How Hot Is Too Hot For Aluminum?

Aluminum is an amazing metal with outstanding mechanical properties that make it an ideal choice for a variety of applications. One of the qualities that sets it apart from other materials is its thermal conductivity. Of all the commonly used metals, copper and aluminum have the greatest thermal conductivity, making aluminum a great option for tasks that involve regulating or moving heat.

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While certain aspects of aluminum tend to get all the attention, such as it’s high strength to weight ratio, excellent corrosion resistance and extreme formability, thermal conductivity often gets overlooked. With the ability to conduct heat much greater than stainless steel and other metals, aluminum has become a great option for manufacturers in many industries, including electronics, plastics and aerospace.

One of the questions we often get asked is how hot can aluminum get before it becomes a problem. People want to know how much heat can be applied to aluminum parts and machinery before the material fails. What these questions all come down to is two main principles: thermal conductivity and melting point; that’s what we’ll be discussing today.

How do we measure thermal conductivity?

What we mean when we talk about a material’s thermal conductivity is its ability to conduct heat. In scientific terms, this is specified as a number based on what is known as Fourier’s law, which states that the rate of heat transfer through a material is proportional to the negative gradient in the temperature and to the area, at right angles to that gradient, through which the heat flows. This is a complicated way of saying that thermal conductivity tells us how fast heat gets transferred through material. In general, the higher the number, the faster the heat transfer.

It’s also important to note that even with pure aluminum, the actual number varies depending on the amount of heat; calculating conductivity can be even more complicated with various alloys. You should never assume that the lab number for thermal conductivity is correct, as you will need to test your application in a variety of scenarios to be sure of how it handles various temperatures.

Let’s look at some real world examples. Styrofoam, which is often used as an insulating material, has very poor thermal conductivity. A Styrofoam cup is good for holding hot coffee because it does not allow the heat of the liquid to transfer to your hand holding the cup. On the other hand, a metal like aluminum has excellent thermal conductivity. That means that if you had an aluminum cup filled with extremely hot coffee, the cup itself would be hot to the touch and hard to hold onto.

A heat sink refers to a passive heat exchanger in which heat generated by an electronic or mechanical device is transferred to either the air or a liquid coolant, thereby preventing the device from overheating. A common use for heat sinks is in CPUs and GPUs, which tend to run hot and which can be damaged by excess heat. Aluminum is commonly found in such devices thanks to its thermal conductivity and light weight.

Another industrial application that benefits from aluminum’s high thermal conductivity is plastics processing. When melted plastic is solidified into a finished part through the injection molding or blow molding process, the curing time in the mold is dependent on the thermal conductivity of its material. Using aluminum instead of steel reduces the cycle time to make the part, increasing productivity and reducing valuable press/machine time.

What is aluminum’s melting point?

Of course, this thermal conductivity is only good to a point. If metal is heated too much it will start to deform, so it’s essential that you know your material’s melting point and how much heat it will need to withstand before using it in an application. There are other situations where knowing aluminum’s melting point is essential, such as when welding or heat treating an aluminum alloy.

What is aluminum’s melting point? If you look it up in a textbook the answer will be 1,221°F (660.3°C), but manufacturers almost never work with pure aluminum. Every alloy has a different melting point, and some are created specifically to thrive in high temperature environments. There are high strength aluminum alloys with Zn, Mg, Cu and Sc as alloying elements that have a melting point as high as 1275°F.

On the other hand, it’s necessary to realize that melting point is not the only factor that must be considered when trying to understand how a metal will handle high temperatures. For example, if you weld an aluminum work piece using 5356 aluminum alloy as weld rod, then the finished part would be highly susceptible to stress corrosion cracking at as little as 150 degrees. The same is true of 5183 and 5556 aluminum alloys. While the melting point might never be reached, you need to be aware that other problems can occur when some alloys are exposed to even moderately high temperatures.

Another major concern with high temperature applications of aluminum is the point at which the mechanical properties of the metal will be affected. Richly alloyed grades that have been strengthened by heat treatment processes will lose these higher mechanical properties when exposed to elevated temperatures. Exposure to excessive heat will temper back and weaken the heat treated metal.

If you have an application that will experience high heat, it’s important that you carefully test it in the prototyping stage, especially if durability is an important consideration. Selecting the right alloy that will perform correctly under your particular set of conditions is imperative to ensuring your bottom line. That’s why working with an experienced material provider can help save you both time and money.

Your Technical Resources Provider

Distinguishing between the many different and diverse aluminum alloys available on the market today isn’t easy. At Clinton Aluminum, we pride ourselves on working closely with our clients to match the right material to every application. Our goal is to be more than just a supplier but a full technical resources partner. We strive to help each of our customers to extract the maximum value from their purchasing decisions.

That’s made possible because our staff averages nearly 13 years of working for us. For this reason and others, Clinton has become the Midwest’s leading supplier of aluminum and stainless steel products. Contact us today to learn more about which aluminum alloy is right for you; we’ll help you answer the question of how much heat is too much heat.

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Modern metallurgy, and the alloys that it produces are truly amazing. An example can be found in a 1983 revision of the 1974 PACER proposal. Both propose to generate electrical power through a fusion reaction, specifically the repeated detonation of thermonuclear bombs in a closed chamber, and using the resulting heat to generate steam to turn a turbine. There are many issues with this proposal, but remarkably finding an alloy that is capable of resisting the 500°C—achieved through vapor cooling—heat while maintaining the strength to resist the blast pressure isn’t one of them. There is actually a range of suitable alloys including mundane 304 and 316 stainless steels.

The best metal for high heat applications though is a bit more than finding a metal that can resist high heats without melting. The metal will need to balance a range of unique factors specific to the application at hand in service to the desired result. For example, titanium might be the best material for the example given above because not only does it have the required heat resistance and strength it’s low density means there are fewer atomic nuclei to interface with all the neutrons being released by the repeated fusion explosions. Similar, although less extreme factors apply in industries like petrochemical refining, manufacturing, combustion engines for aerospace, and more mundane power generation among many others.

Factors to Consider in High Heat Applications

To many people the best metal for high heat applications are by default heat resistant superalloys, a range of nickel-based superalloys that have melting points that start at 1,200°C and range as high as 2,470°C. These are extreme levels of heat resistance. Although the heat is not the only consideration.

In addition to the temperature considerations there are also mechanical and chemical considerations when choosing the best metal for high heat applications. These are a direct result of the elevated operating temperatures of high heat applications. The environment creates the potential for:

• Thermal Expansion: Heat—with rare and limited exception—causes materials to expand. In many high heat applications this expansion can put pressure on the joints and materials.
• Thermal Fatigue: In addition to heat expansion when materials cool they contract, and while there are high heat applications that run constantly most do shut down periodically. This causes repeated stressing that can lead to distortion.
• Thermal/Heat Creep: Distortion over time can lead to creep which is a permanent alteration of the dimensions of an alloy or material due to heat load or thermal fatigue.
• Thermal Corrosion: Often chemicals behave differently at higher temperatures. This leads to circumstances like high temperature hydrogen attack and high temperature sulfidic corrosion which are not completely understood, and can occur with little warning or indication.
• Stress Rupture: Finally, and most seriously all of the above elements can lead to a final catastrophic failure of material. Splitting or cracking of a material that leads to total mechanical failure. Given that many high heat applications are maintaining high pressure on compressible fluids or gasses, stress rupture can be lethal.

The reason that heat resistant superalloys are so often first to mind for high heat applications is that these alloys have proven resistant not only to heat but these other secondary effects of the heat. However, in most applications the heat resistance of heat resistant superalloys is overkill. Petrochemical crackers that turn crude into fuels operate at 400 to 450°C, the steam in normal thermal electric plants is only 600°C and in typical nuclear plants runs from 540°C for the turbine steam to 950°C for fluids running directly from the core through a heat exchanger.

The demands of most high heat applications fall well short of the temperatures that really require the use of heat resistant superalloys. Given the expense of heat resistant superalloys, their use may not be necessary for all high heat applications.

The Types of Metal Used for High Heat Applications

Not all heat resistant alloys are superalloys. The need to manage high heat in industrial applications has been a long term issue, and metallurgy over the decades has developed an array of metals that are capable of dealing with incredibly high heat levels that fall short of the several thousand degrees of heat resistant alloy without succumbing to corrosion, creep, or fatigue.

The first of these are comparatively inexpensive chromium and chrome-molybdenum steel alloys, or as they more commonly known stainless steel and chrome-moly steels. Pure molybdenum takes things a step beyond either of these two metal alloys. The thermal profile of these metals can be seen in the table below.

Stainless Steels Metal Melting point Max Service Temp Coefficient Thermal Expansion 302 1,400-1420°C 870°C 18.7μm/m°C 304 1,400-1455°C 790°C 18.7 μm/m°C 316 1,370-1,400°C 870°C 17.5 μm/m°C Chromoly Steels AISI 4130 1,432°C 816°C — AISI 4140 1,416°C 538°C 12.2 μm/m°C Molybdenum 2,617°C 1,550°C —

These numbers are very comparable to what are traditionally thought of as heat resistant superalloys, and exotic or rare metals that are famous for their ability to resist heat. As shown in the table below.

Nickel Superalloys Hastelloy X 1,260-1355°C 1,095°C 16.3 μm/m°C Hastelloy S 1,335-1,380°C 982°C 13.7 μm/m°C Inconel 600 1,354-1,413°C 1,095°C 13.3 μm/m°C Inconel 625 1,290-1,350°C 982°C 15.1 μm/m°C Inconel 718 1,260-1,336°C 704°C 7.22 μm/m°C Exotic/Rare Metals Titanium 1,650-1,670°C 600°C 8.9 μm/m°C Tungsten 3,422°C 1,925-2,500°C 4.4 μm/m°C

THe biggest difference between the two classes of metals is that superalloys for the most part have a lower coefficient of thermal expansion. Indicating that the biggest risk in high heat applications isn’t the heat as much as the distortion from that heat, and the risk of catastrophic failure it raises. Similarly, although these metals are well known as heat resistant alloys they are better known as corrosion resistant alloys.

The Best Metal for High Heat Applications.

When it comes to deciding what the best metal for high heat applications is, the choice depends on the application. Only the most demandingly heat environments, those with intense heat and rapid heating then cooling cycles are superalloys or exotic metals really needed. Even then certain metal properties would deter use. Titanium, in addition to being heat resistant and pretty good at shielding against high energy radiation, is also a thermal insulator rather than a conductor.

So while the first two properties would seem to make it an excellent choice for something like a nuclear reactor vessel the fact that these can sometimes need rapid cooling without opening up the reactor makes it a far from ideal choice. Reactor vessels are instead made from chrome-moly steels due to their strength, ability to resist fatigue over time, and reasonable ability to absorb heat. The best metal for high heat applications can be a surprise, but there is no shortage of options.

Whatever the best metal for your high heat application is, Industrial Metal Service can help you find the answer. IMS is a supplier of precision sawn aluminum, stainless steel, superalloys, and titanium with new aluminum and remnant superalloy and exotic metals. Call us at (408) 294-2334 to find out what is in stock, or contact us to request a quote.

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