Aluminium casting notes, for buyers and engineers

HPDC: fast, but it traps gas

HPDC injects liquid aluminium into the die at high pressure and high speed, often filling the cavity in milliseconds. It suits thin walls of 1–3 mm, complex shapes and high annual volumes. Dimensional tolerance typically reaches CT4–CT6, surface finish is good and machining stock can be small. Tooling is expensive, but high volume spreads it.

The price is gas entrapment. Turbulent filling can fold cavity air into the metal, creating porosity. Ordinary die castings are therefore generally not T6 heat treated: gas expands during solution treatment and blisters the surface. Welding is also difficult. The thicker the section, the harder it is to feed the last metal to solidify, and the greater the risk of porosity and shrinkage.

Gravity casting: slower, but denser

Gravity casting fills the mould under the metal's own weight and can use sand cores for complex internal passages. Filling is calm, gas entrapment is low, and thick sections can be fed through risers. Internal soundness and ductility are usually better than ordinary HPDC, and the parts can be T6 heat treated and welded. That makes gravity casting suitable for cylinder heads, intake manifolds and bearing caps — parts that must seal, carry pressure or take load.

Dimensional tolerance is usually CT6–CT8, the surface is rougher and machining allowance must be sufficient. Cycle time is measured in minutes, which suits small-to-medium volumes and high-mix work. Consistency depends heavily on the foundry: gating, risering and chills decide whether shrinkage stays under control.

A short checklist

1. Thinnest wall under 3 mm, high volume, dimensions and appearance dominate: evaluate HPDC first.
2. Wall over 8 mm, T6, welding or tight leak requirements: prefer gravity casting, or evaluate low-pressure and squeeze casting.
3. Complex oil or water galleries: sand-core gravity casting is the direct route.
4. Volume too low to amortise a die: gravity casting is often cheaper overall.
5. When in doubt, ask the supplier to run filling and solidification simulation before comparing tooling and piece price.

How Guoyang runs it

Guoyang keeps both routes in house: 25 gravity casting machines and 10 high-pressure die casting machines (DC280T–DC1200T). Customers are not asked to fit one line; the process follows wall thickness, volume, heat treatment and leak requirements. Cylinder heads and intake manifolds that need T6 and pressure tightness run on the gravity line; thin-wall, high-volume parts such as oil pans, gear chambers and brackets run on HPDC.

Three steps: solution, quench, age

1. Solution treatment: heat below the eutectic temperature (about 540°C for A356) and hold for several hours so Mg and Si dissolve into the α-Al matrix.
2. Quench: cool rapidly, usually in water, to freeze the high-temperature state and create a supersaturated solid solution.
3. Ageing: hold at 150–180°C for hours to tens of hours so fine Mg₂Si precipitates form. Temperature and time change strength and ductility noticeably.

Published work on A356 commonly uses solution treatment around 540°C followed by ageing at 155–180°C. Holding time depends on wall thickness, furnace load and the mechanical properties the customer requires; one number cannot be copied to every part. Under-solution leaves strengthening elements undissolved; over-solution grows the grains and can hurt ductility and surface quality.

Why ordinary die castings often cannot be treated

HPDC parts contain entrapped gas. At solution temperature, above roughly 500°C, that gas expands: the surface blisters, the part distorts and internal cracks can form. Ordinary HPDC parts are therefore generally not T6 treated. Structural parts that must be heat treated usually move to gravity, low-pressure, squeeze or vacuum die casting so the gas content is reduced first.

ADC12, an Al-Si-Cu die casting alloy, is not designed for T6 either: its strengthening mechanism, iron content and porosity level differ from A356. Applying an A356 heat treatment recipe to ADC12 rarely gives the result the drawing asks for.

What to watch after T6

• Distortion and straightening: uneven quenching distorts thin-wall parts; leave straightening allowance and use dedicated fixtures.
• Machining sequence: normally heat treat before finish machining so distortion does not destroy the final dimensions.
• Hardness is not the whole story: hardness can be checked batch by batch, but it does not replace tensile strength and elongation. Critical parts need coupon or cast-on test bars as agreed with the customer.
• Cost: another process step brings energy, distortion control and lead-time management. Not every part earns its T6.

How Guoyang runs it

Guoyang's T6 line keeps solution, quench and ageing in one flow, follows the mechanical properties specified by the customer and checks hardness batch by batch. Treated parts then move to precision machining, where critical faces are finished in one clamping wherever the geometry allows. For cylinder heads and intake manifolds that need T6, heat treatment is part of the in-house chain, not an outsourced step.

Note: the A356 chemistry above follows the AA/ASTM standard. Chinese ZL104 carries a slightly higher silicon content; the two are in the same Al-Si-Mg family but are not identical. The drawing and the current standard govern the actual grade.

ADC12: easy to cast, easy to machine, lower cost

High silicon gives fluidity and low shrinkage, copper raises as-cast strength, and iron improves die release and machinability. It suits complex, thin-wall, high-volume die castings. The trade-off is lower ductility and toughness; ordinary die castings contain gas, are generally not T6 treated and are difficult to weld.

A356: heat-treatable, tougher

Lower silicon and a controlled magnesium addition allow T6 precipitation strengthening. Low iron content gives better ductility and fatigue performance, and corrosion resistance is better than ADC12. Fluidity is lower and thin-wall filling is harder, so A356 is mostly used in gravity, low-pressure or sand casting — cylinder heads, intake manifolds and bearing caps that must seal, carry pressure or show some elongation.

Four questions before choosing

1. Is T6 required? If yes, start with a heat-treatable alloy such as A356.
2. Is the wall under 3 mm and the volume high? If yes, a die casting alloy such as ADC12 has the cost advantage.
3. Are welding, leak tightness or long fatigue life required? If yes, evaluate A356 or a lower-gas process first.
4. Does the customer drawing specify an alloy? Then follow the drawing and its standard. Do not substitute on the grounds that “the properties are close enough”.

Guoyang's two routes

Guoyang's gravity line mainly runs ZL104 / ZL114A parts that need T6; the HPDC line runs thin-wall, high-volume ADC12 parts. Alloy and process are decided together, rather than choosing an alloy first and forcing a process to fit. For a buyer, the wrong alloy is difficult to recover later: no amount of parameter tuning will satisfy cost, leak tightness and mechanical properties at the same time.

Gas porosity: gas that did not escape

• Precipitation porosity (hydrogen): molten aluminium absorbs hydrogen during melting, transfer and pouring. Hydrogen solubility drops sharply on solidification, and gas that cannot escape remains in the casting. Wet charge, damp tools, damp refining agents and high humidity all raise the risk.
• Entrapment porosity: turbulent filling folds cavity air into the metal. Common in high-speed HPDC and poorly designed gating.
• Reaction porosity: the metal reacts with sand, coating or die lubricant and generates gas.

Gas pores usually have smooth, rounded walls and clear boundaries on an X-ray. They can appear alone or mixed with shrinkage; one radiograph is sometimes not enough to tell which dominates.

Shrinkage: the last liquid was not fed

Metal contracts as it solidifies. If a hot spot — the last thick section to freeze — cannot be fed from a riser or gate, dispersed shrinkage forms between dendrites. If the feeding path is blocked, a concentrated shrinkage cavity appears.

Shrinkage is irregular and rough-edged. It tends to sit at thick-to-thin transitions, under bosses and in hot-spot centres. It may never reach the surface, but it reduces leak tightness and fatigue life. By the time machining opens it on a sealing face, it is too late.

How to find it

• Visual and after machining: only finds defects that reach the surface.
• X-ray: sample inspection to GB/T 11346 or a customer-specified standard such as ASTM E505; shows the position and size of internal gas and shrinkage cavities.
• Leak / pressure test: for water jackets, oil galleries and intake chambers, as specified by the customer.
• Metallography and sectioning: confirms defect type, dendrite arm spacing and modification.
• CMM and dimensions: sometimes the problem is not a hole at all, but distortion or insufficient stock.

Prevention beats sorting

• Melting: dry charge, preheat tools, rotary argon/nitrogen degassing, hydrogen or density-equivalent checks — do not let gas enter the mould.
• Filling: optimise slow/fast shot profiles, overflows and vents in HPDC; control pouring temperature and fill speed in gravity casting to avoid turbulence.
• Solidification: place risers, chills and gates so thick sections are fed first; use CAE to locate hot spots.
• Tooling: keep vents and overflows clear, avoid excess coating, prevent entrapment and reaction gas.
• Process: first-piece approval and batch sampling; record defect type and position and trace it back to the heat and the mould.

How Guoyang runs it

Incoming ingots are checked by grade and chemistry is spectrographed batch by batch. Every heat is degassed and hydrogen-checked. First pieces are confirmed on a CMM, X-ray sampling looks for internal defects, and sealing faces are leak tested as the customer requires. When a defect appears, the first question is whether it is gas or shrinkage; the fix then goes back to melting, filling or feeding — not to removing more material in machining.

What the standard says

GB/T 6414—2017 (Castings — dimensional tolerances, geometric tolerances and machining allowances) grades machining allowance from A to K (RMA), looked up by the largest finished contour dimension. Typical aluminium grades are D–F for permanent mould (gravity/low pressure), E–G for machine-moulded sand and F–H for hand-moulded sand. HPDC parts are more precise and can use less.

Note that the standard gives a single-side allowance; circular or double-sided machined faces double it. The drawing also has to consider datum selection — you cannot simply apply one number based on the largest dimension.

Practical ranges

• Small HPDC parts: about 0.3–0.5 mm per side; medium parts about 0.5–1.0 mm.
• Permanent-mould gravity castings: commonly 1–2 mm per side; more for large or distortion-sensitive parts.
• Sand castings: commonly 2–5 mm per side, with the top face one grade larger than the bottom and sides.

These are practical ranges, not a substitute for the drawing, the process and the supplier's proven capability. A finer grade is not automatically better: it demands more from the tooling, cores and process control.

When the allowance is too small

• Scale, sand burn-on, cold shuts or depressions remain as black skin after machining.
• Wall variation, mismatch or heat-treatment distortion consumes local stock and the dimension goes out.
• The defective layer is not removed; porosity or shrinkage opens on a sealing face and the leak test fails.

When the allowance is too large

• More machining time and tool wear raise the piece cost.
• Cutting through the sound skin can expose internal porosity.
• Thin walls lose stiffness; clamping and cutting distortion risk rises.
• More chips and lower material utilisation.

How to set it more accurately

1. Fix the datum first: use a large face or a hole pattern as the common datum to reduce re-clamping.
2. Allow for distortion: T6 parts, thin walls and long parts need straightening allowance.
3. Measure the first piece: CMM the raw casting to see the actual stock distribution, then correct the tool or the machining offset.
4. Agree it with the supplier: when casting and machining sit under one roof, the allowance can be tightened to the real process capability; when they are separate, the drawing must say so explicitly.

How Guoyang runs it

Guoyang keeps casting and precision machining on the same site, so raw stock, heat-treatment distortion and machining datums can be validated together at the sample stage. With 96 vertical drilling and milling centres, critical faces are finished in one clamping wherever possible, reducing position error from re-clamping.

Where aluminium has already won

• Engine: cylinder heads, intake manifolds, valve covers, gear chambers, oil pans, bearing caps, EGR valve seats.
• Transmission: clutch housings, gearbox housings, rear covers.
• Structure: brackets, supports, pedals, battery-pack housings (electric vehicles).

These parts share a pattern: complex shape, uneven wall thickness and stable volume, which suit one-piece casting; at the same time they need sealing, dimensions or NVH performance, which brings in precision machining.

Where the boundary sits

• Temperature: ordinary Al-Si alloys lose strength rapidly above about 200°C. Hot engine components still need cast iron, steel or heat-resistant aluminium. The industry is developing Al-Cu-Mn-Zr alloys for medium- and heavy-duty truck blocks and heads; an ORNL–GM collaboration reports around 15% weight reduction and more than 10% fuel-efficiency improvement at research stage — not a production promise.
• Stiffness and wear: aluminium has a low elastic modulus. Sliding surfaces usually need a cast-in liner, bearing seat or surface treatment.
• Cost: ingot price, casting yield and tooling investment decide whether replacing iron pays back. Not every kilogram removed earns its cost.
• Recycling: aluminium has high scrap value, but mixed grades complicate chemistry control in recycled metal; closed-loop recycling and sorting matter more every year.

Where the process is heading

• Vacuum and structural die casting: less entrapment, allowing welded and heat-treated castings to enter body and chassis structures.
• Gravity/low-pressure casting plus T6: still the route for cylinder heads and intake manifolds that need soundness and ductility.
• CAE and process simulation: predict entrapment, shrinkage and distortion before the tool is cut, reducing trial cycles.
• Recycled aluminium and low carbon: OEMs are asking for material carbon footprint and recycled content, and foundries need traceable charge.

Where Guoyang sits

Guoyang does not make “aluminium castings” in general; it goes deep on aluminium parts for commercial-vehicle diesel engines. Gravity and HPDC lines, T6 heat treatment, precision machining and inspection sit in one closed loop, covering cylinder heads, intake manifolds, EGR parts, valve covers, gear chambers, oil pans and brackets. For the customer, that means one quality system from raw casting to finished part, instead of a chain of outside processors.