Method of friction welding

A method of friction welding wherein the parts to be welded, of materials with inherent Young's moduli, are jigged coaxially, set to rotate with respect to each other under an axial pressure recurrently varying with a frequency which is in inverse proportion to the lower value of the Young's moduli and to the diameter--or to the smaller diameter--of the parts while these are being heated up for welding, whereby, the value of the frequency varies over a range between + and -15% and the parts are then upset-forged together.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to methods of pressure welding and has specific 
reference to friction welding. 
The present invention may be of utility in mechanical and electrical 
engineering, tool manufacture and in the chemical industry. 
2. Description of the Prior Art 
The invention holds out special promise in friction welding of materials 
with distinctly different mechanical and thermal properties and those 
which tend to form scale at their end faces. Friction welding under 
constant axial pressure is inapplicable in the above cases due to 
non-uniform distribution of the temperature pattern over the cross section 
of the parts welded, which impairs weld quality. The known methods of 
friction welding under a pulsatory axial pressure provide for a better 
distribution of the temperature pattern over the heat-affected zone. 
However, no adequate uniformity in the distribution of the temperature 
pattern can be obtained in welding materials with distinctly different 
mechanical and thermal properties. Poor penetration at the centre and 
periphery of the weld is likely to occur in this case. 
Known in the art is a method of friction welding (cf. V.I. Vill "Friction 
Welding of Metals", Leningrad, Mashinostroenie, 1970, p. 61 wich is 
realized practically in all friction welding machines. It consists in 
jigging coaxially the parts to be welded, setting them to rotate with 
respect to each other under a constant axial pressure and upset-forging 
the parts together on heating up the weld area adequately. 
The known method fails to provide for a uniform redistribution of the 
temperature pattern over the weld area when dissimilar materials having 
distinctly different mechanical and thermal properties are being welded. 
Weld quality and reliability consequently suffer. A high axial pressure 
applied during the welding may bring about an X-shaped distortion of the 
heat-affected zone resulting in poor penetration at the centre. A low 
axial pressure may cause convexity of the heat-affected zone accompanied 
by poor penetration at the periphery of the weld. 
Also known is a method of friction welding (cf. USSR Inventor's Certificate 
No. 1209398, IPC B23K 20/12, published in 1986) consisting in jigging 
coaxially the parts to be welded the materials whereof have inherent 
Young's moduli, setting the parts to rotate with respect to each other 
under an axial pressure which varies recurrently in order to heat the 
parts up and upsetforging the parts together on heating them up. 
In this method of welding, the axial pressure applied in order to heat up 
the work changes from a minimum value to a maximum one during every period 
of a different frequency. Consequently the value of the maximum pressure 
rises incessantly. This fact widens the field of application of the 
method, rendering the machines capable of welding the parts with a 
diameter which is larger than one for which they have been designed. 
However, a variable frequency with which the axial pressure is recurrently 
changed fails to provide for a redistribution of the temperature pattern 
in a way eliminating poor and non-uniform heating up of the weld zone. 
A rise in the axial pressure to a maximum value in the course of several 
applications brings about a thermal effect during the heating up which 
changes in the direction from the centre to the periphery of the weld in 
materials with distinctly different mechanical and thermal properties. 
Poor penetration at the periphery and an unjustified lengthening of the 
heating up period are likely to occur. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a method of friction welding 
which would intensify the process. 
Another object of the invention is to improve weld quality, preferably in 
welding materials with distinctly different mechanical and thermal 
properties. 
These objects are realized in a method of friction welding effected by 
jigging coaxially the parts of materials with inherent Young's moduli for 
welding, setting them to rotate with respect to each other under an axial 
pressure varying recurrently in the course of heating up and upset-forging 
the parts together whereby in accordance with the invention the 
recurrently variable axial pressure is applied to the parts in the course 
of heating them up with a frequency which is in inverse proportion to the 
lower value of the Young's moduli and to the diameter--or to the smaller 
diameter--of the parts and the value of the frequency varies over a range 
between + and -15%. 
It is expedient to measure--in the event of scale formation at the end face 
of at least one of the parts--the friction torque set up in the course of 
heating up the parts, discontinue the application of the axial pressure 
varying recurrently with a set frequency when the friction torque acquires 
a steady-state value and apply a constant axial pressure to the parts 
until they begin to become upset-forged together. 
Owing to the disclosed method of friction welding the temperature pattern 
is redistributed towards and away from the centre alike. As a result, the 
process is intensified and the parts are uniformly heated up over the 
entire cross-sectional area. A uniform thermal impact the heat-affected 
zone is exposed to improve weld quality, specifically when the metals 
welded have distinctly different properties--as this is the case in 
welding vanadium and steel, molybdenum and tungsten--or when scale is 
formed on the work.

DESCRIPTION OF THE PREFERRED METHOD 
The disclosed method of friction welding can be realized on any known 
machine, a conventional or an inertia-type machine, and is essentially as 
follows. 
The parts to be welded of materials with inherent Young's moduli are jigged 
coaxially and set to rotate with respect to each other. They may have the 
same or different diameters and their materials may be dissimilar with 
distinctly different properties such as melting point, ultimate strength, 
hardness, Young's modulus, etc. An axial pressure is applied to the parts 
welded which varies recurrently with a frequency determined by the 
mechanical and thermal properties of the materials of the parts and by 
their diameters. To be more precise, the frequency is inversely 
proportional to a the lower value of the Young's moduli of the materials 
of the parts and to the diameter--or to the smaller diameter--of the parts 
which is decided by the required strength of the weld, whereby the value 
of the frequency also varies over a range between + and -15%. The value of 
the frequency, f, of applications of the axial pressure P.sub.1 is given 
by the formula derived experimentally: 
##EQU1## 
where E is the lower value of the Young's moduli, MPa, and d is the 
smaller diameter of the parts welded, mm. 
FIG. 1 illustrates the relationship between such variables as the axial 
pressure P.sub.1 applied during the heating up of the parts subjected to 
friction welding (segment OA, curve a), the pressure P.sub.2 applied in 
order to upset-forge the parts together (segment BC, curve a), the speed 
of rotation n during inertia-type welding (curve b.sub.1) and conventional 
welding (straight line b.sub.2) which are all plotted on the ordinate and 
the time t plotted on the abscissa. It will be noted that the axial 
pressure P.sub.1 rises in the beginning from zero to a maximum P.sub.max 
which is constant during every period and is decided by the mechanical and 
thermal properties of the materials of the parts and by their dimensions, 
roughly equalling the value of the constant axial pressure used during 
friction welding. After that the pressure decreases to a value P.sub.min 
which is chosen on condition that the oxide films be removed from the weld 
zone. Variations in the axial pressure P.sub.1 go on to recur with a set 
frequency f until an instant of time t.sub.1 when, in the case of 
inertia-type friction welding, the relative rotary motion of the parts is 
discontinued (the speed n decreases from a maximum to zero, curve b.sub.1) 
or, in the case of conventional friction welding, the parts are stopped 
(the speed n is constant up to the instant t.sub.1 and becomes then zero, 
straight line b.sub.2). The number of periods of application of the axial 
pressure P.sub.1 is determined experimentally for given materials. 
The variable axial pressure can be applied on any known friction welding 
machine by any means: hydraulic, pneumatic or electromagnetic. 
By applying a recurrent axial pressure with the above-indicated frequency 
in the course of heating up the parts, the zone of maximum heat generation 
travels over the surface of friction between the parts welded. Owing to 
that the parts welded. Owing to that the flow stress decreases, the oxide 
and greasy films are destroyed and a weld with a fine-grained homogeneous 
structure is formed. The period of heating up the parts for welding 
shortens, for a uniform temperature pattern is achievable not only by 
virtue of heat conduction in the materials but by a travelling zone of 
maximum heat generation which displaces from the centre of the weld to its 
periphery and in the reverse direction. 
The effect of the recurrently variable axial pressure decreases if the 
frequency exceeds the calculated value by more than 15% because of the 
rate of displacement of the zone of maximum heat generation failing then 
to coincide with that of the axial pressure variation. 
On ceasing to apply the axial pressure P.sub.1, a constant pressure P.sub.2 
(segment BC, curve a) causing the parts to upset-forge together is 
applied. It equals or exceeds the maximum pressure P.sub.max and is 
applied up to an instant t.sub.2 which is determined experimentally. 
If scale is formed at the end face of at least one of the parts welded, the 
friction torque M set up in the course of heating up is measured by any 
known means. FIG. 2 illustrates the way in which M varies with time (curve 
b) and also the relationship between the axial pressure P.sub.1 (segment 
OA, curve a) and the upset-forging pressure P.sub.2 (segment BC, curve a) 
which are laid off as ordinates and time t laid off as abscissa. 
Referring to curve b of FIG. 2, the friction torque M sharply rises in the 
beginning and gently lowers to a constant value which becomes evident at 
the time t.sub.1 '. The recurrently variable axial pressure P.sub.1 
applied with the set frequency f before the instant t.sub.1 ' as described 
hereinabove destroys the layer of scale and disposes it of from the weld 
zone. Further heating of the parts up to the instant t.sub.1 is effected 
under a constant axial pressure P.sub.1 which equals or approaches the 
value of P.sub.max. The upset-forging pressure P.sub.2 is then applied up 
to the instant t.sub.2 (segment BC, curve a) as described hereinabove. 
The invention will be best understood from the examples of its preferred 
embodiment given below. 
EXAMPLE 1 
Vanadium and low-carbon steel parts with a diameter d=30 mm were friction 
welded in the conventional way. The composition of the steel in wt % was 
as follows: C, 0.25-0.28; impurities, .ltoreq.1; Fe, the balance. Assuming 
that Young's modulus of vanadium was E.sub.1 =1.33.multidot.10.sup.5 MPa 
and that of steel was E.sub.2 =2.0.multidot.10.sup.5 MPa, the value of 
E.sub.1 was used to compute the frequency f of axial pressure variations. 
The parameters of welding were as follows: n=100 s.sup.-1 ; P.sub.max =100 
kN; P.sub.min =20 kN; P.sub.2 =200 kN; f=2 Hz. The tensile test of the 
work which had been welded under the axial load applied with a frequency 
within the range of .+-.15% of the computed value of f ended in failure of 
the vanadium part. The tests of the work welded by using a frequency 
outside the specified 15-percent range, e.g. with a frequency f.gtoreq.2.3 
Hz and f.ltoreq.1.7 Hz, ended in failure of the weld. 
EXAMPLE 2 
Two structural steel parts with diameters d.sub.1 =16 mm and d.sub.2 =50 mm 
were friction welded in the conventional way whereby scale was formed at 
the end faces. The composition of the steel in wt % was as follows: C, 
0.4; Cr, 1; Fe, the balance. Young's modulus of the steel was 
E=2.0.multidot.10.sup.5 MPa. The smaller diameter, d.sub.1 =16 mm was used 
to compute the frequency. The parameters of welding were as follows: n=100 
s.sup.-1 ; P.sub.max =25 kN; P.sub.min =15 kN; P.sub.2 =40 kN; f=2.5 Hz. 
No satisfactory welds were obtained when the value of f was greater than 
2.8 Hz and smaller than 2.1 Hz. The friction torque M was measured and a 
constant axial pressure of 25 kN was applied after the friction torque had 
become constant. 
EXAMPLE 3 
A part of heat resistant steel (composition in wt %: C, 0.45; Cr, 14; Ni, 
14; V, 2; Fe, the balance) with a diameter d=16 mm was friction welded on 
an inertia type machine with a part of structural steel (composition in wt 
%: C, 0.4; Cr, 1; Fe, the balance) and with the same diameter. Young's 
modulus of the heat-resistant steel was E.sub.1 =2.3.multidot.10.sup.5 MPa 
and that of the structural steel was E.sub.2 =2.0.multidot.10.sup.5 MPa. 
The frequency f of axial pressure variations was computed on the basis of 
E.sub.2. The parameters of welding were as follows: n=270 s.sup.-1 ; 
P.sub.max =20 kN; P.sub.min =13.5 kN; P.sub.2 =35 kN; f=2.5 Hz. The weld 
was completed in t.sub.2 =4 s whereas the welding on an inertia type 
machine under a constant axial pressure P.sub.1 =20 kN took a period of 
t.sub.2 =5.1 s.