Aluminum alloys are widely used for both exterior paneling and interior parts like gears because of its relatively high strength compared to its weight. While it's possible to machine aluminum alloy parts with tolerances as tight as steel, the metal may not have the ductility or strength for the job after machining alone. Unfortunately, the thermal processing to improve its characteristics can also cause it to distort and warp. Maintain your tolerances while improving the structure of the aluminum by trying one of these four heat treating tricks.
Keeping the metal firmly pressed into a mold as it undergoes the quenching process goes a long way in preventing warping and distortion. This is accomplished through specially made presses that focus pressure on the points most likely to warp due to the stress of thermal processing. These presses are less expensive to create than full molds, and they work with a wide range of quenching methods, since different quenches create varying changes in the properties of the aluminum piece. Even delicate pieces like large-scale screw presses and spiraling gears cool more evenly when press quenched, despite the differences in exposed surface area in the teeth versus the body of the part.
No matter if the aluminum alloy part is pressed or not during quenching, it's essential that the cooling process proceeds as slowly as possible with this type of metal. Rapid quenching helps build a stronger composition of particles and results in higher strength, but it also creates serious internal stress that leads to warping and distortion of the tolerance of the piece. Options for relieving these stresses before they can distort the metal include
- Using high-tech quenching liquids with advanced temperature buffering characteristics, allowing the cool down to start slow and accelerate as the metal stabilizes
- Combining sub-zero conditions and blasts of heat in a technique known as uphill quenching, which is particularly powerful at relieving stresses from forging and other hot machining methods
- Trying re-aging heating procedures shortly after quenching, which gets rid of internal stress without having much effect on the mechanical properties like strength and ductility.
Sometimes the complexity in the bends and thickness changes of an aluminum alloy part makes it practically impossible to evenly cool the piece by quenching in a bath of liquid. In these cases, spraying the quenching liquid on the surface provides better control and more even results in regards to internal stress. Reducing stress this way can prevent distortion on pieces that would be difficult or impossible to machine again after heat treatment. However, spray quenching is complicated because the system needs to constantly monitor the heat throughout the piece and adjust the spraying accordingly to maintain even temperatures through the thickest and thinnest areas.
Finally, sometimes it's best to plan another machining run after heat treatments and quenching are all over with instead of aiming to get a finished product at that point. A finish machining pass can bring the piece back into tolerance without the need for advanced press dies and specialty quenching liquids or sprayers. Depending on what tempering method was chosen to give the aluminum extra characteristics, finish machining may not work. Carburizing in particular is incompatible with secondary surface work because it involves using heat to bond extra carbon to the surface alone, and machining would remove a lot of the carbon added during thermal processing.
This kind of secondary machining must be done quickly after heat treatment because the aging process, whether heated or at room temperature, greatly hardens many aluminum alloy mixtures. Machining before aging allows the piece to regain some of the positive effects from thermal processing it may have lost while being shaped yet again. Contact companies like Pacific Metallurgical Inc for more information.