Analysis of the Surface Integrity in Ultra-precision Cutting of Cp-titanium by Investigating the Chip Formation

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Procedia CIRP 13 ( 2014 ) 55 – 60

Selection and peer-review under responsibility of The International Scientific Committee of the “2nd Conference on Surface Integrity” in the person of the Conference Chair Prof Dragos Axinte [email protected]

doi: 10.1016/j.procir.2014.04.010

ScienceDirect

2

nd

CIRP Conference on Surface Integrity (CSI)

Analysis of the surface integrity in ultra-precision cutting of

cp-titanium by investigating the chip formation

F. Schneider

a,

*, R. Lohkamp

b

, F.J.P. Sousa

a

, R. Müller

b

, J.C. Aurich

a

a_{University of Kaiserslautern, Institute for Manufacturing Technology and Production Systems, 67663 Kaiserslautern}

Gottlieb-Daimler-Str., Germany

b_{University of Kaiserslautern, Institute for Applied Mechanics, 67663 Kaiserslautern, Gottlieb-Daimler-Str., Germany}

* Corresponding author. Tel.: +49-631-205-2868; fax: +49-631-205-3238; E-mail address: [email protected].

Abstract

The microstructuring of component surfaces is a method to optimize the adhesion of cells in biological systems. Ultra-precision cutting represents a technology to manufacture microstructured surfaces. When compared to conventional cutting processes additional effects influencing the material surface have to be considered, such as the size effect between the cutting edge of the tool and the depth of cut or the inhomogeneous structure with anisotropic properties. The aim of the presented research is to investigate the surface integrity in ultra-precision cutting of cp-titanium in the micro- and nanoscale. Quick-stop tests were conducted in orthogonal cutting using an ultra-precision turning lathe in order to understand the chip formation process. These investigations of the chip formation process allow to understand the resulting surface integrity. The analysis of the surface integrity concentrated on the burr formation and the surface topography.

Keywords: Ultra-precision; Surface Integrity; Chip formation

1. Introduction

Microstructuring of component surfaces have a high potential in tribological or biological systems [1]. The potential of optimizing tribological systems by microstructuring the friction partners was reported by Costa and Hutchings [2]. In biological applications, the capability of optimizing the adhesion of microbial cells was revealed by microstructuring [3].

Ultra-precision cutting processes are adequate technologies to manufacture such microstructures due to the high number of possible geometries and the high material removal rate. Moreover, the possibility of increasing the amount of free surface with higher level of surface integrity are usually desirable for a myriad of modern applications. However, in contrast to conventional manufacturing processes, further influences inherent to the micro-scale have to be considered. These influences, such as the size ratio

between the cutting edge and the undeformed chip thickness, may be summarized under the term size effect. [4, 5]. In the ultra-precision machining of surface integrity has an important meaning. Even the smallest deviations from the desired properties can lead to failure of whole systems. Investigations on the process of chip formation may be a useful way to describe the surface integrity. There are different methods to examine the chip formation, varying strongly in cost and effort. In the ultra-precision machining for example the ”SEM Direct Observation Method” is used [6]. This method represents an expensive technology concerning coasts and time. In conventional machining the Quick-Stop-Method is already an adequate technique to investigate the chip formation.

In the quick-stop-method the relative motion between workpiece and tool is suddenly interrupted so that the chip is “frozen” on the workpiece. This interruption can basically be implemented by two different ways: by either acting on the tool, or by acting on the workpiece © 2014 The Authors. Published by Elsevier B.V. Open access under CC BY-NC-ND license.

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[7]. Previous investigations show the potential of the quick-stop-method in the ultra-precision area. In this research a quick-stop device for investigating the chip and burr formation in fly-cutting is used [8].

Up to now the investigations of chip formation of titanium were focused on titanium alloys such as Ti-6Al-4V and on conventional cutting conditions [9, 10].

The aim of this research is to investigate the surface integrity in ultra-precision cutting of cp-titanium by analyzing the process of chip formation, as well as the resulting burrs and texture.

Remaining burrs experience severe plastic deformation during a component´s lifecycle, becoming accordingly harder. Therefore, the presence of burrs may lead to premature failures of components. The originated debris may act like abrasive, exposing the surface to additional stresses and surface damages.

Similarly, the final micro-texture generated by the ultra-precision processes of components must be taken into account. The presence of structures affecting the surface integrity and the subsequent use behavior.

To observe the influence of the uncut chip thickness different thicknesses from nano- to microscale are adopted.

2. Experimental Setup

The cutting experiments as well as the quick-stop tests were carried out using an ultra-precision turning lathe. A crystalline commercial pure (cp-) titanium bar (grade 2) with a diameter of d = 50 mm was used as workpiece material. Cp-titanium is used for example in biological applications due to its biocompatibility [11].

Tools containing a monocrystalline diamond (MCD) tip, which is fixed to a cemented carbide substrate, were used in order to achieve the small undeformed chip thicknesses. The well-known susceptibility to diffusion between titanium and carbon was neglected due to the low material removal level, for which only low temperatures are expected. The tool geometry and the process conditions are presented in Table 1 and Figure 1. Tab. 1 Tool geometry

Fig. 1. Process Conditions

To accomplish the orthogonal cutting process separated ridges were prepared in several parallel sections of the sample, as shown in Figure 2.

To ensure a subsequent transfer of the results to other processes such as micro end milling, all investigations were carried out at low cutting speeds. The chip thicknesses are chosen from the nano- to the microscale in order to cover a wide range.

Fig. 2. 3D Modell of Quick-Stop-Device (a), Detail Modell of the Quick-Stop-Device (b) and Experimental Setup

Tool geometry

tool orthogonal clearance α = 10°

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The quick-stop-tests were performed with a self-developed quick-stop-device (Figure 2) adapted to the requirements of an ultra-precision orthogonal cutting process, such as high stiffness. The quick-stop-device was mounted on a three axis force dynamometer for detecting the process forces. The diamond tool was fixed in a tool holder which was locked by a carriage. When the release mechanism is actuated the carriage slides horizontally rearward and releases the tool holder. The system is accelerated to such an extent by a tensioned spring in the direction of the cutting speed that a sudden interruption of the cutting operation occurs, causing the chip formation process to “freeze”.

3. Results

3.1. Chip formation

To describe the chip formation in ultra-precision cutting of cp-titanium initially the measured forces acting during the process were analyzed. In this context the specific cutting force kc and the force ratio μ, which

are characteristics of the chip formation are analyzed in dependence on the uncut chip thickness h. To study the chip formation the conducted quick-stop tests were analyzed by scanning electron microscopy. This allows the qualitatively and quantitatively analysis of the chip form and geometry. In the following the results of three cutting conditions are presented. The considered cutting conditions possess a cutting speed of 2.6 mm/s and undeformed chip thicknesses of h = 0.1 μm, h = 1 μm and h = 10 μm.

In Figure 3a the specific cutting force kc as a function

of the undeformed chip thickness is presented. It can be seen that the specific cutting force decreases with increasing uncut chip thickness. This results from the fact that some sources of cutting force are independent of the chip thickness. For example, while the friction increases with increasing depth of cut the force component arising from the separation material is independent.

The investigation of the force ratio μ (Figure 3b) revealed that this parameter decreases with decreasing undeformed chip thickness. For h = 1 μm and h = 10 μm the force ratio has a value μ higher than μ = 1. For h = 0.1 μm the value of the force ratio μ was below the μ = 1 mark. This show that the trust force become the leading force component with decreasing undeformed chip thickness.

Fig. 3 Influence of the uncut chip thickness h on the specific cutting force kc (a) and the force ratio μ (b)

Based on the gradients seen in Figure 3, the following statement can be made about the process of chip formation: The decrease of the specific cutting force is a necessary condition for a more efficient separation process. The same applies to an increase in the cutting force ratio. Due to the fact that a major proportion of the force is pointing in the direction of the material separation less friction is occurred.

A parameter to characterize the material behavior in chip formation is the chip compression ratio λ. This parameter is the ratio between the chip thickness and the undeformed chip thickness. The behavior of the chip compression ratio λ in dependence on the undeformed chip thickness h is presented in Figure 4. It is shown that from h = 1 μm to h = 0.1 μm the compression ratio rises markedly. This may be due to the strong decrease in the force ratio when μ < 1. The trust force as the leading force component leads to an increase of friction so the deformation of the material increases as well. Furthermore, the influence of the cutting edge radius increases for undeformed chip thicknesses in the dimension of the cutting edge radius. Between h = 1 μm and h = 10 μm a small rise of the chip compression ratio was detected. This behavior was less intense than the increase aforementioned. This may be attributed to the

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change in chip form during the transition from h = 1 μm to h = 10 μm.

Fig. 4 Influence of the deformed chip thickness h on the chip compression λ

In Figure 5 the scanning electron microscope images of the quick-stop-tests are shown for the three undeformed chip thicknesses: h = 0.1 μm, h = 1 μm, and h = 10 μm. The qualitative analysis of the images reveals curled continuous chips in the macro view for h = 0.1 μm and h = 1 μm. In a more detailed view small segments could be detected. While the chip with h = 1 μm having a continuous uniform shape the chip with h = 0.1 μm, shows a corrugated shape. In contrast, the chip with h = 0.1 μm have imperfections such as fringed edges or dents. For h = 10 μm the chip form changes into a segmented chip. This can be attributed to the fact that for larger undeformed chip thicknesses the state of stress changes and exceeds the deformation capacity of the material. To describe the curvature in chip formation the measurement of the radius of curvature was used. By using this technique it could be detected that with rising undeformed chip thickness the radius of curvature decreases. Although many studies have been conducted on the topic chip curling there is still no comprehensive understanding of its mechanisms [12]. Nevertheless, experimental investigations show that with increasing chip contact length the radius of curvature increases [13].

Fig. 5. SEM-pictures of chips generated in the quick-stop-test

3.2. Surface integrity

In the context of ultra-precision cutting the main criteria for evaluating the surface integrity are the burr formation and the surface topography, both depending on the later purpose of use. In this work, incident-light-, confocal- as well as scanning electron microscope images were qualitatively and quantitatively analyzed to evaluate the burr formation and the surface topography of the ridges,

The images in Figure 6 show the surfaces of the three different undeformed chip thicknesses h = 0.1 μm, h = 1 μm and h = 10 μm. It can be seen that the ridge with the uncut chip thickness of h = 10 μm in particular exhibits large and unsteady edges.

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Fig. 6. Microscope images of the surface topography (a) and the burr formation (b)

For a description of the border area the burrs formed along the ridges after cutting is presented in detail in Figure. Considering a metallographic cross section of the ridge side burrs, the so called poisson burrs, can be seen as described by [14]. The penetration of the cutting tool during machining results in plastic and elastic deformation of material. In addition to the chip formation, material is three-dimensionally deformed in front of the cutting edge. When the cutting edge performs the separation of material, the lateral displaced material as well as the material under the resulting surface remains at the workpiece as a burr.

Taking into account the process of chip formation the unsteady ridge border can be described. Contrary to the continuous chip formation with the minor undeformed chip thicknesses showed for h = 10 μm a segmented chip as seen in part 3.1. In contrast to that the border area of the ridges machined with the minor undeformed chip thicknesses are steady. This may be due to the continuous chip formation for the h = 1 μm. Looking at the border area of the ridge with the undeformed chip thickness of h = 0.1 μm this may be a result of relative slight deformations lateral to the feed direction.

To study the workpiece microstructure images with a confocal microscope were analyzed. The examination of the surface of the workpiece shows a texture of the surface (Figure 7) both in the feed direction and

transversely to the feed direction. This texture in the feed direction could also be detected on the backside of the chips (Figure 8). Therefore, it can be assumed that this structure in the feed direction is formed by the roughness of the cutting edge.

Fig. 7. Confocal microscope images of the resulting surface

The texture lateral to the feed direction was a consequence of the small cutting speed adopted. Its generation may be explained by the chip formation shown in 3.1. As seen in Figure 8 is the surface of the workpiece with the undeformed chip thickness of h = 10 μm the indefinite one. The chip formation occurred in segmented fashion. The surfaces with the minor h show a more uniform trend. The chip formation has a continuous flow relative to the undeformed chip thickness. The deformation for h = 0.1 μm was higher than that one for h = 1 μm.

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4. Conclusion and Outlook

This paper showed the characteristics of the chip formation and the surface integrity obtained by ultra-precision turning of cp-titanium, according to the uncut chip thickness, and under low cutting speed.

First, the influence of the chip thickness on the chip formation was examined. For this purpose, the characteristics of the specific cutting force and the force ratio on the chip formation were investigated. To consider the deformation behavior properly the chip compression was examined more closely. The analyses of the chip form showed that for the undeformed chip thickness of h = 10 μm a segmented chip could be observed. Beyond this the chip form for the minor h is a continuous one. The detailed view of the minor undeformed chip thicknesses showed a certain extend of segmentation. Nevertheless, in both cases, the chip could be described as a continuous chip.

The analysis of the surface integrity was focused on the study of the border area of the ridge after cutting, the burr formation and the surface microstructure. Considering the characteristics of the chip formation the surface integrity could be considered and described. The burr formation show poisson burrs as disclosed in conventional machining processes. A texture in feed direction as well as lateral to the feed direction could be detected.

In future studies, the findings are to be transferred to other areas such as the micro end milling.

Acknowledgements

This research was funded by the German Science Foundation (DFG) within the Collaborative Research Center 926 “Microscale Morphology of Component Surfaces” and the State Research focus “Advanced Materials Engineering (AME)” at the University of Kaiserslautern.

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