A large number of liquid systems, such as Al-Bi, AlIn and Al-Pb, exhibit limited miscibility. They are often referred to as the immiscibles and characterized by their unique thermodynamic features, such as large positive enthalpy of mixing [1]. The thermophysical properties of some immiscible systems are summarized in Table I [2]. These systems have great potential applications in bearing systems, electrical contacts and superconducting devices [3]. Unfortunately, it is technically difficult to produce these alloys [1, 4]. During the early stage of the cooling of a homogeneous liquid, the frequently encountered large density difference between two liquids leads to a rapid spatial separation by buoyant force, resulting in a two-layer structure. Owing to this phenomenon, no casting process under terrestrial conditions has yet been able to produce the desired dispersion microstructure, even when extremely high solidification rates were achieved [1]. Many tests have been performed during space experiments to achieve an appropriate phase distribution under microgravity conditions [3]. The results, however, are rather disappointing, because even under microgravity conditions a coarse phase separation occurs [4]. The development of new processing techniques, therefore, becomes an inevitable task facing both the materials community and the automobile industry. As a consequence, several new techniques emerged where controlled Marangoni motion—strip castings [5] or magnetic fields [6] were introduced to counterbalance the gravity influence. But the results are still unsatisfactory, the immiscible particles were found to concentrate in the middle of strip castings. High energy processes have also been developed to achieve the desired microstructure [7–9] and all are limited in the laboratory scale due to high processing cost and difficulties in controlling the microstructure. More recently, a novel rheomixing machine (twinscrew extruder) has been developed and employed to study the behavior of monotectic alloys by Fan et al. [10]. Due to the limitation of the barrier and screw materials (currently made of steel), this laboratoryscale study is further limited to such immiscible systems that have a low melting point and are non-reactive with the barrier and screw materials, for example, Ga-Pb. A solution to this problem comes from modern composite theory, learning from the metal-ceramic composite in combining the properties of two distinct phases (normally a ceramic phase having limited wettability with the matrix alloy), it is thus a logical evolution to develop immiscible systems by employing those techniques used for composites production. Among those techniques, the squeeze casting method is the most commonly used method which allows rapid production of near net-shape components with good tolerance and surface finish [11]. The key step for this process is to obtain the desired preforms for the later melt infiltration. It is the recently well-developed metallic foams that provides the optimum opportunity [12]. This refers to metallic foam that is suitable for squeeze casting to produce a two-phase system. The differences in the melting temperature of the phases in most immiscible systems (Table I) make it technically feasible to prepare the desired system by infiltration of a reticulated metallic foam which has a higher melting temperature with an alloy having a low melting temperature. This is the origin of the present study that aims to develop a novel process to produce the immiscible system by squeeze casting of engineered metallic foams. Al-Pb was chosen as a model immiscible system to demonstrate the principles. The commercial open-cell aluminum foam (Duocel, ERG, Okland, CA) and pure lead were used in the squeeze casting process. The foam was placed in a steel die and preheated together with the die to a temperature of 200–220 ◦C followed by infiltration with the molten lead with a temperature of 380– 400 ◦C using a squeeze caster fitted with an evacuating system. After the infiltration process a maximum pressure of 50 MPa was maintained until the die cooled and the lead phase solidified. The samples were polished and examined using a Jeol JXA840 scanning electron microscope (SEM). The microstructure of the commercial foam with a relative density of 7% is shown in Fig. 1a and the microstructure of Al-Pb alloys produced therefrom is shown
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May 19, 2023
Annalisa Fortini + 1
