Comparison of the EHLA and LPBF Process in Context of New Alloy Design Methods for LPBF

Abstract

Additive Manufacturing (AM) processes are becoming more and more important for production of parts with increasing geometrical complexity and functionality. However, to draw on the full potential of AM technologies, alloys that exploit process inherent particularities such as extremely high cooling rates (ca. 106 K/s) have to be developed. One of most important AM-processes is Laser Powder Bed Fusion (LPBF), a batch-wise process. This complicates experimental alloy development and increases the use of powder resources since only one chemical composition can be tested within one test job and the process chamber has to be cleaned carefully in between. The process Extreme High-Speed Laser Material Deposition (EHLA) has been found to have similar cooling rates as LPBF, however it uses an in situ supply of powders which allows an easy switching between materials and has potential for rapid alloy development methods. Since the mechanical properties of materials primarily depend on chemical composition and microstructure, which in turn depends heavily on the cooling rates in the production process, the EHLA-process could be used as a means for an accelerated alloy development for LPBF. However, to explore this possibility, a thorough comparison of the two processes has to be performed.In this work, EHLA and LPBF processes are compared and evaluated regarding the following characteristics: process parameters, laser intensities and volume energy densities, resulting microstructure (primary dendrite arm spacing, DAS), melt pool size and shape. The reference samples were manufactured using one set of LPBF process parameters and EHLA samples were manufactured using three different sets of process parameters.The volume energy densities Ev [J/mm³] of the processes were found to differ by a factor 2.4 with higher Ev observed in LPBF. However, considering that approximately 2 to 3 layers are remelted with each pass of the laser beam, the introduced Ev per pass approximates the Ev introduced in the EHLA process. The melt pool size as seen in a cross section in the EHLA-manufactured samples is approximately 25 times larger than in the LPBF-manufactured samples and its depth to width ratio (d/w ratio) can be attributed to a heat conduction welding process while the d/w ratio observed in the LPBF-manufactured sample suggests a transition process between heat conduction welding and deep welding. The observed DAS is in the same order of magnitude for both processes ranging from 0.55 to 1.15 µm in EHLA-manufactured samples and 0.73 µm in the LPBF-manufactured reference sample. Since the resulting microstructures of samples manufactured with both processes show this common feature and EHLA process parameters can be adjusted to control the cooling rates, the transferability between EHLA- and LPBF-processes is supported in this first investigation. Research for a more efficient alloy development for LPBF using EHLA will be continued by e.g. examining chemical compositions and performing mechanical testing.