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Composite Structures

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Friction spot welding of carbon fiber-reinforced polyetherimide laminate

Yongxian Huang⁎, Xiangchen Meng, Yuming Xie, Zongliang Lv, Long Wan, Jian Cao, Jicai FengState Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, PR China

A R T I C L E I N F O

Keywords:Friction spot weldingCarbon fiber-reinforced compositesThermoplastic compositesPolyetherimide

A B S T R A C T

Here, carbon fiber-reinforced polyetherimide (PEI) laminate was joined via friction spot welding (FSpW). Thefeasibility and fracture mechanism of the FSpW joints were investigated. The sound joint with smooth surfaceand without hook defect was achieved. The strong bonding formed at the sleeve stirring zone and thermo-mechanically affected zone, resulting from the macromolecular interdiffusion and interlocking of the smashedcarbon fiber at the bonding interface by thermo-mechanical behavior. Increasing rotational velocity enhancedthe mixing degree of the carbon fiber at the interface and strengthened the bonding interface, which improvedtensile shear properties. The maximum tensile shear load of the FSpW joint with a joining area of 66.4 mm2

reached 1600 N, which was comparable to the strength of state-of-the-art welding. The fracture surfacemorphologies revealed a typical ductile fracture containing the deformation of polymer and the pull-out of thecarbon fiber. The FSpW has feasible and potential to join carbon fiber-reinforced thermoplastic compositeslaminate.

1. Introduction

Global trends in CO2 emission and gas price have attracted extensiveattentions from the manufacturing fields of automotive, aerospace andso on, to produce lighter, safer and environmental friendly vehicles.Carbon fiber-reinforced thermoplastic composites have become thepotential candidates to replace traditional metal materials due to stress-to-weight ratios and toughness [1]. Inevitably, it becomes necessary tothe joining process for the production of larger and complex work-pieces. Adhesive bonding and mechanical joining are the main joiningtechniques of thermoplastic composites [2,3]. The adhesive bondingowns simplified process and excellent fatigue properties, while theadhesive bonding joint is susceptible to temperature or other environ-mental conditions [3]. The mechanical joining presents a high sus-ceptibility to stress concentration, and a bonding part, such as screwbolt or rivet, is detrimental to light weight, as described by Huang et al.[4]. Moreover, the state-of-the-art welding techniques including ultra-sonic welding [5] and laser welding [6] are restricted due to the typesof polymers or polymer matrix composites and geometries of work-pieces to be welded.

Friction spot welding (FSpW) is invented and patented by theHelmholtz Zentrum Geesthacht (HZG) research center of Germany,which owns the advantages of short welding times, high joint qualityand low energy consumption [7–10]. The welding tool of FSpW consistsof a clamping ring, sleeve and pin. During FSpW process, friction be-tween the welding tool and thermoplastic composites generates

frictional heat. The frictional heat is beneficial to softening or meltingthermoplastic composites and then improving macromolecular inter-diffusion. Meanwhile, the material flow induced by the plastic de-formation enhances mechanical interlocking [8]. Moreover, a keyholedefect during conventional friction stir spot welding (FSSW) can beeliminated under the synthesis effects of the clamping ring, sleeve andpin, which increases the area of load bearing and reduces the stressconcentration associated with the hole’s notch effect. Oliveira et al. [7]validated the feasibility of FSpW in poly methyl meth acrylate (PMMA)and stated that the joint quality of the FSpW is comparable and thejoining time is equivalent or shorter compared with other weldingtechniques. Amancio-Filho et al. [8] also employed FSpW to join carbonfiber-reinforced poly amide 66 laminate (CF-PA 66). They expoundedthat a long holding time guaranteed the enough time to cool downunder pressure, which was propitious to obtaining a sound joint withgood surface finishing due to a low polymer shrinkage. Up to present,the researches on defect formation, microstructural evolution, joiningfeatures and fracture mechanism for the FSpW joint of thermoplasticcomposites are infancy.

In this study, carbon fiber-reinforced polyetherimide (CF-PEI) la-minate, extensively utilized in the aerospace skin and stringer partsbecause of high strength, rigidity and chemical resistance as well as lowwater absorption, was chosen as the research object. The feasibility,joining and fracture mechanisms for the FSpW joint of thermoplasticcomposites were investigated to provide technical support for en-gineering application of the FSpW in thermoplastic composites

https://doi.org/10.1016/j.compstruct.2018.02.004Received 25 November 2017; Received in revised form 25 January 2018; Accepted 5 February 2018

⁎ Corresponding author.E-mail address: yxhuang@hit.edu.cn (Y. Huang).

Composite Structures 189 (2018) 627–634

Available online 06 February 20180263-8223/ © 2018 Elsevier Ltd. All rights reserved.

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laminate.

2. Materials and methods

The base material (BM) was PEI laminate sheet reinforced with 45%of 5-harness (5H) satin weave carbon fibers, whose dimensions were2mm×80mm×20mm. The stacking sequence of carbon fiber layerswere configuration of [(0.90)/(± 45)]3/(0.90). The tensile strengthand in-plane shear strength of the BM were 600MPa and 115MPa,respectively. Welding tool is made of titanium alloy to reduce the heatloss, which consists of a clamping ring, sleeve and pin. The clampingring with an outer diameter of 18 mm and a width of 9mm was em-ployed to improve the clamping effect. The outer and inner diameters ofthe sleeve were 9mm and 6mm, respectively. The diameter of the pinwas 6mm. A plunge depth of the sleeve of 2.2mm and dwelling time of2 s were constant. Rotational velocities of the both pin and sleeve were800 rpm, 1000 rpm and 1200 rpm, respectively. Schematic of the FSpWof CF-PEI is displayed in Fig. 1.

The metallographic specimens were prepared in accordance withstandard grinding and polishing procedures and then observed by anoptical microscope (OM). Aiming at the composite, the pores and voidsas well as smashed carbon fiber in the molten and re-solidified PEIresult in the impossible to measure microhardness. The bigger indentermaybe contact with the fiber or pores, leading to inaccurate hardnessvalues. Therefore, nanoindentation was employed to evaluate the localmechanical properties of the composites at the joining interface. Thenanoindentation experiment was carried out using a nano indenter(G200, USA) with a load capacity of 10 N. A Berkovich diamond in-denter was applied. The maximum indentation depth of 1 μm wasconstant and the maximum indentation load corresponding to themaximum indentation depth was recorded. Both the hardness andstiffness of the molten and re-solidified PEI matrix were calculated.Worth mentioning is that all the indents were far from the carbon fiber.Shear tensile specimen was prepared referenced with ASTM D3163[11]. A binding fixture was fabricated and employed to guarantee theaccuracy of tensile shear test. Schematic of the tensile shear test withauxiliary fixture is exhibited in Fig. 2. The shear tensile test was per-formed at room temperature under a constant crosshead speed of1mm/min. A nominal joining area of the sleeve was determined ac-cording to the designed tensile shear specimen since the real joining

area of the shear tensile specimen were unknown. Therefore, the dif-ference between the maximum tensile shear load and the sleeve areawas used for the calculation of shear stress. The fracture surface of thetensile shear specimen was observed by a scanning electron microscope(SEM). For the SEM observation, a very thin gold layer was coated onthe fracture surface.

3. Results and discussion

Fig. 3a exhibits the surface appearance of the typical joint and 3Dmorphologies of the FSpW joints using different rotational velocities aredisplayed in Fig. 3b, c and d. An uncompleted filling defect forms at thesurface with a low rotational velocity of 800 rpm. The thermoplasticmaterials cannot be completely softened at a low frictional heat andthen present brittle feature, which are difficult to be refilled and thenresult in the surface groove featured by a maximum unfilled depth of580 μm, as shown in Fig. 3b. With increasing rotational velocity to1000 rpm, the size of the unfilled defect significantly reduces and onlyappears at the partial border of the sleeve, and the maximum unfilleddepth is 180 μm (Fig. 3c). The softened and plasticized materials areachieved due to the increase in frictional heat induced by increasingrotational velocity. Sound and flat surface morphology without theunfilled defect is obtained with increasing rotational velocity to1200 rpm (Fig. 3d). This is because that sufficient heat input canguarantee the refill of sleeve stirring zone (SSZ) by the adequatelysoftened and plasticized materials as well as smashed carbon fiber.Meanwhile, the micro heave appears at the surface of joint due to theexpanding with heat and contracting with cold. Amancio-Filho et al. [8]discussed that a proper holding time was beneficial to obtaining goodsurface finishing.

The macrostructures in cross-section of the FSpW joints using dif-ferent rotational velocities are displayed in Fig. 4. Different from FSSW[12–14], the keyhole defect is successfully refilled by the FSpW, whicheliminates the hole’s notch effect and then improves the area of loadbearing significantly. The unfilling defect or thickness reduction ap-pears at the pin refilling zone (PRZ) on the surface of joint at a rota-tional velocity of 800 rpm due to insufficient softened or molten ma-terials, as presented in Fig. 4a and b. Meanwhile, the cavity defectappears at the middle of surface, which results from that the loss of thesoftened or molten materials leads to the reductions of axial forces ofthe sleeve and pin, which are difficult to compact the materials in thestirring zone (SZ). The unfilling defect is eliminated with increasingrotational velocity to 1000 rpm because of the improvement of materialflow induced by the increase in frictional heat (Fig. 4c and d). However,the cavity defect in the SZ of the joint is not completely avoided. This isbecause that the insufficient softened or molten polymer at the refillingstage cannot be completely pushed by the pin to refill the cavity left bythe sleeve due to the relative low frictional heat. The expanding withheat and contracting with cold may be another main reason, attributingto the formation of the cavity defect. Sound joint without the cavitydefect and thickness reduction can be achieved at a rotational velocityof 1200 rpm, which benefits from the improvement of tensile proper-ties, as displayed in Fig. 4e and f.

In addition, the lack of adhesive joining defect appears at the

Fig. 1. Schematic of the FSpW process of the CF-PEI laminate.

Fig. 2. Schematic of the tensile shear test for the FSpW joint.

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interface between the upper and lower sheets, resulting from the tensilestress induced by the polymer shrinkage at the cooling stage, as markedby red line in Fig. 4a, c and e. During FSpW process, the rotational pinbegins to retract when the sleeve plunges into the composites, and thecavity is filled with the softened or molten polymers induced by theretraction of the rotational pin (Fig. 1b). The sleeve and pin return tothe original position when the sleeve reaches the designed plunge depthand then dwells several seconds. A plunge force induced by the pinsqueezes the softened materials flow into the cavity caused by the re-traction of the sleeve, completing the joining process (Fig. 1c). How-ever, since the rotational pin does not contact with the materials in the

lower sheet, only the pressure force induced by the rotational pin exertsat the softened or molten polymers. Meanwhile, the lower thermalconductivity of the polymer is difficult to make frictional heat conductto the middle interface between the upper and lower sheets, and thenthe lack of adhesive joining forms. The length of the lack of adhesivejoining gradually decreases with the increase of rotational velocity dueto the improvement of thermo-mechanical behavior (Fig. 4), which ispropitious to enhancing the ability of the load bearing of the FSpWjoint. Meanwhile, the voids defect forms at the SZ due to air entrapmentin the molten polymer during solidification. The viscosity of the moltenpolymer reduces and air can become entrapped in the molten layer

Fig. 3. (a) Surface appearance of the typical joint; 3D morphology of the FSpW joints with different rotational velocities: (b) 800 rpm, (c) 1000 rpm and (d) 1200 rpm.

Fig. 4. Macrostructures of the FSpW joints under different rotational velocities: (a) and (b) 800 rpm, (c) and (d) 1000 rpm, (e) and (f) 1200 rpm.

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since the peak temperature during the joining cycle is relatively high.During the solidification phase, extremely fast cooling rate causes thatthe entrapped air is difficult to escape from the molten layer, leavingthe voids of air pockets after solidification. The formation of the regularvoids as a result of thermal degradation and decomposition is especiallyreported in the laser joining between thermoplastics and metal [6].Moreover, the hook defect formed at the FSpW of aluminum alloys doesnot occur in this study due to the low plunge depth of 0.2mm in thelower sheet, which is propitious to delaying the rapid initiation andpropagation of crack at the joining area of thermo-mechanically af-fected zone (TMAZ) during tensile shear test.

The macrostructure of the FSpW joint in cross-section is divided intoSSZ, PRZ and TMAZ, in which the SSZ and PRZ belong to the SZ, asdisplayed in Fig. 4e. The microstructure of the CF-PEI composites pre-sents a laminate of long or circular fiber due to stacking sequence, asexhibited in Fig. 5. The SSZ contains complex mixtures with the moltenand re-solidified polymer as well as the smashed carbon fibers in ar-bitrary orientation (Fig. 6a). Meanwhile, some incomplete smashedcarbon fibers appear at the PRZ due to without the stirring action of the

Fig. 5. Microstructure of the FSpW joint of the CF-PEI laminate composites.

Fig. 6. Microstructures at the SSZ and TMAZ of the FSpW joints using different rotational velocities: (a) SSZ and (b) TMAZ at the 800 rpm; (c) SSZ and (d) TMAZ at the 1000 rpm; (e) SSZand (f) TMAZ at the 1200 rpm.

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pin, as displayed in Fig. 6c. Moreover, rotational velocity plays noobvious influence on the distribution of the smashed carbon fibers atthe SSZ, as depicted in Fig. 6a, c and e. The TMAZ displays the de-formed and bended carbon fibers along the material flow directioninduced by the sleeve motion under the thermal cycle and mechanicalstirring (Fig. 6b, d and f). No obvious microstructural evolution is ob-served outside the TMAZ due to the low thermal conductivity. It is the

fact that the thermal conductivity of conventional polymer is lowerthan 0.5Wm−1K−1, and thermal conductivities of aluminum or alu-minum alloys are higher two orders of magnitude than conventionalpolymer [15]. Strand [16] also reported that no heat affected zone(HAZ) was observed for FSW joint of polypropylene polymer. Thejoining area forms near the TMAZ besides the SSZ (Fig. 6b, d and f),which is attributed to interdiffusion of molecular caused by thermo-mechanical behavior and then benefits from increasing the area of loadbearing. Schematic of the joining area is displayed in Fig. 7. Thethermo-mechanical joining zone, sleeve joining zone and lack of joiningzone are abbreviated as the TMJZ, SJZ and LoJZ, respectively. Thejoining area consists of the TMJZ outside the sleeve and the partial SJZinduced by insufficient refill. With increasing rotational velocity from800 rpm to 1200 rpm, the joining area gradually increases due to theimprovement of thermo-mechanical effects (Fig. 8), which benefitsfrom the ability of load bearing. The LoJZ inside the sleeve graduallydecreases due to the sufficient material flow induced by the improve-ment of frictional heat. Moreover, Fig. 9 shows the joining interfacebetween the upper and lower sheets at the SSZ. An effective adhesivebonding without the crack induced by the molten and re-solidifiedpolymer forms at the joining interface. Importantly, the smashedcarbon fibers dispersedly distribute at the joining interface and thenimproves mechanical intermixing, which strengthen the joining inter-face strength and then enhance the ability of load bearing. Moreover,increasing rotational velocity is beneficial to improving the distributiondensity of carbon fibers at the interface, further raising mechanicalinterlocking.

Fig. 10 exhibits the results of nanoindentation taken from differentregions owning the thermoplastic PEI in the typical FSpW joint to ex-plain the microstructural and mechanical variations. Therein, Pmax

presents the achieved maximum load in the experiment, and hmax ex-presses the maximum indentation displacement [17,18]. The ability ofload bearing, modulus and hardness of the molten and re-solidifiedpolymer at the SJZ and TMJZ slightly decrease compared with BM,which are attributed to the relatively low crystallinity or thermal de-gradation. Friction between the sleeve and the hard carbon fiber pro-duces instant higher peak temperature during FSpW of CF-PEI laminate.The higher peak temperature may be higher than the degradation-commencement temperature of the PEI and then results in the thermaldegradation, leading to the decrease in hardness value. Oliveira et al.[7] reported that a significant decrease in hardness about 5–10% duringFSpW of polymethyl methacrylat (PMMA) was obtained due to thethermal degradation and the reduction in molecular weight. However,there is only slight variation for the polymer of the SJZ and TMJZ afterexperiencing thermo-mechanical cycle, which results from that thesmashed carbon fiber acts an efficient nucleating agent to improve thecrystallization of the polymer and then increase hardness value. Gaoet al. [19] also stated that the dispersed multi-walled carbon nanotubes(MWCNTs) led to the increase of crystalline phases from the surface ofthe MWCNTs when preparing MWCNTs/ high density poly ethylene(HDPE) composites by submerged friction stir processing. Moreover,

Fig. 7. Schematic of joining area for the FSpW joint.

Fig. 8. Lengths of the kissing bond and joining area in cross-section of the FSpW joint.

Fig. 9. Joining interface characteristics of the FSpW joints using different rotational velocities: (a) 800 rpm, (b) 1000 rpm and (c) 1200 rpm.

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there is no obvious variation for the modulus and hardness of thepolymer at the border of TMJZ (Fig. 10b), which means that no HAZappears and then validate the results of Strand [16].

Fig. 11 shows the results of tensile shear test of the FSpW joints.With the increase of rotational velocity, the tensile properties graduallyincrease (Fig. 11a), which are closely correlated with the welding de-fect, joining area induced by the molten and re-solidified PEI, andmechanical interlocking induced by the smashed carbon fiber at theinterface. Low frictional heat results in small joining area and the oc-currence of the insufficient joining at a rotational velocity of 800 rpm,and then leads to low tensile shear properties. With increasing rota-tional velocity to 1000 rpm, the improvement of effective joining lengthbenefits from joint strength compared with the rotational velocity of800 rpm. Moreover, the cavity defect forms at the middle of the PRZrather than the joining interface, paralleled to the direction of tensileshear force, which does not affect the joint strength. Sound joint withbig joining length attributes to the superior strength with further in-creasing rotational velocity to 1200 rpm. As present, the maximumfracture stress is 45MPa, which is comparable or higher to the state-of-the-art welding techniques [20]. It indicates that the FSpW has poten-tial to join carbon fiber-reinforced composites laminate.

Meanwhile, the fracture energy of the FSpW joint by calculating thearea under the load-extension curve until maximum fracture force isachieved, which shows the ability to absorb mechanical energy ofmaterial in unit volume up to failure [15,21,22]. The increase of rota-tional velocity results in the improvement of the fracture energy be-cause of the augments of the fracture force and extension, as exhibitedin Fig. 11b. This is attributed to the larger joining area and the sharperintermixing of the smashed carbon fiber at the joining interface. Thefracture energy at a rotational velocity of 1200 rpm reaches the

maximum value of 296mJ, revealing the higher toughness of the joint.Fig. 12 displays the fracture surface morphologies of the tensile

shear specimens. Fracture locations of all the samples locate at thejoining interface between the upper and lower sheets (Fig. 12a, e and i).The fracture surface of the joint on the composites laminate looks like acircle, in which un-molten polymer surface at the center of the partialcircle is surrounded by the molten and re-solidified polymer, as in-dicated in Fig. 12a, e and i. Meanwhile, the crack initiates at the LoJZoutside the TMJZ, and propagates along the SJZ (Fig. 12b). The fracturesurface morphologies indicate the two fracture modes at the differentjoining regions. The marginal of the joining area outside the TMJZshows relatively smooth surface, which means that no plastic de-formation happens and the main crack rapid initiation, indicatingbrittle fracture (Fig. 12b). The joining areas at the main TMJZ and SJZexhibit the rough surfaces and the pull-out of carbon fiber, which areattributed to the large plastic deformation under the tensile force,presenting ductile fracture (Fig. 12c). Moreover, the middle at thefracture surface in Fig. 12d of the FSpW joint describes the smoothersurface than Fig. 12c due to the lack of joining, which easily becomesthe crack rapid propagation before the complete fracture and then de-teriorates the load bearing of joint. Meanwhile, worth emphasizing isthat increasing rotational velocity accelerates the plastic deformationdegree of polymer and the pull-out of carbon fiber, benefiting from theload bearing of the joint at a rotational velocity of 1000 rpm, as in-dicated in Fig. 12f, g and h. With increasing rotational velocity to1200 rpm, the amounts of the pull-out of carbon fiber appear at thefracture surface morphologies at the different joining areas (Fig. 12j, kand l), revealing the improvement of mechanical interlocking andtensile properties. These fracture surfaces also conform to the results oftensile shear properties.

Fig. 10. Nanoindentation results at different regions owning the PEI of the typical FSpW joint at a rotational velocity of 1200 rpm: (a) load-displacement curves and (b) average andstandard deviation of nanoindentation modulus and hardness.

Fig. 11. Tensile shear results of the FSpW joints: (a) force-displacement curves and (b) tensile shear properties.

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4. Conclusions

CF-PEI composites laminate was joined via FSpW to eliminate the

restriction of certain types and geometries of polymer matrix compo-sites during the state-of-the-art laser welding or ultrasonic welding. Thejoint formation, joining mechanism and mechanical property were

Fig. 12. Fracture surfaces of the typical FSpW joint at different rotational velocities: (a) macro fracture surface, micro surfaces of (b) marked by “b”, (c) marked by “c” and (d) marked by“d” at the rotational velocity of 800 rpm; (e) macro fracture surface, micro surfaces of (f) marked by “f”, (g) marked by “g” and (h) marked by “h” at the rotational velocity of 1000 rpm; (i)macro fracture surface, micro surfaces of (j) marked by “j”, (k) marked by “k” and (l) marked by “l” at the rotational velocity of 1200 rpm.

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investigated. Based on the present investigation, the conclusions can beextracted.

(1) The FSpW has feasible and potential to join carbon fiber-reinforcedthermoplastic composite laminate. The sound joint with smoothsurface was achieved at a high rotational velocity of 1200 rpm anda small plunge depth of 0.2mm.

(2) The macromolecular interdiffusion and intermixing of carbon fiberat the interface between the upper and lower sheets attributed tothe main joining mechanism. The improvement of thermo-me-chanical behavior induced by increasing rotational velocity wasbeneficial to enhancing the joining area and the degree of me-chanical intermixing of the carbon fiber, promoting tensile shearproperties.

(3) The maximum fracture stress at the rotational velocity of 1200 rpmreached the maximum value of 45MPa due to the bigger joiningarea and the stronger intermixing of the carbon fiber, comparableto the state-of-the-art ultrasonic welding joint.

(4) The LoJZ outside the joining zone at the TMAZ tended to be thecrack source and rapidly propagated along the joining interface,resulting in the joint fracture. The fracture surface presented mixingfracture modes, containing brittle fracture at the TMAZ and ductilefracture at the SSZ.

Acknowledgements

The work was jointly supported by the National Natural ScienceFoundation of China (No. 51575132) and the Fund of NationalEngineering and Research Center for Commercial AircraftManufacturing (No. COMAC-SFGS-2016-33214).

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