Comparison of Wear Properties of GTA and Friction Stir ... · Comparison of Wear Properties of GTA and Friction Stir Welded Lm25-sic Metal... F F 61 products are not easily produced;

Comparison of Wear Properties of GTA and Friction Stir Welded Lm25-sic Metal...F F

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Comparison of Wear Properties of GTA andFriction Stir Welded Lm25-sic Metal Matrix

Composites

P. Venugopal1, N. Murugan2 and V. Balasubramanian3

1Assistant Professor, Department of Mechanical Engineering, KarpagamCollege of Engineering, Coimbatore – 641032, Tamilnadu, India,

E-mail: [email protected], Department of Mechanical Engineering, Coimbatore

Institute of Technology, Coimbatore, Tamilnadu, India.3Professor, Department of Manufacturing Engineering, Annamalai

University, Chidambaram, Tamilnadu, India.

ABSTRACT: Metal Matrix Composites (MMCs) based on particulatereinforcements are particularly attractive due to their lower productioncosts and the possibility to be processed by conventional secondaryprocesses. LM25-SiC Metal Matrix Composites were produced by stircasting process. These composites were welded by two weldingprocess, a conventional Gas Tungsten Arc (GTA) welding and a solidstate welding process namely Friction Stir (FS) welding. The weldedspecimens were treated at elevated and cryogenic temperatures forexperimental studies. Wear rate of GTA welded and FS welded plateswere evaluated and compared. In both GTA and FS welding,specimens treated with cryogenic temperature had lesser wear ratecompared to as welded and specimens treated at elevated temperature.FS welded specimens had lesser wear rate compared to GTA weldedspecimens.Keywords: Metal matrix composites, friction stir welding, tungsteninert gas welding, Cryogenic temperature, Wear.

1. INTRODUCTIONMetal matrix composites (MMCs) based on aluminum alloys arecharacterized by superior properties with respect to the correspon-

International Journal of Material Research, Electronics and Electrical SystemsJanuary-December 2011, Volume 4, Number 1-2, pp. 59– 76

mailto:[email protected]

International Journal of Material Research, Electronics and Electrical SystemsF F

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ding monolithic alloys such as increased specific strength andstiffness, improved properties at high temperature, lower thermalexpansion and better wear resistance, lesser part weight, low thermalshock, high electrical and thermal conductivity compared toconventional metals and alloys [1, 2].

There are several fabrication techniques available for the manu-facture of MMC materials. According to the type of reinforcement,the fabrication technique varies considerably. These techniquesinclude stir casting [3-6], liquid metal infiltration [7], squeeze casting[8], etc. Compocasting involves the addition of particulate rein-forcement into semisolid metal (SSM) by means of agitation. Theadvantage of compocasting lies in a lower processing temperature[9], leading to a longer die life and high production cycle time [10,11].The production can be carried out by conventional foundry methods[12]. If the parameters like molten temperature, stirring time, % ofSiC, etc. are not adequately controlled, it will lead to non-homo-geneous particle distribution resulting in sedimentation andsegregation [13]. Although compocasting is generally accepted as acommercial route for the production of MMCs [14], there is, however,a technical challenge associated with producing a homogeneous,high density composite. The silicon carbide SiC reinforced Alcomposite is perhaps the most successful class of MMCs producedto date. They have found widespread application for aerospace,energy, and military purposes, as well as in other industries – forexample, they have been used in electronic packaging, aerospacestructures, aircraft and internal combustion engine components, anda variety of recreational products [15].

In all these applications, welding plays a vital role. Most of thestudies [16] on the joining of particulate reinforced aluminum matrixcomposites have dealt with gas shielded metal arc welding, laserwelding, gas tungsten arc welding, electron beam welding and frictionwelding but until recently, little attention has been paid to SiCreinforced aluminum matrix composites joined by friction stirwelding.

Friction stir welding [17-21] is a promising process for joiningparticulate reinforced aluminum matrix composites since thismethod is a solid state process. Therefore, the brittle solidification


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products are not easily produced; the energy input and distortionare significantly lower than in fusion welding techniques, thusimproving the welding properties. Since FS welding involves largeplastic flow at elevated temperatures, the MMC must be capable ofsustaining such flow without suffering damage to the reinforce-ments. This requirement is difficult to satisfy with fibre reinforcedMMC, but is feasible with particulate MMC.

These considerations led to selection of LM25 containingdifferent vol % of SiC reinforcement particles as the materials forthe present experiments. Also very limited works have been carriedout in welding of LM25 aluminum alloy, which is commerciallyavailable and used for lot of applications. The composition of LM25is given in Table 1.

Table 1Composition of LM 25 Aluminum Alloy

Elements Si Fe Cu Mg Mn Cr Pb Ti Co V Al

% by 7.475 0.462 0.084 0.432 0.024 0.020 0.014 0.013 0.011 0.004 Rema-Weight inder

The tribological behavior of particle-reinforced aluminum matrixcomposites has been extensively studied for more than last 3 decades[22–27]. However, few studies have been performed so far on tribo-logical properties of welded aluminum matrix composites treated atdifferent temperatures. Prediction of the wear behavior of the slidingcomponents is of utmost importance avoiding huge economic losses.

Therefore it is essential to analyze the wear properties of weldedLM25-SiC metal matrix composites.

In this paper, the wear properties of GTA welded composites arecompared with that of FS welded LM25-SiC metal matrix composites.

2. EXPERIMENTAL PROCEDURE

2.1. Stir CastingAluminum Composites were produced by the commonly used andeconomical method known as stir casting. The stir casting set upused is shown in Fig. 1.


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Cylindrical ingots of size 80 mm diameter and 150 mm heightwere cast. In this process, the base aluminum alloy was stirred in anelectrical resistance holding furnace above liquidus temperatureusing a three bladed mechanical impeller and the reinforcementswere slowly added to the melt after degassing with nitrogen.A constant temperature was maintained using a temperaturecontrolled bimetallic thermocouple composed of Invar and tin. TheSiC particles, which were used to fabricate the composite, had anaverage particle size of 25 µm and average density of 3.2 g/cm3.

Three different weights, 5%, 10% and 15% of SiC were added toLM25 for preparing composites by stir casting process. A holdingtime of 15 minutes at 750º C was given to the melt and then stirringwas continued till the slurry got solidified. Cast composites weresolidified at a cooling rate of 3K/s which was aimed to enhanceincorporation of reinforcement particles in the matrix alloy. Thecomposites of 4 mm thickness were prepared for welding from thecylindrical ingot.

Figure 1: Stir Casting Setup.

2.2. GTA WeldingGTA welding process was used to join composite specimen plates.Detailed process parameters and their levels are given in Table 2.


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Table 2Process Parameters and Its Levels for GTA Welding and FSW.

Code Process Parameters Levels–1 0 1

A % Composition of SiC 5 10 15B Post Weld Heat Treatment –140 27 194C Welding Current (GTAW) 150 175 200

Rotational Speed of tool (rpm) (FSW) 1000 1200 1400

Square butt weld joints were made by GTA welding as per BoxBenkehn design provided in Table 3. Rods of 5356 Al 5% Mg having5 mm diameter was used as filler material. GTA welded compositesare shown in Fig. 2. The range of arc voltage employed was from16.5 volts to 22.1 volts and welds were shielded by argon gas with agas flow rate of 9.5 l/min.

Table 3Design Matrix and Measured Properties of GTA Welded Composites.

Design Matrix Measured Properties

Run A B C Weight Wear Co-(Composition (Post Weld (Welding Loss rate x 10-5 efficient

of SiC) Heat Current) mm3/Nm of Friction% Treatment)ºC Amps

1 5 –140 175 0.0150 3.711 0.392 5 194 175 0.0213 5.269 0.513 15 –140 175 0.0127 3.124 0.334 15 194 175 0.0187 4.612 0.485 5 27 150 0.0209 5.178 0.586 5 27 200 0.0207 5.118 0.577 15 27 150 0.0184 4.546 0.548 15 27 200 0.0177 4.364 0.539 10 -140 150 0.0139 3.439 0.40

10 10 -140 200 0.0129 3.187 0.3911 10 194 150 0.0178 4.412 0.4912 10 194 200 0.0172 4.256 0.4713 10 27 175 0.0198 4.886 0.5414 10 27 175 0.0193 4.773 0.5115 10 27 175 0.0201 4.971 0.53


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2.3. Friction Stir WeldingDetailed process parameters and their levels for joining thecomposite plates by FSW process are given in Table 2. The tool(carbon steel C40) shoulder diameter was 18 mm and the pin was atruncated cone (6.5 mm base and 5.5 mm head diameter) having 2.9mm height. Friction stir welding of composites was carried out asper the design matrix provided in Table 4. A constant axial load of0.5 kN was applied with a feed of 0.67 mm/sec. The FS weldedcomposites are shown in Fig. 3.

Figure 2: GTA Welded Composites.

Table 4Design Matrix and Measured Properties of FS Welded Composites.

Design Matrix Measured Properties

Run A B C Weight Wear Co-(% (Temperature Rotational Loss rate x 10-5 efficient

Composition Treatment) Speed (gms) mm3/Nm of Frictionof SiC) ºC (rpm)

1 5 –140 1200 0.0146 3.605 0.362 5 194 1200 0.0211 5.209 0.493 15 –140 1200 0.0119 2.938 0.31

Contd. Table 4


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4 15 194 1200 0.0186 4.593 0.455 5 27 1000 0.0205 5.062 0.576 5 27 1400 0.0201 4.963 0.567 15 27 1000 0.0182 4.494 0.528 15 27 1400 0.0173 4.272 0.519 10 –140 1000 0.0136 3.358 0.38

10 10 –140 1400 0.0124 3.062 0.3711 10 194 1000 0.0175 4.321 0.4712 10 194 1400 0.0168 4.148 0.4513 10 27 1200 0.0194 4.790 0.5314 10 27 1200 0.0189 4.667 0.5215 10 27 1200 0.0197 4.864 0.53

Figure 3: Friction Stir Welded Composites.

2.4. Heat TreatmentThe corresponding GTA and FS as-welded composites as per thedesign matrix were subjected to a heat treatment at 300ºC (elevatedtemperature) using a muffle furnace for 1 hour and were air cooled.The corresponding GTA and FS as-welded composites as per designmatrix were also subjected to cryogenic treatment at –140ºC usingliquid nitrogen in a cryostat chamber, CTS H-82 CHEST Model with82 liters volume capacity. The temperature of the chamber could be


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varied from ambient to –190ºC with a control accuracy of ± 5ºC. Thedry nitrogen cylinder was provided with a pressure regulator andsolenoid valve for controlling the flow. The welded composites werecryo-treated for 96 hours.

2.5. MicrostructureThe microstructures on the cross section of welded composites wereexamined using optical microscope and photomicrographs weretaken along various zones such as HAZ, weld metal and base metal.The composite specimens were etched with Kellers reagent to revealthe macro and microstructures. Optical microstructures of compositescontaining 5% SiC, 10% SiC and 15% SiC are shown in Fig. 4.

A typical macrostructure of GTA welded composite is shown inFig. 5. Optical microstructures of the FS welded joint for differentzones of composite containing 15 % SiC is shown in Fig. 6.

2.6. Wear TestWear test specimen were machined from welded samples to formcylindrical pins having a diameter of 4 mm and a height of 25 mm.The specimens were metallographically polished and dry slidingwear tests were conducted using a pin-on-disc apparatus at roomtemperature. The details of the wear test conditions are given inTable 5. The initial weight of the specimen was measured in a singlepan electronic weighing machine with an accuracy of 0.0001 g. Thewear of the welded composites were studied for the stated slidingspeed, normal load, sliding distance and track radius. After runningthrough the fixed sliding distance of 3000 m, the specimen wasremoved, cleaned with acetone and weighed to determine the weightloss. The difference in weight gives the wear of the specimen andthen the wear rate was calculated [28].

3. DEVELOPMENT OF WEAR MODELINGThe selected design matrix, shown in Table 3 and 4 were BoxBenkehn Design Matrix of Three-factor, Three-level techniqueconsisting of 15 sets of coded conditions. All input variables at theintermediate level (0) constitute the central points, and the


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combinations of each of the variables at its lowest level (–1) or highestlevel (1) constitute the star points. Thus, the 15 experimental runsallowed the estimation of the linear, quadratic and two-way inte-ractive effects of the input variables.

Figure 4: Typical Optical Micrograph of Al MMC (a) 5% SiC (b) 10% SiC(c) 15% SiC.

Figure 5: Typical Macrostructure of GTA Weld (15 % SiC): (1) Weld Bead;(2) HAZ; (3) Unaffected Base Metal.

Table 5Details of the Wear Test Conditions used in This Study.

Pin material LM25 /5SiCp, LM25 /10SiCp, LM25 /15SiCpDisc material EN36 steel with a hardness of 65 HRCPin dimensions Diameter 4 mm and height 25 mmSliding speeds (m/s) 3Normal load (N) 50Sliding distance (m) 3000Track radius (mm) 55


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Figure 6: Typical Optical Microstructures of the FSW Joint of 15 % SiC Composite(Specimen 8): (a) Parent Material; (b) Thermo-Mechanically Affected Zone

(TMAZ); and (c) Weld Nugget.

Figure 7: Typical Optical Microstructures of (a) GTA Welded MMC SurfaceBefore Wear and (b) GTA Welded MMC Surface After Wear (c) FS Welded MMC

Surface Before Wear and (d) FS Welded MMC Surface After Wear.


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The response function representing any one of the wearcharacteristics can be expressed as y = f (A, B, C). The second orderpolynomial (regression) equation used to represent the responsesurface for K factors is given below.

Y = bo +1

k

i ii

b x=∑ +

1

k

ii iiii j

b x=≠

∑ +, 1

k

ij iji j

b x=

∑ ...(3.1)

For three factors, the selected polynomial could be expressed as

Y = b0 + b1 A + b2 B + b3 C + b11 A2 + b22 B2 + b33 C2

+ b12 AB + b13 AC + b23 BC ...(3.2)The values of the coefficients were calculated by regression

analysis with the help of the MiniTAB15 software package fordifferent responses. The adequacy of the models was then tested bythe analysis of variance technique (ANOVA) and given in Table 6and Table 7. From the tables, it is evident that, all models areadequate.

The final mathematical models developed are given below. Theinput control variables are in their coded form.

For GTA WeldingWear rate = 4.87667 – 0.32875A + 0.63600B – 0.08125C + 0.14017A2 –0.83783B2 – 0.21533C2 – 0.01750AB – 0.03050AC + 0.02400BC

Coefficient of friction = 0.526667 – 0.021250A + 0.055000B –0.006250C + 0.009167A2 – 0.108333B2 + 0.019167C2 + 0.007500AB –0.002500BC.

For FS weldingWear rate = 4.77367 – 0.31775A + 0.66350B – 0.09875C + 0.14404A2

– 0.83146B2 – 0.21996C2 + 0.01275AB – 0.03075AC + 0.03075BC

Coefficient of friction = 0.526667 – 0.023750A + 0.055000B –0.006250C – 0.000833A2 – 0.123333B2 + 0.014167C2 + 0.002500AB –0.002500BC.


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Table 6Analysis of Variance For Testing Adequacy of Models (GTA).

Details of Source Sum of D Mean F-Ratio Square ModelsResponses squares F squares Multiple R

Wear Regression 7.01028 9 0.77892 4.46 97.88% AdequateResiduals 0.15170 5 0.03034Lack of Fit 0.13197 3 0.04399Pure Error 0.01973 2 0.00987

Coeff.of Regression 0.075418 9 0.008380 1.82 97.74% Adequatefriction Residual 0.001742 5 0.000348

Lack of Fit 0.001275 3 0.000425Pure Error 0.000467 2 0.000233

F-Ratio = Mean Square of Lack of Fit/Mean Square of Pure Error.S.S. = Sum of squares.

D.F. = Degrees of Freedom.Mean Square = Sum of Squares/Degrees of Freedom.

Table 7Analysis of Variance For Testing Adequacy of Models (FSW).

Details of Source Sum of D Mean F-Ratio Square ModelsResponses squares F squares Multiple R

Wear Regression 7.23806 9 0.80423 6.26 97.24% AdequateResiduals 0.20563 5 0.04113Lack of Fit 0.18582 3 0.06194Pure Error 0.01980 2 0.00990

Coeff.of Regression 0.087498 9 0.009722 12.75 98.49% Adequatefriction Residual 0.001342 5 0.000268

Lack of Fit 0.001275 3 0.000425Pure Error 0.000067 2 0.000033

4. RESULTS AND DISCUSSIONSOptical micrographs of the composites containing 5, 10 and 15 vol.%of SiC reveal that the uniformity of distribution of SiC improveswith an increase in the SiC volume fraction. This effect has beenattributed to the presence of multiple solidification fronts in thehigher volume fraction composites, leading to geometric capturingof the particles and a correspondingly more uniform particledistribution as reported by Asthana R. [29].


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From the microstructural studies it was found that GTA weldedcomposites had a wider HAZ. This is clearly shown in the typicalmacrograph of the GTA welded specimen in which parent agehardening precipitate experienced over-ageing, causing phasetransformations. The typical microstructure of FS welded compositecontaining 15% SiC reveals that the thermo-mechanically affectedzone (TMAZ), which is adjacent to the weld nugget either theretreating side or the advancing side, has been plastically deformedand thermally affected. The distribution of the SiC particles in thenugget region is more homogeneous, suggesting that re-arrangementof the particles had taken place during friction stir welding due tothe high deformation and stirring. Microstructural examination ofFS welded composites using optical microscope showed none ofthe typical defects, generally observed in the welded zone of compo-sites joined by conventional fusion arc welding processes (such asporosity, or reinforcement segregation).

Wear tests were carried out for GTA welded specimens withconstant welding current of 175 amps and constant post weld heattreatment temperature at –140ºC. Table 3 indicates that GTA weldedcomposites containing 15% SiC and treated at cryogenic temperatureshows decreased weight loss as compared to as-welded andcomposites containing 5% SiC and 10% SiC treated at elevatedtemperatures.

Wear tests were carried out for FS welded specimens withconstant tool rotational speed of 1200 rpm and constant post weldheat treatment temperature at –140ºC. The measured propertiespresented in Table 4 indicates that FS welded composite specimenscontaining 15% SiC, treated at cryogenic temperature shows decreasedweight loss as compared to as-welded and composites containing5% SiC and 10% SiC treated at elevated temperatures. FS weldedcomposites have lesser wear rate than GTA welded composites. Thetest results also show that wear rate decreases with increase inpercentage of SiC.

From Fig. 4.1, it is evident that with the increase in % SiC, thewear rate decreases in both GTA and FS welding process. The wearis found to be low in 15% SiC, because of the presence of the more


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hard SiC particles present in the MMC which acts as the load bearingmember and abrasive nature.

Figure 4.1: Direct Effect of % SiC on Wear Rate andCoefficient of Friction.

The wear rate is slightly less in FS welded specimens comparedto specimens welded by GTA welding. The coefficient of frictionalso reduces with increase in % SiC in both GTA and FS weldingprocess. The coefficient of friction observed is high for 5% SiC andreduced for increase in % SiC. At lower % SiC, temperature rise islow, whereas at higher % SiC, temperature rise is high because ofmore ceramic particles in the matrix.

The coefficient of friction is slightly less in FS welded specimenscompared to specimens welded by GTA welding.

Figure 4.2: Direct Effect of Post Weld Heat Treatment on Wear Rate andCoefficient of Friction.


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Figure 4.2 clearly shows that the wear rate and coefficient offriction decreases in specimens treated with cryogenic temperaturecompared to as weld and specimens treated at elevated temperaturein both GTA and FS welding.

Figure 4.3 a shows that wear rate decreases with increase inwelding current and coefficient of friction decreases with increasein welding current. Figure 4.3 b shows that wear rate decreases withincrease in tool rotational speed.

Figure 4.3: (a) Direct Effect of Welding Current on Wear Rate and Coefficientof Friction - GTA.

Figure 4.3: (b) Direct Effect of Tool Rotational Speed on Wear Rate andCoefficient of Friction - FSW.


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4. CONCLUSIONSThe following conclusions were drawn based on the wear testscarried out on GTA and FS welded LM25-SiC Metal matrix composites:

• Increase in % SiC, decreases the wear rate and coefficient offriction in both GTA and FS welding process.

• The wear rate and coefficient of friction decreases inspecimens treated with cryogenic temperature comparedto as weld and specimens treated at elevated temperaturein both GTA and FS welding.

• Wear rate and coefficient of friction decreases with increasein welding current in GTA welding

• Wear rate and coefficient of friction decreases with increasein tool rotational speed in FS welding.

• Coefficient of friction is high in specimens with 5% SiCcompared to specimens with 10% SiC and 15% SiC in bothGTA and FS welding.

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Comparison of Wear Properties of GTA and Friction Stir ... · Comparison of Wear Properties of GTA and Friction Stir Welded Lm25-sic Metal... F F 61 products are not easily produced;