Why should aluminum alloy parts be heat treated? As we know, many aluminum castings meet the performance requirements under casting conditions and do not need further processing. However, in order to improve the properties and strength and ductility, aluminum castings and aluminum alloy parts are usually processed through a series of heating and cooling cycles called heat treatment. This heat treatment involves three basic operations: solution, quenching and aging. Solution treatment includes heating the casting to near eutectic temperature to dissolve eutectic components and form a solid homogeneous solution.
After this solution treatment, castings can be quenched or rapidly cooled, usually in boiling water, which helps to maintain a uniform solution at room temperature. The third step for heat treatment of aluminum castings is natural or artificial aging, which increases strength and hardness. The age hardening principle can also be used to customize the heat treatment for each application. The combination of these three heat treatments is called mild. The main purpose of heat treatment of aluminum castings is to develop the best combination of mechanical properties that can meet the key requirements of component applications.
Three basic thermal operations are usually combined into a heat treatment cycle that provides various properties. Although aluminum casting related books provide "typical" or "recommended" solutions, the quenching and aging time and temperature of each alloy and tempering, these heat treatment cycles are often variable and manipulated to change the mechanical properties of castings to meet the strength and ductility requirements of specific components. Recent research has included the use of fluidized beds to reach solution temperatures quickly and to provide faster heat treatment cycles.
The benefits of processing and heat treatment of aluminum castings and aluminum alloy parts include:
• Homogenization of alloy elements - this is an element uniformly distributed in the whole matrix, so the performance of the casting will be uniform;
• Stress relief - residual stress generated during high temperature casting and solution temperature cooling; Heating the casting to the middle temperature can reduce these residual stresses;
• Improved dimensional stability and machinability - changes in microstructure may cause castings to grow over time; Maintain tight dimensional tolerance during and after processing, and castings shall be subject to heat treatment to form stable precipitates;
• Mechanical property improvement - The greatest use of heat treatment is to enhance mechanical and corrosion properties by spheroidizing constituent phase particles and precipitation hardening.
Few of the required properties are optimized in a single casting. More often, heat treatment is a compromise that maximizes the use of other properties. For example, tensile strength and yield strength can be improved, but this results in lower elongation. Conversely, higher elongation results in lower tensile and yield strengths.
