Modeling and Simulation of Machining-induced Surface Integrity Characteristicsof NiTi Alloy

AbstractNiTi shape memory alloys have gained increased interest in various industries, including biomedical and aerospace applications due to their unique properties such as shape memory effect and superelasticity. Martensitic phase transformation in NiTi significantly affects the surface integrity characteristics. This phase transformation needs to be better understood to control and enhance the shape memory and microstructural properties of NiTi shape memory alloys. This study presents results of combined experimentation and simulation of cutting-induced phase transformation in orthogonal machining of NiTi shape memory alloys. A phenomenological modeling approach was utilized to model machining-induced phase transformation. NiTi shape memory alloys alloy were in austenite phases at room temperature. The transformation during dry machining process from austenite to martensite phases, and the resulting volume fraction was successfully simulated using DEFORM 2-D software by implementing a user-defined subroutine. The developed model is capable of capturing the trend of variationsinvolume fracture and the depth of transformed layer as a function of cutting speed

Finite Element Simulation of Residual Stresses in Cryogenic Machining of AZ31B Mg Alloy

AbstractMagnesium alloys are lightweight materials primarily used in transportation industry, and are also emerging as a potential material for biodegradable fixation implants. However, unsatisfactory corrosion resistance largely limits the application of these materials. Residual stresses were reported to have significant influence on corrosion resistance of Mg alloys. In this study, a finite element model was developed to simulate the residual stresses in cryogenic machining of AZ31B Mg alloy. After calibration using experimental data, numerical simulations were conducted to study the influence of cutting edge radius and cooling method (dry vs. cryogenic) on residual stresses. The model can be used to establish proper cutting conditions to induce compressive residual stresses to enhance the corrosion resistance of Mg alloys

Improved Surface Integrity from Cryogenic Machining of Ti-6Al-7Nb Alloy for Biomedical Applications

AbstractTi-6Al-7Nb alloy is emerging as an alternative biomedical material for replacing Ti-6Al-4V alloy used in dental implants and femoral stem prosthesis applications. In cryogenic machining using liquid nitrogen, the surface integrity characteristics of Ti-6Al-7Nb alloy significantly improved compared to dry and flood-cooled machining. This study shows that surface roughness improved in cryogenic machining by 35% and 6.6% respectively, compared with dry and flood-cooled machining. Also, the hardness in the cryogenically-machined surface layer increased, by 33.6% and 14.7%, respectively, compared to dry and flood-cooled machining, with the formation of a severe plastic deformation (SPD) layer with less volume fraction of α-phase

Finite Element Modeling of Microstructural Changes in Turning of AA7075-T651 Alloy and Validation

The surface characteristics of a machined product strongly influence its functional performance. During machining, the grain size of the surface is frequently modified, thus the properties of the machined surface are different to that of the original bulk material. These changes must be taken into account when modeling the surface integrity effects resulting from machining. In the present work, grain size changes induced during turning of AA 7075-T651 (160 HV) alloy are modeled using the Finite Element (FE) method and a user subroutine is implemented in the FE code to describe the microstructural change and to simulate the dynamic recrystallization, with the consequent formation of new grains. In particular, a procedure utilizing the Zener-Hollomon and Hall-Petch equations is implemented in the user subroutine to predict the evolution of the material grain size and the surface hardness when varying the cutting speeds (180 - 720 m/min) and tool nose radii (0.4 - 1.2 mm). All simulations were performed for dry cutting conditions using uncoated carbide tools. The effectiveness of the proposed FE model was demonstrated through its capability to predict grain size evolution and hardness modification from the bulk material to machined surface. The model is validated by comparing the predicted results with those experimentally observed

Cryogenic Machining of Biomedical Implant Materials for Improved Functional Performance, Life and Sustainability

AbstractCryogenic cooling is known to provide a very sustainable machining process because of its environmentally benign, and economically and societally-beneficial nature. This keynote paper will focus on recent findings on producing functionally-superior engineered surfaces for improved product quality, performance and sustainability in cryogenically-processed biomedical implants. Results from cryogenic processing of Ti alloys, Co-Cr-Mo alloy, and AZ31B Mg alloy for achieving enhanced surface and sub-surface integrity will be summarized. Experimental results are compared with numerical/analytical simulations. Encouraging findings from this extensive study shows the tremendous potential for challenging broader applications of cryogenic machining technology for biomedical components

The impact of novel material processing methods on component quality, life and performance

AbstractSurface and subsurface characteristics of structural components for use in production equipment and machines, depending on the functions and their usage in service, can be the critical aspect from the service life viewpoint. Generated surface and subsurface characteristics of manufactured components affect functional performance with progressively deteriorating wear, corrosion and fatigue resistance, and consequently determine the effective life of components of such machines and equipment in various industries including aerospace, automotive and power industries. Developing advanced processing methods and predictive models to control surface integrity characteristics of components for achieving improved product life and performance has been an area of significance in advanced manufacturing.This paper summarizes and highlights recent advances in developing novel manufacturing techniques involving cryogenically-assisted processing (machining, burnishing and friction-stir processing) on a range of aerospace, automotive and biomedical metal alloys (Co-Cr- Mo, AZ31BMg, NiTi, Inconel 718, SS 303 stainless steel, and Al 7050) for achieving enhanced product quality, life and performance at component level. This study presents an analysis of surface integrity involving severe plastic deformation (SPD) of these materials induced by cryogenically-assisted manufacturing processes, by showing the resulting product/component performance enhancement through the generation of controllable ultra-fine/nano grain structures in the surface layers of the products/components. This grain refinement is also often accompanied by improved wear and corrosion resistance properties and the generation of compressive residual stresses enabling improved fatigue life, along with more favorable phase transformation in these cryogenically-processed materials. Experimental results are compared with predictions obtained from numerical models and simulations. Encouraging trends are observed with potential for applications in industry