Defects in a Laser Powder Bed Fused Tool Steel

Abstract

Compared to conventional fabrication methods, additive manufacturing (AM) introduces new opportunities in terms of design freedom and part complexity due to the incremental layer-by-layer process. For tooling applications, higher cutting speeds can be realized by implementing of internal cooling channels in tools that could not be fabricated otherwise. However, processability of high-alloyed tool steels with laser powder bed fusion (LPBF) faces certain restrictions. In addition to pore formation, severe cracking caused by a combination of process-related stresses due to the high thermal gradient and susceptible materials may occur. This work aims to clarify the occurrence of process-related defects in dependence of the applied energy input of a high-alloyed cold-work tool steel and to correlate it to the evolution of microstructure respectively solidification structure. Defect surfaces and structural evolution are investigated. The results exhibit that with increasing energy input porosity changes from lack-of-fusion to keyhole porosity. Most recently published investigations suggest cold cracking as predominant failure mechanism during LPBF of tool steels. However, for the investigated material, the present study clearly reveals that, irrespective of the chosen energy input, hot cracks are formed. Crack propagation can be connected to the solidification structure and possible thermal stress accumulations caused by the process.