Current technologies for aluminum castings and their machinability

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Current technologies for aluminum castings and their machinability

Metal casting is one of the leading production techniques for three-dimensional parts in manufacturing industry. The casting process consists of many stages such as design, determination of chemical composition (alloying), mold preparation, melting, pouring, solidification, and finishing. These stages in the process need to be supervised time to time to ensure that the product is made as per the requirement. The problems during these stages also affect machinability of the cast material. This chapter covers topics related to the difficulties encountered in the production of metal alloys by casting, and further their machinability study and its enhancement.
In conventional liquid alloy solidification processes, several processes occur that change the formation of the molten metal structure. With increasing time and decreasing temperature, the liquid metal changes into solid form. This process can be broken into a number of stages. At the beginning of this process, the dendrites (treelike in morphology) form independent of one another. After this, the dendrites grow and eventually begin to spread through the whole area of the bulk metal and form interconnecting networks. Once the solidus temperature is reached, any remaining liquid solidifies instantaneously and traps the networks and their structures in place. At the end of the solidification process for small components, there would typically be hundreds of millions to trillions of dendrites, with the actual quantity depending on the exact cooling conditions, composition, and volume considered. This dendritic microstructure typically results from conventional liquid metal solidification processes. The exact size, shape, and number of primary phase components have a strong influence on the fluid consistency and processability within the semisolid state. These characteristics depend on the processing technique and associated parameters applied during the solidification stage. This formation of the primary phase in turn determines the final properties of the material and the produced component.
In 1971, during experimental work on the measurement of semisolid metal (SSM) viscosity of Sn-15wt%Pb alloys, it was noted that the viscosity was much reduced within the semisolid state if shearing of the melt was continued during solidification (1–5). In this early work, the molten metal was sheared during cooling from above the liquidus temperature into the semisolid temperature regime and the associated microstructure alteration from dendritic to spheroidal was found to be the cause of the reduced viscosity. In some cases, it was noted as being orders of magnitude less viscous after this alteration. It was concluded that the spheroidal microstructure was formed due to the shearing action leading to the release of the secondary dendrite arms from the dendrite arm roots and thence to the structure fragmentation and spheroidization (5). Later, the process of obtaining these structures and thence forming a component shape was termed SSM forming. To date, much research work has been carried out in order to improve the methods of obtaining the required spherical microstructure. Figure 1 shows schematically how the grain size and shape vary irreversibly with time during mechanical shearing in the semisolid state.
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SSM processing occurs between the liquidus and solidus temperature, a range within which the fluidity of molten metal can change greatly. Instead of a dendrite microstructure from the conventional liquid casting, a globular or spheroidal microstructure can be achieved by controlling process parameters, such as temperature of the melt, cooling rate, stirring time, stirring type, stirring speed, etc. (6,7). Both dendritic microstructure and spheroidal microstructures are shown in Figure 2.

SSM casting has been developed as a niche casting process where high mechanical properties or complex shape, or both, are required. It is an advanced technology that offers the ability to produce various components to be used in different industries, mostly within the automotive and aerospace industries. This process has many advantages, including low processing temperature, making it energy efficient, which contributes to part cost saving (7–9). Due to increased fluidity during forming, provided from the spheroidal microstructure, parts formed have lower porosity levels and associated improved mechanical properties (10). Other benefits include die life extension, less filling defects, and faster solidification. From a productivity viewpoint, SSM processing can provide the same or a greater production rate compared to conventional high-pressure die-casting processes. SSM forming technology consists of three main stages (11):
1.
Feedstock preparation: feedstock billets with a fine-grained spheroidal microstructure are developed using an appropriate method and technique.
2.
Reheating or holding within the semisolid condition: the feedstock is reheated or held in the semisolid condition with a heating regime to ensure homogeneous temperature throughout the billet using a heating mechanism such as induction heating.
3.
Forming operations: the final product is produced generally with a casting or forging method of component shaping, termed, respectively, thixocasting or thixoforging.
This chapter presents several techniques that have been used either within the laboratory or within industry in recent years for producing feedstock for SSM processing. All these techniques have the particular aim of producing the spherical microstructure required for SSM processing. The methods of producing this feedstock are presented in the three following sections describing liquid metal routes, solid-state routes, and routes that utilize a combination of methods.
Metal casting consists of two main stages: (1) pouring or forcing liquid metal into a mold cavity with the desired shape and (2) allowing for some additional time for solidification. Although this process sounds simple, producing high-quality castings is a challenging task. This is because of defects that may arise during filling or solidification. There are many reported types of defects in castings (Campbell, 2015). However, most of the defect types share some common characteristics and they will be here categorized according to their mechanism of formation. The discussion will be limited to entrainment, shrinkage defects, and gas porosity.
The two most common types of entrainment defects are oxide bifilms and bubbles while the main driving force for their formation is surface turbulence. Oxide bifilms are formed as the metal front surface folds or two melt fronts impinge (Mirak, Divandari, Boutorabi, & Campbell, 2007). These bifilms might float on the top surface of the melt or are often submerged into the bulk. In the latter case, they may trigger the initiation of cracks and shrinkage cavities during solidification. Gas bubbles are formed as air gets entrapped within the interstices of bifilms. Bubble entrapment might also deteriorate the quality of castings. Bubbles with a diameter smaller than 5 mm are considered to be responsible for microporosity formation. However, larger bubbles are occasionally buoyant enough to levitate themselves up to the top surface of the melt (sometimes even by passing through the dendrite structure). This counter gravity motion is accompanied by traces looking similar to long oxide bifilms and are addressed as “bubble trails.” When the number of bubbles entrapped and consequently the density of trails is large, the end result is a complex network of bifilm trails and bubbles, which is called “bubble damage.” Bubble damage has been identified as the most common defect in castings (Campbell, 2015).
Shrinkage defects can be categorized into two main types, namely, (1) open shrinkage defects and (2) closed shrinkage defects as illustrated in Fig. 7.5. Open shrinkage defects are located at the casting surface being in contact with the atmospheric air and occur during liquid metal contraction or solidification. Open shrinkage defects have the form of caved surfaces and pipes. On the contrary, closed shrinkage defects occur in the bulk of the casting and can be categorized into macro- and microshrinkage. Macroshrinkage refers to pores generated due to insufficient feeding during solidification and is visible during unaided radiographic inspection. The mechanism of microshrinkage formation is similar to the one of macroshrinkage. Micropores are formed in the bulk of the casting due to the blockage of liquid metal flow owing to bubble inclusions or solid metal in the interdendritic regions. Micropores are not visible without magnification during inspection.

Gas porosity is different from shrinkage defects and is formed when floating gas inclusions (air or hydrogen) are entrapped in the dendritic network. These pores are more common in aluminum castings due to the lower solubility of hydrogen in aluminum. During solidification, hydrogen dissolved in aluminum is expelled and gas bubbles are formed. Unlike microshrinkage pores which have a random shape, gas porosity is characterized by its spherical shape (Kenney et al., 1988).