In recent years, semi-solid metal processing (SSMP) of commercial alloys and metal–matrix composites has received considerable interest in manufacturing of near-net-shape products, particularly in automotive industries. The SSMP technique takes the advantage of thixotropic behavior of partially solidified alloys, i.e., becoming fluid when stirred or shaken, and setting back to solid state again when allowed to stand still [1]. A range of reviews are available on SSMP describing its significant advantages over conventional casting routes such as enhanced die life, lower shrinkage and reduced porosity. In addition, lower forming pressure, higher deformation homogeneity and enhanced mechanical properties in the components are some advantages of SSMP over conventional forming routes [2] and [3]. The two major routes of SSMP are the ‘rheo-casting’ and ‘thixoforming’ [3]. However, in both the processes, the key step is to obtain a feedstock material with nearly globular microstructure [4]. Amongst the important routes for obtaining non-dendritic feedstock for SSMP processes the Cooling slope (CS) casting process is a simple rheocasting process which has minimal equipment requirements [5] and [6]. It is emerging as a viable route for generation of feedstock material in both alloys and metal matrix composites with desirable characteristics for further thixoforming [7].
Al–Si–Mg based cast alloys have been used widely for thixoforming processes. Amongst them A356 and A357 are the most common alloys used to obtain near-net shape products because of good casting characteristics, weldability and corrosion resistance [8]. Amongst the thixoforming processes available, forging has considerable potential for severe plastic deformation and improvement in mechanical properties [9]. The investigations by Tahamtan et al. [10] and [11] showed significant improvement in tensile properties of thixoforged A356 alloy compared to that of rheocast and gravity-cast samples. Recently, in-situ reinforced aluminum based metal matrix composites (AMMCs) are emerging as one of the most promising alternatives for eliminating the inherent defects associated with ex-situ reinforced AMMCs. It has been reported that Al based composites reinforced with TiB2 particles have clean interfaces, and moreover TiB2 particles refine the size of eutectic Si by restricting its growth and thereby enhancing the mechanical properties significantly [12], [13] and [14]. In addition, metal forming in the mushy state zone has gained importance for manufacturing of AMMCs [15].
The solution treatment followed by ageing plays a key role in improving the mechanical properties and durability of metallic components [16]. The strengthening mechanisms in the Al–Si–Mg alloy system has been attributed to the precipitation of phases from a supersaturated solid solution (SSS) elucidated by different techniques such as XRD and TEM reported elsewhere [17]. The precipitation sequence and reactions in Al–Si–Mg alloy can be described by Eq. (1) as follows:
equation(1)
SSS→GP−Ispherical→GP−II(β″)needle→(β′)rod→β(Mg2Si)platelet
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The decomposition process begins with the formation of two types of Guinier Preston (GP) zones. One of them is coherent and spherical, called GP-I zone or pre-clusters, with no internal order. As ageing proceeds, the GP-I zones become ordered and acquire an acicular or needle shape. At this stage, they are called GP-II zones or β″. Prolonged ageing results in the transformation of coherent β″ needles into semi-coherent β′ rods. In the later stages, it loses coherency and the equilibrium β (Mg2Si) phase is formed. The age hardening behavior of A356 alloy subjected to thixoforming have been investigated by various researchers, reporting the enhancement in tensile properties after T6 treatment [18] and [19]. The principal strengthening mechanisms operative in case of Al matrix composites is through the dislocations punched out at reinforcement–matrix interfaces to relax the residual stresses owing to the mismatch between the coefficients of thermal expansion (CTE) of Al matrix and reinforcement [20] and [21]. The interest in the age hardening behavior of Al based composites arises from its accelerated kinetics of ageing when compared to that of the unreinforced alloy due to the much greater dislocation density in the former material as reported in the literature [22]. Roy et al. [23] have reported that lattice defects such as dislocations and voids can be generated during plastic deformation of metallic materials, resulting in greater tensile properties. Earlier, Mandal et al. [12] have investigated the effect of TiB2 particles on ageing response of Al–Cu alloy. They found that a continuous increase in the yield strength and ultimate tensile strength without any significant loss in ductility with increase in the amount of TiB2 in both as-cast and peakaged conditions. In addition, TiB2 particles enhance the ageing kinetics. Some earlier works in our research group on ageing studies of A356 alloy dispersed with TiB2 particles indicated that the ageing time comes down from 12 h in the base alloy to 4 h in the composite [14]. Recently, Siddhalingeshwar et al. [15] have studied the effect of mushy state rolling on the ageing kinetics of in-situ Al–4.5Cu–5TiB2 composite and found that peak aging times of mushy state rolled composites reduced significantly compared to that of as-cast alloy.
An extensive review of the literature suggests that very limited information is available on ageing behavior of Al alloy based composites subjected to thixoforming. Although some work exist on the ageing behavior of as-cast Al–Si, Al–Cu alloys reinforced with TiB2 particles, reports on the age hardening behavior of A356–TiB2 composite subjected to thixoforming is still lacking. Therefore, an attempt is being made to study the age hardening behavior of thixoformed A356–5TiB2 composite.