Electric heating – Metal heating – By arc
Reexamination Certificate
1999-08-03
2001-07-03
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121450, C219S121590, C219S13700R, C219S137200
Reexamination Certificate
active
06255618
ABSTRACT:
TECHNICAL FIELD
The present invention relates to plasma arc welding, and more particularly to a plasma arc welding technology suitable for welding a metal, the surface of which is coated with a substance, which has a boiling point that is lower than a melting point of a base metal.
BACKGROUND ART
Plasma arc welding, for example, plasma arc spot welding is being utilized in a variety of fields. This is because plasma arc welding enables metal workpieces laid one on top of the other to be welded together from one side, which is advantageous in that it enables the welding of an intricately-shaped workpiece, and a large workpiece.
However, conventional plasma arc welding proves problematic when welding a metal coated on a surface with a substance that has a lower melting point than a melting point of a base metal, such as galvanized sheet steel, in that satisfactory welding results cannot be obtained because a surface coating material vaporizes, and a vapor thereof penetrates a molten pool of a weld causing an explosion, and/or leaving a hole in a weld.
To solve for this problem, the applicant of the present case proposed in Japanese Patent Application No. 6-061138 a welding method, wherein 2 steps of vapor escape and main welding are performed consecutively by adjusting the power of a plasma arc. That is, initially, using a relatively high power plasma arc, a through-hole is bored in at least one of the steel sheets to be welded together, and a vapor of a coating material is allowed to escape via this through-hole. Next, the power of a plasma arc is lowered to an appropriate level, and two steel sheets are welded.
In the welding method of Japanese Patent Application No. 6-061138, when there is a gap between 2 steel sheets to be welded, because a molten metal is expended in the gap, there are cases in which a vapor escape hole cannot be flattened by subsequent welding steps, or high weld strength cannot be achieved, or, when a gap is large, welding itself cannot be performed. According to a test of the inventors, it was determined that welding substantially cannot be performed when a gap between steel sheets exceeds around 0.2mm.
Therefore, an object of the present invention is, in plasma arc welding, to make it possible to achieve satisfactory welding even when there is a gap between welding materials.
An additional object of the present invention is, in plasma arc welding, to make a vapor of a low-boiling-point material coating a welding material effectively escape, and to make it possible to achieve satisfactory welding even when there is a gap between welding materials.
And yet an additional object of the present invention is, in plasma arc welding, make it possible to measure a gap between welding materials automatically.
DISCLOSURE OF THE INVENTION
In plasma arc welding in accordance with a first aspect of the present invention, there is performed a boring step for boring a hole in work, and subsequent thereto, a plasma arc is controlled so as to perform a main welding process for welding the work, and a filler is supplied to the plasma arc at the least when a main welding process is performed.
According to the plasma arc welding thereof, since a vapor of a coating material can effectively escape via a hole bored in the work, and in a main welding process of subsequent thereto, a molten filler is supplied to a hole, satisfactory welding results can be obtained by effectively filling a gap and a hole with filler. Furthermore, a vapor escape hole can either be bored in only one, or can be bored in all of a plurality of welding materials of the work.
In a preferred embodiment, a filler is not supplied in a boring step, but rather a filler is supplied only at a main welding process. This is so a hole can be bored rapidly.
A filler can be supplied in accordance with either a previously-determined feed quantity or feed time. For example, a filler feed quantity or feed time which conforms to an anticipated maximum gap size is set beforehand, and a filler can be supplied in accordance therewith. Or, a filler feed quantity or feed time can be determined in accordance with the size of a gap between pieces of the work each time welding is performed, and filler can be supplied in accordance therewith.
And as well as filler feed quantity or feed time, another welding condition (for example, arc current, arc voltage, plasma gas flow, plasma gas type, shielding gas flow, shielding gas type, standoff, or the like) can be changed in accordance with the size of the gap. In that case, the various welding conditions to be adjusted can also be collectively determined in accordance with the size of the gap (for example, selecting one set from among a variety of previously-prepared sets of welding conditions that correspond to gap sizes).
When welding conditions are determined in accordance with gap size, it is necessary to measure some physical value or another related to gap size to find out the size of the gap. For example, a plasma arc voltage value at the end of boring, or a rise rate of a plasma arc voltage during boring can be cited as a physical value therefor. In the case of the latter, the above-mentioned measurement and condition determination can be processed in parallel with boring.
When measuring the rise rate of the arc voltage to find out the size of the gap, a plurality of arc current pulses can be repeatedly made to flow during boring. And then, if an arc voltage is measured at individual points in time of the series of current pulses, and the rise rate thereof is calculated, since this rise rate constitutes a value that favorably reflects the gap size, it is relatively easy to find out the size of the gap.
When implementing this method using current pulses, the shielding gas in the boring process can be controlled so as to be slightly less than same in the main welding process. By so doing, because the change in an arc voltage rise rate in accordance with a gap size becomes large, it becomes easy to accurately find out the size of the gap.
In general, with plasma arc welding, the change of an arc voltage resulting from electrode wear during a single welding operation is so small that it can be ignored, enabling a stable arc voltage to be achieved during welding. Further, in plasma arc welding, because a filler must be provided separate from an electrode, the control of a filler feed quantity can be performed independent of the control of a plasma arc, which is a heat source. Therefore, in plasma arc welding, gap size can be accurately grasped on the basis of arc voltage, a filler feed quantity can be optimally controlled in accordance with that gap size, and as a result thereof, satisfactory welding results can be obtained.
In plasma arc welding according to a second aspect of the present invention, a filler feed quantity or feed time is determined in accordance with the size of a gap between workpieces, and a plasma arc is generated for welding the workpieces, and then, a filler is supplied to the plasma arc in accordance with the determined feed quantity or feed time.
According to the plasma arc welding thereof, because a filler of an amount that accords with the gap size between pieces of work is supplied to a welding location during welding, the gap is effectively filled up by the filler, making it possible to obtain satisfactory welding results.
When this plasma arc welding is performed on work coated with a low-boiling-point substance, it is desirable to bore a hole for vapor escape (using either a plasma arc, or a drill, or some other boring machine) at the welding location of the work either before welding or during welding. In this case, according to this plasma arc welding, the gap and hole are filled up by the filler, enabling satisfactory welding results to be obtained. Further, when welding pieces of work not coated with a low-boiling-point substance, there is no need to bore a vapor escape hole, but in this case as well, since a filler of an amount that accords with a gap size can be supplied to a molten pool at the welding location in accordance with this plasma
Imai Yohsuke
Kamiya Kengo
Kurokawa Iwao
Niigaki Yoshitaka
Shintani Toshiya
Gallagher & Lathrop
Komatsu Ltd.
Lathrop David N.
Paschall Mark
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