Materials Science Forum Vols. 836-837

Abstract: In order to solve the poor cutting performance for the titanium alloy and the serious residual tensile stress distribution on the machined surface in cutting titanium alloy, the utilization of prestressed cutting method is proposed to actively control the residual stress distribution status on the machined surface in machining process. Titanium alloy ring parts were pre-stretched at different condition by a lathe-specific pretension device respectively. By the cutting experimental, the cutting force ,chip formation and surface integrity indexes are compared and studied. The results show that in suitable compressive residual stress on machined surface are achieved by utilizing the prestressed cutting method ,meanwhile procedures of residual stress adjustment after machining could be omitted. Furthermore, the magnitude of compressive residual stress could be actively controlled by adjusting the magnitude of prestressed force in certain extent. And uniform saw-tooth chip are generated in prestressed cutting, meanwhile there’s no significant increment of cutting force. Prestressed cutting method could generate good surface integrity.

Abstract: In this paper, the chip morphologies and sizes of hardened PM steel S790 (68 HRC) using four types coated end mills (TiAlN TiCrAlN TiSiN and TiAlSiN) under different cutting parameters in dry milling condition were studied. During the process of high speed milling hardened steel S790, the chip morphologies change from continuous strip to sawtooth with the increase of cutting speeds. When cutting speed reaches 100 m/min, obvious shear slip occoured on the top surface of the chips, which are formed by multilayer metal unit piled by shear slip plane segmentation. The bottom of the chips, which squeeze and slid with rake face of end mills, show shiny smooth surface. TiAlSiN and TiSiN coatings are more suitable for high speed milling than TiAlN and TiCrAlN coatings. With the increase of cutting parameters such as cutting speed, feed rate and axial cutting depth, the chips morphology of S790 change from continuous strips to trapezoidal and triangle sawtooth, and the chips trend easy separation.

Abstract: In order to improve the rough machining efficiency of titanium alloy, experiments were carried out to investigate the influence of feed per tooth on cutting force and cutting power with index-able coated carbide inserts. The curves of cutting parameters, including cutting force and cutting power, were obtained by single factor test. The results showed that, as the feed per tooth increases, the cutting force increases, especially in the direction of cutting width. All forces almost changed linearly with the changing of feed, and the cutting force of feed direction was the smallest force among the three directions of cutting force. The analytical model of tangential cutting force in the x-y plane was established. By calculating average chip thickness and relationships between tangential cutting force and measurements of cutting force to predict the cutting power, the calculation results were accurate which compared with the actual output power of the machine tool.

Abstract: In this paper, time domain simulation has been carried out to study the chatter stability of milling process. Dynamic chip thickness is calculated by analyzing the kinematics of the cutter, and thus dynamic governing equation revealing the dynamic behaviors between the cutter and workpiece is established. Solving framework is constructed by using the Simulink module and S-Function of Matlab software, and dynamic deflection is achieved with the four-order Runge-Kutta algorithm. With the simulated cutting forces, a criterion for the construction of the stability lobe is suggested. At the same time, algorithm for the prediction of the surface topography involving the dynamic response of the machining system is developed.

Abstract: Iron-based alloy GH2132 is a kind of difficult-to-machine material. In this study, the experiments were processed to research the effect of feed per tooth, axial cutting depth and radial cutting depth on milling force. Variance analysis was made on the three factors. The results reveal that axial cutting depth affects milling force significantly, followed by feed per tooth and radial cutting depth has little influence on it. Two types of empirical model of milling force were established by the result of orthogonal experiment and multiple linear regression analysis. It was verified that both (h_m, a_p) model and (h_m, a_p, a_e) model had good prediction accuracy compared with the experimental data. By calculating specific cutting force using the (h_m, a_p) model, a modified coefficient of the specific cutting force for 1mm² chip cross section was proposed. The study would provide guidance to improve the machining precision and machining efficiency of high temperature alloy materials.

Abstract: As a new type of material, Ti₂AlNb based alloy has a broad application prospects in aerospace field due to its excellent properties such as low density, high specific strength at elevated temperature, excellent oxide and creep resistance. Many studies involving its material composition and structural properties have been published. However, there are relatively few literatures available on the machinability of Ti₂AlNb based alloy. In this work, the turning experiment of Ti₂AlNb based alloy was carried out with coated cemented carbide tools. The machinability of Ti₂AlNb based alloy was investigated through the cutting force, cutting temperature, tool wear and surface roughness. The result shows that the cutting force is lower and surface quality is better under condition of higher cutting velocity, lower feed rate and smaller depth of cut. The life of tool is longer when they were cutting under the high cutting velocity, low feed rate and small depth of cut. The research could provide some instructive information and basic data for the turning process parameters optimization of Ti₂AlNb based alloy.

Abstract: Orthogonal-to-oblique transformation model, which is formulated based on the cutting database including shear stress, shear and friction angles, can be used to predict cutting forces in high speed milling process and any other machining process. The involved shear stress, shear and friction angles are traditionally identified from abundant number of turning experiments. For the purpose of saving experimental cost, this paper presents a novel method to identify these parameters directly from flat end milling processes. Identification procedures are established by transforming the cutting forces measured in Cartesian coordinate system into a local system. The advantage lies in that in spite of the cutter geometries and cutting conditions, only a few tests are required to develop the model, which is experimentally validated to be effective for predicting the cutting force in terms of magnitude and shape in other machining cases.

Abstract: The dynamic process of cracks initiation in brittle materials during high speed cutting is primary for the analysis of material fracture mode and finished surface. Cast iron was considered in this paper and discrete element method (DEM) was employed to create a density packed bonded-particle model for studying its machining cracks. The numerical tests of split Hopkinson pressure bar (SHPB) and oblique plane impact were conducted to calibrate the dynamic behaviour of the model. The cutting simulations were carried out for a range of rake angles and cutting speeds. The results showed that both the processes of cracks initiation and the cracks distribution were different greatly under different cutting conditions and the cracks number increased with the cutting speed. In addition, within the detection zone in the model, the duration of crack initiation decreased dramatically with the increase of cutting speed in the lower range. But for the high speed cutting, the duration tended to be stable and was not affected by cutting speed and it was very close to the time of stress wave produced by cutting arriving at the zone. Furthermore, the stress state of particles’ connectors under the action of stress wave was analysed, which suggests that the stress wave is the key factor causing cracks initiation of brittle materials during high speed cutting.

Abstract: Cutting force, cutting heat and tool wear are closely related to the friction characteristics of tool-chip interface in the process of metal cutting. The variation of the cutting speed and temperature have been analyzed by the distribution of stress and strain in the primary and the secondary shear zone. A new friction model has been established to analyze the contact length between cutting tool and chip and local friction coefficient at the sliding zone. Cutting experiments have been performed in the process of carbide tools cutting the titaniumTi6Al4V, the contact length of sticking zone and sliding zone and friction coefficient have been analyzed by measured cutting force. The simulation of cutting process has been carried out based on Deform software, then the simulation results have been compared with the test ones, which verifies the accuracy of the established model.

Abstract: Tool wear is easy occurred in titanium alloy milling process which will affect the surface quality. Surface roughness and surface morphology as an important index to describe and evaluate the surface quality has a great influence on service performance. Therefore, the study on the effect of tool wear on surface qualities is important to improve the surface integrity of titanium alloy parts. Cutting radius of ball-end milling cutter is solved to analyze the effect of tool wear on the cutting radius. The tool wear and the surface qualities of TC4 are achieved through wear experiment. And then the influence law of tool wear on surface qualities and chip morphology are analyzed. The results show that surface roughness value decrease firstly and then increases and that chip morphology with flank wear increase from the unit chip to the serrated chip.