Grain Morphology
In ordering hand forgings, after the size of the block is discussed and specified, the next issue is grain orientation. Grain morphology is perhaps the second most important physical characteristic of a metal, after the basic alloy composition is established.
The word "grain" addresses the size and direction of the grains in relation to the block of aluminum. Metals are built up from an orderly arrangement of crystals — a basic characteristic of a metal is its ability to seek out like molecules and organize into a structure called "crystalline." This basic crystalline form makes up the building blocks for a larger structure we call "grains."
Making Grains
The whole process of making grains begins when liquid or molten metal is cooled enough to become solid. For very pure metals, their atoms — upon slow cooling from a liquid — prefer to assemble in an orderly solid structure, and if left undisturbed, would grow into one very large crystal grain as it slowly cooled.
In some electronic and laboratory uses, "big" crystals in metals are neither desired nor useful. It's the job of a metallurgist to properly devise and introduce controlled "disturbances" in the solidification process, breaking the grains up into the proper size. The next step is to direct the manufacturing (working) process to establish the basic directional structure of the grains — the second requirement, controlling the final grain alignment to match the drawing's call-out.
Mixing Metals
The initial key to controlling grain starts in the casting process. Producing aircraft aluminum alloys, for example, starts with pure aluminum and continues with the addition of other metals in the liquid state.
This initial mixing of different metals disturbs the solidification process enough that the grains already tend to form small "cells." This isn't foolproof, though — additional manufacturing steps are required in the casting process, such as the addition of grain refiners and controlled cooling rates.
The initial grain control in hand forgings starts with the applied forge stock. If you were to examine the cast stock, it would give the visual appearance that the grains are basically the size of little "bb's" squeezed tightly together, with no space between them. In the ingot, no matter which direction you look, the grains appear all the same size and don't have a significant preferred orientation — which is why you really can't describe grain orientation in cast stock with reference to the original cast form.
Refining the Grain
The subsequent process of hand forging transfers the un-aligned, random grain orientation of the cast stock into a controlled "wrought" structure with enhanced metallurgical properties. This wrought working operation "refines" the grains and transforms them into a structure that, through thermal processing, will exhibit high strength and toughness. The forging process involves controlled heating of the stock and, while hot, working it between the flat dies of a powerful forge press into the smaller size and dimensions required for the final product.
Care must be exercised in this process to prevent secondary growth of the grains, which may lower strength. An initial engineering decision by the customer selects the size and preferred orientation of the grains, reflecting the different load paths the finished part will be subjected to. Normally, the short transverse direction is the smallest dimension — though often, due to overriding engineering considerations based on load paths, that's not the case. It's the metallurgist's job to schedule the processing to orient the grains so they conform to the ordered part directions.
Naming the Grain Directions
The forging industry names grain directions differently depending on the shape of the forging.
Three Directions
Longitudinal, Long Transverse, and Short Transverse.
Two Directions
Longitudinal and Radial.
Three Directions
Longitudinal, Long Transverse, and Tangential.
For biscuits, it's wise to include the special testing directions that should be controlled or tested for lot release directly on the drawing.
How Forging Shapes the Grains
These direction names originate from how the stock was initially forged and predominantly worked. Remember, the grains were almost spherical to begin with — at the completion of the forge process, they take on a distinct shape reflecting how the stock was worked. For a rectangle, the grains in the L direction are typically elongated the most, the LT grains are next, and so on. Because of this, the mechanical properties — strength — differ for each grain direction.
You intuitively pick lumber for your home projects with grain direction in mind — the strongest direction is always the longitudinal grain.
With wood, the other grain directions result from how a particular board was cut from the tree. With aluminum hand forgings, that's not the case — the other directions are created based on hot working the forging to its final dimensions. From all this, the job of the hand forger is to take the originally random grain orientation of the cast stock and hot work it into an ordered block with the characteristics needed to meet the designer's specification.
The Standard Call-Out Order
For rectangular hand forgings, the grain direction call-out follows a consistent order:
(Thickness)
(Width)
(Length)