Spot Weld Growth on 304L Austenitic Stainless Steel for Equal and Unequal Thicknesses

Caspian Journal of Applied Sciences Research, 1(11), pp. 83-91, 2012

Available online at http://www.cjasr.com

ISSN: 2251-9114, ©2012 CJASR

83

Spot Weld Growth on 304L Austenitic Stainless Steel for Equal and Unequal

Thicknesses

Nachimani Charde

Department of Mechanical, Material and Manufacturing Engineering, The University of Nottingham Malaysia Campus,

Jalan Broga, 43500 Semenyih, Selangor Darul Ehsan , Malaysia

[email protected]

The austenitic stainless steel (304L) is common material that used in spot welding research. It has been welded

and analyzed for many types of conditions and also for many situations, previously. In this research, unequal

and equal thicknesses were welded and also analyzed for tensile shear strength, hardness changes and

macrograph views. Generally the weld nugget is determined by the process parameters such as welding current,

weld time, electrode force and electrode tips. Noticeably the welding current is the main parameter which creates

the heating effects between metal sheets and unites them together. The current flow is, however, allowed by

sufficient weld time in the welding process. In this experiment, the welding current and weld time were varied

for equal and unequal thicknesses. The results are showing different macro structures and also slightly different

characteristics in terms of hardness and strength.

Key words: Austenitic stainless steel, Stainless steel welding, Spot welding of stainless steel

1. INTRODUCTION

Joining the similar materials with different

thicknesses become common in spot welding. By

joining the unequal thicknesses of base metals, the

characteristic of welded region varies and also

differs in certain characteristic (Dursun, 2011). In

this experiment, the austenitic stainless steel of 1

and 2 mm thicknesses was used to analyze the

changes that affect the weld growth after the

welding process takes place. So the weld region is

separately analyzed for 1mm, 2mm and, of course

for both; the mixed thicknesses. Basically the

growth of weld nuggets is determined by the

welding current, welding time, electrode force and

electrode tips’ areas. The welding current and weld

time were increased while the force and electrode

tip remained constant throughout the experiments

(Shamsul and Hisyam, 2007).

2. MATERIALS AND METHODS

Fig.1: Test sample

The test samples were prepared in rectangular

shape (200mm x 25mm x 1&2mm) as shown in

figure 1 and its chemical and mechanical

properties are tabulated in table 1. A pair of water

cooled copper electrodes with truncated tips

(diameters of 5 mm) was used to join these base

metals and it was simulated using Ansys 4.0 The

melting temperature was found to be more than

1500°C and the heat distributive pattern across the

weld areas is shown in figure 2. It was calculated

from the Joule’s law of heat that the heat was given

by; Heat, Q=I2Rt; where R is the total resistance of

base metals, I is the welding current and t is the

welding time (Marashi et al., 2008). Thus, the well prepared base metals were

initially placed on the top of lower electrode (static

electrode) of the spot welder (75kVA) as

overlaying 60mm on each other and then the

initiating pedal was pressed. The weld process was

started right after with squeezing cycles and; once

the electrodes are well pressed the base metals

according to preset force values; the welding

current is released then. Thereafter the electrode

pressing mechanism (pneumatic based) consumes

Charde

Spot Weld Growth on 304L Austenitic Stainless Steel for Equal and Unequal Thicknesses

84

some time for cold work and returns to the home

position of upper electrode. The basic process

controlling parameters (the current and weld time)

were set before the welding process starts with

constant electrode pressing force (3kN) and round

electrode tips (19.63μm2). By varying the welding

current and weld time from lower to higher values

or from poor welds to good welds and also beyond

good welds; three sets of weld lobe curves were

obtained for equal and unequal base metals

(Pouranvari, 2008).

Table 1: Chemical and mechanical properties 304L (2B) Austenitic Stainless Steel

1mm 2mm

Element Weight % Weight %

C 0.048 0.046

Cr 18.12 18.14

Ni 8.11 8.13

Mn 1.166 1.205

Si 0.501 0.506

S 0.006 0.004

N 0.053 0.051

P 0.030 0.030

Hardness 81.7 (HRB) 86.2 (HRB)

Fig. 2: Simulation of welding process

Fig. 3(a): Weld lobe of 1&1mm base metals


85

Fig. 3(b): Weld lobe of 1&2mm base metals

Fig. 3(c): Weld lobe of 2&2mm base metals

Figure 3 (a, b and c) show the 1&1mm,

1&2mm, and 2&2mm sample thicknesses’ weld

lobe curves. An acceptable region of weld nuggets

are joined using lines in the weld lobe curve and

the combination of colors represent the quality of

welded areas. The green color boxes that

encapsulated with black color represent moderate

good welds; the full green color boxes represent

very good welds; the red color boxes that

encapsulated with green color represent light

expulsion welds; the red color boxes represent

heavy expulsion welds and the black color boxes

represent poor welds joints. Based on the weld

lobe curves’ values; a weld schedule (table2) was

developed to avoid poor and expulsion welds. So

the combinations of the nine (9) weld schedules

were developed for the current and weld time

variations within reasonable good welds region.

Every weld schedule is accomplished of seven

welded pairs. The first five samples were used for

tensile test-averaging; the sixth sample was used

for hardness measurement. The last welded sample

was cut at the line of its diameter and mounted

using resin powder on hot press mount. The

mounted samples (figure 4) were roughly polished

using silicon papers 1200/800p and 600/200p and

also continuously further polished using Metadi

polishing cloth with suspension liquid of 0.05

micron. This polishing process has been conducted

about thirty minutes to one hour on each sample

until the shining surface was seen. At last the ferric

chloride (500ml) was used to saturate these well

prepared (shining surface) samples in a pot about

30-45 minutes. After that the samples were rinsed

off using plain water; dried using air blower;

applied anti-corrosion liquid; and kept in vacuum

chamber.

Charde


86

Table 2: Weld schedule

Sample

No

Weld

Schedule

Thickness

(mm)

Electrode

Tip

Diameter

(mm)

Force

(kN)

Time

(cycle)

Current

(kA)

1-7 1 1&1, 1&2, 2&2 5 3 10 6

8-14 2 1&1, 1&2, 2&2 5 3 10 7

15-21 3 1&1, 1&2, 2&2 5 3 10 8

22-28 4 1&1, 1&2, 2&2 5 3 15 6

29-35 5 1&1, 1&2, 2&2 5 3 15 7

36-42 6 1&1, 1&2, 2&2 5 3 15 8

43-49 7 1&1, 1&2, 2&2 5 3 20 6

50-56 8 1&1, 1&2, 2&2 5 3 20 7

57-63 9 1&1, 1&2, 2&2 5 3 20 8

a) 1&1 mm b) 1&2 mm c) 2&2 mm

Fig. 4: Hot pressed-cross sectional area of welded samples

3. RESULTS AND DISCUSSION

3.1. Tensile shear results

The tensile test was conducted using 100kN tensile

machine. The ultimate tensile strength was taken at

the final value of pulling force at which the

fracture begins. Five samples were used to average

weld strength of a weld schedule. When the

welding current was increased from 6 to 7 and 7 to

8 kA; more heat was developed across the weld

areas and resulting diameter increments regardless

of thicknesses (Jamasri et al., 2008). So the nugget

diameters’ increment has caused strong bounding

between base metals and also requires high tensile

force to break the joints. Similar increment is

noticed when the weld time was increased from 10

to 15 and 15 to 20 cycles. Figure 5 shows the

tensile strength of all; 1&1mm, 1&2mm and

2&2mm. Although the strength incremental pattern

seemed to be almost same for all the samples, the

required breaking strength varies. The 2&2mm

thickness sheets require the pull-to-break force

from 12.5 to 19.5kN. The 1&2mm mixed-

thickness sheets require 7 to 9.5kN while 1&1mm

thickness sheets require 6.7 to 9kN.

3.2. Failure modes

The failed tensile-shear test pieces showed three

main fracture modes. Poor welds (low fracture

force and small nugget diameter) were

characterised by interfacial fracture (IF) – fracture

through the nugget in the junction plane between

the faying sheets. Moderate to good welds showed

tearing on either side of the nugget in the base

metal (partial-tearing-failure - PF). Very good

welds were strongly bonded to both sheets and

therefore it tears on both sides (TF) or a perfect

button pull out will results (Martín, 2009). Figure 6

indicates the common failure modes and the

corresponding tensile strength curve. These types

of curves were seen for base metals fracture during

tensile test regardless of thicknesses. Moreover the

equal thicknesses (1&1 mm or 2&2 mm) weld

fractures seemed to be well balanced on the joint

areas. However the unequal thickness weld

samples were always broken at 1mm sheet sides

rather than 2mm sheet sides.


87

Fig. 5: Tensile-shear test results

Fig. 6: The common failure modes

Fig. 7: Macrograph of stainless steel fractures

3.3. Diameter of Weld Nuggets

A simulation of welding process was carried out

using Ansys 4.0 for stainless steels of various

thicknesses. Figure 8 shows the simulation images

during the complete molten stages of welding

process. The molten and heat affected areas are

clearly visualized for all the thicknesses without

electrodes appearance (Yang et al., 2011).

Charde


88

a) 1&1 mm b) 1&2 mm c) 2&2 mm

Fig. 8: Simulation results of austenitic stainless steel (Electrodes deleted)

Having considered the real macro graph of

welds for current increment (figure 9, 10 and 11);

three noticeable regions are existed after the

completion solidification process[7]

. Firstly the

fusion zone (FZ) undergoes complete melting

during welding process and solidify once the

welding process is over. It was appeared with

coarse grains and the distance expanded

horizontally in all direction is the exact width

(diameter) of the weld joint. The upper electrode

that pressing the welding region has created the

micro indentation during the molten metal’s

expansion. The heat affected zone (HAZ) is existed

due to the heat expansion in all direction from the

fusion zones while heating process is going on.

This leads to the slight alteration of grain sizes

when solidification happens. It is seen with finer

coarse grain as compare to fusion zone. It starts

from the outer border of exact fusion zone and

spreads outwards in all direction to certain extends.

The base metal (BM) is not affected during the

welding process as the microstructures remain

unchanged (Fukumoto, 2008). Figure 12 shows the

width (diameter) and height (indentation) that

obtained for current increments.

a) 6kA b) 7kA c) 8kA

Fig. 9: Macrograph of 1&1 mm austenitic stainless steel






89

Fig. 12: Nugget size of all the thickness

When the current was increased from 6 to 7 and

7 to 8 kA, the width (diameter) was increased and

the height was decreased accordingly. This

phenomenon was noticed for all the thicknesses of

base metals. However the 2&2mm sheets have

higher thicknesses and therefore the indentations

were seen to be slightly higher than the 1&1mm

and 1&2mm sheets. Moreover higher the welding

current then easier the porous existed within the

fusion zones.

3.4. Hardness test results

The hardness test was carried out for all the

thicknesses’ weld joints (Aravinthan et al., 2011).

The hardness was measured from left hand sides

through the heat affected zones; then fusion zones;

again the other side heat affected zones and ended

with the right hand side of base metals. The

hardness of welded areas seemed to be noticeably

increased but slightly varied from one thickness to

another. Thus: when the 1mm base metals are

concerned then the unwelded areas (BM) seemed

to be an average hardness of 81.5 (HRB) but the

welded areas seemed to be about 92 (HRB).

Furthermore the hardness of heat affected zones

(HAZ) was lower than the fusion zones but higher

than the base metals about 87 (HRB). Figure 13

shows the hardness distribution of 1mm weld

joints.

Fig. 13: Hardness of 1&1mm thickness sheets

Similar pattern of hardness distribution was

seen for the other two thicknesses of weld joints.

Figure 14 and 15 show the hardness distribution of

1&2mm and 2&2mm sheets, respectively.

Charde


90



The unwelded areas (BM) of 2mm thickness

was measured to be an average hardness of 86

(HRB) but the welded areas seemed to be about 96

(HRB). Meanwhile the hardness of heat affected

zones (HAZ) was lower than the fusion zones but

higher than the base metals about 88 (HRB) as

how the 1mm thickness welds joint resulted.

Moreover the average hardness of dissimilar

thickness welded zone was about 96 (HRB)

approximately. The heat affected zones was

resulted an average of 89 (HRB) in the dissimilar

weld joints. In overall the hardness of welded

zones are slightly increased regardless of

thicknesses due to the solidification process

(Aravinthan et al., 2011).

4. CONCLUSION

This paper looks into the spot weld growth on

304L (2B) austenitic stainless steel with various

thicknesses and; it concludes that

1. Increase in welding current and welding time

within the welding lobe have resulted increment of

weld nuggets width but simultaneously reduced the

height of weld nuggets regardless of thicknesses.

2. The increments of tensile strength were

observed due to the enlargement of welded areas

within the welding lobe regions.

3. The failure modes of tensile tested samples were

seen to be as normal as other materials’ failures

except dissimilar thicknesses weld joints. The

dissimilar thicknesses were fractured at the lower

thicknesses sheets sides, always.

4. The common three failure modes were seen as

poor weld produces interfacial fracture (IF);

medium weld produces tear form one side (PF);

and a good weld produces button pullout or tear

from both sides (TF).

5. The simulation and experimental weld bead

shapes were almost same to one another except the

intersecting lines in the weld nuggets.


91

6. The hardness values of welded areas were

slightly increased due to the cold work of stainless

steel. However the hardness distributions along the

welded areas were seen to be fluctuating regardless

of thicknesses. Furthermore the increase in

welding current or welding time does not influence

the changes in hardness distribution along the

welded areas.

7. Macro structure exhibits the coarsened grains at

fusion zone, refined grains at heat affected zone

and unchanged grains at base metal; obviously

regardless of thicknesses.

REFERENCES

Aravinthan A, Nachimani C (2011). Analysis of

Spot Weld Growth on Mild and Stainless

Steel. Welding Journal, 143-147.

Aravinthan A, Nachimani C (2011). Metallurgical

Study of Spot Weld Growth on Mild Steel

with 1mm and 2mm Thicknesses. Journal -

The Institution of Engineers, Malaysia, 72

(4): 46-54.

Dursun O (2008). An Effect of Weld Current and

Weld Atmosphere on the Resistance Spot

Weld Ability of 304L Austenitic Stainless

Steel. Materials and Design 29: 597-603.

Fukumoto S, Fujiwara K, Toji S, Yamamoto A

(2008). Small Scale Resistance Spot

Welding of Austenitic Stainless Steels.

Materials Science and Engineering A, 492:

243-249.

Jamasri MN, Ilman R, Soekrisno , Triyono (2011).

Corrosion Fatique Behaviour of RSW

Dissimilar Metal Welds Between Carbon

Steel and Austenitic Stainless Steel with

Different Thickness. Procedia Engineering,

10: 649-654.

Marashi P, Pouranvari M, Amirabdollahian S,

Abedi A, Goodarzi M (2008).

Microstructure and Failure Behavior of

Dissimilar Resistance Spot Welds between

Low Carbon Galvanized and Austenitic

Stainless Steels. Materials Science and

Engineering, A: 175-180.

Martín O, Tiedra PD, Lopez M, Juan MS, García

C, Martín F, Blanco Y (2009). Quality

Prediction of Resistance Spot Welding

Joints of 304 Austenitic Stainless Steel.

Materials and Design, 30: 68-77.

Pouranvari M (2011). Effect of Welding Current

on the Mechanical Response of Resistance

Spot Welds of Unequal Thickness Steel

Sheets in Tensile-Shear Loading Condition.

International Journal of Multidisciplinary

Sciences and Engineering, 2(6): 178-189.

Shamsul JB, Hisyam M M (2007). Study of Spot

Welding of Austenitic Stainless Steel Type

304. Journal of Applied Sciences Research,

3(11): 1494-1499.

Yang Y, Qu X, Luo Y, Yang A (2011). Effect of

Resistance Spot Welding Parameters on the

Austenitic Stainless Steel 304 Grade by

Using 23 Factorial Designs. Advanced

Materials Research, 216: 666-670.

Spot Weld Growth on 304L Austenitic Stainless Steel for Equal and Unequal Thicknesses

Documents

Transcript of Spot Weld Growth on 304L Austenitic Stainless Steel for Equal and Unequal Thicknesses