optimization of process in friction stir welding

Introduction 
Agitated friction welding (FSW) is a solid state bonding process that uses a non-consumable tool to join two work pieces that face without melting the workpiece material. The heat is generated by friction between the rotating tool and the workpiece material, which leads to a smoothed region near the FSW tool. While the tool is traversed along the joining line, it mechanically intermixes the two pieces of metal, and forges the hot and smoothed metal by the mechanical pressure, which is applied by the tool, as well as joining the clay, or mass. It is mainly used in forged or extruded aluminum and particularly for structures which require a very high weld resistance. It was invented and tested experimentally at the Welding Institute (TWI) in the United Kingdom in December 1991. TWI held patents in the process, The most descriptive.

Operating principle
Schematic diagram of the FSW process: (A) Two discrete metal parts touched together, together with the tool (with a probe). (B) The progress of the tool through the joint, also showing the welding zone and the region affected by the tool shoulder.
A rotating cylindrical tool with a profiled probe is fed into a stop link between two clamped workpieces, until the rim, which has a larger diameter than the pin, touches the surface of the workpieces. The probe is slightly shorter than the required weld depth, with the tool shoulder on the work surface. After a short residence time, the tool moves forward along the attachment line to the pre-set welding speed.

Heat of friction is generated between the wear-resistant tool and the workpieces. This heat, together with that generated by the mechanical mixing process and the adiabatic heat inside the material, cause the agitated materials to melt without melting. As the tool moves forward, a special profile in the probe forces plastified material from the front face to the rear, where high forces assist in a forged weld consolidation.

This process of the tool passing through the welding line in a plasticized tubular metal tube results in a severe deformation of the solid state which involves a dynamic recrystallization of the base material.

Microstructural features
• The stirring zone (also nugget, dynamically recrystallized zone) is a region of heavily deformed material that approximately corresponds to the location of the pin during welding. The grains within the stirring zone are approximately equiaxial and often an order of magnitude less than the grains in the parent material. A unique characteristic of the stirring zone is the common appearance of several concentric rings which has been termed "onion" structure. The exact origin of these rings has not been firmly established, although variations in particle number density, grain size and texture have been suggested.
• The flow arm region is on the top surface of the weld and consists of material that is drawn by the shoulder from the weld withdrawal side, around the back of the tool and deposited on the advancing side.
• The thermo-mechanically affected zone (TMAZ) occurs on both sides of the shaking zone. In this region, the deformation and temperature are lower and the effect of the weld on the microstructure is correspondingly smaller. Unlike the agitation zone, the microstructure is recognizably that of the parent material, although significantly deformed and rotated. Although the term TMAZ refers technically to the entire deformed region, it is often used to describe any regions not covered by the terms "agitation zone" and "flow arm".
• The area affected by heat (HAZ) is common to all welding processes. As indicated by the name, this region is subjected to a thermal cycle but does not deform during welding. The temperatures are lower than those of the TMAZ but can have a significant effect if the microstructure is thermally unstable. In fact, in age-hardened aluminum alloys this region commonly exhibits the poorest mechanical properties.

Advantages and limitations
The solid state nature of FSW leads to several advantages over the fusion welding methods since the problems associated with cooling from the liquid phase are avoided. Issues such as porosity, solute redistribution, solidification cracking and leachate cracking do not arise during the FSW. In general, FSW has been found to produce a low concentration of defects and is very tolerant of variations in parameters and materials.
Advantages

·        Good mechanical properties under welding conditions

·        Improved safety due to the absence of toxic vapors or splashes of molten material.

·        No consumables - A threaded pin made of conventional tool steel, eg hardened H13, can weld over 1 km (0.62 mi) of aluminum, and no filler or gas shield is required for aluminum.

·        Easily automated in simple milling machines - lower installation costs and less training.

·        It can work in all positions (horizontal, vertical, etc.), since there is no welding pool.

·        Generally good welding appearance and minimum under / over-thickness thickness, thus reducing the need for costly machining after welding.

·        You can use thinner materials with the same joint strength.

·        Low environmental impact.
·        Overall performance and cost benefits of switching from fusion to friction.
Disadvantages

• Exit the left hole when the tool is removed.

• Large downhill load required for heavy duty work required to hold boards together.

• Less flexible than manual and arc processes (difficulties with thickness variations and non-linear welds).
• Scroll speed is often slower than some fusion welding techniques, although this can be compensated for if less welding passes are required.
Welding Forces

• A downward force is required 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 and therefore the load will vary during welding.

• The displacement force acts parallel to the movement of the tool and is positive in the transverse direction. Since this force arises as a result of the resistance of the material to the movement of the tool, this force could be expected to decrease as the temperature of the material around the tool increases.

• The lateral force can act perpendicular to the direction of travel of the tool and is defined here as positive toward the welding feed side.
• A torque is required to rotate the tool, the amount of which depends on the lowering force and the coefficient of friction (slip friction) and / or the material flow resistance in the surrounding region (stiction).