Contribution to the study of mechanisms involved in the formation of rheocast structure

The work presents some results obtained when producing rheocast structures of Al-(4.5wt.%)Cu alloys, by heat treatment of dendritic material tin as-cast and cold deformed conditions) at temperature above solidus. Macro and microstructures were analysed in order to investigate the evolution mechanisms involved in the structure modifications. Results show that coarsening and coalescence phenomena can lead to a significant difference between the sizes of grain and globule, therefore being necessary to distinguish between micro and macroconstituents. Results also show that, for deformed dendritic structure, a phenomenon of recrystallisation plus boundaries wetting leads to structures where one globule of the primary phase corresponds to one grain in the macrostructure. (C) 2001 Elsevier Science B.V. All rights reserved.10941671SI21521

Semi-solid behavior of new Al-Si-Mg alloys for thixoforming

There is a lack of specific alloys for thixoforming processes. Usually alloys from conventional casting processes were fitted to the thixoforming purpose, e.g. A356, A357. The aim of this study is to produce new Al-Si-Mg alloys specially tailored for thixoforming purpose. The as-rheocast: Al-7 wt%Si-0.5 wt%Mg, Al-4 wt%Si-0.5 wt%Mg, Al-1 wt%Si-0.5 wt%Mg, Al-7 wt%Si-1 wt%Mg, Al-4 wt%Si-1 wt%Mg and Al-1 wt%Si-1 wt%Mg were produced by grain refining. Thus, the effect of Si and Mg content on structure and rheological behavior of these new alloys was analyzed. To simulate the semi-solid behavior at a high temperature, the samples were re-heated to the semi-solid state to produce 45% solid fraction and their structural evolution was analyzed after 0, 90 and 210 s at this solid fraction. At the same temperature and holding time, the samples were submitted to a compression test using a parallel platen viscosimeter. Results showed that the grain/globule size increased as the Si and Mg content increased. Al-7 wt%Si-0.5 wt%Mg, the Al-4 wt%Si-0.5 wt%Mg and the Al-1 wt%Si-0.5 wt%Mg are suitable for thixoforming purposes. Al-Si alloys with 1.0 wt%Mg showed low apparent viscosity values. (c) 2005 Elsevier B.V. All rights reserved.40641671637

Thixo-formability of the AA6063 alloy: Conventional production processes versus electromagnetic stirring

Cold-formed AA6063 alloy has an excellent mechanical behaviour, and is used in a wide range of structural applications. In Brazil this alloy is produced in large amounts, due to wide application at low cost. The present work aims to present a new use for this material, allowing the subsequent manufacture of metal parts by thixo-forming. The objectives of this work are (a) to establish a suitable condition for the production of thixo-formable structures using electromagnetic stirring during semi-continuous casting, with and without grain refiner, and (b) to compare this to the same alloy produced by the conventional process. The alloy was supplied by Companhia Brasileira do Aluminio (CBA), and processed in a semi-continuous casting equipment at the Thixo-Forming Laboratory of Unicamp, which has a controllable electromagnetic field from 600 to 2000 W/kg of material, induced into the alloy during solidification. The final alloy structure was characterized in terms of grain size, primary particle size and shape and rheological behaviour for 45% solid fraction. Furthermore, results were compared to the chemical refining (Al-1 wt%Ti-0.5 wt%B), as well as the combination of both production methods. According to the thixo-formability evaluation protocol, the commercially produced AA6063 alloy has an excellent degree of thixo-formability, presenting a very low-apparent viscosity (10(5) Pa s), despite the difficulty of its processing, i.e., a very narrow semi-solid range (only 23 degrees C). In fact, the most important feature for thixo-formability is to have raw material presenting a very refined structure, as well as, the most globular structure. In this case, the commercial alloy presented grain size of 86 mu m and rheocast quality index 0.60. For commercial processing better equipment and a better temperature control will be indispensable in the commercial development of thixo-forming operations for this type of material. (c) 2007 Elsevier B.V. All rights reserved.1984169915516

Influence of the processing route on the microstructure of aluminum alloy A356 for thixoforming

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Raw materials for thixoforming processes can be obtained by various routes. An analysis was made of the influence of five different processing routes on the production of thixotropic A356 alloy: 1) direct casting in a water-cooled mold; 2) direct casting as in (1), plus enhancement by electromagnetic stirring and grain refining; 3) as in (1), but enhanced by mechanical vibration; 4) as in (1), followed by one ECAP pass (Equal Channel Angular Pressing); and 5) using a commercial product as comparison. The alloy was characterized by both conventional and color microstructure for a comprehensive understanding of its structure prior to and after heating to 580 degrees C, which is the ideal processing temperature for this specific raw material in the semi-solid state. B&W (black and white) and polarized light color metallography of the microstructure were employed to better characterize these structures. Among the five tested routes, the samples prepared by the ECAP route presented the most suitable characteristics for thixoforming, showing smaller grain sizes (measured by polarized light color metallography), smaller globule size and greater sphericity (both measured by B&W metallography) in the semi-solid state. Interconnections in the residual solid fraction in the semi-solid state are usually visible only by polarized light color metallography. However, the use of both color and B&W metallography enabled the best morphological structures for thixoforming to be clearly identified, confirming the promising potential of the ECAP technique in processing raw materials for thixoforming. (C) 2013 Elsevier Inc. All rights reserved.852637Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)FAPESP [2009/17260-0]CNPq [470572/2011-6

An investigation on the effect of intensive shearing on the grain refinement of A5754 aluminium alloy

Grain refinement of aluminium alloys has been an established method of improving mechanical properties of aluminium alloys. A5754 wrought aluminium alloy offers a number of attractive superior properties, however, refinement of this alloy and other wrought alloys is still a matter of concern. A novel technique is introduced for the refinement of wrought and cast alloys, using A5754 alloy as a model material. This method not only provides fine and uniform microstructure throughout the products but it also offers possibilities of industrial applications. This technique eliminates the problems associated with chemical grain refinement and introduces physical refinement as a better option for achieving fine grain sizes. (C) 2009 Elsevier B.V. All rights reserved.48141671358364Innoval Technology Ltd.DTI (UK

Effect of macrostructure and microstructure on the viscosity of the A356 alloy in the semi-solid state

An A356 alloy was processed from liquid state in different conditions to obtain globular as-cast structures instead of the conventional dendritic ones. These structures were produced in a semi-continuous casting system coupled to an electromagnetic power source. This device allows the variation of the intensity of the stirring by controlling the electromagnetic field, as well as of the cooling rate by controlling the casting velocity. Different cast conditions were used: the alloy was poured at 600 degreesC and left to solidify under the action of a 600, 900 and 1200 W electromagnetic field. Cast materials produced in these conditions are suitable for thixo-forming processes after a short re-heating treatment to complete spheroidisation of the primary phase. The generated structures were characterised by macrostructure (grain size) and microstructure (globule size and shape factor). The smaller the grain size, the smaller the globule size and shape factor. The Rheocast Quality Index (RQI = globule size/grain size x shape factor) was used to evaluate the morphological evolution during the partial melting treatment necessary to complete spheroidisation of the primary phase. Viscosity of thixotropic slurries obtained by re-heating the cast materials at 580 degreesC for times of 0, 30, 90, 210 and 600 s, was also determined by hot compress test. A comparison with a commercial ALTHIX(R)67S is described. (C) 2004 Elsevier B.V. All rights reserved.1531SI30030

Thixoformability evaluation of AA2011 and AA2014 alloys

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)This paper examines the feasibility of thixoforming two commercial aluminum copper alloys: AA2011 and AA2014. This evaluation was performed in three steps: 1) characterization of the raw materials, including an analysis of their constituents, partial melting range temperatures, and microstructures; 2) characterization of the samples' morphological evolution during partial melting to observe their globularizadon at two different sets of temperatures, (solid fractions of 45% and 60%), and four holding times: 0, 30, 90 and 210 s; and 3) tests of the morphological behavior under the same partial melting conditions of temperature and holding times. The results indicate that, despite the difficulty of controlling the temperature of the semi-solid transition, these two alloys are suitable for thixoforming processes. Their morphology is stable in the semi-solid range and the corresponding viscosity values are lower than 10(6) Pa . s, resulting in a maximum conformational stress of 2.4 MPa.1041211821196Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)FAPESP [2008/03 946-4]CNPq [470081/2006-6

Macro- and microstructure analysis of SSM A356 produced by electromagnetic stirring

The objective of this work is to present the relationship between macro- and microstructure of the rheocast alloy A356 obtained by electromagnetic stirring. There is a direct relationship between grain size (macrostructure) and globule size/shape factor (microstructure): the smaller the grain size, the smaller are the globule size and shape factor. The A356 alloy was processed in a simple device, which constitutes a semi-continuous casting system, coupled with a power source. The A356 (Al-7.0 wt.%Si) alloy was electromagnetically stirred under different cooling rates (controlled by the casting speed of 1 and 3 cm/s) and different power levels (600-1200 W). Ingots with 45 mm of diameter and 3 kg with very refined and homogeneous rheocast structures were obtained. The direct relationship between macro- and microstructures of the ingots were exploited. A quality index is proposed to better characterise the rheocast structure. (C) 2002 Elsevier Science B.V. All rights reserved.1204169936537

EValuation of the thixoformability of AA7004 and AA7075 alloys

This study involved a complete evaluation of the thixoformability of AA7004 and AA7075 alloys, from their microstructural characterization to their viscous behavior. The alloys were subjected to globularization heat treatments for 0, 30, 90 and 210 seconds in two conditions of solid fractions, 45 and 60%, and to viscosity assays under the same conditions. Heat treatments promote the globularization of primary phase particles; hence, the best viscosity results were achieved for alloys with low solid fractions heat-treated for 210 seconds. Alloys AA7004 and AA7075 showed an apparent viscosity of 10(4) to 10(5) (Pa.s). The behavior of materials in this range is similar to that of molten glass and they show high formability. However, the AA7075 alloy showed a better performance than the AA7004 due to the smaller size of its primary particles and original grains, their lower growth during reheating, and depending on the condition, their viscosity of 10(4) Pa.s, which is extremely low for thixoforming standards