Mechanism Study Of Micro-cutting Based On Dislocations And Strain Gradient Theory

There is an urgent growing need for the precision/ultra precision parts andcomponents in the engineering fields of national defense, micro-electronics, modernmedical science and biological science. Micro-cutting process is one of the mostimportant techniques in machining these components. But it can be easily affected bythe bottleneck issues of unsteady cutting forces, tough surface quality and so on due tosize effect and minimum cutting depth which characterize the micro machining process.Traditional plasticity mechanics theory featuring no material length scale can notexplain well to these problems, so that micro plasticity mechanics theory and materialdislocations have to be put forwarded to analyze and solve these abnormal issues.The most critical different point between micro-cutting and traditional cutting isconsidered as machining characteristic scale. Due to the small edge radius ofmicro-cutting, the material characteristic wage is located at micro meter level which isvery close to the material intrinsic length, and this directly results that shear strengthand plastic flow stress of the material is influenced by the strain gradient effectspresenting in the uneven strain field. This can be regarded as the fundamental reason tothe material hardening, increase of cutting force phenomenon. In order to study andsolve these issues, this paper resorts to the methods of theory analysis, comparison withexperimental results and numerical modeling to study the mechanism of micro-cuttingprocess, the primary content is as follows:Firstly, the strain gradient theory based on the dislocation mechanism is used toreflect the material micro mechanical behavior, and a numerical method to the solutionof strain gradient in the micro-cutting process is come up with to build a materialconstitutive model which can reflect the size effect，tool radius effect and machinedsurface quality.Secondly, a constitutive model suitable for the micro scale is implemented by usingthe FEM user material subroutine (vumat) and applied to the finite element modeling ofdynamic micro-cutting process. This FEM model takes advantage of arbitrary Lagrangeand Euler mesh constraint technique which is effective in modeling the tool edge radiuseffect and is validated through the tests.At last, based on the proposed finite element model of micro cutting process and combined with experimental data, the reflection mechanism between outside appearanceas size effect, surface quality, tool radius effect etc with internal micro-scale plasticitybehavior is studied. The mainly conclusion is as follows:(1) The influence of strain gradient to cutting forces, stress, specific cutting energyand temperature distribution on the work piece is well studied. The outcomes indicatethat the strain gradient effect is one of the most important causes to the size effect.(2) The stress distribution status of the work piece in front of the tool edge isinvestigated, and the underlying reason to the growing trend of cutting force is lying onthe increasing stress in whole.(3)The stagnant zone in front of the tool edge is studied. The stagnant can bediscovered through the cutting speed contour, and the zone area will increase with therise of tool edge radius which results in the transition of a point of material damageposition to a curve position. This transition remarkably deteriorates the surface qualityin the cutting process.This paper reveals the fundamental reasons of size effect and mechanisms of microcutting process, and provides the theoretical basement to the optimization of the wholemachining process.