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D.S. Sahota et al. / Mechanica Confab ISSN: 2320-2491

Vol. 2, No. 2, February-March 2013

STUDY OF EFFECT OF PARAMETERS ON RESISTANCESPOT WELD OF ASS316 MATERIAL

D.S. Sahota #1, Ramandeep Singh &2, Rajesh Sharma $3, Harpreet Singh *4

#1Indo Global Engineering College, Assistant Professor, Department of MechanicalEngineering, Punjab, India.

&2CT Engineering College, Associate Professor, Department of MechanicalEngineering, Punjab, India.

$3CCET, Sec-26, Chandigarh, Assistant Professor, Department of MechanicalEngineering, Punjab, India.

*4RBIEBT, Assistant Professor, Department of Mechanical Engineering, Punjab,India.

Corresponding Email: [email protected]

Abstract

The objective of this work is to study the effect of parameters on resistance spot weld of ASS316 material. In order to study the significance of the process parameters i.e. current,electrode force and weld cycles, towards the percentage improvement in material hardness.

It is clear from the results that parameters significantly affect both the mean and thevariation in the percentage improvement in Hardness values of the ASS316 material. The S/N

ratio analysis suggests third level of weld current, third level of weld cycle and third level ofelectrode force as the best levels for maximum percentage improvement in hardness of ASS316 work-piece in spot welding operation. This study helps us to find out the optimumvalues for three parameters for Resistance Spot Welding for 0.5mm thickness of ASS316 sheetThe average value of the response characteristic obtained through the confirmationexperimentations lies within the 95% confidence level.

Keywords: Resistance Spot Welding (RSW), Austenitic Stainless Steel (ASS) Analysis ofvariance (ANOVA),Electrode pressure, Electric Current, Number of Welding cycles.

1. Introduction

Resistance Spot Welding (RSW) is a high speed process, wherein the actual time ofwelding is a small fraction of second and it is one of the cleanest and most efficientwelding process that has been widely used in sheet metal fabrication. The high speed of process,the case of operation and its adaptability for automation in the production of sheet metalassemblies are its major advantages. Limitations of RSW are equipment cost and powerrequirements, difficulty of disassembly for maintenance or repair of RSW joints, and thenature of the design needed for the process (lap joints are required). Resistance Spot Weldinghas steadily gained importance over the years because of its ability to join the variety of





materials and complicated shapes with high accuracy and great precision. Resistance SpotWelding (RSW) is a high speed process, where the actual time of welding is a smallfraction of second and it is one of the cleanest and most efficient welding process that hasbeen widely used in sheet metal fabrication. The high speed of process, the case of operation andits adaptability for automation in the production of sheet metal assemblies are its majoradvantages. Over the last few years, the weight of automobiles has increased considerably due

to the addition of safety related items, such as impact resistance bumpers and door impactbeams, emission control equipment and convenience items, such as air conditioning. At thesame time fuel consumption has increased significantly primarily due to emission controlequipment.

2. Literature Survey

Waller et al. [1] studied the relationship between the quality of resistance weldingspots and signal curves measured during the welding event. One parameter has been used tostudy the relationship between the variables which allows the examination of the effectsof different variables on the diameter in the form of easily understandable probabilities.Aidun et al. [2] described spot weld strength monitoring system based on the current andvoltage. A change in the welding schedule that comprises change in current with time atconstant pressure shows changes in the amplitudes in both the current and voltage waveforms.Atzori et al. [3] investigated the effect of nucleus diameter, heights of nucleus, nucleus size on mechanical properties i.e. tensile shear and tensile peel strength in electrical resistance spot welding of galvanized chromided micro alloyed steel sheets. Hirch et al. [4]have studied on the resistance spot Weldability of galvanized interstitial free steelsheets with austenitic stainless steel sheets. In micro hardness measurements , themaximum hardness values were in the middle of the weld nugget Darwish et al. [5] hasdeveloped a mathematical model to study the influence of spot welding parameters (weldingcurrent, welding time, electrode force and sheet thickness) on the strength of spotwelded Stainless steel sheets with commercial purity. Experiments were planned onthe basis of Response Surface Methodology (RSM) technique. The mathematical models(failure load and nugget area) co-relating process parameters and their interactions with theresponse parameters have been established. These models have been used in selecting theoptimum process parameters for obtaining the desired spot welding quality at the least possibleconsumed power. Vural et al. [6] investigated the effect of nugget diameter on the fatiguestrength of resistance spot welded joints of galvanized steel and austenitic stainless steel(AISI 304) welded as lap joints. Bouyousfi et al. [7] have studied the effect of spot weldingprocess parameters (are intensity, welding duration and applied load) on themechanical properties and characteristics of the spot joints between two stainless steelsheets (304 ASS) having the same thickness. Micro hardness and tensile test results haveshown that the weld resistance is important and highly correlated to the value of theprocess parameters especially the applied load. The applied load seems to be the

control factor of the mechanical . Characteristics of weld joint compared to the weldingduration and the current intensity. Kim I.S. et al. [8] developed the mathematical models forthe selection of process parameters and the prediction of bead geometry in robotic GMA (Gasmetal arc) welding. Factorial design can be employed as a guide for optimization of processparameters. The results obtained show that developed mathematical models can be applied toestimate the effectiveness of process parameters for a given bead geometry, and a change ofprocess parameters affects the bead width and bead height more strongly thanpenetration relatively. Emin Bayraktar et al. [9] have contributed their research on the selection





of optimal welding conditions and developed new grade steels for automotive applications.The study is based on impact tensile testing to spot welding sheets. Zhang et al [10] Has proposedcombined application of Taguchi method to determine optimal condition with control factors(size of electrode tip, weld current, electrode force and weld time) for improving the RSWprocess quality of high strength steel sheet Nizamettin K. et al. [11] has focused his study on theinfluence of welding parameters on the joint strength of resistance spot-welded titanium sheets.

The results indicated that increasing current time and electrode force increased the tensile shearstrength and the joint obtained under the argon atmosphere gave better strength. Hardnessmeasurement results showed that welding nugget gave the highest hardness. The argon gasused during the welding process was seen to have no influence on the hardness values. J.B.Shamsul et al. [12] studied the influence of welding current on nugget size and hardnessdistribution in the plates of Austenitic Stainless Steel (AISI 304), which was placed as a lap

joint and spot welded under varied welding current. The results have shown thatincreasing welding current increased the nugget size. The nugget size does not influence thehardness distribution. Also increasing welding current does not increase the hardnessdistribution. Dursun Ozyurek [13] investigated the effect of the different welding mediumand welding current on the resistance spot weld quality of 304L stainless steel and forthis purpose, samples are welded in nitrogen at different currents and then cooled in

atmosphere. Samples welded in atmosphere at different currents are then compared with theabove samples to study the influence of different welding medium and current. M. Hamedi etal. [14] worked on numerical study of nugget formation in resistance spot welding. Theyconcluded that as the welding process continues (welding time increases), temperature ofsheets interface rises quickly until this area melts and nugget forms. After the formation ofnugget, rate of temperature rise is reduces. At the end of welding time, as current switchesoff, the weldment starts to cool dawn. And at low electrode forces and high welding currents,the formed fusion zone is large. Oscar Martin et al. [15] predicted the quality level of aresistance spot welding (RSW) joint of 304 austenitic stainless steel (ASS). It is estimatedfrom its tensile shear load bearing capacity (TSLBC). The quality levels are set by ultrasonicnondestructive testing. The weld nugget grows at the expense of heat input that increaseswith increasing WT and WC. The hardness values of the weld nugget do not decrease andeven may be higher than that of HAZ and base metal due to the strain hardening (revealed bythe presence of slip bands in the austenite cast microstructure of the weld nugget), caused byEF (electrode force) during holding time. Tomaz Kek et al. [16] worked on the analyses ofAE (acoustic emission) during resistance spot welding. The research conducted that theanalysis of the acoustic emission signal provides useful information on nugget formation. Themeasurements showed that the AE outbursts are a stronger indicator of expulsion than theelectrical and/or mechanical parameters measured. Furthermore, the frequency spectrum ofthe continuous signal occurring during current flow is also a potential feedback controlparameter as distinctive differences in the frequency spectrum were detected in certain casesin the interval 30 ms before the expulsion. Ugur Esme [17] investigated on the optimizationand the effect of welding parameters on the tensile shear strength of spot welded SAE 1010steel sheets. The level of importance of the welding parameters on the tensile shear strength isdetermined by using ANOVA. Based on the ANOVA method, the highly effectiveparameters on tensile shear strength were found as welding current and electrode force,whereas electrode diameter and welding time were less effective factors. The results showedthat welding current was about two times more important than the second ranking factor(electrode force) for controlling the tensile shear strength. A. Ambroziak et al. [18]Technology of resistance spot welding of Aluminium alloys was discussed. Automotiveindustry tends to use light alloys like Aluminium and magnesium alloys. Companies more





often use Aluminium as an alternative material for vehicle body. The most popular method of joining body sheets is resistance spot welding. Aluminium and its alloys series 5xxx and6xxx can be connected by this technique. It requires high power welding gun and precocioussteering of current and time. The aspects of rapid deterioration of tips must be taken intoaccount. Majid Pouranvari [19] Failure mode of AISI304 resistance spot welds is studiedunder quasi static tensile-shear test. He concluded that failure location for AISI 304 RSW in

pullout failure mode is at HAZ, adjacent to the weld nugget Low fusion zone hardness tofailure location hardness ratio increases the tendency of spot weld failure to occur in theinterfacial failure mode during the tensile-shear test. Metallurgical characteristics of weldsshould be considered to predict and analyze the spot weld failure mode more precisely. C.V.Nielsen et al. [20] worked on three-sheet Spot Welding of Advanced High-Strength Steels.By examining micrographs of the welds, it was found that bonding between the low carbonsteel and high-strength steels predominantly appeared in solid state rather than by a fusionnugget The solid-state bonding is facilitated by the high temperatures and plastic deformationduring the welding process breaking the oxide layer to form metallic bonds. The weldstrengths measured by tension shear tests were found to be relatively high compared withprevious investigations utilizing the same low carbon steel. Furthermore, fracture wastypically in the form of ductile tearing of the low-carbon steel around the formed plug failure.

Only a few of the weaker welds failed in a brittle manner through the interface, and this wasmainly observed for interfaces with zinc coating involved, i.e., for the TRIP steel. S.M.Hamidinejad et al. [21] In this study, the resistance spot welding (RSW) process of thegalvanized interstitial free (IF) steel sheets and galvanized bake hardenable (BH) steel sheets,used in the manufacturing of car bodies, has been modeled and optimized. The qualitymeasure of a resistance spot welding joint is estimated from the tensile–shear strength. Theyconcluded that there is a non-linear relationship between the welding parameters and thetensile– shear strength of the RSW joints. Hessamoddin Moshayedi et al. [22] worked onnumerical and experimental study of nugget size growth in resistance spot welding ofaustenitic stainless steels. In this study, two-dimensional finite element simulation ofresistance spot welding process was performed using fully coupled electrical–thermal andincrementally coupled thermal–mechanical analysis on SS304L sheets to predict weld nuggetformation through temperature distributions at different welding cycles and currentintensities. Finite element modeling was used to investigate the effects of welding time andwelding current on nugget size. When the welding current exceeds a critical value for nuggetformation (6 kA for this work), it causes a rapid growth of nugget Good agreement has beenobtained between the predicted results and the measured data, such that the maximum errorshave been around 13%.

2.1. Resistance Spot Welding (RSW)

Resistance spot welding is a welding process wherein coalescence is produced bythe heat obtained from resistance to the flow of electric current through the work parts held

together under pressure by electrodes. The size and shape of individually formed welds arelimited primarily by the size and contour of the electrodes. The control function of the weldingmachine defines the welding cycle. The particular steps controlled are squeeze time, weldingtime, hold time and off time [2]. The process is used extensively for joining low and mildcarbon steel sheet metal components for automobiles, cabinets, furniture and similarproducts. Stainless steel, aluminum and copper alloys are also spot welded commercially [2].Excessive heating in resistance welding results in metal expulsion during the weldingoperation. Since accurate method for selection of welding variables i.e. welding current,





welding time and electrode force, thickness of sheet, electrode type, electrode tip diameter,gap in the electrodes, shape of electrode tip, electrode material etc. are lacking, we have toadopt a trial and error approach whenever we come across a new-type of metal. Fieldexperience has shown that in resistance spot welding the basic welding variables arepractically linear functions of metal thickness [23]. These greatly simplify the choice ofwelding variables. Automotive manufacturers are therefore greatly interested in reducing

weight by substituting materials with a higher strength to weight ratio than low carbon steel.Austenitic Stainless steels fulfill all of the requirements. Due to this, material ASS316 is chosenfor Resistance Spot Welding studies.

However, there are situations in which these are being used under fatigue loading.Over last more than 45 years, many researchers have investigated the problem of the designof spot welded lap-shear joints in relation to their fatigue strength. They analyzed the stressfield around the joint and also studied the experimental determination of the fatigue behaviorof joints with special geometries. None of the presently known studies, however, manages toprovide an exhaustive and general interpretation of the phenomenon, able to correlate thedata available and to set up the bases for reliable forecasts of the fatigue behavior of lap

joints [24]. Because of the limitation of time, cost and availability of material, the study hasbeen confined to austenitic stainless steel strip of 0.5mm thicknesses. Mechanicalcharacteristics of spot welds have been assessed by performing hardness test.

Figure 1. Resistance Spot Welding [26]

3. Materials and Methodology

3.1. Specification and Compositing of Material Used

Figure 2. Configuration and Dimensions of Specimen (all dimensions in mm)





The specimens were cut from a sheet of 1m x 2.5m. The specimens were cut parallel to therolling direction of the sheets. The dimensions are 300 mm length and 25 mm width, theoverlap being equal to the width of the specimen. This overlap was chosen as per AWSrecommendation [25].

In order to synthesize an appropriate set of welding process parameters on a givenwork piece, a computer-aided system based on basic design principles is required. Thesystem has some optimization procedures in order to polish the process parameters. Thefailure load directly influences the welding quality and therefore, is considered one of themost important features in the welding process. The proper selection of spot weldingparameters is essential in carrying out a convincing failure load analysis. The material used inthe present work is Austenitic stainless steel (Grade 316) sheets of 0.5 mm thickness. Thenominal chemical composition for each element of the above material is listed below inTable 1:

Table 1. Material ASS316 Composition

Element Wt. %age

C 0.08

Cr 16/18

Ni 10/14

Mn 2.0

Si 1.0

S 0.03

P 0.04

Mo 2.0/3.0

3.2. Joining Procedure

Since a number of set of tests has been performed, in which each set was designedto determine the effect of single variable for a specific geometry. Welding has been organizedin such a way that a full set would be done on a single occasion. This has been done to

minimize variation within any one set of samples that is not due to the factor being tested.Similarly, within each set all specimens with a single combination of variables have beenwelded before sequentially changing the test variables. Overlapping has been kept equal to thewidth of the specimen. Size of electrode diameter has been kept as per AWS recommendation[25].





3.3. Experimentation

The three process parameters viz. Current, Electrode Force and Weld cycles were selectedas given in Table 2. The parameters which were kept constant are also listed in this Table.Experiments were conducted according to the test conditions specified by the L 9 (Table 2).Each experiment was repeated three times in each of the trial conditions. Thus, twenty sevenwork-pieces were selected of thickness 0.5mm. In each of the trial conditions and for everyreplication, the hardness characteristics were measured.

3.4. Response Characteristics

The effect of selected process parameters was studied on the Nugget Hardnessresponse characteristics for the material ASS316. Nugget hardness was measured fromthe center of the nugget from three different specimens using Rockwell Hardness Testingmachine.

3.5. Scheme of Experiments

The experiments were designed to study the effect of some of parameters on responsecharacteristics of resistance spot welding process. Taguchi parametric designmethodology was adopted. The experiments were conducted using appropriate orthogonalarray (OA).

Table 2. Selected Process Parameters and Their Range

Sr.No.

ProcessParameters Range Unit

1 Current 4 - 5 KA

2 Electrode Force 2 – 3 Kg/cm 2

3 Weld Cycles 6 - 10 Nil

4 Thickness ofASS316 sheet 0.5 mm

5 Electrode Type Straight Nil

6 Gap in theElectrodes 22 mm

7 Electrode Tip

Diameter3 mm

8 Shape ofElectrode Tip Circular Nil

9 Electrode Material ChromiumCopper Nil





10 Type of CurrentUsed AC KA

11 Temperature 30 ± 2 °C

Parameters and their levels are given in Table 4. Each three level parameter has 2degree of freedom (DOF = Number of levels-1), overall mean has a degree of freedom of 1,and the total DoF required for three parameters each at three levels is 7 =1+ [3 x (3-1)]. Asper Taguchi’s method the total DoF of selected OA must be greater than or equal to the totalDOF required for the experiment. So an L 9 (a standard 3-level OA) having 8 = (9-1) degreeof freedom was selected for the present analysis.

Table 3. Process Parameters and Their Range

Sr. No. Symbol Process Parameters Range Unit

1 C Current 4- 5 KA

2 P Electrode Force 2- 3 Kg/cm 2

3 N Weld Cycles 6-10 Nil

Table 4. Process Parameters and their Values at Different Levels

Symbol ProcessParameters Unit Level 1 Level 2 Level 3

C Current KA 4 4.5 5

P Electrode Force Kg/cm 2 2 2.5 3

N Weld Cycles Nil 6 8 10

Thickness of ASS316 sheet: 0.5mm, Electrode Type: Straight, Gap in theElectrodes: 22mm, Electrode Tip Diameter: 3mm, Shape of Electrode Tip:Circular, Electrode Material: Chromium copper, Type of Current Used: AC,Temperature: (30 ± 2)°C

6. Analysis and Discussion of Results

The main effects of process parameters both for raw data and S/N data are plotted.The response curves (main effects) are used for examining the parametric effects on theresponse characteristics. The analysis of variance (ANOVA) of raw data and S/N data isperformed to identify the significant parameters and to quantify their effect on the responsecharacteristics. The most favourable conditions (optimal setting) of process parameters interms of mean response characteristic are established by analyzing response curves. Theresults of experiments provide insight into the hardness characteristics of the ASS316 sheet





of thickness 0.5mm. The effect of independent process parameters of Current, Pressure andWeld cycle (while keeping other parameters constant) on the selected response characteristicsdiscussed. The average values of response characteristics and S/N ratio (dB) for eachparameter at level one (L 1), level two (L 2) and level three (L 3) are calculated.

It can be seen from Figure 3 that hardness of ASS316 increases as the value of thecurrent, from level 1 to level 3. It means ASS316 is a material which becomes harder at weldnugget when temperature increases during welding process. Actually, Austenite stainlesssteels change its phase at high temperatures. It transforms into martensite. However, thetransformation of austenite ( γ ) into martensite ( α ) cannot be the single cause of the increaseof hardness. It is also influenced by the ratio between the main elements of stainless steelchemical composition: Cr, Ni and Mo. The martensite transformation involves a suddenlyreorientation of C and Fe atoms from the FCC solid solution of γ -Fe to a Body CenteredTetragonal (BCT) solid solution, which is martensite. The phase transformation isaccompanied by volume expansion. The martensite presence in the steel structure contributesto a special hardness of steel. Zones with martensitic structure are stronger and harder thanaustenitic ones and consequently less resistant to corrosion.

Figure 3. Effect of Welding Current on Hardness

Figure 4 favours the results as written above and we know bigger value of weld cycle willincrease the heat at nugget and consequently will transform austenite ( γ ) into martensite ( α ).In ASS 316 austenitic stainless steel deformation - induced α ’ martensitic transformationoccurs by plastic deformation. However this transformation does not occur in the whole part.It occurs only locally where there are favorable conditions for transformation and it causes achange in mechanical and physical properties of austenitic stainless steel. As stated thatAustenite stainless steels change its phase at high temperatures. It transforms into martensite.

However, the transformation of austenite ( γ ) into martensite ( α ) cannot be the single cause ofthe increase of hardness. It is also influenced by the ratio between the main elements ofstainless steel chemical composition: Cr, Ni and Mo. The martensite transformation involvesa suddenly reorientation of C and Fe atoms from the FCC solid solution of γ -Fe to a BodyCentered Tetragonal (BCT) solid solution, which is martensite. The phase transformation isaccompanied by volume expansion. The martensite presence in the steel structure contributes





to a special hardness of steel. Zones with martensitic structure are stronger and harder thanaustenitic ones and consequently less resistant to corrosion.

Figure 4. Effect of Weld Cycles on Hardness

Results of Figure 5 also are overlapping with the results of Figure 3 and Figure 4written above. As we can see that hardness of the material is increasing as the value ofelectrode force is increasing from level 1 to level 3. Material becomes harder at the nuggetwith the increase of electrode force. Probable reason is transformation of Austenite toMartensite and with the increase of electrode force the material becomes more dense andhard at the nugget

Figure 5. Effect of Electrode Force on Hardness

7. Conclusions

Austenitic Stainless Steel 316 is an extremely important commercial alloy due to itsexcellent corrosion resistance, high strength, good ductility and toughness. An increase inweld current, weld time and electrode force results in an increase in weld nugget diameterand width. An increase in weld current, weld time and electrode force results in an increase inelectrode indentation.





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