Wear of PVD Coated and CVD+PVD Coated Inserts in Turning

Transcription

1 Wear of PVD Coated and CVD+PVD Coated Inserts in Turning Paper No.: 65 Session No.: Author Name: Title: Affiliation: Address: 56 (Dynamics and Vibrations in Experimental Mechanics) M.A. Zeb Assistant Professor NWFP, University of Engineering & Technology Peshawar-Pakistan. Department of Mechanical Engineering NWFP, University of Engineering & Technology Peshawar-Pakistan. Phone: (0092)-(91) Fax: (0092)-(91) ABSTRACT The purpose of the research was to find the optimum parameters for turning Ramax-2 stainless steel. The cutting forces were measured using Kistler lathe tool dynamometer, first using PVD coated inserts and then CVD+PVD coated inserts. The flank wear of these inserts was observed by machining Ramax-2 at the optimum parameters. The surface finish was also measured with portable surface roughness meter. The results showed that Ramax-2 can be economically cut if CVD+PVD coated inserts are used. The tool life with CVD+PVD coated inserts was higher as compared to turning with PVD coated inserts, although the surface roughness and cutting forces were almost the same for both kinds of inserts. Keywords: Turning; CVD; PVD; Stainless steel; Cutting forces; Roughness; Wear

2 1. INTRODUCTION Cemented carbide cutting tools coated by physical vapor deposition (PVD) technique give confirmed performance advantages over CVD coated tools [1-3]. The first generation PVD coated carbide tools featured TiN as the hard coating. Their continued success led to the commercial development of second and third generation PVD coatings (TiCN and TiAlN) which offer even higher machining productivity [4]. Physical vapor deposition (PVD) processes are carried out at lower temperatures, which allows elimination of the substantial decarbonization of carbide substrates and maintenance of their toughness and strength. In spite of this, the tool life time of coated carbide inserts obtained when using conventional PVD process is far less than that of CVD coated carbide tools, which is associated with relatively poor adhesion and the higher level of internal stresses of the PVD coatings. On these grounds the PVD coated carbide tools have only limited applications and the PVD processes are not so wide-spread in the cemented-carbide industry as the CVD process [5]. On the other hand commercial Al 2 O 3 coatings are presently produced by chemical vapor deposition (CVD) at temperatures in excess of 1000 C which causes the formation of thermal cracks in CVD Al 2 O 3 during cooling after deposition as a result of the tensile stress built up due to different thermal expansion coefficients of the coating and the substrate material [6]. Therefore for this study the commercially available CVD Al 2 O 3 coated insert was further coated with 3µm AlTiN layer by PVD cathodic arc deposition technique to give increased toughness and tool life. Ramax-2 has a hardness of 340 HB and it is martensitic steel with excellent machinability and good corrosion resistant properties. It can be machined at increased cutting speeds. It is mostly used for holders/bolsters for plastic moulds. In this study it was intended to machine it with two different kinds of inserts at various cutting conditions and find the cutting forces, surface finish and tool wear. The Ramax-2 workpiece was machined first with commercially available PVD coated carbide insert and then with the tailor made insert as stated above. 2. EXPERIMENTAL PROCEDURE The machining tests were performed on Ramax-2, martensitic stainless steel comparable to AISI 420F stainless steel by single point continues turning of cylindrical specimen of 105 mm diameter and 310 mm length on a Boehringer VDF180 Cm CNC turning centre. The specimen was held by the tail stock. No coolant was used in the test. The cutting forces were measured with a Kistler 9121 three component piezoelectric dynamometer and associated 5010 dual mode charge amplifiers connected to a PC employing a force measuring program written in Labview software. Surface roughness measurement was carried out on the machined surface using a Mitutoyo Surface SJ-201 instrument. The tool wear was inspected under Mitutoyo TM microscope and the tool wear photographs were taken with Nikon SMZ1000 microscope incorporating a digital camera. The chemical composition of the workpiece material is given in Table 1. The mechanical and physical properties are given in Table 2. The samples were taken from a round bar of 28 mm diameter. C Si Mn Cr S Ramax Table 1. Chemical composition of the workpiece material

3 Testing Modulus of Tensile strength Yield strength Hardness temperature elasticity 20 C 1100 MPa 910 MPa 340 HB 200 GPa Table 2. The mechanical and physical properties of the workpiece material Two different grades of cemented carbide inserts were used. First kind of insert was commercial PVD multi-layer (TiCN/TiN) coated cemented carbide inserts produced by Sandvik with the specification VNMG MF, VNMG 331-MF. These inserts are recommended for finishing of stainless steel and have GC1025 Sandvik grade. Second kind of insert was commercial CVD having a thick layer of Al 2 O 3 on top of a medium sized layer of Ti(C,N). The total thickness of the coating is approximately 10 microns. These inserts are also by Sandvik with the specification VNMG MF, VNMG 331-MF. These inserts are recommended for cutting stainless steel at relatively high cutting speed and have GC4025 Sandvik grade. These inserts were further coated with 3µm AlTiN layer by PVD cathodic arc deposition technique. With this kind of composite CVD+PVD coatings harder materials can be machined at elevated speeds because the thermal cracking of CVD Al 2 O 3 coated inserts is offset by this PVD coating at lower temperature. A negative rake angle of -4 was used for both kinds of inserts. The nose radius of both of these inserts was 0.4 mm. First test was carried out at fixed cutting speed of 100 m/min and at fixed depth of cut of 1 mm while the feed rate was varied from 0.1 mm/rev to 0.5 mm/rev, in steps of 0.1 mm/rev. Second test was carried out at fixed cutting speed of 100 m/min and at fixed feed of 0.2 mm/rev while the depth of cut was varied from 0.2 mm to 1 mm, in steps of 0.2 mm. Third test was carried out at fixed depth of cut of 1 mm at fixed feed of 0.2 mm/rev while the cutting speed was varied from 50 m/min to 250 m/min, in steps of 50 m/min. The length of cut was 20 mm and the cutting forces were recorded for 1 second of cut after the tool was in stable cutting process. The forces for the first 100 msec were averaged to draw the graphs in each case. 3. RESULTS AND DISCUSSION 3.1. Cutting forces Figure 1 shows the variation of three cutting forces using PVD insert and CVD+PVD inserts. All these forces increase with increasing the feed rate. It is seen that the cutting forces for PVD coated inserts are lower than the cutting forces for CVD+PVD coated inserts. The slope of these forces is smaller at lower feeds as the surface roughness is lower and as the feed rate increases beyond 0.3 mm/rev the forces increase with a larger slope as surface roughness increases and forces endured by the tool tip lead to a rapid tool-wear. Figure 2 shows the trend of the cutting forces for the two inserts used. All these forces increase with increasing the depth of cut. The effect of the depth of cut is same on each cutting force. At 0.4 mm depth of cut the thrust force (feed force) becomes high (158 N) involving a rapid flank wear and chipping. Also the forces are higher in case of CVD+PVD coated inserts.

4 Force (N) CVD-PVD coated Force (N) CVD-PVD coated Fig. 1. Cutting 0.10 forces 0.20 data 0.30 vs. 0.40Feed 0.50 for Feed (mm/rev) turning Ramax-2 using PVD TiCN/TiN and CVD TiCN/Al 2 O 3 +PVD AlTiN coated cemented carbide inserts at a depth of cut of 1 mm and at a speed of 100 m/min Depth of cut (mm) Fig. 2. Cutting forces data vs. Depth of cut for turning Ramax-2 using PVD TiCN/TiN and CVD TiCN/Al 2 O 3 +PVD AlTiN coated cemented carbide inserts at a feed rate of 0.2 mm/rev and speed of 100 m/min. Figure 3 shows that all the three cutting forces i.e., main cutting force, the feed force, and the radial force have a decreasing trend with the increasing cutting speeds. This decrease in the cutting forces when the cutting speed increases, indicates a higher cutting temperature leading to a higher thermal softening of the work material and consequently a decrease in the cutting forces. It can be seen from force lines of figure 3 that between 100 m/min and 150 m/min the thermal softening effect was dominating but after 150 m/min the rate of decrease of these forces is lesser which could indicate that a compromise between work-hardening and thermal softening was prevalent. Again the forces are higher in case of CVD+PVD coated inserts. Force (N) Radial force Feed cutting force Main cutting force CVD-PVD coated Surface speed (m/min) Fig. 3. Cutting forces data vs. Speed for turning Ramax-2 using PVD TiCN/TiN and CVD TiCN/Al 2 O 3 +PVD AlTiN coated cemented carbide inserts at a depth of cut of 1 mm and at a feed rate of 0.2 mm/rev Surface roughness The roughness parameter R a was used in measuring the surface roughness. R a is the universally recognized, and most used, international parameter of roughness. It is the arithmetic mean of the absolute departure of the roughness profile from the mean line. R a 1 = l l 0 Z ( x) dx (3.2.1)

5 Figure 4 shows the increase in the surface roughness with the increase in the feed rate for both kind of inserts. In comparison to Figure 5 and Figure 6 which show the surface roughness variations vs. depth of cut and vs. speed respectively, the surface deterioration rate is much rapid and higher when the feed rate is increased. It reaches to 22 microns for feed rate of 0.5 mm/rev. As long as the feed rate is below 0.4 mm/rev the surface roughness is about 7 microns it is because the tool nose radius is 0.4 mm therefore below 0.4 feed rate the surface finish is good. Ra (micron) CVD-PVD coated Feed (mm/rev) Fig. 4. Surface roughness data vs. Feed for turning Ramax-2 using PVD TiCN/TiN and CVD TiCN/Al 2 O 3 +PVD AlTiN coated cemented carbide inserts at a depth of cut of 1 mm and at a speed of 100 m/min. It can be seen in Figure 5 that the surface finish remains below 4 microns with the increase in depth of cut and there is no significant change in the surface finish for both kinds of inserts. It is a fact that in machining the surface deterioration and tool life is directly affected by feed rate then by cutting speed and then by depth of cut, that is the reason that we see constant good surface finish with increasing depth of cut. Ra (micron) CVD-PVD coated Depth of cut (mm) Fig. 5. Surface roughness data vs. depth of cut for turning Ramax-2 using PVD TiCN/TiN and CVD TiCN/Al 2 O 3 +PVD AlTiN coated cemented carbide inserts at a feed rate of 0.2 mm/rev and at a speed of 100 m/min. It is interesting to note in Figure 6 that the surface finish improves with the increase in cutting speed. The tool endures lesser and lesser forces with the increase in the cutting speed consequently the tool wear is lesser leading to improved surface finish.

6 Ra (micron) CVD-PVD coated Surface speed (m/min) Fig. 6. Surface roughness data vs. Speed for turning Ramax-2 using PVD TiCN/TiN and CVD TiCN/Al 2 O 3 +PVD AlTiN coated cemented carbide inserts at a depth of cut of 1 mm and at a feed of 0.2 mm/rev Tool wear Wear on the flank of a cutting tool is caused by friction between the newly machined workpiece surface and the contact area on the tool flank. The width of the wear land is usually taken as a measure of the amount of wear and can be readily determined by means of a microscope. The wear curve is divided into three regions. Region AB where the sharp cutting edge is quickly broken down and a finite wear land is established. Region BC where wear progresses at a uniform rate. Region CD where wear occurs at a gradually increasing rate. The ISO criteria for flank wear of carbide tools is VB=0.3 mm if the flank is regularly worn in zone B. The same type of general tool wear curve was obtained for the inserts used in this experiment and the VB=0.3 mm criteria was adopted to predict the maximum length cut by each insert. On comparison between Figure 7 and Figure 8 we can appreciate the advantage of having a CVD+PVD composite coating. The length of cut for CVD+PVD composite coated insert is about two and a half times greater then the length cut by the PVD coated insert for identical values of depth of cut, feed rate and cutting speed. Flank wear- VB (microns) Tool wear during turning of Ramax-2 using PVD coated inserts C B 50 0 A Total Length of cut (m) D Fig. 7. Flank wear VB vs. Length of cut for turning Ramax-2 using PVD TiCN/TiN coated insert at a depth of cut of 1 mm, feed 0.2 mm/rev and at a speed of 250 m/min. Flank wear (microns) Tool wear during turning of Ramax-2 using CVD-PVD coated inserts C B A Total Length of cut (m) Fig. 8. Flank wear VB vs. Length of cut for turning Ramax-2 using CVD TiCN/Al 2 O 3 +PVD AlTiN coated cemented carbide inserts at a depth of cut of 1 mm, feed 0.2 mm/rev and at a speed of 250 m/min. D

7 4. CONCLUSION PVD coating makes the cutting tool tougher and the possibility of thermal crack development after cooling is less as seen in CVD coated inserts. However the tool is less wear resistant in case of PVD coatings as proved by the experimental results. The tool life of composite CVD+PVD coated inserts was two and a half times greater than the PVD coated inserts. The appreciable advantage was not seen with CVD+PVD coated inserts as far as the cutting forces are concerned. This is due to the reason that the CVD+PVD coated inserts are best suited to work under speeds higher then 250 m/min as such a coating can take up higher temperature and pressure loads. The CVD+PVD coated inserts are therefore required to cut at more severe cutting conditions e.g., at speeds beyond 250 m/min and at depths of cut higher than 1 mm with lower feed rates. For Ramax-2 (martensitic stainless steel) the composite CVD+PVD coated inserts will give higher production rates than commercially available PVD coated inserts and the production process will be economical as lesser inserts will be utilized.

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