Our Guide to Aluminum Casting

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Humans have always produced tools and parts in various ways, and some of the oldest methods leverage the flexibility of metal at or near its melting temperature, such as casting and forging. Aluminum casting is a centuries-old manufacturing process that produces aluminum parts by pouring molten aluminum into a mold, then allowing it to cool and solidify into a final product. Many industries commonly use this production method due to its versatility and comparatively low cost.

Aluminum casting is used in aerospace, automotive, construction, and consumer goods industries, where lightweight and durable materials are required. It&#;s also commonly used to create intricate designs and shapes that cannot be realized with other manufacturing processes. 

Read on to learn more about aluminum casting. 
Pro-Tip: Check out our aluminum materials page to learn more about the different types of aluminum alloys.

Definition of Aluminum Casting 

Aluminum casting is a method of making things from molten aluminum, and here&#;s how it works. First, a pattern is made to create a mold, then melted aluminum is poured into the mold and left to cool down and harden. Finally, the finished product is removed, trimmed, and finished. 

Cast aluminum is a useful material because it&#;s lightweight, strong, and food consumption-safe, making it an excellent fit for numerous applications. In addition, casting aluminum can create both large, complex parts in massive amounts and small and simple parts in low quantities.

The different aluminum casting processes include sand casting, die casting, investment casting, and permanent mold casting, and each has advantages and disadvantages.

Types of Aluminum Casting

Each type of casting method works in a unique way, and has an optimal fit in the product development process, and is optimized for different parts in a given assembly. The aluminum casting process includes a few stages: design making, shape making, softening and pouring, and wrapping up and finishing. The type of casting process used depends on the specific application and requirements, and each process has unique advantages and disadvantages.

Sand Casting 

Sand casting is the oldest and most prevalent casting method. In sand casting, a mold form is made by pressing sand mixed with a binder around an example of the ideal shape, called a pattern. Liquid aluminum is poured into the pattern and allowed to harden, producing both small and large parts alike. Since sand casting can&#;t be used for repeated production (some patterns are single use), it&#;s commonly utilized for small runs, test samples, and prototypes

Investment Casting

Investment casting uses a wax prototype of the ideal shape that is covered with a clay or ceramic shell. The wax is softened or melted, then removed, to create an empty ceramic shell, which is then filled with liquid aluminum. Investment casting produces intricate details, complex shapes, and various surface finishes. 

3D printing technology allows investment cast molds to be produced from 3D printed stereolithography (SL) prototypes made of thin resin layers. This allows for the creation of highly complex geometric shapes, improved dimensional accuracy, reduced lead time, and more design flexibility due to the capabilities of 3D printing. Investment casting is also commonly called lost-wax casting for the lost wax used to create the mold. 

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Die Casting 

This casting method relies on a metal mold called a die; hence the name die casting. In die casting, a metal mold is manufactured with CNC machining or another process, and liquid aluminum is injected into that mold, or &#;die&#;, and parts are formed quickly at high-pressure. There are two halves of a die-cast, the cover, and the ejector die, with the former being the fixed half and the latter being the moving half of the die. 

Die casting, or high-pressure die casting, produces parts with excellent dimensional accuracy, high production efficiency, complex geometries, accurate and thin walls, and a good surface finish. Die casting is an optimal process for manufacturing high-volume production parts. 

Permanent Form Casting

Permanent form casting, also known as permanent mold casting or gravity die casting, also uses a metal mold, and liquid aluminum is poured into the mold. Permanent form casting differs from die casting because it uses gravity or low pressure to feed the molten metal into the mold, while die casting uses high pressure to force molten metal into the mold. 

Permanent form casting is good for low to medium volume production runs of parts and typically has a lower up-front cost when compared to die casting molds. 

Pro-Tip: To design an aluminum casting mold, check out our best tips for CNC machining complex parts. 

Importance of Aluminum Casting

Here are some reasons aluminum casting is an critical manufacturing process:

• Cast aluminum parts are strong and lightweight, which makes them ideal for applications where weight is a critical design feature. For instance, aviation and automotive companies depend upon aluminum casting because it decreases the weight of vehicles, without sacrificing strength.
• Aluminum casting can produce complex shapes and features, which makes it popular for various products, including engine parts, aerospace components, and consumer goods.
• Aluminum casting produces durable parts that can endure harsh environments and high temperatures. This makes it an ideal choice for industrial applications that require reliability and long-lasting performance.
• Aluminum casting can deliver large quantities of parts for a minimal price, which makes it attractive to organizations hoping to create quality parts at a minimal cost.
• Aluminum is a sustainable material that can be reused numerous times without losing its properties, and recycling aluminum is a relatively low energy process. This makes it an environmentally friendly choice for companies looking to reduce carbon emissions and operate more sustainability.

Aluminum Casting Applications

Aluminum casting has various applications in different fields, and is used to make engine parts, wing components, heat exchangers, valves, and landing gear in the aerospace industry. In the automotive industry, it produces motor blocks, chamber heads, and other components. Aluminum castings make window and entryway outlines and enhance components in the construction industry, and they create consumer cookware, lighting apparatuses, and furniture.

Aluminum casting is a versatile and cost-effective manufacturing process that offers a few benefits over different techniques. It&#;s fundamental in present-day fabricating, because it permits organizations to create items that satisfy strict execution guidelines. It&#;s also environmentally friendly, as aluminum is a sustainable material that can be reused multiple times without mechanical property degradation.

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By overengineering I mean taking a design that might be suitable for manufacture in a professional foundry and just adding an additional safety factor to account for inconsistencies related to the home shop setup. My research puts the fatigue strength of non-tempered A356 aluminum around psi, so in theory a casting of this type should be able to hold lb indefinitely if it has an area of say 1 inch x 1/8th inch in the load direction. The question is how much larger should my casting be to be considered "safe". That is just for example by the way, I certainly am not planning on suspending a lb load from a homemade aluminum casting.

I was not aware of the grain structure issues with larger castings, so I'm already glad about posting this thread. I'm curious what the maximum wall thickness should be for sand casting? I'm getting a range of answers online from 0.2 to 1 inch. In any case adding ribs and/or increasing contact area will be my main focus in design.

I read through the thread on sawdust degassing, seems like a viable method and I'm looking forward to giving it a shot. I have heard about the pool shock method but I'm just not very keen on standing over a pot of molten metal that's spewing chlorine gas. After refining a mold I would have no problem purchasing some clean casting alloy for functional parts. The difference between even assorted scrap castings and regular extrusions is staggering.

This is a good point, luckily most of what I have in mind currently are things like brackets, bases, and other custom mounting solutions like your motor mounts. Nothing that will cause some catastrophe if it fails, but I'd like to do enough engineering to have confidence in my work. I'm not concerned about appearance necessarily, in fact a little evidence of the process can be a point of pride for me, I just want to confirm that some porosity isn't going to impede functionality.

I like the cantilever idea for assessing tensile strength, especially that it will give me a value for yield strength, as well as breaking. I have a stock of casting alloy from a full set of wheels I picked up a while back, so I have plenty of material to refine the process. I've been wanting to get a baseline of this material as it should be plenty strong enough for low load applications. Of course once I have a good pattern for the coupon I can make 1 or 2 with each pour to check for consistency with the design. I'm really curious to see how close it will be to the published values.

It's certainly a subject I've considered at length and have had the benefit of having friends that were professional foundrymen and ability to observe their practices. There certainly is no reason that industry best practice cannot be achieved by a hobbysist, but whether that is practical or not in that setting may be another matter. If you find degassing to be too cumbersome, you have a long way to go to emulate the pros.

Knowing the strength of material is certanly valuable information assuming you can accurately predict the state of stress to begin with. Can you? You need to know the character and nature of loading and have a very accurate FEA model and analysis.

Managing metal quality and H2 porosity in particular is certainly one of the prime issues, but the other, is one-off versus the development work typically done to put a part into production in commercial manufacturing enviroment. That will invlove iterative prototyping, destructive testing, and the abilty to reliably measure your results.

You can make and test all the sample coupons you like, but that doesn't mean your castings will uniformly exhibit the same qualities. Often times, the design features of a casting will mean that it won&#;t freeze uniformly or in a directionally consistent manner (thick sections/intersections for example), and those areas are the ones most likely to exhibit H2 porosity and related shrink defects and experienced foundrymen go right to them when they section and analyze the casting. If those areas happen to coincide with a critically stressed location, it can be bad news or at minimum require some iterative development of the part and feed system to achieve satisfactory results. Then there are usually production sampling plans to insure that everything stays in control along the way.

Most foundries will have the means of testing metal samples directly for H2 content and that would be done before any mechanical testing. There still is value to going to the effort of tensile testing or polishing and sectioning if nothing more as a means of verifying you can control the consistency of your mold media, furnace tune, and as far as using scrap metal, I'd say forget that unless you have a highly reliable source of information as to the alloy.......and don't assume all wheels are 356, because that is not always so. You might argue why do I have to know the metal composition if I know it's strength? If yield and tensile is all that is important, maybe, but then there&#;s fatigue, post heat treating, etc...

If you have a fuel fired furnace, you will move a very large mass of combustion air through your furnace and there will be a large amount of water along for the ride, and if you live in a very humid enviroment, that will be a very large amount of water. You should do the calculation but you will be surprised at that mass of water. Commercial foundries will purge and blanket the melt in a holding furnace just to prevent H2 infiltration when there is no flow!

I use a resistive electric furnace because it has no air flow. In fact, for reasons I'm not quite certain, it seems to have less than atmospheric levels of O2, because if I melt scrap and lift the lid, it will often flash when exposed to air. But I largely avoid exposing the melt to large amounts of water. If you have a fuel fired furnace, crucible hat and purge gas may help, but can also be cumbersome.

I'm a lost foam caster and my mold media is dry sand. I dont have to control or worry about the moisture or other binder content, because it's not there. As far as lost foam being more prone to defects and porosity, I'd say that has not been my experienec, at least no more or less than conventional sand casting. The byproducts of decomposed foam are not soluable in Aluminum. In fact most things are not to any appreciable level. It's just H2 that is the bugger. However I will concede when the castings become thick, I do see more defects, but so do conventional sand castings. I have my coating permeability tuned to produce best results at 1/4" wall thickness.

In the end, because of all the things that need to be controlled, most engineers avoid castings for critcally stressed parts and will opt for machined wrought/billet or forgings. If it's a one-off, it's almost always a CNC'd part from wrought unless you need the strength of a forging and ability to validate a design for production. It is generally excepted that the mechanical properties of castings are derated by process with die cast being the best, then shell/investment, and sand.

Commercial aircraft production spars and ribs are CNC machined from large Aluminum billets. 95% of the billet becomes chips. One example I see from time to time is the triple tree clamp on a motorcycle. Why on Earth would you cast a small part tlike that when it will need finsihed maching anyway? Especially if you are making one?

Best,
Kelly