Improving Build Rate of Low Alloy Steels Produced by Laser Powder Bed Fusion: Influence of layer thickness on processability
Typ
Examensarbete för masterexamen
Program
Publicerad
2020
Författare
Gunnerek, Rasmus
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
One of the largest issues with laser powder bed fusion (L-PBF) is the limited number of alloys that have been qualified for the process. This is especially true for low alloyed steels, as the carbon content can adversely affect processability by increasing number of defects as cracking and/or porosity. Previous work in the Powder Metallurgy and Additive Manufacturing group at Chalmers in the frame of CAM2 established a set of processing windows for two low alloy steels (4130 & 4140) that could produce of defect-free, high density components (>99.8%). The current work focused on increasing the build rate of these alloys by varying the layer thickness. Specimens were produced at layer thicknesses of 40 μm and 60 μm, across surface energy densities (SED) of 2.4 to 3.8 J/mm2. From this investigation, a 40 μm layer thickness yielded a wider processing window in comparison to a 60 μm layer thickness. Examinations of the microstructure revealed a greater number of lack-of-fusion defects at a larger layer thickness, which were the cause of this narrower process window. The melt pool depth at a 60 μm layer thickness was 100-240 μm, indicating that this lack of fusion porosity stemmed from instabilities within the powder bed. It was also found that an increased layer thickness yielded a lower hardness. This was connected to a more pronounced in-situ tempering that occurred at a 60 μm layer thickness. As a result, processing windows for 4130 and 4140 alloys, at layer thicknesses of 40 and 60 μm, were established, allowing to produce defect free, high density (>99.8%) components. Increasing layer thickness allowed to improve the build rate of these materials by up to ~165% in comparison to the previously developed parameters.