| (2.4) |
where
| N | = | rotation speed, rev/min. |
G-factor is the ratio of centrifugal force divided by the gravitation force:
| (2.5) |
where
| GF | = | gravitation factor. |
Solving further, we get
| (2.6) |
where
| D | = | inside diameter of the mold, m (ft). |
If the G-factor is too low in centrifugal casting, the liquid metal will not remain forced against the mold wall during the upper half of the circular path, but will drop inside the cavity. Too high a speed results in excessive stresses and hot tears in the outside surface—on an empirical basis of GF = 50 to 100 for the metal mold and GF = 25 to 50 for a sand cast mold.
True centrifugal casting is characterized by better mechanical properties of the cast than is true in conventional static casting: nonmetallic impurities that segregate toward the bore can be machined off; the casting is relatively free from defects; there is less loss of metal compared to that in conventional sand casting; production rate is high; there are no parting lines; the scrap rate is law; and the process can be used to manufacture bimetallic tubes.
b) Semicentrifugal Casting
During semicentrifugal casting (Fig. 2.17), the mold is rotated around its axis of symmetry.
Fig. 2.17 Semicentrifugal casting
The molds used can be permanent or expendable and may contain cores. The detailed shape is given by the shape cavity of the rotating mold.
The centrifugal force is utilized for slag separation, refilling of melt metal, and increase of the filling power in order to cast parts with thin walls. In general, the rotational speed is lower than that used in true centrifugal casting and is usually set so that G-factors of around 15 are obtained. The mold is designed with risers in the center to supply the feed metal. The central zone of the parts (near the axis of rotation) has inclusion defects and thus is suitable only for parts where these can be machined away. Cogwheels are an example of parts that can be cast using this method.
c) Centrifuging Casting
The principle of centrifuging casting is illustrated in Fig. 2.18. The mold is designed with parts cavities that are symmetrically grouped in a ring located away from the axis of rotation. From a central inlet the molten metal is forced outwards into the mold cavity by centrifugal force. The method is extensively used for casting smaller parts. It is also used in the dental industry to cast gold crowns for teeth.
Fig. 2.18 Centrifuging casting: a) mold for eight castings, b) the casting.
Squeeze casting, also known as liquid metal forging, is a die casting method that is a combination of casting and a forging process. It is based on slower continuous die filling and high metal pressures.
This interesting process aims to improve product quality by solidifying the casting under a metallostatic pressure head sufficient to:
1.prevent the formation of shrinkage defects, and
2.retain dissolved gases in solution until freezing is complete.
This method was originally developed in Russia in the 1960s and has undergone considerable improvement in the U.S.
The sequence of operations in squeeze casting is shown in Fig. 2.19. Liquid metal is poured into an open die, just as in a closed die forging process. The dies are then closed and pressure is applied. The amount of pressure thus applied is significantly less than that used in forging, and parts of great detail can be produced. Coring can be used with this process to form holes and recesses. The porosity is low and the mechanical properties are improved.
During the final stages of closure, the liquid is displaced into the further parts of the die. No great fluidity requirements are demanded of the liquid, since the displacements are small. Both ferrous and nonferrous materials can be utilized with this method. This process also can cast forging alloys, which generally have poor fluidities that normally preclude one taking the casting route. The method produces heat-treatable components that can also be used in safety-relevant applications and are characterized by higher strength and ductility than conventional die castings. For example, in comparison with nonrein-forced aluminum alloy, aluminum alloy matrix composites manufactured by squeeze casting techniques can double fatigue strength at 300°C. Hence, such reinforcements are commonly used at the edges of the piston head of a diesel engine where requirements are particularly high. Because squeeze casting is relatively new, much work needs to be done to better understand the fundamentals of the process. By improving the process, the casting industry will discover new options for producing the complex, lightweight aluminum parts that are increasingly demanded in the automotive industry.
Fig. 2.19 Sequence of operations in the squeeze casting process: a) pouring molten metal into die, b) closing die and applying pressure, c) ejecting squeeze casting.
2.5 MELTING PRACTICE AND FURNACES
Melting the metal and handling liquid metal are two of the most critical components in the overall metal casting operation. The manner in which the metal is melted, the way the metal is transferred into the casting cavity, and the whole liquid metal-handling process have a significant impact on productivity, on the cost of operations, and certainly on the quality of the resultant cast components.
Processing the metal in its molten start is the activity wherein the most gains can be achieved. Molten metal processing is an opportunity for refining and for quality enhancement. For example, processes such as alloying, degassing, filtration, fluxing, and grain refinement and modification in aluminum are usually carried out in the liquid metal prior to casting. The mass transfer rates and the kinetics are such that these reactions are carried out much more effectively in the melt.
Today in the foundry industry there is extensive awareness and recognition of the importance of molten metal processing. It is well understood that the highest quality can be obtained by working metals in the molten state. The level of hydrogen can be reduced and liquid and solid inclusions can be removed and controlled, thus improving the quality of the overall casting. The practice of degassing the metal prior to casting is carried out as a normal step throughout the industry. Filtration and removal of inclusions is another molten metal processing technology that has evolved over the past decade or so, and today most foundries