Research On Dislocation Density And Grain Size Refinement On Machined Surface Of Aluminium Alloys

The microstructure of the machined component has a criticalinfluence on the functional performance of the component such as fatiguelife, corrosion and wears resistance. The microstructure of the machinedsurface changes greatly comparing with the initial microstructure of thematerial after machining process. Researches of the surface integrity, suchas white layer, micro-hardness and residual stress, from the point view ofthe microstructure have great significance to study the relation of thesurface integrity and the machining conditions. However, few studies canbe found to predict microstructure evolution and its relation with thesurface performance of the component.This thesis studies the dislocation density and grain size evolution onthe machined surface of the Aluminium alloy Al6061-T6. A finite elementmodel of the orthogonal cutting process is established based on Abaqussoftware, coupled with a microstructure evolution model based ondislocation density, to predict cutting force, chip morphology, cuttingtemperature, dislocation density and grain size of the workpiece.Machining experiments and metallographic experiments are conducted tovalidate the FE model under the corresponding boundary conditions. Theresearch and main conclusions are as follows:Firstly, a FE model of orthogonal cutting Al6061-T6is establishedusing the software of Abaqus/Explicit6.10.1. Reasonable material propertymodel, friction model and FE modeling method is adopted considering thematerial property and machinability of the Aluminium alloy. The influenceof critical shear yield stress, friction coefficient, heat distributioncoefficient on the predicted force, chip morphology and cutting temperature is discussed. A microstructure evolution model is adopted todescribe dislocation density and grain size evolution in the chip and on themachined surface. This dislocation density based microstructure model isembedded into the FE model of orthogonal cutting in the form ofsubroutines written in FORTRAN to establish a FE model couplingmechanical force, heat and microstructure.Secondly, orthogonal cutting experiments of Al6061-T6areconducted and cutting forces and are measured and chips are collected.Metallographic experiments are conducted to observe microstructure of themachined surface and chips, and the changes of their microstructure areanalyzed. Comparing experimental results and predicted results, criticalshear yield stress, friction coefficient, heat distribution coefficient andparameters of the microstructure evolution predicted model are adjusted tovalidate the reliability of the FE model. Results show that critical shearyield stress and friction coefficient has significant influence on the cuttingforce, chip thickness and cutting temperature. Larger values of the shearyield stress and friction coefficient generate larger cutting force, thickerchip thickness and higher cutting temperature. The values of critical shearyield stress and friction coefficient under different cutting conditions areobtained by the method of parameter sensitivity analysis. Deformationfield and temperature field determines dislocation density and grain sizedistribution in the chip and on the machined surface.Lastly, influence of the cutting parameters, cutting tool andtemperature on the dislocation density and grain size distribution isanalyzed base on this FE model. Conclusions are as follows: the seconddeformation zone has the denser dislocation density under different cuttingvelocities. Dislocation density on the machined surface is similar with thatin the first deformation zone which gradually decreases along the depthdirection of the machined surface. The distribution of grain size iscorresponding to the dislocation density. Dislocation density on themachined surface decreases with increasing cutting speed while grain size increases with that at a certain feed rate, thickness of deformation layerdecreases with increasing cutting speed; Dislocation density on themachined surface decreases firstly and then increases with increasing feedrate, grain size has corresponding tendency at a certain cutting speed,thickness of deformation layer first increases and then decreases withincreasing feed rate. Plastic deformation in the first deformation zoneincreases significantly when the rake angle of the cutting tool is negativewhich lead to denser dislocation density and finer grain size on themachined surface with decreasing rake angle, thickness of deformationlayer decreases with increasing rake angle; plough effect becomes severerwith increasing edge radius of the cutting tool, thus, dislocation densityincreases with increasing edge radius of the cutting tool and grain sizedecreases with that, thickness of deformation layer increases withincreasing edge radius. Increasing the heat convection coefficient canlower cutting temperature which weakens temperature softening effect andaffects the deformed layer on the machined surface on the one hand andhinders dynamic recovery of the grain on the other hand. Thus, Dislocationdensity increases with increasing heat convection coefficient while grainsize decreases with that, thickness of deformation layer increases withincreasing heat convection coefficient.