Additive manufacturing (AM) of ferrous alloys has achieved a technological maturity level that allows for the production of high-performance steel components. The possibility to produce complex shapes and overhanging structures makes AM an interesting process for hot forming tools with integrated conformal cooling channels. Tool materials have successfully been produced using different AM techniques, with selective-laser-melting (SLM) and laser-metal-deposition (LMD) being the most popular ones.

Hot stamping of high strength steel is the most commonly used production process for safety and structural components in the automotive industry. The workpiece is usually an AlSi-coated high strength boron steel. This coating provides several advantages for the hot stamped components such as protection against scaling and decarburization during the heating processes. However, the AlSi-coating has been shown to be a tribological challenge for the dies arising from a complex combination of adhesive and abrasive wear. Thus, when novel tool materials are being considered for hot stamping dies, it is important to fully understand their tribological behavior in this taxing contact.

Despite advances in process techniques and material characterization of AM tool materials, there have been limited attempts at exploring their tribological behavior, particularly at high temperature. Therefore, there is a need to understand the high temperature tribological response of AM tool materials, both from fundamental as well as application point of views. 

In this context, the aim of this work is to investigate the high temperature tribology of additively manufactured tool materials produced through different AM techniques.

The tool materials include a hot-work tool steel produced by SLM, LMD and conventional manufacturing (SLM-TS, LMD-TS and Ref-TS, respectively); a low-nickel maraging steel produced by SLM (SLM-MR); and a high hardness conceptual tool material produced by LMD (LMD-HH). A high temperature reciprocating sliding tribometer was used to run tests at temperatures up to 400°C with alumina and cemented carbide (WC-Co) as counter-bodies. A hot strip drawing tribometer was used to simulate hot stamping conditions, i.e., sliding against Al-Si-coated boron steel at 600°C and 700°C. Different surface finishes (ground, milled and shot-blasted) were also investigated for SLM-TS. 

The reciprocating sliding tests revealed that SLM-TS and Ref-TS showed very similar tribological behavior to each other irrespective of counter-body. SLM-TS, LMD-TS and Ref-TS were tested against the alumina counter-body and showed similar friction and wear behavior at temperatures up to 200°C. However, at 400°C, while the SLM tool steel and the reference tool steel resulted in rapid formation of protective tribolayers, the LMD tool steel showed a much higher degree of wear due to poor tribolayer formation. SLM-MR showed higher and more unstable friction level, as well as a much larger wear volume at all temperature. Formation of stable tribolayer was the dominant aspect determining the wear volume in the tool materials.

In hot strip drawing tests, the friction behavior was similar for all tested tool materials (SLM-TS, LMD-TS, Ref-TS and LMD-HH) with a stable friction level at 600°C and an unstable and higher friction level at 700°C. The sudden increase in friction at 700°C was due to the rupture of the AlSi-coating on the counter-body, which resulted in severe localized material transfer on the tool material surface. The different surface finishes for SLM-TS did not affect the friction behavior. The typical wear mechanism observed in this tribosystem was a combination of adhesive and abrasive wear, where material transfer and groove formation affected each other. LMD-HH tool material showed a significantly less abrasive wear, which in turn affected the nature of adhesive wear. The milled SLM-TS also showed a lower degree of abrasive damage, as a result of the higher strain-hardening in the subsurface region due to the finishing process. The very rough surface of the shot-blasted SLM-TS resulted in deformation and flattening of the load-bearing asperities as well as the accumulation of different debris in the valley regions. 

In summary, the tool steel produced by SLM showed a very similar tribological behavior to conventionally produced tool steel, regardless of test temperatures and other test conditions. Thus, from a tribological point of view, SLM can be used to replace conventional manufacturing processes for tools in hot stamping and other high temperature applications. The tool steel produced by LMD showed a similar tribological response in the simulative hot stamping tribotests, however a worse behavior in the high temperature reciprocating sliding tests, indicating that LMD tool material performance is more system dependent.