August 2021 Volume 3

MATERIALS

to the die used to form the finished part will require tonnage to force fit the part. That action will yield the aluminum and that yielding lowers the residual stress by again matching the crystal lattice sizes between the core and the surface. This technique is not commonly used since most design engineers do not have design data for it. It happens to be a bit stronger than the Txx52 temper also. The technique only works on some geometries as mentioned where the perpendicular ribs can act across a fairly uniformweb width. There are a few other means of controlling residual stress in aluminum that work by preventing it. The most common is the use of glycol to cool the aluminum at a more consistent, but slower rate to room temperature. Residual stress can be reduced up to 60% compared to cold water quenching but there are limits to how thick of a forging can use this quenchant. When thicknesses go above 4”, many specifications prohibit the use of glycol due to loss of toughness and corrosion properties that are not normally measured. However, tensile and hardness minimums, which are tested on the material as quality control tests, can still be met. Therefore, the damage from the slow cooling of thick sections can degrade the long-term life of the part so use glycol with caution and thorough life prediction testing. In summary, control of distortion in aluminum is different than the technique used for steel or titanium. Thermal stress relieving processes do not work if the alloy is a 2000, 6000, or 7000 series aluminum where solution heat treatment to achieve high mechanical properties is required. For aluminum, the best technique for reducing residual stress is a slower quench rate using glycol and tension or compression beyond the yield strength of the alloy after solution heat treatment and before aging. This applied tension or compression is most readily and effectively applied using a single stress relieving blow from a large hydraulic press like the 60,000-ton press referenced previously. ■ Mark Timko Executive Technical Advisor Weber Metals, Inc. Email: marktimko@webermetals.com

There is one caution on cold compression stress relieving from a safety standpoint. You should not use a hammer or screw press for Txx52 stress relieving of aluminum. The problem is the very tight window between moving the material only 1-5% and not moving it at all. Applying pressure and not moving the material because it is within the elastic limit (below the yield point) happens often. Applying a slightly greater force of a few additional tons is then used and the part over-reduces. The amount of force is difficult to control because the displacement gauges on a screw press or hammer cannot resolve the tiny dimensional change between being just below the elastic limit and yielding the part a large amount. The flexure of the press and dies is similar inmagnitude to the displacement of the part being cold compressed. You can use stop blocks to mechanically limit the part’s compression deformation, but this practice requires careful engineering to prevent a block from squeezing out from the sides of the die and injuring someone. If the part is over stress relieved, re-solution heat treatment and another round of stress relieve can fix the over-stress problembut usually there is not enough oversize condition to allow for a re-heat treatment. If the forging does distort from quenching or from stress relieving, straightening before starting the age cycle is required. It is important that straightening is never performed after age unless approved by the final customer. One other technique for reducing residual stress is to stretch the part. This temper is known as the Txx54 temper condition. It is not used often but can be very effective if the part’s geometry provides ribs that are perpendicular to the long axis of the part. The aluminum contracts from forging temperature down to room temperaturemore than the steel die that was used to produce it. At room temperature, the aluminum is about 0.0013” per inch smaller than the part was when hot. The steel die is about 0.0005” per inch smaller at room temperature than when hot. Heat treatment and quench does not change the dimensions of aluminum since it is a single-phase material. Using the differential between the smaller forging to fit in

FIA MAGAZINE | AUGUST 2021 45