The Effect of Welding Process Parameters on the nilon-6 Plate Friction Stir Welded Joint

Nylon-6 is one of the most extensively used engineering thermoplastics. Due to its durability, reduced weight, low coefficient of friction, and abrasion resistance, this polymer is an excellent substitute for various materials, including metals and rubber. Despite being those, nylon-6 materials are joined together utilizing multiple welding processes. The current work examined the viability of the friction stir welding (FSW) method on nylon 6 plates that were 4 mm thick to investigate the effect of tool diameter ratio and feed rate on nylon-6 sheets with the FSW method on the tensile properties of the joints. The 4 mm thickness of the nylon-6 plate was cut into the dimensions of 115 mm x 100 mm. The sliced plates were then joined through the FSW process using ST80-based tools with various parameters in the shoulder to pin diameter ratio (D/r ratio) of 10/3, 15/3, and 20/3 and feed rates (FR) of 4 mm/minute, 6 mm/minute, and 8 mm/minute at a rotational speed (RS) of 5800 RPM. Afterward, the weld joint is cut using water jet cutting according to the tensile testing standard ASTM D638 type IV. Following this, characterization of the speci-mens, including macrograph examination, hardness testing, and tensile testing, was carried out. The results show that the highest tensile strength was obtained at a tool diameter ratio of 10/3 mm with a feed rate of 6 mm/minute, with a tensile strength value of 19 MPa reaching up to circa 90% of joint efficiency. The strength of the welded joint then decreases as the diameter ratio of the tool increases with each feed rate. Some defects appeared at the higher tool diameter ratios welded joint, including incomplete fusion, flash, thinning, and lack of bonding. In conclusision the combination of a D/d ratio of 10/3 and a FR of 6 mm/min at RS 5800 RPM can provide the optimal conditions for a strong nylon-6 friction stir welded joint.


Introduction
Polymers are known to have advantages in terms of design and formation compared to metals.One of the most frequently used types of polymers is nylon 6 Nylon-6 is a high-performance thermoplastic used in a variety of applications including automotive, aerospace, and consumer products [1].However, due to its high melting temperature, low thermal conductivity, and tendency to form voids and defects, joining nylon-6 using traditional welding techniques has proven difficult [2].Friction stir welding (FSW) is a solid-state joining process that has gained significant attention in recent years due to its ability to join a wide range of materials, including thermoplastics [3].FSW is a promising technique for joining nylon-6 because it can avoid these issues, resulting in high-quality welds with minimal thermal damage [4].Previous research has shown that FSW can produce strong and durable welds in nylon-6 [5][6] [7], but process parameters have a significant impact on weld quality [2].
In recent years, there has been a surge of interest in optimizing FSW parameters for nylon-6 in order to improve weld mechanical properties [8][9].Several studies [10] [11] [12] [13] investigated the effect of FSW parameters such as tool geometry, rotational speed, feed rate, and axial force on weld strength and microstructure.
The feed rate is defined as the tool's linear velocity along the welding path, and it influences heat input, plastic deformation, and the microstructure of the welded joint The D/d ratio in friction stir welding (FSW) refers to the ratio of the tool diameter (D) to the pin diameter (d).In the case of nylon-6 FSW joints with variation of feed rates, several studies have conducted to investigate the effect of the D/d ratio.Mehdizadeh et al. [14] studied the effect of D/d ratios ranging from 5/2 to 15/2 on the mechanical properties of nylon-6 FSW joints.They found that increasing the D/d ratio improved the joint strength, with the highest strength achieved at a D/d ratio of 15/2.However, the authors also noted that increasing the D/d ratio above a certain point can lead to excessive heat input and degradation of the material.
Based on the research described above, the investigation into the FSW process is still broad enough to be further investigated.Feeding speed (feed rate) is a significant parameter in the FSW process based on research that has been carried out because it affects the particle distribution of materials in the stir zone [6].Research on FSW with feed rate variation parameters using nylon-6 material has been done.Still, research using feed rate parameters coupled with the ratio of tool diameters using nylon-6 material has never been done.The tool geomtery also greatly affects the heat, affecting the tensile strength and hardness of the material used.Therefore, research on the effect of the ratio of tool diameter and feeding speed on the tensile properties of nylon-6 joints with the FSW method was carried out to to identify the best welding conditions for producing high-quality welds.

Experimental Method
In this study, two pieces of nylon-6 plates with dimensions 115 x 100 x 4 mm were joined by the FSW technique with feed rates of 4, 6, and 8 mm/min at a rotational speed of 5800 RPM, as shown in Table 1.The friction stir welding tools were made of hardened ST80 with a 3.8 mm pins length, 3 mm diameter pin tool, and a D/d ratio of 10/3, 15/3, and 20/3.All welding processes were carried out using the same load.After being welded, the nylon-6 plate was cut into a tensile test specimen using water jet cutting according to ASTM D638 type IV standards, as shown in figure 2. Following this, the specimens were characterized, including macrostructure examination, an Olympus stereo optical microscope, hardness testing using a Shore D hardness tester, and tensile testing using Zwick Roell universal testing machine.

Results and Discussion
Figure 3 displays the appearance of the nylon-6 welded joints.All welded joints from the top view show the thinning dan flash, on the other hand, all the bottom view presented that they have been joined smoothly and evenly.In Figure 3 a, b, c, the variation D/d ratio of 10/3 with a feed rate of 4, 6, 8 mm/min, the appearance of the weld is smooth with small flash, although it tends to be slightly uneven, both on the top and bottom views.At a feed rate of 6, the flash looks rougher at the joints but shows that there has been more material diffusion.In In Figure 4, the macrographs of the nylon-6 welded joint can be observed that in the variation in the D/d ratio of 10/3 (Figure 4. A and b) there are slight flash and incomplete fusion defects.Flash is an excess or imperfect melt of material on the surface of the stir zone caused by high heat due to the large shoulder friction area.Incomplete fusion is not filled within the stir zone caused by a less-than-perfect material cooling process.Although there are still small defects, the appearance of the weld is quite good and looks homogeneous.In variations in D/d ratios of 15/3 and 20/3, the weld joint shows flash defects, incomplete fusion, lack of bonding, and thinning.The lack of bonding is a small cavity between the stir zone and the parent material generated by the poor distribution of particles so that it cannot blend appropriately between these two regions.This can be caused by the FR that is too fast during the FSW process.Thinning is a defect that occurs in the weld area or stir zone, which is in the form of reduced material that results in cavities such as valleys in the FSW connection.From the macrographs, quite good results are shown in the variation in the D/d ratio of 10/3 than those of 15/3 and 20/3.The hardness of the welded joints was examined at the advancing side zone, stir zone, and retreating side zone.In general, the advancing side zone demonstrated the highest hardness value followed by the retreating side zone and the stir zone.In the advancing side zone, there is the most significant deformation of its mechanical and thermal properties, making the material denser and harder [16].This zone experiences higher temperatures and shear deformation rates than the retreating side.This is due to the combined effects of the rotational and traverse speeds on the advancing side, resulting in a higher rate of heat generation due to frictional heating and shear deformation.Hence, the advancing side experiences more extensive plastic deformation and recrystallization than the retreating side, resulting in a finer and more uniform microstructure.In addition, the advancing side also undergoes a higher cooling rate compared to the retreating side due to the rotation direction of the tool.This results in a finer and more uniform microstructure on the advancing side due to the higher rate of nucleation of new grains.In the stir zone area, material melt occurs in this area alone so that the hardness value is the lowest, which is 54.5 HD for parameter variations of 10/3 and 6 mm/min and 48.5 HD for parameter variations of 20/3 and 8 mm/min.Several studies have shown that the hardness of the weld zones in FSW joints is closely related to the microstructure and the level of plastic deformation during the process [17,18].The hardness of the weld zones is an important indicator of the strength and toughness of the joint, as well as its resistance to wear and fatigue.Increasing the D/d ratio from 10/3 to 20/3 resulted in a decrease in the hardness of the stir zone.This can be attributed to the fact that a higher D/d ratio leads to a larger heat input and a longer residence time of the material in the stir zone, resulting in more extensive plastic deformation and grain refinement.However, when comparing the hardness of the weld zones between nylon-6 FSW joints with a D/d ratio of 10/3 and 20/3, with a feed rate of 6 mm/mm and rotational speed of 5800 rpm, it was found that the joints with a D/d ratio of 10/3 had a higher hardness.This may be due to the fact that a lower D/d ratio results in a smaller heat input and a shorter residence time of the material in the stir zone, resulting in less extensive plastic deformation and less grain refinement.As a result, the material in the stir zone maintains a higher degree of crystallinity and thus a higher hardness.Tensile testing has been carried out, including raw materials, plastic steel glue joints, and FSW joints.From the strain-stress curve (Figure 6), we can observe the material's tensile properties from tensile strength, strain, modulus of elasticity, yield strength, etc.The nylon-6 raw material shown by the black curve, this material exhibits a tensile nature behavior typical for polymeric materials where it has a large plastic region.On the other hand, all welded joints and glued joints demonstrated low strain and brittle fracture, except the D/d ratio of 10/3 6mm/min specimen, which possesses reasonably high strain.
The D/d ratio of 10/3 6mm/min specimen show the highest tensile strength achieving 19 MPa which is 89.2% of the raw material (21.3 MPa).In general, the strength of the welded joint then decreases as the diameter ratio of the tool increases with respective feed rates (Figure 8).It is found that the joint strength with D/d ratio 20/3 and FR 8 mm/min demonstrated lower strength.One possible explanation for this difference in strength could be the difference in heat generation and material flow during the welding process.When the D/d ratio is smaller, the contact area between the tool and the material is smaller, leading to a higher contact pressure and higher frictional heat generation at the tool-workpiece interface.This results in more plasticized material, which can lead to better mixing of the materials and formation of a strong bond.Additionally, a smaller D/d ratio results in a larger effective strain rate, which can enhance material flow and reduce the occurrence of voids and defects in the joint [17].On the other hand, a higher feed rate results in a shorter residence time of the plasticized material in the stirred zone, which can result in incomplete mixing and formation of voids that weaken the joint.The feed rate was increased from 6 mm/min to 8 mm/min in the current study, which could have resulted in less material being effectively stirred and less heat input, resulting in a weaker joint This can explain why the joints with a higher feed rate of 8 mm/min in the D/d ratio of 20/3 group showed lower strength compared to the joints with a lower feed rate of 6 mm/min in the D/d ratio of 10/3 group [14].Moreover, the hardness of all zones, including the advancing side zone, stir zone, and retreating side zone, was found to be higher in the D/d ratio of 10/3 group with a feed rate of 6 mm/min compared to the D/d ratio of 20/3 group with a feed rate of 8 mm/min.This is consistent with the strength results, as the hardness is an indicator of the material's strength and resistance to deformation [19].Overall, the results suggest that a smaller D/d ratio of 10/3 with a feed rate of 6 mm/min is optimal for achieving high strength and hardness in nylon-6 FSW joints at a rotational speed of 5800 rpm.However, other parameters such as tool geometry, tilt angle, and tool traverse speed can also affect the joint quality and should be considered when optimizing the FSW process for a specific material and application.The strain value is inversely proportional to the tensile strength value [9].In Figure 8, the average value of the highest modulus of elasticity is found in the variation of plastic steel glue, which is 645.5 MPa.While in the FSW welded joint, the highest elasticity modulus value is found at process parameters 15/3 and 8 mm/min, which is 554 MPa.The lowest elasticity modulus value is found in the variation in the tool diameter ratio of 20/3 with a feed rate of 6 mm/min, which is 310.5 MPa.The greater the value of the modulus of elasticity, the greater the stress required for a particular strain, and vice versa Figure 8. Elastic modulus of the welded joint at various processing paramater compared to raw material and the glued joint specimen Figure 8 shows the fractographs of the specimen.Excessive material melting in the weld zone (flash) causes the specimen not to be completely cut off, commonly called pullout failure (Figure 9.a-f).Meanwhile, the raw material shown a reasonably long stretching of the material (Figure 9.g), and in the plastic steel glue joint demonstrated by Figure 9.h, the material is completely cut off.This is in accordance with the elongation load curve in Figure 7 where the strain in raw material specimens looks large, and at the plastic steel glue joints, the strain looks small.All of the welded joint including the glued joint presented brittle fracture.The brittle fracture of the nylon-6 FSW joints maybe attributed to a number of factors, including deformation-induced crystallization, high polymer viscosity, and insufficient mixing of the plasticized material [17,20].According to Karthikeyan et al. [19], nylon-6's low ductility and toughness make it prone to cracking and brittle fracture, particularly when subjected to high deformation rates.Furthermore, the lower melting temperature of nylon-6 compared to other thermoplastics can result in incomplete melting and inadequate bonding during FSW.Mukherjee et al. [17] also noted that the brittle fracture of the FSW joints may be exacerbated by the high rotational speed and low feed rate, which may result in excessive heat generation and insufficient plasticization.Inadequate plasticization can lead to poor mixing and bonding, resulting in a weak joint with low ductility.Furthermore, Cui et al. [20] proposed that the high strain rate and low strain may contribute to the brittle fracture of nylon-6 FSW joints.The high strain rate may cause deformation-induced crystallization, resulting in brittle material.Low strain, on the other hand, may not provide enough mixing and plasticization to form a strong bond.

Figure 5 .
Figure 5.The hardness value of the advance side, stir zone and retreating side of welded joint with D/s ratio and FR of : 10/3 & 6 mm/min, and 20/3 & 8 mm/min

Figure 6 .
Figure 6.Stress strain curved of the of the welded joint at various processing paramater compared to raw material and the glued joint specimen

Figure 7 .
Figure 7. Tensile strength of the welded joint at various processing paramater compared to raw material and the glued joint specimen

Figure 9 .
Figure 9. Failure tensile testing specimens with variations a. 10/3 & 6 mm/min, b. 10/3 & 4 mm/min, c. 15/3 & 8 mm/min, d. 15/3 & 6 mm/min, e. 20/3 & 4 mm/min, f. 20/3 & 8 mm/min, g.raw material, and h.plastic steel glue4.ConclusionsBased on the findings of the research on nylon-6 friction stir welded joints with varying D/d ratios and feed rates, it can be concluded that :• Choosing the best welding parameters is critical for achieving a strong joint.The strength of the joint is affected by several factors, including rotational speed, D/d ratio, and feed rate.• The optimal welding parameters for achieving the strongest joint were discovered to be a rotational speed of 5800 rpm, a D/d ratio of 10/3, and a feed rate of 6 mm/min.This combination resulted in the joint having the highest tensile strength, ultimate strength, and hardness.• It was discovered that increasing the D/d ratio from 10/3 to 20/3 at an 8 mm/min feed rate resulted in brittle joint failure.Furthermore, increasing the feed rate from 6 to 8 mm/min with a D/d ratio of 10/3 resulted in a weaker joint with lower tensile strength and hardness.• The welding parameters for nylon-6 friction stir welding must be carefully chosen based on the desired joint strength and properties.