Aluminum and steel are two of the most common metals used in engineering and manufacturing. In this article we’ll discuss their strengths and weaknesses to help you choose which is best for your project.
What is the Difference Between Aluminum and Steel?
Fundamentally, aluminum and steel are made up of different materials. Both are most often used in alloy form (a mixture of more than one material to achieve desired properties). Aluminum is sometimes used in its pure form, but rarely is that the best option. Steel doesn’t have a “pure” form, it’s already an alloy of iron and carbon in its most basic form..
Some of the most common alloys of aluminum are 2024, 5052, 6061 and 7075. Each of those have their own unique strengths and weaknesses (more detail about those here). If that wasn’t enough, you can further differentiate those alloys with different tempers such as -T6 and -H32, again to manipulate the properties of the material.
As many alloys as there are for aluminum, steel has many times more options. There are low carbon (often referred to as mild steel) and high carbon steels, there are high strength low alloy (HSLA) steels, there are spring steels, stainless steels and on and on.
Steel designations can be a little more confusing than aluminum because it’s common to refer to steels using standards from different organizations. For example the American Iron and Steel Institute (AISI) has standards like AISI 1008, and the American Society for Testing and Materials (ASTM) has standards like ASTM A36. While both of those are extremely common in America, other countries that produce steels have their own standards and designations.
To take it a step further, steel can also be split into categories depending on how it’s made or processed, for example there are hot rolled (HR), colled rolled (CR), hot rolled pickled and oiled (HRPO), etc.
Physical Properties and Characteristics
Each of the different grades and tempers of both steel and aluminum have their own set of unique properties like density, strength, etc. To see the specific values for any of the properties discussed here and more, check out the materials pages. Let’s get into some of the key differences when looking at steel vs aluminum.
Aluminum vs Steel Weight
Weight can be a tricky trait to compare because it depends on the material density AND the volume of the part. Density is easy, aluminum is nearly ⅓ the density of steel. That is basically true for all alloys of both materials (with some slight variation). For two parts that are exactly the same geometry, a steel part will weigh almost 3x as much.
An important factor to consider when comparing materials to optimize weight is a materials strength-to-weight ratio. Two materials may be similar in strength, but have different densities, or they may have similar densities but a difference in their strength.
Aluminum vs Steel Strength
There is a wide range of strengths across all the alloys of steel and aluminum, and there is some overlap between the two. There are steel alloys that are stronger than aluminum alloys and there are some aluminum alloys stronger than some steel alloys. That said, at the highest ends of the range, the strongest steels are much stronger than the strongest aluminum alloys.
Let’s look at an example, one of the most common aluminum alloys is 6061-T6, and one of the most common steels for structural applications is A36. Looking at their tensile strength values, they are fairly similar with A36 being a few percent higher, so depending on the application you may be able to use either one. Maybe to get even more strength (and probably stiffness) into your part you make it from aluminum that’s 10 percent thicker than if it were made from steel. In that case, a thicker aluminum part will be much lighter than a thinner steel part, even though the aluminum part is stronger. You could make a similar comparison using 7075 aluminum and 4130 steel, but moving up to those you’d also want to consider cost.
Along the same lines as weight and strength, a budget often factors in when designing parts. In general, mild steels (1008 and A36) are less expensive than lower grades of aluminum (5052 and 6061), but high strength steels (4130) are more expensive than higher strength aluminum (7075). Stainless steels will typically fall above aluminum but below higher strength steels in cost.
Speaking of stainless steels, let’s discuss corrosion resistance. One of the major weaknesses of steel is corrosion. Because steel is mostly iron, when exposed to oxygen and moisture steel will quickly start to turn into iron oxide or rust. Left unchecked, that corrosion will continue until all the steel is gone. This is where stainless steels like 304 stainless steel and 316 stainless steel come in. Of the enormous variety of steel alloys, some include ingredients (like chromium and nickel) to help improve corrosion resistance. Stainless steels have their trade-offs, like cost and sometimes strength, but they can often handle corrosion much better than non-stainless steels.
Aluminum may be non-ferrous (without iron), but that doesn’t mean it escapes oxidization. Aluminum does have a trick up its sleeve however. When aluminum is exposed to oxygen, it forms an oxide layer of aluminum oxide. Unlike iron oxide which is weak and flakes off, aluminum oxide is hard and acts like a layer of armor over the exposed aluminum. Because of this, aluminum typically survives much better than steel where corrosion is a concern.
If your project requires steel and you’re concerned about corrosion, zinc plating and powder coating are two great options for improving its corrosion resistance.
Aluminum vs Stainless Steel Durability
How durable a material is depends on the conditions in which the part is used. As we’ve discussed, aluminum and certain grades of stainless steel should be much more durable than steel where corrosion is a contributing factor. If abrasion or wear resistance is a big factor, that’s where aluminum struggles. Aluminum is a relatively soft metal (it can easily be cut with woodworking tools) and doesn’t handle wear as well as steel. Fatigue in aluminum parts also typically happens sooner than in steel.
Aluminum being a softer material isn’t always a disadvantage. Because of its malleability, aluminum is often used as fixturing, or work holding applications where it’s critical to being able to hold a part, but not scratch or damage it. Vice soft jaws are commonly made from aluminum. If you do need to improve the durability of aluminum, anodizing can be beneficial.
The thermal properties of steel and aluminum are another area where they differ. Neither are good thermal insulators, but the high thermal conductivity of aluminum makes it by far the most common material used in heat exchangers and heat sinks. Alternatively, aluminum has a much lower melting point than steel. Stainless steels typically have a higher heat resistance than regular steel.
Steel is magnetic, while aluminum is not. Some types of stainless steels are magnetic, and some are not. Some sources incorrectly claim that if a metal is magnetic, it must not be real stainless. The truth is that it depends on the grade. Some stainless steels are mildly magnetic and can change their level of magnetism based on their heat treatment.
Steel and aluminum can both be welded, though aluminum may require slightly more specialized equipment to weld. Aluminum is welded using an AC process, where steel is more commonly welded using DC. When MIG welding aluminum a special spool gun is used to pull the aluminum wire rather than push it, to prevent kinking the more malleable wire.
Aluminum vs Stainless Steel Applications
Below are some common applications and how steel vs aluminum would do in each. It’s important to realize that any part can be designed poorly and fail, regardless of the material selected.
|Structural Parts (Frames, Brackets, Beams)||Excellent||Excellent||Good|
|Fasteners (Bolts, Washers, Pins)||Excellent||Poor||Good(threads can gall easily)|
|Decorative Parts||Poor(unless coated)||Excellent||Excellent|
|Abrasive or High Wear Applications||Excellent||Poor||Very Good|
|Cooking and Food Processing Equipment||Poor||Excellent(should be coated)||Excellent|
|Cutting Edges (Knives, Lawnmower blades, etc.)||Excellent(depending on grade)||Poor||Excellent(depending on grade)|
|Heat Transfer (Radiators, Heat Sinks)||Poor||Excellent||Poor|
Environmental Impact of Aluminum vs Stainless Steel
Both aluminum and steel are virtually 100% recyclable. While mining, refining and recycling processes and technologies are constantly improving, it’s safe to say as of now that it’s more environmentally friendly (requires less energy) to recycle existing aluminum and steel products than to create new materials from raw ore. Both aluminum and steel are significantly more environmentally beneficial than plastics.
Choosing the Best Material for Your Project is Easy with SendCutSend
Which material you choose for your next project will depend on your requirements, what your parts need to do and what your budget is. In applications like aerospace, where weight and stiffness are primary drivers, aluminum is tough to beat. If you need a cutting edge on a tool, steel is your best bet.
SendCutSend offers a variety of grades of steels, stainless steels and aluminums. When it’s time to decide which material is best for your project, check out the detailed specifications on each of our materials pages.