Laser Directed Energy Deposition of tool steels; Investigating the heat treatment strategies and other potentialities of LDED

Laser additive manufacturing (LAM) has enabled the fabrication of complex and highly customizable metallic parts from a computer-aided design (CAD) file through layer-by-layer construction of the components via local melting of powder/wire using a focussed laser beam. The significant reduction in lead time and the design freedom that allows for substantial savings through weight reduction and the introduction of additional functionality to parts have piqued the industry's interest in these technologies. However, the complicated thermal histories experienced by the components and the very high cooling rates inherent to these processes, lead to different microstructures in the parts, and not all conventional alloys are proper for additive manufacturing (AM). In particular, tool steels with medium to high amounts of carbon are extremely difficult to process due to their susceptibility to cracking. This implies that heat treatments should also be tailored for the LAM parts apart from alloy modification. Therefore this work tries to study in more detail some of the classic tool steels available in the market, along with two recently utilized tool steels for LAM. The first part of this work is devoted to the microstructural characterization of two carbon-bearing tool steels, namely AISI H13, and X35CrMoMn7-2-1 (PLASWeld Ferro55), fabricated through Laser Directed Energy Deposition (LDED). The as-built microstructure of these steels comprises a martensitic matrix with ~10 vol.% retained austenite (RA) as a consequence of microsegregation at cellular\dendrite boundaries. The tempering behavior is studied consequently, using two heat treatment strategies of direct tempering of the as-built part (DT), and austenitization followed by quenching and tempering (QT). It was demonstrated that in the DT scenario, the secondary hardening peak was shifted to a higher temperature (i.e., 525°C vs. 500°C) which is attributed to the decomposition of RA, producing carbides and fresh martensite in the first tempering step in DT samples. This proved advantageous for tempering resistance of H13 samples tempered to an equivalent hardness, where the hardness values for specimens of 500 HV dropped to 360 HV vs. 300 HV for the DT and QT scenarios, respectively. The apparent fracture toughness for H13 was 70 and 89 MPa.m1/2 in the DT and QT specimens, respectively. Similar behavior was also observed for the Ferro55 grade. The Apparent fracture toughness for Ferro55 specimens was considerably higher than H13 specimens tempered to equal hardness (i.e., 110 &106 MPa.m1/2 vs. 70 &89 MPa.m1/2 for the Fe55-DT&QT and H13- DT&QT, respectively). On the other hand, H13 specimens demonstrated better tempering resistance. This was attributed to the presence of vanadium carbides which have much lower coarsening rates compared to Cr carbides which are the main constituents in Fe55. Overall, although the DT scenario demonstrates lower Kapp values compared to the QT scenario, the values are still comparable to the conventionally manufactured (CM) parts, implying that depending on the application, the DT scenario might be a more suitable and economical solution for post-processing. Moreover, for applications where the tool is not subject to high temperatures, Fe55 seems to be a suitable alternative to H13, considering its easier processability and lower tempering temperature required to obtain similar hardness. In the second part of the work, other potentialities of LDED are investigated on two carbon-free maraging tool steels, namely Osprey® 18Ni300 and Osprey® MAR-60HRC. It is well known that parts undergo a so-called Intrinsic Heat Treatment(IHT) during LAM processes due to thermal cycles that previously deposited layers undergo upon deposition of successive layers. Osprey® MAR-60HRC was selected to investigate IHT due to its higher Co and Mo content which gives rise to faster aging kinetics and higher peak hardness. It was demonstrated that through tailoring the thermal history cycles by applying inter-layer dwell times (IDT), it was possible to trigger the precipitation events during the deposition process and to increase the hardness in the as-built part from 360 HV (no IDT) to 520 HV (IDT-250s). This is particularly advantageous in repair applications where it may be possible to avoid heat treating the entire part. The comparatively inferior wear resistance of maraging steels compared to tool steels is another typical concern. One solution to this issue is to enhance the hardness. However, this strategy would result in a reduction in fracture toughness. The last part of this work demonstrates that the production of bimetallic specimens or compositionally graded (CG) material can be a successful approach in this regard. Bimetal specimens with a hard surface (Osprey® MAR-60HRC) and tough core (Osprey® 18Ni300) were prepared successfully. The fracture toughness of the sample was significantly increased from ~56 MPa.m1/2 (Osprey® MAR-60HRC) to 70 MPa.m1/2 with a similar surface hardness.