Study on the effects of tool tile angle,offset and plunge depth on friction stir welding of poly(methyl methacrylate) T-joint

Abstract

The effects of tilt angle (TTA), plunge depth (TPD) and offset (TO) of tool in friction stir welding of poly(methyl methacrylate) T-joint were investigated. To understand better the effects of process parameter, thermomechanical simulation of joint was assessed. The results seem to show that at higher TPD and TTA, frictional heat increases. Woven tissue structure joint line forms after friction stir welding of poly(methyl methacrylate) sheets. The distance of woven layers was affected by TPD and TTA, while TO do not significantly affect heat generation of joint. The best material flow and adequate heat are generated at 0 mm TA, 2° TTA and 0.2 mm TPD, respectively. The highest flexural and tensile strength of friction stir welded joint were approximately 93% and 90% of as-received poly(methyl methacrylate), respectively. Crack forking was detected on the fractured surface of flexural samples and crack path was detected in the vicinity of shrinkage holes at fracture surface of tensile samples. These holes and degradation of poly(methyl methacrylate) during friction stir welding process decrease strength and hardness of the joint.

Friction stir welding, tool plunge depth, tool tilt angle, material flow, mechanical propertiesIntroduction

Using modern joining technology for production of advanced low-weight structures requires comprehensive experimentation. After the emergence of friction stir welding (FSW) technique, many researchers have been working on this process to weld polymeric material. It seems that low heat input in FSW joint area and absence of filling material help properties of joint area near base material.^1–3 For these reasons, industrial and academic groups have produced modern polymeric structures with FSW process.^4–6 In this process, frictional heat is generated by tool shoulder and pin causing material change into plastic phase. Rotational and traverse direction of the tool causes the plasticized material extrude from the leading edge (LE) into stir zone (SZ) and form joint area. Previous research demonstrates that the main heat in this process is created by tool shoulder and internal flow stir action produced by tool pin.⁷ The main mechanical parameters in FSW technique are tool rotational velocity, tool traveling velocity (welding speed), TPD, TTA and tool offset (TO).⁸ To increase the mechanical efficacy of FSW joints, researchers investigated FSW tool rotational and traveling velocity on high-density polyethylene (HDPE), polycarbonate (PC), poly(methyl methacrylate) (PMMA) and acrylonitrile butadiene styrene (ABS) to find the defects of free joint with highest tensile strength.^9–13 In order to achieve better material flow of polymeric FSW joints, some researches were done by preheating the joint line, using stationary aluminum shoulder which is sometimes called “shoe” and stationary heated shoe for polytetrafluoroethylene (PTFE), polyethylene (PE) and ABS.^14–16 The effects of tool pin profile and threaded pin were also investigated on ABS, polyamide 6, polypropylene (PP) and PMMA.^17–19

Among the mechanical aspect of FSW parameters, TPD is a key factor for material flow and heat generation, which determines the amount of axial force into the material in SZ.⁷ TPD defines the amount of tool shoulder distance with the top face of workpiece. According to the Cartesian coordinate system right-hand rule, the FSW tool upward and downward direction can be defined by “Z” axis movement and forward and backward direction by “X” axis movement. In real situation, the plunge direction is “-Z” but to simplify explanations, the TPD is represented by positive numbers which means the shoulder distance falls in the workpieces. Some researchers have reported that appropriate TPD on metallic material causes an improvement in the internal flow.²⁰ However, low TPD causes lack of SZ filling and formation of root tunnel defects.²⁰ Excessive TPD results in material explosion from joint line and decrease in mechanical properties of FSW joint. TTA is also defined as backward axis from base material normal axis, measured by degree.²⁰ However, TTA is a parameter which straightly affects material flow by forging force and material extrusion from LE into trailing edge (TE). By increasing TTA, the root defect in metallic material can be controlled. Research indicates that by tilting the FSW tool, the shear strength of base material decreases and better material flow forms.⁷ Excessive TTA causes the material to explode and formation of flash around joint line.⁷ TO defines the distance of tool axis with normal weld line axis. TO is a significant factor during welding of dissimilar materials, however, in similar joint can affect surface material flow and filling of joint line.²⁰ The schematic view of TTA, TPD and TO is depicted in Figure 1. Despite the existence of some researches on effects of TTA, TPD and TO on the welding of metallic materials, comprehensive documentation of the effects of these parameters on polymers is unavailable.

Figure 1.

Schematic view of TTA, TPD and TO.

Arici and Selale²¹ studied on double pass FSW of PE and showed increasing TTA decreases the joint tensile strength. Saeedy and Besharati Givi²² used 1° and 2° tool tilt angles to find an appropriate surface material flow of PE FSW lap joint. Aghajani Derazkola et al.¹⁰ showed that controlling surface flow and filling of SZ strongly depend on TTA and TPD. The importance of the above factors varies in the geometry of the various joints. In some cases such as T-joint, the main role of mixing material is played by FSW tool pin. During FSW of polymeric material, due to the low heat transfer coefficient of polymer materials, transference of the heat produced by the tool shoulder to the lower regions of the SZ is very low. For this reason, the probability of defect formation or inappropriate flow in the lower area of the SZ increases. This phenomenon is of particular importance in polymeric T-joint since it determines the quality of mixing between the upper sheets and the T-appendages.

In this situation, the selection of proper TTA, TPD and TO is of utmost importance as it determines the heat production and the quality of the material stirring in the lower area of T-joint. Various TTA, TPD and TO are selected to find the highest strength and flow of PMMA T-joint. To understand better the stirring action and internal material flow, thermomechanical modeling of FSW parameters is used.

Process modeling

The steady-state single-phase flow with temperature-depended properties defines for welding process. The continuity equation with index notation for 1, 2 and 3 (representing x, y and z directions) is given by^23–25

V_{i, i} = 0

(1)

where V indicates the plastic flow velocity. The three-dimensional (3D) plastic flow is represented by the momentum conservation equation as^23–25

ρ {(V_{i} V_{j})}_{, i} = - P_{, i} + λ {(V_{i, j} + V_{j, i})}_{, i} - ρ U_{1} V_{j, 1}

(2)

λ = \frac{σ_{e}}{3 \overset{\cdot}{ε}}

(3)

Physical properties of PMMA in different temperatures are shown in Figure 2,^26–28 and the thermal properties of steel FSW tool are defined as²⁹

C_{p} = 468.3 - 8.5 T + 3.0 \times 10^{- 4} T^{2} + 1.8 \times 10^{- 7} T^{3}

(4)

K = 3.8 + 0.092 T - 1.8 \times 10^{- 4} T^{2} + 7.8 \times 10^{- 8} T^{3}

(5)

Figure 2.

(a) Thermal conductivity, (b) specific heat, (c) viscosity of PMMA and (d) FSW tool model.

The steady single-phase momentum conservation equations with reference to a coordinate system attached to the heat source may be represented as²⁹

ρ C_{p} {(V_{i} T)}_{, 1} = - ρ C_{p} U_{1} T_{, 1} + {(k T_{, i})}_{, i} + T_{Total}

(6)

The total heat source produced by tool shoulder, pin body and pin beneath are presented. The heat generation at interface of tool shoulder and PMMA is defined as follows²⁹

T_{ss} = [δ τ + (1 - δ) μ P] \cdot \frac{2}{3} π ω ({R_{1}}^{3} - {R_{1}}^{2})

(7)

The µ and δ are considered 0.1 and 0.5, respectively.²⁸δ ranges between 0 ≤ δ ≤ 1. δ = 0 represents the pure sliding condition, while δ = 1 involves pure sticking.

The generation of heat between bodies of pin-PMMA is defined as²⁹

T_{pss} = \frac{2 δ π ω τ}{3 \tan α} (R_{2}^{3} - R_{3}^{3}) + \frac{2}{3} (1 - δ) \frac{π μ P ω}{\sin α} (R_{2}^{3} - R_{3}^{3})

(8)

The generated frictional heat at beneath of pin-PMMA interface is defined as³⁰

T_{pbs} = \frac{(2 δ π τ ω + 2 (1 - δ) π μ P ω) R_{3}^{3}}{3}

(9)

The total heat would be defined as³⁰

T_{Total} = T_{ss} + T_{pss} + T_{pbs}

(10)

The heat transfer at the bottom of PMMA is determined by³⁰

k {\frac{\partial T}{\partial Z} |}_{Bottom} = h_{b} (T - T_{a})

(11)

The heat transfer coefficient at the bottom face depends on the local temperature and is given by the following relation³⁰

h_{b} = h_{b 0} {(T - T_{a})}^{0.25}

(12)

At the top surface, heat transfer is due to both convection and radiation and is given by³⁰

- k {\frac{\partial T}{\partial Z} |}_{Top} = B ϵ (T^{4} - T_{a}^{4}) + h_{t} (T - T_{a})

(13)

In this model, an FSW tool and a frustum pin were designed (Figure 2(d)). The PMMA is assumed as non-Newtonian fluid with viscoplastic behavior and density based on PMMA. For mesh generation of tool and workpiece, tetrahedral/hybrid elements with T-grid combination shape were used. The region close to the pin tool and the tool itself required a much finer mesh to evaluate the heat transfer model and viscous flow. A sizing function of the tool and workpiece was used to generate the different volume sizes by means of the region close to the pin tool, and the tool itself meshes were finer compared to other areas. The sizing function uses a start size, growth rate and maximum size. For the fine mesh pictured in Figure 3, the start size was 0.1 mm, growth rate was 1.4 mm and a maximum size was 1.8 mm. Finally, the total number of volumes for the lateral case was 9,102,547 volumes.

Figure 3.

Meshed model.

Experimental procedure

PMMA sheets of 4 mm thickness were cut into required sizes to use as raw material. The PMMA properties are presented in Table 1. A steel made clamping system was considered the positioning of PMMA for T-joint. To record thermal changes during the FSW process, J-type thermocouples were placed on top sheets and T-flange. The thermocouples were placed into predetermined holes by Testor’s cement and held for half day before to set. The schematic view of thermocouples places is depicted in Figure 4(a). A high-speed steel (HSS) steel frustum pin tool was applied for joining, which is shown in Figure 4(b). The experimental plan that implemented in this study as process parameters is presented in Table 2. Tensile and flexural strength of joints were assessed according to ASTM EM08 and AWS B4.0:2007 standard, respectively (Figure 4(c) and (d)) to find relation with process parameters and mechanical properties. The hardness test according to the polymeric materials (shore D) was used to record change of hardness along joint area. Internal material flow analyses have been carried out using a video visual measurement machine.

Table 1.

Properties of PMMA.

Parameters	Value
Density, g/cm³	2.58
Tensile strength, MPa	70
Young modulus, GPa	72.3
Elongation, %	4.8
Poison’s ratio	0.2
Melting pint, °C	160
Glass transition, °C	92

PMMA: poly-methyl methacrylate.

Figure 4.

Schematic view of (a) thermocouple places, (b) FSW tool, (c) tensile sample and (d) flexural sample.

Table 2.

FSW process parameters.

Parameters	Value
Rotational speed, r/min	1600
Traveling speed, mm/min	25
TPD, mm	0, 0.1, 0.2, 0.3 and 0.4
TTA, °	0.5, 1, 1.5, 2, 2.5, 3 and 3.5
TO, mm	−0.4, 0 and +0.4

TPD: FSW tool plunge depth; TTA: FSW tool tilt angle; TO: FSW tool offset.

Results and discussion

Material flow

Effects of TPD

TPD has a direct impact on amount of frictional heat and state of friction between the tool and sheets.³¹ It has been approved that friction between tool and workpiece can change from sliding to sticking. With an increase in TPD, the friction between the tool sheets and axial force increases leading to higher amount of heat in tool–sheets interface.³¹ Figure 5(a), (c) and (d), respectively, shows simulation results of internal heat distribution of joints that FSWed with 0, 0.2 and 0.4 mm TPD. According to the simulation results, the frictional heat generation was more in the advancing side (AS) compared to the retreating side (RS) at all TPD. This type of heat generation and flow are results of tool rotation direction. The results show that the highest proportion of frictional heat was produced by FSW tool shoulder.

Figure 5.

Temperature distribution (degree in Kelvin) in joint that FSWed with (a) 0 mm, (b) 0.2 mm and (c) 0.4 mm TPD. Internal material flow of joint that FSWed with (d) 0 mm and (e) 0.2 mm TPD and (f) comparison between TPD and dimension of SZ.

Due to the low contact surface of pin with surrounded material and low heat transfer confident of PMMA, diffusion of heat into lower area of SZ decreased. This phenomenon is perceptible on internal surface flow. Figure 5(d) and (e) shows internal flow of joints that FSWed with 0 and 0.2 mm TPD, respectively. Narrow heat-affected zone (HAZ) and incomplete mixing between T-flange and top surface forms at 0 mm TPD joint and bigger SZ and HAZ were forms in 0.2 mm TPD joint. Geometric dimension of SZ (Figure 5(f)) showed that with increasing TPD, the size of SZ increased, which is related straightly with increase in heat generation in SZ.

Comparison of recorded temperature and simulation result is depicted in Figure 6(a). The rising trend in maximum heat with increasing TPD is the result of more axial force at higher TPD. The results of two-dimensional (2D) geometric dimension of HAZ area around joint line at AS and RS are presented in Figure 6(b). With rising frictional heat, the size of HAZ area increases in both sides. Figure 6(c)–(e) shows pictures of surface flow and HAZ dimension in top view of joints that FSWed with 0, 0.2 and 0.4 mm TPD. The surface flow of joints is woven with tissue structure shape. With TPD, distance between tissue structure layers decreases. As seen, the surface layer distance formed in 0 mm TPD (0.14 mm) decreased at 0.4 mm TPD (0.08 mm). Low heat generation at 0 mm TPD causes incomplete mixing of PMMA during joining process (Figure 6(c)). Increasing TPD till 0.2 mm, uniform composition of material was created (Figure 6(d)), which indicates that TPD helps more mixing of T-flange and top sheets. At higher TPD (0.4 mm), tool body prevents stirring action during welding process and causes plasticized PMMA explodes from SZ and sticks on AS (Figure 6(e)). These cycles repeat during tool traveling and form flash on AS. Figure 6(f)–(h) shows simulation results of heat distribution on the top surface of joints, which were welded with 0, 0.2 and 0.4 mm TPD. Due to the low heat transfer coefficient of PMMA, the distribution of temperature was low and the frictional heat was concentrated on joint line.

Figure 6.

Comparison between TPD and (a) temperature and (b) width of HAZ. Surface material flow of joint that FSWed with (c) 0, (d) 0.2 and (e) 0.4 mm TPD. Surface heat distribution of joint that FSWed with (f) 0, (g) 0.2 and (h) 0.4 mm TPD.

Effects of TTA

TTA helps the forging force and causes material extrusion from LE of FSW tool into TE, which leads filling of the SZ.³² This parameter can change the heat production and flow of material. The results of heat generation in SZ at 0.5°, 2° and 3.5° TTA are shown in Figure 7(a)–(c), respectively. As seen, the maximum heat was generated in the back side of tool axis at all TTA. The frictional heat generation heightens with increasing the TTA. The cross-sectional macrostructure of joints that FSWed with 0.5° and 2° are depicted in Figure 7(d) and (e). At low TTA, the size of SZ is low and mixture of material flow is not appropriate. During the FSW process, the material from front of FSW tool stirred T-flange (with tool pin) and upper sheet (with tool shoulder) mixed and extruded from LE into TE and filled the SZ. At low TTA, the extrusion force is low, which cannot extrude the mixed material from T-flange from LE into TE. For this, contribution of T-flange on creation of SZ is low, which causes the formation of weak joint. However, at high TTA, the contact area between T-flange and pin tool decreases and consequently the joint may not form. The comparison between TTA and dimension of SZ in two dimensions is shown in Figure 7(f). The size of SZ increased slightly from 0° till 2° and after that decreased sharply. At 2.5° TTA, the material explosion and weak mixing of T-flange and upper sheets caused smaller size SZ, but at more than 2.5° the joint did not form completely.

Figure 7.

Heat generation in SZ of joint that FSWed with (a) 0.5°, (b) 2.5° and (c) 3.5° TTA. Internal material flow of joint that FSWed with (d) 0.5° and (e) 2.5° TTA and (f) comparison between TTA and dimension of SZ.

The longitudinal view of heat distribution from simulation result is depicted in Figure 8(a). As shown, the contact area of pin with T-flange decreased with increasing TTA. The effects of TTA on surface material flow of joints which were FSWed at 0.5°, 2° and 3.5° tilt is pictured in Figure 8(b)–(d), respectively. An inappropriate flow pattern in 0.5° and 3.5° joint is visible. The distance between flow rings of sound joints decreased due to higher heat generation at higher TTA. For example, the distance of flow rings from 0.2 mm in 0° decreases till 0.1 mm at joint FSWed at 3.5° TTA. The comparison between maximum heat generation and TTA is depicted in Figure 8(e). Despite the formation of incomplete joint at TTA higher than 2.5°, the heat generation has an increasing trend from 0.5° till 3.5°. The maximum and minimum temperatures in these joints were produced at 0.5° tilt (∼127 °C) and 3.5° tilt (∼135 °C), respectively. This trend repeated for the width of HAZ area around joint line in both AS and RS.

Figure 8.

(a) Longitudinal view of simulation result of heat distribution. Surface material flow of joint that FSWed with(b) 0.5°, (c) 2° and (d) 3.5° TTA. Comparison between TTA and (e) temperature, (f) width of HAZ.

Effects of TO

TO is a key factor in material mixing during FSW of dissimilar materials. In T-weld joints, the TO can directly affect the surface material flow. Due to the similarity of joints between PMMA sheets, the TO on AS or RS does not have significant effects on maximum heat generation, but the TO can help the formation of a more uniform surface material flow. It seems that this parameter has more effects on metallic materials, especially in dissimilar joints compared to polymeric material.³ The internal and surface heat distribution and surface flow of joint with 0.4 mm offset on AS are shown in Figure 9(a)–(c), respectively. The internal and surface heat distribution and surface flow of joint with 0 mm offset on AS are shown in Figure 9(d)–(f), respectively. The relation between TO, maximum temperature (Figure 9(g)), SZ dimension (Figure 9(h)), and surface HAZ (Figure 9(i)) showed very slight changes.

Figure 9.

(a) Internal heat distribution, (b) surface heat distribution and (c) surface material flow of joint that FSWed with 0.4 mm offset in AS. (d) Internal heat distribution, (e) surface heat distribution and (f) surface material flow of joint that FSWed with 0 mm offset. Comparison between TO with (g) recorded and simulated temperature, (h) size of SZ and (i) width of HAZ.

Mechanical properties

Flexural strength

During the flexural test, the samples fractured from two regions, middle of SZ (type 1) and RS of SZ (type 2). Highest strength of joint fractured from type 2 (Figure 10(a)) and weak joint fractured from type 1. The results of the flexural strength of joints are shown in Figure 10(b)–(d), respectively. Appropriate material flow, dimension size of SZ, and defect formation are the main reasons which affect strength of joints. The highest flexural strength produced at 2° TTA. The formed joints at TTA lower than 2° had smaller SZ size, and material explosion from the joint line and incomplete joint area at higher TTA causes low flexural strength. The flexural strength of samples that FSWed with 0.5°, 1°, 1.5°, 2°, 2.5°, 3° and 3.5° TTA, respectively, were 72, 75, 79, 84, 76, 0 and 0 MPa. Appropriate material mixing between T-flange and upper sheets caused that the best TPD was 0.2 mm. The flexural strength of joints that FSWed with 0, 0.1, 0.2, 0.3 and 0.4 mm TPD was 79, 81, 84, 70 and 42 MPa, respectively.

Figure 10.

(a) Flexural sample after test and relation between flexural strength and (b) TTA, (c) TPD and (d) TO. Fracture surface of flexural sample that indicates (e) crack branching, (f) cracks origins, (g) crack path and (h) rapture area.

The flexural strength of joints which were welded with −0.4, 0 and +0.4 mm TO were 81, 84 and 80 MPa, respectively. It seems that the TO does not have significant effects on mechanical properties of joint. The strongest flexural strength was achieved at 2° TTA, 0.2 mm TPD and 0 mm TO that had 93% (84 MPa) PMMA base material strength. Fractographic of flexural samples is shown in Figure 9(e)–(h). Forking (crack branching) was seen on the fractured samples (Figure 10(e) and (f)). The crack branches regularly occur when the release rate of stored energy in the PMMA exceeds the amount of energy released due to the increase in surface area.³³ During the flexural test, the bending stress mode makes the bifurcations to crack path that moves away from origins of crack (Figure 10(g)) and a final stage where the rapture occurs (Figure 10(h)).

Tensile strength

The results of tensile test reveal that all samples fracture path was SZ lower area (Figure 11(a)). The results of tensile strength are presented in Figure 11(b)–(d). With increasing TTA from 0.5° till 2°, the strength of joints increases and after that decreases sharply due to incomplete formation of SZ. The tensile strength of joints that FSWed with 0.5°, 1°, 1.5°, 2°, 2.5°, 3° and 3.5° TTA, respectively, were 50, 54, 60, 63, 47, 0 and 0 MPa. The tensile strength of joints which were FSWed with 0–0.2 mm had slight increase in trend and after that decreased till 0.4 mm TPD. The improved mixing of materials till 0.2 mm TPD leads to increasing trend in tensile strength and material explosion and excess stirring action of tool due to the high axial force and decrease in tensile strength of joints more than 0.2 mm TPD. The tensile strength of FSWed joints that welded with 0, 0.1, 0.2, 0.3 and 0.4 mm TPD was 57, 59, 63, 56 and 38 MPa, respectively. The tensile strength of joints which were welded with −0.4, 0 and +0.4 mm TO was 62, 63 and 62 MPa, respectively. The fracture surfaces of highest tensile strength sample are depicted in Figure 11(e)–(g). The high-magnification scanning electron microscope (SEM) image from fracture surface of tensile sample shows some holes that seem as the results of shrinkage properties of PMMA during cooling stage.³⁴ The results reveal that brittle fracture mode was accrued in all tensile samples. Some crack paths were formed in the vicinity of shrinkage holes (“I” points), which indicates that these holes play crack initiation role during tensile test. These holes are one of the effective parameters on fall down of mechanical properties of joints.

Figure 11.

(a) Tensile sample after test. Relation between tensile strength and (b) TTA, (c) TPD and (d) TO. (e)–(g) Fracture surface of tensile sample that FSWed with 2.5° TTA and 0.4 mm TPD.

Hardness

During FSW, frictional heat changes the hardness of base materials.^35–37 The hardness changes can be completely different between metallic materials and polymeric materials. For this reason, differential scanning calorimetry (DSC) analysis was carried out on PMMA sample to find root of hardness changes in SZ. The result shows that after FSW process, the molecular weight and crystallinity of PMMA decrease (Figure 12(a)), which cause decline in the hardness of SZ. The change in molecular weight and hardness of SZ has straight relation with heat input of joint line. According to the results, all FSWed samples lose their hardness comparing to as-received (90 ShoreD) with increasing heat input (TTA and TPD), even samples with incomplete formation. According to the results, the harness of joints that FSWed with 0, 0.1, 0.2, 0.3 and 0.4 mm TPD were 79, 77.2, 75.5, 71 and 69 ShoreD, respectively (Figure 12(b)). The hardness of joints that FSWed with 0.5, 1, 1.5, 2, 2.5, 3 and 3.5 TTA were 81.5, 78, 76, 75.5, 73, 70.4 and 68.9 ShoreD, respectively (Figure 12(c)). Due to the low heat generation changes in FSW TO, the hardness of joints welded with −0.4, 0 and +0.4 mm TO was approximately same (Figure 12(d)).

Figure 12.

(a) DSC analysis results of pure PMMA and FSWed sample (2.5° tilt angle and 0.3 and 0.4 mm tool plunge depth). Relation between harness changes (Shore D) in SZ with (b) tool tilt angle, (c) tool plunge depth and (d) tool offset.

Conclusion

The effects of TTA, TPD and TO on the FSW of PMMA T-joint were investigated. The results of the investigation on properties of this joint are presented as follows:

Woven tissue structure joint line forms after FSW of PMMA sheets. Due to the low heat distribution of generation, the narrow HAZ was formed around joint line. Rising trend in maximum heat with increasing TPD is a result of more axial force at higher TPD. The surface of woven layer distance which was formed at 0 mm TPD (0.14 mm) decreased at 0.4 mm TPD (0.08 mm). At high TPD, the plasticized PMMA was exposed from SZ and sticks on AS that decreases dimension of SZ.

TTA can change the production of heat and flow of PMMA. The simulation results reveal that the hottest area forms in backing side of tool axis at all TA. At low TTA, the size of SZ is low and mixture of material between upper sheets and T-flange is not appropriate, and at high TA, the contact area between T-flange and pin tool decreases and consequently the joint may not form. The distance between flow rings of sound joints decreases due to the more heat generation and extrusion force at higher TTA. The TO on AS or RS has no significant effect on maximum heat generation, but the offset on tool axis can help the formation of more uniform surface material flow.

During flexural test, the FSWed PMMA T-joint fractured from middle of SZ and RS of SZ. The highest joints fractured from RS of SZ and weak joint fractured from middle of SZ. Appropriate material flow and defect-free SZ were formed at 2° TTA, 0.2 mm TPD and 0 mm TO that had 93% (84 MPa) PMMA base material strength. Inappropriate material blending between T-flange and upper sheets and material explosion from SZ decrease flexural strength of joint. Fracturographic study of flexural sample reveals crack branching (forking) on the fracture surface of flexural samples.

The tensile strength of PMMA joint was affected by TTA and TPD, but not by TO. The tensile strength of joints which were FSWed with 2° TTA, 0.2 mm TPD and 0 mm TO was stronger than other joints. The improved mixing of materials and stirring action of tool due to the appropriate axial force increased tensile strength of PMMA T-joint near 63 MPa (90% of as-received). During tensile test, some crack paths were formed in the vicinity of shrinkage holes, which indicate that these holes play crack initiation role during tensile test. These holes and degradation of PMMA during FSW process decrease hardness and strength of joint.

Footnotes

Appendix 1 Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Hamed Aghajani Derazkola

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