Microstructural prediction in metal cutting and improvement of machinability and surface integrity via laser-assisted machining
Abstract
This study is concerned with the predictive modeling of surface microstructure alterations in terms of grain refinement due to mechanical deformation and thermally driven phase transformation during the machining process. To model grain refinement, a dislocation density-based numerical framework is developed to simulate the chip formation, cutting temperature and grain size during orthogonal cutting of Al 6061 T6 and OFHC Cu; to model phase change, a truly coupled metallo-thermo-mechanical scheme is proposed to considerate mechanical deformation, thermal history, and phase transformation kinetics in orthogonal cutting of AISI 1045 steel under various conditions. The developed metallo-thermo-mechanical coupled analysis is then applied to the three-dimensional (3D) hard turning process for bearing steels to investigate the surface microstructure alteration, particularly the white layer formation mechanisms incorporating both the thermally driven phase transformation and mechanical grain refinement due to severe plastic deformation. To carry on the microstructural evolution simulation and improve computational efficiency, a coupled Eulerian-Lagrangian (CEL) model is developed to simulate steady-state chip formation in two-dimensional (2D) orthogonal cutting by using the commercial software Abaqus. 3D hard turning simulations are undertaken via AdvantEdge FEM software incorporating the material user subroutine for various hard turning conditions. A novel, arbitrary-Lagrangian-Eulerian (ALE) based finite element scheme is developed in ABAQUS to simulate the micro-milling cycles, and a strain gradient constitutive material model is incorporated to model the size effect in micro-milling. Through a quantitative assessment using the experimental data, the model simulations demonstrate the essential characteristics of the deformation field and microstructural evolution mechanism during cutting. Microstructure and surface integrity is further studied experimentally and numerically for difficult-to-machine materials during laser-assisted machining. One-step laser-assisted machining process is proposed for hardened AISI 4130 steel to replace the hard turning and grinding operations. A heat transfer model is developed to predict the temperature field inside the workpiece of complex geometry undergoing laser-assisted profile turning. Microstructure of 4130 steel workpiece is simulated using the 3D nose turning option in AdvantEdge FEM by considering both phase transformation kinetics and grain refinement. The surface integrity analysis is experimentally studied by changing heating and operating conditions, viz., average material removal temperature, cutting speed and feed.
Degree
Ph.D.
Advisors
Shin, Purdue University.
Subject Area
Mechanical engineering
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