Heat treatment of aluminum (solution heat treatment, quenching and aging) is a critical process to ensure the desired mechanical and corrosion properties. Of these steps, quenching is probably the most critical of all operations. If the quench is too fast, the properties are satisfactory, but the part may have excessive deformation or residual stress. This can lead to shortened life due to residual stresses or to additional non-value-added straightening components.
Typical heat treatment of aluminum involves solution heat treatment at about 525°C to ensure that all solutes are in solution. Then, the parts are usually quenched into water or polymer quenchants. After quenching, the parts are straightened again. If the part cannot be straightened immediately after quenching, place the part in a sub-zero freezer (usually at -28°C) to prevent hardening due to natural aging . Once you've had enough time, take the parts out of the freezer and let them warm up to room temperature. The parts are then straightened. Parts are naturally aged, depending on the alloy and required temper.
The parts are then artificially aged at elevated temperatures (121°C to 176°C) to achieve the desired final properties and tempered. Quenching is a critical step in aluminum heat treatment. The purpose of quenching is to preserve the solid solution formed at the solution heat treatment temperature by rapidly cooling to room temperature. Quenching is a balance of supersaturation and diffusion rate. If the quench is too fast, performance is improved, but the part may be deformed or warped. If quenching is too slow, excessive grain boundary precipitation will occur. This eliminates solute aging, which can adversely affect corrosion performance.
In general, the highest strength and corrosion resistance are associated with very fast quenching rates. However, the amount of warping or deformation that occurs during quenching tends to increase as the cooling rate increases. In general, the best quench rate is the slowest quench rate to achieve performance.
Aluminum is extremely susceptible to deformation. During solution heat treatment, the temperature used is very close to the liquidus temperature. This results in very high ductility and low strength at typical solution heat treatment temperatures. In addition to poor strength at high temperatures, aluminum also has a large linear expansion coefficient. This results in massive aluminum growth during solution heat treatment and shrinkage during quenching.
If the part is constrained, the part experiences high strains and stresses. If these stresses exceed the yield strength at temperature, then permanent solidification of the part may occur, causing the part to deform. This shows that the location and constraints of the part are very important to control the deformation of the part. Residual stress before heat treatment Aluminum parts are usually heat treated before forging, casting, or forming. Each of these processes produces significant tensile or residual stress.
The machining of these forgings or castings also produces significant residual stresses. Using a stress relief process after the main fabrication steps will also reduce distortion during heat treatment. If the heating rate is too high, a thermal gradient will appear where the temperature exceeds the yield stress. This is especially true when there are very thick and very thin parts. The flakes will heat up rapidly, while the larger flakes will lag the temperature of the flakes. If the thermal gradient is high enough, the part will deform. If residual stresses from previous operations are present on the part, the removal of these stresses produces deformation.
The water immersion rate during quenching plays an important role in reducing the deformation of aluminum after quenching. Aluminum parts should be quickly immersed in the quenching liquid. This soak rate is often confused with quench delay; however, soak rate is the rate at which the part enters the quenchant. The immersion rate should be between 0.15 m/s and 3 m/s.
Water is a common quenching agent for all aluminum alloys. It is readily and inexpensively obtained, and it is easy to dispose of unless heavily contaminated. Water is used as a quenching medium from ambient temperature to 90°C for castings and thick forgings to reduce quenching thermal stress. As the temperature of the water increases, the stability of the gas phase increases and the onset of nucleate boiling in a stagnant fluid is suppressed. The maximum cooling rate is reduced, as is the total cooling rate.
This leads to uneven quenching of the part, resulting in distortion and high residual stresses. Polyalkylene Glycol Quenches (PAGs) were developed to provide quench rates intermediate between water and oil. By controlling agitation, temperature and concentration, quenching rates such as water can be obtained.
The importance of part positioning and shelving of bracket parts cannot be overemphasized. Due to the poor creep strength of aluminum, the part should be well supported and the load distributed over a large area. Parts, especially sheet metal parts, are usually held in place with steel or stainless steel wire to hold the part in place. This practice can increase distortion or cause damage to the part. Aluminum has a very high coefficient of thermal expansion, while steel has about half that (13x10-6mm/mm-°C for aluminum and 8x10-6mm/mm-°C for carbon steel). This means that aluminum components will grow twice as fast as steel bound wires. The wiring of the aluminum piece should be loose, allowing free expansion without restriction.
