Casting vs. Rod Alloys: Which Aluminum Alloy is Right for Your Project?

Common casting alloys are 356, 319, and A357. Similarly, the examples of aluminum Rod alloys are 6061, 7075, and 2024. In casting alloys there are aluminum, zinc, magnesium, and copper alloys. In Rod alloys, Aluminum, Steel alloys, Copper alloys, Brass and bronze, Titanium alloys, and Nickel alloys are the most popular alloys.

In this article, you will cover the following topics:

• The difference between casting and rod alloys

• How the Aluminum Association numbering system works

• Castability: What does that really mean for an alloy?

• The most common foundry alloys

• The impact of each alloying element
• The significance of impurities in foundry alloys

Let’s begin by exploring the difference between casting alloys and rod alloys. To understand casting alloys, you first need to examine rod alloys. Rod alloys are designed to be wrought, meaning they must be workable, which imparts their final properties. 

These mechanical properties are derived from a combination of the alloy’s chemistry. These are:

• Impurities

• The DC casting or belt casting process

• Hot and/or cold rolling

• Extrusion or forging processes

• and the final forming processes like stamping or high-speed blow forming

• Heat treatment and aging, such as during paint curing or baking in a car body.

Casting Alloys

Casting alloys must be castable into a shape without intermediate steps. This requires:

• Good fluidity to fill the mold
• Effective feeding behavior to address solidification
• Resistance to hot tearing and cracking
•  and mechanical properties

This involves controlling solidification, microstructure, and defect structure, to achieve better properties such as ductility through fine microstructures.

Casting Properties

The first law of materials engineering states that the properties of any material are a direct function of the process used to produce it. Therefore, laboratory specimens are not the same as castings, though they may come relatively close. 

Tensile bars are not castings either; they simulate properties but are not representative of the entire casting. Even excise test bars are just machine bars from specific regions and do not fully represent the casting.

Castability

This term encompasses the ability to fill the mold with the liquid alloy. Fluidity, measured as flow length per wall thickness, is crucial. Once the cavity is filled, the alloy must be able to feed the shrinkage and compensate for it during solidification.

Porosity and Fluidity in Aluminum Foundry Alloys

 

If I don’t properly manage the alloying process, I may encounter porosity, often referred to as shrinkage porosity, as seen in the top right of the images. Additionally, macro segregation can occur. In cases where I have a hot short alloy and cannot adequately feed it, and if it shrinks while restrained in the mold, hot tearing may result.

Fluidity and Alloying Elements

Let’s discuss fluidity and the impact of alloying elements. Silicon is a common alloying element, particularly in the 300 series of aluminum-silicon alloys. With pure aluminum, fluidity is excellent, though properties may not be ideal. 

Adding silicon initially decreases fluidity, reaching its worst at around 3-4%. However, fluidity improves after this point, peaking at the eutectic composition. For optimal fluidity, an 18% silicon level is ideal, typical of hypereutectic alloys in the 390 family.

Magnesium Fluidity

Magnesium shows a similar trend. Adding magnesium initially reduces fluidity significantly. To reach the eutectic point, around 33% magnesium would be needed, resulting in a mish metal rather than a practical alloy. Optimum fluidity would require almost 40% magnesium, though this is rarely practical.

Also Read: Top 10 CNC Machining Companies in UK

Copper Fluidity

Copper exhibits a comparable effect. Adding copper decreases fluidity up to about 10%, after which it increases again, but the eutectic point remains distant, so aluminum-copper eutectic alloys are uncommon.

Combining these observations, a graph can be created showing fluidity changes in millimeters as alloying elements are added.

Feeding Mechanisms

Four different feeding mechanisms are observed:

1. Liquid Feeding: While pouring metal, the mold cavity fills with liquid metal.
2. Mass Feeding: As the metal solidifies slowly, away from the walls, mass feeding occurs.
3. Intergranular or Interdendritic Feeding: Solidified metal feeds through the grain boundaries.
4. Solid Feeding: Important in high-pressure die casting or squeeze casting.

Hot Tearing

Hot tearing occurs because metal expands when heated and contracts upon cooling. Most aluminum foundry alloys shrink by 6-8% during solidification. Ideally, a part should solidify from one end to the other, maintaining liquid metal at the feeding point while the rest solidifies. 

In practice, thick sections opposite the feeding point can cause problems, leading to cracks during solidification when the part is still hot.

Castability of Foundry Alloys

Categorizing castability is challenging, but many have attempted to rate it. For instance, a 201 alloy (aluminum-copper with a bit of silver) is very sensitive to hot tearing, prone to shrinkage, and corrosion but easy to machine. 

Conversely, A356 alloy is highly castable, with excellent fluidity and minimal shrinkage, and can be very corrosion-resistant depending on impurity levels of copper and nickel.

Comparative Mechanical Properties

Mechanical properties are crucial after casting. For A356 or A357 alloys, typical values are approximately 30-40 MPa yield strength, 275-300 MPa tensile strength, and 5-10% elongation. As temperature increases, strength decreases rapidly, while elongation increases.

Selecting Aluminum Alloys for Specific Applications

Property Requirements

 

Alloy	Condition	Yield Strength	Tensile Strength	Elongation
A356	F (as-cast)	20 ksi (140 MPa)	Comparable to A356-T6	13%
535	F (as-cast)	20 ksi (140 MPa)	Comparable to A356-T6	13%
A206	T4	38 ksi (262 MPa)	Not specified	17%
A206 (T7)	T7	50 ksi (345 MPa)	63 ksi (434 MPa)	11.70%
242	Not specified	34 ksi (234 MPa) at room temperature, 33 ksi (228 MPa) at 150°C	Not specified	Not specified

 

For certain applications, you might need alloys with exceptional properties beyond what A356 offers. For improved corrosion resistance or to avoid solution heat treatment, a 500 series alloy like 535 may be preferable.

 In the F (as-cast) condition, 535 can achieve a yield strength of 20 ksi (140 MPa), comparable tensile strength to A356-T6, and higher elongation at 13%, making it a valuable option.

For high-strength requirements, such as in aerospace or suspension castings, A206 alloy is noteworthy. Although it may present challenges with corrosion resistance and hot tearing, it offers significant strength advantages.

 In T4 condition, A206 can reach 38 ksi yield strength and 17% elongation. In T7 condition, it provides 50 ksi yield strength, 63 ksi tensile strength, and 11.7% elongation. 

This alloy maintains high strength at elevated temperatures, which is beneficial for various applications.

If high-temperature strength is critical and ductility is less important, alloys like 242 can be suitable. 

Used in aerospace cylinder heads and similar applications, this alloy provides a yield strength of 34 ksi at room temperature and 33 ksi at 150°C, indicating excellent temperature stability.

Property Selection and Trade-offs

Selecting the appropriate alloy involves balancing multiple criteria to meet application-specific needs. Mechanical properties are crucial, but other factors such as corrosion resistance, castability, and heat treatment requirements also influence the choice. Often, a compromise is necessary to satisfy all criteria.

Casting Properties

Castings rarely exhibit a single, uniform set of properties. Instead, properties vary due to local microstructures, solidification rates, and inclusions. Consequently, mechanical properties typically show a range of values. 

Design properties must account for this variability, and castings may change over time due to aging, chemistry shifts, or seasonal variations. Achieving absolute perfection in castings is economically unfeasible.

North American Aluminum Association Numbering System

The North American Aluminum Association uses two distinct numbering systems for rod and casting alloys:

1. Rod Alloys: These are identified by four-digit numbers, such as 6061 or 6082.
2. Casting Alloys: These use a different system with three digits, occasionally prefixed with a letter and sometimes followed by additional characters. The system works as follows:

• The first digit denotes the alloy family (e.g., 3 for aluminum-silicon alloys).
• The subsequent digits are largely arbitrary, assigned sequentially by the first registrant.
• Letters (A, B, C, etc.) may be used to differentiate variations of a base alloy.
• A dot and a subsequent number specify the composition:
  • Dot 0: Represents the casting composition.
  • Dot 1: Indicates an ingot specification with slightly higher purity.
  • Dot 2: Refers to a higher purity specification.

Understanding these designations helps in selecting alloys that meet specific application requirements and processing conditions.

Aluminum Alloy Modifications and Series

Alloy Modifications

In alloy designations, specific letters indicate modifications:

• S: Strontium modified
• N: Sodium modified
• C: Calcium modified
• P: Phosphorus modified

For example, in the case of alloy 356:

• Dot 0: The silicon content remains unchanged from the casting to ingot form.
• Dot 1: The ingot specification has slightly higher copper content compared to the casting specification.
• Dot 2: Represents a higher purity ingot, often with more magnesium and lower iron compared to the casting specification.

Series of Foundry Alloys

1. 100 Series:

• Composed primarily of pure aluminum (99% minimum).
• Typical alloys include 100, 130, 150, and 170.
• Characteristics: High conductivity, low strength, high ductility, and good corrosion resistance.
• Applications: Electric motors (rotor alloys), bus bars, and decorative castings.

1. 200 Series:

• Mainly contains copper.
• Typical alloys include 201 (with silver), 203 (with cobalt), 206 (pure aluminum copper), and 242 (copper with a little nickel).
• Characteristics: High strength, fracture toughness, heat treatable, prone to casting difficulties, hot tearing, and stress corrosion cracking (SCC).
• Applications: Aerospace castings, speed brakes, thrust reversers, motorcycle parts, diesel and aircraft pistons, and generator housings.

1. 300 Series:

• Composed of aluminum-silicon alloys, with some containing magnesium and/or copper.
• Includes alloys like 356 and 319.
• Characteristics: Excellent castability, heat treatable, versatile with varying properties based on aging and chemistry.
• Applications: Engine components, wheels, aerospace structural castings, and general castings with minimal special requirements.

1. 400 Series:

• Binary alloys with aluminum and silicon.
• Typical alloys include 413, 443, and 444.
• Characteristics: Non-heat treatable, good fluidity, moderate strength, high corrosion resistance, used for intricate and thin-walled castings.
• Applications: Charger cooler housings, wheels, architectural and ornamental castings, garden furniture, and marine pistons.

1. 500 Series:

• Composed mainly of aluminum and magnesium.
• Typical alloys include 514, 518, 520, and 535 (also known as Almac 35).
• Characteristics: Excellent surface finish, corrosion resistance, good machinability, can be anodized, moderate to high strength, and toughness.
• Applications: Marine applications, hardware, ornamental castings, food processing equipment, and optical equipment.

1. 600 Series:

• Not widely used in North America but is available in Australia.
• Contains aluminum and magnesium, often with additional elements.
• Characteristics: Heat-treatable, good strength, and toughness.

     7.  700 Series

• Contains aluminum and zinc, sometimes with magnesium.
• Typical alloys include 705, 709, 711, 712, 713, 771, and 772.
• Characteristics: Good mechanical properties, heat treatable, excellent surface finish, and dimensional stability.
• Applications: Furniture, marine castings, farm machinery, machine tool parts, and aerospace castings.

   8.  800 Series

• Composed of aluminum, tin, copper, and sometimes silicon.
• Typical alloys include 850, 851, 852, and 853.
• Characteristics: Tin provides excellent tribological properties, ideal for bearings and bushings.

International Numbering Systems

 

There is no universal international numbering system for aluminum alloys. Different countries and regions have their own designations. North American and Australian designations may appear similar but are not always identical. Cross-referencing tables are available but can be extensive and may not always have direct equivalents.

So be careful if Australians talk about their three-digit number that doesn’t mean that is the one that you think of now let’s have a look at foundry alloys.

Alloying Elements and Impurities

Influence on Properties

First of all, you need to talk about alloying elements and impurities, the influence on properties, especially strength and elongation, toughness, corrosion resistance, conductivity, and the same element can be both. It can be an alloying element giving it strength, and in a different alloy, it might be an impurity.

The Casting Process

Then you need to talk about the phase diagrams, and here is one example for aluminum and silicon. This would be a binary alloy phase diagram, so there are only two alloying elements: aluminum and silicon. If we’re talking about three elements, then it is called ternary, and if it’s for quaternary alloys.

Key Areas of the Aluminum-Silicon Phase Diagram

Here you can see the aluminum silicon phase diagram with just between 0 and 20 percent silicon, which is where most of our alloys are. 

So you distinguish on top of the line you have the liquid phase. yousee the eutectic point at 12.7 silicon below the liquid and to the left of the eutectic we’ll find liquid and aluminum.

 To the right in the same triangle, you find liquid and silicon that solidifies first. If you fall below the 577°C line where the eutectic liquid is all frozen, you find aluminum and silicon, and this time now it’s all solid.

Common Alloy Ranges

So if you look at the most important areas of this phase diagram, you can see between five and eight percent silicon, you have the most common ones, the 356 but also 355, 319, 320. So most of the sand and permanent mold casting alloys. 

If you move more to the right between eight and ten percent silicon, you find typical die casting alloys like 380 of course, but also 360 or 383. 

And then between 10 and 13 silicon, you find the close to eutectic alloys, so that would be the 336, 339, and of course the 413 which is pretty much the eutectic alloy. 

So you find piston and some die casting alloys and then you have pretty much nothing until you get too far hypereutectic, so in the range of 16 to 20 percent silicon. 

And that’s where you have the 390 type alloy with some variants like the 392 the mercurial, they are used for high wear resistance.

Micrographs of Aluminum-Silicon Alloys

Microstructure Analysis

Here are a few example micrographs from the four regions of the aluminum silicon diagram.

A356 Alloy Microstructure

 On the left top, you see the A356 alloy, the light gray areas are actually the alpha aluminum, so this is pure aluminum that’s solidified first. You remember when we’re falling below this curve liquid, and then you have the alpha phase solidifying, and in this case it would be aluminum first. 

Eutectic Liquid Structure

The other area that you can see here, the darker gray with a funny pattern there, is now what you call the eutectic liquid. So this is a mix between aluminum and silicon, and in this case some magnesium and other elements in there, and you can see this has a very fine structure so this is actually nicely modified eutectic. 

380 Die Casting Alloy Microstructure

To the right, you see the 380 die casting alloy.This looks not as nice at all, and you see big chunky dark parts in there. Needle-like structures in there, so those are big intermetallics, iron needles, and all sorts of impurities that are in there because this is a typical secondary die casting alloy.

 If you look down on the left side, you see a piston alloy, and you can just imagine that this is not an alloy designed for a lot of ductility. 

And just like the 380 die casting, you have a lot of elements in there that just make it strong and hard but not very ductile, you don’t have nice round shapes like on top. 

And then to the right, you see the Mercure 392 and here you can see of course these darker gray spots, they are silicon.

 And as we’ve seen before in the phase diagram here, the silicon actually solidifies first as the alpha phase and then the other elements are freezing after.

Aluminum-Silicon-Copper and Magnesium Alloys

Alloy Composition and Properties

Now let’s have a look at the aluminum silicon copper and some magnesium family, so that means the A380, the most common foundry alloy, the die casting alloy, and the 319 alloy. They are among the most widely used aluminum casting alloys.

 In fact, they represent probably easily two-thirds of the total foundry alloy market in North America.

Elemental Composition and Contributions

 There are very different amounts of both elements, and some also contain some magnesium. Copper contributes here to the strengthening and the machine ability.

 It of course also makes it strong at higher temperatures, that’s why you can see those alloys used or a lot of engine castings. 

Silicon improves the castability and reduces hot shortness, and the aluminum silicon copper alloys with less than 5.6 percent copper are actually heat treatable.

 Although most A380 castings of course are not heat treated because they’re die castings.

Magnesium’s Role and Heat Treatment

There are many alloys of this family that also contain some magnesium, which will improve their heat treatment response giving us the maxillocyte hardening. 

And many hyper eutectic silicon alloys with 12 to 30 percent silicon, also contain copper. 

The primary silicon phase imparts excellent wear resistance, you were able to see before in the microstructure, and the copper contributes to matrix hardening at elevated temperature.

High-Temperature Strength and Applications

So you get high temperature strength combined with wear resistance, and that’s what you see in the 390 or 391 for example. 

There are a multitude of applications, but a lot of them are precisely where you have exposure to high temperatures, so in powertrain castings and also of course many general-purpose high pressure die castings.

Alloy Advantages and Disadvantages

Advantages

The aluminum silicon magnesium family, so that’s the 356 357 type, the advantages are of course moderate to high strength and ductility for structural components. So the magnesium lets us really tailor the property package.

Magnesium gives us excellent castability, are largely immune to hot tearing and cracking, and have a large volume fraction of eutectic which allows us very good feeding. 

And there is the largest database on the mechanical performance of any casting alloy group, so it makes it very easy for a product designer to use one of those alloy types to design their parts. 

They also have high thermal conductivity in comparison to the aluminum copper or aluminum silicon copper alloy, which makes it very attractive for certain applications.

Similarly, they have very good corrosion resistance because they have no copper in there or very low, it’s just an impurity. They have good fatigue properties and machinability,

Disadvantages

They must generally be heat treated to achieve high strength and hardness. The large volume fraction of eutectic gives a microstructure that will always be a compromise and will always require careful control.

So you have to add either strontium or sodium to modify it, or you need extensive thermal modification through heat treatment.

Common Uses Of Aluminum-Silicon-Magnesium Alloys

The major uses here are really for A356, its automotive wheels and in suspension parts. The 357 types are usually aerospace castings that require higher strength, and you see all sorts of general and automotive structural parts. 

You can also see a lot of general-purpose applications, for example, in for street lamps, for all kinds of castings that have no other special requirements like they have to be anodized or so.

 If you look at this family now in more detail, you see here two common representatives. The A356, and you see here again the chemistries for A356.2.1 and 0.0. 

Remember 0.2 would be the high purity ingot, 0.0 is in the casting, and the F357 again the 0.21 and 0. And you can see that the silicon is the same, and you have the magnesium as the major difference. A356 is the lower magnesium alloy, and 357 is the higher magnesium alloy. 

So that means in 356 I will get higher ductility, in 357 I will get higher strength. Now the major alloying elements are the silicon and magnesium, but also you need strontium for modification, and titanium as a grain refiner.

Major impurities are for example copper and then iron, but also calcium and phosphorus.

Variants

Special Variants

Specialties here are for example the F356 is actually a very low magnesium version of the 356 family for basically high ductility that’s traded in for strength. Another oddity is the C356 which is a premium low iron 356 variant and the ingots are actually usually required to have below 0.04 iron.

 Related alloys here would be the 354 and 355. They add a little bit of copper or the 360, which is a die-casting variant.

Role of Silicon

Silicon’s Influence on Alloy Properties

Now let’s have a look at the role of each element and let’s of course start with silicon as it’s the biggest alloying element in all foundry alloys. Higher silicon content in the alloy means it promotes fluidity and cast ability, it reduces shrinkage.

Silicone improves hot tear resistance and feeding characteristics, it also increases strength and stiffness even on its own. Moreover, silicon reduces the specific gravity and the coefficient of thermal expansion.

Silicone improves pressure tightness and corrosion resistance of castings, and in hyper eutectic alloys. So above the 12.7 percent eutectic point, it is used to increase wear resistance. Most die-casting alloys are usually in the 8 to 13 percent range.

Visualizing Silicon’s Impact

Now there are diagrams that really show all the different effects of each element, and here for silicon you can see that. So fluidity, feeding behavior, shrinkage sensitivity, uniformity of shrinkage, and hot tear tendency.

Aluminum Silicon Alloys and Magnesium

Magnesium’s Role in Strength and Heat Treatment

In aluminum silicon alloys, magnesium is used for strength and hardness, especially after heat treatment. It actually gives us the heat treatment response in the alloy. If I have aluminum and silicon as a binary alloy, I will not really get a heat treatment response. So the Mg2Si or max silicide based precipitation hardening displays a useful solubility limit of approximately 0.7 percent magnesium. 

After that, it doesn’t really do anything anymore; it actually softens the matrix. So that’s why usually you see most alloys specified up to 0.6 percent magnesium.

 It was also found that in aluminum silicon alloys, higher magnesium can reduce corrosion resistance slightly, so you have to be careful with that. Precise control improves chip formation and removal and machining operation.

Magnesium and Aging Response

Now if you look at the aging response, you can see this very nicely here with increasing magnesium content. If the magnesium content is low in F temper, the aging response isn’t much. You can see here increasing magnesium does a little bit, and we’re just looking at very low contents up to 0.1 percent. 

But see that at 120°C for 1,000 hours, so over time, it would age naturally. Or if you go to a higher temperature, you can artificially age.

You can see that really as soon as I surpass here you say 0.06 percent, I would say 0.05 approximately, you see very quickly the yield strength goes up. So that shows us the heat treatment response coming from the maxillocite hardening.

Role of Magnesium in Aluminum Magnesium Alloys

Magnesium’s Impact on 500 Series Alloys

If you look at the role of magnesium in aluminum magnesium, the 500 series binary alloys, there it actually gives us excellent mechanical properties, especially high ductility as cast. So no heat treatment needed, wouldn’t really do that much, and as cast, very nice property packages. I especially get very nice physical properties and very nice finishing characteristics. So usually, you will pick a 500 series alloy if I want to polish or anodize the part.

 They also machine very well; they show excellent corrosion resistance. And usually, it’s more difficult to cast than any 300 or even the 400 series alloys, at least at the typical current levels. Relatively poor fluidity and high solidification shrinkage. You also need to make sure you have special handling and control the selective oxidation of the magnesium phase when in the liquid condition. So therefore, very often beryllium is added to these alloys.

Role of Copper

Copper’s Influence on Alloys

Now if you look at the role of copper, you can find this, of course, in the 200 series alloys as the main alloying element, but also in some of the 300 series alloys.

It’s usually a strengthening element, especially at elevated temperatures. Otherwise, I only need it if I want to go to very high strength. 

In the 200 series, it provides solid solution strengthening and creates an age-hardening effect through the aluminum copper precipitate. It increases hardness and density; it improves machining characteristics as well. It actually reduces shrinkage but also ductility and corrosion resistance. 

That’s why it is usually in 300 series, a problem for any application that needs to be corrosion-resistant and is therefore minimized. Lower content means it imparts higher general atmospheric corrosion resistance.

Copper also needs to be limited in structural castings if they are welded, because as soon as you go beyond 0.3 or some would say 0.35 percent copper, you get into trouble when you try fusion welding the parts.

Role of Zinc

Zinc’s Role in Alloys

Now let’s have a look at zinc as an alloying element, as you can find it in the 700 series alloys. Zinc has actually a very wide solid solubility range in aluminum. It can provide solution-strengthening effects at higher concentrations, but excessive amounts are actually not really good and cause cracking.

And you see that sometimes, for example, in an A380 alloy. Zinc is not an alloying element but is limited to three percent. 

So if I’m actually high in silicon and in copper already, so I have a very strong alloy, and now I’m adding three percent zinc, I might actually just crack the casting right before it comes out of the die. 

Zinc can improve machinability, and in small amounts up to 0.2 to 0.3 percent in the aluminum silicon magnesium alloys, zinc has actually very little effect in most alloys.

But if you go a little higher, and you see some alloys with 0.5 to 0.85 percent, it is used as strengthening alloys in combination with special heat treatments. But you can find some problems if we’re trying to bond these castings with other components. 

Challenges and Considerations

When I talk about special heat treatments here, you have to think about not the usual single or two-stage heat treatment solution, heat-treated at 520°C followed by artificial at 180°C or something like that. 

Here we’re talking about several more stages, so you might have solution heat treatment at two different temperatures, and then aging again at different temperatures for different times. 

So you really need to know what you’re doing here, and it takes a lot of fine-tuning to achieve the response that you would like to get. Now in combination with copper or magnesium, it can result in attractive heat-treatable in natural aging compositions. 

But in this case, silicon is impure, so you need to keep it very low. And zinc is also said to enhance surface corrosion, so sometimes it is added a little bit to prevent deeper corrosion, like filiform corrosion. Just give the aluminum its natural corrosion oxide layer on the surface that prevents any kind of harmful deep corrosion.

Role of Iron

Iron as an Impurity

So now let’s talk about iron. And there are actually aluminum iron alloys that use it as an alloying element. You pretty quickly get to the eutectic point, I think it’s around 1.6 percent. But really, in most other alloys and pretty much all other alloys. 

It is just impurity and has to be limited. But it helps reduce die soldering in die casting; that’s where you can find most of the iron. So most die-casting alloys will be at one to up to even 1.3 percent iron.

 But you know it negatively impacts ductility. Iron improves fluidity and hot shortness, and this effect is even stronger the slower the solidification rate of the casting processes. 

So aerospace casting alloys, you will find a lot of them specified with, let’s say, a 0.06 percent iron max to achieve very good properties, especially elongation. And permanent mold, let’s say the typical wheel alloy A356, will be specified with ingots at 0.12 percent max.

 If you look at high-integrity die castings, because they freeze so quickly with very thin walls, they can tolerate easily up to 0.25 percent iron. Iron also improves fluidity and hot shortness, and as I said before, this is used in certain alloy types that use iron as an alloying element.

Example of Iron in Die Casting Alloy

Here you can see a very nice example, or an extreme example I should say, of iron in a die casting alloy. You probably recognize this; it looks similar to what we’ve seen before. It’s an A380, very high iron level. 

And I have seen 380 die castings sometimes with up to three percent iron because very few die casters use the spectrometer to check it. This was just cast, and you can see the crack is along these iron needles as well, which are actually platelets in 3D. And so if you have any stress riser, the crack will just go straight through your casting.

Iron’s Effect on Properties

If you look at what iron really does, a lot of studies have been made. And I’m showing here a picture from Jeff Siegworth’s study, a couple of years old, but that was a very nice demonstration. And you can see it started with 0.5 percent iron, and you have, of course, a trade-off between elongation and strength. 

So you move up the curve depending on how long you age, and you see you’re on one curve and you just have this trade-off. If you want to move up and go to a higher curve, you have to reduce the iron. 

You can see that already a reduction to 0.30 percent moves the curve quite a bit. And then you see another nice move going to 0.15 percent. And then another big move going to 0.03 percent iron in the same type of alloy.

Iron as an Alternative Eutectic Alloy

If you look at iron as an alternative eutectic alloy, you can see this here. And really the strength is not coming from iron. Iron is just used here to make it very testable, but the strength is coming from the magnesium. 

You see it’s about four to four and a half percent magnesium. Silicon is now an impurity and has just a maximum, and so is pretty much everything else. So the only other element you have in there is titanium as a grain refiner, and you have beryllium for oxidation of the magnesium.

Role of Manganese

Manganese in Die Casting Alloys

So now let’s have a quick look at manganese. Manganese is used in, of course, the rod alloys in the 3000 series alloys, but in casting alloys, it’s mainly used in die casting alloys or in secondary alloys, aluminum silicon alloys. And there it is usually added as half of the iron content. 

In die casting, it is used to reduce die soldering and replace iron for higher ductility. Manganese has a limited solid solubility in liquid aluminum but is actually higher than iron.

The higher solubility in aluminum gives us a reduced tendency to form brittle phases and helps improve the properties. It is typically used to correct the iron intermetallic phase and minimize die soldering.

Microstructure Improvement with Manganese

Here you see a typical microstructure. You see the alpha aluminum, the light gray islands surrounded by the eutectic liquid as you have seen it before. Then you see these needle-like structures in between. Those needles are bad for ductility, especially. You call them iron needles (FeSi Al5), and that is beta iron.

 Therefore, what you want to do is add the manganese. On the right side here, you have converted these needles into what you call Chinese script. Now it is alpha iron. There is a lot of interaction between iron and manganese, and you have to watch out when you add those two elements.

Manganese and Iron Solubility

you already know the sludge factor. Manganese goes in with a factor of two of iron, so you have to be careful how much manganese you can add. There is another factor: whether you create large primary crystals, which are also bad. you look at iron content and silicon content. 

On the left side, this blue curve goes up; the more silicon I have, the more iron content I can tolerate without forming these large primary crystals. But this blue line actually goes down the more manganese I am adding. 

For example, if I am adding 0.6 manganese and I am at, let’s say, 10 percent silicon content, I should be well below the 0.2 percent iron content.

Iron Solubility and Manganese Content

If you plot that iron solubility against the manganese content, you can see here on the right side that pretty much all the typical structural die casting alloys are just below this curve. You want to be relatively close to it, have enough manganese to prevent die soldering, and avoid the formation of large primary crystals.

The Role of Other Elements

Titanium as a Grain Refiner

Titanium is used as a grain refiner in many foundry alloys, often in combination with small amounts of boron, typically in a five-to-one ratio. This combination is often referred to as TiB grain refiners.

Chromium

Chromium is not a very popular element to add but is included in 7000 series alloys to improve corrosion resistance. It can also help modify iron needles and reduce die soldering in high-pressure die casting, though it greatly contributes to sludge formation, so its use must be limited.

Nickel

Nickel is typically added in combination with copper to improve elevated temperature properties, providing high-temperature strength. It is used in the 242 alloy and in piston alloys but can cause filiform corrosion even at low levels (above 100 ppm), making it an impurity in most alloys that needs to be minimized.

Modifiers

Strontium

Strontium is the most common silicon eutectic modifier used in the industry, improving the ductility of the alloy. While it can help reduce die soldering in die casting, higher levels can be associated with porosity, particularly in thicker sections of castings and slow solidification processes. It burns off quickly, especially if the melt surface is exposed to air or stirred.

Sodium

Sodium is a very effective modifier for aluminum-silicon eutectic alloys but embrittles aluminum-magnesium alloys due to its rapid fading. It is not typically used because of its short-lived effectiveness.

Phosphorus

Phosphorus is an effective modifier for hyper-eutectic alloys but negatively impacts the modification effects of both strontium and sodium on eutectic alloys. Therefore, phosphorus-modified parts should not be mixed with those modified with strontium or sodium.

Vanadium and Zirconium

These elements are used in high-temperature alloys, such as cylinder head alloys. They are chemically similar to titanium and used in very small concentrations. For an appreciable effect, concentrations need to be above 0.1 percent.

Molybdenum

Molybdenum is virtually insoluble in solid aluminum and has limited solubility in liquid aluminum. It forms intermetallic compounds during solidification, strengthening the alloy as a dispersoid hardening agent.

Scandium

Scandium is a rare and expensive element that inhibits recrystallization, strengthens without reducing ductility, and acts as a grain refiner. Its high cost makes it prohibitive for widespread commercial use.

Other Impurities

Iron

Iron forms detrimental iron needles and must be minimized, but manganese can correct this issue at half the iron content.

Calcium

Calcium is a weak eutectic modifier that increases hydrogen solubility and porosity, negatively affecting surface tension and hydrogen bubble formation.

Lithium

Lithium encourages buildup in aluminum transfer systems, dip tube plugging, and must be minimized in low-pressure systems.

Antimony

Antimony is a good modifier for silicon eutectic, but it reacts with sodium and strontium, forming intermetallics that negatively affect castability and eutectic structures. It is difficult to analyze due to its spectrum overlapping with iron.

Beryllium

Beryllium is used in high magnesium alloys to reduce oxidation losses. However, exposure to beryllium fumes, smoke, and dust can pose health risks during melt handling and subsequent processes like blasting, CNC machining, and welding.

Conclusion

In conclusion, there is a large variety of foundry alloys offering different properties. Different processes require different alloys, and the nomenclature can vary globally, so caution is needed when discussing alloys. Elements can have different roles as either essential alloying components or harmful impurities, necessitating strict control over the quality of foundry ingots and their chemistry.