During the cold-working process, copper screws accumulate significant residual stresses within the metal. These stresses arise from the forced deformation and dislocation accumulation of grains, directly impacting the material's mechanical properties. Annealing, by precisely controlling the heating temperature and cooling rate, provides copper atoms with sufficient energy for diffusion and rearrangement, gradually eliminating these internal stresses. When the copper screw is heated to a specific temperature, the atoms free themselves from the constraints imposed by the cold working process and migrate along grain boundaries and dislocations, gradually restoring the previously distorted crystal lattice to a stable state. This release of internal stress effectively reduces stress concentration points within the material, laying the foundation for subsequent improvements in ductility and shear strength, and preventing sudden fracture caused by excessive localized stress when the screw is subjected to load.
After cold working, the internal grain structure of the copper screw exhibits significant fibrosis and fragmentation. While this structure increases the material's hardness, it significantly reduces its plasticity and toughness. One of the core functions of annealing is to reshape the grain morphology through recrystallization. Under appropriate temperature conditions, the fragmented grains gradually form new nuclei and grow into uniform, fine, equiaxed grains. Equiaxed grains are more tightly bound together, and the grain boundaries are more regularly distributed. This microstructure provides a more balanced load-bearing space within the material, reducing the anisotropy caused by a single grain orientation and fundamentally improving the copper screw's deformation capacity.
The improvement in ductility is closely related to the material's internal plastic deformation mechanism, and annealing enhances the effectiveness of this mechanism by optimizing grain boundary properties. In cold-worked copper, numerous dislocations accumulate at the grain boundaries. These accumulated dislocations hinder atomic slip, making it difficult for the material to undergo significant plastic deformation under load. After annealing, the dislocation density is significantly reduced, and the grain boundaries become more distinct and resilient. When the copper screw is stretched or bent, atoms can slide more smoothly along the grain boundaries, enhancing the coordinated deformation ability between the grains. This allows the screw to withstand greater deformation before breaking, avoiding brittle fracture in the cold-worked state.
The improvement in shear strength is closely related to the uniformity of stress transmission within the material after annealing. In actual copper screw use, shear forces primarily act at the contact point between the thread and the base material. If the material contains numerous internal defects or stress concentration points, shear forces can easily accumulate in these weak areas, leading to premature localized material failure. Annealing eliminates internal defects such as microcracks and porosity, making the copper structure denser. Furthermore, recrystallized grains can more evenly transfer stress. When shear forces act, stress diffuses through grain boundaries to surrounding grains, preventing a single grain from bearing excessive loads, thereby improving the shear resistance of the entire screw structure.
Temperature control during annealing plays a crucial role in regulating the distribution of precipitated phases in the copper alloy, which directly affects the stability of shear strength. The brass or bronze alloys commonly used in copper screws contain alloying elements such as zinc and tin. After cold working, these elements can form unstable supersaturated solid solutions. During the annealing heating process, alloying elements precipitate in an orderly manner, forming uniformly distributed fine strengthening phases. These strengthening phases pin grain boundaries, preventing excessive sliding under load and enhancing the strength of the grains themselves. This strengthening effect, combined with good plasticity, enables copper screws to maintain a certain degree of deformation under shear forces while preventing them from breaking due to insufficient strength.
Annealing also improves the copper screw's processing properties, indirectly ensuring the consistency of the final product's mechanical properties. Cold-worked copper material has a high hardness, making it prone to cracking or burring during subsequent threading and forming processes. These processing defects can become stress concentration sources during use, reducing the actual shear strength. Annealed copper material has a moderate hardness and good toughness, making it easier to obtain a smooth surface and a complete thread structure during processing, reducing defects introduced by improper processing. This structural integrity ensures more uniform stress distribution under load, further enhancing the copper screw's resistance to shear failure.
Annealing's improvement in ductility and shear strength is not an isolated effect; rather, it achieves a synergistic enhancement of both properties through optimized microstructure. Cold-worked copper screws often face the dilemma of being "hard yet brittle": high hardness but poor ductility, and unstable shear strength due to their brittleness. Annealing eliminates internal stress, reshapes grains, and optimizes the distribution of precipitated phases, ensuring that copper maintains a certain level of strength while also imparting good plasticity. This combination of strength and toughness ensures that copper screws can withstand long-term shear loads without plastic deformation and failure, while also absorbing energy through appropriate deformation during unexpected overloads, preventing sudden fracture and meeting the demands of complex working conditions.
