Bulk Nano-Crystalline Manufacturing

Machining, particularly orthogonal cutting, has recently been extensively studied as a severe plastic deformation (SPD) process for the manufacture of ultra-fine grained (UFG) and nanocrystalline materials, which often possess higher strength, hardness and wear-resistance than their coarse grained counterparts.  Compared to other SPD processes such as equal channel angular processing (ECAP), high pressure torsion (HPT) and cold rolling, orthogonal cutting only needs one pass to create large enough strain required for the creation of sub-micron grain sizes in the chip and can be performed at near-ambient temperature for high strength alloys.  The level of plastic strain imposed during orthogonal cutting can be modulated by an appropriate choice of the rake angle of the cutting tool, while the material processing rate and the strain rate of the plastic deformation can also be easily controlled by regulating the cutting speed and/or depth of cut.  It has been shown that orthogonal cutting is a flexible and controllable method for producing UFG microstructures for various metals and alloys, such as copper, aluminum alloys, titanium, nickel-based superalloys and steels. 

To effectively design the cutting process parameters for materials with varying thermo-mechanical properties and improve the quality of the resultant microstructures, it is necessary to investigate the microstructure refinement mechanism during severe deformation and is desirable to have an analytical model for predicting the grain sizes produced by orthogonal cutting.  

Orthogonal cutting of titanium (journal paper)

Grain refinement in aluminum and copper (journal paper)

Cold rolling (journal paper)


 

Dislocation density-based grain refinement modeling of orthogonal cutting of titanium

 

This study is focused on modeling of orthogonal cutting of a commercially pure titanium (CP Ti) material in order to assess the validity of the numerical solution through comparison with experiments.  The dislocation density-based material plasticity model is calibrated to reproduce the observed material constitutive mechanical behavior of CP Ti under various strains, strain rates and temperatures in the cutting process.  Simulation results are presented in chip formation, strains, strain rates, temperatures, grain sizes and dislocation densities in comparison with the actual measurements during orthogonal cutting. 

Grain refinement

Nanocrystalline microstructure resulting from deformation: the microstructural evolutions of CP Ti produced by SPD processes have all shown a similar pattern: at the early stage of deformation, a very high dislocation density is introduced, which leads to the formation of lamellar structure consisting of dislocation cells with thick cell walls and low angles of misorientation.

Coupled Eulerian-Lagrangian (CEL) model 

A two-dimensional (2D), coupled Eulerian-Lagrangian (CEL) model was developed to simulate steady-state chip formation during the orthogonal cutting process using the commercial software Abaqus 6.10.1.  Fully coupled thermo–mechanical Abaqus/Explicit analysis was carried out.   

Material Modeling

Dislocation density-based material plasticity model predictions for CP Ti. 

Strain Rate

Strain rate predictions by the CEL model for orthogonal cutting of CP Ti with a rake angle of 20°

 

Strain

Effective strain predictions by the CEL model for orthogonal cutting of CP Ti with a rake angle of 20°.

Microstructural Evolution 

(a) Predicted total dislocation density (b) homogeneous, loosely distribution of dislocations in the bulk material (c) elongated dislocation cell in the chip primary shear zone, with dense dislocations on the cell walls  and blocked dislocations by subgrain boundaries (d) well developed sub-micron grains in the chip, by break up and reorientation of subgrains. 


 

Grain refinement in aluminum and copper subjected to cutting

 

In this work, dislocation density-based material models are developed to model grain size refinement and grain misorientation during cutting of Al 6061 T6 and OFHC Cu under various cutting conditions.  It is shown that the developed CEL finite element model embedded with the dislocation material models captures the essential features of the deformation field and grain refinement mechanism during cutting.   The model predicts the grains in the machined chips are refined from an initial size of 50~100 µm to about 100~200 nm for Al 6061 T6 and OFHC Cu at a low cutting speed of about 0.02 m/s with negative rake angle tools.  It is shown that a small applied strain, high cutting speed or high cutting temperature will all contribute to a coarser elongated grain structure during cutting.

 

Microstructural Evolution

Predicted microstructural evolution for cutting of OFHC Cu. 

(a) Equivalent strain (b) total dislocation density, mm-2 (c) grain size, mm. 

I, loosely distribution of dislocations in the bulk material prior to cutting; II, elongated dislocation cell in the primary deformation zone; III, equiaxed sub-micron grains in the chip. 

 

 

Copper

Grain size in the chips for OFHC Cu under conditions with varying cutting speeds.

   

Aluminum

Average shear strain and chip thickness for cutting of Al 6061 T6 under conditions Al1-3 (a) Shear strain (b) predicted machined chip thickness (µm).


 

Grain refinement during multi-pass cold rolling

 

This study is focused on multi-pass cold rolling processes of commercially pure titanium (CP Ti) and aluminum (AA 1200). The dislocation density-based material models are developed for CP Ti and AA 1200, which reproduce the observed material constitutive mechanical behavior under various strains, strain rates and temperatures occurring in the cold rolling process.  It is shown that the developed model captures the essential features of the material mechanical behaviors and predicts a minimum grain size of below 100 nm after five-pass cold rolling of CP Ti with equivalent strains up to 2.07 and the average incidental dislocation boundary (IDB) misorientation angle increased to 4.6° after six-pass cold rolling of AA 1200 with equivalent strains accumulated to 5.77.

Process Modeling

Eulerian-type FE model was developed to simulate six-pass cold rolling of AA 1200, in which the workpiece mesh was fixed in space.  During the simulation, material flows into the workpiece mesh from the left inlet surface, and exits the simulation domain from the right outlet surface.

 

Modeling Results

Deformation and dislocation fields predicted in pass-3 cold rolling of CP Ti.

 

 

Grain Refinement

Histograms of predicted (a-c) and measured (d-f) grain size for cold rolled of CP Ti

Misorientation Angle

Measured (a) and predicted (b) IDB misorientation angle distribution after the second and fourth passes cold rolling of AA 1200