Cast alloys and products
- A single operation process; casting is the least costly manufacturing route for a wide range of parts.
- Complex three-dimensional shapes can be produced with intricate internal passages. Powertrain components take maximum advantage of this feature for cylinder heads and blocks. Many such components cannot be made economically otherwise.
- Part consolidation is a large advantage in that a single casting can replace an assembly of parts produced by other means. This reduces tooling costs, eliminates joining operations, and ensures close tolerances.
- The properties of a casting are - apart from alloy composition and heat treatment - strongly influenced by the casting process and part design.
- Mechanical properties will vary with the local freezing rate from location to location within a single casting. Exact mechanical performance data can thus be difficult to obtain in forms other than minimum and typical values and FE analysis is difficult.
- Close co-operation between the part designer, foundryman, and metallurgist is needed very early in the design process in order to ensure a successful outcome.
Aluminium Foundry Alloys can be tailored to cover a wide range of mechanical performance via chemistry and/or process control.
- There are four main families based on Al-Si(-Cu), Al-Cu, Al-Mg(-Si), and Al-Zn(-Si)- Mg systems
- The vast majority of cast Al components are based on the Al-Si system because of its good castability.
- Some Al-Cu, Al-Mg and Al-Zn alloys exhibit better properties than Al-Si alloys, but their casting properties are generally poor. In particular, they show a high tendency to hot tearing.
- Highly corrosion resistant, these alloys polish to a high shine and can be anodised.
Good strength/ductility compromise without need for high temperature heat treatment.
- Used to produce high-pressure die cast automotive steering wheels and structural components.
- Used to avoid quench distortions, as these alloys are subject to age hardening at room temperature without previous solution treatment.
- The addition of Si to these alloy systems in adequate quantities gives better casting properties. Various alloys have been developed or are under development.
- The strongest of the casting alloys, the Al-Cu family sees use mostly in the aerospace industry.
- They also see service as automotive suspension knuckles, high-end turbocharger impellers, etc.
- The alloys are very hot short and difficult to cast in complex shapes.
- Limited to low volume production for sports cars and other exotic vehicles.
- The most popular alloys of the casting industry world-wide.
- Exhibiting excellent castability these alloys feed well, resist hot tearing, and are in general the most manufacturable casting alloys.
- Addition of Mg results in good mechanical properties after heat treatment, addition of
Cu gives a better machinability and increases strength at temperature.
- Al-Si-Mg alloys are used for wheels, structural castings and suspension parts
requiring moderate to high strength and good ductility.
- Al-Si-Cu(-Mg) alloys dominate the market for power train components such as engine blocks, cylinder heads, pistons, and die castings where strength at temperature and/or wear resistance is more important than ductility.
See also:
- AAM – Materials – 1 Resources > Secondary aluminium
- Primary alloys are produced from pure aluminium, melted with addition elements.
- Secondary alloys are produced at a lower cost from scrap (end-of-life recycled) aluminium materials, which are remelted after classifying, with adjustment for main elements.
Secondary alloys have relatively high levels of impurities, esp. Fe, as most available scraps are contaminated with iron or steel components (e.g. bolts, rings, etc.). Fe is detrimental to many properties, mainly castability and ductility, and must be kept at levels as low as possible, with an exception for pressure die casting.
For applications requiring high ductility, Fe content is generally specified < 0.20 %, which requires primary alloys.
When ductility is not a criterion, the Fe level may be raised, and the criterion can be castability or machinability, and secondary alloys can be selected with Fe content as high as 0.5 %, or higher if acceptable.
- In this case Fe is a "wanted impurity", as it lowers the tendency of aluminium alloys to stick to the mould surface.
- For this reason Fe content is specified with a minimum value, generally 0.6 % or more, and secondary alloys are quite suitable.
- The maximum value depends on ductility requirements, it is generally around 1 %, up to 1.3 %.
See also:
- AAM – Products – 6 Cast alloys and products > Alloys > Typical application areas
- AAM – Applications – 1 Power train > Cylinder block > Alloys: composition and heat treatment
- Fluidity as a foundry term differs from the scientific definition. In science fluidity is the reciprocal of viscosity and, as such is an exact measurable quantity.
- Foundry Fluidity is defined as the distance that a liquid metal will flow into a mould cavity and is thus a relative term.
- As molten metal is introduced into a mould cavity it loses thermal energy to the walls of the mould as it flows. Eventually it begins to freeze and at some point the metal will cease to flow.
- The initial metal temperature.
- The heat extracting power of the mould material including the effect of any insulating die coatings that are applied.
- Kinetic energy of the metal. Gravity die casting, sand casting, etc., all rely on the metal flowing downhill under its own weight and momentum. Low pressure or high pressure diecasting conditions impart a different, and adjustable, pressure to be applied to the metal to cause it to flow.
- Metal Cleanliness also has a large impact and, while a quality of the metal, is largely process dependent.
- Insufficient fluidity may result in misruns (incompletely filled castings) and poor replication of details.
- Excessive fluidity may cause sand penetration or excessive flashing of molten metal along parting lines.
- In practice it is controlled via alloy choice, casting and mould temperatures.
- Changes in fluidity may be an indication of dirty metal.
- Changes in fluidity can be expected when trace impurities change, most important are changes in grain refining or modifying practice.
- The oldest of the fluidity tests, the spiral is moulded in sand. Metal at a carefully controlled temperature is poured into the mould and, after shakeout, the distance that the metal flows is measured.
- A pyrex or quartz tube is connected to a vacuum system. The test melt is presented to the tube at the same time that the vacuum is applied; again, at a set temperature. The distance up, or along, the tube that the metal flows is measured as the fluidity.
- Alloying elements such as Cu or Si significantly influence the foundry fluidity of aluminium alloy melts
- The fluidity of aluminium is very sensitive to chosen purity level
- This is most important when casting electrical motor rotors or conductors.
Aluminium shrinks during solidification, as its density in the liquid state is 6.5 % less than in the solid state. Cast parts generally solidify from the surface to the centre. Therefore, any lack of material will appear there if not compensated by feeding (mould design, alloy selection, temperature regime, etc.).
The composition of the alloy has an influence on the volume and type of the shrinkage, see table:
- silicon is the only suitable additional element that reduces shrinkage as silicon expands 8 % when solidifying;
- alloys having a low eutectic ratio present scattered "micro-shrinkage" and "collapsing";
- elements such as phosphorus, sodium, antimony and strontium, which determine the shape of silicon particles during solidification of the aluminium-silicon alloys, have at the same time an effect on the type of shrinkage observed in alloys with high silicon contents.
Cracks (hot tearing) may appear in cast parts after solidification.
Stresses applied on still "mushy" areas are generated by the contraction of already solidified zones. Cracks appear when the "mushy" zone is too thick and the quantity of liquid available is too small to fill the resulting gap.
The composition of the alloy has a major influence on this phenomenon, as the selected alloy determines the possibility for the liquid eutectic phase to progress through the solidifying metal -"mushy" zone - to feed stressed areas.
There are also other factors that the foundry man will take into account to control the "hot tearing" defect, among which are:
- design of the part,
- grain refinement of the alloy,
- stiffness of the mould.
See also:
- AAM – Materials – 3 Designation system > Cast alloys > Designation system for casting alloys – EN 1780-1
The sensitivities of main alloys to hot tearing, coded from 0 to 6, are compared in the table below. The relation between sensitivity and the ratio of eutectic phase and the freezing range of the alloy is obvious.
Silicon plays a major role in this respect, as it is the only practical alloying element that reduces the contraction of the alloy in the solid state.
Mould sticking is a defect observed in pressure die casting which leads to surface defects on the part and progressive deterioration of the mould.
"Mould sticking" may be in fact the consequence of four different phenomena, as shown below:
- cavitation
- erosion
- chemical soldering
- friction
The composition of the alloy may have an influence on mould sticking with two elements:
- Iron is generally considered to be a harmful impurity in gravity casting, mainly towards ductility. For pressure die casting applications, on the other hands, the solubility for iron originating from the mould in liquid aluminium lowers with the iron content of the molten alloy, and 0.5 % is considered as a minimum towards problems of sticking by soldering of the steel mould.
In newer alloy developments, iron is partially replaced by manganese which has a similar effect and reduces also mould sticking with less deterioration of the mechanical properties.
- Silicon content may have an effect on sticking by friction as it modifies the contraction factor: a high content lowers the contraction of the part.
Different other properties of cast parts are also depending on the composition of the alloy. Typical values for electrical resistivity, thermal conductivity and thermal expansion are indicated in table for the as cast condition.
Thermal conductivity and electrical resistivity, which are correlated, depend also on the temper:
- thermal conductivity is maximal in the as cast (and annealed) condition, and minimum after a solution treatment, and the opposite for resistivity.
- Fe is the main trace impurity responsible for degrading the ductility of conventional casting alloys.
- The phase which forms from the liquid in high Si foundry alloys is -FeSiAl5.
- Acicular, or needle-like, in morphology, these particles are commonly referred to as - AlFeSi needles, but they are actually plates.
- Adding Mn at levels equal to roughly half the Fe content corrects the phase to script (Fe,Mn)3Si2Al15 or -AlFeSi.
- The length of the -AlFeSi needles is a function of the cooling rate (of which the secondary dendrite arm spacing is a measure).
- The -AlFeSi needle length also increases with the Fe content.
- The presence of -AlFeSi needles will degrade the mechanical properties of the alloys.
- The ductility is the most strongly impacted property, particularly in secondary alloys with high Fe-content.
- Castings are generally not strengthened by work hardening.
- Solid solution strengthening occurs and is largely determined by the alloy composition.
- Precipitation hardening is the most commonly applied and flexible strengthening mechanism.
- The alloy is solution annealed at high temperature to dissolve the elements which will be used to create the precipitates.
- The alloy is then hot-water quenched to create a super-saturated solid-solution with minimal quench-distortion.
- Sub-microscopic intermetallic compounds are precipitated throughout the Al alloy matrix. The concentration of the hardening elements determines the strength of response.
- The super-saturated solid solution is heated to a low temperature to alloy enough atomic mobility for the pre-cipitation hardening elements to form into precipitates.
- The mechanical result is an ageing curve in which the yield and ultimate strengths gradually increase as the ductility comes down. The process is halted when the desired properties are achieved.
- Al-Cu; end stage is CuAl2
- Al-Mg; end stage is Mg2Si
- Al-Cu-Mg; end stage is CuMgAl2
- Al-Zn-Mg; end stage is MgZn2 (Zn > Mg)
- Al-Mg-Zn; end stage is Mg3Zn3Al2 (Zn < Mg)
- Quenching of cast parts from temperatures in the vicinity of 500°C – 540°C may cause warpage of the castings. A straightening operation before artificial ageing, when the casting is in it's softest condition, is not uncommon.
- The precipitation hardening process may cause growth of the castings. This occurs because of the volume change associated with the formation of precipitates from solid solution. Alloys may be aged for strengthening reasons or they may be aged to stabilize them dimensionally.
- Note that the Al-Zn-Mg and Al-Mg-Zn systems do not grow.
As shown in the schematic Al-Si phase diagram, the majority of commercial foundry alloys are based on this alloy system.
- Hypoeutectic Alloys (<11% Si) are composed of a forest of Al dendrites surrounded by interdendritic Al-Si eutectic (1:1 ratio depending on alloy)
- Eutectic or near eutectic alloys (11 <Si <13) may consist almost entirely of eutectic.
- Hypereutectic alloys (>13% Si) will consist of a eutectic matrix together with wear resistant primary Si.
- Aluminium is a ductile metallic element which can be alloyed to give a wide range of combinations of strength and ductility.
- Silicon is a glassy, hard, brittle semi-conductor whose presence in foundry alloys is important during solidification as it makes the alloy castable.
- Once solid, these alloys are in-situ composites.
- An alloy in which the Al-Si eutectic has been well modified may show a ductility up to three times higher than the same alloy unmodified.
- The shape of the Si may be changed by cooling rate. At high freezing rates this is called quench modification
- The shape of the Si may be changed by a solutionizing heat treatment. This is
- Chemical additions of elements like Na, Sr, Sb, or P may be made to the melt. This is
- One of the first chemical modifiers discovered.
- Na is the strongest of the chemical modifiers in terms of amount required to modify.
30 ppm will completely modify most Al-Si alloys.
- The major disadvantage is the fade rate. Na is lost from the melt by volatilisation in as little as 20 minutes.
- Na can be added elementally via Na metal. Packed in sealed cans or foil packs, the master alloy must be kept dry and handled carefully as it is a hazardous material.
- Na can be added by fluxes as well.
- Strontium is a slightly weaker modifier and amounts ranging from 80ppm to 200ppm may be required depending on the alloy and the cooling rate.
- Generally available in two master alloys: Al-10%Sr and Sr-10%Al. The first is the most popular with foundries as it is inert and easy to handle while the second is reactive and flammable.
- Sr is known to cause a reduction in the feeding distance and a greater tendency towards porosity.
- Sr is more popular with many foundries as it fades much more slowly: hours rather than minutes.
- Sb refines the Si phase as opposed to modifiying it. I.e. the microstructure remains acicular but becomes much finer; thermal modification then renders it indistinguishable from modified material.
- Sb does not fade but is incompatible with Sr or Na.
- Phosphorous poisons the other modifiers/refiners where the Al-Si eutectic is concerned.
- P is added to hypereutectic alloys to refine primary Si for wear resistance.
Literature:
- Tynelius K., Major J.F., and Apelian D.: A Parametric Study of Microporosity in the A356 Casting Alloy System, AFS Transactions, 93-166, 1993, pp. 401-413
- The amount and size of porosity which will actually form at any given gas level is a function of metallurgical factors such as the Sr content and use of particulate grain refiners.
- Local process parameters such as the total freezing time (tf) will also influence the quantity of porosity.
Literature:
- Terjesen, G.: Effects of elevated temperatures on tensile properties and fatigue strength of an AlSi10Mg alloy. ALUMINIUM 78, 2002, p. 464-469
Aluminium castings have had for long a reputation of low fatigue strength.
However, aircraft builders were interested since the 80' in replacing riveted multi-components structures by single castings and tested them: it was demonstrated every time that the casting had a better fatigue resistance than the equivalent riveted part.
A wide spread application has been for years designed and tested upon fatigue criteria: car wheels.
- The actual fatigue performances of castings, compared to values obtained on test- bars, can be lowered mainly by surface or sub-surface defects that act as local stress raisers and crack initiators.
- The more detrimental defects are oxides, when in the shape of films, and cold shuts.
- The critical dimension of defects is about 300 µm, defects under 100 µm have usually no detrimental influence.
"Hot spots" of maximal stresses must be indicated to the foundry, so that particular care be taken in these areas.
Source: https://www.european-aluminium.eu/media/1544/aam-products-6-cast-alloys-and-products.pdf
Web site to visit: https://www.european-aluminium.eu
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