Metal Fabrication Materials
Fabrication is about what we can make using steel, aluminum and,in some cases, copper.
This generic definition needs to take into account the wide variety of steel and aluminum alloys we are forced to work with in today's industry. Let's look at some of these alloys and how their different specs affect our daily work.
Steel: We tend to forget or not think about it, but steel (any steel) is an alloy made using Iron and other elements. Carbon being the most important element after Iron since it increases the alloys' strength (ability to resist deformation) and hardness (resistance to penetration). Carbon accounts for something in between 0.05% to 2.4% of the weight on the alloy. Not much really, right?.
Most steel alloys have less than 0.45% of carbon and those that have a higher percentage are both very hard and very brittle at the same time. Carbon reduces ductility on the steel, and ductility is essential for fabrication.
Higher % of Carbon makes steel harder & stronger but more brittle.
Other elements are melted together in the furnaces to create different steel alloys. Chromium, Silicon, Nickel , etc. each one provides different mechanical and chemical properties to each alloy. The temperature of the melt and the amount of oxygen blown into it have their impact on each steel specific properties
Strength = resistance to traction (or compression)
Hardness = resistance to penetration
Both increased by % of carbon in the alloy
Ductility = capacity to deform greatly before failing
Decreased by % of carbon in the alloy
Aluminum on the other hand is the third element on earth per quantity (thanks Wikipedia, did not know that).
The aluminum we use in fabrication is an alloy of natures' AL element plus Zinc, Silicon, Copper or other less typically used elements.
This tells us that Aluminum is a non-ferrous metal (there's no Iron in it) used because is light and has a good resistance to corrosion.
Lamination has it's part in complicating things in the fabrication process. Whether we bend along the grain or across the grain we will get two different resistances from the same sheet. This happens because when we bend along the grain we are putting pressure "between" grain filaments. On the other hand, when we bend across the grain we have the resistance of each of the grain filaments in the sheet.
Not going into detail of each steel alloy because this is not the place for it and I'm not a chemical expert on the matter. What I'd like us all to remember is that we deal with a man-made alloy of elements, mixed through a high temperature and extremely dangerous process.
Actually what goes through our hands everyday is a marvelous of mans' genius.
We must remember that every alloy has it's own properties and we should know them before working with it.
We should also remember that, as everything that is man-made, there's a tolerance on the information we get. Simply because there's a tolerance involved in the process of making steel or aluminum, laminating it, cutting it and getting it to our job-shops.
There are two basic properties that interest us on sheet-metal fabrication
Strength: The strength of a given material is its' capacity to absorb energy (compression or traction) before breaking.
Ductility: the capacity of a material to sustain plastic deformation while under tensile stress (traction) before fracture (or failure)
Metals have a great capacity in absorbing stress (force) while still being very ductile. This is actually why we use them in fabrication.
As the following diagram shows, steel can take a lot of stress before getting to its' yield point,
From the yield point on, while force keeps increasing, steel starts deforming but it does not fracture.
What happens before the yield point is actually called Elastic Deformation. Meaning that the material goes back to its' original shape once the stress is released. We can notice that steel takes a lot of stress (or energy) before reaching the yield point.
From the yield point until the fracture point what we see is called: Plastic Deformation.
Under this pressure the material suffers a deformation in its shape while still not completely fracturing.
The more ductile a material is, the more it will deform before cracking.
Fabrication is about deforming sheet-metal without cracking it and achieving a desired angle in the process.
Ultimate Tensile Strength (UTS) is the maximum stress or force a material can absorb while deforming but not fracturing
A material can be both very hard and very strong but we need to know if it's ductile enough for us to bend it.
The following test tells us not only the UTS of a material but also something about it's ductility. We need to mind carefully how the fracture happens on the tested material.
Steel, being ductile, has a substantial difference between the Yield Point and the UTS. The video below shows how the UTS is actually measured, using a traction stress test.
Keep in mind that the UTS is the piece of information we need in order to calculate how many "tons" we need to apply on our part
In today's Press Brake Fabrication we mostly use air bending method, which implies plastic deformation, but without reaching its limits.
Doing so, since we do not apply great force, we allow some fibers to still behave elastically. That's the reason why we have spring-back in the sheet.
Different materials = different UTS = different fabrication needs and results
While every supplier should provide a yield strength and a UTS for every different steel we purchase. Sometimes this information is lost or never reaches the operator in front of the press brake.
A simple Google search will save the day, provided we know what to look for (and now we know!!).
Knowing the UTS on the material we re fabricating will help us understand a couple things:
A) the force required to brake into that material given a certain V opening
B) the relation between strength, hardness and the radius obtained on our part.
Most common used steel UTS are:
Mild Steel (1020): 420 MPa
Stainless Steel (304): 700 MPa
Hardox 450: 1400 MPa
A36: 550 Mpa
Aluminum 5052: 230MPa
Aluminum T6: 310 MPa
From all the above we can easily understand the concept of elongation. We can actually see it on the video showing the UTS test.
The elongation of material indicates a % for how much the material elongates (deforming) before cracking. Elongation % would be a good way to indicate ductility.
Of course the outside part of the bend is the one being stressed under tension. which means it goes under elongation. Once the ductility ends, we see crack on the outside of our bend