Chapter 1Introduction1.1PREFACE

In this chapter an introduction about the friction stir welding process, Principle of Operation, microstructure features, submerged friction stir welding and aluminium alloys will be discussed.

1.2FRICTION STIR WELDING Friction-stir welding (FSW) is a solid-state joining process that uses a third body tool to joining aluminium alloys for aerospace, marine automotive and many other applications of commercial importance. Heat is generated between the tool and material which leads to a very soft region near the FSW tool. It then mechanically intermixes the two pieces of metal at the place of the joint, then the softened metal can be joined using mechanical pressure. Important process parameters that control the quality of the weld are a) rotation speed (1200&1400rpm) b) transverse speed (40&60mm/min) c) tool angle 900 and these process parameters were optimized to obtain defect free welded joints. The tool geometry was carefully chosen and fabricated to have a nearly welded interface (threaded cylindrical) pin profile. It is primarily used on aluminuim and most often on extruded aluminum, and on structures which need superior weld strength without a post weld heat treatment.

1.2.1 PRINCIPLE OF OPERATION A constantly rotated non consumable threaded cylindrical-shouldered tool with a profiled probe is transversely fed at a constant rate of 40&60mm/min into a butt joint between two clamped pieces of aluminium material. The probe is slightly shorter (0.4mm) than the weld depth required, with the tool shoulder riding atop the work surface.

Frictional heat is generated between the non consumable tool shoulder and the work pieces. The welding tool is rotated along its longitudinal axis in a conventional milling machine and the work piece material is firmly held in the fixture. The shoulder is passed pressed against the surface of the metal generating frictional heat while containing the softened weld metal. The pin causes the plastic flow in the work piece material on either side of the butt joint. This heat, along with that

generated by the mechanical mixing process and the adiabatic heat within the material, cause the stirred materials to soften without melting. As the pin is moved forward, a special profile on its leading face forces plasticised material to the rear where clamping force assists in a forged consolidation of the weld.

1.2.2 MICROSTRUCTURAL FEATURES

The Stir Zone (SZ) also known as nugget zone or dynamically recrystallised zone.It 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 the grains in the parent material. A unique feature of the stir zone is the common occurrence of several concentric rings which has been referred to as an "onion-ring" structure. The precise origin of these rings has not been firmly established, although variations in particle number density, grain size and texture have all been suggested.

The Thermo Mechanically Affected Zone (TMAZ) is the region in which the FSW ttool has plastically deformed the material, and the heat from the process has also exerted some influence on the material. It 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. Although the term TMAZ technically refers to the entire deformed region it is often used to describe any region not already covered by the terms stir zone.

The Heat Affected Zone (HAZ) is common to all welding processes. HAZ is the region, which lies closer to the weld-center; the material has experienced a thermal cycle that has modified the microstructure and the mechanical properties. However, no plastic deformation occurs in this area. The temperatures are lower than those in the TMAZ but may still have a significant effect if the microstructure is thermally unstable.

1.3FRICTION STIR WELDING EQUIPMENTS

The major equipment’s used in FSW are:

1.3.1 TOOL

The tool used in FSW is threaded cylindrical in shape with concave area with a pin which is coaxial with the axis of rotation of the tool. The tool consists of two main parts i.e. a pin and a shoulder. The function of the tool is mixing of the work piece material and heating by frictional hear and hence material selection of tool is very important factor. The good tool material should feature the following properties:

Good wear resistance. High hardness elevated temperatures and should retain the hardness for

an extended period. Good static and dynamic properties at welded temperature. Mechineability and fracture toughness

1.3.2 MACHINE

A conventional vertical milling machine can be used to carry out the FSW process. The machine must have the ability to apply significant pressure into the work piece, should offer wide range of tool rotation and feed rate speeds, provides enough space for its working table to holding the welding assembly and rigidly during the welding operation.

1.3.3 FIXTURE

The workpiece to be welded have to be securely clamped to prevent the joint phase from being forced apart. Fixture provides the medium in which work piece are rigidly clamped. Special types of fixture can be designed as per the requirements. Workpiece are clamped on to fixture which is further mounted on the vertical milling machine.

1.4 PROCESS PARAMETERS IN FSW:

Some of the critical parameters of FSW

1.4.1 TOOL ROTATION SPEED

One of the crucial parameters that have significant impact on the quality of the weld in FSW is tool rotation speed. The speed at which fixed tool rotates in vertical milling machine is referred to as tool rotation speed. Clockwise rotation can be viewed from above the tool, looking down onto the workpiece. Vertical milling machine offers different tool rotation speeds ranging from very low level to quite high level. Appropriate selection of the tool rotation speed is very much necessary to cause extensive plastic flow of the material to be welded.

1.4.2 TRAVELLING FEED/ FEED RATE

In vertical milling machine, tool is fixed but only rotates at its position and workpieces that are clamped on to fixture travels. By travelling feed/feed rate, we mean the speed with which workpieces that are clamped onto fixture travels. It may also be termed as welding speed i.e. speed at which welding occurs. The relation between travelling feed/feed rate and the generation of heat during welding is complex but it is generally said that raising the tool rotation speed and reducing the feed rate will result in increasing the temperature at weld surrounding which intern leads to production of successful weld.

1.4.3 TOOL TIP SHAPE

The shape of the tool is another very important factor that can lead to improvement in both the quality of the weld and maximum possible welding speed. Optimizing tool shape for producing more heat and achieving more efficient stirring offers two main advantages: (i) Improved breaking and mixing of oxide layer. (ii) More efficient heat generation, yielding higher welding speeds and, of course, enhanced quality. Different

tool tip shapes as used in FSW can be cylindrical, square, triangular, and trapezoidal.

1.4.4 TOOL TIP PLUNGE DEPTH

The part of the tool, which paniterates in workpiece during welding is referred to as the probe or tip of the tool. The plunge depth needs to be correctly set, both to ensure that the necessary downward pressure is achieved and to ensure that the tool fully penetrates into the weld region. Plunge depth should be always less than the thickness of the workpiece.

1.4.5 SHOULDER DIAMETER

The part of the tool, which is pressed onto the surface of the workpiece during welding is referred to as ‘shoulder’. Tool shoulders are designed to produce heat to surface and subsurface region of the workpiece. So one of the most important parameters of the shoulder is diameter because it has significant impact on the amount of frictional heat generated. Greater shoulder diameter increases the pressure force and the weld shape changes which depreciates the mechanical properties of the welds. So the choice of shoulder diameter requires due consideration.

1.5 SUBMERGED FRICTION STIR WELDING

In recent years, under water friction stir welding has come into existence. Underwater Friction Stir Welding (UFSW) is a solid state welding process. It takes place at temperatures below the melting point of the material. In this method, a non-consumable rotating tool runs over the substrates under the water which results into the generation of heat due to friction. This leads to plastic deformation and softening of substrates near the tool area. Then, the substrates can be easily joined. It also minimizes various welding defects like porosity, shrinkage, splatter, embrittlement, solidification, cracking etc. UFSW is one of the advanced welding techniques in the present era.This process doesn’t require shielding gas and filler material for welding which make this process cheaper. It consumes less energy and gives improved mechanical properties. It also provides well defined variation in grain size between different zones along the high quality weld joint produced.

1.6 INTRODUCTION TO ALUMINUM ALLOYS

Aluminum alloys are alloys in which aluminum (Al) is the predominant metal. The typical alloying elements are copper, magnesium, manganese, silicon, tin and zinc. There are two principal classifications, namely casting alloys and wrought alloys, both of which are further subdivided into the categories heat-treatable and non-heat-treatable. About 85% of aluminum is used for wrought products, for example rolled plate, foils and extrusions. Cast aluminum alloys yield cost-effective products due to the low melting point, although they generally have lower tensile strengths than wrought alloys. The most important cast aluminum alloy system is Al–Si, where the high levels of silicon (4.0–13%) contribute to give good casting characteristics. Aluminum alloys are widely used in engineering structures and components where light weight or corrosion resistance is required.

Alloys composed mostly of aluminum have been very important in aerospace manufacturing since the introduction of metal skinned aircraft. Aluminum-magnesium alloys are both lighter than other aluminum alloys and much less flammable than alloys that contain a very high percentage of magnesium.

Aluminum alloy surfaces will formulate a white, protective layer of corrosion aluminum oxide if left unprotected by anodizing and/or correct painting procedures. In a wet environment, galvanic corrosion can occur when an aluminum alloy is placed in electrical contact with other metals with more negative corrosion potentials than aluminum, and an electrolyte is present that allows ion exchange. Referred to as dissimilar metal corrosion this process can occur as exfoliation or interagranular corrosion. Aluminum alloys can be improperly heat treated. This causes internal element separation and the metal corrodes from the inside out. Aircraft mechanics deal daily with aluminum alloy corrosion.

Aluminum alloy compositions are registered with The Aluminum Association. Many organizations publish more specific standards for the manufacture of aluminum alloy, including the Society of Automotive Engineers standards organization, specifically its aerospace standards subgroups, and ASTM International

1.6.1 CLASSIFICATION OF ALUMINUM ALLOYS

1.6.1.1 1XX.X: Controlled unalloyed composition.

1.6.1.2 2XX.X: Aluminum alloys containing copper as the major alloying element.

1.6.1.3 3XX.X: Aluminum-silicon alloys are also containing magnesium or copper.

1.6.1.4 4XX.X: Binary aluminum-silicon alloys.

1.6.1.5 5XX.X: Aluminum alloys containing magnesium as the major alloying element.

1.6.1.6 6XX.X: Currently used.

1.6.1.7 7XX.X: Aluminum alloys containing zinc as the major alloying element, usually also containing Additions of either copper, magnesium, chromium, manganese or combination of these Elements.

1.6.1.8 8XX.X: Aluminum alloys containing tin as the major alloying element.

1.6.1.9 9XX.X: Currently unused.

1.7 SUMMARY

The final motivation of this project is to weld the aluminium alloy with the help of submerged friction stir welding and find out the mechanical characteristics and study the microstructure features of the aluminium alloy and compare the results with friction stir welding.