Modeling and experimental verification of the dead metal zone for cutting force prediction in micro milling

In micro-cutting process, ploughing phenomenon occurring due to the dead metal zone (DMZ) leads to substantial ploughing force resulting in an obvious contribution to the total cutting force. To improve accuracy of the cutting force predicted, this paper aimed to explore the DMZ geometry related to the cutting depth and tool edge radius and thereof effect on cutting force. The prominent contribution of this research is to establish a new DMZ model by employing the slip-line field theory of the plastic formation. Based on the proposed model, DMZ are divided into shearing-dominated, mixed shearing and ploughing, and pure ploughing according to the minimum uncut chip thickness (MUCT). It is firstly proposed that the inner vertex of DMZ is the separation point of shearing effect and ploughing effect during metal cutting. The shape of the DMZ is theoretically calculated by an analytical way and verified by the simulation software. Finally, a cutting force model including shear force and ploughing force is established and verified by a series of experiments. The predicted cutting forces show remarkable agreement with those measured. The result proves that the separation point model is correct and can effectively demonstrate the ploughing force to accurately predict cutting force.

On the Steady-State Workpiece Flow Mechanism and Force Prediction Considering Piled-Up Effect and Dead Metal Zone Formation

The manufacturing of miniaturized components is indispensable in modern industries, where the uncut chip thickness (UCT) inevitably falls into a comparable magnitude with the tool edge radius. Under such circumstances, the ploughing phenomenon between workpiece and tool becomes predominant, followed by the notable formation of dead metal zone (DMZ) and piled-up chip. Although extensive models have been developed, the critical material flow status in such microscale is still confusing and controversial. In this study, a novel material separation model is proposed for the demonstration of workpiece flow mechanism around the tool edge radius. First, four critical positions of workpiece material separation are determined, including three points characterizing the DMZ pattern and one inside considered as stagnation point. The normal and shear stresses as well as friction factors along the entire contact region are clarified based on slip-line theory. It is found that the friction coefficient varies symmetrically about the stagnation point inside DMZ and remains constant for the rest. Then, an analytical force prediction model is developed with Johnson–Cook constitutive model, involving calibrated functions of chip-tool contact length and cutting temperature. The assumed tribology condition and morphologies of material separation including DMZ are clearly observed and verified through various finite element (FE) simulations. Finally, comparisons of cutting forces from cutting experiments and predicted results are adopted for the validation of the predictive model.

Investigation on the Strain Distribution in Tube High-Pressure Shearing

The Finite-element method (FEM) and experiments were used to investigate the geometric factors and material parameter on the strain distribution during tube high-pressure shearing (t-HPS). The results show that t-HPS could be realized successfully either by pressurizing on both ends of the tube, or by pressurizing using the wedge effect; and in both cases, the “dead metal zone” could be found at both ends of the tube. The grain size distribution from the experiment confirmed this strain distribution feature. In the case of t-HPS pressurized using the wedge effect, the half cone angle has little effect on the strain distribution. Decreasing the strain-hardening exponent leads to increased deformation inhomogeneity in both the ideal t-HPS described by theoretical equations and the close to practical t-HPS described by FEM. This feature of t-HPS stands out from other SPD processes like HPT, and makes practical t-HPS behavior more predictable using the analytical formation than any other SPD processes, and places it an advantageous position in understanding the basics of deformation physics through the coupling between practical experiments and theoretical approaches.