Influence of cutting conditions and tool coatings on the surface finish of workpieces of magnesium obtained by dry turning.

De Pipaon, Jose Saenz ; Rubio, Eva ; Villeta, Maria 等

1. INTRODUCTION

The magnesium is the lightest metallic material that can be used in industry. Namely, it is ideal for applications in industries such as aeronautical, aerospace, automotive, medical, electronic or sports, where the ratio density to resistance must be low (Caton, 1991, Gil et al., 2001, Tonshoff et al., 1997).

However, this material presents problems with the heat generated in the machining process, since it has a tendency to be flammable. Autoignition temperature (430[degrees]C) is lower than the melting temperature (650[degrees]C), so any spark can cause the ignition of chips or dust. Besides, the use of water based coolants in the machining of magnesium alloys is dangerous in case of chip ignition; burning magnesium will decompose water to form hydrogen atmospheres which are highly explosive. If a fire occurs, water should never be used to extinguish magnesium fires, dry sand or a suitable extinguisher for fire involving metals should be used.

On the other hand, the use of lubricants or coolants during the machining constitutes an undesirable factor owing to economical and environmental factors, being necessary to develop cleaner manufacturing technologies such as the dry machining. Then, the machining of the magnesium should be under dry conditions, not only because of the economical and environmental factors but, as well, of the security of the process.

In such situation, it is necessary to combine the cutting parameters and types of tools to optimize machining processes that involves magnesium. This allows obtaining pieces with a good dimensional precision and a high quality of the surface finish with a cost as low as possible, and of course with security conditions for workers and equipment.

In industries such as aeronautical and motor because of the high cost of manufacturing components, repair and maintenance operations are usually required. In such operations are used low values of cutting speeds, feeds and depths of cut, maintaining stringent requirements of surface roughness (usually 0.8 < Ra < 1.6[micro]m).

This study is focused on the measure of the surface finish (in terms of Ra) of bars of magnesium UNS M11311 obtained by dry turning varying the cutting parameters and type of tools used. Low cutting conditions have been used as it is usually in repair operations. Besides, three different types of tool with identical geometry and different coatings have been used. Concretely, one specifically for non ferrous metals and two for steels since magnesium normally forms part of hybrid materials (magnesium-aluminium and magnesium-steel).

2. METHODOLOGY

This work is framed within a study involving different light alloys, types of cutting tools (geometries and coatings) and cutting conditions. The methodology used is as follows (Rubio et al, 2005):

* Previous activities to machining process. These activities consist of the design of experiments, namely a Taguchi L27 collected in Table 1 (Taguchi, 1987), the preparation of the test material and the protocols to calculate the cutting parameters values and to registry data and observations of the machining process.

* Turning tests. In these tests workpieces of different light alloys are mechanized under certain conditions of feed, cutting speed, depth of cut and types of tools.

* Monitoring processes. In order to get graphics documents that can be analyzed after the process, all the turning tests described before have been photographed and recorded by video and both the chips obtained and tools used have been photographed with a camera of high resolution.

* Roughness measurement. Measurements of the surface roughness have been made using a surface roughness tester in three generatrices separated one from each other 120 and denoted by G1, G2, G3. In each one of them, the roughness, in terms of Ra, has been measured in four different sections of the length of the workpiece denoted by L1, L2, L3, L4.

* Data processing and analysis of results. The data thus obtained have been treated with mathematical techniques according to the Taguchi L27 design of experiments.

3. APPLICATIONS

For this study, the workpieces used in the turning tests were cylindrical bars with a diameter of 40mm and length of 125mm (useful 100 mm) of magnesium alloy UNS M11311.

The cylindrical bars were dry turned on an EMCO Turn 120 CNC lathe equipped with an EMCO Turn 242 numerical control. The cutting conditions, collected in Table 2, have been expressed in units usually employed in manufacturing workshop. Although they are not System International units (S.I.), they give a more intuitive idea of the values used.

Three different types of tool, from SECO manufacturer, with identical geometry and different coatings have been used. Concretely, one specifically for non ferrous metals and two for steels with a coating of Ti(C,N) + [Al.sub.2][O.sub.3] + TiN. The manufacturer references are: HX, TP200 and TK2000 respectively. They have been denoted by T1, T2, T3 in Table 1.

To observe the machining tests carried out, videos and photographs of the tools were systematically taken during the tests using a Sony Cibershot DSC-P100 digital camera of high resolution. To measure the roughness of the workpiece were used a surface roughness tester Mitutoyo Surftest SJ401. The roughness was measured on three generatrices separated 120[degrees] in four sections L1 = 0-25mm, L2 = 25-50mm, L3 = 50-75mm, L4 = 75-100mm (Table 3).

4. RESULTS

Once the results of the machining tests have been analysed, it can be seen that the best surface finish depend crucially on the feed and, as well although to a lesser extent, on the tool, the cutting speed and especially on its relationship. The best surface finish has been obtained for a feed of 0.05 mm/rev.

Taking only into account the coating factor but no its interactions, the tool shows slightly differences in the surface finish depending on the type used, so for the coatings TP200 and TK2000, similar average values have been obtained while for the HX the surface finish is of a slightly better quality. However, the coating has a little influence at feed level. The effect of the coating increases as the feed grows.

5. CONCLUSIONS

The best surface finishes are obtained for low feeds. Once the lower feed value is fixed the minimum roughness value is obtained for certain combinations of cutting speed and coating. This allows affirming that for repairing operations the machine tool is not a critical issue.

Cutting tools used in the machining of other types of materials (aluminium, steel) can be used obtaining a quality of the surface finish similar to that obtained with tools for specific use of non-ferrous metals. This is important if hybrid materials of magnesium alloys with inserts of other materials such as steel are machined.

Finally, it is important to optimize the cutting parameters to obtain a good quality of the surface finishes, but also to get a chip that does not present high risk of ignition.

6. ACKNOWLEDGMENTS

Funding for this work was provided in part by the Spanish Ministry of Education and Science (Directorate General of Research), Project DPI2005-09325-CO02-02 and the material used in conducting the machining tests by Engine Overhaul Shop of the company IBERIA L.A.E.S.A.

7. REFERENCES

Caton, P.D., Magnesium: an old material with new applications (1991). Materials & Design, 12(6), 309-316.

Gil, F.J.; Manero, J.M.; Rodriguez, D.; Aparicio, C. (2001). Light Alloys, Edciones de la Universidad Politecnica de Cataluna S.L., ISBN: 84-8301-480-7, Barcelona

Taguchi, G. (1987). System of experimental design, American Supplier Institute. Vol 2, ISBN: 0-527-91621-8, New York.

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 and Technology, 162-163C May 2005, 682-689.

Tonshoff, H.K.; Winkler, J., The influence of tool coatings in machining of magnesium (1997). Surface and Coatings Technology, 94-95, 610-616.

Tab. 1. Tests conditions

 N. v(m/min) f(mm/rev) Tool
Test

 1 v3 f2 T1
 2 v1 f3 T2
 3 v3 f3 T1
 4 v1 f1 T3
 5 v3 f3 T2
 6 v2 f1 T3
 7 v1 f1 T1
 8 v3 f2 T2
 9 v3 f1 T3
 10 v3 f2 T3
 11 v2 f2 T1
 12 v2 f3 T2
 13 v2 f2 T3
 14 v3 f3 T3
 15 v1 f3 T1
 16 v3 f1 T2
 17 v2 f3 T1
 18 v2 f1 T1
 19 v1 f2 T2
 20 v3 f1 T1
 21 v1 f2 T3
 22 v2 f3 T3
 23 v1 f3 T3
 24 v2 f1 T2
 25 v2 f2 T2
 26 v1 f1 T2
 27 v1 f2 T1

 Ra ([micro]m)
 N.
Test L1 L2 L3 L4

 1 G1 G3 G2 G1
 2 G2 G2 G1 G1
 3 G2 G1 G1 G3
 4 G2 G2 G2 G2
 5 G1 G3 G3 G2
 6 G3 G1 G3 G1
 7 G1 G1 G1 G1
 8 G3 G2 G1 G3
 9 G1 G3 G1 G3
 10 G2 G1 G3 G2
 11 G3 G1 G1 G2
 12 G3 G1 G2 G3
 13 G1 G2 G2 G3
 14 G3 G2 G2 G1
 15 G3 G3 G2 G2
 16 G2 G1 G2 G1
 17 G1 G2 G3 G1
 18 G2 G3 G2 G3
 19 G1 G1 G2 G2
 20 G3 G2 G3 G2
 21 G3 G3 G1 G1
 22 G2 G3 G1 G2
 23 G1 G1 G3 G3
 24 G1 G2 G1 G2
 25 G2 G3 G3 G1
 26 G3 G3 G3 G3
 27 G2 G2 G3 G3

Tab. 2. Cutting conditions

Tool coating HX TP200 TK2000

v(m/min) 75 150 225
f(mm/rev) 0.05 0.10 0.15
d(mm) 0.25

Tab. 3. Roughness obtained in each test in terms of Ra ([micro]m).

 Ra ([micro]m)

 L1 L2 L3 L4
N.Test 0-25 mm 25-50 mm 50-75 mm 75-100 mm

1 0.563 0.522 0.510 0.462
2 1.425 1.162 1.212 1.176
3 0.966 0.838 0.898 0.885
4 0.415 0.455 0.390 0.420
5 1.340 1.316 1.295 1.287
6 0.392 0.390 0.396 0.525
7 0.474 0.475 0.507 0.374
8 1.041 1.119 1.075 1.110
9 0.309 0.255 0.412 0.422
10 0.574 0.807 0.725 0.726
11 0.516 0.631 0.444 0.676
12 0.985 1.223 1.020 1.032
13 0.534 0.465 0.526 0.577
14 0.852 0.927 0.891 0.961
15 1.099 1.193 0.958 0.988
16 0.460 0.514 0.475 0.540
17 0.724 0.762 0.901 0.681
18 0.376 0.279 0.454 0.323
19 0.622 0.785 0.702 0.689
20 0.361 0.324 0.342 0.437
21 0.844 0.821 0.781 0.804
22 1.287 1.296 1.197 1.237
23 1.103 1.009 1.397 1.334
24 0.330 0.322 0.350 0.378
25 0.712 0.571 0.668 0.498
26 0.420 0.398 0.390 0.329
27 0.679 0.497 0.773 0.561