How does cold-working make copper harder and stronger?

Cold-working increases the hardness and strength of copper through a process known as work hardening.

Although copper is naturally ductile and not particularly hard, physical deformation processes—such as rolling, drawing, or hammering—alter its internal structure. As these stresses are applied, the metal's crystals slip against one another along specific planes. This movement generates and moves line defects within the crystalline structure called dislocations.1

As deformation continues, these dislocations multiply and crisscross each other, creating complex tangles that act as obstacles to further slippage. Because these tangles inhibit the movement of the crystal planes, the metal becomes increasingly resistant to further deformation, effectively increasing its yield stress and overall hardness. This process results in the formation of elongated crystals while maintaining the same face-centred cubic structure found in softer, annealed copper. If desired, this increased hardness can be reversed through annealing, a heat-treating process that restores the metal's original softness.2134