Building with metal is great — it’s strong, versatile, and looks cool. When you’re getting started on a metal project, the first step is determining what type of material will work best. For most projects, steel, stainless steel, aluminum, and copper have a good range of properties and are easily available at a reasonable price. Although many other alloys and metals exist (titanium, bronzes, pewter, etc.), they are usually used in specialized applications. It is easier to stick with one of the discussed alloy groups if you are a beginner.
METAL GEOMETRIES AND STRUCTURAL SHAPES
The type of metal used for a project is a key decision; the shape of the metal for structural purposes is also integral to your project. These are the most common geometries available, and each has its own advantages and disadvantages:
Flat bar:
Strong along flat axis only, easy to bend into curves along other axis. Simple to bolt to.
C-channel:
Retains ease of bolting that flat bar presents, with greater structural strength along other axis.
Angle iron:
Generally used to create flanges off of flat plate.
Box tube/round tube:
Structurally the best shape offered — 4 walls make these shapes strongest in any direction.
I-beam:
Steel-saving design for applications requiring high vertical loading with limited torsional loads for the structure’s weight (buildings, bridges, static structures).
Four Properties of Metals
Ductility/Formability:
The ability to draw metal into a wire, or form it into different shapes. Tools that bend metal exploit formability, and these operations will be harder/not possible with less formable metals.
Weldability:
The ease of welding a material. Easier to weld materials require less prep, and less expertise with a welder. Harder to weld materials require high welder dexterity, more advanced welding techniques, and perhaps special considerations like additional gas purging, preheating, or exotic weld gases.
Machinability:
The ability to cut a metal with a blade. Highly machinable metals can be cut at high speeds and with less expensive band saw blades, milling tools, or drill bits. Cutting tools that are based on friction (anytime sparks fly — abrasive cut off wheels, angle grinders) do not exploit machinability and can be used to cut less machinable metals.
Tensile strength:
The amount of force you need to exert to snap a metal. When newcomers are looking for a “stronger” metal, they usually mean they want a metal with a higher tensile strength.
Material Considerations:
Steel
+ Cheapest of metals
+ Right down the middle of almost all properties
+ Readily available in structural shapes
– Susceptible to rust, needs to be protected
– Harder to cut than aluminum — cannot use the same tools you would use for wood
Stainless steel
+ Most protected metal of the class — will not rust unless exposed to acid due to chromium or nickel content — forms a transparent oxide layer
– Heavy
– Difficult to fabricate and maintain stainless status — other metals affect corrosion resistance
– Warps easily on welding, and can blow off molybdenum causing rust
Aluminum
+ Most practical lightweight material
+ Oxide layer protects from rust as long as it is protected from other metals
+ Very easy to machine
+ Readily available in structural shapes
– Hard to weld for beginners
– More expensive to buy, fewer number of CNC technologies for plate cutting
Copper
+ Potential for beautiful patinas and aging
+ Inert to many other metals
+/– Easily formable, but work hardens quickly — need to use a torch to temper metal while hammering
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