May 2021 Volume 3

MATERIALS

Where Does Non-Uniform Residual Stress Come From in Aluminum? By Mark Timko

The residual stress in all metals occurs during the rapid cooling process from high temperature also known as quenching. Residual stress can also come from bending or twisting of the part after heat treatment. More on that in a future article. Any stresses from forming done prior to solution heat treatment are removed during the heating process. The part may move from non- uniform heating but once the part is at one temperature throughout its cross-section, the residual stress state is zero. See Figure 1.

forgings. Once the metal is immersed into the quenching media, the surface cools rapidly as shown in Figure 2.

Figure 2: Stress state of metal immediately after immersion into the quenchant. • Surface cools very rapidly at 100s of degrees per second. • Temperature difference between surface and core can be as much as 700 degrees for very thick parts. • Surface shrinks and yields. Figure 2 is the metal’s stress state a few seconds after immersion into the quenchant. As shown, the surface in contact with the water cools more quickly than the core which is conducting thermal energy towards the surface but not as quickly as the near surface. Because aluminum has a high amount of size change for a given temperature change (known as the coefficient of thermal expansion), the surface contracts but is held back by the still thermally hot and expanded core. This differential is worst when the cooling media is low temperature such as with a cold water quench. The springs above show expansion because the crystal lattice in the core is intimately bonded to the surface crystals. Each area is trying to achieve equilibrium. The hot core wants to expand but is restrained by the cold, contracted surface. The result is that the stress on the surface is temporarily in tension because it wants to contract but cannot because of the hot expanded core. In addition, the yield strength of the freshly cooled aluminum is very low because precipitation of the atoms/compounds has not had time to occur. The level of stress on the surface exceeds the yield strength of the surface and stretches the atomic spacing (plastic deformation). When metals are stretched beyond the yield point, they are longer and wider on a crystalline

Figure 1: Shown is a side view of a flat straight piece of metal. The springs represent the stress state of heated metal during stable heating within the furnace for the top surface, core, and bottom surface. • Part is very soft. • Residual stress is zero. • Aluminum part is expanded as much as 1.5% due to heat. The theoretical springs at the surfaces and in the core of Figure 1 represent the atomic spacing within the crystal lattice that makes up any metal. As shown, all springs are equally displaced. This means that the stress between the core and surface springs is the same. Therefore, little to no residual stress is present at this stage of the heat treatment phase. But after the atoms have arranged themselves in a dispersed manner from adequate time at temperature (for steel or titanium the phases are stable), the rapid cooling process begins by opening the furnace door and quenching the parts into water, glycol, oil, air, gas, salt, etc. For aircraft grades of aluminum, the quenching media is always water or a glycol/water mixture due to the low thermal mass and the need for rapid cooling to prevent solute from precipitating during cooling. Some softer aluminum alloys can use forced air cooling in thin cross-sections, but it is not common for

FIA MAGAZINE | MAY 2021 29