August 2021 Volume 3

MATERIALS

Controlling Residual Stress in Aluminum Forgings By Mark Timko

All metals can be quenched in water or some cooling media to either control a phase change (as with carbon and alloy steels) or to hold a super saturated solute in solution at room temperature. Aluminum alloys fall into the latter category. The popular 2000, 6000, and 7000 series develop increased strength due to elevated levels of copper, magnesium, silicon, and/or zinc depending upon the alloy series. During quenching, the alloying elements are dissolved into the aluminum substrate. By cooling the metal quickly, the solute is held in suspension instead of precipitating out. During aging, the alloying elements or a compound phase can precipitate in a controlled amount and in a fine dispersion. When done correctly, these precipitates distort the lattice which impedes dislocation movement and thereby increases yield and ultimate tensile strength. The good news is this increases strength. The bad news is this same quenching which allows for increased strength also creates stresses within the crystal lattice that are non-uniformly distributed through the cross-section. As quenched, the outside of the forging is in compression and the inside is in tension. The sum of tension and compression across the cross section always adds up to zero but only when the part is not restrained. This non-uniformity of residual stress fromquench causes lots of problems. Machining the part with un-even removal of material from the surface and into the core will remove chips which had varying levels of tension or compression. Once released from the machining fixture, the part will then find a new shape to balance the stresses. The solution to minimizing residual stress is different for aluminum than for other common metals. For steel and titanium, the residual stress from quenching can be thermally stress relieved to reduce the difference between the tension and compressive forces which cause machining distortion. These materials happen to have tempering or aging temperatures as high or higher than the temperature needed to reduce the internal stress in the material. For more detail on residual stress, see the article in the May 2021 issue of FIA magazine titled, “ Where Does Non- UniformResidual Stress Come from in Aluminum? ” Because aging of aluminum is performed at very low temperatures and even room temperature for some alloy - temper combinations, the residual stresses cannot be relieved using an elevated temperature process without ruining the strength increase that is desired. Instead, the residual stresses need to be reduced by mechanically yielding the aluminum so that those areas in tension, such as the core of the forging, can yield slightly. This process is known as cold compression stress relief. When the forging is released from pressure (tonnage), the permanent set of the crystal lattice size in

the core matches those areas on the surface. Once this is done, the differential between the tension and compression areas by varying depth in the material is lower. With that lower stress differential, distortion from non-uniform metal removal does not cause as large of a shape change during machining compared to non-stress relieved aluminum. The ideal method of compression of aluminum beyond its yield point is in one compression stroke across the entire plan view area. This avoids problems with edge effect fromusing too small of a stress relieving die. If using a die that is smaller than the full plan view, the edges of the die where multiple blows overlap have intermittent high and then low amounts of residual stress along the part’s length. If the forging is large, smaller bites with proper overlapping are needed. While forging a large plan view with a small die is superior to not stress relieving at all, there will always be areas between the compression strokes with high residual stress. Therefore, the single- blow cold compression technique is best as it avoids the edge effect completely. The amount of force needed to overcome the compressive yield strength varies by alloy and how much natural aging has occurred before the cold compression is applied to the part. The tonnage needed to yield the aluminum during cold compression by alloy is shown.

Stress Relieve Force in tons/sq inch

Alloy 2219 2024 6061 7075 7050

20-25 40-50 12-15 25-30 25-35

The aluminum is cold or at room temperature and in the solution- treated (-W) temper at the time of stress relieve. This level of force means that a far larger press is needed to stress relieve the same part than was used to forge the aluminum part at an elevated temperature. For a typical medium sized 7050 -W part of 200 square inches in plan-view, a press of 6000 tons is required to permanently move the aluminum by 1-5%. Some parts used for many commercial aircraft are far larger than 200 sq inches and need up to 60,000 tons to accomplish this amount of cold compression. [See photo of a 60,000-ton press.]

FIA MAGAZINE | AUGUST 2021 44