M.D. Salvador

Influence of processing variables on mechanical characteristics of sunlight aged polyester–glass fibre composites

Abstract Unsaturated polyester–glass fibre reinforced composites have been exposed to sunlight radiation by means of special lamps. Tensile mechanical properties decrease with exposure time. The evolution fits a damped exponential model. Ageing degree varies between 15 and 56%, depending on composite and mechanical property. The influence of cure temperature, kind of resin and reinforcing fabric has been investigated. Toughness property losses are greater than strength ones. The kind of resin had a very small influence on losses in the orthophthalic polyester family. Higher cure temperature decreased losses, mainly in toughness properties. Fabric type showed no influence on strength properties losses but taffeta reinforcement showed higher loss of toughness properties.

Analysis of Boron Carbide Aluminum Matrix Composites

This study deals with the feasibility of using boron carbide (B4C) as reinforcement for aluminum matrix composites (AMCs) obtained by solid-state processes (powder metallurgy and extrusion). Two different reinforcements were considered: B4C as the object of this study and SiC for direct comparison of results. Aluminum alloy AA6061 was used as matrix in all cases. Comparative analysis between both SiC and B4C composites was focused on mechanical and tribological properties and correlated to microstructural features. Hardness and strength increased for composites together with volume fraction of reinforcement, reaching its maximum value to 10% B4C. Regarding tribology, composites showed an increased dynamic friction coefficient but a lower wear rate than the unreinforced aluminum alloy. Applications as brake disks for automotive industry are foreseen.

Effects of Long-term Exposure on E-glass Composite Material Subjected to Stress Corrosion in a Saline Medium

This work provides an insight on very long-term degradation of polyester-fiber glass composites immersed more than 30,000 h in saline medium under service stresses. Samples were loaded under bending conditions with stresses both in the elastic and plastic fields, with the result that characteristics in a flexural mode were able to be determined and the ensuing decrease in characteristics was fitted to an exponential model. The degree of losses ranged from 25 to 31% for the bending modulus, from 28 to 35% for the flexural strength, and from 40 to 51% for the specific fracture energy. The most notable losses were for specimens immersed in artificial sea water under a continuous stress of 140 MPa, corresponding to the plastic behavior of the material. Although the existence of matrix plasticization is doubtful, the osmotic effects of the diffusion on the matrix and the junction to the fibers, the presence of microcracks, and the effects of chemical ions in the medium on the surface fiber composition became evident in the strength degradation of the material.

Multilayer and Particle Size-Graded YSZ Coatings Obtained by Plasma Spraying of Micro- and Nanostructured Feedstocks

Abstract This study was undertaken to attempt to achieve a better balance between zirconia coating properties and high-temperature performance by combining the characteristics of coatings obtained from a micro- and a nanostructured feedstock having the same YSZ composition. First, two single-layer coatings were obtained as reference coatings, using the micro- and the nanostructured feedstock, respectively. Four different composite coatings were then obtained by combining these two feedstocks. Two double-layer (multilayer) YSZ coatings were prepared by depositing the microstructured feedstock on the nanostructured layer and vice versa, while two coatings with different particle size gradients (graded coatings) were prepared by depositing various mixtures of the micro- and the nanostructured feedstock in alternate layers. The microstructure and hardness of the resulting coatings were determined. In the multilayer coatings, each layer exhibited a clearly different microstructure, whereas in the graded coatings the microstructural characteristics changed gradually. Coating hardness developed analogously, each layer displaying a marked change in hardness in the multilayer coatings in contrast to a gradual change in the graded coatings. The microstructure and hardness of the individual layers were thus quite well preserved in the developed composite coatings.

Mechanical Properties of Double-Layer and Graded Composite Coatings of YSZ Obtained by Atmospheric Plasma Spraying

Double-layer and graded composite coatings of yttria-stabilized zirconia were sprayed on metallic substrates by atmospheric plasma spray. The coating architecture was built up by combining two different feedstocks: one micro- and one nanostructured. Microstructural features and mechanical properties (hardness and elastic modulus) of the coatings were determined by FE-SEM microscopy and nanoindentation technique, respectively. Additional adherence and scratch tests were carried out in order to assess the failure mechanisms occurring between the layers comprising the composites. Microstructural inspection of the coatings confirms the two-zone microstructure. This bimodal microstructure which is exclusive of the layer obtained from the nanostructured feedstock negatively affects the mechanical properties of the whole composite. Nanoindentation tests suitably reproduce the evolution of mechanical properties through coatings thickness on the basis of the position and/or amount of nanostructured feedstock used in the depositing layer. Adhesion and scratch tests show the negative effect on the coating adhesion of layer obtained from the nanostructured feedstock when this layer is deposited on the bond coat. Thus, the poor integrity of this layer results in lower normal stresses required to delaminate the coating in the adhesion test as well as minor critical load registered by using the scratch test.

Microwave Technique: A Powerful Tool for Sintering Ceramic Materials

This work has been carried out under a programme that supports research and development at the Polytechnic University of Valencia under multidisciplinary projects PAID2011/059, SP20120621 and SP20120677. A. Borrell acknowledges the Spanish Ministry of Science and Innovation for her JdC contract (JCI2011-10498). A. Borrell and F. L. Penaranda-Foix want to thank the Generalitat Valenciana for the grant obtained in the frame of the Program BEST/2012.

Mechanical and Microstructural Properties of Titanium Matrix Composites Reinforced by TiN Particles

Particulate reinforced titanium composites were produced by PM route. Different volumetric percentages of TiN reinforcements were used, 5,10,15 vol%. Samples were uniaxially pressed and vacuum sintered at different temperatures between 1200-1300°C. Density, porosity, shrinkage, mechanical properties and microstructure were studied. Elastic properties and strength resistance were analysed by flexural strength and tension tests, and after the test, fractured samples were analysed as well to obtain the correlation between the fracture, interparticular or intraparticular, and the level of reinforcement addition. Hardness and microhardness test were done to obtain a better understanding of its mechanical properties. In order to study wear resistance pinon- disc tests were conducted. In addition, the influence of temperature, the reactivity between matrix and reinforcement on microstructural development were observed by optical and electron microscopy.

Effect of welding on the microstructure and stress corrosion cracking susceptibility of AA7028 alloy

Summary The microstructural and stress corrosion cracking changes due to welding are studied for AA7028 aluminium alloy. Special attention is paid to the characterisation of what is known as the ‘white zone’. The influence of delay between quenching and ageing in T73 treatment on the microstructure and on the characteristics of the heat affected zone (HAZ) is also studied. Finally, the effect of thermal treatments applied to this zone after welding is analysed.

Fabrication of ultrafine and nanocrystalline WC-Co mixtures by planetary milling and subsequent consolidations

Abstract In this work ultrafine and nanocrystalline WC–Co mixtures were obtained by low energy milling in planetary ball mill. The effect of the processing conditions on the reduction and distribution of the grain sizes and the internal strains level were studied. The characterisation of the powder mixtures was performed by means of scanning and transmission electron microscopy and X-ray diffraction analysis. Observations through SEM and TEM images showed a particle size below 100 nm, after milling. The X-ray diffraction profile analysis revealed a WC phase refined to a crystallite size of 19 nm. The mixtures obtained have been consolidated and mechanical and microstructurally characterised. The results show improvements in resistant behaviour of the material consolidated from nanocrystalline powders, in spite of the grain growth experienced during the sintering. The best results were found for the material obtained by wet milling during 100 h, which presents values of hardness higher than 1800 HV.

Aluminium 7020 Alloy and Its Welding Fatigue Behaviour

Since Alfred Wilm discovered the aluminium alloys hardening precipitation phenomena at the begining of the last century (Polmear 1996), the use of aluminium alloys has increased, owing their advantages against corrosion and good strength weight ratio. The Aluminium Zinc Magnesium ternary alloys are getting more relevance every day. These alloys are commonly called Al-Zn-Mg. In this family the most used are the AA7005 and AA 7020, which are nearly the same alloys. Although the most remarkable difference between them is the slightly better mechanical behaviour of the 7020 one after welding. The ageing development of these alloys follows a simple precipitation phenomena summarized as: