Aluminum is a metal that defies expectations. It is often thought of as weak, possibly due to its association with beverage cans that can easily be crushed on a forehead. Despite this, it is one of the strongest materials for its weight. It’s strong enough to make the wings that carry a jetliner and scarce enough that gathering discarded cans for recycling is a worthwhile activity. And yet, it is the most abundant metal in the Earth’s crust and the third most abundant element overall.
The reason for these contradictory perceptions is aluminum’s reactivity with oxygen. It binds with oxygen to the point that it’s impossible to find metallic aluminum in nature. This means the metal’s behavior is non-intuitive compared to other metals. When heated instead of entering a molten metallic state—like steel, copper, or many other metals—aluminum wants to become a fine particulate of aluminum oxide. This has meant that casting aluminum vs. rolling or forging has involved a fair amount of clever chemistry that has compromised the ways the metals are used in some roles. However, a succession of 5000 series aluminum alloys has been changing the industry as cast aluminum tooling plates with long desired properties like weldability. The reason why 5000 series aluminum cast well can seem mysterious to the unfamiliar, but the answer has to do with the chemistry behind the metallurgy.
Challenges of Casting Aluminum
Cast aluminum is simply aluminum that is melted down and poured into a mold where it cools into a shape. Aluminum’s incredible reactivity with oxygen poses some unique challenges in its casting. Aluminum’s tendency to bond with a common atmospheric element tends to lead to a lot of porosity as the aluminum oxide disrupts the crystalline structure of the metal. This porosity has meant aluminum castings tend to lack structural strength and are mostly used for ornamental work, occasionally cookware and other uses where a lack of strength isn’t an impediment.
There are ways of reducing and nearly eliminating this porosity, though: One way is adding certain chemical compounds that can bond with oxygen and other gasses in the molten mix and float to the surface of the puddle. Additionally, alloying elements can further reduce this porosity, increase the structural strength and enhance the quality of cast aluminum overall. These improvements can be further improved by the processes used to melt the aluminum. The use of an electric arc furnace to melt the aluminum rather than a combustion one can help minimize exposure to atmospheric gasses. The result of all these improvements is a cast aluminum plate that bears very little resemblance to porous and rather coarse aluminum castings.
Advantages of Cast Aluminum Plate
Cast aluminum plate is a strong aluminum plate that is precision engineered from its formulation to its casting and final machining. It is a dimensionally stable material specifically because it is cast rather than forged or rolled. Forging or rolling involves physical manipulation of the plate while it is in a semi-molten state: hammering it, extruding it through a form that shapes it, placing it under pressure, or squeezing it between two rollers. These processes induce internal stresses into the cast aluminum that actually help to hold it in the desired shape. The issue is that when material is removed, these internal forces can cause distortion in the plate, and, in very precise machining, this potential distortion can throw the resulting components out of specification.
The longstanding solution to this potential problem has been the MIC-6 aluminum plate (a 7000 Series alloy)—a cast aluminum plate that is cast, stress-relieved, and ground to its final dimensions. It has outstanding dimensional stability when machined, making it very desirable for a range of precision aluminum components. It also has outstanding properties of flatness making it as much loved as a tooling plate or a steady rest that holds materials in place during machining. It is also used as a mold plate due to its ability to deliver a truly flat surface.
Despite the advantages of MIC-6 vs. 6061 or other forged structural aluminum, it does have a major disadvantage in that it isn’t readily weldable, as compared to Alca 5, Max 5, and Alimex ACP 5080. This can be a serious drawback as if a tooling plate or mold is damaged there is no ready way to repair and a new one will need to be manufactured. It can also be inconsistent in how it accepts anodizing. This lack of weldability and inconsistency in electroplating is due to the alloy’s silicon content—a constituent component that is part of how it can be cast in the first place. The new range of 5000 series aluminum alloys does not share these issues when they are cast.
Casting 5000 Series Aluminum Alloys
The defining trait of aluminum grades is their main alloying agents. For 5000 series aluminum, this major alloying element is magnesium, on average anywhere from a little under 1% to as high as 6% of the alloy’s composition. Typically, for most alloys in the series, magnesium is around 1% – 3% of the mix. When it comes to the 5080 range of 5000 series aluminum from which all new cast aluminum formulations come, the proportion of magnesium in the blend takes a significant jump anywhere from 4% – 6% of the mix. The proportion of silicon in the blend drops sharply in these blends. It is quite likely that this significant increase in magnesium is part of why 5000 series aluminum cast so well.
The rise in the proportion of magnesium to silicon in these formulations is certainly why these new 5080 series cast plates are so much more weldable than MIC-6. They are also notably more friendly to anodization and other surface treatments than MIC-6, which is also likely due to their proportionally small silicon content. The fact that these new 5000 series aluminum cast metals have the traditional advantages of cast aluminum without the disadvantages opens up a lot of options in manufacturing and machining. Although, these are proprietary metals, and enjoying the full advantages requires a reliable supplier.
