Influence of cutting fluid conditions and cutting parameters on the chip form in turning of titanium and steel alloys
Bogdan SŁODKI
1,a, Grzegorz STRUZIKIEWICZ
1,b*, Łukasz ŚLUSARCZYK
1,c1Cracow University of Technology, Production Engineering Institute Al. Jana Pawła 37, Cracow, 31-864, Poland, Tel.: +48 12 374-3212
E-mail: a[email protected], b[email protected]*, c[email protected]
Keywords: Turning, stainless steel, titanium alloy, HPC, chip form.
Abstract. The paper presents the results of turning tests of Ti6Al4V alloy with a sintered carbide tool. For selected sets of cutting data, two kinds of coolant supply were compared. Conventional coolant supply with the pressure of 7 bar was compared with HPC (High - Pressure Coolant) system working with the pressure of 70 bar. The tests revealed the fact that HPC system is useful for small values of feed taking into account chip form. Photographs of chips and their form analysis are presented. The results of tests performed by Sandvik Coromant concerning turning stainless steel were compared and discussed.
Introduction
Problems concerning chip breakage and control have been intensively investigated by many researcher [1,2,3] especially when CNC machining was introduced [4,5,6]. From many material groups difficult-to-cut materials such as HRSA alloys [7,8,9,10], titanium alloys or stainless steel [1] can be distinguished. Titanium alloys usage has increased significantly in recent years. There are widely used in aviation industry, chemical industry and medicine. Titanium is lighter than steel, corrosion resistant, has good mechanical properties in high temperature. Although cutting forces are only slightly higher than the cutting forces for steels, titanium alloys have properties that make them more difficult to machine than a steel of equivalent hardness. [1,11]. Cutting causes high temperature field in cutting zone (titanium has a low heat-transmission coefficient, e.g. for titanium Ti6Al4V this coefficient is 7 W/m K, for carbon steel is 50 W/m K). Titanium alloys are prone to chemical reaction with tool material when the temperature exceeds 500 C. That causes a chemical crater wear [12]. The contact length between the chip and the rake face is short what causes the concentrated/high stress field. It is also possible for the heat of machining to cause some titanium alloys to ignite and burn. Titanium's low modulus of elasticity makes the part susceptible to deflection and vibration, particularly during heavy cuts. Stainless steel can be divided into the following groups according to its structure: ferritic, martensitic and austenitic. High cutting forces and a high level of heat generated in the cutting process lead to fast cutting edge wear [12,13,14].
Out of these three groups austenitic steel is the most demanding for cutting operation. It has a high work hardening rate and low thermal conductivity. It bonds to the cutting edge and can cause unpredictable tool performance. It is extensively described in the Sandvik Coromant [15] technical reference book. Machining of ferritic steel can be compared with machining of low carbon steel.
Martensitic steel contains more carbon than ferritic steel and is mainly machined in an annealed condition. Additionally, it is difficult to achieve correct chip form and roughness for variety of cutting data [16,17,18,19]. Chip breakers located on the rake face of carbide inserts has limited range of application. Traditionally titanium as well as other alloys is machined with coolant supply which can be delivered in traditional way with the pressure about 7 bar or when HPC (High Presure Coolant) systems were introduced with the pressure about 70 bar. In the paper HPC system supporting chip forming and breakage produced by Sandvik Coromant [15] was tested and the results were compared with conventional pressure supply.
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Machining experiments
The experiments were conducted using titanium alloy Grade5 and stainless steel Sanmac 316L as workpiece material. Sandvik Coromant tool holder type C6-PCMNN-00115-12HP and carbide insert CNMG 12 04 08-SMC 1115 were used [15]. The chemical composition of a workpieces are shown in Table 1 and Table 2. The tests were carried out with Mazak Integrex 200-IV lathe equipped with a system for high pressure coolant supply. Research plan was developed for the three variables, assuming two levels of depth of cut (ap), pressure (p), and four levels of feed (f). Cutting data are presented in Table 3. Cutting speed was constant, vc = 50 m/min. Machining parameters was selected according to the manufacturer tool recommendation.
KAPR = 50 [deg]
DCON = 63 [mm]
LF = 115 [mm]
IC = 12.7 [mm]
S = 4.7625 [mm]
L = 12.8959 [mm]
RE = 0.8 [mm]
Fig. 1 C6-PCMNN-00115-12HP tool holder and CNMG 12 04 08-SMC 1115 carbide insert
Table 1 Composition of Grade5 (Ti-6Al-4V) titanium alloy
Material [%] Al Fe O Ti V
Ti-6Al-4V 6 0.25 0.20 90 4
Table 2 Composition of stainless steel Sanmac 316L
Material [%] C Si Mn P S Cr Ni Mo
Sanmac 316L 0.03 0.3 1.8 0.04 0.03 17 10 2.1
Table 3 Cutting data
Symbol Cutting data Level
A Feed f [mm/rev] 0.15 0.20 0.25 0.30
B Depth of cut ap [mm] 1.0 3.0
C Cutting pressure p [bar] 7 70
Chips evaluation and classification was carried out after each test. Fig. 2 shows the method used for chip classification and determination the chip form classification coefficient.
Fig.3 presents examples of chips achieved in turning of Sanmac 316L. This is presented on the basis of research results presented by Sandvik Coromant [15].
Table 4 presents photographs of chips forms achieved in performed tests together with the classification of their forms. The following assigns were accepted:
"+" - chips correct
"-" - chips incorrect
"0" - chips acceptable.
Fig. 2 Chip form classification principle
a) b)
conventional 70 bar
Fig. 3 Chips form achieved in turning of Sanmac 316L a) incorrect, b) correct (70 bar)
ap = 1.0 mm, f = 0.2 mm/rev
Table 4 Chip classification in particular tests, Ti6Al4V alloy Cutting fluid
conditions
vc = 50 [m/min]; ap = 1.0 [mm]
f [mm/rev] f [mm/rev]
0.15 0.20 0.25 0.30 0.15 0.20 0.25 0.30
70 bar
+ + + +
Conventional
- - 0 0
Cutting fluid conditions
vc = 50 [m/min]; ap = 3.0 [mm]
f [mm/rev] f [mm/rev]
0.15 0.20 0.25 0.30 0.15 0.20 0.25 0.30
70 bar
+ + + +
Conventional
0 0 + +
It should be noticed that in tests with the conventional coolant supply, chip form changes when the feed value increases. Table 4 shows that for the feed value greater than f = 0.2 mm/rev chip form changes from incorrect to acceptable. The conclusion is that the decisive factor in the selection of the way of coolant supply (conventional or HPC) could be the quality of machined surface. Fig. 4 presents the results of surface quality measurement (Ra) for conventional and HPC coolant supply.
Fig. 4 Surface roughness after turning
On the base of measurement results, functions describing various dependencies concerning roughness and cutting data were build. They are presented in Table 5.
Table 5. Roughness function formulas
Formula Coefficient Description
21.085 ∙ . R2 = 0.985 ap = 1.0 mm, conventional
28.237 ∙ . R2 = 0.993 ap = 1.0 mm, 70 bar
14.056 ∙ . R2 = 0.983 ap = 3.0 mm, conventional
14.183 ∙ . R2 = 0.958 ap = 3.0 mm, 70 bar
Test results show that HPC system does not significantly influence the surface roughness. Thus, the optimal implementation of HPC system in longitudinal turning of titanium alloy is a finishing turning with low values of feed (below f = 0.2 mm/rev). For greater values of feed and depth of cut (rough turning) HPC implementation for achieving correct chip form can be a too expensive solution.
Moreover, it can be seen that HPC system has significant influence on increase of metal removal rate Qv for feed value f < 0.2 mm/rev. For example (Fig. 5) with the assumption of achieving surface roughness Ra = 1.0 µm, implementation of HPC in turning with depth of cut ap = 1.0 mm increases Qv about 9.7 %. For the roughness value Ra = 1.5 µm this increase is about 7.7% in comparison with conventional coolant supply.
In rough machining implementation of HPC gives only minor increase of Qv value.
For example, for surface roughness value Ra = 1.0 µm and the depth of cut ap = 3.0 mm the increase of Qv value is only about 3.3% and for Ra = 1.5 µm is about 2.5% in comparison with conventional coolant supply.
a)
b) c)
Fig. 5 a) Surface roughness calculated according formulas presented in Table 5 b) metal removal rate for ap = 1.0 mm, c) metal removal rate for ap = 3.0 mm Conclusions
The paper presents research results of Ti-6Al-4V turning with the implementation of HPC system and some examples of stainless steel turning Sanmac 316L using the same system of coolant supply. The analysis of results showed that:
- HPC system could be effectively used for finishing turning in both cases for the feed values equal or lower than f = 0.2 mm/rev.
- It can be stated that HPC system does not significantly influence the surface roughness of machined parts.
- In the case of semi roughing or roughing turning the effectiveness of HPC system supporting the chip breakage decreases significantly and the implementation of HPC system can only increase production costs.
References
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Influence of Cutting Fluid Conditions and Cutting Parameters on the Chip Form in Turning of Titanium and Steel Alloys
10.4028/www.scientific.net/KEM.686.74
DOI References
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10.1016/j.ijmachtools.2009.02.010
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2015. 1018530.
10.1080/10910344.2015.1018530
[3] R. Maruda, S. Legutko, G. Krolczyk, P. Raos, Influence of cooling conditions on the machining process under MQCL and MQL conditions, Tehnicki Vjesnik - Technical Gazette, 22 (4) (2015) 965-970.
10.17559/tv-20140919143415
[4] W. Zębala, M. Plaza, Comparative study of 3-and 5-axis CNC centers for free-form machining of difficult-to-cut material, International Journal of Production Economics, 158 (2014) 345-358.
10.1016/j.ijpe.2014.08.006
[5] AG. Mamalis, J. Kundrak J, M. Horvath, On a novel tool life relation for precision cutting tools, Journal of Manufacturing Science and Engineering - Transactions of the Asme 127, 2 (2005) 328-332.
10.1115/1.1794158
[6] B. Mikó, J. Beňo, Effect of the Working Diameter to the Surface Quality in Free-form Surface Milling, Key Engineering Materials 581 (2014) 372-377.
10.4028/www.scientific.net/kem.581.372
[8] B. Karpuschewski, K. Schmidt, J. Prilukova, J. Beňo, I. Maňková, N. Hieu, Influence of tool edge preparation on performance of ceramic tool inserts when hard turning, Journal of Materials Processing Technology 213, 11 (2013) 1978-(1988).
10.1016/j.jmatprotec.2013.05.016
[9] W. Zębala, R. Kowalczyk, Cutting Data Influence on Cutting Forces and Surface Finish During Sintered Carbide Turning, Key Engineering Materials 581 (2014) 148-153.
10.4028/www.scientific.net/kem.581.148
[10] J. Kundrák, G. Varga, Possibility of reducing environmental load in hard machining. Key Engineering Materials 496 (2011) 205-210.
10.4028/www.scientific.net/kem.496.205
[12] B. Karpuschewski, K. Schmidt, J. Beňo, I. Maňková, J. Prilukova, Measuring procedures of cutting edge preparation when hard turning with coated ceramics tool inserts, Measurement 55, 9 (2014) 627-640.
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