2212-8271 © 2013 The Authors. Published by Elsevier B.V.Selection and peer-review under responsibility of The International Scientifi c Committee of the “14th CIRP Conference on Modeling of Machining Operations” in the person of the Conference Chair Prof. Luca Settineridoi: 10.1016/j.procir.2013.06.146

Procedia CIRP 8 ( 2013 ) 534 – 539

14th CIRP Conference on Modeling of Machining Operations (CIRP CMMO)

SOM based Methodology for Evaluating Shrinkage Parameter of the Chip Developed in Titanium Dry Turning Process

M. Batistaa,*, J. Salguerob, A. Gomez-Parraa, S. Fernández-Vidala, M. Marcosa aFaculty of Engineering, University of Cadiz. C/ Chile 1, E11002 – Cadiz (SPAIN)

bPolytechnic High School, Univ. Cadiz. Avda. Ramon Puyol s/n, E11202 – Algeciras, Cadiz (SPAIN) * Corresponding author. Tel.: +34 956 015123; fax: +34 956 015100.E-mail address: moises.batista@uca.es.

Abstract

In the most of machining processes, chip morphology and microstructure contain deep information about the cutting mechanisms, their evolution and, even, about both the process and the workpiece. However, usually, changes in the process are related only to changes in the tool morphology and in the surface integrity hiding relevant information. This is commonly due to the chip characterization is not an easy task. In effect, chip geometry and dimensional features are habitually hardly measurable and evaluable. In this work, a methodology for characterizing the chip developed in the dry turning processes of Titanium alloys (Ti6Al4V - UNS R56400) has been proposed. This characterization has been achieved by considering different geometrical and dimensional parameters. Thus, shrinkage parameter has been evaluated through the changes in length and compared with the corresponding values determined through variations measured in thickness and width. However, the partially discontinuous form of the Ti chips makes difficult the dimensional evaluation. Stereoscopic Optical Microscopy (SOM) techniques can help to evaluate the chip shrinkage parameter through the measurement of the shear angle. In parallel, SOM techniques can assist for measuring other geometrical parameters of the chip. In this context, the evolution of the shrinkage parameter with the length of machining and the changes of the chip geometry with the cutting parameters (cutting speed and feed) has been also analyzed. © 2013 The Authors. Published by Elsevier B.V. Selection and/or peer-review under responsibility of The International Scientific Committee of the 14th CIRP Conference on Modeling of Machining Operations" in the person of the Conference Chair Prof. Luca Settineri Keywords: Chip morphology; Ti6Al4V; dry turning; shrinkage parameter; SOM

1. Introduction

The use of Titanium alloys has growth in the last years [1-4], especially in the airship building sector [2,5,6]. Figure 1 shows the growth of Ti alloys consumption in the civil airship building industry for the main aeronautical world companies [2,5,6]. This increase is caused by the excellent physicochemical properties of these alloys [7].

Currently, Titanium ASTM Grade 5 (Ti6Al4V) is the most commonly used titanium alloy in the Aerospace industry.

Ti alloy based airship structural and auxiliary elements need to be machined before to be incorporated direct or indirectly -as a preform- to the aircrafts structure. a considerable number of airship. These parts

must to be machined under very strict quality requirements are demanded because of, commonly, these pieces are placed in critical zones of the aircrafts. Turning, drilling and contour milling are the main machining process applied to the manufacturing process of those elements.

Fig. 1. Evolution of consumption of Ti in aircrafts building [2]

Available online at www.sciencedirect.com

© 2013 The Authors. Published by Elsevier B.V.Selection and peer-review under responsibility of The International Scientifi c Committee of the “14th CIRP Conference on Modeling of Machining Operations” in the person of the Conference Chair Prof. Luca Settineri

Open access under CC BY-NC-ND license.

Open access under CC BY-NC-ND license.

brought to you by COREView metadata, citation and similar papers at core.ac.uk

provided by Elsevier - Publisher Connector

535 M. Batista et al. / Procedia CIRP 8 ( 2013 ) 534 – 539

However, Ti alloys are considered as low

machinability materials [7,8]. On the other hand, the current legislation on environment warn about the need to avoid the use of materials and compounds that can affect negatively the environment. This is the case of the cutting fluids (lubricants, coolants) and/or can damage health [7-9]. Because of this, dry or near to dry machining processes are promoted [7,8]. This in-creases the difficulty of machining Ti alloys.

In the specialized literature, some studies about the dry cutting of Ti alloys can be found [7,8,10-15]. Most of these studies are focused either on the analysis of the surface quality of the workpieces or on the analysis of tool wear.

Usually, chip arrangement and chip morphology can contain deep information about the machinability of an alloy in an cutting process. However, it is not easy to find papers where the chip analysis is used for characterising the machining process of alloys. So, in the case of Ti alloys, only some isolated studies have been carried out, most of them based on FEM studies for obtaining numerical models [16-18]. In this sense, there is a lack of studies devoted to the determination of chip geometric and dimensional parameters and their relationship with the cutting parameters.

In this work, a study of chip shrinkage factor, , as a function of the cutting parameters has been achieved for the case of dry turning of Ti6Al4V alloy. This study has been carried out on the basis of the image analysis of chips developed during the dry machining process. These images were acquired by Stereoscopic Optical Microscopy (SOM).

2. Shrinkage Factor

Chip shrinkage factor can be considered as a measure of the material strain by compression during the cutting process. Thus, this parameter not only provides a geometric information, but also supplies information about other parameters directly related to physical phenomena that occurs during the machining process (the springback of the chip or the amount of energy accumulated by the effect of friction between the chip and rake face of the tool,...) [19].

Shrinkage Factor is defined as the ratio of the length of the chip after (LC) and before the cutting (Lm). Thus:

m

c

LL

(1) (1)

In a first approximation, a constance of volume during the cutting process can be considered. This allows calculating as the cocient between the

transversal sections SC (after machining) and Sm (before machining).

et (2)

where t is the chip thickness before cutting and e the

chip thickness after cutting. It is well known that.

sinft (3)

is the cutting edge position angle -directly

measurable and considered constant during the process- and f is the feedrate. Finally, equation [2] can be written as:

cos

sin (4)

In this equation, represents to the shear angle and

is the rake angle. Shear angle can be measured through the image analysis and rake angle is an input cutting parameter. So, shrink-age factor can be calculated from this equation and, even, its value can serve to estimate the average value of the chip thickness after cut-ting:

sincossinft

e (5)

In this work, all the aforementioned hypotheses have

been assumed.

3. Experimental

Ti6Al4V (UNS R56400) alloy bars were used as workpieces in the experimental stage of this work. Weight percent composition of this alloy is shown in Table 1.

Table 1. Composition of UNS R56400 (% wt)

C Fe H N O Al V Ti 0.10 0.20 0.015 0.03 0.20 6.0 4.0 re

536 M. Batista et al. / Procedia CIRP 8 ( 2013 ) 534 – 539

The turning tests were carried out on a turning center

EMCO Turn 242 equipped with a numerical control EMCOTRONIC TM02.

Neutral WC-Co uncoated turning inserts (55º tool point angle) were used as cutting tools, Figure 2. The ISO nomenclature for this tool is DCMT11T308. A new cutting insert was used in each test in order to avoid the wear influence on the process and, by that, in the geometry of the chip.

Test workpieces were dry turned applying cut-ting speeds (vc) from 50 to 100 m/min, feed-rates (f) from 0.05 to 0.30 mm / rev and a constant cutting depth (d) of 1 mm. Samples of the chip obtained were recorded after each test.

Fig. 2. Geometry and dimensions that define the tool used

Chip samples were prepared for their observation by Stereoscopic Optical Microscopy (SOM). In this metallographic preparation process, chip samples were embedded in resin and, later, they were polished to obtain a mirror finish. STRUERS Labopress and STRUERS Tegrapol equipments were used for it. Chips are placed in a longitudinal disposition in order to determine longitudinal section parameters. KROLL reagent was used (80% of H2O, HNO3 10% and 6% HF) in order to facilitate the evaluation of angular parameters of the chip.

Chip longitudinal section measurements were made by processing the SOM images, using ClaraVision Perfect Image v7.

SOM images were acquired from a Stereo-Microscope NIKON SMZ800 through an Optikam B5 Camera (5 Mp), using a LEDs ring as the lighting system. Calibration/resolution software allows reaching an accuracy of 0.1 μm/px.

Figure 3 shows a SOM image of a longitudinal section of a Ti6Al4V chip after a dry cutting process. In this figure, the chip geometric parameters measured in this work are plotted. So, it can be distinguished:

• ’: complementary of shear angle ( = /2 - ’); • h: height of peaks; • v: height of valleys; • e: thickness of chips.

Fig. 3. Measured chip parameters

4. Results

Figure 4 shows the macro-evolution of the chip arrangement as a function of machining time for three machining instants defined as initial, medium and final. In this figure, it can be noticed as the evolution is markedly different depending on the feed rate. In effect, shorter chip is ever obtained when feed is increased. For higher feeds the changes are less evident. However, for the lowest values of f, larger chips are disposed, which evolve to chip nest forms. On the one hand, these differences can be related to the less value of the chip thickness -equation [5]- that makes chip less rigid and more susceptible to geometric changes. On the other hand, they can be related to the changes in the rake face of the cutting tool as a consequence of the TiOx formation [20], which changes the friction behavior of the chip.

Stereoscopic Optical Microscopy (SOM) can help to understand this evolution. In effect, SOM images includes in Figure 5 show as the saw-teeth form of chip is more accused for higher feed-rates. Higher compression stresses are associated to higher feeds and so, for these values, shear-sliding effect is more intense. Moreover, for these values changes during the machining process are less noticeable, once the material shear limit is reached. For that reason, this limit is not so easy for reaching when feed values. This is in good agreement with the results obtained by other authors [17-21]. In addition, no substantial change is seen by the change of cutting speed [21].

537 M. Batista et al. / Procedia CIRP 8 ( 2013 ) 534 – 539

25 m/min 0.1 mm/rev 0.3 mm/rev

Initi

al

Med

ium

Fi

nal

Fig. 4. Macrogeometrical evolution of the chip during the machining

25 m/min 0.1 mm/rev 0.3 mm/rev

Initi

al

Med

ium

Fi

nal

Fig. 5. Evolution of the chip with the feed using SOM techniques

However, everything seems to indicate than the chip geometrical features are maintained practically constant over time machining considered, Figure 6.

Initial Chip Medium Chip Final Chip

Fig. 6. Evolution of the chip parameters considered as a function of machining time

Thus, chip macrogeometrical evolution can be associated to the existence of distributed stresses in the shear-sliding planes, which can make that chip be fragmented more quickly for lower values of feed-rates.

As a consequence of all this, for the metallographic processing and the later microscopic study, a significant fragment of the chip in the intermediate zone of the machining has been selected. Thus, the added effects that may occur during the process -such as wear- will be taken in consideration at this stage of the re-search.

Values of the parameters measured from SOM images of longitudinal sections of chips have allowed confirming the above commented observations, Figure 6.

On the other hand, Figure 7 plots the evolution of height of peaks as a function of feed for the different cutting speeds applied. Looking at this figure it can be observed as the value of peaks height increases with feed, in good accordance with the arguments above exposed.

Moreover, the effect of the cutting speed changes for the different feeds. In effect, as it can be seen in Figure 7, for the lowest values of feed, peak height decreases with cutting speed. This tendency is reversed for the highest feed values.

This can be explained in similar terms of the above observed features. Chip formation and arrangement is due to a combination of the effects of feed and cutting speed.

538 M. Batista et al. / Procedia CIRP 8 ( 2013 ) 534 – 539

Fig. 7. Evolution of the peak height as a function of feed for thedifferent cutting speeds applied

Figure 8 plots the evolution of the distance peaks-valleys as a function of cutting speeds. Notice as, inaccordance with the above commented, the values aresignificantly different for highest feeds, where,moreover, the effect of the cutting is noticeable.

Fig. 8. Evolution of the peak-valley distance as a function of thecutting speed

Figure 9 plots the evolution of the values of chipthickness (measured, e, and calculated, t) as a function of the cutting parameters. Notice as there is not significant difference between them for each value of feed and cutting speed, except when cutting speed is higher than50 m/min. This is logical because processes at cuttingspeeds upper than 50 m/min are considered high speed machining for titanium alloys and may present a different behavior, and vibrations may begin to appear inthe process due to the increase of the thrust forcerequired for chip detachment.

Therefore, it could be to say that in this case it isworking in a region where the Stabler Hypothesis isverified, so that the measured valuesshould be close tothose obtained in orthogonal cutting. This can beconfirmed by plotting the shear angle evolution as a function of cutting speed for the different feeds applied.Thus, the effect of vC and f are compensated. A potential model can be constructed for it in the form:

= 0.87 · f-0.0355ff · vC-0.0625 (6)

Fig. 9. Evolution of the Chip Thickness measured (e) and calculated (t)as a function of the cutting parameters

According to this model, soft changes can be related to the cutting parameters.

Fig. 10. Evolution of shear angle as a function of cutting speed

Chip geometric parameters evolution can be reflected in the evolution of the Shrinkage Factor as a function of the cutting parameters, vC and f. In this case, theShrinkage Factor can be adjusted to a potential model:

= 0.87 · f-0.0376ff · vC-0.0723 (7)

Notice as the evolution is very similar to thecalculated for f, according to equation [5].

5. Conclusions

Titanium is a very important element in the present industry and due to increasing consumption, it isnecessary to find techniques that allow the machiningwith efficiency.

539 M. Batista et al. / Procedia CIRP 8 ( 2013 ) 534 – 539

The chip is an element that can extract important

information about the process. The geometry of titanium chip is directly related cutting parameters used.

[12] P.J. Arrazola, A. Garay, L.M. Iriarte, M. Armendia, S. Marya, F. Le Maître, 2009. Machinability of titanium alloys (Ti6Al4V and Ti555.3). Journal of Materials Processing Technology 209, p. 2223–2230

The apparition of saw-tooth chip is directly dependent on the progress and the geometric changes can be observed and measured by processing SOM images.

The evolution of the chip with the machining time isof little significance, since -although occur morphological differences- these changes do nottranslate into significant microgeometrical changes.

[14] S. Sun, M. Brandt, M. S. Dargusch, 2009. Characteristics of cutting forces and chip formation in machining of titanium alloys. International Journal of Machine Tools & Manufacture 49, p. 561–568 [15] H.A. Abdel-Aal, M. Nouari, M. El Mansori. 2009.

Furthermore, the greatest differences are in the chips obtained when higher feeds are applied, which increasesthe peak-valley height by increasing the cutting speed.

[16] M. Calamaz, D. Coupard, F. Girot, 2008. A new material model for 2D numerical simulation of serrated chip formation when machining titanium alloy6 Ti–6Al–4V. International Journal of Machine Tools & Manufacture 48, p. 275–288 The Shrinkage Factor can evaluate the deformation of

the chip. Evolution of this variable as a function of thecutting parameters is comparable with that obtained for shear angle, according to equation [5] under an orthogonal cutting.

[17] M. Sima, T. Ozel, 2010. Modified material constitutive models for serrated chip formation simulations and experimental validation in machining of titanium alloy Ti–6Al–4V. International Journal of MachineTools & Manufacture 50, 943–960

In both of cases, similar potential models can be obtained for analyzing their evolution.

Acknowledgements

This work has been funded by the Spanish Government (Project DPI2011-29019), from theEuropean Union (FEDER/FSE) and from the Andalusian Government.

[21]

References

[1] Titanium Metals Corporation. Annual Report for 2011 [2] C. Chunxiang, H. BaoMin, Z. Lichen, L. Shuangjin, 2011. Titanium

alloy production technology, market prospects and industry development. Materials and Design 32, p. 1684–1691

[3] Critical raw materials for the EU. Report of the Ad-hoc Working Group on defining critical raw materials. European Commission Enterprise and Industry. 2010

[4] Avanced Techniques in aerospace manufacturing from Maniko. Sevanco Strategic Solutions 2012 Report

[5] Boeing Comercial Aircraft Group. http://www.boeing.com/ [6] Airbus, an EADS Company. http://www.airbus.com/ [7] M. Alvarez, J. Salguero, J.A. Sanchez, M. Huerta and M. Marcos.

2011. SEM and EDS Characterisation of Layering TiOx Growth onto the Cutting Tool Surface in Hard Drilling Processes of Ti-Al-V Alloys, Advances in Materials Science and Engineering, Article ID 414868, doi:10.1155/2011/414868

[8] J.L. Cantero, M.M. Tardio, J.A Canteli, M. Marcos, M.H. Miguelez, 2005. Dry drilling of alloy Ti–6Al–4V, Int. J. Mach. Tools and Manuf. 45, p. 1246-1255

[9] S.M.A.Sulliman, M.I.Abukari, E.F. Mirghani, 1997. Microbial contamination of cutting fluids and associated hazards, Tribology Int. 30, p. 753-757

[10] M. Álvarez, A. Gómez, J. Salguero, M. Batista, M. Huerta, M. Marcos, 2010. SOM-SEM-EDS Identification of Tool Wear Mechanisms in the Dry-Machining of Aerospace Titanium Alloys, Advanced Materials Research 107, p. 77-82

[11] E. O. Ezugwu, Z. M. Wang, Titanium alloys and their machinability—a review, 1997. Journal of Materials Processing Technology 68, p. 262-274

[13] M. Calamaz, J. Limido, M. Nouari, C. Espinosa, D. Coupard, M. Salaün, F. Girot, R. Chieragatti, 2009. Toward a better understanding of tool wear effect through a comparison between experiments and SPH numerical modelling of machining hard materials. Int. Journal of Refractory Metals & Hard Materials 27, p. 595–604

Influence of thermal conductivity on wear when machining Titanium alloys. Tribology International 42, p. 359-372

[18] J. Hua, R. Shivpuri, 2004. Prediction of chip morphology and segmentation during the machining of titanium alloys. Journal of Materials Processing Technology 150, p. 124–133

[19] M. Batista, M. Calamaz, F. Girot, J. Salguero, M. Marcos, 2012. Using Image Analysis Techniques for Single Evaluation of the Chip Shrinkage Factor in Orthogonal Cutting Process. Key Engineering Materials 504-506, p. 1329-1334

[20] J. Salguero, M. Batista, J.A. Sánchez, M. Marcos, 2011. An XPS Study of the Stratified Built-Up Layers Developed onto the Tool Surface in the Dry Drilling of Ti Alloys. Advanced Materials Research 223, p. 564-572 M.J. Bermingham, J. Kirsch, S. Sun, S. Palanisamy, M.S. Dargusch, 2011. New observations on tool life, cutting forces and chip morphology in cryogenic machining Ti-6Al-4V, International Journal of Machine Tools and Manufacture 51, p. 500–511