Induction assisted friction stir welding

Induction-assisted friction stir welding

Induction assisted friction stir welding

ABSTRACT

Induction-assisted friction stir welding:  Friction stir welding (FSW) is limited to use in metals having high melting points due to the inadequate tool life. Tool life is decreasing due to the high wear rate. Welding high melting point metals reduces the tool life. The alternative changing of the tool may cost too high. The best way to increase the tool life for friction stir welding of high melting point alloy is to preheat the metal.  Preheating reduces the tool wear rate and increases the tool life. One of the best methods to heat ferromagnetic metal is using the induction heating process. This chapter aims to review on induction-assisted FSW process. To study the effect of additional heating on FSW in hard metals and to determine its influence on the strength of joining.

1. Introduction

Induction-assisted friction stir welding

Friction stir welding (FSW) is a solid-state welding process that has gained much consideration in research areas as well as the manufacturing industry since its prologue in 1991 (Lakshminarayanan and Balasubramanian 2011). For almost 20 years, FSW has been used in high technology applications such as aerospace, automotive, marine, and boilers.   The main feature of a solid-state welding process is the work material is only converted to semi solidus state, which allows a lower temperature and a lower heat input welding process (Oudni, Mohellebi, and Feliachi 2008). This is beneficial over conventional fusion welding where excessive high heat input is required to melt the work material.

FSW

Induction-assisted friction stir welding

 Much less heat input required for FSW translates into financial benefits, safer and less complicated welding procedures. FSW does not produce sparks or flames (Kansab and Feliachi 2012; Kohlia and Singh 2011). Thus, safety and environmental issues are not of major concern.

 FSW process

 

Induction-assisted friction stir welding

 FSW process provides proven good quality and strong weldment with the smaller expense and lesser number of equipment eliminates the use of filler metal and improved weldability. been employed in aerospace, automobile, and shipbuilding industries. FSW process continues to propagate in many applications such as oil pipe welding and boiler tank welding (Álvarez et al. 2014).

friction stir welding

The friction stir welding makes it possible to join lightweight materials such as aluminum alloy, magnesium alloy, copper, and titanium alloys which are very difficult to weld by conventional welding.

laser heating

One of the main drawbacks of FSW was it is not able to weld very hard metals. Now, by providing additional heating sources like induction heating, laser heating, and arc heating it is possible to weld hard metals too. These clear advantages have greatly increased the usage of these materials in structural applications.

 2. Electromagnetic induction

Induction heating occurs due to electromagnetic force fields producing an electrical current in a part. The parts heat due to the resistance to the flow of this electric current.

2.1. Electric resistance

All metals show electric resistance while conducting electricity. The resistance to this flow of current causes losses in power that result in the formation of heat. Some metals, such as silver and copper, have very low resistance and, as a result, they are very good conductors. 

Copper wires

Copper wires are used to carry electricity through power lines because of the low heat losses during transmission. Other metals, such as steel, have high resistance to an electric current so that when an electric current is passed through steel, substantial heat is produced (Bhadeshia and DebRoy 2009).

steel heating

 The steel heating coil on top of an electric stove is an example of heating due to the resistance to the flow of the household, 60 Hz electric current. In a similar manner, the heat produced in a part in an induction coil is due to the electrical current circulating in the part.

2.2. Electromagnetism and alternating current

Induction heaters use alternating electric current to an induction coil. The induction coil becomes the induction heating source, which induces an electrical current into the metal part of the workpiece. No contact is required between the workpiece and the induction coil as the heat
source and the heat are limited to a small area or surface zones nearby to the coil.

induction coil

This is because the alternating current (ac) in an induction coil produces electromagnetic flux around it. When the induction coil is subject
next to or around a workpiece, the lines of force focus in the air gap between the coil and the workpiece. The induction coil actually functions as a primary coil, with the workpiece to be heated becoming the secondary coil.
The force field surrounding the induction coil induces an equal and opposing electric current in the workpiece. Due to the resistance to the flow of this induced electric current, the workpiece gets heated (DobrzaĔski 1997; Sadeghipour, Dopkin, and Li 1995).

heating of the workpiece

The rate of heating of the workpiece is dependent on the frequency of the induced current, the intensity of the induced current, the specific heat of the material, the magnetic permeability of the material, and the resistance of the material to the flow of current. The induced currents are referred to as eddy currents, with the highest intensity current being produced within the area of the intense magnetic fields. A schematic diagram for the working of induction is shown in Figure 1.

Induction heating

Induction heating involves heating a workpiece from room temperature to a higher temperature, such as is required for induction tempering or induction austenitizing. The rates and efficiencies of heating depend upon the physical properties of the workpieces as they are being heated (DobrzaĔski 1997).   These properties are temperature-dependent, and the specific heat, magnetic permeability, and resistivity of metals change with temperature.

Steel

Steel has the ability to absorb more heat as temperature increases. This means that more energy is required to heat steel when it is hot than when it is cold. At 760 °C steel exhibits an increase in resistivity of about 10 times larger than when at room temperature. Finally, the magnetic permeability of steel is high at room temperature, but at the Curie temperature, just above 760 °C, steels become nonmagnetic with the effect
that the permeability becomes the same as air.

stir welding

2.3. Skin effect and reference depth

Induction heating occurs when an eddy current is induced into a workpiece that is a deprived conductor of electricity. For the induction heating process to be efficient and practical, certain relationships of the frequency of the electromagnetic field that produces the eddy currents, and the properties of the workpiece must be fulfilled.

nature of induction heating

The basic nature of induction heating is that the eddy currents are produced on the outside of the workpiece in what is often referred to as ‘skin effect’ heating. Because almost all of the heat is produced at the surface, the eddy currents flowing in a cylindrical workpiece will be most concentrated at the outer surface, while the currents at the center are minor.

frequency

 The depth of heating depends on the frequency of the ac field, relative magnetic permeability, and the electrical resistivity of the workpiece. The skin healing effect is defined as the depth at which approximately 86% of the heating due to resistance of the current flow occurs. The reference depths reduce with higher frequency and amplify with higher temperature (Lienert et al. 2003).

3. Induction heating process

Induction heating is a widely used method for heat treatment of steel. Its main advantages compared to other methods are quick and accurate localized heating, low consumption of energy, and eco-friendly (Cannale, Mesquita, and Totten 2008). 

electromagnetic field

Induction heating is a complex interaction of thermal, electromagnetic, and metallurgic phenomenon. An alternating electric current (AC) induces an electromagnetic field, which in turn induces eddy currents in the workpiece. The currents liberate energy in the form of heat. This is then transferred throughout the workpiece. During the induction heating process, the temperature of the heated material changes on such a huge scale depending on material properties.

frequency of the electric

To determine the optimal coil position, frequency of the electric current, and the optimal amplitude in order to achieve the required temperature profile in a non-ferromagnetic steel cylinder workpiece. The finite-element numerical modeling grouping with the genetic algorithm optimization method was used for this purpose.

heated workpiece

The heated workpiece was bounded by a copper coil consisting of four loops. Using a genetic algorithm the parameters were the optimized position of the single-coil loop, the amplitude, and the frequency of the electric current in the coil.

numerical model

The numerical model was experimentally validated by assessment of measurement and simulation results. The optimized solution and the global optimum had comparable temperature profiles due to a better selection of the electric current parameters compared to the profile obtained by solving the problem naturally. 

planning induction heating of steel

This approach can be used for planning induction heating of steel materials at low-energy consumption and high time efficiency (Meran, Kovan, and Alptekin 2007). The effect of process parameters such as feed rate, dwell time, current, and the gap between the material and inductor coil on the hardness of AISI 1040 steel under two dissimilar conditions i.e. rolled and normalized throughout induction hardening.

hardness

The optimal values for hardness obtained were 56.4 HRC and 57.8 HRC, respectively, for the rolled and normalized conditions at the feed
rate of 3.21 mm/s, dwell time 5 s, current 135 Amperes, and the gap between the material and inductor coil 5.29 mm as the optimum value of process parameters.

manufacture shafts

To manufacture shafts, automobile components, or axles from medium carbon steel, the raw material must be first normalized and then induction hardened so that uniform hardness of the material can be obtained (Mishra and Ma 2005).

efficient

This work presents an efficient approach for getting better the energy efficiency of the induction heating process, by process parameter optimization. Instead of focusing on improving the hardware or equipment (such as power supplies, and induction coils) variable process parameters like voltages and frequencies of the heaters in the induction are optimized.  

simulation method

The numerical simulation method was used as an initial investigation method, instead of performing a set of expensive physical experiments to save experimental costs. A computer-aided optimization approach in combination with Design of Expert (DOE), meta-model, and multi-objective optimization based on a genetic algorithm (GA) technique to methodically optimize the induction heating process parameters. The result shows that the energy efficiency can vary 10% in the suitable input range of process parameters (Aissani et al. 2010).

4. Induction-assisted friction stir welding

The successful application of FSW in aluminum and its alloys, during recent years and the FSW technique, has been extended to many other metallic materials. Nowadays, the main problem found in FSW was the difficulty to weld high melting point metals or its alloys are the tool
material cost, appropriate design, and the scarcity of high melting point material welding tools.

friction stir welding

It is well known that friction stir welding can provide strong and reliable welded joints it can apply to a variety of materials including those that are difficult to be joined by other welding methods. When the seam throughout the welding process or the melting point of the workpiece is high, then the tool can be traveled too rapidly along with the welding.  The frictional heat generated between the tool shoulder and the structural members may not be sufficient to plasticize the structural members. 

traveling speed

The main consequence is the formation of tunnel defects in the stir zone, which will inevitably decrease the quality of the weld joint. The traveling speed of the tool is generally restricted by the rate at which frictional heat is generated between the tool and the workpiece.  For the low melting point metals like Al and Mg alloys, the frictional heat is high enough to soften the materials and a sound weld can be obtained after the FSW process. When the FSW was applied for a higher melting point alloys such as steels, Ti, or Ni alloys, much larger heat input is required to plasticize the materials so that the plastic stirring can proceed.

PCBN, WC alloy, Si3N4

The tool shoulder can reach a temperature above 8600c and the weld seam behind the tool stays around this temperature for up to 25 mm behind the tool when the FSW is applied to the steels. On the other hand, although the rotation tool has also been developed and can be made of highly durable materials like PCBN, WC alloy, Si3N4, etc., the wear of the rotation tool is still very severe.

quality

The tool surface was damaged rigorously and the tool became greatly deformed and would worsen the weld quality if it uses for the further welding process.

welding starts on the metal

Preheating of the work materials before the welding starts on the metal is one of the best and economic methods for gaining better welding strength and increase tool life (Thomas et al., 1991a). Induction and laser-assisted FSW carried out on marine steel plate grade A was successfully welded. 

forge

A decreasing forge force has been observed and it helps to enhance the tool life. The preheating treatment slightly increases the hardness of the stir zone and it does not have any effect on tool travel speed (Thomas et al., 1991b).

FSW for welding

An induction-assisted friction stir welding method was used to attain sound welds in cast Sloan Digital Sky Survey (SDSS) plates. The welding speed is twice while comparing to the optimal value of conventional FSW for welding SDSS plates.

other parameters

The other parameters such as rotation speed and axial force were the same as the conventional FSW process. The microstructural changes induced in welding under normal FSW and induction-assisted FSW were investigated. A very fine-grained banded structure is formed in the stir zone, with dissimilar dynamic recrystallization extents in ferrite and austenite (Kawase, Miyatake, and Hirata 2000). The hardness and strength
of the joints increase with the significant reduction in grain size, particularly in induction-assisted FSW where the structure refining is more intense (Sato et al. 1999; Sun and Fujii 2010). 

5. Laser-assisted friction stir welding

Laser heating is also used as a preheating source for welding hard metals. A laser heating source was used as a preheating source for friction stir welding of S45C steel plates having 3.2 mm thick. With respect to the laser focal point, a different preheating effect was gained. It shows changes in the microstructure and mechanical properties of the weldment. The best joints were produced using the following parameters; laser beam was focused 10 mm ahead at the joint line with welding speed 600 mm/min. In a normal FSW process, the maximum welding speed achieved is 400 mm/min, the achieved result is much higher compared to normal FSW. The preheating helps to prevent the formation of the brittle martensite phase.

induction-assisted FSW

While comparing with induction-assisted FSW, the laser-assisted FSW is much more costly. The flexibility of welding is much higher in induction-assisted FSW. Due to the higher welding cost, laser-assisted FSW is less preferable. (Campanelli et al. 2013).

6. Concluding remarks

This review on induction-assisted friction stir welding having many recent journal papers shows how preheating helps to apply FSW on harder metals (Acero et al. 2005; Aissani et al. 2010; Bhadeshia and DebRoy 2009; Cannale, Mesquita, and Totten 2008; DobrzaĔski 1997; Kranjc et al. 2009; Lienert et al. 2003; Mahoney et al. 2010; Meran, Kovan, and Alptekin 2007; Mishra and Ma 2005; Nandan, Debroy, and Bhadeshia 2008; Robson, Kamp, and Sullivan 2007; Sadeghipour, Dopkin, and Li 1995; Selvaraj, Murali, and Koteswara Rao 2013; Sinclair 2009). This induction
heating enhances the tool life, increases the welding speed, and can it decreases the grain size while plasticizing. This decrease in grain size improves the joint strength (Dang
2013; Gopi and Manonmani 2012a). Induction heating-assisted FSW can be adopted in all ferromagnetic materials and by comparing to other heating sources, it provides quick heating (Gopi and Manonmani 2012b). Preheating causes hardening on the stir zone and it may awfully affect the tool pin profile (Gopi and Manonmani 2012c; Thomas et al., 1991a). So, it requires considerable further development for avoiding the hardening in the stir zone and produces much more welded joints (Sun and Fujii 2010).

 

 

 

 

 

 

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