The improvement of the mechanical strength of aluminum alloy nuts relies on the precise control of microstructure through heat treatment processes. The core of this process lies in the synergistic effect of solution treatment, aging treatment, and annealing to optimize the distribution of alloying elements and the precipitation behavior of the second phase, thereby forming a high-density strengthening phase and eliminating internal defects.
Solution treatment is the primary step in improving the strength of aluminum alloy nuts. By heating the nut to a temperature above the solution line, the strengthening elements in the alloy (such as copper, magnesium, and zinc) fully dissolve in the aluminum matrix, forming a supersaturated solid solution. This process requires strict control of heating temperature and holding time: too low a temperature will lead to insufficient element dissolution, and residual second-phase particles will become crack initiation points; too high a temperature may cause overheating, resulting in localized melting of grain boundaries and significantly reducing the material's toughness. The holding time needs to be adjusted according to the nut thickness to ensure uniform element diffusion while avoiding grain coarsening. Rapid quenching is crucial for solution treatment. Cooling with water or a polyether solution "freezes" the supersaturated solid solution to room temperature, providing the microstructure basis for subsequent aging treatment.
Aging is a core step in strengthening aluminum alloy nuts, and it is divided into natural aging and artificial aging. Natural aging relies on the slow diffusion of solute atoms at room temperature to form nanoscale GP regions (copper atom clusters), gradually increasing strength, but the process is lengthy (usually several days to weeks). Artificial aging accelerates the precipitation of strengthening phases (such as θ' and η' phases) by heating to 120-200℃. Staged aging processes can further optimize performance: first, low-temperature pre-aging forms high-density GP regions, then high-temperature final aging promotes uniform precipitation, balancing strength and stress corrosion resistance. For example, using 120℃ pre-aging for 2 hours combined with 175℃ final aging for 8 hours can increase tensile strength by more than 20% while maintaining a reasonable elongation.
Annealing plays a supporting role in improving the strength of aluminum alloy nuts, mainly by eliminating work hardening and internal stress, creating conditions for subsequent strengthening treatments. Homogenization annealing is used for cast nuts. Through high-temperature, long-term holding (450-520℃, 15-20 hours), dendritic segregation is eliminated, ensuring a uniform distribution of elements such as copper and magnesium, reducing low-melting-point eutectic structures, and improving the material's hot workability. Intermediate annealing is used for nut blanks before cold heading. Holding at 320-350℃ for 1.5-2 hours eliminates work hardening caused by cold deformation, restores plasticity, and prevents cracking during heading. Finished product annealing adjusts the state according to requirements: high-temperature annealing (300-350℃) produces soft nuts, while low-temperature annealing (150-200℃) produces semi-hard nuts, balancing strength and assembly requirements.
The cooling rate control in the quenching process directly affects the final properties of the aluminum alloy nut. High-speed cooling (such as quenching at 5-15 m/min in an air-cushion furnace) can suppress the coarsening of strengthening phases, forming fine and dispersed precipitates and improving strength; however, excessively rapid cooling may lead to increased residual stress, causing nut deformation or cracking. For nuts with complex cross-sections (such as those with flanges or internal threads), differentiated cooling strategies are required: upper spray intensity should be greater than lower spray intensity to offset uneven cooling caused by gravity; or pre-cooling quenching technology can be used, pre-cooling in air for 30-60 seconds to lower the surface temperature to 350℃ before water cooling, reducing quenching warpage by 70%.
Precise control of heat treatment process parameters is crucial for improving the strength of aluminum alloy nuts. Furnace temperature uniformity must be controlled within ±3℃, using a zoned temperature-controlled resistance furnace to avoid localized overheating or overcooling. Thermocouple density must meet the requirement of at least one monitoring point per cubic meter for real-time temperature data feedback. Holding time error must be controlled within ±5%, with quenching delay monitored by an automatic recorder to ensure process repeatability. For 7-series aluminum alloy nuts (such as 7075), cryogenic treatment at -18℃ is required to further refine the grains and improve strength and fatigue resistance.
Through the synergistic effect of solution treatment, aging treatment, and annealing, the yield strength of aluminum alloy nuts can be increased by 200-300%, and fatigue life can be increased by 5-8 times. Modern intelligent heat treatment systems can achieve temperature control accuracy of ±1℃ and full data traceability, providing high-strength and high-reliability aluminum alloy nut products for aerospace, new energy vehicles, and other fields.
