How grain flow affects the design of a forged part

Aluminium leaves the mill with its internal structure already worked in one direction. Forging moves that structure around the shape of the part, so the fibre tends to follow the contour rather than run straight through it.

Why the designer needs to care

Where the alloy, heat treatment, geometry and process parameters suit it, aligning the grain with the primary load path can help a part’s behaviour under load and fatigue. That is a conditional statement rather than a guarantee: the result depends on all of those factors together, and on the loads the part actually sees in service.

What it means practically is that grain direction is something you can design for, in the same way you design a section or a radius.

Three decisions that follow from it

Where the parting line sits. The parting line is where the two die halves meet. Its position determines how the material has to flow to fill the cavity, and therefore where the grain ends up.

Draft angle. Enough draft lets the part release cleanly. Too little makes the tooling harder to run; too much adds stock you then machine away.

Machining allowance. Machining after forging cuts across whatever grain the forging produced. On a critical face it is worth asking how much stock is being removed and where.

What to ask a supplier

Ask where they intend to put the parting line, and how they expect the grain to run relative to the primary load. A shop that can discuss that with you is engaging with your part rather than just quoting a shape.