Pulse welding equipment and its control method

Abstract: Comprising means for feeding a welding wire, means for feeding a pulse current between the welding wire and base metal, and means for controlling this pulse current feeding means, the control means controls the pulse current feeding means so as to flare up the welding wire upon arc starting, and then controls the pulse current feeding means until the pulse current becomes lower in frequency than the pulse frequency of the main welding current.

Description: The present invention relates to pulse welding equipment for welding by using a welding wire and outputting pulse peak current and base current alternately, and a method of controlling the same. BACKGROUND OF THE INVENTION A control method of pulse welding equipment of a prior art disclosed in Japanese Laid-open Patent No. 4-270069 is described below while referring to Fig. 4A and Fig.4B. Fig. 4A shows the lapse of time relating to the position of the welding wire tip end upon arc starting and the arcing. When forming an arc 4 between a welding wire 2 supplied to a nozzle 1 at the tip end of a welding torch and a base metal 3, it shows the process consists of the following subsequent steps: (a) a state before start of welding, (b) a short-circuited state of welding wire 2 and base metal 3 just before arc starting, (c) a flare-up state of welding wire 2, (d) a transitional state until reaching a stable state of appropriate arc length for main welding, and (e) the stable state of appropriate arc length for main welding. On the other hand, Fig. 4B schematically shows the lapse of time of the welding current corresponding to Fig. 4A, that is: the state (b) just before arc starting is immediately followed by a hot current output period (TH period), an initial current output period for flaring up the welding wire 2 continues (TINI period), being followed by a transition period to main welding current by changing from high frequency to low frequency in gradual steps (TSLP period), and a main welding current output period (TW period). The welding currents outputted in each period of TH, TINI, TSLP, and TW are represented by IH, IINI, ISLP, and IW respectively. In such conventional control method of pulse welding equipment, in the process of transition of arc length from state (c) to state (e), the pulse welding frequency is changed in gradual steps from high frequency to the frequency of main welding state, and in the TSLP period (d), the arc length is gradually shortened. It helps to lower the incidence of short-circuiting between the welding wire and base metal in the course of transition from right after arc starting until the arc length of main welding condition, and decrease the generation of spatter by short-circuiting. However, in the conventional control method of pulse welding equipment, because of the provision of TSLP period for changing from high frequency to low frequency in gradual steps, it took a long time until the arc was stabilized at a proper length for main welding. By this method, generation of spatter by short-circuiting is decreased. But, to the contrary, spatter was generated due to abnormal growth of droplets at the tip end of the welding wire 2. Further, as the welding wire 2 flares up, the state of long arc length continues for a while, and the area of the arc 4 contacting with the base metal 3 is wider. Therefore the deposition quantity of the wire 2 on the base metal 3 in unit area decreases, and the heat affected zone of the base metal 3 is widened. Document GB-A- 2269949 discloses a pulse welding apparatus. The welding current is formed by superimposing a pulse current on a continuous base current. A separation detector provides a separation signal when a molten droplet separates from the tip of a wire electrode. An integrator measures the amount of charge of the pulse current from a point in time coincident with the separation signal and a comparator cuts off the pulse current when the measured charge amount is equal to a predetermined valve. The current pulses may be arranged in groups, each group causing separation of one droplet and being terminated by the comparator. The melting and separation of droplets from the welding wire is arranged to occur in a regular manner and to be controllable to affect good welding. SUMMARY OF THE INVENTION The pulse welding equipment of the present invention comprises: means for feeding welding wire; and means for feeding welding current between said welding wire and a base metal; characterised by means arranged to control said welding current, including an initial current period pulse frequency command unit arranged to output welding current at a first frequency immediately after an arc is formed and during an initial current output period; means arranged to output welding current at a second frequency, lower than said first frequency during a main welding transitional current output period; a timer arranged to determine a duration of said initial current output period and said main welding transitional current output period, and a main welding pulse frequency command unit arranged to output welding current at a main welding frequency, higher than said second frequency during main welding; wherein immediately after an arc is formed, said means for controlling causes said welding current to transition from said first frequency to said second frequency lower than said first frequency, and then to said main welding frequency higher than said second frequency. The present invention provides a method of pulse welding comprising the steps of: feeding a welding wire; and feeding welding current between said welding wire and a base metal; characterised in that immediately after an arc is formed, causing said current transition from a first frequency to a second frequency lower than said first frequency, and then to a main welding frequency higher than said second frequency. From flare-up of welding wire following arcing when starting welding, until output of main welding current, it is designed to output a pulse current of lower frequency than the pulse frequency of main welding current. The present invention therefore provides pulse welding equipment for starting an arc smoothly, and its control method. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A is an explanatory diagram of lapse of time relating to the position of the welding wire tip end upon arc starting and the arcing in embodiment 1 of the invention; Fig. 1B is a schematic diagram of lapse of time of welding current upon arc starting in embodiment 1 of the invention; Fig. 2 is a block diagram of pulse welding equipment in embodiment 4; Fig. 3 is a block diagram of pulse welding equipment in embodiment 5; Fig. 4A is an explanatory diagram of lapse of time relating to the position of the welding wire tip end upon arc starting and the arcing in a prior art; and Fig. 4B is a schematic diagram of lapse of time of welding current upon arc starting in the prior art. DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS Referring now to the drawings, preferred embodiments of the invention are described in detail below. Embodiment 1 Fig. 1A shows the lapse of time relating to the position of the welding wire tip end upon arc starting and the arcing in embodiment 1, and Fig. 1B schematically shows the lapse of time of welding current corresponding to Fig. 1A. Same elements as in the prior art shown in Fig. 4A and Fig. 4B are identified with same reference numerals and detailed description is omitted. Fig. 1A and Fig. 1B show the arc starting state. In Fig. 1A and Fig. 1B, state (a): first a welding wire 2 is fed to a base metal 3 at low speed, state (b): it contacts with the base metal 3 in a hot current output period (TH period), and an arc is formed, state (c): the welding wire is flared up in the initial current output period (TINI period), state (d): the welding wire melt speed is decreased in the transitional period to main welding current (TLQ period) for controlling the welding output at a lower pulse frequency than the pulse frequency in the main welding current output period (TW), and state (e): the TW period at pulse frequency of main welding current output time (hereinafter called main welding) starts. What this embodiment 1 differs from the prior art lies in the following points: (a) there is the TLQ period for welding at a lower pulse frequency than the pulse frequency of main welding, and (b) the welding wire feed speed at this time is slower than the main welding speed until TH period and TINI period, and is changed to the feed speed of main welding time after TINI period, in TW period including TLQ period. By this means, the arc length is (a) long in the TINI period as the welding wire 2 flares up, (b) suddenly becomes short in the TLQ period, and (c) is appropriate in the TW period. Accordingly, the time from the welding wire 2 contacting with the base metal 3 until the arc is stabilized at an appropriate length is shorter, and smooth arc starting is realized. Further, since the period of the state (c) of long arc length is shorter, the heat affected zone of the base metal 3 decreases. As the period of flare-up state of the welding wire 2 is shorter, moreover, the contacting area of the arc 4 with the base metal 3 is narrower, and the arc 4 is concentrated on the base metal 3, so that the deposition quantity in the unit area of the base metal 3 is increased. In addition, it is effective to decrease the spatter due to abnormal growth of droplets at the tip end of the welding wire 2. For example, when the welding wire material is aluminum, the welding wire diameter is 1.2 mm, and the output current is 60 A to 90 A, in the prior art, it takes about 420 ms from contact of the welding wire with the base metal until the arc is stabilized at an appropriate length; whereas in embodiment 1 it takes about 300 ms, and the time until stabilization is shortened, and the arc length is changed smoothly. Still more, as the welding wire flares up and the period of long arc length is shortened, the period of excessive output voltage is shortened, and the heat affected zone of the base metal is narrower. In the example presented herein, the changeover time of pulse frequency is, in the case of output current of 70 A, TH period of 15 ms, TINI period of 90 ms, and TLQ period of 200mns. The pulse frequency in the TLQ period is about half pulse frequency of the pulse frequency in the main welding time. (Evaluation review 1) Embodiment 1 of the invention was evaluated. Results are discussed while referring to Table 1. Table 1 shows the best results obtained by current command value. Current command value [A] ∼60 60 ∼100 100 ∼160 160∼ Welding wire diameter [mm] 1.2 Pulse frequency 0.75 0.50 0.60 0.80 Duration (ins) 100 200 150 150 1.0 Pulse frequency 0.90 0.60 0.70 0.80 Duration (ms) 100 150 140 150 In Table 1, the welding wire material was aluminum, and welding was done at welding wire diameter of 1.2 mm and 1.0 mm. The operation was fillet welding. This is the evaluation result of pulse frequency in the TLQ period and duration of TLQ period in Fig. 1B at each current command value in this case. The numerical value of the pulse frequency in Table 1 shows the rate when the pulse frequency at the main welding time is supposed to be 1. In the case of higher frequency than the pulse frequency of TLQ period recorded in Table 1, it is known that the effect of the invention is reduced. The effect of the invention was also sacrificed when the duration was set shorter than the TLQ period in Table 1. Or, when set shorter than the pulse frequency of the TLQ period in Table 1 or set longer than the duration of TLQ period, it is known that short-circuiting is more likely to occur between the welding wire and the base metal, possibly causing spattering. In the prior art of control method of pulse welding equipment, there is a current region likely to induce flare-up depending on the current command value, and the arc length varies when flaring up. Depending on the welding wire diameter, moreover, the welding wire feed speed varies, and a similar flare-up current region exists, and the arc length varies when flaring up. It is hence known from Table 1 that the effects are enhanced by varying the duration of TLQ period and pulse frequency depending on the output current. (Evaluation review 2) Embodiment 1 of the invention was further evaluated from a different viewpoint than in Evaluation review 1. Results are explained by referring to Table 2. Pulse frequency 0.5 1.0 2.0 Bead appearance Δ o ○ × Bead shape Δ o ○ ○ Vibration × ○ o ○ Table 2 is an evaluation table by varying the pulse frequency of TINI period in Fig. 1B, at welding wire diameter of 1.2mm, using aluminum wires. The pulse frequency in Table 2 shows the rate when the pulse frequency in main welding condition is supposed to be 1. The evaluation in Table 2 shows the bead appearance (heat affected zone) upon arc starting, bead shape (penetration), and vibration transmitted to the torch due to contact between the welding wire 2 and the base metal 3. The evaluation is ranked in four levels: excellent (o ○), good (○), fair (Δ), and poor (×). The pulse frequency and duration of the TLQ period in Fig. 1B were evaluated according to Table 1. The duration of TH period in Fig. 1B was fixed at 15 ms, and the duration of TINI period was 90 ms, and fillet welding was conducted. It is known from Table 2 that the starting bead appearance and bead shape are improved when the pulse frequency of the TINI period is set at the pulse frequency of the main welding as compared with other frequency. Embodiment 2 Embodiment 2 of the invention is described below while referring to Fig. 2. This embodiment relates to a control method of starting the arc of pulse MIG (Metal Inert Gas) welding equipment by detecting the arc voltage, controlling the peak width or base width, and maintaining the arc voltage at a constant level. In Fig. 2, the pulse MIG welding equipment of this embodiment comprises: (a) a driving circuit 18, (b) a current detector 25 for detecting the welding current output, (c) a voltage detector 10 for obtaining the welding voltage output, (d) a first switching element 11 for driving by obtaining a current detection signal from the current detector 25, (e) a hot current command unit (IH) 12, (f) a pulse peak current control unit (IP) 14, (g) a base current control unit (IB) 15, (h) an output voltage command unit (VA) 17, (i) a main current transitional period pulse frequency command unit (ILQ) 20 in the TLQ period in Fig. 1B, (j) a timer (TM) 22 for determining the duration of TINI period and TLQ period in Fig. 1, (k) an initial current period pulse frequency command unit (IINI) 23 in the initial current output period of TINI period, (l) a main current pulse frequency command unit (IW) 24 for outputting current during main welding, (m) a second switching element 21 and a third switching element 19 driven by the TM 22, (n) a pulse waveform operator 16 for selecting the IP 14 or IB 15 by the feedback value of output voltage command unit 17 and voltage detector 10 and the output signal of the third switching element 19, (o) a fourth switching element 13 driven by the pulse waveform operator 16, and (p) a welding wire feeder 100 for feeding a welding wire 2 to a base metal 3 through a nozzle 1. In this constitution, the driving circuit 18 is composed of: (a) a three-phase alternating-current power source 5, (b) a primary diode 6 for rectifying alternating-current voltage, (c) a switching element 7 for converting the rectified direct current into an alternating-current output, and driving the welding output, (d) a transformer 8 for transforming the alternating-current output, and (e) a secondary diode 9 for rectifying the transformed alternating-current output. In thus constituted pulse MIG welding equipment of this embodiment 2, the operation is described below. First, when starting welding, a no-load voltage is applied between the welding wire 2 and the base metal 3 by the operation of the driving circuit 18. During output of no-load voltage, the first switching element 11 selects the hot current command 12. Next, when the welding wire 2 contacts with the base metal 4, the current detector 25 detects the output current, and sends a signal to the first switching element 11. The first switching element 11, receiving a signal from the fourth switching element 13, sends it out to the switching element 7 to drive it. It is the TH period in Fig. 1 from the time of the welding wire 2 contacting with the base metal 3 until the first switching element 11 is changed over. Then, the TH period is changed over to the TINI period. In the TINI period, the fourth switching element 13 receives a signal from the pulse waveform operator 16, and outputs IP 14 and IB 15 by changing over alternately. At this time, by the signal of the TM 22, the second switching element 21 issues an output signal of the IINI 23, and the third switching element 19 issues a command signal sent out from the second switching element 21. Consequently, the duration of the TINI period is counted by the TM 22, and upon lapse of a specific time, the third switching element 19 selects the ILQ pulse frequency signal 20, and sends a signal to the pulse waveform operator 16. At this time, the TINI period is changed over to the TLQ period. In the TLQ period, the pulse waveform operator 16 receives the ILQ pulse frequency signal and the fed-back signal, and determines the peak period and the base period. The pulse frequency during the TLQ period is the frequency of the output signal of the ILQ 20. Then, the duration of the TLQ period is counted by the TM 22, and after a specific period, the third switching element 19 is changed over to the signal issued from the second switching element 21, and the second switching element 21 selects the output signal of IW 24 which is the main welding pulse frequency. At this time, the TLQ period is changed over to the TW period. In this operation, the hot current is outputted in the TH period, the IINI 23 is outputted in the TINI period, and ILQ pulse frequency is outputted in the TLQ period. The transition of arc length at this time is same as in embodiment 1. By fixing the output signal of the second switching element 21 at the IW 24, the pulse frequency in the TINI period is kept same as in the main welding. Thus, the same effects as in embodiment 1 are obtained. Embodiment 3 Embodiment 3 of the invention is described below while referring to Fig. 3. This embodiment 3 relates to a control method of starting the arc of pulse MIG welding equipment by detecting the arc voltage, controlling the pulse frequency, and maintaining the arc voltage at a constant level. Same elements as shown in Fig. 2 are identified with same reference numerals, and detailed description is omitted. In Fig. 3, reference numeral 27 is a low voltage output command unit (VIG) for issuing a low voltage command to the voltage command in the TW period during main welding in the TLQ period, and reference numeral 26 is an output voltage adjusting circuit (VW) for determining the output voltage by receiving a voltage signal from the third switching element 19. The operation is described below by reference to Fig. 3, while omitting the same operation as in Fig. 2. In the TH period, a hot current is outputted. In the TINI period, receiving the pulse frequency condition from the IINI 23, the fourth switching element 13 issues command signals to the pulse peak current command unit 14 and base current command unit 15. In the TW period, receiving the peak width from the IW 24, the pulse frequency is controlled by changing the base current period by the feedback voltage, so that the arc voltage is kept constant. Herein, in the TLQ period, the third switching element 19 selects the signal of the VIG 27, and sends a signal to the VW 26. The pulse waveform operator 16 receives the feedback signals of the VW 26 and voltage detector 10. At this time, since the pulse waveform operator 16 receives a feedback signal for setting lower than in main welding, the base period is extended so as to suppress the output voltage. The pulse frequency at this time is lower by the portion of extension of the base period. Thus, the arc starting in embodiment 3 is same as the operation in embodiment 2. At the same time, the same effects as in embodiment 1 are obtained. As clear from the description herein, according to the pulse welding equipment and its control method of the invention, since the time from the flare-up state until reaching the appropriate arc length for main welding can be shortened, transition from arc starting to main welding is smooth. Moreover, since the period of flare-up state is short, the arc is concentrated on the base metal, so that the deposition quantity per unit area can be increased.

Claims: Pulse welding equipment comprising: means for feeding welding wire (100); and means for feeding welding current (18) between said welding wire and a base metal; characterised by means arranged to control said welding current, including an initial current period pulse frequency command unit (23) arranged to output welding current at a first frequency immediately after an arc is formed and during an initial current output period; means (20, 27) arranged to output welding current at a second frequency, lower than said first frequency during a main welding transitional current output period; a timer (22) arranged to determine a duration of said initial current output period and said main welding transitional current output period, and a main welding pulse frequency command unit (24) arranged to output welding current at a main welding frequency, higher than said second frequency during main welding; wherein immediately after an arc is formed, said means for controlling causes said welding current to transition from said first frequency to said second frequency lower than said first frequency, and then to said main welding frequency higher than said second frequency. The pulse welding equipment of claim 1, wherein the speed at which said welding wire is fed by said means for feeding welding wire is changeable. The pulse welding equipment of claim 1, wherein the speed at which said welding wire is fed by said means for feeding welding wire is changed after said welding current frequency changes to said second frequency. The pulse welding equipment of claim 1, wherein said first frequency and said main welding frequency are substantially equal. The pulse welding equipment of claim 1, wherein said welding current is applied at said first frequency before an arc has been formed. The pulse welding equipment of claim 1, wherein said means for controlling causes said welding current to transition from said first frequency to said second frequency after said welding wire has been fed to contact with said base metal and an arc has been formed. A method of pulse welding comprising the steps of: feeding a welding wire; and feeding welding current between said welding wire and a base metal; characterised in that immediately after an arc is formed, causing said current transition from a first frequency to a second frequency lower than said first frequency, and then to a main welding frequency higher than said second frequency. The method of pulse welding of claim 7, further comprising the step of changing the speed at which said welding wire is fed after said current is caused to transition to said second frequency. The method of pulse welding of claim 7, wherein said first frequency and said main welding frequency are substantially equal. The method of pulse welding of claim 7, wherein said welding current is applied at said first frequency before an arc has been formed. The method of pulse welding of claim 7, wherein said current is caused to transition from said first frequency to said second frequency after said welding wire has been fed to contact with said base metal and an arc has been formed. The pulse welding equipment of claim 1, wherein said means for controlling includes a main welding transition pulse frequency command unit (20), as said means arranged to output welding current at said second frequency and said means for controlling further includes. a current detector (25) for detecting the welding current output, a voltage detector (10) for obtaining the welding voltage output, a first switching element (11) for driving by obtaining a current detection signal from said current detector, a hot current command unit (12) for issuing a hot current during a hot current output period right after short-circuiting of welding wire and base metal, a pulse peak current control unit (14), a base current control unit (15), an output voltage command unit (17), a second switching element (21) and a third switching element (19) driven by said timer, a pulse waveform operator (16) for selecting said pulse peak current control unit and base current control unit by said output voltage command signal, the feedback value obtained from said voltage detector and the output signal of said third switching element, and a fourth switching element (13) driven by said pulse waveform operator. The pulse welding equipment of claim 1, wherein said means for controlling includes a low voltage output command unit (27), as said means arranged to output welding current at said second frequency, for issuing a lower voltage command to an output voltage adjusting circuit, and said means for controlling further includes a current detector (25) for detecting the welding current output, a voltage detector (10) for obtaining the welding voltage output, a first switching element (11) for driving by obtaining a current detection signal from said current detector, a hot current command unit (12) for issuing a hot current during a hot current output period right after short-circuiting of welding wire and base metal, a pulse peak current control unit (14), a base current control unit (15), an output voltage command unit (17), a second switching element (21) and a third switching element (19) driven by said timer, a pulse waveform operator (16) for selecting said pulse peak current control unit and base current control unit by said output voltage command signal and the feedback value obtained from said voltage detector, a fourth switching element (13) driven by said pulse waveform operator, and an output voltage adjusting circuit (26) for determining the output voltage by receiving a voltage signal from said third switching element.

Inventors: HONGU TOSHINORI, UEDA AKIKO, YONEMORI SHIGEKI, HONGU, TOSHINORI, UEDA, AKIKO, YONEMORI, SHIGEKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1999
Main classification: B23K9
IPC codes: H02M9, B23K9