# Wire Arc Additive Manufacturing of Stainless Steels: A Review
## 6 Mechanical Properties
### Hardness Distribution  
[PERSON] et al. concluded that duplex stainless steel (wire 2209) WAAM-made parts had a lower hardness value than the base metal (a 2205 duplex stainless steel substrate) due to the lower ferrite content in the layers [105]. According to [PERSON] et al., the Vickers hardness values of 316L stainless steel components exceed 175 HV, obtained by both SpeedArc (the primary metal transfer mode is the short-circuited transfer mode) and SpeedPulse (the primary metal transfer mode is a projected spray transfer mode) additive manufactured process [90], as shown in Figure 12. Though scanning speeds and deposition rates are the same for both processes, the SpeedArc WAAM can provide a more exceptional solidified structure (smaller secondary dendrite arm spacing) in the deposition parts owing to a lower heat input and higher cooling rate. As a result, the hardness values of SpeedArc WAAM parts are higher than those of SpeedPulse WAAM parts. However, this article does not address the hardness distribution of different layers of the same component.  
[PERSON] et al. found that the hardness value of the 316L stainless steel sample reduced slowly from the bottom to top because of the heat accumulation. As described in Figure 13, the average hardness in the bottom region ranges from 177.9 HV to 182.75 HV, which is slightly higher than that in the top region, i.e., in the range of 169-174.35 HV [96].  
On the other hand, process parameters--such as the transient and gradual reduction of bottom current, scanning speed, and cooling time--have little influences on the hardness of the deposited samples. As seen in Figure 14, there is no significant difference in hardness values between different samples (316L stainless steel) [96]. In the study of [PERSON] et al. [100], the scanning speed also barely affect the hardness of WAAMed stainless steel samples (H00 Cr21 Ni10 stainless steel) (see Figure 13 in [100]). However, the average microhardness values of samples G3510 and G3015 are slightly higher than that of others, since these two samples have relatively small heat inputs [96]. As reported by [PERSON] et al. [91], post processing heat treatment could adjust the final hardness of WAAM 17-4 PH stainless-steel components., specimens underwent H900 ageing treatment (AD+H900 and ST+H900) have higher hardness than the ones treated without further ageing (as presented in Figure 15 [91]). The maximum hardness value acquired for H900 ageing treatment is 448 HV30, while the maximum hardness value obtained for AD or ST treatment is about 340 HV30, the hardness value increased 32% after heat treatment [91].  
Figure 12: Vickers hardness of 316L stainless steel components prepared by SpeedPulse and SpeedArc additive manufacturing. A 500 g load was applied for all indentations with a dwell time of 10 s [90].  
Figure 13: Micro-hardness of bottom, middle, and upper parts of sample G3010 (gradual reduction of bottom current, scanning speed is 30 cm/min, cooling time is 10 s) [96].  
In summary, post heat treatments usually have much more significant influence on the hardness of WAAMed samples than printing process parameters.