Nonexpendable (Permanent) Mold Gravity Feed Casting

3.5.1 Permanent Mold Casting

In principle, permanent mold casting is analogous to expendable mold casting processes. In this case the molds are machined cast, wrought or nodular iron, cast steel, or wrought steel and can be reused repetitively until damage, wear, or the effects of thermal fatigue necessitate repair or replacement (Fig. 3.8). The ability to form internal passages involves metallic or sand cores. Intricate details and undercuts can often be cast using segmented steel cores.

Sand cores become necessary when the design prohibits drawing the core after the casting has solidified. When sand cores are used, the process is referred to as “semipermanent mold” casting.

Permanent mold tooling is typically more expensive than that required for sand casting and other expendable mold processes and is justified by the volume of production. The volume of production also dictates the extent of process automation. Molds can be manu-ally operated or extensively automated. Production rates of auto-mated multimold operations are high, and parts display consistent dimensional characteristics and properties.

While the principles and mechanics of gravity casting are similar, the metallurgical structure of permanent mold castings reflects the refinement of higher solidification rates. Typical and specified mini-mum mechanical properties including ductility are higher than those of expendable mold castings. The improved mechanical prop-erties of permanent mold castings provide part of the justification for selecting this process over competing gravity casting options.

The same terminologies used in expendable mold gating apply.

There is a downsprue into which molten metal is introduced from a pouring basin, from which the metal flows into runners, risers, in-feeds, and casting cavity. Directional solidification is promoted by selective chilling of mold sectors by air, mist, or water. An insulating coating is used to protect the mold from the molten aluminum and to facilitate removal of the casting from the mold after solidification is complete. Typical mold dressings or washes are suspensions of talc, various metal oxides such as zirconia, Fig. 3.6 Alloy 224.0 impellers produced by low-pressure plaster casting

Fig. 3.7 Vacuum molding unit. Source: Ref 1

chromia, iron oxide and titania, colloidal graphite and calcium carbonate in water, and sodium silicate. The thickness and thermal characteristics of these coatings are used to locally increase or decrease heat absorption during solidification. As a result of wear, these coatings must be periodically repaired or replaced to ensure consistent process performance and casting results. Mold surfaces are periodically blasted with dry ice or mild abrasives to remove coatings and scale after which new mold coatings are applied.

The permanent mold process is less alloy tolerant than most expendable mold processes. The most popular permanent mold alloys display superior castability such as those of the Si, Al-Si-Mg, and Al-Si-Cu (Mg) families. Mold rigidity is a challenge in the casting of hot-short alloys in which liquidus-solidus range and elevated-temperature strength combine to increase the ten-dency for cracking during and after solidification. Determined ef-forts to cast even the most difficult foundry alloys such as low-iron aluminum-copper alloys have nevertheless been successful, and alloys with limited castability are routinely cast in permanent molds.

Permanent mold castings can be produced in sizes ranging from less than a pound to more than several hundred pounds. Surface finish typically varies 150 to 400 μin. (3.8 to 10 μm). Basic linear tolerances of about⫾0.01 in./in. (⫾10 mm/m), and minimum wall thicknesses are about 0.100 in. (2.5 mm).

3.5.2 Low-Pressure Die Casting (LP), Pressure Riserless Casting (PRC)

In this process, permanent molds are mounted over a sealed furnace. A tube extends from the mold cavity into the molten metal below. By pressurizing the furnace, metal is forced through the tube into the mold cavity (Fig. 3.9). When the metal has solidified, the pressure is relieved, the mold is opened, and the casting is removed in preparation for repeating the cycle.

Most low-pressure casting has been confined to radially sym-metrical designs, but a wide range of nonsymsym-metrical parts have also been produced. Nearly all automotive wheels are cast by this process (Fig. 3.10, 3.11).

Process parameters include (a) the rate at which pressure is applied, which regulates mold filling, (b) pressure, which is rela-tively unimportant once solidification begins, and (c) thermal gra-dients, which are essential for establishing directional solidifica-tion. As in conventional permanent mold, these gradients are established by the selection and controlled thicknesses of mold coatings and by selective chilling of mold sections. Since most low-pressure castings are produced using only one metal entry point and since risers normally necessary to avoid internal shrink-age voids are not typically used, the gross-to-net weight ratio is low and trimming and finishing operations associated with gating are minimized.

The low-pressure casting cycle is dictated by the solidification of metal at the junction of the fill-tube and mold cavity.

While countergravity metal flow into the mold cavity is quies-cent, the process does present the risk of inclusion contamination.

When the mold is opened and the casting is removed, the vacuum seal that existed at the liquid-solid interface is broken and molten metal remaining in the tube falls to the furnace metal level. The cycling of metal flow vertically in the fill tube can result in the buildup of oxides on the inner surfaces of the fill tube whether the

Fig. 3.8 Permanent mold machined from steel. Source: Ref 1 Fig. 3.9 Low-pressure permanent mold. Source: Ref 1

Chapter 3: Aluminum Casting Processes / 27

tube is ceramic or coated metal. To minimize this condition, back pressure can be maintained in the system so that molten metal is retained at an elevated level in the fill tube at all times. It is also possible to replenish metal in the furnace with each cycle by valv-ing rather than periodically refillvalv-ing when the metal in the furnace is nearly depleted. Filtration of metal at the point of entry into the mold is routinely used to prevent included matter from

contami-nating the casting. Filtration may consist of steel screens, ceramic strainers, or fused or foamed porous ceramics.

3.5.3 Vacuum Riserless Casting (VRC)

The use of vacuum rather than pressure to introduce molten metal into steel dies has significant advantages over low-pressure casting.

The molten metal bath is open and accessible. Molten metal level can be maintained within a narrow range in close proximity to the mold entry point so that the vertical dimension between the metal surface and the mold cavity can be minimized.

Limitations are in the size and cost of molds that can be engi-neered to apply and retain vacuum pressures.

A high degree of mold chilling has been used to enhance met-allurgical structures, improve mechanical properties, and shorten cycle times. The VRC process is ideally suited for automation and high production rates to produce castings with exceptional surface quality and metallurgical properties. Examples of VRC products are shown in Fig. 3.12.

3.5.4 Centrifugal Casting

Centrifugal force in aluminum casting involves rotating a mold or a number of molds filled with molten metal about an axis.

Cylindrical or tubular shapes may be centrifugally formed in ver-tically or horizontally rotated drums, while conventional castings are produced by the rotation of one or more molds about a vertical axis. Metal may be introduced before or during rotation.

Baked sand, plaster, or graphite molds have been used, but iron and steel dies are most common. Centrifugal castings are generally, but not always, denser than conventionally poured castings and offer the advantage of greater detail.

Wheels, wheel hubs, motor rotors, and papermaking and printing rolls are examples of aluminum parts produced by centrifugal cast-ing. Aluminum alloys suitable for permanent mold, sand, or plaster casting can be cast centrifugally.

Fig. 3.10 Alloy A356.0 alloy automotive wheels produced by low-pressure casting

Fig. 3.11 Variety of parts, including automotive pistons, metallurgically bonded diesel engine pistons, compressor pistons, cylindrical and journal bearings, anodes, and cookware, produced by the low-pressure casting process

Fig. 3.12 Examples of castings produced by the vacuum riserless casting (VRC) process include rocker arms, compressor pistons, connect-ing rods, trowel handles, valve components, and other parts

3.5.5 Squeeze Casting

Although a number of process developments have been referred to as squeeze casting, the process by which molten metal solidifies under pressure within closed dies positioned between the plates of a hydraulic press is the only version of current commercial interest.

The applied pressure and retained contact of the metal with the die surface improves heat transfer and inhibits hydrogen precipitation and shrinkage void formation. The result is a denser, fine-grained casting with excellent mechanical properties.

Squeeze casting has been successfully used for a variety of ferrous and nonferrous alloys in traditionally cast and wrought compositions. Applications of squeeze-cast aluminum alloys in-clude reciprocating engine pistons, brake rotors, automotive and truck wheels, and structural automotive frame components (Fig.

3.13). Squeeze casting is simple and economical, is efficient in its use of raw material, and has excellent potential for automated operation at high rates of production.

3.5.6 Semisolid Forming

Semisolid forming incorporates elements of casting, forging, and extrusion. It involves the near-net-shape forming of metal parts from a semisolid raw material that incorporates a uniquely non-dendritic microstructure.

Mechanical or electromagnetic force is employed during billet solidification to fragment the solidifying structure. The result is a spherulitic structure that behaves thixotropically in the liquidus-solidus range. The billet retains its shape at closely controlled temperatures above the melting point at which the shear strength is low, even at relatively high percent fraction solid. When the billet has been reheated, it is forced into dies under pressure to form a casting that retains the characteristics of the starting billet micro-structure. Just as important, the mold cavity is filled without the turbulence associated with gravity pouring or the injection of mol-ten metal, and internal porosity formation is minimized by reducing the volume of liquid metal that solidifies from the semisolid con-dition (Fig. 3.14).

A number of alternative approaches to the production of the semisolid raw material have been or are being developed. A process in which particle ingots are continuously fed and mechanically stirred to provide the required semisolid state and microstructure has been developed and used in magnesium alloy casting produc-tion. The incompatibility of materials of containment with suffi-cient strength for this process in molten aluminum remains to be overcome. Attempts to eliminate expensive thin-cast billet through slurry approaches in mold filling have also been undertaken.

Semisolid forming is more costly than conventional casting, but offers unique properties and consistently excellent quality. In ad-dition, the viscous nature of semisolid alloys provides a natural environment for the incorporation of third-phase particles in the preparation of reinforced metal-matrix composites.

Specialized billets are commercially available, and semisolid-formed applications are broadening in the aerospace, automotive, military, and industrial sectors. The process represents an alterna-tive to conventional forgings, permanent mold, investment and die castings, impact extrusions, machined extrusion profiles, and screw machine products. Applications include automotive wheels, master brake cylinders, antilock brake valves, disk brake calipers, power steering pump housings, power steering pinion valve housings, compressor housings, steering column parts, airbag containment housings, power brake proportioning valves, electrical connectors, and various covers and housings that require pressure tightness.