Titanium and nickel alloys represent a significant metal portion of the aircraft structural and engine components. When these critical structural components in aerospace industry are manufactured with the objective to reach high reliability levels, surface integrity is one of the most relevant parameters used for evaluating the quality of finish machined surfaces. The residual stresses and surface alteration (white etch layer and depth of work hardening) induced by machining of titanium alloys and nickel-based alloys are very critical due to safety and sustainability concerns. This review paper provides an overview of machining induced surface integrity in titanium and nickel alloys. There are many different types of surface integrity problems reported in literature, and among these, residual stresses, white layer and work hardening layers, as well as microstructural alterations can be studied in order to improve surface qualities of end products. Many parameters affect the surface quality of workpieces, and cutting speed, feed rate, depth of cut, tool geometry and preparation, tool wear, and workpiece properties are among the most important ones worth to investigate. Experimental and empirical studies as well as analytical and Finite Element modeling based approaches are offered in order to better understand machining induced surface integrity. In the current state-of-the-art however, a comprehensive and systematic modeling approach based on the process physics and applicable to the industrial processes is still missing. It is concluded that further modeling studies are needed to create predictive physics-based models that is in good agreement with reliable experiments, while explaining the effects of many parameters, for machining of titanium alloys and nickel-based alloys.

Machining induced surface integrity in titanium and nickel alloys: A review

Surface integrity in material removal processes: Recent advances

Nickel-based super alloys are gaining more significance, now-a-days, with extensive applications in aerospace, marine, nuclear reactor and chemical industries. Several characteristics including superior mechanical and chemical properties at elevated temperature, high toughness and ductility, high melting point, excellent resistance to corrosion, thermal shocks, thermal fatigue and erosion are primarily responsible for wide domain of application. Nevertheless, machined surface integrity of nickel-based super alloys is a critical aspect which influences functional performance including fatigue life of the component. This review paper presents state-of-the-art on various surface integrity characteristics during machining of nickel-based super alloys. Influence of various cutting parameters, cutting environment, coating, wear and edge geometry of cutting tools on different features of surface integrity has been critically explained. These characteristics encompass surface roughness, defects (surface cavities, metal debris, plucking, smeared material, redeposited material, cracked carbide particles, feed marks, grooves and laps), metallurgical aspects in the form of surface and sub-surface microstructure phase transformation, dynamic recrystallisation and grain refinement and mechanical characteristics such as work hardening and residual stress. Microstructural modification of deformed layer, profile of residual stresses and their influence on fatigue durability have been given significant emphasis. Future research endeavour might focus on development of new grades, advanced processing techniques of the same to ensure their superior stability of microstructure and thermo-mechanical properties along with advanced manufacturing processes like additive manufacturing to achieve highest level of fatigue durability of safety critical components while maintaining acceptable surface integrity and productivity.

State-of-the-art in surface integrity in machining of nickel-based super alloys

Tension–compression asymmetry of the stress–strain response in aged single crystal and polycrystalline NiTi

An experimental investigation of the micro and macromechanical transformation behavior of polycrystalline NiTi shape memory alloys was undertaken. Special attention was paid to macroscopic banding, variant microstructure, effects of cyclic loading, strain rate and temperature effects. Use of an interference filter on the microscope enabled observation of grain boundaries and martensitic plate formation and growth without recourse to etching or other chemical surface preparation. Key results of the experiments on the NiTi include observation of localized plastic deformation after only a few cycles, excellent temperature and stress relaxation correlation, a refined definition of “full transformation” for polycrystalline materials, and strain rate dependent effects. Several of these findings have critical implications for understanding and modeling of shape memory alloy behavior.

Stress-induced transformation behavior of a polycrystalline NiTi shape memory alloy: micro and macromechanical investigations via in situ optical microscopy

Controlled orthogonal and controlled oblique machining of annealed AISI 4340 have been undertaken in a design of experiments framework to investigate the machining-induced residual stresses resulting from these processes. The experimentation demonstrates significant simplifications in the machining-induced residual stress problem when the stresses are expressed in a coordinate system fixed in the tool and also indicates that the directions along the cutting edge and normal to the cutting edge of the tool are principal directions of the machining-induced residual stresses. Based on the experimental results, a plane strain thermoelastoplastic model of metal flow under the flank of a cutting tool is developed to predict the full in-plane biaxial residual stress profiles existing at and beneath the newly created surface. Calibrated results show favorable agreement with the experimental machining-induced residual stresses in annealed AISI 4340.

Machining-Induced Residual Stress: Experimentation and Modeling

The effect of stress state on the character and extent of the stress-induced martensitic transformation in polycrystalline Ni-Ti shape memory alloy has been investigated. Utilizing unique experimental equipment, uniaxial and triaxial stress states have been imposed on Ni-Ti specimens and the pseudoelastic transformation strains have been monitored. Comparisons between tests of differing stress states have been performed using effective stress and effective strain quantities; a strain offset method has been utilized to determine the effective stress required for transformation under a given stress state. Results of the tests under different stress states indicate that (1) despite the negative volumetric strain associated with the austenite-to-martensite transformation in Ni-Ti, effective stress for the onset of transformation decreases with increasing hydrostatic stress; (2) effective stressvs effective strain behavior differs greatly under different applied stress states; and (3) austenite in Ni-Ti is fully stable under large values of compressive hydrostatic stress.

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Effect of stress state on the stress-induced martensitic transformation in polycrystalline Ni-Ti alloy

The effect of different uniaxial and triaxial stress states on the stress-induced martensitic transformation in CuZnAl was investigated Under uniaxial loading, it was found that the compressive stress level required to macroscopically trigger the transformation was 34 pct larger than the required tensile stress The triaxial tests produced effective stress-strain curves with critical transformation stress levels in between the tensile and compressive results It was found that pure hydrostatic pressure was unable to experimentally trigger a stress-induced martensitic transformation due to the large pressures required Traditional continuum-based transformation theories, with transformation criteria and Clausius-Clapeyron equations modified to depend on the volume change during transformation, could not properly predict stress-state effects in CuZnAl Considering a combination of hydrostatic (volume change) effects and crystallographic effects (number of transforming variants), a micromechanical model is used to estimate the dependence of the critical macroscopic transformation stress on the stress state

Stress-induced martensitic phase transformations in polycrystalline CuZnAl shape memory alloys under different stress states

Residual stress measurements from endmilling of annealed AISI 4340 have been made at multiple points within the cut geometry to investigate the effects of location on the machining-induced residual stresses from endmilling. In the same experiments the effects of axial depth of cut and feed on the residual stresses induced in the machined surface have been investigated in a design of experiments framework. The experimentation demonstrates that location, feed and axial depth of cut have strong influences on the machining-induced residual stresses from end-milling when expressed in a workpiece coordinate frame. In addition, expression of the residual stresses in a coordinate frame fixed in the tool demonstrates a simplification in the residual stresses from end-milling in that the stresses at multiple locations within the cut geometry show strong similarities when expressed in this coordinate frame.

Experimentation on the Residual Stresses Generated by Endmilling

A model to predict the full biaxial surface and subsurface residual stresses from the turning process is presented. The model formulation includes thermomechanical coupling, plastic heating, frictional heating, convection, conduction, thermal softening and strain hardening. Calibration and validation of the model are undertaken. The predictive model stands as a tool both to optimize the turning process based on the resulting residual stresses and also to gain an in-depth understanding of the physics of residual stress development from the turning process. In addition, thorough analysis of the experimental results reveals insight into the effects of feed and depth of cut on the surface and subsurface machining-induced residual stresses.

Kurt Jacobus

Papers

Machining-Induced Residual Stress: Experimentation and Modeling

Effect of stress state on the stress-induced martensitic transformation in polycrystalline Ni-Ti alloy

Stress-induced martensitic phase transformations in polycrystalline CuZnAl shape memory alloys under different stress states

Experimentation on the Residual Stresses Generated by Endmilling

Predictive model for the full biaxial surface and subsurface residual stress profiles from turning