May 2021 Volume 3

MATERIALS

level than before the application of stress. This means the surface crystals of aluminum and alloying elements do not match up in size one-for-one with those in the core of the part. After a few seconds longer in the quench media and with the high conduction of heat within the part, the temperature is evening out. Eventually, the part and water are the same temperature and can be removed from the quenchant. See Figure 3 for how the stress state has changed.

Figure 4: The upper section of the surface has been removed by machining. The part is still restrained in a holding fixture. • A milling cutter removes surface from the top only. • The forging is restrained (held in a vice or vacuum chuck) during machining in this graphic. • Stresses inside are not yet balanced to zero during machining because of the restraint.

Figure 3: Stress state after metal and quenchant have equalized in temperature. • Deformed (yielded) surface is now at the same temperature as the undeformed core. • Surfacemust contract and coremust stretch to avoid separating. • Sumof tensile and compressive stresses adds to zero across full cross section. Now that the core of metal has cooled, it has also contracted. Because the outer surfaces cooled faster than the core and contracted to such an extent that the stress exceeded its yield strength and stretched, the crystal lattice in the core is smaller than the lattice at the surface. This means the larger crystals in the outer surface are pulling on the crystals in the core. Likewise, the inner core crystal lattice is pushing outward on the outer crystals. Therefore, after quenching and equalizing in temperature, the inner structure is now in tension (pulled) and the outer layer is in compression as shown by the springs. The outer layer in compression is much thinner than the core lattice which is in tension. Since the tension and compression stresses need to balance across the full cross-section, the compressive stresses are quite high at negative 20-25 ksi. The tension stresses in the core are on the order of positive 5-10 ksi but withmore volume to balance the higher but smaller amount of compressive surface stress. These differences in stress direction (positive versus negative) are what cause the distortion during machining. See Figures 4 and 5 for a graphic on howmachining operation redistributes these stresses.

Figure 5: The machined part is now released from its fixture. The stresses inside must balance across the remaining thickness, so the part moves to allow this redistribution to occur. The machinist sees this change as distortion because the cutter likely moved in a straight, flat tool path. • Upon release from the restraint, the internal residual stresses balance. •The part bows upward to allow the stresses to equalize.

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FIA MAGAZINE | MAY 2021