A study on mechanisms of saw-tooth chip formation in hard turning under hybrid nanofluid-assisted MQL environment

Saw-tooth chip generated from hard turning is one of the most distinguishing features with conventional turning process. The presented work aims to contribute an extensive understanding the formation mechanism and main parameters of saw-tooth chips in oblique hard turning of 90CrSi hardened tool steel (60–62 HRC). The effects of dry, Al 2 O 3 nanofluid MQL, Al 2 O 3 /MoS 2 hybrid nanofluid MQL conditions on the serrated chip formation were experimentally investigated in terms of chip morphology, chip color, segment spacing, serrated segmentation frequency, and shear angle. The obtained experimental results indicated that, due to the better cooling and lubricating performance of nanofluid and hybrid nanofluid, the shear angles ϕ increase about 91.73% and 94.88% respectively when compared to dry cutting, which makes the formation of saw-tooth chip more favorable. Moreover, the analysis of microstructure image of the back side of chip proves the better lubrication from ball roller and tribo-film formation of Al 2 O 3 and MoS 2 nanoparticles. Based on the chip morphology analysis, the shear angle ϕ, the degree of chip segmentation on the top side in the chip’s transverse section K 2 , and the chip deformation coefficient in the transverse section K 3 should be used as the additional criterions to evaluate the frictional improvement in cutting zone.