No 4 (2018)

The issue of the study of the porous bodies’ deformation and the plasticity of their materials is important as it is related to the production of complex parts using the wide opportunities of drop stamping (DS). In this case, the increased requirements are imposed on the porous material plasticity as the emergent defects may be not removed at the final DS stages what impairs the strength properties. The information about the plasticity properties of a porous material in the heated state allows properly developing the technology of complex parts’ DS.

The paper investigates such structural defect of powdered materials as the porosity. The authors studied its influence on the mechanical properties of materials and analyzed the regularities of these properties’ changes in the interval of brittleplastic transition temperatures. The mechanical tests of the powdered specimens were carried out with the identification of the main dependencies of mechanical properties on the porosity in the interval of brittle-plastic transition temperatures. The authors selected the main deformation modes when the crack resistance is studied, identified the main factors influencing the peculiarities of brittle-plastic transition in the powdered materials, described the phenomena where the crack formation takes place, as well as defined the details of this process.

The study identified the nature of change of the strength coldbrittleness temperature (T_CL) and the plasticity coldbrittleness temperature (T_CH): if T_CL increases with the increase in porosity, T_CH decreases under the same condition. Thus, at the specimen porosity of 3–5 %, its value, depending on the powder type, is 100–150 K, and at the porosity of 10– 15 %, it is lower than 77 K for all powders.

The paper presents the results of the experiment on the processing of 105WCr6 steel ring blanks prehardened up to the HRC 55 hardness. This material is applied to produce cutting and measuring tools with the high requirements for the accuracy in size and flatness after the thermal treatment. The experiment involved the ring facing using the 16B16T1C1 machine with the CBN plate cutter without the use of a lubricating and cooling fluid. The cutting rate and the advancement and depth of cutting were varied during the three-factor experiment. For the wider industrial application, hard turning requires the additional research related to the study of special aspects of chip formation, the identification of the dependencies of cutting forces and temperature in the cutting zone on the processing mode. The authors studied the chip formation process and the quality of processed surface and carried out the systematization of chip types depending on the cutting modes. The main technology factor determining a chip type is the cutting rate. When it increases the chip type changes from a continuous chip through a transition form to a segmental chip. When zooming in a chip, the welldefined chip segmentation can be seen. When increasing the cutting rate the segments become more defined that causes the change of a chip type. At the critical value of the cutting rate, the chip comes from a discontinuous one to a segmental chip. In this case, the dynamic component of cutting force related to the chip formation process grows. Such change in the cutting process dynamics is accompanied by the corresponding traces of a tool on a processed surface. Stable type of chip formation promotes the formation of a surface with the regular minimum height microrelief. The growth of chip formation dynamism, when increasing the cutting rate, causes the formation of a moire effect on a processed surface. The study identified the processing modes optimal in terms of efficiency, surface condition and chip type. The results obtained can be used to organize an automated manufacturing with the use of CNC machines and automatic lines.

The modern software systems simulating the welding process do not cover all its specific features. For example, they are too cumbersome to be applied in the penetration automatic regulation according to the mathematical model. For this purpose, the authors proposed using a mathematical model of heat distribution within the parts from a normal circular heat source acting on the flat layer surface. The coefficients of such a model should be determined by the experiments close to the data of a problem to be solved (the reduction method). The paper presents the results of hanging surfacing of spots on the 12H18N10T high-alloy steel plate of 4 mm thick. To record in time the welding arc current in argon medium with the non-consumable electrodes, the authors used the recorder. The relative deviations of the spots’ diameters from the average value were checked for the compliance with the normal distribution law. It is determined that the deviations of the spots’ shape from the circle do not meet this law. The average diameters of the obtained spots were used to predict the penetration depth, which was determined by macrosections with the 20 amplification. In this case, the authors used several values of axial heat flux of the heat source: 2800, 3500 and 4200 ^W/cm2. The values of thermal diffusivity were taken from literature data averaged a=0.04 cm2/s. Using the diameters of spots, the effective power of the welding arc and the specific effective power per 1A of welding current were calculated. The penetration of the spots was calculated using the average power density. The best convergence of calculated and experimental data was obtained at the axial heat flux of 2800 W/cm²; it averages about 5 % in absolute value. Similar results were obtained when predicting the diameters of spots using penetration depth. Thus, the authors developed the technique for determining three coefficients of the model to apply them for the welding process automatic control.