What Is Friction Stir Welding

What Is Friction Stir Welding - Friction Stir Welding (FSW) is a method of joining solid-state that utilizes a consumable tool to join two opposing workpieces without melting the material of the workpiece. 

The heat is produced by friction between the tool's rotating mechanism and the material of the workpiece, that results in an area that is soft close to where the tool is. 

What Is Friction Stir Welding

As the tool moves along the joint line it mechanically mixes the two metal pieces and then heats the softened and hot metal due to the mechanical pressure that is created by the tool similar to joining clay or dough. 

It is typically used on extruded or wrought aluminum and is particularly useful for structures that require extremely strong weld strength. 

FSW can be used to join aluminum alloys and copper alloys the titanium alloy, mild steel magnesium alloys and stainless steel. 

Recently, it was successfully employed in welding of polymers. Additionally, the welding of metals that are not compatible such as aluminum and magnesium alloys has been recently accomplished with FSW. 

The application of FSW is found in the modern shipbuilding, train and aerospace projects.

It was created and tested in The Welding Institute (TWI) in the UK in the month of December in 1991. TWI was the first to patent the method, with the first of which was the most descriptive.


Principle of operation

FSW is carried out using an elongated tool that rotates and is fitted with a an engraved pin (also called as a probe) with a diameter less than the shoulder's diameter. 

The tool is inserted into an elongated butt joint between two workpieces that are clamped, until the probe penetrates the workpiece and the shoulder touches the workpiece's surface. 

The probe is slightly smaller than the required weld depth with the tool shoulder resting on top of the work surface. 

After a brief time of dwell the tool is then moved ahead across the joint, at the welding speed that is pre-set.

Frictional heat is created in the contact between the tool that is wear-resistant and workpieces. 

The heat, in conjunction with the heat produced through the mixing process and the adiabatic heating within the material, causes the heated materials to soften, without melting. 

When the probe is moved forward, a specific shape on the probe pulls the plasticized material from the front surface to the rear in which the strong forces aid in the formation of a solidification of the welding.

The process of the tool moving across the weld line within a tubular shaft that is plasticised metal causes massive deformation in the solid state that involves the recrystallisation of the material base.


Micro-structural characteristics

Its solid state nature in the FSW process, in conjunction with its unique tool shape and asymmetrical speed profile, result in an extremely unique micro-structure. 

The micro-structure is divided into the following zones:

Stir zone (also called the dynamically recrystallised area) is a zone of material that has been deformed to a degree which roughly corresponds to the position of the welding pin. 

The grains in the stir zone are roughly equivalent and are often less than the grains of the main material.

One unique characteristic of the zone of stir is the frequent presence of concentric rings. 

This has been described as the "onion-ring" design. 

The exact origin of these rings have not been established with certainty, but the variations in particle density or grain size, as well as texture have been proposed.

The flow arm area is located on the upper side of the weld. 

It is made up of a material pulled by the shoulder away from the side that is retreating from the weld to the back of the tool, then placed on the side that is moving.

The zone that is affected by thermomechanical forces (TMAZ) is found on either side of the zone. 

In this zone, the temperature and strain are less and the impact from welding to the structure of the material is less. 

In contrast to the stir zone the micro-structure can be recognized as being similar to the material of the parent, though significant deformed and rotated. 

Although TMAZ technically is used to describe the entire region that has been deformed however, it is commonly utilized to refer to any part that is not included in those terms, stir zone or flow arm.

The zone that is affected by heat (HAZ) is the common element in the majority of welding processes. 

It is evident by its title, this area is subjected to a heat cycle but it does not undergo deformation in the course of welding. 

The temperatures are less than those of the TMAZ however, they could still be significant when the micro-structure is unstable. 

In reality, in aged-hardened aluminum alloys this region usually has the weakest mechanical properties.


Advantages and disadvantages

Solid-state properties of FSW provides a number of advantages over other fusion welding techniques because the problems that arise from cooling in fluid phase problems are eliminated. 

Porosity issues as well as redistribution of the solute, cracking, and liquation cracking do not occur in FSW. 

It is generally accepted that FSW was found to have a very low amount of flaws and is resistant to changes in the parameters and the materials.

However, FSW is associated with several unique flaws in the event that it's not performed correctly.

Insufficient weld temperatures because of low rotational speed or large traverse speed, for instance, means that the weld materials are not able to handle the massive deformation caused by welding. 

This could result in tunnel-like cracks that run through the weld. 

They can appear on the surface or beneath the surface. 

The low temperatures can also hinder the forging ability of the tool and lessen the durability of the bond between the materials from both sides that the weld is. 

The contact of light between the two materials can be attributed to the term "kissing bonds". 

This is a particularly alarming defect as it can be difficult to identify by nondestructive methods like ultrasonic or X-ray tests. 

If the pin isn't long enough, or the tool is able to rise out of the plate, then the connection to the bottom of the weld could not be harmed and formed through the device, which could result in the defect of not having penetration. 

This is basically an abrasion in the material which could be a cause fractures due to fatigue.

There are a variety of advantages of FSW over conventional fusion welding methods have been identified.

Good mechanical properties in the welded condition.

Increased safety because of lack of fumes that can cause harm, or the speading of molten materials.

No consumables - A threaded pin constructed of standard tools steel e.g. Hardened H13 is able to weld up to 1 km (0.62 miles) of aluminum, and there is no gas shield or filler is needed for aluminium.

It is easy to automatize simple milling machines - cost savings on setup and education.

The ability to operate in all directions (horizontal and vertical) because there isn't any weld pool.

Welds that look good generally have low thickness over-matching or under-matching, thereby cutting down on the expense of grinding after welding.

Use thinner materials with the similar joint strength.

Environmental impact is minimal.

General performance and cost savings when switching from fusion to friction.

However, some negatives of the method have been identified:

Exit hole is left after the tool is removed.

The force of the down is huge, which is why the clamping system that is heavy-duty to keep the plates.

Flexible than arc and manual processes (difficulties with variations in thickness and non-linear welding).

Sometimes, traverse rates are slower than other fusion welding methods, but this can be compensated for if there are fewer welding passes required.


The most important welding parameters


Tool design

Design of the device is a crucial element, as a properly designed tool will enhance both your quality welding and also the maximum speed at which welding can be done.

It is essential that the material used for welding is strong, tough and wear-resistant in the heat of welding. 

Furthermore, it should possess an excellent oxidation resistance as well as low thermal conductivity in order to reduce damage to thermal and heat to the machinery further down to the transmission. 

Tool steel that has been hot-worked, such as AISI H13 is suitable for welding aluminum alloys with thicknesses of 0.5-50 millimeters. 

However, higher-end tool materials are required for more demanding uses like highly abrasive metallic matrix composites, or more melting-point-rich materials like titanium or steel.

Improved design of tools have led to significant improvements in quality and productivity. 

TWI has created tools designed to improve the penetration depth, and consequently increase the thickness of the plates which can be successfully weld. 

A good illustration can be "whorl," a "whorl" design which makes use of tapered pins with the ability to re-entrant, or with a variable-pitch thread to increase the downwards flow of the material. 

There are other designs, including those of the Tri flute as well as the Trivex series. 

The Tri flute design features a sophisticated system that includes three tapering threaded flutes that are designed to enhance the motion of materials in the vicinity of the device. 

The Trivex tools employ a more simple pin, which is non-cyclinderical and have been shown to decrease the force acting on the tool when welding.

Most tools feature an elongated shoulder that acts as an escape space for the material that is displaced by the pin. 

It also keeps the material from expanding from shoulders' sides, and ensures downward pressure and an excellent forging of the materials behind the tool. 

The Tri flute tool employs a different technique that features a number of concentric grooves that are machined into the surface. 

They are designed to increase the motion of the material in the upper layer of the weld.

Commercial applications that are widely used in the friction stir welding processes for steels as well as other hard alloys, such as titanium-based alloys require creation of durable and cost-effective tools.

Selection of materials, design, and cost are key factors in the search of commercially-effective tools that can be used to weld tough materials. 

The research continues to comprehend the effect of the materials' composition, structure, characteristics and the geometry of their properties regarding their efficiency, endurance, and cost.


Tool rotation speeds and traverse speeds

There are two speeds for the tool that must be taken into consideration when doing friction-stir welding: how fast the tool turns and how fast it travels through the interface. 

Both of these parameters are of great importance and should be selected carefully to ensure a smooth and efficient welding process. 

The relationship between the traverse speed, the speed at which the weld is made and the amount of heat that is absorbed when welding is a bit nebulous however, in general it can be stated that increasing the speed of the rotation or reducing the speed of traverse will result in a more hot weld. 

To ensure an effective weld, it is essential that the surrounding material tool is sufficiently hot to permit the wide plastic flow needed and reduce the forces that exert pressure upon the device. 

In the event that the tool's material gets cold there could be voids or imperfections could be found in the stir zone , and in extreme instances, the tool could break.

The excessively high temperature input in contrast could be harmful to the final characteristics for the weld. In theory, it could result in defects due the melting-point phase liquation (similar to cracks caused by liquation occurring in fusion welding). 

The competing demands are the basis for the idea of the concept of a "processing window" which is the spectrum of parameters used in processing, viz. the speed of traverse and rotation which will result in the best quality weld. 

In this time frame, the weld will be sufficiently high temperature input to provide sufficient plasticity of the material, but not too high that weld properties become severely damaged.


The tilt of the tool and the depth to which it plunges

The depth of the plunge is defined as the length of the lowest point on the shoulder that is below the level of the plate. 

It is believed to be an important element to ensure the quality of the weld. 

By lowering the shoulder to beneath the surface of the plate increases the pressure beneath the tool, which helps to ensure an adequate forging process of the material to in the back. 

A tilt of the tool between 4 or 5 degrees, so that the back side of the machine is higher that the front has also been shown to aid in the forging process. 

The plunge depth must be properly set to ensure that the required downward pressure is reached as well as to make sure that the tool completely gets into the weld. 

Due to the heavy loads that are required by the welding equipment, it is possible for the device to be able to deflect, reducing the depth of the plunge compared to the set-up that could cause defects on the surface of the weld. 

However an excessive depth can cause the pin to rub against the surface of the backing plate or an undermatch in the thickness of the weld compared with the material used for base. 

Variable-load welding has been designed to automatically adjust for adjustments in the tool's displacement and TWI have tested the use of a roller system to maintain the tool's position above the weld surface.


Forces of welding

While welding range of forces are able to act on the tool:

A downward force is required to keep the position of the tool below the surface of the metal. 

Certain friction-stir welding equipment operate using load-control, however in most cases, the vertical location of the tool is set, and the load can change when welding.

The force of the traverse is in the opposite direction to the tool's motion which is positive when it's in direction of the tool. 

Because this force is a due to its resistance to the movements of the instrument, it can be expected that the force will decrease as temperature of the material surrounding the tool increases.

The force lateral can be in a perpendicular direction, and is described here as positive to the side that is advancing of the weld.

Torque is the force that drives it, and the magnitude of which will be determined by the force at the bottom and the its coefficient (sliding friction) or the flow strength of the material within the area surrounding (stiction).

To prevent the tool from breaking and to reduce the wear and tear that is caused to the machinery and the tool The welding process is adjusted to ensure that the forces exerted against the device are as minimal as they can be, and that abrupt changes are kept to a minimum.

To find the optimal mix of weld parameters it's likely that compromises must be found, since circumstances that favor low forces (e.g. high heat input, slow speed of travel) could be unsuitable from the standpoint of efficiency and weld properties.


Material flow

The initial research on the mechanism of flow of materials around the tool utilized inserts of a different material that had a distinct appearance to the standard material when examined under a microscope to find out the direction in which material moved when the tool moved. 

The results were interpreted as being a representation of extrusion in situ, where the tool, the backing plate and the base material are what is known as the "extrusion chamber" that the hot, melted material is pushed. 

In this scenario, the tool's rotation does not draw any material towards the probe's front and instead the material moves in ahead of the probe and moves across to either side. 

When the material is finished passing into the tool, pressure created on the "die" causes the material to be pushed back together. 

Then, the it is then consolidated when the back of the shoulder of the tool moves over the top and the massive down force is used to repress the material.

In recent times, a different theory has been proposed that suggests substantial material movement in specific locations. 

The theory suggests that some material can move around the probe for at most one rotation in the probe, and it is this motion that results in an "onion-ring" structure within the stirring zone. The researchers employed a combination of copper strip inserts made of thin material as well as the "frozen pin" techniquein which the device is quickly stopped from moving.

They concluded that motion in the material takes place through two steps:

The material on the advancing side of a welding process enters an area that is rotated and moves along with the profiled probe.

The material was extremely deformed and then sloughs away behind the pin, forming an arc when observed at from the top (i.e. along the tool's axis). 

It was discovered that the copper was found to enter the zone of rotation near the pin, from where it was split into pieces. 

The fragments were discovered in the arc-shaped areas of the metal behind the tool.

The lighter material was sourced from the side that was retreating that was in front of the pin. 

It was moved towards the back of the tool. 

It filled the gaps between the arcs of the moving side material. 

This material didn't spin around the pin and the less deformation led to a greater grain size.

The main benefit for this theory is the fact that it gives an plausible explanation for the formation in the structure of onion rings.

The marker method for friction stir welding gives information regarding the initial and final locations of the marker within the weld material. 

The flow of the material is then calculated from these locations. 

The detailed material flow field that occurs during friction stir welding could be calculated using the mathematical considerations that are based on basic science-based principles. 

Material flow calculations are frequently employed in a variety of engineering applications. 

Calculating the material flow fields during friction stir welding can be accomplished using both extensive numerical simulations or with simple but powerful analytical equations. 

The complete models used in the calculation of the material flow fields can also provide crucial details such as the shape of the stir zone as well as the torque that is applied to the tool. 

Simulations using numerical methods have demonstrated the ability to accurately forecast the outcomes of studies with markers as well as the geometry of the stir zone that is observed during friction stir welding tests.


The generation of heat and its flow

For any welding procedure the process is generally, recommended to speed up the process and minimize the amount of heat input to increase efficiency and lessen the impact from welding to the properties mechanical of the welding. 

In the same way it is essential to make sure that the temperature surrounding the tool is high enough to allow adequate flow of the material and to prevent flaws and tool damage.

If the speed of traverse increases, for a given input of heat it is more chance for it to move ahead of the tool as well as the gradients of thermal become greater. 

At some point, the speed will become so fast that the material that is ahead of the tool is too cold and the stress on the flow is too high, preventing adequate material movement. 

This can result in tool fractures or flaws. 

In the event that there is a concern that the "hot zones" is too wide it is possible to improve the speed of traverse and thus efficiency.

The welding process may be broken down into various stages, where the flow of heat and thermal profile are different Dwell. 

The material is heated up through a rotating, stationary tool in order to reach a suitable temperature prior to the tool that it can allow for the tool to traverse. 

This could also involve that the tool is pushed into the material.

Transient heating.

When the tool starts to move it will experience an in-between period when the production of heat and the temperature around the tool will change in a complex way until a stable condition is attained.

Although fluctuations in the production of heat may occur, the heat field surrounding the tool is in a steady state, at the very minimum on a macroscale.

Stable state post. At the end of the welding it is possible for heat to "reflect" away from the edge of the plate, which can lead to further heating around the tool.

The generation of heat in friction-stir welding is a result of two primary sources: friction on the edges of the tools as well as its deformation materials around the tool. 

The source of heat is typically thought to occur mostly beneath the shoulder because of its greater surface area and to be equivalent to the force needed to counteract the force of contact in the area between the tool's surface and workpiece.

One of the biggest challenges when applying the equations is finding appropriate parameters for the friction coefficient, or the herring stress interfacial. 

The parameters under the tool are extremely and difficult to determine. 

So far they have been employed in the form of "fitting parameters" which is where the model draws on the measured temperature data to create an acceptable simulated thermal field. 

Although this method is effective in the creation of models for processes to anticipate, for example residual stresses, it's less effective for providing insights into the actual process.


Applications

The FSW procedure was first licensed by TWI in a majority of industrialized nations and is licensed to more than 183 users. 

Friction stir welding as well as its variations, such as spot welding with friction stir and processing using friction are utilized in various industrial applications like offshore and shipbuilding automobile rolling stock for general fabrication, railways, robotics, as well as computers.


Shipbuilding and offshore

Two Scandinavian aluminum extrusion businesses have been the very first companies to use FSW commercially to the manufacturing of freezers for fish in Sapa in 1996. Then, they also manufactured deck panels and landing platforms for helicopters in Marine Aluminum Aanensen. 

Marine Aluminum Aanensen subsequently merged with Hydro Aluminum Maritime, which later became Hydro Marine Aluminum. 

A few of these freezer panels are manufactured in the UK by Riftec along with Bayards. 

In 1997, two-dimensional friction-stitch welds on the bow section that is hydrodynamically flared on the bottom of Ocean Viewer ship The Boss were produced at Research Foundation Institute with the first portable FSW machine. 

This Super Liner Ogasawara at Mitsui Engineering and Shipbuilding is the largest friction stir-welded ship to date. 

Its Sea Fighter of Nichols Bros and the Freedom-class Littoral Combat Ships include prefabricated panels made by Fabricators of FSW Advanced Technology and Friction Stir Link, Inc. respectively.

Houbei's missile boat is Friction Stir Welded rocket launch containers from China Friction Stir Centre.

HMNZS Rotoiti in New Zealand includes FSW panels manufactured by Donovans with a milling machine.

Numerous companies use FSW on armor plating on amphibious assault vessels.


Aerospace

United Launch Alliance applies FSW to the Delta II, Delta IV and Atlas V expendable launch vehicles and the first one equipped with a friction stir welded interstage component was introduced in the year 1999.

The technique was also utilized to construct Space Shuttle's external tank.

Space Shuttle external tank, for Ares I and for the Orion Crew Vehicle test article at NASA and SpaceX, as well as Falcon 1 and Falcon 9 rockets from SpaceX.

The toe nails that support the ramps of the Boeing C-17 Globemaster III cargo aircraft manufactured by Advanced Joining Technologies and the beams that protect cargo for The Boeing 747 Large Cargo Freighter were the first commercially manufactured parts for aircrafts.

The FAA-approved wings and fuselage panels from Eclipse 500 Eclipse 500 aircraft were made by Eclipse Aviation, and this firm delivered 259 friction stir-welded business jets prior to when they were compelled to Chapter 7 liquidation.

The floor panels of the Airbus A400M military aircraft are being manufactured through Pfalz Flugzeugwerke and Embraer used FSW on Legacy 450 and 500 Jets.

Legacy 450 and 500 Jets Friction stir welding is also employed to make fuselage panels for aircrafts like the Airbus A380.

The BROTJE Automation system uses friction stir welding to make manufacturing gantry machines designed for aerospace and different industrial processes.


Automotive

Aluminum engine cradles, as well as suspension struts to stretch Lincoln Town Cars were the first automotive components that were made by friction stir welding at Tower Automotive, who use this process for an engine tunnel for Ford GT. 

A spin-off of this business is known as Friction Stir Link, Inc. and has been able to successfully utilize FSW technology. 

FSW method, e.g. to make the trailer flatbed "Revolution" that is part of Fontaine Trailers. 

For the flatbed trailer "Revolution" of Fontaine Trailers. Japan FSW is used on suspension struts in Showa Denko and for joining of aluminum sheets with galvanized steel brackets to form the boot (trunk) lid of the Mazda MX-5. 

Friction stir spot welding has been extensively used to join the bonnet (hood) and the rear door of the Mazda RX-8 and the boot lid of the Toyota Prius. 

Wheels are friction stir-welded in Simmons Wheels, UT Alloy Works and Fundo. 

The rear seats of Volvo V70 Volvo V70 are friction stir welding at Sapa and HVAC pistons are welded in Halla Climate Control and exhaust gas recirculation coolers are made at Pierburg. 

Blanks that are custom-made for the customer are friction stir welded to an Audi R8 at Riftec. 

In the B column of the Audi R8 Spider is friction stir welded using two extrusions manufactured by Hammerer Aluminum Industries in Austria.


Railways

Since 1997, roof panels have been constructed from aluminum extrusions by Hydro Marine Aluminum, using the custom 25 m FSW machine e.g. in the case of DSB Class SA-SD trains from Alstom LHB.

Side and roof panels that are curved for the Victoria line trains of the London Underground, side panels for Bombardier Electro star trains at Sapa Group and side panels for Alstom's British Rail Class 390 Pendolino trains are produced in the Sapa Group. 

Japanese Express and commuter trains,60 as well as British Rail Class 395 trains are friction stir-welded by Hitachi and Kawasaki, respectively.

Kawasaki uses spot welding using friction stir to roof panels as well as Sumitomo Light Metal produces Shinkansen floor panels. 

Innovative floor panels made of FSW are manufactured in Hammerer Aluminum Industries in Austria for the Stadler Kiss double decker rail cars to achieve an internal height of 2 meters on both floors as well as for the car bodies that are being built for the Wuppertal Suspension Railway.

Heat sinks to cool the engines with high-power electronics are produced by Sykatek, EBG, Austerlitz Electronics, Euro Composite, Sapa and Rapid Technic, and are the most popular application for FSW because of its superior heat transfer.


Fabrication

Cathode sheets and facade panels are friction stir-welded at AMAG in addition to Hammerer Aluminum Industries, including friction stir lap welding of copper to aluminum. 

Bizerba Slicers for meat, HVAC systems from Okolufter, and Siemens vacuum vessels x-ray are friction stir welded by Riftec. 

Vacuum vessels and valves are produced from FSW in Japanese as well as Swiss companies. 

FSW is also used in the packaging of radioactive waste SKB in copper canisters that are 50mm thick.

Pressure vessels made of o1 m semispherical forgings that are 38.1 millimeters thick aluminum alloy 2219 from Advanced Joining Technologies and Lawrence Livermore Nat Lab. 

Friction Stir processing is used to make propellers of ships by Friction Stir Link, Inc. and to hunting knives manufactured by Diamond Blade. 

Bosch employs it in Worcester to make heat exchangers.


Robotics

KUKA Robot Group has adapted its KR500-3MT heavy-duty robotic system to friction stir welding with the DeltaN FS software. 

The robot made its debut official appearance at the Euro BLECH exhibition in November 2012.


Personal computers

Apple used friction stir welding to the 2012 iMac to securely connect the bottom and the back of the device.


Joining of Aluminum 3D Printing Material

FSW has been demonstrated to be employed in conjunction with other ways to join metal 3D printing material. 

Utilizing the correct FSW tools and the correct parameters, a solid and clean weld may be created to join the metal 3D printer materials. 

In addition being a good tool, FSW tools should be more durable than the materials they need to be weld. 

The most critical parameters of FSW are the probe's rotation speeds, traverse speed as well as spindle tilt angle and the depth of the target. 

The efficiency of the weld joint in FSW on 3D printing metal could reach as high as 83.3 percent when compared to its base materials ' strength.

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