Castable aluminum alloys offer diverse mechanical properties. Common types include A356, A357, and A380. Heat treatments like T6 enhance strength. They are used in automotive, aerospace, and general engineering. Key properties are tensile strength, elongation, and hardness. Alloy selection depends on application needs.

In this article, you will get the basic knowledge of different types of castable aluminum alloys.

Overview of Castable Aluminum Alloys

Definition and Importance

There are many alloys for casting. Some of these are aluminum alloys. Aluminum contains a mixture of other elements (such as magnesium or silicon). That produces its different alloys.

Aluminum melts at temperatures around 660 °C. This melted form then feeds into the mold to take on a profile shape.

Because of their lower weight with a density of 2.7 g/cm³, these alloys are 3x lighter than steel. This is the reason for their suitability across many of the applications. For instance, cars, planes, and machines.

Also, they resist corrosion well and can be made into any shape while being strong.

Composition and Properties of Castable Aluminum Alloys

Ötvözet összetétele

The formation of castable aluminum alloys includes the composition of many elements. For example, aluminum can contain silicon (5–12%), magnesium (0.2–10%), copper (1–4%), or zinc (1–3%).

Silicon melts at 577°C, which makes it fill molds better. Meanwhile, magnesium aids in getting more strength as it forms fine precipitates. The copper develops hardness.

For instance, there is 8–10% silicon and 3–4% copper in alloy A380. That’s why they offer high fluidity, spreading over mold gaps as narrow as 0.1 mm.

Role of Trace Elements

Small amounts of elements like iron (<1%) prevent sticking to molds, and manganese (0.5%) improves corrosion resistance.

Casting aluminum alloys avoids mold sticking simply because of the contribution of small amounts of trace elements like iron (<1%).

Similarly, they resist corrosion well and have smaller grain sizes because of manganese (0.5%) and titanium (0.2%), respectively. That also reduces cracks as a result.

Microscopic Structure and Composition

The mentioned image depicts the effect of composition on grain structure. For example, the coarse grain (50–100 µm wide) occurs in the casting’s initial phase.

Silicon elements (10–20 µm) get high strength, blocking dislocations. Meanwhile, nucleation forms at a point of grain boundaries (GBs). Nuclei grow to the matrix’s state throughout cooling.

Mechanikai és fizikai tulajdonságok

Strength and Ductility:

Castable alloys have tensile strengths that vary between 200 and 380 MPa. For example, a356 gets 280 MPa and is stretchable (10%) right before breaking.

Likewise, heat treatment (e.g., T6 temper) induces 20% increased strength and 8% ductility in A356. It achieves this through heating alloys to 500°C, quenching them with water, and aging them at 150°C for 5 hours.

Korrózióállóság

When an alloy contains magnesium (e.g., 520.0 with 10% Mg), it gets a protective oxide layer. This means they can compete against corrosion lasting 15-20 years in marine environments.

In LM6, the composition of high silicon content prevents saltwater corrosion. As a result, they continue to function at 50 Mpa pressure in underwater products.

Thermal and Electrical Conductivity

Alloys like 319 have 6% Si and 3% Cu. So they conduct heat at 150 W/m·K. That is what makes them a top choice among engine components.

Also, they exert low density, which is around 2.7 g/cm³. It improves fuel efficiency (10%) because of its lower weight, especially in cars.

Impact of Heat Treatment

Manufacturers perform heat treatment to reduce voids and refine grains. For example, the aging step increases hardness from 80 HB to 95 HB in alloys like ADC12.

Additionally, the recrystallization process (which occurs in the image) causes grain shrinkage up to 10–20 µm. So, the alloy gets increased fatigue resistance (30%) as a result.

Common Aluminum Alloy Grades

A380

There is 8–10% silicon, 3–4% copper, and less than 1% iron present in A380 castable aluminum alloy. The silicon particle has a lower melting point. This means they flow smoothly in molds, filling each gap.

The hardness is better because of the copper elements, which make them ideal for high-stress parts. That’s why this alloy with 320 MPa creates engine brackets and electronic housings.

It fills gaps properly and also causes fewer defects. Furthermore, this alloy cools faster, resulting in a reduced production time of around 15%.

A356

A356 alloy has silicon and magnesium content (7% and 0.3%, respectively). This is why it offers better fluidity and strength.

There is roughly 280 MPa tensile strength in this alloy and 12% ductility. Therefore, they can stretch up as much as 12% before breaking.

You can use a356 alloy to make aircraft and automotive wheels, suspension parts, aircraft landing gear, and wing frames.

It also has the unique property of withstanding 200°C temperatures for 500 hours. Even at this point, it doesn’t crack easily.

After heat treatment, this alloy has increased strength (20%). So, they are ideal to use in aerospace applications.

6061

6061 alloy is composed of 1% magnesium, 0.6% silicon, and 0.3% copper. These elements, like magnesium, increase its ability to stop corrosion. This alloy lasts 20 years outdoors and does not rust.

Olvassa el: 6061 vs 6063 alumínium ötvözetek

The tensile strength (310 Mpa) of 6061 alloy makes it useful in bridges, beams, and chassis parts like suspension arms and marine frames. Its low density further aids in making it 60% lighter than steel.

ADC12

Containing 10-20% silicon and 2-3% copper, ADC12 alloy is quite popular. Silicon in this metal enables smooth casting, while copper raises its hardness.

Heat treatment brings changes in hardness, exceeding it to 95 HB (Brinell). This alloy is used to make parts like car engine blocks, smartphone housings, and gearbox cases.

It also takes on complex shape profiles within 30 seconds, saving production up to 25%.

LM25

LM25 alloy is made by adding 0.3% magnesium and 7% silicon. Moreover, its tensile strength hits 260 MPa at 200 °C.  It gives good castability to manufacture pump housings, marine structures, and hydraulic valves.

Its sand-cast walls are as thin as 1 mm; that’s what its unique feature does. So, the LM25 goes well with the production of lightweight designs.

LM6

10–13% of the silicon content and less than 0.1% of magnesium are present in the LM6 alloy. Because of the higher amount of silicon, this alloy does not corrode easily. It can survive 50 MPa of pressure in seawater.

The manufacturers use it for casting boat propellers and dock fittings. Completing years, they function in saltwater without rusting. That also reduces maintenance costs by up to 40%.

520.0

The castable aluminum alloy of 520.0 has 10% magnesium and 0.1% silicon. This proposition of elements increases its tensile strength (380 MPa) and also reduces its weight by up to 50%.

You can use them to make aerospace brackets, rocket housings, and military gear. Another feature is to handle vibration around 500 Hz, showing no cracks.

319

This alloy has 6% silicon along with 3% copper. Silicon improves fluidity, but copper achieves better thermal conductivity (150 W/m·K).

They are cast to make cylinder heads and heat sinks. It gives maximum precision in parts such as ±0.02 mm and fits tightly in engines.

413

The alloy 413 includes 12% silicon and 2% iron. This is why it gives pressure tightness around 50 MPa. The parts, like hydraulic pumps and valves, are their applications.

Also, this alloy seal leaks in gaps, which can be as tiny as 0.5 mm, saving fluid.

535

Manufacturers add 7% magnesium and 0.15% silicon in the composition of 535 alloy. Its corrosion resistance conforms to exactitude in pH 8–10 environments like seawater.

They are used to produce ship hulls and offshore rigs, along with welding at 300°C. They do not crack under this temperature and can last 25 years in marine conditions.

Casting Processes for Aluminum Alloys

öntvények

The die casting process includes melting the aluminum alloy and injecting it into the mold under high pressure. It casts parts in no more than 10-30 seconds, and the output result meets fast and precise parameters.

Among the other alloys, A380 and ADC12 are the most commonly used to manufacture car engine brackets and airplane door handles.

Homoköntés

The sand casting process uses sand molds. Their grain size fluctuates between 0.10 and 0.5 mm. The process is suitable and goes well with manufacturing parts like engine blocks. It is much more affordable (50%) than die casting but does not give smooth surfaces.

Beruházási öntés

Investment casting includes wax patterns with ceramic coatings. They produce parts adding detailed elements, like turbine blades.

Also, it effectively gets tighter tolerances about ±0.05 mm but takes a very long time (48 hrs per mold).

Challenges and Solutions in Aluminum Casting

Casting Defects

Castable aluminum alloys cause defects like porosity and shrinkage, which weaken parts. That is exactly why you need to use vacuum casting to reduce porosity by up to 70%.

In order to improve surface roughness (up to 12.5 µm), picking shot peening with 0.5 mm beads at 80 psi is important.

Also, preheating molds before injecting molten alloy avoids stickiness and fills internal areas with better fluidity.

Ötvözet kiválasztása

Choosing the wrong alloy can ruin the whole project. For instance, the A380 is best suited for thin-walled vehicle parts.

Meanwhile, LM6 creates a resistance bearer that faces seawater. ADC12 fills smaller gaps in no more than 30 seconds but can crack if overheated above 600°C.

Heat Treatment Challenges

The hot zone in furnaces heats alloys. The use of 500 °C temperature and operating for 4 hours provides strong features in them.

The quench tank in this process aids in preventing cracks as it cools down parts at 10°C/min. The cooling temperature, however, must not be poor. This is because it can risk brittleness. For example, right quenching brings 20% strength to alloy 6061.

Következtetés:

Castable aluminum alloys are very important across many industries like automotive, aerospace, industrial, or even consumer products. They are light in weight, which is why they cut down fuel usage.

Also, their ability to stop corrosion means they can last more than 25 years. This is also the reason they are more demanding in harsh environments.

If there are challenges like shrinkage or porosity, you can fix them using particular techniques. So, make versatile products using aluminum alloys of your choosing.