Electric heating – Metal heating – By arc
Reexamination Certificate
2001-01-26
2003-01-28
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S130210, C219S130330
Reexamination Certificate
active
06512200
ABSTRACT:
FIELD OF INVENTION
The present invention relates generally to welding and more specifically to a control system for use in arc welding operating in the short circuit (or dip) transfer mode.
BACKGROUND OF THE INVENTION
In a typical arc welding system operating in the dip transfer mode, a welding circuit is established which includes a consumable electrode, a workpiece and a power source, The electrode is generally a solid wire and not only conducts the electric current and sustains the arc, but also melts and supplies filler material into the joint. A shielding gas such as carbon dioxide or blends of argon with carbon dioxide and/or oxygen may be supplied during the welding process to support the arc and prevent the molten metal reacting with oxygen and nitrogen in ambient air.
During the arcing phase of the welding cycle, a molten metal droplet forms on the end of the electrode. As the electrode is advanced from the contact tip, the metal droplet engages the molten metal pool formed in the workpiece creating a short circuit. The arc is actually quenched at this point and the current rises at a rate determined by the power source characteristics. The increase in current causes an electromagnetic pinch force to be applied to the molten droplet material that forms a bridge between the electrode and the workpiece. The applied pinch force assists in the promotion of the bridge rupturing, so that molten material is transferred to the workpiece and the arc is re-established. However, the high current at the time of rupture often causes the bridge to rupture with an explosive force, thereby resulting in welding spatter. Spatter is undesirable in the welding process as it diminishes the weld quality and results in additional cleaning of the weld site, thereby increasing both the cost and time of production of the weld.
Sophisticated power sources have been developed with spatter control systems which minimise the spatter by ensuring the current is turned off immediately before an impending bridge rupture is detected. However, these control systems are not widely applicable as the majority of power sources are not capable of switching the current off fast enough prior to rupturing occurring.
SUMMARY OF THE INVENTION
An aim of the present invention is to provide a control method and system for use in welding which can reduce spatter and which is able to be used with conventional power supplies, A further aim of the invention is to provide a welding power supply incorporating this improved control method and system.
In a first aspect, the invention provides a method of controlling an arc welding system operating in the dip transfer mode, the welding system including a power source and a consumable electrode which in operation is operative to be advanced into contact with a workpiece, the welding system being operative to create a welding circuit which is energised by said power source and which has a welding cycle comprising an arcing phase where the electrode is spaced from said workpiece and an arc is generated across said space, the arc being operative to form a molten droplet on the end of the electrode, and a short circuit phase where the electrode is in contact with said workpiece, the welding cycle changing from the arcing phase to the short circuit phase on contact of the molten droplet with the workpiece, and changing from the short circuit phase to the arcing phase after rupturing of a bridge of molten material formed between said electrode and said workpiece, the method including the steps of:
(i) conditioning the welding system to form a molten droplet on the electrode end during the arcing phase which is above a predetermined threshold size so that bridge rupturing can occur during the short circuit phase without requiring high current during the short circuit, and
(ii) controlling the current output from the power source during the short circuit phase.
In a second aspect, the invention provides a system for controlling an arc welding system operating in the dip transfer mode, the welding system including a power source and a consumable electrode which in use is operative to be advanced into contact with a workpiece, the welding system being operative to create a welding circuit which is energised by said power source and which has a welding cycle comprising an arcing phase where the electrode is spaced from said workpiece and an arc is generated across said space, the arc being operative to form a molten droplet on the end of the electrode, and a short circuit phase where the electrode is in contact with said workpiece, the welding cycle changing from the arcing phase to the short circuit phase on contact of the molten droplet with the workpiece, and changing from the short circuit phase to the arcing phase after rupturing of a bridge of molten material formed between said electrode and said workpiece, the control systems including:
(i) conditioning means operative to condition the welding system to form a molten droplet on the electrode end during the arcing phase which is within a predetermined threshold size range so that bridge rupturing can occur during the short circuit phase without requiring high current during the short circuit, and
(ii) current control means operative to control the current output from the power source during the short circuit phase,
In accordance with the present invention, the control system uses the size of the droplet to obviate the need for a high current to initiate bridge rupturing. In the present system, because of the size of the droplet, rupturing is able to result primarily from the surface tension at the droplet-pool interface. Therefore, with the control system, the current is able to be maintained at a relatively low level throughout the short circuit phase. In this regard, it has been found that utilising the systems and methods of the invention, bridge rupturing can occur using current levels of 30-40% of the natural short circuiting current. As the natural short circuiting current is typically in the vicinity of 350 amps, using the present invention bridge rupturing may occur in the vicinity of 150 amps.
A control system according to the present invention is therefore able to significantly reduce spatter. Further, the system has the substantial benefit over existing spatter control systems as it does not require fast “turn-off” of the current prior to bridge rupture to reduce spatter and is therefore capable of being used on a wide range of power sources.
The molten droplet formed during the arcing phase should not be excessively big as this can lead to process instability. In particular, if the droplet is too big during the arcing phase, it may shift from a central position on the electrode tip which will adversely affect its positioning in the contact area of the workpiece and may also lead to localised arcs forming particularly when CO
2
shielding gas is used. Further, if the droplet is too big, it may separate prematurely from the electrode.
In a preferred form, the control system is operative to condition the welding system to produce a molten droplet within the threshold size range by applying a current pulse during the arcing phase. This current pulse is specifically designed to increase the size of the molten droplet but is less than required to cause detachment of the droplet from the electrode. Preferably following the current pulse, the current is maintained during the arcing phase at a level which will produce the required heat input to ensure that the droplet remains molten and workpiece plate fusion occurs.
Preferably during the short circuit phase, the current is maintained or clamped below a predetermined level. If required, the current may be reduced after the short circuit is detected to assist the wetting in of the molten droplet. Once the wetting in period is completed, the current may then be increased to its clamp level where it is held until the bridge ruptures. After detecting the rupture, the welding process enters the subsequent arcing phase where the current pulse is again applied to grow the droplet to
Cook Christopher David
Cuiuri Dominic
Dean Gary
Norrish John
CRC for Welded Structures, Limited
Shaw Clifford C.
Wolf Greenfield & Sacks P.C.
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