Projection for resistance welding of soft metals

A projection for use in electrical resistance welding of sheet metals and thin-wall parts, a method of fabricating such projection and the use of the same are disclosed. The flanks of the projection possess internally thereof an angle of about 40.degree. to 75.degree. and externally an angle of about 40.degree. to 100.degree., and the strength of the material at the region of the flanks is about 50% greater than the strength of the neighboring basic material. The method of forming the projection contemplates fabricating the same through plastic deformation of the basic material and the dimensionally exact shaping is accomplished by compression molding of the flowable metal between a punch and die of a tool.

BACKGROUND OF THE INVENTION 
The present invention relates to an improved design of projection for use 
in electrical resistance welding of sheet metal and thin-wall parts, a 
method of fabricating such projection and to the use of such projection. 
During the mass production of parts by resistance welding it is projection 
welding of ferrous metals which constitutes one of the most widespread 
used welding techniques for obtaining high quality connections. In 
contrast to spot welding during projection welding the expanse or extent 
of the weld connection is not governed by the cross-sectional area of the 
electrode, rather by the cross-sectional area of the projection. On the 
one hand, there is thus exactly defined and maintained constant the 
welding surface and therefore the current path, the current density and 
the surface compression and, on the other hand, there also exists the 
possibility of simultaneously welding a multiplicity of projections and 
under exactly the same conditions and of preventing current shunts. 
In the case of non-ferrous metals, especially, aluminium and aluminium 
alloys, it is not possible to positively perform the known projection 
welding when working with thin sheets. This is predicated upon the fact 
that aluminium, in contrast to iron, opposes the welding current with 
considerably less electrical resistance and at the same time appreciably 
better conducts the thermal energy which is produced by the current, and 
hence, conducts such away from the site which is to be welded. A further 
difficulty in addition thereto is that upon application of the force 
needed for welding at the workpiece, i.e. the projection, it is necessary 
for such to be larger than in the case of iron and such projection 
collapses before the current can be turned-on. If there is used a smaller 
electrode force then the projections can score or burn before there arises 
the actual welding operation because at the contact region between the 
projection and sheet metal there is present too great resistance. 
Therefore it has already been proposed to undertake measures wherein the 
electrode is applied extremely gently at the workpiece and by means of an 
adjustable timing mechanism the complete electrode force is first then 
applied directly prior to the presence of the welding current. 
Also in the case of machines working with program controls for the mutual 
timewise coordination of the course of the current and pressure the 
heretofore known methods have not up to now enabled any satisfactory 
randomly reproducible projection welding for the mass production of parts. 
Equally, in the pertinent more recent publications, for instance 
"Resistance Welding Manual", Volume 1, page 44 and Aluminium-Taschenbuch 
13, Aufl. 1974, page 581, there is repeatedly mentioned that projection 
welding of aluminium can not be positively carried out. Significant in 
this respect is also the work of S. A. Westgate and R. M. Rivett, entitled 
"Effect of Projection Design when Welding Single Projections in Aluminium 
Autobody Sheet", March 1979, 89/1979, of the Welding Institute. 
Nonetheless while a further publication authored by Pfeifer, entitled 
"Fachkunde des Widerstandsschweissens", states broadly at page 44 thereof 
that projection welding of light metals is possible "with suitable 
machines", yet there is not given any specific teaching as to how such 
type welding is capable of being performed. 
From the prior art publications and from practice it is known to weld 
aluminium with embossed projections and this technique also has been found 
to be useful. Embossed projections only can be provided at solid parts, 
for instance handle projections or extensions of pans; at sheet metal 
parts or other thin-wall parts it is totally impossible to economically 
produce solid projections. 
SUMMARY OF THE INVENTION 
Therefore, with the foregoing in mind it is a primary object of the present 
invention to overcome the aforementioned drawbacks and limitations 
prevailing in the projection welding art. 
Another and more specific object of the present invention is to provide a 
novel construction of projection by means of which it is possible to 
positively projection weld sheets formed of aluminium, aluminium alloys 
and the like, especially for performing multiple projection welding. 
A further important object of the invention is to provide a new and 
improved method of fabricating a projection used in resistance welding and 
which projection design is extremely suitable for industrial mass 
production. 
Another significant object of the present invention is to weld aluminium 
and aluminium alloys with a minimum expenditure of energy and with minimum 
loading of the power network. 
Still a further significant object of the invention resides in providing 
projection weld connections of aluminium and aluminium alloys so as to 
have essentially metallurgically faultless structure and reproducibility. 
Now in order to implement these and still further objects of the invention, 
which will become more readily apparent as the description proceeds, the 
invention contemplates a projection for the electrical resistance welding 
of sheet metals or sheets, wherein the flanks of such projection 
internally possesses an angle of about 40.degree. to 75.degree., 
preferably 45.degree..+-.5.degree., and externally an angle of about 
40.degree. to 100.degree., preferably 45.degree..+-.5.degree., and the 
strength of the material at the region of the flanks is in the order of 
about 50% greater than the strength of the neighboring base or basic 
material. It is particularly desirable if both angles are the same, and 
specifically amount to 45.degree..+-.5.degree.. 
According to the invention the projection may be ring-shaped or annular and 
its central circular surface is located in the same plane as the 
neighboring base material. Further, the projection may have a 
substantially line-like configuration, in other words, can constitute a 
longitudinal projection. 
There is also disclosed a method of fabricating an annular or longitudinal 
projection, especially a projection at sheets and thin-wall parts, wherein 
the projection is produced by plastic deformation of the base material and 
the dimensionally exact shaping is obtained by compression molding of the 
flowable metal between a punch and die of a tool.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Describing now the drawings, in FIG. 1 there is shown a solid embossed 
projection 1 as the same can be produced at solid light metal workpieces 2 
by deformation, for instance by embossing or by being affixed thereat. 
Such projections 1 are capable of withstanding the electrode force needed, 
on the one hand, for maintaining a uniform electrical transition 
resistance between the electrode surfaces 3 and the workpieces 2 and 4 to 
be welded and, on the other hand, between such workpieces 2 and 4. 
FIG. 2 illustrates another conventional hollow embossed circular or round 
projection 5 as the same is generally employed for iron sheets. When 
working with light metals the projection 5 tends to collapse before the 
welding current can be turned-on and there cannot be produced a useful 
weld connection. 
FIG. 3 illustrates in cross-sectional view a longitudinal or substantially 
line-shaped or linearly extending projection 16 for aluminium and 
aluminium alloys which can be used for welding narrow flanges where there 
is not available sufficient space for an annular projection. Both of the 
flanks 18 and 19 have an approximately 50% greater strength than that of 
the base or basic material by virtue of the great plastic deformation of 
the base material 17 arising in the forming or shaping tool. The flanks 18 
and 19 of the wedge-shaped projection 16 which is form pressed out of the 
base material 17, in other words the sheet metal, internally enclose an 
angle .alpha. of about 60.degree. and externally an angle .beta. of about 
90.degree.. 
FIG. 4 illustrates a construction of an annular or annular corrugated 
projection 7 as the same has been found to be particularly advantageous 
for the welding of aluminium and aluminium alloys. The projection 7 
distinguishes itself externally from the annular projections which are 
used when working with steel particularly by virtue of the smaller 
relationship d/b.about.1.5 to 3 which, in this case, if desired, produces 
a completely welded nugget according to FIG. 9. In the foregoing equation 
d is the mean or intermediate diameter of the annular projection and b is 
the flange width. 
At the regions 11 and 12 of the flanks there is also obtained for such 
projection configuration or shape, due to the deformation work, an 
increase in the strength of the base material by about 50%. Not merely the 
strength increase at the projection flanks 11 and 12, but rather with this 
shape particularly also the central circular surface 13 which remains 
locally unaltered in relation to the surrounding material 10, can transmit 
an appreciable portion of the electrode force to the inner flank 12 of the 
projection 7. The total electrode force therefore is uniformly distributed 
within the projection 7 over both of the flanks 11 and 12. The force lines 
in the projection 7 extend approximately parallel and are only slightly 
inclined with respect to the vertical through the welding plane. At the 
incipient phase of the compression of the projection 7 under the action of 
the electrode force F and the heating by virtue of the welding current I 
there is appreciably reduced the inclination of the force lines. The 
magnitude of the specific surface compression between the workpieces is 
essentially maintained. 
Of course the diameter d of the projection 7 may not have any random size, 
since the welding current requirement increases approximately as the 
square of the diameter d. In the case of a sheet thickness amounting to 
1.05 mm it has been found to be advantageous to provide a diameter d=3 mm 
and a projection height h=0.7 mm, for instance when working with a 
material AlMg 0.4 Si 1.2. A comparison with a circular projection having a 
height h of 1.1 mm and the same diameter d has shown that upon loading 
with a force F=200 daN of the projection of FIG. 2 there arises a 
permanent deformation of only 8% of h; on the other hand in the case of 
the circular projection there is present a permanent deformation of 54% of 
h. 
In contrast to the heretofore known mode of fabrication of the projections 
when working with iron materials, wherein such are freely flow formed 
(FIG. 7), the projections fabricated according to the invention are form 
pressed or compression molded (FIG. 8). The geometric shape of the 
projections, especially the flanks 11, 12 (FIG. 4) and 18, 19 (FIG. 3) is 
thus identical for all projections at a workpiece. The plastic deformation 
of the base material is accomplished with a shape-imparting punch 20 and a 
shape-imparting die 21. 
In particular, it is not only possible to maintain by means of the forming 
or shaping tools 20 and 21 of FIG. 8, on the one hand, the angles .alpha. 
and .beta., rather these angles can be even first obtained and, on the 
other hand, the desired deformation and thus strengthening of the material 
at the flanks 11, 12 and 18, 19 can be produced. What is of significance 
is that in the case of an annular projection the central circular surface 
13 possesses at least the level of the unaltered remaining surface of the 
sheet adjacent the projection. It has been found to be particularly 
advantageous to have both angles .alpha. and .beta. of the same magnitude, 
and in particular values of .alpha. and .beta. of each 
45.degree..+-.5.degree. have to be preferred. 
It should be apparent to those skilled in the art that a qualitatively good 
weld is not only solely dependent upon the shape or configuration of the 
projections, but rather also upon the material which is to be welded and 
the course as a function of time of the electrode force, and particularly 
the welding current plays an appreciable role. 
The high thermal conductivity of aluminium and its alloys requires that the 
welding energy, i.e. the welding current be applied in an extremely short 
amount of time. In addition thereto there is present the fact that 
aluminium materials, for the stated reasons (high electrical conductivity, 
lower internal electrical resistance) as is well known require greater 
currents than iron materials. 
With the described construction of the projections in the forms shown in 
FIGS. 3 and 4 together with a course of the welding current and electrode 
force as shown in FIGS. 5 and 6 it is possible to produce in a most 
surprising manner qualitatively extremely upright, randomly reproducible 
weld connections according to the micrograph of FIG. 9. 
In FIGS. 5 and 6 reference character 14 designates the current course 
during the time t.sub.1 t.sub.2 and t.sub.1 t.sub.3, respectively. 
Reference character 15 designates the course of the electrode force at the 
time t.sub.o to t.sub.4. The current curve 14 of FIG. 6 possesses in 
relation to that of FIG. 5 a so-called post glowing or incandescence phase 
of t.sub.2 t.sub.3. Such post incandescence phase can be useful or 
necessary when working with certain materials, in order to prolong the 
cooling time, and thus, to obtain an improved recrystalization. An 
additional increase of the welding pressure, demonstrated by the chain-dot 
course of the electrode force 15 shown in FIGS. 5 and 6, at the end of the 
welding phase contributes to preventing the formation of blow holes and 
fissures in the welding nugget. 
FIG. 9 illustrates in cross-sectional view a welding nugget 22 of an 
annular projection weld between two sheet metal elements or sheets 23 and 
24 of different aluminium alloys. In neither of both sheets 23 and 24 have 
the surfaces been freed of the oxide layer. Nonetheless the welding nugget 
is completely homogeneous, symmetrical and without inclusions. A further 
factor which is to be observed is the configuration of the electrode and 
its moved mass, upon which depends the so-called follow-up behaviour. The 
electrode must be able to follow the fusing or melting projection free of 
delay during the extremely short welding time t.sub.1 t.sub.2, in order to 
prevent any spattering away of the liquid material. 
The electrodes are preferably designed so as to have large surface areas, 
so that between the workpiece and the electrode there appears a low 
current density, and thus, the electrode is subjected to extremely low 
wear. It is particularly advantageous to use a frequency converter-welding 
machine for performing the described welding operation. Through the use of 
direct current-like single pulse welding operation it is possible to apply 
to the workpiece the welding energy in the shortest amount of time. 
During the simultaneous welding of a multiplicity of projections the 
electrodes are advantageously mounted to be movable and resiliantly 
suspended, so that each individual projection can be impinged under 
exactly the same conditions with the electrode force and the welding 
current. 
In contrast to the heretofore more or less positively carried out weld 
connection of aluminium and aluminium alloys, among other things, by spot 
welding, the inventive weld connection affords a multiplicity of 
advantages which, by way of example and not limitation, can be enumerated 
as follows: 
(A) Production increase by a multiple because it is possible to 
simultaneously weld a plurality of projections without there being formed 
current shunts and there being required uneconomically high welding 
currents, and as a consequence of which there is obtained: 
(i) uniform mechanical strength of the connections; 
(ii) no change of the strength upon current fluctuations; 
(iii) additional production increase since there is low negligible 
electrode contamination and wear by virtue of the lower current density at 
the contact location; 
(iv) constant uniform welding quality due to the omission of alloy 
depositions at the electrode; 
(v) proper position of the weld connection at the workpiece due to the 
prior formed projection; and 
(vi) no cleaning of the oxidized sheet metal surfaces by pickling, brushing 
and so forth prior to welding. 
While there are shown and described present preferred embodiments of the 
invention, it is to be distinctly understood that the invention is not 
limited thereto, but may be otherwise variously embodied and practiced 
within the scope of the following claims. ACCORDINGLY,