MECHANICAL AND TRIBOLOGICAL PROPERTIES OF COATED STEEL BLANKING PUNCH

INTRODUCTION

Blanking of steel sheets is extensively applied in the automotive industry. It is necessary to develop the wear resistance of the surface of the punch, [1, 2]. The quality of blanked parts is strongly affected by the wear of the tool, punch and die, [3]. Wear of the tool increases the exerted force and decreases the tool life. Besides, increase of clearance between the punch and die damages the quality of the parts surface, [4]. There is an increasing demand to coat the surfaces of the tool to reduce wear, [5]. Physical vapor deposition (PVD) process is applied to coat the surfaces of the tool, [6]. The highest tool purity can be obtained by PVD coating technique due to low pressures, [7, 8]. The PVD coating of the punch with a surface layer of improved hardness and low friction may reduce wear.

Wear resistance of AlCrN coatings was investigated, [9]. The results were compared to    uncoated punches under the same operating conditions. The and compared with the wear resistance of uncoated punching tools. Wear observed for the cutting edge of the punch was reduced by AlCrN coatings. TiAIN and AlCrN coatings were tested, [10, 11]. Examination of the worn surface of the tested coatings by scanning electron microscope (SEM) showed that abrasion and adhesion were the major wear mechanisms that deformed plastically the punch cutting edge. AlCrN coatings showed promising results than TiAlN coatings.

In the present work, four types of coating material TiN, TiCN, TiAlN and CrN were applied to coat steel punch to investigate their performance to increase fatigue life, hardness and decrease wear. A mathematical model was introduced based on the experimental results.

EXPERIMENTAL

X155CrMoV12 cold work tool steel was used as punch material, while St.  37 was used as work piece. Blanking punches were hardened and tempered, where the hardness was 60 - 62 HRC. The surface roughness was Ra = 0.4. Ion plating process was applied to deposit the coating layers, Fig. 1, where the substrate material was vaporized and bombarded by high-energy gas ions.

Fig. 1 Ion Plating Process.

The coating materials were TiN (Titanium Nitride), TiCN (Titanium Carbon Nitride), TiAlN (Titanium Aluminum Nitride) and CrN (chromium Nitride) deposited through LD-600A PVD coating machine. The specification of the coating materials is shown in Table 1.

Table 1: Coatings Materials Characteristics

Examination of microstructure, hardness, wear, fatigue and tensile tests were carried out according to ASTM- A892, [12]. Chemical etchant (3 %NTAL + 97 % Alcohol) was used to prepare the examined surfaces. The measurement of the micro-hardness of coated surface was carried out according to ASTM-E384, [13]. Pin-on-disk (ASTM-G99), [14], was used determine wear. The pin was coated cylinder of 7.8 mm diameter, where the load was 200 N and the disc rotated by  200 r.p.m for 15 minutes. The fatigue test was performed according to ASTM-E466, [15], at 500 N applied load. For every punch, steel (A10) washer stamp was used using hydraulic press, where 10000 blanking processes were done.

RESULTS AND DISCUSSION

The microstructures of the punches coating layer and substrate material are showing in Fig. 2.

Fig. 2 Microstructure of the tested Punches (X1000), (a) TiN Coated Punch, (b)TiCN Coated Punch, (c)TiAlN Coated Punch, and (d) CrN Coated Punch.

Fig. 3 Vickers Hardness of the tested surfaces.

The photomicrographs show the microstructure of the tested coatings, where the thickness of coating is equal for all the tested punches, homogeneous and well adhered to the substrate. The measurement of the Vickers hardness of the punches versus distance from edge of punch is shown in Fig. 3. The TiAlN coated punch showed the highest hardness (3044 HV). The uncoated punch has the lowest hardness (764 HV). This behavior can be attributed to the presence of iron carbide, iron nitride, molybdenum carbide and titanium aluminum nitride in TiAlN coated punch. Wear measured by weight for the tested coatings is shown in Fig. 4. It is clearly seen that TiAlN coated punch has the lowest wear, while the uncoated punch exhibited the highest wear value.

Figure 5 shows the alternating stress amplitude versus number of cycles to failure (S-N curve) for the tested punches. TiAlN coated punch displayed the highest number of cycles to failure that reached to 1.5 × 10⁶ cycles while the uncoated punch has the lowest number of cycles to failure that approached 0.5 × 10⁶ cycles. The weight loss of the tested punches after blanking is shown in Table 2, while wear values of the tested coated punches are shown in Fig. 6. TiAlN coating displayed the lowest wear (0.50 g), while uncoated punch showed the highest wear (1.50 g). It was observed that the coated punches gave lower noise during the blanking than that observed for uncoated one, where TiAlN coated punch was the favorite one.

Fig. 4 Wear of the tested Punches.

Fig. 5 S-N Curve of the tested Punches.

Table 2. Weight Loss of the tested punches after Blanking Process.

Fig. 6 Weight loss of the tested punches after blanking.

The experimental results carried out in the present work for coated and uncoated punches are shown in Table 3. It is shown that the hardness of TiAIN represented the highest values among the other coating materials due to the relatively higher content of titanium and molybdenum carbide in the TiAlN coated punch. The hardness increase decreased the wear.

Table 3. Tests Values for Coated and Uncoated Punches

In addition, TiAlN punch coated material gained the highest number of cycles to failure compared to the other coatings.

The chemical analysis was carried to determine the contents of the compounds in the coatings using XRD (X-Ray Diffraction). The contents of the compounds are shown in Table 4, where increasing contents of iron carbide (Fe₃C), and molybdenum carbide (Mo₂C) increased  hardness,  [16 - 17].

Table 4. X-Ray Diffraction Analysis of Coated and Uncoated Punches.

As seen from analysis of XRD the tested coating TiAIN owned the highest values of Fe₃C, and Mo₂C. Increasing the iron nitrate Fe₂N and decreasing ferrite -Fe content increased hardness, fatigue life and wear resistance. The table values confirmed that TiAIN had the best performance among the others coatings.

Mathematical Modeling

The correlation between mechanical properties (hardness, wear and fatigue) with chemical composition of the tested PVD coatings, Minitab 17 package (Statistical software for analysis in statistics and econometrics) was used, [18], as follows:

Hardness = 2495 - 40.63 Fe₃C + 146.8 Mo₂C - 13.79 α-Fe - 23.25 Fe₂N

Wear = 0.000136 + 0.000005 Fe₃C - 0.000016 Mo₂C + 0.000005 α-Fe + 0.000005 Fe₂N

Fatigue = 673625 - 2365 Fe₃C + 68804 Mo₂C - 10457 α-Fe - 5556 Fe₂N

The values of hardness, wear and fatigue number of cycles extracted from Minitab 17 are shown in Table 5.

Table 5. Minitab 17 Software Analysis Data.

CONCLUSIONS

1. TiAlN coating showed the highest value of hardness (3044 Hv), while the uncoated surface displayed the lowest value (764 Hv).

2. The lowest wear was displayed by TiAlN coating.

3. TiAlN coatings displayed the highest number of cycles to failure (1500000 cycles).

4. Values derived from the mathematical model shows an agreement with the experimental results, where TiAIN coating have the best values among the other types of coated punch materials.