V. Amigó

Microstructural evolution of Ti-6Al-4V during the sintering of microspheres of Ti for orthopedic implants

Abstract The sintering of microspheres for porous coating is made in titanium alloys at temperatures close to 1400xa0°C, thus substantially modifying the microstructure of the bases alloy. This changes is fundamentally due to grain size enlargement and α+β Widmanstaetten structure. Therefore it is necessary to make a deep study of grain growth effect on the hardness of the base material and the component of the sintered balls, in order to evaluate mechanical properties variation due to changes in the microstructure. There are also important complementary stabilization treatments of the microstructure by means of controlled cooling at high speeds it avoids the formation of β phases in grain boundaries, at the same time that avoid the broken-up of the Widmanstaetten structure, developing in an increase in mechanical resistance and fatigue resistance of the prosthesis.

Microstructure and mechanical behaviour of Al–Si–Mg alloys reinforced with Ti–Al intermetallics

Abstract MMCs of aluminium alloy matrix reinforced with Ti–Al intermetallics were prepared in the laboratory by following a powder metallurgy route and hot extrusion or forging were used as consolidation processes. Different studies were carried out in the composites developed. In order to evaluate possible reaction products and interphases between the matrix and reinforcement, isothermal heat treatments regarding time were carried out at a fixed temperature of 530xa0°C and at different times of 1, 6, 12 and 24xa0h. In this study, optical and scanning electron microscopy with X-ray microanalysis, were used to determine the growth of the reaction layer. The influence of production process was also estimated. Ultimate strength and strain to failure were studied. There were performed tests at room temperature of the material in different precipitation state and at high temperature of the material in extrusion state. The type of damage was analysed by scanning electron microscopy. Ultimate strength trends to grow with reinforcement addition in both T1 and T6 conditions for low temperature. Strain to failure remains the lowest on T6 state. The reinforcement addition affect decrease for high temperature testing and matrix influence was demonstrated. Tribological properties were analysed in conditions of dry friction and lubrication. It was studied the evolution of the coefficient of friction in front of the speed of sliding, applied pressure, regime of lubrication and content of reinforcement in the composites. The matrix has shown a great plasticity, whereas the reinforcement has a hardening effect also showing a certain plasticisation.

Tribocorrosion behavior of beta titanium biomedical alloys in phosphate buffer saline solution

The tribo-electrochemical behavior of different β titanium alloys for biomedical applications sintered by powder metallurgy has been investigated. Different mechanical, electrochemical and optical techniques were used to study the influence of the chemical composition, Sn content, and the electrochemical conditions on the tribocorrosion behavior of those alloys Ti30NbxSn alloys (where x is the weight percentage of Sn content, 2% and 4%). Sn content increases the active and passive dissolution rate of the titanium alloys, thus increasing the mechanically activated corrosion under tribocorrosion conditions. It also increases the mechanical wear of the alloy. Prevailing electrochemical conditions between -1 and 2V influences the wear accelerated corrosion by increasing it with the applied potential and slightly increases the mechanical wear of Ti30Nb4Sn. Wear accelerated corrosion can be predicted by existing models as a function of electrochemical and mechanical parameters of the titanium alloys.

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 bending strength of porous titanium processed by space holder method

Abstract Porous titanium specimens have been produced by means of the space holder method. Titanium grade 3 (TiCP3) has been used as material and ammonium bicarbonate as spacer. Process parameters (compaction pressure, metal grain size range and spacer volume) influence on porosity morphology and distribution and bending strength has been analysed. The results denote an important loss of strength when comparing samples sintered without and with spacer. On the other hand, a higher bending strength was observed in the porous samples with a smaller size of ammonium bicarbonate particles. Finally, the evolution of bending strength with the compaction pressure depends on the spacer volume, having a direct dependency for reduced amounts and inverse for higher contents.

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.

Development of Al-Si-Mg alloys reinforced with diboride particles

Abstract A series of aluminium composites, with varying volume fractions of diboride particles, were made by mixing various proportions of aluminium alloy and titanium diboride (TiB2) powders followed by hot extrusion. Different studies were carried out in the composites developed in order to obtain their general properties. The possible reaction between the matrix and reinforcement was evaluated by performing heat treatments regarding temperature at a fixed time of 1xa0h and at different temperatures of 350, 400, 450 and 500xa0°C. In this study, optical and scanning electron microscopy (SEM) with X-ray microanalysis, were used to determine the growth of the reaction layer. Mechanical properties such as ultimate strength and strain to failure were studied. The tests were performed at room and high temperature in the material in state T1 and T6. The type of damage was analysed by SEM. Tribological properties were analysed in conditions of dry friction as well as lubrication with standard lubricant in automotive industry of the type SAE 250. The evolution of the coefficient of friction in front of the speed of sliding, applied pressure, regime of lubrication and content of reinforcement in the composites was studied. The contribution also describes results from corrosion tests on aluminium composites with different percentages of reinforcement. Specimens were examined after exposure to the corrosive solution employing SEM and the type and amount of corrosion was observed.

Electrochemical behavior of near-beta titanium biomedical alloys in phosphate buffer saline solution

The electrochemical behavior of three different near-β titanium alloys (composed by Ti, Nb and Sn) obtained by powder metallurgy for biomedical applications has been investigated. Different electrochemical and microscopy techniques were used to study the influence of the chemical composition (Sn content) and the applied potential on the microstructure and the corrosion mechanisms of those titanium alloys. The addition of Sn below 4wt.% to the titanium powder improves the microstructural homogeneity and generates an alloy with high corrosion resistance with low elastic modulus, being more suitable as a biomaterial. When the Sn content is above 4%, the corrosion resistance considerably decreases by increasing the passive dissolution rate; this effect is enhanced with the applied potential.

Stiffness variation of porous titanium developed using space holder method

Abstract The excellent properties of Ti have resulted in its generalised use for bone implants. However, Ti is very stiff in comparison with human cortical bone, and this creates problems of bone weakening and loosening of the implant. This article discusses the mechanical properties (flexural and compressive strength, and stiffness) of porous Ti–6Al–4V specimens developed using the space holder method. These properties are examined relative to the production process parameters: compacting pressure and sintering time, as well as temperature, and the addition of spacer and its particle size. It is seen that when spacer is added, compressive strength decreases with the application of compacting pressure and that these are the most influential parameters. The developed pieces show a closed and unconnected porosity. Small additions of spacer (25 vol.-%) reduce stiffness to around half of that shown by the solid material, and the resulting pieces are strong enough to be used as bone substitute.

Evaluation of chemical degradation of commercial polypropylene

Abstract One of the polymers most commonly used in the automotive industry is polypropylene. Despite its importance and the number of experiments that have been carried out, its chemical degradation in different environments is still a subject of debate. Degradation experiments were carried out to evaluate resistance behavior with polypropylene samples exposed to basic acids or organic mixtures. Polypropylene was subjected to a degradation process for 0 and 1000xa0h. Also two atmospheric conditions were analyzed: polypropylene immersed in a liquid and polypropylene exposed to saturated atmosphere. The environments were solutions of water and NaCl, H 2 SO 4 , HNO 3 , HCl and AcH, NaOH and NaClO, and finally solvent as diesel engine and unleaded gasoline. A reduction of resistance properties and high plasticity was observed in strongly solvent environment as unleaded gasoline. On the other hand, a different tendency was seen in the rest of environments, but in general the resistance properties were similar with respect to original polypropylene. On the other hand, thermal studies also were performed. Microstructural changes of polypropylene were analyzed by dynamic mechanical thermal analyzer (DMTA).