Key words: Friction stir welding, Tool geometry, load bearing, Tool material, FEM

1. Introduction

Friction stir welding (FSW) doesn’t require electrodes and is safe for the environment. A lightweight method of bonding metal, feathered stir welding may be applied to copper, zinc, magnesium, and aluminium alloys¹⁾. Friction stir weld joints have a shorter lifespan than original material. Despite being relatively new, this welding technology has proven to be quite profitable when it was first introduced in 1991²⁾. During friction stir welding, the metal is heated to a temperature below its melting point. Of course, using a friction stir welding (FSW)1-5 tool is necessary for optimal process efficiency^2,3). The tool is constructed from a rotating circular shoulder and a cylindrical pin with threads. Friction warms the work piece mainly, and the softened alloy is pushed to form the connection⁴⁾. FSW avoids the primary fusion welding issues of porosity, solidification and liquidation cracking, and volatile alloying component loss by keeping the workpiece from melting too much^3-5). The main reasons why welding aluminium and other soft metals is profitable are these advantages. But the FSW tool is under a lot of heat and stress when welding heavy metals like titanium and steel alloys^4,5). Fig. 1. FSW is not very practicable for these metals now because of the high cost and short equipment life. Although there have been various attempts recently to create inexpensive, reusable tools, more work in tool design is required to advance the use of FSW to hard metals⁵⁾. Most of these projects have an empirical focus. This study reviews the literature on several important FSW tool subjects, such as process economics, load bearing capabilities, failure reasons, material selection, microstructure, and tool form^5,6). Multiple designs are used in a non-consumable alternating device composed of materials that can endure temperatures higher than the components that need to be coupled⁶⁾. To achieve frictional intensity, the instrument’s test basically entails applying pressure to the adjacent sides of the work components and rotating it. This causes the area surrounding the wet test and the point where the work piece and device shoulder meet to create a third body, or mellowed, plasticized zone⁷⁾. The shoulder gives the work piece more friction treatment and prevents plasticized material from escaping the weld^5-7). As the temperature rises, plasticized material is ejected onto the instrument’s following side by the spinning device’s main side because the metal’s strength at this point lowers to less than the applied shear pressure. The welding is then made persistent by pushing the tool continually along the joint line^6,7).

Fig. 1

Friction-stir welding tools⁷⁾

2. Tool Material Selection

Aluminium alloys’ FSW demonstrates less tool wear and tear. Tool steels and other similar materials are therefore often utilised. But it has been noted that tool wear presents a significant challenge to the FSW of materials that will ultimately wear out, including metal matrix composites (MMCs), as well as materials with high melting temperatures, like steel and titanium⁸⁾. It was also said that choosing the right tool material is crucial for FSW of composites, titanium, and steel. It has been determined that as comprehensive research on the topic of tool material selection has not yet been carried out, more study is necessary in this area^8,9). Stated the characteristics that might be considered while selecting hardware for FSW/P procedures. They asserted that the desired device’s material was influenced by the characteristics and attributes of the welded metal^7-9). The interaction with the broken instrument material might have an impact on the eventual weld’s micro- structure. The amount of stress applied to the device depends on the work material’s strength. The device’s material characteristics affect the heat age, and, in turn, the temperatures reached¹⁰⁾. For instance, warm conductivity is important for selecting the appropriate device material to achieve specific junction qualities. The coefficient of warm development affects how a gadget perceives warm loads. The choice of device material may also be influenced by the work materials’ pliability, reactivity, and hardness^8-10).

3. Desired FSW/P Tool Material Characteristics

To create a high-quality FSW joint, accurate instrument material identification is essential. When selecting the device material for FSW/P, the following elements need to be considered^9-11).

• (a) Excellent machinability for testing and constructing intricate highlights on the shoulder.

• (b) No harmful reactions with the weld metal,

• (c) Good mechanical properties

• (d) Dimension stability at high temperature, good chemi- cal properties.

• (e) Low coefficient of thermal expansion

4. Tool Materials

A few different device materials have been used in the FSW/P technique. Fast steel (HSS), ceramics, metal carbides, Ni-composites, and equipment preparations are a few of these materials¹²⁾.

4.1 Tool steels

4.1 Tool steels

Tool steel is one of the most often used device materials for handling and welding magnesium, copper, and aluminium. It can fuse metal combinations up to 50 mm thick. Warm exhaustion blocking, high machinability, and good accessibility are the characteristics of these materials^10-12). The instrument preparations are shielded from damage by the twisted and scraped portions present in the FSW of aluminium alloys and other materials with low melting temperatures^11,12). It is possible to weld both distinct and similar welds, such as lapped joints and butt junctions, with the correct instrument preparations. Welding a butt joint design is more challenging^12,13). The device is shifted towards the softer material and out of the butt contact area. Materials are positioned on the pressing side (AS) while the softer material is positioned on the withdrawing side (RS). A few materials, such as high carbon high chromium (HCHCr), high velocity steel (HSS), hardened steel (SS), H13, and C40, have been connected using the appropriate instrument preparations. Device preparers have successfully welded mild steel, aluminium, copper, and magnesium^11-13).

4.2 Nickel Alloys and Cobalt Base Alloys

4.2 Nickel Alloys and Cobalt Base Alloys

Super alloys based on nickel and cobalt exhibit remarkable resistance to erosion and drag in addition to remarkable strength, pliability, hardness, and solidity. Accelerators provide the power for these combinations. As a result, throughout the exercise, their body temperature should be kept below 600-800 degrees Celsius, which is the range at which precipitation typically happens^10-13). However, their unfavourable machinability prevents them from being used, making it difficult to create complex parts like woodwinds and instrument profile pads. These components are frequently used in copper composites’ FSW/P. Co-based amalgam equipment has been used to join zircaloy-4 atomic grade material, which has a thickness of 3.1 mm¹⁴⁾.

4.3 Polycrystalline Cubic Boron Nitride (PCBN)

4.3 Polycrystalline Cubic Boron Nitride (PCBN)

Preparing with FSW/P and other high-temperature materials has proven to be challenging. This directly related to the lack of authorised equipment. a part composed of materials that can withstand the harmful effects of any high temperatures the device could experience^13,14). As stated in the FSW expert guide, there should be no equipment material left in the joint, therefore wear prevention is essential. PCBN is one of the most promising instrument materials that can meet these requirements¹⁵⁾. Originally intended for use in hardware preparation machining, PCBN is now used as a material for FSW/P equipment because to its high mechanical and warm exhibition. PCBN is selected in the FSW/P of prepared and Ti combinations due to its high temperature solidity, strength, and hardness at higher temperatures^13-15).

4.4 Ceramic Materials

4.4 Ceramic Materials

Additionally, pottery has been used to make FSW/P equipment. Despite this, the material is brittle and breaks rapidly when submerged in water. Despite being weaker than carbides, fired cutting tools are more authentic and safer¹⁶⁾. These materials can be used for the machining of super alloys and hard preparations. When making earthenware, two types of cutting tools that are considered revolutionary are silicon nitride and alumina^15,16).

Fig. 2

Tool Geometry.

5. Parameters and Materials

Tool 3D model prepare on Solid works with following parameters¹⁷⁾.

Probe diameter = 25mm

Length of probe = 45mm

Length of shoulder = 15mm

Length of pin = 3.7mm

Diameter of tool pin = 4.0mm

Plunge diameter = 10mm

A tool with a circular probe was used. The dimensions of the tools are as follows:

We have selected three materials.

(a) Structural Steel

(b) Titanium

(c) Ceramic (Alumina (Al2O3))

Fig. 3

3D Model of tool

Table 1

Properties of Structural steel¹⁸⁾

Young’s modulus	2*105 mpa
Poison’s ratio	0.3
Density	7.85e-006 kg/mm3
Thermal expansion	1.2e-005 1/c
TYS	250mpa
CYS	250mpa
TUS	460mpa

Table 2

Properties of Titanium¹⁹⁾

Density	4.5 g/cm3
Melting point	1668 d C
Tensile strength	220mpa
Poison’s ratio	0.34
Modulus	116 gpa
Hardness	70
Shear modulus	43 gpa

Table 3

Properties of ceramic¹⁹⁾.

Density	3.97 g/cm3
Hardness	2000 kg/mm2
Tensile strength	270 mpa
Poison’s ratio	0.27
Thermal conductivity	35 w/m-k
Specific heat	0.21 c/g- dC
Young modulus	393 gpa

6. Modeling & Simulation

The friction stir welding process is simulated for a range of welding parameters, including transverse and rotational speeds of the tool. When the tool’s rotational speed increases, the work piece’s temperature rises; yet, when the tool’s traversal or welding speed increases, the temperature falls^19,20). The sites closest to the stirred zone record the highest temperatures, and as one moves farther away from the stirred zone, the temperatures continue to drop²⁰⁾. While transverse residual stresses showed no discernible effect on the traverse speed, longitudinal residual stresses rose as the tool traverse speed increased. It is notable that FSW has minimal residual stress because of reduced heat input and stress accommodation through recrystallization^20,21).

Table 4

Specifications of materials²¹⁾

Material	Structure steel, Titanium, Ceramic
Force apply	50 N
Temperature	500 degree C
Room temperature	30 degree C
Acceleration	1G

Fig. 4

Total deformation of structural steel tool²¹⁾

Fig. 5

Total deformation of titanium tool²¹⁾

Fig. 6

Total deformation of ceramic tool²¹⁾

Fig. 7

Total heat flux of structural steel tool²¹⁾

Fig. 8

Total heat flux of titanium tool²¹⁾

Fig. 9

Total heat flux of ceramic tool²¹⁾

Table 5

Modal analysis of structural tool²²⁾

Mode	Frequency [Hz]
1.	11738
2.	11744
3.	24920
4.	33258
5.	33301
6.	36814
7.	56019
8.	56050
9.	57235
10.	81001

Table 6

Modal analysis of titanium tool²²⁾

Mode	Frequency [Hz]
1.	10649
2.	10656
3.	22003
4.	29914
5.	29952
6.	33460
7.	50467
8.	50496
9.	50553
10.	71531

Table 7

Modal analysis of ceramic tool²²⁾

Mode	Frequency [Hz]
1.	23107
2.	23117
3.	49697
4.	65715
5.	65800
6.	72384
7.	1.1061e+005
8.	1.1067e+005
9.	1.1413e+005
10.	1.5988e+005

Modal analysis is used to display the ten unique modes of the tools for each of the three materials²²⁾. The frequency or rate at which an object vibrates when it is naturally disturbed is known as its natural frequency^21,22). Harmonic oscillators are often used to simulate an object’s inherent frequency as objects might have several intrinsic frequencies²³⁾.

6.1 Load & Boundary Condition

6.1 Load & Boundary Condition

In this example, a 50N force is applied to the tool tip or pin and the ambient temperature is 30°C. A 500°C force is applied to the tool pin’s temperature. Here, tetrahedral meshing is employed using 21023 elements, 42564 nodes, an automatic mesh generation technique, and an element size of 2 mm^23,24). The tool’s upper face is fastened to the machine’s chuck for frequency response and accelerates 1G in a top-to-bottom manner²⁵⁾.

7. Result & Discussion

We analyse static structures, frequency response analysis, and thermal analysis to calculate the total heat flux when 1G of acceleration is applied to the tool’s fixed geometry. These investigations show how different material tools work and/or reduce the vibration waves that go from the machines to the tools, which affects the weld quality. Increased vibration attenuation efficiency and material characteristics are indicators of better weld quality. Throughout the whole welding operation, the computed temperature values and the observed ones coincide rather well. The temperature distribution in the lateral direction at the exact moment the shoulder centre crosses this place is displayed in Fig. 14. A high degree of agreement between the computed and observed temperatures suggests that the model that was created to forecast temperature history is producing useful outcomes. The simulated findings show that, although the lateral force has a poor relationship with the tool’s rotating speed, the mechanical forces in the longitudinal and vertical directions may be decreased by increasing the tool’s rotational speed within a specific range. It is observed that because of the discretization required for step movement and plate meshing in the FEM simulations, the expected force values fluctuate.

Fig. 10

Frequency response of structural steel tool²³⁾

Fig. 11

Frequency response of titanium tool²³⁾

Fig. 12

Frequency response ceramic tool²³⁾

Fig. 13

Total deformation of different material tools

Fig. 14

Total Heat flux of different material tools

Fig. 15

Frequency response

8. Conclusions

This is where the best thermal effects for structural steel might be obtained via friction stir welding: Improper weld quality and tool life will occur if the friction between the tool and the work piece prevents the tool from dispersing force as the temperature rises. (a) Given that structural steel has a maximum heat flux of 45 W/mm², it is possible to increase welding performance by using structural steel tools to heat welding plates to the appropriate temperature. (b) The lowest heat flux is 18 W/mm³ for the titanium tool and 28 W/mm² for the second ceramic tool. (c) Titanium tools have a maximum amplitude of more than 22,000 mm/sec², which adversely impacts the welding process, whereas structural steel has a minimum amplitude of less than 4000 mm/sec², (d) This suggests that structural steel is the material that attenuates vibration waves during the FSW process the best. Analysis of all the data revealed that structural steel outperforms the other two of these three FSW tool materials. A prototype finite element modelling approach for the simulation and analysis of the friction welding process is implemented using an explicit code that incorporates adaptive meshing and advection techniques. (e) The fully coupled thermal-mechanical FE model has an effective analysis of the peak temperature, temperature fields, deformation, stresses, and strains; maximal values are mostly anticipated to be located around the edge of the rubbing surface. The FE model utilised in this work can help with the formulation of weld parameters and improve knowledge of the friction welding process, hence minimising the need for expensive and time-consuming experimental procedures, even if a more precise analysis will always be required.

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