FIELD OF THE INVENTION
The present invention relates to a method of friction stir spot welding (FSSW) of
CMn440 cold rolled and annealed (CRCA) steel of 1.4 mm thickness within 4
second production time. More particularly, the invention relates to a method of
friction-stir spot welding of high strength automotive grade CMn440 steel sheets
by determining appropriate process parameters.
BACKGROUND OF THE INVENTION
Friction Stir Spot Welding (FSSW) is a process that has recently received
considerable attention in automotive and other industries. A variant of the
"linear" friction stir welding (FSW) process is the friction stir spot welding (FSSW)
process that produces a spot weld in two overlapping metal sheets. FSSW has
proven to be cost effective and productive in joining light weight structural alloys
such as aluminium. Unlike resistance spot welding, FSSW avoids severe heating
and cooling cycles. Furthermore, the success of the solid state approach has
made FSSW an attractive method for welding sheet metal.
In FSSW process, a cylindrical rotating tool with a protruding pin plunges at a
specific rate into the overlapping sheets to a predetermined depth. It is then
retracted at a rapid rate either immediately or after a dwell period. The frictional
heat generated softens the metal and the rotating pin causes material flow in
both circumferential and axial directions. The forging pressure applied by the tool
shoulder results in the formation of an annular, solid state bond around the pin.
The retraction of the pin leaves a characteristic exit hole in the spot welds. The
four main FSSW parameters are tool rotational speed, plunge rate, plunge depth
and dwell time (time prior to retraction of the tool). Although the feasibility of
joining steel sheets using FSSW has been recently considered, information on
how these parameters affect the weld quality is limited. The known prior art
generally deals with tool life rather than proposing a process that meets
acceptable quality and productive standards. According to literature [1], DP600
steel of 1.6 mm thick have been welded applying 1500rpm and different plunge
rates to establish metallurgical bonding within 1.6 to 3.2s. There is however, no
mention about the nugget diameter criteria, performance requirement and actual
properties for steel sheet joints. Hovanski [2] successfully lap joined hot stamped
boron steel by FSSW using PCBN tool. By varying the welding parameters, such
as rotational velocity, plunging velocity, and dwell time, a variation in lap shear
tensile strength from 6-12MPa was obtained. Increased dwell time yielded
between 40-90% direct increases in lap shear strength for all plunge rates.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method of friction-stir spot
welding of high strength automotive grade CMn440 steel sheets by adopting
specific process parameters.
Another object of the invention is to propose a method of friction-stir spot
welding of high strength automotive grade CMn440 steel sheets by producing
weld diameters that match the nugget diameter criteria of resistance spot welds
(i.e. BS1140:1993).
A further object of the invention is to propose a method of friction-stir spot
welding of high strength automotive grade CMn440 steel sheets that displays
mechanical performances better than that of resistance welding.
A further object of the invention is to propose a method of friction-stir spot
welding of high strength automotive grade CMn440 steel sheets in a reasonably
acceptable production cycle time.
SUMMARY OF THE INVENTION
Accordingly, a method to produce lap joints in high strength CMn440 steel sheets
by friction stir spot-welding process using a polycrystalline boron nitride (PCBN)
tool, applying a high rotational speed of 2400rpm, high feed rate of 300
mm/min, and depth of penetration of 2.2 mm is being proposed. Two cold rolled
steel sheets of 1.4mm thickness and with chemical composition (wt%) C= 0.09,
Mn = 0.047, S= 0.0091, P=0.024, having mechanical properties of UTS (453
MPa), YS (350 MPa), elongation (10%) were clamped in lap configuration on the
operating platform of a friction stir welder. A compressor was used to generate
the plunging force during the welding, and a chiller unit was used to maintain
effective cooling of the tool and the anvil. The polycrystalline boron nitride tool,
selected for the purpose, carried-out friction stir spot welding with a rotational
speed of >2400rpm, feed rate of >300 mm/min and depth of penetration of
2.2mm producing weld diameters less than 6mm. The entire welding cycle was
completed within 4 seconds time.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS AND
TABLES:
FIGURE 1 - Shows a photographic view of a Friction Stir Welder.
FIGURE 2 - Shows a pictorial view of a polycrystalline boron nitride tool
used for establishing the present invention.
FIGURE 3 - Shows a specimen with dimension used for friction stir spot welding
according to the invention.
Table 1 - Shows composition (wt%) of the base material used for the
invention.
Table 2 - Shows mechanical properties of the base material used for the
invention.
Table 3 - FSSW parameter and tensile testing data of the welded product of
the invention.
DETAILED DESCRIPTION OF THE INVENTION
The experiments were conducted using CMn440 steel sheets of 1.4mm thickness.
The base material composition and properties are given below.
Table 1 :

Friction stir spot welding was carried out using Friction Stir Welder as shown in
figure 1. An attached compressor generates the plunging force during welding. A
chiller unit maintains effective cooling of the tool and anvil.
In absence of a procedure qualification standard for FSSW process, a Resistance
Spot Welding (RSW) standard BS1140:1993 was considered. Tensile shear test
was made from lap joint sheets of dimension 175 x 45 mm with an overlap of 35
x 45 mm, which is shown in figure 3.
The FSS process parameters include: Feed Rate; Force applied along the Z-axis;
Tool rotation speed (RPM); Depth of penetration; Dwell time or Holding time.
Successful welding was carried out by clamping tow overlapping sheets in the
FSW machine table. The parameters used are given in Table 3. The nugget
diameter is an average of three samples and produced according to BSl 140
standard. Tensile tests of different FSS welded joints were carried out in a
Universal Testing Machine, results of which are given in Table 3.
Table 3 : FSSW parameter and tensile testing data of welded CMn440 steel.
WE CLAIM :
1. A friction stir spot-welding process to produce Lap joints of high strength
CMn440 steel sheets adapting a polycrystalline boron nitride (PCBN) tool,
comprising the steps of :-
- providing at least two metal sheets of base material with desired
thickness and with chemical composition (wt%) c= 0.09, Mn = 0.047,
S= 0.0091, P=0.024, having mechanical properties of UTS (453 MPa),
Ys (350 MPa), elongation (10%);
- disposing the metal sheets on an operating platform of a friction stir
welder having a compressor to generate the plunging force during the
welding, and a chiller unit to maintain effective cooling of a tool and
the anvil, the tool being selected is a polycrystalline boron nitride;
- carrying-out friction stir spot welding with a rotational speed of
2400rpm, feed rate of 300 mm/min and depth of penetration of 2.2
mm. wherein the nugget diameters less than 6mm are maintained,
and wherein the entire welding cycle is completed in 4 seconds time.
2. The process as claimed in claim 1, wherein a fine grain microstructure in the
stirred zone (SZ) and thermo-mechanically affected zone (TMAZ), is
produced, which enables acceptable lap shear strength properties.
3. The process as claimed in claim 1 or 2, wherein the lap shear tensile loads of
the joints are higher than the minimum acceptable loads for the known
resistance spot welds.

The invention relates to a friction stir spot-welding process to produce Lap
joints of high strength Mn440 steel sheets adapting a polycrystalline boron
nitride (PCBN) tool, comprising the steps of providing at least two metal
sheets of base material with desired thickness and with chemical
composition (wt%) c= 0.09, Mn = 0.047, S= 0.0091, P=0.024, having
mechanical properties of UTS (453 MPa), Ys (350 MPa), elongation
(10%); disposing the metal sheets on an operating platform of a friction
stir welder having a compressor to generate the plunging force during the
welding, and a chiller unit to maintain effective cooling of a tool and the
anvil, the tool being selected is a polycrystalline boron nitride; carrying-
out friction stir spot welding with a rotational speed of 2400rpm, feed rate
of 300 mm/min and depth of penetration of 2.2 mm. wherein the nugget
diameters less than 6mm are maintained, and wherein the entire welding
cycle is completed in 4 seconds time.