Additive manufacturing by laser-based powder bed fusion is a promising technique for bulk metallic glass production. But, reheating by deposition of subsequent layers may cause local crystallisation of the alloy. To investigate the crystalline phase evolution during laser scanning of a Zr-based metallic glass-forming alloy, a simulation strategy based on the finite element method and the classical nucleation theory has been developed and compared with experimental results from multiple laser remelting of a single-track. Multiple laser remelting of a single-track demonstrates the crystallisation behaviour by the influence of thermal history in the reheated material. Scanning electron microscopy and transmission electron microscopy reveals the crystalline phase evolution in the heat affected zone after each laser scan. A trend can be observed where repeated remelting results in an increased crystalline volume fraction with larger crystals in the heat affected zone, both in simulation and experiment. A gradient of cluster number density and mean radius can also be predicted by the model, with good correlation to the experiments. Prediction of crystallisation, as presented in this work, can be a useful tool to aid the development of process parameters during additive manufacturing for bulk metallic glass formation.

The development of process parameters and scanning strategies for bulk metallic glass formation during additive manufacturing is time-consuming and costly. It typically involves trials with varying settings and destructive testing to evaluate the final phase structure of the experimental samples. In this study, we present an alternative method by modelling to predict the influence of the process parameters on the crystalline phase evolution during laser-based powder bed fusion (PBF-LB). The methodology is demonstrated by performing simulations, varying the following parameters: laser power, hatch spacing and hatch length. The results are compared in terms of crystalline volume fraction, crystal number density and mean crystal radius after scanning five consecutive layers. The result from the simulation shows an identical trend for the predicted crystalline phase fraction compared to the experimental estimates. It is shown that a low laser power, large hatch spacing and long hatch lengths are beneficial for glass formation during PBF-LB. The absolute values show an offset though, over-predicted by the numerical model. The method can indicate favourable parameter settings and be a complementary tool in the development of scanning strategies and processing parameters for additive manufacturing of bulk metallic glass.

The work presented in this thesis aims to develop a modelling approach to predict crystalline phase evolution in bulk metallic glass during additive manufacturing with laser-based powder bed fusion (PBF-LB). Metallic glasses are non-crystalline metallic materials that generally possess exceptional properties because of its amorphous struc-ture. Manufacturing of metallic glass is possible by rapid cooling of a liquid metal alloy. The required cooling rates to avoid crystallisation generally limits traditional manufac-turing techniques to small/thin samples. The desirable properties of metallic glasses motivate manufacturing of larger samples. PBF-LB is one promising method by which bulk metallic glass potentially can be produced without size limitation. Cooling rates in this process are generally several orders of magnitude higher than critical cooling rates to bypass crystallisation in glass forming alloys. Crystalline structures may still evolve within the solid material because of thermal cycling during the manufacturing process. Numerical simulation can assist development of process for bulk metallic glass formation by predicting the phase evolution. Simulations can also help to increase the understand-ing of where and when crystalline structures develop with respect to process parameters and scanning strategy. Simulation of bulk metallic glass formation during PBF-LB is a challenge. The thermodynamic and kinetic properties of the material and the large variations in time and length scales in the process makes accurate simulations difficult. This thesis address these challenges by developing a modelling approach for simulation of the temperature history and crystalline phase evolution. The objective is to assist the development of process parameters for bulk metallic glass formation. The approach includes finite element modelling to compute the temperature history in the heat affected zone. The modelling includes approximations of the energy input and approaches to sim-ulate the large variations in time and length scales associated with PBF-LB. Computed temperature histories acts as input in calculations of the crystalline phase evolution in the metallic glass. The phase transformation modelling approach includes a modified isothermal model and classical nucleation and growth theory. The result is a coupled thermal and phase transformation model that can predict the trend in crystalline phase evolution in a bulk metallic glass with respect to the process parameters. The predictions show very good agreement to experimental estimates of the crystalline volume fraction. Comparison of simulations makes it possible to evaluate the process parameters in terms of crystalline size distribution. The model is a powerful tool that help the development and fine tuning of process parameters to produce bulk metallic glass.

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.

The crystallization rate during selective laser melting (SLM) of bulk metallic glasses (BMG) is a critical factor in maintaining the material's amorphous structure. To increase the understanding of the interplay between the SLM process and the crystallization behavior of BMGs, a numerical model based on the classical nucleation theory has been developed that accounts for the rapid temperature changes associated with SLM. The model is applied to SLM of a Zr-based BMG and it is shown that the transient effects, accounted for by the model, reduce the nucleation rate by up to 15 orders of magnitude below the steady-state nucleation rate on cooling, resulting in less nuclei during the build process. The capability of the proposed modelling approach is demonstrated by comparing the resulting crystalline volume fraction to experimental findings. The agreement between model predictions and the experimental results clearly suggests that transient nucleation effects must be accounted for when considering the crystallization rate during SLM processing of BMGs.

A finite element model for prediction of the temperature field in the powder bed fusion process is presented and compared to measurements. Accurate temperature predictions at the base plate are essential to accurately predict the formation of crystals in a metallic glass forming material. The temperature measurements were performed by equipping the base plate with thermocouples during manufacturing of a cylinder with the glass forming alloy AMZ4. Boundary conditions for heat losses through the base plate/machine contact interfaces was calibrated to fit the measurements. Additional heat losses was used to account for radiation at the top surface and conduction through the powder bed. An interface boundary condition based on conservation of heat flux was examined to match the heat flow to the machine structure and the temperature predictions was satisfying. Still, temperature predictions with a constant heat transfer coefficient matched the measurements within 1.5^oC during the entire building process of about 9 hours.

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. 

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.

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.

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.