Procedia Engineering 132 ( 2015 ) 593 – 599

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1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the Scientific Committee of MESIC 2015doi: 10.1016/j.proeng.2015.12.536

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The Manufacturing Engineering Society International Conference, MESIC 2015

Analysis of the effects of tool wear on dry helical milling of Ti6Al4V alloy

S.R. Fernández-Vidala*, P. Mayueta, A. Riverob, J. Salgueroa, I. del Sola, M. Marcosa aUniversity of Cadiz, Faculty of Engineering. Mechanical Engineering and Industrial Design Dept. Av. Universidad de Cádiz 10, E-11519 -

Puerto Real, Cádiz, SPAIN. bFundacion Tecnalia Research & Innovation. Scientific and Technological Park of Guipuzkoa. Paseo Mikeletegui 7, E-20009, Donostia-San

Sebastián (Guipuzkoa), Spain

Abstract

It is not easy to find papers that contain studies about the holes development in Ti alloys by alternative machining processes such as helical milling. The increased competitiveness and legislative requirements in the aeronautical sector has caused the necessity to develop products and processes seeking the greatest profitability and sustainability, taking as reference the so-called four axes for manufacturing performance: economy, energy, environment and functionality. This work reports on the first results of a study about the influence of cutting speed and feedrate on the hole finishing in the oriented helical dry milling process of alloy Ti-6Al-4V. © 2016 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of MESIC 2015.

Keywords: milling; drilling, dry, Ti6Al4V, wear; quality;

1. Introduction

In the airship building industry, all elements are assembled to form the final product [1]. The joining method by rivets is the most widely used in the aerospace assembly of structural elements [2]. This requires prior drilling operations with high quality to prevent future structural damages [3].

* Corresponding author. Tel.: +034 956 01 51 23; fax: +034 956 01 52 57.

E-mail address: raul.fernandez@uca.es

© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the Scientific Committee of MESIC 2015

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Legislative requirements and the increased competitiveness and in the aeronautical sector has caused the necessity to develop products and processes seeking the greatest profitability and sustainability, taking as reference the so-called four axes for manufacturing performance: economy, energy, environment and functionality [4].

The dry machining is being currently tested in aircraft manufacture to meet the required environmental conditions.

Ti6Al4V alloy is highlighted, among the most representative metal alloys in this sector –mainly as structural components and engine parts. This is due to its excellent toughness, light weight, and high resistance to fatigue and corrosion in service at high temperatures.

Among the most negative features in dry drilling of titanium alloys is a progressive tool wear. This involves a reduction in the process efficiency caused by a rise in cutting forces and cutting edge temperatures. So, while tool wear increases, vibrations appears and reduce the finishing quality of generated surfaces. These negative features are emphasized in dry drilling of titanium alloys.

While in the conventional drilling process is carried out by a rotating cutting tool with axial feedrate. In helical milling the hole is produced by a milling cutting-tool, with rotational movement, and following a spiral tool path to machine the work piece. Consequently, the frontal edges are constantly drilling and the peripheral edges are milling. The machining length depends on the axial distance in each turn and the orbital path (2·π·ROrbital tool).

Although the operation time can be higher in helical milling. However, tool edges is under lower pressures and temperatures when helical milling processes are achieved. It can be proposed for avoiding some negative thermal-mechanical effects of classical drilling. In the four axes sustainability, this involves a benefit for energetic, environmental and, even, economical performances. Thus, at any cases, surface quality and tool life could be favorably affected by the use of orbital drilling.

2. Experimental Methodology

In this preliminary study, helical milling (HM) tests were performed using a 6 mm diameter mill (see geometry in Fig. 1) for drilling holes of diameter 7.92 mm on Ti6Al4V (UNS R 56400) alloy plates of 297x210x8mm . HM tests were carried out in a horizontal three-axis CNC Agil-2G machine. A clamping system has been especially designed for the machining operations. This system is based on a plane support frame with nine fixing elements, and it makes easier the assembly process between pieces and dynamometer, minimizing deviations in parallelism and homogeneity in forces application.

Fig. 1. (Left) Helical milling test. (Right) Geometry of the tool

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Different combinations of speeds and feedrates have been applied (Table 1). Ten holes have been machined in each condition in order to determine the main characteristic changes in the first stages of the process.

Dimensional tolerance and surface roughness have been used for evaluating the holes quality. Some plastic-form replicas have been made for the roughness evaluation. For this phase the equipment used is a

Perthometer PAV-CV with a step rate PGK 120. Diameters have been measured with a three-contact internal micrometer. The diameter value has been taken as the average between four measurements taken in each hole. These values have been recorded at different depths into the holes (Fig. 2).

Table 1. Cutting conditions for 7.92 mm diameter holes

Fig. 2. (Left–Center) Hole roughness evaluation with replica. (Right) Three-contact internal micrometer

3. Results and discussion

3.1. Geometric and compositional alterations in the tool.

Tool wear has been monitored using SOM-SEM-EDS techniques to identify the main characteristic tool wear mechanisms, which take place in the HM process. The changes on the tool was conducted by applying Stereoscopic Optical Microscopy (SOM) techniques, through a Nikon SMZ800 Stereoscopic Microscope. A deeper study has been achieved by using Scanning Electron Microscopy (SEM) through a Phillips QUANTA microscope and Energy Dispersive Spectrometry (EDS) through an EDAX Analyzer attached to the cited microscope.

So, at least in the first 10 holes machined, it has been able to distinguish two main wear mechanisms: secondary adhesion and diffusion.

Test Tool diameter

Hole diameter

Orbital rate

Axial tool path

Rotational Movement

Vc Sspindle f F Translation Movement

Helicoidal

mm mm mm mm m/min rpm mm/rev mm/min nº rev1 6 7,92 0,96 13 Down Milling 24 1299 0,050 65 Counterclockwise 122 6 7,92 0,96 13 Down Milling 24 1299 0,075 97 Counterclockwise 83 6 7,92 0,96 13 Down Milling 24 1299 0,100 130 Counterclockwise 64 6 7,92 0,96 13 Down Milling 49 2599 0,050 130 Counterclockwise 65 6 7,92 0,96 13 Down Milling 49 2599 0,075 195 Counterclockwise 46 6 7,92 0,96 13 Down Milling 49 2599 0,100 260 Counterclockwise 37 6 7,92 0,96 13 Down Milling 73 3898 0,050 195 Counterclockwise 48 6 7,92 0,96 13 Down Milling 65 3465 0,075 260 Counterclockwise 39 6 7,92 0,96 13 Down Milling 73 3898 0,100 390 Counterclockwise 210 6 7,92 0,96 13 Down Milling 98 5197 0,050 260 Counterclockwise 311 6 7,92 0,96 13 Down Milling 98 5197 0,075 390 Counterclockwise 212 6 7,92 0,96 13 Up Drilling 73 3898 0,100 390 Clockwise 2

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Secondary adhesion wear appears due to high pressures and high chemical affinity between workpiece (Ti) and tool materials [5]. TiOx precursors are formed onto the tool surface, which facilitates the mechanical adhesion of alloy to the tool, Fig. 2. When adhesion appears the tool consistency decrease in Ti-6Al-4V helical milling.

The diffusion between carbon atoms (tool) is faster than metal atoms (Co and W) [6]. This is one of the reasons of the fragility produced in cutting-tools during the machining of titanium alloys [7] (Fig. 3).

Fig. 3. Titanium adhesion onto the tool surface. Diffusion wear in the tool

Subsequent effects of adhesion and diffusion wear mechanisms can be observed in peripheral edge wear, which is the most related with surface quality. These effects have been distinguished as chipping and flank wear: Chipping is the term used to describe the apparition of fractures, increasing in the union between frontal and

peripheral edges. The main reason of this is the particularities of this zone, the larger cutting speeds is localized along the frontal edge and the larger stress is along the peripheral edge. In this cross there is a temperature increase [6]. Furthermore, the g no-constant cutting process is the origin of thermal cycles that benefits tool wear [8]. This tool wear mechanism is more common in frontal than in peripheral edges. This kind of tool wear increase with the machining time generating the fracture of the tool [9] (Fig. 4).

Fig. 4. Different observed forms of tool wear in the drilling process

3.2. Helical milling holes

SOM observations of holes reveal that it must be carefully planned the full trajectory of the helical milling in order to avoid loss of circularity in the final form of the holes as a consequence of an incomplete milling. It is caused when the tool has not exceeded the machining plate the same distance of the feed rate per revolution (Fig. 5). Metrological analysis of those holes have not been included in this study.

Adhesion

Difussion wear

Fracture Flank Wear Chipping

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Fig. 5. (Left) Loss of circularity (diameter changes) during the orbital drilling process. (Right) Defects when up milling strategies are used

On the other hand, the average roughness (Ra) values for 7.92 mm diameter holes have been found between 0.46 and 2.33 µm, as it is plotted in Fig. 6. In this figure, it can be observed as, in spite of the detected tool wear, all the Ra values can be considered as acceptable, and the most of them are so far of the Ra permitted values in the airship building industry.

Notwithstanding, looking at Fig. 6, it can be appreciated how the roughness values show a lighttrend to increase as a result of evolution of the tool wear in the peripheral cutting edges.

Fig. 6. Average Roughness, Ra, in the orbital drilled holes

The dimensional tolerances in diameter for 7.92mm holes are between 7.877 and 7.908mm (Fig. 7). Most of the holes are under the nominal diameter. This can be explained by heat generation during the process. Even under low cutting speeds, the temperature in the cutting zone for Ti alloys can reach 1000 °C [5]. Consequently, there is a thermal expansion of the workpiece that has a direct influence into the diameter size. The tool wear behaviour has to be also taken into account, there is a continuous tool diameter reduction during drilling [10]. All this makes necessary to re-consider the milling strategies to reach diameters values into the required tolerances interval.

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Fig. 7. Diameter orbital drilling tests

The best results in diameter and roughness were obtained in tests 4 and 5 with a speed of 49 m/min and feed of 0.05 and 0.075 mm/rev, but the wear on the tool is more pronounced in test 4 with fractures at the limits of the front edge. Because of this, intermediate cutting speeds and low feedrates are recommended. Intermediate cutting speeds compensate for effect of low Vc -avoiding excessive compressive stresses- and high Vc -avoiding excessive high temperatures. In both of causes, it favors a better approximation to the nominal diameter. In a similar way, low feedrates allow obtaining better values of Ra [4] favoring the surface quality of the holes. Both effects are in good agreement with the observations made on the tool.

4. Conclusions

Different drilling strategies have been followed. It has been determined the necessity of taking into account the space covered by the tool from the hole. It must be at least equal to feed per helical turn in the orbital milling to prevent finishing defects.

Increased cutting temperature resulting from the friction produced by the front edge of the tool in the forward and high speed peripheral edge cuts, causes the bond areas in the front edge with the peripheral edge there is a greater wear mechanisms of adhesion and diffusion. As a result increases the likelihood of fracture of the tool during machining.

Higher cutting speeds worsen the quality of hole caused by thermal damage (fatigue, plastic deformation, chemical reaction) and lower speeds to increase the possibility of mechanical damage to the tool (chipping and fracture). On the other hand, low feedrates allows reaching very good average roughness values.

With the increase of the cutting parameters, an acceleration occurs in the tool wear and, therefore, the quality of the finished drill. While progress with increasing the forces produced by the front edges occurs, the cutting speed produces increased wear on the peripheral edges defining the finish of the hole.

This work has been focused on a preliminary study of the dry orbital drilling process of Ti-6Al-4V alloy. The holes quality has been analysed from the dimensional deviations of diameter and from the average roughness evaluated through plastic replicas. Results have been also related with the tool wear during the machining process.

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Acknowledgements

The UE (Project COSSTA), from the Andalusian Goverment (PAIDI) and from Fundacion Tecnalia R&I

(through the CATEDRA TECNALIA of the UCA) have given financial support to this work.

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