Properties of Additively Manufactured Electric Steel Powder Cores with Increased Si Content

Abstract

In this paper, the best laser powder bed fusion (L-PBF) printing conditions for FeSi steels with two different Si content (3.0% and 6.5%) are defined. Results show very strict processing window parameters, following a lack of fusion porosity at low specific energy values and keyhole porosity in correspondence with high specific energy values. The obtained microstructure consists of grains with epitaxial growth starting from the grains already solidified in the underling layer. This allows the continuous growth of the columnar grains, directed parallel to the built direction of the component. The magnetic behaviour of FeSi6.5 samples, although the performances found do not still fully reach those of the best commercial electrical steels (used to manufacture magnetic cores of electrical machines and other similar magnetic components), appears to be quite promising. An improvement of the printing process to obtain thin sheets with increased Si content, less than 0.5 mm thick, with accurate geometry and robust structures, can result to an interesting technology for specific application where complex geometries and sophisticated shapes are required, avoiding mechanical machining processes for electrical steel with high silicon content.

Keywords: FeSi steels; additive manufacturing; magnetic properties.

Figure 1

Particle size distribution of the as-received powders of FeSi3 and FeSi6.5 steels, used in the L-PBF system.

Figure 2

Powder steel morphology of FeSi3 (a) and FeSi6.5 (b), SEM images.

Figure 3

Absorptivity, printing modes’ limits (conduction and keyhole), and porosity, derived through numerical modeling of the processes described in [10,13], referring to steels with 6.7% and 6.9% Si.

Figure 4

Internal section of sample-ring manufactured for magnetic characterization of FeSi3 and FeSi6.5.

Figure 5

Experimental set-up for the magnetic property measurement of the samples in FeSi3 and FeSi6.5.

Figure 6

Effect of the specific laser energy E [Jm^-1] on the densification of FeSi3 steel samples. (a) sample S1 (E = 150 Jm⁻¹, v = 0.5 ms⁻¹, P = 75 W), relative density of the sample 99.93% and pores with irregular shape; (b) sample S7 (E = 250 Jm⁻¹, v = 1 ms⁻¹, P = 250 W), relative density of the sample 99.99%; (c) sample S18 (E = 350 Jm⁻¹, v = 0.5 ms⁻¹, P = 175 W), relative density of the sample 99.98% and pores with spherical shape.

Figure 7

Effect of the specific laser energy E (Jm⁻¹) on the densification of FeSi6.5 steel samples. (a) sample S1 (E = 150 Jm⁻¹, v = 0.5 ms⁻¹, P = 75 W), relative density of the sample 99.93% and pores with irregular shape; (b) sample S3 (E = 200 Jm⁻¹, v = 0.835 ms⁻¹, P = 167 W), relative density of the sample 99.99%; (c) sample S18 (E = 350 Jm⁻¹, v = 0.5 ms⁻¹, P = 175 W), relative density of the sample 99.98% and pores with spherical shape.

Figure 8

Example of transition from conduction melting to keyhole formation. (a) optical micrograph of the longitudinal section to the build direction (BD) of S1-FeSi6.5 sample (E = 150 Jm⁻¹, v = 0.5 ms⁻¹, P = 75 W), conduction melt mode; (b) optical micrograph of the longitudinal section to the build direction (BD) of S18-FeSi6.5 sample (E = 350 Jm⁻¹, v = 0.5 ms⁻¹, P = 175 W), keyhole mode.

Figure 9

Columnar microstructure of longitudinal sections along the built direction (BD) of the highest density samples: (a) S7-FeSi3 (E = 250 Jm⁻¹, v = 1 ms⁻¹, P = 250 W) and (b) S3-FeSi6.5 (E = 200 Jm⁻¹, v = 0.835 ms⁻¹, P = 167 W).

Figure 10

Example of non-columnar microstructure in the high porosity samples. (a) S1-FeSi3 (E = 150 Jm⁻¹, v = 0.5 ms⁻¹, P = 75 W) and (b) S20-FeSi6.5 (E = 400 Jm⁻¹, v = 0.6 ms⁻¹, P = 240 W).

Figure 11

XRD spectra of FeSi3 and FeSi6.5 steels samples, in as-built condition and after heat treatment at 1150 °C for one hour.

Figure 12

Dynamic hysteresis loops at 50 Hz, samples FeSi3, (a) sample with full-section; (b) sample with optimized Section 1; (c) sample with optimized Section 2.

Figure 13

Dynamic hysteresis loops at 50 Hz, samples FeSi6.5, (a) sample with full-section; (b) sample with optimized Section 1; (c) sample with optimized Section 2.

Figure 13

Dynamic hysteresis loops at 50 Hz, samples FeSi6.5, (a) sample with full-section; (b) sample with optimized Section 1; (c) sample with optimized Section 2.

Figure 14

(a) Anhysteretic curves of the samples with optimized sections; (b) power losses of the samples with full-section; (c) power losses of the samples with optimized sections.