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Materials Science and Engineering A 529 (2011) 237–245

Contents lists available at SciVerse ScienceDirect

Materials Science and Engineering A

journa l homepage: www.elsevier .com/ locate /msea

Mechanical properties and microstructure of resistance spotwelded severely deformed low carbon steel

F. Khodabakhshi, M. Kazeminezhad∗, A.H. Kokabi

Department ofMaterials Science andEngineering,Sharif University of Technology, Azadi Avenue, Tehran, Iran

a r t i c l e i n f o

Article history:

Received 25 May 2011

Received in revised form 6 August 2011Accepted 8 September 2011

Available online 16 September 2011

Keywords:

Mechanical properties

Microstructure

Lowcarbon steel

Severe plastic deformation

Resistance spot welding

a b s t r a c t

The welding of nanostructured low carbon steel sheets produced by severe plastic deformation (SPD) has

been considered in the present paper. Constrained groove pressing (CGP) method is used for imposing

the severe plastic deformation to the steel sheets asa large pre-strain. The SPDed sheets are joined using

resistance spot welding (RSW) process. The results show that severe plastic deformation can effectively

increase the electrical resistivity of steel sheets; therefore it can affect the microstructure and mechanical

properties of spot welds. Microstructure and mechanical properties of fusion zone, heat affected zone

(HAZ), recrystallized zone and base metal of SPDed sheets are investigated and the results are compared

with those of as-received specimens. The results show that with increasing the large pre-strain in sheets,

at constant welding parameters (welding current and time), the fusion zone size, electrode indentation

and nugget diameter are increased. Thus, peak load and hardness in fusion zone and HAZ are increased

with increasing the CGP pass number. Also, the microstructures of fusion zone and HAZ are refined to

lower sizes for larger pre-strained specimens.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Severe plastic deformation (SPD) has many advantages in pro-cessing of nanostructured materials [1]. The principle of thismethod includes increasing the dislocation density by heavily uni-

form deformation, forming of dense dislocation walls and at followconverting to ultra fine grain (UFG) or nano-scale microstructure[2]. Materials processedvia SPD have some advantages dueto theirnon-porous structure, great mechanical properties such as high

strength and proper dimensions [3,4]. Several methods have beeninvented for imposing thesevere plastic deformation to themetalsinformsofbulkandsheet [4–10]. The process of constrained groovepressing (CGP) has been introduced by Shin et al., is a major SPD

process for manufacturing of UFG sheets that involves repetitivecorrugating and flattening stages [8,11]. One pass of this processincludes four stages and after one CGP pass, a strain magnitudeof 1.16 is imposed to sheets. Stages of one CGP pass are shown in

Fig. 1(a). By imposing more CGP pass numbers, the higher strainsare achieved.

Low carbon steel sheets have been widely used in fabricationindustry due to their excellent strength to weight ratio. Some of

the typical applications of these sheets are in automobile indus-try. Previous works carried out by the present researchers [12,13],show that constrained groove pressing process can effectively

∗ Corresponding author. Tel.: +98 21 66165227; fax: +98 21 66005717.

E-mail address: [email protected] (M. Kazeminezhad).

improve the mechanical properties of low carbon steel and refine

its microstructure to nano-scale. The CGPed sheets have higherstrength to weight ratio than that of as-received sheets which ismore applicable in industries.

However, considering the size and shape limitations of CGPed

sheets and other SPDed materials, the applications of these sheetsfor producing of large or complex parts such as car bodies arelimited. Therefore, in order to develop the applications of thesematerials, their joining is essential. Here minimizing the difference

between strength of the weld zone and that of base metal is themain purpose. The microstructural sensitivity of SPDed materialsto temperature increscent due to extremely high internal energy isa main challenge in welding of these types of materials [14]. Pre-

vious researches imply that friction stir welding (FSW) is one of thebest methods to join SPDed andnanostructured materials. Sev-eral works have been done on joining of severely deformed metalsby FSW process [15–21]. However, in this method, the hardness

values of weld zone show a decrease in comparison with thosereported for base metal, dueto recrystallization and grain coarsen-ing.

Considering the prior works [15–21], there are some works on

joining of some SPDed metals such as aluminum and magnesiumalloys, but still there is no work on SPDed steel. Due to very highstrength of severely plastic deformed steels, FSW process may beso difficult. Also,in some industrial applications usingFSW method

for joining is not possible. It seems that altering the process can bean interesting point for joining of severely plastic deformed steelsheets.

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238 F. Khodabakhshiet al. / MaterialsScience and Engineering A 529 (2011) 237–245

Fig. 1. Schematic of constrained groove pressing (CGP) stages and resistance spot welding (RSW) process.

Resistance spot welding (RSW) is thedominant process forjoin-

ing the steel sheet, particularly in automotive industry. Simplicity,lowcost, high speed (low process time) and automation possibilityare some advantages of this process. Since, the mechanical proper-ties of spot weldsare so important, the joining of SPDedsteel sheets

is interesting. The variation of SPDed low carbon steel propertiesdue to spot welding process is a determinant factor. This subjectcan be illuminated that is spot welding a good process for this caseor not? For answering, following works are carried out.

In thepresent work, severeplastic deformation is carried outonlow carbon steel sheets by CGP technique. Different strain magni-tudes are imposed to the sheets. Then the SPDed sheets are joined

using resistance spot welding process and their mechanical prop-erties and microstructures are investigated.

2. Experimentalmaterial andprocedure

2.1. Constrained groove pressing

A 3 mm thick uncoated low carbon steel sheet used in automo-tive industry (St-12) was used in this study. The yield strength and

ultimate tensile strength of this steel sheet at as-received condi-tionwere 204and 288MPa, respectively. Thechemical compositionof the studied steel was Fe–0.0527C–0.203Mn–0.0229Si–0.006P.Before welding, these steel sheets were subjected to severe plas-

tic deformation by constrained groove pressing method and large

magnitudes of pre-strains were imposed to the sheets. In this pro-cess, the sheets were deformed between corrugated and flat dies.One pass of CGP process includes four stages that is shown in

Fig. 1(a). In the first stage, flat sheet is corrugated which resultsin a 0.58 effective strain in deformed regions. Then in the secondstage, flat dies flatten the corrugated sheet by imposing an extra0.58 strain in reverse direction to the previously deformed sec-

tions. After 180◦ rotation of the sheet around its perpendicular axis,the successive pressings with the corrugated and flat dies (thirdand fourth stage) result in a large homogeneous effective strainthroughoutthe sample. At theend of one CGPpass,a uniform effec-

tive strain of 1.16 has been imposed throughout the specimen. Byrepeating the CGP process, a large amount of plastic strain can beaccumulated in the work piece without changing its initial dimen-

sions. In this research, CGPprocess was applied to lowcarbon steel

sheets up to four passes and the strain magnitude up to 4.64 was

achieved.

2.2. Resistance spot welding

Resistance spotwelding is a process of joiningtwo or moremetalparts by fusion at discrete spots in the interface of work pieces(Fig. 1(b)).Resistance to current flowthroughthe metal workpiecesand their interface generates heat; therefore, temperature rises at

the interface of the work pieces. When the metal is melted, themetal will begin to fuse and a nugget begins to form. The current isthen switched off and the nugget is cooled down to solidify under

pressure.Low carbon steel samples that processed from previous section

were joined via resistance spot welding process with conditionsand parameters as presented below.

Before doing the welding tests, surface finishing treatments

were applied to CGPed steel sheets and their thicknesses werereduced to 2.5 mm. Spot welding was performed using a 120 kVAAC pneumatic type resistance spot welding machine. For weldingof2.5mm thick steelsheetat this work,according to ANSI/AWSD8-

9 1999 standard [22], water cooling copper–beryllium electrodeswith a face diameter of 6.5 mm was used. In all of the experiments,electrodeforce,pulsation andsqueeze time werekept at4 kN,3 and5 cycles, respectively. In fixed weldingtimesof 12,16 and 20 cycles,

welding current was changed from 9.5 to 17.3kA as presented in

Table 1. For characterizing the spot welds, following examinationsshould be applied.

Electrical resistivity values of SPDed low carbon steel sheets for

as-received condition, one, two, three and four passes of CGP weremeasured by standard 4-point probe technique at room tempera-ture.

There are generally three indexes for quality control of resis-

tance spot welds: weld fusion zone size (FZS), weld mechanicalperformance andfailure mode. Weld fusion zone size is dependenton weld penetration and nugget diameter. The tensile-shear testis the most widely used test for evaluating the spot weld mechan-

ical performance in static condition. Peak load obtained from thetensile-shear load–displacement curve is often used to describe themechanical behaviors of spot welds. The static tensile-shear test

samples were preparedaccordingto ANSI/AWSD8-9 1999standard



F. Khodabakhshiet al. / MaterialsScience and Engineering A 529 (2011) 237–245 239

Table 1

Welding samples and parameters.

Specimen Welding current (kA) Welding

time (cycle)

As-received

– 10.6 11.6 12.4 13.2 13.8 14.4 15 15.4 15.9 16.3 16.7 17 17.3 12

– 10.6 11.6 12.4 13.2 13.8 14.4 15 15.4 15.9 16.3 16.7 17 – 16

9.5 10.6 11.6 12.4 13.2 13.8 14.4 15 15.4 15.9 16.3 16.7 – – 20

One pass

– 10.6 11.6 12.4 13.2 13.8 14.4 15 15.4 15.9 16.3 16.7 17 17.3 12

– 10.6 11.6 12.4 13.2 13.8 14.4 15 15.4 15.9 16.3 16.7 17 – 16

9.5 10.6 11.6 12.4 13.2 13.8 14.4 15 15.4 15.9 16.3 – – – 20

Two passes

– 10.6 11.6 12.4 13.2 13.8 14.4 15 15.4 15.9 16.3 16.7 17 – 12

9.5 10.6 11.6 12.4 13.2 13.8 14.4 15 15.4 15.9 16.3 16.7 17 – 16

9.5 10.6 11.6 12.4 13.2 13.8 14.4 15 15.4 15.9 16.3 – – – 20

Three passes

9.5 10.6 11.6 12.4 13.2 13.8 14.4 15 15.4 15.9 16.3 16.7 17 – 12

9.5 10.6 11.6 12.4 13.2 13.8 14.4 15 15.4 15.9 16.3 16.7 – – 16

9.5 10.6 11.6 12.4 13.2 13.8 14.4 15 15.4 15.9 – – – – 20

Four passes

9.5 10.6 11.6 12.4 13.2 13.8 14.4 15 15.4 15.9 16.3 16.7 – – 12

9.5 10.6 11.6 12.4 13.2 13.8 14.4 15 15.4 15.9 16.3 – – – 16

9.5 10.6 11.6 12.4 13.2 13.8 14.4 15 15.4 – – – – – 20

Fig. 2. Spot welded tensile-shear test sample.

[22] that the sample dimensions are shown in Fig. 2. Tensile-sheartests were performed at a cross head speed of 1 mm/min with an

Instron machine. Fig. 3 shows the schematic of tensile-shear test, ashim part was used for preventing sheets bending during test.

Failure mode is the manner in which spot weld fails. Generally,the resistance spot weld failure occurs in two modes, interfacialfailure (IF) and pullout failure (PF). In the interfacial mode, fail-ure occurs via crack propagation through the fusion zone; while,

in the pullout mode, failure occurs via complete (or partial) nuggetwith drawl from one sheet. Spot weld failure mode is a qualitativemeasurement of the weld quality. Spot welds that fail in nuggetpullout mode lead to higher peak loads level than those of spot

welds which fail in interfacial failure mode. After complete sepa-ration in the tensile-shear test, cross-section of the samples wasvisually examined and failure mode was determined.

For metallographic observation, samples were cut along the

center line of the spot welds and were examined from thicknessplane. Subsequently, standard metallographic procedure and opti-cal microscopy observation were carried out for microstructuralinvestigations.

The micro-hardness profiles across the investigated spot-weldswere determined using a Vickers hardness testing machine. Thehardness values were measured along the weldment at regularintervals of 0.25mm as shown in Fig. 4.

3. Results and discussion

3.1. Electrical resistivity

The changes into electrical resistivity of low carbon steel sheets

versus CGP pass number are presented in Table 2. As can be seen,

Fig. 3. Schematic of tensile-shear test accomplish.

Fig. 4. Vickers micro hardness measurement profile along the weldments.




Table 2

The variation of electrical resistivity of low carbon steel versus CGP pass number

and imposed strain.

CGP p ass n umber Strain Electrical r esistivity ( cm)

0 0 57.44

1 1.16 97.22

2 2.32 100.41

3 3.48 100.46

4 4.64 106.06

imposingthe onepassof CGPprocess canrapidlyincrease the resis-tivity of low carbon steel sheets. However in the following passes,the rateof electrical resistivity increscent is reduced. This trend canbe interpreted by considering the effects of grain boundaries and

dislocations on electron scattering [12].Dueto imposing of thesevere plastic deformationto lowcarbon

steel sheets by CGPmethod,the dislocation densityis increased andcoarse grained microstructure is refined to nano-scale. Therefore,

electron scattering due to grain boundaries and dislocations areincreased and the electrical resistivity is increased.

3.2. Tensile-shear test

The experimental results indicate that welding current has asignificant effect on the load carrying capacity and energy absorp-tion capability of the spot welds under tensile-shear static test.

The effects of welding current and welding time on peak load of tensile-shear test are shown in Fig. 5 for sheets with differentpasses of CGP. As can be seen in this figure, increasing the weld-ing current and time for each pass leads to increasing the peak

load of steel welds due to increasing of fusion zone size (FZS) andfusion penetration depth. For example at one CGP pass specimen,increasing the welding current from 9.5 to 16.7 kA at welding timeof 12 cycles, leads to increasing of peak load from 0 to 26.4kN.

This behavior can be observed with increasing the welding timeup to 20 cycles. At welding current range of 16.7–17.3kA, a drop

in peak load occurs. This maybe due to the effect of expulsionon peak load that occurs at higher welding current [23]. Simi-

lar trends can be observed for as-received and other passes of CGP. Also, if the results for different CGP passes are compared, itcan be seen that with increasing the CGP pass number from as-received to four passes specimens, the level of peak load curve

is increased and peak load magnitude of failure curve at constantwelding parameters (welding current and time) is increased. The

reasons of this behavior will be explained in details in Section 3.6.Since the results for three CGP passes and four CGP passes are sim-ilar; therefore the results of three CGP passes are not shown in thisfigure. Fig.5 also shows that failure mode types arechanged during

increasing the welding current and welding time in tensile sheartest. At low welding current, IF failure mode is dominant for allCGP passes (that involves low failure energy absorption), but withincreasing welding current, PF failure mode is dominant. Where

failure mode is changed and PF failure occurs at specimens, thewelding parameters are introduced as an optimum condition. Thelimits of welding parameters required for failure mode changes aredecreasedwithincreasing the CGPpass number that is explained in

Section 3.3. Optimum welding parameters, i.e., welding current of 14.4 kA andweldingtimeof 16 cycles, areselected forinvestigatingtheeffect of CGPprocesson somespot welds characteristicssuch as,electrode indentation (electrode dip), nugget diameter, peak load,

microstructural changes and micro hardness variations (Sections3.4–3.8).

3.3. Failure behavior

Two distinct failure modes were observed during static tensile-shear testing: interfacial fracture (IF) and nugget pullout (PF), seeFig.6. Theeffectof thewelding current onthe failure modeis shown

in Fig. 5. As can be seen in this figure, failure mode has a significantinfluence on the peak failure load of spot welds.

Fig. 7 shows a simple model describing stress distribution atthe interface and circumference of a weld nugget. Driving force for

the interfacial failure mode is shear stress and for pullout failure istensile stress. Tensile stress is mainly induced by bending momentdue to overlapping of two sheets and weld nugget rotating duringshear-tensiletest.In fact,tensile bendingstressesplay an important

role in pullout failure mode. It should be noted that normal stressin T sites is tensile while in C sites is compressive. In the pullout

failure mode, increase of experienced tensile stress in T sites leadsto local plastic deformation through thickness direction.

For spot welds made at low welding currents, low fusion zonehardness and smallfusion zonesize leadto experiencinginterfacialfailure mode during tensile-shear test. While for spot welds madeat high welding currents, higher hardness of fusion zone due to

Fig. 5. Effects of welding current and time on thetensile-shear test peak load of differentCGP passesspecimens.




Fig. 6. Types of failure mode in tensile-shear test specimens: (a)IF mode and (b) PF mode.

martensite formation and larger fusion zone leads to experiencingpullout failure mode during tensile-shear test.

The failure mode is a qualitative criterion for weld reliabilitywhich is widely used in the manufacturing environment. As can be

seen in Fig.5, f or small welding current, the dominant failure modeis interfacial for all passes. However, when the welding current is

increased beyonda critical value, the failure mode is changed to thepullout one. These critical values are different for different passesand its magnitude is decreased with increasing the CGP pass num-ber. Since, the changes of failure mode to PF mode is due to the

increasing of weld strength and weld quality; therefore it can berepresented that with increasing theCGP pass number, the magni-tude of welding parameters for giving an optimum weld quality isreduced. In other words, it canbe claimed that severeplastic defor-

mation of low carbon steel sheets by constrained groove pressingprocess can improve the resistance spot weldability and an opti-mum quality of weld is achieved at lower welding parameters.

3.4. Electrode indentation

Fig. 8 shows the effects of CGP pass number at constant welding

current and welding time (14.4 kA and 16 cycles, respectively) onthe nugget diameter and electrode indentation depth. Accordingto Fig. 8, electrode indentation depth is increased with increas-ing the magnitude of pre-strain (or the number of CGP passes) at

constant welding parameters. For this welding current and time,electrode indentationis increasedfrom 0.3mm at zeropre-strainto0.5mm at pre-strain of 4.64. Electrode indentation,whichcan affectthe mechanical and microstructure properties of the spot welds,

depends on electrode pressure and temperature of electrode/sheetinterface. The general trend of Fig.8 can be explained as follow. Thetotal electrical resistance of a sheet can be attributed to the con-

tributions of the contact resistance values at the electrode-sheetinterfaces (R1and R5 in Fig.1(b)),at the sheet fayinginterface (R3),and the bulk resistance (R2and R4). Resistance valuesof R2,R3 andR4 have themain role for spot welding process andprovision of the

Fig.7. A simplemodeldescribingstressdistributionin interfaceand circumferential

of a weld nugget.

required heat at sheets interface. Resistance of R3 is dependent onchemical composition of sheets and their surface conditions. But,

bulk resistance values (R2 and R4) can be expressed as [24]:

R = l

s (1)

where is the electrical resistivity, l is thethickness of sheets andsis the electrodes tip diameter. As can be shown in Section 3.1, withincreasing the CGP pass number electrical resistivity is increased.

Therefore, at constant l and s, bulk resistance (R) is increased withincreasing the number of passes. Total heat generated at sheetsinterface can be obtained as [24]:

Q = RI 2t (2)

where Q is the heat input, R is the bulk resistance, I is the weld-

ing current and t is the welding time. Using Eqs. (1) and (2), andelectrical resistivity data (Table 2), it can be seen that at constantwelding current and time, heat input is increased with increas-

ing of the CGP pass number. Increasing the heat input leads tothe temperature rise of the electrode/sheet interface which in turnincreases the degree of plastic deformation that can occur in thesheet surface under electrode pressure. Therefore, the magnitudeof electrode indentation is increased with increasing the CGP pass

number at constant welding parameters. Also, spot welds withexpulsion exhibit severe electrode indentation [23]. Increasing theelectrode indentation leads to increase in contact surface of watercooled copper electrode system to welding zone, and therefore the

rate of cooling is increased with increasing the CGP pass num-ber. This subject is the main reason of microstructure refining andhardness increasing at welding zone and HAZ of CGPed sheets will

Fig. 8. Effects of pre-strain andCGP pass numberon nuggetdiameterand electrode

indentation depth.




Fig. 9. Tensile-shear peak load versus CGP pass number.

be discussed in details in Sections 3.7 and 3.8. Refining of weldmicrostructureleads to increasingof weldstrength and thissubject

can affect the peak load variation (Sections 3.2 and 3.6).

3.5. Nugget diameter

Fig.8 alsoshowsthat thenugget diameter(that is correspond tofusion zone size) of low carbon steel sheet spot welds is increased

with increasing the CGP pass number at constant welding currentand welding time (14.4kA and 16 cycles, respectively). At these

welding parameters, nugget diameter of as-received specimen isabout 7 mm, but itis increased upto 9.75mm atfourCGPpasses. Asdiscussed in previous section, heat input is increased with increas-ingthe number of passes. Therefore, it leads to increasing of nugget

diameter (or fusion zone size). This subject is the main reason of peak load increasing with increasing of CGP pass at constant weld-ing parameters (Sections 3.2 and 3.6).

Fig. 10. Microstructure of weld nugget, heat affected zone (HAZ) and base metal of as-received specimen at constant welding parameters (welding current: 14.4 kA, and

welding time: 16 cycle)in two magnifications.




3.6. Peak load

The peak point in the load displacement plot of tensile-sheartestcorresponds to thepoint of crackpropagationthroughthe weldnugget for interfacial mode and to the necking point at the failurelocation for the pullout mode [23]. The variation of peak load at

constant welding parameters (welding current 14.4 kA and weld-ingtime16 cycles) versus CGPpass number is shownin Fig.9. Itcanbe seen from this figure that peak load is generally increased withincreasing the CGP pass number at similar welding condition. At

this welding current and welding time, peak load of as-receivedspecimen is about 17.798kN, but it is increased up to 24.12kNat four CGP passes. Similar trends can be observed for peak loadvariation at others welding parameters. As discussed in previous

sections, microstructure refining at weld nugget due to electrode

indentation is increased with increasing the CGP pass numberat constant welding parameters. Also, weld nugget diameter is

increased with increasing pass number at similar condition. Con-sidering microstructure refining and nugget diameter increasing,the peak load changes at different CGP pass numbers are reason-able.

3.7. Microstructural changes

Fig. 10 shows a typical microstructure of the as-receivedspot weld at two distinct magnifications (at constant welding

parameters of 14.4kA and 16 cycles) that indicates three distinctmicrostructuralzones in the joint region: (i)fusion zone (FZ) or alsocalled weld nugget, (ii)heat affected zone(HAZ) and(iii) basemetal(BM).

Fig. 11. Microstructure of weld nugget, heat affected zone (HAZ), recrystallized zone and base metal of four passes specimen at constant welding parameters (welding

current: 14.4 kA, and welding time: 16 cycle) in twomagnifications.




The microstructure of the as-received low carbon steel sheetobserved on the surface plane of sheet consists of approximately

6 vol.% pearlite (dark contrast) and the remainder is ferrite (brightcontrast). It is clearly shown in Fig. 10 thatthe initial microstructureor base metal microstructure contains equiaxed grains in size of 30m. Welding nugget has a ferrittic–martensitic microstructure.

The microstructure gradient in the HAZ can be observed in Fig. 10.Microstructure is more heterogeneous in the HAZ than that in thefusion zone as it will be verified by hardness profilein thenext sec-tion. The material in the HAZ experiences a peak temperature and

a cooling rate which are inversely proportional to its distance fromthe fusion line. The HAZ microstructure near the fusion boundaryconsists of martensite, grain boundary ferrite and Widmanstattenferrite. Martensite formationin the weld regionand HAZof thelow

carbonsteel is attributed to high cooling rate of resistance spot pro-cessdue to thepresence of water cooled copper electrodes andtheirquenching effect as well as short welding cycle. It has been shownthrough modeling works [25–30] that even at 500 ◦C the cooling

rates in spot welding are more than 1000◦C/s (that is required formartensite formation). Martensitic formation in weld region andHAZ is due to rapid cooling rate and occurs for all passes of CGP,but the sizes of martensitic packs are different for different large

pre-strains.

For comparison of SPDed steel sheet welds with as-receivedone, some spot weld microstructures for four CGP passes spec-imens at two magnifications and same welding parameters are

shown in Fig. 11. There are some differences between microstruc-tures of CGPed steel sheet welds and those of as-received ones thatdescribed as below. As canbe seen in Fig. 11, weldsof CGPed sheetshave threedistinct microstructural zones: FZ, HAZ and BM as same

as before. Moreover, a new region named recrystallized zone isobserved in CGPed sheets. X-ray diffraction (XRD) and transmis-sion electron microscopy (TEM) investigations in previous workof present authors [12] indicate that microstructure of low car-

bonsteel duringCGP process is evaluated to 200–300 nm grainsizerange. Nano-metric structure in base metal of SPDed steel sheetsis changed during spot welding and due to heating, the restora-

tionmechanismscan be activated[31]. Microstructures of weldandHAZ inSPDed steelsheets arethesameas those ofas-received sheetwhich are ferrittic–martensitic. Also,it can be seenthat microstruc-tures of weld and HAZ in severely deformed steel sheet are finerthan those of weld and HAZ for as-received sheet. As discussed,

this subject in electrode indentation section is due to increasingof cooling rate with imposing of severe plastic deformation into

steel sheets. High thermal cycle gradient coupled with the largepre-stored energy due to severe plastic deformation can explain

the observation of recrystallized microstructure at a distance fromHAZ. In other words, time and temperature in this region are suf-ficient for occurring of recrystallization phenomenon, but due torapid cooling, time is not sufficient for occurring of grain growth

phenomenon. The size of recrystallized grains is proportional toamount of pre-strain and with increasing the pre-strain level byapplying more CGP pass numbers, the numbers of recrystallizednuclei are increased and therefore microstructure of recrystal-

lized zone is refined to lower size and the length of this zone isincreased. For different passes and different welding parameters,similar trends can be observed.

3.8. Micro hardness measurements

Typical variations of micro-hardness value across thespot welds

are shown in Fig. 12 for as-received and different passes CGPedlow carbon steel (at welding current 14.4kA and welding time 16cycles). In as-received specimen,the micro-hardness values remainin a plateau for the base metal and the fusion zone, but a sharp

increase in the HAZ from the base metal to the fusion zone sidecan be observed. The factors that are commonly attributed to theincrease in micro-hardness along the spot-welds of low carbonsteels are (a) variation in the morphology of ferrite and marten-

site at these zones [32], (b) residual stress dueto thermal cycling of the welding process [33], and (c) internal stress generated due totheload applied by electrodes duringthe welding process [34]. The

variations in the morphology of microstructure at the base metal,HAZ and in the fusion zone for as-received steel sheet are illus-trated in Fig. 10. The residual stress induced due to thermal cyclingand the electrode pressure is considered to be released. Hence, the

variationin micro-hardnessprofilealongthe weld canbe attributedto the above factors only. It can be seen from Fig. 12, micro hard-ness level at weld nugget is higher than base metal and decreasessharply through HAZ from weld to base metal. The variations of

micro hardness at weld nugget are dueto thesubject that ferrite or

martensite phase is placed under hardness indenter tip.Fig. 12 also shows the Vickers micro hardness profile of spot

welds at welding current 14.4kA and welding time 16 cycles for

one, two, threeand four passesCGPed steel sheets. Itis obvious thatthe variation in micro-hardness profile along the weld in all casesis similar to as-received sheet, but as discussed in microstructuresection, a newregionin CGPed sheets appears named recrystallized

Fig. 12. Vickers micro hardness profilefor spot weld of differentpasses CGPed specimens (welding current of 14.4 kA and welding time of 16 cycles).




Fig. 13. Effect of CGP pass number on micro hardness of base metal and welding

zone at a constant welding current and welding time.

zone. Due to existence of this zone, variation of micro hardnessfrom weld to base metal is not sharp which is a good advantage.As the same as zero pre-strain sheets, for all passes, micro hard-

ness level at weld nugget is higher than that of base metal anddecreased through HAZ and recrystallized zone from weld to base

metal. With increasing the CGP pass number, the level of microhardness value at all zones is increased. Increasing the hardness

of base metal during severe plastic deformation via CGP methodis common and arises from the work hardening and grain refiningmechanisms [13]. Increasing the hardness level at other regionsis due to increasing the cooling rate and refining the microstruc-

ture as discussed before. The strength of a zone in a spot-weld canbe proportional to the magnitude of its hardness governed by itsmicrostructural details. Fig. 13 shows mean micro hardness valueat weld nugget andbase metalas a function of CGPpass number. As

canbe seen in this figure, micro hardnessof as-received sheet (basemetal) is about 130 VHN and after four CGP passes is increased upto 180 VHN. Micro hardness at welding zone of as-received spec-imen is about 220 and is increased up to 280VHN for four passes

CGPed specimen. It is obvious that with increasing the hardness of base metal during SPD, hardness of welding zone is increased andthe rates of increscent are the same approximately. Therefore, it isshown that due to resistance spot welding of severely deformed

steel sheets, strength andhardness at welding zone andit’s aroundare not decreased, but also it is increased significantly. This subjectalso can be as a major advantage for using resistance spot weldingprocess in joining of severelyplastic deformedsteel sheets. At other

welding parameters, similar trends for micro hardness variationsat different passes can be observed.

4. Conclusions

The main interest of this research is using the resistance spotwelding as a safe process for joining of severely plastic deformedsteel sheets. Mechanical properties and microstructures of welds

were investigated and the conclusions can be summarized as fol-lows:

1. Severe plastic deformation via CGP method can efficiently

increasethe electrical resistivity of thelow carbon steel sheets upto 100% and this subject can change the spot welds mechanicaland microstructural properties.

2. Fusion zone size and electrode indentation depth are the main

factors controlling mechanical properties such as peak load and

micro hardness for all specimens and they are changed withimposing the constrained groove pressing to sheets.

3. With increasing the welding current and time for CGPed speci-mens, the peak load is increaseddue to increasing of fusionzonesize and electrode indentation depth.

4. Microstructures of fusion zone, heat affected zone and base

metal of CGPed low carbon steel sheet are finer than those of as-received sheet and the refining is increased with increasingthe CGP pass number.

5. At a certain welding parameters, micro hardness level at weld

nugget is higher than that at base metal and decreased throughHAZfrom weld to base metal. Also, with increasing the CGP passnumber the level of micro hardness in each region is increased.

Acknowledgements

The authors wish to thank the research board of Sharif Univer-sity of Technology for the financial support and theprovision of theresearch facilities used in this work.

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