Microstructure and Properties of Semi-Solid Aluminum Alloys: a Literature Review
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metals Review Microstructure and Properties of Semi-Solid Aluminum Alloys: A Literature Review Annalisa Pola 1,* ID , Marialaura Tocci 1 and Plato Kapranos 2 1 DIMI—Mechanical and Industrial Engineering Department, Via Branze, 38-25123 Brescia, Italy; [email protected] 2 Department of Materials Science & Engineering, The University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, UK; p.kapranos@sheffield.ac.uk * Correspondence: [email protected]; Tel.: +39-030-371-5576 Received: 4 February 2018; Accepted: 6 March 2018; Published: 13 March 2018 Abstract: Semi-solid processing of aluminum alloys is a well-known manufacturing technique able to combine high production rates with parts quality, resulting in high performance and reasonable component costs. The advantages offered by semi-solid processing are due to the shear thinning behavior of the thixotropic slurries during the mold filling. This is related to the microstructure of these slurries consisting of solid, nondendritic, near-globular primary particles surrounded by a liquid matrix. This paper presents a review on the formation of this nondendritic microstructure, reports on the different proposed mechanisms that might be responsible, and illustrates the relationship between microstructure and properties, in particular, tensility, fatigue, wear, and corrosion resistance. Keywords: semi-solid; microstructure; mechanical properties; wear; corrosion 1. Introduction Semi-solid metal (SSM) processing is a manufacturing technique where an alloy, in the form of a slurry of near-globular primary particles in a liquid matrix, is injected into a die, allowing the production of near-net-shape components. The main advantage of this technology is related to the flow properties of the metal in the form of a slurry, which, in the semi-solid state, is non-Newtonian and exhibits shear thinning behavior [1,2]. The viscosity of the SSM slurry is higher than when fully liquid, reducing the risk of turbulent [3] or spray flow, which is more typical of conventional pressure die-casting [3,4]. However, thanks to the shear thinning behavior of the slurry, under the influence of a shear force acting on it when it flows into the die, the viscosity decreases and the metal slurry is able to fill the cavity completely in a nonturbulent manner. As a consequence, semi-solid cast parts are almost free of gas porosity. Injecting a partially solidified alloy slurry has the added benefit that shrinkage porosity is virtually absent [5]. The low or even absent porosity allows the production of structural parts with good mechanical properties that can also undergo subsequent heat treatments or welding operations. Semi-solid processing guarantees higher performance than die-casting, while maintaining a number of the advantages of die-casting, such as good dimensional tolerances, high production rates, high surface quality, complex near-net-shape parts, and thin sections with very limited need of any finishing operations [6]. In addition, when compared to conventional die-casting, SSM processing increases die life because of the lower stress associated with the lower injection temperatures and speeds (i.e., lower mold attack and erosion, lower thermal shock), reduced cycle times and risk of hot tearing due to the lower temperature of the metal slurry (no over-heating), and associated lower energy consumption [4,6,7]. On the other hand, SSM manufacturing requires specialized equipment for alloy preparation, combined with strict control of process parameters, particularly the alloy temperature, i.e., the solid/liquid Metals 2018, 8, 181; doi:10.3390/met8030181 www.mdpi.com/journal/metals Metals 2018, 8, 181 2 of 17 fraction. Unfortunately, all these tend to increase the production costs [8], even though recent investigations have shown that SSM processing is only slightly more expensive than conventional die-casting and cheaper than some other competing foundry processes [9]. Since its development in the early 1970s at MIT [10], much research has been performed worldwide, aimed mainly at developing new and alternative routes of feedstock production for different alloys suitable for SSM processing. At present, there are a number of different techniques used to produce semi-solid castings, differentiated by the percentage of liquid/solid fraction they employ and the way they produce the alloy in the semi-solid state. SSM methods can be divided in two main categories according to their processing route, known as rheocasting and thixocasting [11]. In rheocasting, the semi-solid slurry is prepared in situ from the liquid state down to a certain percentage of solid fraction (usually between 10 and 30% [12]), and then directly transferred into the shot sleeve for being injected into the die. In thixocasting, a billet, characterized by an almost globular or rosette-like microstructure developed through some specific route, is reheated in the mushy zone (semi-solid region) to an appropriately chosen solid fraction (usually between 50% and 60% [13]), placed in a modified shot sleeve and finally injected into the die [14]. Another classification methodology distinguishes the SSM methods according to the initial step used for obtaining the semi-solid feedstock, i.e., from the liquid state, by controlled solidification or from the solid state, via heavy plastic deformation and recrystallization [11]. Some of the most common routes for feedstock material preparation are: mechanical stirring, such as the SSR [15,16] or GISS [17] processes, electromagnetic stirring (EMS or MHD) [18,19], ultrasonic stirring (UTS) [20,21], New Rheocasting (NRC or UBE) [22,23], cooling slope [24], twin screw [25], Rheometal [26], liquid mixing method [27], SEED [28], thermomechanical [29], and SIMA [14,30]. A number of interesting reviews about the different technologies available for obtaining nondendritic slurries can be found in the literature [11,13,14,31–33]. Nevertheless, independently from the chosen technique, the fundamental concept is based in developing feedstock with a microstructure of a solid, near-globular phase surrounded by a liquid matrix when in the semi-solid state. As already mentioned, when a shear stress is applied, the near-globular solid particles move easily between and over each other, reducing the viscosity and making the material behave like a liquid. On the contrary, when a shear stress is applied on a dendritic microstructure, the liquid remains entrapped between dendrite arms and prevents them from moving freely, thus increasing the viscosity of the alloy [32]. This paper presents a review on the formation of nondendritic microstructures, microstructures that have a key role in semi-solid processing, and discusses the different proposed mechanisms together with ways to analyze SSM microstructures. In addition, the review provides information on the variation of mechanical properties and corrosion behavior through modification of microstructures that are typical of SSM processing. 2. Formation of Nondendritic Microstructures in SSM Processing During the early experiments performed by Spencer et al. on a Sn–Pb15 alloy [10], it was found that the microstructure of the material was strongly affected by the constant shearing of the alloy when in semi-solid state. Particularly, it was shown that shearing action causes the formation of a nondendritic grain structure, which is the distinctive characteristic of semi-solid alloys. Moreover, with further shearing during cooling, it is also possible to obtain spheroidal particles, typically with some entrapped liquid [4]. The authors also reported that high shear rates and slow cooling rates can promote the formation of spherical particles instead of rosette-like ones [10]. The steps for the formation of nondendritic microstructures have been extensively studied over the years and one of the first proposed mechanisms is shown in the schematic illustration of Figure1. Metals 2018, 8, x FOR PEER REVIEW 3 of 17 Metals 20182018,, 88,, x 181 FOR PEER REVIEW 33 of of 17 Figure 1. Globule formation during stirring in the semi-solid range: (a) initial dendritic fragment, (b) dendriticFigure growth, 1. Globule (c) formationrosette, (d during) ripened stirring rosette, in the and semi-solid (e) spheroid range: [4]. (a) initial dendritic fragment, (b) Figure 1. Globule formation during stirring in the semi-solid range: (a) initial dendritic fragment, (b) dendritic growth, (c) rosette, (d) ripened rosette, and (e) spheroid [4]. dendritic growth, (c) rosette, (d) ripened rosette, and (e) spheroid [4]. 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