# Anti-Galling Cold, Dry Forging of Pure Titanium by Plasma-Carburized AISI420J2 Dies
## 1 Introduction  
Pure titanium and titanium alloys have been highlighted as structural biomedical parts and tools because of their high specific strength and good biocompatibility [1]. Their shaping and forging processes were strictly limited to minimum reduction in thickness due to their risk of galling to stainless steel, WC (Co), and monolithic ceramic dies [2]. Die material selection and design is needed to find a way to enable anti-galling manufacturing to shape and forge raw titanium and titanium alloy workpieces into biomedical parts. The authors proposed a thick \(\beta\)-SiC-coated SiC die material for anti-galling forging [3]. As proven by [4], no mass transfer of metallic titanium was seen on the contact interface of a \(\beta\)-SiC-coated punch to a pure titanium workpiece. The debris particles of TiO\({}_{2}\) were prevented from depositing onto this coating surface. This anti-galling process was sustained under in situ solid lubrication by the free carbon isolated from the carbon-supersaturated \(\beta\)-SiC coating surface and under the in situ formation of intermediate titanium oxide tribofilms on the contact interface. Owing to this galling-free mechanism, a pure titanium wire was forged under cold and dry conditions with a 70% reduction in thickness [5]. Hence, simple shaping and upsetting can be put into practice in industries by using these \(\beta\)-SiC-coated SiC die systems. However, the low toughness of the \(\beta\)-SiC coating becomes a barrier in further applications of this die material system in the near-net shaping and precise forging of raw titanium and titanium alloys into biomedical parts and tools. Plasma carburizing has developed as one of the most reliable surface treatments to harden tool steel and stainless steel dies [6; 7]. After [8], a thick carburized layer with a thickness of 30 \(\upmu\)m was formed at 743 K for 72 ks onto austenitic stainless steel substrates. Without the precipitation synthesis of chromium and iron carbides, carbon solute content higher than 2 mass% was attained at the surface of the plasma-carburized layer. This high carbon supersaturation resulted in high hardness, wear toughness, and corrosion resistance. As discussed in [8; 9], a fully carbon-supersaturated layer without precipitates was formed by decreasing the holding temperature down to 673 K; however, the carburized layer thickness also decreased down to a few \(\upmu\)m.  
A radio-frequency (RF)-direct current (DC) plasma processing system was developed to build up the experimental setup for low-temperature plasma nitriding and carburizing. The low-temperature nitrogen supersaturation process was performed using this RF-DC plasma processing system [10]. In the case of this RF-DC plasma nitriding with the use of a hollow cathode, a nitrogen-supersaturated layer with a thickness of 60 \(\upmu\)m formed at 673 K for 14.4 ks. Its average hardness reached 1400 HV, and its nitrogen solute content became 4 mass%. In the present study, this high-density plasma processing system is utilized to supersaturate martensitic stainless steel die substrates with carbon. The case-hardened AISI420J2 is employed as a die material, whereas chromium carbide precipitates are used to strengthen the stainless steel matrix. The carburized AISI420J2 punch is characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron dispersive X-ray spectroscopy (EDX). For the carbides, the homogeneous carbon mapping by EDX as well as the peak shifts of \(\alpha^{\prime}\)-iron by XRD prove that the AISI420J2 punch is carbon-supersaturated. This punch is fixed into the upper die set for cold, dry forging of pure titanium wires. The power-stroke relationship is monitored in situ to describe this cold, dry forging behavior. The smaller bulging deformation of the forged wires proves that the titanium workpiece is homogeneously upset with low friction.