Friction Stir Welding Tool Design Pdf

Abstract

Since its inception no one has done analysis and design of FSW tool. Initial dimensions of FSW tool are decided by educated guess. Optimum stresses on tool pin have been determined at optimized parameters for bead on plate welding on AZ31B–O Mg alloy plate. Fatigue analysis showed that the chosen FSW tool for the welding experiment has not ∞ life and it has determined that the life of FSW tool is 2.66×10⁵ cycles or revolutions. So one can conclude that any arbitrarily decided FSW tool generally has finite life and cannot be used for ∞ life. In general, one can determine the suitability of tool and its material to be used in FSW of the given workpiece materials in advance by this analysis in terms of fatigue life of the tool.

1 Introduction

FSW was invented by TWI England and has become widespread. By FSW one can weld similar materials (Al, Mg, Steel, etc.) as well as, dissimilar materials (Al to Mg, steel to Al, etc.) [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. Advancing side (AS) of the tool is the side where linear velocity vector of tool and welding direction are one and same, retreating side (RS) is the side where these directions are opposite to each other. Weld nugget (WN) is the central core of the FSW volume, in which material has dynamically recrystallized. Thermomechanically affected zone (TMAZ) is beside WN and has experienced plastic deformation but not recrystallized, and beside TMAZ is the heat affected zone (HAZ) which has experienced only thermal cycle; beside HAZ is the base metal on either side of weld volume.

No study has been done to determine the strength of FSW tool during (after fixing the dimensions of FSW tool, by educated guess) FSW process. If the tool material is costly then one has to ensure the intact of the tool. Traverse force opposite to welding direction is the main force which causes the breakage of pin from the shoulder of the tool. So if one fixes the optimum traverse force, then that itself ensures the rigidity of the tool. It has observed that traverse force reaches highest at low rpm compared to that of at high rpm. If the tool is safe for low rpm then in turn it is safe for higher rpm, since workpiece material flow stress will be lower for higher rpm.

Prado et al. [15] has set the optimum shape of the tool experimentally; this type of tool has preferred for all welds. Here the author has made a maiden attempt to analyse and design of FSW tool.

2 Experimental

Indigenously developed computer controlled FSW machine (BiSS Bangalore) was used for FSW experimentation. The base metal used is AZ31B–O magnesium alloy; composition of AZ31B–O Mg alloy: 2.5%–3.5%, aluminum; 0.7%–1.3%, zinc and 0.20%–1.0%, manganese. This has tensile strength=240 MPa, yield tensile strength=140 MPa, elongation=10%, Young's modulus=45 GPa. HDS material was used for FSW tool; HDS tool material has ultimate tensile strength=1200–1590 MPa, yield strength=1000–1380 MPa, melting point=1427°C, Young's modulus=215 GPa.

Size of Mg alloy plate used was 250 mm×80 mm×5 mm where 5 mm was the thickness of Mg alloy plate. A few 200 mm length plates were sufficient to determine optimum parameters. Later FSW was carried out by using these optimum parameters by noting various stresses or forces and torques on the tool at these optimum parameters.

FSW tool has pin with top diameter 6 mm, bottom diameter 4 mm, average diameter 5 mm, pin length 4.7 mm, tool shoulder diameter 15 mm. Bead on plate FSW was conducted on Mg alloy plate for the following parameters: rotational speed varied from 300 to 500 rev min⁻¹, welding speed was 60 mm min⁻¹, backward tool tilt angle was 2° and plunge depth of tool was 4.9 mm.

After welding, plate were cut transversely to the weld line, along the length of weld at equal length intervals; samples polished, etched by using solution of 10 ml nitric acid in 100 ml distilled water and macro image was taken. For Mg alloy plate defect free weld was obtained at 340 rev min⁻¹ (Figure 1). For this rpm, the following readings corresponding to the above optimized parameters have been taken from FSW machine computer; traverse force on FSW tool perpendicular to tool axis (F_tr)=1.838×10³ N, total torque on FSW tool (T_t)=19.66×10³ N·mm, total axial force acting on FSW tool (F_A)=12.326×10³ N.

Figure 1: Macro image of Mg alloy plate welded cross section sample at 340 rpm.

Figure 1:

Macro image of Mg alloy plate welded cross section sample at 340 rpm.

3 Results and discussion

Here average pin diameter taken=5 mm, which is equal to thickness of workpiece=5 mm. Tool geometry has to be decided by thickness of workpiece. Same tool geometry (but may be different tool material) for a particular thickness of workpiece material (or for different workpiece materials). It is assumed that the tool shoulder with overhanging length L and diameter d_sh is rigid enough to withstand all type of forces and torque and moments since pin has been designed to be safe, in turn shoulder is always safe, since d_sh>d_p.

One has to determine the temperature (T) of tool during FSW. T of tool depends on rpm, workpiece material and tool material. T can be measured by inserting thermocouple inside the tool pin through centre of shoulder or by other means. T is the maximum temperature of tool pin during plunging of tool pin into the workpiece and then dwell and afterwards tool just starts to traverse after giving a very slow (2 mm/min) traverse speed. For harder materials (such as steel) highest rpm yielding highest weld nugget temperature should be selected.

Case study

FSW tool is simultaneously subjected to axial force, transverse (in the direction opposite to welding direction) and torque. The following applies to experiment in experimental Section 2.

From Figure 2,

Figure 2: FSW tool with various quantities.

Figure 2:

FSW tool with various quantities.

F_Ap+F_Ash=F_A=12.326×10³ N (from FSW machine computer and see Figure 2)

(F_Ap/F_Ash)=(A_p/A_sh)=(π×5²/4)/[π (15²–5²)/4]=0.125

F_Ap=0.125 F_Ash

F_Ash=10956.444 N

F_Ap=1369.555 N

σ_Ap=1369.555/(π×2.5²)=69.75 N/mm² (compressive) (taking=70 MPa)

Total torque T_t=19.66×10³ N·mm, from FSW machine computer.

In Figure 2,

A₁=π×2.5²=19.634 mm²

A₂=π×5×4.6=72.256 mm²

A₃=π×(7.5²–2.5²)=157.079 mm²

A_s=A₁+A₂+A₃=248.97 mm²

T₁=T_t×(A₁/A_s); T₂=T_t×(A₂/A_s); T₃=T_t×(A₃/A_s);

Torque on only pin=T_tp=T₁+T₂=19.66×10³×(19.634+72.256)/248.97=7.256×10³ N·mm

Maximum shear stress (τ) occurs at the root of the pin τ=T_tp×r/J=7.256×10³×2.5×32/(π×5⁴)=295.6411 N/mm² (=296 MPa). This estimate could be a higher value, since shoulder area A₃ is located at a higher radius; torque shared by shoulder is higher than that of pin.

In Figure 2, maximum bending moment at the root of pin of length L is M=F_tr×L/2:

M=1.838×10³×4.7/2=4319.3 N·mm.

Completely reversed maximum bending stress (σ_b) occurs at points 1, 2 (Figure 2):

σ_b=M×y/I=4319.3×2.5×64/(π×5⁴)=351.968 N/mm² (= ± 352 MPa).

All the following equations are extracted from [16]. We know that,

(S_ut)_T=(S_T/S_RT)_T×(S_ut)_RT

If T°C is the highest temperature of the friction stir welded volume and of the FSW tool, then (S_ut)_T is the tensile strength of the tool material at T°C (=400°C), (S_ut)_RT is the minimum tensile strength of the tool material (=1200 MPa) at room temperature and (S_T/S_RT)_T is the ratio at 400°C=0.9; then

(S_ut)₄₀₀=0.9×1200=1080 MPa.

τ=296 MPa; σ_b=352 MPa; σ_Ap=69.75 MPa.

S_e ^|=0.5×1080=540 MPa.

k_a=4.51 (1080)^−0.265=0.708; k_b=k_c=k_d=k_e=1.

S_e=k_a×k_b×k_c×k_d×k_e×S_e ^|=0.708×540=382.57 MPa.

From Figure 2, filet radius=1.5 mm; (D/d)=(15/5)=3; (r/d)=(1.5/5)=0.3.

Therefore K_t=1.3; q=0.95, for bending.

(K_f)_bend=1+q (K_t–1)=1+0.95 (1.3–1)=1.3 for bending stress;

(K_f)_Ax=1+q (K_t–1)=1+0.95 (1.55–1)=1.52 for axial stress;

(K_fs)_tor=1+q_s (K_t–1)=1+0.95 (1.2–1)=1.2 for torsion stress;

where K_f's are fatigue stress concentration factors.

(σ_a)_bend=σ_b=352; (σ_a)_ax=0.0; (τ_a)_tor=0.0;

(σ_m)_bend=0.0; (σ_m)_ax=σ_Ap=69.75; (τ_m)_tor=τ=296.

σ_a ^|={[(K_f)_bend (σ_a)_bend+(K_f)_Ax (σ_a)_ax/0.85]²+3 [(K_fs)_tor (τ_a)_tor]²}^1/2=1.3×352=457.6 MPa.

σ_m ^|={[(K_f)_bend (σ_m)_bend+(K_f)_Ax (σ_m)_ax]²+3 [(K_fs)_tor (τ_m)_tor]²}^1/2

={[1.3×0.0+1.5×69.75]²+3 [1.2×296]²}^1/2

=624.057 MPa.

Since S_e=S_a the fatigue factor of safety n_f is n_f=(S_a/σ_a ^|)= (382.57/457.6)=0.836; so pin design is not safe for ∞ life of pin.

Stress concentration at filet can be reduced by increasing the filet radius. Tool can be made stronger by increasing the pin diameter. But both of these decrease the effective flow of material around the tool, as the path length around the tool pin at the shoulder increases by the increase of these (filet radius and pin diameter) two quantities. This will result in formation of defects in weld volume, so both of these cannot be changed.

Life cycles of the tool, if the tool has been used at the above stated optimum parameters can be found as follows [17]. See Figure 3,

Figure 3: S–N curve on a log–log graph paper.Sf is the fatigue strength. N is number of cycles.

Figure 3:

S–N curve on a log–log graph paper.

S_f is the fatigue strength. N is number of cycles.

0.9 S_ut=0.9×1080=972

Log₁₀ (0.9 S_ut)=Log₁₀ (972)=2.987

Log₁₀ (S_e)=Log₁₀ (382.57)=2.5827

Log₁₀ (S_f)=Log₁₀ (457.6)=2.66

EF=(DB×AE)/AD=(6–3)(2.987–2.66)/(2.987–2.5827)=2.426

Log₁₀N=3+EF=3+2.426=5.426

N=10^5.426 cycles=2.66×10⁵ cycles.

4 Conclusions

Initial dimensions of FSW tool are assumed by educated guess.
Optimum stresses on tool pin have been determined at optimized parameters for bead on plate welding on AZ31B–O Mg alloy plate.
Fatigue analysis showed that the chosen FSW tool for the welding experiment has not ∞ life and it has determined that the life of FSW tool is 2.66×10⁵ cycles or revolutions.
So one can conclude that any arbitrarily decided FSW tool generally has finite life and cannot be used for ∞ life.

In general, one can determine the suitability of tool and its material to be used in FSW of the given workpiece materials in advance by this analysis in terms of fatigue life of the tool.

Acknowledgements

The author is grateful to DRDO, India (grant no. DRDO/MME/SVK/0618), and Indian Institute of Science, for their financial backing for this work.

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Published Online: 2017-5-25

Published in Print: 2016-12-20

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