Galling-Free Dry Near-Net Forging of Titanium Using Massively Carbon-Supersaturated Tool Steel Dies

Massively carbon-supersaturated (MCSed) tool steel dies were developed to make galling-free forging products from titanium bar feedstocks in dry conditions without lubricating oils. Two types of tool steel dies were used, SKD11 and ACD56, following the Japanese Industrial Standard (JIS). The plasma-immersion carburizing process was employed to induce massive carbon supersaturation in two kinds of tool steel dies at 673 K for 14.4 ks. A pure titanium bar was upset in a single stroke up to the reduction of thickness of 70% using the MCSed SKD11 die. Very few bulging displacements of the upset bar proved that μ = 0.05 on the contact surface of the MCSed SKD11 die to pure titanium work. Two continuous forging experiments were performed to demonstrate that an in situ lubrication mechanism played a role to prevent the contact surface from galling to titanium works in both laboratory- and industry-scaled forging processes. After precise microstructure analyses of the contact surface, the free-carbon film formed in situ acted as a lubricating tribofilm to reduce friction and adhesive wear in continuous forging processes. The MCSed ACD56 dies were also used to describe the galling-free forging behavior of manufacturing eyeglass frames and to evaluate the surface quality of the finished temples. The applied load was reduced by 30% when using the MCSed ACD56 dies. The average surface roughness of the forged product was also greatly reduced, from 4.12 μm to 0.99 μm, together with a reduction in roughness deviations. High qualification of forged products was preserved together with die life prolongation even in dry manufacturing conditions of the titanium and titanium alloys.