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1

Effect of casting procedures in the structure and flow behaviour of semisolid A356 alloy

Proni C. T. W., Robert M. H., Zoqui E. J.

Archives of Materials Science and Engineering

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2015

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Vol. 73, nr 2

82--93

EN

Purpose: Evaluation of different casting methods to produce raw material for thixoforming, aiming costs reduction in the production of thixoformable alloys. Reduction of costs may stimulate the commercial use of the semisolid processing technology (SSM). Design/methodology/approach: It is analysed the effect of different casting routes in the microstructure features in semisolid A356 alloy, and in its rheological behaviour. Different casting procedures were investigated: a) pouring in water cooled Cu mould; b) same as „a)” adding electromagnetic stirring; c) same as „a)” adding mechanical vibration; d) same as „b)” with addition of grain refiner; e) same as „c)” with addition of grain refiner. Cast materials were reheated to the semisolid sate and the effect of different holding times upon the globularization of the primary phase was analysed for each cast structure. The semisolid material in each condition was evaluated concerning rheological behaviour. Mechanical properties of thixoformed products were evaluated using flexion tests. Findings: Despite the several methods currently in use to produce raw material for thixoforming, this work shows that the best combination of quality of thixoformable material/ production cost /process operationality can be achieved using casting in permanent mould, under water cooling and mechanical vibration. Resulting cast material under this condition presents grain size smaller than 100 μm, ideal for SSM. Lower the grain size, lower the primary globule size and higher the roundness of the primary phase particle and lower the apparent viscosity of the semisolid. In the best condition achieved, apparent viscosity measured was circa 105 Pa.s (similar to the working range for glass), leading to a probable homogeneous die filling during thixoforming in high pressure die casting machines (HPDC). Research limitations/implications: Regardless the best microstructure for SSM resulting from casting under mechanical vibration, it is still necessary to reach the optimum casting condition in terms of vibration in order to improve, even more, refinement of the microstructure. Practical implications: The suggested process is a simple technique to reduce costs in the production of raw material for thixoforming. The technology is easily implementable in industries. Originality/value: The development of a simple, original, low cost method to produce raw material for SSM technology.

2

Design of micro porous Al foams by high energy milling

Robert M. H., Silva R. R.

Archives of Materials Science and Engineering

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2015

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Vol. 71, nr 1

5--15

EN

To explore a new route to produce metallic foams which results in a structure of closed micro porous. High energy milling is employed to incorporate particles of foaming agents in metallic powders to promote homogeneous distribution of micro gas bubbles during foaming. Design/methodology/approach: AA2014 powders were mixed with TiH2 particles as gas releasing agent, through high energy milling, producing composite powders. Powders were compacted and obtained compacted precursors were heated to promote foaming of the metal. Effect of processing conditions in the expansion of the metal, structural characteristics, density and mechanical properties under compression, of obtained foams was analyzed. Findings: Foaming composite powders of AA2014/TiH2 produced by high energy milling is a promising route to produce micro porous aluminium foams. The best foaming condition among the conditions investigated, occurs for the highest milling time (17 h) and highest heating rate (3°C/s) imposed during foaming, resulting in 140% of maximum expansion and foams with relative density of 0.44. Research limitations/implications: Main limitation of the proposed process is the long time required to produce composite powders by high energy milling, which can justify the process for specific purposes where micro porous are required. However, as all new development, further works can lead to the optimization of processing parameters, mainly concerning reduction of processing time, to make the process compatible to wider industrial applications. Practical implications: New products can be developed for specific applications requiring porous with micro scale. Originality/value: The use of the foaming agent structurally incorporated in the metal powder to produce precursors for foaming is original.

3

Processing and properties of AA7075/ porous SiO2-MgO-Al2-O3- composite

Robert M. H., Jorge A. F.

Journal of Achievements in Materials and Manufacturing Engineering

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2012

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Vol. 54, nr 1

7--15

EN

Purpose: the work presents a new composite based in Al matrix reinforced with porous, lightweight and low cost SiO2-/MgO/Al2-O3- ceramic particles. The new material can present a unique combination of properties: those related to metal/ceramic composites and still associating some characteristics of cellular materials, as the low density and high plastic deformation under compression stresses. Design/methodology/approach: processing technique involves the infiltration of AA7075 alloy in the semisolid thixotropic state into a layer of porous ceramic particles. Products were analyzed by X-ray tomography, optical and electronic microscopy to observe microstructure and metal/ceramic interfaces. Density was measured by He picnometry; semi-static compression tests were performed to evaluate the deformation ability of the material. Thermal properties were theoretically evaluated. Findings: concerning the production method, thixoinfiltration is a feasible processing route, with no difficult control of parameters and does not rely on specific and onerous equipment. Moreover, it is flexible to different alloys. Concerning the product, low density composites can be produced with good dispersion of reinforcement and reliable internal quality; this material presents a plateau of plastic deformation at low stresses under compression, signalizing a potential application as energy absorbers. Theoretical simulations show also good thermal insulation ability. Research limitations/implications: as a new product, the full characterization of properties of the new composite is still to be achieved. Therefore, the full potential of commercial application is to be determined. The product can present limitations for application involving tensile stresses due to poor metal/ceramic interfaces. Practical implications: the thixoinfiltration can represent an alternative, low cost processing route for low density composites. The new composites presented are low weight and low cost material, presenting a unique combination of properties which can bring a whole new application field, as low cost, low density components for energy absorption and thermal insulation. Originality/value: both the processing route and the material produced - a low density metal/ceramic composite, using porous ceramic particulates as reinforcement, are new concepts under development by the proposing group at FEM/UNICAMP.

4

Thixoinfiltration: a new approach to produce cellular and other low density metallic materials

Robert M. H., Jorge A. F., Gatamorta F., Silva R. R.

Journal of Achievements in Materials and Manufacturing Engineering

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2010

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Vol. 40, nr 2

180-187

EN

Purpose: the work presents an innovative approach for the production of cellular metallic materials as well as low density metal matrix composites, by using thixoforming techniques; thixotropic semisolid metal is infiltrated into removable and non-removable space holder preforms. Different kinds of preforms are tested to obtain open cell material (sponges), syntactic foams and low density composites. Products are evaluated concerning relative density and mechanical behavior under compressive stresses. Design/methodology/approach: Al alloy AA2011 was infiltrated in the semisolid state into preforms of sintered NaCl particles, sintered glass spheres, vermicular ceramic particles and porous ceramic granulates. After solidification, preforms were either removed by leaching (NaCl) resulting in open cell cellular product, or not (all others), resulting in composites of low density. Tomography tests were used to observe internal quality, and semi-static and dynamic compression tests were performed to evaluate the deformation ability of the material. Findings: results show that thixoinfiltration is a simple and low cost technique to produce different types of low density, porous material. Open cell material as well as syntactic foams and low density composites can be produced with reliable internal quality and dispersion of cells and reinforcement. Composites containing porous reinforcements can present some mechanical characteristics of the conventional cellular metals. Research limitations/implications: as all new developments, the complete understanding of the influence of processing variables upon the final quality of the product, as well as its consistency, must be provided before the technology can be widely used commercially. Practical implications: the technique can represent an alternative, low cost processing route for the fabrication of sponges, foams and low density composites, which can avoid restrictions and operational difficulties of presently available manufacturing processes based in liquid manipulation or powder sintering methods. Originality/value: infiltration of appropriate preforms by thixotropic metallic alloys to produce low density composites and cellular material is a new technique under development by the proposing group at FEM/UNICAMP.

5

Investigations on the suitability of some ferrous alloys for semi-solid processing

Robert M. H., Galdino A. G. S.

Journal of Achievements in Materials and Manufacturing Engineering

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2009

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Vol. 34, nr 2

188-195

EN

Purpose: the work analyses the thixoability of SAE 1070, SAE 1548 and SAE 4340 steels; the possibility of producing thixotropic semi-solid by partial melting of these alloys and the phase transformations taking place during the process. Design/methodology/approach: thixoability was characterized by differential scanning calorimetry (DSC) to determine transformation temperatures involving liquid formation and dfl/dT within the solidification range. Thixotropic slurries were produced by heat treatments at different temperatures above Ts, and distinct holding times. Microstructures were analysed by RX diffractometry, optical and electronic microscopy and EDS microanalysis. Findings: results show that the three alloys investigated present high thixoability, given by wide solidification ranges associated with affordable sensitivity of liquid fraction with temperature within these ranges. Higher thixoability is presented by SAE 1070, followed by SAE 1548 and SAE 4340. Results show also that is perfectly feasible the production of thixotropic slurries of all investigated alloys, by simply heating to temperatures where a liquid phase can be present. Spheroidisation of solid primary phase is fast and increasing holding time at the semi-solid temperature leads to excessive growth of the globules in the thixocast material. Research limitations/implications: thixoability prediction models rely on sensitive experiments as thermoanalysis, with results strongly dependent on experimental conditions; and on thermodynamic data, sometimes not available or reliable for a specific alloy composition. Practical implications: the prediction of the thixoability of a certain alloy can make it more effective its thixoprocessing, allows better control of processing parameters and of the quality of final product. The viability of producing thixotropic semi-solid of ferrous alloys by simple partial melting can definitively insert these families of metallic alloys in the semi-solid processing field. Originality/value: ferrous alloys have become part of the thixoforming scenario more recently, when compared to aluminium and magnesium alloys. Therefore, the study of the thixoability of three commercial steels can bring important information as far as their utilization in thixoforming processing is concerned.

6

Analysis of the thixoability of ASTM A536 ductile iron

Robert M. H., Cristofolini R.

Journal of Achievements in Materials and Manufacturing Engineering

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2008

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Vol. 28, nr 2

115-122

EN

Purpose: Thixoability of the ASTM A536 nodular cast iron is analyzed, it meaning its ability to hold a thixotropic semi-solid state and to be formed as such. Thixoability can be characterized by the solidification range, fraction of primary phase and sensitiveness of liquid fraction with temperature (df1/dT) within the solidification range. It is also investigated the effect of thixocasting in the microstructure of the considered alloy. Design/methodology/approach: Differential thermal analysis, differential scanning calorimetry and thermodynamic calculation package THERMOCALC were used to predict transformations temperatures involving liquid formation and df1/dT within the solidification range. Microstructures of thixotropic slurries produced by partial melting were observed. Findings: Thixoforming of ASTM A536 nodular iron can be considered in a narrow window of about 28° C, were some dissolution of graphite nodules can still be afforded; this window meaning the range of temperatures of co-existence of austenite + graphite + liquid were the eutectic transformation is taking place. At higher temperatures the dissolution of graphite nodules in liquid can be significant. Research limitations/implications: Thixoability prediction models rely on sensitive experiments as thermoanalysis, with results strongly dependent on experimental conditions; and on thermodynamic data, sometimes not available or reliable for a specific alloy composition. Practical implications: The prediction of the thixoability of a certain alloy can make it more effective its thixoprocessing, allows better control of processing parameters and quality of final product; can also subsidize modifications in the alloy to make it more suitable to semi-solid processing. Originality/value: The study of the thixoability of a nodular hypereutectic cast iron is an original subject, not available in the specialized literature, however absolutely necessary if thixoprocessing of this family of alloys is to be considered.

7

Producing thixotropic semi-solid A356 alloy: microstructure formation x forming behaviour

Robert M. H., Zoqui E. J., Tanabe F., Motegi T.

Journal of Achievements in Materials and Manufacturing Engineering

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2007

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Vol. 20, nr 1-2

19-26

EN

Purpose: The work discuss the phenomena involved in the formation of the microstructure of semi-solid thixotropic alloy A356 produced by different techniques and the relation between microstructure and forming behaviour of the material. Design/methodology/approach: Thixotropic slurries of A356 alloy were produced from liquid and from solid conditions; in the first case either by stimulating nucleation rate or crystal multiplication during solid growth; in the second case by recrystallization and partial melting of deformed structures. Flow behaviour was analysed by viscosity measurements or by flow ability in forging operations. Findings: Results show that different production techniques activate different mechanisms, leading to distinct structure features in the semi-solid slurry, and, as consequence, in its forming characteristics. Techniques which promote formation of isolate globules of primary phase, like those involving nucleation stimulation and recrystallization, result in a semi-solid with better flowing behaviour. On the other hand, techniques based on crystal multiplication during growth lead to more interconnected globules in the slurry and poorer forming behaviour. Research limitations/implications: Semi-solid processing is suitable only for alloys with appropriate liquid fraction x temperature relation. Practical implications: the konowledge of the phenomena involved in the formation of thixotropic metallic slurries produced by different techniques, and their consequences in the material structure and flow behaviour, allow the decision of the adequate slurry production method for a specific application, in order to take the best advantage of the semi-solid technology. Originality/value: The analysis of the relation between production process x flow behaviour of semi-solid thixotropic A 356 is original.

8

Manufacturing of cellular A2011 alloy from semi-solid state

Robert M. H., Delbin D.

Journal of Achievements in Materials and Manufacturing Engineering

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2007

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Vol. 24, nr 1

115-122

EN

Purpose: The work presents a new method to produce cellular metallic material by pressing the alloy in the thixotropic semi-solid state into a layer of space holder particles, which are removed from the product after the forming operation. Design/methodology/approach: It is investigated the influence of the thixoforming temperature and the size of space holder particles, in the ability of penetration of the slurry in the porous preform as well as the structure of the obtained porous material (general aspect, quantitative and qualitative characterization of porosity microstructure of cells walls and density of the product). Findings: Cylindrical samples presenting three different ranges of porosity were produced. The cellular material obtained contains open porosity, being characterized as sponge. Products were analyzed by tomography and metallographic techniques. Results show that the proposed process is able to produce acceptable porous material, in a simple and low cost technique. The quality of the product depends rather on the processing temperature than on the size of space holder particles. Low liquid fraction in the thixotropic slurry can lead to incomplete infiltration and deformation of the preform. In the analyzed conditions influence of the size of space holder particles could be observed neither in the processing ability nor in the quality of the product. Density of produced porous material increases as processing temperature increases, due to the increase of cells walls thickness. Research limitations/implications: The investigated process is suitable only for alloys with a significant solidification range. Practical implications: The new method to produce cellular metals can represent energy savings and is highly operational when compared to conventional methods based on liquid infiltration, since lower temperatures are involved and no need of liquid handling is required. Originality/value: the process proposed is a new one; no techniques based on thixoforming of the alloy into porous preforms are known so far.

9

Production of cellular A2011 alloy from semi-solid state

Robert M. H., Delbin D.

Journal of Achievements in Materials and Manufacturing Engineering

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2006

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Vol. 17, nr 1-2

137--140

EN

Purpose: The work investigates a new method to produce cellular A2011 alloy and analyses the influence of processing parameters on the quality of the product. Design/methodology/approach: The proposed process involves pressing the alloy in the thixotropic semi solid state into a layer of space holder particles, which are removed after the forming operation. It is investigated the influence of the thixoforming temperature and the size of space holder particles, in the ability of penetration of the slurry in the porous pre-form as well as the structure of the obtained porous material. Products were analyzed by tomography and metallographic techniques. Findings: The proposed process showed to be able to produce acceptable porous material in a simple and low cost technique; the cellular material produced was characterized as sponge, as presented open and interconnected porosity. The quality of the product depends rather on the processing temperature than on the size of space holder particles. Low liquid fraction in the thixotropic slurry can lead to incomplete infiltration and deformation of the pre-form. In the analyzed conditions influence of the size of space holder particles could be observed neither in the processing ability nor in the quality of the product. Density of produced porous material increases as processing temperature increases, due to the increase of cells walls thickness. Research limitations/implications: The investigated process is suitable only for alloys with a significant solidification range. Practical implications: The new method to produce cellular metals can represent energy savings and is highly operational when compared to conventional methods based on liquid infiltration, since lower temperatures are involved and no need of liquid handling is required. Originality/value: The process proposed is a new one; no techniques based on thixoforming of the alloy into porous pre-forms are known so far.