Eugênio José Zoqui

Structural modifications in rheocast Al–Cu alloys by heat treatment and implications on mechanical properties

Abstract The work shows the effect of heat treatment T6 (solution and controlled precipitation of CuAl2) on the microstructure of rheocast Al-4.5wt.% Cu alloy. Rheocast structures were obtained by partial melting to 40% liquid fraction from dendritic material with initial different grain sizes. Globular structures with grain sizes varying from 15 to 170 μm were produced and characterized for grain size, morphology, and Cu distribution and were mechanically tested, in both as-rheocast and T6 conditions. Results showed that increasing grain size results in a decrease in tensile and yield strengths, the latter according to a Hall–Petch type of relation. Elongation and constriction increase as grain diameter increases to a certain extent; for higher increase in grain diameter, these properties decrease. Concerning the effect of heat treatment in the structure, rheocast material requires more time for complete dissolution of the eutectic phase than cast structures, can keep higher Cu content in solution and prevents grain growth during holding. Greater improvement in mechanical properties is achieved by heat treatment in rheocast material.

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

Abstract The work presents some results obtained when producing rheocast structures of Al–(4.5wt.%)Cu alloys, by heat treatment of dendritic material (in 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.

Alloys for Semisolid Processing

The objective of this chapter is to understand what semisolid alloys are and what are their requirements in terms of liquid to solid transition (in rheocasting processes) or solid to liquid transition (in thixoforming), key to understanding semisolid materials technology. The chapter exploits the few commercial alloys available for semisolid metals (SSMs) and describes the concepts involved in the development of SSM materials.

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 104 to 105 (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 104 Pa.s, which is extremely low for thixoforming standards.

The effect of heating rate on the microstructural breakdown required for thixoformability

Abstract Thixoforming processes depend upon three basic features of the semisolid slurry: the thermodynamics of the liquid-to-solid transition; the morphology of the microstructure; and the slurrys rheological properties. By studying the thermodynamics of the solid-to-liquid transition of a specific alloy, one can determine whether the entire thixoforming process can be controlled when the alloy is used as the raw material. The rheology of the semisolid material is determined not only by the morphology of the microstructure of the primary solid phase immersed in the liquid, but also by the relative amounts of liquid and solid. In thixoforming operations the raw material is heated up to the semisolid state and injected into the die. Hence, the heating procedure has a significant impact on the morphology of the remaining solid and, consequently, its viscous behaviour, as shown in this paper. A heating rate close to 50 K min−1 was found to produce the most suitable morphology for semisolid processing as the mechanisms involved (Ostwald ripening and coalescence) were present to an equal extent.

Characterization of the Microstructure and Rheological Behavior of Al-4wt%Si-2.5wt%Cu Alloy Produced by Direct Chill Casting and Electromagnetic Stirring

This work involved an evaluation of a Al-4wt%Si-2.5wtCu alloy to be used as raw materials in the thixoforming process. The alloy was produced by direct casting under electromagnetic stirring to obtain ingots of 250 mm length and 30 mm diameter. The alloy was analyzed to determine their morphology and rheological behavior in the semi-solid range. The tests included characterization of the microstructural evolution by subjecting them to re-heating treatment in two conditions of solid fractions, 45% and 60%, for 0, 30, 90 and 210 s. A compression testing device designed specifically to evaluate semi-solid materials was used to determine the rheological behavior, and the tests were performed at the same heating rate and hold times to determine the apparent viscosity. The use of electromagnetic stirring to produce the raw material was effective in producing alloy with very small grain/primary particle sizes (80 up to 120 µm). Based on the morphological evolution in the semi-solid state, the alloy showed only minor variations in grain/primary particle size and surface factor (SF) as a function of the different globularization heat treatment times, suggesting that all these alloys are suitable for production. The Al-4.0wt%Si-2.5wt%Cu alloy with 45% solid fraction presented apparent viscosity about 1.5 x106 Pa.s.

Microstructure of Thixoformable Hypoeutectic Cast Iron

The use of a specially designed hypoeutectic cast iron as a potential raw material for the thixoforming process is described in this paper. Thixoforming technology normally uses aluminum-silicon alloys such A356 and A357 as raw materials. Iron-based alloys are less common, despite the lower cost of the raw material. The paper describes the semi-solid behavior and corresponding final microstructure of a hypoeutectic gray cast iron after thixoforming tests. The Fe-2.6wt%C-1.5wt%Si alloy was prepared via conventional casting in sand molds. Samples were heated to the semi-solid state at 1160 and 1180oC and held at these temperatures for 0, 30, 90 and 120s, and then subjected to compression tests. Two-platen compression tests were carried out in an instrumented eccentric press in order to determine the semi-solid behavior. The holding time in the semi-solid range simulates the industrial heating process that is time-controlled rather than temperature-controlled. The semi-solid behavior indicated that the semi-solid cast iron behaves like aluminum-silicon alloys, presenting a stress of up to 24MPa under 80% strain and a corresponding apparent viscosity of up to 1.5*105 Pa.s at 1180oC. The final microstructure after compression testing was essential in determining the material’s morphological evolution. Tests revealed that heating up to the semi-solid range followed by thixoforming changes the material’s graphite morphology from type A to B (or E), but does not significantly affect the interdendritic arm spacing between graphite lamellae. The resulting structure is composed of fine graphite and pearlite.

Thixoforming of Aluminium-Silicon Alloys in a Mechanical Eccentric Press

The semisolid processing technology is not widely used due to the high cost of raw material and the equipment it requires. New low-cost raw materials and processes could be the key to expand the use of this technology. This paper describes an initial effort to develop new Al-Si-Mg in terms of raw material production and processing. The morphological evolution of all the alloys produced was characterized during their reheating to the semisolid state at 45 and 60% solid fraction, as well as the semisolid behaviour in terms of viscosity versus shear rate. The adaptation of the semisolid technology to the thixoforming process via eccentric press was tested using an equipment up to 25 tons. This type of equipment is not commonly employed in this kind of processing. Results indicate that alloys with low silicon content, e.g., 2 or 4wt%Si, behave similarly to alloys with 7wt%Si, which are normally used in the thixoforming process, with a viscosity of about 2 * 105 Pa.s. The semisolid behaviour of low silicon alloys indicates the potential expansion of the range of raw materials for this application. Thixoforming of semisolid materials in an eccentric press appears to be a very promising technology, yielding parts that, despite their simplicity and restricted shape, display a very good final mechanical behaviour.

Electromagnetic Stirring Versus Ecap: Morphological Comparison Of Al-si-cu Alloys To Make The Microstructural Refinement For Use In Ssm Processing

This work evaluates the morphological evolution at the semisolid state of the Al-4.0wt%Si-2.5wt%Cu alloy produced by direct chill casting under electromagnetic stirring (EMS) and by one equal channel angular pressing (ECAP) pass. The ECAP emerged as a promising technique capable of reduction and homogeneous metals microstructure imposing large deformations occurs in a matrix that contains two channels of the same cross-sectional area and forms an angle of 120°. The materials were submitted to reheating treatment in condition of 60% solid fraction at treatment times of 0, 30, and 90 s. Comparing the two cases, we have the presented ECAP process that had an excellent response to the recovery and recrystallization mechanisms, and refined microstructures ideal for thixoforming were produced. Primary particle sizes of about 45 μm and grain sizes of about 75 μm and a circularity shape factor of more than 0.60 were obtained. The low silicon alloy, Al-4.0wt%Si-2.5wt%Cu, presented excellent refinement when processed via equal channel angular pressing, presenting good morphological stability at the semisolid state, without significant changes in size or shape of the solid particles. This fully globular structure is favourable for thixoforming processes.

Morphological Evolution of Al-2wt%Si-2.5wt%Cu Alloy Produced by EMS and EMS Enhanced by ECAP

This work evaluates the morphological evolution at the semisolid state of the Al-2wt%Si-2.5wt%Cu alloy produced by direct chill casting under electromagnetic stirring (EMS) and this same raw material enhanced by one equal channel angular pressing pass (ECAP), specially designed to produce raw material for thixoforming processes. The traditional EMS production method is compared to ECAP in order to determine if a second procedure is necessary for this particular alloy. The ECAP process emerged as a promising technique capable of imposing severe homogeneous deformation in metals, which could improve thixoforming processes. The ECAP deformation occurs in a matrix that contains two channels of the same cross-sectional area and form an angle of Φ = 90° and Φ = 120°, the process can be repeated many times allowing to control the microstructure and properties of the materials. Both raw materials, EMS and EMS + ECAP were submitted to re-heating treatment in two conditions of solid fractions, 45% and 60% at soaking times of 0s, 30s and 90s. With this procedure primary particle sizes of about 60 μm were obtained, which exhibit favourable characteristics for the thixoforming process.