The benefits of coating the tools with titanium nitride in the deep drawing of mild steel cups

Abstract

The purpose of this research is to look into the beneficial effects of coating the punch, the die, or both, used for deep drawing mild steel cups starting with blanks cut out from sheet metal. This paper describes a series of experiments in which cups of diameter 60 mm were deep drawn using blanks of cold-rolled mild steel which were of 0.85 mm thickness. Two identical sets of tools, one coated with titanium nitride and the other uncoated, were used. Each set of tools consisted of punches and dies of various edge radii, as well as one blankholder of fixed edge radius. The tools were made of ASSAB DF-2 steel and hardened to about Rc 60. The titanium nitride coating was applied by the PVD (physical vapour deposition) method in order not to lose the required hardness of the tool by annealing. The thickness of the coating ranged from 5 to 8 microns. Tests were conducted using various combinations of punches, dies and blankholders. In order to assess the benefits of the titanium nitride coating, measurements were made of the punch force required to draw a cup, as well as the limiting draw ratio in each case. The effects of geometrical parameters such as punch and die edge radii were also observed. Results show that the titanium nitride coatings on the punch, the die, as well as the blankholder, do significantly reduce the punch force required in all cases. This is attributed to the lubricating effect of the coatings. However, in order to increase the limiting draw ratio, it was found to be more advantageous to leave the punch uncoated whilst coating only the die for any given combination of edge radii. Surprisingly, coating the blankholder had only a negligible effect on the limiting draw ratio in each case. Geometrical parameters were also found to affect the punch force as well as the limiting draw ratio, the former being reduced and the latter being increased as the punch edge radius or the die edge radii or both are increased.