Patent Description:
The present invention also relates to a device for joining a joining element to a component, especially for carrying out the above-mentioned method, with a joining head, which comprises a retaining device for a joining element and by means of which the joining element can be moved along a joining axis in relation to a component, and with a cleaning device to carry out a cleaning process on a joining surface of the component and/or on a joining surface of the joining element.

Methods and devices of the above-mentioned kind are widely known, especially in the field of so-called stud welding or stud gluing.

In these methods, joining elements such as studs are joined to components such as plates in such a way that the studs protrude perpendicular to a surface of the component. Such joined arrangements can be used to attach clips made from plastics material, for example, to the stud. The clips may, for example, be used to fix pipes or cables in relation to the component, such as, for example, fuel pipes, brake pipes or electrical cables. The generic joining method is therefore used in particular in the field of bodywork manufacturing for motor vehicles.

In stud welding, an electrical current flow is established between the joining element and the component, the joining element being raised above the component so that an arc is generated between said components. The arc causes the opposite joining surfaces of the component and the joining element to melt. The joining element is then lowered onto the component so that the electrical joining current is short-circuited. The entire molten mass solidifies and the joining process is complete.

In stud gluing, an adhesive which can be activitate is generally applied to one joining surface of a joining element beforehand. Stud gluing then takes place by activating the adhesive. The joining element and the component are then pressed against one another and finally the adhesive is cured. This can be achieved by a variety of factors, such as by applying heat, for example.

The joining process itself is not the only factor responsible for the quality of such joints. The material properties and the surface quality of the component, and also the joining element in some cases, also play a not insignificant role in this process. This applies if the component and the joining element are manufactured from a steel material. Besides, this problem applies if the component and the joining element are each manufactured from an aluminum alloy.

Changes in the characteristic properties of the component are particularly noticeable in joints based on aluminum alloys. Such properties may include whether the aluminum alloy is a recycled material. In addition, there may also be problems with regard to irregular grain sizes on the upper layer, which may be up to <NUM> deep, and in particular when using extruded material.

Irregular grain sizes may lead to different conductivity values. As a result, this may affect the current flow through the arc.

Many components are also manufactured using casting processes. In such cases, the surface is coated with release agents, which may include waxes, oils, polysiloxanes, hydrocarbons, polymers, etc. If the coating or the coat comprising such release agents is unevenly distributed over the surface, it is particularly difficult to adapt the joining parameters appropriately. If coated with carbon, this can lead to pores or cavities in a welded joint, or in other words to a higher porosity of the welded joint overall, which may have a detrimental effect on its strength.

In addition, alloy components may also have an effect on weldability.

As a general rule, components with defined surface specifications are required, but practice suggests that these surface specifications, to which a joining process is then specifically adapted in relation to joining parameters, are not always observed satisfactorily.

In stud welding, the use of an arc cleaning ("clean flash") process before the actual stud welding process is already known in the art. In this case, an arc is created between the joining element and component with alternating polarity before the welding process, causing impurities to be ionized and detached from the component surface. The problem with this process is that such impurities may accumulate on the joining surface on the stud, as a result of which problems may still arise, even in this case, with regard to the consistency of the joints.

<CIT> (showing the features of the preamble of claims <NUM> and <NUM> respectively) discloses a stud joining method with weld studs having at least one tip, which is dimensioned in such a way that a small explosion to cleanse a weld-on region can be achieved. When the tip comes into contact with the weld-on region, it is vaporized explosively by a surge of power from a welding power source, and an explosive wave thus generated and/or the resultant plasma cleanse the weld-on region of the structure of impurities.

<CIT> discloses a welding method to create a weld joint with a TIG/plasma torch having an electrode which creates a plasma pulse or TIG arc pulse. The arc or plasma pulse can be utilized in a number of ways, and for example, to fully penetrate the work pieces as shown in the weld region. Alternatively, the TIG/plasma arc can be used to carve out a molten trench in the work piece. Additionally, the pulsed plasma or TIG arc (which is a transferred arc) can be used to clean the welding surface of the work pieces from contaminants prior to welding.

<CIT> discloses a plasma arc welding method using a cleaning action of the plasma arc.

In the light of the above, one object of the invention is to provide an improved method for joining a joining element to a component and an improved device for this purpose.

The above object is achieved on the one hand by a method for joining joining elements to components, particularly for stud welding or stud gluing, as defined in claim <NUM>, steps including providing a joining element which comprises a first joining surface, providing a component which comprises a second joining surface, preparing said first and/or second joining surface, and carrying out the joining process, in which the joining element is joined to the component, wherein the preparatory step comprises at least one of the following cleaning methods for cleaning the joining surface: a TIG arc method, a plasma gas cleaning method and a snow jet method.

The object is also achieved by a device for joining a joining element to a component, for carrying out the method according to the invention, as defined in claim <NUM>, a joining head, which comprises a retaining device for a joining element and by means of which the joining element can be moved along a joining axis in relation to a component, and with a cleaning device to carry out a cleaning process on a joining surface of the component and/or on a joining surface of the joining element, said cleaning device comprising a TIG arc cleaning device, a plasma gas cleaning device and a snow jet cleaning device.

The cleaning methods according to the invention each differ from the clean flash cleaning method mentioned initially, in which an arc with alternating polarity is created between the joining surfaces using stud welding equipment, which at least causes the joining surface of the component to be cleaned.

The cleaning method according to the invention, which differ from such a clean flash method, allows only one of the joining surfaces or both joining surfaces, one after the other, to be cleaned in a targeted manner.

The cleaning step preferably entails cleaning the joining surface using a physical cleaning medium, which differs from an arc created between the joining element and the component.

In other words, and according to the present invention, the cleaning step entails a cleaning process which is carried out independently of the joining process.

The cleaning step is preferably carried out in one stage, in which a joining element is already in a retaining device of a joining head and is assigned to a specific position (joining position) on the component. This is particularly advantageous if a cleaning device for carrying out the cleaning process is arranged on the joining head.

Alternatively, it is possible to carry out consecutive cleaning processes at each of these joining positions on a component on which a plurality of joining elements, for example, are to be fixed, in some cases even before a joining element is placed in a retaining device of a joining head if applicable. The cleaning process or cleaning processes may thus be performed on one component or jointly, so that all joining elements can then be attached to the component, no further cleaning process being required between the joining processes.

As mentioned above, the cleaning process is preferably carried out using a cleaning medium.

The cleaning medium may be a gas, a liquid or a solid. The cleaning medium is preferably applied to the component by means of a separate cleaning device, which directs the cleaning medium onto a joining surface, particularly onto a joining position on the component. The cleaning device used to apply the cleaning medium is according to the present invention designed such that it is separate and independent from the technology used to carry out the joining process.

The method and the device according to the present invention can preferably be combined by a step in which at least one characteristic variable of the component and/or the joining element is recorded and subsequently evaluated. The evaluation may, in some cases, allow for the fact that it is necessary to carry out a preliminary cleaning process according to the invention if a joining position is evaluated negatively. In other cases, in which a joining position receives a good preliminary evaluation, it may not be necessary to carry out such a cleaning process before carrying out the joining process.

The characteristic variable in this case may relate to the material, the surface quality, surface processing, carbon coating on the surface, cleanliness, and may relate to release agents in the case of a cast workpiece, but may also include relative variables such as the component material in relation to the joining element material, for example.

According to an embodiment, only at least one characteristic variable of the component is recorded and only one joining surface of the component is cleaned, if this is necessary. All subsequent references to recording and evaluating a variable of a component and to cleaning a joining surface of the component should, however, relate equally to recording or evaluating a variable of the joining element and cleaning a joining surface of the joining element unless otherwise explicitly specified.

Such a characteristic variable is preferably recorded automatically and specifically preferably by means of an appropriate recording device. This recording device or these recording devices may include suitable sensors which either work on a purely passive basis or where the workpiece actively undergoes a physical process, in which the subsequent reaction to this process is recorded by sensors.

Such an active recording process may, for example, entail an electrical conductivity measurement using an eddy current measurement method, or even a surface coating measurement using fluorescence excitation, or contact resistance measurement.

In the case of fluorescence measurement, light in the visible range or in the UV range may be applied to the joining surface, and the resulting excited fluorescence radiation (usually in a different frequency range) is then recorded. Individual photons can be "counted" in particular, the number of recorded photons or light quanta usually being in correlation with the thickness or density of a coating on the joining surface of the component.

In the case of electrical conductivity measurements, an alternating magnetic field may, for example, be induced in the component surface. As the component is preferably a non-magnetic material such as an aluminum alloy, this gives rise to eddy currents in the component, which in turn generate a magnetic field. This reaction field can then be recorded. The magnitude and intensity of the reaction field may be an indicator of specific material properties, such as hardness, thermal conductivity, homogeneity or similar properties of such a component. In particular, the reaction field correlates to electrical conductivity.

In the case of contact resistance measurements, a contact is placed on the joining surface and a voltage between the contact and the component is increased and/or a force by means of which the tip of the contact is pressed onto the component is increased. The thickness and/or density of a coating on the surface can be deduced as a result of the alternating electrical resistance resulting from this action.

The object of the invention is thus achieved in its entirety.

According to the present invention, the plasma gas cleaning method entails generating a non-transmitted (or non-transferable) arc between a tungsten electrode and an anode surrounding the tungsten electrode, said arc generating plasma when using a plasma gas, said plasma being directed onto the joining surface. In this case a tungsten electrode is understood to mean an electrode manufactured from a metal with a very high melting point, or in other words, in particular an electrode made from a material such as tungsten, which does not melt when an arc is generated.

In this case, the arc is generated between the tungsten electrode and an anode made from an electrically conductive material surrounding the tungsten electrode. In other words, an electric arc is not generated between the tungsten electrode and the component or its joining surface in this step. As a result, this is a non-transmitted (or non-transferable) arc. The plasma or the "plasma arc" cannot be electrically conductive as a result of this measure and can therefore preferably not be deflected by magnetic means. Accordingly, the plasma arc can be focused satisfactorily and is preferably not deflected or only slightly deflected from a cleaning axis (joining axis) as a result.

The plasma or the plasma arc which is directed onto the joining surface causes any surface coatings on the joining surface to evaporate without these materials subsequently accumulating on the joining surface of the joining element.

Standard impurities such as oil films, grease, etc. can be removed particularly well.

In this process it is particularly preferable if the plasma gas is passed under pressure into an intermediate space between the tungsten electrode and the anode, the plasma being discharged from the intermediate space towards the joining surface.

The gas pressure also ensures that a coating of this kind on the joining surface is also eliminated from the surface as a result of the gas pressure, or in other words, an oil film can be driven outwards in the form of a ring.

According to another preferred embodiment, the anode is connected to a plasma gas nozzle at an end located downstream in the direction of the plasma gas discharge direction, said nozzle combining the plasma or plasma arc emerging from the intermediate space.

A very narrow plasma arc can be produced as a result, said arc preferably comprising a conical shape when it emerges from the plasma gas nozzle with a cone angle of < <NUM>°, particularly < <NUM>°.

The plasma emerging onto the joining surface, which is also referred to as the plasma arc, is therefore very directionally stable. Position deviations between a programmed position and an arc deflection due to blowing effects are therefore very minor. The plasma arc may also be stable if the distance between the plasma gas nozzle and the component fluctuates. Such a plasma arc can also continue to function in a stable manner even with low electric currents.

An inert gas or similar is preferably not generated around the plasma arc, as the joining surface is not melted by means of the plasma arc in the region of the joining surface, or in other words, the presence of oxygen or similar at the cleaning region is not generally a problem.

According to a preferred embodiment, a distance ranging from <NUM> to <NUM> is adjusted between the plasma gas nozzle and the joining surface during the cleaning step. The distance preferably ranges from <NUM> to <NUM>, or particularly from <NUM> to <NUM>.

It is also advantageous if the ratio between a nozzle diameter of the plasma gas nozzle and a distance adjusted between the plasma gas nozzle and the joining surface during the cleaning step ranges from <NUM>:<NUM> to <NUM>:<NUM>.

In this case the nozzle diameter of the plasma gas nozzle is preferably the internal diameter of the plasma gas nozzle, or in other words the effective diameter through which the plasma emerges from the plasma gas nozzle.

In particular, this ratio may range from <NUM>:<NUM> to <NUM>:<NUM>.

It is also advantageous if the anode and/or a plasma gas nozzle connected to the anode is cooled by means of a cooling device.

As a result, the plasma jet formed by the tungsten electrode and the anode can be produced such that it is thermally stable. The cooling device may preferably be water cooling.

It is also preferable if an electrical voltage ranging from <NUM> V to <NUM> V is applied between the tungsten electrode and the anode to generate the plasma. The electrical voltage may in particular range from <NUM> V to <NUM> V, particularly from <NUM> V to <NUM> V.

It is also preferable if an electric current ranging from <NUM> kA to <NUM> kA flows between the tungsten electrode and the anode to generate the plasma.

When generating the plasma to clean the joining surface, a stable arc can be produced with relatively low voltages and relatively high currents.

The diameter of the plasma gas nozzle preferably ranges from <NUM> to <NUM>.

In an embodiment, the joining method further comprises the step of generating an ignition tip on the joining surface.

In an embodiment, the joining element is joined to the component through arc welding, with drawn-arc ignition, and wherein the joining process comprises:.

It is assumed that the above-mentioned features and the features still to be explained below can not only be used in the respective specified combination, but also in other combinations or in isolation, without deviating from the scope of the present invention as defined in the appended claims.

Embodiments of the invention are shown in the drawings and explained in greater detail in the following description. These drawings are as follows:.

<FIG> is a schematic representation of a joining device for joining joining elements to components, generally referred to as <NUM>.

The joining device <NUM> comprises a joining head <NUM>, which can be moved freely in the space by means of a robot <NUM>, said joining head <NUM> preferably being mounted on one arm <NUM> of the robot <NUM> in this case.

A carriage <NUM> can preferably be moved along a joining axis <NUM> on the joining head <NUM>. The maximum stroke of the carriage <NUM> is preferably larger than a maximum joining stroke.

A retaining device <NUM> to retain a joining element <NUM> is arranged on the carriage <NUM>. The joining element <NUM> may, for example, be designed as a stud, with a shaft portion which is not shown in greater detail, and a flange portion which is not shown in greater detail, a first joining surface <NUM> being formed on one side of the flange portion facing away from the shaft portion. The joining element <NUM> is preferably made from aluminum or aluminum alloy.

The joining element <NUM> can be joined to a component <NUM> such as a plate by means of the joining device <NUM>, the component <NUM> preferably also being made from aluminum or an aluminum alloy.

A second joining surface <NUM> is formed on the component <NUM>, said surface having a diameter DFB, which approximately corresponds to the diameter of the flange portion of the joining element <NUM>.

A coating <NUM> may be formed on the joining surface <NUM>, said coating being formed of release agents or waxes, oils, polysiloxanes, hydrocarbons, polymers, etc..

The joining device <NUM> is in particular designed as a stud welding device, but may also be in the form of a stud bonding/ stud gluing device.

The joining device <NUM> comprises a cleaning device <NUM>, by means of which the second joining surface <NUM> can be cleaned before carrying out the joining process. The cleaning device <NUM> is preferably designed to direct a cleaning medium onto the second joining surface <NUM>, and specifically along a longitudinal axis <NUM>, which is oriented at an angle a with respect to the second joining surface <NUM>. The angle α may, for example, range from <NUM>° to <NUM>°, and particularly from <NUM>° to <NUM>°.

In an embodiment (not shown in the figures), the first joining surface can be cleaned before carrying out the joining process by the joining device <NUM>. In another embodiment, the first and second joining surfaces might be cleaned simultaneously and/or both surfaces might be cleaned by the cleaning device <NUM>.

As illustrated, the cleaning device <NUM> is attached to the joining head <NUM>.

Furthermore, the joining device <NUM> may comprise a recording device <NUM>, which is able to record the status of the second joining surface <NUM> and/or a surface coating on the second joining surface <NUM>. In particular, the recording device <NUM> is designed to record a characteristic variable of the component <NUM>.

In this case the cleaning device <NUM> is attached to the joining head <NUM>.

In order to provide high quality joints, and especially to provide consistent joints, it is preferable for each joining surface <NUM> to be first processed by the recording device <NUM> before carrying out a joining process on said surface, after which the characteristic variable thus recorded is evaluated. A decision can be made on the basis of this variable whether a joining process can be performed immediately afterwards, or whether it is desirable or necessary to perform a cleaning process using the cleaning device <NUM> beforehand.

<FIG> shows a cleaning device <NUM>-<NUM> in the form of a plasma gas cleaning device.

The plasma gas cleaning device <NUM>-<NUM> comprises an elongated tungsten electrode <NUM>, which preferably extends coaxially in relation to a joining axis <NUM> or cleaning axis <NUM>.

The cleaning device <NUM>-<NUM> also comprises an anode sleeve <NUM>, an annular intermediate space <NUM> being formed between the tungsten electrode <NUM> and the anode sleeve <NUM>.

A plasma gas <NUM> is admitted to the intermediate space <NUM>. An arc voltage U is applied between the tungsten electrode <NUM> and the anode sleeve <NUM>, causing a corresponding current I to flow.

Plasma <NUM> is generated between the tungsten electrode <NUM> and the anode sleeve <NUM> from the plasma gas <NUM> as a result of this arc voltage U and the current I, said plasma emerging from a plasma gas nozzle <NUM> arranged at one downstream end of the anode sleeve <NUM>.

As a result, a kind of plasma arc (or plasma jet) is generated from the plasma gas nozzle <NUM> towards the second joining surface <NUM>, this arc being, according to the present invention, a non-transmitted arc (or non-transferable arc), and preferably not undergoing any magnetic deflection due to ground effects.

The space A between the plasma gas nozzle <NUM> and the second joining surface <NUM> may, for example, range from <NUM> to <NUM>. The internal diameter DD of the plasma gas nozzle may, for example, range from <NUM> to <NUM>.

<FIG> also shows that the arrangement of the tungsten electrode <NUM> and the anode sleeve <NUM> may be cooled by a cooling device <NUM>, for example by water cooling. As a result, this arrangement can be made more thermally stable.

As a general rule, it is not necessary to supply an inert gas around the plasma arc <NUM>, as is known from TIG welding, for example. If this is still necessary for specific reasons, an inert gas sleeve <NUM> may be arranged around the outside of the anode sleeve <NUM> such that an inert gas <NUM> can be supplied between the inert gas sleeve <NUM> and the anode sleeve <NUM>.

<FIG> shows a snow jet cleaning device <NUM>-<NUM>, aspect not covered by the present invention, in which a gas <NUM> such as CO<NUM> and compressed air are passed into a snow jet nozzle <NUM> from a compressed air generator <NUM>. In this process the gas <NUM> is first compressed and then expanded in the snow jet nozzle such as to produce snow or ice crystals <NUM> in the snow jet nozzle <NUM>.

The internal diameter DD' of the snow jet nozzle may, for example, range from <NUM> to <NUM>.

The snow crystals <NUM> carried by the compressed air flow impact on and break up a coating <NUM>, as illustrated schematically in <FIG>.

In the snow jet cleaning device <NUM>-<NUM>, it may be preferable if a joining or cleaning axis <NUM> is oriented at an angle a in relation to the joining surface <NUM>, said angle ranging from <NUM>° to <NUM>°.

<FIG> shows a TIG arc cleaning device <NUM>-<NUM>, aspect not covered by the present invention, In this case, an arc voltage is applied between a tungsten electrode <NUM>' and the component <NUM> such that a TIG arc <NUM> is created between the tungsten electrode <NUM>' and the component <NUM> in the region of the joining surface <NUM>. If applicable, an inert gas sleeve <NUM>' may be provided around the tungsten electrode <NUM>' such that the TIG arc <NUM> can be surrounded by an inert gas <NUM>.

<FIG> shows a plan view of a joining surface <NUM> of a component <NUM>, said joining surface having a diameter DFB.

A radius of the joining surface <NUM> is shown as r.

Various positions on a plasma arc <NUM> (or a snow jet) directed onto the joining surface <NUM> are shown as <NUM>.

It is evident that the diameter DR of this plasma arc <NUM> (or the snow jet) may be greater than or equal to the diameter DFB, but may also be smaller. An effective overall cleaning surface can be achieved by moving the plasma arc <NUM> (or the snow jet) in relation to the second joining surface <NUM>, for example on a circular path <NUM>. It is also possible to position the plasma arc <NUM> (or the snow jet) at an angle in relation to the joining surface <NUM> such as to produce an overall tumbling motion.

<FIG> show another embodiment of a joining device <NUM>' which generally corresponds to the joining device <NUM> shown in <FIG> with regard to its structure and mode of operation. The same components are therefore identified by the same reference numerals.

The joining device <NUM>' comprises a motor <NUM>, which is fixed to the joining head <NUM>, a cleaning device <NUM> being able to rotate around an axis of rotation, which is oriented transversely with respect to the joining axis <NUM>. In this case the motor <NUM> is connected to the cleaning device <NUM> via an interface <NUM>. The direction of rotation <NUM> around the axis of rotation is shown in <FIG>. A displacement measurement device <NUM> is preferably assigned to the cleaning device <NUM> and used to record the angle of rotation.

The angle a at which a cleaning medium is directed onto a joining surface <NUM> of the component <NUM> can be adjusted by means of the motor <NUM> as a result.

<FIG> show different steps of a joining method according to the invention. The cleaning device <NUM> is a cleaning device <NUM>-<NUM> in the form of a plasma gas cleaning device.

As illustrated in <FIG>, the plasma <NUM> or a plasma jet is used to clean the joining surface <NUM>, <NUM>, and in particular the second joining surface <NUM> as described above. The plasma <NUM> or plasma jet will first clean the joining surface (in particular the second joining surface <NUM>). Any lubricant or contamination provided on the joining surface are removed through the plasma <NUM> or plasma jet. Through the thermal effect of the plasma, the coating <NUM> (which can be as previously mentioned oils, polymers, contaminations. ) is vaporized, burnt and/or removed.

In an aspect not covered by the present invention, the plasma <NUM> or plasma jet is further applied in order to create a local melting of the joining surface, as shown in <FIG>. The parameters used to generate the plasma during the cleaning step might be modified to provide the melting area. The pressure applied by the plasma on the melting area generates a projection or ignition tip <NUM>. The projection or ignition tip <NUM> has a circular shape or a circular cross section. For example, the projection or ignition tip <NUM> has a crater-like shape.

The ignition tip <NUM> enables a better welding of the joining element on the component, as already known from the prior art. The generation of the joining tip <NUM> on the component <NUM> and not on the joining element <NUM>, allows to avoid a preforming of the joining element <NUM>. Thus, the shape of the joining element <NUM> might be randomly chosen and its end face (or joining surface) may not need to be prepared.

More particularly, after forming the ignition tip <NUM>, the joining element <NUM> may be joined to the component <NUM> through arc welding, with drawn-arc ignition. In a first step, the first joining surface <NUM> is placed adjacent the ignition tip of the second joining surface <NUM>. An electric pilot current is switched on. The joining element <NUM> is then lifted away from the component <NUM> with the retaining device <NUM>. The welding current flows through the arc in such a manner that the first joining surface <NUM> and second joining surface <NUM> start to melt. More particularly, the second joining surface starts to melts from the ignition tip, which allows a better repartition of the melting. The ignition tip <NUM> allows the arc to remain in a precise location.

Claim 1:
Method for joining by welding joining elements (<NUM>) to components (<NUM>) with the following steps:
- providing a joining element (<NUM>), which comprises a first joining surface (<NUM>), and providing of a component (<NUM>), which comprises a second joining surface (<NUM>);
- providing a joining device (<NUM>) adapted to join the joining element to the component through arc welding;
- preparing the first and/or second joining surface (<NUM>; <NUM>); and
- carrying out a joining process with the joining device, in which the joining element (<NUM>) is joined to the component (<NUM>);
wherein the preparing step comprises a plasma gas cleaning method characterized in that the plasma cleaning method consists of applying plasma to the component by means of a cleaning device separate and independent from the technology used to carry out the joining process,
wherein the plasma cleaning method includes generating a non-transferable arc (<NUM>) which is directed onto the joining surface (<NUM>) and wherein the non-transferable arc (<NUM>) is generated between a tungsten electrode (<NUM>) and an anode (<NUM>) surrounding the tungsten electrode (<NUM>), wherein the non-transferable arc (<NUM>) generates a plasma (<NUM>) when using a plasma gas (<NUM>), and wherein said plasma (<NUM>) is directed onto the joining surface (<NUM>).