某型號接線盒支架沖壓模具設(shè)計(jì)-復(fù)合模含開題及9張CAD圖,型號,接線,支架,沖壓,模具設(shè)計(jì),復(fù)合,開題,cad INFLUENCE OF SUBSTRATE MATERIAL ON WEAR PERFORMANCE OF STAMPING DIES UTILIZING A NEW DIE WEAR TEST SYSTEM mer N. Cora and Muammer Ko NSF I/UCR Center for Precision Forming / Department of Mechanical Engineering Virginia Commonwealth University Richmond, VA KEYWORDS U/AHSS, Stamping, Die Wear Test, Die Materials, Coatings ABSTRACT Stamping of Ultra/Advanced High Strength Steel (U/AHSS) sheets results in high contact stresses and non-uniform strains. These, in turn, cause high wear rates and springback. In order to prevent the excessive wear effect in stamping dies, various countermeasures have been proposed such as alternative coatings, modified surface enhancements in addition to the use of newer die materials including cast, cold work tool, powder metallurgical tool steels. A new, slider type of test system was developed to replicate the actual stamping conditions including the contact pressure state, sliding velocity level and continuous and fresh contact pairs (blank-die surfaces). A vertical machining centre, with vertical and normal force sensors mounted on its spindle, was employed to generate the contact pressure and controlled movement of die samples on sheet blank. Several alternative die materials in coated or uncoated conditions were tested against different AHSS and stainless steel blanks under certain load, sliding velocity, and lubrication circumstances. This paper briefly describes the test system and experimental methodology; and presents wear resistance performance of four different substrate materials namely DC 53, SKD 11, DRM 3, DRM 51 coated with same thermal diffusion (TD) technique. All the die samples were tested against a type of advanced high strength steel (AHSS) which is dual-phase (DP) 600 (UTS is 600 MPa). INTRODUCTION Die wear is an undesired and unpredictable failure and downtime reason in metal forming operations. It directly affects the part formability and surface quality, and causes production loss, cost increase and delays. AISI D2 die material has been a widely used tool steel for various forming applications in the stamping industry. However, it is found to be not suitable for stamping of Advanced High Strength Steel (AHSS) grades (DP, TRIP, etc.) because of excessive wear/galling and toughness issues. Several attempts have been sought to find alternative solutions for die wear issues including development of different die materials, coatings, and surface enhancements. The literature is abundant in terms of various die wear test methods developed for different Transactions of NAMRI/SME 325 Volume 37, 2009 applications and wear conditions. However, they either do not reflect the actual stamping conditions or require special, cumbersome and costly preparations that are not applicable in small spaces such as laboratory environment. For example, in pin-on-disk test Rabinowicz 1995; Blau and Budinski 1999, the die material (in form of a very small pin) is in contact with the same disc surface (sheet metal of interest) during the entire the testing duration, which is not a true representation of the actual stamping operation conditions, because at every stamping stroke the die material gets in contact with new sheet metal surfaces (i.e., virgin surface conditions). In addition, as opposed to the actual stamping conditions, the contact surface area is very small in the pin-on-disk wear tests. SRV (Schwingung Reibung Verschlei: reciprocating friction and wear) tester is one of several configurations of pin-on-disk test systems, and the same surfaces of the die and sheet materials of interest are in contact during the whole test Wan et al. 1995; Hardell 2007. Similarly, the twist-compression test (TCT) Lenard, Medley, and Schey 1996; Costello and Riff 2005 is based on the repeated contact tracks on the same sheet material surface, and it is found to be suitable for comparing the test variables (such as lubricant) rather than obtaining an absolute measure of wear Dalton 2002. Likewise in load-scanner test, a stationary test cylinder is used as a tool sample, and it is contacting with another rotating cylinder which is the sheet material of interest Podgornik, Hogmark, and Pezdirnik 2004. In this test, the same contact surface is scanned repeatedly in every cycle; however, in a real stamping operation a die/punch is in contact with untouched blank surfaces continuously. On the other hand, other wear tests such as (a) strip pulling Attaf et al. 2002; Boher et al. 2005, (b) u-bending/deep drawing Sato and Besshi 1998; Schedin 1994; Nilsson, Gabrielson, and Stahl 2002, (c) strip drawing Schedin 1994; Jonasson et al. 1997; Hortig and Schmoeckel 2001, (d) draw bead Sanchez 1999, (e) combined draw-bead and strip pulling Dalton 2004, and (f) bending under tension or radial strip drawing Schedin 1994; Eriksen 1997 are more representative of the actual stamping conditions; however, they are lengthy (e.g. 15-70 km strip length is needed in combined draw-bead and strip pulling test), costly and require special arrangements such as specially slit coils, large test area and extra equipment like hydraulic clamps, presses, coilers/de-coilers, etc. With an increasing demand to introduce new lightweight materials into the auto body components, newer and alternative die materials, coatings, surface treatment and enhancements, lubricants become necessary to ensure prolonged die life, competitive part cost and consistent high part quality. Accurate and rapid testing of all possible combinations of die material, coating and surface treatment using the existing wear testing methods is not feasible in terms of time, cost and reliability. The main motivation in this study was to establish a test system that provides reliable results, in a rapid manner and also to simulate/control the parameters as much as possible to real conditions. The test system we proposed eliminates the repeated contact surface issues by continuous sweeping of fresh/untouched blank surface by means of tool/die sample. This paper presents the results of a study for which the aim was to investigate the effect of different substrate materials, recently introduced tool steel DC 53, conventional tool steel SKD 11 (equivalence of AISI D2/DIN 1.2379), cold forging tool steel DRM 3, DRM 51, on die wear resistance under stamping conditions of DP 600 AHSS grade sheet blanks. The test system, experimental conditions, die material, and sheet blank properties are presented in detail. Experimental measurements and results are described and conclusions and recommended future work discussed. TEST SYSTEM An alternative die wear test system was developed with the premise of rapid and accurate wear performance assessment of alternative die materials for newer stamping materials. Its earlier design and comparison with existing die wear test methods were discussed in previous work of the authors Cora, Usta, and Ko 2007, a brief description of the updated test system is as follows: This die wear test system is based on the use of precise and controlled motion of a vertical machining centers (Haa VF- 3 CNC) x, y, and z-axes and spindle (no rotation). A load sensor was mounted on the Transactions of NAMRI/SME 326 Volume 37, 2009 spindle through a holder that also houses the die sample of interest. AHSS sheet blanks are laid on the x-y table with clamps at four corners as can be seen in Figure 1. CNC was programmed for the precise pressing of die sample and one- way scratching/sweeping on the AHSS sheet blank. Normal and friction force occurring at the die and blank interface was recorded during the tests. Figure 2 shows the die sample dimension and an actual picture with the wear tracks on the sheet blank. FIGURE 1. TEST SYSTEM. FIGURE 2. DIE SAMPLE DIMENSIONS (TOP) AND ITS ACTUAL PHOTO ON WEAR TRACKS (BOTTOM). Experimental Procedure and Test Materials Each die sample was tested along 2.2 km track distance under an average normal load of 220N with a sliding speed of 0.33 m/s utilizing the above mentioned system. DP 600 (Dual- Phase, 600MPa UTS) sheet blanks of 330 x330 x1mm were used in tests. Chemical compositions for die and sheet blanks, tested material-coating combination, and hardness values for the die samples and sheet blanks are tabulated in Tables 1 through 4. TABLE 1. CHEMICAL COMPOSITIONS OF THE TESTED DIE SAMPLES. Material C Cr Mo W V Cr Mo DC 53 0.95 8 2 - 0.3 8.0 2.0 SKD 11 1.50 12 1 - 0.3 8.0 2.0 DRM 3 0.60 4 2Mo+W 1.0 8.0 2.0 DRM 51 Patent pending by DAIDO TABLE 2. TESTED MATERIAL CONFIGURATION, SUBSTRATE HARDNESS, AND DENSITY VALUES. TABLE 3. TYPICAL CHEMICAL COMPOSITION OF DP600 STEEL SHEET BLANKS Cuddy et al. 2005. TABLE 4. HARDNESS AND AVERAGE SURFACE ROUGHNESS (RA) VALUES FOR DP 600 SHEET BLANK. Die specimens provided by DAIDO Steel Co. Ltd were prepared with the following procedure: First, all the samples are roughly machined before pre-heat treatment. In the heat treatment DC 53 die samples were exposed to gas Substrate material + Coating configuration Substrate Hardness (HRC) Substrate Density (kg/m 3 ) DC 53 + TD Coating 60.4 7870 SKD 11+TD Coating 57.2 7730 DRM 3 +TD Coating 62.7 7920 DRM 51+TD Coating 60.0 7970 Chemical Composition Material Grade C Mn Si Al S P DP 600 0.106 0.800 0.310 0.044 0.005 0.01 Hardness Measured (HV 1 ) Average Surface Roughness Ra (m) 203 0.24 Transactions of NAMRI/SME 327 Volume 37, 2009 quenching at 1030C, then tempered for 1 hour at 550C. After the heat treatment applied; the die samples are machined to final dimensions and polished prior to thermal diffusion (TD) coating process. TD coated samples are heat treated after coating process again for improved performance. The final procedure for the sample preparation is polishing. EXPERIMENTAL RESULTS AND DISCUSSION Performance evaluation of die samples was based on the following measurements (1) mass loss, (2) surface profile (roughness) and (3) microscopic evaluations. To have information about surface roughness, contact surface of die samples are measured with a stylus (Ambios XP-1, Ambios Tech., CA), which is a contact- type of profilometer. All the measurements are taken normal to the sliding direction which was followed during the test. Figures 3 to 6 show the micrographs for contact surfaces of the tested materials before and after tests. Sliding directions were unidentifiable on the contact surfaces except barely visible tracks on DRM 3 sample. FIGURE 3. MICROGRAPHS FOR DC 53 DIE SAMPLE CONTACT SURFACE BEFORE TEST (LEFT) AND AFTER TEST (RIGHT). FIGURE 4. MICROGRAPHS FOR SKD 11 DIE SAMPLE CONTACT SURFACE BEFORE TEST (LEFT) AND AFTER TEST (RIGHT). FIGURE 5. MICROGRAPHS FOR DRM 3 DIE SAMPLE CONTACT SURFACE BEFORE TEST (LEFT) AND AFTER TEST (RIGHT). FIGURE 6. MICROGRAPHS FOR DRM 51 DIE SAMPLE CONTACT SURFACE BEFORE TEST (LEFT) AND AFTER TEST (RIGHT). Specific wear rate (k) was used to asses the wear resistance performance. It is defined as follows: n V k sF = (Eq. 1) where V is wear volume, s stands for sliding distance, F n is applied normal load, and k is for specific wear rate (mm 3 /N.m). The specific wear rates of the test samples are given in Figure 7. The smaller value stands for higher wear resistance and as can be inferred from the table that the performances of the DRM 3 and DRM 51 are close to each other, slightly higher than DC 53, and less than SKD 11. For repetition purposes, 3 replications were performed with the DC 53 sample. Based on good results obtained from replications as depicted in Figure 7, repetitions for other cases were not performed. The coating was not removed from the substrate completely in any replication test, which strengthened the consistency of test results. This test group is one of planned test phases in our test matrix. The effect of different substrate hardness for DC 53 material, effect of different coating types on DC 53 substrate material on wear resistance were investigated prior to this study Cora and Ko 2008a,b. In general, increasing mass losses are expected Transactions of NAMRI/SME 328 Volume 37, 2009 for the same material with the increasing contact normal forces/stresses. It is concluded that the combination of tested substrate material and coating type performed better then the previous tests samples. FIGURE 7. SPECIFIC WEAR RATES FOR TESTED MATERIALS. Tables 5 and 6 show the average and root- mean-square surface roughness values before and after experiments. Similarly, Figure 8 depicts the variation of average surface roughness value (Ra) with error bars. Variation tendency of average and root mean square roughness values is the same and this is an expected situation in most cases. The surface roughness values (Ra,Rq) for die samples contact surfaces are improved when compared with the surface roughness values measured before tests. TABLE 5. AVERAGE SURFACE ROUGHNESS VALUES (Ra) PRIOR TO AND AFTER TESTS. Die sample Ra Before Test (m) Ra After Test (m) DC 53 0.035 0.033 SKD 11 0.032 0.018 DRM 3 0.020 0.014 DRM 51 0.030 0.029 TABLE 6. ROOT-MEAN SQUARE ROUGHNESS VALUES (Rq) BEFORE AND AFTER TESTS. Die sample Rq Before Test (m) Rq After Test (m) DC 53 0.05 0.043 SKD 11 0.048 0.023 DRM 3 0.028 0.019 DRM 51 0.047 0.041 FIGURE 8. VARIATION OF AVERAGE SURFACE ROUGHNESS VALUE FOR TESTED DIE SAMPLES. Improved surface roughness is verified when the evolution of coefficient of friction during the tests is examined. In particular, friction coefficient for SKD 11 die sample was measured as 0.04 (mean value). It is noticed that the friction coefficient decreases with the increasing load. The stability of coefficient of friction can be explained by the lack of coating removal from the substrate and insignificant topography change on the contact surface. SKD 11 is Japanese standard tool steel and is known as equivalent of AISI D2 (DIN 1.2379). It has been one of the most commonly used tool steel materials in cold forming, blanking, trimming, and calibration dies in the past 3-4 decades. When forming of advanced high strength steels are started, using D2 as a tool /die material caused excessive problems in forming the material such as surface quality defects because of excessive wear galling issues. The other experienced problems were shortened tool life, unexpected failures of tools, shutdowns, and production losses consequently. These problems initiated quests for alternative die materials and surface treatment technologies in industry. Several alloyed steels and powder metallurgically produced tool steels have been developed for extended tool life applications without or reduced severe wear problems. As reported in the previous study of the authors Cora and Ko 2008a bare D2 is the least wear resistant tool steel among the tested uncoated alternative die materials. Contrary to weak performance of uncoated D2 tool steels, extended tool lives have been practiced with the coated D2 samples Miller 2008. Similar Transactions of NAMRI/SME 329 Volume 37, 2009 improved performance for the SKD 11 sample was witnessed after the tests. The acceptable upper limit of specific wear rate for engineering sliding surfaces is regarded by some researchers as 1x10 -6 mm 3 /m.N, and all the tested samples performed well above when compared to given value van der Heide et al. 2006. Conclusion and Future Work The effect of substrate material on die wear resistance has been investigated for four different steels against advanced high strength steel sheet blanks of DP 600. The proposed test system enables fast, cost-effective, reliable wear resistance assessment especially for the die materials used in forming of advanced high strength steel sheet blanks. Test results showed that the combination of substrate material and coating technique applied can significantly change the wear resistance compared to performance of the bare/uncoated material. The optimum hardness value for the substrate material and the coating technique applied are the other important factors for improved performance. As can be seen from Table 3, the substrate hardness values for the tested materials varied from 57 to 62 HRc, however, the superiority of the one of tested die sample to another is undistinguishable. It is also worth to mention that TD coating contributed to improved performance of the tested materials as experienced in previous test stages Cora and Ko 2008a,b. In application of TD coating, two separate heat treatments are applied before and after coating process. It is reported by the sample provider that the secondary heat treatment provides higher performance. The future studies will include the effect of lubricated /unlubricated tests, and performance assessment of different lubricants in addition to continuation of testing other alternative die materials. ACKNOWLEDGMENT This study is partially funded by NSF IIP grant #0638588 (NSF I/UCRC Center for Precision Forming). The authors are grateful to International Mold Steel Inc. and Daido Steel Inc. for providing die samples and to US Steel and SSAB for providing AHSS sheet blanks. REFERENCES Attaf, D., L. Penazzi, C. Boher, and C. 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Schedin (1997). “Comparative Study of Shotblasted and Electrical-discharge-textured Rolls.” Wear, Vol. 207, pp. 34-40. Lenard, J.G., J.B. Medley, and J.A