RU2311273C2 - Heat release at contact spot welding automatic measuring and controlling method - Google Patents

Abstract

FIELD: contact welding processes and equipment, namely automatic control of welding machines.

SUBSTANCE: method comprises steps of determining power coefficient cosφ in each period of welding current and value of heat release in zone electrode-electrode. Value of heat release is calculated according to expression q_i = (I₂)² x r _el-el x 0.02, where r_el-el - resistance of zone electrode-electrode; I₂ -acting current value in secondary circuit. If calculated value differs from preset one, decision for performing correction of thyristor ignition angle in next period of welding current is assumed. Angle of thyristor ignition is determined according to given formula α_i+1=α_i·(√q/q_i) +(a₀/a₁)·(√q/q_i-1) where q - preset heat input for period; a_{1 ,} a₀ - coefficients determined empirically on base of power coefficient; α_i - thyristor ignition angle in current period.

EFFECT: improved quality of spot-welded joint having predetermined geometry size regardless of state of electrode and welded part surface in the result of stable heat release in interval electrode-electrode.

2 cl, 3 dwg, 1 ex

Description

The invention relates to the field of resistance welding and can be used for automatic control and control of resistance spot welding machines.

Obtaining a welded joint with stable quality in the process of resistance welding is possible while ensuring control over the geometric parameters of the spot-welded joint, which are the main criterion for its quality. Moreover, these parameters depend on the amount of heat released during welding in the electrode-electrode gap.

A known method of automatic measurement and regulation of electric heating, which includes measuring the thermo-EMF between the welding electrodes and the outer surfaces of the parts, calculating the temperature of the welding zone and adjusting the mode of electric heating [USSR Author's Certificate No. 764898, cl. B23K 11/24, 1980].

This method allows to increase the temperature stability directly in the welding zone, however, it does not allow to control the penetration depth of the parts to be welded, which does not eliminate the risk of partial incomplete penetration or gluing of parts. In addition, this method (and other similar methods) does not take into account the heating of welding electrodes during prolonged intensive work, which leads to a decrease in thermo-EMF and a decrease in the accuracy and quality of control.

A known method of quality control of contact welding, in which measure the current values of the parameters of the welding mode and calculate the geometric dimensions of the core of the weld point and the depth of mutual penetration of parts using the mathematical model of the process [USSR Author's Certificate No. 550253, class. BK 11/24, 1977].

This method allows you to directly control and adjust the size of the spot-welded joint, however, it has a significant error due to a possible violation of the correspondence between the values of the region of the controlled process parameters and the domain of the model when the welding conditions change. In addition, this method provides for the simultaneous measurement of the welding force, welding current and heat dissipation in the welding section, which significantly complicates the control system constructively.

Closest to the invention according to the technical solution, there is a method for controlling the welding current in contact spot welding on single-phase machines, which consists in determining the turn-on angle of the thyristors of the welding machine, depending on the obtained value of the heating value N, taking into account the effective value of the welding current, the complex resistance of the welding circuit and the network voltage at that, at each moment of time, the power factor cosφ is determined, and the value of the heating value is calculated by the formula

where N _e and cosφ _e are the heating value and the power factor of the system, determined during welding in the absence of disturbing factors [USSR Author's Certificate No. 1611642, cl. B23K 11/24, 1990].

This method allows to improve the quality of welding by compensating for disturbing factors in the welding circuit, but does not exclude lack of penetration during electrode wear due to a decrease in current density, and cannot prevent splashes in conditions of deterioration in surface preparation due to increased heat generation in the electrode-electrode gap.

The problem to which the invention is directed is to improve the quality of contact spot welding by obtaining the specified geometric dimensions of the spot-welded joint, regardless of the state of the electrodes and the surface of the welded parts by stabilizing heat generation in the electrode-electrode gap.

This problem is solved by the fact that in the method of automatic measurement and regulation of heat generation in contact spot welding, which in each period of the welding current provides for determining the power factor cosφ and the value of q _i heat generation in the electrode-electrode section, in the case of deviation of this value from the given decision the adjustment of the opening angle of the thyristors α _{i + 1} in the next period, the opening angle of the thyristors α _{i + 1} in the next period of the welding current is determined by the formula

where q is the specified heat release for the period; and ₁ and a ₀ are the coefficients determined empirically based on the value of cosφ; α _i - the opening angle of the thyristors in the current period.

The value of q _{i is} calculated by the formulas

where r _e-e is the resistance of the electrode-electrode section; I ₂ - the current value of the current in the secondary circuit; R2 and X ₂ are the total active and inductive resistances of the secondary circuit; U ₂₀ - secondary open circuit voltage of the welding transformer; K _i - coefficient of regulation of the welding current.

The coefficients a ₁ and a ₀ can be determined depending on cosφ from the following expressions:

It should be noted that to determine the heat release q _{i, a} family of approximating dependencies can be obtained, the use of which will simplify the calculations according to formulas (1) ... (5) when they are carried out on a microprocessor control system.

Calculation of heat in the current period of the welding current using formulas (2) ... (5) allows you to obtain data on the thermal situation in the electrode-electrode section without using direct measurements, which greatly simplifies the control process.

The decision to adjust the opening angle of the thyristors α _{i + 1} in the next period of the welding current using formula (1) allows, by taking into account disturbing influences, to stabilize the heat generation in the electrode-electrode gap.

The determination of the coefficients a ₁ and a ₀ depending on cosφ from expressions (6) and (7) makes it possible to calculate K _i with an error of up to 2%, which is sufficient to ensure the control of the contact spot welding process.

Thus, the calculation in each period of the welding current of heat generation in the electrode-electrode gap and adjusting the opening angle of the thyristors according to the proposed formulas allows to improve the quality of the spot-welded joint, to avoid lack of fusion and splashes regardless of the quality of the surface preparation and the degree of wear of the electrode working surface (an increase is possible diameter up to 50% of the initial).

The invention is illustrated by drawings, in which:

figure 1 is a functional diagram of the regulation of heat during resistance spot welding;

figure 2 is a graph of changes in the values of r _e-e resistance in the gap electrode-electrode during the welding cycle by the example of spot welding of 08kp mild steel with a thickness of 1 + 1 mm;

figure 3 - changes in the calculated values of the amount of heat during the welding cycle in the same example.

The method is implemented on standard resistance welding machines operating from an industrial frequency alternating current network and containing (Fig. 1) a welding transformer 1, the primary winding of which is connected to the network through a thyristor unit 2, the operation of which is controlled by the welding regulator 4 through the ignition unit 3. The control system is also equipped with a feedback sensor 5, which measures the value of the power factor cosφ.

The method of automatic measurement and regulation of heat during contact spot welding is as follows.

Before welding, the regulator 4 is laid predetermined total active R ₂ and X ₂ inductive secondary circuit resistance of the secondary voltage U the transformer _20, the idle stroke and the duration t _{communications} flow of the welding current and a predetermined value q for a period of heat welding current. During the welding process, the sensor 5 measures the value of the power factor cosφ in the positive half-cycle of the welding current and sends a signal to the regulator 4. During the negative half-period of the welding current, the regulator 4, using the formulas (2) ... (5), calculates the amount allocated in the welding gap for the given period heat q _i and by the formula (1) calculates the required angle α _{i + 1 of the} opening of the thyristors for the next period.

Example. When welding sheet parts of low-carbon steel with a thickness of 1 + 1 mm on a contact welding machine of the MT-4017 type, the following values were entered into the regulator: the total active resistance of the circuit R ₂ = 60 μOhm, the total inductive resistance of the circuit X ₂ = 180 μOhm, the secondary open circuit voltage stroke U ₂₀ = 2.50 V, the specified heat release for the period of the welding current q = 180 J and the welding time t _St = 0.12 s (6 periods). The change in resistance r _{uh of the} electrode-electrode gap (Fig. 2) during the welding process with a nominal diameter of the working surface of the electrodes equal to 5 mm and increased due to wear diameter equal to 8 mm is represented by curves 1 and 2, respectively. q _i for the period of the welding current (figure 3) using the proposed method (curves 1 and 2) allows to obtain high-quality connections in both cases. At the same time, welding without the use of heat stabilization (curves 1 'and 2') is accompanied by a decrease in heat during worn electrodes.

The criterion for the quality of the process in this case is the absence of traces of metal splash from the weld point and from under the welding electrodes, the formation of a zone of mutual melting with a diameter of at least nominal for a given thickness of samples.

After completion of the welding process, a visual inspection of the samples showed no signs of splashes. The destruction of the samples showed the formation of the required in accordance with GOST 15878-79 zone of mutual melting of parts on all samples, regardless of the quality of the surface preparation of the parts to be welded and the wear of the welding electrodes for samples welded using the proposed method. And on samples welded with worn electrodes and at fixed values of phase adjustment, the formation of a spot-welded joint occurred with the formation of lack of penetration.

Thus, the proposed method for the automatic measurement and regulation of heat generation in contact spot welding allows you to stabilize the amount of heat generated in the welding gap, to avoid fusion and splashes and to obtain high-quality spot-welded joints regardless of the welding machine, preparation of the surface of the welded parts and wear of the working surface of the welding electrodes up to 50% of the nominal.

Claims (2)

1. A method for automatically measuring and regulating heat generation in contact spot welding, which provides for determining the power factor cosφ and the value of heat generation in the electrode-electrode section q _i in each period of the welding current, if this value deviates from the given one, a decision is made to adjust the opening angle of the thyristors in the next period α _{i + 1} , characterized in that the opening angle of the thyristors in the next period α _{i + 1 of the} welding current is determined by the formula

where q is the specified heat input for the period, and ₁ and a ₀ are coefficients determined empirically based on cosφ, α _i is the opening angle of the thyristors in the current period, while the value of q _{i is} calculated as q _i = (I ₂ ) ² · r _e-e · 0.02;

K _i = a ₁ · α _i + a ₀ , where r _ee is the resistance of the electrode-electrode section, I ₂ is the effective current value in the secondary circuit, R ₂ and X ₂ are the total active and inductive resistances of the secondary circuit, U ₂₀ is the secondary idle voltage of the welding transformer, K _i is the coefficient regulation of the welding current, and the coefficients a ₁ and a _{0 are} determined from the following expressions: a ₀ = 2.138 cosφ ² -3.443 cosφ + 2.872; a ₁ = -0.900 cosφ ² + 1.490 cosφ-1.173. 2. The method according to claim 1, characterized in that the welding current regulation coefficient K _i is determined with an error of up to 2%.

Images