PRODUCING IT

The invention relates to a process for producing a multi-part assembly, to a multi-part assembly, to a connecting part for a multi-part assembly and to a device for producing a multi-part assembly.

A process for connecting parts by a friction welding process is known from US-PS 3,477,115. This document describes the superimposition of two metal plates, a rotating connecting part being driven under pressure by the upper plate either through a pre-existing bore or by melting away and discharging the content of the bore by means of a rotating rive -like part. The rotating connecting part makes frictional contact with the lower plate so a friction welded connection is produced between the lower plate and the connecting part. The upper plate is not included in the resultant material connection but merely held positively between the lower plate and the connecting part roughly in the manner of a riveted joint. Such a connection between two plates can loosen and does not withstand high stresses.

A process for the material connection of rotationally symmetrically designed workpieces made of metals having markedly differing heat resistance is also known from DE 31 01 227 AI . The process is a friction welding process. It is proposed according to DE 31 01 227 AI that the metallic workpiece with the higher heat resistance be faced before the actual welding process. The facing of the workpiece with the higher heat resistance should ensure that the surface to be welded is orientated exactly perpendicularly to the axis of rotation of the friction welding machine and uniform abrasion of the welded surface on the workpiece with lower heat resistance and

consequent, more uniform heating are therefore to be effected. The oxide skin which adversely affects perfect welding is also to be removed by facing.

It is an object of the present invention to provide a process for producing a multi-part assembly by means of which the strength of the multi-part assembly is increased. A multi-part assembly with increased strength is also desired. A further aim of the invention is to provide a connecting part for a multi-part assembly by means of which a multi-part assembly of this type can be reliably formed. A constructionally simple device for producing a multi-part assembly is also to be provided.

The present invention provides a process for producing a multi-part assembly, in particular a three-part assembly, characterised in that at least one flat metal part and one metallic base are pressed onto one another to form a structure to be connected a connecting part with a tapering end portion and of a material with a higher melting point than that of the flat metal part and the structure are rotated relative to one another to form a molten material and are pressed against one another until the connecting part has penetrated the flat metal part, so that molten material for forming a material welded joint between the base and the flat metal part is brought therebetween.

The present invention further provides a multi-part assembly, comprising at least one flat metal part and one metallic base which are placed on one another to form a structure, a connecting part which is partially driven into the structure by rotation and of which the end portion penetrating into the structure forms a friction welded joint with the base and/or the flat metal part at a connecting point, characterised in that the base and the flat metal

part penetrated by the connecting part have a common material connection lying substantially in a plane of contact and annularly surrounding the connecting part .

The present invention further provides a connecting part for a multi-part assembly, which connecting part has a tapering end portion which projects at least partially into the structure.

The present invention further provides a device for producing a multi-part assembly, comprising a holding unit for holding a structure formed by at least two superimposed parts, a rotatable clamping unit which is connected to a drive unit and is drivable at least in the direction of the axis of rotation, for the fixing of a connecting part, a propulsion unit which moves the clamping unit with the connecting part so the connecting part can be brought onto and into the structure, a clutch arranged between the clamping unit and the drive unit and with a brake system connected to the clamping unit.

In the process according to the invention for producing a multi-part assembly, it is proposed that at least one flat metal part and a metallic base are superimposed to form a structure. A connecting part which has a tapering end and is made of a material with a higher melting point than that of the flat metal part and the structure are rotated relative to one another to form a molten material and are pressed against one another. During this process, the connecting part penetrates into the structure. Owing to the advance and the rotation of the connecting part into the structure after the melting of the flat metal part and the surface of the base, at least a proportion of the molten material passes between the two parts. After the molten material has cooled, an annular direct material welded joint

is produced between the base and the flat metal part and, generally, also a further material connection between the connecting part and the two parts of the structure in each case. This procedure results in a multi-part assembly which, in contrast to formerly known multi-part assemblies, has increased strength as the parts themselves which form the structure are materially connected to one another.

The structure is preferably arranged stationarily and the connecting part arranged rotatably during the process. The molten material is also set into rotation by rotation of the connecting part, so that the molten material is displaced between the parts to be connected. The stationary arrangement of the structure is desirable if the structure consists of relatively large parts as is the case, for example, with parts of a vehicle body. The energy required to form the connection increases with the speed of movement, allowing faster processing.

In the process according to the invention, the connecting part is pressed rotatably against the structure. During movement of the connecting part, dry friction occurs between it and the structure. The tapering end portion of the connecting part easily penetrates into the structure. The area of contact between the connecting part and the structure increases during the penetration process. The frictional moment between the connecting part and the structure also increases as the area between the connecting part and the structure increases. The frictional energy is converted into heat so that the structure and possibly the connecting part melt in the frictional region. Owing to the tapering design of the connecting part, in particular a very flat tip, the molten material is pressed radially outward by the connecting part. This displacement process causes the molten material to be pressed between the parts of the

structure. The introduction of the molten material between the parts is also assisted by capillary action in the narrow gap between the parts .

The friction between the connecting part and the structure decreases during the connecting process. It passes from dry friction into mixed friction. With mixed friction, liquid and dry friction exists between the connecting part and the structure. The molten material collects under pressure in the peripheral region in a cone-like annular bead and does not escape upwardly. As the energy introduced into the connection continues to increase, the friction passes from mixed friction, into liquid friction. At the transition from dry friction into liquid friction, the coefficient of friction decreases so a rise in temperature is only possible to a limited extent at a constant speed of the connecting part. It is therefore proposed that the speed of the connecting part and/or of the structure be abruptly reduced after a predetermined state of the molten material has been reached, preferably until the connecting part comes to a standstill. Therefore, a rigid connection is also created between the connecting part and the structure when the molten material cools.

The state of the molten material can be defined by the temperature or, indirectly, by the frictional force or frictional moment prevailing between the connecting part and the structure. The connecting part is preferably then monitored, in particular decelerated in less than one second, when the timed change in the frictional force or frictional moment between the connecting part and the structure falls below a predetermined desired limit value or the temperature of the molten material has reached a quasi stationary state.

According to the invention, the connecting part is pressed into the structure under a predetermined force or with predetermined propulsion. It has proved advantageous if the connecting part is pressed against the structure with a greater force during and/or after a deceleration process than before the deceleration process. Owing to the penetration of the connecting part during and/or after a deceleration process, the rotating molten material is pressed more markedly between the parts to be connected. The connecting part is pressed in the last deceleration phase in particular with a force which is 1.3 to 2.5, preferably 1.5 times as great as the force before deceleration.

According to a further advantageous idea it is proposed that the connecting part should consist of a material having higher strength than the body of the structure. The connecting part preferably consists of steel or a steel alloy which can also be provided with surface protection. One or both parts which form the structure preferably consist of a light metal, in particular of aluminium or an aluminium alloy. A design of the structure in which the parts to be connected consist of the same material is preferred.

When carrying out the process according to a preferred embodiment of the invention for forming a multi-part assembly, the flat metal part and the base are superimposed to form a structure and secured in a holding unit of a device. The connecting part is fixed in a rotatable clamping unit of the device connected to a drive unit and is then positioned above the structure. The connecting part is then set by the clamping unit into a rotational movement to a predetermined speed with a predetermined force and/or predetermined propulsion and into the structure. Once the

molten material which has formed during the connecting process has reached a specific state at a predetermined connecting point, the clamping unit is uncoupled from the drive unit and is decelerated in a controlled manner and pressed against the structure while applying an additional force .

A further aspect of the invention concerns a multi-part assembly, in particular a three-part assembly, in particular an assembly produced by the process according to the invention. The multi-part assembly comprises at least one flat metal part and a metallic base which are superimposed to form a structure and a connecting part which is partially driven into the structure by rotation and of which the end portion projecting into the structure forms a friction welded connection at a point of connection to the base and/or the flat metal part, the base and the flat metal part penetrated by the connecting part having a common material welded joint which essentially lies in a plane of contact and annularly surrounds the connecting part.

The structure is preferably formed by two similar superimposed parts, in particular two superimposed metal sheets. The connecting part can at the same time be a fastening element. The connecting part preferably consists of a material having higher strength than the body of the structure. For example, it can consist of steel or a steel alloy.

The end portion of the connecting part is preferably substantially conical in design. The base of the conical end portion of the connecting part can also be smaller than the cross-sectional area of a portion of the connecting part adjoining the end portion. The conical end portion can be designed in the form of a pointed cone or a blunt cone.

To increase the friction between the connecting part and the structure during the process of producing a multi-part assembly, the face of the tapering end portion preferably has increased roughness.

According to a further advantageous idea it is proposed that the end portion should have radially outwardly extending grooves. The radially outwardly extending grooves are preferably designed in the form of saw teeth when viewed in the circumferential direction. The grooves form receiving pockets in which the resultant molten material is stored so the transition from dry friction into mixed friction is delayed. Furthermore, the molten material is discharged radially outwardly through the radially outwardly extending grooves.

According to a further advantageous embodiment of the connecting part, the connecting part has at least one tool engagement region. The tool engagement region can be designed in the form of a polygon, preferably an externally or internally located hexagon, so the connecting part can be set into rotation by introduction of a tool into the tool engagement region of the connecting part .

According to a further idea of the invention, a device for producing a multi-part assembly, in particular a three-part assembly is proposed. The device comprises a holding unit for holding a structure formed by at least two superimposed parts. The device also comprises a rotatable clamping unit which is connected to a drive unit, can be driven at least in the direction of the axis of rotation and serves to fix a connecting part. The clamping unit is connected to a propulsion unit so the connecting part can be brought into the structure with a predetermined force and at

a predetermined propulsion rate. A clutch is arranged between the clamping unit and the drive unit so the clamping unit can be rapidly uncoupled from the drive unit . A brake system which decelerates the clamping unit is connected to the clamping unit. The device preferably comprises a closed-loop and an open-loop control system by means of which the speed of the drive unit, the coupling process and the deceleration process are controlled.

The clamping unit is preferably driven by an electrically driven motor, or pneumatically.

It is mentioned at this point that the volume of molten material is relatively small when metal sheets are connected to a fastening stud, so that the molten material solidifies relatively rapidly. To ensure that the molten material maintains a desired state during the shifting process, the shifting process has to take place relatively rapidly. To this end it is proposed that the clutch be electromagnetically, pneumatically or hydraulically actuable. To allow a controlled deceleration process which is as rapid as possible, the brake system preferably comprises a disc brake. Alternatively, the brake system can be designed in the form of an eddy-current brake.

Further advantages and features of the invention will be described with reference to the embodiments illustrated in the drawings.

Figures 1, 2 and 3 each schematically show a stage during the process for producing a multi-part assembly;

Figure 4 shows a stage of the process during the production of a multi-part assembly, on an enlarged scale,-

Figure 5 is a graph of the characteristic values during the production of a multi-part assembly;

Figure 6 shows a first embodiment of a connecting part in a front view;

Figure 7 is a perspective view from below of the connecting part according to Figure 6; Figure 8 shows a second embodiment of a connecting part in a front view;

Figure 9 is a perspective view from below of the connecting part shown in Figure 8;

Figure 10 shows a third embodiment of a connecting part in a front view;

Figure 11 is a perspective view from below of the connecting part shown in Figure 10;

Figure 12 shows a fourth embodiment of a connecting part; Figure 13 is a perspective plan view of the connecting part shown in Figure 12;

Figure 14 is a schematic view in perspective of a device for forming a multi-part assembly;

Figure 15 schematically shows a detail of the device and;

Figure 16 shows a detail of a multi-part assembly frame .

A process for producing a multi-part assembly is described hereinafter with reference to a preferred embodiment. The illustrated embodiment is a three-part assembly. The descriptions can accordingly be transferred to a multi-part assembly comprising more than three parts.

Figure 1 shows sc.ematically a first stage during the process of producing a multi-part assembly. Figure 1 shows a stage in which a flat metal part 1 and an (also flat) metallic base 2 are superimposed. They form a structure 4. A connecting part 3 is positioned above the structure 4. The connecting part 3 is substantially circular in cross

section. It comprises a tapering end portion 5. The end portion 5 is designed in the form of a truncated cone and orientated toward the body 1.

The structure 4 is held stationary. The connecting part 3 is rotatably drivable round its axis 6. The connecting part 3 is set into rotation round its axis 6 by means of a driving unit (not shown) . The connecting part 3 is pressed into the structure 4 by the rotation round its axis 6.

Figure 2 shows a stage of the process in which the connecting part 3 penetrates with its end portion 5 into the flat metal part 1 of the structure 4. During the rotation of the connecting part 3, the material of the flat metal part 1 is initially plasticised and then melted. The same also applies, in part, to the connecting part 3.

Figure 3 shows a final stage of the process. The connecting part 3 penetrates the flat metal part 1 and penetrates in part into the base 2 of the structure 4. A material connection has formed between the connecting part 3 and/or the two parts 1 and 2 to be connected. As indicated in Figure 3, a proportion of the molten material 7 has penetrated between the parts 1, 2 so that the molten material 7 forms a material connection 9 annularly surrounding the connecting part 3 after solidifying in a common plane of contact 8 of the parts 1, 2.

Reference is made to the illustration in Figure 4 for a more detailed description of the process. Figure 4 shows a connecting part 3 which penetrates in part into a structure 4. The connecting part 3 has a conically tapering end portion 5. During formation of the connection, the connecting part 3 rotates round the axis of rotation 6. The

axis of rotation 6 coincides with the longitudinal axis of the connecting part 3. If the tapering end portion 5 of the connecting part 3 comes into contact with the flat metal part 1, friction is produced between the connecting part 3 and the flat metal part 1 of the structure 4. This is initially dry friction between the flat metal part 1 and the connecting part 3. The connecting part 3 is shifted in the longitudinal direction of the axis 6 so that it penetrates successively into the structure 4. During the penetration process, the frictional surface between the connecting part 3 and the metal part 1 initially increases until the surface of the conically tapering end portion 5 completely comes into contact with the metal part 1. Heat is generated by the friction between the connecting part 3 and the part 1, this heat initially resulting in plasticisation of the flat metal part 1 and leading to the liquefaction thereof in the region of the area of contact of the tapering end portion 5 during the continued supply of energy in the form of frictional heat. The dry friction passes into mixed friction containing both dry friction and liquid friction. The molten material 7 is pressed radially outwardly owing to the conically tapering end portion 5, so dry friction invariably occurs substantially in the centre of the tapering end portion 5 while mixed friction occurs in the outer region of the tapering end portion 5. Owing to several effects, the molten material 7 penetrates between the flat metal part 1 and the base 2 of the structure 4. On the one hand, the molten material is conveyed into the gap between the parts 1, 2 by capillary action. Furthermore, the molten material 7 is also set into rotation owing to the rotation of the connecting part 3 so the molten material 7 is pressed between the parts 1, 2 as the result of centrifugal forces . Once the molten material 7 has reached a predetermined state at a predetermined connecting point, the connecting part 3 is abruptly decelerated and pushed

into the structure 4 with increased force. The molten material 7 is pressed between the parts 1, 2 of the structure 4 during this process. Since the molten material 7 passes between the parts 1, 2, these parts 1, 2 are materially connected to one another so the two parts 1, 2 are generally also materially connected to the connecting part 3.

The graph shown in Figure 5 shows some values which characterise the process as a function of time.

The curve A describes the timing of the frictional moment during production of a multi-part assembly.

As shown by the course of the curve A, the frictional moment increases steeply to a moment t . The increase in the frictional moment recedes to the increase in the dry friction during penetration of the connecting part 3 into the flat metal part 1 of the structure 4. The frictional moment increases because, owing to the conically tapering formation of the end portion of the connecting part 3, the area of contact between the connecting part 3 and the metal part 1 is increased. At moment t _u , the dry friction passes into mixed friction. The transition between the dry friction and the mixed friction can be more or less pronounced. The transition between dry friction and mixed friction is usually continuous. Solid and liquid friction exists in the case of mixed friction. The frictional moment decreases continuously. The reduction in the frictional moment is due to the reduction in the dry friction content of mixed friction, so mixed friction passes into pure liquid friction which leads to an almost constant trend of the frictional moment.

The curve B shows schematically the temperature trend during the process of producing a multi-part assembly. The temperature trend shows that a pronounced rise in the temperature is recorded in the region of the connecting point up to moment t _u . The rise in temperature is due to the increasing evolution of heat caused by dry friction. A smaller rise in temperature is noted after moment t , as less evolution of heat due to friction occurs from moment t _u owing to the decreasing frictional moment.

As shown by the connection between the temperature trend as a function of time and the frictional moment as a function of time, continuation of the process beyond moment t does not lead to a significant change in temperature. At moment t , mixed friction also passes into liquid friction. A state of quasi equilibrium is achieved in the molten material at moment t _p . At this moment, the connecting part is pressed into the structure 4 with increased force.

Figures 6 and 7 show a first embodiment of a connecting part 3. The connecting part 3 has a substantially hexagonal collar 10. A shank 11 adjoins the collar 10. The shank 11 has a substantially conical end portion 5. The surface of the conical end portion 5 is inclined by the angle α. The angle α is preferably between 5° and 10°, in particular 7°.

As shown in Figure 6, the shank 11 is also slightly conical in design. It tapers toward the end portion 5, the angle of inclination β of the surface of the shank 11 being substantially smaller than the angle of inclination β. The angle of inclination β is preferably 1° .

The hexagonal collar 10 is used for tool engagement so the connecting part 3 is held on the collar 10 and set into rotation.

Figures 8 and 9 show a further embodiment of a connecting part 3. The connecting part 3 has a collar 10 which is hexagonal in cross section. A shank 11 adjoins the collar 10. The shank 11 tapers from the collar 10. An end portion 5 in the form of a truncated cone adjoins the shank 11. The inclination of the surface of the end portion 5 which tapers in the form of a truncated cone is preferably between 10° and 15°, in particular 13°.

Figures 10 and 11 show a further embodiment of a connecting part 3. The connecting part 3 has a substantially hexagonal collar 10. A shank 11 adjoins the collar 10. The shank 11 has an end portion 5 comprising a centrally formed truncated cone-shaped portion 12. Radially outwardly extending grooves 13 emanate from the truncated cone-shaped portion 12. The grooves 13 are saw tooth-shaped in design, when viewed in the circumferential direction. The depth of each groove 13 increases radially outwardly. The envelope of the grooves 13 forms a truncated cone with the portion 12. The inclination of the theoretical envelope substantially corresponds to the angle of inclination β, as shown in Figure 6.

A modification of the connecting part 3, as shown in Figure 6, is shown in Figures 12 and 13. The connecting part 3 has a collar 10 which is circular in cross section. A shank 11 having a conical end portion 5 at its end opposite the collar 10 adjoins the collar 10. Four tool engagement regions 15 formed equidistantly from one another are formed over a common periphery in the end face 14 of the collar 10 opposing the shank 11. The tool engagement regions 15 are designed in the form of recesses which are triangular when viewed in the circumferential direction. Each tool engagement region 15 has a terminal support face

16 for a tool. The design of the tool engagement regions 15 ensures that the connecting part 3 can only rotate in a clockwise direction.

The embodiments of a connecting part 3 shown in Figures 6 to 13 have a central recess 17 through which the mass of the connecting part 3 and therefore also the heat storage capacity are reduced.

Figure 14 shows a device for producing a multi-part assembly, in particular a three-part assembly. The device comprises a frame 18. A travelling slide 19 is arranged on the frame 18. A drive unit 20 which is connected to a clamping unit 21 is arranged on the travelling slide 19. The clamping unit 21 is capable of travelling in the direction of the axis of rotation 22 with the slide 19. The clamping unit 21 is used to clamp a connecting part 3, not shown. A propulsion unit, not shown, is connected to the slide 19 so a connecting part held in the clamping unit 21 can be brought onto and into a structure 4 held in a holding unit 23. The clamping unit 21 is connected to a clutch/brake unit 27.

The clutch/brake unit 27 is shown schematically in Figure 15. The driving shaft 28 coming from the drive unit 20 has, at its front end, a clutch disc 29 which can be brought into frictional contact with an output disc 30. To improve the transmission of the driving moment from the shaft 28 via the clutch disc 29 to the output disc 30, the output disc has an annular friction lining 31. The output disc 31 is formed on a shaft 32 connected to the driving unit 21. The shaft 32 is connected positively and non-positively to the clamping unit 21. The shaft 32 is rotatably mounted in a housing 33 of the clutch/brake unit. The bearings 34, 35 are provided for this purpose.

If the clutch disc 29 makes frictional contact with the output disc 30, the clamping unit 21 is rigidly coupled to the drive unit 20 so the clamping unit 21 is set into rotation when the drive unit is started up. The clutch disc 29 and the output disc 30 can be taken out of contact to allow free running of the shaft 32. For this purpose, the shaft 32 with the output shaft 30 is movably mounted in the direction of the axis of rotation 22.

A brake 34 which is shown schematically in Figure 15 is provided to decelerate the clamping unit 21. The brake 34 is designed in the form of a drum so the outer edge of the output disc 30 can be brought into frictional contact with the internal surface of the drum 34. The drum of the brake 34 is rotationally engaged in the housing 33. It is preferably arranged in the housing 33 in such a way that a braking process is triggered only when the clutch disc 29 and the output disc 30 have been completely separated.

Figure 16 shows the connection between two hollow profiles 24, 25 by the process according to the invention. The hollow profiles 24, 25 can be, for example, legs of a frame connected to one another at an angle of 90. An L-shaped connecting piece 26 penetrates into the hollow profile 24 and 25. The connecting piece 26 is designed in the form of a H-profile in the illustration. The connecting piece 26 is preferably designed to be introduced into the profile 25 or 26 by a clamping effect. The profile 24 is connected to the connecting piece 26 by a connecting part 3 comprising a collar 10 from which a shank 11 extends to the connecting piece 26. An end portion 5 designed in the form of a cone adjoins the shank 11. The end portion 5 penetrates into the connecting piece 26. The collar 10 of the connecting part 3 adjoins the hollow profile 24. The

hollow profile 24 is materially connected to the connecting piece 26 in the region of the common plane of contact 8. The material connection annularly surrounds the shank 11 of the connecting part 3. The connecting part 3 is also materially connected both to the wall of the hollow profile 24 and to the wall of the connecting piece 26.

The hollow profile 25 is connected to the connecting piece 26 by a connecting part 3a. The connection of the connecting part 3a to the hollow profile 25 and the connecting piece 26 as well as the material connection between the hollow profile 25 and the connecting piece 26 are similar to the connection of the connecting part 3 to the hollow profile 24 or of the hollow profile 24 to the connecting piece 26. The connecting part 3a differs from the connecting part shown in figure 16 in that it does not have a collar. Therefore, the connecting part 3a can end with the surf ce of the hollow profile 25.