A Control Method for the Ultrasonic Spot Welding of Fiber-Reinforced Thermoplastic Laminates Through the Weld-Power Time Derivative

Journal of Manufacturing and Materials Processing

Article A Control Method for the Ultrasonic Spot Welding of Fiber-Reinforced Thermoplastic Laminates through the Weld-Power Time Derivative

Shahan Tutunjian 1,2 , Martin Dannemann 2,* , Fabian Fischer 1, O˘guzhanEro˘glu 1 and Niels Modler 2 1 BMW Group, 80788 Munich, Germany; [email protected] (S.T.); [email protected] (F.F.); [email protected] (O.E.) 2 Institute of Lightweight Engineering and Polymer Technology (ILK), Technische Universität Dresden, 01307 Dresden, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-351-463-38134

Received: 24 October 2018; Accepted: 24 December 2018; Published: 30 December 2018

Abstract: It was found that the ultrasonic spot welding may serve as an efficient method to join relative large thin-walled parts made of fiber-reinforced thermoplastics. In this study, a new control method for the ultrasonic spot-welding process was investigated. It was found that, when welding fiber-reinforced thermoplastic laminates without energy directors, overheating and decomposition of the polymer at the weld spot occurred. The occurrence of the overheating took place at unpredictable times during welding. It was observed that the time trace of the consumed power curve by the welder follows a similar pattern as the time trace of the temperature in the weld spot center. Based on this observation, a control system was developed. The time derivative of the welder power was monitored in real time and, as soon as it exceeded a critical value, the ultrasonic vibration amplitude was actively adjusted through a microcontroller. The controlling of the ultrasonic welding process forced the temperature in the weld spot to remain in an adequate range throughout the welding duration for the polymer diffusion to occur. The results of the controlled welding process were evaluated by means of weld temperature measurements, computed tomography scans, and microscopic analysis of the weld spot fracture surfaces.

Keywords: ultrasonic welding; differential ultrasonic spot welding; ultrasonic weld control system; ﬁber-reinforced thermoplastic laminates; joining of ﬁber-reinforced thermoplastic composites

1. Introduction The well-established methods used in joining of fiber-reinforced polymers in general are mechanical fastening, adhesive bonding, and fusion bonding (sometimes referred to as welding) [1,2]. From the many already existing welding technologies for fiber-reinforced thermoplastics, a remarkable one is ultrasonic welding [3]. Mechanical vibrations with a frequency above the audible spectrum (usually between 20 kHz and 45 kHz) are applied through a metallic horn on the work-piece accompanied by static pressure. The vibrations cause intensive intermolecular and boundary friction heating at and around the joint interface, which in turn heats up locally and rapidly until the polymer diffusion temperature is exceeded; then, the vibrations are stopped and the joint is healed under static pressure [4]. In practice, energy directors are intentionally inserted in the weld interface at the required joint location to focus the ultrasonic vibration energy [4]. Benatar and Gutowski [5] investigated the ultrasonic welding process of carbon fiber-reinforced polyetheretherketone (PEEK) with the presence of energy directors. They concluded in their work that a good bond quality was achieved when the

J. Manuf. Mater. Process. 2019, 3, 1; doi:10.3390/jmmp3010001 www.mdpi.com/journal/jmmp J. Manuf. Mater. Process. 2019, 3, 1 2 of 17 melt fronts of the adjacent energy directors met. This event was traced by the quick rise in the welder power. However, molding of the energy directors on relatively large thin-walled and thermoformed thermoplastic sheet laminates demands additional processing steps, which might be undesirable in high-volume-production [2]. Villegas [6] investigated an ultrasonic welding approach, in which a neat polymer thin film was placed separately in the weld interface as a flat energy director. To monitor the process, the displacement and power curves of the welder were used. The energy-directing approach using thin-film placement in the weld interface was expanded to include continuous ultrasonic seam welding of stiff thermoplastic composites plates [7]. Li et al. [8] investigated a spot welding method through focusing the ultrasonic energy between the mating fiber-reinforced thermoplastic plates with the absence of the energy directors. It was found that applying high static welding forces during the weld initiation stage helps concentrate the ultrasonic energy at the desired spot under the horn and improve the weld strength, and may eliminate the melting at the contact edges. Zhi et al. [9] monitored the quality of the ultrasonic welds done without energy directors between carbon fiber-reinforced thermoplastic plates through the horn displacement and dissipated power. The welding process was analyzed using temperature measurements by placing K-type thermocouples at the weld interface and the horn contact interface. A similar approach was used in Reference [10]. Grewell [11] reported in research on the ultrasonic welding of neat thermoplastic parts that the joint quality can be enhanced when using a well-defined weld force and amplitude profile during the ultrasonic welding. Fischer et al. [12] implemented the welder energy to determine the end of the weld process when welding thermoplastic plies during a fully automated patch-preforming. Several methods for controlling and monitoring the ultrasonic welding process were proposed and investigated in the past decades; however, the need for an active process control system is essential. It is known that the heat generation during the ultrasonic spot welding of thermoplastics or thermoplastic composites without energy directors or through thin-film energy directors is dominated in the initial stages by friction energy dissipation. At the following stages, the friction energy dissipation gradually diminishes, and heating due to viscoelastic dissipation dominates [6,9,13]. Ideally, it may be assumed that the energy dissipated by friction is converted to heat. The generated heat flux by the friction movement may be averaged to the amount of energy dissipated per unit time per unit area . q f ric. Nevertheless, the hammering effect α should be introduced to the equation [14]. The hammering effect represents the average portion of the cycle in which the horn applies the displacement on the laminates. If the interfacial friction is considered to be Coulomb damping, then the amount of energy . dissipated q f ric per unit area is the result of the relative movement amplitude u0 multiplied by the interfacial pressure amplitude σ0 and the friction coefficient µ (given in Equation (1)). The viscoelastic heating may be taken from literature as the dissipated energy by viscoelastic damping [5,13–16]. This . 00 heat qvisc is volumetric and may be resembled by the loss modulus of the viscoelastic material E multiplied with the square of the strain amplitude ε0, and is given in Equation (2) [17,18].

. µω q = α2 u σ (1) f ric π 0 0

00 2 . ωE ε q = α2 0 (2) visc 2 In this work, a welding method without energy directors was used. The ultrasonic spot welding configuration is eligible for the automated application of spot welds on large thermoplastic composite thin-walled parts. One of the main problems, which often occurs during the ultrasonic welding of fiber-reinforced thermoplastic laminates without energy directors, is the unpredictable overheating and consequent decomposition of the matrix at the weld spot. The randomness of the overheating leads to a lack in process stability. This overheating occurs because there is no sufficient matrix in the weld zone to flow and carry the excessive heat away. In the case of welding the composite parts with energy directors, as soon as the temperature exceeds the melting (softening) temperature of the energy director, the melt fronts flow and carry the heat away from the weld zone [15,19]. However, in the case J. Manuf. Mater. Process. 2018, 2, x FOR PEER REVIEW 3 of 17 J. Manuf. Mater. Process. 2019, 3, 1 3 of 17 of spot welding without energy directors, the rate of flow of the matrix becomes lower than the rate of spotheating. welding As a withoutresult, the energy weld directors,spot reaches the dec rateomposition of flow of the temperatures matrix becomes rapidly lower and than unexpectedly. the rate of heating.To overcome As a result,this drawback, the weld spotan algorithm reaches decomposition for a logical weld-process temperatures control rapidly method and unexpectedly. was developed To overcomeand its influence this drawback, on the weld an spot algorithm overheating for a logical was investigated. weld-process control method was developed and its influence on the weld spot overheating was investigated. 2. Material and Methods 2. Material and Methods 2.1. The Ultrasonic Spot Welding Configuration 2.1. The Ultrasonic Spot Welding Configuration The ultrasonic spot welding was done without energy directors in the interface; the energy focusingThe was ultrasonic achieved spot by welding applying was the donewelds without amid a energyflat-end directors anvil and in a theflat-end interface; ultrasonic the energy horn. focusingThe anvil was and achieved the horn by had applying circular the contact welds amidto the a welded flat-end parts, anvil andin which a flat-end the ultrasonicanvil had horn.a smaller The anvilcontact and diameter the horn 𝐷 had with circular the welded contact parts to the than welded the horn parts, 𝐷 in. A which combination the anvil of had a relative a smaller high contact static diameterweld forceD [8]a with and the geometry welded parts of the than anvil/horn the horn enDdsh. facilitate A combination the focusing of a relative of the ultrasonic high static energy weld forceat a spot [8] andin the the weld geometry interface. of the The anvil/horn configuration ends facilitateof the ultrasonic the focusing welding of the is ultrasonic given in energyFigure 1; at a spotdetailed in the view weld of interface.the horn/anvil The configurationends and the ofessent the ultrasonicial dimensions welding are illustrated is given in in Figure the detail1; a detailed view. view of the horn/anvil ends and the essential dimensions are illustrated in the detail view.

Figure 1. The conﬁguration of the ultrasonic spot welding. Illustration of the main components andFigure dimensions. 1. The configuration of the ultrasonic spot welding. Illustration of the main components and dimensions. One of the main problems which was observed during the ultrasonic spot welding of ﬁber-reinforcedOne of the thermoplasticmain problems laminates which was without observed energy during directors the ultrasonic was overheating spot welding and of thermal fiber- decompositionreinforced thermoplastic of the thermoplastic laminates matrixwithout in en theergy weld directors interface. was It overheating was observed and that, thermal at an unpredictabledecomposition time,of the the thermoplastic temperature inmatrix the weld in the spot weld rose veryinterface. rapidly It was and exceededobserved thethat, thermal at an decompositionunpredictable time, temperature the temperature of the matrix. in the The weld decomposition spot rose very of the rapidly matrix and caused exceeded defects the in thethermal weld spotdecomposition in the form temperature of cavities, of delamination, the matrix. The and decompo gas entrapments,sition of the which matrix in caused turn reduced defects in the the quality weld andspot strengthin the form of the of weldcavities, spot. del Toamination, avoid overheating and gas andentrapments, create a uniform which weldin turn spot, reduced a control the methodquality wasand essential.strength of the weld spot. To avoid overheating and create a uniform weld spot, a control method was essential. 2.2. The Process Controlling Hypothesis 2.2. TheThe Process power Controlling consumed Hypothesis by the ultrasonic welder P(t) during the ultrasonic spot welding follows, in mostThe cases, power a typicalconsumed time-trace by the ultrasonic pattern, which welder is plottedP(t) during as the the solid ultrasonic line in Figurespot welding2. Parallel follows, to it, plottedin most ascases, the brokena typical line, time-trace is the temperature pattern, which curve is measured plotted as in the the solid center line of in the Figure spot weld.2. Parallel The plotsto it, illustratedplotted as the in Figurebroken2 line,are real is the measurements temperature whichcurve measured belong to ain single the center ultrasonic of the spotspot weld.weld. The plots illustrated in Figure 2 are real measurements which belong to a single ultrasonic spot weld.

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Figure 2. An example plot of the weld power and the weldweld spot temperature as functions of time for an exemplary ultrasonic spot weld applied on a carboncarbon ﬁber-reinforcedfiber-reinforced thermoplasticthermoplastic laminatelaminate pair.pair.

By observing the temperature and the power curvecurve patterns, one might clearly notice that the weld power P(t) and the weld center temperature T(t) have a clear correlation. In the weld initiation phase, the power and the temperature curves rise rapidly;rapidly; then, at a transition phase, the temperature and the powerpower curvescurves undergoundergo only only slight slight changes. changes. At At a a certain certain instance instance of of time, time, both both curves curves show show an abruptan abrupt rise rise before before stabilizing stabilizing again. again. This This second second rise rise of theof the temperature temperature leads leads to theto the decomposition decomposition of theof the matrix matrix at at the the weld weld spot. spot. A A similar similar observation observation was was made made byby TolunayTolunay [[19]19] whilewhile measuringmeasuring the temperature in thethe interfaceinterface ofof twotwo disc-shapeddisc-shaped specimensspecimens duringduring ultrasonicultrasonic welding.welding. It was assumed in this work that, if the second rise of the powerpower curve is prevented, then this may prevent the sudden increase in temperature up to the decompositiondecomposition zone. It may lead instead to the stabilization of the temperature and, consequently, the weld spot will have sufﬁcientsufficient time in the optimum temperature zone to undergo a uniform diffusion process. The prevention of the second rise of temperature was achieved by means of monitoringmonitoring the power time derivative in real-time.real-time. If the derivative in the expected time period exceeded a certain pre-set value, then the applied vibration displacement amplitude was reduced. The pre-set power derivative value to trigger the amplitude drop and the amount of amplitude drop werewere determineddetermined experimentally.experimentally. It was reported that the power P(t) consumed byby the weld is the sum of the total power dissipated in the welded welded material material as as heat heat through through friction friction and and viscoelastic viscoelastic mechanisms mechanisms plus plus the thepower power lost lostin the in therig rigand and system system [6,14]. [6,14 The]. The viscoelastic viscoelastic dissipated dissipated po powerwer is isa afunction function of of the the square square of of the strainstrain amplitude (Equation (2)) (2)) and, thus, a function of the square ofof thethe deformationdeformation amplitudeamplitude [[15].15]. The friction heating on the other hand is a function function of the the average average slippage slippage amplitude in the weld interface interface u𝑢0 andand bothboth components components of theof the interfacial interfacial pressure pressure amplitude amplitudeσ0, which 𝜎 , arewhich the averageare the static average interfacial static pressureinterfacialσ staticpressureand the𝜎 average and the dynamic average interfacial dynamic pressure interfacial amplitude pressureσ amplitudedyn0. By a closer 𝜎. observation By a closer ofobservation the friction of dissipationthe friction (Equationdissipation (1)), (Equation both the (1)), slippage both the and slippage the dynamic and the pressure dynamic are pressure functions are of the displacement amplitude A0. Thus, the0 friction power dissipation becomes a function of the functions of the displacement amplitude A . Thus, the friction power dissipation becomes a function. squareof the square of the ultrasonicof the ultrasonic vibration vibration amplitude. amplitude. The parameters The parameters and variables and variables which correlate which withcorrelateq f ric . arewith given 𝑞 in are Equation given in (3) Equation and the parameters (3) and the which parameters correlate which with q correlatevisc are given with in 𝑞 Equation are given (4). The in variablesEquation which(4). The mainly variables inﬂuence which the mainly dissipated influence or consumed the dissipated power or are consumed given in Equationpower are (5). given in Equation (5). . 2 q f ric ∝ u0, σdyn0, σstatic, ω ∝ A 0, σstatic, ω ; (3) 𝑞 ∝𝑢,𝜎,𝜎,𝜔∝𝐴,𝜎,𝜔; (3)

. 2 2 q ∝ ε, ω ∝ A , ω ; (4) 𝑞visc ∝ 𝜀0,𝜔 ∝ 𝐴,𝜔0 ; (4)

𝑃𝑡 ∝𝑞,𝑞∝𝐴,𝜎,𝜔. (5)

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. . 2 P(t) ∝ q f ric, qvisc ∝ A0, σstatic, ω . (5) In this discussion, only the adjustable variables were taken into consideration. The static pressure (the static welding force), in addition to its influence on the friction heating, influences the heating rate through effecting the hammering effect as reported in References [14,15]. Therefore, adjusting it might give unpredictable influence on the power dissipation. The frequency is assumed by several researchers to be used as a weld control input because it is in a linear correlation with the weld power; however, this in turn is constant in the given welder and can only be adjusted in a narrow range. Therefore, to control the weld power P(t), the active adjusting of the vibration amplitude is most applicable to the design of the control system.

2.3. The Control System and Instruments The control system used for the experimental study was attained by connecting an external microcontroller through a commercial high-speed digital amplifier/data recorder to the main welder controller. The latter in turn was the standard built-in controller of the ultrasonic generator. The used weld generator was capable of outputting a nominal maximum power of 2400 W with a nominal mean frequency of 30 kHz. The ultrasonic stack consisted of a 30-kHz converter, a 1.5:1 titanium booster and a 4:1 steel stepped flat-end horn. The horn-end had a diameter of Dh = 18 mm and the anvil had a diameter of Da = 10 mm. The anvil was constructed in a way that it had a natural frequency of about 42 kHz to avoid resonance vibrations within the rig. According to the laser vibrometer measurements, the stack provided a maximum base-to-peak displacement amplitude A100% = 0.028 mm at the horn-end. The displacement amplitude at the end of the horn could be adjusted through the internal controller of the welder in terms of percentage from the maximum available amplitude A = 100 A0/A100%. During welding, it can be varied through an external analog signal; a 10-V analog signal from the external controller sent to the generator corresponds to A = 0% amplitude and 0 V corresponds to A = 100%. The welder generator can provide, in real time, the consumed electrical power value as an analog output signal in the range of 0–10 V, where 0 V corresponds to 0 W and 10 V corresponds to the maximum welder power of 2400 W. The measurement and amplification unit consisted of a temperature signal amplification module, an amplification module for input signals, an amplification module for the output signals, and a data recorder. The sample rate at which the measurement unit was operated at was 300 Hz. The implemented controlling unit was a commercial programmable microcontroller. The wiring and connections are schematically illustrated in Figure3. The algorithm for the controller is illustrated in Figure4. The microcontroller read the amount of power consumed by the welder in terms of an analog voltage signal (0–3.3 V); then, it converted the voltage into the power value in terms of watts, where 0 V corresponded to 0 W and 3.3 V corresponded to 2400 W. The discrete time derivative of the measured power value dP/dt was calculated in the microcontroller in real time by dividing the difference in the power value between two successive controller loops over the duration of the complete loop. In the same internal cycle, the controller checked within three if-loops for certain conditions of the power derivative. In the first if-loop, the controller checked if dP/dt was within the range between dPset1 and dPset2; then, the pre-set initial amplitude A was dropped with a value of ∆A = a. This means that the new amplitude percentage after the loop would be A = A − ∆A. In a similar manner, the amplitude was dropped in the second and third if-loops, although with larger step sizes of b and c, respectively. At the end of each control loop, the microcontroller output the target vibration displacement amplitude percentage in terms of an analog 0–3.3 V signal at A, which was further amplified by the digital amplifier to 0–10 V and fed to the welder internal controller. The amplitude drop repeated for each loop of the controller until the if-loop conditions were not true; then, the amplitude remained constant until the pre-set weld time was reached and the trigger signal was set to LOW. Then, the ultrasonic vibrations were stopped and the weld healing took place under static pressure for a duration of 2 s. J. Manuf. Mater. Process. 2018, 2, x FOR PEER REVIEW 5 of 17

In this discussion, only the adjustable variables were taken into consideration. The static pressure (the static welding force), in addition to its influence on the friction heating, influences the heating rate through effecting the hammering effect as reported in References [14,15]. Therefore, adjusting it might give unpredictable influence on the power dissipation. The frequency is assumed by several researchers to be used as a weld control input because it is in a linear correlation with the weld power; however, this in turn is constant in the given welder and can only be adjusted in a narrow range. Therefore, to control the weld power P(t), the active adjusting of the vibration amplitude is most applicable to the design of the control system.

J. Manuf. Mater. Process. 2018, 2, x FOR PEER REVIEW 6 of 17 complete loop. In the same internal cycle, the controller checked within three if-loops for certain conditions of the power derivative. In the first if-loop, the controller checked if dP/dt was within the range between dPset1 and dPset2; then, the pre-set initial amplitude A was dropped with a value of ΔA = a. This means that the new amplitude percentage after the loop would be A = A − ΔA. In a similar manner, the amplitude was dropped in the second and third if-loops, although with larger step sizes of b and c, respectively. At the end of each control loop, the microcontroller output the target vibration displacement amplitude percentage in terms of an analog 0–3.3 V signal at A, which was further amplified by the digital amplifier to 0–10 V and fed to the welder internal controller. The amplitude drop repeated for each loop of the controller until the if-loop conditions were not true; then, the amplitude remained constant until the pre-set weld time was reached and the trigger signal was set to LOW. Then, the ultrasonic vibrations were stopped and the weld healing took place under static pressure for a duration of 2 s. FigureFigure 3. 3.A A schematic schematic representation representation of of the the welder, welder, the the data data recording, recording, and and the the control control system. system.

The measurement and amplification unit consisted of a temperature signal amplification module, an amplification module for input signals, an amplification module for the output signals, and a data recorder. The sample rate at which the measurement unit was operated at was 300 Hz. The implemented controlling unit was a commercial programmable microcontroller. The wiring and connections are schematically illustrated in Figure 3. The algorithm for the controller is illustrated in Figure 4. The microcontroller read the amount of power consumed by the welder in terms of an analog voltage signal (0–3.3 V); then, it converted the voltage into the power value in terms of watts, where 0 V corresponded to 0 W and 3.3 V corresponded to 2400 W. The discrete time derivative of the measured power value dP/dt was calculated in the microcontroller in real time by dividing the difference in the power value between two successive controller loops over the duration of the

(a) (b)

Figure 4. (a) The algorithm of the control program in the microcontroller. (b) Schematic illustration of Figurethe algorithm. 4. (a) The algorithm of the control program in the microcontroller. (b) Schematic illustration of the algorithm. This may be expressed as follows: if the slope of the power curve exceeds a critical value, then the amplitudeThis may beA isexpressed reduced withas follows: a constant if the pre-set slope ∆ofA the. With power this curve approach, exceeds the amounta critical of value, heat beingthen thegenerated amplitude in the A is weld reduced zone with is actively a constant reduced pre-set and Δ controlledA. With this within approach, an optimum the amount range. of Inheat previous being generatedwork in the in framethe weld of zone this research, is actively it reduced was reported and controlled that only within one if-loop an optimum is sufﬁcient range. to In control previous the workweld powerin the frame [20]. In of this this current research, work, it was the algorithmreported that was only expanded one if-loop to include is sufficient three parallel to control if-loops, the weldthus power allowing [20]. the In system this current to react work, to small the algorithdP/dt valuesm was withexpanded smaller-amplitude to include three drop parallel steps if-loops, and for thuslarger allowingdP/dt values the system with higher-amplitude to react to small dropdP/dt steps.values The with controller smaller-amplitude started checking drop steps for the and power for largerderivative dP/dtonly values after with adelay higher-amplitude of t0 from the drop weld steps. start The trigger controller point. started This delay checking served for to the avoid power the derivativeamplitude only drop after in the a earlydelay phase of t0 from of the the welding, weld start i.e., during trigger the point. weld This initiation delay whereserveddP/dt to avoidbecomes the amplitudevery large. drop The controlin the early principle phase is of explained the welding, further i.e., during in the later the weld paragraphs. initiation where dP/dt becomes very large.In order The to control verify principle the process, is ex theplained temperature further in vs. the timelater inparagraphs. the center of the weld spots was measuredIn order using to averify thermocouple the process, (twisted the temperature and arc welded vs. K-Typetime in 0.08-mmthe center wire) of the and weld the temperature spots was measuredsignal value using was a thermocouple ampliﬁed and (twisted stored paralleland arc towelded the power K-Type and 0.08-mm amplitude wire) values and the using temperature the data signal value was amplified and stored parallel to the power and amplitude values using the data recorder with a sample rate of 300 Hz. The measured values of the power, for the calculation of dP/dt, were filtered using a low-pass Butterworth filter with a frequency of 5 Hz, because the noise in the

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J. Manuf. Mater. Process. 2018, 2, x FOR PEER REVIEW 7 of 17 recorderJ. Manuf. Mater. with Process. a sample 2018, 2 rate, x FOR of PEER 300 Hz.REVIEW The measured values of the power, for the calculation 7 of of 17 powerdP/dt, werevalues filtered may usingresult a low-passin undesired Butterworth peaks in filter its withderivative a frequency curves, of whereas 5 Hz, because the measured the noise temperatureinpower the power values values values may were mayresult filtered result in undesired in using undesired a low- peakspass peaks inBessel inits its derivativefilter derivative with acurves, curves,frequency whereas whereas of 50 Hz the. measuredmeasured temperaturetemperature valuesvalues werewere filteredfiltered using using a a low-pass low-pass Bessel Bessel filter filter with with a a frequency frequency of of 50 50 Hz. Hz. 2.4. The Specimen and the Specimen Fixture 2.4.2.4. TheTheThe SpecimenSpecimen laminates andand were thethe SpecimenpositionedSpecimen FixtureFixture as illustrated in Figure 5, and the spot weld was applied on their coincidingTheThe laminateslaminates center of were weresymmetries. positionedpositioned asas illustratedillustrated inin FigureFigure5 5,, and and the the spot spot weld weld was was applied applied on on their their coincidingcoinciding centercenter ofof symmetries.symmetries.

(a) (b) (a) (b)

Figure 5. A schematic drawing of the test specimen illu illustratingstrating the the laminate laminate placement, placement, dimensions, dimensions, andFigure the 5.thermocouple A schematic location locationdrawing in inof relarelation thetion test to tospecimen the the designated designated illustrating weld weld the spot. spot. laminate ( (aa)) Top Top placement, view view of of the the dimensions, specimen, specimen, andand ( theb) side thermocouple view of the location specimen. in relation to the designated weld spot. (a) Top view of the specimen, and (b) side view of the specimen. The fixture ﬁxture of the specimens is illustrated in FigureFigure 66.. ItIt waswas constructedconstructed toto allowallow thethe preciseprecise positioningThe fixture of the of laminates. the specimens It was isdesigned illustrated to consconstrainin Fitraingure all 6. the It was relative constructed in-plane to movements allow the precisebut, at thepositioning same time, of theensure laminates. the parallelism It was designed of the lamina laminate to conste trainsurfaces all the with relative the horn in-plane and the movements anvil ends. but, The at anvilthe same end time,was built ensure to resemble the parallelism a freely of rotatingthe lamina semi-sphere te surfaces which with the was horn cut andat one the side anvil to ends.form Thethe contactanvil end surface.surface. was built TheThe parallelism toparallelism resemble of aof thefreely the anvil anvilrotating and and the semi-sphere horn-endthe horn-end faces which faces played was played a cut crucial at a one crucial role side in theroleto form uniform in thethe uniformdistributioncontact surface.distribution of the The vibration ofparallelism the energy,vibration of thus the energy, avoidinganvil thus and the avoidingthe melting horn-end the of themelting faces laminate played of the surfaces laminatea crucial at their surfacesrole contact in theat theirwithuniform thecontact horndistribution with and the anvil. hornof the and vibration anvil. energy, thus avoiding the melting of the laminate surfaces at their contact with the horn and anvil.

Figure 6. The tool for positioning and fixing ﬁxing the lamina laminatestes during the DUS welding (installed on the ultrasonicFigure 6. Thewelder). tool for positioning and fixing the laminates during the DUS welding (installed on the ultrasonic welder). The specimens for the experimental studies were cut from 960 mm × 800 mm × 2.1 mm thermoplasticThe specimens laminate for sheetsthe experimental made form studiesa 12 K were 2 × cut2 twill from carbon 960 mm woven × 800 fabric-reinforced mm × 2.1 mm thermoplastic.thermoplastic Thelaminate matrix sheets was a madehigh-fluidity form a Poly12 amideK 2 × 6.62 twill(Nylon carbon 6.6). Basedwoven on fabric-reinforced the differential thermoplastic. The matrix was a high-fluidity Polyamide 6.6 (Nylon 6.6). Based on the differential

J. Manuf. Mater. Process. 2019, 3, 1 8 of 17

J. Manuf.The Mater. specimens Process. 2018 for, 2, the x FOR experimental PEER REVIEW studies were cut from 960 mm × 800 mm × 2.1 8 mmof 17 thermoplastic laminate sheets made form a 12 K 2 × 2 twill carbon woven fabric-reinforced thermoplastic.scanning calorimetry The matrix test, the was recorded a high-fluidity glass transition Polyamide temperature 6.6 (Nylon of 6.6). the composite Based on thewas differential Tg = 69 °C, ◦ scanningthe melt onset calorimetry temperature test, the was recorded 243 °C, glass and the transition melt point temperature was Tm = of259 the °C. composite The thermal was decompositionTg = 69 C, the ◦ ◦ meltinitiation onset temperature temperature was was taken 243 fromC, and literature the melt as T pointdi = 358 was °C accordingTm = 259 toC. Reference The thermal [21]. decomposition The laminates ◦ initiationwere consolidated temperature with was the takenhot-plate from press literature approach. as T diThe= 358finalC thickness according of toh = Reference 2.1 ± 0.05 [21mm]. Thewas laminatesresulted by were stacking consolidated seven prepregs with the in hot-plate the cross-ply press sequence approach. with The a final nominal thickness fiber ofvolumeh = 2.1 fraction± 0.05 mmof 50%. was The resulted specimens by stacking were dried seven in prepregsan oven at in 75 the °C cross-ply for a minimum sequence duration with a of nominal 96 h before fiber welding volume fractionto ensure of that 50%. the The laminates specimens were were free dried from in excessive an oven atwater 75 ◦ Ccontent. for a minimum duration of 96 h before weldingFor tothe ensure temperature that the measurement, laminates were the freethermocouples from excessive were water prepared content. by twisting and arc-welding the endsFor theof a temperature fine gauge measurement,40 (ϕ = 0.08 mm) the thermocouplesK-type thermocouple were prepared wire pair. by The twisting thermocouples and arc-welding were thecarefully ends placed of a fine in gaugethe center 40 ( φof= the 0.08 laminate mm) K-type pairs at thermocouple the weld interface wire (see pair. Figures The thermocouples 5 and 7); then, werethey carefullywere incorporated placed in theinto center the matrix of the by laminate applying pairs a atsh theort weldpulse interface(0.2 s) of (see ultr Figuresasonic 5vibration and7); then, with theyan adequate were incorporated static force into on it. the This matrix approach by applying ensured a short that pulsethe sensing (0.2 s) ofend ultrasonic of the thermocouple vibration with was an adequatefixed in the static desired force onlocation; it. This later, approach during ensured welding, that thethe sensingstrain concentration end of the thermocouple at the matrix was in fixed the indirect the desiredvicinity location; of thermocouple later, during was welding, significantly the strain reduced. concentration It is a well-known at the matrix challenge in the direct to measure vicinity ofthe thermocouple temperature was in significantlythe weld spot reduced. during It is aultrasonic well-known welding. challenge During to measure the initial the temperature phase, the in themeasured weld spot temperature during ultrasonic might welding.be higher During than theexpected. initial phase,This might the measured be due temperatureto high local might strain be higherconcentration than expected. around Thisthe mightthermocouple be due to sensing high local end. strain The concentration K-type thermocouple around the may thermocouple also give sensingmisleading end. measurements The K-type thermocouple under static maypressure also if give no misleadingappropriate measurements filters are applied. under Nevertheless, static pressure in ifliterature, no appropriate the use filters of thermocouples are applied. Nevertheless,to analyze and in monitor literature, the the ultrasonic use of thermocouples welding process to analyzeis often andencountered monitor [9,10,19]. the ultrasonic welding process is often encountered [9,10,19].

Figure 7.7.Thermocouple Thermocouple placement placement on theon laminate the laminate and a magniﬁed and a magnified photo of the photo K-type of thermocouple the K-type sensingthermocouple end. sensing end.

2.5. Experiment Plan The experiments were divided into two groups. In the firstfirst group (UC-x), the influenceinfluence of the weld duration t was monitored under constant parameters and without an active weld control. In weld duration tww was monitored under constant parameters and without an active weld control. In thethe secondsecond groupgroup (C-x),(C-x), the weld process control was turnedturned on andand thethe weldweld durationduration was set asas constant at t = 2.5 s. The weld control system was bypassed for the first group by setting the dP constant at tww = 2.5 s. The weld control system was bypassed for the first group by setting the dPsetset values veryvery highhigh (considered(considered asas inf.).inf.). This high value during the welding was not exceeded, and the amplitude remained constant throughout the entire weldweld duration.duration. In the C-x cases, different values and scenarios for the dP were investigated; in the first three cases, the second and third if-loops and scenarios for the dPsetset were investigated; in the first three cases, the second and third if-loops were ignored by setting the dP very high and, for the fourth and the fifth cases, the second and third were ignored by setting the dPsetset very high and, for the fourth and the fifth cases, the second and third if-loopsif-loops werewere takentaken intointo consideration.consideration. The relevant process control parameters which were used for thethe experimentalexperimental studiesstudies areare givengiven inin TableTable1 1..

J. Manuf. Mater. Process. 2019, 3, 1 9 of 17

Table 1. Weld parameters used for the experimental analysis of the uncontrolled and the controlled ultrasonic spot welds (the represented parameters are taken from the control algorithm in Figure4).

Weld Start Amplitude Weld Duration dP dP dP set1 set2 set3 a (%) b (%) c (%) Case d (%) tw (sec) (W/s) (W/s) (W/s) UC-1 80 1 Inf. Inf. Inf. 0 0 0 UC-2 80 1.8 Inf. Inf. Inf. 0 0 0 UC-3 80 2.5 Inf. Inf. Inf. 0 0 0 C-1 80 2.5 100 Inf. Inf. 1 0 0 C-2 80 2.5 200 Inf. Inf. 1 0 0 C-3 80 2.5 300 Inf. Inf. 1 0 0 C-4 80 2.5 100 300 500 1 2 3 C-5 80 2.5 200 600 4000 1 2 3

The additional relevant process variables, which were set constant throughout the investigations, were as follows: the weld press average static force, 650 N; the static force at the solidification phase, 600 N; the solidification time, 2 s. Several repetitions for each weld case (7–10 repetitions) were done to gather statistical data. The time traces of the temperature and the power were plotted for the UC-3 case, in order to discuss and illustrate the behavior of the temperature development in relation to the power curve during the DUS weld process. The results of the temperature measurement in the weld spots of the controlled welds were analyzed to identify the influence of the control system on the temperature development vs. time at the center of the weld spot. A reference weld spot was chosen from each case and was scanned through the computed tomography method. The scans were done in the laboratories of the Institute of Lightweight Engineering and Polymer Technology (ILK) at Technische Universität Dresden, using an in situ FICT 160; the X-Ray microfocus was 80 kV at 80 µA. The scans were done with a resolution of 12 µm with a duration of 15 min/scan. The computed tomography (CT) scans delivered insight into the quality of the weld spots. The quality was determined by the presence or the absence of the irregularities caused by the overheating of the matrix.

3. Results and Discussions

3.1. The Power and Temperature Curves In Figure8, typical temperature and power curves for an uncontrolled weld are plotted against time. The curves are the result of the UC-3 with a weld duration of tw = 2.5 s. The dashed line is the time trace of the temperature measured in the weld spot for one of the repetitions, and the solid line is the corresponding consumed weld power curve. The gray shaded areas around the curves represent the ranges covered by the time traces of temperature, while the hatched area represents the range of the power covered by the repetitions under similar conditions. In most of the observed cases, as the oscillations were applied, the power rose rapidly and reached a peak value at ta = 0.1 s. At this stage, the temperature increased rapidly; then, it stabilized ◦ around 200 C. At a certain instance of time (ts = 1.3 s), as soon as the interfacial layer of the welded spot initiated to melt, the entire applied vibrational strain energy focused there. Consequently, the temperature only in this thin local zone underwent a rapid increase, while the remaining layers away from the interface in the thickness direction remained relatively cold and solid. From this point on, the overall energy dissipation was higher than the total energy consumption in the previous stages, and this was observed in the rise of the power curve between ts = 1.3 s and td = 1.58 s. The increase in ◦ temperature above Tdi = 358 C led to the gradual thermal decomposition of the matrix. The stage between the decomposition td and the temperature rise initiation ts is referred to as the second jump. The decomposition process and the heat transferred by the escaping hot gases resulted in a thermal equilibrium, and the temperature remained almost stable in a range around the mean value of 400 ◦C. J. Manuf. Mater. Process. 20192018, 32, 1x FOR PEER REVIEW 10 of 17

Figure 8. The power and temperature curves of thethe uncontrolleduncontrolled weldswelds casecase UC-3.UC-3. Illustrating the characteristic times and temperaturestemperatures during the DUS welding.welding.

This behaviorbehavior of of the the temperature temperature and theand power the power curves werecurves observed were observed for most of for the most uncontrolled of the welds.uncontrolled However, welds. the However, onset time the for onset the second time for jump the ( tseconds) in the jump curves (ts) deviated in the curves strongly deviated from one strongly weld fromto another. one weld It can to beanother. observed It can in Figurebe observed8 that thein Figure typical 8 temperature that the typical curve temperature (dashed line) curve underwent (dashed line)the second underwent jump the at the second onset jump time ofat aboutthe onsetts = time 1.34 sof and about reached ts = 1.34 the s decompositionand reached the temperature decomposition at td temperature= 1.58 s. Meanwhile, at td = 1.58 the s. earliest Meanwhile, time for the the earliest onset of time the temperaturefor the onset jump of the was temperature recorded at jumpts = 0.75 was s recordedand the latest at ts time= 0.75 was s andts = the 2.1 latest s. A similar time was behavior ts = 2.1 for s. theA similar power curvebehavior was for observed the power in almost curve allwas of observedtheJ. Manuf. recorded Mater. in almost Process. cases. 2018 Theall of, 2 power ,the x FOR recorded curvePEER REVIEW jump cases. onset The precededpower curve the jump temperature onset preceded jump onset. the temperature The onset 11 of of17 jumpthe power onset. jump The onset can be of detectedthe power by jump its positive can be detected derivative by atits the positive mark-line derivative (1) in at Figure the mark-line9. (1) in Figure 9. One may notice that the attempts to reach an optimum weld duration with statistical approaches may not be practical. The deviations of ts from the mean value of an optimum weld case were quite large. The ts was influenced unpredictably by many uncontrollable factors such as local matrix concentration, surface imperfections, and ultrasonic signal interferences. The suggestion of Benatar and Gutowski [5] to stop the vibrations when the power curve reaches a peak might work well for some ultrasonic weld configurations. However, in the differential spot welding, as can be inferred from Figure 9, when the power curves reached the peak value (mark-line (3)), the temperature had already passed the thermal decomposition temperature of the matrix and the weld suffered. To overcome these limitations, it is possible to deduce from the above-discussed phenomena that the power curve exhibits a positive slope just before the initiation of the second temperature jump. The weld power is most sensitive as seen in Equation (7) to the vibration amplitude A0. It is assumed that, by detecting the second power jump initiation and reacting to it by actively reducing the vibration displacement amplitude, it is possible to control the temperature at the weld spot within an optimum range and for a sufficient duration to allow the weld spot to fully diffuse without overheating. Figure 9.9. The time traces of the power, temperature,temperature, andand thethe powerpower derivativederivative for the uncontrolleduncontrolled weld casecase C-3-3.C-3-3. TheThe initiationinitiation of of the the second second jump jump of of the the power power and and the the temperature temperature are are marked marked as the as bluethe blue (1) and(1) and the greenthe green (2)lines (2) lines respectively. respectively.

OneThe traditional may notice PID that (proportional–integral–derivativ the attempts to reach an optimume) weldcontrol duration method with was statisticalnot an option approaches for this mayapplication. not be The practical. PID needs The a deviations certain value of t soffrom the weld the meanpower value to regulate of an optimumaround it and, weld in case the wereDUS quitewelding, large. there The doests was not inﬂuenced exist a known unpredictably power value by many which uncontrollable can be taken factors as a set such point as localfor the matrix PID concentration,controller. For example, surface imperfections, if a minimum and safe ultrasonic value for power signal interferences.is set, and the weld The suggestion spot coincides of Benatar with a andhigh Gutowskimatrix content [5] to stoplocation the vibrationsof the laminate, when thethen power the amplitude curve reaches would a peak be adjusted might work lower well than for needed and the weld will never reach the necessary melting temperature for the diffusion to occur.

J. Manuf. Mater. Process. 2019, 3, 1 11 of 17 some ultrasonic weld conﬁgurations. However, in the differential spot welding, as can be inferred from Figure9, when the power curves reached the peak value (mark-line (3)), the temperature had already passed the thermal decomposition temperature of the matrix and the weld suffered. To overcome these limitations, it is possible to deduce from the above-discussed phenomena that the power curve exhibits a positive slope just before the initiation of the second temperature jump. The weld power is most sensitive as seen in Equation (7) to the vibration amplitude A0. It is assumed that, by detecting the second power jump initiation and reacting to it by actively reducing the vibration displacement amplitude, it is possible to control the temperature at the weld spot within an optimum range and for a sufﬁcient duration to allow the weld spot to fully diffuse without overheating. The traditional PID (proportional–integral–derivative) control method was not an option for this application. The PID needs a certain value of the weld power to regulate around it and, in the DUS welding, there does not exist a known power value which can be taken as a set point for the PID controller. For example, if a minimum safe value for power is set, and the weld spot coincides with a high matrix content location of the laminate, then the amplitude would be adjusted lower than needed and the weld will never reach the necessary melting temperature for the diffusion to occur.

3.2. The Controlled DUS Welding Process The used controller detected the second jump of the temperature through monitoring sudden increases in the power time derivative above a preset value (the mark-line (2) in Figure9). If the condition was true, then the amplitude was reduced with a pre-set percentage as described in the Section2. The effectiveness of this control method lies in the fact that it detects the initiation of the weld spot melting and reacts to it by reducing the intensity of the viscoelastic heating. If the drop of the amplitude is relatively small, it takes several loops of amplitude reduction until the power curve stabilizes. This phenomenon gives time for the temperature to flow and distribute with a less sharp gradient in the thickness direction than in the initial phase. However, if the weld duration is prolonged, the inner layers will get hot enough to squeeze-flow under the static weld force into the interface and away from the weld spot, which in turn causes a second kind of weld spot degradation. The experiments showed that, in most of the controlled welds, the measured temperature in the weld center remained below the decomposition temperature. The time traces of the temperature measured in the weld spots are plotted in Figure 10a–e for the five investigated controlled cases (C-1~5) respectively. The shaded grey areas in Figure 10 represent the temperature spectrum which resulted from the repetitions in each controlled case. The broken line is the time trace of a representative temperature curve. Parallel to each temperature curve the adjusted vibration displacement amplitude percentage is plotted. For the C-1 welds with dPset = 100 W/s, the measured temperature in some of the weld spot remained below 200 ◦C throughout the weld duration and the diffusion of the spot was not complete. An exception in one of the welds of the C-1 case was observed; the temperature in the weld spot continued increasing after the amplitude reduction occurred and it exceeded the Tdi limit. The problem of insufficient heat energy was not observed in the C-2 case where dPset = 200 W/s. However, for some of the repetitions, the temperature in the weld center and for the last 0.5 s of the weld duration exceeded the Tdi limit. The observed welds of the C-3 remained well within the optimum temperature zone; the temperature of the welds remained almost above the melting and mostly below the decomposition temperatures with the highest temperature measured at 308 ◦C. J. Manuf. Mater. Process. 2018, 2, x FOR PEER REVIEW 12 of 17 the optimum temperature zone; the temperature of the welds remained almost above the melting and mostly below the decomposition temperatures with the highest temperature measured at 308 °C. For the controlled welds with the three if-loops (C-4 and C-5), a significant improvement in the process stability was observed. The system reacted with a higher level of flexibility to the power curve changes, and this was reflected in the temperature curves of the weld spots. The C-4 case had a low dPset1 and, as a consequence, the amplitude drop occurred in the early stage of the weld process; hence, the maximum measured temperature was around 304 °C. In some weld spots, the temperature remained throughout the weld duration just below the melting point of the matrix, such as the highlighted broken line in C-4. By increasing the power derivative limits for all three levels, such as in C-5, the reaction to the temperature second jump was delayed. Thus, in some of the repetitions of the C-5 case, it was observed that the temperature increased in the early stages of the weld to almost 340 °C; then, the control system reacted and the temperature dropped gradually and stabilized at J.around Manuf. Mater.300 Process.°C. Nevertheless,2019, 3, 1 in both of the tested cases, the temperature remained below12 ofthe 17 decomposition initiation temperature.

(a) (b)

(e)

Figure 10. The time traces of the temperatures measured in the center of the weld spot (dotted lines) and the corresponding adjusted amplitude for the controlled weld cases and the shaded areas represent the temperature spectrum from the repetitions. The results of the each control parameter case are plotted in (a) C-1, (b) C-2, (c) C-3, (d) C-4, (e) C-5.

For the controlled welds with the three if-loops (C-4 and C-5), a significant improvement in the process stability was observed. The system reacted with a higher level of flexibility to the power curve changes, and this was reflected in the temperature curves of the weld spots. The C-4 case had a low dPset1 and, as a consequence, the amplitude drop occurred in the early stage of the weld process; hence, the maximum measured temperature was around 304 ◦C. In some weld spots, the temperature remained throughout the weld duration just below the melting point of the matrix, such as the highlighted broken line in C-4. By increasing the power derivative limits for all three levels, such as in C-5, the reaction to the temperature second jump was delayed. Thus, in some of the repetitions J. Manuf. Mater. Process. 2019, 3, 1 13 of 17 of the C-5 case, it was observed that the temperature increased in the early stages of the weld to almost 340 ◦C; then, the control system reacted and the temperature dropped gradually and stabilized at around 300 ◦C. Nevertheless, in both of the tested cases, the temperature remained below the decomposition initiation temperature. Example power curves from each of the controlled weld cases are plotted vs. time in Figure 11. J. Manuf. Mater. Process. 2018, 2, x FOR PEER REVIEW 13 of 17 It can be seen here that the power curves did not undergo the second jump; instead, they were almost flat at the last second of the welding duration. This is because, whenever the weld power Figure 10. The time traces of the temperatures measured in the center of the weld spot (dotted lines) initiated to increase, the controller reacted to it by reducing the weld vibration displacement amplitude. and the corresponding adjusted amplitude for the controlled weld cases and the shaded areas Consequently,represent the less temperature ultrasonic spectrum vibration from energy the repeti wastions. input The to the results weld of spotthe each and control the heating parameter rate was reduced.case are The plotted elimination in (a) C-1, of (b) the C-2, second (c) C-3, power (d) C-4, jump (e) C-5. eventually led to the elimination of the second temperature jump in the weld spot center.

C-1 1000 C-2 C-3 800 C-4 C-5 600

400 Power (W)

200

0 0.00 0.50 1.00 1.50 2.00 2.50 time (s) Figure 11. The time traces of the weld power for the controlled weld cases (the plotted curves are for the weld spots whose temperature curvescurves areare plottedplotted asas thethe dotteddotted lineline inin FigureFigure 1010).).

ExampleThe investigations power curves prove from that each the of the control controlled method weld can cases be are useful plotted to eliminate vs. time in the Figure overheating 11. It can problembe seen here during that the the ultrasonic power curves spot did welding not undergo of the composite the second laminates. jump; instead, If the controlthey were system almost is well flat atcalibrated, the last second the temperature of the welding of the weldduration. spot centerThis is might because, be controlled whenever to the remain weld within power the initiated adequate to weldingincrease, temperature.the controller reacted to it by reducing the weld vibration displacement amplitude. Consequently, less ultrasonic vibration energy was input to the weld spot and the heating rate was 3.3. The Computed Tomography Analysis reduced. The elimination of the second power jump eventually led to the elimination of the second temperatureIn order jump to determine, in the weld on onespot hand, center. the impact of the overheating on the weld spots and, on the otherThe hand, investigations the effect of theprove weld that control the control on the weldmethod spot can quality, be useful CT scans to eliminate were done the of overheating the volume problemaround the during weld the spots. ultrasonic The temperature spot welding measurements of the composite were laminates. restricted toIf the control weld center; system thus, is well the calibrated,CT scans may the givetemperature insight into of the the weld state spot of the center weld might spot atbe locationscontrolled away to remain from thewithin thermocouple. the adequate It waswelding observed temperature. from the section images produced by the scans that the uncontrolled welds which were stopped before the second jump (tw < ts) were not fully formed and had a small diameter. An example 3.3.of this The is Computed the UC-1 Tomography weld case. TheAnalysis welds which were stopped far after the second jump, such as in the UC-3In case, order had to noticeabledetermine, defectson one hand, in the the volume impact at theof the direct overheating vicinity ofonthe the weldweld spot.spots Theand, defects on the wereother mosthand, intensive the effect inof thethe weld spotcontrol at theon the weld weld plane. spot Thequality, observed CT scans defects were were done in of the the shape volume of aroundcavities the in the weld composite, spots. The which temperature in turn were measurements left behind were by the restricted thermally to decomposedthe weld center; and thus, evaporated the CT scansmatrix. may In onegive ofinsight the CT into scans the ofstate a weld, of the which weld spot by chance at locations was stopped away from just the shortly thermocouple. after the second It was observedjump (tw ≈fromts), athe uniformly section images diffused produced weld spot by was the observed.scans that Thethe imagesuncontrolled from thewelds CT which scans atwere the weld plane and at the cross-section through the weld spot for the uncontrolled welds are given in stopped before the second jump (tw < ts) were not fully formed and had a small diameter. An example ofFigure this is12 the; the UC-1 cavities weld are case. marked The welds with arrowswhich were in the stopped CT scan far images after ofthe UC-3 second (tw jump,> ts). such as in the UC-3 case, had noticeable defects in the volume at the direct vicinity of the weld spot. The defects were most intensive in the weld spot at the weld plane. The observed defects were in the shape of cavities in the composite, which in turn were left behind by the thermally decomposed and evaporated matrix. In one of the CT scans of a weld, which by chance was stopped just shortly after the second jump (tw ≈ ts), a uniformly diffused weld spot was observed. The images from the CT scans at the weld plane and at the cross-section through the weld spot for the uncontrolled welds are given in Figure 12; the cavities are marked with arrows in the CT scan images of UC-3 (tw > ts).

J. Manuf. Mater. Process. 2018, 2, x FOR PEER REVIEW 14 of 17 J.J. Manuf. Manuf. Mater. Mater. Process. Process. 20182019,, 23,, x 1 FOR PEER REVIEW 1414 of of 17 17

Figure 12. SectionSection imagesimagesimagesfrom fromfrom the thethe computed computedcomputed tomography tomographytomography (CT) (CT)(CT) scans scansscans of threeofof threethree uncontrolled uncontrolleduncontrolled welds. welds.welds. The Thetop sectionstop sections are atare the at weldthe weld plane plane (B–B) (B–B) and and the bottomthe bottom sections sections are acrossare across the weldthe weld spot spot (C–C). (C–C). The Theschematic schematic drawings drawings on the on leftthe illustrateleft illustrate the location the location of the of sections. the sections.

The time time traces traces of the of temperatures the temperatures measured measured in the controlled in the controlled weld spots, weld which spots, were CT-scanned, which were areCT-scanned, plotted as arethe plottedbroken aslines the at broken each equivalent lines at each case equivalentin Figure 10. case The in images Figure of10 two. The sections images of of a spottwo sectionsweld from of aeach spot controlled weld from weld each case controlled are plo weldttedtted caseinin FigureFigure are plotted 13.13. TheThe in CTCT Figure scansscans 13 .ofof The mostmost CT ofof scans thethe controlledof most of thewelds controlled revealed welds uniformly revealed diffused uniformly weld diffused spots with weld almost spots withno cavities almost noand cavities negligible and defects.negligible From defects. the CT From scans, the no CT significant scans, no signiﬁcant differences differences between the between weld spots the weld done spots with done different with controldifferent parameters control parameters (C-2 to C-5) (C-2 were to C-5) observed. were observed. Only the Only CT thescan CT of scan the ofweld the spot weld for spot the for C-1 the case C-1 showedcase showed wide widespread spread delamination delamination and cavities and cavities at thethe at weldweld the weld spot.spot. spot.TheThe scannedscanned The scanned weldweld weldofof thethe of C-1C-1 the casecase C-1 hadcase a had temperature a temperature which which exceeded exceeded the T thedidi ofofT thethedi of matrixmatrix the matrix duedue toto due inappropriateinappropriate to inappropriate controlcontrol control parameters.parameters. parameters.

Figure 13. TheThe imagesimagesimages from fromfrom the thethe CT CTCT scans scansscans of ofof ﬁve fivefive controlled controlledcontrolled welds. welds.welds. The TheThe top imagestoptop imagesimages are at areare the atat weld thethe planeweldweld plane(B–B) and(B–B) the bottomand the ones bottom are at aones cross-section are at througha cross-section the weld spotthrough (C–C) (forthe theweld section spot locations, (C–C) (for(forrefer thethe to section Figuresection 12 locations,locations,). referrefer toto FigureFigure 12).12).

To further support the observations done using th thee CT scans, some of the welds were separated and the fracture surfaces were analyzedanalyzed underunder aa stereo-microscopestereo-microscope withwith aa magnificationmagnification ofof 22×.×. The microscopic images images of of fracture fracture surfaces surfaces from from each each controlled controlled weld weld case case (C-x) (C-x) are are given given in in Figure Figure 14.14 . Traces ofof fiber fiber pull-up pull-up can can be seenbe seen in the in fracture the fracture surfaces, surfaces,surfaces, which is whichwhich in turn isis an inin indication turnturn anan of indicationindication the cohesive ofof thethefailure cohesivecohesive mode offailurefailure the weld modemode spot. ofof thethe However, weldweld spot.spot. it was HoweveHoweve difficultr, toit tracewas anddifficult identify to trace the cavities and identify caused the by cavitiesoverheating caused and by thermal overheating decomposition and thermal of the decomposition thermoplastic matrixof the fromthermoplastic the microscopic matrix images.from the microscopic images.

J.J. Manuf. Manuf. Mater. Mater. Process. Process. 20182019,, 23,, 1xx FORFOR PEERPEER REVIEWREVIEW 15 15 of of 17 17

Figure 14. Microscopic images of the fracture surfaces (at the weld plane B–B) of the controlled weld spots. An exemplary fracturefracture surfacesurface waswas chosenchosen from from each each controlled controlled weld weld case case (images (images taken taken with with a aresolution resolution of of 1024 1024× ×768 768 pixels pixels using using a a stereo-microscope stereo-microscope with with a a magniﬁcation magnification of of 2 ×2×).).

It isis suspectedsuspected thatthat thethe presencepresence ofof thethe thermocouplethermocouple may interfere with thethe ultrasonicultrasonic spot welding process. The The presence presence of of the the thermocouple in in thethe weldweld spotspot may,may, in in some some cases, cases, act act as as an energy director, andand thethe meltmelt mightmight initiateinitiate there.there. Therefore,Therefore, ultrasonicultrasonic spot weldswelds werewere performedperformed without thermocouples and under similar conditions ofof the C-5 weld case. Then, the volume around thethe weldweld spotspot spot waswas was scannedscanned scanned usingusing using CT.CT. CT. TwoTwo Two cross-sectcross-sect cross-sectionionion imagesimages images fromfrom from thethe the CTCT CT scanscan scan areare are illustratedillustrated illustrated inin Figurein Figure 15, 15one, one at the at theweld weld plane plane (B–B) (B–B) and the and second the second at a cross-section at a cross-section through through the weld the spot weld (C–C). spot (C–C).The images The images show showa uniform a uniform diffused diffused weld weld spot spot withwith with nono no tracestraces traces ofof of overheatingoverheating overheating andand and thermalthermal decomposition. The The image at the C–C cross-section shows some minor cavities away from the weld interfaceinterface (marked(marked byby white white arrows). arrows). It It was was unclear unclear at at thatthat stagestage ifif thesethese smallsmall cavitiescavities werewere causedcaused byby thethe welding welding processprocess oror werewere defectsdefectsdefects ininin thethethe laminateslaminateslaminates dueduedue tototo production.production.production.

Figure 15. CT scan of a controlled weld spot without the presence of the thermocouple. The weld was Figure 15. CT scan of a controlled weld spot without the presence of the thermocouple. The weld was Figuredone under 15. CT similar scan of conditions a controlled and weld with spot parameters without asthe the presence C-5 case. of the thermocouple. The weld was done under similar conditions and with parameters as the C-5 case.

J. Manuf. Mater. Process. 2019, 3, 1 16 of 17

It was observed from both the temperature measurements and the CT scan analysis that the control method might be a useful tool to eliminate or reduce the overheating and thermal decomposition of the thermoplastic matrix at the weld spot. The different investigated parameter sets show that the dPset value has a signiﬁcant impact on the degree of success of the weld control system. The investigation proves that the control system is not inﬂuenced strongly by the presence of the thermocouple in the weld spot.

4. Conclusions The influence of the new developed logical control method for the differential ultrasonic welding of fiber-reinforced thermoplastic laminates on welding process stability was investigated. The investigations were done by monitoring the time traces of the temperature in the weld spot and were verified through computed tomography scans of the weld spots. Three uncontrolled weld sets and five controlled weld sets were observed with several repetitions for each case. The three uncontrolled welds were done with different weld durations, which were 1 s, 2 s, and 2.5 s. The controlled welds, however, were carried out using a constant weld duration of 2.5 s, yet with varying weld control criteria. The suggested control system relied on the power time derivative of the welder to detect the initiation of the melting of the weld spot, and reacted to it by reducing the ultrasonic vibration amplitude to ensure a uniform diffusion temperature in the weld spot. Through the investigated cases, it was observed that a complete elimination of the temperature jump up to the decomposition temperature was possible by implementing the logical control system. The welder-power time derivative may be used to indirectly detect the melting of the matrix and control the temperature in the ultrasonically welded spots of fiber-reinforced thermoplastic laminates. The control parameter dPset was found to have a high influence on the weld temperature. Controlling with small dP/dt limit values caused a rapid drop in the ultrasonic amplitude and resulted in an insufficient heat energy generation for the matrix to reach the melting temperature. Increasing the dPset or using three if-loops with three levels of dPset such as the C-5 case led the temperature of the weld to remain within the optimum boundaries for all the recorded cases. The CT scans of the volume around the weld spots delivered a better understanding of the influence of the temperature on the weld spot. The CT scans revealed a presence of cavities in the weld spots, in which the temperature reached or exceeded the thermal decomposition temperature of the matrix. However, the CT scans of the controlled weld spots showed mostly uniformly diffused and defect-free weld spots. It is essential to investigate the influence of the logical weld control system on the weld strength and the process stability in comparison to the uncontrolled DUS welding process. The tests are to be repeated for DUS welds without the thermocouple, whose presence might influence the weld spot formation. Eventually, a more sophisticated control system needs to be developed.

Author Contributions: Conceptualization, S.T. and O.E. Methodology, investigation, and writing—original draft preparation, S.T. Writing—review and editing, M.D. Supervision, N.M. Project administration, F.F. Funding acquisition, O.E. Funding: This research received no external funding. Acknowledgments: Special thanks to Robert Carl, Kerem Ucar, Florian Buchner, and Albert Fellermayer for their contributions to the research work and their support in conducting the numerous time-intensive measurements. Thanks to Thomas Forstner for his contribution to the method development. Thanks to Hans Julius Langeheinecke for his support and contribution to the reviewing of the written work. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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