Titanium to steel joining by spark plasma sintering (SPS) technology

Abstract

The bonding of Ti–6Al–4V to low alloy steel (AISI4330) using SPS technique in the 850–950 °C temperature range was examined. The formation of a thin (∼1 μm) titanium carbide interfacial layer was observed with a thickness only slightly dependent on the joining temperature. This layer separates the joined metals and prevents the formation of Fe–Ti intermetallics in the bonding zone. The maximal tensile strength of the joints (of about 250 MPa) was achieved for bonding at 950 °C for 3.6 ks. The formation of the titanium carbide layer and its evolution are discussed based on the isothermal section of the ternary Fe–Ti–C phase diagram.

Introduction

Conventional fusion welding procedures are not applicable in Ti-to-steel joining on account of the formation of brittle Fe–Ti intermetallic compounds at the interface and shape distortion of the parent materials. Solid state joining, such as diffusion bonding (DB) by hot pressing, friction stir welding (FSW) and explosive welding (EXW) are possible alternatives for Ti-to-steel bonding that provide adequate mechanical properties to the joints.

Qin et al. (2006) have reported that DB of Ti alloy (4.5 wt.% Al and 2.2 wt.% V) to austenitic stainless steel with low carbon content (0.04 wt.% C) results in a relatively high shear strength (300 MPa) of the joints. Kurt et al. (2007), however, reported for Ti–6Al–4V alloy and ferritic stainless steel (0.24 wt.% C) joints a significantly lower shear strength (190 MPa) that was attributed to the formation of Fe–Ti intermetallics in the joined region. In order to prevent the formation of Fe–Ti intermetallics a foil of nickel or copper was used as an interfacial layer. Elrefaey and Tillmann (2009) have reported that the presence of a Cu interlayer leads to the formation of Cu–Ti intermetallics in the joined region, yielding relatively low (90 MPa) tensile strength of the joints. Kundu and Chatterjee (2008) have pointed out that joining with Ni as an interlayer provides 270 MPa tensile and 194 MPa shear strength. Church and Wild (1998) have reported that DB of Ti–6Al–4V to low-carbon steels (0.15 and 0.25 wt.% carbon) at 950 °C allowed them attaining a compressive shear strength of the joints higher than 300 MPa. The formation of a thin (100 nm) titanium carbide layer at the interface was observed.

The FSW technique was applied by Fazel-Najafabadi et al., 2010, Fazel-Najafabadi et al., 2011 in joining 304 stainless steel to commercially pure (CP) Ti. The shear strength of the joints (119 MPa) was attributed to a mechanical interlocking at the bond region associated with the massive deformation of the parent metals.

Chiba et al. (1995) discussed the microstructural aspects and bonding characteristics of explosively welded CP-Ti and high-carbon steel (0.82 wt.% carbon) in both as-welded and post-annealed conditions. In the as-welded state, thin amorphous layers were detected along the contact surface of both parent materials. The formation of these layers was attributed to melting and rapid solidification of the metals in the vicinity of the interface during the EXW process. A thin titanium carbide layer was formed at the interface during post-annealing and served as a barrier for diffusion of the species across the interface. The shear strength of the joints after heat treatment in the 800–1050 °C temperature range was about 300 MPa.

Recently, the SPS technique was applied for joining similar and dissimilar materials. He et al. (2012) effectively joined two samples of Ti-alloys and clearly established that the SPS technique allows obtaining significantly stronger joints of similar metals (Ti–6Al–4V), as compared to those bonded by hot pressing.

Successful attempts were reported by Nakamura et al. (2005) for joining dissimilar materials, titanium or aluminum alloys, with steel and by Hirose et al. (2006) for pure tungsten and steel.

No information is yet available on using the SPS technology for joining Ti–6Al–4V alloy to carbon steel. Considering the widespread use of these structural materials the issue is of certain interest. In this study we examine the processing parameters of the SPS technology, present and discuss the microstructure and the mechanical properties of the Ti–6Al–4V alloy/carbon steel (0.3 wt.% C) joints that were obtained.