Research On Some Fundamental Mechanical Problems In Heating-Assisted Micro-Part Blanking And Numerical Simulation

With the requirement of the development of science and technology, the parts with small size and large aspect ratio have been more and more extensively used in various industries and consumers. It is known that, although blanking is an efficient and environmental friendly method for mass production of micro-parts compared with etching and electrochemical micromachining, blanking of the micro-parts with larger aspect ratio remains unsolved, due to that it results in excessive tool loads, with consequent tool failure or decrease in tool life. Two parameters dominate the attempt to use large aspect ratio - the strength of the currently available tool-materials and the shear strength of the work-material. Of the two, the availability of blanking would naturally opt to reduce the latter by making use of heating-assisted technology than developing high-strength tool-materials. On the basis of the progress in the research of the heating-assisted micro-parts blanking, some fundamental problems related to micro-parts blanking are investigated in this dissertation. The main contribution of the dissertation is listed as follows:(1) Since a heating-assisted micro-part blanking process may involve both high temperature and large heating-rate, the effect of heating rate on the mechanical properties of work-materials should also be investigated. For LY12 aluminum alloy, experiment shows that, the local thermal inconsistency and residual stress field at moderate heating rate may result in hardening, while the severe local thermal inconsistency and residual stress field at high heating-rate may accelerate the initiation and growth of microdefects, resulting in damage and property degradation of the material. SEM observation and analysis reveal distinct difference between the metallographs of the materials undergoing different heating-rate histories. The materials undergoing higher heating-rate history appears more brittle, attributed to more damage and less recrystallization.(2) Based on a simple thermomechanically consistent mechanical model, a constitutive model is proposed for finite elastoplastic strain and deformation of materials. It takes into account the effects of temperature on the properties, and the effects of strain, heating-rate, strain-rate and recrystallization on the hardening and damage of materials. The incremental form of the constitutive model, the corresponding numerical algorithm and the finite element formalism are also developed. Based on variable-temperature Johnson-Mehl equation, the volume fraction transformed is calculated and it is involved in hardening function.(3) The user defined material subroutines of the proposed constitutive model are developed based on the subroutine interface of commercially available finite element code ABAQUS. The coupled transient thermal-mechanical processes of the upsetting of a mild steel specimen are simulated. The responses of the aluminum alloy LY12 specimens heated at different heating-rate to a prescribed temperature followed by tension until fracture are also simulated. The computed results are in satisfactory agreement with the experimental results, and the effects of the main influencing factors can be well described, demonstrating the validity of the proposed constitutive model in the analysis of heating-assisted finite elastoplastic deformation and damage processes.(4) The coupled thermal-electric simulations, with copper, LY12 aluminum alloy and 304 stainless steel as workpiece materials, and tungsten carbide and plain carbon tool steel as tool materials, respectively, are numerically simulated with the proposed constitutive model and the corresponding approach. The results show that the shear zone of workpiece can be heated by electrical current to an appropriate blanking temperature in a short time interval, which enables a sufficient reduction of the shear strength of the work material as well as the blanking force. Analysis also shows that, in order to avoid improper deposition of energy in a workpiece-tool system, the electrical and thermal conductivities of the tool material should be superior to that of the work-material as much as possible.(5) The laser-assisted and electricity-assisted heating micro-part blanking processes, with FeNi42 alloy, 304 stainless steel and LY12 aluminum alloy as workpiece materials, respectively, are numerically simulated with the proposed constitutive model and the corresponding approach. The results show that assisted heating can provide an appropriate blanking temperature in a short time interval, and sufficiently reduce the shear strength of the work material and the corresponding blanking force, to meet the requirement of the blanking with large aspect ratio and mass production.