An accurate dead metal zone (DMZ) model can well understand the material separation and flow mechanism in the cutting process. Most available DMZ models considered the cutting state steady with complete chip formation and sufficient material piled up. To investigate unsteady-cutting state material separation, this paper proposed an improved DMZ model considering the chip fracture. The uncut chip thickness (UCT), which causes the instantaneous maximum pressure, is chosen as the criterion of the DMZ generation at an unsteady-cutting state. The DMZ model is divided into four types, whose geometry and position were theoretically calculated and verified by finite-element method (FEM) and turning experiments corresponding to Al7075-T6 with a maximum error of 12.5 %. Based on the DMZ modeling, the instantaneous shearing and ploughing effects are quantitatively analyzed. Furthermore, metallographic analyses were carried out for further observing the material flow mechanism. The results showed that different UCTs can result in distinct cutting states. Lower UCT led to the pure ploughing or ploughing dominating the cutting process, resulting in trivial chip generation. With the UCT increasing, the shearing dominates gradually and produces continuous chips. This research can contribute to in deeply investigating the instantaneous material flow and further material removal mechanism in the machining process.