were performed at 3000, 6000 and 10000 cycles in Fusayama´s artificial saliva at 5 and 60 oC. Results: An improvement of compressive strength was noticed on glass-ionomer reinforced with alumina fillers in comparison with the commercial glass ionomer. SEM images revealed the morphology and distribution of alumina or zirconia in the microstructure of glass-ionomers. Also, defects such as cracks and pores were detected on the glass-ionomer cements. The materials tested were not affected by thermal cycling in artificial saliva. Conclusion: Addition of inorganic particles at nano-scale such as alumina can increase the mechanical properties of glass-ionomer cements. However, the presence of cracks and pores present in glass-ionomer can negatively affect the mechanical properties of the material because they are areas of stress concentration.