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Effect of surface preparation on the strength of vibration welded butt joint made from PBT composite
Journal Pre-proof Effect of surface preparation on the strength of vibration welded butt joint made from PBT composite Ezzat A. Showaib, Ammar H. Elsheikh PII:
S0142-9418(19)31805-7
DOI:
https://doi.org/10.1016/j.polymertesting.2019.106319
Reference:
POTE 106319
To appear in:
Polymer Testing
Received Date: 3 October 2019 Revised Date:
9 December 2019
Accepted Date: 29 December 2019
Please cite this article as: E.A. Showaib, A.H. Elsheikh, Effect of surface preparation on the strength of vibration welded butt joint made from PBT composite, Polymer Testing (2020), doi: https:// doi.org/10.1016/j.polymertesting.2019.106319. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Effect of Surface Preparation on The Strength of Vibration Welded Butt Joint Made from PBT Composite
Ezzat A. Showaib, Ammar H. Elsheikh Production Engineering and Mechanical Design Department, Faculty of Engineering, Tanta University, Tanta, Egypt.
Abstract Vibration welding technique has been widely used to weld molded surfaces parts produced by injection or compression molding techniques. However, the majority of early studies used machined surfaces to eliminate the complication associated with molded surfaces. Different process parameters such as the welding pressure, frequency, and amplitude have been investigated to determine their optimal values that maximize the welding strength. However, some other parameters such as joint design and the welding interface preparation were leftover for real application test or for technology transfer studies. Most of molded parts from semi-crystalline materials and their composites usually have skin layer that was exposed to thermal history differs from that of the core. Moreover, the amount and the orientation of fibers in the skin layer differ from that of core and shell regions. Therefore, this work investigates and explores the effect of the molded surfaces with skin on tensile strength of vibration welded butt joints made from polybutylene terephthalate reinforced with 30% glass fiber (PBT GF30). The effect of fibers orientation on the welded joint strength has been also investigated.
Keywords: Vibration Welding; Molded Surfaces; Applied Technology; PBT Composite; Skin/Core.
1. Introduction Polymer-based composites have shown promising applications in automotive industry [1-3]. Many investigations have been carried out to improve the mechanical and thermophysical properties polymer-based composites [4-6]. Welding of final products made of polymerbased composites is the bottle neck of the manufacturing process to obtain high quality products [7-9]. Vibration welding (VW), also called linear friction welding, has been widely used in welding all thermoplastics and their composite parts. It can be used for welding medium and large parts such as cars instrument panel, automotive bumper, air intake manifolds, and resonators [10-12]. Some of the problems associated with molding large parts of thermoplastic materials could be overcome using VW technique. In a typical VW process, the initial thermal energy is produced by frictional works due to an oscillatory motion as shown in Figure 1. The friction work (initial heating) is generated by rubbing the surfaces of the welded joint against each other, using an oscillatory motion under normal force. The main process parameters of VW are depicted in Figure 1 (welding pressure, amplitude, and welding frequency). Penetration, which defined as the decrease in the distance between the two parts being welded due to outflow of molten material at the welding interface, is another important parameter that should be investigated [13, 14]. The penetration time relationship in conjunction with the interface temperature is schematically presented in Figure 2 in which this relationship could be divided into four regions. The first region (phase I) is the Coulomb friction in which no penetration is detected and the interface temperature rises up almost to the melting temperature. The second region (phase II) is the transition region in which the thermal energy due to coulomb friction decreases, while the viscoelastic and viscous thermal energy increase till become the dominant energy toward the end of this phase. The temperature at the weld interface reaches the melt temperature of the welded material in this phase. The molten material at the welding interface flashes out and leads to increase the penetration with a rapid rate. This region lasts to critical penetration value called a threshold value after which the penetration rate stays constant in region three as shown in Figure 2.The third region (phase III), which called steady state region, in which the penetration rate remain constant. This phase stays until the vibration (oscillation) motion stopped. It has been reported that the welding strength of welded materials reaches its maximum value under certain vibration parameters if this phase is reached [15-20]. Last region (phase IV), is the solidification phase, starts with the end of vibration motion. Before full solidification, the molten film was exposed to the normal pressure which leads to additional penetration. This normal pressure results in deforming the molten film after solidification as well as the surrounding heated region which consequently produces residual stresses at the welding interface [12]. The parameters which may have effect on the VW strength could be classified into three major categories: material properties (amorphous or semi-crystalline material, percentage of additives, type and shape of reinforced materials, and the orientation of reinforced materials), welding process parameters (vibration amplitude, welding pressure, frequency, and penetration), and the joint shape parameters (weld thickness, joint shape design, defects in the welded area, and surface preparation). The above parameters have been investigated and optimized for different materials except the last three parameters that need to be investigated and optimized using advanced metaheuristic optimization techniques [21-23]. Most of the VW studies prepare the welded surfaces by machining however in the real applications welded surfaces produced by injection molding or similar machines, in which semi-crystalline materials and their composites usually have skin that has thermal history differs from that of the bulk material. Also, it is well known that thermoplastic composites exhibit skin with different fiber amount and orientations which is not similar to that found in core or shell of parts produced by injection molding machines.
During molding process of composite materials, fibers are forced to be oriented with the mold flow direction (MFD) which affected by thickness of molded part, number and location of gates, and fiber type and length [24-27]. The fibers orientation has a significant impact on the strength of thermoplastic composites and consequently the VW joint strength. Typically any molded part has a skin that has lower fiber contents (polymer rich) than the bulk material, and shell regions in which the majority of fiber will be oriented in MFD direction [28-31]. A rule of thumb: the fibers orientation has a significant impact on the strength of thermoplastic composites [32-34]. However, in the middle between the two shell regions there is a core region in which the majority of fibers oriented in a direction differs than that in the shell regions [35]. Based on the abovementioned analogy, it is expected that the two skins will met together first during welding of two parts with molded faces. So in VW process it is important to get rid of or to wipe out the skin layers to reach the shell regions. The skin layer thickness depends on the thickness of the part being molded for example it was found to be 0.2 mm for molded part thickness of 3.2 mm for glass-reinforced polybutylene terephthalate [36]. Most of investigations carried out on the VW process used machining to prepare the welded surfaces of the specimens. This is reasonable in order to study the VW process and parameters affecting its performance. Very few researchers have included in their work samples with molded surfaces [37-39]. But no one took into account the existence of skin, shell and core and their effect on VW performance. Therefore, this work will investigate and explore the effect of the molded surfaces with skin on tensile strength of vibration welded butt joints made from polybutylene terephthalate reinforced with 30% glass fiber (PBT GF30).
Figure 1 A schematic presentation of the vibration welding process.
Figure 2 A schematic presentation of the penetration time curve and corresponding temperature curve.
2. Experimental Procedures The main aim of this study is to explore the influence of surface preparation on vibration welded butt joint strength. The main surface preparation factors that could affect the VW strength are the thermal history and the fiber orientation and concentration at the welded surfaces. Therefore, it was suggested to use semi-crystalline polymer reinforced with short glass fibers as a composite material. Plaques produced by injection molding machine of Poly (butylene terephthalate) (PBT) reinforced by 30% weight chopped glass fiber (short E-glass fiber) was used in this study. This PBT GF30 produced by SABIC Innovative Plastics, which is known commercially by VALOX 420. The PBT GF30 was supplied in the form of pellets which were used to produce the plaques by injection molding machine. The properties of the used glass fibers are listed in Table 1. The common way for describing the fiber length distribution is the weighted average fiber length (Lw) which can be calculated by, Lw =
∑
(1)
∑
It was found that the weighted average fiber length (Lw) for the row material equal 327 µm with average length of 254 µm and median length of 233µm. Table 1 Properties of the glass fibers.
Density (g/cc)
2.57
Tensile strength (Gpa) 2.01
Coefficient of
Specific Heat
Thermal
Poisson's
thermal
(J/kg.K)
Conductivity
Ratio
expansion 7
(W/m.K)
-1
(10 K ) 0.23
5.1x10-6
805
1.35
Weighted average fiber length (µm) 327
The specimens were cut from plaque with thickness of 6.35 mm (0.25 inch) in a rectangle shape. The shape and dimensions of the plaque is shown in Figure 3. Fibers orientations in the specimens, which depend on specimen and gate location on the molded plaques and also
its thickness, were not characterized in this study. However, for sake of comparison, four 152.4 mm (6 inch) by 25.4 mm (1 inch) specimens, corresponding to locations 1to 4, were cut from the plaques as shown in Figure 3 (a). These specimens were tensile tested under the same loading conditions as the welded specimens; thereby they provide a basis for evaluating the performance of welded joints these specimen are called control samples. Figure 3 (b) shows the weld samples that were cut from the plaque to eight rectangular shape with the following dimension 76.2 mm (3 inch) long, 25.4 mm (1 inch) width and 6.35 mm (0.25 inch) thickness. Two of them were welded such that the machined surface assigned S1 welded to the surface S5 and so on. The welded samples will have the same dimension as of the control samples. As the welded surfaces produced by machining, these samples called the welded machine surfaces (WMS). Figure 3(c) shows the weld samples that were cut from the plaques to eight rectangular shape with the following dimension 76.2 mm (3 inch) long, 25.4 mm (1 inch) width, and 6.35 mm (0.25 inch) thickness. Note that the surface of the rectangle assigned by F is the surface produced by injection molding machine. Two of these rectangles were welded such that the molded surface assigned F1 welded to the surface F5 and so on. The welded samples will have the same dimension as of the control samples. As the welded surfaces produced by injection molding therefore these samples called the welded molded faces (WMF). There are two different gates locations one is blue and the other one is red as shown in Figure 3. When using the red gate the samples will be oriented in the direction parallel to the mold flow direction and will be called P-flow samples. While, when using the blue gate the samples will be oriented in direction perpendicular to the mold flow direction and will be called X-flow samples. The welded samples as shown in Figure 4 (a) become like the control samples in their shape and dimensions. Both sides of the welded samples and the control samples were cut to the dimensions as shown in Figure 4 (b) with 19.0 mm width. This cut was get rid of defects at both edges of the welded samples [15]. The shape in Figure 4 (b) was routed down to the ASTM D638 standard tensile test specimen shape as shown in Figure 4 (c). The tensile test was conducted under constant displacement rate 0.5 mm/s which, is corresponding to nominal strain rate of 10-2 S-1. The welding machine used in this study was built using a closed loop servo hydraulic system design by MTS as a special research machine. This machine has a capability of welding at different frequencies, amplitudes, and welding pressures Po. The welding time is controlled based on a designed penetration value ζo, which could be set before starting the welding process. The machine was connected to a personal computer for acquiring the penetration time data during the welding process. A detailed information about this welding machine and its capabilities were described elsewhere [15].
Figure 3 Layout of specimens cut from injection molded plaques and the gate location, (a) four control specimens and their location and orientation, (b) eight specimens with machined surfaces welded at the machined surfaces (S1to S5, …...), (c) eight specimens with molded faces welded at the molded faces (F1to F5, …...).
Figure 4 Shows a shape and dimensions for VW tensile tests specimens: a) dimensions of the welded samples; b) dimensions of the welded samples after cleaning the edges; c) final dimensions of the tensile tests specimens according to ASTM D638.
The welding frequency (N) and amplitude (a) for PBT GF30 were chosen as 120 Hz and 1.59 mm, respectively, according to [16]. The vibration motion is automatically shut down based on the intended penetration value ζo, which was assigned to the machine. The penetration was measured during the welding process using an LVDT fixed on the upper and the lower grip bases. The machine automatically realigns itself such that the two welded surfaces come to their initial position after the vibration motion was stopped. Thin sections were prepared as shown in Figure 5, in which schematic presentations for the thin section location and direction (X-Y plane) are shown. The thin section plane which is in X-Y directions remains in all kind of samples, such that X direction represents the specimen thickness (6.35mm) while Y direction represents the longitudinal direction of the samples (the test direction). This means that, the Y direction is in the direction of MFD in case of the P-flow direction samples, however Y direction will be perpendicular to the MFD in case of the X-flow direction samples. Recall that in case of X-flow direction samples the mold gate changes its location from red to blue location. It is important to mention that the X-Z plane is the welding plane and Z direction is the vibration motion direction. These sections were used to examine the fibers orientations by X-ray image (CMR) and transmission optical microscope to measure of the core to shell ratio. Scanning electron microscope (SEM) was used to explore the morphology of the fracture surface and to examine the fiber matrix interaction at the welding interface plane.
Figure 5 Schematic presentations for the thin section location and direction (X-Y plane) used in X ray imaging (CMR).
3. Results and Discussion 3.1. Comparison Between Penetration Curves The penetration/time curve gives good understating to the welding process during the welding procedures. This relationship for both molded (WMF) and machined (WMS) is shown in Figure 6 (a). The result shows that the penetration in region I (phase one) tacks more time in case of molded surfaces to be rise compere to the machined surfaces. This could be attributed to the orientation of the fibers at the weld surface. In case of machined surface, a large amount of fibers usually be perpendicular to the welded surface therefor one can expect that the coefficient of friction will be higher than that in case of molded surface as the fibers usually be parallel to the mold surface. That could explain why the first and second regions take more time in case of molded WMF specimens to reach phase three compared with that of machined surfaces (WMS). One more reason is the amount of fibers in the skin layer is usually lower than that in the bulk material (core and shells). However, in the steady state phase it was found that the penetration rate appears to have very similar slope in both WMF and WMS. This could be due to that the amount of polymer required to be melted and the amount of fibers to be heated up are the same in both cases. Keeping in mind the amount of heat dissipated to the surrounding is same in welding both of machined and molded surfaces specimens. Figure 6 (b) shows the slope of the penetration time curves for both WMS (black lines) and the WMF specimens. The slope of the curves was discrete and calculated using the acquired data therefore the results of related to the slope are affected by the noise and does not show smooth lines. However, the behavior of the samples welded with machined surfaces (the black lines) takes of faster than the samples welded with molded surfaces which represented by the colored lines. Also, WMS samples reach the third region (the steady state region) faster than the WMF samples. The value of the slope of the steady state region is almost the same in all welded samples as shown in Figure 6 (b). It is important to note that the total time required to weld the WMF is longer than that required time to weld WMS for the same penetration value. This means that the time required to stop the vibration motion in case of WMF is longer than that for WMS. This is an important issue for the commercial machines to reduce and readjust the overall production time.
Figure 6 a) Penetration time curve for machined and molded surface specimens; b) shows the slope of the penetration time curves for machined (black line) and molded surface specimens.
3.2. Control Samples results For the sake of comparison, two different configurations were prepared as shown in Figure 3 (two different gate locations). These different gate locations produce two set of samples: P-flow and X-flow. The MFD is parallel to the longitudinal direction (tension test direction) for the former and perpendicular to the longitudinal direction for the latter. These two different orientations have been considered to figure out the effect of fibers orientations on the tensile strength. That is because of that most of the fibers are oriented with MFD as shown in Figure 7. This figure shows an X-ray image (CMR) for thin section (took from P-
flow sample and prepared as mentioned above). The two shell regions have fibers oriented parallel to the red arrow which indicates the direction of MFD, and the core region has the fibers oriented perpendicular to the thin section plane; while the top and the bottom layers are the skins. Figure 8 shows a side view for different specimens at different locations in the plaque showing shell and core regions, the light color represents the core region while the dark area represents the shell regions. The difference between shells to core ratios is not significant between samples that is because of discarding 25.4 mm from all sides of the molded plaque as shown in Fig. 3(a). This means that the four samples located in the middle of the molded plaque and that leads to almost similar core with a ratio range from 0.3 to 0.4 of the specimen thickness. Figure 9 shows the maximum tensile strength for the four control specimens (blue hatched bars) with blue dashed line represents their average 91.25 MPa (X-flow samples). This value will be considered as the tensile strength in the direction perpendicular to MFD. Note that the locations that far away from the gate has a lower tensile strength compared with that of the location in vicinity of the gate. Due to the difference in the core to shell ratio between samples the tensile strength for specimen No. 1 is higher than that in the specimen No. 4. However, this behavior is out of the scope of this work, so the average will be considered as a representative to the tensile strength in this direction (X-flow direction). As it is known that the welded layer usually has fibers oriented in the welding plane which is perpendicular to the test direction, the average of the tensile strength for control specimens in the direction perpendicular to the flow field (X-flow direction) (91.25 MPa) was used to refer to the tensile strength of the welded joints. The tensile strength in the direction parallel to the MFD (P-flow direction) is presented by red hatched bars in Figure 9. The tensile strength in parallel direction is higher than that in the perpendicular direction due to the diversity of fibers orientations in these two directions. The average of the tensile strength in the direction parallel to the MFD is represented by red continues line as in Figure 9 and it is around 133.4 MPa. As shown in Figure 7 and Figure 8 the assumption of the core region approximately ranges from 0.3 to 0.4 of the specimen thickness is appropriate for presenting the samples of this work. Keeping in mind, the direction of fibers orientation in the core region usually is perpendicular to the fibers orientation in the shell regions [25]. With a simple mathematic computation using the role of mixture one can predict the approximate tensile strength in the core area in which the fibers oriented in direction perpendicular to the shell direction. For example in case of specimen in the direction of MFD which is (P-flow) type, it has two shell layers with fibers oriented parallel to the test direction. And in the meantime it has one core region ranged from 0.3 to 0.4 of the specimen thickness with fibers oriented in direction perpendicular to the test direction. The P-flow type samples have an average strength of 133.4 MPa. However, for X-flow samples, two shell layers with fibers oriented perpendicular to the test direction are observed. And it has one core region ranged from 0.3 to 0.4 of the specimen thickness with fibers oriented in direction parallel to the test direction. The X-flow samples have an average strength of 91.25 MPa. From the above two analogy the tensile strength of region with fibers oriented perpendicular to the test direction was found to be around 49 MPa. While the calculated tensile strength in the region with the majority of fibers oriented in the test direction was found to be in the range of 177 to 190 MPa. It is important to mention here that the tensile strength of the polymer matrix mentioned in the data sheet provided by supplier and tested according to ASTM-638 is around 48 MPa. This means that the tensile strength of the VW sample can expected to be very close to this value, because of the fibers oriented in VW plane are perpendicular to the test direction, similar behavior was observed in many researches [40].
Moreover, the bending strength of the standard samples according to ASTM- D790 was found to be 195 MPa. The predicted bending strength for the shell is close to the bending strength of the standard samples according to ASTM- D790 (195 MPa). Because of the convergent evaluation between the calculated strength results (for the matrix material and the PBT glass fiber reinforcement) and that reported by the supplier data sheet, this gives confidence to the aforementioned calculations and also the predicted value of the core to shell ratio.
Figure 7 X ray image (CMR) for thin section took from P-flow sample showing the two shell regions in which fibers oriented parallel to the black arrow which indicates the direction of MFD, and the core region in which the fibers oriented perpendicular to the thin section plane. The top and the bottom layers are the skins.
Figure 8 Side view for three different specimens showing shell and core regions, the light color represents the core while the darker area represents the shell regions, Note the difference between samples in the core to shell ratio.
Figure 9 Maximum tensile strength for the four control specimens (blue pares) with the dashes line represents their average and anther four specimens parallel to the MFD (hatched pares) with continues line represents their average.
3.3. Welding Pressure
There are contradictory results found in the literature regarding the effect of welding pressure on the strength of joints welded by VW. Some researchers claimed that welding pressure has a significant effect on the weld strength of butt joints [25]. While others claimed that high welding pressure generally has a negative impact on weld strength [38]. That because of the pressure results from the formation of a very thin layer at the welding interface. This will lead to force fibers to be oriented in the welding plan and that will result in low weld strengths. However, in some other work, it was found that the pressure has no effect or little effect on the strength of VW samples. On the contrary to above behavior polyetherimide shows increase in VW strength with increasing welding pressure [16]. Therefore, two different pressures were used (0.9 and 3.45 MPa) to produce welded samples to test the effect of welding pressure on the tensile strength of the welded joints. The machined surfaces samples were used to investigate the effect of the welding pressure, because of their surfaces were prepared under controlled conditions and had good flatness. Figure 10 shows the relative tensile strength of two set of samples one at 0.9 MPa welding pressure and the other at 3.45 MPa. The samples welded at 3.45 MPa have an average relative tensile strength (58%) higher than the samples welded at 0.9 MPa (52%). This means that the average tensile strength of the samples welded at 3.45 MPa (which is 53 MPa) is very close to or little higher than the strength of the polymer matrix material (48 MPa). It is well known from literature that the VW butt joints made of thermoplastic composites are usually have strength lower than that of the bulk material, and may reach the strength of the polymer matrix [38]. Therefore, it is was believed that the welding pressure of 3.45 MPa is appropriate for welding PBT GF30. While, in case of welding at low pressure the average tensile strength (47 MPa) was lower than the strength of the polymer matrix. This reduction could be attributed to the presence of some defects in the welded area. Also, the welded samples that were welded at 3.45 MPa are more consistent than that welded at lower pressure. Therefore, the pressure of 3.45 MPa was used for welding all welded samples.
Figure 10 Effect of welding pressure on relative tensile strength for the machined welded specimens.
3.4. Flow and Cross Flow Welded Samples Machined samples have been used to investigate the effect of fibers orientations in the welded samples. There are two examined configurations considered in this study: P-flow and X-flow. In order to examine the effect of these two configurations on the welding samples thin sections were prepared as stated above in X-Y plane to explore the difference in the morphology of these two welding configurations. X direction represents the specimen thickness direction. While Y direction is parallel to the test direction (the specimen longitudinal direction) and represents the flow direction in case of P-flow samples and represents the cross flow direction in case of X-flow samples. Figure 11 shows the X-ray images for these thin sections. Figure11 (a) Shows the whole width for P-flow specimen in which the shell regions have most of the fibers orientated in the Y direction which means they are parallel to the flow direction and perpendicular to the welding plane (X-Z plane). While most of the fibers in the core region orientated in the direction perpendicular to the thin section which means they are in Z direction. It is very difficult to identify the welding line in the core region because of the fibers in the welding plane have the same orientation as in the core. This could give wicked welding strength because of there is no bridging between the welding surfaces and the fibers laying parallel to each other. A higher magnification for shell region contoured by in white rectangle is shown in Figure 11 (c). It is clear from this image that the welding layer has thickness at the edge of the specimen larger than that far from the edge [41]. The welding layer especially close to the specimen edge shows that some of the fibers oriented toward the specimen edge and some of them tilted to the welding plane and that will produce a lot of bridging between welding surfaces. While some of them stay perpendicular to the welding plane and they produce more bridging which lead to enhance the welding strength. While Figure 11(b) shows the whole width for X-flow specimen. From this figure the two shell regions have most of the fibers oriented perpendicular to the section plane which means they were parallel to the vibration motion direction (Z direction). It is very difficult to identify the welding line. A higher magnification for the white rectangle area in the shell
region is shown in Figure 11(d). A crack was detected at the edge of the specimen which leads to reduction in the welded joint strength such crack was detected in many different penetration values. This crack affects the strength of the welded joints in case of X-flow specimen. Moreover, the fibers oriented parallel to each other with no interference or bridging between welding surfaces is the main cause of impairing joint strength and the inconsistent results. Because of the aforementioned behavior of X-flow samples, the remaining work has been conducted on the P- flow samples. Also, it is recommended to make the welding plane perpendicular to the MFD in order to eliminate the problems associated with X-flow direction.
Figure 11 X-ray image for thin section. (a) Whole width for P-flow specimen. (b) Whole width for X-flow specimen. (C) and (d) are higher magnification for the white rectangle in (a) and (b) respectively.
3.5. Welded Joint Strength
Penetration is one of most important factors affecting VW strength. However, it shows a very little effect on the strength of VW of thermoplastics and their composites if the penetration value passes a threshold value ζt (the penetration at starting of the steady state phase) [42]. Most of previous work prepared their samples by machining the welded surfaces. In this study all welded samples with machined surfaces WMS were welded at a preset penetration value equal to 0.5 mm which is more than the threshold value (0.25mm) as stated in different works [19, 40, 43]. Samples with molded surfaces were welded under the same conditions as in the machined surfaces samples, except the penetration values used with molded samples were set at different values starting from 0.5 mm to 2.5 mm. This range was chosen because of 0.5 mm is larger than the threshold value and is equal to that used in WMS. And the maximum penetration value of the 2.5 mm was used because of it is in the same range of welded intake-manifold with molded surfaces as recommended by [38]. This value was chosen, so it can get rid of the skin founded at the molded surfaces and reaches the shell region. Increasing the penetration more than 2.5 mm is not recommended as it will create large amount of flash which is not tolerable. Figure 12 shows the relative tensile strength for the WMS, with the dashed line represents their average value at penetration 0.5 mm. While the blue symbols represent the relative tensile strength for welded specimens with molded surfaces WMF at different penetrations range from 0.5 mm to 2.5 mm. It is important to note that the relative strength for WMF
increases with increasing the penetration value. The blue curve represents the fitting equation for the mean tensile strength values at different penetrations. The blue curve starts at a relative tensile strength of 0.39 and then ramping up with increasing the penetration value. However, it has an asymptotic value almost equal to the mean value of the relative tensile strength of WMS (58%).
Figure 12 The relative tensile strength for the machined welded specimens with the dashes line represents their average. Blue line represents the average relative tensile strength for welded specimens with molded surfaces at different penetration.
3.6. Molded Surface Behavior
The initial value of VW strength of WMF at penetration equal to 0.5mm was the driving force of this work. That because of this strength is very low compared with the strength of WMS and also it is lower than the strength of the polymer raw material (matrix material). However, many researchers found that the strength of VW of short glass fibers reinforced thermoplastics is approximately equal to matrix material strength [42]. Therefore, more penetrations value were tested to pass the skin of the two sides of the welded parts and to reach the shell regions. Surprisingly, it was found that the VW strength of WMF increases with increasing the penetration value. There may be many causes lead to such phenomenon need to be addressed in order to understand and justify this behavior. One of these causes could be the thickness of the samples tested in this work. Because of the specimens’ thickness were 6.35 mm which is larger than the thickness of the samples in other works. And also it is expected that the molded surfaces with large thickness may have irregularities and they show more non flatness than that in the thin molded surfaces. This cause consumes part of penetration value of molded surface to reach a reasonable strength. Other causes could be the skin layer thickness which may vary from one side of the mold to another and also the gate location may affect the skin layer thickness. Also the shape of the molded part and its thickness has influence on the skin layer thickness. It is known from literature that the skin layer is polymer reach and has fibers orientations unlike to that in shell regions and that consequently affect the VW strength of WMF [36]. Therefore, in order to pass the skin layer during VW of WMF, more penetration is required to reach the shell region. Also because of there is no sharp transition from skin to shell region the VW of WMF may require more
penetration value to get to a higher strength. All the above causes lead to enhance the strength of WMF with increasing the penetration value for VW butt joint made from PBT GF30 with thickness and specimen orientation as stated above. 3.7.
Failure Mechanism of Welded Joint
As stated above the welded samples were P-flow type. Figure 11 (a & c) is an X-ray image for thin section from P-flow type specimen in which a good interaction or interference between fibers in the shell regions is observed. And this gives indication that the weld specimen will have good strength. Unfortunately, the welded joint strength was not high as it was expected. Therefore it is required to understand the mechanism of failure in the welded samples. Examining the fracture surface of welded specimen may help to figure out the reasons or causes for such behavior. Figure 13 shows a photo by stereo optical microscope for part of fracture surface for VW specimen with whole width showing fibers orientations in both core and shell regions. It is clear from the figure that, for core region, the fibers laying in the welding plane has the same direction of the welding vibration motion. The fibers in this region appear to be stacked beside each other however in the shell regions it is difficult to recognize any fiber except very small white spots which indicate that the fibers are perpendicular or tilted to the fracture surface in these regions. A higher magnification for the white rectangle is shown in Figure 14 (a) in which an SEM micrograph for part of VW fracture surface is shown. The core region shows very little material drawn which indicates poor connection or adhesion between the welding surfaces in this region. While the shell region shows large amount of matrix material has been drawn. This gives indication for good welding strength in contradiction with the measured strength for welded joint which is found to be 58% of the control strength. Therefor a higher magnification for the shell region was needed to understand or to solve this contradiction. Figure 14 (b) shows SEM micrograph with high magnification for fracture surface of VW specimen at the shell region in which an evidence for bad adhesion between fibers and the polymer matrix is noticeable. This bad adhesion could explain why the welded specimen exhibits low strength compared with the strength of the control samples with the presence of good interaction and bridging between fibers in the shell regions. It is well known that any parameter affects the composite strength affects the VW strength of composite materials. It is recognized that the efficiency of the fibers matrix adhesion plays an important role in composite action behavior. This means that the interaction or the adhesion between fibers and the matrix material play a major factor in the weld strength. Figure 14 (b) shows some evidence or traces for poor adhesion between fibers and the matrix material. For example the figure has 6 arrows each of them points out to a reason for producing poor welding strength. Arrows with number one points to missing fiber without any trace of adhesion between the missing fiber and matrix material left behind (smooth surface). Also some fibers show poor or no adhesion with the polymer matrix which can be seen in dark area around some fibers indicating poor or no adhesion between fibers and the polymer matrix. The other reason is noted by arrow with number two which points out to broken fibers. These broken fibers indicate that they exposed to high shear force due to the oscillatory motion and that leads to break these fibers. Also because of the orientation of fibers in shell region are perpendicular to the welded surfaces, they will break due to rubbing the welded surfaces against each other. This could explain why the weighted average fiber length Lw of the welding flash (241µm) is lower than that found in the center of the molded plaques (289µm). The arrow with number three points out to high drawing matrix material which indicates to a poor composite action between fibers and polymer matrix material. These three reasons which mentioned above all
of them are results of high shear forces due to the oscillatory motion which lead to break some fibers and break the adhesion between some fibers and the matrix material. The remaining fibers act as a reinforced material to the polymer matrix and this phenomenon leads to some of the fibers in the welded material at the shell regions to act as composite with good strength. The outcome from the collection of these mechanisms leads to a better strength of the VW joint if it has the same configuration and same material.
Figure 13 A photo by stereo optical microscope for part of fracture surface for VW specimen with whole width showing fibers orientations in both core and shell regions.
Figure 14 a) SEM micrograph for part of VW fracture surface shows the fiber orientations in the core and shell region. White arrow indicates the vibration direction of VW motion; b) SEM micrograph with high magnification for fracture surface of VW specimen at the shell region shows evidence for bad adhesion between fiber and the polymer matrix.
4. Conclusions This study presents an experimental investigation to explore the effect of the molded surfaces with skin on tensile strength of vibration welded butt joints made from polybutylene terephthalate reinforced with 30% glass fiber (PBT GF30). The following conclusions could be drawn: The fiber orientation plays a major role in defining the strength of the VW joints; as the welded surface with perpendicular oriented fibers has a better joint strength comared with that of parallel oriented fibers. When fibers oriented parallel to the mold surface, cracks were detected at the welded surfaces which lead to reduce the VW joint strength. Molding conditions, gate location and part thickness have a significant impact on MFD which accordingly affect the fibers orientations and consequently the VW joints strength. The existence of the skin in molded surfaces leads to a week VW joints; therefore it should be removed either by machining or by increasing the welding penetration tell reach the shell region. A penetration of 2.5 mm is required to reach maximum welded joint strength. For the same penetration VW of molded surfaces takes more time than the machined surfaces.
The oscillation motion in VW process produces very high shear force which breaks some of the fibers especially the long one. This was measured by weighted average fiber length Lw of the welding flash (241 µm) which is lower than that found in the center of the molded plaques (289 µm).
Acknowledgments The author gratefully acknowledges the financial supports of the General Motors Research and Development Center Polymer Department and Science and Technology Development Fund of Egypt. Also, the author wishes to thank Dr. V. K. Stokes and Mr. L. P. Inzinna in GE Corporate Research and Development for their help in welding the samples in their laboratory. Also the author gratefully acknowledges Dr. Michael G. Wyzgoski for his useful discussion.
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Highlights •
The effect of surface preparation on the strength of vibration welded butt joint made from PBT composite has been investigated.
•
Penetration for machined and molded surfaces during welding process is demonstrated.
•
The effects of fibers orientation and welding pressure on the joint strength have been investigated.
•
An average tensile strength of welded samples close to or little higher than the strength of the polymer matrix material could be achieved by selecting the optimal welding pressure.
Conflict of interest The authors of the submitted manuscript “Effect of Surface Preparation on The Strength of Vibration Welded Butt Joint Made from PBT Composite” declared that there is no conflict of interest.
S0142-9418(19)31805-7
DOI:
https://doi.org/10.1016/j.polymertesting.2019.106319
Reference:
POTE 106319
To appear in:
Polymer Testing
Received Date: 3 October 2019 Revised Date:
9 December 2019
Accepted Date: 29 December 2019
Please cite this article as: E.A. Showaib, A.H. Elsheikh, Effect of surface preparation on the strength of vibration welded butt joint made from PBT composite, Polymer Testing (2020), doi: https:// doi.org/10.1016/j.polymertesting.2019.106319. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Effect of Surface Preparation on The Strength of Vibration Welded Butt Joint Made from PBT Composite
Ezzat A. Showaib, Ammar H. Elsheikh Production Engineering and Mechanical Design Department, Faculty of Engineering, Tanta University, Tanta, Egypt.
Abstract Vibration welding technique has been widely used to weld molded surfaces parts produced by injection or compression molding techniques. However, the majority of early studies used machined surfaces to eliminate the complication associated with molded surfaces. Different process parameters such as the welding pressure, frequency, and amplitude have been investigated to determine their optimal values that maximize the welding strength. However, some other parameters such as joint design and the welding interface preparation were leftover for real application test or for technology transfer studies. Most of molded parts from semi-crystalline materials and their composites usually have skin layer that was exposed to thermal history differs from that of the core. Moreover, the amount and the orientation of fibers in the skin layer differ from that of core and shell regions. Therefore, this work investigates and explores the effect of the molded surfaces with skin on tensile strength of vibration welded butt joints made from polybutylene terephthalate reinforced with 30% glass fiber (PBT GF30). The effect of fibers orientation on the welded joint strength has been also investigated.
Keywords: Vibration Welding; Molded Surfaces; Applied Technology; PBT Composite; Skin/Core.
1. Introduction Polymer-based composites have shown promising applications in automotive industry [1-3]. Many investigations have been carried out to improve the mechanical and thermophysical properties polymer-based composites [4-6]. Welding of final products made of polymerbased composites is the bottle neck of the manufacturing process to obtain high quality products [7-9]. Vibration welding (VW), also called linear friction welding, has been widely used in welding all thermoplastics and their composite parts. It can be used for welding medium and large parts such as cars instrument panel, automotive bumper, air intake manifolds, and resonators [10-12]. Some of the problems associated with molding large parts of thermoplastic materials could be overcome using VW technique. In a typical VW process, the initial thermal energy is produced by frictional works due to an oscillatory motion as shown in Figure 1. The friction work (initial heating) is generated by rubbing the surfaces of the welded joint against each other, using an oscillatory motion under normal force. The main process parameters of VW are depicted in Figure 1 (welding pressure, amplitude, and welding frequency). Penetration, which defined as the decrease in the distance between the two parts being welded due to outflow of molten material at the welding interface, is another important parameter that should be investigated [13, 14]. The penetration time relationship in conjunction with the interface temperature is schematically presented in Figure 2 in which this relationship could be divided into four regions. The first region (phase I) is the Coulomb friction in which no penetration is detected and the interface temperature rises up almost to the melting temperature. The second region (phase II) is the transition region in which the thermal energy due to coulomb friction decreases, while the viscoelastic and viscous thermal energy increase till become the dominant energy toward the end of this phase. The temperature at the weld interface reaches the melt temperature of the welded material in this phase. The molten material at the welding interface flashes out and leads to increase the penetration with a rapid rate. This region lasts to critical penetration value called a threshold value after which the penetration rate stays constant in region three as shown in Figure 2.The third region (phase III), which called steady state region, in which the penetration rate remain constant. This phase stays until the vibration (oscillation) motion stopped. It has been reported that the welding strength of welded materials reaches its maximum value under certain vibration parameters if this phase is reached [15-20]. Last region (phase IV), is the solidification phase, starts with the end of vibration motion. Before full solidification, the molten film was exposed to the normal pressure which leads to additional penetration. This normal pressure results in deforming the molten film after solidification as well as the surrounding heated region which consequently produces residual stresses at the welding interface [12]. The parameters which may have effect on the VW strength could be classified into three major categories: material properties (amorphous or semi-crystalline material, percentage of additives, type and shape of reinforced materials, and the orientation of reinforced materials), welding process parameters (vibration amplitude, welding pressure, frequency, and penetration), and the joint shape parameters (weld thickness, joint shape design, defects in the welded area, and surface preparation). The above parameters have been investigated and optimized for different materials except the last three parameters that need to be investigated and optimized using advanced metaheuristic optimization techniques [21-23]. Most of the VW studies prepare the welded surfaces by machining however in the real applications welded surfaces produced by injection molding or similar machines, in which semi-crystalline materials and their composites usually have skin that has thermal history differs from that of the bulk material. Also, it is well known that thermoplastic composites exhibit skin with different fiber amount and orientations which is not similar to that found in core or shell of parts produced by injection molding machines.
During molding process of composite materials, fibers are forced to be oriented with the mold flow direction (MFD) which affected by thickness of molded part, number and location of gates, and fiber type and length [24-27]. The fibers orientation has a significant impact on the strength of thermoplastic composites and consequently the VW joint strength. Typically any molded part has a skin that has lower fiber contents (polymer rich) than the bulk material, and shell regions in which the majority of fiber will be oriented in MFD direction [28-31]. A rule of thumb: the fibers orientation has a significant impact on the strength of thermoplastic composites [32-34]. However, in the middle between the two shell regions there is a core region in which the majority of fibers oriented in a direction differs than that in the shell regions [35]. Based on the abovementioned analogy, it is expected that the two skins will met together first during welding of two parts with molded faces. So in VW process it is important to get rid of or to wipe out the skin layers to reach the shell regions. The skin layer thickness depends on the thickness of the part being molded for example it was found to be 0.2 mm for molded part thickness of 3.2 mm for glass-reinforced polybutylene terephthalate [36]. Most of investigations carried out on the VW process used machining to prepare the welded surfaces of the specimens. This is reasonable in order to study the VW process and parameters affecting its performance. Very few researchers have included in their work samples with molded surfaces [37-39]. But no one took into account the existence of skin, shell and core and their effect on VW performance. Therefore, this work will investigate and explore the effect of the molded surfaces with skin on tensile strength of vibration welded butt joints made from polybutylene terephthalate reinforced with 30% glass fiber (PBT GF30).
Figure 1 A schematic presentation of the vibration welding process.
Figure 2 A schematic presentation of the penetration time curve and corresponding temperature curve.
2. Experimental Procedures The main aim of this study is to explore the influence of surface preparation on vibration welded butt joint strength. The main surface preparation factors that could affect the VW strength are the thermal history and the fiber orientation and concentration at the welded surfaces. Therefore, it was suggested to use semi-crystalline polymer reinforced with short glass fibers as a composite material. Plaques produced by injection molding machine of Poly (butylene terephthalate) (PBT) reinforced by 30% weight chopped glass fiber (short E-glass fiber) was used in this study. This PBT GF30 produced by SABIC Innovative Plastics, which is known commercially by VALOX 420. The PBT GF30 was supplied in the form of pellets which were used to produce the plaques by injection molding machine. The properties of the used glass fibers are listed in Table 1. The common way for describing the fiber length distribution is the weighted average fiber length (Lw) which can be calculated by, Lw =
∑
(1)
∑
It was found that the weighted average fiber length (Lw) for the row material equal 327 µm with average length of 254 µm and median length of 233µm. Table 1 Properties of the glass fibers.
Density (g/cc)
2.57
Tensile strength (Gpa) 2.01
Coefficient of
Specific Heat
Thermal
Poisson's
thermal
(J/kg.K)
Conductivity
Ratio
expansion 7
(W/m.K)
-1
(10 K ) 0.23
5.1x10-6
805
1.35
Weighted average fiber length (µm) 327
The specimens were cut from plaque with thickness of 6.35 mm (0.25 inch) in a rectangle shape. The shape and dimensions of the plaque is shown in Figure 3. Fibers orientations in the specimens, which depend on specimen and gate location on the molded plaques and also
its thickness, were not characterized in this study. However, for sake of comparison, four 152.4 mm (6 inch) by 25.4 mm (1 inch) specimens, corresponding to locations 1to 4, were cut from the plaques as shown in Figure 3 (a). These specimens were tensile tested under the same loading conditions as the welded specimens; thereby they provide a basis for evaluating the performance of welded joints these specimen are called control samples. Figure 3 (b) shows the weld samples that were cut from the plaque to eight rectangular shape with the following dimension 76.2 mm (3 inch) long, 25.4 mm (1 inch) width and 6.35 mm (0.25 inch) thickness. Two of them were welded such that the machined surface assigned S1 welded to the surface S5 and so on. The welded samples will have the same dimension as of the control samples. As the welded surfaces produced by machining, these samples called the welded machine surfaces (WMS). Figure 3(c) shows the weld samples that were cut from the plaques to eight rectangular shape with the following dimension 76.2 mm (3 inch) long, 25.4 mm (1 inch) width, and 6.35 mm (0.25 inch) thickness. Note that the surface of the rectangle assigned by F is the surface produced by injection molding machine. Two of these rectangles were welded such that the molded surface assigned F1 welded to the surface F5 and so on. The welded samples will have the same dimension as of the control samples. As the welded surfaces produced by injection molding therefore these samples called the welded molded faces (WMF). There are two different gates locations one is blue and the other one is red as shown in Figure 3. When using the red gate the samples will be oriented in the direction parallel to the mold flow direction and will be called P-flow samples. While, when using the blue gate the samples will be oriented in direction perpendicular to the mold flow direction and will be called X-flow samples. The welded samples as shown in Figure 4 (a) become like the control samples in their shape and dimensions. Both sides of the welded samples and the control samples were cut to the dimensions as shown in Figure 4 (b) with 19.0 mm width. This cut was get rid of defects at both edges of the welded samples [15]. The shape in Figure 4 (b) was routed down to the ASTM D638 standard tensile test specimen shape as shown in Figure 4 (c). The tensile test was conducted under constant displacement rate 0.5 mm/s which, is corresponding to nominal strain rate of 10-2 S-1. The welding machine used in this study was built using a closed loop servo hydraulic system design by MTS as a special research machine. This machine has a capability of welding at different frequencies, amplitudes, and welding pressures Po. The welding time is controlled based on a designed penetration value ζo, which could be set before starting the welding process. The machine was connected to a personal computer for acquiring the penetration time data during the welding process. A detailed information about this welding machine and its capabilities were described elsewhere [15].
Figure 3 Layout of specimens cut from injection molded plaques and the gate location, (a) four control specimens and their location and orientation, (b) eight specimens with machined surfaces welded at the machined surfaces (S1to S5, …...), (c) eight specimens with molded faces welded at the molded faces (F1to F5, …...).
Figure 4 Shows a shape and dimensions for VW tensile tests specimens: a) dimensions of the welded samples; b) dimensions of the welded samples after cleaning the edges; c) final dimensions of the tensile tests specimens according to ASTM D638.
The welding frequency (N) and amplitude (a) for PBT GF30 were chosen as 120 Hz and 1.59 mm, respectively, according to [16]. The vibration motion is automatically shut down based on the intended penetration value ζo, which was assigned to the machine. The penetration was measured during the welding process using an LVDT fixed on the upper and the lower grip bases. The machine automatically realigns itself such that the two welded surfaces come to their initial position after the vibration motion was stopped. Thin sections were prepared as shown in Figure 5, in which schematic presentations for the thin section location and direction (X-Y plane) are shown. The thin section plane which is in X-Y directions remains in all kind of samples, such that X direction represents the specimen thickness (6.35mm) while Y direction represents the longitudinal direction of the samples (the test direction). This means that, the Y direction is in the direction of MFD in case of the P-flow direction samples, however Y direction will be perpendicular to the MFD in case of the X-flow direction samples. Recall that in case of X-flow direction samples the mold gate changes its location from red to blue location. It is important to mention that the X-Z plane is the welding plane and Z direction is the vibration motion direction. These sections were used to examine the fibers orientations by X-ray image (CMR) and transmission optical microscope to measure of the core to shell ratio. Scanning electron microscope (SEM) was used to explore the morphology of the fracture surface and to examine the fiber matrix interaction at the welding interface plane.
Figure 5 Schematic presentations for the thin section location and direction (X-Y plane) used in X ray imaging (CMR).
3. Results and Discussion 3.1. Comparison Between Penetration Curves The penetration/time curve gives good understating to the welding process during the welding procedures. This relationship for both molded (WMF) and machined (WMS) is shown in Figure 6 (a). The result shows that the penetration in region I (phase one) tacks more time in case of molded surfaces to be rise compere to the machined surfaces. This could be attributed to the orientation of the fibers at the weld surface. In case of machined surface, a large amount of fibers usually be perpendicular to the welded surface therefor one can expect that the coefficient of friction will be higher than that in case of molded surface as the fibers usually be parallel to the mold surface. That could explain why the first and second regions take more time in case of molded WMF specimens to reach phase three compared with that of machined surfaces (WMS). One more reason is the amount of fibers in the skin layer is usually lower than that in the bulk material (core and shells). However, in the steady state phase it was found that the penetration rate appears to have very similar slope in both WMF and WMS. This could be due to that the amount of polymer required to be melted and the amount of fibers to be heated up are the same in both cases. Keeping in mind the amount of heat dissipated to the surrounding is same in welding both of machined and molded surfaces specimens. Figure 6 (b) shows the slope of the penetration time curves for both WMS (black lines) and the WMF specimens. The slope of the curves was discrete and calculated using the acquired data therefore the results of related to the slope are affected by the noise and does not show smooth lines. However, the behavior of the samples welded with machined surfaces (the black lines) takes of faster than the samples welded with molded surfaces which represented by the colored lines. Also, WMS samples reach the third region (the steady state region) faster than the WMF samples. The value of the slope of the steady state region is almost the same in all welded samples as shown in Figure 6 (b). It is important to note that the total time required to weld the WMF is longer than that required time to weld WMS for the same penetration value. This means that the time required to stop the vibration motion in case of WMF is longer than that for WMS. This is an important issue for the commercial machines to reduce and readjust the overall production time.
Figure 6 a) Penetration time curve for machined and molded surface specimens; b) shows the slope of the penetration time curves for machined (black line) and molded surface specimens.
3.2. Control Samples results For the sake of comparison, two different configurations were prepared as shown in Figure 3 (two different gate locations). These different gate locations produce two set of samples: P-flow and X-flow. The MFD is parallel to the longitudinal direction (tension test direction) for the former and perpendicular to the longitudinal direction for the latter. These two different orientations have been considered to figure out the effect of fibers orientations on the tensile strength. That is because of that most of the fibers are oriented with MFD as shown in Figure 7. This figure shows an X-ray image (CMR) for thin section (took from P-
flow sample and prepared as mentioned above). The two shell regions have fibers oriented parallel to the red arrow which indicates the direction of MFD, and the core region has the fibers oriented perpendicular to the thin section plane; while the top and the bottom layers are the skins. Figure 8 shows a side view for different specimens at different locations in the plaque showing shell and core regions, the light color represents the core region while the dark area represents the shell regions. The difference between shells to core ratios is not significant between samples that is because of discarding 25.4 mm from all sides of the molded plaque as shown in Fig. 3(a). This means that the four samples located in the middle of the molded plaque and that leads to almost similar core with a ratio range from 0.3 to 0.4 of the specimen thickness. Figure 9 shows the maximum tensile strength for the four control specimens (blue hatched bars) with blue dashed line represents their average 91.25 MPa (X-flow samples). This value will be considered as the tensile strength in the direction perpendicular to MFD. Note that the locations that far away from the gate has a lower tensile strength compared with that of the location in vicinity of the gate. Due to the difference in the core to shell ratio between samples the tensile strength for specimen No. 1 is higher than that in the specimen No. 4. However, this behavior is out of the scope of this work, so the average will be considered as a representative to the tensile strength in this direction (X-flow direction). As it is known that the welded layer usually has fibers oriented in the welding plane which is perpendicular to the test direction, the average of the tensile strength for control specimens in the direction perpendicular to the flow field (X-flow direction) (91.25 MPa) was used to refer to the tensile strength of the welded joints. The tensile strength in the direction parallel to the MFD (P-flow direction) is presented by red hatched bars in Figure 9. The tensile strength in parallel direction is higher than that in the perpendicular direction due to the diversity of fibers orientations in these two directions. The average of the tensile strength in the direction parallel to the MFD is represented by red continues line as in Figure 9 and it is around 133.4 MPa. As shown in Figure 7 and Figure 8 the assumption of the core region approximately ranges from 0.3 to 0.4 of the specimen thickness is appropriate for presenting the samples of this work. Keeping in mind, the direction of fibers orientation in the core region usually is perpendicular to the fibers orientation in the shell regions [25]. With a simple mathematic computation using the role of mixture one can predict the approximate tensile strength in the core area in which the fibers oriented in direction perpendicular to the shell direction. For example in case of specimen in the direction of MFD which is (P-flow) type, it has two shell layers with fibers oriented parallel to the test direction. And in the meantime it has one core region ranged from 0.3 to 0.4 of the specimen thickness with fibers oriented in direction perpendicular to the test direction. The P-flow type samples have an average strength of 133.4 MPa. However, for X-flow samples, two shell layers with fibers oriented perpendicular to the test direction are observed. And it has one core region ranged from 0.3 to 0.4 of the specimen thickness with fibers oriented in direction parallel to the test direction. The X-flow samples have an average strength of 91.25 MPa. From the above two analogy the tensile strength of region with fibers oriented perpendicular to the test direction was found to be around 49 MPa. While the calculated tensile strength in the region with the majority of fibers oriented in the test direction was found to be in the range of 177 to 190 MPa. It is important to mention here that the tensile strength of the polymer matrix mentioned in the data sheet provided by supplier and tested according to ASTM-638 is around 48 MPa. This means that the tensile strength of the VW sample can expected to be very close to this value, because of the fibers oriented in VW plane are perpendicular to the test direction, similar behavior was observed in many researches [40].
Moreover, the bending strength of the standard samples according to ASTM- D790 was found to be 195 MPa. The predicted bending strength for the shell is close to the bending strength of the standard samples according to ASTM- D790 (195 MPa). Because of the convergent evaluation between the calculated strength results (for the matrix material and the PBT glass fiber reinforcement) and that reported by the supplier data sheet, this gives confidence to the aforementioned calculations and also the predicted value of the core to shell ratio.
Figure 7 X ray image (CMR) for thin section took from P-flow sample showing the two shell regions in which fibers oriented parallel to the black arrow which indicates the direction of MFD, and the core region in which the fibers oriented perpendicular to the thin section plane. The top and the bottom layers are the skins.
Figure 8 Side view for three different specimens showing shell and core regions, the light color represents the core while the darker area represents the shell regions, Note the difference between samples in the core to shell ratio.
Figure 9 Maximum tensile strength for the four control specimens (blue pares) with the dashes line represents their average and anther four specimens parallel to the MFD (hatched pares) with continues line represents their average.
3.3. Welding Pressure
There are contradictory results found in the literature regarding the effect of welding pressure on the strength of joints welded by VW. Some researchers claimed that welding pressure has a significant effect on the weld strength of butt joints [25]. While others claimed that high welding pressure generally has a negative impact on weld strength [38]. That because of the pressure results from the formation of a very thin layer at the welding interface. This will lead to force fibers to be oriented in the welding plan and that will result in low weld strengths. However, in some other work, it was found that the pressure has no effect or little effect on the strength of VW samples. On the contrary to above behavior polyetherimide shows increase in VW strength with increasing welding pressure [16]. Therefore, two different pressures were used (0.9 and 3.45 MPa) to produce welded samples to test the effect of welding pressure on the tensile strength of the welded joints. The machined surfaces samples were used to investigate the effect of the welding pressure, because of their surfaces were prepared under controlled conditions and had good flatness. Figure 10 shows the relative tensile strength of two set of samples one at 0.9 MPa welding pressure and the other at 3.45 MPa. The samples welded at 3.45 MPa have an average relative tensile strength (58%) higher than the samples welded at 0.9 MPa (52%). This means that the average tensile strength of the samples welded at 3.45 MPa (which is 53 MPa) is very close to or little higher than the strength of the polymer matrix material (48 MPa). It is well known from literature that the VW butt joints made of thermoplastic composites are usually have strength lower than that of the bulk material, and may reach the strength of the polymer matrix [38]. Therefore, it is was believed that the welding pressure of 3.45 MPa is appropriate for welding PBT GF30. While, in case of welding at low pressure the average tensile strength (47 MPa) was lower than the strength of the polymer matrix. This reduction could be attributed to the presence of some defects in the welded area. Also, the welded samples that were welded at 3.45 MPa are more consistent than that welded at lower pressure. Therefore, the pressure of 3.45 MPa was used for welding all welded samples.
Figure 10 Effect of welding pressure on relative tensile strength for the machined welded specimens.
3.4. Flow and Cross Flow Welded Samples Machined samples have been used to investigate the effect of fibers orientations in the welded samples. There are two examined configurations considered in this study: P-flow and X-flow. In order to examine the effect of these two configurations on the welding samples thin sections were prepared as stated above in X-Y plane to explore the difference in the morphology of these two welding configurations. X direction represents the specimen thickness direction. While Y direction is parallel to the test direction (the specimen longitudinal direction) and represents the flow direction in case of P-flow samples and represents the cross flow direction in case of X-flow samples. Figure 11 shows the X-ray images for these thin sections. Figure11 (a) Shows the whole width for P-flow specimen in which the shell regions have most of the fibers orientated in the Y direction which means they are parallel to the flow direction and perpendicular to the welding plane (X-Z plane). While most of the fibers in the core region orientated in the direction perpendicular to the thin section which means they are in Z direction. It is very difficult to identify the welding line in the core region because of the fibers in the welding plane have the same orientation as in the core. This could give wicked welding strength because of there is no bridging between the welding surfaces and the fibers laying parallel to each other. A higher magnification for shell region contoured by in white rectangle is shown in Figure 11 (c). It is clear from this image that the welding layer has thickness at the edge of the specimen larger than that far from the edge [41]. The welding layer especially close to the specimen edge shows that some of the fibers oriented toward the specimen edge and some of them tilted to the welding plane and that will produce a lot of bridging between welding surfaces. While some of them stay perpendicular to the welding plane and they produce more bridging which lead to enhance the welding strength. While Figure 11(b) shows the whole width for X-flow specimen. From this figure the two shell regions have most of the fibers oriented perpendicular to the section plane which means they were parallel to the vibration motion direction (Z direction). It is very difficult to identify the welding line. A higher magnification for the white rectangle area in the shell
region is shown in Figure 11(d). A crack was detected at the edge of the specimen which leads to reduction in the welded joint strength such crack was detected in many different penetration values. This crack affects the strength of the welded joints in case of X-flow specimen. Moreover, the fibers oriented parallel to each other with no interference or bridging between welding surfaces is the main cause of impairing joint strength and the inconsistent results. Because of the aforementioned behavior of X-flow samples, the remaining work has been conducted on the P- flow samples. Also, it is recommended to make the welding plane perpendicular to the MFD in order to eliminate the problems associated with X-flow direction.
Figure 11 X-ray image for thin section. (a) Whole width for P-flow specimen. (b) Whole width for X-flow specimen. (C) and (d) are higher magnification for the white rectangle in (a) and (b) respectively.
3.5. Welded Joint Strength
Penetration is one of most important factors affecting VW strength. However, it shows a very little effect on the strength of VW of thermoplastics and their composites if the penetration value passes a threshold value ζt (the penetration at starting of the steady state phase) [42]. Most of previous work prepared their samples by machining the welded surfaces. In this study all welded samples with machined surfaces WMS were welded at a preset penetration value equal to 0.5 mm which is more than the threshold value (0.25mm) as stated in different works [19, 40, 43]. Samples with molded surfaces were welded under the same conditions as in the machined surfaces samples, except the penetration values used with molded samples were set at different values starting from 0.5 mm to 2.5 mm. This range was chosen because of 0.5 mm is larger than the threshold value and is equal to that used in WMS. And the maximum penetration value of the 2.5 mm was used because of it is in the same range of welded intake-manifold with molded surfaces as recommended by [38]. This value was chosen, so it can get rid of the skin founded at the molded surfaces and reaches the shell region. Increasing the penetration more than 2.5 mm is not recommended as it will create large amount of flash which is not tolerable. Figure 12 shows the relative tensile strength for the WMS, with the dashed line represents their average value at penetration 0.5 mm. While the blue symbols represent the relative tensile strength for welded specimens with molded surfaces WMF at different penetrations range from 0.5 mm to 2.5 mm. It is important to note that the relative strength for WMF
increases with increasing the penetration value. The blue curve represents the fitting equation for the mean tensile strength values at different penetrations. The blue curve starts at a relative tensile strength of 0.39 and then ramping up with increasing the penetration value. However, it has an asymptotic value almost equal to the mean value of the relative tensile strength of WMS (58%).
Figure 12 The relative tensile strength for the machined welded specimens with the dashes line represents their average. Blue line represents the average relative tensile strength for welded specimens with molded surfaces at different penetration.
3.6. Molded Surface Behavior
The initial value of VW strength of WMF at penetration equal to 0.5mm was the driving force of this work. That because of this strength is very low compared with the strength of WMS and also it is lower than the strength of the polymer raw material (matrix material). However, many researchers found that the strength of VW of short glass fibers reinforced thermoplastics is approximately equal to matrix material strength [42]. Therefore, more penetrations value were tested to pass the skin of the two sides of the welded parts and to reach the shell regions. Surprisingly, it was found that the VW strength of WMF increases with increasing the penetration value. There may be many causes lead to such phenomenon need to be addressed in order to understand and justify this behavior. One of these causes could be the thickness of the samples tested in this work. Because of the specimens’ thickness were 6.35 mm which is larger than the thickness of the samples in other works. And also it is expected that the molded surfaces with large thickness may have irregularities and they show more non flatness than that in the thin molded surfaces. This cause consumes part of penetration value of molded surface to reach a reasonable strength. Other causes could be the skin layer thickness which may vary from one side of the mold to another and also the gate location may affect the skin layer thickness. Also the shape of the molded part and its thickness has influence on the skin layer thickness. It is known from literature that the skin layer is polymer reach and has fibers orientations unlike to that in shell regions and that consequently affect the VW strength of WMF [36]. Therefore, in order to pass the skin layer during VW of WMF, more penetration is required to reach the shell region. Also because of there is no sharp transition from skin to shell region the VW of WMF may require more
penetration value to get to a higher strength. All the above causes lead to enhance the strength of WMF with increasing the penetration value for VW butt joint made from PBT GF30 with thickness and specimen orientation as stated above. 3.7.
Failure Mechanism of Welded Joint
As stated above the welded samples were P-flow type. Figure 11 (a & c) is an X-ray image for thin section from P-flow type specimen in which a good interaction or interference between fibers in the shell regions is observed. And this gives indication that the weld specimen will have good strength. Unfortunately, the welded joint strength was not high as it was expected. Therefore it is required to understand the mechanism of failure in the welded samples. Examining the fracture surface of welded specimen may help to figure out the reasons or causes for such behavior. Figure 13 shows a photo by stereo optical microscope for part of fracture surface for VW specimen with whole width showing fibers orientations in both core and shell regions. It is clear from the figure that, for core region, the fibers laying in the welding plane has the same direction of the welding vibration motion. The fibers in this region appear to be stacked beside each other however in the shell regions it is difficult to recognize any fiber except very small white spots which indicate that the fibers are perpendicular or tilted to the fracture surface in these regions. A higher magnification for the white rectangle is shown in Figure 14 (a) in which an SEM micrograph for part of VW fracture surface is shown. The core region shows very little material drawn which indicates poor connection or adhesion between the welding surfaces in this region. While the shell region shows large amount of matrix material has been drawn. This gives indication for good welding strength in contradiction with the measured strength for welded joint which is found to be 58% of the control strength. Therefor a higher magnification for the shell region was needed to understand or to solve this contradiction. Figure 14 (b) shows SEM micrograph with high magnification for fracture surface of VW specimen at the shell region in which an evidence for bad adhesion between fibers and the polymer matrix is noticeable. This bad adhesion could explain why the welded specimen exhibits low strength compared with the strength of the control samples with the presence of good interaction and bridging between fibers in the shell regions. It is well known that any parameter affects the composite strength affects the VW strength of composite materials. It is recognized that the efficiency of the fibers matrix adhesion plays an important role in composite action behavior. This means that the interaction or the adhesion between fibers and the matrix material play a major factor in the weld strength. Figure 14 (b) shows some evidence or traces for poor adhesion between fibers and the matrix material. For example the figure has 6 arrows each of them points out to a reason for producing poor welding strength. Arrows with number one points to missing fiber without any trace of adhesion between the missing fiber and matrix material left behind (smooth surface). Also some fibers show poor or no adhesion with the polymer matrix which can be seen in dark area around some fibers indicating poor or no adhesion between fibers and the polymer matrix. The other reason is noted by arrow with number two which points out to broken fibers. These broken fibers indicate that they exposed to high shear force due to the oscillatory motion and that leads to break these fibers. Also because of the orientation of fibers in shell region are perpendicular to the welded surfaces, they will break due to rubbing the welded surfaces against each other. This could explain why the weighted average fiber length Lw of the welding flash (241µm) is lower than that found in the center of the molded plaques (289µm). The arrow with number three points out to high drawing matrix material which indicates to a poor composite action between fibers and polymer matrix material. These three reasons which mentioned above all
of them are results of high shear forces due to the oscillatory motion which lead to break some fibers and break the adhesion between some fibers and the matrix material. The remaining fibers act as a reinforced material to the polymer matrix and this phenomenon leads to some of the fibers in the welded material at the shell regions to act as composite with good strength. The outcome from the collection of these mechanisms leads to a better strength of the VW joint if it has the same configuration and same material.
Figure 13 A photo by stereo optical microscope for part of fracture surface for VW specimen with whole width showing fibers orientations in both core and shell regions.
Figure 14 a) SEM micrograph for part of VW fracture surface shows the fiber orientations in the core and shell region. White arrow indicates the vibration direction of VW motion; b) SEM micrograph with high magnification for fracture surface of VW specimen at the shell region shows evidence for bad adhesion between fiber and the polymer matrix.
4. Conclusions This study presents an experimental investigation to explore the effect of the molded surfaces with skin on tensile strength of vibration welded butt joints made from polybutylene terephthalate reinforced with 30% glass fiber (PBT GF30). The following conclusions could be drawn: The fiber orientation plays a major role in defining the strength of the VW joints; as the welded surface with perpendicular oriented fibers has a better joint strength comared with that of parallel oriented fibers. When fibers oriented parallel to the mold surface, cracks were detected at the welded surfaces which lead to reduce the VW joint strength. Molding conditions, gate location and part thickness have a significant impact on MFD which accordingly affect the fibers orientations and consequently the VW joints strength. The existence of the skin in molded surfaces leads to a week VW joints; therefore it should be removed either by machining or by increasing the welding penetration tell reach the shell region. A penetration of 2.5 mm is required to reach maximum welded joint strength. For the same penetration VW of molded surfaces takes more time than the machined surfaces.
The oscillation motion in VW process produces very high shear force which breaks some of the fibers especially the long one. This was measured by weighted average fiber length Lw of the welding flash (241 µm) which is lower than that found in the center of the molded plaques (289 µm).
Acknowledgments The author gratefully acknowledges the financial supports of the General Motors Research and Development Center Polymer Department and Science and Technology Development Fund of Egypt. Also, the author wishes to thank Dr. V. K. Stokes and Mr. L. P. Inzinna in GE Corporate Research and Development for their help in welding the samples in their laboratory. Also the author gratefully acknowledges Dr. Michael G. Wyzgoski for his useful discussion.
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Highlights •
The effect of surface preparation on the strength of vibration welded butt joint made from PBT composite has been investigated.
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Penetration for machined and molded surfaces during welding process is demonstrated.
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The effects of fibers orientation and welding pressure on the joint strength have been investigated.
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An average tensile strength of welded samples close to or little higher than the strength of the polymer matrix material could be achieved by selecting the optimal welding pressure.
Conflict of interest The authors of the submitted manuscript “Effect of Surface Preparation on The Strength of Vibration Welded Butt Joint Made from PBT Composite” declared that there is no conflict of interest.