The semisolid microstructural evolution of a severely deformed A356 aluminum alloy

Abstract

A cast A356 aluminum alloy was successfully processed through applying a new severe plastic deformation method, accumulative back extrusion (ABE), at the temperature of 300 °C. In consequence, a severely deformed microstructure consisting of round and fine Si particles has been obtained. Microstructural evolution of the ABE-processed material soaked for various times at different isothermal temperatures in semisolid state was investigated. It has been shown that the proposed semisolid thermomechanical route would lead to a fine semisolid microstructure, where the spheroidal α-Al globules are surrounded by continuous liquid film. The results indicate that the ABE processing followed by 10 min holding at 580 °C and/or 6 min at 590 °C has led to the best thixotropic characteristics, i.e., the high shape factors and low globule sizes. The thixotropic structure characteristics have been also properly discussed in terms of globularization and coarsening mechanisms as well as the kinetics of microstructural evolutions. The almost linear semisolid coarsening behavior of the processed material indicates the outstanding role of Ostwald ripening mechanism in the growth of globules.

Introduction

In order to fulfill the customers’ requirements on quality of the products (i.e., satisfying dimensional accuracy, geometrical complexity, and functional flexibility), thixoforming has been thoroughly put into practice by many manufacturers instead of conventional casting and forming routes [1], [2]. In comparison with conventional liquid state and solid state forming processes, the thixoforming (i.e., forming the materials in the semisolid state to desired near-net-shape products) presents significant potential upsides, a comprehensive review of which would be found in [3], [4]. The general difficulties in thixoforming, however, centre around the wide interval over which the solidification occurs which may eventually result in hot tearing. This is added by the steep slope of the fraction liquid versus temperature curve, which may lead to a narrow processing window [4], [5]. Accordingly, only particular grades of Al–Si alloys are being used in high volume production route due to their excellent thixoformability and the high fluidity in the semisolid region along with appropriate volume fraction of dispersed eutectic phase which melts in the semisolid state [6], [7].

To apply any thixoforming procedure, the starting microstructure needs to be globular in the semisolid state. Following to this, the thixotropic behavior of the material is strongly contingent upon its semisolid microstructural characteristics such as solid particle size and morphology as well as the thickness and spatial distribution of the liquid phase [8]. In this regard, many methods have been proposed to produce the globular microstructures, which are generally classified in two main categories: liquid metal and solid state routes [4]. The liquid metal routes consist of techniques such as cooling slope (CS), mechanical stirring, and magneto hydro dynamic stirring (MHD) processes, while solid state routes (i.e., thermomechanical treatments) include strain-induced melt-activated (SIMA) method and recrystallization and partial melting (RAP). Appreciating the advantages of the solid state methods over the liquid metal routes (e.g., the reduced amount of entrapped liquid and more spherical particles [9], [10]) which subsequently result in enhanced thixoformability, many researchers have been devoted to study the microstructural evolution of different materials during SIMA and RAP processing [11], [12], [13], [14]. In a general point of view, these processes consist of plastic deformation and subsequent semisolid soaking of work-pieces. During soaking, recrystallization takes place, and once the liquid starts to form, grain boundaries are wetted and penetrated by it, leading to the formation of fine equiaxed particles.

To introduce the required strain prior to isothermal holding (soaking), different procedures have been employed such as rolling, forging, upsetting, and extrusion. All of these methods, nonetheless, have their own drawbacks, among which the deformation inhomogeneity, the restricted value of imposed strain, and the radical changes in the dimensions of the work-pieces are the most important ones [15], [16]. To well overcome the aforementioned disadvantages, severe plastic deformation (SPD) methods have been proposed as an alternative solution by which the required strain energy can be enforced to the work-pieces without any change in their geometry. In this regard, equal channel angular pressing (ECAP) has been the most common SPD route successfully employed in developing the thixotropic microstructures of magnesium (AM (Mg–Al–Mn) [17], ZK (Mg–Zn–Zr) [18], AZ (Mg–Al–Zn) [19], [20], and ZW (Mg–Zn–Y) [21] series) as well as aluminum (Al–Si–Mg [22]) alloys. It has been shown that the ECAP followed by semisolid soaking yields a desirable thixotropic structure in terms of fineness and roundness of globules as well as the lack of entrapped liquid. Nevertheless, it is worth mentioning that ECAP is accompanied by some limitations such as deformation heterogeneity particularly after first few passes, relatively high required load, and the need of interpass operations (e.g., machining) .

Understanding the main issues which have been considered in the previous involved studies, the present work has been conducted to study the evolution of microstructure during accumulative back extrusion (ABE) process followed by partial remelting to fabricate the fine and globular A356 aluminum feed-stocks that are appropriate for thixoforming. ABE is a recently developed SPD method, the capability of which in imposing high amounts of strain per each pass has been previously established [23]. Moreover, lower required load and its continuity (i.e., no need of interpass-operation), has made it potentially suitable for mass production [24]. The present results are drawn to evaluate the microstructural evolution during the proposed semisolid thermomechanical treatment.