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SECTOR FOCUS: QUENCHING FLUIDS


The application of Polymer Quenchants to control residual stress and distortion in aluminium


D. Scott MacKenzie, PhD, FASM


The heat treatment of aluminium (Solution Heat Treatment, Quenching and Aging) are critical processes to ensure that the desired mechanical and corrosion properties are achieved. Of these steps, quenching is perhaps the most critical of all the operations. If quenching is too fast, the desired properties may be met, but the material may suffer excessive distortion or stresses. This can result in shortened life, or in additional non-value added straightening of the component. This increases cost and cycle time.


The typical heat treatment for aluminium consists of solution heat treatment to approximately 525°C to insure that all solute is in solution. Parts are then typically quenched into water or polymer quenchants. Following quenching, parts are then straightened. If parts cannot be straightened immediately after quenching, the parts are placed into a sub-zero freezer (typically at -28°C) to prevent hardening due to natural aging. Once time is available, the parts are removed from the freezer and straightening of the parts is then performed. Parts are naturally aged for a period of time, depending on the alloy and the desired temper. Parts are then artificially aged at an elevated temperature (121°C to 176°C) to the desired


10 LUBE MAGAZINE NO.149 FEBRUARY 2019


final properties and temper. The process is often controlled by process specification, such as AMS 2770 Heat Treatment of Aluminium Alloy Parts [1]


.


The objective of quenching is to preserve as nearly intact as possible the super saturated solid solution formed at the solution heat treating temperature, by rapidly cooling to a lower temperature. Quenching is a balance of supersaturation and diffusion rate. As mentioned, if quenching is too fast, then properties are achieved, but distortion or warping of the parts may occur due to high thermal gradients. However, if quenching is too slow, then excessive grain boundary precipitation can occur. This removes the solute from aging, and has detrimental effects on corrosion properties. Generally, the highest strengths and corrosion resistance attainable are those associated with the fastest quenching rates. In general, the best quench rate is the slowest quench rate that just achieves properties.


Of all the possible “defects” occurring during the heat treatment of aluminium, distortion during quenching is the most common. It is probably responsible for most of the non-value-added work (straightening) and costs associated with aluminium heat-treating.


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