Zhipeng Pan

The effects of dynamic evolution of microstructure on machining forces

A material microstructure-mechanics-affected machining scheme is proposed to account for the influence of material microstructural evolution on cutting mechanics. Explicit calculation of material microstructural evolution path is provided. To blend the material microstructure states into the thermo-mechanical coupling process, the material microstructure terms are introduced into the traditional Johnson–Cook model. As an application, the machining forces and average grain size are predicted in the orthogonal turning of titanium alloys. This method provides a more comprehensive way to explore microstructure-thermo-mechanical coupling in the machining process.

Turning induced residual stress prediction of AISI 4130 considering dynamic recrystallization

ABSTRACT Machining-induced residual stress distribution is strongly influenced by the machining process condition, tool geometry and workpiece material mechanical properties. The high temperature, large strain and high strain rate environment will promote the material micro-structural attribute changes. The material micro-structural attribute changes could directly affect the material mechanical properties. An analytical model is proposed for the residual stress prediction in the orthogonal turning by considering the material dynamical recrystallization induced grain growth effect. The grain size effect on the material flow stress behavior is included by adding a grain size dependent term into the traditional Johnson–Cook model. The Johnson–Mehl–Avrami–Kolmogorov model calculates the recrystallized volume fraction and grain size as a function strain, strain rate and time. The average grain size is calculated with a rule of mixture by volume. Then the modified Johnson–Cook model is embedded into a classic residual stress prediction model for the machining induced residual stress profile prediction on the machined workpiece surface. Experimental tests are conducted for the model validation. The predicted residual stress shows good approximation with the measurement in both the trend and magnitude of the residual stress. Also, the effects of cutting speed and feed rate on the residual stress profile are investigated.

Residual stress prediction for turning of Ti-6Al-4V considering the microstructure evolution:

An analytical model for residual stress prediction considering the effects of material dynamic recrystallization under process-induced mechanical and thermal stresses is proposed. The effect of microstructure evolution on residual stress generation during the turning process is considered. The Johnson–Mehl–Avrami–Kolmogorov model is used to calculate grain size evolution due to thermal mechanical effects in the machining process. A modified Johnson–Cook flow stress model is developed by introducing a material grain growth–induced softening term. The classic Oxley’s cutting mechanics theories are implemented for machining forces calculation. A hybrid algorithm accounting for thermal, mechanical, and microstructure evolution effects is used to predict the residual stress profile on a machined workpiece surface. The proposed method is implemented for the orthogonal turning of Ti-6Al-4V material. Comparison is conducted between the model prediction and the literature measurement residual stress data. The general trend of the machining-induced residual stress on the machining surface is accurately captured by the proposed model. Also, the parametric study is conducted to investigate the effect of rake angle and depth of cut on the residual stress profile.

Residual stress prediction based on MTS model during machining of Ti-6Al-4V

The material microstructure attributes are largely ignored in the machining community for the machining mechanics modeling. A physical-based mechanical threshold stress (MTS) model is proposed for the orthogonal turning application of Ti-6Al-4V material. The MTS model takes the material internal state variables, such as dislocation to dislocation interaction and dislocation/interstitial resistance, into the flow stress consideration. The MTS model is embedded into an analytical residual stress prediction model for machining induced residual stress prediction. The experimental data are provided for the model validation. The prediction generally captures the trend of the residual stress profile compared with experiments. The proposed model provides a microstructure insight of the workpiece material in the machining process modeling.

Finite element simulation of residual stress in machining of Ti-6Al-4V with a microstructural consideration:

Machining-induced residual stress plays significant role in the corrosion resistance and fatigue life of the manufacturing end-product. The high temperature condition in the turning process could induce microstructure changes of titanium alloys, which would directly influence the residual stress generation in the machining process. A prediction model is proposed to calculate the microstructure evolution in orthogonal turning process. Based on the explicit calculation of grain size growth and phase transformation, a microstructure-sensitive Johnson–Cook model considering the material microstructural attributes is developed for the machining-induced residual stress prediction of Ti-6Al-4V. Grain size characterization with electron backscatter diffraction test is conducted on the machined workpiece surface. Experimental measurement on force and residual stresses are conducted for model validations. The predicted force value agrees well with the measurement data. The general trend of residual stress profile is captured by the prediction model. The mechanisms of how the residual stress would be influenced by the different machining conditions, such as surface cutting speed, and feed rate, are also studied. The proposed method provides an in-depth understanding on the machined workpiece residual stress distribution as influenced by material microstructural attributes evolution in the machining process.

Process and Microstructure in Materials-Affected Manufacturing

The fundamental understanding of manufacturing processes has been long focused on the geometric, mechanic, and thermal aspects leading to the product shape and finish. However, the effects of process mechanics attributing to material microstructural properties and constitutive characteristics are essential but not yet well understood due to the intricacy of multiple scale process-materials interaction physics. Further, the effects of materials mechanics on the process behaviors, in the context of stress and heat generations, carries significant practical relevance but has not been fully addressed in science. This is to state that manufacturing processes, such as metal forging, polymer compression modeling, 3-D printing, et al., commonly involve a significant amount of mechanical, thermal, and even chemical loadings that interact strongly with part material microstructural evolutions, which in turn determine the performance and functionality beyond just the shape and finish of the end products. On the other hand, the materials microstructure in terms of grain size, texture, phase field, etc. can also change the stress and heat generation mechanics of the manufacturing process. The scope of this paper is to present the “materials-affected manufacturing” connotation in exploring how process mechanics and materials mechanics interact retroactively with each other, and based upon this connotation better predictions of force, temperature, residual stress, and final part properties and functionalities can be possible. The materials-affected manufacturing analysis methodology involves an iterative blending scheme in combining microstructural synthesis and material homogenization analysis to allow for the interactive effects of materials dynamics and processing mechanics to be considered simultaneously. This paper discusses the basic formulation, computational configuration, and experimental validation in the example cases of machining operations with material recrystallization, grain size variation, recrystallization, texture, and phase field in consideration. Explicit calculation of material microstructure evolution path is provided as functions of process parameters and materials attributes. To factor the material microstructure states into the thermo-mechanical coupling process, the material microstructure terms are introduced into the traditional material constitutive model with hardened steels and titanium alloys as examples. Results show that residual stresses and machining forcescan be better modeled and predicted in the materials-affected manufacturing analysis platform.