Hongtao Ding

DISLOCATION DENSITY-BASED GRAIN REFINEMENT MODELING OF ORTHOGONAL CUTTING OF COMMERCIALLY PURE TITANIUM

Recently, machining has been exploited as a means for producing ultra-fine grained (UFG) and nanocrystalline microstructures for various metal materials, such as aluminum alloys, copper, stainless steel, titanium and nickel-based super alloys, etc. However, no predictive, analytical or numerical work has ever been presented to quantitatively predict the change of grain sizes during machining. In this paper, a dislocation density-based viscoplastic model is adapted for modeling the grain size refinement mechanism during machining by means of a finite element based numerical framework. A novel Coupled EulerianLagrangian (CEL) finite element model embedded with the dislocation density subroutine is developed to model the severe plastic deformation and grain refinement during a steady-state cutting process. The orthogonal cutting tests of a commercially pure titanium (CP Ti) material are simulated in order to assess the validity of the numerical solution through comparison with experiments. The dislocation density-based material 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. It is shown that the developed model captures the essential features of the material mechanical behavior and predicts a grain size of 100-160 nm in the chips of CP Ti at a cutting speed of 10 mm/s.

Dislocation Density-Based Grain Refinement Modeling of Orthogonal Cutting of Titanium

Recently, orthogonal cutting has been exploited as a means for producing ultrafine grained (UFG) and nanocrystalline microstructures for various metal materials, such as aluminum alloys, copper, stainless steel, titanium and nickel-based super alloys, etc. However, no predictive, analytical or numerical work has ever been presented to quantitatively predict the change of grain sizes during plane-strain orthogonal cutting. In this paper, a dislocation density-based material plasticity model is adapted for modeling the grain size refinement mechanism during orthogonal cutting by means of a finite element based numerical framework. A coupled Eulerian–Lagrangian (CEL) finite element model embedded with the dislocation density subroutine is developed to model the severe plastic deformation and grain refinement during a steady-state cutting process. The orthogonal cutting tests of a commercially pure titanium (CP Ti) material are simulated 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. It is shown that the developed model captures the essential features of the material mechanical behavior and predicts a grain size of 100–160 nm in the chips of CP Ti at a cutting speed of 10 mm/s. [DOI: 10.1115/1.4027207]

Surface Micropatterning of Pure Titanium for Biomedical Applications Via High Energy Pulse Laser Peening

Pure titanium is an ideal material for biomedical implant applications for its superior biocompatibility, but it lacks of the mechanical strength required in these applications compared with titanium alloys. This research is concerned with an innovative laser peening-based material process to improve the mechanical strength and cell attachment property of pure titanium in biomedical applications. Evidence has shown that engineered surface with unsmooth topologies will contribute to the osteoblast differentiation in human mesenchymal pre-osteoblastic cells, which is helpful to avoid long-term peri-abutment inflammation issues for the dental implant therapy with transcutaneous devices. However, surface quality is difficult to control or mechanical strength is not enhanced using conventional approaches. In this paper, a novel high energy pulse laser peening (HEPLP) process is proposed to both improve the mechanical strength and introduce a micropattern into the biomedical implant material of a commercially pure Titanium (cpTi). The strong shock wave generated by HEPLP presses a stainless steel grid, used as a stamp, on cpTi foils to imprint a micropattern. To understand the basic science during the process, the HEPLP induced shock wave pressure profile and history are modeled by a multiphysics hydrodynamic numerical analysis. The micropatterns and strength enhancement are then simulated using a dislocation density-based finite element (FE) framework. Finally, cell culture tests are conducted to investigate the biomedical performance of the patterned surface.

Optimization of Friction Stir Extrusion (FSE) Parameters Through Taguchi Technique

In this study, a novel friction stir extrusion (FSE) process is investigated for fabrication of fully-consolidated wires. The process parameters of rotational speed (RS), plunge rate (PR), and extrusion hole size are optimized using the Taguchi L8 orthogonal design of experiments. The optimum process parameters are determined with reference to the average grain size of the wire. The analysis of variance shows that the RS of plunge die is the most dominant factor in deciding the soundness of joint, while PR also plays a significant role. The microstructural studies reveal that initial grains of Mg ingot undergoes significant refinement in the specimens produced by the FSE process. Mechanical tests show that almost all recycled specimens can achieve higher strength and elongation than parent material at room temperature. This study shows that defect free, high quality wires can be produced using a proper combination of process parameters and recommends parameters for producing the best wire properties.

IMPROVING MACHINABILITY OF HIGH CHROMIUM WEAR-RESISTANT MATERIALS VIA LASER–ASSISTED MACHINING

The machinability of high chromium wear resistant materials is poor due to their high hardness with a large amount of hard chromium carbides. This study is focused on improving the machinability of high chromium wear resistant materials with different microstructures and hardness levels via laser-assisted machining (LAM). A laser pre-scan process is designed to preheat the workpiece before LAM to overcome the laser power constraint. A transient, three-dimensional LAM thermal model is expanded to include the laser pre-scan process, and is validated through experiments using an infrared camera. The machinability of highly alloyed wear resistant materials of 27% and 35% chromium content is evaluated in terms of tool wear, cutting forces, and surface integrity through LAM experiments using cubic boron nitride (CBN) tools. With increasing material removal temperature from room temperature to 400°C, the benefit of LAM is demonstrated by 28% decrease in specific cutting energy, 50% improvement in surface roughness and a 100% increase in CBN tool life over conventional machining.

Ultrasonic Cavitation Peening of Stainless Steel and Nickel Alloy

Ultrasonic cavitation peening is a peening process utilizing the high pressure induced by ultrasonic cavitation in liquids (typically water). However, the relevant previous investigations in the literature have been limited. In this paper, ultrasonic cavitation peening on stainless steel and nickel alloy has been studied, including the observation or characterization of the surface hardness, morphology, profile, roughness and oxygen contamination of treated workpiece samples. It has been found that for the studied situations, ultrasonic cavitation peening (at a sufficiently high horn vibration amplitude) can obviously enhance the workpiece surface hardness without significantly increasing the surface roughness, changing surface morphology observed by scanning electron microscope (SEM), or contaminating the surface by oxygen.© 2013 ASME

Physics-Based Microstructure Simulation for Drilled Hole Surface in Hardened Steel

For a fully hardened steel material, hole surface microstructuresare often subject to microstructural transition because of theintense thermomechanical loading. A white layer can be formedon the surface of a drilled hole of hardened carbon steels, whichresults from two mechanisms: thermally driven phase transforma-tion and mechanical grain refinement due to severe plastic defor-mation. In this study, a multistep numerical analysis is conductedto investigate the potential mechanism of surface microstructurealterations in hard drilling. First, three-dimensional (3D) finiteelement (FE) simulations are performed using a relative coarsemesh with

INDUCTION-HEATED TOOL MACHINING OF ELASTOMERS—PART 2: CHIP MORPHOLOGY, CUTTING FORCES, AND MACHINED SURFACES

ABSTRACT The induction-heated tool and cryogenically cooled workpiece are investigated for end milling of elastomers to generate desirable shape and surface roughness. Elastomer end milling experiments are conducted to study effects of the cutting speed, tool heating, and workpiece cooling on the chip formation, cutting forces, groove width, and surface roughness. At high cutting speed, smoke is generated and becomes an environmental hazard. At low cutting speeds, induction heated tool, if properly utilized, has demonstrated to be beneficial for the precision machining of elastomer with better surface roughness and dimensional control. Frequency analysis of cutting forces shows that the soft elastomer workpiece has low frequency vibration, which can be correlated to the surface machining marks. The width of end-milled grooves is only 68 to 78% of the tool diameter. The correlation between the machined groove width and cutting force reveals the importance of the workpiece compliance to precision machining of elastomer. This study also explores the use of both contact profilometer and non-contact confocal microscope to measure the roughness of machined elastomer surfaces. The comparison of measurement results shows the advantages and limitations of both measurement methods.

Cryogenic Cutting of AZ31B-O Mg Alloy for Improved Surface Integrity: Part II — Physics-Based Process Modeling of Surface Microstructural Alteration

This is the Part II of a two-part series numerical study which investigates the improvement of surface integrity of AZ31B-O magnesium (Mg) alloy by cryogenic cutting. In Part I, material constitutive behavior and grain refinement mechanism of AZ31B-O Mg alloy under cryogenic cutting conditions were modeled based on both slip and twinning mechanisms. In this study, the material model is implemented in the two-pass cryogenic cutting finite element simulations using a commercial machining simulation software package of AdvantEdge 6.4. The microstructural evolution by nanocrystalline grain refinement and other improvement of the surface integrity of AZ31B-O Mg alloy after cryogenic cutting are simulated. With quantitative assessments, simulation results are further discussed in grain refinement, microhardness change, residual stress, and slip/twinning mechanism in the machined surface of Mg alloy by cryogenic cutting. The results show that the surface integrity of Mg components can be significantly improved by using cryogenic cooling and a larger tool edge radius.Copyright

INDUCTION-HEATED TOOL MACHINING OF ELASTOMERS—PART 1: FINITE DIFFERENCE THERMAL MODELING AND EXPERIMENTAL VALIDATION

ABSTRACT Experiments and finite difference thermal modeling of the induction-heated tool for end milling of elastomers are investigated. Three sets of experiments are designed to calibrate the contact thermocouple for the tool tip temperature measurement, study the effect of tool rotational speed on induction heat generation and convective heat transfer, and measure the tool temperature distribution for finite difference inverse heat transfer solution and validation of modeling results. Experimental results indicate that effects of tool rotation on induction heat generation and convective heat transfer are negligible when the spindle speed is below 2000 rpm. A finite difference thermal model of the tool and insulator is developed to predict the distribution of tool temperature. The thermal model of a stationary tool can be expanded to predict the temperature distribution of an induction-heated rotary tool within a specific spindle speed range. Experimental measurements validate that the thermal model can accurately predict tool tip peak temperature.