Research On Mechanism And Surface Integrity For Micro Cutting Al7050-T7451 Alloy

High-accuracy miniaturized components are increasingly in demand for various industries, such as aerospace, biomedical, electronics, environmental, communications, and automotive. Numerous researchers have investigated the feasibility of using other fabrication processes, such as LIGA, laser, ultrasonic, ion beam and micro-electro discharge machining methods, to manufacture commercially viable micro-components. However, the majority of these methods are slow, and limited to a few silicon-based materials and essentially planar geometries. Micro cutting bring many advantages to the fabrication of micro-sized features. It is capable of fabricating 3D free-form surfaces, which is essentially important for the production of micro-injection molds. Moreover, it can process a variety of metallic alloys, composites, polymers and ceramic materials to form functional devices. Micro cutting raises a great number of issues mainly due to size or scale. In the last few decades, there have been extensive studies of tool life, edge radius effect, surface generation, size effect, minimum chip thickness, micro structural effects as well as finite element modeling and molecular dynamics simulation of micro cutting. However, fundamental understanding and general consensus on the mechanism that dominates mechanical machining at the micro scale is lacking.Cutting force is an important parameter in metal cutting process. Research of the cutting forces has significant meaning for clarifying the cutting mechanism, calculating of power consumption, designing of cutting tools, machine tools and fixtures, selecting reasonable cutting parameters and optimizing tool geometry, etc. Simplify the cutting process from 3D micro-milling to 2D orthogonal cutting, built a model for 2D cutting forces which embody the characteristics of micro-cutting process, and make new divisions for the cutting zone. In order to facilitate the use of Kistler 9257B type dynamometer for cutting force measurement, the workpiece was pretreated, then the orthogonal turning experiments can be realized in CNC milling center. Considering the influence of tool edge radius in micro cutting process, various depths of cut under different cutting speeds were selected in the single factor experimental scheme. The cutting forces were measured during the experiment, and then the average cutting forces were calculated. The cutting forces and thrust forces present different patterns of change with various depth of cut under different cutting speeds. The ratio of thrust force to cutting force was calculated, and the ratio increases nonlinearly as the depth of cut decreases. The same trend was found under different cutting speeds, especially when the depth of cut less than 25μm. The so called frictional size-effect exists in micro cutting. The specific cutting force and specific thrust force were calculated. The cutting speed has little influence on both specific cutting forces, while the depth of cut has large effect. Both specific cutting forces increase as the depth of cut decreases, especially when the depth of cut less than 25μm. This phenomenon indicates that the size effect existing in micro cutting process and the tool edge radius does influence the specific cutting force.In micro cutting process, the chip formation mechanism is different from the conventional cutting process because of the existence of the minimum chip thickness. The chip geometry also has its own characteristics. Different chips under various cutting parameters were derived from the orthogonal cutting experiment. It is noticed that the curl of the chip increases with increased depth of cut at different cutting speeds. The chips produced under very small depth of cut are very thin and fragile, and wavy, while under large depth of cut the chips are smooth and continuously. Under very small depth of cut, the cutting forces present periodically vibrations at different cutting speed, while under large depth of cut, the cutting forces are stable. This phenomenon reflects the procedure that the workpiece material piles up before the tool edge till a specific thickness and then the chip formed. An effective shear angle model was established to describe the mechanism of micro cutting. The coefficient of chip deformation was calculated and the influence factors such as cutting parameters and tool edge radius were studied.In micro cutting process, size effect and minimum chip thickness caused by the big round tool edge have significant influence on the distribution of plastic strain, strain rate and temperature around the tool edge and in the cutting zone. It's difficult to directly observe the complex distributions via experiments. The finite element analysis is a good supplement to experiments and analytical methods. The constitutive model of workpiece material is essential for FEM simulation. The constitutive model for A17050-T7451 was established based on Power Law from the data derived from Hopkison bar experiment. The CAE model was established for simulation and the results were compared to the experiments to verify the model. It is found from the simulation results that the stable build-up edge does exist and the minimum chip thickness for micro cutting A17050-T7451 with carbide tool at about 30%of the tool edge radius. Size effect was also found in simulation result. The effect of tool edge radius and cutting parameters on the distribution of the maximum temperature, the plastic strain and strain rate were also study.Surface integrity is the state of the surface texture after machining or processing, its evaluation indicators including surface roughness, work hardness, residual stress, microstructure, and so on. Different machined surfaced were derived and saved from the single factor orthogonal cutting experiment. The surface roughness, microstructure, work hardness, and residual stress of each machined surface under different cutting parameters were measured, and the effect of tool edge radius and cutting parameters are also investigated. It is confirmed that the tool edge was at good conditions during the experiments from the measurement result of surface morphology. It is also found that the residual stress of the machined surface is tensile stress in orthogonal cutting process. The cutting edge radius does affect the residual stress. The tensile stress decreases as the depth of cut increases, especially when the depth of cut is smaller than the cutting edge radius (25μm). The formation mechanism of residual stress was investigated, and work hardening model was also established.Micromilling process is a significant way for manufacturing small features or miniaturized components. For small parts, the ratio of surface area to volume is much larger than traditional components. The effect on the accuracy and performance of surface integrity and burr is much greater. The surface roughness prediction model for the bottom surface in slot milling was proposed based on the theoretical analysis. The minimum chip thickness, size effect and cutting parameters were considered in the model. The full factor orthogonal micromilling experiment was conducted. It if found from the result that the axial depth of cut has greatest effect on the bottom surface roughness than other parameters. The bottom surface morphology was measured. The top burr was clearly observed via SEM. The minimum chip thickness and size effect has obvious influence on burr formation, and the size of top bur was also influenced by cutting parameters and tool wear. Further more, the top burr formed in down milling process is much smaller than that in up milling process.