Papers by Author: Maria Helena Robert

Abstract: The work explores the production of Al/SiC and Al/SiC/NiAl₂O₄ composites by thixoforming compacted mixtures of AA7075 machining chips and reinforcing particles. It is analyzed the influence of processing parameters in the various processing steps, such as mixing, compacting, heating and thixoforming, in the final quality of products. Results showed the general viability of producing composites by the proposed technique; the utilization of recycling material is particularly important in the high demanding energy processing sector, as the Al industry. NiAl₂O₄ particles and, in less extent, also SiC particles, can penetrate into the liquid phase present in the thixotropic microstructure within the chip, promoting their disaggregation; reinforcement distribution in the composite depends on appropriate choose of processing parameters.

Abstract: The present study deals with a new design of aluminium alloy based composites reinforced with SiC particles and Si/Al₂O₃ powders through combined methods of powder metallurgy and thixoforming. Moreover, recycled machining chips are used as raw material, specifically AA7075 chips generated in the aeronautical industry. The proposed method is based on forming at high temperatures a compacted mixture of metal chips and reinforcing particles, with the metal in thixotropic semi-solid condition. Composites containing different SiC weight fractions (10, 20 and 30%) were produced and had their microstructure analyzed. Mechanical properties were evaluated by means of micro-indentation tests. General results show the feasibility of producing composites by the proposed route. Products with good mechanical properties could be obtained. The process, even still not completely optimized as some improvement still must be achieved, can bring a new possibility for the production of a noble material from recycled wastes, particularly important in the high energy spending Al industries.

Abstract: Shaping materials containing porous constituents is a critical issue. This work analyses the process of abrasive water-jet (AWJ) cutting applied to low density metal matrix composites (LD.MMCs) consisting of AA7075 alloy as matrix and porous, foamed particles of SiO₂/Al₂O₃/Fe₂O₃ or vermicular SiO₂/Al₂O₃/MgO, as reinforcements. Results show that although presenting lower density (from 1.09 g/cm³ to 1.59 g/cm³, depending on the particle type) and lower strength than the non-reinforced AA7075 alloy, the composites present some difficulties in AWJ cutting. Deflection of the jet was noticed, being more evident on vermiculite-reinforced samples. Anyhow machinability numbers for the composites analyzed are in the order of 20% higher than the AA7075 matrix alone. Jet deflection, surface quality and machinability number showed to be dependent on the internal structure of the porous ceramic particles: vermiculite can pull out under the jet while cinasite is more susceptible to suffer fracture. This mechanisms result in lower jet deflection, better surface and better overall machinability in the case of AA7075 / cinasite LD.MMC.

Abstract: The high cost of processing of Ti alloys is an important limitation to their utilization in a number of applications. An approach to overcome such difficulties would be their processing by precision casting. The purpose of this work is to evaluate the centrifugal casting process of Ti-Cu and Ti-Nb alloys, analysing the influence of alloying element type and content in the obtained microstructure. Samples with different compositions were prepared in arc furnace with non-consumable tungsten electrode and water-cooled copper hearth under high purity argon atmosphere and cast in a copper mould. Microstructures were analysed by SEM, characterized by EDS and X-ray diffraction. Vickers hardness measurements helped the analysis of phases.

Abstract: This work investigates the viability of production of thixotropic semisolid Ti-Cu alloys by controlled partial melting. Samples were submitted to different holding times at temperatures above solidus, from different initial microstructure conditions. In some cases, deformation by rolling before heating to semisolid was imposed. It is investigated the influence of deformation ratio and holding time in the obtained structure. Results show that thixotropic semisolid can be easily produced; pre-deformation leads to fine, globular primary phase in the liquid for short holding times. Therefore, thixoforming of Ti alloys is feasible, and can bring a whole new concept on forming complex shapes of these alloys, at low energy consumption, and new possibilities in engineering components with special properties.

Abstract: Different techniques used to produce semi-solid alloys can result in different structures in the material and, therefore, in distinct rheological behaviours which determine its thixo-forming ability. Suitable raw materials to be used for SSM forming must present non-dendritic, very fine or fragmented structure in order to globularize without excessive agglomeration when re-heated to the semi-solid state. This work analyses the influence of raw material production route on the rheological behaviour of semi-solid A356 alloy. Techniques used were: electromagnetic stirring (EMS) and chemical ultra-refining (UR). Samples were re-heated to 580oC (~ 0.45 solid fraction) and hold for 0, 90 and 210s to allow the observation of the structure evolution. After structures characterization, the samples were submitted to compression tests, at δH/δt = 10mm/s, in the same temperature/holding time conditions. Viscosity of the differently prepared raw material was related to the grain size, primary particle size, geometrical factor (roundness shape factor and contiguity).

Abstract: This work analyses the production of Al based composites with particulate reinforcement, via mechanical alloying. Composites were produced by mixing Al and NbAl3 powders by high energy mechanical alloying, under liquid nitrogen atmosphere, followed by cold pressing and hot sintering; and by controlling NbAl3 phase precipitation in liquid Al (in situ formation of the reinforcement). Results on composite produced from powders showed better distribution and incorporation, besides finer dispersion of particles in the matrix when mechanical alloying is employed. In this case, high dispersion on particulate phase was found despite predominance of small particles; there are no evidence of interface formation. When composites are produced by in situ formation of NbAl3 intermetallics, results showed that the formation of the reinforcement directly from the liquid matrix and the peritectic reaction between NbAl3 and liquid Al, provide a perfect reinforcement/matrix interface. Products showed good mechanical properties, good wear behavior and reduced thermal expansion.