JOINING OF DISSIMILAR ALLOY SHEETS (AL 6063&AISI 304) DURING
RESISTANCE SPOT WELDING PROCESS: A FEASIBILITY STUDY FOR AUTOMOTIVE INDUSTRY
Reddy Sreenivasulu
Department of Mechanical Engineering
R.V.R. & J.C.College of Engineering (Autonomous)
Chowdavaram, Guntur, AndhraPradesh, India
E-mail: rslu1431@gmail.com
Submission: 10/05/2014
Revision: 24/05/2014
Accept: 25/05/2014
ABSTRACT
Present design trends in automotive
manufacture have shifted emphasis to alternative lightweight materials in order
to achieve higher fuel efficiency and to bring down vehicle emission. Although
some other joining techniques are more and more being used, spot welding still
remains the primary joining method in automobile manufacturing so far. Based on
the literature survey performed, venture into this work was amply motivated by
the fact that a little research work has been conducted to joining of
dissimilar materials like nonferrous to ferrous. Most of the research works
concentrated on joining of different materials like steel to steel or aluminium
alloy to aluminium alloy by resistance spot welding. In this work, an
experimental study on the resistance spot weldability of aluminium alloy (Al
6063) and austenitic stainless steel (AISI304) sheets are lap joined by using a
pedestal type resistance spot welding machine. The weld nugget diameter, force
estimation under lap shear test and T – peel test were investigated using
digital type tensometer attached with capacitive displacement transducer (MIKROTECH;
BANGALORE; Model: METM2000ER1). The results shows that joining of Al 6063 and
AISI 304 thin sheets by RSW method is feasible for automotive structural joints
where the loads are below 1000N act on them, it is also observed that by
increasing the spots per unit length, then the joint with standing strength to
oppose failure is also increased linearly in case of interfacial failure mode
and nonlinear for pullout failure mode.
Keywords: Dissimilar alloys, Resistance
Spot Welding, Failure mode, Al 6063 alloy sheet, AISI 304 sheet.
1. INTRODUCTION
Materials
for motor vehicle applications are required to maintain the integrity of the
structure (i.e. to be sufficiently robust to withstand their service
environment) and to be inert (i.e. corrosion resistant). Stainless steels are
used in motor vehicle applications because they are resistant to corrosion and
high temperature oxidation, offer energy absorption properties and maintain
their mechanical properties over a wide temperature range (ETAL, 2012).
During
recent years, the use of joints between dissimilar materials has considerably
increased. Conventional structures made of steel have been replaced by lighter
materials, capable of providing high mechanical strength, lower volume of
material and good corrosion resistance. In the developing of new technologies
for the aerospace industry, these junctions are of great importance, because
they allow the systems, subsystems and components manufactured in stainless
steel and aluminum alloy to be structurally united.
The
difficulties in the welding of aluminum alloy with stainless steel by fusion
welding processes have been a great challenge for engineering, because they
result from hard and brittle intermetallic phases that are formed between
aluminum and steel at elevated temperatures (KHAN; KUNTZ; ZHOU, 2008).
Spot
welds made by resistance welding are the primary method of joining in
automobile industry, quality of resistance spot welds is one of the major
concerns. In
resistance spot welding overlapping sheets of metal are joined by applying
electric current and pressure in the zone to weld with copper electrodes, as
illustrated in Figure1, copper is used for electrodes because it has low
electrical resistance and high thermal conductivity.
Spot welding operation is composed
of three steps that are the squeezing, welding and holding stages. Squeezing
consists of applying the weld force to the work-pieces in order to obtain the
appropriate amount of pressure, prior to welding. During welding, the electric
current passes through the work-pieces, while the welding force is maintained,
generating heat (SHAMSUL; HISYAM, 2007).
In the course of the holding stage
current is switched off and weld force maintained, allowing the weld to forge
and cool under pressure. Although some other joining techniques are more and
more being used, spot welding still remains the primary joining method in
automobile manufacturing so far.
Figure 1: Principle of Resistance Spot Welding
Source: Shamsul and Hisyam (2007)
1.1.
Relation between Welding Current and Time
Heat developed during welding is
proportional to time and to square of current. Though both parameters are
responsible for heat generation, the weld heating rate is determined only by
current, because heat lost to the work-piece and to copper electrodes increases
with weld time.
Heat lost to the work-piece
increases heat affected zone and thermal distortion, while heat in the
electrodes can degrade them, all being undesirable effects. The level of
current required for any metal tends to be inversely proportional to its
electrical and thermal resistivity’s (POURANVARI, et al., 2008).
Figure 2: Schematic representation of current-time relationship and change in resistance during RSW
Source: Pouranvari, et al. (2008)
1.2.
Failure Modes in RSW
There are two types of failure modes
possible in resistance spot welds while doing static tensile-shear test. They
are NUGGET PULLOUT and INTERFACIAL FRACTURE. When the welding current is varied
failure mode changes. During tensile-shear test, the shear stress at the sheet/sheet
interface is the driving force for the interfacial mode, and the tensile stress
at the nugget circumference is the driving force for the pullout failure mode (MAJID,
2011).
Each driving force has a critical
value and the failure occurs in a mode when its driving force reaches its
critical value, sooner. The Fusion zone size is the governing parameter
determining stress distribution. For small weld nuggets, the shear stress
reaches its critical value before the tensile stress causes necking; thus,
failure tends to occur under interfacial mode. Therefore, there is a critical
weld Fusion Zone size beyond which, the pullout failure mode is expected (KAH; MARTIKAINEN, 2012).
Figure 3: Types of Failure modes in RSW
Source: KAH and MARTIKAINEN
(2012)
Spot welds for automotive
applications should have a sufficiently large diameter, so we require nugget
pullout failure mode. Interfacial mode is unacceptable due to its low load
carrying and energy absorption capability. Interfacial Failure (or nugget fracture)
of the weld nugget through the plane of the weld- the dominant failure mode for
small diameter spot welds. When the load is increased, localized necking
occurs.
2. LITERATURE STUDY
Car designers today seek materials
with the very best stiffness, mass reduction, and safety performance. The
competition between different materials for structural applications in cars is
intense. Choice centers on mass saving, formability, weldability, corrosion
resistance, fatigue resistance, cost, and environmental factors. Safety and
crashworthiness, especially, should take priority.
Austenitic stainless steels are
preferred materials for structural frameworks and body paneling of buses and
coaches. Experience gained in these contexts can be readily transferred to the
automotive sector. Stainless steel is an excellent candidate for car body
structural applications. Besides offering weight savings, enhanced
crashworthiness, and corrosion resistance, it can also be recycled (HAYAT, 2011).
The material blends tough mechanical
and fire-resistant properties with excellent manufacturability. Under impact,
high-strength stainless steel offers excellent energy absorption in relation to
strain rate. It is ideal for the revolutionary “space frame” car body-structure
concept. Weldability of a material is one of the key factors governing its
application in the auto industry. Resistance spot welding is widely used to
join sheet metals in the automotive industry.
The quality and performance of the
spot welds significantly affect the durability and safety design of the
vehicles. Therefore, the failure characteristics of spot welds are very
important parameters for the automotive industry. Failure mode of resistance
spot welds (RSWs) is a qualitative measure of mechanical properties. Demands
for improved productivity, efficiency, and quality pose challenges to the
welding industry.
As materials become ever more
sophisticated in their chemical composition to provide ever-better functionally
specific properties, a more complete and precise understanding of how such
materials can be joined for optimal effectiveness and efficiency will become
essential. Traditional options for welding will surely evolve, sometimes to
provide unimagined capabilities. In addition, totally new methods will almost
certainly emerge as evolution of materials gives way to revolution to meet
unimagined new designs and design demands. Kah and Martikainen (2012) discussed
some of the role and future direction of welding technology, welding materials,
productivity and efficiency, education and safety having an impact on future
growth in welding technology.
Analysis of drivers and the key
needs of some manufacturing industries have been researched, giving general
trends and strong indications as to expected trends in technology that will be
seen in the future. It also provides a good foundation for future research and
creates awareness of the developmental direction of welding processes and
materials in manufacturing industries (SCHUBERTH, et al., 2008).
Figure 4: Requirements
for welding production technology permitting its integration to automatization
Source:
Schuberth, et al. (2008)
Failure mode of AISI304 resistance
spot welds is studied by Majid Pouranvari etal under quasi-static tensile-shear
test. Their results showed that the conventional weld size recommendation of
4t0.5 is not sufficient to guarantee pullout failure mode for AISI304 steel
RSWs during tensile-shear test. Shamsul and Hisyam (2007) studied on austenitic
stainless steel types 304 were welded by resistance spot welding.
The relationship of nugget diameter
and welding current was investigated. Hardness distribution along welding zone
was also investigated. The results indicated that increasing welding current
gave large nugget diameter. The welding current did not much affect the
hardness distribution. Hayat (2011) studied on resistance spot weldability of
180 grade bake hardening steel (BH180), 7123 grade interstitial free steel
(IF7123) and 304 grade austenitic stainless steel (AISI304L) with each other
was investigated. it was determined that with increasing weld time, tensile
shear load bearing capacity (TLBC) increased with weld time up to 25 cycle and
two types of tearing occurred.
It was also determined that while
the failure occurred from IF side at the BH180+IF7123 joint, it occurred from
the BH180 side at the BH180+AISI304L joint. R.K Rajkumar, Fatin Hamimi etal
discussed in their paper about spot welding of dissimilar materials. A good
weld from spot welding mechanism is what most of the manufacturers preferred
and desired for mechanical assemblies in their systems.
The robustness is mainly relies on
the joining mechanism of mechanical parts; especially when combining two
different materials and therefore this paper analyzes the spot weld growth on
302 austenitic stainless steel and low carbon steel of 1mm of thickness.
Ladislav Kolarik etal presented an analysis of the properties of resistance
spot welds between low carbon steel and austenitic CrNi stainless steel. The
thickness of the welded dissimilar materials was 2 mm (HAYAT, 2011).
A DeltaSpot welding gun with a
process tape was used for welding the dissimilar steels. Resistance spot welds
were produced with various welding parameters (welding currents ranging from 7
to 8 kA). Light microscopy, micro hardness measurements across the welded
joints, and EDX analysis were used to evaluate the quality of the resistance
spot welds. The results confirm the applicability of DeltaSpot welding for this
combination of materials.
3. EXPERIMENTAL SET UP
An austenitic stainless steel
(AISI304) and Al 6063 sheet of 1.2 mm thick was used as the materials (samples
size as per ANSI/AWS/SAE/D8.9-97
standards are shown in Figure 8). Resistance spot welding was performed using a
pedestal type resistance spot welding machine. Welding was conducted using a
45-deg truncated cone electrode with 10-mm face diameter.
Welding current was varied from 5 kA
to 10 kA and welding time, electrode pressure and holding time were fixed at 10
cycles, 2 bar and 30 cycles, respectively. The tensile-shear tests were
performed at a cross head of 2 mm/min with a tensometer. The Failure mode was determined from the
failed samples.
Figure
5: Resistance Spot Welding Equipment (pedestal type) & Specimens during resistance spot welding for lap and
T-joints
Figure
6: Digital Tensometer attached with capacitive type displacement transducer
(Mikrotech, Bangalore, model: METM2000ER1)
Figure
7: failure mode of lap shear test specimen & failure mode of T-peel test
specimen
Table 1: Typical chemical composition
and physical properties for aluminium alloy 6063
|
Element |
% Present |
|
|
Si |
0.2 to 0.6 |
|
|
Fe |
0.0 to 0.35 |
|
|
Cu |
0.0 to 0.1 |
|
|
Mn |
0.0 to 0.1 |
|
|
Mg |
0.45 to 0.9 |
|
|
Zn |
0.0 to 0.1 |
|
|
Ti |
0.0 to 0.1 |
|
|
Cr |
0.1 max |
|
|
Al |
Balance |
|
Property |
Value |
|
Density |
2700 kg/m3 |
|
Melting Point |
600°C |
|
Modulus of Elasticity |
69.5 GPa |
|
Electrical Resistivity |
0.035x10-6 Ù.m |
|
Thermal Conductivity |
200 W/m.K |
|
Thermal Expansion |
23.5 x 10-6 /K |
Table 2: Typical chemical composition
and physical properties for Austenitic Stainless Steel (AISI304)
|
Component |
Wt.
% |
|
|
||
|
C |
Max 0.08 |
|
|
Cr |
18 - 20 |
|
|
Fe |
66.345 - 74 |
|
|
Mn |
Max 2 |
|
|
Ni |
8 - 10.5 |
|
|
P |
Max 0.045 |
|
|
S |
Max 0.03 |
|
|
Si |
Max 1 |
|
|
Mechanical
Properties |
||||
|
||||
|
Hardness,
Brinell |
123 |
|||
|
Hardness,
Rockwell B |
70 |
|||
|
Hardness,
Vickers |
129 |
|||
|
Tensile
Strength, Ultimate |
||||
|
Tensile
Strength, Yield |
||||
|
Elongation
at Break |
||||
|
Modulus
of Elasticity |
193 - 200 GPa |
|||
|
Poisson's
Ratio |
0.29 |
|||
|
Charpy
Impact |
||||
|
Shear
Modulus |
||||
|
|
|
Figure
8: Samples geometry and dimensions: (a) Lap shear, (b) T-peel
3.1.
Lap shear Test
In the lap-shear geometry, a shear load is applied. The weld
nugget rotates to align with the loading line. When the load is increased,
localized necking occurs (see Figure 9 below). Fracture initiates at one of the
localized necking points, when the ductility of the sheet metal is reached (see
Figure 18, below). Although a shear load is applied, the failure mechanism is tensile.
Figure
9: Failure mechanism for lap-shear
sample
Observation during tensile test of lap
shear samples reveals the failure process as schematically demonstrated in Figure
7. As the sample is pulled initially, the weld nugget experiences a rotation,
which essentially aligns the nugget with the loading line.
In first stage the material
surrounding the nugget is subjected to a predominantly tensile load and the
deformation near the nugget is similar to a rigid button embedded in a ductile
sheet. As the load increases, localized necking of the sheet metal occurs at
the two apices, at locations near the juncture of the nugget and the base metal.
Note that these two points are on
the two different pieces of the coupons. Fracture then initiates at one of
these two points, when the ductility of the sheet material is reached.
Eventually pullout failure of the weld occurs as the initial crack grows around
the circumference of the weld nugget.
3.2.
T- Peel Test
The peel test is a simple test for
measuring the nugget size. When welding samples, the second weld nugget (B)
should be marked as shown in Figure 10. In the peel test, the sheets are first
separated on one end of a lap joint, and the roller rolls up one sheet while
the other is gripped. As the roller rolls over the weld, half of the workpiece
is torn off at the weld and a weld (A) button is left.
Figure.10 Peel test for measuring nugget diameter
With continued
peeling, the whole workpiece is torn off and another weld (B) button is left.
The nugget size can be estimated and recorded as a parameter for welding
quality by measuring the diameter of pullout button (B). If the button shape is
irregular, the button diameter is determined by taking an average of the
maximum and minimum dimensions. Manually measured the nugget diameter for each
of the pullout buttons (B) for each welding sample, and an enlarged view shown
in Figure 11 is taken for each to be able to measure the diameter for each
nugget.
Figure.11 Schematic showing joint
failure modes during peel test
Interface
failure is due to lack of bonding or only weak bonding between sheets. Once a
weld nugget formed, joints generally failed through the nugget when the nugget
diameter is small or by a button pullout when it is above a certain size, which
is called weld failure or button pullout. The failure modes usually serve as a
rough indicator of whether a specimen size is adequate or not.
4. EXPERIMENTAL OBSERVATIONS
4.1.
Tensile- Shear Test:
Table 3: Observations measured under lap tensile shear
test on Tensometer up to failure
|
Sample
1 |
Sample
2 |
Sample
3 |
|||
|
welding
time=30s |
welding
time=35s |
welding
time=40s |
|||
|
Load(Kg) |
Elongation(mm) |
Load(Kg) |
Elongation(mm) |
Load(Kg |
Elongation(mm) |
|
0 |
0 |
0 |
0 |
0 |
0 |
|
32 |
0.1 |
20 |
0.1 |
20 |
0.1 |
|
46 |
0.2 |
32 |
0.2 |
32 |
0.2 |
|
67 |
0.3 |
41 |
0.3 |
45 |
0.3 |
|
82 |
0.4 |
64 |
0.4 |
----- |
----- |
|
98 |
0.5 |
75 |
0.5 |
----- |
----- |
|
----- |
---- |
78 |
0.6 |
----- |
----- |
4.2.
T-Peel Test:
Table 4: Observations measured under T-peel test on
Tensometer up to failure
|
Sample
1 |
Sample
2 |
Sample
3 |
|||
|
welding
time=30s |
welding
time=35s |
welding
time=40s |
|||
|
Load(Kg) |
Elongation(mm) |
Load(Kg) |
Elongation(mm) |
Load(Kg) |
Elongation(mm) |
|
0 |
0 |
0 |
0 |
0 |
0 |
|
1 |
0.1 |
2 |
0.1 |
3 |
0.1 |
|
3 |
0.2 |
4 |
0.3 |
4 |
0. 4 |
|
4 |
0.4 |
5 |
0.6 |
5 |
0.7 |
|
5 |
0.7 |
6 |
0.8 |
6 |
0.9 |
|
6 |
1.3 |
7 |
1.2 |
7 |
1.5 |
|
7 |
1.8 |
8 |
1.6 |
8 |
1.8 |
|
8 |
2.2 |
9 |
1.9 |
9 |
2.2 |
|
9 |
3.2 |
10 |
2.4 |
10 |
2.8 |
|
10 |
3.8 |
11 |
2.7 |
11 |
3.6 |
|
11 |
4.5 |
12 |
3.2 |
12 |
4.9 |
|
12 |
5.6 |
13 |
4.5 |
13 |
5.6 |
|
----- |
6.2 |
14 |
5.6 |
14 |
6.6 |
|
----- |
----- |
15 |
6.8 |
14 |
7.5 |
|
----- |
----- |
------ |
7.9 |
14 |
10 |
|
----- |
----- |
------ |
------ |
15 |
10.6 |
|
------ |
----- |
------ |
------ |
16 |
11.9 |
|
------ |
----- |
------ |
------ |
17 |
12.8 |
|
------ |
------ |
------ |
------ |
18 |
13.9 |
|
----- |
------ |
------ |
------- |
19 |
14.8 |
|
----- |
------ |
------- |
------- |
19 |
17.2 |
|
----- |
------ |
------ |
------- |
--------- |
18.5 |
4.3.
Nugget diameter
Table 5: Nugget diameters (average) measured manually
on failed samples under lap tensile shear test on Tensometer
|
Tensile- Shear Test |
||||||
|
sample |
Al 6063 |
AISI304 |
||||
|
d1(mm) |
d2(mm) |
d average(mm) |
d1(mm) |
d2(mm) |
d average(mm) |
|
|
1 |
4 |
4 |
4 |
4 |
4 |
4 |
|
2 |
11 |
14 |
12.5 |
7 |
7 |
7 |
|
3 |
8 |
13 |
10.5 |
8 |
7 |
7.5 |
|
T-Peel Test |
||||||
|
1 |
7 |
5 |
6 |
5 |
5 |
5 |
|
2 |
7 |
4 |
5.5 |
5 |
5 |
5 |
|
3 |
4 |
4 |
4 |
4 |
4 |
4 |
5. RESULTS AND DISCUSSION
The
results as shown in Table 5 indicate that the nugget diameter varies between
4.0 to 12.5 mm and from table 3, tensile shear force varies between 480 to 980
N by taking welding time 40s, 35s and 30s. It was found that tensile shear
force decreased with increase in welding time. The curves are depicted in Figure
12 and in table 4 under T-peel test, tensile shear force varies between 120 to
190 N by taking welding time 30s, 35s and 40s.
It
was found that tensile shear force increased with increase in welding time. The
curves are depicted Figure 13 clearly shows an increasing trend within the
investigation range of both nugget diameter and tensile shear force for an
increase in welding time. The nugget diameter is a critical response in
determining the quality of the spot weld. Also it was observed that the
elongation of Al6063 material is more than AISI304 it is generally due to
softness of material in both tests.
The
load verses elongations during both tests are depicted in Figure 14 & Figure
15 using tensometer. When there is an increase in nugget diameter, of course
increase in cross sectional area, the load carrying capacity also increases
leading to increase in tensile shear force.
Figure 12: Relation between welding
time and nugget diameter in Lap tensile shear test (sample 1-welding time=30s,
sample2-welding time=35s, sample3- welding time=40s)
Figure13: Relation between welding
time and nugget diameter in T-peel test (sample 1-welding time=30s,
sample2-welding time=35s, sample3- welding time=40s)
Figure 14 graphs show load vs
elongation in lap tensile-shear test
Figure .15 graphs shows load vs
elongation in T-Peel test
6. CONCLUSIONS
Ø With
constant welding current and welding time, nugget size and mechanical strength
are improved. However, increasing the welding time and the joint strength
should be controlled to avoid expulsion in the welding zone especially on Al6063
alloy.
Ø The
highest internal tensile residual stress occurs in the center of the nugget
zone and decreases slightly towards the nugget edge. The behavior of the
surface residual stress is different, and decreases from center of the weld
zone towards the edge of the nugget. However, after this area, the value of the
residual stress falls down.
Ø With
increasing the welding time, the residual stress is decreased in the weld zone.
However increasing the welding time and the welding current boil down to increase
input heat to the weld zone, the maximum temperature doesn’t change by much,
while the size of the weld nugget increases; and thus, the rate of temperature
reduction (despite a fixed amount of holding time) decreases and as a result,
the maximum residual stress diminishes.
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