investigating friction stir welding on thick nylon 6 plates€¦ · friction stir welding (fsw),...

Introduction The use of polymeric materials hasgrown widely in various sectors suchas packaging, building, electronic, au-tomotive, and aerospace industries.Particularly, Nylon 6 has wide engi-neering applications and is used inlarge quantities in automotive oilpans, gears, slides, cams, bearings, flu-id reservoirs, and the sports industry(Ref. 1). Polymeric materials offermany advantages over metal and its al-loys owing to certain distinct proper-ties: light weight, high specificstrength, high specific modulus, de-sign flexibility, low production costs,

good corrosion and environmental re-sistance, thermal and electrical insula-tion, and durability (Ref. 2). Increase in use of a particular materi-al, on the other hand, increases the im-portance of the joining process. In engi-neering, the production of a single piecefrom molding is an ideal situation be-cause it eliminates many steps of as-sembling. However, due to the complex-ity of parts and dissimilarity in joiningcomponents in some cases, efficientjoining processes are necessary. Friction stir welding (FSW), amongmodern joining techniques, is nowwidely considered as a joining process,owing to its low-cost production and

eco-friendly process. Moreover, by us-ing this technique, the weld joint canbe accomplished by controlling fewerparameters, such as rotational speed,feed rate, plunge depth, and tool di-mension (Ref. 3). Since its inception, avariety of metal alloys has been suc-cessfully joined by this process. Inves-tigations regarding their mechanicaland microstructural properties arewidely reported in literature. However,in the case of polymers, application ofFSW to produce joints is a relativelynew approach and requires further in-vestigation. The nonconductor natureof polymers makes it challenging toweld them, especially in the case of athick workpiece. However, some poly-meric materials have been successfullyfriction stir welded up to the maxi-mum workpiece thickness of 10 mm.The results, in order to achieve themaximum strength, are characterizedthrough various means and reportedin literature. In order to avoid the root defectfracture and to enhance the weldstrength, different approaches havebeen adopted to eliminate this partic-ular defect. For this purpose, Arici etal. (Ref. 4) used double passes of thetool on 5-mm-thick polyethylene (PE)sheets. Pirizadeh et. al. (Ref. 5) used abobbin tool on 5-mm-thick acryloni-trile butadiene styrene (ABS) sheets.However, in the case of polymers withlow melt viscosity or low melting tem-peratures, such as Nylon 6, which hasa melt viscosity of 300 Pa-s at 250C(low shear rate) (Ref. 6), double passes

WELDING RESEARCH

Investigating Friction Stir Welding on Thick Nylon 6 Plates

Exploring the effects of rotational speed on micromechanical properties, flow behavior, and thermal variations

BY A. ZAFAR, M. AWANG, S. R. KHAN, AND S. EMAMIAN

ABSTRACT Polymeric materials, despite being thermal insulators, are now being welded usingdifferent welding techniques. In the current work, the feasibility of the friction stir welding (FSW) process on 16mmthick Nylon 6 plates was studied. The effects of rotationalspeed on the weld quality were investigated by the temperature development, micromechanical properties, crystallization growth, and fracture analysis of the joints. Resultsshowed the dependence of temperature and tensile values on rotation rates was insignificant. However, appearance of considerable defects at higher rotation rates, observed invisual and microscopic analysis, indicated that Nylon 6 is weldable only at lower rotationrates due to its low melt viscosity. Moreover, identical fracture locations during tensiletests revealed that the interface of weld zone on the retreating side was the weakestpart of the joint. It can be attributed to the lack of bonding at the interface of the weldzone on retreating side and relatively low crystallinity in the retreating side region. Dueto different rheological and physical properties of polymers than metals, the flowphenomenon in Nylon 6 was found to be different from that of metals, resulting in a distinct isolated pin plunged zone.

A. ZAFAR ([email protected]), M. AWANG ([email protected]), S. R. KHAN, and S. EMAMIAN are with the departmentof mechanical engineering, Universiti Teknologi Petronas, Perak, Malaysia.

WELDING JOURNAL / JUNE 2016, VOL. 95210-s

KEYWORDS Friction Stir Welding Nylon 6 Polymer Material Flow Threaded Pin

Zafar Supplement June_Layout 1 5/13/16 2:51 PM Page 210

of the tool or bobbin tool may result inexcess heat due to the dual friction onupper and lower surfaces of the work-pieces, eventually increasing the flash.The preweld heating, performed byAydin (Ref. 7) on 4-mm-thick ultra-high molecular weight (UHMW) poly-ethylene sheets, may not be suitablefor polymers with low melt viscosityand can intensify the flash formationdue to increasing heat. The influence of pin shape, studiedon PE sheets by Bilici et al. (Ref. 8) andAhmadi et al. (Ref. 9) made them con-clude that the truncated cone pin hasthe highest tensile strength as com-pared to other pin shapes. However,due to large rheological and propertydifferences among polymers, one pinprofile cannot be assumed as optimumfor all polymers. Another tool consist-ing of the hot shoe was investigatedby Bagheri et al. (Ref. 10) on ABSsheets. The main reason to use thisshoe was to heat the polymer throughthe shoulder during the weldingprocess. Their results were comparablewith the work carried out by Mendeset al. (Ref. 11), who used a stationaryshoulder tool on the same materialwithout external heating. However,squeezing out of the plasticized mate-rial below the shoulder, particularly inlow melt viscosity polymers still re-mained a problem. Inaniwa et al. (Ref. 12) and Pan-neerselvam et al. (Ref. 13) joined Ny-lon 6 sheets with thicknesses of 5 and10 mm, respectively. In their studies,they eliminated the primary heatsource by keeping a small opening be-tween the shoulder and workpiece topsurface. Comparing their approaches,it was found that Panneerselvam et al.(Ref. 13) joined Nylon 6 at a quitehigher revolution pitch (ratio between

welding speedand rotation rate)compared toInaniwa et al.(Ref. 12) work.However, considering the Nylon 6properties especially low melt viscosi-ty behavior, it is believed that higherrevolution pitch will produce enor-mous flash. Flash formation has beenreported by Panneerselvam et al. (Ref.13) as well. On the other hand, the gapbetween shoulder and workpiece willcertainly lead to the formation of acrown above the weld zone. Material flow, due to its direct rela-tion with weld quality, has been thor-oughly investigated on metals by vari-ous means. Lorrain et al. (Ref. 14) andLi et al. (Ref. 15) used foil insert tech-nique, Edwards and Ramulu (Ref. 16)used powder as a tracer material, Sei-del and Reynolds (Ref. 17) utilized themarker material insert technique,whereas Colligan (Ref. 18) insertedsmall steel balls in aluminum to studymaterial flow. Colligan (Ref. 18) con-cluded that material moved behind thepin and deposited on the retreatingside. In another study on aluminum6061, Guerra et al. (Ref. 19) reporteddifferent flow on advancing side (AS)and retreating side (RS). Seidel andReynolds (Ref. 17) observed that themajority of the material in the weldnugget simply moved around the pinand displaced behind the pin. The material flow in polymeric ma-terials has been studied on polymethyl methacrylate (PMMA) bySimoes et al. (Ref. 20). They comparedtheir flow study with the Arbegast(Ref. 21) flow model and observedthat the pin-affected zone remainedisolated and straight along the pin.Their results showed no cross flow

from the weld zone to the base materi-al. Similarly, a clear distinction be-tween the shoulder-affected zone andpin-affected zone could be seen. Current work involved studying theprocess on 16-mm-thick Nylon 6plates by investigating the tempera-ture development, micromechanical,and thermal properties of the joint.With the aim to reduce the flash for-mation, a small-diameter shoulder toolwith right-hand threaded pin wasused. Moreover, marker material in-sert technique was utilized to examinethe flow phenomenon and stirringuniformity in the weld.

Materials and Method In the present investigation, 180-mm-long weld passes were made on16-mm-thick Nylon 6 (Polyamide-6)plates in butt joint configuration atroom temperature. Bridgeport VMC2216 CNC machine was utilized forFSW of specimens welded at a 0-degtilt angle and FSW-TS-F16 FSW ma-chine was used to prepare welds at a 3-deg tilt angle. The FSW tool used in this studywas machined from H13 tool steel rod Fig. 1. The pin of the tool was maderight-hand threaded for uniform stir-ring, while rotating in a clockwise di-rection (Ref. 13). The tool was heattreated before being used for weldingand, therefore, its hardness was in-creased to 56 HRC from 24 HRC. Rotational speed, due to its maincontribution in the FSW process, hasbeen studied (Ref. 22). It was there-

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JUNE 2016 / WELDING JOURNAL 211-s

Fig. 1 Friction stir welding tool with a schematic of dimensions.

Fig. 2 A schematic of the thermocouple locations.

Fig. 3 Configuration of the tensile test specimen.


fore varied between 300 and 1000rev/min. The other parameters, suchas feed rate or welding speed, dwelltime, and tilt angle, were kept con-stant and were selected based on pre-vious studies and preliminary tests(Refs. 12, 13). Feed rate in polymerswas usually kept low so that the mate-rial in front of the pin could get suffi-cient time to plasticize. Parametersused in this study are shown in Table1. A tilt angle of 3 deg, reported as an

optimum FSW angle for HDPE byBozkurt (Ref. 22), was also used at theoptimum rotation speed. The opti-mum rotation speed, used in the pres-ent work, was obtained from per-formed tests. Furthermore, a K-type thermocou-ple was also placed at 1 mm below thepin tip to estimate the weld zone (WZ)temperature at each rotation speed Fig. 2. In all experiments, a clockwiserotating tool was plunged into theworkpiece with a 10-mm/min plungerate to the depth equal to the pinlength. All the welding operationswere performed at room temperature. Cross sections of weld specimens,perpendicular to welding direction,

were made and were observed visually.The mechanical strength of the jointswas analyzed by tensile testing onspecimens that were obtained perpen-dicular to the welding direction. Thetensile tests were performed in accor-dance to ASTM Standard D638-10 us-ing a Zwick-Roell UTM machine at thecrosshead speed of 1 mm/min. Aschematic of the D638-10 specimen isshown in Fig. 3. Weld zone and frac-tured surfaces during tensile testswere analyzed by scanning electronmicroscope (SEM). In order to deter-mine the crystalline content in theWZ, a differential scanning calorime-ter (DSC) test was carried out using aPerkin-Elmer differential scanningcalorimeter at a heating rate of10C/min. Furthermore, material flowduring the welding process was stud-ied by marker material insert tech-nique at achieved optimum parame-ters. Subsequently, due to the markermaterials difference in color from thebase material, its movement was visu-ally analyzed by different sectioning ofthe specimens.

Results and Discussion

Morphological andMicrographic Analysis

In order to observe any visual de-fects, friction stir welded specimenswere cross-sectioned perpendicular tothe welding direction. Figure 4A Eshows the different morphologies oftop surfaces of the welded specimens. Although the flash formation can beobserved in all specimens, Fig. 4A, Bspecimens show comparatively lessflash formation. It is also noted that theflash in Fig. 4B is on retreating side(RS). For this reason, it is believed thatthe temperature on the RS is alwayshigher than the advancing side (AS),which leads to the formation of flash onthe RS (Ref. 23). Further increase in ro-tation speed to 500 rev/min resulted in

WELDING RESEARCH


Fig. 4 Cross sections of specimenswelded at the following: A 300rev/min with 0deg angle; B 400rev/min with 0deg angle; C 500rev/min with 0deg angle; D 1000rev/min with 0deg angle; E 300rev/min with 3deg angle.

A

C

E

D

B

Table 1 Friction Stir Welding Parameters

Tilt Angle (Deg) Feed Rate (mm/min) Dwell Time (s) Rotation Rate (rev/min)

0 25 15 1000 0 25 15 500, 400, 300 3 25 15 300


higher heat input, leading to excessflash in the form of bubbles, which ap-peared at some intervals Fig. 4C. 300rev/min with a 3-deg-angle specimenalso showed bubble-like flash, which in-dicates that tilting of the angle at 3 deggenerated high heat during the process Fig. 4E. It may be due to the tiltedtool trailing edge of the shoulder thatapplies higher compressive load in thesurface of material increasing frictionand resulting in higher heat generation. The formation of a crown-likeshape was also observed in all speci-mens except the 1000-rev/min speci-men. Crown peak was less in 300rev/min with 0-deg specimen anglecompared to 400, 500, and 300rev/min with 3-deg-angle specimens.A specimen welded at 1000 rev/minshowed a different morphology Fig.4D. Excess semimolten Nylon 6squeezed out of the WZ, and afterswirling around the shoulder, ap-peared to have solidified on the shoul-der. As a result, the WZ decreased inthickness, which would eventually re-duce the tensile strength. It is believedthe appearance of weld defects at high-er rotation rates is due to overheatgeneration, which eases the plasticized

polymer to flow out. Low magnification SEM images ofweld zones, taken on cross sections,are shown in Fig. 5. The specimenwelded at 1000 rev/min was excludedfrom further results and discussiondue to its poor weld quality. The weldin Fig. 5A, which was made with thelowest rotation speed of 300 rev/minwith 0-deg angle, presented an excel-lent superficial appearance, unlikewelds produced at higher rotationrates (Fig. 5BD), which showed dif-ferent defects, likely caused by excessheat input. However, a minor defect inthe form of improper bonding was ob-served at the bottom of the RS inter-face. A specimen welded at 400rev/min rotation rate showed varioussmall porosities with improper bond-ing on the RS border line Fig. 5B. Likewise, increasing the rotationalspeed to 500 rev/min resulted in a ma-jor defect called tunnel defect with poorbonding on the RS border line Fig.5C. Considering the good weld appear-ance of 300 rev/min with a 0-deg-anglespecimen, and in order to remove theminor weld defect at the bottom, it wasinvestigated using a 3-deg tilt angle aswell. However, the micrograph result

showed a large cavity with a slight im-proper bond on the RS in Fig. 5D. Moreover, weld zones were also ana-lyzed at higher magnification. Micro-graphs shown in Fig. 6AD are the high-er magnifications of Fig. 5AD, respec-tively. It is clear rotation speed has a sig-nificant effect on the microstructure ofthe welds. The 300 rev/min with 0-degangle specimen in Fig. 6A showed uni-form and perfect surface quality, where-as the specimens at higher rotationrates or a 3-deg tilt angle showed a rela-tively rough surface Fig. 6BD. It isassumed that fracture is easy to occur inrough surface compared to smooth sur-face, as it can provide stress concentra-tion points. From the above detailed descrip-tion, it can be deduced that, due to lowmelt viscosity compared to other poly-mers, such as ABS (Ref. 11), PE (Ref.22), PP (Ref. 24), and PMMA (Ref. 25),Nylon 6 is weldable only at lower rota-tion speeds.

Tensile Test Results

Figure 7 exhibits the peak stressvalues of tensile specimens, obtainedfrom stress strain curves. The trendindicates the increase in tool rotation-al speed leads to a decrease in tensilestrength. However, considering thevisible aforementioned defects, Fig. 7shows the difference in strength val-ues is not that large. The tensilestrength obtained at 500 rev/min isalso comparable to that of the speci-men at 400 rev/min, even though itcomprises tunnel defect in it. More-over, the strength of each specimen, interms of value, was quite less thanthat of the base material. The highesttensile strength, obtained at 300rev/min, 0-deg tilt angle, was 27.21MPa, which corresponds only to about32% of base material. At the same ro-tational rate when the angle is tilted to3 deg, the strength decreased to itslowest value of 18.08 MPa. Compared with Panneerselvam et al.(Ref. 13) and Inaniwa et al. (Ref. 12),who studied 10- and 5-mm-thick Nylon6, respectively, almost similar resultswere found. Although Nylon 6 FSW re-sults are lower in tensile strength com-pared to other polymers, such as PE(Ref. 4), ABS (Ref. 10), and polypropy-lene (PP) (Ref. 24), it is believed thatthis process, due to its certain advan-

WELDING RESEARCH


Fig. 5 Micrographs of welds at a rotational speed of the following: A 300 rev/minwith 0deg angle; B 400 rev/min with 0deg angle; C 500 rev/min with 0deg angle;D 300 rev/min with 3deg angle.

A

C D

B


tages over other techniques, can be suit-able for this material.

Fracture Analysis

The fractured area in any place of thewelded joint is a direct indication of theweakest part of that joint. In the pres-ent investigation it was noted that allspecimens of each set of parameters ex-hibited identical fracture location,which is at the interface of the WZ onthe retreating side (IW-RS). However,interface of the WZ on the advancingside (IW-AS) remained intact. One frac-tured specimen for each rotation rate isshown in Fig. 8. It is also important tonote here the specimen welded at 500rev/min (Fig. 8C) also showed fractureat the IW-RS, despite the fact it con-tains a tunnel defect. It indicates thatthe IW-RS is weaker than the tunneldefect. Inaniwa et al. (Ref. 12) and N.Mendes et al. (Ref. 11) also observedthe same fracture locations in theirstudy on FSW of different polymers.This prefered fracture location in thesespecimens can be related to the forma-tion of flash preferrentially on the re-treating side. In general, flash forma-

tion causes the lack of material, whichultimately results in cavities and blow-holes. In addition to it, other phenome-na in the WZ make the RS weaker thanthe AS include higher temperature onthe RS (Ref. 23), low shear velocity onthe RS (Ref. 12), and difference in flowon both sides (Ref. 10). Scanning elec-tron microscopy results of fracturedspecimens observed toward the WZ,shown in Fig. 9, exhibit the same frac-ture phenomenon in all specimens.

Temperature Analysis

Temperatures measured at 1 mm be-low the pin tip are shown graphically inFig. 10. An increase in temperaturewith the increase of rotation speed isobvious and can be observed in thegraph. Although the temperature dif-ferences at low rotation rates are notsignificant, noticeable differences inweld quality were observed. Cavities,tunnel defect, and smoke during theprocess were seen above 400 rev/minrotation speeds. A maximum tempera-ture of 167C at 1000 rev/min showedlarge amounts of flash formation withthe emission of smoke. Smoke is a com-bination of different volatiles, majorly

carbon dioxide (CO2), water (H2O), andammonia (NH3), evolved due to en-dothermic reaction during the FSWprocess. Evolution of volatiles is direct-ly linked to the weight, suggesting a de-crease in polymer weight (Ref. 26).Therefore, it is believed that at relative-ly higher rotation rates, a small de-crease in weight also occurred. Considering the melting point ofNylon 6 is 220C, it can be assumed allrotation rates used in the presentstudy are applicable for Nylon 6, asthermal degradation of polymer oc-curs at 350C (Ref. 27). It was ob-served that temperature for optimumweld parameter 300 rev/min with 0-deg angle is 119C. There was lowflash formation and no visible defectthat could be attributed to the weldingin plasticized condition due to lowheat input avoiding low melt viscosity.Low heat input results in avoiding thedeterioration in the properties of basematerial. Increase in temperatureabove this point may cause the forma-tion of excess flash due to lower meltviscosity, which eventually results inlower weld quality.

Analysis of Crystallinity

In order to analyze the postweldthermal conditions of the joint and toinvestigate the reason of identical frac-ture locations, the degree of crys-tallinity of different sections of theWZ was analyzed using DSC curves.The specimens, for this purpose, weretaken from the base material (BM),weld center (WC), AS, and RS. The de-gree of crystallinity of any polymerhas a direct relation to its mechanicaland physical properties. Increase inthe degree of crystallinity has shownincrease in tensile strength, stiffness,

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Fig. 6 SEM microstructures of welds, observed in weld nugget, at rotational speedof A 300 rev/min with 0deg angle; B 400 rev/min with 0deg angle; C 500rev/min with 0deg angle; D 300 rev/min with 3deg angle.

Fig. 7 Effect of rotational speed on tensilestrength of Nylon 6 FSW specimens.

A

C

B

D


yield point, and hardness, but a reduc-tion in impact strength (Refs. 2830).

Figure 11 shows the DSC curves of aspecially chosen 300 rev/min with 0-

deg-angle specimen, due to its compar-atively good micro-mechanical results.The peak value of the curve gives themelting enthalpy (Hm) of the poly-mer, which is directly proportional tothe crystallinity according to this for-mula (Ref. 31)

Wc = Hm/Hm 100% (1)

where Hm is the enthalpy of fusionand Hm is the enthalpy of fusion of100% crystalline Nylon 6, of which thevalue is 230 J/g (Ref. 32). Keeping in view Equation 1 and Fig.11, it can be concluded the crystallini-ty of the specimen is reduced in theWZ compared to the base material. Itreduced further in the RS and becamelowest in the WC by making the AS thecomparatively highest crystalline re-gion in the WZ. However, low crys-tallinity along with some defects onthe borderline of the RS made the RSthe preferred fracture location. It isalso important to mention here the re-duction in crystallinity is basicallylinked to the rapid cooling of the ma-terial (Ref. 32). In this case, the cool-ing rate for the whole joint was thesame, as postweld specimens wereplaced in an open environment atroom temperature. Therefore, it is be-lieved that it is mainly the tempera-ture differences at different regions ofthe WZ that lead to the cooling differ-ence and, therefore, the difference incrystallinity.

Material Flow duringFriction Stir Welding Material flow during the weldingprocess was studied by employing a1.5-mm-thin ABS sheet as a markermaterial. For this purpose, specimenswere friction stir welded at optimumparameters of 300 rev/min rotationspeed, 0-deg tilt angle, and 25-mm/min feed rate, selected based onprevious results. The set of parametersis shown in Table 1. Postweld speci-mens were cut in different sections,such as longitudinal-section (parallelto welding direction), vertical crosssection (perpendicular to welding di-rection), and horizontal cross section,to visually analyze the displacement ofmarker material in x, y, and z direc-tions. The sectioning scheme is alsoshown in Fig. 12.

WELDING RESEARCH


Fig. 8 Fractured specimens during tensile tests welded at the following: A 300 rev/minwith 0deg angle; B 400 rev/min with 0deg angle; C 500 rev/min with 0deg angle; D 300 rev/min with 3deg angle.

A

C D

B

Fig. 9 SEM micrographs of fractured surfaces, observed at the interface of the RS towardthe weld zone, welded at the following: A 300 rev/min with 0deg angle; B 400 rev/minwith 0deg angle; C 500 rev/min with 0deg angle; D 300 rev/min with 3deg angle.

A

C D

B


Figure 13A shows the specimen be-fore welding with marker material inthe longitudinal direction (parallel towelding direction). After welding, ver-tical cross sections of the specimenwere made to observe the flow inx-direction, which is perpendicular to

the welding direction (WD). It can beclearly seen in Fig. 13B that thin mark-er material, when placed on the RS (I),spread all over the welding zone,equivalent to pin diameter. Similarly,in Fig. 13C, marker material placed onAS (II) stirred and spread in completewelding zone. This spreading indicatesa uniform stirring, either marker ma-terial is on the AS or the RS. However,in both cross sections it can be ob-served that marker material at the topof the specimen is not well stirred andpositioned toward the AS. It is be-lieved this unstirred area is due to asmall, unthreaded part of pin near theshoulder. Furthermore, it is also ob-served the depth of the WZ is equiva-lent to the pin plunged length. Itshows there was no cross flow fromthe plunged area to the unplungedzone at the bottom of the pin. A simi-

lar phe-nomenonwas ob-served onthe adja-cent rightand leftsides of theplungedarea. Thisrestrictionof WZ with-

in the plunged area is also mentionedby Simoes and Rodrigues (Ref. 20) intheir study of PMMA.

In order to observe the y-directionflow (parallel to WD) of the specimen,marker materials were placed trans-versely (perpendicular to WD) on theAS (I), weld interface (II), and RS (III),as shown in Fig. 14A. After welding,horizontal sections were prepared. It isclear from Fig. 14BD marker materialafter stirring was displaced behind thepin. The maximum displacementmeasured in Fig. 14C, D was remark-ably very long, 11 mm, whereas the di-ameter of the pin is 7.5 mm. However,distribution of marker material(shown in Fig. 14C, D) was uniform,but narrowing of marker material atthe end was observed. It is believedthe farthest narrow part is squeezedand extruded by the pin. This extru-sion phenomenon is similar to Colli-gans (Ref. 18) material flow study onFSW of aluminum alloy, in which heconsidered the welding process due tostirring and extrusion. As the materialof the AS in Fig. 14B is prone to flow

on sides, it can be said the vacant sidesat the end in Fig. 14C, D can be filledby material from the AS. Thereby, itcovers a complete welding zone andleaves no defects. In order to understand the completeflow during welding, flow in the z-direction was also observed. For thispurpose, marker materials were placedat bottom, middle, and top of the ASand RS. It is shown in Fig. 15A, B, re-spectively. After welding, longitudinalsections of welded specimens weremade to observe vertical movement.These are shown in Fig. 15C, D. It isclear from the sections that material atthe bottom expanded up to the surfacesof both specimens. A similar case wasobserved for middle marker materials.Marker material at the top expelled outfrom the specimen and resulted in for-mation of flash. No difference in thisupward movement of marker materialswas found either on the AS or RS ofspecimens. A large, vertical movementof material during welding was also ob-served by Guerra et al. (Ref. 19), Li et al.(Ref. 15), and Seidel and Reynolds (Ref.17) in their study on aluminum.

Conclusions

Systematic work was carried out onthe friction stir welding of 16-mm-thick Nylon 6 plates using a threadedpin tool with a small-diameter shoul-der. Based on the aforementioned re-sults and discussion, the followingconclusions can be made:

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Fig. 10 Effect of the rotational speed on temperature; measured 1 mmbelow the pin tip.

Fig. 12 Sectioning scheme of material flow specimens.

Fig. 11 DSC curves of 300 rev/min with a 0deg angleNylon 6 sample of different locations.


Nylon 6, due to its low melt vis-cosity, is weldable only at lower rota-tion rates. Therefore, a 300 rev/minrotation speed and 25-mm/min feedrate has given comparatively goodweld results at 0-deg tool angle. At higher rotation rates, squeez-ing out of excess plasticized materialand defects formation in the weldzone were observed. A small-diameter shoulder, on theother hand, reduced the amount offlash by reducing the primary heat. The processing temperatures,measured 1 mm below the pin plungedzone, were quite below the thermaldegradation temperature of Nylon 6(350C). Thus, it is assumed the Nylon6 in the stirring zone did not undergoextreme thermal degradation, al-though minor reduction in molecular

weight due to smoke evolution is be-lieved to have occurred. The tensile strength of all jointswas quite lower than that of the basematerial. As the result of microstructureobservation in the weld zone regions,relatively smooth and uniform mi-crostructure was observed at the opti-mum set of parameters. DSC results showed the crys-tallinity of the weld zone decreasedcompared to base material. Moreover,the retreating side compared to the ad-vancing side was found to have lowcrystallinity. During tensile tests, all specimensfractured at the interface of the weldzone on the retreating side. It can beattributed to the lack of bonding atthe interface of the weld zone on the

retreating side, and low crystallinecontent in the retreating side region. Material flow examination re-vealed large (more than pin diameter)backward displacement of plasticizedmaterial. However, overall a uniformlymixed distinct isolated pin plungedzone was found.

The authors would like to thankUniversiti Teknologi PETRONAS forthe research facilities and financialsupport.

1. A Guide to nylon. Available:ptsllc.com/intro/Nylon_intro.aspx. 2. Magnus, C. Feasibility Study of Metalto Polymer Hybrid Joining. 2012. 3. Liu, F. C., Liao, J., and Nakata, K.2014. Joining of metal to plastic using fric-tion lap welding. Materials & Design 54(2):236244. 4. Arici, A., and Sinmazelk, T. 2005.Effects of double passes of the tool on fric-tion stir welding of polyethylene. Journal ofMaterials Science 40(6): 33133316. 5. Pirizadeh, M., Azdast, T., Ahmadi, S.R., Shishavan, S. M., and Bagheri, A. Fric-tion stir welding of thermoplastics using anewly designed tool. Materials & Design54(2): 342347. 6. Olabisi, O., and Adewale, K. 1997.Handbook of Thermoplastics. Vol. 41. CRCpress. 7. Aydin, M. 2010. Effects of weldingparameters and pre-heating on the friction

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Fig. 13 A Specimen before welding with a marker material parallel to the WD; B vertical cross section of postweldspecimens when marker material is on the RS; C the AS.

Fig. 15 Specimens before welding with marker material at the bottom, middle, and top ofthe following: A AS; B RS; C longitudinal section of postweld specimen AS; D RS.

Fig. 14 A Specimen before welding with marker material perpendicular to the WD; B horizontal cross section of postweld specimenswhen marker material is on the AS; C weld interface; D RS.

References

Acknowledgments

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investigating friction stir welding on thick nylon 6 plates€¦ · friction stir welding (fsw),...

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