Hélder Puga

Auxetic materials — A review

Auxetic materials are endowed with a behavior that contradicts common sense, when subjected to an axial tensile load they increase their transverse dimension. In case of a compression load, they reduce their transverse dimension. Consequently, these materials have a negative Poisson’s ratio in such direction. This paper reviews research related to these materials. It presents the theories that explain their deformation behavior and reveals the important role represented by the internal structure. Their mechanical properties are explored and some potential applications for these materials are shown.

On assessment of processing variables in vertical centrifugal casting technique

Abstract The aim of the present study is to investigate the influence of the vertical centrifugal casting technique over mechanical and metallurgical properties of a hypereutectic Al–18Si alloy. Due to the inherent vibration of the centrifugal casting technique, and in order to study and understand the individual effects of the equipment vibration and the centrifugal force itself (pressure or fluid dynamics), as well as the combined effect of both, three different tests were performed: gravity casting, gravity casting with vibration and centrifugal casting. It was concluded that the metallurgical and mechanical properties of castings obtained by the centrifugal casting process depend on the combined effect of the centrifugal pressure and/or fluid dynamics and on the inherent vibration of the technique itself. Correlations between the different casting techniques and obtained mechanical and metallurgical properties are presented.

Characterisation of metal/mould interface on investment casting of γ-TiAl

Abstract This paper describes the evaluation of different refractory compounds – SiO2, ZrO2 and Y2O3 – as face coats of investment casting shells for γ-TiAl. The effect of the different refractories on the metal–mould interaction is studied in different aspects. Experimental results include characterisation of the constituents present at the metal/mould interface, the segregation profiles of residual elements, namely oxygen, from the interface to the inner part of the samples, the extension and microhardness of the samples external hard case and the samples surface finishing.

Grain refinement of Al-Mg-Sc alloy by ultrasonic treatment

In foundry practice, ultrasonic treatment has been used as an efficient technique to achieve grain refinement in aluminium and magnesium alloys. This article shows the strong effect of pouring temperature and ultrasonic treatment at various temperatures on the grain refinement of Al-1 wt% Mg-0.3 wt% Sc alloy. Without ultrasonic treatment, a fine grain structure was obtained at the pouring temperature of 700 °C. The average grain size sharply decreases from 487 ± 20 to 103 ± 2 μm when the pouring temperature decreases from 800 to 700 °C. Ultrasonic vibration proved to be a potential grain refinement technique with a wide range of pouring temperature. A microstructure with very fine and homogeneous grains was obtained by applying ultrasonic treatment to the melt at the temperature range between 700 and 740 °C, before pouring. Cavitation-enhanced heterogeneous nucleation is the mechanism proposed to explain grain refinement by ultrasound in this alloy. Moreover, ultrasonic treatment of the melt was found to lead to cast samples with hardness values similar to those obtained in samples submitted to precipitation hardening, suggesting that ultrasonic treatment can avoid carrying out heat treatment of cast parts.

Effect of ultrasonic degassing on performance of Al-based components

The effect of high intensity ultrasound based on the novel multi-frequency multimode modulated technology on the final density, porosity, mechanical and fatigue properties of an AlSi9Cu3(Fe) alloy after different processing time was studied. Reduced pressure test was used to evaluate the density of alloys. The tensile and fatigue tests were used to evaluate the static and dynamic properties for the different time of ultrasonic degassing, respectively. It is found that ultrasonic degassing is effective in reduction porosity as well as to improve the final density of castings. Furthermore, the experimental results suggest that the porosity level does not have a substantial influence on the static properties contrary to what is observed on fatigue properties.

Effect of grain and secondary phase morphologies in the mechanical and damping behavior of Al7075 alloys

The present study evaluates the role of the microstructure in the static and dynamic mechanical behavior of as-cast Al7075 alloy promoted by ultrasonic treatment (US) during solidification. The characterization of samples revealed that US treatment promoted grain and intermetallics refinement, changed the shape of the intermetallic phases (equilibrium phases of soluble M and/or T (Al, Cu, Mg, Zn) and their insoluble Al-Cu-Fe compounds) and lead to their uniform distribution along the grain boundaries. Consequently, the mechanical properties and damping capacity above critical strain values were enhanced by comparison with values obtained for castings produced without US vibration. This results suggest that the grain and secondary phases refinement by US can be a promising solution to process materials to obtain high damping and high strength characteristics.

Modeling and elastic simulation of auxetic magnesium stents

Auxetic materials are characterized by getting thiner/ larger in tension/compression. This counterintuitive behavior is advantageous in specific applications such as self-expandable stents. There are currently some stents that make use of this behavior, nevertheless there are still auxetic geometries that are not explored in this field. Additionally, Pure Magnesium is a promising material to manufacture bioabsorbable stents. This study presents the modelation of novel auxetic self-expanding stents based in Reentrant and Chiral geometries. They are simulated using Finite Element analysis to determine the presence of negative Poissons ratios and if they are a possible solution for further stent development. It is concluded that such modelations show low values of Poissons ratio and may be a viable possibility to obtain a new generation of self-expanding stents.

Comparison of the Poisson's Ratio of Simulated Rigid and Elastic Auxetic Models Using Kinematic and Finite Element Analysis

Materials that possess a negative Poissons ratio are called Auxetics. They are characterized by the counterintuitive behavior of expanding in tension and contracting in compression. To justify this deformation behavior, there have been developed theoretical modelations like the reentrant and bowtie models. However, the most generalized models are based on geometries that possess rigid trusses and hinging nodes. This does not portrait the reality of the application of these models. In this study, there have been performed simulations, using kinematic analysis to characterize the auxetic behavior of the theoretical rigid/hinging models and finite element analysis to characterize elastic models that represent real bodies. There were determined and compared the Poissons ratios of the theoretical and elastic reentrant and bowtie. Additionally it was shown that there is a significant difference between the results. In conclusion the theoretical models predict lower values of Poissons ratio, while the elastic models that simulate a real body show less auxetic behavior.

Use of Acoustic Energy in Sand Casting of Aluminium Alloys

During the last years, some researchers have focused the development of ultrasonic microstructure refinement /modification techniques of die-casting aluminium alloys, to improve their properties. The developed techniques are highly efficient when applied to the die-casting process, but their capability with sand and ceramic moulding are unknown. Sand/ceramic aluminium castings are prone to coarse microstructure, porosities and inclusions due to low cooling rates and turbulent gravity pouring, and suitable processing techniques are required to eliminate those drawbacks. This article reports some results of a research work aiming the development of a reliable, low-cost and environmentally friend casting process, for geometrically complex and massive high strength sand/ceramic aluminium castings, to eliminate traditional soundness related defects and simultaneously promote the development of refined microstructures. The article presents the effect of applying ultrasound to AlSi9Cu3 alloy during solidification on a sand mould on the resultant microstructure. Results include microstructure characterization and its relationship with thermal analysis data collected from the center of the cast samples during cooling.

Sustainable Reverse Engineering Methodology Assisting 3D Modeling of Footwear Safety Metallic Components

One main objective of the technique of reverse engineering is focused on development of new products based on the improvement of existing products. The present work aims to demonstrate a sustainable methodology exploring the capabilities of reverse engineering, applied to produce brand new geometric solutions for safety metallic components incorporated in footwear. The data acquisition is done using different techniques, contact methods (CMM – Measuring Coordinate Machine) and non-contact methods (Laser Scanning). Those measuring techniques for data acquisition are the key entry for the 3D shape recovery, boosting the development of new components based on the improvement of existing products. Despite these techniques being widely explored in multiple engineering sectors, author’s contribute was focused on the proposal and validation of a sustainable methodology based on an algorithm in MATLAB that performs the surface generation under user control. Such methodology has been tested through a real model of a toecap component used in safety footwear.Copyright