Directed Energy Deposition of Low‐Alloyed Steels: An Insight on Microstructural and Mechanical Properties

Abstract

Low‐alloyed steels are used for a variety of different applications like bearings or gears. Additive manufacturing technologies like directed energy deposition (DED‐LB/M) allow for a fast and close‐to‐contour fabrication of sophisticated products without excessive waste of material. However, the DED‐LB/M process cannot be considered as state‐of‐the‐art for this group of materials. This study presents findings on the material properties of the additively manufactured low‐alloyed steel Bainidur AM by means of DED‐LB/M. This includes studies on the mechanical properties (hardness, compression strength) as well as the microstructural properties (scanning electron microscopy [SEM]). The microstructure in the as‐built state appears like a bainitic–martensitic one with shares of retained austenite which is not fully transformed during cooling. As a differentiation is barely possible from the SEM images, a plethora of investigations is further used to assess the microstructure. As‐built samples possess a good combination of ductility and hardness. Furthermore, the specimens are characterized by a good tempering stability up to 600 °C. This tempering stability is characterized by a homogeneous hardness of around 400 HV1 for all temperatures. In contrast, the conventionally hardened specimens show a drop‐off in material hardness, further indicating the excellent material properties of additively manufactured Bainidur AM.

Effect of Volumetric Energy Density and Part Height on the Material Properties of Low‐Alloyed Steels Manufactured by Laser‐Based Powder Bed Fusion of Metals

“…Similar results were observed when processing Bainidur AM by means of DED-LB/M. [12] The microstructure formation of steels upon cooling from the austenitization temperature can be distinguished into continuous cooling and isothermal transformation. [13,14] A continuous cooling is typically defined by specific cooling rates.…”

Material Characterization of Hybrid Components Manufactured by Laser-Based Directed Energy Deposition on Sheet Metal Substrates

“…The YS = (413.0 ± 46.2) MPa is on average 54 % higher and the TS = (576.0 ± 22.8) MPa is on average 36 % higher, compared to the pure substrate material with YS = (267.9 ± 4.8) MPa and TS = (424.5 ± 1.5) MPa. High cooling rates may lead to the formation of a fine-grained, mostly dendritic microstructure of the coating applied in DED-LB/M [12]. For the selected alloys and process parameters, this coating exhibits a bainitic-martensitic microstructure, with about three times the hardness of the soft-annealed base material and a transition zone in between [12].…”

“…High cooling rates may lead to the formation of a fine-grained, mostly dendritic microstructure of the coating applied in DED-LB/M [12]. For the selected alloys and process parameters, this coating exhibits a bainitic-martensitic microstructure, with about three times the hardness of the soft-annealed base material and a transition zone in between [12]. Thus, it can be assumed that the increase in strength is primarily caused by the coating and not by a thermally induced microstructural transformation in the substrate material observed for higher laser powers of 800 W [12].…”

“…For the selected alloys and process parameters, this coating exhibits a bainitic-martensitic microstructure, with about three times the hardness of the soft-annealed base material and a transition zone in between [12]. Thus, it can be assumed that the increase in strength is primarily caused by the coating and not by a thermally induced microstructural transformation in the substrate material observed for higher laser powers of 800 W [12]. The higher strength of the coating also increases that of the material composite.…”

“…A parameter set as shown in Table 2 was used for all specimens. The parameters are based on preliminary investigations of the same material combination and DED-LB/M system and ensure a defect-free processing of these alloys [12]. Tensile tests with optical strain measurement.…”