WEAR AND BLANKING PERFORMANCE OF AlCrN PVD-COATED PUNCHES

Transcription

1 Materials Science, Vol. 48, No. 4, January, 2013 (Ukrainian Original Vol. 48, No. 4, July August, 2012) WEAR AND BLANKING PERFORMANCE OF AlCrN PVD-COATED PUNCHES M. Çöl, 1 D. Kir, 2 and E. Erişir 1,3 Blanking of sheet steels is one of the most frequently used processes in the manufacturing industry. During blanking processes, the wear of punches occurs due to the cyclic contact of the opposite surfaces. Therefore, the punch is exposed to high stresses and requires high resistance against wear. When worn tools are used, the tool surface directly affects the quality of sheet products and causes some surface defects. The PVD coating of the punch with a surface layer of improved hardness and low friction may reduce wear. The aim of the present study is to characterize the tool wear of AlCrN coated punch used for the sheet-steel blanking. The worn punch surfaces were examined by scanning electron microscopy. The results showed that wear take place due to the abrasion, adhesion, and delamination along sliding flank surfaces. Edge rounding and fracture of cutting edges were also observed. After blanking tests, the tool wear volume of punches and the burr height of blanked parts were also measured. The wear volumes were determined by means of profile measurements of the worn punch carried out after the completion of a large amount of blanked parts. Keywords: Blanking, punch, tool wear, PVD coatings, wear measurements, burr height. The research into wear-resistant coatings is continuously developing in many applications like punching, blanking, and trimming tools. The development of wear resistant surfaces by hard coatings arises from the need of increasing the tool life and performance. In a blanking tool, the wear of punch and die directly affects the quality of parts by causing some defects during blanking [1, 2]. It is well known that the errors in the produced parts caused by blanking are: dimensional, positional, form and surface errors [3]. However, the most common errors are form errors, such as the edge draw-in, the depth of crack penetration, and the burr height [1, 3, 4]. Hence, the form errors should be prevented to improve the quality of sheet products. It is well known that PVD coatings are now available to increase the tool wear performance. According to the experimental studies on the punchability of ultrahigh-strength steels and tool wear carried out by Högman [5], wear can be reduced by applying the PVD coating to the punch. They further established that the cutting edge and punch corner are critical worn areas during the blanking process. The mechanical and thermal conditions near the coating surface determine the location of wear. Klocke and Raedt [6] studied the PVD coated tools and discovered that the highest mechanical stresses appear on the cutting edge and on the surface of the coating, where the tribological conditions of blanking lead to temperatures of about 400 C and contact pressures of more than 3000 MPa. The numerical simulations of Vaz and Bressan [7] also showed that the maximum shear stresses occur near the punch corner. It should be noted that the type of coating layer is also crucial. The TiN coating is used specially for machining tools [8]. However, blanking is a cold-working process and the high-temperature durability is not necessarily required. 1 Department of Metallurgical and Materials Engineering, Kocaeli University, Turkey. 2 Vocational School Gebze, Kocaeli University, Turkey. 3 Corresponding author; eerisir@kocaeli.edu.tr. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 48, No. 4, pp , July August, Original article submitted April 2, X/13/ Springer Science+Business Media New York

2 WEAR AND BLANKING PERFORMANCE OF AlCrN PVD-COATED PUNCHES 515 Fig. 1. SEM micrograph showing the microstructure of an X155CrVMo 12-1 punch tool ( 2500). Hence, the AlCrN coatings with better toughness properties are preferred to get higher wear resistance [9]. The industrial trials and test results obtained by Wang, et al. [10] show that the tool life of AlCrN coated punches is more than 2.5 and 5 times longer than the tool lives of TiN-coated and uncoated punches, respectively. The PVD coatings are applied to the punch tools in the operations of sheet-steel blanking. The wear of punch tools and the appearance of form errors can be delayed by AlCrN coatings through PVD deposition. The purpose of the present study is to characterize the tool wear of AlCrN-coated punches used for sheet-steel blanking. Materials and Methods In the experiments, DIN (X155CrVMo 12-1) cold-work tool steel was used as a punch material. As a workpiece material, we used 1-mm-thick and 75-mm-wide cold-rolled DIN (S235JR) low-carbon steel strips. The chemical compositions (wt.%) of the punch and strip steels are as follows: X155CrVMo 12-1 punch steel (1.55 C, 0.30 Mn, 0.25 Si, 12.0 Cr, 0.70 Mo, and 1.0 V); S235JR strip steel (0.1 C, 0.20 Mn, and 0.40 Si). The punches were hardened and then tempered to get a core hardness of 60 HRC. The microstructure of the punch consists of primary and secondary carbides of Cr, Mo, and V in the fine-tempered martensitic matrix, as shown in Fig. 1. After heat treatment, the punches were ground to reach a surface roughness (R a ) of ± 0.2 μm. The experimental study was performed for the AlCrN-coated and uncoated states of tool steel. The AlCrN coatings were deposited by using a Balzers Rapid-Coating-System (RCS) deposition machine. In Fig. 2, we show the AlCrN coating with a thickness of ca. 2 μm and a hardness of 3200 HV0.05. The blanking tests were carried out by using a blanking test machine with 250 kn eccentric shaft press. The punch speed was 70 rpm and the maximum strokes were about 65,000 for the uncoated punch and 120,000 for the AlCrN-coated punch. All blanking tests were carried out under dry cutting without using any lubrication materials. The distance between blanks was 5 mm and the dimensions are schematically shown in Fig. 3. A scanning electron microscope (SEM, JSM-6060) was used to observe the microstructures of the cross sections and worn surfaces. The clearance Cl (%) was found from the following expression as a percentage of the strip [3, 11]:

3 516 M. ÇÖL, D. KIR, AND E. ERIŞIR Fig. 2. SEM micrograph of the cross section of an AlCrN-coated X155CrVMo 12-1 punch tool ( 4000). Fig. 3. The dimensions of the strip and punching arrangement (all dimensions are in mm). Cl = D d 2t 100, (1) where D and d are the diameters of the die and the punch and t is the thickness of the strip. The diameters of the die and the punch were 10 mm and 9.9 mm, respectively. As a result, the value of clearance considered in the present work was 5%. The tool wear volume of the punch and the burr height of the blanked strip were measured every 5000 and 1000 strokes, respectively. The tool wear volume of the punch was derived from the data of measurements of wear radius by using a stereo microscope and can be described as the wear volume (V) [1]:.V = 5π 4 (r punch ) 3 + 3π 2 R p(r punch ) 2 (mm 3 ), (2) where R p is the punch diameter and r punch is the radius of the cutting edge of the punch.

4 WEAR AND BLANKING PERFORMANCE OF AlCrN PVD-COATED PUNCHES 517 Fig. 4. SEM micrograph of the coated punch prior to blanking tests ( 150). The average value of four positions around the punch was measured to analyze the amount of volume wear after every 1000 strokes. On the other hand, the burr height was measured from the cross sections of blanked parts by using a light microscope (LM). Five blanked parts were taken in order to determine the burr height after every 1000 strokes. The measurements were performed with the help of a LM. Results Figure 4 shows the cutting edge and flank surface of the AlCrN-coated punch prior to blanking tests. The sharp cutting edge was observed before using. Moreover, it can be seen that the coating surface retains some droplets of the PVD coating process. Cutting Edge and Flank Wear. The cutting edge and flank wear of AlCrN-coated and uncoated punches were investigated by using SEM after blanking tests. The worn surfaces were taken from the AlCrN-coated punch after 120,000 strokes and from the uncoated punch after 65,000 strokes. In Fig. 5, we can clearly see that wear is more severe for the uncoated punch, although the number of strokes was twice larger for the AlCrNcoated punch. Figure 5a shows that the AlCrN coating suffers slight wear after 120,000 strokes. Some microattrition and wear tracks were seen above the coating layer. Edge rounding also occurred due to wear. It is known that the edge rounding increases the tool radius and leads to an increase in the burr height in the blanked parts [12]. For the uncoated punch depicted in Fig. 5b, the edge rounding is more severe after blanking tests with 65,000 strokes. Edge fracture was also observed. The cause of the edge fracture may be connected with the repeated impact loads after edge rounding. Therefore, the loss of material is not uniform but strongly irregular along the cutting edge. As can be seen in Fig. 6, the flank surfaces of the uncoated and AlCrN-coated punches show adhesive and abrasive wear. The axial shear force is very large in the punching process. It may cause wear of the flank surfaces of the punch during shear [13]. The flank surface close to the cutting edge presents some wear tracks (grooves and valleys) due to abrasion (Fig. 6). On the other hand, adhered or welded layers were observed on the flank surface of the uncoated punch in Fig. 6b. This corresponds to a typical adhesive wear. Cross Section of the Worn AlCrN-Coated Tool. Another wear mechanism observed on the worn surface of the AlCrN-coated punch is delamination wear.

5 518 M. ÇÖL, D. KIR, AND E. ERIŞIR Fig. 5. SEM micrographs of the edge wear after blanking tests: (a) AlCrN-coated after 120,000 strokes, (b) uncoated after 65,000 strokes ( 150). Fig. 6. SEM micrographs of the flank wear after blanking tests: (a) AlCrN-coated after 120,000 strokes, (b) uncoated after 65,000 strokes ( 150). Suh [14] claimed that plastic deformation near the region of the worn surface or large primary carbides with dislocation pileups may produce crack initiation. In Fig. 7, plastic deformation and fracture appear near the worn surface. This involves the initiation of surface and subsurface cracks, crack propagation, and the formation of wear particles. The cracks initiated in the subsurface region develop toward the surface. Measurement of the Burr Height and Wear Volume. To measure the burr height as a function of strokes, five blanked samples were taken at the end of the intervals (each of 1000 strokes). The blanked samples were mounted in resin to the cross section and fixed on the polished surface for metallographic investigations. The burr heights of different strokes were measured. The burr height was measured for each blanked sample according to the LM micrographs of burr samples by taking four specified points on the perimeters and then the average burr height was calculated. The values of burr height are measured in % of the thickness of blanked sheets. The wear volumes of the uncoated and AlCrN-coated punches were found according to Eq. (2). The measured burr height and wear volume can be seen as a function of the number of strokes in Fig. 8. In the coated and uncoated punches, the burr height and wear volume increase with the number of strokes. Here, we can also see that the wear volume of the uncoated punch is larger than for the coated punch. The deviations of the values of wear volume for the uncoated punch depend on the inhomogeneous wear of the cutting edge (Fig. 5b).

6 WEAR AND BLANKING PERFORMANCE OF AlCrN PVD-COATED PUNCHES 519 Fig. 7. SEM images of the AlCrN-coated and uncoated punch surfaces after blanking tests: (a) AlCrN-coated punch after 120,000 strokes ( 2200), (b) uncoated punch after 65,000 strokes ( 4300). Fig. 8. Measured burr height (I) and wear volume (II) depending on the number of strokes: (, ) coated; (, ) uncoated. CONCLUSIONS In the present study, the blanking performances of the AlCrN-coated (produced by Arc PVD) and uncoated punches were evaluated under the actual operating conditions for sheet-steel blanking. The wear resistance of AlCrN coatings was investigated and compared with the wear resistance of uncoated punching tools. The degrees of wear at the cutting edge and on the flank of the punch proved to be reduced by the AlCrN coatings obtained by PVD. As for the wear mechanisms investigated by using SEM, it was revealed that abrasion and adhesion are the main responsible factors and result in the edge rounding and fracture at the cutting edge of the punch. Evidence of delamination was also observed in the cross sections of worn surfaces of the punch. The measurements of the wear volume and burr height also gave the conclusive results. The wear volume at the punch edge linearly increases with the number of strokes. The wear volume is larger for the uncoated punch (as expected). The SEM investigations also indicate that the process of wear of the uncoated punch causes some edge fractures at the cutting edges. The burr height seems to depend on the rounding of edges of the worn punch. The burr height is low for the lower numbers of strokes, especially for the AlCrN-coated

7 520 M. ÇÖL, D. KIR, AND E. ERIŞIR punch, with increasing the number of strokes above 10,000 for the uncoated and AlCrN-coated punches, and then continues as a linearly increasing function. Acknowledgement. The authors are thankful to the Kocaeli University Research Fund for supporting this study under Project No. 2008/026. REFERENCES 1. R. Hambli, BLANKSOFT: a code for sheet metal blanking processes optimization, J. Mater. Process. Technol., 141, (2003). 2. G. Monteil, F. Gréban, and X. Roizard, In-situ punch wear measurement in a blanking tool, by means of thin layer activation, Wear, 265, (2008). 3. J. J. Hernandez, P. Franco, M. Estrems, and F. Faura, Modeling and experimental analysis of the effects of tool wear on form errors in stainless steel blanking, J. Mater. Process. Technol., 180, (2006). 4. R. Hambli, Design of experiment based analysis for sheet metal blanking processes optimization, Int. J. Adv. Manuf. Technol., 19, (2002). 5. B. Högman, Steel for press tools: blanking of ultra high strength sheet steels, in: Sixth Internat. Tooling Conf. (Karlstad, Sweden, September, 10 13, 2002), Karlstad, Sweden (2002), pp F. Klocke and H. W. Raedt, Formulation and testing of optimized coating properties with regard to tribological performance in cold forging and fine blanking applications, Int. J. Refract. Met., 19, (2001). 7. M. Vaz, Jr., and J. D. Bressan, A computational approach to blanking process, J. Mater. Process. Technol., , (2002). 8. M. Uchida, N. Nihira, A. Mitsuo, et al., Friction and wear properties of CrAlN and CrVN films deposited by cathodic arc ion plating method, Surf. Coat. Technol., , (2004). 9. M. Okumiya and M. Griepentrog, Mechanical properties and tribological behavior of TiN CrAlN and CrN CrAlN multilayer coatings, Surf. Coat. Technol., 112, (1999). 10. L. Wang, X. Nie, J. Housden, et al., Material transfer phenomena and failure mechanisms of a nanostructured Cr Al N coating in laboratory wear tests and an industrial punch tool application, Surf. Coat. Technol., 203, (2008). 11. R. Hambli, Design of experiment-based analysis for sheet metal blanking processes optimization, Int. J. Adv. Manuf. Technol., 19, (2002). 12. W. Klingenberg and T. W. de Boer, Condition-based maintenance in punching/blanking of sheet metal, Int. J. Machine Tools & Manufacture, 48, (2008). 13. S. Y. Luo, Studies on the wear conditions and the sheared edges in punching, Wear, 208, (1997). 14. N. P. Suh, The delamination theory of wear, Wear, 25, (1973).