Study on thermal-assisted underwater friction stir welding for carbon fiber-reinforced polyetherimide

Abstract

Friction stir welding (FSW) of carbon fiber-reinforced thermoplastic has shown attractive applications in industrial fields. However, the heat input during the FSW is hard to control, which leads to significant material loss and carbon fiber damage. In this paper, a carbon fiber-reinforced polyetherimide (CF-PEI) plate is joined via thermal-assisted underwater friction stir welding (UFSW). The evaluation of heat-assisted UFSW joints is conducted by studying the surface morphology, microstructure of the weld interface, the state and distribution of carbon fibers, and tensile analysis. The findings indicated that water can reduce heat input, and the water temperature has a significant impact on the weld properties. Specifically, at a water temperature of 60°C, there is a considerable reduction in material loss and carbon fiber damage. The study also reveals that this temperature facilitates the interweaving of carbon fibers into a stable structure at the interface and their integration with the PEI substrate. This reduces the inhibition of the material on carbon fiber elongation, ultimately enhancing the bonding interface and improving tensile shear properties. The maximum tensile strength value of the welds reached 33.6 MPa, equivalent to 79.6% of the CF-PEI sheet. Compared to FSW, joint tensile strength is increased by 45.5%.

Introduction

Carbon fiber-reinforced thermoplastic composites (CFRTP) are widely used in automobiles, railways, and aerospace due to their high strength-to-weight ratio and excellent mechanical properties. ¹ The joining process of CFRTP is inevitably studied to produce workpieces adapted to more application scenarios. ² Adhesive bonding and mechanical fastening are typically joining techniques of polymer composites. ³ However, the adhesive bonding joints are susceptible to temperature and environmental conditions. ⁴ Mechanically fastened joints are more sensitive to stress concentrations, and bonding parts such as bolts or rivets are not conducive to lightweight. ⁵ Ultrasonic and laser welding have been applied in polymer bonding but are expensive and require skilled technicians. ⁶ Additionally, ultrasonic welding is restricted by the types of polymer and geometries of the material. ⁷ Laser welding is prone to forming porosity defects.^8,9 Therefore, it is necessary to find other joining means for polymers.

Friction stir welding (FSW) was developed to bond lightweight metals with high efficiency, superior joint quality, and low energy consumption.^10,11 FSW between polymers has become an effective way to manufacture lighter, more environmentally friendly workpieces. ¹² Some researchers have studied the application of FSW to CFRTP. Huang et al. ¹³ successfully welded PEI laminate sheet reinforced with 45% of 5-harness satin weave carbon fibers using friction stir spot welding and obtained the best joints at 1200 r/min. Moreover, adjusting the rotational speed can change the bonding area of the joint, resulting in a superior joint. H. Ahmadi et al.^14,15 conducted welding experiments on plates of polypropylene (PP) composites with 20 wt% carbon fibers and evaluated the effect of welding parameters on the tensile shear strength using the Taguchi method. They also explored the influence of different tool pin profiles on the strength of joints and found that the best joint is obtained using a threaded cylindrical taper tool. However, there is limited research on enhancing the strength of joints with CFRTP.

Underwater friction stir welding (UFSW) has proven to be an effective method to enhance the strength of joints. UFSW suppresses peak temperatures, improves the thermal cycling of the weld, and increases welding stability. ¹⁶ Gao et al. ¹⁷ used UFSW to weld polyethylene and obtained a high-quality joint with a tensile strength of 12.3 MPa, which is higher than the strength of FSW joints. Derazkola et al. ¹⁸ established a coupled 3D thermo-chemical-mechanical numerical model to simulate the UFSW process of polycarbonate (PC). The results show that the heat input is reduced due to the higher cooling rate of the water, which leads to an increase in the viscosity of the material and a decrease in the shrinkage and crystallinity of the polymer. In addition, the strength of the UFSW joint is increased by 8% compared with FSW. Sun et al. ¹⁹ successfully bonded polycarbonate sheets using underwater friction stir welding with an external auxiliary heat source. The effect of welding parameters on weld strength is analyzed using the Taguchi method. The results indicate that the external auxiliary water temperature significantly impacts the strength of the weld seam. However, these studies only applied UFSW to polymers, and application to CFRTP needs to be mentioned. Meanwhile, the state of carbon fibers at different heat-assisted water temperatures and their effect on strength have also yet to be studied.

In this paper, carbon fiber-reinforced polyetherimide (CF-PEI) is welded by the thermal-assisted UFSW. Optical microscopy (OM) is applied to observe the evolution law (Changes in the location and distribution of carbon fibers at different water temperatures) of carbon fibers in the stirred zone (SZ). The effect of heat-assisted water temperature on the material flow and carbon fiber state is analyzed. Tensile tests were utilized to explore the relationship between carbon fibers and tensile strength, thus revealing the formation mechanism of the joint. The state of the carbon fibers at the fracture interface after tensile testing is investigated by scanning electron microscopy (SEM). The results showed that a joint with a tensile strength of 33.6 MPa is obtained at 60°C, which reaches 79.6% of the tensile strength of CF-PEI base material. The feasibility and potential of UFSW in carbon fiber-reinforced polymer are verified.

Material and Experiments

CF-PEI, comprising PEI with 30 wt% of randomly distributed short carbon fibers, is utilized in the UFSW experiments. The specimen possessed a thickness of 2 mm, a length of 150 mm, and a width of 100 mm. The main physical properties of CF-PEI are presented in Table 1.

Table 1.

Selected physical and mechanical properties of the CF/PEI.

Tensile strength (MPa)	Glass transition temperature (°C)	Melting temperature (°C)	Transversal thermal conductivity (W m-1 K-1)	Longitudinal thermal conductivity (W m-1 K-1)
42.2	215	260	0.28	4.42

The thermal-assisted UFSW experiments are performed on the HT-JM20 × 8/2 machine. The UFSW pin tool is made of tool steel (H13) featuring a tool shoulder diameter of 10 mm and a length of 50 mm, a pin length of 3 mm, and a diameter and length of clamping end of 20 mm and 40 mm, as shown in Figure 1.

The special fixture has been installed in the welder machine. The fixture comprises a clamping device, a water tank, a temperature sensor, and a temperature control module with a heater, as depicted in Figure 1. The individual functions of the dedicated fixture are listed in Table 2.

Figure 1.

Thermal-assisted UFSW clamping tool.

Table 2.

Part function of the dedicated fixture.

Part number	Part name	Function
1	Clamping device	Fixed workpieces
2	Water tank	Creating a UFSW environment
3	Temperature sensor	Detect real-time water temperature
4	Heater	Heating of water
5	Temperature control module	Control of heating temperature

Two CF-PEI sheets are fixed together using Part 1 to form a lap-welded structure and immersed in water. The water temperature is regulated in real-time and maintained at a constant level by Parts 3, 4, and 5. Representative parameters are selected after several experiments in the same experimental environment, as shown in Table 3. The rotational and welding speeds are set to 1400 r/min and 10 mm/min, respectively. Welding tests are executed out in the air and at different water temperatures (20°C, 40°C, 60°C, 80°C) as shown in Table 3. The tilt angle of the tool is 2° along the longitudinal axis, and the plunge depth is 3 mm.

Table 3.

Specific parameters for CF-PEI welding experiment.

Serial number	Speed (rpm)	Welding speed (mm/min)	Underwater (yes/no)	Temperature (°C)
1	1400	10	Yes	20
2	1400	10	Yes	40
3	1400	10	Yes	60
4	1400	10	Yes	80
5	1400	10	No	20

High-definition magnifying glasses are used to analyze the formation of macroscopic defects. Metallographic specimens for microstructural analysis are made by cutting the weld seam perpendicular to the welding direction. Cross-sections of the welded joints are polished using sandpaper with different particle diameters and a 0.01 μm diamond polishing agent. The optical microscope (OM, model: Olympus) is used to inspect the microstructure of the welded joint. Tensile samples are prepared according to ASTM D3163, and the tensile test is conducted on a universal testing machine with a constant crosshead speed of 0.5 mm/min. A scanning electron microscope (SEM, model: Quanta 2000) is used to observe the microstructure of the weld fracture. For the SEM observation, a very thin gold layer was coated on the fracture surface.

Results and Discussion

Welding Surface Appearance

The surface morphology of the FSW joint is shown in Figure 2. Collapse defects are generated in the center of the weld seam. Large flashes and material buildups are found on the advancing side (AS). During welding, heat input is generated by friction between the rotating tool and the material. Frictional heat on the welding surface is increased by the expanded contact area between the tool shoulder and the material. Excessive heat input causes the material to melt. The molten material has low thermal conductivity, cannot be cooled in time. It is carried to the AS by the tool from the receding side (RS) and the center of the weld. Therefore, flashes and material buildups are formed. A higher number of flashes indicates more material loss from the weld seam, resulting in significant surface collapse.

Figure 2.

Typical morphology of FSW joint.

The surface morphology of UFSW joints at room temperature is shown in Figure 3(a). Compared with Figure 2, the collapse and flash of the weld seam surface are less pronounced, but tunnel and fish-scale patterns are observed. A thermal cycle is formed between the weld seam and the cooling water via the stirring action of the tool. Since the frictional heat is transported out from the weld seam by the circulating water, the heat input to the weld seam is insufficient. The material in the weld does not get more heat, which can lead to inadequate material softening. Insufficient softening of the material means poor material flow, which can result in small holes not being filled. Two or more small holes on the surface of the weld are joined together to form a tunnel. In addition, insufficiently softened material flows unevenly, resulting in discontinuous contact between the tool and the surface. Therefore, discontinuous fish scale patterns are formed on the weld seam surface. However, the water molecules increase the viscosity of the material, stopping excessive loss of material.

Figure 3.

Weld surface morphology of UFSW joints at different water temperature.

The typical surface morphology of the weld seam at 40°C is shown in Figure 3(b). From Figure 3(b), surface roughness and fish scale pattern density are significantly reduced, tunnels are eliminated, but circular pits are detected. The heat input increases as the water temperature rises, making further softening of the material possible. The softened material flows into the tunnel under pressure from the tool shoulder. However, some materials are still not sufficiently softened due to the high-water cooling rate of 40°C. Insufficiently softened and sufficiently softened materials are present at some locations in the weld seam. The increased unevenness in material flow results in a deeper fish scale pattern, accompanied by the formation of circular pits.

Compared to other water temperatures, at 60°C, the weld seam surface is flat and without defects such as grooves, fish scales and flashes, as shown in Figure 3(c). The cooling effect of water on the weld seam is weakened compared to 40°C. At this temperature, the material receives sufficient heat to achieve adequate softening. The molten material in the weld seam is mixed more thoroughly via the stirring action of the tool. Uniformly mixed materials are less prone to porosity during cooling and shrinkage. Therefore, no material loss occurs and the weld seam surface quality is high.

The surface morphology of the weld seam at a water temperature of 80°C is shown in Figure 3(d). The surface quality of the weld seam is compromised. Pin flash, fish scale patterns, and pits are again detected. The weld seam appearance is similar to that in the air. The cooling rate of the water is further reduced, which leads to the molten material not being sufficiently cooled. Materials in the molten state are highly mobile. Highly mobile materials are prone to being lost from the weld due to the influence of the tool. The molten material adheres to the carbon fibers and flows out of the weld seam, making pin flashes. In addition, the ability of the tool to carry material is reduced due to the lubricating effect of water. Some material does not flow from the RS to the AS and spilled at the RS. Therefore, flash defects are also observed in RS. This material loss reduces the contact between the shoulder and the workpiece surface, resulting in an irregular fish scale pattern due to the lack of axial pressure. However, the peak temperatures during welding are still suppressed by the cooling water. At the same time, water and tool shoulders exert pressure on the weld seam. Therefore, a collapse similar to that in the air did not occur.

Microcosm of the Weld Cross-Section

Macroscopic morphologies of the weld cross-section at air and 60°C water temperature are shown in Figure 4(a) and (b), respectively. The thickness of the joint in the air is reduced by more than 1.4 mm. Based on previous studies, collapse always occurs even though welding parameters such as rotation and welding speed are adjusted. Moreover, the joint strength is essentially no more than 50% of the BM.^14,15,19 In general, there are no visible regional differences in the cross-section of the polymer joints. However, significant regional divisions are found in the CF-PEI joint at 60°C due to the presence of carbon fibers, as shown in Figure 4(b). The weld seam cross-section is divided into the base material (BM), stir zone (SZ), and thermo-mechanically affected zone (TMAZ) based on the length and distributions of the carbon fibers. The carbon fiber in BM remains unchanged. The carbon fibers in SZ are elongated due to the stirring action of the tool. Although the length of carbon fibers increases at high temperatures, this dimensional change is small. Therefore, the increase in length of the carbon fibers in this paper is obtained by comparing it to the carbon fibers at the BM within the cross-section. It is not a change in the length of the carbon fibers themselves, but a change in the apparent length of the carbon fibers within the field of view. The change in mapping length means that the carbon fibers have been reoriented. A region with a carbon fiber density significantly lower exists on the outer side of the SZ, serving as the boundary between the SZ and other areas. These two regions, the outer side of the boundary and the lower side of the weld seam surface, are called the thermo-mechanical affected zone (TMAZ). TMAZ is not only subjected to the stirring action of the tool but also to frictional heat, with the latter having a predominant effect. In this region, the distribution of carbon fibers has changed, and only a smaller amount of carbon fibers are elongated.

Figure 4.

Macroscopic morphology of weld seam cross-section, (a) air; (b) 60°C water temperature.

The macroscopic morphology of the weld seam cross-section at different water temperatures is shown in Figure 5. At lower water temperatures, the cooling effect of environmental media reduces the effect of frictional heat on TMAZ, resulting in a smaller bonding area. The root defect arises when the material, after being transported from the lower plate to the upper plate by the tools, fails to enter the cavity below the tool. In addition, regions cannot be classified based on the state of the carbon fibers due to the small area of the TMAZ. As the temperature increases, the defects inside the weld seam are improved, and the bonding area of the weld seam is magnified. The enhancement of thermal-mechanical motion is beneficial for softening more materials in TMAZ. The ability of the material in the TMAZ to fill defects is enhanced, so the bonding area is large, as shown in Figure 4(b) and Figure 5(c). However, canyon-shaped defects are produced on the receding side (RS) at excessively high- water temperatures. The presence of internal defects in the weld seam, primarily on the RS, suggests a flow of molten material from the RS towards the AS. The macroscopic morphology of the weld seam cross-section corresponds to the surface morphology of the weld, which proves the correctness of the above analysis.

Figure 5.

Macroscopic morphology of the weld cross-section at different water temperatures, (a) 20°C; (b) 40°C; (c) 60°C; (d) 80°C.

The microscopic morphology of the SZ after being magnified 10 times is shown in Figure 6. The degree of mixing between carbon fiber and PEI substrate in SZ increases as the water temperature rises. Since insufficient heat input at low temperatures, the mapping length of most carbon fibers remains unchanged, and tiny defects still exist inside the SZ. Increased temperatures cause more material to melt. The molten material has a less inhibitory effect on the flow of carbon fibers. Therefore, the mapping length of carbon fibers at 60°C increases compared to 20°C and 40°C. At this time, the material flow situation can be inferred from the state and distribution of the carbon fibers. The arrows in Figure 6 mark the direction of the carbon fibers. Carbon fiber and PEI substrate flow from the RS to the AS and then to the SZ to fill defects. Additionally, carbon fiber flows counterclockwise and exhibits a layered distribution similar to onion rings. This property of the carbon fiber indicates a uniform mix of materials from the upper and lower panels.

Figure 6.

Microstructure of joints at different water temperatures, (a) 20°C; (b) 40°C; (c) 60°C; (d) 80°C.

As shown in Figure 6(d), there are serious defects in the weld seam at 80°C. The viscosity of the PEI substrate is reduced due to the higher peak temperatures. Meanwhile, the lower cooling rate at 80°C is not conducive to material being sufficiently cooled. The molten material is lost from the weld by thermo-mechanical movement, which leads to the formation of cavities. Cooling water may enter the cavities. The water pressure makes it difficult for entrained air to escape from the weld during shrinkage and solidification. Excessive air in the weld seam promotes the formation of canyon-shaped defects. The direction of the force from the tool is changed because of the presence of defects. Therefore, long carbon fibers are found in TMAZ. Moreover, some carbon fibers point to defects, which suggests that the material has the behavior of filling defects during the flow process. This distribution of carbon fibers adequately supports the previous analysis of material flow.

The State of Carbon Fiber

The state of carbon fiber at different water temperatures is shown in Figure 7. Carbon fibers lengthen as the temperature increases. At low temperatures, the insufficiently softened material restricts the tensile behavior of the tool on the carbon fibers, resulting in low carbon fiber elongation. In Figure 7(b), the mapping length of the carbon fiber is increased compared to the carbon fiber in the BM, but there are still many carbon fibers at SZ in the cross-section that are similar in state to those at the BM. This phenomenon indicates that the carbon fibers do not flow vigorously at low temperatures. Therefore, the carbon fibers are not subjected to interweaving behavior. The PEI substrate softens more at 60°C as compared to 40°C and 20°C. The carbon fibers in the softened substrate have a high degree of freedom. The intense thermo-mechanical motion promotes the formation of an interwoven structure of the carbon fibers. In addition, the diameter of stretched carbon fiber shrinks, making it easier to be shredded by tools. Thus, interwoven and broken carbon fibers are observed in Figure 7(c). At high temperatures, the number of punctate carbon fibers (P-CF) increased, indicating that more carbon fibers are shredded by the tool during vigorous material flow. Compared to 60°C, the interweaving degree of carbon fibers decreases at 80°C. The high degree of freedom of the carbon fibers in the molten material facilitates their regular flow within the material. Therefore, carbon fibers that are parallel to each other are observed.

Figure 7.

Macro state of carbon fibers at different temperatures: (a) 20°C; (b) 40°C; (c) 60°C;(d) 80°C.

The microstructures of carbon fibers at 60°C are shown in Figure 8. The length of a carbon fiber is about 150 μm, and its diameter is approximately 7 μm. Although some carbon fibers are sheared by the tool, they are not separated. The broken carbon fibers contact each other and form new bonds during the cooling and shrinkage, as displayed in the circles in Figure 8. Some carbon fibers are interwoven into polygonal structures, as marked in the quadrilateral in Figure 8. This structure disperses the shear forces within the material. At the same time, the deformation and flow of the material are hindered by the interwoven carbon fibers after cooling. In addition, some carbon fibers do not display fracture trends, illustrating that they still have toughness and retain reinforcing properties to the PEI matrix. This structure and distribution of carbon fibers increases the tensile strength of the joint.

Figure 8.

State of carbon fibers in UFSW joints at 60°C water temperature.

Mechanical Properties

The tensile test results of the thermal-assisted UFSW joint are shown in Figures 9 and 10. In order to ensure the reliability of the data, three tensile specimens are prepared for each parameter, and the tensile strengths are averaged, as shown in Table 4. Most joints are fractured at RS, as shown in Figure 9(b). The microstructure of the weld seam cross-section in Figures 5 and 6 exhibits that RS is slightly lower than AS. Moreover, most defects are located at RS, proving the rationality of the fracture at RS. The tensile strength first increases and then decreases with rising water temperature, as indicated in Figure 10. The tensile strength is closely related to the size of the defect, the bonding area, the degree of material mixing, and the degree of carbon fiber interweaving. The low tensile strength of the joint at 20°C is attributed to the small bonding area of the weld seam. The higher tensile strength at 40°C compared to 20°C can be attributed to fewer defects in the weld seam. When the water temperature rises to 60°C, the weld seam has the fewest defects, the largest bond area, and the highest degree of carbon fiber interweaving. At this time, the tensile strength of the joint reached 33.6 MPa, which is 79.6% of BM strength. This result is comparable to advanced welding levels and far exceeds ordinary FSW. The low strength of the joints at high temperatures is caused by increased internal defects and excessive material loss. The coefficient of variation (COV) is the lowest at a water temperature of 60°C, which indicates high welding stability at an auxiliary water temperature of 60°C.

Figure 9.

Schematic diagram of weld tensile test: (a) tensile test conditions; (b) tensile specimen and fracture position.

Figure 10.

Results of thermal-assisted UFSW joint tensile testing: (a) tensile stress–displacement curves; (b) summary of the tensile properties for the joints.

Table 4.

Tensile strength of welds at different water temperatures.

	air (MPa)	20°C (MPa)	40°C (MPa)	60°C (MPa)	80°C (MPa)
Specimen1	13.2	19.3	24.4	31.4	20.9
Specimen2	15.1	21.4	29.9	34	20.1
Specimen3	14.4	25.8	30.7	35.4	26.2
Average strength	14.2	22.2	28.3	33.6	22.4
COV	0.06	0.13	0.10	0.05	0.12

The joint strength is analyzed based on the state of the carbon fibers. The thermo-mechanical effect is enhanced with increasing temperature, thus promoting an increase in the elongation of the short-cut carbon fibers. The length of the carbon fibers at low temperatures is less than 50 μm, which means that their dispersion is not intense. Therefore, no interweaving behavior is found within the cross-section. The elongation of the carbon fibers increased when the water temperature increased. The carbon fibers at the SZ are distributed at the joints of the upper and lower plates. Meanwhile, most carbon fibers are angled to the dividing line between the top and bottom panels. These carbon fibers bear the tensile and shear forces of the interface between the upper and lower plates. Carbon fibers are interwoven into a stable structure to prevent deformation during stretching. In addition, the material in the TMAZ is sufficiently affected by heat, thereby benefiting the adequate mixing of the carbon fibers with the PEI substrate. These behaviors of carbon fibers are conducive to joint strength enhancement. However, the length of the carbon fibers exceeds 270 μm, and the diameter further decreases in high-temperature water, as shown in Figure 7(d). At this time, the carbon fibers exhibit a state of imminent fracture, indicating its lowest toughness. The load-carrying capacity of individual carbon fibers is weakened. Meanwhile, excessive rupture of the carbon fibers also reduces their reinforcement ability. Therefore, the tensile strength is reduced.

The typical morphology of carbon fibers at the weld fracture with different water temperatures is shown in Figure 11. At room temperature in water, the carbon fibers at the fracture are rarely interwoven, which indicates that the position change of carbon fibers is not drastic, as shown in Figure 11(a). As the auxiliary water temperature is raised, the carbon fibers are pulled out from the fracture surface. The carbon fiber pullout phenomenon occurs due to the interaction between interwoven carbon fibers during stretching. At an auxiliary water temperature of 60°C, the number and length of pulled-out carbon fibers increased, indicating that the position of the carbon fibers changed more dramatically and the degree of interweaving increased. In Figure 11(d), the carbon fiber pullout phenomenon is not observed due to defects in the weld seam. Therefore, the weld exhibits the best tensile strength at an auxiliary water temperature of 60°C. The carbon fiber morphology at the fracture corresponds to the cross-sectional morphology of the carbon fibers, which proves the correctness of the previous analysis.

Figure 11.

SEM image of the carbon fibre state at the fracture: (a) 20°C; (b) 40°C; (c) 60°C; (d) 80°C.

Referring to the published literature, in this paper, a relatively high level of binding efficiency of CFRTP joints is obtained. The maximum tensile strength of various CFRTP joints is shown in Table 5.

Table 5.

Maximum tensile strengths of various CFFTP joints.

Reference	Process	BM	Max. Strength (MPa)	% of BM strength
Huang 13	FSSW	PEI laminate with 45% of 5-harness satin weave carbon fibers	45	39.1
H. Ahmadi 14	FSW	PP with 20 wt% carbon fibers	6.06	30.3
W. Lucas T 20	FSW	Carbon fiber reinforced low-melt polyaryletherketone	73.8	50
This work	UFSW	PEI with a 30 wt% random short carbon fiber	33.6	79.6

Conclusion

In this paper, heat-assisted UFSW is applied to bonds CF-PEI. Weld seam surface morphology, tensile properties and macro/microstructure of the joints are studied. Macroscopic analysis of the weld seam surface showed that the surface quality of the joints improved and then deteriorated as the water temperature increased. Adjusting the water temperature parameters ensures that the material in the weld seam melts, flows, and cools at the appropriate temperature, leading to a flat and smooth weld seam. Microanalysis and tensile test results indicate that the formation mechanism of the joint is elongation, intertwining, fracture, and dispersion of carbon fibers at the joint. The carbon fibers intertwine through thermo-mechanical movement, forming a stable structure capable of withstanding internal shear forces and preventing material deformation during stretching. High-quality joints with sufficient bonding area and uniform mixing between interwoven carbon fibers and PEI matrix are acquired at 60°C. The maximum tensile strength of the joint is 33.6 MPa, equivalent to 79.6% of the tensile strength of the CFRTP sheet. UFSW has the potential to be used in CFRTP and to enhance the strength of welded joints.