Metal casting and solidification

The KG-40 housing will be sand cast in gray iron by a foundry in southern Illinois that has supplied Kestrel Drive Systems for years. Before signing off the pattern, Jonah and Amira visit to review the gating, the risers and the cooling simulation. Casting is the oldest way of shaping metal and still one of the most economical for complex shapes: engine blocks, pump bodies, valve housings, turbine blades and machine bases are all cast. In one step it can produce hollow interiors, varying wall thicknesses and integral bosses that would take hours to machine. This lesson covers the main casting processes, what happens when metal freezes, and how foundries design molds that produce sound castings.

The basic steps of casting

Every casting process follows the same outline. A mold is made with a cavity in the shape of the part, slightly larger to allow for shrinkage. Metal is melted, its chemistry adjusted, and it is poured or injected into the cavity through a gating system. The metal solidifies, the casting is removed, and the gates and risers are cut off. The casting is then cleaned, inspected, often heat treated, and machined on its functional surfaces.

A sand mold is made in two halves held in a flask: the top half is the cope and the bottom half is the drag, meeting at the parting line. A pattern, a full-size model of the part made of wood, plastic or metal, is packed in sand to form the cavity and then withdrawn. Internal passages, such as the oil gallery and the main cavity of the KG-40 housing, are formed by cores: shapes of bonded sand placed in the mold that the metal flows around. After the casting cools, the core sand is shaken out, leaving the hollow.

The gating system controls how metal enters. The pouring basin catches the stream; the sprue is a vertical tapered channel that accelerates it; the runner carries it horizontally; and ingates lead it into the cavity. Good gating fills the cavity quickly enough that no section freezes early, but gently enough to avoid turbulence, which folds oxide films and sand into the metal. Filters made of ceramic foam often sit in the runner to trap oxides. Risers are reservoirs of liquid metal attached to the casting; they feed metal into it as it shrinks during solidification, and are cut off afterward.

Expendable-mold processes

Green sand molding uses silica sand bonded with clay and water. It is cheap, reusable after reconditioning and suitable for parts from a few grams to many metric tons. Most iron castings, including the KG-40 housing, are green sand castings. Chemically bonded sands, set with resin binders, give stronger, more accurate molds for larger or more precise parts. Shell molding coats a heated metal pattern with resin-coated sand to form a thin, rigid shell, giving better accuracy and finish at higher tooling cost.

Investment casting, also called the lost-wax process, starts with a wax pattern injected into a metal die. Patterns are assembled onto a wax tree, dipped repeatedly in ceramic slurry and coated with stucco to build a shell. The wax is melted out, the shell is fired, and metal is poured into the hot shell. Investment casting gives excellent detail, thin walls and good tolerances, and it handles alloys too hard to machine easily. Turbine blades, surgical implants, golf club heads and small precision valve parts are investment cast. Lost-foam casting is similar but uses an expanded polystyrene pattern that vaporizes as the metal fills the unbonded sand around it.

Permanent-mold processes

Permanent molds are made of metal and used thousands of times. Gravity permanent-mold casting pours metal into a preheated steel or iron mold; it gives finer grain and better finish than sand, and suits aluminum pistons and wheels. Pressure die casting injects molten metal, usually aluminum, zinc or magnesium, into a hardened steel die at high pressure, often 30 to 100 MPa, and at high speed. Cycle times are tens of seconds, tolerances are tight and walls can be under 2 mm thick. The trade-offs are very expensive dies, limits on part size, and trapped gas porosity that makes many die castings unsuitable for heat treatment or welding.

Low-pressure die casting pushes metal up from a sealed furnace into the die through a fill tube with a modest gas pressure, giving calm filling and sound castings; aluminum car wheels are made this way. Centrifugal casting spins a mold so that metal is flung outward against it, producing dense pipes, cylinder liners and bushings with impurities pushed toward the bore, where they are machined away. Continuous casting, used at steel mills, solidifies metal as it is drawn from a water-cooled mold into an endless strand that is later rolled.

How metal solidifies

A pure metal freezes at a single temperature. If we plot temperature against time as it cools, the curve falls, levels off at the freezing point while latent heat is released, and then falls again once all the metal is solid. Alloys usually freeze over a range of temperatures, between the liquidus and the solidus that we met on phase diagrams in MEC-250. During that range the casting is a mushy mixture of growing solid crystals and remaining liquid.

Solidification starts at the mold wall, where heat is removed fastest. A thin chill zone of fine, randomly oriented grains forms first. Behind it, grains with favorable orientations grow inward as long columnar crystals, parallel to the direction of heat flow. In the center, where the remaining liquid cools more uniformly, equiaxed grains may nucleate. Most alloys grow as dendrites, tree-like crystals whose arms reach into the liquid; the spacing between dendrite arms is smaller when cooling is faster, and finer spacing gives better strength and ductility. Grain refiners, such as titanium-boron additions in aluminum, promote fine equiaxed grains throughout.

Gray cast iron, the KG-40 housing material, solidifies with carbon forming graphite flakes in an iron matrix. The flakes give gray iron its excellent damping and machinability, and they also expand slightly as they form, which partly offsets the shrinkage of the iron. That is one reason gray iron is so easy to cast soundly. Ductile iron, with graphite as spheres instead of flakes, is stronger and tougher but needs magnesium treatment and more careful feeding.

Solidification time and Chvorinov's rule

How long a section takes to freeze depends on how much heat it holds compared with how fast it can lose heat through its surface. Chvorinov's rule captures this with the ratio of volume to surface area, called the casting modulus.

Here \(t_s\) is the solidification time, \(V/A\) is the modulus and \(C_m\) is a mold constant that depends on the metal, the mold material and the pouring temperature; it is found from test castings. A thick, compact section has a large modulus and freezes last; a thin fin or rib freezes first. This is the key to feeding: a riser must freeze after the section it feeds, so its modulus must be larger, typically by 20 percent or more.

# Solidification times and riser sizing for the KG-40 housing (gray iron, green sand)
Cm = 2.2          # mold constant from test castings, s/mm^2
n = 2

def modulus(V, A):
    return V / A

def t_solid(M):
    return Cm * M ** n

# Housing sections: (volume mm^3, cooling area mm^2); joined faces are left out of the area
sections = {
    "wall 8 mm (per 100 x 100 patch)": (100 * 100 * 8, 2 * 100 * 100),
    "mounting rib 12 x 40": (12 * 40 * 200, 2 * 40 * 200 + 12 * 200),
    "bearing boss 120 x 100 x 60": (120 * 100 * 60, 120 * 100 + 2 * 120 * 60 + 2 * 100 * 60),
}
print("Section                            modulus (mm)   time (s)")
for name, (V, A) in sections.items():
    M = modulus(V, A)
    print(f"{name:34s} {M:10.2f}   {t_solid(M):8.0f}")

# Top riser, cylinder with H = D: modulus = D/5 (side and top cool, base joined)
M_boss = modulus(*sections["bearing boss 120 x 100 x 60"])
factor = 1.2
D = 5 * factor * M_boss
print(f"\nRiser for the boss: D = H = {D:.1f} mm, modulus {D/5:.2f} mm, "
      f"freezes {t_solid(D/5) - t_solid(M_boss):.0f} s after the boss")

# Pattern dimensions with a shrinkage allowance
allowance = {"gray iron": 0.010, "aluminum alloy": 0.013, "steel": 0.020}
part_length = 280.0   # mm, housing length on the drawing
for metal, a in allowance.items():
    print(f"Pattern length for {metal:15s}: {part_length * (1 + a):.1f} mm")

Section                            modulus (mm)   time (s)
wall 8 mm (per 100 x 100 patch)          4.00         35
mounting rib 12 x 40                     5.22         60
bearing boss 120 x 100 x 60             18.75        773

Riser for the boss: D = H = 112.5 mm, modulus 22.50 mm, freezes 340 s after the boss
Pattern length for gray iron      : 282.8 mm
Pattern length for aluminum alloy : 283.6 mm
Pattern length for steel          : 285.6 mm

Shrinkage and the defects it causes

Metals shrink in three stages as they cool from the pouring temperature: liquid contraction as the liquid cools to the liquidus, solidification shrinkage as liquid turns to denser solid, and solid contraction as the solid casting cools to room temperature. Solidification shrinkage is typically 3 to 7 percent by volume for steels and aluminum alloys and much less for gray iron. If liquid cannot reach a freezing region to make up the loss, a shrinkage cavity forms, either a large pipe near the top or scattered microporosity in the last regions to freeze.

Solid contraction does not cause cavities, but it changes dimensions. Patterns are made oversize by a shrinkage allowance, roughly 1 percent for gray iron, 1.3 percent for aluminum alloys and 2 percent for steel, scaled into every dimension. A pattern maker's rule, longer than a normal rule by exactly that allowance, makes the job automatic. Patterns also get a machining allowance on surfaces to be cut and a draft angle, usually 1 to 3 degrees, so the pattern can be drawn from the sand without tearing it.

Feeding works only if solidification is directional: thin regions far from the riser freeze first, and the freezing front advances toward the riser. Foundries arrange this by placing risers on the heaviest sections, by adding chills (metal inserts in the mold that pull heat out fast) to sections that would otherwise freeze late, and by tapering walls so they thicken toward the riser.

Designing parts for casting

Designers can make a foundry's job far easier. Keep wall thicknesses as uniform as possible, because isolated heavy sections become hot spots that need their own risers. Where thickness must change, change it gradually. Use generous fillets at internal corners, both to avoid hot spots where walls meet and to reduce hot tearing. Avoid large flat areas, which distort, and add ribs to stiffen them. Provide draft on surfaces parallel to the pattern's draw direction, and put the parting line where it is easy to make and where flash can be ground off without harm. Specify machining stock only where needed, and locate datum features on surfaces that the foundry can hold consistently.

Today almost every new casting is simulated before the pattern is made. Software models the filling, the temperature field during solidification and the likely porosity. Jonah's review of the KG-40 simulation found a small hot spot where a mounting boss met the main wall. Adding a 6 mm fillet and a chill on the outside of the boss moved the last-to-freeze region into the riser, and the predicted porosity disappeared.

Safety and the environment in foundries

Foundries combine several serious hazards. Molten metal at 1300 to 1600 °C causes severe burns, and a small amount of water trapped under it can turn to steam and eject metal violently, so ladles, tools and molds must be dry and preheated. Workers wear aluminized clothing, face shields and spats. Silica sand produces respirable crystalline silica dust during shakeout and cleaning, which causes silicosis; dust collection, wet methods and respiratory protection are required, and both Canadian and US exposure limits for respirable silica are strict. Noise, heat stress and lifting heavy ladles add further risks. Environmentally, foundries recycle most of their sand and scrap metal, but they must control emissions from melting and binders.