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  • 1.
    Lindwall, Johan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Boundary conditions for simulation of powder bed fusion for metallic glass formation: measurements and calibration2019Conference paper (Refereed)
    Abstract [en]

    This work aims at simulation of the powder bed fusion process with an alloy that have the capability to become amorphous. The powder bed fusion process enables rapid solidification such that crystallization can be bypassed. This enables formation of bulk metallic glass. However, elevated temperatures and thermal cycling of solidified material may be destructive to the metallic glass by formation of nuclei. Thermal simulation of the process combined with a phase model can reveal if and where a crystalline structure may form during the printing process.

    The work includes temperature measurements on the base plate during printing for calibration of bound- ary conditions for heat losses for the PBF process, Figure 1. The measurements are performed with thermocouples located at three positions: at the center, half radius and at the edge of the circular base plate, five millimeters from the top surface. The temperature of the protective gas is also measured during the printing process. This measurement is performed with thermocouples located in the gas flow, both at the inlet and outlet.

    An axisymmetric two dimensional thermal model was used to simulate the printing of a 60 mm tall cylinder with a diameter of 10 mm and a layer thickness of 20 μm using the layer-by-layer approach [1]. Boundary conditions for heat losses was calibrated to match the temperature measurements on the base plate. A phase model was calibration based on DSC measurements on amorphous samples (so far at low temperatures and low heating rates) and the crystalline phase fraction is predicted with a JMAK expression.

  • 2.
    Lindwall, Johan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Modelling of Bulk Metallic Glass Formation in Powder Bed Fusion2018Conference paper (Other academic)
    Abstract [en]

    Additive manufacturing by the powder bed fusion process can provide cooling rates high enough to avoid crystallization, i.e. create bulk metallic glasses. The small melting pool connected to a relatively large volume of cooling material gives cooling rates many orders of magnitude larger than the critical cooling rate for the studied glass forming alloy AMZ4. However, subsequent reheating of built material may cause devitrification, i.e. crystallization of the amorphous phase. The present work aims to simulate the thermal cycles of the powder bed fusion process in order to evaluate and mitigate the risk of devitrification. This is done by combining finite elements simulations with a phase transformation model for the amorphous and crystal phases.

    The response of AMZ4, in the present case limited to heating of amorphous material from room temperature, was evaluated using DSC measurements with varying low heating rates. This limited set of information is used to construction the lower part of the crystallization diagram based on a JMAK-model.

    Previous work has developed simulation techniques for efficient simulations of glass formation in powder bed fusion. Temperatures can be computed with sufficient accuracy and considerable reduced computational time compared to a fully detailed model. The simplifications were based on temporal reduction by consolidating the heat source to strings or entire layers by assuming infinite scanning speed in one or two directions. The JMAK- model will now be used combined with these techniques. Further understanding of when and where crystals may be formed can be acquired by the presented work.

  • 3.
    Lindwall, Johan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Modelling of Bulk Metallic Glass formation in Powder Bed Fusion2019Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis discusses a model for simulation of the Powder Bed Fusion (PBF) process of metallic powder with the capability to become amorphous. The temperature field in the PBF process is predicted by a three-dimensional thermal finite element model in three dimensions using a layer-by-layer approach, meaning that the scanning strategy of the moving laser spot is consolidated into a single heat source acting on the entire layer momentarily. This temporal reduction enables simulations of large domains and many layers while it becomes less computational demanding compared to a detailed transient model that incorporates a scanning sequence. Predictions of the amorphous and crys- talline phase fractions are performed with a phase model coupled to the temperature field simulation. The phase model is based on differential scanning calorimetry measure- ments and optimized to fit continuous heating transformation into a crystalline state of an amorphous sample. The simulations are performed on the commercial available glass forming alloy AMZ4.

    Bulk Metallic Glass (BMG) have an amorphous structure and possesses desirable me- chanical, magnetic and corrosion properties such as high yield stress, low magnetic losses and high corrosion resistance. Glass forming alloy has the potential to become amorphous provided that the solidification rate is rapid enough to avoid crystallization. However, traditional manufacturing techniques, such as casting, limits the cooling rates and size of components due to limited heat conduction in the bulk. With Additive Manufacturing (AM) on the other hand, it is possible to produce BMG’s as the melt pool is very small and solidification can be achieved very rapid to bypass crystallization. Yet, crystals may form by devitrification (crystal formation upon heating of the amorphous phase) because of thermal cycling in previous layers. Simulation of the process will aid the understanding of glass formation during AM and the development of process parameters to control the level of devitrification. 

     

  • 4.
    Lindwall, Johan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lundbäck, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Thermal FE-simulation of PBF using adaptive meshing and time stepping2017In: Simulation for Additive Manufacturing 2017, Sinam 2017, Technische Universität München (TUM), ECCOMAS, , 2017, p. 62-63Conference paper (Refereed)
  • 5.
    Lindwall, Johan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Malmelöv, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lundbäck, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Efficiency and Accuracy in Thermal Simulation of Powder Bed Fusion of Bulk Metallic Glass2018In: JOM: The Member Journal of TMS, ISSN 1047-4838, E-ISSN 1543-1851, Vol. 70, no 8, p. 1598-1603Article in journal (Refereed)
    Abstract [en]

    Additive manufacturing by powder bed fusion processes can be utilized to create bulk metallic glass as the process yields considerably high cooling rates. However, there is a risk that reheated material set in layers may become devitrified, i.e., crystallize. Therefore, it is advantageous to simulate the process to fully comprehend it and design it to avoid the aforementioned risk. However, a detailed simulation is computationally demanding. It is necessary to increase the computational speed while maintaining accuracy of the computed temperature field in critical regions. The current study evaluates a few approaches based on temporal reduction to achieve this. It is found that the evaluated approaches save a lot of time and accurately predict the temperature history.

  • 6.
    Lindwall, Johan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Pacheco, Victor
    Ångström Laboratory, Uppsala University, Uppsala.
    Sahlberg, Martin
    Ångström Laboratory, Uppsala University, Uppsala.
    Lundbäck, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Thermal simulation and phase modeling of bulk metallic glass in the powder bed fusion process2019In: Additive Manufacturing, ISSN 2214-8604, Vol. 27, p. 345-352Article in journal (Refereed)
    Abstract [en]

    One of the major challenges with the powder bed fusion process (PBF) and formation of bulk metallic glass (BMG) is the development of process parameters for a stable process and a defect-free component. The focus of this study is to predict formation of a crystalline phase in the glass forming alloy AMZ4 during PBF. The approach combines a thermal finite element model for prediction of the temperature field and a phase model for prediction of crystallization and devitrification. The challenge to simulate the complexity of the heat source has been addressed by utilizing temporal reduction in a layer-by-layer fashion by a simplified heat source model. The heat source model considers the laser power, penetration depth and hatch spacing and is represented by a volumetric heat density equation in one dimension. The phase model is developed and calibrated to DSC measurements at varying heating rates. It can predict the formation of crystalline phase during the non-isothermal process. Results indicate that a critical location for devitrification is located a few layers beneath the top surface. The peak is four layers down where the crystalline volume fraction reaches 4.8% when 50 layers are built.

  • 7.
    Marattukalam, Jithin James
    et al.
    Department of Physics, Materials Physics, Uppsala University.
    Pacheco, Victor
    Department of Chemistry- Ångström Laboratory, Uppsala University.
    Karlsson, Dennis
    Department of Chemistry- Ångström Laboratory, Uppsala University.
    Riekehr, Lars
    Department of Chemistry- Ångström Laboratory, Uppsala University.
    Lindwall, Johan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Forsberg, Fredrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Jansson, Ulf
    Department of Chemistry- Ångström Laboratory, Uppsala University.
    Sahlberg, Martin
    Department of Chemistry- Ångström Laboratory, Uppsala University.
    Hjörvarsson, Björgvin
    Department of Physics, Materials Physics, Uppsala University.
    Development of process parameters for selective laser melting of a Zr-based bulk metallic glass2020In: Additive Manufacturing, ISSN 2214-8604, Vol. 33, article id 101124Article in journal (Refereed)
    Abstract [en]

    Parameters for selective laser melting of Zr59.3Cu28.8Al10.4Nb1.5 (trade name AMZ4), allowing crack-free bulk metallic glass with low porosity, have been developed. The phase formation was found to be strongly influenced by the heating power of the laser. X-ray amorphous samples were obtained with laser power at and below 75 W. The as-processed bulk metallic glass was found to devitrify by a two-stage crystallization process within which the presence of oxygen was concluded to play an essential role. At laser powers above 75 W, the observed crystallites were found to be a cubic phase (Cu2Zr4O). The hardness and Young’s modulus in the as-processed samples was found to increase marginally with increased fraction of the crystalline phase.

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