Accepted Manuscript

Resistance spot welding joints of AISI 316L austenitic stainless steel sheets:

Phase transformations, mechanical properties and microstructure characteriza-

tions

Danial Kianersi, Amir Mostafaei, Ahmad Ali Amadeh

PII: S0261-3069(14)00350-1

DOI: http://dx.doi.org/10.1016/j.matdes.2014.04.075

Reference: JMAD 6466

To appear in: Materials and Design

Received Date: 30 January 2014

Accepted Date: 29 April 2014

Please cite this article as: Kianersi, D., Mostafaei, A., Amadeh, A.A., Resistance spot welding joints of AISI 316L

austenitic stainless steel sheets: Phase transformations, mechanical properties and microstructure characterizations,

Materials and Design (2014), doi: http://dx.doi.org/10.1016/j.matdes.2014.04.075

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers

we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and

review of the resulting proof before it is published in its final form. Please note that during the production process

errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

1

Resistance spot welding joints of AISI 316L austenitic stainless steel sheets:

Phase transformations, mechanical properties and microstructure

characterizations

Danial Kianersi a, Amir Mostafaei b,* , Ahmad Ali Amadeh c

a Department of Materials Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran

b Young Researchers and Elites Club, Tehran North Branch, Islamic Azad University, Tehran, Iran

c School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran

Abstract

In this paper, we aim to optimize welding parameters namely welding current and time in

resistance spot welding (RSW) of the austenitic stainless steel sheets grade AIAI 316L.

Afterward, effect of optimum welding parameters on the resistance spot welding properties and

microstructure of AISI 316L austenitic stainless steel sheets has been investigated. Effect of

welding current at constant welding time was considered on the weld properties such as weld

nugget size, tensile-shear load bearing capacity of welded materials, failure modes, failure

energy, ductility, and microstructure of weld nuggets as well. Phase transformations that took

place during weld thermal cycle were analyzed in more details including metallographic studies

of welding of the austenitic stainless steels. Metallographic images, mechanical properties,

electron microscopy photographs and micro-hardness measurements showed that the region

between interfacial to pullout mode transition and expulsion limit is defined as the optimum

*Corresponding author: Amir Mostafaei, Tel.: +989126216801;E-mail address:amir.mostafaei@gmail.com and a_mostafaei@sut.ac.ir

2

welding condition. Backscattered electron scanning microscopic images (BE-SEM) showed

various types of delta ferrite in weld nuggets. Three delta ferrite morphologies consist of skeletal,

acicular and lathy delta ferrite morphologies formed in resistance spot welded regions as a result

of non-equilibrium phases which can be attributed to the fast cooling rate in RSW process and

consequently, prediction and explanation of the obtained morphologies based on Schaeffler,

WRC-1992 and Pseudo-binary phase diagrams would be a difficult task.

Keywords: Resistance spot welding; AISI 316L austenitic stainless steel; Welding parameters

optimization; Mechanical properties; Microstructure characterization; Phase transformations.

1. Introduction

In recent years, iron and its derivations have been used in wide range of applications such as

vehicles, ships, bridges and buildings due to their machining abilities, mechanical properties, and

low prices. Different welding methods have been utilized in manufacturing industries such as gas

tungsten arc welding (GTAW), submerged arc welding (SAW), shielded metal arc welding

(SMAW), flux core arc welding (FCAW), and resistance spot welding (RSW). Among them,

RSW is extensively used in manufacturing processes due to its repeatability, inexpensive

equipment, and easily controllable process [1] for joining of metal sheets such as iron and steels

[2], aluminum alloys [3], magnesium alloys [4], titanium alloys [5]. Also RSW has been utilized

to investigate physical and mechanical properties of different joint metals (dissimilar materials)

such as iron-aluminum [6] and/or steel-galvanized iron [7]. In RSW method, overlapping sheets

are placed between two electrodes and heat is obtained by passing a large electrical current for a

short period of time [8]. Afterward, electrical resistance at the metals interface causes a localized

3

heating for joining and finally, the resistance spot welded region will be produced by the

combination of heat, pressure and applied process time [9].

Stainless steels, especially low carbon grades, have been widely used in various industrial

applications such as appliance manufacturing due to their corrosion resistance and proper

decorative appearance at ambient atmosphere. Stainless Steel grade 316L is one of the most

important types of austenitic stainless steels which consists of austenite and ferrite phases in the

microstructure. As it was reported, this grade is widely used in various industrial applications

due to its high corrosion resistance [10], decorative appearance, and excellent weldability [11].

Moreover, low carbon content declines formation of carbides in the grain boundaries within

welding [12].

However, one major drawbacks of the austenitic stainless steel is the heat affected zone

(HAZ) in which intergranular Cr-rich carbide precipitation forms during welding process and

leads to the reduction of corrosion resistance in welded joint [13]. In order to solve this issue,

two ways were suggested [14, 15]: (1) use of austenitic stainless steel with low carbon content

such as AISI 316L and (2) rapid welding process of RSW which can decrease the formation of

these undesired carbides. Based on Fukumoto et al. report, hot cracking is another issue which

occurs in the welded region [16]. It is reported by Fukumoto et al. that a small amount of delta

ferrite in the microstructure of the weld nugget can prevent hot cracking problem and may

reduce sensitization in HAZ welded regions [16].

There are some predictive diagrams for evaluation of the microstructure of welds such as

conventional Schaeffler and WRC-1992. It has been mentioned that the majority of these

diagrams have been developed based on the chemical composition of the base metal and

solidification rate of the weld metal has not been considered. Since rapid solidification may

4

change microstructure of the welded regions [14, 15], it would be quite difficult to predict

morphology of weld based on these diagrams. Pseudo-binary phase diagram can predict

solidification modes and the microstructure of welded zones of the austenitic stainless steel

versus changes in the proportion of Creq/Nieq or cooling rate. According to this diagram, the

microstructure and solidification modes of welded zones will change from fully austenite (A) to

fully ferrite (F) mode with variation of Creq/Nieq or cooling rate [14, 15].

In this study, for improving multiple welding characteristics including nominal weld nugget

size, smaller HAZ, tensile-shear load bearing capacity of welded materials, failure modes, and

failure energy, the optimization studies were devised. Obtained optimum condition was chosen

for mechanical and microstructure investigations. The mechanical properties were investigated

by microhardness profile and tensile-shear tests. Additionally, the microstructure of the welds

was studied by metallographic investigations and scanning electron microscopy (SEM) methods.

Chemical composition at different regions of weld nuggets was evaluated by Energy Dispersive

Spectroscopy (EDS). The predictive Schaeffler, WRC-1998 and pseudo-binary phase diagrams

have been also used to investigate the welds microstructure. As it was expected, rapid welding

process led to the formation of complex microstructures in HAZ and weld regions and

consequently, it elaborated prediction of the formed phases in welds.

2. Materials and experimental methods

2.1. Materials and methodologies

Austenitic stainless steel sheet grade 316L (UNS 31603), with a thickness of 1 mm and

chemical composition given in Table 1 was used as the base metal. Fig 1a shows dimension of

the specimen which was welded by RSW method (linear overlapping of plates). Fig. 1b

5

demonstrates geometric morphology of the nugget and Fig. 1c depicts selected resistance spot

welded 316L austenitic stainless steel sheets. Austenitic stainless steel sheet was cut into pieces

in dimensions of 105 mm ×45 mm according to ANSI/AWS/SAE/D8.9 standard [17] and were

spot welded by using a 150 kVA pedestal-pneumatic RSW machine. The welding procedure was

carried out by using water cooled conical Cu–Cr electrodes with a constant surface of electrodes

(4 mm face diameter) according to ANSI/AWS/SAE/C1.1-66 standard [18]. Initially, in order to

determine optimum manufacturing parameters on the morphological feature and mechanical

properties, changes in welding current from 4 to 9 kA, and welding time from 4 to 7 cycles, were

considered. After determination of optimum welding condition, the welding currents were

selected between 4 to 9 kA with 1 kA step-rise and other welding parameters such as squeeze

time, welding time, holding time, electrode force were kept constantly which were given in

Table 2.

2.2. Mechanical studies and metallographic evaluations

All specimens were prepared for tensile–shear test according to ANSI/AWS/SAE/D8.9

standard [17]. The tensile-shear tests were performed with Santam universal testing machine

with 5 tons capacity at a constant cross head displacement rate of 2 mm.min-1. Failure modes

were studied from failed specimens and it was calculated by measuring the area under the load-

displacement curve up to the maximum tensile-shear load [19, 20]. Failure energy, which is the

area under the stress-strain curve up to the maximum load, and peak load, which is measured as

the maximum point in the tensile-shear curve, were extracted from the obtained load-

displacement curves. Additionally, the Vickers microhardness examinations across the weld

6

nugget, HAZ and base metal were carried out on the metallographic specimens by Struers

(Duramin model) under indentation load of 50 g for 10 s.

The longitudinal sections through the weld nuggets as well as the similar sections of the base

metals were prepared according to metallographic procedure in ANSI/AWS/SAE/D8.9 standard

[17]. The solution of 10 mL nitric acid, 15 mL hydrochloric acid, 10 mL acetic acid and two

drops of glycerin was used to etch specimens [17]. The optical microscopic studies of the welded

samples were performed by Olympus (model BX60) and scanning electron micrographs of the

samples were analyzed using SEM (Philips model XL30), equipped with an energy dispersive X-

ray (EDS) to investigate the microstructure and chemical composition of the weld. Image

analyzer computer program was used to measure macroscopic diameter of weld nuggets.

3. Results and discussion

3.1. Optimization studies on welding current and time

In the first place, the effect of welding current and time on various parameters including

nugget diameter size, welding penetration, width and thickness of HAZ, peak load, failure

energy, indentation depth, and penetration rate were meticulously considered and obtained

results were presented in Fig. 2.

Fig. 2a showed discrepancies of nugget diameter size versus welding current and time. As it

can be seen, nugget size increased within increasing of welding current and time. In fact, the

main reason can be attributed to the higher heat input amount which leads to the formation of

nugget with bigger fusion zone. In the other words, when the welding current and/or time

increases, plates expose to higher amount of head input which may lead to the formation of

width and thick nugget. In addition, welding penetration increased by the growth of fusion zone

7

(Fig. 2b). Width and thickness of HAZ were measured on the metallographic cross sections.

Obtained results revealed that width of HAZ increased by increasing welding current and time

(Fig. 2c), in contrast, thickness of HAZ decreased by increasing welding current and time (Fig.

2d). In fact, when fusion zone increases, weld with larger nugget size will form and

consequently, thickness of HAZ declines. Variations of two important output results including

tensile-shear load bearing capacity (peak load) and failure energy, which were obtained from

stress-strain diagram, versus welding current and time were presented in Fig. 2e and 2f. Since

higher failure energy is desired in RSW manufactured samples, so based on the Fig 2f, the best

welding condition can be obtained in 8 kA welding current and 4 cycles welding time. What is

more, Fig. 2e revealed that the higher amount of tensile-shear load bearing capacity or peak load

can be attained when welding current is 8 kA. Another important manufacturing parameter

which affects appearance of the welded sample is indentation depth (Fig. 2g). In fact, this item is

quite important when decorative issues are one of the most important factors in the

manufacturing process. The amount of indentation depth may affect nugget size and more

importantly mechanical properties of the final products. Thus, this parameter should be

optimized as well and in this study, range of 0.4 to 0.5 mm was in the acceptable amounts. The

other important factor governing the mechanical performance of the manufactured samples with

RSW is penetration rate and it can be calculated based on the following equation:

Fig. 2h showed variations of penetration rate versus welding current and time. As it can be

seen, welding current has an important role in this parameter and obtained results showed that

acceptable condition can be achievable in 8 kA welding current.

8

At the end, since higher amount of failure energy can be the other important parameter in

RSW process and depicts that mechanical properties of the manufactured samples are reliable,

thus in this study, 4 cycles welding time and 8 kA welding current are the optimum

manufacturing conditions. In the following, welding time was kept constant at 4 cycles in order

to deeply investigate the influence of welding current on the mechanical and microstructural

properties of weld nuggets.

3.2. Effect of welding currents on weld nuggets diameter size, tensile-shear load bearing

capacity and failure energy of welded materials

There are several factors which influence physical and mechanical properties of welded

structure namely weld nugget diameter, weld penetration, strength and ductility of the welded

region, surface appearance and sudden internal discontinuities caused by presence of pits and

cracks [11]. Hasanba•o• lu et al. reported that among all these factors, the most important

parameter affecting tensile-shear load bearing capacity is weld nugget diameter size [21].

Fig. 3 shows the variation of weld nugget diameter versus different welding currents. As can

be seen in Fig. 3, weld nuggets diameter increased when the welding current increased from 4 to

9 kA. What is more, based on the presented results in Fig 2a, welding time has the same effect on

the nugget size.

Fig. 4a indicates schematic of the stress-strain curve (load-displacement) of the welded

samples. Two areas are determined which are peak load and failure energy. In Fig. 4a, the area

under the load-displacement (stress-strain curves) up to the peak load is the amount of failure

energy. Fig. 4b illustrates stress-strain curves of spot welds during tensile-shear test and Fig 4c

summarized changes in tensile-shear load bearing capacity of welded materials with different

9

welding currents. As seen in Fig. 4d, the amount of energy absorption increases within

increasing in welding current up to 8 kA. As it was reported by Pouranvari et at., when welding

current increases, the amount of failure energy increases as well [22]. Fig 4d illustrates variation

of failure energy versus welding current. However, in the case of 9 kA welding current, energy

absorption declines and it depicted that mechanical property of the sample which was welded by

9 kA welding current decreased. This reduction in the amount of energy absorption and tensile

shear load can be attributed to drifting out of the molten materials of the weld nugget due to

generation of high heat input in 9 kA welding current.

According to Fig. 3 and Fig. 4, it is obvious that the weld nugget diameter and tensile-shear

load bearing capacity of welded materials were increased by stepping up a welding current from

4 kA to 8 kA. In the other word, tensile-shear load force of welded specimens increased by

increasing of heat input which was related to welding currents. It was reported by Pouranvari et

al. [8] that the enhancement in tensile-shear force of weld with increasing of welding current is

related to the weld nugget diameter size. Additionally, Vural et al. mentioned that the more

enlargement of nugget diameter size caused to achieve specimen with the higher tensile–shear

load bearing capacity of welded materials [23]. Furthermore, by increasing in welding current

from 8 kA to 9 kA, the weld nugget diameter increased continuously while the tensile-shear

force decreases (Fig. 4b-c). This phenomenon may be attributed to the drifting out of the molten

materials followed by thinning the cross section of the weld nugget due to high heat input of 9

kA welding current. In order to simplify the conception of the event, cross sectional image of the

spot weld nugget joint by high heat input (9 kA welding current) was shown in Fig. 5. It is clear

that nugget was formed within larger diameter size and narrower thickness. Totally, the

10

minimum and maximum tensile-shear forces or peak load were 2450 and 8070 N which belongs

to 4 and 8 kA welding currents, respectively.

3.3. Effect of welding current on failure modes

Failure mode, fracture characteristics and sheet separation of the welds are features which

influenced quality of the resistance spot welded specimens. Fig. 6 presents macrographs of six

different failure modes observed in tensile tests. As it is expected and seen from Fig. 6, within

increasing the welding current, these modes can be characterized from solely interfacial failure

mode (IF) to completely tearing around weld nugget and HAZ. Fig. 6a shows the failure mode

obtained from sample which was welded at 4 kA welding current. Due to low heat input, the spot

weld was separated through the weld nugget in the minimum tensile-shear load bearing capacity

of the welded materials. The minimum tensile-shear load bearing capacity of weld represented

the minimum nugget diameter size which was measured and shown in Fig. 3.

As it can be seen in Fig. 6b, by increasing welding current from 4 kA to 5 kA, although the

tensile-shear force increased, the separation takes place through weld nugget which can be

attributed to enlargement in weld nugget diameter size. When welding current increases,

enlargement in nugget diameter happens and consequently the specimen will fail in higher

tensile-shear force. In the other words, the higher energy is needed for complete rupture in

welded sheets.

Failure mode will be changed from interfacial failure (IF) to pullout failure (PF). In the case

of third sample welded by using 6 kA welding current (Fig. 6c), the specimen failed in the

combination of separation in weld nugget and HAZ. As it was shown in Fig. 3 and 4, further

increasing in welding current up to 7 kA and 8 kA resulted in manufacturing of specimens with

11

higher weld nugget diameter and tensile-shear forces, and consequently, they tore from HAZ and

base metal around the weld nugget with button pullout failure which is named button pullout

with tearing from base metal (Fig. 6d and 6e). With the comparison of two latter specimens, it

was predicted that the specimen which has been produced within 8 kA should have higher

tensile-shear force. Finally, by increasing the welding current up to 9 kA, although the weld

nugget size increased, tensile-shear force sharply decreased. It could be attributed to the

generation of high heat input by using 9 kA welding current which led to molten materials

expulsion during resistance spot welding. Fig. 6f shows that this specimen completely tore from

HAZ around weld nugget due to the generation of high heat input, excessive grain growth and

weakening in HAZ region [19, 24]. Marashi et al. investigations reveal that there is a relationship

between failure mode and energy absorption and in fact, when failure energy amount is higher,

failure would be pullout failure mode [7].

Specimens which had interfacial fractures during tensile-shear tests were rejected due to low

peak load and separation through weld nuggets (Fig. 6a-c), in contrast, specimens which torn

from HAZ, base metal or mixed modes were accepted (Fig. 6d-f). The other way for

clarification of failure mode can be considered by failure energy. As it was shown it Fig. 4c,

energy absorption of failed sample increases when specimen was welded by using higher

welding current. In the other word, it can be seen in Fig. 4b-d that the average peak load and

absorbed energy of those welds which failed in PF mode are higher than those samples failed in

IF mode. Thus, tensile-shear load of the welds increases in the order of IF, IF-PF and PF modes

[7]. Totally, the best welded sample was attained at 8 kA welding current which characterized by

button pullout with tearing from the base metal failure mode.

12

3.4. Microstructure studies of welds

3.4.1 Metallographic investigations of resistance spot welded region

Cross-section macroscopic images of the welded specimens joined at various welding

current from 4 kA to 9 kA at the constant welding time (4 cycles) are shown in Fig. 7. It can be

easily observed that weld nugget size increases within increasing welding current.

Morphology of the welded regions was investigated by optical microscopic analysis (OM)

and scanning electron microscopy (SEM). The microstructure of the sample welded by 8 kA

welding current is shown in Fig. 8. Morphological images taken from HAZ regions depicts in

Fig. 8a and 8b. As seen, grain size increased in HAZ in comparison to base metal due to short

cycle of RSW [14, 15]. In addition, sensitization problem caused by the formation of chromium

carbides in HAZ area was not observed in this study. Two main reasons include: (1) use of

stainless steel with low content carbon and (2) RSW process with short cycles [15]. Fig. 8c

shows microstructure of the weld nugget with lower magnification. It is obvious that morphology

is suitable based on its appearance. Fig. 8d and 8e illustrates welded region and it can be easily

seen that grain growth has occurred in weld metal zones. As seen in Fig. 8a and 8d, grains are

elongated parallel to electrode compression direction in the weld nugget which similar results

was reported by Kocabekir et al. [24]. Also, it can be seen in Fig. 8e that there are ferritic

microstructure with various morphologies.

In order to consider effect of welding current on the shape and microstructure of the weld

nuggets, specimen which was produced with lower welding current (4 kA) was chosen and

evaluated by optical microscopy and results were shown in Fig. 9. As it was expected, an

incomplete weld nugget was formed due to lack of heat input. Fig 9c showed that insufficient

heat input led to the formation of weld nugget with shortest diameter. What is more, this

13

deficiency can be confirmed by the extracted results from tensile-shear test. Morphological

evaluations taken from various regions of the specimen revealed that the major reason for lower

mechanical properties could be attributed to the small nugget size which influenced the quality of

spot welded materials. It was reported by Fukumoto et al. [16] and Kearns [9] that internal

discontinuities such as large cavities, cracks, and porosity which are common defects in RSW

process and usually caused by low electrode force or high welding current might lead to weld

failure under severe condition. Fig. 9 depicted that although various regions (base metal, HAZ,

weld nugget and their interfaces) formed without any defects, the main reason for the weak

mechanical properties of the welds made at 4 kA is its small fusion zone size. In the other word,

inadequate heat input resulted in the formation of small nugget which cannot provide a weld with

proper mechanical properties.

To investigate the effect of disproportionate use of welding parameter on the obtained

manufactured samples, morphological studies were carried out on the sample welded by using 9

kA welding current and 4 cycles welding time and metallographic images were present in Fig.

11. Typical microstructure image of the welded sample by applying higher welding current and

welding time is shown in Fig. 11c. As can be seen if Fig. 11c, weld nugget with large welded

area was form and also as the generated heat input was more than usual, unwanted electrode

indentation took place which might lead to the formation of thin weld nugget. Weld metal with

ferritic microstructure is shown if Fig. 11a and 11b. Microstructure photo of transition zone is

illustrated in Fig. 11d and metallographic image taken from weld metal and HAZ is depicted in

Fig. 11e.

Scanning electron microscopic images (SEM) was used to evaluate morphology and

chemical composition of the welds. Fig. 12 shows SEM images of specimen welded at 8 kA

14

welding current. Fig. 12a and 12b show SEM micrographs taken from center of the weld nugget

with two distinctive areas including HAZ region and weld metal. Columnar structure in weld

nugget can be easily seen. Additionally, Fig. 12c shows SEM image with two zones including

base metal and HAZ. It can be seen that there are cavities in the microstructure in the interface of

HAZ and base metal. This kind of defects is generally appeared in welded samples due to high

welding current or low electrode force. As it was reported by Harlin et al. [25] and Kocabekir et

al. [24], high welding shrinkage can be responsible for this kind of internal defects which may be

caused by high thermal expansion. However, if these defects were produced in the center of weld

nugget, they might not significantly affect mechanical properties [24]. The tensile shear results

proved this discussion as well (Fig. 4).

3.4.2. Prediction of welds microstructure

Standard austenitic stainless steel weld metal contains two phases including austenite and

ferrite [24] . In this study, the conventional Schaeffler, WRC-1992 and pseudo-binary phase

diagrams were used to predict microstructures of weld nugget. Since cooling rate is not in

equilibrium condition in RSW process and conventional Schaeffler and WRC-1992 phase

diagrams do not work at all cooling rates, it is essential to apply another diagram which is based

on schaeffler and WRC-1992 predictive graphs which is named pseudo-binary predictive phase

diagram. Fig. 13a, 13b and 13c show the Schaeffler, WRC-1992 and pseudo-binary predictive

phase diagrams of austenitic stainless steels, respectively.

First of all, equivalent amount of Cr and Ni were calculated according to Schaeffler

predictive phase diagram and attained as 19.293, for Creq and 11.715, for Nieq. It is shown

schematically in Fig. 13a that conventional Schaeffler diagram anticipates that austenite and

15

ferrite phases will form in weld nugget based on the base metal chemical composition. In the

second step, the equivalent amount of Cr and Ni were calculated based on WRC-1992 diagram

and obtained as 18.745, for Creq and 11.585, for Nieq. It is schematically demarcated in Fig. 13b

that WRC-1992 solidification mode phase diagram anticipated FA (primary ferrite + austenite)

solidification mode for AISI 316L sample based on its chemical composition.

Pseudo-binary phase diagram illustrates that during welding process, morphology and

microstructure of austenitic stainless steel will be changed in weld nugget regions. Thus,

different morphologies such as fully austenitic to eutectic delta ferrite, skeletal delta ferrite,

acicular delta ferrite, lathy delta ferrite, Widmanstatten austenite and fully delta ferrite can be

observed in welds based on changes in proportion of Creq/Nieq. According to Fig. 13c,

solidification mode may change from austenite (A) to primary austenite + eutectic ferrite (AF),

primary ferrite + austenite (FA) and fully ferrite (F) within increasing the ratio of Creq/Nieq.

Based on this diagram, when cooling rate is moderate and/or the Creq/Nieq is low, which in this

study is approximately 1.618 (based on WRC-1992 phase diagram’s formulas), skeletal delta

ferrite will be formed in weld nugget. In contrast, when cooling rate and/or the Creq/Nieq is high,

lathy delta ferrite will be dominated phase in the microstructure of weld nugget [14, 15]. So,

pseudo-binary phase diagram predicts skeletal delta ferrite for moderate cooling rate and lathy

delta ferrite for high cooling rate. However, there were a few contradictions in this study when

metallographic examinations were performed on welded specimens.

Fig. 13 depicts various types of delta ferrite in weld nuggets at 4 kA and 8 kA welding

currents. SEM image taken from specimen welded by using 4 kA welding current is shown in

Fig 13a. It can be seen that skeletal and lathy delta ferrites have formed in weld nugget region.

By increasing welding current from 4 kA to 8 kA, which leads to the generation of higher heat

16

input, delta ferrite with morphology of acicular will form as well as skeletal and lathy delta

ferrite in weld nugget areas and consequently results in slightly coarser microstructure of weld

nugget. In the other word, this coarse structure could be caused by formation of acicular delta

ferrite beside formation of skeletal and lathy delta ferrite in the microstructure of the weld.

Formation of delta ferrite with various morphologies in different regions of weld nugget

illustrate different cooling rate caused by unequal cooling rate of water-cooled copper electrodes.

According to the pseudo-binary phase diagram, formation of skeletal delta ferrite (in some

regions of Fig. 13a and Fig. 13b) can be attributed to the moderate cooling rate. At this cooling

rate, austenite consumes nickel, carbon, and nitrogen which are austenite-promoting elements

and consequently, ferrite is adequately depleted from these components and enriched in ferrite-

promoting elements such as chromium and molybdenum. Formation of lathy delta ferrite in some

regions (Fig. 13a and Fig. 13b) can be explained by the fact that cooling rate is high (based on

the pseudo-binary phase diagram). Lathy delta ferrite was replaced by skeletal delta ferrite due to

restriction in diffusion phenomenon during ferrite-austenite transformation. Additionally, when

cooling rate was dramatically increased, a few regions in weld nugget were quickly solidified

and then led to the formation of acicular delta ferrite in these areas (Fig. 13b or Fig. 13c). Thus,

this morphology was formed due to fast cooling rate in welding process and as a result,

solidification mode and proportion of Creq/Nieq in some regions of weld nugget were changed

[21]. Also, another reason for formation of the acicular delta ferrite can be attributed to the

restriction of long-range diffusion at lower transformation temperature and consequently

transformation occurs over shorter distances in fast cooling rates [15]. It can be concluded that

major microstructure formed in resistance spot weld regions are non-equilibrium phases due to

fast cooling rate in RSW process.

17

Energy dispersive X-ray (EDS) analysis was performed to investigate if there were changes

in the chemical composition of the welded samples. Fig. 14 shows SEM micrographs taken from

specimens which were welded by using 4 kA and 8 kA welding currents and Table 3 summarizes

EDS elemental analysis results. As seen, chemical composition in different regions of weld

nugget is approximately as similar as the base metal and no remarkable variations were detected.

However, weight percent of molybdenum increased in weld regions in case of the manufactured

samples within higher welding current. As it is given in Table 3, the amount of ferrite-promoting

elements such as molybdenum (Mo) and silicon (Si) are higher in the case of point numbers 5 to

8 which is related to the sample welded by using 8 kA welding current. While, in the case of

sample welded by applying lower welding current (4 kA), weight percent of the ferrite-

promoting elements, specifically Mo and Si, is lower and it confirms that there is lower amount

of ferrite in the microstructure of the welds. EDS results proved that higher heat input which is

caused by higher welding current led to the formation of ferritic microstructure in the welded

samples. Also, image analysis for quantitative microscopy with clemex software showed that the

ferrite volume fraction in the Fusion zone in higher that HAZ and base-metal.

3.5. Effect of welding current on microhardness

For further investigation, Vickers microhardness measurements were performed on the weld

nugget, HAZ and base metal on specimens which were welded by applying low (4 kA) and high

(8 kA and 9 kA) welding current and/or heat input. Fig. 15 shows direction of the microhardness

pro applied on specimens. Initial microhardness of the AISI 316L sample was around 275

HV. Effects of various welding currents on welds hardness were determined. Hardness profile of

the resistance spot welded areas is shown in Fig. 16. According to the obtained results,

18

microhardness of the weld nugget is lower in comparison to HAZ and base metal at both low and

high welding currents. This behavior arises from the fact that when welding process is performed

in atmosphere and the specimens are cooled in air, heat dissipation rate is high [11]; grains

growth might occur in weld nugget and result in lower microhardness in weld nugget area [24].

In addition, results show that microhardness in weld nugget region of specimen which is welded

under low heat input (4 kA welding current) is slightly higher than the ones which were welded

under high heat input (8 kA and 9 kA welding current). It can be attributed to grain growth with

increasing in generated heat input during RSW process. By the comparison of three specimens

welded at low and high welding current, it revealed that microhardness in HAZ area was not

significantly affected by increasing in welding current up to 8 kA, while in the case of 9 kA

welding current, microhardness value declined to the lower amount which could be attributed to

the grain growth due to high heat input. In the other word, as can be seen in Fig. 16, hardness

values for HAZ region obtained around 250 HV and negligible variation can be detected for the

samples welded by 4 kA and 8 kA, however, in the case of 9 kA it reduced to 240 HV. At the

end, hardness value of the weld metal attained 197 to 218 HV depending on the applied welding

current.

According to metallographic evaluations, grains growth took place in HAZ and weld metal

and consequently, it might lead to reduction in microhardness values in comparison to base

metal. In the other word, the reduction of the microhardness can be associated to the formation of

delta ferrite morphology in weld nugget and somehow HAZ regions. What is more, hardness

value gradually increased in the HAZ from near the fusion zone to near the base metal which can

be attributed to the fact that cooling rate near the fusion zone is low which leads to grain growth

and in contrast, the area near the base metal experiences higher cooling rate and showed fine

19

grained microstructure and higher hardness value. In this study, reduction in microhardness value

of weld metal can be justified by formation of delta ferrite morphology. As ferrite structure

provides lower microhardness in comparison to austenite morphology, observed variations in

welds microhardness seems to be rational.

4. Conclusions

Since the main objective of manufacturing process is always to improve overall quality of a

product, it is necessary to optimize multiple quality characteristics simultaneously. In this

research, microstructural and mechanical properties of resistance spot welding joint of AISI

316L austenitic stainless steel were investigated. The results have been enumerated in the

following:

1- Tensile-shear load bearing capacity of welded materials increased within increasing the

welding current up to 8 kA due to the enlargement of weld nugget size whereas further increase

in weld current up to 9 kA had an adverse effect. The minimum and maximum tensile-shear

forces or peak load obtained 2450 N and 8070 N for 4 kA and 8 kA welding currents,

respectively.

2- By increasing welding current at constant welding time, various failure modes occurred

from solely interfacial failure mode to completely tearing around weld nugget from HAZ.

Specimens that have interfacial fractures were rejected and ones which tore from HAZ, base

metal or compound modes were accepted. The best weld had button pullout with tearing from the

base metal failure mode. Also, calculated failure energy from load-displacement curves showed

that welded specimen with 8 kA welding current has the highest plastic deformation with pullout

failure mode.

20

3- In weld nugget region, grains were found to be elongated with columnar structure,

parallel to the electrode compression direction. The size of grains increased in HAZ region in

comparison to the base metal.

4- SEM-BE micrographs and optical microscopy analyses revealed that skeletal and lathy

delta ferrite morphology are the main microstructures in the weld nugget area of specimen which

was welded under low heat input (4 kA welding current). Moreover, acicular delta ferrite as well

as skeletal and lathy delta ferrite morphology was found in weld nugget of the sample which was

welded under high heat input (8 kA welding current). In fact, increasing in generated heat input

may lead to slightly coarsening of weld nugget microstructure by forming acicular delta ferrite

morphology in this region. EDS results confirmed formation of ferrite in the welded sample

made by 8 kA welding current due to presence higher weight percent of ferrite-promoting

elements such as molybdenum (Mo) and silicon (Si) in chemical composition of weld metal.

5- Microhardness studies showed that hardness of weld nugget was lower in comparison to

HAZ and base metal. Also, hardness of weld nugget and HAZ areas was slightly higher in the

specimen welded under low heat input (4 kA welding current) in comparison to the specimen

welded under high heat input (8 kA welding current). It can be related to the grain growth

phenomenon which may occur during RSW process of AISI 316L stainless steel.

21

References

[1] Vural M, Akkus A. On the resistance spot weldability of galvanized interstitial free steel

sheets with austenitic stainless steel sheets. J Mater Process Technol. 2004;153-154:1-6.

[2] Xiaoyun Z, Guanlong C, Yansong Z, Xinmin L. Improvement of resistance spot weldability

for dual-phase (DP600) steels using servo gun. J Mater Process Technol. 2009;209:2671–5.

[3] Han L, Thornton M, Shergold M. A comparison of the mechanical behaviour of self-piercing

riveted and resistance spot welded aluminium sheets for the automotive industry. Mater Des.

2010;31:1457–67.

[4] Sun DQ, Lang B, Sun DX, Li JB. Microstructures and mechanical properties of resistance

spot welded magnesium alloy joints. Mater Sci Eng A. 2007; 460–461:494–8.

[5] Kahraman N. The in -

welded titanium sheets. Mater Des. 2007;28:420-7.

[6] Qiu R, Iwamoto C, Satonaka S. Interfacial microstructure and strength of steel/aluminum

alloy joints welded by resistance spot welding with cover plate. J Mater Process Technol.

2009;209:4186-93.

[7] Marashi P, Pouranvari M, Amirabdollahian S, Abedi A, Goodarzi M. Microstructure and

failure behavior of dissimilar resistance spot welds between low carbon galvanized and austenitic

stainless steels. Mater Sci Eng A. 2008;480:175-80.

[8] Pouranvari M, Marashi SPH. Failure mode transition in AHSS resistance spot welds. Part I.

Controlling factors. Mater Sci Eng A. 2011;528:8337-43.

[9] Kearns WH. Welding Processes, AWS Welding Handbook, 3. Seventh ed: American

Welding Society, Macmillan Press Ltd., London, England; 1980.

22

[10] Cui Y, Lundin C, D. Austenite-preferential corrosion attack in 316 austenitic stainless steel

weld metals. Mater Des. 2007;281:324-8.

[11] Ozyurek D. An effect of weld current and weld atmosphere on the resistance spot

weldability of 304L austenitic stainless steel. Mater Des. 2008;29:597-603.

[12] Trigwel S, Selvaduray G. Effects of welding on the passive oxide film of electro polished

316L stainless steel. J Mater Process Technol. 2005;166:30-43.

[13] Park SHC, Sato YS, Kokawa H, Okamoto K, Hirano S, Inagaki M. Corrosion resistance of

friction stir welded 304 stainless steel. Scr Mater. 2004;51:101-5.

[14] Lippold J, C., Kotecki D. Welding metallurgy and weldability of stainless steels: Wiley

Interscience Publication, Chapter3: Alloying Elements and Constitution Diagrams. pp. 19-52.

Chapter 6: Austenitic Stainless Steels, pp. 143-225.; 2005.

[15] Lippold J, C., Kotecki D. Welding metallurgy and weldability of stainless steels: Wiley

Interscience Publication, Chapter 6: Austenitic Stainless Steels, 2005, pp. 143-225.

[16] Fukumoto S, Fujiwara K, Toji S, Yamamoto A. Small-scale resistance spot welding of

austenitic stainless steels. Mater Sci Eng A. 2008;492:243-9.

[17] Weld button criteria. recommended practices for test methods for evaluating the resistance

spot welding behavior of automotive sheet steel materialsAmerican National Standard, 1997,

ANSI/AWS/SAE/D8.9, 3.1 and 10.6.

[18] Weld button criteria. Recommended Practices for Resistance Spot WeldingAmerican

National Standard, 1997, ANSI/AWS/SAE/C1.1-66, Section 1.

[19] Pouranvari M, Mousavizadeh SM, Marashi SPH, Goodarzi M, Ghorbani M. Influence of

fusion zone size and failure mode on mechanical performance of dissimilar resistance spot welds

23

of AISI 1008 low carbon steel and DP600 advanced high strength steel. Mater Des.

2011;32:1390-8.

[20] Alizadeh-Sh M, Marashi SPH, Pouranvari M. Resistance spot welding of AISI 430 ferritic

stainless steel: Phase transformations and mechanical properties. Mater Des. 2014;56:258–63.

[21] Hasanba•o• lu A, Kaçar R. Resistance spot weldability of dissimilar materials (AISI 316L–

DIN EN 10130-99 steels). Mater Des. 2007;28:1794-800.

[22] Pouranvari M, Marashi SPH, Safanama DS. Failure mode transition in AHSS resistance

spot welds. Part II: Experimental investigation and model validation. Mater Sci Eng A.

2011;528:8344-52.

[23] Vural M, Akkus A, Eryurek B. Effect of welding nugget diameter on the fatigue strength of

the resistance spot welded joints of different steel sheets. J Mater Process Technol.

2006;176:127–32.

[24] Kocabekir B, Kacar R, Gunduz S, Hayat F. An effect of heat input, weld atmosphere and

weld cooling conditions on the resistance spot weldability of 316L austenitic stainless steel. J

Mater Process Technol. 2008;195:327-35.

[25] Harlin N, Jones T, B., Parker J, D. Weld growth mechanism of resistance spot welds in Zinc

coated steels. J Mater Process Technol. 2003;143-144:448-53.

24

Table captions

Table 1. Chemical composition of AISI 316L Austenitic stainless steel (wt %).

Table 2. Welding parameters for resistance spot welding.

Table 3. EDS elemental analysis of the welded samples with 4 and 8 kA welding current which

are shown in Fig. 12 (wt. %).

Figure captions

Fig. 1. (a) Dimensions of tensile-shear test specimens in mm, (b) geometric morphology of the

nugget, and (c) resistance spot welded 316L austenitic stainless steel sheets.

Fig. 2. Effect of welding current and time on (a) weld nugget diameter size, (b) welding

penetration, (c) width of HAZ, (d) thickness of HAZ, (e) peak load, (f) failure energy, (g)

indentation depth, and (h) penetration rate.

Fig. 3. Variations weld nugget diameter size as a function of different welding currents.

Fig. 4. (a) Schematic of stress-strain or load-displacement curve (b) stress–strain curves of the

samples welded at constant 4 cycles welding time, (c) tensile-shear load bearing capacity of

resistance spot samples welded at constant 4 cycles welding time, and (d) Energy absorption

versus welding current and constant 4 cycles welding time.

Fig. 5. Optical image of resistance spot weld region welded at 9 kA welding current.

Fig. 6. Various types of failure modes obtained from tensile-shear test samples, (a) 4 kA, (b) 5

kA, (c) 6 kA, (d) 7 kA, (e) 8 kA, and (f) 9 kA welding currents at constant welding time.

Fig. 7. Cross-section images of the joint welded by (a) 4 kA, (b) 5 kA, (c) 6 kA, (d) 7 kA, (e) 8

kA, and (f) 9 kA welding currents at constant 4 cycles welding time.

25

Fig. 8. Cross section micrographs of sample welded by using 8 kA welding current and 4 cycles

welding time: (a-b) transition zone between weld nugget, HAZ and base metal, (c) resistance

spot welded region, (d) center of weld nugget, and (e) weld nugget area.

Fig. 9. Cross section micrographs of sample welded by using 4 kA welding current and 4 cycles

welding time: (a) border between base metal and HAZ, (b) border between weld nugget and

HAZ, (c) resistance spot welded region, (d) HAZ area, and (e) center of weld nugget.

Fig. 10. Cross sectional micrographs of sample welded by using 9 kA welding current and 4

cycles welding time: (a) and (b) weld metal regions, (c) resistance spot welded region, (d) and (e)

weld metal, border between welded nugget, HAZ and base metal.

Fig. 11. SEM photographs of resistance spot welded regions manufactured at 8 kA welding

current: (a-b) HAZ and weld nugget, and (c) interface image of base metal and HAZ.

Fig. 12. (a) Schaef , (b) WRC-1992 diagram, and (c) Pseudo-binary phase diagram.

Fig. 13. Scanning electron photographs taken from samples (a) welded at 4kA welding current,

(b-c) welded at 8kA welding current.

Fig .14. SEM images taken from weld nugget area (a-b) welded by using 4kA welding current

and (c-d) welded by using 8kA welding current. EDS elemental analysis were carried out at the

determined points.

Fig. 15. Schematic of the microhardness measurements taken from cross section of the samples.

Fig. 16. Microhardness profiles of welds manufacture by using various welding current.

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

Research Highlight

Resistance spot welding of AISI 316L stainless steel sheets.

Microstructure prediction by the use of Schaeffler and Pseudo-binary diagrams.

Non-equilibrium phases including skeletal, acicular and lathy delta ferrite formed.

Mechanical characterization of weld nuggets including peak load and failure energy.

Different failure modes were found at various welding currents.

54

Table 1. Chemical composition of AISI 316L Austenitic stainless steel (wt.%).

Table 2. Welding parameters for resistance spot welding.

Table 3. EDS elemental analysis of the welded samples with 4 and 8 kA welding current which

are shown in Fig. 13 (wt. %).

Point number Fe Cr Ni Mo Mn Si Ti

1 67.07 15.27 9.87 4.37 1.27 1.14 0.55

2 66.45 15.96 9.99 4.93 1.10 1.07 0.50

3 66.60 16.13 9.34 4.78 1.87 0.86 0.41

4 65.68 15.33 10.56 5.85 0.90 1.13 0.55

5 62.66 14.77 10.38 8.47 2.02 1.43 0.27

6 63.05 14.92 9.51 8.39 1.92 1.89 0.32

7 62.51 15.19 9.67 8.24 1.97 2.23 0.19

Fe

68.490

C

0.029

Ni

10.020

Cr

16.670

Mn

1.650

Mo

2.050

Si

0.370

Co

0.210

Cu

0.280

Nb

0.037

Ti

0.022

V

0.048

W

0.060

Al

0.002

P

0.034

N

0.024

Squeeze time

(cycle)

Welding time

(cycle)

Holding time

(cycle)

Welding current (kA)

Electrode force(N)

Welding condition

Weld cooling

condition

20 4-5-6-7 30 4-5-6-7-8-9 4000 Atmosphere Atmosphere

55

8 63.17 15.42 10.20 7.93 1.82 1.33 0.12