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Dela Cruz et al. Microstructures 2023;3:2023012 https://dx.doi.org/10.20517/microstructures.2022.33 Page 19 of 25

Figure 10. The relationship between residual strain (average KAM value) and hardness for both the reference alloy and the LPBF alloy
fabricated at different LEDs.

The residual strain in the LPBF alloy, according to the average crystal misorientation data, was also shown
in Figure 10 to decrease with increasing LED. This was reported to increase the hardness , and when the
[127]
[128]
residual strain was relieved, the hardness decreased . For this reason, the relationship between the average
KAM value and hardness in both the reference alloy and LPBF alloy at different processing conditions was
investigated. However, the decrease in residual strain corresponded to an increase in hardness, as seen in
Figure 10, which differed from the previous reports [126-128] . Therefore, the residual strain may have indeed
been relieved from the LPBF alloy through the formation of cracks, particularly in 0.88 J/mm LED.

The influence of grain size, presence and volume of phases, and residual strain was analysed to identify the
possible factor affecting the hardness of the LPBF-fabricated Fe-30Mn-6Si alloy. Hardness is known to
increase as the grain size decreases, ε-martensite volume fraction increases, and residual strain increases. It
was observed that the increase in hardness was mainly influenced by ε-martensite at high LEDs of
0.44 J/mm and 0.88 J/mm [Figure 9], while grain size and residual stress were not seen to influence their
hardness according to the accepted theories [104,105] and observations [126-128] . The sub-grain phase boundaries
between the different variants of ε-martensite increased the hardness in those large-grained microstructures.
Also, high hardness value was found in the LPBF alloy with low residual strain [Figure 11]. This suggests
that the residual strain at the high LED was relieved after cracks were formed. At 0.25 J/mm LED, hardness
has increased relative to the reference alloy because of the low grain size and high vol.% martensite
(Figures 8 and 9, respectively). However, the hardness for 0.29 J/mm LED slightly decreased (2%) despite
the increase in martensite vol.% (26%) as compared to that of 0.25 J/mm LED. Concurrently, the grain size
of the former increased by 65%. From the given correlations, the increase in grain size was likely the main
mechanism for the slight decrease in hardness from 0.25 to 0.29 J/mm LED.

CONCLUSIONS
A LPBF technique is normally carried out using pre-alloyed powder, but the supply of pre-alloyed powder is
limited, thereby confining this technique’s adaptability to readily available raw materials. It was