Characterisation of laser welded DMD parts made of soft martensitic steel 1.4313
OPEN ACCESS
Loading...
Author / Producer
Date
2025
Publication Type
Doctoral Thesis
ETH Bibliography
yes
Citations
Altmetric
OPEN ACCESS
Data
Rights / License
Abstract
This thesis explores the combination of Additive Manufacturing (AM) and Conventional Manufacturing (CM) to produce large and complex hybrid parts made of soft martensitic steel. The overall goal is to reduce weight by part segmentation. Furthermore, it addresses the challenge in manufacturing of closed and hollow structures using Direct Metal Deposition (DMD) by applying part segmentation. A design guideline, which applies part segmentation and assists in deciding whether a subpart should be manufactured by AM or CM, was developed. AM refers to DMD and CM to hot forging (HF) or casting within this thesis. A practical application of these findings is demonstrated through a piston for a horizontal gas compressor characterised by a large, hollow, closed structure with interior strengthening ribs.
Weight reduction is accompanied by small wall thicknesses which can be manufactured by DMD. However, they bear the risk of thermal distortion, which is the major obstacle during manufacturing, especially after the heat treatment. The thermal strain during the heat treatment was higher than the critical buckling strain and the yield strain and the residual stresses were higher than the yield strength, which explains the plastic deformation during the heat treatment. This was enhanced by an inhomogeneous temperature distribution of a very large part in which the heat requires a long time to diffuse from the interior to the exterior. The lower thermal diffusivity in DMD parts compared to HF parts strengthens the effect of residual stresses and inhomogeneous temperature distribution which makes DMD parts more sensitive to buckling. The design optimisation proposed design recommendations including compliant mechanisms and reinforcement. To achieve weight reduction without major distortion, it is essential to reinforce the functional relevant elements while minimising the weight in less critical sections.
Furthermore, the microstructure and mechanical properties of hybrid specimens were analysed which mainly define the functionality apart from distortion. The hybrid specimens have a heterogeneous microstructure with variations in toughness and hardness. The highest ultimate tensile strength (UTS) and rupture strain is observed in the heat treated DMD specimens. All laser-welded samples show a reduced toughness due to the stresses induced by laser welding. Additionally, HF-DMD specimens suffer lower strength, since they break in the weaker HF segments. The as built DMD and laser weld segment show grain refinement, which can be explained by the rapid cooling of the melt pool. However, the largest grain refinements are present after heat treatment in the laser weld segment, since a high dislocation density provokes grain refinement. The heat treated DMD segment has a reduced dislocation density due to the cyclic reheating which yields in a heterogeneous dual-phase microstructure. The HAZ of the hybrid laser weld in the HF segment is clearly visible and is characterised by grain refinement and carbides. It marks the weakest point in the heat treated condition which should be considered during the design of a part.
The hardness is lowered after heat treatment despite the grain refinement, which means that the Hall-Petch relationship cannot be applied on the block size. Due to the complex microstructure of soft martensitic steel, consisting of substructures with blocks, packets and laths, the definition of the grain size is controversially discussed in literature. The reduction in hardness can be explained by the formation of stable finely dispersed austenite during the heat treatment that lower the martensite fraction. Furthermore, in martensitic steels, other strengthening mechanisms, alongside grain boundary hardening, play a significant role, such as solid solution hardening and dislocation hardening. Further, the precipitation of Si-Mnoxides was investigated, which plays a minor role for hardening, but an essential role for surface oxidation.
In conclusion, a design guideline, manufacturing process chain, thermal properties, distortion simulation, and mechanical characterisation of hybrid parts are presented showcasing the potential of hybrid parts for weight reduction of closed and hollow structures. Future research should consider improving simulation accuracy and quantifying strengthening mechanisms of soft martensitic steel.
Permanent link
Publication status
published
External links
Editor
Contributors
Examiner : Wegener, Konrad
Examiner : Schleifenbaum, Johannes Henrich
Examiner : Meboldt, Mirko
Book title
Journal / series
Volume
Pages / Article No.
Publisher
ETH Zurich
Event
Edition / version
Methods
Software
Geographic location
Date collected
Date created
Subject
Organisational unit
03943 - Meboldt, Mirko / Meboldt, Mirko
03641 - Wegener, Konrad (emeritus) / Wegener, Konrad (emeritus)