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Friction welding of R6M5 steel in the superplastic condition. Changes in the structure, phase composition and properties of welded joints during welding

In the production of tools, the main defects in welded joints between steel 45 and R6M5 steel, produced by conventional friction welding, are ‘shiny rings’ (or ‘shiny slip bands’) and the ferritic interlayer on the side of the high-speed and structural steel, respectively. The thickness of these defects in the cross-section of the welded joints may change from 1–2 μm to several tens or even hundreds of micrometres. The ‘shiny rings’ in the cross-section may have the form of both straight and wavy lines, close to or far away from the welded joints. Both defects have a negative effect on the properties of the welded joints. In Ref. 4, investigations were carried out into the mechanism of formation of ‘shiny slip bands’ and the possible conditions of welding in the superplastic temperature range, preventing the formation of these bands, have been determined. The results of investigations carried out in Ref. 5 were used to construct the time diagram of the variation of axial pressure in the friction welding process in the superplastic condition of R6M5 steel (Fig. 1). According to this diagram, the pulsed variation of the heating pressure takes place when reaching a temperature of 495– 500 °C. The duration of the breaks and pulses of the heating pressure is 2–3 s. The duration of the process from the moment of the pulse variation of heating pressure to the completion of forging should not exceed 10–12 s. The proposed algorithm of the process of welding bimetallic components in the temperature range of superplasticity of the R6M5 steel makes it possible to increase the heating time in order to ensure smooth transition to the temperature of phase transformations. Consequently, this results in the formation of a transition region of the material in which the transition from the structure of steel 45 to the structure of the high-speed steel takes place smoothly, without the formation of ‘shiny rings’. In order to substantiate the parameters of the welding process, investigations were carried out into the structure, phase composition and mechanical properties of bimetallic components during the entire welding cycle (from the moment of the start of operation of the photosensor to the completion of forging). The most interesting moments from this viewpoint are the start of the break and start of the pressure pulses in heating and forging. In the investigations, the initial materials in the friction welding were represented by cylindrical components, i.e. bars of steels 45 and R6M5 with a diameter of 15 mm in the as-received condition. Welding was carried out in a modernised MF-327 machine, which makes it possible to change the heating and forging pressure in accordance with the diagram in Fig. 1, at a fixed speed of rotation of the spindle of 90 rpm. The temperature of the specimen directly in the welding zone and the distribution of temperature in the perimeter were investigated on the basis of the readings of chromel– alumel thermocouples, installed in relation to each other at an angle of 120° in the components of R6M5 steel at a distance of 2 mm from the weld edges. To determine the intensity of heating in the cross-section of the contacting surfaces, thermocouples were also installed at a depth of R, 0.5 R, and on the surface of the specimen. The temperature of the surface of the specimens was evaluated using a specially produced photosensor of temperature consisting of the infrared photodiode FD-263 and an amplification unit. In order to record the parameters of the welding conditions (pressure in the pneumatic cylinders of the carriage and the number of rotations of the spindle), sensors operating on the basis of the principle of the optical couple were used. The parameters of the welding conditions and of the signal from the photosensor and from the thermocouples were recorded in a 12-beam loop S F G N Y U S O V, E A T R U S H C H E N K O and V F S O V E T C H E N K O Tomsk Polytechnic Institute Friction welding of R6M5 steel in the superplastic condition. Changes in the structure, phase composition and properties of welded joints during welding