TY - BOOK

T1 - Cracking Mechanisms of High-Alloyed Tool Steels processed with Laser Powder Bed Fusion

AU - Platl, Jan

N1 - embargoed until 31-12-2026

PY - 2021

Y1 - 2021

N2 - High-alloyed tool steels are used for several tooling applications such as hot-forming, cutting or stamping. Due to their high alloying element contents, they represent a material group with extraordinarily high hardness respectively wear resistance that withstands extreme loads during these applications. However, the high carbon equivalent numbers that come along with these high alloying element contents, make them prone to cracking during microwelding, i.e., during processing with laser powder bed fusion (LPBF). Based on the almost unconstrained design freedom, additive manufacturing facilitates economic advantages in comparison to conventional manufacturing processes, i.e., casting, rolling, forging or subtractive post-processing. This doctoral thesis aims to shed light on the processability of tool steels with LPBF without platform preheating. The ultimate goal is to correlate the occurrence of cracks with the microstructural evolution. Hence, two alloy types, firstly a carbon-containing cold-work tool steel and secondly a carbon-free FeCoMo alloy, which is a potential candidate for LPBF due to the lack of carbon, were manufactured and subsequently investigated with regard to the predominant cracking mechanism and potential cracking causes in the course of this work. Due to the fine morphology (microstructure and crack surfaces), besides conventional metallographic methods, i.e., light optical and scanning electron microscopy (SEM), high-resolution techniques were carried out. Stress and phase evolution resulting from the complex thermal cycle during LPBF has been determined usng high-energy X-ray diffraction (HEXRD) with synchrotron radiation. Besides high-resolution energy dispersive X-ray spectroscopy, atom probe tomography (APT) and transmission electron microscopy (TEM) have been utilized to unambiguously determine the crack surface chemistry and microstructural constituents that contribute to crack formation. This thesis also aimed to illuminate material properties that arise due to rapid cooling during LPBF, i.e., high supersaturation and an extremely fine microstructure. Results show that besides conventional characterization techniques, high-resolution methods such as HEXRD, APT and TEM are mandatory to unambiguously determine the presence of microstructural constituents that contribute to or are responsible for failure of these high-alloyed tool steels during LPBF. In addition, SEM parameter adaptions (mainly a minimization of the electron beam’s interaction volume) are necessary to validly analyze the microstructure and crack surfaces. Synchrotron strain measurements yielded the presence of stress accumulations that lead to cracking of an eutectic carbide network, which formed during solidification and has been unambiguously determined with APT and TEM in the carbon-containing tool steel. A potential adaption of the alloying system to prevent the formation of eutectic solidification carbides has been proposed based on thermodynamic calculations. With regard to the carbon-free FeCoMo alloy, four potential brittle failure causes, i.e., the formation of brittle ordered domains, epitaxially oriented coarse grains, precipitation reactions during LPBF and the presence of non-metallic inclusions in the microstructure and on the crack surface, were found. As the former three were excluded or only merely contribute to cracking, the fourth cracking cause has been assigned as most probable failure reason for this promising alloy.

AB - High-alloyed tool steels are used for several tooling applications such as hot-forming, cutting or stamping. Due to their high alloying element contents, they represent a material group with extraordinarily high hardness respectively wear resistance that withstands extreme loads during these applications. However, the high carbon equivalent numbers that come along with these high alloying element contents, make them prone to cracking during microwelding, i.e., during processing with laser powder bed fusion (LPBF). Based on the almost unconstrained design freedom, additive manufacturing facilitates economic advantages in comparison to conventional manufacturing processes, i.e., casting, rolling, forging or subtractive post-processing. This doctoral thesis aims to shed light on the processability of tool steels with LPBF without platform preheating. The ultimate goal is to correlate the occurrence of cracks with the microstructural evolution. Hence, two alloy types, firstly a carbon-containing cold-work tool steel and secondly a carbon-free FeCoMo alloy, which is a potential candidate for LPBF due to the lack of carbon, were manufactured and subsequently investigated with regard to the predominant cracking mechanism and potential cracking causes in the course of this work. Due to the fine morphology (microstructure and crack surfaces), besides conventional metallographic methods, i.e., light optical and scanning electron microscopy (SEM), high-resolution techniques were carried out. Stress and phase evolution resulting from the complex thermal cycle during LPBF has been determined usng high-energy X-ray diffraction (HEXRD) with synchrotron radiation. Besides high-resolution energy dispersive X-ray spectroscopy, atom probe tomography (APT) and transmission electron microscopy (TEM) have been utilized to unambiguously determine the crack surface chemistry and microstructural constituents that contribute to crack formation. This thesis also aimed to illuminate material properties that arise due to rapid cooling during LPBF, i.e., high supersaturation and an extremely fine microstructure. Results show that besides conventional characterization techniques, high-resolution methods such as HEXRD, APT and TEM are mandatory to unambiguously determine the presence of microstructural constituents that contribute to or are responsible for failure of these high-alloyed tool steels during LPBF. In addition, SEM parameter adaptions (mainly a minimization of the electron beam’s interaction volume) are necessary to validly analyze the microstructure and crack surfaces. Synchrotron strain measurements yielded the presence of stress accumulations that lead to cracking of an eutectic carbide network, which formed during solidification and has been unambiguously determined with APT and TEM in the carbon-containing tool steel. A potential adaption of the alloying system to prevent the formation of eutectic solidification carbides has been proposed based on thermodynamic calculations. With regard to the carbon-free FeCoMo alloy, four potential brittle failure causes, i.e., the formation of brittle ordered domains, epitaxially oriented coarse grains, precipitation reactions during LPBF and the presence of non-metallic inclusions in the microstructure and on the crack surface, were found. As the former three were excluded or only merely contribute to cracking, the fourth cracking cause has been assigned as most probable failure reason for this promising alloy.

KW - Selektives Laserschmelzen

KW - Werkzeugstähle

KW - Rissmechanismen

KW - Mikrostrukturelle Charakterisierung

KW - Versagensanalyse

KW - Hochenergetische Synchrotron-Röntgenstrahlung

KW - Atomsondentomographie

KW - Transmissionselektronenmikroskopie

KW - Laser powder bed fusion

KW - Tool steels

KW - Cracking mechanisms

KW - Microstructure characterization

KW - Failure analysis

KW - High-energy X-ray diffraction

KW - Atom probe tomography

KW - Transmission electron microscopy

M3 - Doctoral Thesis

ER -