Electric heating – Metal heating – For bonding with pressure
Reexamination Certificate
2002-01-09
2004-07-13
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
For bonding with pressure
C219S098000, C219S130500
Reexamination Certificate
active
06762392
ABSTRACT:
The invention relates to a lift-and-strike welding process as well as to a lift-and-strike welding apparatus. The process and the apparatus are suitable in particular for welding a weld stud onto an aluminium surface or steel sheet surface, which have in each case a surface coating, e.g. a lubricant coating.
A lift-and-strike welding process, in particular a stud lift-and-strike welding process, has the advantage of industrial-scale capability combined with processing reliability with regard to, for example, reliable ignition of an arc. The lift-and-strike welding process is also less noisy than other welding processes. The lift-and-strike welding process is therefore used in numerous fields especially on account of its being economical to operate. Especially in the automobile industry, lift-and-strike welding has become an established technique. Aluminum and aluminum composite components are becoming increasingly popular as materials in the automobile industry on account of their low weight. From DE 195 244 90, for example, a lift-and-strike welding process is known, wherein an aluminum weld stud is welded to a workpiece made of aluminum. In such a process, the height of lift of the weld stud will vary depending upon the measured arc voltage. It is also known from said document that, to prevent a short circuit from being caused by melted material dripping from the weld stud, by reversing the polarity at either the weld stud or the workpiece during the welding operation. It is also known that, by reversing the polarity, the formation of the molten bath may be varied.
The object of the present invention is therefore to provide a lift-and-strike welding process and a corresponding lift-and-strike welding apparatus, with which an element may be welded reliably and with a high quality onto a surface, even if a coating should be disposed on the surface.
The lift-and-strike welding process is such that, as a first step, a surface of a component is cleaned, namely by applying a first voltage so as to strike an arc between a stud to be connected to the surface, and the surface. In a second step, a polarity of the first voltage is reversed. Then the element is welded on by means of at least one second voltage.
This process is particularly suitable for use with both steel sheets and aluminium sheets, which have either an organic or zinc coating. The zinc coating may be electro-plated or galvanised or may be Bonazink. The coating may also consist of accumulated dirt or the like. For example, it has proved particularly suitable to use the process for the welding of steel sheets having a sheet thickness of 1 to 0.5 mm and less which has a zinc protection layer, e.g. in the case of hot galvanising, of 70 &mgr;m and less, e.g. also in the case of thin zinc protection layers, of 30 to 3 &mgr;m or even less. The cleaning process is very precisely adaptable to the surface to be cleaned and is also suitable for very thin coatings. There now follows a detailed description of the mode of operation of the invention with reference to a machining of a component made of aluminium. The features described below are however also applicable to the welding of a corresponding steel sheet.
Aluminium components which are cold formed, in particular deep drawn, have a surface coating in the form of a lubricant. Said lubricant prevents cold welding between the aluminium workpiece and a machining tool. The lubricant moreover reduces the friction force which arises. An organic coating, in particular a wax or oil-based coating is often used. By striking a cleaning arc, the effect is achieved that the organically based coating as a result of overheating by the arc volatilises leaving no significant residues, in particular leaving no residue, and/or is displaced from the welding region. The subsequent actual welding operation by means of e.g. a pilot current and subsequent welding current of the lift-and-strike welding process allows the element, which is to be welded on, to be dipped into a weld pool of the aluminium surface which is not contaminated with the previous coating.
For cold-formed aluminium sheets a wax-based lubricant coating is customary. During an arc welding process, the wax releases hydrogen which would bond with the molten aluminium during the welding operation. The moment the molten aluminium hardens again, the ability of the aluminium to bond with the released hydrogen is lost. The hydrogen is exhaled and leaves behind a high porosity in the region of the joint zone. Said porosity leads to an enormous deterioration of the welding quality. Through use of the aluminium lift-and-strike welding process it is possible to avoid a poor welding quality. The process also allows its users to dispense with previous cleaning of the aluminium components used. Cold deep-drawn sheets, for example, prior to subsequent welding previously had to be sent through a washing lane in order to prepare the surface of the aluminium sheets for the welding process. Said cleaning operation is now no longer necessary. As a result, aluminium-containing components having a coating may even without basic preliminary cleaning be reliably welded e.g. with a weld stud. The quality of the weld joint therefore depends on the ambient conditions in the joint zone which are created by the cleaning arc, wherein the surface is advantageously rendered dry and metallically pure.
It is advantageous when, after the first step of applying the cleaning voltage, in a second step a polarity of the first voltage is reversed. By said means the cleaning may be influenced by altering the arc. It is further advantageous when, after the first voltage used as a cleaning voltage has dropped, the actual lift-and-strike welding process ensues after a specific period of time. The process is improved by reversing the polarity between the first and subsequent second voltage. Said reversal is effected preferably in the period of time, during which the first voltage has dropped, in particular to zero. In said case, during the cleaning phase a positive polarity is preferably adjusted for the first voltage. This means that at the aluminium surface of the component there is a negative potential, while the weld-on element has a positive potential. It is therefore possible to heat the aluminium sheet up to temperatures at which the coating is volatilised. The aim is in particular to clean a region of the surface which, for example, approximately corresponds to, or is optionally slightly smaller or slightly larger than, the subsequent joint zone. Given use of a weld stud, the aim is to achieve a circular cleaned surface having a diameter which preferably corresponds to the diameter of the weld stud. Given a different geometry of the weld-on component, e.g. an oval or angular cross section, the cleaned surface is advantageously of a corresponding sise. This is assisted by a polarity of the type described above. For the subsequent welding operation a negative polarity is preferably selected. A negative polarity during the cleaning operation might give rise to the problem of rust particles arising and/or remaining in the region of the surface to be cleaned.
The first voltage is moreover preferably set higher, in terms of its magnitude, than the immediately following voltage of reverse polarity. It is set, for example, by appropriate adjustment of the height to which the weld-on element is lifted above the surface. By increasing the distance, the voltage may likewise be increased while the current intensity, for example, remains constant. This enables a requisite energy density to be produced for the cleaning operation while, e.g. given use of a subsequent pilot current for the lift-and-strike welding process with reverse polarity, the aluminium surface is heated up and the arc stabilised in such a way that a weld pool of suitably required depth is produced when the subsequent welding voltage is applied.
The application of a first cleaning voltage may be effected separately from a subsequent application of a pilot welding voltage. For reversal of the polarity of the arc
Broehl Reinhold
Gottwals Haymo
Krengel Michael
Schmidt Klaus Gisbert
Schmidt Wolfgang
Harness & Dickey & Pierce P.L.C.
Newfrey LLC
Shaw Clifford C.
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