Investigation on the Mechanism and Failure Mode of Laser

materials Article

Investigation on the Mechanism and Failure Mode of Laser Transmission Spot Welding Using PMMA Material for the Automotive Industry Xiao Wang *, Baoguang Liu, Wei Liu, Xuejiao Zhong, Yingjie Jiang and Huixia Liu School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, China; [email protected] (B.L.); [email protected] (W.L.); [email protected] (X.Z.); [email protected] (Y.J.); [email protected] (H.L.) * Correspondence: [email protected]; Tel.: +86-511-8879-7998; Fax: +86-511-8878-0276 Academic Editor: Daolun Chen Received: 22 November 2016; Accepted: 26 December 2016; Published: 1 January 2017

Abstract: To satisfy the need of polymer connection in lightweight automobiles, a study on laser transmission spot welding using polymethyl methacrylate (PMMA) is conducted by using an Nd:YAG pulse laser. The influence of three variables, namely peak voltages, defocusing distances and the welding type (type I (pulse frequency and the duration is 25 Hz, 0.6 s) and type II (pulse frequency and the duration is 5 Hz, 3 s)) to the welding quality was investigated. The result showed that, in the case of the same peak voltages and defocusing distances, the number of bubbles for type I was obviously more than type II. The failure mode of type I was the base plate fracture along the solder joint, and the connection strength of type I was greater than type II. The weld pool diameter:depth ratio for type I was significantly greater than type II. It could be seen that there was a certain relationship between the weld pool diameter:depth ratio and the welding strength. By the finite element simulation, the weld pool for type I was more slender than type II, which was approximately the same as the experimental results. Keywords: laser technique; laser transmission spot welding; welding mechanism; thermoplastic polymer; morphology of weld pool

1. Introduction In recent years, with the increasingly severe issues of global resources and environmental protection, innovative research of lightweight products for energy savings and environmental protection is becoming a new trend of development. Especially in the automotive industry, researchers are working to reduce the weight of the body for alleviating the problem of energy consumption and tail gas pollution caused by the growing number of cars. Thermoplastic polymer materials have the advantages of being light weight, having a high level of strength, low density, easy molding, low cost, good flexibility, corrosion resistance, etc. [1,2]. Therefore, the thermoplastic polymer materials become the first choice for replacing steel and cast iron. Spot welding technology, as a kind of high efficient polymer connection technology, has been widely used in automobile manufacturing [3]. The traditional methods of spot welding include friction stir welding, ultrasonic spot welding, friction lap joining, etc. [4–6]. Ultrasonic welding, now an important welding method, has the best welding precision. However, there are limitations due to it being contact welding. Mustafa et al. [4] studied the process parameters of friction stir welding using high-density polyethylene (HDPE). The depth, velocity and time of rotation was controlled, the new theory of the Taguchi method was used to optimize the experimental parameters. Jeng et al. [7] studied the relationship between different ultrasonic spot welding parameters and welding quality. The research also found that the temperature rise during the welding process also affected the initial

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bonding strength of the welding parts. Paoletti et al. [8] analyzed the force and torque developing during friction stir spot welding (FSSW) of thermoplastic sheets varying the main process parameters. According to the achieved results, using low values of the plunging speed has beneficial effects on both the process (reduction in the force and torque) and the mechanical behaviour of the joints. Increasing the tool rotational speed results in reduced processing forces and higher material mixing and temperature. Jeng et al. [9] studied polycarbonate sheets in order to assess the influence of the tool geometry on the joining loads and material flow by friction stir spot welding. An instrumented drilling machine was used to measure the plunging load and the torque developing during the process. The analysis of material flow enabled understanding the behavior of the load and torque trends measured during the process. Okada et al. [6] proposed a Friction Lap Process to join a metallic material with a polymer and investigated mechanical and metallurgical properties of this dissimilar joint. In this paper, the joining mechanism was discussed with evaluation of the microstructure at the interface between aluminum alloy and polymer. Lambiase et al. [10] analyzed the influence of the processing speeds and processing times on mechanical behaviour of friction stir spot welding joints produced on polycarbonate sheets. The analysis involved the variation of rotational speed, tool plunge rate, pre-heating time, dwell time and waiting time. Compared to the traditional polymer spot welding technology, laser spot welding is non-contact welding and has the advantages of high welding quality, being eco-friendly, having a small heat affected zone, etc. [11,12]. In the laser spot welding technology, laser transmission spot welding uses the light transmissions of thermoplastics to weld components. Thus, the laser transmission spot welding is the most potential welding method in the trend of substituting steel for plastic, and this technology is gradually applied to all walks of life. In recent years, the research on laser transmission spot welding of polymers is rarely involved. Visco et al. [12] used an Nd:YAG pulsed laser welding of polymers to optimize the welding parameters, and the experiment mainly focused on the single factor optimization regarding the action time. Yusof et al. [13] studied the welding performance on laser transmission welding of plastics with different metals using microscopic observations and tensile tests. The difference of the connection strength between different welding materials was obtained by controlling the welding parameters. However, there are few studies on the mechanism and failure mode of laser transmission spot welding, especially for the laser transmission spot welding of polymers. Lambiase et al. [14] investigated Laser-Assisted Metal and Plastic bonding (LAMP) of AISI304 sheets with polycarbonate sheets, which introduced an integrated experimental approach aimed at understanding how the main process conditions influence welding quality, dimensions and presence of defects. With the development of society, the lack of scientific and reasonable connection mechanism analysis will affect the application of laser transmission spot welding technology in the industry In this paper, the widely used thermoplastic polymer polymethyl methacrylate (PMMA) in the automotive industry was chosen as the research object [15]. The Nd:YAG pulse laser with a wavelength of 1064 nm was used to weld materials during laser transmission spot welding. This research mainly adopted two kinds of welding types. Through the comparative study of the bubble and the failure mode, the welding mechanism was further understood. Then, through the optical microscope observation of the weld pool, the weld pool diameter:depth ratio corresponding to two welding types was analyzed, which helped us understand the effect of the melt pool diameter:depth ratio on the welding strength. Finally, the influence of laser energy density on the welding pool was obtained by the finite element simulation. The welding pool for different welding types was compared and analyzed. 2. Materials and Methods In this experiment, the upper and lower materials are thermoplastic polymers PMMA, and the sample size is 50 mm × 20 mm × 1.5 mm. The research adopted the Nd:YAG pulsed laser with a wavelength of 1064 nm and the peak power was 7 kW (Rofin, München, Germany), which can directly set up peak voltages and frequency et al. In the laser transmission welding process,

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Materials 2017, 10, 22must be transparent. The underlying material should have higher absorption 3 of 14 the upper material properties [16]. The transparent material must have good transparency. The transmittance of PMMA upper material must be transparent. The underlying material should have higher absorption in the wavelength of 1064 nm was 86%, measured by ultraviolet visible near infrared absorption properties [16]. The transparent material must have good transparency. The transmittance of PMMA spectroscopy (Spectrograph Cary 5000, Varianby Corporation, Palo Alto, CA, USA). Therefore, in the wavelength of 1064Type nm was 86%, measured ultraviolet visible near infrared absorption it wasspectroscopy necessary to(Spectrograph add absorbent Clearweld (c/o Crysta-Lyn Chemical Company, Inc., Binghamton, Type Cary 5000, Varian Corporation, Palo Alto, CA, USA). Therefore, it NY, USA) on the surface the lower layer material in the experiment. Since the Inc., amount of absorbent was necessary to addofabsorbent Clearweld (c/o Crysta-Lyn Chemical Company, Binghamton, USA) process on the surface of an theimportant lower layer material in the experiment. Since the amount of on in theNY, welding was also factor, the amount of absorbent Clearweld painted absorbent in the welding process was also an important factor, the amount of absorbent Clearweld each sample should remain approximately the same. painted the on each sample should remain approximately same. During experiment, the pneumatic clamping the device was adopted to ensure the uniformity During the experiment, the pneumatic clamping device was adopted to ensure the uniformity of the clamping force in the process of laser transmission spot welding. After many experiments, of the clamping force in the process of laser transmission spot welding. After many experiments, the the clamping force of 20 N is a relatively ideal value. The K9 glass was used as the clamping layer. clamping force of 20 N is a relatively ideal value. The K9 glass was used as the clamping layer. The The schematic diagram of the experimental device of laser transmission spot welding was shown in schematic diagram of the experimental device of laser transmission spot welding was shown in FigureFigure 1. 1.

Figure 1. Schematic diagram of a laser transmission spot welding experiment device.

Figure 1. Schematic diagram of a laser transmission spot welding experiment device.

In this experiment, the pulse width of the laser was 2 ms for all welded samples, the pulse frequency’s duration was Hz. In addition, 0.6laser s waswas recorded type pulsesamples, frequency,the andpulse In this experiment, the 25 pulse width of the 2 msasfor allI and welded the duration was 5 Hz. Furthermore, 3 s was recorded as type II (see Table 1), the pulse frequency frequency’s duration was 25 Hz. In addition, 0.6 s was recorded as type I and pulse frequency, and the was was reduced five times and duration times. Two Table parameters peak was duration 5 Hz. Furthermore, 3 s was increased recorded five as type II (see 1), thewere pulsevaried: frequency voltage and defocusing distance. Three peak voltages (440 V, 460 V, 480 V) and three defocusing reduced five times and duration increased five times. Two parameters were varied: peak voltage and distances (6 mm, 8 mm, 10 mm) were studied. defocusing distance. Three peak voltages (440 V, 460 V, 480 V) and three defocusing distances (6 mm, After welding, the tensile test was carried out by the microcomputer control electronic 8 mm,universal 10 mm) testing were studied. machine (Instron Type UTM 4104, Shenzhen, China). The maximum tensile After welding, the welding tensile test was carried out by and the microcomputer electronic universal strength of the two methods were obtained analyzed. In ordercontrol to get better results for testing machine (Instron Type UTM 4104, Shenzhen, China). The maximum tensile strength of the the tensile tests, the tensile speed should try to be controlled at less than 0.5 mm/min during shear tensile experiments. Then, obtained the microstructure of the solder jointsto was by Kean's two welding methods were and analyzed. In order getobserved better results forelectron the tensile Corporation, Japan), and the cross morphology of the solder tests, microscope the tensile(Keyence speed should try to Osaka, be controlled at less than section 0.5 mm/min during shear tensile joints was analyzed under the two welding methods. The weld depth and weld diameter were experiments. Then, the microstructure of the solder joints was observed by Kean’s electron microscope measured by a polarizing (Axio Carlmorphology Zeiss, Oberkochen, The was (Keyence Corporation, Osaka,microscope Japan), and theLab.A1 cross pol, section of theGermany). solder joints relationship between the weld diameter:depth ratio and the maximum tensile strength was analyzed under the two welding methods. The weld depth and weld diameter were measured by obtained. Finally, the influence of laser energy density on the welding temperature was obtained by a polarizing microscope (Axio Lab.A1 pol, Carl Zeiss, Oberkochen, Germany). The relationship finite element simulation. The weld pool in different welding types is compared and analyzed.

between the weld diameter:depth ratio and the maximum tensile strength was obtained. Finally, the influence of laser energy density on the welding temperature was obtained by finite element simulation. The weld pool in different welding types is compared and analyzed.

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Table 1. Experimental parameters of spot welding. Group

Sample Size (n)

Weld Type

Peak Voltage (V)

Defocusing Distance (mm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3

type I type I type I type I type I type I type I type I type I type II type II type II type II type II type II type II type II type II

440 440 440 460 460 460 480 480 480 440 440 440 460 460 460 480 480 480

6 8 10 6 8 10 6 8 10 6 8 10 6 8 10 6 8 10

3. Results and Discussion 3.1. The Effect of Bubbles in the Spot Welding Area Laser transmission spot welding is a kind of hot melt welding, and the polymer absorbs some energy and converts it into heat, so that the upper layer and lower layer material can be melted and mixed to form the weld. However, during the welding process, the heating zone often produces bubbles and ablation due to defects such as welding conditions and process, which affects the appearance and welding performance. Figure 2 shows the welding morphology of PMMA in the type I, 460 V, 8 mm and type II, 460 V, 8 mm. Figure 2b,e shows the weld morphology of type I, 460 V, 8 mm and type II, 460 V, 8 mm when the magnification is 100 times. A certain difference in the morphology between the two analyzed cases is appreciable in the higher magnification pictures. A small number of large bubbles and many small bubbles can be seen in Figure 2c. For the high input laser energy density, the degradation and bubbles occur in the weld. Meanwhile, the short action time leads to the escape of bubbles, and it is easy to generate many bubbles [17]. For the same peak voltage and defocusing distance, long laser action time and low pulse frequency make the molten pool of small bubbles have enough time to converge and escape, which leads to some very small bubbles occurring, as shown in Figure 2f. In the process of thermal degradation of plastics, these bubbles mainly consist of water vapor, carbon dioxide, carbon monoxide, and hydrocarbons [18]. These tiny bubbles will cause the material surface to generate pits and cracks, which can trigger the micro-anchor mechanism and increase the joining strength. Liu et al. [19] studied the friction lap welding between the aluminum alloy and polyethylene glycol terephthalate, and they found that these bubbles produce high pressure and make the fused plastic flow onto the pits or holes of the metal surface, so as to provide more mechanical binding to achieve a tight connection between metals and polymers. Therefore, to a certain extent, the bubbles have certain benefits to improve the welding quality.

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Figure Figure2.2.Weld Weldmorphology morphologyof oftype typeI,I,460 460V, V,88mm. mm.(a) (a)Magnified Magnified50 50times; times;(b) (b)magnified magnified100 100times; times; Figure 2. Weld morphology of type I, 460 V, 8 mm. (a) Magnified 50 times; (b) magnified 100 times; (c) (c) magnified magnified 500 500 times. times. Weld Weld morphology morphology of of type type II, II, 460 460 V, V, 88 mm; mm; (d) (d) magnified magnified 50 50 times; times; (e) (e) (c) magnified 500 times. Weld morphology of type II, 460 V, 8 mm; (d) magnified 50 times; (e) magnified magnified 100 times; and (f) magnified 500 times. magnified 100 times; and (f) magnified 500 times. 100 times; and (f) magnified 500 times.

3.2. 3.2.Shear ShearFailure FailureAnalysis Analysis 3.2. Shear Failure Analysis Failure Failure analysis analysis plays plays aa very very important important role role and and isis significant significant in in the the manufacturing manufacturing of of Failure analysis plays a very important role and is significant in the manufacturing of mechanical mechanical mechanical parts. parts. Through Through the the failure failure analysis, analysis, itit isis helpful helpful to to formulate formulate the the process process parameters parameters parts. Through the failurethe analysis, it is helpful to formulate [20]. the process parameters reasonably and reasonably reasonably and and improve improve the adaptability adaptability of of the the application application [20].During During the the tensile tensile shear shear tests, tests, the the improve the adaptability of the application [20]. During the tensile shear tests, the failure modes of failure failure modes modes of of plastic plastic welding welding parts parts are are shown shown in in Figure Figure 3,3, which which have have been been adapted adapted from from plastic weldingused parts are shown in Figure 3, which [21,22]. have been adapted from classifications used with classifications classifications used with with adhesive-based adhesive-based joints joints [21,22]. There There are are two two main main types types of of failure: failure: adhesive-based joints [21,22]. There are two main types of failure: interfacial and substrate. The failure interfacial interfacial and and substrate. substrate. The The failure failure of of the the interface interface isis mainly mainly due due to to the the tensile tensile strength strength of of the the of the interface is mainly due to than the tensile strength of the welded joint, which is smaller than that of welded joint, which is smaller that of the base plate. When the welding intensity is getting welded joint, which is smaller than that of the base plate. When the welding intensity is getting the basethe plate. When the welding along intensity is getting larger, the fracture will generate along the weld larger, larger, thefracture fracturewill willgenerate generate alongthe theweld weldseam seamand andeven eventhe thecenter centerof ofthe thebase baseplate platewill willbe be seam and even the center of the base plate will be broken. broken. broken.

Figure Failure Figure3. Failuremodes modesof ofplastic plasticwelding weldingparts. parts.(a) (a)Interface Interfacefailure; failure;(b) (b)the thebase basesheet sheetfracture fracturealong along Figure 3.3.Failure modes of plastic welding parts. (a) Interface failure; (b) the base sheet fracture along the welding joint; (c) bulk base sheet fracture the welding joint; (c) bulk base sheet fracture the welding joint; (c) bulk base sheet fracture

In order to analyze the different fracture modes of plastic welding parts, the pulse voltage used in the experiments was 480 V, and the defocusing distance was 6 mm. The morphology changes of

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the different fracture modes of plastic welding parts, the pulse voltage6used of 13 in the experiments was 480 V, and the defocusing distance was 6 mm. The morphology changes of the welding spots after shearing tests were observed by the KEYENCEVHX-1000C digital the welding (Keyence spots afterCorporation). shearing testsUnder were observed by the KEYENCEVHX-1000C digital microscope the above-mentioned process parameters (480microscope V, 6 mm), (Keyence Corporation). Under the above-mentioned process parameters (480 V, 6 mm), Figure shows Figure 4 shows the two different fracture forms of plastic welding parts, namely type I and4type II, the two different fracture forms of plastic welding parts, namely type I and type II, which were which were conducted by using a controlled electronic universal testing machine (Instron Type conducted using aChina). controlled universal testing machine (Instron Type UTM 4104, UTM 4104, by Shenzhen, Due electronic to the welding strength being higher than the tensile strength of Shenzhen, China). Due to the welding strength being higher than the tensile strength of the base plate, the base plate, the failure mode of type I is the base plate fracture that generates along the welding the failure modethe of type I is the base plate fracture that generates along However, joint. However, bonding strength of type II is obviously smaller thanthe thatwelding of type joint. I, which results the bonding strength of type II is obviously smaller than that of type I, which results in the interfacial in the interfacial failure. Then, further analysis has been carried by the KEYENCEVHX-1000C digital failure. Then, further analysis has been carried by the KEYENCEVHX-1000C digital microscope. microscope.

Figure 4. The fracture modes of 480 V, 6 mm. (a) type I; (b) type II. Figure 4. The fracture modes of 480 V, 6 mm. (a) type I; (b) type II.

Figure 5 shows post-failure images of the bond interface for the samples (type II, 480 V, 6 mm), Figure 5 shows post-failure images of the bond interface for the 5a, samples (type II, 480 V,ablation 6 mm), and the tensile failure mode is interface failure. As shown in Figure there is an obvious and the tensile failure mode is interface failure. As shown in Figure 5a, there is an obvious ablation phenomenon in the central area of laser irradiation, which is caused by the high density of energy phenomenon in the area of laser irradiation, which region, is caused by the density of energy input. Materials willcentral become more brittle in the ablation where it high is easy to produce the input. Materials will become more brittle in the ablation region, where it is easy to produce the brittle brittle fracture phenomenon. Nevertheless, the energy input density becomes smaller at the edge of fracture phenomenon. Nevertheless, the energy inputweaker. density becomes smaller at the edge of the weld the weld area, and the pyrolysis effect becomes At the same time, the diffusion and area, and the pyrolysis effectand becomes At chains the same time, the diffusion andcan entanglement of the entanglement of the upper lower weaker. molecular become stronger, which produce obvious upper and lower molecular chains become5b,c. stronger, which can produce obvious ductile tearing, local ductile tearing, as shown in Figure During the stretching process, thelocal fracture surface of as shown in Figure 5b,c. During the stretching process, the fracture surface of the sample is a mixed the sample is a mixed type of fracture including the brittle fracture and ductile fracture. According to type of fractureresults, including brittle fracture ductile fracture. According to the measuring results, the measuring thethe maximum load is and about 268 N. the maximum load ispost-failure about 268 N. Figure 6 shows images of the bond interface for the samples (type I, 480 V, 6 mm). Figure 6 shows post-failure of the interface theissamples 480 V, 6 along mm). As can be seen from Figure 6a, it images is obvious thatbond tensile failure for mode the base(type sheetI, fracture As be seen fromwhich Figureshows 6a, it isthat obvious that tensile failure is the even base sheet fracture along the thecan welding joint, the welding strength ismode very high, higher than the tensile welding joint, which shows that the welding strength is very high, even higher than the tensile strength strength of the substrate. Figure 6a is the failure morphology of welded sample, when the of the substrate. 6a isThe the phenomenon failure morphology of welded sample, when magnification is magnification is Figure 200 times. of tensile deformation cannot bethe seen in this figure. 200 times. The phenomenon of tensile deformation cannot be seen in this figure. However, it can be However, it can be seen that there are a lot of bubbles generated. Furthermore, the number and size seen there are are closely a lot of related bubblestogenerated. Furthermore, the number andThese size of the bubbles of thethat bubbles the peak voltage and pulse frequency. bubbles make are the closely related to layers the peak voltage andachieve pulse frequency. Thesewhich bubbles make the the welding upper and lower upper and lower of the material micro riveting, increases strength. layers of the material microlaser riveting, which increases the welding Fromand Figure 6b,c, From Figure 6b,c, dueachieve to the large energy density, the serious ductilestrength. deformation the local due to the large laser energy density, the serious ductile deformation and the local brittle fracture brittle fracture occur in the specimen during the tensile process. The failure of the materials can be occur in thetospecimen the tensile process. failure of the materials be attributed to attributed the stressduring concentration induced by The bubbles, ablation and othercan defects during the the stress concentration induced by bubbles, ablation and other defects during the welding process. welding process. The unbalanced stress was caused by delamination in the welding seam. However, The unbalanced stress was caused by and delamination in the welding seam. occurs However, owing to the owing to the materials being melted fully combined, intense action in the molecular materials being melted and fully combined, intense action occurs in the molecular chain. The maximum chain. The maximum load of the welding joint is higher, which is about 350 N. load of the welding joint is higher, which is about 350 N.

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Figure 5. Post-failure images of the bond interface for the samples (type II, 480 V, 6 mm), Figure 5. Post-failure images of the bond interface for the samples (type II, 480 V, 6 mm), (a) magnified 5. Post-failure of the 500 bond interface for the 1000 samples (type II, 480 V, 6 mm), (a)Figure magnified 100 times; images (b) magnified times; (c) magnified times. 100(a) times; (b) magnified (c) magnified times. 1000 times. magnified 100 times;500 (b)times; magnified 500 times; 1000 (c) magnified

Figure 6. Post-failure images ofbond the bond interface the samples (type 480 V, mm), Figure 6. Post-failure images of the interface for thefor samples (type I, 480 V, I, 6 mm), (a) 6magnified Figure 6. Post-failure images of the bond interface for the samples (type I, 480 V, 6 mm), (a) magnified 200 times; (b) magnified 500 times; (c) magnified 1000 times. 200 times; (b) magnified 500 times; (c) magnified 1000 times. (a) magnified 200 times; (b) magnified 500 times; (c) magnified 1000 times.

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3.3. The Effects of Pulsed Laser Parameters on the Weld Dimensions 3.3. The of Pulsed zone Laser (HAZ) Parameters on thedefined Weld Dimensions TheEffects heat-affected is well in the welding of metals. It is the region of the material near the weld and microstructural occur a result The heat-affected zonewhere (HAZ)property is well defined in the weldingchanges of metals. It isasthe regionofofheat the conduction from weldwhere seam [23]. An HAZ also occur inchanges polymeroccur welding material near thetheweld property and can microstructural as [24,25]. a resultInoforder heat to analyze the relationship between welding strength and in weld diameter:depth of weld in two conduction from the weld seam [23]. An HAZ can also occur polymer welding [24,25]. In order to forms, the KEYENCEVHX-1000C digital microscope (Keyence Corporation) was used to observe analyze the relationship between welding strength and weld diameter:depth of weld in two forms, the of the weld pool. The morphology the weld pool shown Figurethe 7. the microstructure KEYENCEVHX-1000C digital microscope (Keyence of Corporation) wasisused to in observe When measuringofthe of the weld pool, due to welding joint, the weld microstructure thediameter weld pool. The morphology ofthe theirregularity weld pool of is the shown in Figure 7. When diameter is calculated as: of the weld pool, due to the irregularity of the welding joint, the weld measuring the diameter diameter is calculated as: weld diameter = (transverse diameter + conjugate diameter)/2. (1) weld diameter = (transverse diameter + conjugate diameter)/2. (1) The The parameters parameters of of weld weld diameter diameter and and weld weld depth depth were were measured measured thrice thrice and and average average value value was was obtained for each set of experiments. obtained for each set of experiments.

Figure 7. 7. Morphology Morphology of of weld weld pool pool (a) (a) weld weld diameter; diameter; (b) (b) weld weld depth. depth. Figure

It is very important to improve the quality of the connection by studying the interactive effects It is very important to improve the quality of the connection by studying the interactive effects of of process parameters on the welding pool forming. Figure 8 shows the effect of peak voltage on the process parameters on the welding pool forming. Figure 8 shows the effect of peak voltage on the weld weld diameter, weld depth and the weld diameter:depth ratio of type I and type II, where the diameter, weld depth and the weld diameter:depth ratio of type I and type II, where the defocusing defocusing distance is 8 mm. As shown in Figure 8a,b, with the increase of the peak voltage, the distance is 8 mm. As shown in Figure 8a,b, with the increase of the peak voltage, the weld diameter weld diameter and weld depth gradually increase. However, under the same peak voltage, the weld and weld depth gradually increase. However, under the same peak voltage, the weld depth of type I is depth of type I is significantly smaller than that of type II, while the weld diameter of type I is always significantly smaller than that of type II, while the weld diameter of type I is always greater than that greater than that of type II. This is because the action time of these two types is different, even of type II. This is because the action time of these two types is different, even though the total output though the total output energy of type I and type II is largely the same. The action time of type II is energy of type I and type II is largely the same. The action time of type II is five times as much as that of five times as much as that of type I. In addition, the upper and lower materials are the transparent type I. In addition, the upper and lower materials are the transparent PMMA. The absorption of laser PMMA. The absorption of laser energy mainly depends on the absorbent called Clearweld. energy mainly depends on the absorbent called Clearweld. Therefore, the melting part of type I mainly Therefore, the melting part of type I mainly concentrates in the vicinity of the absorbent, which concentrates in the vicinity of the absorbent, which makes type I have a larger weld diameter. On the makes type I have a larger weld diameter. On the other hand, the materials in type II have enough other hand, the materials in type II have enough time to melt through the heat conduction, which time to melt through the heat conduction, which leads to a larger weld depth. It is found that the leads to a larger weld depth. It is found that the quality of connection is greatly affected by the weld quality of connection is greatly affected by the weld diameter:depth ratio, which also has a certain diameter:depth ratio, which also has a certain relationship with the tensile load of the connecting piece. relationship with the tensile load of the connecting piece. Figure 8c shows the relationship between Figure 8c shows the relationship between the peak voltage and the weld diameter:depth ratio, where the peak voltage and the weld diameter:depth ratio, where the weld diameter:depth ratio of type I is the weld diameter:depth ratio of type I is significantly larger than that of type II. However, the effect of significantly larger than that of type II. However, the effect of peak voltage on the weld peak voltage on the weld diameter:depth ratio is very small. As shown in Figure 8d, with the increase diameter:depth ratio is very small. As shown in Figure 8d, with the increase of the peak voltage, the of the peak voltage, the tensile loads of type I and type II increase. Moreover, the tensile load of type I tensile loads of type I and type II increase. Moreover, the tensile load of type I was greater than that was greater than that of type II. of type II.

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Figure 8. The effect of peak voltage on the welding conditions of type I and type II when defocusing Figure 8. The effect of peak voltage on the welding conditions of type I and type II when distance is 8 mm. (a) Weld depth; (b) weld diameter; (c) the weld diameter:depth ratio; (d) tensile defocusing distance is 8 mm. (a) Weld depth; (b) weld diameter; (c) the weld diameter:depth ratio; loading. (d) tensile loading.

Figure 9 shows the effect of defocusing distance on the weld diameter, weld depth and the weld Figure 9 shows effectI of defocusing distance thevoltage weld diameter, depth andfrom the diameter:depth ratiothe of type and type II, where the on peak is 460 V. weld As can be seen weld diameter:depth of type I and type II, where the the peak is 460 V. As can the be seen Figure 9a,b, with theratio increase of defocusing distance, weld voltage depth decreases, while weld from Figure 9a,b, with the increase of defocusing distance, the weld depth decreases, while the weld diameter increases. Under the same defocusing distance, the weld depth of type I is less than that of diameter increases. Under the same distance, the than weldthat depth of type is lessFigure than that type II, while the weld diameter of defocusing type I is always greater of type II. IFrom 9c,d,ofit type II, while the weld diameter of and typethe I isweld always greater than that ofof type II. IFrom Figure than 9c,d, that it can can be seen that the tensile loads diameter:depth ratio type are greater of be seen that the tensile loads and the weld diameter:depth ratio of type I are greater than that of type II type II when the defocusing distance is the same. Nevertheless, with the increase of the defocusing when the defocusing distance is the same. with the increase of the defocusing distance, distance, the tensile load decreases, while Nevertheless, the weld diameter:depth ratio increases. In a certain range, the tensile load decreases, while the weld diameter:depth ratio increases. In a certain range, the tensile the tensile loads decrease with the increase of defocusing distance, which has great influence on the loads decreaseofwith thepool increase of defocusing distance, which has great influence on the morphology morphology weld and welding quality. of weld pool and welding quality.

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Figure 9. 9. The onon thethe welding condition of type I and type type II when peak Figure Theeffect effectofofdefocusing defocusingdistance distance welding condition of type I and II when voltage is 460 isV.460 (a) Weld (b) weld(b)diameter; (c) the weld diameter/depth ratio; (d) tensile peak voltage V. (a) depth; Weld depth; weld diameter; (c) the weld diameter/depth ratio; loading. (d) tensile loading.

3.4. Temperature Temperature Distribution Distributionin inDifferent DifferentJoining JoiningTypes Types 3.4. Laser transmission transmission welding welding is is the the process process of of rapid rapid local local heating heating and and rapid rapid cooling. cooling. Therefore, Therefore, Laser we can call laser transmission welding the nonlinear transient heat conduction process. Field we can call laser transmission welding the nonlinear transient heat conduction process. Field function function of the temperature field isof a function space and time domains. and of the temperature field is a function space andoftime domains. However, theHowever, space and the timespace domains time domains are not coupled. Thus, the finite element equation is established by the local are not coupled. Thus, the finite element equation is established by the local discretization method. discretization The heat method. transfer mainly has three basic forms: heat conduction, heat convection and heat The heat has three basic forms: heat conduction, heat transfer convection heat radiation. For transfer the lasermainly transmission welding of polymers, the main heat formand is heat radiation. For the laser transmission welding of polymers, the main heat transfer form is heat conduction. Liao et al. [26] analyzed variations of shear strength that depend on the fiber laser process conduction. Liao et al. [26]ofanalyzed of shear that depend the fiber laser during micro-spot welding AISI 304 variations stainless thin sheets.strength A preliminary study on used ANSYS 12.0 process during micro-spot welding of AISI 304 stainless thin sheets. A preliminary study used results to obtain initial process conditions. The results show that the response surface methodology ANSYSand 12.0 results to obtainneural initialnetwork/integrated process conditions.simulated The results show that the response surface (RSM) back propagation annealing algorithm (BPNN/SAA) methodology (RSM) andtools back neural network/integrated simulated annealing methods are both effective for propagation the optimization of micro-spot welding process parameters. algorithm (BPNN/SAA) methods are both effective tools for the optimization of micro-spot The thermal analysis model is established based on the volumetric heat source. Accordingwelding to heat process parameters. transfer and energy conservation of Fourier’s law, the temperature field control equation of nonlinear The heat thermal analysisismodel based on the volumetric heat source. According to transient conduction given is byestablished [27]: heat transfer and energy conservation of Fourier’s law, the temperature field control equation of    2  nonlinear transient heat conduction is given ∂T by [27]: ∂ T ∂2 T ∂2 T Q = ρ( T ) c ( T ) − λ( T ) + 2 + 2 (2) 2 ∂y 2 T ∂z 2 T   ∂x T   2T  ∂t Q = ρ (T )c (T )    (2)   λ (T )  t x2 y 2 z2  







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where Q refers to the strength of the internal heat source; λ, ρ, and c refer to heat conduction coefficient, density, specific heatstrength of a material, respectively; and T λ,and t refer to temperature and time where and Q refers to the of the internal heat source; ρ, and c refer to heat conduction variable, respectively. coefficient, density, and specific heat of a material, respectively; and T and t refer to temperature and Astime thevariable, materialrespectively. properties and the geometric condition of the object are known, the initial As and the the material properties andare theneeded geometric condition of the solution. object areThe known, initial is condition boundary condition to obtain the special initial the condition condition and the boundary condition are needed to obtain the special solution. The initial condition is T ( x, y, z, 0) = T0 (3)

T ( x, y , z , 0)  T0 Among, ( x, y, z) ∈ D, the boundary condition is Among, ( x, y, z )  D , the boundary condition is ∂T − q + h( T − T0 ) + σε( T 4 − T 4 ) = 0 kn ∂n k T  q  h(T  T )  σε(T 4  T 4 ) 00 n

n

0

0

(3)

(4) (4)

where D refers to the model range; σ, h and ε refer to the coefficient of thermal radiation where D refers to the model range; σ, h and ε refer to the coefficient of thermal radiation (5.67 × 10−8−8W/m22·K), the coefficient of heat convection and the coefficient of thermal radiation, (5.67 × 10 W/m ·K), the coefficient of heat convection and the coefficient of thermal radiation, respectively; T and q refer to temperature and surface heat flux density, respectively. Due to the heat respectively; T and q refer to temperature and surface heat flux density, respectively. Due to the radiation condition, the temperature field distribution becomes a typical nonlinear problem. heat radiation condition, the temperature field distribution becomes a typical nonlinear problem. The change of temperature field and temperature time relationship of the highest point of PMMA The change of temperature field and temperature time relationship of the highest point of is characterized by the thermal analysis The process roughly divided preprocess, PMMA is characterized by the thermalmodel. analysis model. Thecan process canbe roughly beinto: divided into: solution and postprocessor, as shown in Figure 10. The physical performance parameters needed in preprocess, solution and postprocessor, as shown in Figure 10. The physical performance theparameters temperature field in simulation are shown Table 2. are shown in Table 2. needed the temperature fieldin simulation

Figure 10. Simulation analysis process. Figure 10. Simulation analysis process. Table 2. Physical performance parameters of polymethyl methacrylate (PMMA). Table 2. Physical performance parameters of polymethyl methacrylate (PMMA). Coefficient of Viscous Flow Density [28] Specific Heat Thermal Conductivity [28] MaterialDensity [28] Specific Heat Capacity Coefficient of Thermal Viscous Flow 3) Temperature (°C) (kg/m Capacity [28] (J/(kg·K)) Material (W/(m·K)) (kg/m3 ) [28] (J/(kg·K)) Conductivity [28] (W/(m·K)) Temperature (◦ C) PMMA 1180 1470 0.21 220 220 PMMA 1180 1470 0.21

Figure 11 shows the weld pool of Y–Z cross section of the upper and lower layers of the PMMA 11 shows weld of Y–Z cross section of theisupper and the lower layers ofdistance the PMMA in Figure type I and type II the under thepool condition that the peak voltage 460 V and defocusing is in 8type I and type II under the condition that the peak voltage is 460 V and the defocusing mm. The melting point of PMMA material is 385 K. When the temperature is higher than 385 distance K, the is 8upper mm. and The lower melting pointofofmaterials PMMA can material K.At When the temperature is higher 385 K, layers form is a 385 weld. the same time, the weld pool isthan formed theinside upperthe and lower layers of materials can form a weld. At the same time, the weld pool is formed material. As can be seen from the diagram, the red area is the morphology of the weld inside material. canofbetype seenI is from diagram, area is the of the pool,the and the weldAs pool longthe and thin, butthe thered weld depth ofmorphology type II is larger thanweld that pool, of and theI.weld of type I is long and thin, but in thethe weld depth ofarea type is larger of type type The pool maximum temperature is different connection forII type I andthan typethat II. This is I. the temperature energy density is different these two types. Since thetype peak and thethe Thebecause maximum is different in thefor connection area for type I and II. voltage This is because defocusing distance have for beenthese determined, the Since difference of pulse frequency anddefocusing duration leads to energy density is different two types. the peak voltage and the distance different energies in the area. Therefore, theand energy density of type I was much greater have been determined, the connection difference of pulse frequency duration leads to different energies in the than type area. II. In addition, thethe temperature of type was higher than type II. than type II. In addition, connection Therefore, energy density ofI type I was much greater

the temperature of type I was higher than type II.

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Figure 11. Comparison of weld pool when the peak voltage is 460 V and the defocusing distance is 8

Figure mm. 11. Comparison of weld pool when the peak voltage is 460 V and the defocusing distance is (a) type I; (b) type II. 8 mm. (a) type I; (b) type II. 4. Conclusions

4. Conclusions This study assessed the efficacy of the welding types of PMMA using Clearweld as the absorbing medium. the Three variables were investigated: peak voltages (440 V, 460 V,as480 This study assessed efficacy of the welding types of PMMA using Clearweld theV), absorbing defocusing distances (6 mm, 8 mm, 10 mm) and the welding type: type I (pulse frequency and the medium. Three variables were investigated: peak voltages (440 V, 460 V, 480 V), defocusing distances duration for 25 Hz, 0.6 s) or type II (pulse frequency and the duration for 5 Hz, 3 s)). These two types (6 mm, all 8 mm, 10 mm) and the in welding type I (pulse frequency thestronger duration for 25 IIHz, 0.6 s) had good performance welding,type: and the welding strength of typeand I was than type or type for II (pulse frequency and the duration for 5 Hz, 3 s). These two types all had good performance PMMA. conclusions can be results: than type II for PMMA. in welding,The andfollowing the welding strength of made type Ifrom wasthe stronger The following conclusions can be made from the results: (1) Laser transmission spot welding of type I produced a lot of small bubbles and some big

(1)

(2)

(3)

(4)

bubbles. These bubbles made the upper and lower layers produce a micro anchor, improving

Laser transmission spot welding of type I produced a lot of small bubbles and some big bubbles. the welding strength. However, type II produced few bubbles and the bubbles were very small. These bubbles made the upper lower produce a micro anchor, improving theofwelding (2) The welding strength of typeand I was higherlayers than type II. In addition, the tensile failure mode strength. type II produced few andThis the indicated bubbles that were small. typeHowever, I was the base sheet fracture along thebubbles solder joint. thevery welding strength was higher than the of the than substrate. the tensile failure failure mode ofmode of The welding strength oftensile type Istrength was higher typeFurthermore, II. In addition, the tensile is the interface type I type was IIthe base sheet failure. fracture along the solder joint. This indicated that the welding strength (3) The weld pool diameter:depth ratio of type I was obviously larger than type II. The welding was higher than the tensile strength of the substrate. Furthermore, the tensile failure mode of performance was better. It indicated that the weld pool diameter:depth ratio greatly affected the type IIquality is the of interface failure. the joint. The diameter:depth ratio of type I was than throughout type II. The (4) weld Frompool the result of thermal conductive analysis, heatobviously was rapidlylarger distributed thewelding same material of PMMA for the case of the joints. Because laser energy density type I affected was performance was better. It indicated that weld poolthe diameter:depth ratio of greatly the higher than type II, the weld pool of type I is long and thin, and the temperature of type I was quality of the joint. higher than type II. From the result of thermal conductive analysis, heat was rapidly distributed throughout the same material of PMMA for theis case of joints. Because the laser density type I was higher Acknowledgments: This work supported by the National Natural Scienceenergy Foundation of Chinaof(No. 51275219) and type the Changzhou High-Technology Research Key Laboratory (No. CM20153001). than II, the weld pool of type I is long and thin, and the temperature of type I was higher than type II. Author Contributions: Xiao Wang, Baoguang Liu and Huixia Liu conceived and designed the experiments; Yingjie Jiang performed the experiments; Wei Liu analyzed the data; Xuejiao Zhong contributed materials and

tools; Xiao Wang, Baoguang and Huixia the paper. This work isLiu supported byLiu thewrote National Natural Science Foundation of China (No. 51275219) Acknowledgments: and the Changzhou High-Technology Research Key Laboratory (No. CM20153001). Conflicts of Interest: The authors declare no conflict of interest.

Author Contributions: Xiao Wang, Baoguang Liu and Huixia Liu conceived and designed the experiments; References Yingjie Jiang performed the experiments; Wei Liu analyzed the data; Xuejiao Zhong contributed materials and tools; Xiao Wang, Baoguang Liu and Huixia Liu wrote the paper. 1.

Zhang, S. Laser welding technology of plastics. World Plast. 2007, 253, 56–62.

Conflicts authors no conflict of for interest. 2. of Interest: Cole, G.S.; The Sherman, A.M.declare Light-weight materials automotive applications. Mater. Charact. 1995, 35, 3– 9. 3. Aslanlar, S.; Ogur, A.; Ozsarac, U.; Ilhan, E. Welding time effect on mechanical properties of automotive References sheets in electrical resistance spot welding. Mater. Des. 2008, 297, 1427–1431.

1. 2. 3. 4.

Zhang, S. Laser welding technology of plastics. World Plast. 2007, 253, 56–62. Cole, G.S.; Sherman, A.M. Light-weight materials for automotive applications. Mater. Charact. 1995, 35, 3–9. [CrossRef] Aslanlar, S.; Ogur, A.; Ozsarac, U.; Ilhan, E. Welding time effect on mechanical properties of automotive sheets in electrical resistance spot welding. Mater. Des. 2008, 297, 1427–1431. [CrossRef] Bilici, M.K.; Yukler, A.I.; Kurtulmus, M. The optimization of welding parameters for friction stir spot welding of high density polyethylene sheets. Mater. Des. 2011, 327, 4074–4079. [CrossRef]

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