Introduction to Forming

Let’s move away from casting and powder metallurgy and talk about metal forming.  The whole premise of metal forming is to basically take a piece of metal and beat the shape you want out of it.  This is exactly what metalsmiths do – they take iron or steel pieces and heat them enough that their soft, and then they can easily deform them into the shape they want with tools like hammers and anvils.  Of course, it is quite the skill to be able to do this.  And now we have machines that can do it better than we could, and faster.  This process is called forging.

Forging is just one of the metal forming processes.  Metal forming processes, in general, begin with billets or blanks of material (i.e. basically just rectangles of the material), and plastically deform the material into the desired shape.  If you’ve forgetting what plastic deformation is, then refresh your memory with this materials science page.  If you don’t want to skip to the link, just remember this: elastic deformation isn’t permanent; whatever deformation you’ve imparted will recover once the load is removed (it will spring back just like an elastic).  Plastic deformation is permanent: you’ve applied so much load that you’ve caused a rearrangement of the crystal structure, and the part will stay in the deformed shape once the load is removed.  To form parts in this way, the metal that we begin with must be ductile – it must be able to elongate without tearing and fracturing (kind of like silly putty, but obviously much less extreme).  Another note: anytime you can make a part using these methods, you could probably have done it by casting, or with powder metallurgy.  It really all comes down to cost, required material specs, dimensional tolerances, etc.  I also think it comes down to the who the company’s suppliers are, and what capabilities they have.  If you’re looking to make a new part, and you’ve used a certain supplier in the past that can cast a component, then you might just stick with that method again.  Then again, cost is king.

Hot or Cold?

Forging is the classic way of making something by heating it up and hitting it.  But also, we have presses capable of hitting pieces with such force that we don’t even need to heat the metal up anymore – also known as cold forging.  Cold forging requires more force, but the result can be better, especially in terms of surface finish and dimensional accuracy.  When things are hot they expand, and so forging something while it’s hot and allowing it to cool will result in some changes in dimensions.  Hot versus cold forging is a big distinction that you should know a little bit about.  Let’s put the advantages and disadvantages in a list:

Hot Forging – above the recrystallization temperature, to avoid any strain hardening.

Advantages

• less force required, metal flows more easily
• easier to forge irons and steels hot
• post-machining operations are easy (no work hardening)
• extra step of heating up the material required, costing time and money
• good starting point for heat treating the metal afterwards
• resulting part remains ductile (no work hardening)

Disadvantages

• lower dimensional accuracy/tolerances
• warpage during cooling
• increased reactions between atmosphere and hot part (oxidization)
• can end up with inconsistent grain structure

Cold forging – below the recrystallization point.  Preferred when the metal is already a bit soft.

Advantages

• little to no finishing work, so no extra steps required, saving time and money
• improved dimensional accuracy/tolerances
• better surface finish
• work hardening, strong part
• can impart nice directional properties

Disadvantages

• high quality, strong dies required
• powerful, expensive equipment required
• work hardening, less ductile
• residual stresses
• complex geometries might be difficult

Open and Closed-Die Forging

Somewhat surprisingly, there are a few different ways to forge parts.  It’s not so simple as just hitting a piece of metal into the shape you want.  Well it is, but there’s different ways to do it.  Two common method are closed die forging and open die forging, and they are fairly similar.  In both cases, two halves of the die come together, with extreme force.  The force makes the metal deform, and it’s forced to acquire the shape of the die.  You could do this yourself with something soft, like pizza dough or silly putty and your hands.  It’s more or less the same principle, but with vastly greater forces, obviously.  With closed die forging, the two halves come together and there is nowhere else for the material to go, so it assumes the shape of the die.  With open die forging, the material can spill out the sides, since the dies don’t fully enclose the material.  This is especially good for locally redistributing material – we’ll get to why that can be useful in the paragraph below.

Let’s talk more about the closed die forging process.  Usually the forging process occurs over several steps, so that less force is required in each step.  A drawback to this method is that it will require several intermediate dies between where you start and the final die, and so those dies need to be made and designed.  The starting piece of metal you work with has to be prepped a little bit as well.  You can’t just grab some jumbled mess of metal and keep hitting it until it resembles what you want.  Well, I suppose you could do that, but making connecting rods for car engines with this method might leave you with some very angry customers.  The metal used for closed-die forging is called a billet or blank.  Usually it’s some nicely prepared, uniform piece of metal, like an extruded bar (and we’ll get to extrusions below) or some cast piece.  Now, this is a generic shape that could be used for many forgings. What we may need to do is prep this piece a little bit more, so that it fits the final mold nicely and there is material in all the right places.  This can be done with an open die forging process.  Open die just means that the material isn’t completed confined by the die when it is struck – it can flow out the sides, between the two die halves.  Now, we can use an open-die forging process to kind of shape the material into how we want it.  For example, if the final component has a very thick section, we might need to build up some material over in that section so that there is enough later on.  Conversely, we might want to make some sections of the billet a little bit thinner.  This process of redistributing material in the billet is called fullering (redistributing material away from a certain area, or making it thinner) and edging (moving material towards some section).  It’s kind of intuitive.  Think of play with clay – depending on what shape you’re making, sometimes you’ll gather material by pushing toward some common area, or spread it out and make it thinner.  Same idea.  Now after the billet is prepared and subjected to this preforming stage, the rest of the process is usually automated, as it is transferred from die to die, until the final shape is completed.  Closed die forming is a bit more complicated than open die forging, because you need to design the final die shape and the intermediate dies as well.  More engineering work is required, you can’t just hit the part into the shape you want with a hammer or ram.  But although the starting costs are higher, it becomes cheaper to produce each part, if you’re making a lot of them, because it’s generally quicker and a more efficient process.  It’s favoured by something like the automotive industry.  Many side door hinges are closed-die forged (especially on german cars – it’s a little bit more expensive, but offers better performance than the stamped metal alternatives that you’ll see on many cheaper cars) because production volumes are high, often in the hundreds of thousands of parts.

Now, we can get into some of the benefits of open die forging versus closed die.  Generally speaking, open die forging is more suited to one-off pieces or customized pieces.  I mean,it could be used for high production runs, but generally it’s more of a skill – the operator continues to rotate and move the piece and it gets struck in just the right way.  This is how a significant amount of traditional metal working was done – like making swords, for example.  Anytime an anvil and a hammer is involved, it’s an open-die forging process.  With closed-die forging, you get the exact same shape consistently – the shape of the die.  The open die forging process can generally be better for the microstructure of the material, since you’re more easing the piece into the shape you want, and so you can end up with less voids, and a better microstructure – including smaller grains, and therefore, greater strength.  This isn’t to say this is a hard and fast rule.

With or without Flash

Now there’s a couple variations on the hot forging process that are worth mentioning.  The process above, where the metal is forced into a confined space, is called cold forging without a flash.  Conversely, you can forge with  ‘flash’.  If you forge with flash, basically the die isn’t completely closed – there is a thin region where the metal can flow outside of the die.  Since this region is thin, it cools quickly and solidifies, and acts as a barrier to prevent the rest of the material from flowing out of the die – forcing it to take the desired shape.  Now, what are the advantages of forging with flash versus without flash?  An immediate benefit of the flashless process is that you don’t need any subsequent machining operation to remove the flash – which saves cost – and you use less material as well, because the flash can end up being a significant portion of your starting material.  Naturally, flashless forging is a little bit more difficult.  The die can be a bit trickier to make, and you generally need to be more aware of how the material is flowing, and so things like the placement of the workpiece and the lubrication become important.

General Advantages of Forging

The general advantages of forgings, whether it’s hot, cold, open, or closed?  You can make a part that’s higher strength than the equivalent cast part, or machined part – especially true when cold forging is done, because of the work hardening.  The major disadvantage is the cost of the equipment – the dies, and the large forces required means you need some pretty impressive equipment.  You might also even need special buildings with proper foundations, because of the forces generated.  So the large capital expenditure is definitely a consideration.  Again, it comes down cost mostly.  If the production volume is high, forging might well be a good manufacturing solution.

Design for Forging (or Forgeability)

Just as we talked about castability (or design for casting, or design for manufacturing), we can discuss forgeability.  If you design some part which you would like to be forged, but you don’t actually consider the forging process, it is likely that the supplier will either a) laugh at you or b) politely tell you that the design in it’s current state cannot be forged.  This will require more time to go back and correct the design so that it can actually be forged.  Technically speaking, we need to consider the metal flow within the die, the friction and the heat generated and the interfaces between the die and the material, and the shape of the part.  If the material doesn’t flow properly, you’ll end up with external or internal (particularly insidious) defects, which no one will appreciate.  If the part shape is fairly boxy, then closed die forging will probably be a good option.  If there are long parts with thin sections, you might have more trouble forging it.  if It’s hot, you need to consider temperature gradients and how the material will cool and potentially prevent good flow.  If the part is complex, you’ll probably need a few intermediate dies to gradually form the shape.  Aluminums are easier to forge than steels.  You’ll need to consider future machining operations, so leave enough material in areas you might want to machine.  Remember your draft angles, so that you can actually remove the part from the die.  Never have completely sharp corners, because a) it’s terrible design since it adds a stress concentration, and b) material will have a difficult time flowing around a sharp corner.