Abstract
In the industrial panorama, laser powder bed fusion (LPBF) systems enable the near net shaping of metal powders into complex geometries with unique design features. This makes the technology appealing for many industrial applications, which require high performance materials combined with lightweight design, lattice structures and organic forms. However, many of the alloys that would be ideal for the realisation of these functional components are classified as difficult to weld due to their cracking sensitivity. γ-TiAl alloys are currently processed via electron beam melting (EBM) to produce components for energy generation applications. The EBM process provides crack-free processing thanks to the preheating stages between layers, but lacks geometrical precision. The use of LPBF could provide the means for higher precision, and therefore an easier post-processing stage. However, industrial LPBF systems employ resistive heating elements underneath the base plate which do not commonly reach the high temperatures required for the processing of γ-TiAl alloys. Thus, elevated temperature preheating of the build part and control over the cooling rate after the deposition process is concluded are amongst the features which require further investigations. In this work, the design and implementation of a novel inductive high temperature LPBF system to process Ti–48Al–2Cr–2Nb is presented. Specimens were built with preheating at 800 °C and the cooling rate at the end of the build was controlled at 5 °C min−1. Crack formation was suppressed and apparent density in excess of 99% was achieved.
Highlights
Laser powder bed fusion (LPBF), known as selective laser melting (SLM), employs a highly energetic laser beam to selectively deposit a layer of metallic powder feedstock upon a substrate material
Taking into consideration the available technologies and the need for uniform high preheating temperatures for the processing of crack susceptible alloys, the aim of the present work was the development of a novel high temperature laser powder bed fusion (HT-LPBF) system, namely project Grisù [19]
In the case of the ambient temperature printing of the γ-TiAl alloy, the process had to be interrupted after 10 layers of material had been deposited due to powder bed instabilities and protruding parts which impeded the correct deposition of successive layers
Summary
Laser powder bed fusion (LPBF), known as selective laser melting (SLM), employs a highly energetic laser beam to selectively deposit a layer of metallic powder feedstock upon a substrate material. In order to obtain stable processing conditions and high part density, full melting of the powder bed is required. For the processing of metallic powders, LPBF requires the use of a beam with elevated energy density which has been enabled by the development of high brilliance fiber laser sources in recent years [1]. The main reason behind the limitation in the range of processable materials is linked to the elevated thermal gradients and cooling rates that are experienced locally during the fusion process and are intrinsic to the technology due to the concentrated energy beam required to melt the powder bed. As clearly explained by Hagedorn et al these values are critical when processing metallic powders due to the elevated stress concentrations which are generated [4]
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