Aluminum Alloy Permanent Mold Gravity Casting: Temperature & Velocity Control (Mainly for A356/A357)

Core principles: Match temperature to wall thickness; aim for smooth laminar flow in speed control; avoid air entrapment, oxidation, cold shuts, and shrinkage cavities. Temperature involves two systems: molten aluminum pouring temperature and mold temperature. Speed refers to the ingate filling linear velocity / total pouring time.

I. Temperature Control

1. Molten aluminum pouring temperature (measured temperature at tap-out and at the ingate; thermocouple / infrared temperature measurement)

Liquidus: A356 is approximately 610°C. The pouring temperature is generally 70–100°C above the liquidus.

• Thin-wall parts (<4 mm, complex cavities): 730–750°C to ensure fluidity and prevent cold shuts and misruns.
• Normal wall thickness (4–15 mm, housings, brackets): 710–730°C (most commonly used).
• Thick sections (>15 mm, valve bodies, wheel hubs, etc.): 690–710°C. Too high a temperature will aggravate gas absorption, oxidation, coarse grains, microshrinkage, and hot tearing.
• Tolerance: after production is stable, control within ±5°C; if transfer time is long (ladling, transport), increase appropriately by 5–10°C to compensate for temperature drop.

✅ Defects caused by excessively high temperature: gas porosity, oxidation inclusions, shrinkage cavities, coarse grains, mold burnout, hot cracks.

✅ Defects caused by excessively low temperature: cold shuts, misruns, flow lines, inclusions.

Melting furnace temperature: 20–30°C higher than the pouring temperature to allow for temperature drop during ladling and transport. Degassing + refining + grain refinement must be provided as supporting processes; high-temperature molten aluminum readily absorbs hydrogen.

2. Mold (permanent mold) temperature control (very critical; often ignored by many factories)

Initial mold preheating / first shot: 180–250°C. Never pour directly into a cold mold; rapid chilling of the molten aluminum causes cold shuts, and thermal shock can crack the mold.

Steady-state temperature in continuous production: 200–300°C.

• For thin-wall parts, use the upper limit, 250–300°C, to delay cooling of the molten aluminum.
• For thick-wall parts, use the lower limit, 200–240°C, to accelerate surface solidification of the casting and establish directional solidification (feeding from the far end of the casting toward the riser).

Mold temperature gradient: the temperature at the bottom / far end of the casting should be lower, while the temperature in the riser area should be higher, so as to achieve bottom-to-top directional solidification and reduce microshrinkage. Use mold insulating coatings, water cooling / air cooling, and heating rods for zoned temperature control.

Mold coatings: insulating coatings (risers / thin walls) and chill coatings (thick sections / hot spots) are used to locally adjust the cooling rate.

II. Pouring Speed (Filling Speed) Control

In gravity casting, faster is not always better. The goal is laminar filling: no turbulence or rolling, no splashing, molten aluminum rising smoothly, and air escaping smoothly through the vents.

Two expressions: ingate linear velocity (m/s) and total pouring time (s).

1. Recommended ingate linear velocity

High-quality castings (valve bodies, seals, parts requiring T6 heat treatment): ≤0.5 m/s. This is commonly used in tilt-pouring gravity casting; the flow is laminar, the oxide film is hardly broken, and slag inclusions and gas porosity are greatly reduced. It is the golden threshold for high-end gravity-cast aluminum.

Ordinary housings and brackets: 0.5–0.8 m/s; try not to exceed 0.8 m/s for long periods, as this very easily causes air entrapment and oxide film entrainment, forming inclusion defects.

Direct top pouring: the drop height is large, so the flow velocity easily exceeds the limit. Tilt-pouring gravity casting is the optimal speed-control solution, with a small molten aluminum drop and uniform filling.

2. Reference total pouring time (based on casting weight and cavity complexity)

• Small parts <1 kg: 5–10 s
• Medium castings 1–5 kg: 10–25 s
• Thick / complex parts above 5 kg: 20–40 s

Process debugging approach: start slowly, rise at a uniform speed, and slow down when nearly full. Rapid initial entry will cause the molten aluminum to impact the cavity bottom, splash, and break the oxide film.

3. Speed control methods

Gating system design: ingate cross-section, pouring cup, and slag trap/weir; structurally limit the maximum flow velocity (most fundamental).

Equipment: tilting gravity casting machine (servo tilt-angle control, settable angular velocity, stable and repeatable); robotic ladling and pouring is much more consistent than manual ladling.

Manual pouring: fix the ladle height, maintain a stable ladle tilting angle, and keep a constant flow rate; keep the ladle spout always submerged below the molten aluminum surface (bottom-pour feeding to prevent air entrapment).

✅ Too fast: turbulence, splashing, air entrapment, oxidation inclusions, gas porosity.

✅ Too slow: premature solidification of molten aluminum → cold shuts, misruns.

III. Coupled Temperature + Speed Matching (Core of Process Debugging)

Thin-wall complex parts: higher temperature + moderate, slower filling speed, ensuring molten aluminum fluidity while avoiding air entrapment.

Thick, simple parts: lower temperature + low-speed smooth filling, reducing the risk of microshrinkage and hot tearing.

When the mold temperature is high, the pouring temperature can be appropriately lowered; when the mold temperature is low, the molten aluminum temperature should be raised to compensate.

Filling simulation (AnyCasting / ProCAST): predict flow velocity, air entrapment locations, and temperature field in advance to reduce trial cost.

IV. On-Site Monitoring and Stabilization Plan

Temperature measurement: furnace thermocouple + infrared temperature measurement at the gate; record the molten aluminum temperature for every shot. Use multi-point thermocouples in the mold to monitor mold temperature distribution.

Speed: the tilting machine records tilting time / angle; manual pouring uses fixed weight + timing.

Trial debugging sequence:

① First establish/fix the mold temperature gradient.
② Select the pouring temperature range.
③ Test pouring speed from slow to fast, and find the window with no air entrapment and no cold shuts.

Environment: when humidity is high, hydrogen absorption by molten aluminum intensifies; lower the upper limit of the pouring temperature.

V. Parameter Adjustments for Common Defects

Gas porosity / inclusions: reduce filling flow velocity, reduce molten aluminum drop height, lower pouring temperature, and strengthen degassing.

Cold shuts / misruns: increase molten aluminum temperature, increase mold temperature, and appropriately shorten pouring time.

Microshrinkage / shrinkage cavities: lower pouring temperature, optimize mold temperature gradient, and enhance riser insulation.

Hot cracks: lower pouring temperature, lower mold temperature, and reduce casting shrinkage stress.