Development of a science-based technology for the manufacture of sheet panels with fasteners

. An analysis was made of the application of Stud Welding hardware welding technologies depending on the welded structures used, the duration of the process, the shape of the stud butt and the consumables used. The use of CD Stud Welding technology for welding metalware to thin-sheet panels is substantiated. The essence of the process of formation of a welded joint is described, which consists in the explosive evaporation of a protrusion at the end of a hardware during the discharge of capacitor banks, the excitation and burning of an electric arc in the metal vapors of the parts to be welded, their convergence when the spring mechanism overcomes the pressure of metal vapors, the immersion of the hardware into the weld pool of the part with subsequent crystallization molten metal in the liquid phase. The spring mechanism was calibrated using a compression dynamometer. The flowing pulsed current was registered depending on the input energy of the discharge of capacitor banks. The main parameters of the capacitor welding mode are determined and the dependences on the input energy and preliminary static pressure are established. The indicator of mechanical tests was taken as a quality parameter - the pull-off force. The effect of metal vapor pressure, which counteracts the convergence of welded parts, is revealed. Dependences of opposing pressures to the convergence of welded parts are constructed.


Introduction
The expansion of the range of body products and the steels used to which it is necessary to weld rod elements has expanded significantly, which requires research to develop the technology of their welding, in relation to specific structures, taking into account operating conditions [1][2][3]. In addition, the repair of structures by welding additional hardware is currently practiced [4].
An analysis of the methods for connecting fasteners to sheet structures showed that, depending on the grades of welded alloys, 3 technological methods have been developed: condenser stud welding -SD, electric short arc welding with a short technological cycle -SC and electric arc welding technology -ARC. For each technological method, studs are used with a certain configuration at the end: with a ledge, on a cone, with a flat surface [5,6].
When using SC and ARC technologies, it is necessary to take into account consumables for protecting the weld pool from the environment (shielding gases, flux rings) [7,8].
A variety of technologies is the duration of the arc. To localize heat release during welding of joints in a heterogeneous combination, it is preferable to use the CD Stud Welding technology [9,10].
The process of welding hardware CD Stud Welding is as follows, Figure 1. Details D1 and D2 are statically pressed against each other by a spring mechanism with a compression force Fcomp.=50-200N. At the end D1 there is a cylindrical protrusion of a certain diameter and height.
Energy is stored in capacitor C through charger device CD when turned on block management BM. When the thyristor VS is turned on through details D1 and D2 current pulse flows. In the contact zone of the details, heat is released according to the Joule-Lenz law. The duration of the process is 1 -5 ms. The specific pressure on the parts to be welded, created by the spring mechanism, does not exceed 10 N/mm 2 . The CD Stud Welding technology by discharging capacitor banks is recommended for welding rod elements made of dissimilar metals with a diameter of up to 12mm.
The purpose of the research is to develop the modes of capacitor welding of hardware in relation to thin-sheet panels.

Materials and methods
Sheet panels of different sizes are sheets closed with a rectangular or square hole, along the perimeter of which fasteners are welded. As a rule, panels are laser-cut from low-carbon steel sheets. The thickness of the panels does not exceed 1.5mm.
Copper-plated studs made of steel 10 are used as hardware. In the experimental part of the research, the quality of the obtained welded joints was assessed by the force to separate the stud from the sheet using a UMM 10 tensile testing machine.
The current was measured with a TE1600 device with a Rogowski coil. The pressing force of the hardware in the KP-8 pistol was measured using a DOS-0.1 dynamometer.
To develop the technology and obtain strong welded joints, the areas for determining and effectively varying the energy parameters of the process were established. The main parameters of the CD Stud Welding process and the areas of their variation are presented in Table 1 [11]. The value of the input energy W was determined by the formula: C -Capacity funded block, А; U -Working voltage, V.

Results
In the process of carrying out experimental studies, the charge voltage of the capacitor banks and the force of static pressing of the welded parts by the spring were varied. Taking into account the possibility of the equipment used -the CDP -99 installation, for a rod element diameter of 6 mm, the allowable voltage varied from 110 to 134 V with a resolution of 1V. The charge voltage of capacitor banks in experimental studies was 110 V, 122 V and 134 V. Welding was carried out using a KP-8 gun, in which, due to the rotation of the screw, the force of pressing the hardware to the panel by the spring mechanism was regulated. The experiments were carried out for three adjustment steps : 55N, 100N, and 180N.
The results of calculating the input energy and the magnitude of the recorded current depending on the charge voltage of the capacitor banks are shown in Table 2. The evaluation of the quality of welded joints of experimental samples was carried out according to the results of tests for separation [12,13].
As a result of the tests of experimental samples, the dependences of the force on separation from the input energy Ps=f(W) and effort pressing springs Fs=f(Fpres), figure 3 and figure 4. To create statistical data, 3 samples were welded by varying the charge voltage of the capacitor banks and with different spring pressure. The obtained experimental samples are shown in Figure 5.  Table 3.
The results of testing the strength of welded joints are given in Table 3 [14]. The duration of the arc burning depends on the vapor pressure of the molten metal, as well as on the tension of the spring mechanism. Scheme of the impact of pressure during the burning of the arc coverage in the coverage area of the joints in section 6.
r -heat of vaporization of the welded material, (for steel r=6300000 J/kg); I -capacitor bank discharge current, A. Pressure generated by the spring mechanism: Fpres.
The results of the assessment of the effects of pressures at different input energies are shown in Figure 7.
The total pressure Ptot. is equal to the pressure difference: specific pressure and metal vapor pressure.

Discussion
An analysis of the dependences of the strength of welded joints on the input energy showed the following. With an increase in the energy input into the joint zone, cleaning is improved, because. the area of the cleaned surfaces (from pollution, oxides) increases due to the electric arc. When the input energy is exceeded, the strength decreases, because. overheating of the joint zone occurs due to an excessive increase in specific energy, accompanied by a change in the structure in the welding zone.
An analysis of the dependences of the strength of welded joints on the static pressing force showed the following. With an increase in the static pressure of the parts to be welded by the spring, the burning time of the electric arc required to clean the surfaces to be welded decreases. With a decrease in the pressure Fcont by the spring, due to the formed metal vapor pressure Pv.m., the duration of cleaning increases.
An external examination showed that with increasing Pcont, samples 6, 7, 8 -the volume of molten metal and the area of the near-weld zone with displaced oxides decrease, while the strength decreases.
A change in the charge voltage of capacitor banks in a quadratic dependence affects the input energy and, accordingly, the magnitude of the current, which determines the volume of near-surface melting of the metal.
It was found that with an increase in the charge voltage of the capacitor banks, the protrusion at the end did not completely burn out, it was partially deformed, but the strength of the welded joint, together with the deformation of the plates, also increased [15].
In welded joints with low strength characteristics, penetration was observed not over the entire cross-sectional area of the hardware [16].

Conclusions
On the basis of similar discharge-pulse processes, a theoretical analysis of the production of CD Stud Welding permanent joints was carried out, a hypothesis was put forward, consisting in explosive evaporation of the relief at the end of the rod element, cleaning of the surfaces to be joined due to the burning of an electric arc in metal vapors, joint crystallization of the surfaces to be welded in the liquid phase.
The experimental studies of the CD Stud Welding process made it possible to determine the parameters of the modes that have a significant impact on the quality of the welded joint: the charge voltage of the capacitor banks, the pressing force of the welded parts.