polymers

Review Thermal by the Third Phase Between Polymers: A Review for Ultrasonic Weld Technology Developments

Jianhui Qiu 1,*, Guohong Zhang 1 , Eiichi Sakai 1, Wendi Liu 2 and Limin Zang 3 1 Department of Mechanical Engineering, Faculty of Systems Science and Technology, Akita Prefectural University, Akita 015-0055, Japan; [email protected] (G.Z.); [email protected] (E.S.) 2 College of Transportation and Civil Engineering, Fujian Agriculture and Forestry University, Fuzhou 350108, China; [email protected] 3 MOE Key Laboratory of New Processing Technology for Nonferrous Metal & Materials, College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China; [email protected] * Correspondence: [email protected]

 Received: 30 December 2019; Accepted: 8 March 2020; Published: 31 March 2020 

Abstract: (USW) is a promising method for the welds between dissimilar materials. Ultrasonic thermal welding by the third phase (TWTP) method was proposed in combination with the formation of a third phase, which was confirmed as an effective technology for polymer welding between the two dissimilar materials compared with the traditional USW. This review focused on the advances of applying the ultrasonic TWTP for materials. The research development on the ultrasonic TWTP of (PC) and polymethyl methacrylate (PMMA), (PLA) and polyformaldehyde (POM), and PLA and PMMA are summarized according to the preparation of the third phase, welded strength, morphologies of rupture surfaces, thermal stability, and others. The review aimed at providing guidance for using ultrasonic TWTP in polymers and a basic understanding of the welding mechanism, i.e., interdiffusion and molecular motion mechanisms between the phases.

Keywords: thermal welding by the third phase (TWTP); ultrasonic welding (USW); welding between dissimilar materials; thermoplastic welding technology

1. Introduction Thermoplastic materials have drawn much attention as alternatives to thermoset materials in many fields due to their advantages such as low cost and effective manufacturing [1]. After being damaged, and their composites can join together based hot melt adhesives in the weld lines under pressure, without the degradation of material properties [2]. USW can overcome the problems such as stress concentrations and delamination due to hole drilling in mechanical fastening, and toxicity, surface modification, and environmental damage in adhesive bonding. Moreover, it can solve the main point relating to the cost of productions. Additionally, there are some techniques that are considered for welding thermoplastics and their composites, i.e., resistance, induction, ultrasonic welding [3], laser welding, and others. Ultrasonic molding has emerged as a promising replication technology for low and medium volume production of micro-scale parts recently [4]. During a relatively long-term application, it can produce a wide variety of polymeric materials without any thermal decomposition [5] and with significantly reduced energy consumption [6]. Furthermore, metal products that are employed in aerospace, aircrafts, automotive, and microelectronics industries are increasingly replaced by thermoplastic products [7–12].

Polymers 2020, 12, 759; doi:10.3390/polym12040759 www.mdpi.com/journal/polymers Polymers 2020, 12, 759 2 of 26

The use of polymer-based parts in automotive could reduce their weight, improve their performance characteristics, and reduce carbon emissions into the environments [13]. Many adhesives and adhesion technologies [14–19] are available due to the diversity of used materials, but the toxicity problem during welding remains. This accelerating trend requires a new reliable technology to weld thermoplastics, especially for welding dissimilar materials [20–38] in industry.

1.1. Techniques for Welding Thermoplastics Thermoplastics are employed in a wide range of applications due to their reduced weight, superior performance characteristics, relaxing stress concentration, and reprocessibility. Usually, it is necessary to use a welding technique for the preparation of structurally complex products, although polymer has excellent processability, and thus a high degree of free shape [39]. According to the physical principles of joining, joints in can be classified as follows: one may distinguish betweemn joints held together by force, shape, or material. The latter category (owing to the rapid development of welding technologies) is used more frequently in several industrial applications. Welding technology is not only necessary for the production, but also for the repair and recycling. In order to prolong the service life of the materials and joints, the waste materials should be impacted and discussed again. Obviously, the key problems are whether the products manufactured or repaired with this technology retain the original properties of the raw materials and how it is possible to optimize the quality of the welded joint [40,41]. Using welding to achieve joints, it is important to produce the strongest cohesion-based joints. The necessary preconditions of welding reveal that a rheological adequate state is necessary to achieve a good quality weld [42]. Joining thermoplastic technologies can be divided into three methods, namely [43]: mechanical fastening, adhesive bonding, and thermal welding. Mechanical fastening methods include clipping, clamping, screwing, and riveting. Self-piercing rivets and clinch joints have been identified as two types of fasteners with considerably potential use in automotive bodies [44]. Work pieces (WPs) are mechanically fastened together at room temperature in each process. There may be no requirement for the predrilling of holes in the WPs under both cases. The advantages and disadvantages of mechanical fastening are summarized in Table1[45,46].

Table 1. Advantages and disadvantages of mechanical fastening.

Advantages Disadvantages Reopen ability of WPs Augmented stress concentration Convenient technology and machinery Loosening of WPs due to creep or stress relaxation Controllable volume capability Crazing and cracking Joint of dissimilar materials Enclosure limitation Assurance of structural integrity by well-known residual stresses due to the differences between WPs prediction methods and analysis Ease of joint inspection Loss of properties due to moisture Need to access both sides of the part, increased Little surface preparation and cleaning is required number of process steps Repair or replacement of pieces is facilitated Weight penalty due to thicker sections and fasteners

Adhesive bonding is another way for widely used, which is possible to join dissimilar and incompatible materials [47,48]. It has been used car manufacturers in concept cars and low volume niche products [49–52], e.g., Ford’s AIV, Rover’s ECV3, a limited extent in Honda’s NSX [53] and Mitsubishi. However, no matter how high-performance adhesives such as epoxy or solvent-based adhesive systems are, considerable environmental concerns are remaining. The health and safety hazards require significant costs to provide adequate recycling. Given current environmental concerns, there is the possibility that these substances may be banned in the future. Nevertheless, using polymeric panels to replace the metal ones as an integral part of the injection molding process might not be the best way [54–57]. Furthermore, releasing harmful byproducts is also an important problem for adhesive Polymers 2020, 12, 759 3 of 26 bonding of thermoplastics [58]. The in vivo degradation of medical adhesives has not been clearly understood, and degradation byproducts such as formaldehyde have been shown to be potentially toxic to cells [59]. The advantages and disadvantages of adhesive bonding are shown in Table2[ 60,61].

Table 2. Advantages and disadvantages of adhesive bonding.

Advantages Disadvantages Dissimilar materials bonding ability Difficulty to disassemble Good as repair method Requiring good surface preparation No holes required Assembly rate limitations Assembly of thin or flexible substrates Resistant only to shear loading Low stress concentration High buying and disposal costs Good surface finishing Temperature sensitivity Weight reduction Adhesive may suffer thermal and environmental degradation Sealing Difficulty in predicting bond failure Improvement of fatigue resistance Emission control

1.2. Thermal Welding Methods Thermal welding methods have been defined as the permanent joining of two materials without the use of adhesive or other chemical products at the interface or weld lines [43]. These methods can be applied to many thermoplastics, and some of them have been established methods used in aircraft, automotive, electron device, packaging, and medical fields [62]. Thermal welding methods commonly mention the application of localized heats, times, and pressures. The eventual aim of thermal welding between polymers is the creation of a seamless joint that can own the same mechanical properties with the matrix materials. There are some methods reported to weld between thermoplastics, which are all sharing the same feature of heat generation at the welded interfaces. There are some ways for thermal welding to be employed for the products. Hot-plate welding, laser welding, , and other methods have been studied by many researchers for developing the achievable welded strength. In hot-plate welding, two WPs are set between two heated plates and the weld is expected to achieve according to the melting and solidifying process. Shim [63] used hot-plate welding to weld acrylonitrile butadiene styrene (ABS) polymers by a lap-joint method. They found that welded strength enhanced with the increased plate temperature and contact time. Welded strength was controlled well by the sufficient flow of the molten polymer, while the maximum welded strength of 11 MPa was obtained under 260 ◦C. Watson [64] welded (PP), (PS), and poly (phenylene oxide) (PPO) circular tubing, which confirmed that the plate temperature and contact time are the important factors to obtain the maximum welded strength. The strength could reach one of the matrices once selecting suitable welding conditions. Nonhof [65] welded ABS and PP using hot-plate welding, presenting that it was difficult to determine a single optimal welding parameter around mass of productions. They also reported that a system of factorial experimental design was the best way for hot-plate welding, and a shortcoming of the melt residue adhering to the hot-plate surface was emphasized, especially for the thermoplastics with a high melting point [66,67]. In laser welding, a laser beam is employed for irradiation to give energy at the interfaces of WPs, where the thermoplastics can be welded after melting and solidifying. Both butt-joint and lap-joint ways are used, while the key points for the welding effects are laser power, scan speed, irradiation times and absorption properties of the matrices [68]. Laser welding is a high-speed process and can achieve weld instantaneously. So, the effect of heat on the zones around the weld lines is relatively smaller when compared with other welding methods [69]. Potente [70] welded polyetheretherketone (PEEK) by pigmenting carbon black into the WP to help absorb laser energy at the interfaces. Acherjee [71] conducted a transmission laser welding for PMMA and investigated the welded strength and the widths of the weld lines widths. With the enhancement of laser power, welded strength and weld line widths increased when compared with the decrease of the strength and widths as the increased scan speed. Georgiev [72] also employed the transmission laser welding to weld Teflon fluorinated ethylene Polymers 2020, 12, 759 4 of 26 propylene (FEP) to titanium foil and the interfacial formation was investigated. Amanat [14] reported that, although it cost too much, laser welding was the most suitable technology to weld implantable medical devices due to its instantaneous joint, partial heating, and minimal effect on others. Friction welding is employed utilizing the heat that is generated by rubbing two welding surfaces of WPs together to melt and join them. Like other thermal welding methods, WPs weld can be achieved under welding pressure after the interfaces cool and consolidate [14]. The most common friction welding techniques are vibration, spin, stir, and ultrasonic welding. For the vibration method, one of the WPs is actionless and the other can vibrate along with the welding surface at the desired frequency and amplitude. The heat generated from vibration can melt the surfaces of WPs and then join them together after consolidation. This technique is limited to the parts with flat mating surfaces, and is mostly applied to weld large size parts, such as intake manifolds and bumper assemblies in cars [69,73]. Also, it requires specialized fixtures, and the thermoplastic to be welded must be stiff enough to avoid deformation during the welding process. Stokes [74–76] welded PC, terephthalate (PBT), and polyetherimide (PEI) to themselves, where the maximum welded strength could achieve the tensile strength of the matrices. Welded strength of ABS could reach 90–95% of the tensile strength of the matrices, and the damage mechanism was quite different from PC and PEI [77]. Cakmak [78] gave a report related to the amorphous and semi-crystalline PEN, while the welded strength of the crystalline polyethylene 2,6-naphthalene dicarboxylate (PEN) was two times that of amorphous PEN. Moreover, Chung [79] studied the microstructure of the heat-affected zone during the vibration welding process for butt joints. The heat-affected zone was divided into two distinct areas. One was a central layer which was recrystallized from the molten polymer, and the other was a deformed outer layer that was the result of polymer deformation. The central layer had a higher molecular orientation than the outer layer, and in tensile tests, failure occurred through the interface between the inner and outer heat-affected zone layers.

1.3. Recent Work for Welding Thermoplastics For polymer welding methods, the conventional external heating technology including [80–83], ultrasonic [84,85], and vibration [86,87] welding has become the most important ones in industry, relative to the internal or mechanical heating methods. Induction [88,89] and laser [90–92] welding fall into the category of radiation/electromagnetic technologies, which set new targets in the plastics welding technology. In the last decades, efforts have been made to improve the present processes and to develop new polymer joining techniques [93]. Czigány [94] welded PP and studied the effects of welding conditions on the welded strength by using a hot-gas welding method. They designed a series of experiments with automatic welding station for the welding results. Solvent [95] investigated the welding in the fabrication of micro fluidic devices for thermoplastics, specifically those made of PMMA. It was found that many high-performance polymers were resistant to solvents, and required the use of specialized solvents if they were available. Amancio [10] evaluated the feasibility of the new technology friction of thermoplastic using PMMA plate. The new method was that WPs were first fixed in the welding machine and then the sleeve and pin began to rotate in the same direction. Then, the sleeve was forced against the upper joining partner generating frictional heat. Potente [96] studied the laser transmission welding of thermoplastics due to its advantageous properties and the increasing interest in this technology. Polycaprolactam 6 (PA6) and the quasi-simultaneous laser welding process were used, and a model was introduced using the finite elements method to describe melt displacement and temperature profiles for laser through-welding. Grewell [97] investigated the orbital vibration welding and linear vibration welding methods with employing WPs made of PP, PP/polyethylene (PE) copolymer, PC, ABS, and PA. Standard full-factorial designs with each process were carried out and the differences and benefits between the two methods were reported. Moreover, permeating welding technology depended on infrared such as semiconductor laser, and carbon dioxide laser have attracted much attention in both academic and industrial fields in Polymers 2020, 12, x 5 of 27

(PE) copolymer, PC, ABS, and PA. Standard full-factorial designs with each process were carried out and the differences and benefits between the two methods were reported. Moreover, permeating welding technology depended on infrared such as semiconductor laser, Polymers 2020, 12, 759 5 of 26 and carbon dioxide laser have attracted much attention in both academic and industrial fields in the latest years [98–104]. Also, double color molding and sandwich welding methods have been theproposed latest years with [for98– the104 ]. Also, products double colorwith high-performance molding and sandwich and functionalities welding methods due to have its simply been proposedtechnology with and for low the cost. plastic Naruse products [105] withcarried high-performance out ultrasonic plastic and functionalitieswelding using duea surface to its acoustic simply technologywave device. and There low cost. are some Naruse advantages [105] carried to using out ultrasonic higher frequencies, namely using it amakes surface the acoustic joining wavetime device.shorter, There damages are some of joined advantages parts can to using be avoi higherded, frequencies,and positioning namely accuracy it makes become the joining higher. time In shorter,their study, damages PE films of joined were used parts as can WPs be and avoided, the peak and welded positioning strength accuracy could becomereach the higher. tensile Instrength their study,of the PE matrix films polymers. were used as WPs and the peak welded strength could reach the tensile strength of the matrix polymers. 2. Ultrasonic Weld for Polymers 2. Ultrasonic Weld for Polymers 2.1. Ultrasonic Weld Methods 2.1. Ultrasonic Weld Methods USW for polymer (Figure 1), which uses vibratory energy at high frequencies (20–40 kHz) that are beyondUSW for the polymer range of (Figure human1), hearing which uses to produce vibratory low energy amplitude at high (20–30 frequencies µm) mechanical (20–40 kHz) vibrations, that are beyondis a rapid the rangejoint ofmethod human at hearing the welding to produce interf lowace amplitude due to the (20–30 weldingµm) mechanical pressure [106]. vibrations, USW is for a rapidthermoplastics joint method is a at joining the welding technique interface without due toadditi the weldingonal substances, pressure [such106]. as USW adhesives for thermoplastics or solvents. isUSW a joining possesses technique well-defined without local additional heating substances, of only the such joint as area, adhesives short cycle or solvents. times in USWthe range possesses of just well-defineda few seconds, local and heating comparatively of only the jointlow area,cost-efficie short cyclency times[107]. inDuring the range the of ul justtrasonic a few seconds,consolidation and comparativelyprocess, energy low generated cost-efficiency from [ 107an]. ultrasonic During the transducer ultrasonic consolidationis transferred process, to a WP energy in the generated form of fromultrasonic an ultrasonic oscillation transducer [108]. is transferred to a WP in the form of ultrasonic oscillation [108].

Vibration Horn direction

Work pieces

Base

FigureFigure 1. 1.Schematic Schematic illustration illustration of of USW USW method. method. Adapted Adapted from from Reference Reference [ 106[106].].

ThereThere are are two two di differentfferent USW USW types, types, i.e., i.e., near near field field and and far far field field for for the the traditional traditional ways ways [ 109[109].]. In In thethe near near field field type, type, the the horn horn is is set set close close to to WPs WPs (up (up to to 6 6 mm), mm), while while in in the the far far field field one, one, the the horn horn is is set set atat distances distances starting starting from from 6 6 mm mm [ 110[110].]. ForFor weldingwelding thickthick partsparts ofof lowlow stistiffnessffness oror crystallinecrystalline polymers,polymers, thethe near near field field type type is is suitable, suitable, while while for for thin thin parts parts of high of high stiff nessstiffness or amorphous or amorphous polymers, polymers, the far the field far typefield is type employed is employed only. only. Commonly,Commonly, it it is is necessary necessary to to provide provide enough enough energy energy to to melt melt crystals crystals and and initiate initiate intermolecular intermolecular didiffusionffusion for for the the WPs WPs prepared prepared from from semi-crystalline semi-crystalline polymers polymerssuch suchas as high-densityhigh-density PEPE (HDPE),(HDPE), PP,PP, andand PEPE [ 44[44].]. TheyThey areare alsoalso notnot suitablesuitable forfor thethe far-fieldfar-field typetype becausebecause ofof damping damping inin the the welding welding process.process. Generally, Generally, the the sti stiffnessffness of of WPs WPs can can improve improve ultrasonic ultrasonic vibration vibration transmission transmission and and enhance enhance the weldthe weld ability ability and properties and properties for USW. for Joint USW. design, Join partt design, geometry, part energygeometry, requirements, energy requirements, amplitude, andamplitude, clamping and geometry clamping can geometry influence can the influence welded the strength welded and strength performance. and performance. The major The welding major parameterswelding parameters are welding are time, welding welding time, pressure, welding vibration pressure, amplitude vibration and amplitude holding time, and where holding welding time, time,where welding welding pressur, time, welding and vibration pressur, amplitude and vibration are the amplitude controlling are parameters the controlling [109]. parameters [109]. USWUSW is is applied applied in domesticin domestic appliances appliances and medical and medical industry forindustry manufacturing for manufacturing non-implantable non- medicalimplantable devices, medical where devices, hermetic where seals hermetic and contamination-free seals and contamination-free joints are required. joints are By usingrequired. USW, By Liuusing [111 USW,] welded Liu amorphous [111] welded PS andamorphous semi-crystalline PS and PP,semi-crystalline which indicated PP, that which the weldingindicated time that and the amplitude of vibration were important for the welded strength. The welded strength of semi-crystalline PP was 26 MPa, which was three times higher than that of amorphous PS with only 8 MPa. Certainly, for the maximum strength, more energy was needed to achieve weld for PP compared with PS. Chuah [112] investigated three kinds of energy director shapes in the far field type of USW, which are Polymers 2020, 12, x 6 of 27 welding time and amplitude of vibration were important for the welded strength. The welded strength of semi-crystalline PP was 26 MPa, which was three times higher than that of amorphous PS Polymers 2020, 12, 759 6 of 26 with only 8 MPa. Certainly, for the maximum strength, more energy was needed to achieve weld for PP compared with PS. Chuah [112] investigated three kinds of energy director shapes in the far field typesemi-circular of USW, shape,which rectangularare semi-circular shape shape, and triangular rectangular shape. shape Amorphous and triangular ABS and shape. semi-crystalline Amorphous ABSpolyester and semi-crystalline were welded and the efficiency were waswelded calculated and the as efficiency welded strength was calculated divided as by welded parent strength material dividedstrength. by They parent emphasized material strength. that the welded They emphasiz propertiesed werethat the significantly welded properties affected by were the significantly shape of the affectedenergy director, by the shape in which of the semi-circular energy director, shape wasin which confirmed semi-circular as higher shape up to was 10% confirmed weld efficiency as higher than upothers. to 10% In automotive,weld efficiency instrument than others. panels In were automoti joinedve, with instrument metallic piecespanels usingwere metallicjoined with inserts metallic [113]. piecesThe fabrication using metallic of door inserts fascia, [113]. bumpers, The isolationfabrication pieces of door in the fascia, motor bumpers, housing was isolation current pieces examples in the of motorparts welded housing by was ultrasonic current energyexamples [114 of]. parts welded by ultrasonic energy [114]. Figure 22 showsshows thethe conventionalconventional USWUSW methodmethod forfor polymers.polymers. TheThe hornhorn fixesfixes toto thethe ultrasonicultrasonic transducer works on one of the WPs. That That is is to to say, say, the horn is vibrating out of of the the weld weld line line and and the the ultrasonic energy needs to be transmitted by one of WPs, which are always used between identical polymers [115]. [115].

Vibrating direction

Horn

WPs

Base

(a) Transferring weld (b) Direct weld

Figure 2. ConventionalConventional USW methods. Ad Adaptedapted from from Reference Reference [99]. [99].

Advantages andand disadvantages disadvantages of of USW USW are shownare shown in Table in 3Table. Other 3. majorOther problems, major problems, as observed as observedin vibration in vibration welding, includewelding, the include damage the of damage electronic of electronic components components [112]. [112].

TableTable 3. Advantages and disadvantages of USW.

AdvantagesAdvantages Disadvantages Disadvantages HighHigh production production rates rates Sample sizeSample limitations size limitations (maximum (maximum of 0.23 of 0.23m × m 0.3 0.3m) m) × EnergyEnergy efficiency efficiency Usually Usuallyonly for only compatible for compatible thermoplastics thermoplastics DesignDesign freedom freedom Noise Noiseconcerns concerns EaseEase of assembly of assembly Expensive Expensive equipment equipment Embedding of extra pieces EmbeddingNo filler of materials extra pieces needed No materials needed

Therefore, the most troublesome problem is that ultrasonic weld is difficult difficult to weld dissimilar materials. Moreover, Moreover, once once the the size size and and shape of the WPs are changed, the horn may not remain the previous vibration so that the weldweld cannot be achieved normally. 2.2. Weld Between Dissimilar Polymers 2.2. Weld Between Dissimilar Polymers It was reported [116] in 2007 that weld for dissimilar polymers was the necessary and efficient processing technology for the excellent materials showing their functions efficiently or the situation of when a single kind of product needs to be employed in a different environment (temperature, load, Polymers 2020, 12, 759 7 of 26 etc.). Usually, the welds between dissimilar materials are carried out in these fields: welding related to biology-body and artificial organs of nano-biomedicine, electronics technique, energy technology in generating and storing electricity, automotive field with welding between soft and hard materials (organic-matters and metals, insulators), aerospace field with welding between functional materials (semiconductors and metals, insulators), and so on. That is to say, the welding technique for dissimilar materials will be very important and essential in many industrial fields today. Mechanical fastening and adhesive bonding methods are the traditional ways to weld dissimilar materials. Filho [117] gave the comparison for the bonding methods, where the field of welding for dissimilar plastics was still a new area in joining technology nowadays in several industrial areas. The development of new materials and fabrication techniques has become a matter of success for industrial sectors such as transportation. Polymers, polymeric composites, and polymer–metal structures are being increasingly employed in several products mainly due to the associated weight savings. Conventionally, an ideal component should not show any disadvantages that would deteriorate mechanical strength after welding. Nevertheless, the sizes of WPs are usually limited by their production process. Therefore, the understanding and development of available and new joining techniques is a key subject in industry. For the weld between dissimilar polymers, other techniques such as hybrid joining method and plastic injection over molding in metallic perforated parts are being currently exploited [118,119]. Ageorges [120] reported a review for thermoplastic matrix composites by using fusion-based joining methods. All of the presented welding methods for plastics were almost adequate for thermoplastic composites, in which the resistance welding was the most popular technology for application. But friction welding was not suitable due to the microstructural deterioration of the composite through fiber breaking or realignment. McKnight [121] proposed to weld carbon fiber reinforced polyetherketone composite using resistance welding and the welded performance and weld line structure were analyzed. Ageorges et al. [122] gave a report for welding glass fiber and carbon fiber reinforced polyetherimide composite laminates. Other examples of polymer composites welding can also be found [123,124]. In brief, the welds between dissimilar polymers are almost limited by mechanical fastening and adhesive bonding ways, which bring problems according to their disadvantages. Therefore, it is necessary to develop the novel weld method.

2.3. New Ultrasonic Welding Application for Polymers There are considerable research literatures on the USW process for thermoplastic materials at the coupon level [125–135]. The welded condition parameters including welding time, welding pressure, and vibration amplitude, were investigated and the relationship between the welded ability and the parameters were reported. For energy director (ED) USW technique, solidification force and time and shape of EDs were also discussed [136]. Since the heat generation and ultrasonic power could be dissipated during the welding process, it was essential to make sure the ultrasonic parameters such as ultrasonic amplitude, power and vibration time. Palardy [136] proposed an original experimental and modeling approach towards a better understanding of the hammering effect. It consisted of an experimental static welding setup with a high frequency laser sensor and a finite element model as shown in Figure3. A numerical, multiphysical model based on the framework developed by Levy [137] was employed to evaluate the changed power according to USW process by amplitude transmission measurements. The results showed that it was possible to obtain a good estimation of the vibration transmitted to the upper adherend from laser measurements close to the sonotrode, the effect of which was shown to decrease during the welding process, due to the heating of the interface which directly affected further heat generation. Though these might be potential for evaluating the vibration transmitted of the top WP using the laser sensor, it had been confirmed decrease during the welding process because the heat generated at the interfaces would give effects on the further ultrasonic heat production. Additionally, this might be another way to analyze the welding process, especially for the heat generation, but the welding method was limited and similar to the conventional USW. Polymers 2020, 12, x 8 of 27

Polymers 2020, 12, x 8 of 27 give effects on the further ultrasonic heat production. Additionally, this might be another way to analyze the welding process, especially for the heat generation, but the welding method was limited give effects on the further ultrasonic heat production. Additionally, this might be another way to and similar to the conventional USW. analyze the welding process, especially for the heat generation, but the welding method was limited Polymersand similar2020, 12to, 759the conventional USW. 8 of 26

Figure 3. Schematic drawing of determination of amplitude transmission using a tilted sensor. AdaptedFigure 3. fromSchematic Reference drawing [136]. of determination of amplitude transmission using a tilted sensor. Adapted Figure 3. Schematic drawing of determination of amplitude transmission using a tilted sensor. from Reference [136]. Adapted from Reference [136]. Ultrasonic guided waves (GW) are widely acknowledged to have high potential application [138,139].Ultrasonic Over the guided last two waves decades, (GW) some are studies widely have acknowledged focused on tofinding have reliable high potentialways of Ultrasonic guided waves (GW) are widely acknowledged to have high potential application correlatingapplication changes [138,139 in]. OverGW propagation the last two with decades, realistic some defects studies in joints. have Singher focused [140,141] on finding investigated reliable [138,139]. Over the last two decades, some studies have focused on finding reliable ways of theways interaction of correlating of ultrasonic changes GW in GW with propagation adhesively withbonded realistic single defects lap metallic in joints. joints Singher produced [140 with,141] correlating changes in GW propagation with realistic defects in joints. Singher [140,141] investigated differentinvestigated surface the interaction treatments. of ultrasonicEach surface GW treatment with adhesively resulted bonded in different single lap bond metallic strength, joints while produced the the interaction of ultrasonic GW with adhesively bonded single lap metallic joints produced with strengthwith diff erentshould surface be enhanced treatments. for a Each higher surface level. treatment Pedro [142] resulted presented in diff theerent propagation bond strength, of ultrasonic while the different surface treatments. Each surface treatment resulted in different bond strength, while the GWstrength in ultrasonically should be enhanced welded thermoplastic for a higher level. composite Pedro [joints142] presentedin Figure the4. They propagation studied the of ultrasoniceffects of strength should be enhanced for a higher level. Pedro [142] presented the propagation of ultrasonic weldGW inmanufacturing ultrasonically defects welded on thermoplastic GW transmission composite across jointsthe joint in Figureand triangular4. They studied(EDs) integrated the e ffects into of GW in ultrasonically welded thermoplastic composite joints in Figure 4. They studied the effects of theweld lower manufacturing composite defectsadherents on GWenabled transmission the production acrossthe of jointdefective and triangularjoints in a (EDs) controlled integrated manner. into weld manufacturing defects on GW transmission across the joint and triangular (EDs) integrated into Theythe lower concluded composite that adherents it was only enabled possible the productionto detect and of defectivedistinguish joints the in two a controlled defective manner.scenarios They by the lower composite adherents enabled the production of defective joints in a controlled manner. combiningconcluded thatthe characteristic it was only possible frequency to detect and time and distinguishanalysis at last. the two defective scenarios by combining theThey characteristic concluded that frequency it was andonly time possible analysis to dete at last.ct and distinguish the two defective scenarios by combining the characteristic frequency and time analysis at last.

Figure 4. Power and travel curves as function of energy for the initial weld, produced with a travel of Figure0.5 mm. 4. AdaptedPower and from travel Reference curves [as142 function]. (A) intact of energy EDs; (foBr) collapsethe initial of weld, EDs dueproduced to melting; with (aC travel) partial of 0.5flow mm. of theAdapted EDs and from melt-front Reference arrest; [142]. (D (A) remelting) intact EDs; of ED(B) melt-frontscollapse of EDs and subsequentdue to melting; flow (C out) partial of the Figure 4. Power and travel curves as function of energy for the initial weld, produced with a travel of flowoverlap; of the (E EDs) complete and melt-front ED squeeze arrest; out (D from) remelting the welding of ED interface,melt-fronts with and melt subsequent and partial flow flow out of the the 0.5 mm. Adapted from Reference [142]. (A) intact EDs; (B) collapse of EDs due to melting; (C) partial overlap;matrix beyond (E) complete the first ED adherend squeeze layer. out from the welding interface, with melt and partial flow of the flow of the EDs and melt-front arrest; (D) remelting of ED melt-fronts and subsequent flow out of the matrix beyond the first adherend layer. Nateshoverlap; [(143E) complete] presented ED asqueeze study undertakenout from the with welding an objective interface, towith establish melt and USW partial process flow forof the joining matrix beyond the first adherend layer. polymerNatesh blends [143] expressed presented to aida study the eco-friendly undertaken qualities with an desired objective in manufacturingto establish USW sectors. process PC andfor joiningABS blends polymer were blends welded expressed after creating to aid suitable the eco-friendly parts with qualities energy desired directors inusing manufacturing injection molding sectors. Natesh [143] presented a study undertaken with an2 objective to establish USW process for PCtechniques. and ABS Finally,blends onlywere a welded welded after strength creating of 6.02 suitable N mm parts− was with achieved. energy directors using injection joining polymer blends expressed to aid the eco-friendly· qualities desired in manufacturing sectors. moldingWang techniques. [144] welded Finally, carbon only fiber a welded/PEEK strength (CF/PEEK) of 6.02 composites N·mm−2 was assisted achieved. by ultrasonic as shown PC and ABS blends were welded after creating suitable parts with energy directors using injection in FigureWang5. [144] They welded reported carbon the influence fiber/PEEK of vibration (CF/PEEK) time composites and ED on assisted the joints, by inultrasonic which the as heating shown molding techniques. Finally, only a welded strength of 6.02 N·mm−2 was achieved. inrate Figure could 5. be They accelerated reported by the using influence flat ED, of andvibration the maximum time and temperature ED on the joints, was lowered. in which By the using heating flat Wang [144] welded carbon fiber/PEEK (CF/PEEK) composites assisted by ultrasonic as shown rateED, could the lap be shear accelerated strength by ofusing the jointflat ED, could and reach the maximum 28 MPa, whichtemperature might was be necessary lowered. toBy beusing further flat in Figure 5. They reported the influence of vibration time and ED on the joints, in which the heating ED,enhanced the lap by shear using strength other ways of the such joint as could surface reac modification.h 28 MPa, which might be necessary to be further rate could be accelerated by using flat ED, and the maximum temperature was lowered. By using flat enhancedChan by [145 using] presented other ways a composite such as surface film of PMMAmodification. microspheres in polydimethylsiloxane (PDMS), whichED, the was lap used shear as strength the fusion of layerthe joint to avoid could trapped reach 28 air MPa, and towhich restrict might the flowbe necessary of the melted to be polymer further duringenhanced welding by using (Figure other6), ways compared such as with surface conventional modification. one of (EDs) melt and flow across the surface of the samples, resulting in a thinner fusion layer. Although the welded strength in this methodology is limited by the particle size of the PMMA microspheres, it is a way to change the (EDs) into the layer

without shape modification. Therefore, the welded strength will be focus of the following work. Polymers 2020, 12, x 9 of 27

Polymers 2020, 12, 759 9 of 26 Polymers 2020, 12, x 9 of 27

Figure 5. Schematic illustration of joint configuration USW (dimensions in mm). Adapted from Reference [144].

Chan [145] presented a composite film of PMMA microspheres in polydimethylsiloxane (PDMS), which was used as the fusion layer to avoid trapped air and to restrict the flow of the melted polymer during welding (Figure 6), compared with conventional one of (EDs) melt and flow across the surface of the samples, resulting in a thinner fusion layer. Although the welded strength in this methodology is limited by the particle size of the PMMA microspheres, it is a way to change the (EDs) into FiguretheFigure layer 5.5. Schematic Schematicwithout illustrationshapeillustration modification. ofof jointjoint configurationconfigurat Thereforione,USW USWthe (dimensionswelded (dimensions strength in in mm). mm). will AdaptedAdapted be focus fromfrom of the followingReferenceReference work. [ 144[144]. ].

Chan [145] presented a composite film of PMMA microspheres in polydimethylsiloxane (PDMS), which was used as the fusion layer to avoid trapped air and to restrict the flow of the melted polymer during welding (Figure 6), compared with conventional one of (EDs) melt and flow across the surface of the samples, resulting in a thinner fusion layer. Although the welded strength in this methodology is limited by the particle size of the PMMA microspheres, it is a way to change the (EDs) into the layer without shape modification. Therefore, the welded strength will be focus of the following work.

Figure 6.6. IllustrationIllustration of polymerpolymer using (a) conventionalconventional and (b) composite filmfilm USW. AdaptedAdapted fromfrom Reference [[145].145].

In summary, for all of the latest research on the USW method, it is concluded that the WPs have In summary, for all of the latest research on the USW method, it is concluded that the WPs have been changing from polymeric materials to their composite (or blends). Moreover, (EDs) are popular been changing from polymeric materials to their composite (or blends). Moreover, (EDs) are popular and draw much attention by many researchers. The compounds, structure, shapes and sizes are and draw much attention by many researchers. The compounds, structure, shapes and sizes are different from each other, though they are all melted and welded into the weld lines. Furthermore, different from each other, though they are all melted and welded into the weld lines. Furthermore, energies coming from ultrasonic vibrations are all sent from one WP to another, which might cause energies coming from ultrasonic vibrations are all sent from one WP to another, which might cause the inhomogeneity of the weld line and residual stress at the interfaces. Finally, welded strength is the inhomogeneityFigure 6. Illustration of the of weld polymer line using and residual(a) conventional stress atand the (b )interfaces. composite filmFinally, USW. welded Adapted strength from is important point for welding applications in industry, and so it is necessary to be further enhanced. importantReference point [145]. for welding applications in industry, and so it is necessary to be further enhanced. Therefore, the ultrasonic TWTP method between polymers has been proposed, which is an Therefore, the ultrasonic TWTP method between polymers has been proposed, which is an effective way to solve the above-mentioned problems and introduced to more widely fields. Schematic effectiveIn summary, way to solvefor all theof the above-mentioned latest research on problems the USW method,and introduced it is concluded to more that widely the WPs fields. have drawing of ultrasonic TWTP technology is shown in Figure7[ 146,147]. According to the slide guide Schematicbeen changing drawing from of polymeric ultrasonic materials TWTP technology to their composite is shown (or in blends). Figure 7 Moreover, [146,147]. (EDs)According are popular to the (x direction), WPs are installed on the WPs holders, and the third phases of interposed sheets (IPSs) slideand drawguide much(x direction), attention WPs by aremany installed researchers. on the TheWPs compounds,holders, and structure,the third phasesshapes ofand interposed sizes are are fixed on the sheet holders (z direction), which are connected to an ultrasonic transducer. Then, sheetsdifferent (IPSs) from are each fixed other, on thoughthe sheet they holders are all (zmelt diedrection), and welded which intoare theconnected weld lines. to an Furthermore, ultrasonic IPSs are set between the two WPs and the welding pressures are given by different level (x direction). transducer.energies coming Then, fromIPSs areultrasonic set between vibrations the two are WPs all sentand fromthe welding one WP pressures to another, are which given bymight different cause Abrasive is designed (y direction) to modify the welding surface roughness of WPs. At last, levelthe inhomogeneity (x direction). Abrasive of the weld paper line is designedand residual (y direction) stress at theto modify interfaces. the weldingFinally, surfacewelded roughnessstrength is after the parameters of ultrasonic vibration are set, the weld can be achieved. ofimportant WPs. At pointlast, after for weldingthe parameters applications of ultrason in industry,ic vibration and so are it set,is necessary the weld tocan be be further achieved. enhanced. The effects of the technology on welding parameters have been investigated for both identical TheTherefore, effects ofthe the ultrasonic technology TWTP on welding method parametebetween rspolymers have been has investigated been proposed, for both which identical is an and dissimilar polymers. The ultrasonic TWTP technology can solve the above problems and avoid andeffective dissimilar way polymers.to solve theThe above-mentionedultrasonic TWTP teproblemschnology andcan solveintroduced the above to problemsmore widely and avoidfields. theSchematic disadvantages drawing of of the ultrasonic traditional TWTP USW technology method in Tableis shown3. It mayin Figure be applied 7 [146,147]. widely According in industry, to sothe the slide following guide (x statements direction), are WPs the are application installed ofon this the technology WPs holders, in welding and the dithirdfferent phases pairs of of interposed polymers. sheets (IPSs) are fixed on the sheet holders (z direction), which are connected to an ultrasonic transducer. Then, IPSs are set between the two WPs and the welding pressures are given by different level (x direction). Abrasive paper is designed (y direction) to modify the welding surface roughness of WPs. At last, after the parameters of ultrasonic vibration are set, the weld can be achieved. The effects of the technology on welding parameters have been investigated for both identical and dissimilar polymers. The ultrasonic TWTP technology can solve the above problems and avoid

Polymers 2020, 12, x 10 of 27 Polymers 2020, 12, x 10 of 27 the disadvantages of the traditional USW method in Table 3. It may be applied widely in industry, theso disadvantages the following of statements the traditional are the USW application method in of Table this 3.technology It may be inapplied welding widely different in industry, pairs of sopolymers. the following statements are the application of this technology in welding different pairs of Polymers 2020, 12, 759 10 of 26 polymers. Connecting to ultrasonic transducer Connecting to ultrasonic transducer Interposed Sheet Interposed Sheet vibration direction vibration direction z y z y x Welding pressure x Welding pressure Welding pressure Work pieces Welding pressure Work pieces Figure 7. Welding principle schematic drawing followed by ultrasonic TWTP technology. Adapted Figure 7. Welding principle schematic drawing followed by ultrasonic TWTP technology. Adapted Figurefrom 7.Reference Welding [146,147]. principle schematic drawing followed by ultrasonic TWTP technology. Adapted from Reference [146,147]. from Reference [146,147]. 3.3. Ultrasonic Ultrasonic TWTP TWTP Technology Technology of of PC PC and and PMMA PMMA 3. Ultrasonic TWTP Technology of PC and PMMA 3.1.3.1. IPS IPS Preparation Preparation for for Ultrasonic Ultrasonic TWTP TWTP Between Between PC PC and and PMMA PMMA 3.1. IPS Preparation for Ultrasonic TWTP Between PC and PMMA IPSIPS for for ultrasonic ultrasonic TWTP TWTP weld weld between between PC PC and and PMMA PMMA were were prepared prepared as as functional functional gradient gradient materialsmaterialsIPS for due dueultrasonic to to the the di TWTPdifferentfferent weld physical phys betweenical properties properties PC and of ofPMMA matrices. matrices. were Figure Figu preparedre8 illustrated8 illustrated as functional the the preparation preparation gradient materialsmethodsmethods ofdue of the the to IPS theIPS [ 148[148].different]. PC PC/PMMA/PMMA physical composites composites properties were wereof matrices. prepared prepared Figu by by injectionre injection 8 illustrated molding molding the with withpreparation a a volume volume methodsfractionfraction ofof of PC:PMMAthe PC:PMMA IPS [148].= = 1:1.PC/PMMA 1:1. Then, Then, PC, PC,composites PMMA, PMMA, andwere and PC PC/PMMAprepared/PMMA by composites composites injection molding were were reshaped reshaped with a tovolume to layers layers fractionwithwith the the of thickness thicknessPC:PMMA of of 0.5 = 0.5 1:1. mm mm Then, by by hot-press, hot-press,PC, PMMA, respectively. respecti and PC/PMMAvely. Finally, Finally, thecomposites the three three diff differenterentwere layersreshaped layers were towere usedlayers used to withmoldto mold the and thickness and the IPSthe wasIPSof 0.5 was obtained mm obtained by afterhot-press, after modifying modifying respecti thevely. thicknessthe thicknessFinally, to 1the mm.to three1 mm. different layers were used to mold and the IPS was obtained after modifying the thickness to 1 mm. Pressure Pressure PC 0.5m PC 0.5m PC/PMMA 1.0mm PC/PMMA 1.0mm PMMA PMMA

Pressure Pressure

FigureFigure 8. 8.Schematic Schematic diagram diagram of of IPS IPS preparation preparation for for PC-PMMA PC-PMMA weld. weld. Adapted Adapted from from Reference Reference [148 [148].]. Figure 8. Schematic diagram of IPS preparation for PC-PMMA weld. Adapted from Reference [148]. 3.2. Mechanical Properties of IPS for Ultrasonic TWTP Between PC and PMMA 3.2. Mechanical Properties of IPS for Ultrasonic TWTP Between PC and PMMA 3.2. MechanicalThe mechanical Properties properties of IPS for were Ultrasonic shown TWTP in Figure Between9 with PC the and nominal PMMA stress-strain curves. The The mechanical properties were shown in Figure 9 with the nominal stress-strain curves. The tensile strength of the IPS (PC is yield strength) was 60 MPa. The matrix PC showed a necking tensileThe mechanicalstrength of propertiesthe IPS (PC were is yield shown strength) in Figure was 9 with60 MPa. the nominalThe matrix stress-strain PC showed curves. a necking The deformation and the nominal stress weakened sharply when the strain reached yield strength, showing tensiledeformation strength and of thethe IPSnominal (PC is stress yield weakenedstrength) wassharply 60 MPa. when The the matrix strain PCreached showed yield a neckingstrength, a typical ductile character. PC continued its elongating with the necking deformation, where the stress deformationshowing a typicaland the ductile nominal character. stress PC weakened continued sh itsarply elongating when thewith strain the necking reached deformation, yield strength, where slowly increased and then fractured finally when the strain was above 110%. In contrast, the elasticity showingthe stress a typical slowly ductile increased character. and then PC continuedfractured finaits elongatinglly when the with strain the necking was above deformation, 110%. In contrast,where of PMMA was higher than that of PC, but the necking deformation was not observed, presenting thethe stress elasticity slowly of increasedPMMA was and higher then fracturedthan that finaof PC,lly butwhen the the necking strain deformationwas above 110%. was notIn contrast, observed, a typical brittle character. The IPS made of PC and PMMA showed a brittle character, which was thepresenting elasticity ofa typicalPMMA brittle was higher character. than Thethat IPSof PC, made but of the PC necking and PMMA deformation showed was a brittle not observed, character, similar to PMMA. It was suggested that IPS was a brittle material with a tensile strength of 58 MPa. presentingwhich was a similartypical tobrittle PMMA. character. It was Thesuggested IPS made that ofIP SPC was and a brittlePMMA material showed with a brittle a tensile character, strength Meanwhile, IPS exhibited a good compatible structure, resulting in good compatibility with PC and whichof 58 was MPa. similar Meanwhile, to PMMA. IPS Itexhibited was suggested a good thatcompatible IPS was structure,a brittle material resulting with in gooda tensile compatibility strength PMMA due to the absence of the phase separation [149] at the rupture surface. So, the IPS prepared ofwith 58 MPa. PC and Meanwhile, PMMA due IPS toexhibited the absence a good of the compatible phase separation structure, [149] resulting at the inrupture good compatibilitysurface. So, the from PC and PMMA was considered to be appropriate as the third phase for welding PC and PMMA. with PC and PMMA due to the absence of the phase separation [149] at the rupture surface. So, the

Polymers 2020, 12, x 11 of 27

Polymers 2020IPS, 12 prepared, 759 from PC and PMMA was considered to be appropriate as the third phase for welding 11 of 26 Polymers 2020, 12, x 11 of 27 PC and PMMA. IPS prepared from PC and PMMA was considered to be appropriate as the third phase for welding PC and PMMA. 60

60 40

40 PC 20 PMMA

Nominal Stress/MPa Nominal PC PC/PMMA layer 20 Interposed sheet PMMA0

Nominal Stress/MPa Nominal PC/PMMA0 layer 10 20 Interposed sheetNominal Strain/% 0 Figure 9.0 NominalFigure 9. Nominal stress-strain stress-strain10 curves curves of of PC, PC, PMMA,20 PMMA, PC/PMMA PC/PMMA composite composite and IPS. and Adapted IPS. Adapted from from Nominal Strain/% Reference [Reference149]. [149]. Figure 9. Nominal stress-strain curves of PC, PMMA, PC/PMMA composite and IPS. Adapted from 3.3. Welded3.3. Strength Welded Strength of Ultrasonic of Ultrasonic TWTP TWTP Between Between PC PC and PMMA PMMA Reference [149]. The weldedThe strengths welded strengths at diff erentat different welding welding times times were were shown shown inin FigureFigure 10. 10 The. The tensile tensile stress- stress-strain 3.3. Welded Strength of Ultrasonicstrain TWTP curves Between (Figure PC 10a) and under PMMA a welding pressure (P) of 0.4 MPa indicated that the tensile stress curves (Figureincreased 10a) linearly under with a welding the strain pressure prolonging, (P and) of the 0.4 samples MPa indicated was broken that in the the strain tensile of below stress 4% increased The welded strengths at different welding times were shown in Figure 10. The tensile stress- [149]. This character was same for all the curves in Figure 10a, even though they were welded at strain curves (Figurelinearly 10a) with under the a welding strain pressure prolonging, (P) of 0.4 and MPa the indicated samples that wasthe tensile broken stress in the strain of below 4% [149]. different conditions. Figure 10b illustrated the curves of tensile strengths versus welding time (t), increased linearlyThis with character the strainwas prolonging, same forand allthe thesamples curves was broken in Figure in the 10 straina, even of below though 4% they were welded at different where the welded strengths presented same trend as welding time under different P. Under welding [149]. This characterconditions. was same Figure for all 10theb curves illustrated in Figure the 10a, curves even though of tensile they strengthswere welded versus at welding time (t), where the pressure of 0.3 MPa, welded strengths enhanced with the increase of welding time firstly and then different conditions. Figure 10b illustrated the curves of tensile strengths versus welding time (t), welded strengthsdecreased presented with the time. same However, trend the as effect welding of weld timeing undertime from di ff1 erentto 4 s on P. the Under strength welding were not pressure of where the welded strengths presented same trend as welding time under different P. Under welding significant when compared to that of welding pressure. It was concluded that the strength could be pressure of 0.30.3 MPa, MPa, welded welded strengths strengths enhanced enhanced with the increase with the of welding increase time of weldingfirstly and timethen firstly and then decreased with enhanced with prolonging time, but the change was limited by presses. decreased withthe the time.time. However, However, the theeffect e ffofect weld ofing welding time from time 1 to from4 s on 1the to strength 4 s on were the strengthnot were not significant when significant whencompared compared to that that ofof welding welding pressure. pressure. It was concluded It was concluded that the strength that could the strengthbe could be enhanced with enhanced withprolonging prolonging time, time, but but the change the change was limitedPolymers50 was by limited2020 presses., 12, x by presses. 12 of 27

50 40 50

40 30 40

20 t=1.0s 30 30 t=1.5s t=2.0s 10 t=1.0s 20 Nominal stress/MPa 20 t=3.0s t=1.5s t=4.0s t=2.0s 10 0 10 Nominal stress/MPa 0 1 2 3 4 5 P=0.3MPa t=3.0s strength/MPa Welding t=4.0sNominal strain/% P=0.4MPa 0 0 0 1 2 3 4 5 0 1 2 3 4 5(a) Nominal strain/% Welding time/s

(a) (b) Figure 10. Welded strengths at different welding times. (a) Nominal stresses and strain at different Figure 10. Welded strengths at different welding times. (a) Nominal stresses and strain at different welding times under welding pressure of 0.4 MPa; (b) Welded strengths at different welding times. welding times under welding pressureAdapted of from 0.4 Reference MPa; (b [149].) Welded strengths at different welding times. Adapted from Reference [149]. The SEM images of ruptured surface were shown in Figure 11. The welded areas were The SEM images of rupturedcalculated, surface where were the shown welded in strength Figure was11. enhanced The welded with areasthe increased were calculated,areas. With the increase of where the welded strength was enhancedwelding time, with more the frictional increased heat was areas. produced, With welded the increase areas were of gradually welding increased. time, It could be seen that the strength was weak at 1.5 s and the area was also very low (Figure 11a), and the maximum more frictional heat was produced,strength welded was obtained areas werewhen the gradually areas were increased. full (Figure 11b). It could Additionally, be seen the that air bubbles the at ruptured strength was weak at 1.5 s andsurfaces the area existed was alsowhen verywelding low time (Figure was longer 11a), than and 4 thes (Figure maximum 11c). It strengthwas considered that the was obtained when the areas werepolymers full (Figure decomposed 11b). at Additionally, the local part theand airgas bubbles was released at ruptured because of surfaces the longer ultrasonic existed when welding time wasvibrating longer and than heating. 4 s (Figure The air bu11bblesc). Itmight was be considered the reasons for that the the decreased polymers strength when the welding time was higher than 3 s. decomposed at the local part and gas was released because of the longer ultrasonic vibrating and heating. The air bubbles might be the reasons for the decreased strength when the welding time was higher than 3 s.

Polymers 2020, 12, 759 12 of 26 Polymers 2020, 12, x 13 of 27

500µ a

500µm b

c 10µm

FigureFigure 11.11. SEMSEM imagesimages of of ruptured ruptured surfaces surfaces of of welded welded interfaces interfaces of of PC PC side side under under welding welding pressure pressure of 0.3MPa.of 0.3MPa. (a), ( (ab),), (b (c),) ( arec) are under under welding welding time time of 1.5of 1.5 s, 3 s, s and3 s and 4 s. 4 Adapted s. Adapted from from Reference Reference [149 [149].].

TheThe weldedwelded strengthsstrengths atat didifferentfferent weldingwelding pressurespressures werewere shownshown inin Figure Figure 12 12.. FigureFigure 1212aa illustratedillustrated thethe tensiletensile stress–strainstress–strain curvescurves atat aa weldingwelding timetime ofof 22 s.s. ComparedCompared toto thatthat ofof weldingwelding time,time, thethe eeffectsffects ofof weldingwelding pressurepressure onon thethe tensiletensile stressstress presentedpresented aa samesame trend,trend, showingshowing aa linearlinear eeffectffect andand thethe specimensspecimens brokenbroken in in thethe strainstrain ofof belowbelow 4%4% [[149].149]. FigureFigure 1212bb showsshows thethe dependencedependence ofof tensiletensile strengthstrength on on welding welding pressure, pressure, and and the the strength strength enhanced enhanced as theas the increased increased welding welding pressure, pressure, which which was morewas more significant significant than thosethan those of welding of welding time. time. However, However, when when the pressures the pressures were higherwere higher than 0.4 than MPa, 0.4 theMPa, ultrasonic the ultrasonic plastic weldsplastic oftenwelds displayed often displaye errorsd and errors cannot and weld cannot normally weld normally due to the due excessive to the frictionexcessive force friction and powerforce and [149 power]. Moreover, [149]. Moreover, excellent welding excellent pressure welding was pressure not observed was not due observed to the factdue thatto the the fact rigidity that ofthe the rigidity functional of the gradient functional IPS wasgradient stronger IPS thanwas thatstronger of matrix than PC.that Therefore,of matrix IPSPC. couldTherefore, vibrate IPS together could vibrate with the together ultrasonic with vibration. the ultrasonic vibration. The SEM photos of ruptured surface at a welding time of 3.0 s were shown in Figure 13. As shown in Figure 13a, when the welding pressure was weak (0.1 MPa), there was little frictional heat produced, welding areas were small, and the strength was weak obviously. According to Figure 13b, more heats at the interface would happen with the improved welding pressure. Therefore, the wider the welding areas were, the stronger the welded strengths. According to the analysis above, the welded areas and the strengths were linearly related to each other. Obviously, the strengths could be supposed by the welded areas and the different strengths could also be investigated by the welded areas of the interfaces.

Polymers 2020, 12, 759 13 of 26 Polymers 2020, 12, x 14 of 27

50

40

30

20 P=0.1MPa P=0.2MPa

Nominal Stress/MPa 10 P=0.3MPa P=0.4MPa 0 0 1 2 3 4 Nominal Strain/ %

(a) 50

40

30

20

10

Tensile strength/MPa Tensile t=3s t=2s 0 0 0.1 0.2 0.3 0.4 0.5 Welding stress/MPa

(b)

Figure 12. WeldedFigure 12. strengths Welded strengths at diff erentat different welding welding pressures. pressures. ( a)) Nominal Nominal stress stress and strain and strainat different at di fferent welding pressures under welding time of 2 s; (b) Welded strengths at different welding pressures welding pressures under welding time of 2 s; (b) Welded strengths at different welding pressures under under welding times of 2 s and 3 s. Adapted from Reference [149]. Polymerswelding 2020, 12 times, x of 2 s and 3 s. Adapted from Reference [149]. 15 of 27 The SEM photos of ruptured surface at a welding time of 3.0 s were shown in Figure 13. As shown in Figure 13a, when the welding pressure was weak (0.1 MPa), there was little frictional heat produced, welding areas were small, and the strength was weak obviously. According to Figure 13b, more heats at the interface would happen with the improved welding pressure. Therefore, the wider the welding areas were, the stronger the welded strengths. According to the analysis above, the welded areas and the strengths were linearly related to each other. Obviously, the strengths could be supposed by the welded areas and the different strengths could also be investigated by the welded areas of the interfaces.

a 500µm

b 500µm

FigureFigure 13.13. SEMSEM imagesimages ofof rupturerupture surfacessurfaces underunder weldingwelding timetime ofof 3.03.0 s.s. (a)) andand ((bb)) areare atat weldingwelding pressurespressures ofof 0.10.1 MPaMPa andand 0.40.4 MPa.MPa. AdaptedAdapted fromfrom ReferenceReference[ [149].149].

3.4. Interfacial Structures Analysis Figure 14 illustrated that the decomposition of PC and PMMA started at 450 °C and 300 °C respectively. Air bubbles could be brought easily at the hyperthermal interface between PMMA and IPS, which was proven by the images of the interface.

100

80

60

40 PC Relative weight/% 20 PMMA PC/PMMA 0 100 200 300 400 500 Temperature/℃

Figure 14. TG test of PC, PMMA and PC/PMMA. Adapted from Reference [149].

With the ultrasonic TWTP technology, the welding of dissimilar materials from PC and PMMA was achieved successfully. Based on the analysis for welded strength and interfacial structure, a shorter welding time or weaker welding pressure was inadequate, the frictional heat produced at the interfaces was not enough, welding areas were small, and thus the welds could not be achieved appropriately, and the welded strengths were weak. An excessively long welding time or excessively strong welding pressure is harmful, with much heat produced accompanied by air bubbles at interfaces due to the materials decomposing. This was the reason for the welded strength not reaching the lever of the block materials. Therefore, appropriate welding time and pressure could obtain the optimum value of welded strength.

Polymers 2020, 12, x 15 of 27

a 500µm

b 500µm

Figure 13. SEM images of rupture surfaces under welding time of 3.0 s. (a) and (b) are at welding Polymers 2020, 12, 759 14 of 26 pressures of 0.1 MPa and 0.4 MPa. Adapted from Reference [149].

3.4.3.4. Interfacial Interfacial Structures Structures Analysis Analysis Figure 14 illustrated that the decomposition of PC and PMMA started at 450 °C and 300 °C Figure 14 illustrated that the decomposition of PC and PMMA started at 450 ◦C and 300 ◦C respectively.respectively. Air Air bubbles bubbles could could be be brought brought easily easily at at the the hyperthermal hyperthermal interface interface between between PMMA PMMA and and IPS,IPS, which which was was proven proven by by the the images images of of the the interface. interface.

100

80

60

40 PC Relative weight/% 20 PMMA PC/PMMA 0 100 200 300 400 500 Temperature/℃

Figure 14. TG test of PC, PMMA and PC/PMMA. Adapted from Reference [ 149]. Figure 14. TG test of PC, PMMA and PC/PMMA. Adapted from Reference [149]. With the ultrasonic TWTP technology, the welding of dissimilar materials from PC and PMMA was achievedWith the successfully. ultrasonic BasedTWTP ontechnology, the analysis the for welding welded of strength dissimilar and materials interfacial from structure, PC and a shorter PMMA weldingwas achieved time or weakersuccessfully. welding Based pressure on the was analysis inadequate, for welded the frictional strength heat and produced interfacial at the structure, interfaces a wasshorter not enough, welding welding time or weaker areas were welding small, pressure and thus was the inadequate, welds could the not frictional be achieved heat produced appropriately, at the andinterfaces the welded was strengths not enough, were welding weak. An areas excessively were sm longall, weldingand thus time the or welds excessively could strongnot be weldingachieved pressureappropriately, is harmful, and withthe welded much heatstrengths produced were weak. accompanied An excessively by air bubbles long welding at interfaces time or due excessively to the materialsstrong welding decomposing. pressure This is washarmful, the reason with formuch the heat welded produced strength accompanied not reaching by the air lever bubbles of the at blockinterfaces materials. due to Therefore, the materials appropriate decomposing. welding This time was and the pressure reason couldfor the obtain welded the strength optimum not value reaching of weldedthe lever strength. of the block materials. Therefore, appropriate welding time and pressure could obtain the optimumTherefore, value to of achieve welded the strength. maximum welded strength, the increment of welding area and the removal of air bubble were important. Moreover, shorter welding time and higher welding pressure might be effective for ultrasonic TWTP for PC and PMMA. Air bubbles were at the bottom edge although that was the interface of the top strength specimens for PC and PMMA ultrasonic TWTP. It was suggested that the bubbles could be squeezed out of the interfaces with the melting plastics resin together. Finally, welded strength was more sensitive to welding pressure when compared with welding time.

4. Ultrasonic TWTP Technology of PLA and POM

4.1. Welded Strength for Ultrasonic TWTP Between PLA and POM The ultrasonic TWTP between PLA and POM was achieved because these two matrices were widely employed in industry. The welded strengths of PLA and POM under a welding pressure of 0.4 MPa at different welding time were shown in Figure 15. The welded strengths significantly increased and then decreased with the increase of welding time and the maximum welded strength (43 MPa) was achieved at 2 s and 4 s. It was suggested that the strength could reach the optimum value rapidly under the higher welding pressure and then become weak when the time was too long. It could be considered that ultrasonic TWTP between PLA and POM could be achieved earlier than between PC and PMMA. Polymers 2020, 12, x 16 of 27

Therefore, to achieve the maximum welded strength, the increment of welding area and the removal of air bubble were important. Moreover, shorter welding time and higher welding pressure might be effective for ultrasonic TWTP for PC and PMMA. Air bubbles were at the bottom edge although that was the interface of the top strength specimens for PC and PMMA ultrasonic TWTP. It was suggested that the bubbles could be squeezed out of the interfaces with the melting plastics resin together. Finally, welded strength was more sensitive to welding pressure when compared with welding time.

4. Ultrasonic TWTP Technology of PLA and POM

4.1. Welded Strength for Ultrasonic TWTP Between PLA and POM The ultrasonic TWTP between PLA and POM was achieved because these two matrices were widely employed in industry. The welded strengths of PLA and POM under a welding pressure of 0.4 MPa at different welding time were shown in Figure 15. The welded strengths significantly increased and then decreased with the increase of welding time and the maximum welded strength (43 MPa) was achieved at 2 s and 4 s. It was suggested that the strength could reach the optimum value rapidly under the higher welding pressure and then become weak when the time was too long. ItPolymers could be2020 considered, 12, 759 that ultrasonic TWTP between PLA and POM could be achieved earlier 15than of 26 between PC and PMMA.

60 50 40 30 20

Nominal Stress/MPa Nominal 10 P=0.4MPa 0 0 1 2 3 4 5 6 Welding time/s

Figure 15. Welded strengths of PLA and POM at different welding times under welding pressure of 0.4 MPa. Adapted from Reference [150]. Figure 15. Welded strengths of PLA and POM at different welding times under welding pressure of 0.4Figure MPa. Adapted16 showed from the Reference welded [150]. strengths at different welding time. It could be observed that welded strengths generally increased firstly and then decreased with the increased welding time and Figure 16 showed the welded strengths at different welding time. It could be observed that the maximum strength of 47 MPa was achieved at 3 s. This was quite different from the one under welded strengths generally increased firstly and then decreased with the increased welding time and 0.4 MPa. The optimum strength was not kept for a long time, and was amazingly higher than the the maximum strength of 47 MPa was achieved at 3 s. This was quite different from the one under Polymersreported 2020 results., 12, x 17 of 27 0.4 MPa. The optimum strength was not kept for a long time, and was amazingly higher than the reported results. 50

40

30

20

10 Welding strength/MPa 0.3MPa 0 0 1 2 3 4 5 6 Welding time/s

Figure 16. Welded strengths at different welding times under welding pressure of 0.3 MPa. Adapted Figure 16. Welded strengths at different welding times under welding pressure of 0.3 MPa. Adapted from Reference [150]. from Reference [150]. 4.2. Interfacial Structure and Mechanism Analysis 4.2. Interfacial Structure and Mechanism Analysis Figure 17 showed the rupture surfaces of welded interface between POM and PLA under a weldingFigure pressure 17 showed of 0.4 the MPa rupture and the surfaces welding of timeswelded of 1interface s and 2 s,between respectively. POM Itand was PLA observed under that a weldingmatrices pressure and IPS of were 0.4 MPa melted and enough the welding at the times welding of 1 times and of 2 s, 1 s,respectively. and lots of It non-weld was observed areas that were matricesremained, and supposing IPS were amelted weak weldedenough strength.at the weldin IPS wereg time not of melted1 s, and enough lots of andnon-weld remained areas on were POM remained,side, which supposing was joined a weak to POM welded matrix. strength. The IPS IPS surface were not on melted PLA side enough was veryand tidyremained compared on POM with side,the PLAwhich surface, was joined supported to POM by matrix. the melted The areas IPS surf as shownace on inPLA Figure side 17wasb. Itvery was tidy suggested compared that with POM themolecules PLA surface, could supported diffuse into by IPS the early melted than areas the oneas shown of PLA inside Figure at a 17b. short It weldingwas suggested time. The that melting POM moleculespoint of POMcould was diffuse lower into than IPS thatearly of than PLA. the This one resulted of PLA side in molecular at a short chainswelding of time. POM The beginning melting to pointdiffuse of POM earlier was and lower easier, than and that thus of the PLA. weld This could resulted be carried in molecular partly under chains the of samePOM heatbeginning produced. to diffuseAdditionally, earlier and IPS easier, was melted and thus completely the weld at could the welding be carried time partly of 2 s,under the welded the same area heat was produced. full, and a Additionally,high strength IPS was was obtained. melted completely at the welding time of 2 s, the welded area was full, and a high strength was obtained.

a b

1mm 1mm

c d

1mm 1mm

Figure 17. Rupture surfaces of welded interface between POM and PLA under welding pressure of 0.4 MPa. (a) is POM side at welding time of 1 s, (b) is PLA side at welding time of 1 s, (c) is POM side at welding time of 2 s and (d) is PLA side at welding time of 2 s. Adapted from Reference [150].

5. Ultrasonic TWTP Technology of PLA and PMMA

5.1. IPS Preparation for Ultrasonic TWTP Between PLA and PMMA

Polymers 2020, 12, x 17 of 27

50

40

30

20

10 Welding strength/MPa 0.3MPa 0 0 1 2 3 4 5 6 Welding time/s

Figure 16. Welded strengths at different welding times under welding pressure of 0.3 MPa. Adapted from Reference [150].

4.2. Interfacial Structure and Mechanism Analysis Figure 17 showed the rupture surfaces of welded interface between POM and PLA under a welding pressure of 0.4 MPa and the welding times of 1 s and 2 s, respectively. It was observed that matrices and IPS were melted enough at the welding time of 1 s, and lots of non-weld areas were remained, supposing a weak welded strength. IPS were not melted enough and remained on POM side, which was joined to POM matrix. The IPS surface on PLA side was very tidy compared with the PLA surface, supported by the melted areas as shown in Figure 17b. It was suggested that POM molecules could diffuse into IPS early than the one of PLA side at a short welding time. The melting point of POM was lower than that of PLA. This resulted in molecular chains of POM beginning to diffuse earlier and easier, and thus the weld could be carried partly under the same heat produced. Additionally, IPS was melted completely at the welding time of 2 s, the welded area was full, and a Polymers 2020, 12, 759 16 of 26 high strength was obtained.

a b

1mm 1mm

c d

1mm 1mm

Figure 17. Rupture surfaces of welded interface between POM and PLA under welding pressure of Figure 17. Rupture surfaces of welded interface between POM and PLA under welding pressure of 0.4 MPa. (a) is POM side at welding time of 1 s, (b) is PLA side at welding time of 1 s, (c) is POM side 0.4 MPa. (a) is POM side at welding time of 1 s, (b) is PLA side at welding time of 1 s, (c) is POM side at welding time of 2 s and (d) is PLA side at welding time of 2 s. Adapted from Reference [150]. at welding time of 2 s and (d) is PLA side at welding time of 2 s. Adapted from Reference [150]. 5. Ultrasonic TWTP Technology of PLA and PMMA Polymers5. Ultrasonic 2020, 12, xTWTP Technology of PLA and PMMA 18 of 27 5.1. IPS Preparation for Ultrasonic TWTP Between PLA and PMMA 5.1. IPS Preparation for Ultrasonic TWTP Between PLA and PMMA PLA/PMMAPLA/PMMA blends blends were were prepared prepared as as IPS IPS phase phase to to weld weld PMMA PMMA and and PLA PLA [148]. [148]. PLA/PMMA PLA/PMMA blendsblends with with a avolume volume of of 1:1 1:1 were were molded molded by by injection injection molding molding and and then then modified modified to a to thickness a thickness of 1 of mm1 mm via via a hot-press. a hot-press.

5.2.5.2. Mechanical Mechanical Properties Properties of of IPS IPS for for Ultrasonic Ultrasonic TWTP TWTP Between Between PLA PLA and and PMMA PMMA FigureFigure 18 18 gave gave the the tensile tensile strengths strengths of of matric matriceses and and PLA/PMMA PLA/PMMA blends. blends. PLA PLA and and PMMA PMMA were were typicaltypical brittle brittle polymer polymer with with the the yield yield strengths strengths of of 58 58 MPa MPa and and 61 61 MPa, MPa, respectively. respectively. Their Their nominal nominal strainsstrains were were about about 10%. 10%. With With the the increased increased content content of of PLA, PLA, the the yield yield strength strength of of PLA/PMMA PLA/PMMA blends blends enhancedenhanced (Figure (Figure 19). 19).

80

60

40 PMMA 60%PLA 20%PLA 80%PLA 20

Nominal stress/ MPa 40%PLA PLA 50%PLA 0 0 10 20 Nominal strain/ %

Figure 18. Tensile strengths of matrices and PLA/PMMA blends. Adapted from Reference [148]. Figure 18. Tensile strengths of matrices and PLA/PMMA blends. Adapted from Reference [148].

Dramatically, the yield strength of PLA/PMMA blends with 50% PLA was the highest and the strain was about 180%. According to blend principle, the tensile strengths of blends should be in the range between PLA and PMMA matrices. However, for the PLA/PMMA blends, the tensile strength was enhanced. The maximum yield strength was significantly enhanced to about 73 MPa, which was 18% higher than PLA, showing a typical ductile characteristic. It was indicated that the strength and plasticity of the blends were much enhanced compared with the matrix polymers [148]. The third phase material was the key material of weld for ultrasonic TWTP. High yield strength and plasticity were important and necessary to improve and relax the residual stress inside after weld. For the third materials of ultrasonic TWTP, all PLA/PMMA blends were suitable for employment as IPS. Therefore, the PLA/PMMA (Vol. 50% content of PLA) blend was employed for the following weld of PLA and PMMA due to its optimum properties.

75

70

65

60 Tensile strength/MPa Tensile

55 0 20 40 60 80 100 Content of PLA(V)/%

Figure 19. Tensile strengths of matrices and PLA/PMMA blends. Adapted from Reference [148].

Polymers 2020, 12, x 18 of 27

PLA/PMMA blends were prepared as IPS phase to weld PMMA and PLA [148]. PLA/PMMA blends with a volume of 1:1 were molded by injection molding and then modified to a thickness of 1 mm via a hot-press.

5.2. Mechanical Properties of IPS for Ultrasonic TWTP Between PLA and PMMA Figure 18 gave the tensile strengths of matrices and PLA/PMMA blends. PLA and PMMA were typical brittle polymer with the yield strengths of 58 MPa and 61 MPa, respectively. Their nominal strains were about 10%. With the increased content of PLA, the yield strength of PLA/PMMA blends enhanced (Figure 19).

80

60

40 PMMA 60%PLA 20%PLA 80%PLA 20

Nominal stress/ MPa 40%PLA PLA 50%PLA 0 0 10 20 Nominal strain/ %

Figure 18. Tensile strengths of matrices and PLA/PMMA blends. Adapted from Reference [148].

Dramatically, the yield strength of PLA/PMMA blends with 50% PLA was the highest and the strain was about 180%. According to blend principle, the tensile strengths of blends should be in the range between PLA and PMMA matrices. However, for the PLA/PMMA blends, the tensile strength was enhanced. The maximum yield strength was significantly enhanced to about 73 MPa, which was 18% higher than PLA, showing a typical ductile characteristic. It was indicated that the strength and plasticity of the blends were much enhanced compared with the matrix polymers [148]. The third phase material was the key material of weld for ultrasonic TWTP. High yield strength and plasticity were important and necessary to improve and relax the residual stress inside after weld. For the third materials of ultrasonic TWTP, all PLA/PMMA blends were suitable for employment as IPS. Therefore,Polymers 2020 the, 12 PLA/PMMA, 759 (Vol. 50% content of PLA) blend was employed for the following weld17 ofof 26 PLA and PMMA due to its optimum properties.

75

70

65

60 Tensile strength/MPa Tensile

55 0 20 40 60 80 100 Content of PLA(V)/%

FigureFigure 19. 19. TensileTensile strengths strengths of of matrices matrices and and PLA/PM PLA/PMMAMA blends. blends. Adapted Adapted from from Reference Reference [148]. [148].

Dramatically, the yield strength of PLA/PMMA blends with 50% PLA was the highest and the strain was about 180%. According to blend principle, the tensile strengths of blends should be in the range between PLA and PMMA matrices. However, for the PLA/PMMA blends, the tensile strength was enhanced. The maximum yield strength was significantly enhanced to about 73 MPa, which was 18% higher than PLA, showing a typical ductile characteristic. It was indicated that the strength and plasticity of the blends were much enhanced compared with the matrix polymers [148]. The third phase material was the key material of weld for ultrasonic TWTP. High yield strength and plasticity were important and necessary to improve and relax the residual stress inside after weld. For the third materials of ultrasonic TWTP, all PLA/PMMA blends were suitable for employment as IPS. Therefore, thePolymers PLA /2020PMMA, 12, x (Vol. 50% content of PLA) blend was employed for the following weld19 of 27 PLA and PMMA due to its optimum properties. Figure 20 presented the TG analysis of PLA, PMMA, and PLA/PMMA blends. All of the matrices Figure 20 presented the TG analysis of PLA, PMMA, and PLA/PMMA blends. All of the matrices and blends showed a same decomposition temperature of 320 °C. The thermal properties of the and blends showed a same decomposition temperature of 320 C. The thermal properties of the blends blends were almost same as the matrix, and thus the third phase◦ IPS would not decompose before werethe almostmatrices. same as the matrix, and thus the third phase IPS would not decompose before the matrices.

100

50 10℃/min PLA

Relative weight/ % weight/ Relative PMMA 0 PLA/PMMA(1:1) 0 100 200 300 400 500 Temperature/℃

FigureFigure 20. 20. TG analysis of of PLA, PLA, PMMA, PMMA, and and PLA/PMMA PLA/PMMA blends blends of 1:1. of Adapted 1:1. Adapted from Reference from Reference [151]. [151].

5.3.5.3. Welded Welded StrengthStrength of Ultrasonic Ultrasonic TWTP TWTP Between Between PLA PLA and andPMMA PMMA TheThe welded welded strengths strengths atat didifferentfferent weldingwelding timestimes underunder didifferentfferent welding pressures pressures (P (P) )are are shown inshown Figure in 21 Figure. The 21. welded The welded strengths strengths generally generally increased increased with with an an increase increase of of welding welding timetime from from 1 to 3 s under1 to 3 any s under welded any pressure.welded pressure. The maximum The maximum welded welded strengths strengths of 47 of MPa47 MPa and and 45 45 MPa MPa were were achieved underachieved the weldingunder the pressure welding ofpressure 0.3 MPa of and0.3 MPa welding and welding time of time 3 s, andof 3 weldings, and welding pressure pressure of 0.6 of MPa and welding0.6 MPa time and welding of 4 s, respectively. time of 4 s, respectively.

50

40

30

20 P=0.6MPa 10 P=0.3MPa Welding strength/MPa Welding P=0.1MPa 0 0 1 2 3 4 5 6 Welding time/s

Figure 21. Welded strengths at different welding times under different welding pressures (P). Adapted from Reference [151].

These were similar to the welds between PC and PMMA as well as that between PLA and POM. The rupture surfaces images of welded interface were shown in Figure 22. This indicated the processes of vibrating, heat production, and welded areas enlarging. The decrease of welded strength suggested that too long a welding time or excessively strong welding pressure were harmful due to the interface overheating.

Polymers 2020, 12, x 19 of 27

Figure 20 presented the TG analysis of PLA, PMMA, and PLA/PMMA blends. All of the matrices and blends showed a same decomposition temperature of 320 °C. The thermal properties of the blends were almost same as the matrix, and thus the third phase IPS would not decompose before the matrices.

100

50 10℃/min PLA

Relative weight/ % weight/ Relative PMMA 0 PLA/PMMA(1:1) 0 100 200 300 400 500 Temperature/℃

Figure 20. TG analysis of PLA, PMMA, and PLA/PMMA blends of 1:1. Adapted from Reference [151].

5.3. Welded Strength of Ultrasonic TWTP Between PLA and PMMA The welded strengths at different welding times under different welding pressures (P) are shown in Figure 21. The welded strengths generally increased with an increase of welding time from 1 to 3 s under any welded pressure. The maximum welded strengths of 47 MPa and 45 MPa were achieved under the welding pressure of 0.3 MPa and welding time of 3 s, and welding pressure of Polymers 2020, 12, 759 18 of 26 0.6 MPa and welding time of 4 s, respectively.

50

40

30

20 P=0.6MPa 10 P=0.3MPa Welding strength/MPa Welding P=0.1MPa 0 0 1 2 3 4 5 6 Welding time/s

Figure 21. Welded strengths at different welding times under different welding pressures (P). Adapted Figure 21. Welded strengths at different welding times under different welding pressures (P). from Reference [151]. Adapted from Reference [151]. These were similar to the welds between PC and PMMA as well as that between PLA and These were similar to the welds between PC and PMMA as well as that between PLA and POM. POM. The rupture surfaces images of welded interface were shown in Figure 22. This indicated the The rupture surfaces images of welded interface were shown in Figure 22. This indicated the processesprocesses of vibrating, of vibrating, heat heat production, production, and and weldedwelded areas areas enlarging. enlarging. The The decrease decrease of welded of welded strength strength suggestedsuggested that that too too long long a weldinga welding time time oror excessivelyexcessively strong strong welding welding pressure pressure were were harmful harmful due to due to the interfacethePolymers interface 2020 overheating., 12 overheating., x 20 of 27

a PLA IPS

500µm

b PLA

IPS

500µm

c

PLA

500µm

d

IPS

500µm

FigureFigure 22. SEM 22. SEM images images of of rupture rupture surfaces surfaces underunder weldin weldingg pressure pressure of of0.3 0.3 MPa MPa and and welding welding time timeof of (a) 1 s,(a) ( b1 )s, 2 (b s,) (2c s,) 3(c s) and3 s and (d )(d 5) s.5 s. Adapted Adapted from from ReferenceReference [151]. [151].

Therefore, ultrasonic TWTP between PC and PMMA, PLA and POM, PLA and PMMA were all achieved using the third phases of IPS prepared from the matrices. Welded strength could be more than 80% of the tensile or yield strength of the matrices. Appropriate welding conditions were necessary for the optimum welded strength, but too long a time and high a pressure would make them weak and induce damage due to the overheating and decomposing.

6. Molecular Interdiffusion Analysis of Ultrasonic TWTP Technology

6.1. Interdiffusion Analysis of Ultrasonic TWTP The schematic drawing of macromolecular inter-diffusion was shown in Figure 23. When the temperature was higher than glass transition temperature (Tg), the molecular chains became loos and began to move due to the frictional heat (Figure 23(2)). Then, when the temperature reached above the melting point (Tm), the material softened and melted so that molecular chains became free and could inter-diffuse across the weld interface (Figure 23(3)) [147]. Eventually, the whole chains forgot their initial conformation, and the center of mass diffusion took places, and thus welds could achieve. It could give the welding process of the molecular motion and show the reason for why IPS was partly welded to one matrix side at first. For the most important, they could explain the reason for the successes of weld for dissimilar materials.

Polymers 2020, 12, 759 19 of 26

Therefore, ultrasonic TWTP between PC and PMMA, PLA and POM, PLA and PMMA were all achieved using the third phases of IPS prepared from the matrices. Welded strength could be more than 80% of the tensile or yield strength of the matrices. Appropriate welding conditions were necessary for the optimum welded strength, but too long a time and high a pressure would make them weak and induce damage due to the overheating and decomposing.

6. Molecular Interdiffusion Analysis of Ultrasonic TWTP Technology

Interdiffusion Analysis of Ultrasonic TWTP The schematic drawing of macromolecular inter-diffusion was shown in Figure 23. When the temperature was higher than glass transition temperature (Tg), the molecular chains became loos and began to move due to the frictional heat (Figure 23b). Then, when the temperature reached above the melting point (Tm), the material softened and melted so that molecular chains became free and could inter-diffuse across the weld interface (Figure 23c) [147]. Eventually, the whole chains forgot their initial conformation, and the center of mass diffusion took places, and thus welds could achieve. It could give the welding process of the molecular motion and show the reason for why IPS was partlyPolymers welded 2020, 12 to, x one matrix side at first. For the most important, they could explain the reason for21 theof 27 successes of weld for dissimilar materials.

interface minor

force force vibration vibration

WP IPS WP IPS (a) (b)

interface minor

force vibration force vibration

WP IPS Rg2 Rg1 (c) (d)

Figure 23. Schematic drawing of macromolecular inter-diffusion of IPS and WPs. Adapted from ReferenceFigure 23. [147 Schematic]. Temperatures drawing of of the macromolecular interfaces are (a )inter- underdiffusion room temperature; of IPS and (WPs.b) higher Adapted than glassfrom transitionReference temperature; [147]. Temperatures (c) above of melting the interfaces point; (ared) back(1) under to room room temperature. temperature; (2) higher than glass transition temperature; (3) above melting point; (4) back to room temperature.

7. Conclusions and Work in Future USW is a convenient way for polymers welds, and it is a promising method for welds between dissimilar materials. As a result, the ultrasonic TWTP method has been proposed and confirmed as an effective technology. This review focused on the thermal welding methods for polymers. The ultrasonic TWTP technology of PC and PMMA, PLA and POM, and PLA and PMMA were investigated according to the third phase preparation, welded strengths, SEM images of rupture surfaces, thermal stability and others. The welded strength could reach more than 80% of the tensile or yield strengths of the matrices. Materials for the third phases of IPS were suitable for the weld due to their excellent mechanical and thermal properties, where functionally gradient materials were the most important for the most excellent ultrasonic TWTP. Moreover, appropriate welding conditions were necessary for the optimum welded strength, but too long a time and high a pressure could make them weak and induce damage due to the overheating and decomposing. Finally, it is concluded that ultrasonic TWTP technology can achieve welds evidently. The interdiffusion analysis of the molecular motion is studied carefully, which explains the reason for the successes of weld for dissimilar materials. Composite materials, particularly carbon fiber reinforced polymers (CFRP), have been increasingly used for applications in aircraft structures due to their great potential for weight reduction [152]. In the future, the welding work will be focused on the weld between the composites of CFRC, CNT/polymer composite, and other conductive composites. Also, degradable and medical composites will be considered in future studies due to their needs for the following application.

Polymers 2020, 12, 759 20 of 26

7. Conclusions and Work in Future USW is a convenient way for polymers welds, and it is a promising method for welds between dissimilar materials. As a result, the ultrasonic TWTP method has been proposed and confirmed as an effective technology. This review focused on the thermal welding methods for polymers. The ultrasonic TWTP technology of PC and PMMA, PLA and POM, and PLA and PMMA were investigated according to the third phase preparation, welded strengths, SEM images of rupture surfaces, thermal stability and others. The welded strength could reach more than 80% of the tensile or yield strengths of the matrices. Materials for the third phases of IPS were suitable for the weld due to their excellent mechanical and thermal properties, where functionally gradient materials were the most important for the most excellent ultrasonic TWTP. Moreover, appropriate welding conditions were necessary for the optimum welded strength, but too long a time and high a pressure could make them weak and induce damage due to the overheating and decomposing. Finally, it is concluded that ultrasonic TWTP technology can achieve welds evidently. The interdiffusion analysis of the molecular motion is studied carefully, which explains the reason for the successes of weld for dissimilar materials. Composite materials, particularly carbon fiber reinforced polymers (CFRP), have been increasingly used for applications in aircraft structures due to their great potential for weight reduction [152]. In the future, the welding work will be focused on the weld between the composites of CFRC, CNT/polymer composite, and other conductive composites. Also, degradable and medical composites will be considered in future studies due to their needs for the following application.

Author Contributions: J.Q.: conceptualization, supervision, project administration, review and validation. G.Z.: writing of original draft preparation, methodology, data analysis and editing. E.S.: review and validation. W.L.: review and validation. L.Z.: review and validation. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

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