Additive manufacturing (AM) of ferrous alloys has achieved a technological maturity level that allows for the production of high-performance steel components. One potential application of AM is the production of tools for hot forming processes. In addition to the general advantages of AM, such as reduced material waste and expedited manufacturing, it also allows for producing complex shaped parts (e.g., conformal cooling channels) that are unfeasible to produce by conventional manufacturing processes. In recent years, several studies have reported the successful production of tool materials using different AM processes, with the selective-laser-melting (SLM) and laser-metal-deposition (LMD) being the most popular ones. SLM, also known as laser-powder-bed-fusion (LPBF), is particularly promising for producing dies as this process enables production of high quality complex shaped parts with very high density and microstructural homogeneity. LMD, also known as direct-metal-deposition (DMD), on the other hand is more suitable for tool repair as it is flexible and easier to implement.

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 viewpoints. Thus, this work aims to bridge this gap through investigating the tribological behavior of AM tool materials at different temperatures and in different test configurations, also using a conventionally-produced tool steel as reference. 

To achieve this, several different tool materials produced by SLM and LMD were subjected to different tribological tests. 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. Alumina and cemented carbide (WC-Co) were used 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 were also investigated for SLM-TS. 

The reciprocating sliding tests revealed that SLM-TS and Ref-TS showed similar tribological behavior to each other against both the more inert alumina and the more reactive WC-Co counter-bodies. Sliding against alumina, wear of SLM-TS and Ref-TS exhibited the following trend: increase in wear volume at intermediate temperatures, due to thermal softening and poor tribolayer formation; reduction in wear volume at higher temperatures due to formation of protective tribolayer. On the other hand, LMD-TS did not form a protective tribolayer at the higher temperatures, unlike SLM-TS and Ref-TS; thus, LMD-TS showed significantly more wear at 400°C compared to the other hot-work tool steels. Sliding against the more reactive WC-Co counter-body, SLM-TS and Ref-TS also showed similar behavior to each other, though following different trend in wear: reduction in wear volume at intermediate temperatures; and further reduction at higher temperatures. Protective tribolayers were formed at all temperatures, as material transfer from WC-Co onto the tool steel promoted tribolayer formation. SLM-MR was also tested against WC-Co, the tool material showed significantly higher wear volume at all temperatures due to poor tribolayer formation. Overall, in the reciprocating sliding tribotests, lower and more stable friction levels were associated with lower tool material wear and more pronounced tribolayer formation. 

From the hot strip drawing tests, the friction behavior was similar for all tested tool materials (SLM-TS, LMD-TS, Ref-TS and LMD-HH), 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 a 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, milling and shot-blasting, also showed similar 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 lower abrasive damage, which in turn affected the nature of adhesive wear. The milled SLM-TS also showed a lower degree of abrasive damage, which were associated with 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. SLM-TS, LMD-TS and Ref-TS showed similar friction and wear behavior in the hot strip drawing tests.

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 the production of tools in hot stamping and potentially other 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 materials have limited performance compared to SLM. The results from the high hardness and the strain-hardened tool materials indicate that the use of a high hardness tool material limits adhesive wear.