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1、Wear 274 275 (2012) 355 367 Contents lists available at SciVerse ScienceDirect Wear jou rnal h om epage: Wear at the die radius in sheet metal stamping Michael P. Pereiraa, Wenyi Yanb, Bernard F. Rolfec aCentre for Material fax: +61 3 5227 1103. E-mail addresses: .au (M.P. P

2、ereira), (W. Yan), .au (B.F. Rolfe). has been shown that the transient stage results in unique contact pressure 13, sliding distance 14, and bulk deformation condi- tions 15 at the wearing interface, which differ signifi cantly to those experienced during t

3、raditional wear tests. It has been spec- ulated by Pereira et al. 1315 that the contact and deformation conditions that exist during the transient stage may be critical to the overall sheet metal stamping tool wear response. It is well known that several different wear mechanisms including micro-cut

4、ting, ploughing, ratchetting and galling can occur during sheet metal stamping. The literature reveals that the location of wear on the die radius, the type of mechanism that occurs, and the relative severity of the wear response can vary sig- nifi cantly (for example, see 2,1618). This variation ca

5、n often be observed at different locations over a single die radius surface for a given stamping process. For bending-under-tension processes, the location, type and severity of wear shows close correlation to the characteristic two-peak contact pressure distribution that exists over the tool radius

6、 4,6,8,19,20. However, for sheet metal stamp- ing processes, the correlation between the wear behavior and the contact conditions is currently unknown. Through the use of surface profi lometry, the wear depth over the die radius has been characterized for the case of a cylindrical cup forming proces

7、s 16,21. This method can successfully quan- tify wear mechanisms that involve material removal, such as the micro-cutting wear mechanism. However, when material is not removed from the surface i.e. when ploughing, galling, or a 0043-1648/$ see front matter 2011 Elsevier B.V. All rights reserved. doi

8、:10.1016/j.wear.2011.10.006 356M.P. Pereira et al. / Wear 274 275 (2012) 355 367 combination of abrasive and adhesive mechanisms occurs together the surface profi lometry/wear depth results can be more diffi cult to interpret. In these cases, microscopy or visual observation has been used to identif

9、y the predominant wear mechanisms 17,18. However, to the authors knowledge, there is no published work that details the types of wear mechanism, and quantifi es the loca- tions at which they occur, over the die radius in sheet metal stamping. This knowledge is especially important, considering the v

10、arying contact conditions that have been identifi ed to occur, both over the die radius and throughout the duration of the stamping process. Therefore, the fi rst aim of this investigation is to charac- terize the location, type and severity of wear that occurs over the die radius, for a typical she

11、et metal stamping process. Once the wear response has been characterized, the importance of the macro-scale contact, sliding and deformation conditions over the die radius can be examined. In particular, the conditions at the die radius that are critical to the overall tool wear behavior can then be

12、 determined. Therefore, the second aim of this investigation is to assess whether the recently identifi ed transient stage of the stamp- ing process, and the resulting transient conditions at the die radius (shown in Fig. 1a), are important to the overall wear response. 2. Experimental setup 2.1. Te

13、st method and confi guration The channel forming test shown in Fig. 2 forms the basis of this study. The test confi guration, summarized in Table 1, is based on semi-industrial wear tests previously reported in the litera- ture 17,22,23 and numerical studies conducted by Pereira et al. 13,14. The ge

14、ometric, process and material parameters closely represents a typical wear prone automotive sheet metal stamp- ing operation. An Erichsen Universal Sheet Metal Testing Machine Table 1 Summary of the geometric and process parameters for the channel forming operation. Punch widtha40 mm Draw depthd 50

15、mm Initial holder force Fh20 kN Die-to-punch gap g 2.1 mm Blank length l 150 mm Die corner radius Rd5 mm Punch corner radiusRp5 mm Blank thicknesst2 mm Blank widthw 19.5 mm Punch speed v 1.5 mm/s (Model 145-60) was used as the press system. The custom tooling (shown in Fig. 2) was designed to be com

16、patible with the standard tooling available, while permitting easy removal of the die radius inserts for inspection and interchange. It is worth noting that there is a difference between the width of the punch, a, used in this study (40 mm), compared to that of the numerical studies (30 mm) 13,14. H

17、owever, comparison to the previous numerical study is still valid, as numerical analysis showed that this difference in punch width has negligible effects on the contact and deformation conditions at the die radius 13. Addi- tionally, the punch speed, v, of 1.5 mm/s is signifi cantly slower than the

18、 ram speeds used in typical automotive sheet metal stamping processes 23. The authors appreciate that the effects of deforma- tional and frictional heating, at higher punch speeds, may infl uence the tool wear rate and mechanisms produced. However, investiga- tion of these effects is beyond the scop

19、e of this work and is not considered in this study. As previously stated, one of the primary aims of the tests was to determine the wear behavior that was critical to the overall tool wear response i.e. the type of wear mechanism and the location Fig. 1. Discontinuous and time-dependent contact cond

20、itions experienced over the die radius during a typical sheet metal stamping operation. (a) Evolution of contact pressure, showing the existence of two distinct stages (adapted from 13). (b) Sliding distance distribution experienced over die surface at different magnitudes of contact pressure (adapt

21、ed from 14). M.P. Pereira et al. / Wear 274 275 (2012) 355 367357 Fig. 2. Channel forming wear test and tooling, using an Erichsen Universal Sheet Metal Testing Machine: (a) prior to, and (b) at the end of the stamping process. on the die radius that results in failure of the stamping process. For a

22、utomotive sheet metal stampings, failure is typically judged by the existence of visible scratches on the sidewalls of the formed parts 17,22. Therefore, after each forming operation, the sidewalls of the stamped parts were carefully visually inspected for any signs of wear. It is known that the wea

23、r rate (for galling, in particular) can increase very rapidly once the process has initiated 18,24. Therefore, to obtain accurate information on the type and location of wear, it was important to stop the testing process as soon as the initiation of the critical wear mechanisms was evident. For this

24、 rea- son, the surfaces of the die radius inserts were also closely visually examined after each part was stamped. It is appreciated that a combination of visual inspection and sur- face profi lometry (i.e. the measurement of scratch depths) on the stamped part sidewalls can be used to provide an in

25、dication of the wear state of the process 17. However, in this investigation, the authors found that visual inspection alone provided an effi cient and successful method of identifying the development of the criti- cal wear response. This was especially the case because of the need and desire to sto

26、p the tests when the critical wear mechanisms were fi rst visible. Once the test was stopped, the die radius insert surfaces were cleaned with acetone and examined in detail with surface profi lometry and optical microscopy techniques (as will be described in Section 2.3). Prior to the tests, the ed

27、ges of the sheet metal blank were deburred to remove any sharp edges associated with the guillotine operation that was used to cut the blanks to size. All tools and blank surfaces were wiped with acetone using a lint-free cloth before each Fig. 3. Surface profi le measurements along the blank slidin

28、g direction. sample was formed. Suffi cient time was allowed to ensure that the acetone had evaporated from the active surfaces prior to each test. Lubrication was not used. 2.2. Materials The tools that contacted the blank during the forming stroke (die, die radius inserts and blank holder) were ma

29、nufactured from AISI D2 tool steel and through-hardened to 60 HRC. D2 grade tool steel was chosen as it is commonly used in the automotive industry for stamping press tooling 17. The blank material was an uncoated dual phase (DP600) sheet steel, with a strain hardening index of 0.15, yield strength

30、of 400 MPa and tensile strength 660 MPa, as determined from tensile tests. 2.3. Surface characterization A combination of surface profi lometry and optical microscopy was used to characterize the surface of the die radius inserts prior to and after the stamping wear tests. The form (shape) of the di

31、e radii were measured using 2D profi lometry along the blank sliding direction, while the roughness was measured in both the sliding and transverse directions. 2.3.1. Surface profi le (form) measurements along the sheet sliding direction The surface profi lometry was conducted using a Taylor Hob- so

32、n Form Talysurf Intra (112/347701) instrument with a custom, 120 mm long, 60 degree conical, 2 ?m radius, diamond tipped sty- lus. The stylus sliding speed and sampling rate were 1 mm/s and 2000 Hz, respectively. Each die insert was positioned at an angle of 40 degrees to the horizontal, while the f

33、orm of the die radius surface was measured along the blank sliding direction using a single horizontal traverse of the stylus, as shown in Fig. 3. The region of approximately 5 to 80 degrees on the die radius was measured using a 7 mm measure- ment length. This measurement was repeated at three loca

34、tions 358M.P. Pereira et al. / Wear 274 275 (2012) 355 367 Fig. 4. Excerpt from the engineering drawing of the die radius insert. All dimensions in mm. General linear tolerance 0.2 mm. across the die radius i.e. at distances of 15, 20 and 25 mm, as mea- sured from the edge shown in Fig. 3. Note that

35、 the blank material contacts the die radius insert surface at the region of approximately 1131 mm from the die radius insert edge, as indicated by the shaded region on the die radius insert in Fig. 3. Careful attention was given to the manufacture and measure- ment of the die radius inserts in this

36、study. It was rationalized that the shape of the die radius surface should be manufactured pre- cisely particularly regarding the cylindricity of the radius surface, and the accuracy of the transition between the radius and the fl at surfaces. Therefore, a small profi le tolerance of 0.02 mm was spe

37、c- ifi ed on the active surfaces of the die radius insert (refer to surface BC shown in Fig. 4). However, the tolerance on the size of the die radius was set to 0.2 mm in order to minimize costs. The quality of each die radius was checked by calculating the deviation of the measured profi les with r

38、espect to an ideal profi le shape. The ideal profi le consisted of a horizontal line with a per- fectly round radius exactly tangent to the horizontal line. The error between one of the measured profi les and an ideal profi le is shown in Fig. 5, as calculated using a mathematical solver routine dev

39、el- oped in Microsoft Excel. The horizontal line of the x-axis can be considered as the ideal profi le, while the curve shows the devia- tion of the measured profi le from this ideal. The solver minimized the error between the measured and ideal profi les, through transla- tion and rotation of the m

40、easured profi le in two-dimensional space. As part of the error minimization routine, the radius of the ideal profi le was also permitted to vary within 0.2 mm, in accordance with the manufacturing tolerance specifi ed. For the measured die radius profi le examined in Fig. 5, the best fi t to the id

41、eal profi le was achieved with a radius of 5.048 mm. It is evident that the die surface profi le shown in Fig. 5 is very accurate, with a maximum variation from ideal of less than 0.004 mm for the entire region from approximately 5 to 80 - 0.005 - 0.0025 0 0.0025 0.005 80706050403020100- 10 Deviatio

42、n of measured profile from ideal mm Angle on die radius deg R 5.048 mm Fig. 5. Deviation of a measured profi le (die radius insert II at 20 mm from edge) with respect to an ideal profi le shape. The ideal radius was taken as 5.048 mm for the case shown. degrees on the die surface. For each of the th

43、ree measurements taken across each of the die radius surfaces tested, the maximum profi le deviation was calculated to be less than 0.006 mm for the region between 5 and 75 degrees on the die surface. Note that this region (from 5 to 75 degrees) approximately corresponds to the predicted blank conta

44、ct zone, as shown in Fig. 1. Hence, any effects associated with manufacturing inaccuracies are assumed to have a negligible effect on the wear results obtained in this study. 2.3.2. Surface roughness measurements along the sliding and transverse directions The profi le measurements, described above,

45、 were used to calcu- late the roughness of the die radius inserts in the sliding direction. For the transverse direction, the die insert holder device was used to position the die insert at the angle ? from the horizontal, such that the stylus was positioned at the same angle ? on the die radius sur

46、face (see Fig. 6). As shown, the transverse direction roughness measurements were taken from 15 to 27 mm from the die insert edge. The measurement was repeated at 10 degree increments on the die radius, from 0 to 80 degrees. For each profi le measurement, Fig. 6. Surface profi le measurements transv

47、erse to the blank sliding direction. M.P. Pereira et al. / Wear 274 275 (2012) 355 367359 Table 2 Parameters used for the roughness analysis. Upper cut-off, lc0.25 mm Short wavelength cut-off, ls2.5 ?m Evaluation length, ln Sliding direction7 mm Transverse direction 12 mm Bandwidth 100:1 Filter Gaus

48、sian the Taylor Hobson Ultra software (version 0) was used to calculate the mean arithmetic roughness, Ra, using the parameters detailed in Table 2. For typical Ra calculations, the upper cut-off length (lc, shown in Table 2) is used to suppress irregularities that occur over longer lengths,

49、such that the surface roughness can be determined 25. Therefore, in the same way that the surface roughness is calculated for a nominally fl at surface with some waviness, the Gaussian fi l- ter and fi lter cut-off was used to separate the roughness from the nominal form associated with the curved s

50、urface of the die radius inserts. To choose the most appropriate fi lter cut-off for the cor- rect calculation of Ra, a pragmatic empirical approach was utilised, based on the procedure detailed in 25. Using this approach, a Gaussian roughness fi lter was applied, starting with the longest cut-off a

51、vailable (8 mm) and fi nishing with the shortest cut-off available (0.08 mm), and the resulting modifi ed surface profi le was observed in each case. Considering all the profi le measure- ments in the sliding direction, the upper cut-off length of 0.25 mm gave the best compromise between highlightin

52、g the surface fea- tures of interest (i.e. the roughness), while suppressing the form to a minimal amount. Utilising this empirical approach, it was clear that the smaller cut-off length of 0.08 mm suppressed the rough- ness itself, while the larger cut-off of 0.8 mm did not eradicate the form suffi

53、 ciently hence these were not suitable for the rough- ness calculations. It should be noted that the value of upper cut-off length chosen (0.25 mm) also closely corresponds to the cut-off rec- ommended by the relevant ISO standard 26, for the values of Ra measured in the study. 2.3.3. Optical micros

54、cope imaging The die radius inserts were positioned on the optical microscope stage at angles of 080 degrees from the horizontal; in 10 degree increments (see Fig. 7). At each angle, a series of digital photographs were captured through the 10 optical lens at 1 mm increments along the transverse dir

55、ection. Using this method, the region of the surface from 0 to 80 degrees on the radius and 1428 mm from the die insert edge was examined. A full set of micrographs was Fig. 7. Optical microscope setup for imaging of the die radius insert surfaces. The die insert is positioned at an angle of 40 degr

56、ees to the horizontal for the case shown. captured at the end of the test, corresponding to approximately 120 images for each of the die radius insert surfaces, in order to obtain a detailed understanding of the wear behavior. This was a very time-consuming task; therefore, fewer micrographs were ta

57、ken prior to the tests. The aim, in this case, was to obtain a gen- eral understanding of the initial surface topography, in combination with the roughness measurements described in Section 2.3.2. 2.4. Test conditions The details of the individual channel forming wear tests exam- ined in this study

58、are summarized in Table 3. These are in addition to the general geometric and process parameters specifi ed in Table 1. The values of the die radius, maximum profi le deviation and rough- ness are average values obtained from the surface profi lometry measurements and calculations described in Secti

59、on 2.3. As shown in Table 3, the primary differences between each of the tests were the specifi ed drawing depth and the method used to prepare the die radius insert surfaces. With regards to the surface preparation method employed; die radius inserts I, V and VI were polished by hand, using 1200 grit sili- con carbide wet and dry sandpaper on a fl at sheet of glass. Die radius insert II was also polished by hand, using the same method, but with 2400 grit sandpaper. Die radius insert III was polished us