Indira Dey


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Dey

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Indira

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0000-0003-1259-1218

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2022 - 20259
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Publications 1 - 9 of 9
  • Mechanical properties and microstructure analysis of laser welded hybrid parts made of additively and conventionally manufactured 1.4313 soft martensitic steel
    Item type: Journal Article
    Dey, Indira; Floeder, Raphael; Kunze, Karsten; et al. (2025)
    Materials & Design
    This study investigates laser welding of additively (AM) and conventionally manufactured (CM) parts, aiming to enhance cost and energy efficiency for a diverse product range. In this context, hybrid specimens combining AM/CM subparts were produced, where AM subparts were created using DED, CM parts by hot forming, and the two were joined using laser welding. The material analysed is soft martensitic stainless steel. Mechanical characterisation was performed through tensile testing and hardness measurements and microstructure characterisation through EBSD, SEM, EDS, and light microscopy. The study reveals the presence of ultra-fine grains in the heat treated laser weld segments which suggests grain subdivision due to martensite deformation. As built hybrid specimens exhibited lower toughness due to the laser welds and lower strength due to the CM segments. The weakest point after the heat treatment was the HAZ of the CM segment. The best mechanical performance was observed in homogeneously heat-treated AM specimens. Moreover, the variability in grain size were examined but did not conform grain boundary strengthening, particularly after the heat treatment. This study highlights the critical influence of microstructural variations on the mechanical properties of hybrid welds, emphasizing the need for further investigation into strengthening mechanisms and individual heat treatments.
  • Mechanical Properties and Microstructure Analysis of Additive/ Conventional Manufactured 13cr-4ni Soft Martensitic Steel
    Item type: Working Paper
    Dey, Indira; Floeder, Raphael; Kunze, Karsten; et al. (2023)
    SSRN
    Additive manufacturing (AM) opens new possibilities to manufacture metallic, complex, and customized parts. However, AM also faces geometrical and economical limitations especially in material quality and residual stresses. Joining AM parts with conventionally manufactured (CM) parts will be necessary in order to produce cost-efficiently the vast variety of products and might overcome those limitations. A few studies investigated part segmentation between AM/CM parts but none of them used martensitic steel with its complex microstructure. The analysed material is a soft martensitic stainless steel due to its low carbon content of 0.03 % C and Ni content of 4%. Its metallurgy is characterised by the prior austenite grains, packets, blocks, and laths. In this study, the grain size distribution of martensitic steel is correlated with the mechanical properties of different segments based on the Hall-Petch relationship. The AM segments are produced by direct energy deposition (DED), the CM segment by hot forming (HF), and the joint by laser welding (LW). The mechanical properties and microstructure are examined by Vickers micro hardness measurements, optical microscopy, Electron Backscatter Diffraction (EBSD), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX). Hybrid specimens fail in the HF segment and show a limited elongation due to the laser weld. The best results in terms of ultimate tensile strength and elongation are achieved by the heat treated homogeneous DED specimens. Ultra fine grains are present in the LW segment after heat treatment which implies recrystallisation. Si- and Mn-oxides are present in the LW and DED segments. The martensitic grain size is defined by the length and width of the blocks separated by high-angle-boundaries. The correlation between grain size and hardness does not follow the Hall-Petch relationship. This means that other hardening mechanisms than grain boundary strengthening are dominant in soft martensitic steels.
  • Parameters Development for Optimum Deposition Rate in Laser DMD of Stainless Steel EN X3CrNiMo13-4
    Item type: Journal Article
    Dalaee, Mohammad; Cerrutti, Eduardo; Dey, Indira; et al. (2022)
    Lasers in Manufacturing and Materials Processing
    Laser Direct Metal Deposition (DMD) has been developed as a manufacturing process to deposit coatings on existing materials and proves advantageous in Additive Manufacturing (AM) of complex and precise components. However, it is necessary to carefully determine the proper process parameter combinations to make this method economically viable for industries. The intent of this study is to address enhancement in productivity of laser DMD of stainless steel EN X3CrNiMo13-4. Accordingly, the effects of the main laser process parameters of laser power P, scan speed v, powder flow rate m˙, and spot diameter s on track geometries and build-up rate are discussed. The regression analysis is conducted to derive correlations between the combined set of main parameters and deposition rate. The results show a good linear regression correlation of R2 >0.9 for the geometrical characteristic of aspect ratio, dilution, and deposition rate. The constructed processing map, using linear regression equations, presents proper process parameters selection in connection with deposition rate, aspect ratio, and dilution rate.
  • Model-Based Heat Input Control Validated on Martensitic Steel 1.4313
    Item type: Journal Article
    Dey, Indira; Egorov, Sergei; Soffel, Fabian; et al. (2023)
    Key Engineering Materials
    The ability of direct metal deposition (DMD) to fabricate complex geometries is still limited. Especially in thin-walled structures heat accumulation can lead to intolerable geometric deviation and which has to be avoided. Combining thin walls and massive sections in one layer requires parameter adapting for each section within a layer. An existing semi-empirical model predicts the optimal process parameters for the austenitic steel 1.4404. This study demonstrates the validity of the model for martensitic steel 1.4313 by an experimental campaign. The demonstrators are characterized by a massive inner part attached to a thin-walled rib. They were fabricated by DMD using constant and adapted heat input and were qualified by visual inspection, geometrical accuracy, Vickers hardness, and microstructure analysis. The demonstrators built with the adapted laser power showed enhanced geometrical accuracy which is essential for post-processing. The hardness along the symmetry plane was significantly increased, especially in the thin wall section. The study confirms the applicability of the model for martensitic steel in terms of geometrical accuracy but identifies perspectives to integrate microstructural aspects into the model.
  • Manufacturing a prototype with laser direct metal deposition and laser welding made from martensitic steel 1.4313
    Item type: Journal Article
    Dey, Indira; Fabbri, Maicol; Gemmet, Simon; et al. (2023)
    The International Journal of Advanced Manufacturing Technology
    Burckhardt Compression Holding AG, based in Winterthur, is an internationally active manufacturer of reciprocating com- pressors who uses three-piece pistons in its Laby® reciprocating compressors. Due to their design for casting, the pistons have a high weight, which limits the size of the piston, particularly for the large diameters. For this reason, solutions are being looked for to produce pistons in lightweight design using metal additive manufacturing processes to counteract these challenges. One of the innovative techniques for weight reduction that has been applied in various fields of science and industry is laser direct metal deposition (DMD). Therefore, a project was started with Burckhardt Compression to reduce the mass enabling higher operating speeds. This study presents a workflow to manufacture a lightweight piston from martensitic steel 1.4313 by direct metal deposition (DMD) with a diameter of approximately 342 mm and a height of 140 mm. The piston is characterized by different segments, which are conventionally and additively manufactured to overcome machine limitations. The piston crown was joined to the additive manufactured part and sealed by CO2 laser welding. Reducing the laser power in DMD reduced the temperature, and hence, oxidation of manganese and silicium and reducing the carrier gas flow improved the buildup rate and reduced the turbulence induced oxidation. Alternating the feed direction per layer improved the geometrical accuracy and avoided material accumulation at sharp corners. A method was found to indicate quantitatively the geometrical accuracy of a radius in buildup direction. The welding types and seams for laser welding were selected to enable a good force flow; however, a clamping device was necessary. A double weld strategy was considered in order to reduce a notch effect at the hidden T-joints. The design enabled a 40% weight reduction resulting in a weight of 24 kg compared to the cast piston with a weight of 40 kg. Metallographic analysis and 3D scans were performed in order to evaluate the material quality and geometrical accuracy. The study shows the limitations and challenges of DMD and how to overcome machine limitations by part segmentation.
  • CA single track simulation of laser conduction welding with stainless steel 316L (1.4404)
    Item type: Conference Paper
    Dey, Indira; Schätti, Nic; Gerstgrasser, Marcel; et al. (2022)
    Procedia CIRP
    The study presents a coupled cellular automata (CA) approach for a single track microstructure simulation used for laser conduction welding. A high-power CO2 laser beam (1000W) traverses the substrate, with the beam shaped by conventional optics, which produces a Gaussian profile. The process relies on a shallow melt phase to maintain a conduction limited weld. Laser conduction welding does not require a filler material. The stainless steel material 1.4404 was used as substrate material with an initial grain size of 10µm and 20µm. The melt pool geometry, temperature history, cooling rates and diffusivities define the grain morphology. Temperature-dependent diffusivity coefficients and atomic spacing parameters are suggested. The simulation outputs of the grain morphology are qualitatively and quantitatively compared to experimental results. Initial results have shown that due to the individual melt pool conditions complex microstructures are developed. These fine, complicated microstructures cannot be satisfactorily resolved and quantified using standard optical microscopy. Electron backscatter diffraction (EBSD) has to be used for validation.
  • Comprehensive Distortion Analysis of a Laser Direct Metal Deposition (DMD)-Manufactured Large Prototype Made of Soft Martensitic Steel 1.4313
    Item type: Journal Article
    Dey, Indira; Floeder, Raphael; Solcà, Rick; et al. (2024)
    Journal of Manufacturing and Materials Processing
    Additive manufacturing (AM) by using direct metal deposition (DMD) often causes erratic distortion patterns, especially on large parts. This study presents a systematic distortion analysis by employing numerical approaches using transient–thermal and structural simulations, experimental approaches using tomography, X-ray diffraction (XRD), and an analytical approach calculating the buckling distortion of a piston. The most essential geometrical features are thin walls situated between massive rings. An eigenvalue buckling analysis, a DMD process, and heat treatment simulation are presented. The eigenvalue buckling simulation shows that it is highly dependent on the mesh size. The computational effort of the DMD and heat treatment simulation was reduced through simplifications. Moreover, artificial imperfections were imposed in the heat treatment simulation, which moved the part into the buckling state inspired by the experiment. Although the numerical results of both simulations are successful, the eigenvalue and DMD simulation cannot be validated through tomography and XRD. This is because tomography is unable to measure small elastic strain fields, the simulated residual stresses were overestimated, and the part removal disturbed the residual stress equilibrium. Nevertheless, the heat treatment simulation can predict the distortion pattern caused by an inhomogeneous temperature field during ambient cooling in an oven. The massive piston skirt cools down and shrinks faster than the massive core. The reduced yield strength at elevated temperatures and critical buckling load leads to plastic deformation of the thin walls.
  • Characterisation of laser welded DMD parts made of soft martensitic steel 1.4313
    Item type: Doctoral Thesis
    Dey, Indira (2025)
    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.
  • Thermal Characterisation of Hybrid Laser Welds Made of Conventionally and Additively Soft Martensitic Steel 1.4313
    Item type: Journal Article
    Dey, Indira; Mayer, Thomas; Egli, Bianca; et al. (2025)
    Metals
    Part segmentation can be used to overcome limitations of additive manufacturing (AM) processes such as Direct Energy Deposition of Metals (DED). In this case subparts of soft martensitic steel 1.4313 produced by conventional manufacturing (CM) and AM are joined by laser welding. This paper reports the difference in thermal conductivity of conventional and additive manufactured parts. The thermal conductivity was calculated from the thermal diffusivity, the specific heat, and the bulk density. Furthermore, the temperature was measured during welding and the microstructure analyzed. The far field temperature was measured using eight K-type thermocouples and the microstructure was analyzed by metallography and light microscopy. The results showed that the thermal conductivity of AM material is 8% lower and therefore the heating rate 5% lower compared to CM material. The lower thermal conductivity is explained in the literature by its higher dislocation density, unfavorable alloying element distribution and a lower rest austenite content. AM introduces structural complexity that hampers electron and phonon transport, thereby reducing the thermal conductivity despite similar base chemical compositions. The heat-affected zone is only clearly visible on the CM side due to carbide formation. In DED parts, it comes to different phases in non-equilibrium states, which complicates the identification of carbides and the HAZ. The findings are important for the design of hybrid components to improve the the joint integrity and functionality of hybrid parts.
Publications 1 - 9 of 9