Research On Synergistic Enhancement Of Pure Copper Mechanical Properties By Rolling And Annealing And The Mechanism

Due to the attracting properties,including ultrahigh electrical conductivity,outstanding thermal conductivity and excellent corrosion resistance,was used in a wide range of application prospects,such as electronic and mechanical industries.However,advanced technology and diversification of demand scenarios have led to increasingly demanding requirements for pure copper,with the need for strength-ductility synergy becoming more urgent.Studies have shown that the introduction of nanotwins into the microstructure of materia ls can improve strength-ductility.To do this,the microstructure of T2 pure copper has been modulated via rolling and annealing treatment to synergistically improve the strength-ductility trade-off of pure copper.Firstly,the initial-copper microstructure is modulated via annealing treatment to promote its strength and ductility.The experience showed that the ultimate tensile strength(UTS)and elongation after fracture(ε_f)were 276.9MPa and 12.4%,respectively,for the initial-copper with a small amount of recrystallization.Increasing annealing temperature or time,the recrystallization and annealing twins in initial-copper rise,leading to enhance ductility.For initial-copper,annealed at 150℃for 60 min resulted in UTS and elongation increased up to 309.1 MPa and 16.7%,respectively.The strength of copper was further improved by rolling at room temperature(RTR).97.0%of the microstructure of RTR-copper was deformed microstructure and the percentage of deformed twins was 0.27%with an average twin thickness of 9.2 nm in RTR-copper,thus increasing the UTS of pure copper to 418.7 MPa and theε_f to 7.3%.To release the energy stored in the rolling process and to improve the strength-ductility,RTR-copper was annealed to enhance the content of recrystallizations and twins in the microstructure and to improve the strength-ductility synergy.As a result of the annealing treatment,the deformed microstructure of the specimens decreased significantly and the low-angle grain boundaries(LAGBs)changed to high-angle grain boundaries(HAGBs),which promoted the formation and growth of recrystallizations and increased the content of annealed twins;as a result,the strength of RTR-copper decreased and its plasticity increased.The UTS of RTR-copper was 417.4 MPa after annealing at 100°C for 30 min and theε_f was 7.7%,which increased the UTS by 50.7%and decreased the elongation by 4.7%compared with the initial-copper.The stored energy during rolling at room temperature promotes the occurrence of recrystallization,while the liquid nitrogen environment inhibits the recrystallization process,which enhances the strength of the sample again,so the initial-copper was rolled at a liquid nitrogen environment(-196°C)with90%reduction.The microstructure of the specimens after cold-rolled(CR)was composed by deformed microstructure,recrystallizations and deformed twins,the grain size was reduced to 3.1μm,so the tensile strength value reached 501.3MPa.The CR-copper specimens were annealed to enhance plasticity,resulting to high synergistic strength-ductility copper.The service conditions of pure copper require it to be thermally stable,so high-temperature tensile tests were carried out at 100°C,150°C and 200°C for initial-copper,RTR-copper and CR-copper respectively.It was concluded that the initial-copper and rolled pure copper did not grow in recrystallization grains at the tensile temperatures of 100°C and 150°C,thus the strength did not fluctuate much compared to the room temperature tensile test,but the increased content of annealing twins contributed to the increased plasticity.Therefore,the initial and rolled copper has excellent thermal stability at 100°C and 150°C.In summary,rolling deformation and annealing treatments regulate the microstructure of pure copper,which in turn synergistically promotes the strength-ductility of pure copper.