*3.3. Microstructure Analysis*

The evolution of the microstructures as a function of heat treatment is an important aspect of metallurgical evaluation as it is related to the mechanical properties of the samples. Therefore, the microstructures of the SLM processed AlSi10Mg samples after stress relieving were taken and these are presented in Figure 1a–c, demonstrating the three build directions at 50× magnifications.

**Figure 1.** Optical Microscopy (OM) microstructure of AlSi10Mg (orientation 0◦, 45◦ and 90◦) samples: (**<sup>a</sup>**–**<sup>c</sup>**) at 50× magnification post stress relief.

Figure 1a–c illustrates that post stress relieving, the grain boundaries disappeared completely and a different homogeneous "sand like" structure was observed. The microstructural observation acquired a big change in the morphology compared to the as-built micrographs, which had scale-like morphologies [16], as presented by Mfusi et al. From the observation, this structure exhibited a rough surface, which was speculated to be a result of the silicon (depicted by yellow arrow) and aluminum (depicted by white arrow) being uniformly distributed. Brandl et al. [15] also observed the same change in the microstructure after performing peak hardening heat treatment for SLM processed AlSi10Mg. Finer silicon particles were observed in 45◦ orientation relative to 0◦ and 90◦ orientation, which were anticipated to be attributed by the defects contained in the specimen. Built direction played a role as it was observed also that 0◦ orientation had more courser particles than the rest. Tang determined that there were three stages that silicon undergoes during stress relief heat treatment where silicon and Mg2Si precipitates, spheroidization of precipitates and silicon particles coarsening [20].

Fousova et al. [12], explained the effect as the comparative infringement of the silicon network causing the coarsening of distinct Si particles, which are observed in all the images. Zhang et al. [21] also stated that after stress relieving, there is a diffusion of silicon dendrites into a discontinuous state.

#### *3.4. Electron Backscatter Di*ff*raction (EBSD)*

EBSD was performed on the as-built and stress relieved samples to determine the effect that stress relieving has on the phases and grains produced by SLM processing. Table 3 presents the phase fractions of the main elements of AlSi10Mg alloy for the as built samples, as well as for samples after stress relieve heat treatment respectively, for the three build directions.


**Table 3.** Showing as built and stress relieved Electron Backscatter Diffraction (EBSD) phase fraction.

In Table 3, 0◦ orientation samples, before stress relief had a minimal amount of the main elements and a high percentage of zero solution. After stress relieving an increase in aluminum and especially silicon was observed on the surface of the sample, this supports the observation made in Figure 1 OM images. The 45◦ orientation showed a huge fraction of silicon element and a significant amount of aluminum before stress relieve. After stress, a huge rise to zero solution is seen. On the other hand 90◦ orientation showed to have an insignificant change before and after stress relieve with a drop in the Mg2Si as compared to other orientations. During the SLM process, Mg2Si appeared upon final solidification while appearing beneath solidus temperature below equilibrium circumstances. Mg generally melted into the aluminum matrix considering that it was not present as any intermetallic phases during solidification at equilibrium as stated by Tang et al. [20] then the increase of Mg2Si and silicon after heat treatment was caused by the precipitation onto the surface.

Figure 2 shows the pictures of the electron backscatter diffraction results as-built and after stress relief.

**Figure 2.** EBSD of AlSi10Mg samples at the Energy Dispersive Spectroscopy (EDS layer image, orientation A (**a**) and (**d**), orientation B (**b**) and (**e**) and orientation C (**c**) and (**f**), as-built and post stress relief respectively).

Figure 2a showed cellular and dendritic growth, which was also observed in the optical microstructure. Wu et al. [22] attributed these cellular growths as long cells that are formed as a result of cooling conditions in place of dendrites. The dark region seen in Figure 2b,c was drastically eradicated in Figure 2e,f, where more silicon and aluminum were visible. These dark regions, according to Wu et al. [22] and Mathe et al. [6], are aluminum grains that also contain Al–Si eutectic, which is difficult to distinguish from cell boundaries because of the disappearance of diffraction data. Figure 2d has developed roughness on its surface that looks "pimple" like as compared to Figure 2a before stress relief. The same roughness was observed in the optical microstructures.

Heat treatments at high temperature can encourage the combination of second phases as well as change in distribution of those phases [23]. This was observed in Figure 2c after stress relief. In these samples, the columnar and equiaxed grains were observed before stress relief. These have been proposed to be formed directly by solidification that takes places as cellular dendrites as a consequence of rapid cooling with little information for the mechanism of formation [22,24]. In the optical microstructure, it was stated as silicon segregation to the grain boundaries as a result of rapid cooling [19]. Aboulkhair et al. [25] and Li et al. [23] proposed that these silicon rich boundaries isolated by the aluminum grains and the cellular structure are the fine eutectic composed of aluminum grains with silicon particles. Longer columnar grain sizes after stress relief are seen even though they were covered by the black region. In Figure 2f some Mg2Si precipitation was also observed and the silicon phase that was more visible on the as-built samples, while after stress relief it seemed to fade.

Wu et al. [22] stated that the reason SLM processed AlSi10Mg is optimum in strength is because the larger aluminum regions contains silicon particles, which are surrounded by the thick eutectic boundaries that prevents dislocation movement inside the aluminum grains. In these pictures, it is observed that the dark lines seen before stress relieved are removed. These dark lines were also observed by Mathe et al. [6] for SLM produced AlSi10Mg, which were attributed as shear bands that are caused by the shear strain from the manufacturing process. Grain refinement was also observed on the samples after stress relieving.

#### *3.5. Tensile Strength after Stress Relieving*

To determine the effect that build directions have on the mechanical properties of post stress relief tensile specimen tensile measurements were performed. The average values of triplicate measurements are presented in Figures 3 and 4. In the previous study of the as-built AlSi10Mg SLM samples [16], orientation 90◦ had the highest ultimate tensile strength compared to the other orientations. After stress relieving, the Ultimate Tensile Strength (UTS) values dropped drastically from 420–470 MPa as-built, [16], to 110–160 MPa. The same was observed also for the modulus values in Figure 3b.

**Figure 3.** The illustration of (**a**) Ultimate Tensile Strength UTS variation and (**b**) modulus of elasticity for the Selective Laser Melting (SLM) processed AlSi10Mg samples as a function of build direction.

**Figure 4.** The illustration of (**a**) yield strength and (**b**) elongation for the SLM processed AlSi10Mg samples as a function of build direction.

In Figure 3a, 0◦ and 90◦ orientation exhibited the higher UTS values compared to 45◦ orientation. This is in contrast with the as-built samples where the 0◦ orientation had the lowest UTS value and the 90◦ orientation still had the highest value. The drastic decrease in the mechanical properties of the SLM produced has also been observed by Brandl et al. [15,19], for the same material, which they attributed to the changes in the grain structures and phases of the samples present when heat treatment occurs above the eutectic temperature.

The yield strength results in Figure 4a showed higher values for the 0◦ and 90◦ orientation, with the values ranging from 61–96 MPa. The 90◦ orientation was ductile but could only endure elastic deformation up to just below 88.1 MPa, this might be because of the number of pores suffered by the

material for the as-built samples [16]. The yield strength of the 45◦ orientation was radically dropped by a magnitude of approximately 4.3 times to 108 MPa. Figure 4b showed the elongation results of the samples after stress relieving, where for all the build directions the elongation increased significantly from 6.25–7.25 mm as-built [16] to 12.5–23 mm post stress relief. This means with stress relief the ductility of the material is drastically improved. The e ffect of post build stress relieving in this case had both a negative and positive impact with the most improvement seen for the elongation.

Represented graphically in Figure 5a are the stress–strain curves of the tensile tests after stress relieving. All the samples after stress relieve experienced Lueder's bands during tensile testing (see Figure 5b), which was a result of high total elongation before fracture. This phenomenon occurs when a specimen that cannot yield to the given load, yields, which is also known as discontinuous yielding [3,7], the behavior is typically observed in the area where an increase in strain occurs without an increase in stress. It was observed in the comparison of mechanical properties that before stress relieving the samples had more strength and less ductility [16], however, after thermal treatment, in the attempt to acquire ductility, strength was compromised.

**Figure 5.** Showing Batch A, B and C (0◦; 45◦ and 90◦) samples, (**a**) tensile strain and (**b**) extension.

Zhang et al. [26] determined that stress relieve reduces the strengths and fatigue properties of the alloy significantly due to reduced solid solubility and the precipitation of the fine silicon as well as the demolition of the fine sub-structures within the aluminum grains. Li et al. [23,27] stated that even though heat treatment is required for quality improvement of a part by microstructural refinement, it has been found to decrease the UTS while increasing the ductility. This is as a result of the silicon trapped in the aluminum matric that precipitates to the grain boundaries, reducing the solid solution strength [12].

Rosenthal et al. [28], also reported a decline in the ductility that is inversely proportional to the strain rate, which suggests the e ffects of confined strain rate hardening had not taken place, and that the existence of the silicon phase could be predominantly the reason for this conduct. Brandao et al. [29] used stress relief to prevent the residual stress from distorting the components. The e ffect was the same as in this work, as it led to a drastic decrease in the static yield strength.
