# Influence of Cutting Parameters and MQL on Surface Finish and Work Hardening of Inconel 617
## 4 Results and Discussion
### Study C: MQL vs. Conventional coolant
#### 4.3.3 Material Characterization--Work Hardening  
The work hardening recovery in this paper is characterized by the hardness difference from the point of highest work hardening to the lowest, across the measured depth from the immediate subsurface in the white layer and through the plastic deformation zone, as shown in Figure 8. A sample of the as-received Inconel 617 was measured to have an average hardness of 340 \(\pm\) 40 HV, known as the bulk hardness of the material. All conditions generated similar profiles of work hardening and recovery. The highest level of work hardening occurs just below the machined surface where machining temperatures and pressures are the highest and occurred in the range of 410-550 HV, as seen in Figure 9, before returning to the bulk hardness further into the subsurface.  
The coolant deposition method and the level of tool wear were found to have the largest influence on the level of work hardening recovery, whereas the cutting parameters used had a minor impact in comparison (Figures 10 and 11, through the Taguchi design of the experiments). The main effect plot shows that MQL has a higher mean strength than the flood coolant in the experiments. New tools showed a higher mean strength than the worn tools. However, the impact of the method of coolant deposition on the work hardening is  
Figure 7: Microscope images of the turning tool edge for T1 (DoC = 0.1 mm, feed = 0.075 mm/rev, cutting speed = 50 m/min) showing **(a)** a nose image of the full cutting edge **(b)** the crater wear on the rake face **(c)** the flank wear on the flank face and **(d)** a 3D angle showing the full tool wear.  
also influenced by the cutting parameters used as shown in the interaction between these factors. Additionally, the interaction plot in Figure 11 shows that the parameters have an impact on each others' relationship to the work hardening recovery with the exception of coolant use and tool wear, which are independent of each other.  
It was observed that in every case, MQL generated a greater level of work hardening in the immediate subsurface, with HV values between 495 and 550 HV, while the flood coolant generated HV values between 410 and 500 HV. Due to the location of MQL mist deposition, directed towards the rake face of the tool and generated chip, the flank face had limited cooling and resulted in higher friction and pressure between the tool and the generated surface, causing the high level of initial work hardening. However, in studies performed on  
Figure 8: A polished and etched sample of the cross-section of the machined Inconel 617 under T7 conditions (DoC = 0.5 mm, feed = 0.075 mm/rev, cutting speed = 75 m/min) showing the grain boundary deformation and presence of a white layer caused by machining.  
Figure 9: The bulk and subsurface hardness of the machined Inconel 617 using flood and MQL coolants under T4 conditions (DoC = 0.3 mm, feed = 0.075 mm/rev, cutting speed = 100 m/min).  
MQL machining of other Inconel grades, MQL generated lower initial hardnesses compared to flood cooling [21; 49]. This phenomenon may be caused by the MQL deposition location directing the oil towards the flank face, as explained in their methodology.