Analysis of structure-property gradients in orthogonally machined chips and workpiece subsurface
Introduction
In metal machining, the workpiece material is subject to severe plastic deformation (SPD) during the removal process. As a result, the deformed material in both the machined chips and the subsurface undergoes significant refinement via dynamic recrystallization (DRX), which affects the mechanical properties [1,2]. Machined chips are often a focus of study as the chip microstructure is considered to be representative of the process history [3,4]. The machined subsurface has been shown to also consist of refined microstructure [5]. Studies of the subsurface microstructure, however, are less common as the volume of affected material is significantly smaller in the workpiece surface (depth∼100 μm [6]) compared to the chip, which may be large (thickness ∼1 mm) depending on the material behavior and process conditions. Furthermore, gradients of strain and microstructure refinement are large in the workpiece subsurface whereas the chip tends to be more homogeneous. Nevertheless, gradients are present in the chip due to frictional interactions with the rake face of the cutting tool [7].
Spatial statistics are often used in material science to describe heterogeneous materials [8,9]. In recent years, these methods have been adapted to consider crystallographic texture in single phase materials [10,11]. In recent work, the present authors developed a novel metric for quantifying microstructure evolution during machining [3]. This metric quantifies spatial correlation in images obtained by electron backscatter diffraction (EBSD) microscopy. These correlation statistics capture the evolution of scale and directional anisotropy in machined OFHC Copper (Cu) chips [3].
Spherical nanoindentation protocols that enable the inference of elastic and post-elastic material behavior at small scales have been established [12]. Post-elastic behavior enables the estimation of both yield and hardening properties of the material [13,14]. In machining, these new characterization methods have not been used even though they are invaluable since the length scales involved are too small for traditional mechanical testing. Recently, micropillar compression tests, which, unlike spherical nanoindentation, require significant sample preparation effort, have been used to investigate the post-elastic behavior of machined Ni-based superalloy [7]. In this paper, we present a comparative analysis of the microstructure and mechanical properties in machined OFHC Cu. Microstructure is quantified using EBSD autocorrelation maps and the post-elastic properties are quantified using spherical nanoindentation. Gradients of microstructure and properties in both the machined chip and the workpiece subsurface are analysed, compared, and key findings discussed.
Section snippets
Experimental methods
Dry tube turning experiments that simulate orthogonal cutting were performed. Tubes were manufactured with an outer diameter of 30.48 mm and 2 mm thickness. A high speed steel cutting tool was custom manufactured with rake and clearance angles of 3.39° and 8°, respectively. Both the rake and clearance surfaces of the tool were finely ground to a surface roughness of Rq∼2.5 μm, and a cutting edge radius of < 20 μm. A feed of 0.3 mm and two cutting speeds of 0.20 m s−1and 1.00 m s−1 were used. An
Results and discussion
EBSD maps and the corresponding spatial statistics for micrographs taken at various locations in the machined chips produced at the two cutting speeds are shown in Fig. 2. These results indicate that at the tool-chip interface the microstructure is extremely fine and directionally anisotropic in the direction of chip flow. In the middle of the chip (mid-chip), the microstructure is coarser with a more “smeared” morphology and the direction rotates towards the shearing direction. At the higher
Conclusions
In this paper, microstructure-property gradients in the machined chip and workpiece subsurface were experimentally analysed using EBSD and spherical nanoindentation. The key findings of this work are:
- 1.
Material at the tool-chip interface is of finer scale than the mid-chip material. This is because friction imposes additional strain, which the mid-chip material does not undergo, and therefore grain refinement is more ‘complete’ at the tool-chip interface. Despite the presence of recrystallized
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Financial support of the work by the Morris M. Bryan, Jr. Professorship is acknowledged.
References (15)
- et al.
Surface Integrity in Material Removal Processes: Recent Advances
CIRP Annals-Manufacturing Technology
(2011) - et al.
Sub-Micrometer Structures Generated During Dry Machining of Copper
Materials Science and Engineering: A
(2003) - et al.
A Study of the Interactive Effects of Strain, Strain Rate and Temperature in Severe Plastic Deformation of Copper
Acta Materialia
(2009) - et al.
Control of Deformation Levels on Machined Surfaces
CIRP Annals-Manufacturing Technology
(2011) - et al.
Experimental and Numerical Assessment of Subsurface Plastic Deformation Induced by OFHC Copper Machining
CIRP Annals-Manufacturing Technology
(2015) - et al.
Grain Refinement Mechanism of Nickel-Based Superalloy by Severe Plastic Deformation-Mechanical Machining Case
Acta Materialia
(2019) - et al.
Finite Approximations to the Second-Order Properties Closure in Single Phase Polycrystals
Acta Materialia
(2005)
Cited by (4)
Multi-mechanism-based twinning evolution in machined surface induced by thermal-mechanical loads with increasing cutting speeds
2023, International Journal of Machine Tools and ManufacturePhysical modelling with experimental validation of high ductility metal cutting chip formation illustrated by copper machining
2022, International Journal of Machine Tools and ManufactureCitation Excerpt :Finally, in this Section it is appropriate to introduce a small number of further experimental studies of chip formation even though they do not all meet the criterion of presenting data on all three of cutting and thrust force and chip thickness ratio. All the works [1–9] describe the chip formation at all cutting speeds as continuous. A recent paper [12] describes the chip formation resulting from feeding a tool radially into a rotating disc.
Characterization of material strain and thermal softening effects in the cutting process
2021, International Journal of Machine Tools and ManufactureCitation Excerpt :To apply such approaches, the workpiece behaviors at high plastic strain, strain rate, and temperature as encountered during the cutting process must be determined. However, characterization of the material behaviors under realistic machining conditions is still a challenge because the extremely intense plastic deformation and rapid heating may lead to dynamic recrystallization [5,6] and hinder the microstructure transformation [7], respectively. However, the large strains, high strain rates, and high temperatures concentrated locally in the deformation zones near the cutting tool edge are significantly greater than those achieved by the SHB test.
Multi-physical analysis of the electrochemical behaviour of OFHC copper surfaces obtained by orthogonal cutting
2021, Corrosion Engineering Science and Technology