Processing of high-performance materials by laser beam powder bed fusion (LB-PBF) provides an alternative manufacturing route to, i.e., investment casting and is suitable for production of high-performance materials having complex geometry such as turbine blades. The main benefit of powder bed fusion in general is associated with the fact that increased geometrical complexity does not add any cost. However, the processability of the alloys of interest is closely linked to process parameters where highperformance materials belong to a special class of materials that need substantial attention to avoid problems, not at least with regard to different types of cracking. In this chapter, the relationship between process parameter-microstructure-defect relationship will be discussed and analyzed.

Microstructures of material conditions of nickel-based superalloy Alloy 247LC fabricated using laser powder bed fusion (L-PBF) were investigated. Experiments designed in a prior study revealed the L-PBF process parameters for which the material conditions displayed a reduced susceptibility to cracking. Certain process parameters produced material conditions with an increased susceptibility to cracking. In this study, the material conditions were investigated in detail to reveal their microstructure and to determine the cause of cracking. The reason for the transition between a reduced to an increased susceptibility to cracking was examined. The results revealed solidification cracking occurred at high-angle grain boundaries. Solidification cracking may have been promoted at high-angle grain boundaries because of the undercooling contribution of the grain boundary energy. Furthermore, Si segregation was observed in the cracks. Thus, the presence of Si most likely promoted solidification cracking. It was observed that a high crack density, which occurred in the high energy density material condition, was associated with a large average grain size. The fact that certain combination of process parameters produced microstructures with a low susceptibility to cracking, indicates that reliable Alloy 247LC material may be printed using L-PBF by employing improved process parameters. © 2022

In this study, Alloy 247LC samples were built with different laser powder bed fusion (L-PBF) process parameters. The samples were then subjected to solution heat treatment at 1260 °C for 2 h. The grain size of all the samples increased significantly after the heat treatment. The relationship between the process parameters and grain size of the samples was investigated by performing a design of experiment analysis. The results indicated that the laser power was the most significant process parameter that influenced the grain height and aspect ratio. The laser power also significantly influenced the grain width. The as-built and as-built + heat-treated samples with high, medium, and low energy densities were characterized using a field emission gun scanning electron microscope equipped with an electron backscatter diffraction detector. The micrographs revealed that the cells present in the as-built samples disappeared after the heat treatment. Isolated cases of twinning were observed in the grains of the as-built + heat-treated samples. The disappearance of cells, increase in the grain size, and appearance of twins suggested that recrystallization occurred in the alloy after the heat treatment. The occurrence of recrystallization was confirmed by analyzing the grain orientation spread of the alloy, which was lower and more predominantly <1° in the as-built + heat-treated conditions than in the as-built conditions. The microhardness of the as-built + heat-treated samples were high which was plausible because γ’ precipitates were observed in the samples. However, the L-PBF process parameters had a very low correlation with the microhardness of the as-built + heat-treated samples.

This thesis is about laser powder bed fusion (L-PBF) of the nickel-basedsuperalloy Alloy 247LC. Alloy 247LC is mainly used in gas turbine blades and processing the blades with L-PBF may confer performance advantage over the blades manufactured with conventional methods. This is mainly because L-PBFis more suitable, than conventional methods, for manufacturing the complex cooling holes in the blades. The research was motivated by the need for academia and industry to gain knowledge about the processability of the alloy using L-PBF. The knowledge is essential to eventually solve the problem of cracking encountered when processing the alloy. In addition, dense parts with low void content should be processed and the microstructure and properties should meett he required performance. Heat-treatment is usually performed to acquire final properties, so it is also of interest to study this aspect. Thus, the thesis answered some of the important questions related to process parameter-microstructure- property relationships.

Populärvetenskaplig sammanfattning

Denna avhandling handlar om laserpulverbäddsmältning (L-PBF) av legeringen247LC. Legering 247LC används i gasturbinblad och tillverkningen av bladen medL-PBF ger fördelar i förhållande till bladen tillverkade med konventionella metoder. Detta beror huvudsakligen på att L-PBF är mer lämpad än konventionella metoder för att tillverka de komplexa geometrier som krävs förbladen. Forskningen var motiverad utifrån behovet hos akademi och industri att få kunskap om legeringens processbarhet gällande L-PBF. Kunskapen är nödvändig för att kunna lösa problemet med sprickbildning, vilket är ett stort problem vid tillverkningen av legeringen. Avhandlingen besvarade några av de viktiga frågorna relaterade till förhållandet mellan processparametrar och mikrostruktur.

The manufacturing of parts from nickel-based superalloy Alloy 247LC by laser powder bed fusion (L-PBF) is challenging, primarily owing to the alloy’s susceptibility to cracks. Apart from the cracks, voids created during the L-PBF process should also be minimized to produce dense parts. In this study, samples of Alloy 247LC were manufactured by L-PBF, several of which could be produced with voids and crack density close to zero. A statistical design of experiments was used to evaluate the influence of the process parameters, namely laser power, scanning speed, and hatch distance (inherent to the volumetric energy density) on void formation, crack density, and microhardness of the samples. The window of process parameters, in which minimum voids and/or cracks were present, was predicted. It was shown that the void content increased steeply at a volumetric energy density threshold below 81 J/mm3. The crack density, on the other hand, increased steeply at a volumetric energy density threshold above 163 J/mm3. The microhardness displayed a relatively low value in three samples which displayed the lowest volumetric energy density and highest void content. It was also observed that two samples, which displayed the highest volumetric energy density and crack density, demonstrated a relatively high microhardness; which could be a vital evidence in future investigations to determine the fundamental mechanism of cracking. The laser power was concluded to be the strongest and statistically most significant process parameter that influenced void formation and microhardness. The interaction of laser power and hatch distance was the strongest and most significant factor that influenced the crack density. © 2020 by the authors.

This thesis is about laser powder bed fusion (L-PBF) of the nickel-based superalloy: Alloy 247LC. Alloy 247LC is used mainly in gas turbine blades and processing the blades with L-PBF confers performance advantage over the blades manufactured with conventional methods. This is mainly because L-PBF is more suitable, than conventional methods, for manufacturing the complex cooling holes in the blades. The research was motivated by the need for academia and industry to gain knowledge about the processability of the alloy using L-PBF. The knowledge is essential in order to eventually solve the problem of cracking which is a major problem when manufacturing the alloy. In addition, dense parts with low void content should be manufactured and the parts should meet the required performance. Thus, the thesis answered some of the important questions related to process parameter-microstructure-defect relationships.

The thesis presented an introduction in chapter 1. A literature review was made in chapter 2 to 4. In chapter 2, the topic of additive manufacturing was introduced followed by an overview of laser powder bed fusion. Chapter 3 focused on superalloys. Here, a review was made from the broader perspective of superalloys but was eventually narrowed down to the characteristics of nickelbased superalloys and finally Alloy 247LC. Chapter 4 reviewed the main research on L-PBF of Alloy 247LC. The methodology applied in the thesis was discussed in chapter 5. The thesis applied statistical design of experiments to show the influence of process parameters on the defects and microstructure, so a detail description of the method was warranted. This was given at the beginning of chapter 5 and followed by the description of the L-PBF manufacturing and the characterization methods. The main results and discussions, in chapter 6, included a preliminary investigation on how the process parameters influenced the amount of discontinuity in single track samples. This was followed by the results and discussions on the investigation of voids, cracks and microhardness in cube samples (detail presentation was given in the attached paper B). Finally, the thesis presented results of the microstructure obtainable in L-PBF manufactured Alloy 247LC. The initial results of the microstructure investigation were presented in paper A.

This paper reviews state of the art laser powder bed fusion (L-PBF) manufacturing of γ′ nickel-based superalloys. L-PBF resembles welding; therefore, weld-cracking mechanisms, such as solidification, liquation, strain age, and ductility-dip cracking, may occur during L-PBF manufacturing. Spherical pores and lack-of-fusion voids are other defects that may occur in γ′-strengthened nickel-based superalloys manufactured with L-PBF. There is a correlation between defect formation and the process parameters used in the L-PBF process. Prerequisites for solidification cracking include nonequilibrium solidification due to segregating elements, the presence of liquid film between cells, a wide critical temperature range, and the presence of thermal or residual stress. These prerequisites are present in L-PBF processes. The phases found in L-PBF-manufactured γ′-strengthened superalloys closely resemble those of the equivalent cast materials, where γ, γ′, and γ/γ′ eutectic and carbides are typically present in the microstructure. Additionally, the sizes of the γ′ particles are small in as-built L-PBF materials because of the high cooling rate. Furthermore, the creep performance of L-PBF-manufactured materials is inferior to that of cast material because of the presence of defects and the small grain size in the L-PBF materials; however, some vertically built L-PBF materials have demonstrated creep properties that are close to those of cast materials.© 2020 by the authors. Licensee MDPI, Basel, Switzerland.

Alloy 247LC is sensitive to cracking during laser beam powder bed fusion (PBF-LB) manufacturing. Post processing is thus required to close cracks and achieve desired properties. In this study, samples of Alloy 247LC were manufactured by PBF-LB and subsequently post processed by hot isostatic pressing (HIP), HIP + solution and ageing heat treatments. The microstructure was characterized. Results showed cracks in the as-built condition. Cracks were not detected after HIP. Bright microconstituents were observed in the region between the cells, mainly, because of the partitioning of Hf and Ta into the intercellular region, where they presumably form carbides. What is assumed to be oxides were prominent in the microstructure. Thermodynamic calculations showed rapid formation of ?’ precipitates in the alloy, due to the high total concentration of Al and Ta and this was linked to the high hardness values in the as-built condition. © 2019 MS&T19®