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

Abstract

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.

Microstructural characterization of the A356 alloy in different processing routes used in the thixoforming process

“…The first processing technique was used as comparison parameter, since it is a conventional casting, without electromagnetic stirring and without grain refiner use. The use of electromagnetic stirring [10,11,12,13] has great benefits to obtain refined structures, because promotes the breakdown of structure formation, stimulating like this crystalline multiplication and its consequent globularization. In the whole tests which utilized electromagnetic agitation was used the higher power provided by the system, 8kW that is enough to produce a magnetic field of 13 Gauss [12].…”

“…The field-induced electric currents promote a strong agitation, breaking the structure in formation and stimulating the crystalline multiplication; achieving low values of primary globules sizes and grains that are important factors for the thixoforming process. [10,11,12,13]. In order to, this work aims to evaluate different processing routes, using A356 alloy as base material the in terms of microstructural characterization by optical microscopy, using conventional metallographic (black and white) and polarized metallography (colour).…”

Electromagnetic Stirring versus ECAP: Morphological Comparison of Al-Si-Cu Alloys to Make the Microstructural Refinement for Use in SSM Processing

“…ECAP deformation was performed in a metallic mould with cylindrical channels with cross sections of 30 mm and 120 ∘ outer angle; Figure 1 shows the details of the used ECAP die. A 50-ton hydraulic press operating at maximum speed of approximately 7 mm/s was used to impose a single ECAP pass in ingots lubricated with MoS 2 [10]. Before the ECAP process, the ingots 30 mm in diameter and 150 mm long were heated to 300 ∘ C sufficient to prevent surface cracks but also to allow better passage thereof through the ECAP die.…”

“…Heat treatment globularization aims to verify the stability of alloy at high temperatures in order to evaluate the microstructural evolution and correlate these characteristics with the mechanical and rheological properties of these materials. The treatments were carried out in approximately 6 min (25 mm tall × 30 mm diameter samples) in a Norax induction furnace (25 kW, 8 kHz) with a heating rate of 80 ∘ C to 100 ∘ C/min [3,10]. When the samples reached the temperatures of the 60% solid fraction, they were held at those temperatures for 0, 30, and 90 s, after which they were cooled in water.…”

“…As the cross section of the sample remains approximately constant, the process can be repeated many times allowing for controlling the microstructure and thus the final properties of the materials [8,9]. It is known that Al-Si-Cu alloy content of low silicon does not have a fine structure by ECAP process [10], but alloys with the same content and greater than 5wt%Si have large refinement of the microstructure, with grain sizes/primary globules much lower when compared with the electromagnetic stirring [11].…”