Mechanical and Microstructural Anisotropy of Laser Powder Bed Fusion 316L Stainless Steel.

Zdeněk Pitrmuc,Petr Mikeš,Jan Šimota,Libor Beránek,Vladislav Andronov,František Holešovský,Jiří Sommer

doi:10.3390/ma15020551

Zdeněk Pitrmuc, Petr Mikeš + Show 5 more

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https://doi.org/10.3390/ma15020551

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Abstract

This paper aims at an in-depth and comprehensive analysis of mechanical and microstructural properties of AISI 316L austenitic stainless steel (W. Nr. 1.4404, CL20ES) produced by laser powder bed fusion (LPBF) additive manufacturing (AM) technology. The experiment in its first part includes an extensive study of the anisotropy of mechanical and microstructural properties in relation to the built orientation and the direction of loading, which showed significant differences in tensile properties among samples. The second part of the experiment is devoted to the influence of the process parameter focus level (FL) on mechanical properties, where a 48% increase in notched toughness was recorded when the level of laser focus was identical to the level of melting. The FL parameter is not normally considered a process parameter; however, it can be intentionally changed in the service settings of the machine or by incorrect machine repair and maintenance. Evaluation of mechanical and microstructural properties was performed using the tensile test, Charpy impact test, Brinell hardness measurement, microhardness matrix measurement, porosity analysis, scanning electron microscopy (SEM), and optical microscopy. Across the whole spectrum of samples, performed analysis confirmed the high quality of LPBF additive manufactured material, which can be compared with conventionally produced material. A very low level of porosity in the range of 0.036 to 0.103% was found. Microstructural investigation of solution annealed (1070 °C) tensile test samples showed an outstanding tendency to recrystallization, grain polygonization, annealing twins formation, and even distribution of carbides in solid solution.

Highlights

Additive manufacturing (AM) has experienced unprecedented development in recent years, and it has gained a regular role among production technologies
With laser powder bed fusion (LPBF) technology, units of material feedstock in the form of powder are distributed layer by layer in a powder bed and fused in desired regions together layer by layer using the thermal energy of a scanning laser beam
In the majority of publications and materials of powder manufacturers’ datasheets, the mechanical values obtained from vertically printed test bars are given, so vertical test samples were considered as reference average values in this experiment as well—UTS = 607 ± 1MPa; YTS = 476 ± 20 MPa; E = 44 ± 2%

Summary

Introduction

Additive manufacturing (AM) has experienced unprecedented development in recent years, and it has gained a regular role among production technologies. AM allows the application of completely different, very complex, and non-traditional CAD approaches to the design of parts that are otherwise very limited by conventional manufacturing technologies. With laser powder bed fusion (LPBF) technology, units of material feedstock in the form of powder are distributed layer by layer in a powder bed and fused in desired regions together layer by layer using the thermal energy of a scanning laser beam. LPBF triggers an extraordinary versatility to geometry and material design, being considered a near-net-shape technology as well as an exceptional technique to create functionally graded materials [7] and complex components with individualized local functional requirements [8,9]. Over the last decade, sintering parameters have been optimized, and porosity has been significantly eliminated

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