CHAPTER 1 INTRODUCTION

Transcription

1 1 CHAPTER 1 INTRODUCTION 1.1 ALUMINIUM ALLOYS Aluminium and its alloys offer an extremely wide range of capability and applicability, with a unique combination of advantages that make the material of choice for numerous products and markets due to their attractive properties include low density, appearance, strength, fabricability and corrosion resistance. They are basically classified as wrought alloys and cast alloys. Wrought alloys are designated as 1xxx- pure aluminium, 2xxx-Al- Cu alloys, 3xxx-Al-Mn alloys, 4xxx-Al-Si alloys, 5xxx-Al-Mg alloys, 6xxx- Al-Mg-Si alloys, 7xxx-Al-Zn alloys and 8xxx-Al and other elements where first digit denotes principal alloying elements, second digit refers to the variation of initial alloy and third and fourth digits refer to the individual alloy variations. 1xxx, 3xxx and 5xxx are non-heat treatable alloys and 2xxx, 6xxx and 7xxx series are heat treatable alloys (ASM Hand book 1985). 1.2 CAST ALUMINIUM ALLOYS Cast aluminium alloys are the most versatile of all common foundry alloys and generally have the highest castability ratings. The important characteristics, which include high fluidity, low melting point and shorter casting cycles. Aluminium alloy castings are routinely produced by pressure-die, permanent-mould, green and dry sand and investment casting. Cast aluminium alloys are designated as 1xx.x pure Al, 2xx.x Al-Cu alloys,

2 2 3xx.x-Al-Si-Cu alloys, 4xx.x-Al-Si alloys, 5xx.x-Al-Mg alloys, 7xx.x-Al-Zn alloys, 8xx.x-Al-Sn alloys and 9xx.x-Al with other elements where first digit refers to the principal alloying element, second and third digit refers to the specific alloy designation (number has no significance but unique) and fourth digit refers to the casting (0) or ingot (1,2) designation (ASM Hand book 1990). Further variations in specified compositions are denoted by prefix letters used to define the impurities limits (A-iron content). Silicon is the main alloying element and forms eutectic with aluminium at 11.7% Si. It imparts high fluidity and low shrinkage, which result in good castability and weldability. Cast aluminium-silicon alloys are widely used in automotive, aerospace, chemical and food industries due to their high fluidity, ease of casting, low density and controllable mechanical properties. The applications grow as industry seeks new ways to save weight and improve performance. Generally, cast aluminium alloys are less weldable than wrought aluminium alloys. In this research work, three types of Al-Si alloys, namely low silicon alloy A319, medium silicon alloy A356 and eutectic alloy A413 were chosen for investigation. A319 (Al-Si-Cu) aluminium alloy is widely used for a range of engineering applications where moderate stress is envisaged. Its casting characteristics make it ideal for castings of varying section thickness as it can cope with heavy and moderately thin sections. Castings manufactured from this alloy can be heat treated to improve the mechanical properties. The machining properties are better than the high silicon aluminium alloys and considerably improve with heat treatment. They can be considered where relatively high static loads are anticipated as creep extension at moderately elevated temperatures is modest. They are used in automotive cylinder heads, internal combustion engine crankcases, engine mountings, gearboxes, etc.

3 3 A356 (Al-Si-Mg) aluminium alloy has excellent castability and good mechanical properties that can be varied by heat treatment. The alloy is mostly used for high specification castings in industries where shape and complex form is required and casting soundness is important. Its use is widespread in the automotive industries for alloy wheels, cylinder heads and blocks and in many other industries such as chemical, marine and electrical (John R. Brown 2000). A413 (Al-Si) is a eutectic alloy, which is used in castings where thin, complex castings with large surface areas are required. The alloy is of medium strength with excellent ductility but suffers a rapid loss of properties at elevated service temperatures. This alloy possesses high corrosion resistance and excellent castability making it suitable for marine fittings, water manifolds and road transport applications. On deck castings and other marine applications, water cooled manifolds, jackets, thin walled and intricate instrument cases, switch boxes and motor housings, very large castings such as doors, panels, pumps and other equipment in the chemical and dye industries, castings used in the manufacturing of paint and food industries. 1.3 NEED FOR WELDING OF CAST ALLOYS For a variety of reasons it is necessary to weld aluminium castings to themselves or to wrought alloys (Mike Renshaw 2004). Typical reasons for needed to weld may be: A damaged casting, e.g. an oil sump after driving over an obstruction in the road, Cosmetic dressing of a new casting such as filling in a defect left by a sand inclusion,

4 4 Joining multiple castings together for example sectioned poles, Joining a casting to wrought material for example in a road vehicle where the casting forms a node and extruded and / or sheet materials are joined to the node, Build up of a casting, either to repair worn areas or as part of the manufacturing process, and Joining of aluminium to other metals such as copper and steel. Cast aluminium alloy constructions are used very widely and number of constructions, produced from Al-Si alloys is continually increasing (Višniakov et al 2004) like rims of complicated shape with different wall thicknesses. It was already known that the joint between cast Al alloys has a potential for expanding the usage of economic casting in airframe and missile applications (Lederich et al 2001). 1.4 PROBLEMS DURING WELDING OF CAST ALLOYS Though wrought aluminium alloys are easily weldable, cast alloys are not easily weldable by fusion welding processes. Common defects that are encountered during the welding of these alloys, which will reduce the strength of the weldments are elucidated as follows: i. Porosity: Porosity arises from the gas dissolved in the molten weld metal, which becomes trapped during solidification, forming bubbles in the solidified weld metal. Hydrogen has low solubility in the solid but high solubility

5 5 in molten aluminum, which is a major problem and leads to the above defect. ii. iii. iv. Hot cracking: The basic mechanism behind hot cracking is high temperature cracking mechanism and is a function of how metal alloy systems solidify (Kearney, 1974). It is caused due to difference in the melting points of the different alloying elements added to the pure metal. In the aluminum alloys, the alloying elements form a range of eutectics with varying freezing points substantially lower than the bulk metal. Loss of strength in Heat affected zone (HAZ): Since the HAZ experiences one or more cycles of heating and cooling, the properties of the weldment are different from those of the unaffected base metal. The best option is to weld casting with lower heat input in order to minimize the HAZ. Grain coarsening: Weld fusion zones typically exhibit coarse columnar grains because of the prevailing thermal conditions during weld metal solidification. This often results in inferior weld mechanical properties and poor resistance to hot cracking. 1.5 SOLUTION FOR THE ABOVE PROBLEM Compared with many of the fusion welding processes that are routinely used for joining cast Al alloys, Friction Stir Welding (FSW) is an emerging solid state joining process in which the material that is being welded does not melt and recast. FSW was invented in the year 1991 by The Welding Institute in UK (Thomas et al 1991). The basic principle of FSW process for joining two separate plates is illustrated in Figure 1.1.

6 6 Figure 1.1 Schematic representation of principle of FSW process The key component of the process is the specially designed rotating tool consisting of profiled pin (or probe) extending along the axis of the rotating component and shoulder. The process proceeds by rotating the tool at high angular speeds, and plunging the pin into the workpiece until the shoulder makes full contact with workpiece surfaces. The rotating tool is then moved along the joint line while a relatively high forging force is applied to maintain full contact between the shoulder and the workpiece surface. The heat generated due to friction between tool and workpiece softens the material and allows the rotating tool to mechanically stir the softened material flowing to the backside of the pin where it is consolidated to form a metallurgical bond. FSW creates the weld joint without bulk melting. In addition, the extensive thermomechanical deformation induces dynamic recrystallization and recovery that refine the microstructure of the stir region. Therefore, welds made by FSW are shown to have much improved mechanical properties such as the tensile strength and the fatigue life than the corresponding fusion welds (Sharma et al 2004). FSW is considered to be the most significant invention in the field of metal joining. As compared to conventional welding techniques, it

7 7 consumes considerably less energy. No shielding gas or flux is used, thereby making the process environmentally cleaner. The joining does not involve use of filler metal and therefore any aluminium alloy can be joined without concern for the compatibility of composition, which is an issue in fusion welding (Mishra et al 2005). FSW is successful in welding a wide range of materials namely plastics, metal matrix composites, aluminum, copper, steels, stainless steel, nickel alloys, and titanium (Nakata et al 2006, Ma et al 2004). At present, the process is found to be more efficient to join wrought non-ferrous alloys such as aluminium, magnesium and copper. Researchers throughout the world are working to extend this process to other ferrous and non-ferrous alloys. In accordance with the current trend, the present investigation is also carried out to derive the benefits of FSW process for joining cast aluminium alloys, which is not so easy, by fusion welding processes. 1.6 BRIEF OUTLINE OF THE THESIS The first chapter presents a brief introduction about this thesis, which covers details of wrought aluminium alloys, cast aluminium alloys and friction stir welding (FSW) process. Chapter 2 contains an in-depth review of published literature concerning FSW of aluminium alloys. Chapter 3 deals with the scope and objectives of the present investigation. Chapter 4 gives information about the experimental procedure employed in this research work. Procedures for fabrication of the joints, specimen preparation, and testing are explained. Effect of tool rotation speed, welding speed and axial force on mechanical and metallurgical properties of FSW joints are reported in chapter 5, 6 and 7 respectively. Chapter 8 contains the optimization of FSW process parameters and characterization of joints made with the optimized FSW process parameters. Chapter 9 presents the important conclusions of the present investigation.