*3.2. Hardness Measurements*

The Vickers hardness method was used to measure the hardness of the samples in different orientations after stress relieving and is presented in Table 2.


**Table 2.** Build orientation hardness results after stress relief.

After stress relief the samples suffered an enormous drop in hardness from 126–128 HV for the as-built samples [16], to 46–49 HV as shown in Table 2. This illustrated the uncertainty of the stress relieving method undertaken towards the improvement of the mechanical properties of SLM produced AlSi10Mg, irrespective of the build direction. However, the largest drop in hardness was observed for batch A that was built in the 0◦ direction. This decrease in hardness after stress relief in the SLM produced aluminum alloy study was also observed by Fiocchi et al. [11] and Aboulkhair et al. [16,19], where a negative response was seen after heat treating the AlSi10Mg alloy using the T6 treatment and stress relief at 300 ◦C. In their work, stress relief showed a significant decrease in hardness of up to 66%, compared to the decrease of up to 62% obtained in this investigation. They attributed this behavior to the material softening that was seen to expose the additively manufactured component's weakness for application that requires high hardness properties. Trevisan et al. investigated the effect of stress relieving on SLM produced AlSi10Mg parts and also reported negative results as well [17], which might not render the temperature profiles viable for the improvement of properties, however the process is sometimes necessary in order to relieve the residual stresses obtained in SLM produced parts.
