Precipitation reactions during the intrinsic heat treatment of laser additive manufacturing

Abstract

Laser additive manufacturing (LAM) allows to produce complex and highly customized metallic parts from a computer aided design file (CAD) by melting metallic powder with a focused laser beam. The inherent geometrical design freedom this technique offers enables tremendous weight savings and parts with a complexity that would be impossible to achieve by conventional manufacturing techniques. However, many alloys face problems of either poor processability in LAM or insufficient strength. Even those alloys that are well processable, often do not exploit the full potential of LAM processes as they were typically designed and optimized for conventional processing routes. This work aims at designing new alloys custom-tailored to LAM processes making use of some unique features of these processes. For example, the cyclic re-heating occurring during the process, the so called intrinsic heat treatment (IHT), is used to trigger precipitation reactions already during the process avoiding an aging heat treatment for precipitation strengthened materials. Furthermore, the potential of triggering phase transformations in conventional alloys used in LAM is evaluated and LAM-produced and conventionally-produced parts are compared. The complex microstructures of all samples are characterized at different length scales ranging from cm to nm by light optical microscopy (LOM), scanning electron microscopy (SEM) including electron backscatter diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and atom probe tomography (APT). After showing that during the IHT of a directed energy deposition (DED) processing of a conventional 18Ni-300 Maraging steel slight clustering occurs, simple ternary Fe-Ni-Al and Fe-Ni-Ti steels are developed that respond very well to the in-situ strengthening approach using the IHT. Rapid alloy prototyping approaches using compositionally graded samples are used to efficiently screen a large variation in compositions and find the optimal ones that show the desired microstructure and a strong response to the IHT. In an Fe17Ni10Al (at%) steel, exceptionally high number densities of 1025 NiAl precipitates per m3 are achieved in the as-DED-produced state that lead to a significant increase in hardness and strength. In an Fe18Ni6Ti (at%) steel it is demonstrated how the sequence of two phase transformations (martensite transformation and precipitation) is necessary to obtain precipitation hardening by IHT. Dense networks of Ni3Ti precipitates are triggered in the as-produced state. Furthermore, a detailed understanding of the thermal history of DED-produced materials opens pathways to locally control the microstructure. A combined alloy and process design approach for Al-Sc-Zr alloys allows to produce parts in-situ strengthened by a high number density of thermally stable Al3(Sc,Zr) precipitates. Coarsening of Al3(Sc,Zr) precipitates that takes place already during the IHT in a commercial Al-Sc-Zr alloy can be stopped via the control of solidification conditions together with addition of Zr to the alloy. This allows for enough Zr in the Al matrix to form a Zr-rich shell around Al3Sc precipitates upon IHT and to stop coarsening. The approach of in-situ strengthening via the IHT should be applicable to a wide range of precipitation hardening alloys as well as to further LAM processes such as laser powder bed fusion (LPBF). The IHT cannot only be used for in-situ strengthening precipitation hardening alloys but also be used to trigger phase transformations in general. In a commercial Ti-6Al-4V alloy, the influence of the IHT on the decomposition of the brittle martensitic microstructure is investigated. This thesis shows the tremendous potential of alloy design targeted at laser additive manufacturing (LAM) and the effectiveness of the intrinsic heat treatment (IHT) to trigger phase transformations in-situ already during the LAM process