Life is full of decisions. Tea or coffee? Pepsi or Coke? Sometimes the answer doesn’t matter that much. Other times it matters a great deal.
Take extruded aluminum tube for example. This comes in two forms: structural and seamless. Structural extruded tube is made by the porthole process, and for that reason is sometimes referred to as “porthole.” Seamless is made by the seamless extrusion process.
Generally speaking, seamless extruded tubes are a little more expensive, which might push you to specify structural. However, this is one of those situations where saving a few bucks could end up costing a lot more. Let’s take a look at when it’s okay to use porthole extruded tubes and when you should pay the extra for seamless.
Structural and seamless tubes are both extruded. That means a billet of material is forced through a hole in a die shaped to form the two-dimensional cross-section you need. (We’ve called it the “toothpaste process” in the past and compared it to squeezing icing through a shaped nozzle to decorate a cake.)
Sometimes you want that extruded material to have a hole down the center. The simplest example is a circular tube or pipe. (Read “Your Guide to Hollow Cylinders” for an explanation of the difference.) Porthole and seamless are different ways of forming that hollow internal section.
In porthole, metal is forced around a shape that matches the hollow section you want to extrude. Physics being what it is, the tooling needs ribs to hold that shape in place, and the metal has to flow around them. That separates the metal as it extrudes, so a second die forces those sections back together. That forms a longitudinal seam running the length of the extrusion.
As the name suggests, seamless extruded tubes avoid this. In this process, a forming mandrel is inserted into the billet of material from the rear and pushed through until it’s very close to the opening in the die. The material flows through the gap between the mandrel and die, emerging with both internal and external dimensions fixed and without any seams.
Seams Affect Microstructure and Properties
Looking at the seam through a microscope often reveals a different crystal structure to the aluminum elsewhere in the extrusion. This is a result of the grain distortion caused by the deformation and subsequent pressure that welds the separate pieces together. The extent of the difference depends on how much deformation and pressure the metal undergoes, which in turn is related to extrusion speed, temperature, pressure, and alloy series.
The net effect of this distortion is to create a weaker region along the seam. However, the size and impact of this weakness depend on profile shape and alloy.
Deciding Which to Use
Choosing between these two types of hollow section aluminum extrusion comes down to the application. Structural tubes are preferred in some and seamless work better in others. Here’s a summary of the strengths and weaknesses of each.
Structural Aluminum Tubing Pros:
- Very consistent wall thickness around the cross-sections
- Can be extruded faster than seamless (which brings down the price)
- When extruding small cross-sections it’s sometimes possible to fit multiple opening in a single die, increasing efficiency and lowering the cost per foot
Structural Aluminum Tubing Cons:
- Welded seams create a region of weakness where the tube could rupture if subjected to high internal pressure or significant bending
- Seams may be visible on the surface and will be highlighted by anodizing
- The seam is where corrosion is most likely to start, (although most grades of aluminum don’t corrode to any significant extent.)
Seamless Aluminum Tubing Pros:
- Higher-strength/resistance to deformation and internal pressure
- Uniform appearance when anodized
Seamless Aluminum Tubing Cons:
- Slower extrusion process (which can increase the price)
- Inferior concentricity of internal to the outer diameter
We can summarize the arguments this way: If the application involves internal pressure or the extrusion will be subjected to significant deformation, choose seamless. If neither of those apply, ask how concerned you are about appearance. If the answer is, a lot, seamless is probably the correct route. However, if the highest priority is keeping the cost down, pick structural.
Applications for Seamless and Structural Aluminum Extrusion
Seamless extruded tubes are used in applications that involve internal pressure. Hydraulic cylinders, pipes, and fittings are good examples. They are also used when the extrusion will be put through significant deformation. A good example of this bending electrical conduit or bus bars.
Another group of applications are those involving high loads that will deform the tubing or extrusion if it isn’t strong enough. Examples of this type include aircraft seat frames, baseball bats, and driveshafts.
You might also choose seamless when appearance is important. This is especially true if the extrusion will be anodized as that tends to highlight the seam.
Structural tubing is appropriate for all those applications that don’t apply high forces. Railings and window frames are examples, along with wall framing and other architectural and decorative uses. Frames for enclosures, material handling carts, and machine guards are more industrial examples. Many applications in trailers and RVs are well-served by structural tubing too.
An Important Decision
As structural tubing is less expensive it tends to be the default choice. Before specifying structural tubing though, remember to consider the application. It will work most of the time, but in some situations, it could be an expensive mistake. If you’re unsure which direction to go in, we’ll be happy to discuss your application with you.