Friction Welding: Techniques, Applications, And Merits

Design and Analysis of Bevel Gear in Differential Gear Box and Effects of Bevel Angle on Fatigue Failure for Literature Review.

Introduction to Friction Welding

Friction welding is defined as Solid state or a simply forge whereby the welding process occurs between two mating surfaces of metals through the application of friction between them. The welding technique has been adopted by well-known international companies in in America and Europe such as  and Rockwell international to manufacture their machines.

Friction welding is considered to be very essential in the production industry, by adopting this welding method the high overhead costs which the companies will undergo to have this technique in place will be balanced with the high production rates and lower labour requirements.  

The friction welding technique has many dimensional and hardware which are easily adjustable making it to be very significant in the production of very small parts and components. There are many industrial applications where friction welding can be applied such as the machine and spare part production.

The process of friction welding was first discovered in the late 1920s, however during that time there was very little data which was recorded about its use. A detailed discussion about friction welding was recorded in the USSR in the early 1960s, but that time it was described as ‘very doubtful, as a production technique due to the challenges in generating reciprocating linear motion. The fist well-structured industrial research into friction welding took place in the 1980s at TWI, UK, while the first ever research academic to be to be recorded took place at Ohio State University, and the University Of Bristol, UK. 

The friction welding technique works on the basic principle of friction. In the process of welding, friction is used to create heat at the interfaces surfaces (Anderson, 2016). The heat is used to join the two work pieces through the application of the external pressure at the work pieces surfaces. In friction welding, the friction is applied until the plastic forming temperature is reached which is usually900-1300^oc for steel. After the heating process, pressure which is uniformly increasing is applied to both of the metal work pieces until they make a permanent joint.

There are many merits which are associated with the use of friction we in production. Some of the merits include:

• It easier and convenient to join metals which are not similar and some of which are considered to be unweldabl or incompatible.
• Friction welding is very fast method of welding as compared to other welding techniques(Granjon, 2014).
• Due to their versatility the friction welders are able to join a wide range of materials, shapes and sizes without any challenge.
• In this friction welding joint preparation is not of great importance since, saw cut, machine and even the sheared surfaces are weldabl.
• The resulting joints are of forged quality, with up to 100% butt joint weld through the contact area.
• Human error are eliminated by machine controlled process, and the quality of the weld is independent of the operator skill.
• It is environmental friendly as there is no objectionable fumes, gases or smoke  that are generated  which needs to be exhausted.
• In friction welding there are no consumable that are required, no filler material, flux or shielding gases.
• The power requirements for friction welding as low as 20% of that which is required for conventional welding processes.
• Since there is no melting which occurs there is no solidification defects such as gas porosity, slag or segregation inclusions.

Even though there are many benefits which are associated with friction welding it has also some drawbacks such as:

• This technique of welding is mainly used in round bars which have the same cross section. Due to that the bars which are not of the same cross section cannot be welded. In some industries welding of metals which are not round in shape is very common the application pf friction welding for such cases is limited(Sahin, 2016).
• There are exist holes left when the tools are withdrawn. Most of the metal bars which are to be joined together are held by strong tools which drills holes through them.
• Friction welding requires extensive clamping
• Needs special backing support
• The cost of setting up a friction welding facility are very high. The equipment and machines which are used for friction welding are very expensive to acquire. This makes a lot of people to move to the convention welding which is much easier and cheaper.
• The design of the joints is very limited.
• It involves critical preparations of work pieces(Welding InstituteElsevier Science & Technology, 2010).

Many production industries all over the world are experiencing the high costs of energy due to the amount of energy which is consumed during the production process’s adoption of friction welding can greatly assist in solving this challenges (Yilbas, 2016). The friction welding has been applied in various industrial applications as discussed below: 

Benefits of Friction Welding

The marine and shipbuilding are among the earliest industries to adopt friction welding for commercial applications (Anderson, 2016). Friction welding is suitable for the following applications:

• Panels for side, decks, floors and bulkhead.
• Superstructures and Hulls
• Aluminium extrusions
• Booms and Mast for example for sailing boat

Currently the aerospace industry is welding production parts and prototype by application of friction welding. There exist opportunities to weld, ribs, spars and stringers for civilian and military aircrafts (Vill', 2015). The friction welding process can be considered for the following:

• Fuselage, empennages, wings
• In the case of space vehicle the friction welding can be considered for cryogenic fuel tank
• Scientific and military rockets
• Aviation fuel tanks

The commercial production of high speed trains which are made from aluminium extrusions that can be joined through friction welding has been published and the applications include:

• Trams, underground undercarriages and the rolling stock of railways
• Good wagons and railway tankers
• Container bodies(American Welding Society. Committee on Friction Welding, 2012)

• Electric motor housing
• Kitchens and cooking equipment
• Gas cylinders and gas tanks
• Axle tube, gears, drive line, valves and other components are friction welded.
• Connecting rods, Gear levers, drill bits are friction welded
• It regularly used to replace casting or forging assembly
• Truck rollers, and hydraulic piston rods are friction welded
• In welding shafts and tubes
• In electrical industries it is used for welding aluminium and copper equipment
• In pumps friction welding is used to weld pump shafts(Singh, 2016).

There exists many variations in the process which work on the same principle, due to that there are various types of friction welding as listed below:

1. Continuous induce friction welding
2. Linear friction welding
3. Spin welding/Inertial friction welding
4. Friction surfacing

Inertial Friction Welding refers to a variation of the friction welding in which the required energy to make the weld is provided mainly by the stored rotational kinetic energy of the welding machine (Blau, 2012). In the Inertial Friction welding, one of the two workpieces is connected to a flywheel while the other is prevented from rotating. The flywheel is then accelerated to a given rotational speed which is predetermined, storing the required energy. The motor driver is disengages and then the work pieces are joined together by the friction welding force. This makes the faying surfaces to rub to each other under pressure. The kinetic energy which is stored in the rotating wheel is released as heat through friction at the interface of the weld as the wheel speed decreases. A forge force might be applied before the flywheel completely stops (Wang, 2013). After rotation has stopped the forge force is maintained for a predetermined period. 

In this welding process, the rotor I connected together with band brake. When the plastic temperatures are achieved, the band brake which was connected comes into action with the main role being is to stop the rotor  but at the same time the pressure which is applied  continue to increase until the two work pieces joints well and thus the weld is achieved (Yilbas, 2016).

Linear Friction welding is a solid-sate joining process which works through oscillating one workpiece relative to another while under a large compressive force as shown in fig 1 below. The friction which exist between the oscillating surfaces generates heat that causes the material of the interface to plasticise (Blaga, 2015). The material which is plasticised is then expelled from the interface causing the workpiece to burn-off (shorten) in the compressive force direction. In the process of burn-off the interface contaminants such as foreign oxides and particles that can affect the properties and possibly the service life of a weld are expelled. Once the workpieces are free from contaminants the metal to metal contact occurs which leads to formation of an integral bond (Vill', 2015). Linear friction welding unlike other methods of friction welding it takes place in four phases as listed below:

1. Phase 1: Initial stage
2. Phase 2: transition stage

• Phase 3: Transitionstage

1. Phase 4: Forging and Deceleration phase

Drawbacks of Friction Welding

Phase 1 ( Initial stage):in this stage  contact exist  between asperities on the two surfaces  which are to be joined, then heat is generated  because of friction shown in the figure below.

The asperities deforms after softening , thus increasing the true area of the work pieces which is in contact.axial shortening in the direction of the applied force is observed  even tough it is neligible. 

This makes the true area of the contact between the work pieces to increase to 100% of the cross sectional area. More materials are softened as the heat conducts away. Due to the expulsion of the viscous materials form the interface the burning-off starts to occur (Metallurgists, 2013).

Phase 3 (Transition stage): During this phase axial shortening occurs at a constant  rate  by the rapid  expulsion of the viscous  interface material  and a  quasi-steady-state condition is  also achieved, forming the flash as shown in the figure below (Friction Welding Technical Group, 2010).

Phase 4 (Forging and Deceleration phase): In this phase the workpieces are aligned and the relative motion is stpoed.in some cases an addition forge force may be applied to assist the weld to consolidate well (Ellis, 2013).

During linear friction there are eight are six parameters which are used as listed below.

• Oscillation amplitude
• Oscillation frequency
• Ramp-up time
• Supplied force
• Normal force
• Axial shortening(American Welding Society. Committee on Friction Welding, 2012)

The finite element model

The friction welding is a very complex process which involves the interactions friction force and heat energy (Chaturvedi, 2016). The amount of heat which is generated at the friction interface directly depend of the friction. An increase in the temperature distribution leads to deformation of the body by thermal influences and strains the material thermal and mechanical physical properties (Anderson, 2016) .Then the plastic deformation changes the mechanical work into heat energy by a process which is not reversible.

The figure below shows the weldment. 

R1 is the inner radius of weldment near the frictional interface and R2 is the outer radius

The balance relation of flow of heat is presented in the analysed domain Ὼ by the following axisymmetric heat transfers Equations. 

Where by:

T is current temperature

k(T) is thermal conductivity,

 C (T) p is the heat specific

The expression of q is

Where by:

 is the coupling is factor for the thermo-mechanical action and denotes the rate of internal heat generation during the plastic deformation.

a is thermal efficiency of plastic deformation.

q_i is the internal heat source.

 The initial temperature of the parts is uniform and is described as: 

Industrial Applications of Friction Welding

On the free surface, the boundary condition is given as following: 

and 

Where h₁ is the convection coefficient and h₂ is the radiation coefficient. T_s is the surrounding temperature. On the friction surface, the boundary condition is written in the following equation 

Where q(r,t) is the heat flux at friction surface and described as: 

Where

The friction coefficient during the initial stage of the inertia friction welding process is described as following: 

Where T , P ,V are the temperature, pressure and linear velocity in the frictional interface. a ,b , c , f_o are the constant obtained from experiments. During the steady- friction stage of the inertia friction welding process, the friction coefficient is described as

Where by: 

The calculation model for stress and strain fields of the welding process is described by Kirchhoff balance equation 

Where

 is Kirchhoff stress tensor

 is Green strain tensor,

 is component of virtual displacement,

P_oi and T_oi are unit volume force component acting on the deformed body respectively. The strain increment  consists of the elastic strain increment  and the plastic train increment  

Where  is the elastic-plastic matrix. The nonlinear governing equation group could be obtained when the above two models were dispersed through finite element method.  

This process id developed from friction welding process whereby a coating material is applied to substrate. Simply it can be referred to as a surface coating process. A rod which is made up of the coating material and referred to as mechatrode (Lovell, Chiocago) .It is then rotated under given pressure in order to generate a plastic type of layer in the rod at the interface between the rod and the substrate, through the movement of the substrate across the face of the rod which is rotating a plastic layer is deposited (American Welding Society. Committee on Friction Welding, 2012).

Conclusion

In conclusion, the friction welding technique works on the basic principle of friction. In the process of welding, friction is used to create heat at the interfaces surfaces. The heat is used to join the two work pieces through the application of the external pressure at the work pieces surfaces.

The friction welding has many advantages such as, It is environmental friendly as there is no objectionable fumes, gases or smoke  that are generated  which needs to be exhausted. In friction welding there are no consumable that are required, no filler material, flux or shielding gases. The power requirements for friction welding as low as 20% of that which is required for conventional welding processes. (Blaga, 2015)

Types of Friction Welding

The friction welding technique has many dimensional and hardware which are easily adjustable making it to be very significant in the production of very small parts and components. There are many industrial applications where friction welding can be applied such as the machine and spare part production (Agarwal, 2015).

Friction welding has a wide range of applications such as in the railway systems, shipping building and other marine operations, aerospace industry and the manufacture of spare and other components.

Timeline

no	Duration	Proposed work	resources
1
2
3
4
5
6

References

Agarwal, L. (2015). Spot friction welding of Mg-Mg, Al-Al and Mg-Al alloys. Chicago: University of Michigan-Dearborn.

American Welding Society. Committee on Friction Welding, A. W. (2012). Recommended Practices for Friction Welding. Chicago: American Welding Society,.

Anderson, E. S. (2016). Design of a machine for thin-foil friction welding process development. Paris: Madison.

Blaga, L.-A. (2015). Ths solid phase welding of metals. Paris: John Wiley & Sons,.

Blau, P. J. (2012). Friction Science and Technology: From Concepts to Applications, Second Edition. London: CRC Press.

Chaturvedi, M. C. (2016). Welding and Joining of Aerospace Materials. London: Elsevier Science.

Daniela Lohwasser. (2014). Friction Stir Welding: From Basics to Applications. Chicago: Elsevier, .

Ellis, G. (2013). Friction Welding in Production. Berlin: Society of Manufacturing Engineers.

Filho, S. T. (2011). Joining of Polymer-Metal Hybrid Structures: Principles and Applications. London: John Wiley & Sons,.

Friction Welding Technical Group, W. I. (2010). Recommendations for Friction Welding Butt Joints in Metals for High Duty Application. Texas: Welding Institute,.

Granjon, H. (2014). Underwater Welding Soudage sous l'Eau: Proceedings of the International Conference Held at Trondheim, Norway, 27-28 June 1983, under the Auspices of the International Institute of Welding. Texas: Elsevier,.

Kazem Besharati Givi. (2017). Advances in Friction-Stir Welding and Processing. Chicago: Elsevier Science & Technology,.

Kim, J. K. (2017). A study on plastic flow of metal in inertia friction welding. Chicago: Cornell University.

Lovell, C. M. (Chiocago). Friction Welding for High Performance Aerospace Applications. TYexas: University of Birmingham.

Metallurgists, I. o. (2013). The Metallurgist and Materials Technologist. Paris: the University of Michigan.

Sahin, A. Z. (2016). Friction Welding: Thermal and Metallurgical Characteristics. London: Springer Science & Business Media,.

Singh, R. (2016). Applied Welding Engineering: Processes, Codes, and Standards. Auckland: Elsevier.

Vill', V. I. (2015). Friction welding of metals. Sydney: American Welding Society; trade distributor.

Wang, K. K. (2013). Friction Welding. Chicago: Welding Research Council.

Welding InstituteElsevier Science & Technology. (2010). Recommendations for Friction Welding Butt Joints in Metals for High Duty Applications. Texas: Elsevier Science & Technology, .

Yilbas, B. S. (2016). Friction Welding: Thermal and Metallurgical Characteristics. Auckland: Springer Science & Business Media.