Study On Microstructure Evolution And Properties Of CuNiSi Alloy In Continuous Extrusion

CuNiSi alloy has become an important material for lead frames due to its high strength,electrical and thermal conductivity,excellent machinability and corrosion resistance.At present,the hot rolling process has the disadvantage of long production flow,low yield and high energy consumption.The continuous extrusion process can overcome the disadvantages of the traditional process,but the deformation resistance of CuNiSi materials is high,and the regulation of metal flow during the continuous extrusion process needs to be studied.The influence of die structure on the deformation and the law of microstructure evolution during the continuous extrusion process have not been understood.In this work,the microstructures and properties were regulated during continuous extrusion.The hot deformation behavior of CuNiSi alloy was investigated by compression experiments,and the constitutive equation of hot deformation behavior was derived.The temperature field,stress-strain field and strain rate field of CuNiSi alloy during continuous extrusion were studied by numerical simulation,and the influence of different dies on each physical field was studied.The feasibility of continuous extrusion of CuNiSi alloy was confirmed.Finally,the microstructure evolution of CuNiSi alloy during continuous extrusion process was studied by experiments,and the microstructure and comprehensive properties of CuNiSi alloy strip were controlled by cold rolling aging processes,during which high performance CuNiSi alloy strip were obtained.From this work,the following conclusion can be drawn:（1）In the hot compression experiment,the strain rate range of 0.1410s^-1 at 4004500℃is the unstable zone,while the strain rate range of 0.141s^-1 at 5504600℃is the optimal zone.The linear regression of the stress peak was carried out to obtain the constitutive equation of the CuNiSi alloy material:（?）（2）Numerical simulation analysis of the bite process of continuous extrusion billet shows that the stress above the CuNiSi alloy rod material in contact with the compacting wheel can reach 460MPa.For different mold shapes（7×7mm,5×10mm,4×12.5mm）,the temperature distribution is basically the same in the stable extrusion stage.For 5×10mm scheme,the product flow rate is high and the high strain rate zone is small.（3）The microstructure evolution in the bite stage of strip experiment was investigated and the continuous extrusion stability stage was explored.In the cavity,the fiber flow line organization of upper,middle and lower routes are 40°,90°,55°,respectively.After the CuNiSi alloy flow through the die,the directions turn to be horizontal.The tensile of the strip product can reach 468MPa,which is 101%higher than the original billet.（4）Compared with billet,the hardness changes greatly in the aging experiments is more obvious,indicating that continuous extrusion deformation can promote the aging behavior.The aging condition of different temperatures are determined as 400℃aging for 8h,500℃aging for 1h,600℃aging for 10 min.（5）The hardness peak analysis of strip shows that,the tensile strength is 612MPa,after aging at 400℃f or 8h,the fracture dimples are shallow and the plasticity is poor.After aging at 500℃for 1h,the tensile strength is 632MPa,the recrystallization degree is the highest,and the mechanical properties are good.The diameter of fracture dimple is about 0.5μm.Aged at600℃f or 10min,the strength is 571MPa,but the plasticity is good.（6）The continuous extrusion CuNiSi alloy strip has been carried out in two subsequent processes:one is rolling at 60%reduction rate and aging at 500℃for 049hp the other is pre-aging at 500℃for 30min and rolling at 60%reduction rate and aging at 500℃for 049h.The results shows that the maximum hardness of the two process is 1.5h.During the experiment time of the two process routes,the conductivity increase with aging.60%rolling+500℃aging 1.5h and 500℃p re-aging for 30min+60%rolling+500℃agingfor 1.5h are the best.（7）After rolling at 60%reduction rate and aging at 500℃for 1.5h,the material has a microstrain is 2.153×10^-4,and dislocation density is 1.147×10¹³m^-2,tensile strength is683MPa and conductivity is 44%.TEM analysis shows that the dislocation are entangled by the second phase particles.After rolling at 500℃for 30min,60%reduction rate and 500℃for 1.5h,microstrain is 9.176×10^-4,and dislocation density is 2.33×10¹⁴m^-2.TEM analysis shows that a large number of recrystallized grains were observed,and precipitates are aggregated at the grain boundary,which hinders dislocation movement and the further improves the alloy strength.The tensile strength of this alloy can reach to 763MPa,and the conductivity can achieve 49.6%IACS.