Ninggang Shen

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.

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

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

Surface Micro-Scale Patterning for Biomedical Implant Material of Pure Titanium via High Energy Pulse Laser Peening

Pure titanium (commercial pure cpTi) is an ideal dental implant material without the leeching of toxic alloy elements. Evidence has shown that unsmooth implant surface topologies may 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. Studies have been conducted on the grit blasted, acid etched, or uni-directional grooved Ti surface. However, for these existing approaches, the surface quality is difficult to control or may even damage the implant. A novel idea has been studied in which more complex two-dimensional (2D) patterns can be imprinted into the dental implant material of cpTi by high energy pulse laser peening (HEPLP). The strong shock wave generated by HEPLP press a stainless steel grid, used as a stamp, on Ti foils to imprint a 2D pattern. In this study, the multiple grid patterns and grid sizes were applied to test the cell’s favor. The HEPLP induced shock wave pressure profile and history were simulated by a 2D multi-physics hydrodynamic numerical analysis for a better understanding of this technique. Then, the cell culture tests were conducted with the patterned surface to investigate the contribution of these 2D patterns, with the control tests of the other existing implant surface topography forming approaches.© 2014 ASME

Experimental and Numerical Analysis of Laser Peen Forming Mechanisms of Sheet Metal

Laser peen forming (LPF) is a novel non-contact sheet metal forming process without detrimental thermal defects. High pressure shock waves induced by a focused laser pulse are applied on the workpiece surface to generate deformations. In this study, the deformation mechanisms induced by LPF are experimentally and numerically investigated under different experimental conditions. Experiments have shown that when keeping laser parameters constant, deformation mechanisms vary depending on the sample thickness. The results show that aluminum sheet samples of 0.25 mm in thickness bend concavely for pulse energy ranging from 0.2 to 0.5 J, while 1.75 mm aluminum sheets bend convexly under the same conditions. There is a transition thickness threshold of sheet metal at which the deformation mechanism changes from concave to convex with the increase of the sample thickness with certain levels of laser parameter. This transition thickness threshold is determined to be around 0.7–0.88mm with the studied process parameters. Experiments also show that as the pulse energy increases, the transition thickness of the bending deformation mechanism increases slightly. Under the concave deformation mechanism, the workpiece is more sensitive to pulse energy, while pulse energy is not a critical factor in the convex mechanism. A finite element analysis (FEA) is performed to simulate the LPF deformation process with different specimen thicknesses and loading conditions. The simulation results agreed well with experimental results.Copyright

Experimental and Modeling Analysis of Micro-Milling of Hardened H13 Tool Steel

This study is focused on experimental evaluation and numerical modeling of micro-milling of hardened H13 tool steels. Multiple tool wear tests are performed in a micro side cutting condition with 100 μm diameter endmills. The machined surface integrity, part dimension control, size effect and tool wear progression in micromachining of hardened tool steels are experimentally investigated. A strain gradient plasticity model is developed for micromachining of hardened H13 tool steel. Novel 2D FE models are developed in software ABAQUS to simulate the continuous chip formation with varying chip thickness in complete micro-milling cycles under two configurations: micro slotting and micro side cutting. The steady-state cutting temperature is investigated by a heat transfer analysis of multi micro-milling cycles. The FE model with the material strain gradient plasticity is validated by comparing the model predictions of the specific cutting forces with the measured data. The FE model results are discussed in chip formation, stress, temperature, and velocity fields to great details. It is shown that the developed FE model is capable of modeling a continuous chip formation in a complete micro-milling cycle, including the size effect. It is also shown that built-up edge in micromachining can be predicted with the FE model.Copyright

Cellular Automaton Simulation of Microstructure Evolution for Friction Stir Blind Riveting

Friction stir blind riveting (FSBR) process offers the ability to create highly efficient joints for lightweight metal alloys. During the process, a distinctive gradient microstructure can be generated for the work material near the rivet hole surface due to high-gradient plastic deformation and friction. In this work, discontinuous dynamic recrystallization (dDRX) is found to be the major recrystallization mechanism of aluminum alloy 6111 undergoing FSBR. A cellular automaton (CA) model is developed for the first time to simulate the evolution of microstructure of workpiece material during the dynamic FSBR process by incorporating main microstructure evolution mechanisms, including dislocation dynamics during severe plastic deformation, dynamic recovery, dDRX, and subsequent grain growth. Complex thermomechanical loading conditions during FSBR are obtained using a mesh-free Lagrangian particle-based smooth particle hydrodynamics (SPH) method, and are applied in the CA model to predict the microstructure evolution near the rivet hole. The simulation results in grain structure agree well with the experiments, which indicates that the important characteristics of microstructure evolution during the FSBR process are well captured by the CA model. This study presents a novel numerical approach to model and simulate microstructure evolution undergoing severe plastic deformation processes. [DOI: 10.1115/1.4038576]

Mechanical Ruling of Diffraction Grating: Part I — Aluminum Film Preparation and Characterization

This is Part I of a two part series study on mechanical ruling of diffraction grating. Mechanical ruling is a major method for fabricating diffraction grating. In mechanical ruling, the first step is to prepare the Al film. High quality preparation technique is needed to satisfy the requirements of the thickness and the mechanical properties of the film. The purpose of this paper is to present a complete study on the preparation technique and mechanical properties of the Al film used for mechanical ruling of diffraction grating. XRD and SEM experiments and analysis were conducted to investigate the microstructure of the Al film and the mechanical properties of the Al film were measured using nanoindentation and scratch tests. The Al film exhibits favorable mechanical properties which will the key for the experimental and numerical simulation studies of mechanical ruling of diffraction grating.Copyright

Predictive modeling of surface microstructure of hardened steel subject to drilling

Hole surface microstructures are very critical to the mechanical performance and fatigue life of metallic products from drilling processes. When steel material is drilled at a fully hardened condition, hole surface microstructures are often subject to transition because of the intense thermo-mechanical loading in the drilling process. A white layer can be formed on the surface of a drilled hole of carbon steels with high matrix hardness. The formation of the white layer mainly results from two reasons: thermally driven phase transformation and mechanical grain refinement due to severe plastic deformation on the machined surface. In this study, a multi-step numerical analysis is conducted to investigate the potential mechanism of surface microstructure alterations in the drilling process of hardened steels. First, three-dimensional (3D) Finite Element (FE) simulations are performed using a relative coarse mesh with AdvantEdge for hard drilling of AISI 1060 steel to achieve the steady-state solution for thermal and deformation fields. Defining the initial condition of the cutting zone using the previous 3D simulation results, a multi-physics model is then implemented in twodimensional (2D) coupled Eulerian-Lagrangian (CEL) finite element analysis in ABAQUS to model both phase transformation and grain refinement at a fine mesh to comprehend the surface microstructure alteration. The interaction among surface microstructures, drilling parameters and the hardness of the workpiece material are studied simultaneously. With the comparison to related experimental results, the capabilities of the multi-physics model to accurately predict critical surface microstructural attributes such as phase compositions, grain size, and microhardness during the drilling of carbon steel are demonstrated.