Aluminium Gravity Die Casting: An Engineer's Reference

1. The process, step by step

The term covers any process in which liquid metal fills a metal mould under gravity alone: hand ladle pouring into a static die, machine pouring, and tilt pouring on a rotating machine. It is called permanent mould casting in North America. Low-pressure die casting (metal pushed up a riser tube by gas pressure) and high-pressure die casting (metal injected by a plunger) are different processes with different rules, and this article notes where the differences matter.

1.1 The die

Dies are machined from grey cast iron for short and medium runs and from hot-work tool steel (1.2344, H13) for long runs and thin-walled parts. They open along one or more parting planes and carry ejector pins, loose pieces and retractable metal cores for straight-draw features. Internal passages that cannot be drawn are formed by resin-bonded sand cores set into the die before each pour, in which case the process is sometimes called semi-permanent mould casting, or by salt cores that are dissolved out in water. The need to disassemble the die to release the casting is the main constraint of the process, and it limits both part geometry and the filling and feeding systems. Die life is typically tens of thousands of shots for grey iron and well above that for tool steel, with heat checking (thermal fatigue cracking of the cavity surface) the usual end of life.

1.2 Die coat and die temperature

The cavity is sprayed with a water-based refractory die coat, typically an insulating white coat (mineral filler in a sodium silicate binder) on the cavity and a graphite or thicker insulating coat on runners and risers. The coat protects the die from thermal shock, slows the initial chill so that thin sections fill without cold laps, and gives the impermeable die surface enough "surface permeability" for the melt to flow over it and for volatiles to escape. John Campbell argues that the common practice of a thick rough coat on the runners and a thin smooth coat on the cavity is a mistake, because the metal is in the runner for only a second or two and the roughness adds turbulence; he recommends one thin smooth coat everywhere. Coat thickness and die temperature, normally held at 250 to 400 °C by preheating and by water or air cooling channels, are the two variables that decide the solidification pattern. A die that runs too cold gives misruns and cold laps; one that runs too hot gives coarse structure, draws and long cycle times.

1.3 Melting and melt treatment

Alloy is melted in gas-fired or electric crucible or reverberatory furnaces, normally from certified ingot to EN 1676 plus foundry returns. Melt treatment consists of degassing (usually a rotary impeller flushing nitrogen or argon through the melt), fluxing to lift oxides, and additions of grain refiner (Al-Ti-B rod) and modifier (Al-Sr master alloy or sodium salts). Section 3 explains what each of these actually does. Casting temperature for the Al-Si alloys is typically 700 to 750 °C; higher temperatures are rarely necessary if the running system is good, and they raise hydrogen solubility exponentially.

1.4 Pouring or tilting

In static gravity die casting the operator or a pouring machine fills a basin and the metal falls down the sprue into the cavity, exactly as in sand casting. In tilt casting the die is mounted on a machine that starts horizontal or slightly tilted, the pouring cup is filled while the metal is at rest, and the die is rotated through about 90° so that the metal runs into the cavity as the angle increases. Tilt casting deserves scrutiny before it is specified: the process can transfer metal without surface turbulence, but only when the die starts tilted slightly uphill, the initial rotation is slow so that the metal front never exceeds the critical velocity of about 0.5 m/s, and the rotation speeds up once the runner is primed. Started from horizontal or downhill, the metal runs as a jet down the runner, splashes at the far end and produces a persistent oxide flow tube, and the castings are no better than top-poured ones. Published work shows accurately controlled tilt speed raised the Weibull modulus (a measure of reliability) of aluminium castings from 2 to 55. The practical consequence for the buyer: a foundry with programmable tilt machines and a filling system designed for horizontal transfer will deliver more consistent pressure-tight castings than one with fixed-speed tilt or ladle pouring, and it is a fair question to ask at audit.

1.5 Solidification, ejection and finishing

The die stays closed until the casting is strong enough to eject, typically one to five minutes depending on section. Metal cores are withdrawn as early as possible to release the casting before contraction locks it onto the core and tears it. The casting is then knocked out, sand cores removed by vibration, gates and risers sawn off, and the part trimmed of flash at the parting line and shot blasted. Heat treatment, straightening, machining, leak testing, impregnation where permitted, and surface treatment follow as specified. Cycle times of two to six minutes per cavity and low labour content make the process economic from a few hundred to a few tens of thousands of pieces per year.

2. Alloys and how to specify them

The most common error on casting drawings is to call up a wrought grade (6082, 6061, 5083) or a bare "aluminium". Cast alloys are different alloys, designed around castability, with their own designations and product standards. In Europe specify to EN 1706, which gives chemical composition and mechanical properties for each alloy in each casting process. The material designation has a numeric form, for example EN AC-42000, and a chemical form, EN AC-AlSi7Mg. The casting process is stated by a letter (S sand, K permanent mould or chill cast, D pressure die, L investment; EN 1706 clause 5.4) and the temper by a suffix (F as cast, O annealed, T1 controlled cooling and naturally aged, T4 solution treated and naturally aged, T5 controlled cooling from casting and artificially aged or over-aged, T6 solution treated and fully artificially aged, T64 solution treated and under-aged for ductility, T7 solution treated and over-aged for stability; clause 5.3). The complete designation in the form given by the standard (clause 5.5) is EN 1706 AC-42000-K-T6: alloy EN AC-42000, chill cast, solution treated and fully artificially aged. The shorter "EN AC-42000 KT6" is widely used and understood. Note that the standard gives T5 values for chill casting only for the Al-Si-Cu and Al-Si-Cu-Mg alloys (46200, 47000, 48000), not for the Al-Si-Mg family, and gives T64 values only for the Al-Si-Mg family.

Equivalences in Table 1 are working analogues. Composition limits (especially iron), test bar requirements and property minima differ between EN 1706, ASTM B108 and GB/T 1173, so state one standard and one grade on the drawing. The GB/T column is given because most gravity die castings bought in the UK are now made in China, and a Chinese foundry will quote against a ZL grade unless told otherwise; the mapping is close enough for enquiry but the certificate must be to the standard on your drawing.

2.1 Why the Al-Si alloys dominate

Silicon gives fluidity, low solidification shrinkage and resistance to hot tearing, so an AlSi7Mg or AlSi10Mg alloy will fill a thin, complex die that an Al-Mg or Al-Cu alloy will not. Magnesium at 0.25 to 0.6 percent makes the Al-Si-Mg family heat treatable through Mg2Si precipitation. Copper adds strength, especially at temperature, and machinability, at the cost of corrosion resistance and ductility. The eutectic AlSi12 alloys freeze over a very narrow temperature range, which is why they are chosen for thin-walled and leak-tight parts that will not be heat treated.

2.2 Iron, and why the secondary alloys are cheaper

Iron is the most damaging impurity in aluminium castings. It forms plate-like beta-Al5FeSi intermetallics. In the bifilm framework set out in section 3.1 below, these plates grow on entrained oxide films and straighten them into planar cracks, so the harm done by iron is largely through the oxide bifilms it flattens rather than through any intrinsic brittleness of the intermetallic. The standard remedy is manganese: about 0.5 percent Mn for every 1.0 percent Fe converts the plates into compact alpha-phase "Chinese script" that wraps around the oxide and does far less damage. This is why EN AC-42100 (Fe max 0,19 percent, Mg 0,25 to 0,45 percent; EN 1706:2020 Table 1) costs more than EN AC-42000 (Fe max 0,55 percent, Mg 0,20 to 0,65 percent), and why the recycled Al-Si-Cu alloys are the cheapest and the least ductile: EN AC-46200 AlSi8Cu3 allows 0,8 percent Fe, and the pressure die casting alloy EN AC-46000 AlSi9Cu3(Fe) allows 1,3 percent. Titanium in both AlSi7Mg alloys is a maximum of 0,25 percent with no minimum unless grain refinement is called for (Table 1, note j). If your part is fatigue loaded or pressure retaining, specify the low-iron variant.

3. Melt quality decides the casting

The mechanical properties, leak-tightness and fatigue life of an aluminium casting are governed less by its chemistry than by two things that are invisible on a certificate: entrained oxide films and dissolved hydrogen. A buyer who understands this can ask better questions at audit.

3.1 Oxide films (bifilms)

Liquid aluminium is covered instantly by a thin, solid alumina film (spinel in Mg-containing alloys). Whenever the surface is folded over, by a falling pouring stream, a splash at the end of a runner, a vortex in the degasser or a dropped ladle, the film is entrained dry side to dry side as a double film, a bifilm, that behaves as a crack in the finished casting. Campbell's central argument, repeated throughout his Complete Casting Handbook, is that most porosity, most leaks and most fatigue and tensile failures in aluminium castings originate at these bifilms rather than at anything a metallurgist would find on a chemical analysis. The practical rules that follow are well established: no free fall of metal greater than about 12 mm inside the mould, bottom or side gating with a tapered sprue, metal front velocity below about 0.5 m/s, and no pouring from height into transfer ladles. Leaks in light alloy castings concentrate in the regions of a top-poured gravity die casting that were struck by the falling stream, visible as grey oxide striations on the casting wall, while regions filled uphill are bright and leak free; the textbook example is a sump (oil pan) casting. If a foundry shows you frosted grey streaks below the ingate on a pressure-tight part, the running system is the problem, not the impregnation.

3.2 Hydrogen

Hydrogen is the only gas significantly soluble in liquid aluminium, and its solubility falls by a factor of about twenty on freezing, so dissolved hydrogen precipitates as porosity in the last liquid to solidify. The equilibrium value at 750 °C on a day of 30 percent relative humidity is roughly 0.1 mL per 100 g, which is low enough for most commercial castings; wet refractories, wet charge or damp degasser rotors push it towards ten times that. Hydrogen porosity is fine and dispersed and, contrary to shop folklore, rarely causes leaks: gas pores are held apart by the dendrite structure and only become interconnected at impossibly high porosity levels. Some gravity die foundries deliberately run gassy metal on unfed parts such as inlet manifolds so that the dispersed gas offsets shrinkage; that is acceptable for a decorative part and not for a fatigue-loaded or pressure-retaining one.

The common shop-floor check is the reduced pressure test (RPT): a sample is solidified under partial vacuum, sectioned or weighed, and the result reported as a density index. A density index below about 2 to 3 percent is a reasonable requirement for pressure-tight or T6 structural work. The caution is that the RPT responds to bifilms as much as to hydrogen, because the pores need bifilms to nucleate on; a melt can pass the test after rotary degassing and fail it after being poured into the holding furnace. Treat the RPT as a melt cleanliness test and ask for it to be taken from the holding furnace immediately before pouring, not after degassing.

3.3 Modification

Unmodified Al-Si alloys solidify with the eutectic silicon as coarse plates. Adding sodium or, more usually today, strontium (typically 0.02 to 0.04 percent from an Al-10Sr master alloy) refines the eutectic silicon to a fine fibrous form and improves ductility. Campbell's account is more subtle than the textbook one: he attributes modification to the deactivation of aluminium phosphide nuclei and to the planar (sodium) or cellular (strontium) growth front that results, and notes that sodium is still used in many gravity die shops because the planar front gives a strong, uniform casting skin. The die caster watches each casting for surface "draws" or "sinks" at hot spots and adds sodium to the melt when they appear. Sodium fades within tens of minutes; strontium is more persistent but promotes surface-connected porosity on poorly fed castings and reacts quickly with nitrogen during degassing. For specification purposes, ask for a modified structure (fine eutectic silicon) on a metallographic section rather than specifying a modifier level.

3.4 Grain refinement

Titanium and boron, added as Al-5Ti-1B rod, nucleate the primary aluminium on TiB2 and TiAl3 particles and refine the grain size, which improves hot tearing resistance, feeding and the distribution of intermetallics. In the bifilm framework a large part of the benefit is due to the refiner particles settling on bifilms and carrying them to the bottom of the furnace, which is why refiner added to the furnace and given time to settle works better than refiner added to the pouring stream. Titanium of 0.10 to 0.20 percent is typical of the EN 1706 Al-Si-Mg alloys.

3.5 Dendrite arm spacing

Dendrite arm spacing (DAS) is set by local freezing rate and is the reason gravity die castings are stronger and more ductile than sand castings of the same alloy: a die-cast wall of 10 mm freezes with a DAS of perhaps 25 to 40 µm against 60 to 100 µm in sand. Fine DAS also shortens solution treatment: published work shows 356 alloy with 40 µm DAS can be solution treated at 540 °C in a matter of minutes rather than hours. A DAS requirement, or more usefully a maximum on a named section, is a legitimate thing to put on the drawing of a critical part, and it is checked with a microscope on a sectioned casting.

4. Heat treatment

For the Al-Si-Mg alloys the temper is part of the material specification and must be stated. The choice is between F (as cast, cheapest, properties depend on cooling rate in the die), T5 (artificial ageing from the as-cast condition, a modest strength gain with no quench), T6 (solution treatment, quench and full ageing), T64 (T6 under-aged for higher ductility and impact) and T7 (over-aged for dimensional and property stability at elevated temperature).

4.1 Quench stress and distortion

The quench freezes the solute into solution, but on any casting with unequal sections the outside cools and hardens first, the inside contracts later against a rigid frame, and the result is internal tension in the core of the section and compression at the skin. The quench strain of an aluminium casting is about 1 percent, ten times the yield strain, and in the bifilm framework the combination of bifilms and quench stress, rather than chemistry, accounts for most field failures of aluminium castings. Ageing at 150 to 180 °C does not relieve this stress. The mitigations are a hot water quench (limited benefit), a polymer glycol quench (large benefit, but the polymer must be cleaned from internal passages), forced air quenching for cylinder heads and similar parts, and careful racking so that the quenchant reaches all faces at once. Figures from one study: distortion in cold water taken as 100 percent, 86 percent in 80 °C water, and 3.5 percent in a 20 percent glycol solution. Where a part must be flat or round after T6, ask the foundry what it quenches in and whether it straightens after ageing, and put a flatness or runout requirement on the drawing that applies after heat treatment.

4.2 Blisters and incipient melting

Two other heat treatment defects are worth knowing. Blisters, raised bubbles on the surface after solution treatment, are hydrogen from the furnace atmosphere inflating bifilms close to the surface; they indicate turbulent filling rather than a furnace problem. Incipient melting from an over-temperature furnace opens grain boundary bifilms as the low-melting phases contract again on cooling, and permanently reduces ductility even though the microstructure may look normal. Both are grounds for rejecting the batch, not the individual casting.

4.3 Hot isostatic pressing

HIP (typically 100 MPa argon at around 500 °C for Al-7Si-Mg) collapses internal pores and presses bifilm surfaces into contact. Campbell's reading of the evidence is that HIP does improve fatigue life of Al-7Si-Mg castings, because the intermediate magnesium content lets the entrained alumina film transform to spinel and bond, but that alloys with very low or very high magnesium show little benefit, and that near-surface pores can collapse into sinks that exceed the machining allowance. HIP is an aerospace-grade cost; for most industrial gravity die castings the money is better spent on the filling system.

5. Design rules for gravity die castings

5.1 Size and wall thickness

Routine parts run from about 100 g to 30 kg; specialist foundries go to 100 kg or more with large tilt machines. Minimum wall thickness is 3 to 4 mm as a general design value, with 2.5 mm possible over short distances in the eutectic alloys and thicker walls (4 to 5 mm) advisable for Al-Mg and Al-Cu alloys. Walls should be as uniform as practical; where a thick boss or flange is unavoidable, it should be positioned where a riser or a die cooling insert can reach it, and blended with tapers or generous radii. Isolated heavy sections that no riser can feed are the usual cause of shrinkage porosity and, on a pressure part, of leaks after machining.

5.2 Draft, radii and parting line

Unlike sand casting, the die does not break away, so draft is essential. As a rule of thumb use 1 to 2° on external walls and 2 to 3° on internal walls and cores, more on deep pockets and on textured or heavily coated surfaces; but note that a drawing which invokes ISO 8062-3 general tolerances without stating draft inherits the default draft angles of ISO 8062-3:2023 Table 4 for permanent mould casting, which are steeper on short features (grade A external: 3,5° at 6 to 10 mm feature height, 2,8° at 16 to 25 mm, 1,8° at 63 to 100 mm, 1,2° above 400 mm; internal values are about 1° more). State the draft you can accept, or the grade, on the drawing. Internal radii of 2 to 3 mm and external radii of 1 to 2 mm reduce stress concentration in the casting and thermal fatigue in the die. A flat parting plane with all features drawing perpendicular to it gives the cheapest die and the best dimensional control. Undercuts need loose pieces, retractable slides or sand cores, each of which adds cost and tolerance.

5.3 Cores and holes

Straight-draw metal cores can form holes down to about 6 mm diameter with a length-to-diameter ratio of around 4:1, and larger cores are often tapered to ease withdrawal. Complex passages (water jackets, oil galleries, manifold runners) use sand cores, which cost more per piece, give a rougher internal surface (Ra 12.5 µm or coarser), and introduce moisture and binder gases into the die. Gravity die foundries using sand cores are well aware of the contamination and of the tar-like deposits that block die vents and limit the run length before cleaning. Holes below about 5 mm, and all threaded holes, are drilled and tapped after casting.

5.4 Tolerances

Dimensional tolerances are specified in ISO 8062-3:2023 by dimensional casting tolerance grade (DCTG). Table A.1 of the standard gives DCTG 6 to 8 for light metal alloys in metallic permanent moulds in long-series production, one grade coarser for complex castings, and GCTG 3 to 5 (Table A.3) for geometrical tolerances. The tolerances in Table 7 are total values, disposed symmetrically about the nominal unless the drawing says otherwise (clause 7.2): for a dimension between 63 and 100 mm, DCTG 6 is 0,78 mm total (±0,39), DCTG 7 is 1,1 mm (±0,55) and DCTG 8 is 1,6 mm (±0,8). So a realistic first estimate for a 100 mm dimension on a gravity die casting is ±0,4 to ±0,8 mm, with the finer grade on small, simple parts from a well-developed die. The standard has no separate parting line or core allowance: surface mismatch across the parting line is controlled within the Table 7 linear tolerance and the form tolerances (clause 8), but wall thickness is toleranced one grade coarser than the general DCTG by default (clause 9), which matters for pressure-retaining walls. Geometric tolerances (flatness, concentricity) are quoted separately and apply after heat treatment. Any feature that needs better than process capability is shown as a machined feature. ISO 8062-3 Table B.1 gives required machining allowance grades RMAG D to F for light alloys in permanent moulds, which Table 12 turns into 0,3 to 0,5 mm per surface for castings up to 40 mm, 0,5 to 1 mm at 63 to 100 mm, 0,8 to 1,5 mm at 100 to 160 mm and 1 to 2 mm at 160 to 250 mm; add to the upper end for surfaces at the top of the casting as poured, where dross and oxide gather.

5.5 Surface finish

As-cast surfaces from a coated die are typically Ra 3.2 to 6.3 µm, noticeably better than sand and not as good as pressure die casting. The die coat texture is reproduced on the casting, so a change of coat changes the appearance; if the part is cosmetic, agree a reference sample. Gate and riser witness areas are ground and are visible; identify any faces where a gate stub or ejector mark is not acceptable.

5.6 Inserts and marking

Steel or brass inserts (threaded bushes, wear rings, cooling tubes) can be cast in, provided they are preheated, clean, and located positively in the die. Cast-in identification (alloy, part number, cavity number, date wheel) costs nothing once it is in the die and supports traceability under EN 10204 and PED. Raised characters cast better than recessed ones.

6. Defects, inspection and acceptance

6.1 Defects typical of the process

The defect population differs between processes, so the notes below apply to gravity die casting of Al-Si alloys; sand and pressure die castings have their own. Misruns and cold laps come from a cold die, thin die coat or low pouring temperature and appear on thin walls farthest from the gate. Shrinkage porosity sits under heavy sections and at the junction with the riser neck; the fix is riser and die cooling design, not pouring temperature. Oxide films and the leaks that follow them are traceable to the filling system, as described in section 3.1. Gas porosity indicates melt practice (damp charge, damp rotor, excessive holding temperature). Hot tears occur at constrained corners around metal cores that were withdrawn too late, and in Al-Mg and Al-Cu alloys around any sharp change of section. Die coat inclusions and flaking come from coat breakdown on runners or from a coat that was not fully dried. Soldering (aluminium welding to the die) is chiefly a pressure die casting problem but occurs on uncoated ejector pins and cores in gravity dies. Flash follows die wear at the parting line and ejector pins. Distortion after T6 is covered in section 4.1. Sinks and draws on the surface over hot spots are a feeding and modification problem (section 3.3).

6.2 Chemical and mechanical testing

EN 1706 requires analysis of every element limited in its Table 1, on a spectrometer sample taken from the melt at the time the castings are made and cast into a metallic die (clauses 6.1 and 6.2). Its mechanical property minima (Table 3 for chill castings) apply to separately cast test pieces from the same melt, made by the same process and heat treated with the castings (clause 7.3.2.1), with a chill cast test piece of at least 12 mm diameter (clause 7.3.2.3). Those pieces represent the alloy and the heat treatment, not the section thickness of the part, and the standard says so in numbers: yield and tensile strength measured on the casting itself may be as low as 70 percent of the table values, and elongation up to 50 percent lower in some locations (clause 7.3.3.2). For critical parts specify test coupons cut from a named location on a sample casting, with agreed minima. Hardness (Brinell, HBW 2,5/62,5) on the casting itself is the cheap check that a T6 has actually been done: EN 1706 Table 3 minima for EN AC-42000 are 55 HBW as cast, 90 HBW in T6 and 80 HBW in T64. Electrical conductivity by eddy current (percent IACS) is a second non-destructive check on temper. A metallographic section for modification, DAS and porosity is worth requiring on first article and on periodic samples.

6.3 Non-destructive testing

Visual inspection is to EN 1370 comparators or MSS SP-55 style reference photographs. Dye penetrant testing of aluminium castings is specified in EN 1371-1:2011, whose scope (clause 1) is sand, gravity die and low-pressure die castings in all cast metals except copper-tin alloys, investment and pressure die castings being excluded, and which defines severity levels for linear and non-linear indications (clause 5.3); ASTM E165 with E1417 is the ASTM route. Radiography is evaluated against ASTM E155, the reference radiographs for aluminium and magnesium castings, which give graded plates for gas holes, gas porosity, shrinkage of several types, foreign material and cracks in section thicknesses of 6 mm and 19 mm; EN 12681-1 and -2 cover the radiographic technique and acceptance for castings under the EN system. Ultrasonic testing is little used on aluminium gravity die castings except for thick sections. Leak testing, by air under water, by pressure decay or by helium, is the acceptance test that matters most on housings, and the test pressure, hold time and allowable leak rate should be on the drawing. Impregnation with sealant to seal porosity is common and effective on non-critical housings, but should be either explicitly permitted or explicitly prohibited: a customer that finds impregnated castings in a pressure-retaining application without having agreed it has grounds for rejection.

The critical part of any NDT specification is the acceptance level, not the method. The sound approach is to classify the casting by the consequence of its failure, which sets how many castings are inspected, and separately to state a quality grade for the whole casting or for zones marked on the drawing, with the highly stressed zones at a tighter grade than the rest. Whether you express that with ASTM E155 severity levels, EN 1371-1 severity levels or the leak test alone, the principle is the same: zone the drawing, state a level for each zone, and the foundry can price the inspection and the scrap rate that goes with it. A drawing that says only "free from defects" cannot be priced and will not be enforced.

7. Pressure equipment and certification

Aluminium gravity die castings are used for pressure-retaining housings in pneumatics, hydraulics, refrigeration, compressors and instrumentation. Castings for pressure equipment sold in the UK or EU fall under the Pressure Equipment (Safety) Regulations 2016 or PED 2014/68/EU. Under PED Annex I section 4.3 the material manufacturer must certify that the material complies with the specification, and for main pressure-bearing parts in categories II to IV an inspection certificate to EN 10204 type 3.1 is only acceptable if the manufacturer's quality assurance system for materials has been certified by a competent body established in the EU (or, for UKCA, an approved body). That certificate is specific: it names the alloy, the product standard, the delivery condition, the product form and the size range, and anything outside that range needs a particular material appraisal (PMA). Ask to see the certificate at RFQ stage and check the scope against your part, because many foundries offering "3.1 certificates" for aluminium castings have no such approval and the certificate is then a works certificate in all but name.

Aluminium castings for pressure equipment are almost always EN AC-42000 or 42100 in T6 because the family combines pressure tightness, heat treatability and a long track record. EN 1706 property minima apply to separately cast bars; the pressure equipment code (EN 13445, EN 12952, the AD 2000 rules or ASME VIII) sets the design stress from those minima with the appropriate casting quality factor, so the NDT grade agreed under section 6.3 feeds directly into the allowable stress.

8. Choosing gravity die casting, and specifying it

Gravity die casting wins against sand casting when the quantity justifies a die (usually from a few hundred pieces per year, since the die is far cheaper than a pressure die casting tool), when properties, surface finish or repeatability matter, or when the part must be leak tight. Sand casting wins for very large parts, very low volumes, and geometries too complex to draw from a die. High-pressure die casting wins at high volume for thin-walled parts that will not be heat treated, welded or pressure tested at high pressure. Low-pressure die casting competes directly with gravity die for symmetrical parts such as wheels and cylinder heads and offers better filling control at the cost of slower cycles and a more expensive machine. Metal additive manufacturing (AlSi10Mg by laser powder bed fusion) wins below a few tens of pieces or where internal geometry cannot be cored.

A complete specification for an aluminium gravity die casting contains: the alloy and product standard with process and temper (for example EN 1706 AC-42100-K-T6); tolerance grade to ISO 8062-3 and any machined features with allowance; geometric tolerances that apply after heat treatment; surface finish and any faces where gates, ejector marks or die coat texture are not acceptable; the leak test pressure, medium, hold time and acceptance criterion; whether impregnation is permitted; NDT method, extent and acceptance level by zone; test bar and test coupon requirements, with hardness or conductivity on the casting; metallographic requirements (modification, DAS, porosity) if the part is critical; certification type to EN 10204; and marking. Our earlier article on what to include in a casting RFQ covers the commercial side, and the DDP landed cost article covers what it costs to bring the parts to the UK.

Frequently asked questions

What is the difference between gravity die casting and pressure die casting?

In gravity die casting the metal is poured or tilted into a coated iron or steel die under gravity alone, freezes relatively slowly and can be fed by risers. In high-pressure die casting the metal is injected at high speed and pressure into an uncoated steel die. Gravity die castings can be heat treated to T6, welded and made pressure tight; pressure die castings hold thinner walls and finer detail at higher volumes but normally cannot be solution treated and contain more entrapped gas.

Which aluminium alloys are used for gravity die casting?

Mostly Al-Si alloys to EN 1706: EN AC-42000 AlSi7Mg and EN AC-42100 AlSi7Mg0,3 (A356) for heat-treated structural parts, EN AC-43000 AlSi10Mg and EN AC-43300 AlSi9Mg for general engineering, EN AC-44100 AlSi12 for thin-walled or leak-tight parts that are not heat treated, and EN AC-46200 AlSi8Cu3 for low-cost non-critical parts. Al-Mg alloys such as EN AC-51300 AlMg5 are used for corrosion resistance and anodising.

What does T6 mean for an aluminium casting?

Solution heat treated, quenched and artificially aged. For AlSi7Mg the casting is held at about 530 to 540 °C for several hours, quenched in water and aged at around 150 to 180 °C. It roughly doubles the yield strength compared with the as-cast condition. The quench is the main source of residual stress and distortion, so the quenchant and part orientation matter.

What tolerances can aluminium gravity die casting hold?

ISO 8062-3:2023 Table A.1 gives DCTG 6 to 8 for light alloys in metallic permanent moulds, one grade coarser for complex castings. For a 63 to 100 mm dimension that is a total tolerance of 0,78 to 1,6 mm, so roughly ±0,4 to ±0,8 mm. Wall thickness is toleranced one grade coarser by default. Functional features are machined; ISO 8062-3 Table 12 gives 0,5 to 1 mm machining allowance at 63 to 100 mm and 1 to 2 mm at 160 to 250 mm for grades D to F.

Can aluminium gravity die castings be used for pressure equipment?

Yes, within limits. Under PED 2014/68/EU Annex I section 4.3 the foundry needs a quality assurance certificate for material manufacturers issued by a competent body, and the scope is specific to alloy, temper, product form and size. Our partner foundry's certificate covers EN AC-42000 T6 gravity castings up to 30 mm wall thickness, 350 mm diameter and 20 kg.

Sources and further reading

Sources: Campbell, J., Complete Casting Handbook, 2nd ed., Butterworth-Heinemann, 2020 (paraphrased, chiefly chapters 6, 9, 10, 16 and 19). BS EN 1706:2020+A1:2021, BS EN ISO 8062-3:2023, BS EN 1371-1:2011. Other standards are named for identification only; always work from the edition cited on your drawing.

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