Experimental Study And Forming Mechanism Of Ball Spinning Ni-Cu Alloy Ultrathin Walled Tube

With the development of China’s equipment manufacturing industry,precision plastic forming has gradually become a hot spot in the field of plastic processing.In the field of electronic equipment and precision instrument manufacturing,the demand for ultrathin walled tubes is gradually expanding.In this paper,based on the ball spinning process,the finite element analysis software was used to analyze the spinnability of NCu 40-2-1 Ni-Cu alloy tubes.Ultimately the ultra-thin tube with a thickness of 0.05 mm was successfully prepared by a vertical ball spinning machine.In addition,the forming process,strengthening mechanism,and microstructure evolution of ultrathin walled Ni-Cu alloy tubes were studied by various material observation methods.Firstly,based on the tensile test results of Ni-Cu alloy at room temperature,the calculation models of ball spinning parameters were established.A finite element model was proposed to predict the ultimate thickness reduction ratio during ball spinning.The spinnability of tubes in the selected parameters was analyzed by using the method of gradually increasing the total thickness reduction ratio and keeping the relative thickness reduction ratio of each pass the same.When the wrinkling factor is used as the measurement standard,the fluctuation of the wrinkling factor increases gradually when the total thinning rate increases.When the deformation uniformity coefficient is taken as the measurement standard,the deformation uniformity coefficient of tubes decreases first and then increases with the increase of the thickness reduction ratio.The deformation of tubes is more uniform after the second pass,the first pass is the second,and the third pass is the worst.By reason of the foregoing,it is predicted that the limit thickness reduction ratios of Φ10.8mm Ni-Cu alloy tubes are 80%～90% in the selected technological parameters.Secondly,the spinning test was carried out based on the process model of the ultrathin walled tube established in this paper.The forming qualities of pipe fittings were detected,and the main causes of defects in the spinning process were summarized.The microstructure of the pipe surface(RD-TD plane)was observed by TEM(Transmission Electron Microscope,TEM),and the formation process of the ultrafine lamellar structures(UFL structures)and ultrafine crystals(UFGs)were analyzed.According to the hardness changes of tubes after spinning,the strengthening mechanism during ball spinning was obtained.The results show that when the thickness reduction ratio of Ni-Cu alloy tubes is greater than 80%,the internal microstructure can be strengthened in three ways: dislocation slip,UFL structure refinement,and deformation twin generation.Thirdly,the microstructure evolutions of pipe fitting surface(RD-TD plane)and thickness direction(RD-ND plane)were observed and analyzed by XRD(Xray diffraction)and EBSD(Electron Back-Scattering Diffraction).On the RD-TD plane,the preferred orientation of(111)and(200)crystal planes decreases gradually,while the preferred orientation of(220)crystal planes increases sharply.On the RD-ND surface,after the ball spinning,the tubes are mainly composed of<110>//RD.The RD orientation dominates,and the typical torsional(pure shear)texture of FCC metals is formed — {111}<110>.Finally,based on the distribution density variation of geometric essential dislocations(GNDs)in tubes,the uniformities of the internal structure were characterized.Moreover,the influences of GND density on tube strength were analyzed.It is found that the distribution of grain morphology and dislocation in the thickness direction is obviously different when the thickness reduction ratio of the tube is small because of insufficient deformation.As the thickness reduction ratio increases to 90%,the grain morphology and geometrical essential dislocation distribution tend to be uniform,which means that subsequent deformation will become difficult.