Parametric Optimization of Butt Welded Polycarbonate using Response Surface Methodology

Friction Stir Welding (FSW) is a solid-state joining process for metals, non-metals and polymers. It is carried out with the help of a specially designed, non-consumable tool. The heat required, for creating a softened region at the faying surfaces, is generated by rotation of tool against the work piece material. Being a solid-state welding process, it offers several advantages like inducing minimum effect on the mechanical properties of base material, reduced shrinkage and distortion, no spatter or Ultra Violet (UV) radiations etc. However, developing a sound weld requires an appropriate combination of several process parameters e.g. the design of tool, its rotational and traversing speeds etc. To substantiate this aspect, an approach based on Response Surface Methodology (RSM) is presented during this paper that optimizes the combination of process parameters while investigating their effect on the mechanical properties of a friction stir welded butt joint configuration of Polycarbonate. To minimize the total number of combinations a Central Composite Rotatable Design (CCRD) is used with three factors and two levels. The results have shown that the butt joints fabricated at a traverse speed of 14 mm/min, rotational speed of 1700 RPM and with simple cylindrical conical tool geometry yielded the maximum ultimate tensile strength of 51.299 MPa.


INTRODUCTION
elding is a metal joining process which is generally performed by fusing the adjoining surfaces while adding some additional filler material. Having been performed at very high temperatures, the welded joints always require to be shielded while in molten state to avoid oxidation and any other contamination that may damage the overall strength of joint. Shielded Metal Arc Welding, Gas Metal Arc Welding, Flux Cored Arc Welding, Gas Tungsten Arc Welding, Oxy-Fuel Welding, etc. are some of the examples where the welded joints require proper shielding. Fusion consumable pin profile tool and can be employed to join metals, non-metals and polymers.
FSW, a solid-state joining technique, was initially implemented at The Welding Institute (TWI) in United Kingdom [1]. The basic idea was to generate heat at the tool by the combination of pressure and frictional forces to diffuse and merge the adjoining surfaces. FSW requires a comparatively lesser amount of heat for joining the surfaces thus eliminates the chances of distortion and other welding flaws. Dashatan at al. [2] concluded that FSW, in addition to be environment friendly, has also proved to be economical as it does not require any filler material and shielding gas. The ability of FSW to weld even non-metals and polymers have made this method even more useful and important in industry. This has increased the application of polymers in the automotive, electronics, aerospace and packaging industries due to their lighter weight and higher strength to weight ratio [3].
Modern welding equipment are generally far more expensive than conventional joining techniques i.e. adhesive and mechanical fastening, that is why it limits their scope of application [4]. However, better joining strengths can be achieved with a comparatively lower investment by using FSW as there is no requirement of filler material and shielding gas. Also, through proper positioning and alignment of the welding tool and specimens, better joint formation can be surely achieved [5]. Reynolds et al. [6] carried out FSW of a 3.2mm thick SS alloy at a single welding speed in combination with two different rotational speeds. They concluded that higher rotational speed responds with a higher energy input per unit of weld's length thus making the process applicable to higher melting alloys too. Mishra et al. [7] described that the preferred advantages of FSW are higher joining speeds, autogenous welding, improved metallurgical properties, and reduced skilled manpower dependencies. Scialpi et al. [8] conducted experiments on 9mm Polypropylene (PP) sheets. They used a Design Of Experiments (DOE) based analysis to study the effect of process parameters on the dimensions of Heat Affected Zone (HAZ) and strength of joint developed. They concluded that better quality and strength was achieved in FSW as compared to conventional welding techniques. Eslami et al. [9] studied the effect of different tool geometries on joint's strength of dissimilar PP and Polystyrene (PS) polymers. They observed that a stationary tool with two sleeves produced a stronger weld with good surface quality. They also came up with the conclusion that behavior of polymers is different from metals and therefore similar strength, in both materials, is difficult to obtain even with the same tool geometry.
Mehta et al. [10] developed an approach for the FSW of two dissimilar materials (Copper and AA6061-T651) and observed the effect of process parameters on welded joints. They argued that a defect free joint having a maximum strength of 133 MPa could be developed by using cylindrical tool pin profile. They also observed that with increase in the shoulder diameter and tilt angle, the plunge load also increased. Derazkola et al. [11] stated that rotating pin geometry is the main influence for material flow and attaining the peak temperature for high strength joints. They also observed that heat generated at the tool shoulder, pin surface and pin top have a direct relationship with the number of revolutions. Rane and Yadav [12] effectively employed FSW on two dissimilar materials i.e. Aluminum alloy and Cu alloy sheets of 6mm thickness. They observed that maximum hardness and strength was achieved at mid-level values of both the considered parameters (traversing speed and feed). Oyedemi et al. [13] concluded that response of tri-flute conical tool was better than the other profiles. They also observed that welding force is approximately proportional to pin surface area and inverted tool ensures better results in welding regions. Huanga et al. [14] used a threaded tapered profile tool for triple facets FSW of Polyether Ether Ketone (PEEK) and Al alloy while verifying the soundness of joint formation. Panaskar et al. [15] [18] analyzed the FSW response of ABS sheets by experimental and statistical approaches. RSM with Central Composite Design (CCD) was used utilizing four factors, first with three levels and then with two levels of block parameters (i.e. cylindrical and conical pin). 101% and 99% higher strength was achieved from conical and cylindrical pinned tools, respectively.
Lambiase et al. [19] studied the effect of varying process parameters on FSW of Polycarbonate sheets. They carried out a single lap shear test to determine the mechanical strength of joints. Two full factorial designs were developed for experimental analysis. Also, an ANN model was developed to compare the experimental results and model predictions. The results showed that all the considered parameters did not have a similar impact on the joint strength.
Mostafapour et al. [20] also analyzed the effect of process parameters on HDPE sheets while employing heat assisted FSW and carrying out the structural and mechanical testing of joints. They came up with the conclusions that rotational speed of tool has a significant role in welded joint's strength and a traversing speed of 25mm/min was found to be the optimum value for Polyethylene sheets. Another study that investigated the effect of parameters on joint strength was carried out by Astarita et al. [21]. They butt welded a 3 mm thick Polycarbonate sheet through FSW and achieved a maximum of 75% of the base material's strength.
Hajideh et al. [22] conducted a successful FSW of 8 mm thick sheets of dissimilar materials -Polymethyl Methacrylate (PMMA) and PC. 98% of PC base material joint was achieved. The effect of process parameters on weld strength and hardness followed a similar trend. Hasanzadeh et al. [23] worked on the inclusion of Alumina nano particles in Polycarbonate. Four different percentages of 0, 1, 2 and 3 were experimentally utilized to conclude the effect on strength and hardness of 3.6mm thick specimens. 01% weightage of Nano Alumina resulted in greatest strength, whereas, 0% weightage produced the greatest hardness in welded specimens. Also, it was observed that welding parameters did not have significant role in enhancing the mechanical properties of nano composite specimens. Kim et al. [24] studied the FSW of Aluminum A6005 sheets extruded in 4.8mm thickness in butt joint formation. Process parameters selected to optimize the process were rotational speed, welding speed and tilt angle.
Palanivel et al. [25] applied RSM to optimize the independent parameters in FSW. Model fitness was checked by ANOVA and its F-value depicted the significance of the model and determined the noise factor. The value of R 2 determined the fitness of the model and it ranged from 0 to 1. Scatter diagram was used to compare the observed and predicted values and for checking the empirical model. Dhancholia et al. [26] studied the FSW of AA 6061 and 7039 Aluminum alloys considering the process parameters of rotational speed, welding speed and their effects on tensile, yield, impact strength and hardness. A CCD for RSM using two factors and two levels was applied to optimize the parameters. Optimized rotational and welding speeds were perceived as significant factors. By applying the optimization technique, the mechanical tests of buttwelded, dis-similar joints achieved 95 % confidence level. It was observed that the rotational and welding speeds had a significant impact on the over all results. In the FSW, process parameters influence the weld properties. This behavior of process parameters effecting the mechanical properties, material flow and microstructure formation of Friction Stir Welded joints of metals and composites have been illustrated in numerous research studies [27]. In addition, Payganeh et al. [28] welded the Polypropylene with 30% Glass Fiber (GF) in a square butt joint configuration. It was observed that the tool pin geometry had an outstanding effect on the mechanical strength of joints. Panneerselvam et al. [29] found that right and left-handed threaded pin profile tool and the direction of tool rotation also influence the weld strength.
It is apparent from the literature review, presented here, that FSW has been rarely used for developing butt welded joints in Polycarbonates. Also, the application of FSW to Butt welded configurations of Polycarbonate is even more rare when it comes to using an optimized combination of the parameters, which significantly affect the strength of welded joints, is concerned. Being motivated by this gap in research an approach, based on RSM in combination with CCD, is developed during this research which maximizes the weld strength by optimizing the combination of some significant process parameters like traversing speed, rotational speed, and tool geometry. 3D plot effects and ANOVA are used to verify and present the optimal level for each parameter.

METHODOLOGY
The methodology adopted during this research is presented in the form of a flow diagram as shown in Fig. 1. The flow diagram elaborates the complete procedure followed during the research from identifying and examining the experimental needs till conducting the confirmation experiments of optimum parameters.

PROCEDURE
To implement the overall methodology presented in Fig. 1, stepwise procedure adopted is outlined as follows:  11. Confirmation for the adequacy of model

Manufacturing of Mold
The first step in the methodology was to design and fabricate a mold to get desired dimensions of PC plates. The size of plates was required to be kept at 100mm x 50mm x 4mm keeping in view the application of producing butt welded joint through FSW. First a solid model was designed, created, assembled and analyzed using CAD Modeling software as shown in Fig. 2. Fig. 3 shows the mold, manufactured on a 3-axis CNC milling machine and finally assembled. Further, this research used pure natural polycarbonate material, for molding plates of the required dimensions. An injection molding machine was used to cast and extract the plates from the cavity of mold to avoid sink marks, flow lines and vacuum voids etc.

Manufacturing of FSW tools
In order to investigate the effect of tool geometry on the weld strength, four tools with different pin geometries of 1mm pitch, 1mm depth of thread and a taper of 15  used here, are shown in Fig. 5 and Table 1. The manufacturing of these tools was carried out on a CNC turning machine using high-speed steel.

Selection of Process parameters
The process parameters of FSW, having major influence on the quality and strength of joints, are rotational speed, traverse speed and tool geometry [30]. Therefore, during this research FSW was carried out at a traversing speed from 14 to 35 mm/min and rotational speed of 950 to 1700 RPM while using different tool geometries, as shown in Table 2 and Table 3.

Development of Design Matrix
RSM was used to develop the regression models whereas CCD was used for the analysis of current surface response based on three factors: two levels for two continuous factors and four levels for one categorical factor to allow the estimation of all first order and two-factor interaction terms. The rotation of design remained dependent upon the axial points and distance from the center. The design contained 56 sets of runs and regression models were developed by using these experimental runs. The total number of experiments were calculated using equation (1) No. of experiments = (2 n + 2n + nc) (1) where n is the number of continuous factors. In this research, two continuous factors (Rotational speed and Traversing speed) were used. Also, nc is the number of center points and a total of six center points were used during these experiments. Also, a categorical factor (Tool geometry) was also used in this research work while multiplying each experiment by a number of levels of that factor.

Friction Stir Welding Procedure
To execute the process of FSW, a CNC milling machine was used during this research, to achieve different traversing and rotational speeds. The molded PC plates were clamped in the designed fixture in a butt joint configuration. The tool was allowed to plunge slowly into the plates and weld was produced using the designed parameters. Later on, once the FSW process was completed, a sufficient amount of time was given to the weld plates, while being fastened in the fixture, to cool down in order to avoid any distortion due to the heat generated during the process.

Specimen Preparation
The tensile samples were prepared according to ASTM D638 standard, as shown in Fig. 6. Fig. 7 shows all of the 56 experiments designed and to be tested on the tensile testing machine to determine the strength of joints. Fig. 8 shows the apparatus setup of the tensile testing specimen.

Mechanical Testing
All the experiments were conducted as per CCD. Table 4 presents the response (UTS) obtained from the PC butt joint specimens along with the values of the influencing parameters. Threaded cylindrical, threaded cylindrical conical and simple cylindrical conical tools recorded abrupt changes in tensile strength unlike threaded conical tool.

Development of Empirical Model
Using of the coded factor equations, response for the given level of each factor was predicted. High and low, levels of the coded factor were +1 and -1. Comparison of factor coefficients on the relative impact of factors can be identified by the coded equation. Analyses of the predicted response from regression equations, as calculated from the design of experiments, identified the optimal conditions.

Analysis of Variance (ANOVA)
By performing ANOVA, the significance of process parameters was identified. The model terms are found to be significant if the Value of "Prob > F" is less than 0.0500. The significant model terms are A, B, C, AB, and BC in this case (

Adequacy of Model
The accuracy of the model can be confirmed by the value of R 2 . Model is termed as accurate when the R 2 value is closest to unity. In this case, the value of R 2 = 0.916, which is closest to unity as shown in Table 6, suggests that the model is accurate enough. Fig. 9 shows that all points lie on the trend line, indicating the normal distribution of errors. The predicted response values of the model are well in agreement with the actual values which is shown in Fig. 10, which determines a good fit of model.

RESULTS & DISCUSSIONS
Finally, now it's time to substantiate the effect of different process parameters (traverse speed, rotational speed and tool geometry) on the overall strength of the joint. For this purpose, the discussion is sequentially arranged by considering different tool geometries (one by one) in combination with varying traverse and rotational speeds and discussing their effect on tensile strength of the friction stir welded joints. Fig. 11 shows a 3D interacted graph for traverse and rotational speed against tensile strength when a threaded cylindrical tool was used. It can be observed from the graph (Fig. 11) that the tensile strength initially increases up to a certain limit, with the increase in the values of both corresponding parameters, and then starts decreasing with any further increase in the values parameters (traverse and rotational speed). From the graph it is also evident that the higher value of tensile strength has been achieved when the rotational speed is low, and the traversing speed is high. The reason for this behaviour is that at low rotational and traverse speeds, lesser amount of heat is generated which consequently does not facilitate the material of the two adjoining surfaces to get mixed up adequately and thus results in poor quality of welds. Whereas, at higher rotational and traverse speed, although the material melts quickly due to higher heat generation, but at the same time the higher speeds does not allow the material to settle down properly thus causing porosity in the weld joint.

Threaded Cylindrical Tool
Zafar et al. [31] reported that welding parameters vary from polymer to polymer. Those having higher melting point and viscosity, require higher rotational and low traversing speeds to achieve sufficient heat and good joint strength. Here also, the tensile strength initially increases and then starts decreasing by further increasing the values of both speeds. At lower rotational speed and higher traversing speed and vice versa, the value of tensile strength remains the same. The reason for this behavior is because in both of these situations (lower rotational speed and higher traversing speed or otherwise) the amount of heat generation would be more or less the same thus generating similar results in terms of tensile strength. different aspect of this graph is that maximum tensile strength has been achieved at the highest rotational speed and lowest traversing speed. The reason for this behavior was investigated by Bilici et al. [32]. They concluded that the straight and tapered profiles have different stir zone thicknesses. The straight profile generates a lowest fracture load while tapered profile generates the highest fracture load. Further, excessive taper angle generates additional heat, which lowers the weld strength. Prashant et al. [33] reported that stepped conical tool produces efficient joint as compared to other profiles. It, also, improves material mixing and better material movement in the weld zone.

Threaded Conical Tool
The results of FSW while using threaded conical tool are presented in Fig. 14. From the figure (Fig. 14) it is evident that tensile strength of the welded joint remains maximum when either the traversing speed remains maximum against a corresponding lowest value of rotational speed or the other way around. In case the values of both the speeds i.e. rotational and traversing, are altered simultaneously form minimum to maximum, the variation in tensile strength is approximately the same as described in the previous cases. It (tensile strength) initially increases up to certain limit and then starts decreasing with further increase in the values of rotational and traversing speeds. Zafar et al. [31] and Eslami et al. [9] reported that the quality of welds generated by FSW with threaded conical tool have been very high having no porosity or voids. This is because threads around the probe facilitate the softened material, at the faying surfaces, to get mixed up properly thus generating a uniform weld seam free of voids. Bilici et al. [32] also determined the effects of threaded profile pin on the quality of welds. They observed that with an increase in the pitch length there occurs a corresponding decrease in the fracture load and consequently weakens the weld joints.

OPTIMIZATION OF PROCESS PARAMETERS
The use of optimized combination of process parameters is very essential for the FSW process as it improves the overall quality of welds and largely increases the tensile strength of the welded joints. Therefore, the objective of this research has also been to maximize the tensile strength of a friction stir welded joint by optimizing the combination of some critical process parameters. The results of different combinations are shown in Table 7 and Table 8.
Optimum values of the parameters resulting in maximum tensile strength are mentioned in Table 08 and presented in ramps order in Fig. 15. Plot based on the desirability approach is also shown in Fig. 16.

Confirmation test
In order to confirm the theoretical results, obtained during this research, four different confirmation tests were carried as listed in Table 9. To calculate the percentage of error in each test and confirm the accuracy of the model the following equation (Equation 08) was used.
Percentage error = (8) The samples prepared for confirmation test are shown in Fig. 17 and 18. All confirmation tests were performed on the tensile testing machine. The results obtained are listed in Table10. . It was observed in all the 3D graphs that in case of all four tool geometries the simultaneous increase in both traversing and rotational speeds depicted that the tensile strength initially showed an increasing trend but after certain limit a decreasing trend was witnessed with further simultaneous increase in the values of both the parameters. The final results showed that the ultimate tensile strength of 51.299 MPa was achieved at the traversing speed of 14mm/min, rotational speed of 1700 RPM and with simple cylindrical conical tool geometry.