Study On Drawing Deformation Mechanism,microstructure And Properties Of Platinum-coated Nickel Composites

Thin-film platinum resistance temperature sensors have the advantages of small size,fast response,good vibration resistance,good stability,accurate temperature measurement.They has been widely used for temperature measurement and control in automobile manufacturing,electromechanical,aerospace and other fields,and have become a research hotspot in recent years.Lead line as the key material of thin film platinum resistance component,its performance is directly related to the service life,stability and reliability of thin film platinum resistance temperature sensor.At present,pure platinum wire or platinum nickel-plated wire are generally used as lead materials.There are problems such as the high price of precious metals or short service life.In this paper,cladding drawing method was proposed to prepare low-cost and high-quality platinum-cladding nickel wire.At present,the key scientific problems such as the coordinated deformation mechanism in the deformation process,the interface growth law in the heat treatment process and the failure mechanism in high temperature service are not clear.For this reason,the finite element simulation method to simulate the drawing behaviour of platinum-cladding nickel composite rods prepared by cladding drawing method to obtains better drawing process parameters.Then platinum-cladding nickel wires were prepared under the guidance of the simulation results,and the analysis of the coordinated deformation mechanism,the interfacial growth law and the failure mechanism of high-temperature service were carried out to provide theoretical guidance to improve the quality and performance of platinum-cladding nickel composite wire.The main contents and results are as follows:（1）The finite element simulations of deformation behavior were carried out.the optimal parameter combinations of the first-step drawing were semi-angle of drawing dieα=3°and the original platinum tube D=7.55 mm;the optimal parameter combinations of the drawing after combining were semi-angle of drawing dieα=3°and single pass deformation amount of 10.4%.Under this condition,The platinum layer and nickel rod achieve better coordinated deformation,which is beneficial to obtain higher quality platinum-coated nickel composite rod.（2）The influence of the deformation amount and semi-angle of drawing die on the deformation behaviour is revealed.The equivalent force and equivalent strain on the nickel core and platinum layer increase with the increase of the deformation amount.the platinum layer deforms and needs to consume the deformation work,and the remaining pressure can be smoothly transmitted into the nickel core,so that both of them can reach the yield deformation together to achieve a coordinated deformation.During the drawing process,due to the difference in the flow of metal between the platinum layer and the nickel core,the platinum layer piles up at the inlet of the die,and with the increase in the pile-up amount,the piled-up platinum is pressed again into the nickel core,forming a wavy interface.The evolution of the interface went through three stages of formation,intensification and flattening.（3）The coordinated deformation mechanism in the deformation process was explored.During the drawing process,the platinum-clad nickel composite interface is clear,the thickness is uniform,and there is no fracture phenomenon,the platinum layer,the interface and the nickel core are transformed into an axial fibre organization;the grains at the interface junction are transformed into[111]∥DD orientation,thus achieving the coordinated deformation of the three.The drawing process will lead to the generation of serrated cross-section,and its formation mechanism is as follows:during the drawing process,the interface is distorted by the circumferential compressive stress of the die,and with the increase of deformation,the more serious the distortion is,which eventually transforms into a serrated interface.Due to the existence of the interface,and with the existence of huge mechanical property differences between the inner and outer metal,the generation of serrated interface is inevitable.（4）The growth law of the interface was revealed during the heat treatment process.With the increase of heat treatment temperature,the thickness of the interface layer gradually thickens;Under the heat treatment temperature of 500°C-800°C,the diffusion rate of Ni atoms into Pt is larger than that of Pt atoms into the nickel core.The diffusion constants were D_0Ni→Pt=5.07×10^-9m^2/s and D_0Pt→Ni=3.5×10^-7m²/s,and the diffusion activation energies were Q_Ni→Pt=106×10³J/mol and Q_Pt→Ni=144×10³J/mol,respectively.（5）The failure mechanism of platinum-coated nickel composite wire in high-temperature service was obtained`.The higher the oxidation temperature,the greater the Pt-Ni mutual diffusion rate,nickel through the grain boundary and body diffusion to the surface of the composite wire,and immediately reacted with oxygen to generate Ni O,which is a typical selective external oxidation.The useful life is strongly dependent on the use of temperature and minimum platinum layer thickness.At 500°C,the diffusion coefficient of nickel in platinum is 1.82×10^-17～1.19×10^-17m²/s.（6）The research results have been transformed in enterprises,and low-cost and high-quality platinum-clad nickel composite wires for thin-film platinum resistance temperature sensor leads have been prepared in stable batches.