*2.2. Characterization Techniques*

Density measurements on the samples were performed using the Ohaus densitometer employing the Archimedes principle with ethanol as a liquid medium. For testing the mechanical properties (ASTME8), the 20kN Zwick/Roell Tensile/Compression tester GmbH from Radeberg in Germany was used, while hardness testing was performed on the Zwick micro/macro (Hv) hardness tester GmbH from Radeberg in Germany. The microstructures of the polished samples after etching with Keller's reagen<sup>t</sup> were viewed on the optical microscope. The samples were also analyzed for phase and grain information using a Zeiss LEO 1530-FESEM fitted with the Oxford Energy Dispersive Spectroscopy (EDS) and Hardware Lab Kit (HKL) Electron Backscatter Diffraction (EBSD) detector GmbH from Radeberg in Germany. The fractured tensile samples also underwent fracture analysis on the JEOL JEM-210 SEM from Peabody in USA. A 1342 Instron 30 kN fatigue crack machine from Norwood in US was used to conduct fracture toughness and fatigue crack growth tests according to ASTM399. The dimensions of the samples were as seen in Scheme 1, with a notch length of 7.37 mm for both fatigue crack growth and fracture toughness samples. The load used on the samples was 156 MPa, cycling at a rate of 2.1 mm/min and frequency of 15 Hz.

## **3. Results and Discussion**

#### *3.1. Density and Porosity Measurements*

The density results of the AlSi10Mg as-built samples were investigated and presented by Mfusi et al. [16], where the average density was 2.68 g/cm3, with relative densities above 99% for the samples in different build directions. However, after stress relieving at 300 ◦C the density values dropped drastically to 2.58–2.61 g/cm3, which were 0.07–0.1 g/cm<sup>3</sup> lower than as-built, presented in Table 1. It was also observed that the porosity values were higher at 2.67–3.81%, which was above the accepted levels for applications.

**Table 1.** Showing the orientation density and porosity levels after stress relieve.


This behavior was also observed by Calignano [17], where they also determined that stress relief led to a decline the density and mechanical properties of the specimen. Ahmed [5] determined that the greatest challenge in producing aluminum alloys parts by SLM technique was to minimize porosity, which is the major effect of the relative density as aluminum alloys are easily subjects to oxidation during processing, stimulating pore formation. Therefore, more research is needed to address the impact of heat treatment temperatures on the properties of the SLM produced samples in order to optimize the properties for intended applications [18].
