# Mechanical Properties of White Metal on SCM440 Alloy Steel by Laser Cladding Treatment
## 3 Results and Discussion  
### Micro Vickers Hardness Testing  
Figure 2 plots the hardness testing results at each SCM440 alloy steel depth as the laser power increases from 1.1 kW to 1.3 kW and 1.5 kW, with the powder feed rate, head speed and head height fixed at 2.5 rpm, 10 mm/s and 22 mm, respectively. In the case of the centrifugal casting method (black line), the Vickers hardness of the white metal layer ranged between 24 HV and 26 HV and was measured at about 205 HV at the boundary layer point. The hardness values, depending on the depth toward the core at 0.1 mm, did not vary significantly, and the average hardness value was 155.6 HV. When comparing the hardness values in the cladding layer, the laser cladding process hardness value was close to the centrifugal casting process hardness value, so it seemed that it could be replaced. However, a rapidly increasing hardness value behavior was exhibited at a depth of 0.1 mm. In particular, the hardness values of the HAZ rose to a greater extent with the laser power. At a depth of 0.1 mm, the laser power of 1.3 kW yielded a hardness of 325 HV. This hardness increased to 380-450 HV at 1.5 kW, and a hardened layer of over 270-330 HV appeared down to a depth of 0.2 mm. The higher the laser power of the cladding, the wider the heat-affected zone (HAZ), and the increase in the hardness value was inferred by the difference in the amount of powder absorbed by the base material being affected by the high-power energy beam.  
Figure 3 plots the hardness testing results depending on the powder feed rate (2.5, 3.0 or 3.5 rpm), with the laser power, head speed and head height fixed at 1.3 kW, 10 mm/s and 22 mm, respectively. The highest hardness values were presented at the 0.1 mm depth, with the most hardened layer forming at the lowest powder feed rate. This was ascribable to the influence of the material composition change due to the increased heat input from the laser directly irradiating the substrate, as the powder feed rate decreased under the same laser power and head speed conditions.  
\begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Values** \\ \hline Laser power (\(\mathrm{kW}\)) & 1.1, 1.3, 1.5 \\ Powder supply rate (\(\mathrm{rpm}\)) & 2.5, 3.0, 3.5 \\ Head speed (mm/s) & 10, 15, 20 \\ \hline \end{tabular}
\end{table}
Table 2: Parameters of the laser cladding process.  
Figure 2: Comparison of micro Vickers average hardness according to the variable laser power of 3.1.  
Figure 4 plots the hardness testing results depending on the head speed (10, 15 or 20 mm/s), with the laser power, powder feed rate and head height fixed at 1.3 kW, 2.5 rpm and 22 mm, respectively. It is shown therein that the hardness value decreased with the head speed, with the measured hardness values ranging between 220 and 350 HV.  
As a result of the micro Vickers hardness analysis, it was shown that the higher the laser power, the lower the powder feed rate, or the lower the head transfer speed, the higher the hardened layer was formed. This means that the ratio of the absorbed laser beam energy increased, and it could be estimated that the characteristics of the white metal gradually deteriorated [19]. Therefore, if the hardness of the laser cladding is higher than that of the centrifugal casting process, then the laser cladding conditions are not satisfied, which adversely affects the product quality.  
Figure 4: Comparison of the micro Vickers average hardness values according to the variable head speed.  
Figure 3: Comparison of micro Vickers average hardness according to the variable supply rate.