Surface roughness study of UNS A97050-T7 bars obtained by dry turning.

De Agustina, B. ; Marin, M. ; Rubio, E. 等

1. Introduction

The aluminium alloys are considered strategic materials in the aeronautical, motor and aerospace sectors due to their excellent weight to resistance ratio. Specifically, they are employed in the production of different elements that compose aircraft and aerospace vehicles. For such application an improved surface quality for the mechanized parts is required.

In spite of the important role these materials have from a competitive point of view, they can commonly show problems of machinability associated with the heat generated during the machining process, mainly due to the strong tendency to adhere to the tool these materials have. For this reason, although it is possible to evacuate the heat by means of the chip, the tool and the workpiece, cutting fluids are still widely used (Agustina et al., 2008). This fact seriously degrades the environment quality and increases the cost of machining. As a result, dry machining has been extensively studied in recent years (Nouari et al., 2003).

Such a situation makes it necessary to look for new tool designs or, a cheaper alternative is to look for combinations of cutting parameters and types of tools that optimize the machining process, allow to obtain workpieces with a good dimensional precision and a high quality surface finish, keep the cost as low as possible and, of course, ensure secure conditions for workers and equipment (Rubio et al., 2008)

In this study in order to analyse the evolution of the surface quality of an aluminium UNS A97050--T7 bar with respect to the cutting parameters employed (cutting speed and feed rate) a series of dry turning tests (no longer than 10 seconds) were carried out using tools with coating of TiN.

2. Methodology

This investigation is framed within a series of studies in which different materials, types of tools and cutting conditions are involved. Then, in order to systematize all the steps to follow until obtaining the proposed objectives, a work methodology has been developed. The main steps of the methodology are (Agustina et al., 2007):

Previous activities to the machining operations. These activities consist on the identification of the used resources and the preparation of the protocols both to calculate cutting parameters values and to register data and observations of the machining process.

Turning tests. In each test a workpiece is mechanized during less than 10 seconds (short tests) under certain conditions of feed, cutting speed and depth of cut.

Collection of the chips. Chip or chips obtained in each test are collected, identified and saved so they are perfectly identified and accessible for any later confirmation to the realization of the tests.

Designation of the chips. The different types of chips obtained during the cutting tests have been designated according to their morphology, based on the ISO 3685 standards (table 1).

Classification of the chips. Once chips have been called according their basic form and type they have been cassified as favorable or unfavorable form the point of view of the security of the process, the tool integrity and surface finish.That is, a chip will be considered as favourable with regard to: the security of the process, whether it does not suppose a dangerous for its continuity; the tool integrity, whether if does not produce or can produce the failure/wear of the tool; the surface finish, whether it does not casuse an important deterioration of the workpiece surface.

Monitoring of the process. In order to have graphic documents that can be analyzed after the process, all the turning tests described previously, have been recorded by video and both the chips obtained and the inserts used in each one of them photographed with a camera of high resolution.

Previous activities to the roughness measurement. In order to systematize tested pieces roughness measurements, first of all, the measurement process has to be defined. This is, basically, to measure the roughness along four lines separated n/2 radians in each one of the tested pieces. So, to carry out the measurements it is necessary to dispose properly a series of auxiliary elements, and measure instruments and to verify that all they are in perfect state.

Roughness measurement. According to the measure process defined previously, roughness measurement has to be made along the machined length. In this measure process, a data (xh z) of the surface geometry of the piece are obtained. These data have to be recorded in the suitable format so that later they can be used by the available software.

Data processing and analysis of results. A first approach to the study of the surface quality of the mechanized pieces has been made in this work. The arithmetical average roughness, Ra, has been selected as a parameter to analyse. According to ISO 4288 standards (ISO 4288, 1998), this parameter is defined like the arithmetical average of the absolute values of the deviations of the profile of roughness R and is expressed mathematically by means of the equation (1):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (1)

The data processing consists on calculating and plotting the average value of Ra in the different sections verified in the four lines mentioned before, for those sections, so much the measured Ra value as its average value. The data analysis is made qualitatively, in terms of the evolution shown in the graphs, and quantitatively, by comparison of the obtained values for the different used parameters.

Analysis of tools. From the obtained results, a tools preselection has been made. Then, selected tools should be analysed both macroscopic and microscopic techniques. The first ones, using the taken macrographs and a profilemeter that allows measuring the quantity of the adhered material and the second ones, by means of techniques of Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometer (EDS) in order to verify the alterations of the geometry of the tools.

3. Experimental layout and materials

For this study, the workpiece used for the turning test was a cylindrical bar with a diameter of 54 mm and length of 90 mm of UNS A97050--T7 aluminium alloy. Its composition in percentage of mass has been included in Table 2:

Tab. 2. Composition (% mass) of UNS A97050 alloy

Cu     Mg     Zn      Cr     Fe
1.60   2.37   16.56   0.03   0.09

Mn     Ni    Si     Ti      Zr     Al
0.05   0.01  0.06   0.018   0.10   Rest

The cylindrical bar was horizontally dry turned on an EmcoTurn 120 CNC lathe equipped with an EMCO Turn 242 numerical control. The cutting conditions applied were cutting speeds from 40 m/min (0.66m/s) up to 170 m/min (2.83 m/s) and feeds from 0.05 mm/rev up to 0.30 mm/rev. Cutting depth was maintained at 1 mm in all the tests. TiN coated tools (manufacturer reference SECO DCMT 11T308F2--TP1000) were employed for the tests.

To observe the machining tests carried out, photographs and videos of the tools and the resulting chips were taken systematically during the tests using a Sony Cybershot DSC--P100 digital camera of high resolution.

To define the surface quality of the workpiece it was selected the parameter Ra (the arithmetical average roughness). The cut--off length was taken as 0.8 mm and the sampling length as 4 mm. Additionally, the SurfTest SJ--401 software allows carrying out roughness evaluation by means of protocols where it is possible to select, the roughness profile, R, and different parameters such as Ra, Rz and Rq.

For quantifying the adhered material to the tools a profilemeter TOPCON VP300D has been used and for the SEM/EDS analysis, a Scanning Electronic Microscope, called Quanta 200, which has a system of Energy Dispersive Spectrometer has been used as well.

[FIGURE 1 OMITTED]

4. Results

Table 3 represents the values of the roughness (Ra) measured along four lines separated n/2 radians: G1, G2, G3 and G4, for each cutting parameters applied.

To facilitate the analysis of the surface quality obtained on the workpiece under the different combinations of cutting parameters tested, two similar graphics were designed to show the evolution of the roughness with respect to the feed (figure 2) and with respect to the cutting speed (figure 3). The values of the roughness Ra represented on both graphics, correspond to the arithmetical average of the parameter roughness Ra measured along G1, G2, G3 and G4 for each test.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

From figure 2, it can be seen that at cutting speeds of 40, 85 and 170 m/min, the higher the feed, the higher values of roughness was obtained, as was expected (Rubio et al., 2005), (Kulenovic et al., 2007). Thus, it is important to indicate that at these condition parameters, a larger quantity of material was adhered to the tool, this fact could also contribute to obtain less quality of the surface machined. To illustrate this effect on the tool, figure 4 shows the tools employed at different feeds (0.30 and 0.10 mm/rev) and identical cutting speeds (125 m/min). It can be seen the material adhered to the tool is considerably greater at the higher feed, despite the tests carried out in this study lasted less than ten seconds. In fact, specifically at low cutting speeds, when the adhesion to the tool is mechanical, the continuous sliding of the fragments from the chips to the tool causes an increasing tool wear (list et al., 2005).

[FIGURE 4 OMITTED]

On the other hand, analyzing the evolution of the roughness with the cutting speed, in figure 3, it can be observed that the tendency of the values of the roughness measured was the opposite as it was with respect to the feed, though less accentuated, specially at 0.30 mm/rev for the higher cutting speeds applied 85, 125 and 170 m/min. Under such cutting conditions the values of roughness hardly varied.

5. Conclusions

The main conclusions extracted from this study are:

--The surface quality of the aluminium UNS A97050--T7 bar which is obtained during dry turning short tests (no longer than 10 seconds) improves with the descent of the feed and with the increase of the cutting speed. From these two parameters, the feed is the parameter more influential on the surface roughness.

--Taking into account these results, it would be convenient to carry out new tests using values of the nearer feed to the values that have lead to obtain the best surface quality and compare with the suitability of the chips obtained during the machining in order to complete the work.

--To further study the machinability of the UNS A97050--T7 alloys, it would be convenient to carry out new tests using other types of tools with different geometry and materials, applying other cutting depths and analysing other aspects of the machining such as the cutting force and temperature in the cutting area.

DOI: 10.2507/daaam.scibook.2009.90

6. Acknowledgment

Funding for this work was provided in part by the Spanish Ministry of Education and Science (Directorate General of Research), Project DPI2005-09325 c02--02.

7. References

Agustina, B.; Rubio, E.M.; Sanz, A. & Domingo R. (2007). A classification of the UNS A97050--T7 aluminium alloy chips in short duration tests under dry cutting conditions, Proceedings of the MESIC--CISIF'2007, SIF, Sebastian, M. A. (Ed), pp. 1--8, ISBN: 978--84--611--8001--1, Madrid, July 2007

Agustina, B.; Rubio, E.M.; Marin, M.M.; Sebastian, M.A. (2008) Analysis of the material adhered on inserts with and without TiN coating during the dry turning of the aluminium alloy UNS A97050--T7, Proceeding of the CIRP ICME08, Teti, R. (Ed), pp. 1--6, ISBN: 978--88--900948--7--3, Naples July 2008

Agustina, B.; Rubio E.M.; Marcos, M. (2007) Study of the adhered material to the cutting tools on dry turning of aluminium alloys, Annals of DAAAM for 2007, October 2007, 215--216, ISSN: 1726--9679

Kulenovic, M.; Begic,; Cekic, A. (2007) Experimental Investigation of Carbon Steel in High Speed Cutting, Annals of DAAAM for 2007, Katalinic, B. (Ed), pp. 411--412, October 2007, ISSN: 1726--9679

List, G., Nouari, M., Gehin, D., Gomez, S., Manaud, J.P., Le Petitcorps, Y., Girot, F, (2005). Wear behaviour of cemented carbide tools in dry machining of aluminium alloy, Wear 259 (2005) 1177--1189, ISSN: 0043--1648

Nouari, M.; List, G.; Girot, F.; Coupard, D. (2003). Experimental analysis and optimisation of tool wear in dry machining of aluminium alloys, Wear Vol. 255, No.(7--12), 1359--1368, August--September 2003, ISSN: 0043--1648

Rubio, E.M.; Camacho A.M.; Sanchez--Sola, J.M.; Marcos, M. (2005) Surface roughness of AA7050 alloy turned bars. Analysis of the influence of the length of machining, Journal.of Materials.Processing of. Technology. Vol. 162--163C, pp. 682--689, May 2005, ISSN: 0924--0136

*** ISO 3685:1993, Tool--life testing with single--point turning tools, 1993.

*** ISO 4288:1998, Geometrical product specifications (GPS). Surface texture: profile method. Rules and procedures for the assessment of surface texture, 1998: 1--20

This Publication has to be referred as: De Agustina, B[eatriz]; Research and teaching staff. Marin, M[arta] & Dr. Rubio, E[va] (2009). Surface Roughness Study of UNS A97050--T7 Bars Obtained by Dry Turning, Chapter 90 in DAAAM International Scientific Book 2009, pp. 923--930, B. Katalinic (Ed.), Published by DAAAM International, ISBN 978-3-901509-69-8, ISSN 1726--9687, Vienna, Austria

Authors' data: Research and teaching staff, De Agustina, B[eatriz]; Research and teaching staff. Marin, M[arta]; Dr. Rubio, E[va], National Distance University of Spain (UNED), Department of Manufacturing Engineering, C/ Juan de Rosal 12, E28040 Madrid, bdeagustina@ind.uned.es

Tab. 1. Adapted classification from ISO 3685 (ISO 3685, 1993)

                                                Short/
                      Type/         Long/Flat/   Conical/   Snarled
Cutting               Basic forrn   Conn         Loose

Straight              Ribbon        1.1          1.2        1.3
Mainly up curling     Tubular       2.1          2.2        2.3
                      Spiral        3.1          3.2
Mainly side curling   Washer        4.1          4.2        4.3
Up and side curling   Conical       5.1          5.2        5.3
                      helical
                      Arc           6.1          6.2

Tab. 3. Ra ([micro]m) measured under the different cutting parameters
applied

v(m/min),             Ra ([micro]m)
/(min/rev)   G1      G2      G3        G4

170,0.30     2.796   3.689   2.875   3.616
170,0.20     1.569   1.537   1.522   1.537
170,0.10     0.592   0.576   0.593   0.578
170,0.05     0.316   0.403   0.396   0.424
125,0.30     3.386   3.326   3.313   3.313
125,0.10     0.488   1.383   0.564   1.780
125,0.05     0.482   0.537   0.458   0.627
85,0.30      3.351   3.496   3.398   3.215
85,0.20      1.556   1.633   1.651   1.823
85,0.10      0.812   1.011   1.025   1.058
85,0.05      0.881   0.799   0.905   1.082
65,0.30      3.754   3.669   3.78    3.624
65,0.10      1.385   1.573   1,690   1.730
65,0.05      1.135   1.369   1.127   1.507
40,0.30      4.31    3.935   4.509   4.617
40,0.20      3.225   3.799   2.929   3.628
40,0.10      1.603   1.814   1.583   2.115
40,0.05      1.900   2.215   1.785   2.579