Research On Short Preparation Process,Microstructure And Properties Of High Performance Cu-Ni-Si Alloy Strip For Lead Frame

Cu-Ni-Si alloy has become an important lead frame material because of its high strength,electrical conductivity,thermal conductivity and good processability.At present,the traditional manufacturing process of Cu-Ni-Si alloy strip for lead frame has many defects,such as surface cracks and scratches,inhomogeneous microstructure,disordered grain orientation,high work hardening rate,many times of intermediate annealing,insufficient aging precipitation,single precipitate size,insufficient dispersion distribution,easy growth of precipitation,long process and low comprehensive performance.In view of the above problems,this paper focuses on the research of short preparation process,microstructure and properties of high performance Cu-Ni-Si alloy strip for lead frame.In this paper,the Cu-Ni-Si alloy for the typical lead frame-C70250 copper alloy was used as the research object.The C70250 copper alloy strip with columnar grain structure parallel to the drawing direction were prepared by two-phase zone continuous casting(TZCC)technology,then the cold rolling and aging treatment were carried out.The microstructure of the samples was characterized by the TTRIII multifunctional X-ray diffractometer,OLYMPUS-BX53M optical microscope(OM),ZEISS AURIGA scanning electron microscope(SEM),JEM-2010 transmission electron microscope(TEM)and JEOL-2100 high resolution electron microscope(HRTEM).The mechanical and electrical properties of the samples were evaluated by MTS810 electronic universal tensile testing machine and AT515 DC resistance tester.The causes of surface defects of continuous casting C70250 copper alloy strip were studied and the corresponding control measures were put forward;The influence and mechanism of solidified grain morphology,orientation and precipitation on mechanical properties and electrical conductivity of continuous casting C70250 copper alloy strip were explored;The influence and mechanism of cold rolling reduction rate on microstructure,mechanical properties and electrical conductivity of C70250 copper alloy were revealed;The influences of single large deformation cold rolling-aging and double large deformation cold rolling-aging process on the grain structure,dislocation and precipitates of C70250 copper alloy strip were studied,the mechanism of their effect on the properties was also discussed.The above research results laied a theoretical foundation for the short process preparation of high performance Cu-Ni-Si alloy strip for lead frame.The main innovative achievements are as follows:The mechanism of the surface scratch and crack formation of continuous casting C70250 copper alloy strip was revealed,and a new two-phase zone continuous casting technology with high quality strip was developed.The two-phase zone continuous casting was introduced to prepare C70250 copper alloy strip with columnar grain structure parallel to the drawing direction.The causes of scratch and crack defects were analyzed,and the methods of elimination were proposed.The long-term existed problem of traditional cold mold continuous casting has been solved.The main reason for scratch formation is that the position of solid-liquid interface is too high,which leads to the long friction distance between the billet and the mold;The excessive width of the crack prone zone and the uneven temperature gradient at the front of the solid-liquid interface are the main reasons for the formation of cracks.By simultaneously reducing the position of solid-liquid interface,shortening the width of crack prone zone and improving the uniformity of temperature gradient at the front of solid-liquid interface,the surface scratch and crack defects of continuous casting strip can be eliminated.When the casting speed is constant and the mold temperature is increased,the position of the solid-liquid interface is gradually reduced,the width of the crack prone zone is shortened,and the uniformity of the temperature gradient at the front of the solid-liquid interface is improved;When the mold temperature remains constant between the solidus and liquidus,increasing the casting speed can further reduce the position of the solid-liquid interface,shorten the width of the crack prone zone,and improve the uniformity of the temperature gradient at the front of the solid-liquid interface.The mechanism of high tensile strength,elongation after fracture and electrical conductivity of two-phase zone continuous casting C70250 copper alloy strip was revealed.When the continuous casting speed is 15 mm/min and the mold temperature is 1070?,C70250 copper alloy strip with fine dispersed Ni2Si phase,low-energy small-angle grain boundary and strong[001]orientation columnar grain structure was prepared by two-phase zone continuous casting.The tensile strength,elongation after fracture and electrical conductivity of C70250 copper alloy strip were 328 MPa,40.4%and 24.3%IACS,respectively,which is significantly higher than the properties of C70250 copper alloy prepared by traditional cold mold continuous casting.Due to the strong pinning effect of Ni2Si phase,the dislocation of C70250 copper alloy sample with columnar grain is difficult to slip during the room temperature tensile deformation,which leads to the remarkable improvement of the alloy strength;The deformation between and inside grains of columnar grains with low-energy small-angle grain boundary and strong[001]orientation is more uniform,which is not easy to form high strain concentration,and the elongation after fracture of the strip is high.The precipitation of Ni2Si phase and the columnar grain structure parallel to the drawing direction reduce the electron scattering effect of Ni,Si atoms and transverse grain boundary,which is helpful to improve its electrical conductivity.The direct large deformation cold rolling process of C70250 copper alloy strip with columnar grain structure was developed.The mechanism of synchronous increase of tensile strength and electrical conductivity of cold rolled strip was clarified.The C70250 copper alloy strip with columnar grain structure was directly cold rolled without homogenization and solution treatment,when the accumulative deformation rate reaches 97.5%,the tensile strength and electrical conductivity of the alloy increase by 327 MPa and 0.6%IACS respectively.On the one hand,the shear deformation degree of C70250 copper alloy strip with columnar grain structure increases gradually,and a number of parallel shear bands composed of high-density dislocations are formed,which seriously hinders the slip of dislocations and leads to a substantial increase of tensile strength;On the other hand,a large number of parallel shear bands uniformly cut the matrix,and finally form fibrous microstructure.The effect of transverse grain boundary on electrical conductivity is greatly reduced,which leads to abnormal increase of electrical conductivity.A new compact preparation process of "two phase zone continuous casting-double large deformation cold rolling+aging" was developed to prepare C70250 copper alloy strip.The high performance C70250 copper alloy strip for lead frame was prepared,the mechanism of high tensile strength and high electrical conductivity of the alloy strip was revealed.The C70250 copper alloy strip prepared by two-phase zone continuous casting can be directly subjected to double large deformation cold rolling-aging treatment.When the alloy is primary cold rolled with 97.5%reduction rate,primary aged at 400?/30 min,second cold rolled with 60%reduction rate and second aged at 400?/45 min,the tensile strength and electrical conductivity of the alloy reach 879 MPa and 48.9%IACS respectively,owning excellent comprehensive properties.The incompatibility of high tensile strength and high electrical conductivity of traditional Cu-Ni-Si alloy strip for lead frame is solved.The Ni2Si phases formed by primary cold rolling-aging of C70250 copper alloy strip with columnar grain structure effectively pin the dislocations during secondary cold rolling,resulting in high density tangled dislocation bands,which promotes the further precipitation of Ni and Si atoms during the secondary aging process,the new Ni2Si phases with an average size of 5.1 nm precipitate again,and the average size of Ni2Si phase formed during primary aging grows to 14.2 nm,the transformation rate of precipitated phase is 50.6%,finally produces the fibrous microstructure with high density,multi-scale mixed and fine dispersed Ni2Si phases.The strength of the alloy is significantly improved by dislocation strengthening and Orowan strengthening.The recrystallization is delayed.The scattering effect of solute atoms and transverse grain boundaries on electrons is significantly reduced,and the alloy maintains high electrical conductivity.