The drawn cups formed with the finely polished uncoated die at elevated BHF are summarized in Fig. 6. The lowest BHF limit for a successful drawn cup is determined at 7 kN as wrinkle is observed in the cup formed at BHF = 6 kN. Tearing was observed around the cup bottom due to the stretching by the punch corner during the drawing process under an excessive BHF of 16 kN. Therefore, the BHF range for the successful drawn cups is from 7 ~ 15 kN. Within this range, delayed cracks are observed in all cups except the one for BHF = 12 kN. Most of the delayed cracks are observed around the valley points of the cups due to the large amount of wall thickening resulting from the short height. All drawn cups have four ears consisting of 4 peak and 4 valley points.
The number of cracks and the time taken for its first appearance in the cups formed with the uncoated die are summarized in Table 3. The time is recorded immediately after completing the test until the formations of all cracks are complete. It clearly shows that the duration for the first crack increases with increase in BHF. The formation of the cracks is gradually suppressed with increase in BHF. However, the suppression becomes weak when excessive BHFs are applied i.e. from 14 kN and above. A crack-free cup is obtained under BHF of 12 kN. The highest number of cracks is obtained with BHF = 13 kN, followed by 8 kN and others. The longest duration for the first crack is obtained at BHF of 13 kN. The duration sharply decreases for BHF greater than 13 kN.
Table 3 Number of cracks and time taken for its first appearance for uncoated die
BHF
(kN)
|
No. of Cracks
|
Duration for 1st Crack
(hrs)
|
6
|
1
|
5
|
7
|
1
|
8
|
8
|
2
|
9.5
|
9
|
1
|
9
|
10
|
1
|
11
|
11
|
1
|
13
|
12
|
Crack-free
|
13
|
3
|
14
|
14
|
1
|
4
|
15
|
1
|
4
|
16
|
Tearing at bottom
|
The data in Table 3 are summarized and presented in Fig. 7. The duration for the 1st crack increases with the increase in BHF. The crack is successfully eliminated at BHF = 12 kN. The duration and the number of cracks hit the peak values of 14 hours and 3 cracks at BHF = 13 Kn. Both values sharply reduce to 4 hours and only 1 crack for BHF from 14 ~ 15 kN.
The drawn cups formed with the TiN coated die at elevated BHF is shown in Fig. 8. Wrinkle and tearing around cup bottoms are observed under BHF = 4 kN and 11 kN, respectively. Therefore, the BHF range for successful drawn cups is 5 ~ 10 kN. In comparison with the cups formed with uncoated die, successful BHF range is reduced from 7 ~ 15 kN to 5 ~ 10 kN with the TiN coated die. However, delayed cracks are not observed in the successful drawn cups in its entire BHF range. The crack-free BHF range is successfully lowered and widened from 12 kN to 5 ~ 10 kN by replacing the finely polished uncoated die with the TiN coated die under the same lubrication condition. Lower and wider BHF range is preferred in the industries as it is difficult to maintain a constant and high BHF value with coil springs or die cushion during the process.
The comparison of forming load profiles between cups formed with (a) TiN coated die and (b) Uncoated die is shown in Fig. 9. Since the two dies have the same dimensions, typical bell-shape drawing load profiles are obtained for both dies. The increase in drawing load is very minimum for increase in BHF values in both cases. Peak drawing loads range from 105 ~ 115 kN are obtained around 60% of the total punch travel distance for both dies.
The comparison of peak drawing loads between the TiN coated and uncoated dies at different BHF levels is shown in Fig. 10. Overall, the peak loads for the TiN coated die are lower than that of the uncoated die at the same BHF levels. For coated die, the drawing loads increase with increase in BHF i.e. the frictional forces acting in the blank-die interface increases under higher holding pressure. In contrast, only slight changes in peak drawing loads are observed for uncoated die under elevated BHF. Therefore, the extreme-pressure performance of the TiN coated die is not as good as the uncoated die under the same lubrication condition. For uncoated die, galling or cold welding tends to form between the surface asperities in the interface between the SUS304 blank and the coated die surface. The galling effect or the continuous forming and breaking of the welds has produced some fine particles that facilitate the sliding motion of the tool over the blank, particularly at high BHF. However, galling is reduced with TiN coating resulting in low drawing load at low BHF. Therefore, the extreme-pressure performance of the coated die is reduced due to absence of galling at the interface.
The comparison of the average heights and the average changes in wall thickness between the drawn cups formed with the TiN coated and uncoated dies at elevated BHF is shown in Fig. 11. Overall, the average cup height formed with the coated die is larger than the uncoated die. The peak and valley heights are slightly increased with increase in BHF for both dies. The heights hit peak values at BHF of 8 kN and 12 kN for the coated and uncoated dies, respectively. Since the only crack-free cup is obtained at the peak height for uncoated die with BHF of 8 kN, increase in cup height, particularly in the valleys is favourable for eliminating the delayed crack. By applying TiN coating to the die surface, larger cup heights are obtained with lower BHF values. Overall, the wall thickness in the valleys for the cups formed with the coated die is smaller than the ones formed with the uncoated die. However, the wall thickness in the peaks for the cups formed with the coated die is at the same level with the uncoated die. Due to constant volume, the materials contributing to the elongated height is originated from the side wall below the cup edge. The average changes in wall thickness in the peaks and the valleys are slightly reduced with increase in BHF for both dies. The average thickness hit minimum percentages at BHF of 8 kN and 12 kN for the coated and uncoated dies, respectively.
The relationship between the average changes in wall thickness and the average heights of the peaks & valleys of the crack-free cups formed with the TiN coated die is illustrated in Fig. 12. Overall, the average wall thickness is increased, and the height is reduced with increase in BHF. The largest height and the smallest wall thickness for both points are obtained at BHF of 8 kN. Under excessive BHF i.e. greater than 8 kN, the tribological performance of the lubricant becomes poor, leading to the reverse trend of both values. However, delayed cracks are not observed in the drawn cups up to the upper BHF limit of 10 kN.
The relationship between the average changes in wall thickness and the average heights of the peaks & valleys of the cups formed with the uncoated die is illustrated in Fig. 13. A similar trend of increase in height and decrease in amount of wall thickening under elevated BHF is obtained with the uncoated die. However, the largest height and the smallest wall thickness for both points are obtained at BHF = 12 kN or 50 % higher than one with the coated die. The minimum BHF for obtaining a crack-free cup with the coated die is 140 % (i.e. reduced from 12 kN to 5 kN) less than one with the uncoated die.
The longitudinal distributions of residual hoop stresses passing through the valley points along the outer surfaces of the crack-free cups obtained from the ring-slitting test are shown in Fig. 14. Overall, the amount of tensile residual stresses of the cup formed with the uncoated die is larger than the ones formed with the coated die in the lower half of the cups due to its high BHF value. For coated die, the increase in BHF reduces both the amount of tensile stresses and the slope of the stress for greater than 80 % of its total height. Low tensile residual stress level with less gradient, particularly in the upper portion along the outer surface of cups is favourable for eliminating the delayed cracks. The favourable residual stress distribution is obtained with the TiN coated die at BHF values much lower than that with the coated die.