When selecting a material for a metal fabrication project, it’s important to understand the difference between an aluminum tube and a pipe. Using the incorrect term will have cost implications and almost certainly affect performance.
In this blog, we’ll explain why there are two words for what are essentially hollow cylinders, and why it’s important to know the difference. We’ll also focus on aluminum rather than any other metals used for making tubes and pipes, because it’s one we work with extensively, and where we see the most confusion.
Tubes Vs. Pipes
Think of plumbing, and you’re thinking of pipes. Pipes are hollow cylinders through which fluids flow. Tubes are also hollow in cross-section, and while they could carry fluids if desired, that’s not what they are engineered for.
In general, as the function of pipes is to transport liquids and solids, the most important consideration when designing a piping system is the flow rate. This depends on the internal cross-section, (plus other factors). However, achieving a target flow demands the bore of the pipe be exactly as given in the specifications. It’s also very important that the bore is consistent from length to length.
Additional considerations for pipe are that it must not leak, and for many applications, it should resist high internal pressures. Achieving these goals requires a high level of structural integrity.
Tubes are also hollow cylinders, although tubing can also be square or rectangular in cross-section. Tubing is used extensively in fabrication for its load-carrying capabilities. It’s cut and welded into bracing, supports, and frames of many types.
With tubing, the dimensions of the internal space matter less than the external dimensions. These are important because cut lengths of tubes often need to fit within a defined space: excessive variation can make this difficult.
Readers with an electrical background may wonder where conduit fits in this classification. It is after all a form of a hollow cylinder and can be formed from aluminum. While there’s nothing firm, the consensus is that larger diameter, thicker wall conduit is a form of pipe, and thin wall conduit used indoors is the tube.
Pipe and Tube Manufacturing
There are two principal methods of making both pipe and tube: welded seam and extrusion. Here’s an overview of each.
Welded Seam
The raw material is supplied as a coil of strips. The strip is fed out and the sides progressively rolled up into a tube until the two edges meet. This seam is welded to form a hollow cylinder, which is subsequently cut into lengths.
To produce a non-round tube, the circular form is passed through rollers that push it into the square or rectangular shape needed.
Extrusion
This is where solid material is forced through a die that creates the shape required. (It’s essentially the same process as squeezing toothpaste from a tube or making pasta shapes.)
The complication with extrusion is that the die forms a solid piece with the desired external shape. This leaves the problem of making the inside hollow.
There are two ways of doing this. One is to hold a mandrel in the center as the material is pushed out through the die. The mandrel is located by ribs, which separate the extruded material into two halves. These are then pushed back together as the material leaves the mandrel to form the required hollow cylinder.
The alternative is to make a hole by piercing the material in the center and stretching it over a mandrel.
Process Strengths and Limitations
The welded seam method is the lowest cost, but the joint is a potential weak point. Consequently, while the tube is often made this way it’s not the preferred route for the pipe. However, welded seam pipe can be “drawn over mandrel” (DOM) as a secondary operation. This improves the dimensional consistency of the bore and strengthens the joint.
The highest quality of pipe is obtained by extrusion and piercing. This creates a “seamless” pipe with an accurate bore and high integrity, but the process is more expensive than the welded seam route.
Producing Hollow Cylinders From Aluminum
Aluminum is a popular material choice for many metal fabrication projects. It’s much lighter than steel, but, depending on the series selected, its strength can approach that of steel. This high specific strength makes it a good material for transport applications like trailer components, where less mass means more payload and/or lower fuel costs.
Unlike non-stainless grades of steel, aluminum is naturally corrosion-resistant and, depending on the details of the application, may not need coating. When coating is needed, the anodizing process forms a hard oxide surface that can be colored.
From the pipe and tube manufacturing perspective, the attractive feature of aluminum is its ductility. This makes extrusion possible at loads far less than steel would need. (Steel generally has to be softened by heating, which introduces other complications.)
Most aluminum tube and pipe is extruded from either 6061 or 6063 grade. These tend not to harden, so the extrusion equipment can run faster. The former is significantly stronger, but the latter looks better thanks to a finer grain structure and anodizes well when a colored finish is needed.
Implications for Your Metal Fabrication Project
While we take on work that needs pipe, it’s relatively uncommon. However, we do use a lot of tubes. For the highest strength, and when weight is less of a concern, this is usually welded steel. But for weight-sensitive applications, and those where appearance and corrosion resistance are important, aluminum tube is often a good choice.
As mentioned, aluminum is readily extruded into hollow cylinders, but it can also be formed into a limitless range of other shapes. When the shape isn’t fully closed it’s just called “extrusion”, or sometimes “section”. Aluminum extrusion comes in many profiles, and as the tooling is relatively inexpensive, custom shapes can be quite affordable.
Many fabrications need welding, and this is something to consider during design. Welding aluminum takes a little more skill and experience than steel, but can be done successfully. Of the two grades of aluminum mentioned previously, 6063 is the more weldable, in part because 6061 loses its temper and hardness during welding.
Partner With Fabrication Experts
If you’re designing your fabrication, think carefully whether it needs a pipe or tube. (Probably tube, unless those hollow cylinders are intended to carry a fluid.) Alternatively, if we’re doing the design work for you, the hollow cylinders will be tube, unless the application dictates otherwise.
The tube can be formed from various metals, but there are advantages to using aluminum: it saves weight and adds corrosion resistance, (which can avoid the need to paint or powder coat). It can also be welded, provided the welder has a degree of skill and experience. (Ours are AWS-certified)
If you need more advice on the question of pipe versus tube for hollow cylinders, please contact us. We’ve been in the fabrication business a long time, there’s little we haven’t seen before, and our focus is on ensuring our customers get exactly what they need.