SUS304 stainless steel surface asperities have a strong affinity to tool steel surfaces. Therefore, the flow of material in the flange portion during the deep drawing process is retarded due to adhesive wear, leading to increase in amount of wall thicknening along the cup edge and rising risk for delayed cracks. In this paper, TiN coating is applied to the drawing die surface to weaken the affinity. Under elevated blank holding forces (BHF), the experimental results showed that the crack-free BHF range for the TiN coated and the uncoated dies are 5–10 and 12 kN, respectively. The crack-free BHF magnitude is successfully lowered and the range is enlarged with the coated die. The weakened affinity is evidenced by the low estimated coefficient of friction (COF) obtained from the finite element (FE) simulation. In contrast, the estimated COF of the uncoated die is high even at low BHF due to adhesive wear. Therefore, delayed cracks are observed under BHF range of 7–11 kN. At BHF 12 kN, the amount of wear fragments formed in the boundary layer increases as a result of the continuous polishing of the SUS304 asperities by the uncoated die surface asperities. The COF is sharpyly decreased due to the smooth relative movement of contacting surfaces facilitated by the fragments. Therefore, the cracks are prevented. However, the fragments tend to penetrate into the SUS304 surface under excessive BHF of 13 kN and above resulting in the formation of the cracks again. To eliminate the cracks, drawn cups must achieve threshold values of less than 32.5% wall thickening and more than 33.3 mm cup height in the valley points.
The effect of TiN coated die on eliminating delayed cracks in deep drawing processes of stainless steel SUS304 cylindrical cups under elevated blank holding forces (BHF) using a commercial lubricant at room temperature is investigated in the experiment. For comparison, the experiment is repeated using an uncoated and finely polished die under the same conditions. The results shows that the crack-free BHF range for the coated and the uncoated dies are 5~10 kN and 12 kN, respectively. Both the magnitude and range of the crack-free BHF are successfully lowered and enlarged by applying TiN coating to the die surface. Lower magnitude and wider range for BHF are preferred in the industries as it is difficult to maintain a high, constant and precise BHF during the deep drawing process using coil springs or die cushions. The elimination of the cracks is mainly due to the decrease in amount of strain-induced martensite resulting from the lower amount of wall thickening, particularly in the valley points along the cup earring profiles. The improved tribological performance by the coating enhances the radial flow of the materials into the die cavity resulting in lower amount of wall thickening. The chance for delayed cracks is reduced with decreasing amount of wall thcikening. Overall, the amount of tensile residual stresses along the outer surface of the cup, particularly in the upper portion is reduced with the coated die due to its low BHF. Therefore, the risk for the cracks is reduced.
SUS304 stainless steel has strong affinity to carbon tool steel surfaces. Therefore, the flow of material in the flange portion during the deep drawing process is retarded, leading to increase in amount of wall thicknening along the cup edge and rising risk for delayed cracks. In this paper, TiN coating is applied to the drawing die surface to weaken the affinity. Under elevated blank holding forces (BHF), the experimental results showed that the crack-free BHF range for the TiN coated and the uncoated dies are 5~10 kN and 12 kN, respectively. The entire BHF range for successful drawn cups formed with the coated die are crack-free. The crack-free BHF magnitude is successfully lowered and the range is enlarged with the coating. Crack-free cups having large elongated height and low amount of wall thickening along the cup edge are formed. The weak affinity is evidenced by the low estimated coefficient of friction (COF) obtained from a FE simulation model based on the Coulomb’s law of friction. In contrast, the estimated COF of the uncoated die is high even at the low BHF due to the strong affinity. Therefore, delayed cracks are observed under BHF range of 7~11 kN. At BHF of 12 kN, wear fragments are formed in the boundary layer as a result of the continuous polishing of the SUS304 asperities by the uncoated die asperities. The COF is sharpyly decreased due to the smooth relative movement of contacting surfaces facilitated by the particles and the formation of cracks is prevented. However, the segments tend to penetrate into the SUS304 surface under excessive BHF of 13kN and above. The relative motions of the segments are prohibited, resulting in the reformation of the cracks.
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