Friction Stir Welding Process - Welding Technology

Friction Stir Welding (FSW)

Friction Stir Welding (FSW) is a solid state welding process in which a rotating tool is fed along the joint line between two workpieces. It is used to join two facing surfaces. During welding, heat is generated due to friction and the metal is mechanically stirring to from the weld seam. FSW differs from normal friction welding in such a way by generation friction heat by a separate wear resistant tool instead of the parts between them.

In friction stir welding process, the rotating tool consists of a cylindrical shoulder and a smaller probe or pin projecting beneath it, as shown in image, is used. During welding, the shoulder rubs against the top surfaces of two parts thereby developing friction heat and the pin or probe simultaneously generates additional heat by mechanical mixing of the metal along the butt surfaces. At the same time, the probe las been designed in order to perform the mixing perfectly. Typical tool profiles used in friction stir welding are shown in image. The heat is produced by the combination of friction and mixing. During the process, the metal will not melt but it softens.

The softening of metal takes place up to a highly plastic condition. When the tool moves forward along the joint, the leading surface of the rotating probe is forcing the metal around it. Then, the developed force forges the metal into a weld scam. So, the shoulder helps to limit the plasticized metal flowing around the probe.

Since workpiece does not melt, problems such as porosity, solidification cracks and thermal distortion are non-existent. It is extremely important to identify appropriate combinations of tool geometry, tool rotational speed and welding speed to ensure a proper material flow. The harder is the workpiece material, the stronger has to be the tool material because the tool experiences severe atmosphere of stress and temperature. The commercial use of FSW for hard alloys still remains intangible.

FSW process is used in aerospace, automotive, railway, and shipbuilding industries. The main applications are butt joints on large aluminium parts. Sometimes, steel, copper and titanium as well as polymers and composites are also joined by using FSW. image illustrates various. steps followed in performing friction stir welding which is already described earlier in detail.

Materials Used in Friction Stir Welding Process

Variety of metals can be joined by this process which cannot join by conventional processes.

Aluminum and Aluminum alloys
Brass
Cast iron
Ceramic
Copper
Lead
Bronze
Steel alloys
Magnesium and Magnesium alloys
Vanadium.

Microstructural Features

The solid-state nature of FSW process combined with its unusual tool and asymmetric nature results a highly characteristic microstructure. The microstructure can be divided into the following zones.

The stir zone is a region of heavily deformed material that roughly corresponds to the location of the pin during welding. The grains within the stir zone are roughly equiaxed and often an order of magnitude smaller than grains in the parent material.
A unique feature of the stir zone is the common occurrence of several concentric rings which is referred to an “onion-ring” structure.
The flow arm zone is on the upper surface of the weld and it consists of material which is dragged by the shoulder from the retreating side of the weld around the rear of the tool and deposited on the advancing side.
Thermo-Mechanically Affected Zone (TMAZ) occurs on either side of the stir zone. In this region, the strain and temperature are lower and the effect of welding on the microstructure is correspondingly smaller.

Welding Forces

During welding a number of forces act on the tool. They are as follows:

Downward force: This force is necessary to maintain the position of the tool at or below the material surface. Some friction stir welding machines operate under load control but in many cases, the vertical position of the tool is preset. So, the load varies during welding.

Traverse force: This force acts parallel to the tool motion and it is positive in the traverse direction. Since this force arises as a result of the resistance of the material to the motion of the tool, it decreases as the temperature of the material around the tool increases.

Lateral force: It acts perpendicular to the tool traverse direction and it is defined as positive towards the advancing side of the weld.

Torque: Torque is required to rotate the tool. The amount of torque depends on the downward force and friction coefficient (sliding friction) and the flow strength of the material in the surrounding region.

Flow of Material

During friction stir welding process, some materials rotate around the pin or probe for atleast one rotation and this material movement produces “onion-ring” structure in the stir zone. The material motion occurs by two processes:

Material on the advancing front side of-a weld enters into a zone that rotates and advances with the probe. This material is very highly deformed and sloughs off behind the probe to form arc-shaped features when viewed from top (down the tool axis). The copper entered the rotational zone around the pin where it was broken up into fragments. These fragments are only found in the arc shaped features of material behind the tool.
The lighter material comes from the retreating front side of the probe and it is dragged around to the rear of the tool and fills in the gaps between arcs of advancing side material. This material does not rotate around the probe and lower level of deformation resulted in a larger grain size.

Generation and Flow of Heat

For welding process, it is desirable to increase the travel speed and minimize the heat input as it increases productivity and possibly reduce the impact of welding on the mechanical properties of the weld. At the same time, it is necessary to ensure that the temperature around the tool is sufficiently high to permit adequate material flow and prevent flaws or tool damage.

When the traverse speed is increased, there is less time for heat to conduct ahead of the tool and the thermal gradients are larger. At some point, the speed will be so high that the material ahead of the tool will be too cold and the flow stress is too high to permit adequate material movement. It results flaws or tool fracture. If the “hot zone” is too large, then there is scope to increase the traverse speed. Therefore, the productivity also increases.

Friction Stir Welding Cycle

The welding cycle can be split into several stages.

Dwell: The material is preheated by a stationary rotating tool to achieve a sufficient temperature ahead of the tool to allow the traverse. This period may also include the plunge of the tool into the workpiece.

Transient heating: When the tool begins to move, there will be a transient period where the heat production and temperature around the tool alter in a complex manner until steady-state is reached.

Pseudo steady-state: Although fluctuations in heat generation occur, the thermal field around the tool remains effectively constant.

Post steady-state: Near the end of the weld, heat may “reflect” from the end of the plate leading to additional heating around the tool.

Heat generation during friction-stir welding arises from two main sources such as friction at the surface of the tool and deformation of the material around the tool.

Advantages, Limitations and Applications of Friction Stir Welding

Advantages

It ensures the good mechanical properties of the weld joint.
It avoids toxic fumes, warping, shielding issues and other problems associated with arc welding.
It permits less distortion or shrinkage on joints.
It provides good weld appearance.
Improved safety is obtained due to the absence of toxic fumes or the spatter of molten material.
There is no use of consumables.
It can be easily automated on simple milling machines due to lower setup costs and less training.
It can be operated in all positions as there is no weld pool.
Generally, it ensures a good weld appearance and minimal thickness.
It can be used for thinner materials with same joint strength.
It produces low environmental impact.
General performance and cost benefits are from switching from fusion to friction.
Simplicity of operation and simple equipment are obtained.
It requires less time to perform welding.
Only less surface impurities and oxide films are produced.
Heat affected zone is small as compared to conventional flash welding.
Dissimilar metals can be joined.
High quality weld can be obtained.
Pores are minimized.
There is no use of shielding gases.
There is no surface cleaning required.

Limitations

An exit hole remains the same after the tool is withdrawn from the work.
Large down forces are required with heavy-duty clamping necessary to hold the plates together.
It is less flexible than manual and arc processes.
It produces slower traverse rate than some fusion welding techniques although this may be offset if fewer welding passes are required.
Process is restricted to flat and angular butt welds.
It can be used only for joining small parts.
It requires heavy rigid machine due to high thrust pressure
In case of tube welding process, it becomes complicated.
In case of high carbon steels, it is difficult to remove flash.

Applications

Friction stir welding processes are mainly used in the following industries.
Automobile: Bimetallic engine valve, universal joint yoke, gear, hub etc.
Aerospace: Turbine blade joining, seamless joining etc.
Consumer: Hand tools and sports equipment.
Industrial machines: Spindles, tapers and tools.
Medical: Stainless steel joining of containers.
Mining/Drilling: Twist drills.
It is used in hydraulic equipment.
It is used in rolling stock for railways, general fabrication, robotics and computers.