| Precious metal alloy ultra-fine wires are widely used in high-precision resistors,sensors and potentiometers due to its corrosion resistance,oxidation resistance and resistance stability.However,the processing technology of precious metal alloy ultra-fine wire used in the high-advanced field in China is relatively backward,which can not achieve stable batch production.Some studies have shown that the processing properties of the ultra-fine wires of metal materials are closely related to the factors such as grain shape,size,casting defects,heat treatment and drawing process,in addition,the using property require higher electrical and mechanical properties of annealed state precious metal alloy ultra-fine wire.It is of great significance to study the effects of grain shape,size,casting defects,heat treatment and drawing process on the processing,electrical and mechanical properties with precious metal alloy ultra-fine wires.In this paper,Ag-28Cu-0.75 Ni alloy with different grain shape,size and micro-shrinkage is obtained by horizontal continuous casting,water-cooled copper mold casting and graphite mold casting;The effects of grain shape,size and micro-shrinkage on the process property,resistivity and mechanical property of ultra-fine wires are studied;In addition,the effects of heat treatment and drawing process on the processing,electrical and mechanical properties of Ag-28Cu-0.75 Ni alloy ultra-fine wires are studied,too.The main results are as follows:(1)The Ag-28Cu-0.75 Ni alloy ingot with columnar crystal grain size about 20?m×60?m is obtained by controlling the cooling direction of horizontal continuous casting;The Ag-28Cu-0.75 Ni alloy ingot with equiaxed crystal grain size about 10?m is obtained by water-cooled copper die casting of large super-cooling degree;Based on the slow cooling of graphite die casting,the Ag-28Cu-0.75 Ni alloy ingot with tri-crystal zone is obtained,which consist of 5?m surface fine crystal,columnar crystal with a length diameter ratio of about 3 and30?m core equiaxed crystal;After the same drawing process,Columnar crystal due to its lower dislocation density and micro-hardness increase rate,no cleavage step of as-cast fracture,has the higher axial tensile strength of as-cast,and shows the better ultra-fine wire processing property,its can realize the high-efficiency continuous processing of ultra-fine wires with diameter of0.025 mm and length over 1000 m,and the ultra-fine wire(diameter 0.1mm)has the higher surface finish,with an average surface fluctuation of 0.07?m.(2)Compared with the equiaxed crystal,the columnar crystal and tri-crystal Ag-28Cu-0.75 Ni alloy have larger grain size,After the same technology ultra-fine wires processing,the deformation rate is larger of single grain,the grain is easy to break and produce a large number of sub-crystal,and enhanced the scattering ability of electrons,which make the two kind of ultra-fine wires with processed state have higher resistivity,which are 4.16??·cm and4.08??·cm respectively;Due to the lower dislocation density and work hardening rate,the dislocation change,grain recovery and recrystallization are the lower in the process of annealing,which make the columnar crystal ultra-fine wire maintain the higher resistivity after twice continuous annealing,which is 3.68??·cm.The tensile strength of columnar crystal ultra-fine wire is as high as 1106.5MPa in processed state,the main reason is that during the drawing process,the processing texture and the work hardening can improve the tensile strength;As a results of the lower dislocation density and work hardening rate,after continuous annealing,the columnar crystal ultra-fine wire maintains higher tensile strength,which is 858.9MPa.(3)Due to the larger super-cooling degree,the nucleation rate of the alloy melt is increased and affected the filling ability of the melt,the Ag-28Cu-0.75 Ni alloy ingot with higher micro-shrinkage proportion is obtained;Due to the super-cooling degree of graphite casting is lower,the alloy melt keeps better filling ability during solidification,and the Ag-28Cu-0.75 Ni alloy ingot with lower micro-shrinkage proportion is obtained.Micro-shrinkage is easy to cause stress concentration and produce crack source,and the micro-shrinkage proportion too higher,to higher the probability of generating striation source,as a result,Ag-28Cu-0.75 Ni alloy with lower micro-shrinkage proportion have better ultra-fine wires processing property;Micro-shrinkage can block the continuity of the material,reduce the binding force,enhance the scattering of electrons,and the micro-shrinkage can not be eliminated in the later processing,as a result the higher electrical resistivity of both the processed state and annealed state ultra-fine wires with high micro-shrinkage proportion,which are 4.39??·cm and 3.77??·cm,the tensile strength is 1042.8MPa of processed state,and 807.8MPa of annealed state.(4)With the increase of heat treatment temperature and holding time,the entangled dislocation and residual stress release more complete,the grain recovery and recrystallizationmore thorough,make the micro-hardness decreased of Ag-28Cu-0.75 Ni alloy with the increase of heat treatment temperature and holding time;After drawing with large deformation rate of50mm/s,Ag-28Cu-0.75 Ni alloy is prone to dislocation proliferation and sub-crystal formation,due to the drawing process with large deformation rate plays a role of work hardening,make it has higher micro-hardness;It can be seen that reasonable heat treatment and drawing process can eliminate the work hardening phenomenon,which is beneficial to the ultra-fine wires processing of Ag-28Cu-0.75 Ni alloy.Due to the effect the dislocation,residual stress,grain recovery and recrystallization of heat treatment,after vacuum heat treatment at 450?/30 min and400?/60 min,the resistivity and tensile strength of ultra-fine wires decreased to 3.55??·cm,3.51??·cm,757.1MPa and 748.8MPa respectively. |
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