Thesis - PhD - Dattatraya Parle - IIT Bombay - 1.1 Background

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 Chapter 1

Introduction

1.1 Background

With increased applications of metal cutting in the creation of small objects or small features on large objects, there is a compelling need to understand the mechanics of micro-cutting is envisaged. Micro-cutting is an application of traditional metal cutting processes such as turning, milling and drilling to the micro scale manufacturing. It is also a flexible and efficient way to manufacture micro-scaled parts on a wide range of materials.

In micro-cutting, the uncut chip thickness has a significant influence on the cutting performance. As the uncut chip thickness decreases, the specific cutting energy in micro-cutting increases nonlinearly [1]. This is also known as ‘size effect’ or ‘scaling effect’ in metal cutting, which occurs due to several factors such as material strengthening, subsurface deformation, tool geometry, fracture phenomenon, workpiece microstructure, etc. [2]. The specific cutting energy in micro-cutting is given by [1]:                                                                                                (1.1)

where, 

U = specific cutting energy, 

F_c = cutting force, 

w = width of cut, 

t_o = uncut chip thickness. 

The size effect have implications in various purposes such as to know the machinability of a material, to know the machining efficiency, to design appropriate machine tools for micro-cutting, etc. Initial work related to the size effect was mostly experimental in nature. In the recent past, efforts have been made to model the size effect using analytical or numerical methods. Literature review reveals that various factors that cause size effect in micro-cutting include:

·       material strengthening,

·       subsurface deformation,

·       tool geometry,

·       fracture (microcrack and gross fracture) phenomena, and

·       Workpiece microstructure.

The first three causes have been well studied and reported in the literature, whereas the gross fracture phenomenon is a recent focus of researchers. However, the same time, very limited literature is available on the size effect caused due to microcrack formation and workpiece microstructure during micro-cutting.

It is well-known that material strength is a function of strain, strain-rate, temperature, strain gradient and material inhomogeneity. In literature, the phenomena of size effect has been attributed to material strengthening mechanism [3] because of material inhomogeneity [4-5], strain [6], strain-rate [7-11], strain-gradient [3, 12-15], thermal effects [9, 11, 16], etc. Similarly, formation of new surfaces in metal cutting is inevitable. A cutting tool not only makes contact but also exerts forces on a machined surface leading to subsurface deformation during cutting. The additional force component causes the size effect in metal cutting [17-21]. Many researchers attributed the size effect to parameters related to the tool geometry such as tool edge radius and tool rake angle. Because of the tool cutting edge radius, a significant amount of energy is involved in the flow of material over the cutting edge also known as ploughing in metal cutting [21-28]. The presence of cutting edge radius not only causes ploughing but also alters effective rake angle during micro-cutting, where cutting edge radius is comparable to the uncut chip thickness. Lucca et al. [23] showed that at higher negative rake angles, the specific cutting energy is higher during the micro-cutting experiments using a single crystal diamond tool. Additionally, researchers observed that the size effect in metal cutting at higher cutting speeds is caused either due to strain rate or thermal effects [9].

Traditional studies did not consider fracture (or crack) formation in analytical models as the cracks are usually not evident at the tip of a tool or at the root of a chip during cutting experiments on ductile materials [29]. A review of literature reveals that the following three types of cracks can occur during micro-cutting:

·       cracks can occur at the end of shear plane,

·       gross fracture can occur ahead of tool-tip where material separation occurs, and,

·       microcracks can form in the middle of shear plane.

In the brittle materials, occurrence of a crack or fracture during cutting is well accepted [30-31]. At the same time, the experimental evidences on crack formation in cutting of ductile metals have been presented by several researchers [32-37]. In the recent past, Atkins [38] presented an analytical model to evaluate fracture energy required to separate material from a workpiece in the form of a chip using the concept of fracture mechanics. It was shown that a consideration to the fracture phenomenon in the analytical modeling of metal cutting gives a better estimate of cutting forces and specific cutting energies. The Atkins’s model is widely used by various researchers to calculate fracture energy during cutting with sharp tools [29, 39-42] but significant amount of energy is involved in the flow of material over the cutting edge while cutting with rounded edge tools [43]. Moreover, Astakhov and Xiao [44] proposed that energy balance in metal cutting is further complicated and involves several other energies, besides the energies considered by Atkins [38] and Karpat [43]. Therefore, the fracture energy evaluated using Atkins’s and Karpat’s models would be higher than that of the evaluated using the energy balance proposed by Astakhov and Xiao [44]. Thus, to accurately evaluate the contribution of fracture energy in the total cutting energy, it is understood that some other parameters would be necessary.

Although, the gross fracture phenomenon ahead of the tool has been modeled to some extent, the formation of microcracks along shear zone during micro-cutting has not been modeled adequately. Researchers have observed the presence of microcracks during micro-cutting experiments [32, 45]. In a pioneering work, Shaw [46] suggested that under the influence of unusually high shear stress and shear strain conditions in metal cutting, a localized fracture in the form of microcrack forms along the shear plane. He also suggested that the microcrack formation could be one of the causes of the size effect in metal cutting. Researchers reported [32, 45] that it is not possible to measure the exact size and the number of microcracks along a shear plane. This could be due to their small size and dynamic variation in their number because of unloading effect on the tool. Moreover, in literature, there is no specific analytical or numerical formulation to determine the number, the location and the contribution of the microcracks to the size effect during chip formation.

In micro-cutting process, the uncut chip thickness is typically in the range of grain size of workpiece material which can vary from 100 nm to 100 μm [47]. The process of micro-cutting involves uncut chip thickness that is comparable to tool edge radius (r) and grain size in a work material. A combination of grain size and tool edge radius affects the mechanics of micro-cutting and has a significant influence on the size effect [48]. In the past, researchers confirmed that cutting forces depend on microstructures within work material using experimental approaches. Researchers also used numerical modeling methods to study the effect of microstructure in micro-cutting using phenomenological constitutive material models [49]. Vogler et al. [50] performed finite element based micro-cutting simulations to study the level of cutting forces during micro-cutting of pearlite and ferrite microstructures. Simoneau et al. [51] performed micro-cutting experiments and developed heterogeneous finite element model of orthogonal micro-cutting to study the chip formation and quality of machined surfaces. They approximated workpiece to consists of two materials (pearlite and ferrite) representing the microstructure of AISI 1045 steel. Chuzhoy et al. [52-53] developed a microstructure-based FE model of micro-cutting to predict the cutting forces more accurately than considering the homogeneous material model. They also found that the variation in forces during micro-cutting a multiphase material, is larger than that of in micro-cutting a single phase material.  Most of the past studies have either used phenomological models to predict work material properties or developed approximate multi-phase models without taking into account actual nature, size and distribution of grains along with grain boundaries. Moreover, there are various challenges to model actual grain and grain boundaries using traditional numerical methods. It is further understood that the microstructures have a significant impact on the size effect during micro-cutting depending on the cutting conditions.

1.2 Research gaps

1.3 Thesis outline