Assessment of Wear Failure Modes of Carbide Cutters in High- Speed Milling of Hardened Steels

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1 Failure of Engineering Materials & Structures Code 26 UET TAXILA MECHNICAL ENGINEERING DEPARTMENT Assessment of Wear Failure Modes of Carbide Cutters in High- Speed Milling of Hardened Steels Asif Iqbal 1, Naeem Ullah Dar 2, Iqbal Khan 3 and He Ning MED UET Taxila and 4 College of Mechanical & Electrical Engineering, Nanjing University of Aeronautics & Astronautics, Nanjing, P.R. China ABSTRACT. High-speed milling of hardened steels is blessed with numerous advantages, but the major shortcoming of rapid tool wear has relentlessly outdone its benefits. Ultra-high temperatures created at primary and secondary shear zones, during high-speed cutting, significantly accelerate different wear mechanisms of tungsten carbide tools, especially adhesion and diffusion wears. In this paper an attempt has been presented to investigate the effects of cutting and tooling parameters upon severity of different wear/damage mechanisms in highspeed milling of hardened steels using carbide end-mills. The tooling parameters investigated were ceramic coating and cutter s helix angle, while cutting parameters included cutting speed, feed rate, milling orientation, and minimum quantity of lubrication (MQL). The tool wear/damage modes were studied using Scanning Electron Microscope and Energy Dispersive Spectrometry. It was found that cutting speed increased the intensity of adhesion wear and besides, it also initiated the oxidation wear, while the increase in feed rate changed the dominant mode of tool damage from adhesion to chipping. Changing the milling orientation from up- to down-milling significantly increased the tool life, while application of MQL marginally increased the tool life. Introduction of ceramic coating TiAlN reduced the severity of diffusion and adhesion wear by a great deal and thus tool life was improved. Helix angle did not prove to be influential upon severity of any wear mode but tools of larger helix angle provided slightly longer tool life values. INTRODUCTION High-speed machining (HSM) is characterized by the removal of metal at very high strain rates. For a given workpiece material, the cutting is said to be in high-speed range if the cutting speed lies between 5 to 10 times of conventional cutting speeds range [1]. High-speed milling applied to the machining of hardened steels (hardness > 35HRc) is known as hard-milling process. In addition to the general benefits offered by the HSM process the hard-milling process is blessed with some particular advantages as well, like compressive residual stresses, elimination of grinding process, better dimensional accuracy, minimal micro-structural alterations, and increased fatigue life [2, 3]. Despite of these amazing benefits, hard-milling process comes also with a major demerit of rapid tool wear that drastically reduces tool life to unacceptable values.

2 Asif Iqbal et al FEMS (2007) Though polycrystalline boron nitride (PcBN) outperforms coated tungsten carbide in hardmilling process [4, 5], still carbide cutters are used more than PcBN because of significant difference in cost and trustworthiness in critical machining conditions [6]. The core reason of rapid wear of carbide cutters is the elevated cutting temperatures occurring at primary and secondary shear zones that accelerate adhesion, diffusion, and oxidation modes of wear [7]. The severity of these modes of wear as well as of other tool damage mechanisms depends upon settings of different tooling and cutting parameters in high-speed milling of hardened steels. The influential ones among them are workpiece material s microstructure, composition, and hardness, tool s coating, helix and rake angles, milling orientation, cutting speed and feed, and type of coolant. This paper will analyze the effects of aforementioned parameters upon initiation and severity of different tool damage mechanisms in high-speed end milling of tool steel (AISI D2) and high strength low alloy steel (AISI 4340). Scanning Electron Microscopy (SEM) along with Energy Dispersive Spectrometry (EDS) will be used to get the detailed surface information of worn out tools. EFFECTS OF TOOLING PARAMETERS Important tooling parameters include ceramic coating (Titanium Aluminum Nitride) of tungsten carbide tools, tool s helix angle, and rake angle. The effects of these parameters will be studied upon high-speed milling of two kinds of hardened steels: (1) high strength low alloy steel, AISI 4340 (hardness 47HRc), and (2) high chromium, air-cooled, cold work tool steel, AISI D2 (hardness 56HRc). Table 1 presents the details of half fractional factorial design of experiments. Table 1. Detail of experiments for studying effects of tooling parameters Test No. Coating Helix angle ( ) Rake angle ( ) Material 1 Uncoated 30 0 AISI Uncoated AISI D2 3 Uncoated 45 0 AISI D2 4 Uncoated AISI TiAlN 30 0 AISI D2 6 TiAlN AISI TiAlN 45 0 AISI TiAlN AISI D2 Experimental Details All the experiments were performed on Micron UCP 710, 5-axis, vertical milling center having maximum power of 16kW. Flank wear was measured using 10x tool maker s microscope and SEM pictures of all the tools were taken using Joel JSM 5610LV microscope, while Thermo Noran Energy Dispersive Spectrometer along with Vantage digital acquisition engine were used to conduct micro-analysis at the surfaces of tools.

3 121 Asif Iqbal et al FEMS (2007) 26 The cutting tools used were flat end solid K30 carbide cutters having diameter (D) of 8mm, corner radius (R) of 2mm, flank angle (α) of 6º (primary) and 12º (secondary), and number of flutes (Z) equal to 4. The cutting parameters that were kept constant in all experiments, were: radial depth of cut (a e ) = 0.3mm; axial depth of cut (a p ) = 5mm; tool overhang = 28mm; and milling-orientation = down-milling. No coolant was employed in any of the eight tests. The tool failure criteria employed was the attainment of maximum width of flank wear land (VB max ) of 0.2mm; or excessive chipping of tool s edge or face; or plastic deformation of the tool. Experimental Results From the statistical analysis of tool life results it was found that workpiece material and tool coating are highly influential parameters, while helix angle is marginally influential. The shortest tool life was observed in test number 2 in which the harder of two steels (AISI D2) was milled using uncoated carbide cutter having small helix angle. The longest tool life value was obtained in test number 7. In order to understand the wear failure modes of the tools involved, consider figure 1, which presents the SEM images of selected tools after they had attained tool failure criteria. The image labeled as 1 describes the worn out condition of the uncoated cutter that was used in milling of AISI Chipping of the edge as well as of the flank face is clearly observable even at small magnification level. Image 2 presents even worse condition in which surface of uncoated tool has been shown which was utilized to mill the harder material, i.e. AISI D2. This tool provided the shortest tool life of all the cutters involved. Huge-scale chipping and catastrophic fissure is obvious from the picture. The chipping observed in both the cases is combination of two phenomenon: (1) direct chipping of the tool s edge under shocking load of intermittent cutting of hard workpiece material, imposed upon somewhat brittle carbide substrate; and (2) chipping-off of the flakes of carbide substrate weakened by adhesion of workpiece material particles upon tool s edge, and by diffusion of carbon atoms of tool s substrate into workpiece material. Naerheim et al. [8] have reported that cemented carbide cutters are prone to diffusion wear especially in cutting of steels at high speed. Also Childs et al. [7] have observed that the surface of cutting tool is prone to high rate of diffusion wear if it is not protected by the coated layer. Image 3 shows the worn out edge of uncoated tool used in milling of AISI The edge is not as badly chipped as is one shown in image 1. The difference between two cases is that the latter one has employed larger helix angle and it has been proved by Mutuo et al. [9] that larger helix angle of end mill cutter leads to considerable reduction in cutting forces. The reduction in cutting forces might have led towards reduced rate of chipping in the tool shown in image 3. The next three images represent the tools whose surfaces have had been coated by 8 to 10µm thick layer of a ceramic material TiAlN using physical vapor deposition (PVD) process. The coating is believed to give elevated high temperature hardness to the tool and it also reduces the cutting temperature by trimming down the contact length between chip and tool s rake face [10].

4 Asif Iqbal et al FEMS (2007) Fig.1. SEM images of worn out tools Image 4 presents the surface of the damaged most flute of the coated cutter used in milling of AISI D2. Very small scale chipping is evident from the image. Moreover, minute protrusions joined to the edge can also be observed from the image. These protrusions are the particles of workpiece material that have been pressure welded (adhered) to the surface of the edge. These adhesions weaken the edge and later on are removed out along with some portion of the edge (in form of flake) under action of intermittent cutting, causing damage to the tool. Figure 2 shows the graph representing EDS analysis of surface of tool. From the graph, high proportion of Fe and Cr (major constituents of AISI D2) are observable. This observation verifies occurrence of adhesion wear at the edge of tool. Fig.2. The graph representing EDS analysis of cutting edge of the tool used in test 7 Image 5 presents portion of edge and flank face of one of the flute belonging to the coated tool used in milling of AISI It is clearly observable that layers of workpiece material are adhered to the tool s surface. This observation was also verified by EDS analysis, which revealed presence of high content of Fe at the surface. Image 6 presents an edge of coated tool used in

5 123 Asif Iqbal et al FEMS (2007) 26 milling of harder steel, from which medium scale chipping is evident. The discussion leads to conclusions that uncoated tools are badly damaged by chipping and rupture when high-speed milling hardened steels and that increase in hardness of steel causes the change of dominant tool damage mode from adhesion to chipping. EFFECTS OF CUTTING PARAMETERS In this section the effects of following cutting parameters will be studied upon severity of wear failure modes of coated carbide cutters: cutting speed (V c ); feed per tooth (f z ); milling orientation (up- and down-milling); and coolant (dry and MQL). Table 2 presents the details of half fractional factorial design of experiments. Table 2. Detail of experiments for studying effects of cutting parameters Test No. V c (m/min) f z (mm/tooth) Milling Orientation Coolant Up MQL Down Dry Up Dry Down MQL Up Dry Down MQL Up MQL Down Dry Experimental Details All the fixed parameters were same as described in previous section, except for following. The workpiece material utilized was only AISI D2 (62HRc), having dimensions mm 3. All the tests employed K30 tungsten carbide cutters with PVD coated mono layer of TiAlN. The helix, flank, and rake angles utilized were 50, 6 and -5, respectively. Same tool failure criteria were employed as for previous set of experiments. Experimental Results From the statistical analysis of tool life results it was found that effect of milling orientation was most significant, followed by that of cutting speed and feed rate. The test number 2, i.e. combination of cutting speed of 150m/min, feed rate of 0.08mm/tooth, down-milling, and MQL provided the longest tool life. Figure 3 presents the SEM photographs of the end mills used in eight tests. The image labeled as 1 in figure 3 is the magnified view of damaged most flute of the cutter used in test 1. At the edge, considerable amount of adhesion is visible. The bright spot in the image was analyzed by EDS and it was found that the protrusion contained high proportion of Fe and Cr. This observation verified the occurrence of adhesion wear. Image 2 depicts a very mild

6 Asif Iqbal et al FEMS (2007) situation, in which no signs of intense chipping or adhesion are evident upon surface of the tool which provided longest life. Image 3 shows relatively high-scale intensity of chipping but not much adhesion. The EDS analysis revealed high proportion of W, with small share of Fe, and non-existence of Ti and Al. This observation shows that chipping was so intense that coating was not at all detected in the analysis. Higher level of feed rate, resulting in augmented chip load at the edge, caused increased amount of chipping. Image 4 shows view of edge damaged by lesser intensity of chipping as compared to that shown in image 3. Intensity of adhesion is almost same in both the cases. This observation asserts that changing from dry and up-milling to MQL and down-milling puts check upon chipping rate. In collective terms the last four images show significant increase in adhesion as compared to first four images. This implies that high level of cutting speed causes increase in intensity of adhesion. Ultra-high temperatures generated because of high cutting speed accelerate the deposition of workpiece material particles and layers upon tool s surface. Moreover, in EDS analyses of tools involved in last four tests also revealed the occurrence of oxidation at flank faces. The oxidation occurs because at high cutting speeds the flank face of the tool slides at freshly cut surface of workpiece with higher speed and the resulting high temperatures cause oxygen in the environment to react with carbides of the substrate [7]. In coated carbide tools, the tendency of oxidation wear is less but still it does occur at high temperatures, especially when the coated layer is worn out. The resulting oxides render the tool s surface weaker and the sliding portion of workpiece can easily remove the flakes from the flank face of the tool. In image 5 a very thick adhesive layer of workpiece material can be observed. Adhesion was so thick that W was rarely detected in the EDS analysis. Image 6 shows relatively bleak adhesion as compared to that in image 5. Minute-scale chipping is also observable. Image 7 shows increased scale of chipping along with considerable amount of adhesion. The reason for increased scale of chipping is the higher level of feed rate employed in test 7. Image 8 shows the flank face of the tool used in test 8. Adhesion and oxidation wear were observed. In all of the cases the abrasive wear mode was found almost non-existent. This is because the extreme hardness of ceramic coating and carbide substrate did not allow the hard particles, contained in the workpiece material, to tear or scratch the surface of any cutter. CONCLUSIONS The paper presents experimental study focusing upon investigation of effects of tooling and cutting parameters upon wear failure modes of carbide cutters employed in high-speed milling of hardened steels. From the discussion following conclusions can be drawn: 1. The most dominant failure mode of carbide cutters in milling of hardened steels is chipping; followed by adhesion, oxidation, and diffusion wear 2. Increase in workpiece material hardness leads to shortened tool life because of occurrence of high-scale chipping. 3. Introduction of ceramic coating helps increasing tool life because of its high hardness and reduction of chip-rake contact length. Other tooling parameters do not have significant effect upon severity of different wear failure modes.

7 125 Asif Iqbal et al FEMS (2007) Increased feed rate enhances the chipping rate because of increased chip load on cutting edge, while increased cutting speed accelerates adhesion wear and initiates oxidation wear because of increased cutting temperature. MQL employed down-milling causes reduction in rate of all wear modes (especially chipping) as compared to dry and upmilling. Fig.3. SEM images of tools used in experiment set 2

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