Why a hammered car part beats a cast one by a fifth
Squeeze or pound metal into shape instead of pouring it into a mould, and it comes out tougher. Forged parts typically have about 20 percent more strength for their weight than cast or machined parts of the same material, which is why engines, tools and heavy machinery still depend on a craft blacksmiths practised for millennia.
Forging shapes metal with concentrated compressive force from a hammer or a die. For most of history that meant a smith, a hammer and an anvil turning out pots, hand tools, blades, cymbals and jewellery. In the 12th century water power began driving large trip hammers, allowing much bigger pieces of iron to be worked. Today presses and power hammers driven by compressed air, electricity or hydraulics produce forgings weighing anything from under a kilogram to hundreds of tonnes.
Temperature defines the method. Heat metal above its recrystallisation point and you are hot forging, which is quicker and more precise because new crystals keep forming and cancel out the hardening that deformation causes. Below about 30 percent of that temperature, usually room temperature, cold forging leaves the metal work-hardened. Warm forging sits between the two. Every operation either draws metal out, making it longer and thinner, upsets it, making it shorter and fatter, or squeezes it into a closed shape.
Open-die or smith forging leaves the workpiece free to spread except where the tools touch it, so the operator must keep turning and positioning it. It suits very large items and short runs, and can align the metal's grain for extra strength in a chosen direction. Closed-die forging hammers metal into cavities shaped like the finished part, with contact lasting only milliseconds per blow. Surplus metal squirts out as a rim called flash, which cools and stiffens quickly, helping force the rest into every corner. Flash can waste 20 to 45 percent of the starting metal, so some shops use fully enclosed dies to avoid it.
Dies are costly to design and cut from tough tool steel, but each extra part is cheap, which is why car and tool makers favour the method for big production runs. Press forging trades the hammer's instant blow for slow, steady pressure over seconds, deforming the whole workpiece rather than just its surface, and letting engineers control the internal strain.
Source: Forging
