Luis Vanderlei Torres

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.

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.

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.

Comparison of Morphological Evolution of Al-7wt%Si-2.5wt%Cu Alloy Produced by Direct Chill Casting/Electromagnetic Stirring and ECAP

In semisolid state forming the thermodynamic control of the solid liquid transition is necessary but not sufficient condition for having a good processing. For thixoforming it is critical to have a refined microstructure that during the heating to the semisolid state, achieve a mixture of small spheres immerse in liquid, which is crucial in order to get the best rheological properties for the slurry. This work compares the morphological evolution at the semisolid state of the Al-7wt%Si-2.5wt%Cu alloy produced by a) direct chill casting under electromagnetic stirring (EMS) combined with grain refinement and b) processed via one single pass in an equal channel angular pressing. EMS introduces metal flow during solidification that allows the better distribution of the nuclei provide by the grain refiner. ECAP has emerged as a promising technique capable of imposing large homogeneous deformations in metals, that could leads to a optimal grain refining. The ECAP deformation occurs in a die that contains two channels of the same cross-sectional area and form an angle of 120°. The raw materials were submitted to a heating treatment in order to reach 60% liquid fraction, at soaking times of 0s, 30s, 90s and 210s and the microstructure was evaluated via B&W and color metalography. Comparing the two production processes, it is observed that the ECAP process is highly efficient: while the traditional EMS method produced grains about 150 μm size, ECAP technique achieved grains of 60 μm, providing a fully globular structure, which exhibit favorable characteristics for the thixoforming process reaching to apparent viscosity about 2*104Pa.s.

Rheological Behavior of Semisolid Hypoeutectic Cast Iron Alloyed to Chromium

This paper proposes and tests new Fe-2.6wt%C-2.8wt%Cr-1.8wt%Si specially designed for thixoforming processing. Samples were heated to the semisolid state at 1195 °C and held at this temperature for 0, 30, 60 and 90s, and then subjected to compression tests. Two-platen compression tests were carried out in order to determine the semisolid behavior. The holding time in the semisolid range simulates the industrial heating process that is time-controlled rather than temperature controlled. Morphological characterization pointed to an as cast microstructure composed by pearlite matrix plus chromium based carbides and nodular graphite and heated conditions composed by pearlitic matrix plus chromium based carbides without free graphite. The semisolid behavior indicated that at 1195 °C the thixoforming procedure requires a maximum stress of 2.5 MPa and maximum apparent viscosity of 4.8x105 Pa.s for the 0s soaking time condition, 2.0 MPa and 4.6x105 Pa.s for 30 s, 1.7 MPa and 3.7x105 Pa.s for 60 s and 1.2MPa and 3.3x105 Pa.s for 90 s of holding time, best condition achieved.

Morphological Characterization of Al-2.0wt%Si-2.5wt%Cu Alloy Produced by Direct Chill Casting and Electromagnetic Stirring for Thixoforming Process

This work evaluates, by means of its morphological characterization, a new Al-2.0wt%Si-2.5wt%Cu alloy produced by direct casting under electromagnetic stirring for thixoforming process. The alloy was submitted to re-heating treatment in two conditions of solid fractions, 45% and 60%, for 0, 30, 90 and 210 s. Re-heating treatment times promote solid phase particles globularization, presenting average grain sizes of approximately 90 μm for both solid fraction. These results show that the alloy is viable for thixoforming process, especially in the condition of 90 s of treatment time at 45% and 60% of solid fraction, that presents grain size about 90 μm and shape factor higher than 0.60. Another important fact about the small grain/primary particle size is related to the use of electromagnetic stirring in its production, that promotes the breakdown of the material structure and consequently stimulates globularization.