# Anti-Galling Cold, Dry Forging of Pure Titanium by Plasma-Carburized AISI420J2 Dies
## 3 Experimental Results
### Contact Interface Analysis  
The cold, dry forging experiments up to \(r\) = 70% were continuously performed 10 times to leave a distinct contact interface onto the carburized AISI420J2 punch surface.  
Figure 8 depicts a microscopic image of the contact interface of the carburized AISI420J2 and the forged titanium workpiece. Many stripes were formed on this interface radially, from its centerline to its ends, together with the lateral plastic flow of the work material. Three positions were selected for SEM-EDX analyses between the centerline of the interface and its end. Region A was located near the end of the interface, region B was in its middle, and region C was near the centerline.  
Figure 9 depicts the SEM images of region A and the element mapping of oxygen, carbon, titanium, chromium, and iron, respectively. For the original precipitate of (Cr and Fe) oxides and carbides, the chromium and iron uniformly distribute on the contact interface. Carbon, titanium, and oxygen distribute in stripes radially along the plastic flow direction of the titanium workpiece. Since little carbon is detected, even in trace levels, on the carburized AISI420J2 surface before forging, these carbon stripes come from the carbon solutes isolated from the AISI420J2 matrix. Both the titanium and oxygen stripes are  
Figure 8: Optical microscopic observation of the contact interface of the carburized AISI420J2 surface and the forged titanium workpiece.  
Figure 7: Relationship of the applied power to the stroke for each reduction in thickness.  
exclusively present at the positions without carbon content. This implies that the carbon stripes protect the punch surface from adhesion of titanium.  
Figure 10 also depicts the SEM image of region B with the element mapping in the middle of the contact interface. No change in the chromium and iron maps was detected between regions A and B. The carbon stripes broaden, while the titanium and oxygen stripes become thinner compared with those in Figure 9. The exclusive presence of carbon stripes on titanium and oxygen ones becomes more distinct than those seen in Figure 9.  
Figure 11 shows the SEM and element mapping of region C. A few titanium and oxygen stripes are seen at the centerline of the contact interface. Most of region C is covered with unbound carbon films. Iron and chromium maps are the same as those seen in Figure 4; the punch surface at the center in contact is free from adhesion of titanium. This proves that the punch surface at the initial contact with the titanium workpiece with high static pressure during dry forging is free from galling because of in situ carbon lubrication. Figures 9-11 reveal that the interface area overlapped by the carbon films gradu  
Figure 10: SEM–EDX image and element mapping of region B in the contact interface of the carburized AISI420J2 punch and the titanium workpiece.  
Figure 9: SEM–EDX image and element mapping of region A in the contact interface of the carburized AISI420J2 punch and the titanium workpiece.  
centerline of the interface to its end. This suggests that formation of free carbon stripes and films on the interface is driven by the applied stress during dry forging.