Davi Munhoz Benati

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.

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.

In Situ Observation of Semisolid Fe-2.5C-1.5Si Gray Cast Iron

Fe-2.5C-1.5Si gray cast iron evaluated in previous works exhibited promising potential as semisolid raw material presenting low levels of maximum stress and viscosity, similar to Al-Si alloys. This work is intended to investigate phase transformations and liquid phase formation for the Fe-2.5C-1.5Si gray cast iron in order to understand the performance of the alloy during the semisolid processing. Thus in situ heating experiments via high temperature laser scanning confocal microscopy were performed to analyze the solid-to-liquid transition. At room temperature alloy presented a matrix of pearlite and ferrite with type D flake graphite. During the heating process the main transformations observed were graphite precipitation on the austenite grain boundaries, graphite precipitates and flakes graphite growing and coarsening with the increasing of temperature and the beginning of melt around 1140°C. Coarsened flakes at high temperatures resulted in a liquid continuous network after melting, thereby the liquid phase was formed surrounding and wetting homogeneously the solid phase. This favors the detachment of grains from each other and leads to the intended solid globules immersed in liquid.

Obtaining Martensitic Structures during Thixoforming of Hypoeutectic Gray Cast Iron

The control of parameters such as liquid fraction, holding time, and cooling rate during thixoforming can help control the final microstructure of the thixoformed part, thus improving its mechanical properties. This study intended to investigate conditions required to obtain martensite in hypoeutectic gray cast iron at 3.1% CE (carbon equivalent) deformed in the semisolid state. Samples heated up to 1130, 1135, and 1145°C (liquid fractions of 10, 30, and 45%) were compressed into platens without any holding time (0 s). If a sample presented a martensitic structure for 0 s holding time, new samples were retested at the same temperature for 30, 60, and 90 s holding times. The die casting process was simulated by allowing the platens to become locked after hot compression. Samples that cooled in the locked platens were submitted to higher cooling rates than samples that cooled with the platens open and presented martensite instead of the conventional ferrite and pearlite. Thus, the factor that had the greatest influence on the formation of martensite was the cooling rate rather than stress. The thixoforming process presented good morphological stability, which is highly desirable for industrial applications.

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.