CHAPTER 1 INTRODUCTION

Transcription

1 1 CHAPTER 1 INTRODUCTION In todays modern world there are different welding techniques to join metals. They range from the conventional oxyacetylene torch welding to laser welding (Appendix 1). The two types of welding are fusion welding and pressure welding. The fusion welding process involves the bonding of the metal in the molten stage and may need a filler material if required, such as a consumable electrode or a spool of wire. Some processes may also need an inert ambience in order to avoid oxidation of the molten metal. A flux material or an inert gas shield in the weld zone protects weld pool to avoid defects. Examples of fusion welding are metal inert gas welding (MIG), tungsten inert gas welding (TIG) and laser welding. There are many disadvantages in the welding techniques where the metal is heated to its melting temperatures and let it solidify to form the joint. The melting and solidification causes the mechanical properties of the weld to deteriorate in some cases such as low tensile strength, fatigue strength and ductility. The disadvantages also include porosity, oxidation, micro segregation, hot cracking and other micro structural defects in the joint. Thereby fusion process limits the combination of the metals that can be joined because of the different coefficients of thermal expansion. The solid state welding is a process where coalescence is produced at temperatures below the melting temperatures of the base metal without any need for the filler material or any inert ambience in many cases. Examples of solid

2 state welding are friction welding, explosion welding, forge welding, hot pressure welding and ultrasonic welding. The three important parameters time, temperature and pressure individually or in combinations produce the joint in the base metal. As the metal in solid state welding does not reach its melting temperatures, only fewer defects are caused due to the melting and solidification of the metal. In solid state welding, the metals being joined retain their original properties as melting does not occur in the joint and the heat affected zone (HAZ) is also very small compared to fusion welding techniques where most of the deterioration of the strength and ductility begins. Dissimilar metals can be joined with ease compared to the fusion welding FRICTION STIR WELDING TECHNIQUE Friction stir welding (FSW) is an advanced friction welding process. The conventional friction welding is done by moving the parts to be joined relative to each other along a common interface also applying compressive forces across the joint. The frictional heat generated at the interface due to rubbing softens the metal and the soft metal gets extruded due to the compressive forces and the joint forms in the clear material. The relative motion is stopped and compressive forces are increased to form a sound weld before the weld is allowed to cool. Friction stir welding is also a solid state welding process; this remarkable up-gradation of friction welding was invented in 1991 in The Welding Institute (Thomas 1991). Figure 1.1 shows the schematic representation of the Friction Stir Welding process. The process starts with clamping the plates to be welded to a backing plate, so that the plates do not fly away during the welding process. A rotating wear resistant tool is plunged on the interface between the plates to a predetermined depth and moved forward in the interface between the plates to form the weld.

3 3 Figure Friction stir welding process. The advantages of FSW technique are that it is environmentally friendly, energy efficient, there is no necessity for gas shielding for welding aluminium. Mechanical properties as proved by fatigue, tensile tests are excellent. FSW has been successfully produced on many of the important commercial aluminium alloys including the 1xxx, 2xxx, 5xxx, 6xxx, and 7xxx. Fusion welds in these alloys can have high defect rates. Friction stir welding can be used to join dissimilar aluminium alloys as well. FSW is routinely used to join 2xxx and 7xxx series aluminium alloys. FSW of Al alloys has several advantages over fusion welding processes. FSW offers important advantages over fusion welding which includes welding of aluminium alloys. Aluminium alloys are difficult to weld, better retention of baseline material properties, fewer

4 welding defects, lower residual stresses, and improved dimensional stability of the welded structure. Another important advantage is FSW joins aluminium alloys fairly rapidly, at about four millimeters per second, with low heat input and without shielding gas and costly filler materials WELDING OF ALUMINUM ALLOYS Aluminium is the most abundant metal available in the earths crust. Steel was the most used metals in the 19 th century but aluminium has become a strong competitor for steel in engineering applications. Aluminium has many attractive properties compared to steel as it is attractive and versatile to use. It is used extensively in aerospace, automobile and other industries. The most attractive properties of aluminium and its alloys which make them suitable for a wide variety of applications are their light weight, appearance, fabricability, strength and corrosion resistance. The most important property of aluminium is its ability to change its properties in a very versatile manner; it is amazing how much the properties can change from the pure aluminium metal to its most complicated alloys. There is more than a couple of hundreds alloys of aluminium alloys and many are being modified from them internationally. Aluminium alloys have very low density compared to steel. It is almost one third the density of steel. Properly treated alloys of aluminium can resist the oxidation process which steel cannot resist; it can also resist corrosion by water, salt and other factors. There are many different methods available for joining aluminium and its alloys. The selection of the method depends on many factors such as geometry and the material of the parts to be joined, required strength of the joint, permanent or dismountable joint, number of parts to be joined, the aesthetic appeal of the joint and the service conditions such as moisture, temperature, inert atmosphere and corrosion.

5 Most alloys of aluminium are easily weldable. MIG and TIG are the welding processes which use the most, but there are some problems associated with this welding process like porosity, lack of fusion due to oxide layers, incomplete penetration, cracks, inclusions and undercut. But they can be joined by other methods such as resistance welding, friction welding, and laser welding. During welding, many physical and chemical changes occur such as oxide formation, dissolution of hydrogen in molten aluminium and lack of colour change when heated WELD DEFECTS IN CONVENTIONAL PROCESSES Toe crack Porosity Slag Hot (solidification) cracks Undercut Hot tears Lamellar tears Lack of fusion Micro-fissures Root crack Under bead cracking Figure Various weld defects ( Because of a history of thermal cycling and micro structural Changes, a welded joint may develop certain discontinuities. Welding discontinuities can also be caused by inadequate or careless application of established welding technologies or substandard operator training. Figure 1.2 shows various weld defects that may occur in fusion welding and are listed below. Porosity Slag inclusion

6 6 Cracks Undercut Hot tears Incomplete fusion Incomplete penetration Discontinuities Lamellar tear 1.4 APPLICATIONS OF FRICTION STIR WELDING The use of latest joining techniques such as Friction Stir Welding appears to be increasing, since it does not involve melting. The application of these processes in the past has been restricted, but with the increased recognition of the benefits of automation and the requirement for high-integrity joints in newer materials, it is envisaged that the use of these techniques will grow. This process originally intended for welding of aerospace alloys, especially aluminium extrusions, whereas in conventional friction welding heating of interfaces is achieved through friction by rubbing two surfaces. In friction stir welding process, a third body is rubbed against the two surfaces to be joined in the form of a small rotating non-consumable tool that is plunged into the joint. The contact pressure causes frictional heating. The probe at the tip of the rotating tool forces heating and mixing or stirring of the material in the joint. The FSW process can also cope with circumferential, annular, nonlinear, and three dimensional welds. Since gravity has no influence on the solid-phase welding process, it can be used in all positions: Horizontal Vertical

7 7 Overhead Orbital. The process has been used for the manufacture of butt welds, overlap welds, T-sections, fillet, and corner welds. For each of these joint configuration, specific tool designs are required which are being further developed and optimized (Jhonson and Kallee 1999) Applications of Friction Stir Welding in Various Materials FSW can be used for joining many types of materials and material combinations. Continuing development of the FSW tool, its design and materials have allowed preliminary welds to be successfully produced in: Aluminium alloys 2000 (Al-Cu), 5000 (Al-Mg), 6000 (Al-Mg-Si), 7000 (Al- Zn), 8000 (Al-Li) and other aluminium alloys of the 1000 (commercially pure), 3000 (Al-Mn) and 4000 (Al-Si) series. Copper and its alloys Lead Titanium and its alloys Magnesium alloy, magnesium to aluminium Zinc MMCs based on aluminium (metal matrix composites) Plastics Mild steel. FSW has been successfully developed for many non-ferrous alloys and steels. The process is fully mechanized, needs no traditional welder skills, no consumables, and produces solid state welds with minimal distortion, and excellent mechanical properties.

8 Applications of Friction Stir Welding in Industry FSW is attractive for the friction heating which is generated locally and there is no need for the widespread softening of the assembly. The weld is formed across the entire cross-sectional area of the interface in a single shot process and this technique is capable of joining dissimilar materials. This process is completed in a few seconds with very high reproducibility, and suitable for a mass production industry. Shipbuilding and marine industries include: panels for decks, sides, bulkheads and floors, aluminium extrusions, hulls and superstructures, helicopter landing platforms, offshore accommodation, marine and transport structures, masts and booms, e.g. for sailing boats, refrigeration plant. Aerospace industry includes: wings, fuselages, empennages, cryogenic fuel tanks for space vehicles, aviation fuel tanks, external throw away tanks for military aircraft, military and scientific rockets, repair of faulty MIG welds. Land transportation include engine and chassis cradles, wheel rims, attachments to hydro formed tubes, tailored blanks, e.g. welding of different sheet thicknesses, space frames, e.g. welding extruded tubes to cast nodes truck bodies, tail lifts for lorries, mobile cranes, armour plate vehicles, fuel tankers, caravans, buses and airfield transportation vehicles, motorcycle and bicycle frames, articulated lifts and personnel bridges, skips, repair of aluminium cars, magnesium and magnesium/aluminium joints (Nicholas 1998). 1.5 ADVANTAGES OF FRICTION STIR WELDING The process advantages resulting from the fact that the FSW process takes place in the solid phase below the melting point of the materials to be joined. The benefits therefore include the ability to join materials which are

9 difficult to fusion weld, for example 2000 and 7000 aluminium alloy. These are light reflective in nature. 9 FSW of dissimilar materials will be required in the near future for advanced automotive and aerospace design. In the present research, it was trying to apply the FSW technique to 6082-T6 aluminium alloy of various thicknesses. Then, the metallographic structure and the mechanical properties of the welded joint with various welding speeds were examined. The process advantages are: Low distortion, even in long welds Excellent mechanical properties as proved by tensile and bend tests No fume No porosity No spatter Low shrinkage Can operate in all positions Energy efficient (Cary 2004) Non-consumable tool One tool can typically be used for up to 1000 m of weld length in 6000 series aluminium alloys No filler wire No gas shielding for welding aluminium No welder certification required Some tolerance to imperfect weld preparations-thin oxide layers can be accepted (Hansen 2003) FSW can use existing and readily available machine tool technology. The process is also suitable for automation and adaptable for robot use.

10 LIMITATIONS OF FRICTION STIR WELDING The main limitations of the FSW process are: Welding speeds are moderately slower than those of some fusion welding processes (up to 750 mm/min for welding 5 mm thick 6000 series aluminium alloy on commercially available machines) Weld plates must be rigidly clamped Backing bar required Keyhole at the end of each weld (Hansen 2003) 1.7 SIGNIFICANCE OF WORK Friction Stir Welding facilitates welding of plate to plate joints required for several applications. This process is chosen for the welding of AA 6082-T6 (Al-Mg-Si alloy) of various thicknesses. This alloy finds use for structural and other industrial applications. Since the tool geometry and the interaction of tool with base metal is important for obtaining good quality welds, different tool geometries were designed and manufactured. Since the mechanism of bonding involves efficient movement of plasticized material around tool profile, the joints were investigated thoroughly for defects under varying conditions of process parameters. In this work three important process parameters viz. Spindle speed, welding speed, shoulder penetration and two parameters viz. Shoulder profile, pin profile related to tool geometry were considered. Plate thicknesses of 4 mm to 8 mm will be widely used in structural and fabrication industries. Hence the plate thicknesses of 4 mm, 6 mm and 8 mm were chosen and the friction stir welding is carried out to optimize the parameters. Friction stir welding tool pin diameter is constrained to the thickness of the plate. The same diameter of the pin could not perform welding of all

11 thicknesses. The parameters under study are optimized for each plate thickness. The optimum parameter level of every thickness was interpreted and projected. This facilitates to directly select the parameters for unknown thicknesses without performing the trials. 11 Most Friction Stir Welding processes are carried out on only one side of the parent metal. The tool is made to pass on only one side of the material interface. It creates defects such as a root defect (Dickerson and Przydatek 2003), which is defined as an area at the bottom of the joint that is not welded. The bottom of each joint was not stirred and left un-welded. This defect weakens the region of the welded zone and is responsible for all tensile and bending failure (Arici and Sinmaz 2005). Also a material with greater thickness requires lengthy tool pin and greater force causes deflection of the tool leading to the damage of the tool. To avoid these defects and to prevent the tool damage, double side friction stir welding was carried out. In double side friction stir welding, the first pass is carried out along the interface for approximately half of the material thickness. Then the specimen is turned back and clamped for the second pass of the tool. The reduced weld zone by double side weld is discussed. The optimization of process parameters was carried out using design of experiments approach and the joint properties were evaluated by conducting tensile and hardness tests. Metallographic investigations were also performed on the welded joints. The double ellipsoidal heat source model is effectively used with modified heat source parameters to carry out the numerical investigation. The experiments are conducted to validate the temperature distribution for different weld conditions of different plate thicknesses obtained from numerical investigation.

12 ORGANIZATION OF DISSERTATION The thesis is organized into six chapters. Chapter I present the brief history of friction stir welding and its applications, advantages and limitations. Then the chapter provides the scope of friction stir welding with need for the research and sequence of research adopted. Chapter II includes basic details about the research areas and the earlier works done by various researchers in the fields of optimization technique, mechanical property of joints and metallographic study of joints. Chapter III introduces the experimental setup which includes machine tool setup, selection of material and the type of joint used in this work and describes the design of experiments, testing of weld joints, mathematical modeling to optimize the parameters to achieve the maximum joint strength. Chapter IV describes details about the numerical investigation which includes the basics of thermal and thermo mechanical models. The finite element modeling procedure using ANSYS package, and the assumptions made were detailed. Chapter V gives results of various tests including mechanical and metallurgical investigations of weld joint. This includes tension test, micro hardness survey, metallographic study and numerical investigation. Finally obtained results were discussed. Chapter VI gives conclusions of the investigations conducted in this work and some future works are proposed for further investigations. Furthermore, references and appendices are included at the end of this thesis as supporting materials.