Not too long ago we were getting lofty sounding engineering degrees, and knew nothing about how to actually make anything. What kind of metal should you use? Heat-treat? How do you hold a bearing? What’s the difference between grease and oil? What hack saw blade should be used? Where do you buy metal that isn’t in Home Depot or Lowes? We hope this forum will be a resource for budding engineers and others wanting some practical information, and maybe answer some of our own questions.

What is the difference, if any, in the tensile and shear strength of aluminum 6061-T6 bar stock, which, as I understand, has been rolled, and an extrusion of the same metal, but because of the need to retain the special shape of the extrusion die, has not been rolled? Or am I mistaken in my understanding of how aluminum bar stock has been formed?

I’m not an expert on this, but here’s what I’ve found out:

First, when you actually buy the material, the manufacturer or distributor will tell you the properties of what you’re buying. For instance, 80 / 20 extrusions (the “industrial erector set”, as they call it) have all sorts of tools for figuring out what size to use for a given load. The shape’s probably just as important as the shear and tensile strengths for those.

Secondly, I don’t think bar is necessarily rolled instead of extruded, or that you can infer any conclusions on its strength just because it was rolled or extruded.

“T6” actually refers to a heat treat process, not just a final state of a certain hardness. It means that the metal has been heated up, quenched (dumped in water to rapidly cool), and then artificially aged (kept above room temperature, 100-200C) to harden. So when you see T6, regardless of whether the metal was rolled or extruded, it’s also undergone the above heat treatment, which would likely erase the effects of the original manufacturing (I could be wrong on this). Interestingly, whereas the tempering process with steel makes it less hard, tempering aluminum actually hardens it.

An awesome article all about aluminum, it’s manufacture and heat treatment can be found at this bicycle site.

It may be true in general that rolled aluminum is harder and stronger than extruded, but there are so many variables, it’s probably hard to infer much without knowing other details about the manufacturing process. I suspect it may be possible to apply many different levels of T6 tempering.

I am also not an expert on Al alloys, but I believe that the T6 thermal process involves heating the metal to the point that all phases go into solution. The precipitations that form as a result of the subsequent artificial aging (giving the alloy it strength) are not then dependent on any ?cold? working that may have been done initially.

I am unclear as to whether standard 6061 billet shapes are formed (rolled/extruded) hot or cold. I assume that most forms other than plate are extruded.

speaking of aluminium… anyone know of a good place to get cheap 1/2” aluminium plate discards are fine also. I am in Detroit Mi so if anyone knows of a place in detroit even better…

I’m by no means an authority on the subject, but this is my current understanding on the topic.
6061 T6 Aluminum reffers first to a very specific allow “6061” and second to a specific heat treatment “T6”. The combination of the two produces very repeatable tensile and fatigue strengths in whatever form your aluminum happens to be in. IE two finish machined parts of the same alloy and heat treat regardless of the starting form will produce extremely similar results in tensile and fatigue strength testing. Hopefully that answers the first question.

Second, different manufacturing processes in some cases produce very different results in end strength, rigidity and fatigue life in a finished product. These are usually the difference between casting, billet machining, and forging being the most notable.
The process should fit the function and strength requirements of the finished product.
Typically casting being the weakest because of the alloys used in this process of manufacturing, not that its a “bad” or “cheap” process, but just its general mature and most widely used practices and materials, it is typically weaker. There are several examples of high strength production castings, commonly seen on modern motorcycles.
The next most common process in my opinion would be billet machining, a fancy way of saying that a final part was machined/carved/cut from a larger block of material, typically heat treated and high strength alloys such as 6061/4140/A2,S7 etc, Note here, “billet machining is not limited to any alloy, it can be 6-4 aircraft titanium, or cheap low strength 2024, or “no grade” cast aluminum blocks or sheets.
The final common process again in my opinion would be forging, nowadays nearly as common in aluminum manufacturing as it is in steel. Forging typically starts with a higher grade alloy because it is a more expensive and involved process than the rest so material expense becomes less of a concern compared to manufacturing costs. In forging components a bar/rod/block can go through one or many steps where a close rough shape similar to casting is acheived, the difference is in the forging process instead of being melted into a complete liquid and poured, the material in question is hammered or pressed into shape. This process increases the material density due to the pressures involved thus increasing strength as well as creating a “grain” for lack of a better term much like a piece of wood. Forged items tend to be the strongest and most fatigue resistant of all the aformentioned processes. After being forged into a very close but rough shape, the item is finished machined where necesary to achieve a precision component just the same as it is in the casting process.

Another common after the fact pocess used to achieve higher strength in steel components is “shot peening”. This is a process where thousands of steel “shot” much like small BB’s or blown through a special air gun to “peen” the surface of the material, this again gives a finer and denser surface grain in most steel alloys adding to the strength and fatigue resistance of the finished component.

General words on heat treating. Heat treat is as broad a topic as alloys, and processes in itself. In general terms though for steel, “hardening” is normally done through heating the steel to a set temperature and then rapidly cooling in a specific medium according to a set formula for the specific alloy your working with. “Tempering” Most times reffers to the general process of softening an alloy to a target hardness level. Steel is generally “hardened” to a maximum, or harder than necesary state, and then “drawn back”, “tempered” or “anealed” to achieve a specific finished material hardness/temper. “Temper” in many circles is used to reffer to more than simply hardness, including fatigue resistance and how brittle a finished part is.

And now that I’ve rambled for an extended length of time, one final word of warning on aluminum, while a fantastic material to work with there are a couple of commonly missed pitfalls. Most of the more inexpensive alloys of aluminum have a tendancy over time if they are used in an application that flexes to “work harden” and become brittle leading to eventual failure and breakage. 6061 T6 is not so prone to work hardening, but then again its already heat treated and hardened to a certain extent. 6061 is very difficult, read near impossible to bend into a shape without either cracking or sacrificing a large portion of its strength and resilience. Thing to remember, it can be bent into a shape, but the thicker the material, and the sharper the bend both increase the likelyhood of failure during or shortly after fabrication.

hopefully this helps someone, it certainly has plenty of information to look up, some of which I’m sure may be slightly off target.