1.1 Overview of Friction Stir Welding
Welding is a fabrication process used to join metallic materials or thermoplastics in which the joining edges are heated and fused together with or without filler metal by the application of heat and pressure or both to form a permanent weld nugget for joint strength. The art of welding metals is very ancient and about 3000 years old. Welding process arises from Bronze Age and Iron Age. The present advanced welding methods come through several invention and modification. Nowadays welding techniques have become a very advanced process in technology. The technology continues to develop advanced invention of diffusion bonding, structural adhesive bonding, laser beam welding (LBW), electron beam welding, magnetic pulse welding and friction stir welding (FSW) in the latter half of the century. Out of these welding processes, FSW process is a recent and advanced solid state joining technique.
FSW is an emerging technique for welding similar and dissimilar material with enhanced mechanical properties. The reason for huge attention on this process is to weld almost all non weldable or difficult to weld materials like aluminium (Al) and magnesium (Mg) that are difficult by fusion welding process due to severe oxidation prominent, dangerous spatter formation and bursting of weld pool including inadequate filler material and process parameters. FSW process has ability to produce high quality similar joint like Mg/Mg, Al/Al and dissimilar joint like Al/Cu etc. without melting the parent material and eliminate the defect seen in the fusion welding process. The FSW process also has ability to join plates of different thickness with superior weld quality that is difficult by other welding process.
1.2 Motivation
The expedition of FSW starts with the joining of aluminum and its alloys but with due course it was found to be successful in joining other metals as well as non- metals. Among different metallic materials Mg can be considered the new age material.
Introduction
It has increasing relevance in every field, ranging from the automotive and aerospace industry, to applications in electronics and biology as biodegradable human implants. It is also abundantly available in seawater. However, joining of Mg alloys is one of the challenges that need to be addressed to fully exploit the potential of engineering applications. Fusion welding processes are mostly practiced to weld Mg alloys but introduction of FSW process can change the existing scenario. The process of FSW is governed by many influencing parameters and selection of those parameters to fall within the welding window for Mg alloys is not targeted in great details. This research work is motivated towards exploring the welding parameters ranges for Mg alloy. This work is also motivated towards enhancement of weld qualities of Mg alloy with the inclusion of appropriate alloying material in the joint interface. The findings from the current research work will surely provide benefits to the practitioner and industrial users of FSW to implement the welding process with high success rate and appreciable weld qualities.
Joining of two dissimilar materials is very importance for integrating complementary characteristics of different materials within one single component. Al and copper (Cu) have different material and mechanical advantages and combining these two materials can fetch effective solution in many industrial uses. Joints of Al/Cu have extensive use in electrical conductors where characteristics of both the materials play vital role. However, joining of Al and Cu using fusion welding processes is quite a challenging task due to the great differences in their thermo-physical properties.
Introduction of the FSW process brings effective solution towards obtaining sound Al/Cu joints. However, a need is realized for full exploration of range of influencing process parameters to be adopted during the process. With this motivation the current research work is decided to explore optimized parametric range for successful joining of Al/Cu.
With the optimized parametric range, the research work is extended towards enhancement of weld qualities of A/Cu joints by introducing third material at the joint interface. The experimental investigation carried out in this work would provide a tool for selection of welding parameters for Al/Cu joining using FSW process with improved mechanical properties.
Chapter 1 Automobile and aeronautical structural panels can be designed with variations in the panel thickness i.e., reducing material where it is not needed, which will reduce the weight of the structure. This type of panel can be manufactured by welding plates (or structures) with different thickness (WPDT). In transportation industry using WPDT technique around 50% weight can be reduced without compromising the mechanical properties. The most interesting and effective advantages of the use of WPDT are cost effectiveness and distribution of weight. WPDT is a concept of joining materials with different thickness to get a single structure with improved properties. The process of joining dissimilar thickness plates using FSW is in the early development stage and further details investigation is required for fully exploiting its capabilities. Therefore, to address this need the current research work targets to develop welding technique for joining dissimilar thickness workpieces with different thickness ratio. New weld configurations are proposed for successful joining of dissimilar thickness materials using FSW process with better mechanical properties.
1.3 Research Objectives
Based on the research gap found in the published research work, the objectives of the present work are summarized as follows:
Optimization of process parameters in FSW of Mg alloy using Taguchi grey relational analysis.
Effect of individual process parameter on microstructural and mechanical properties of FSWed Mg alloy.
Enhancement of mechanical and metallurgical properties in joining Mg alloy by introducing alloying elements at the joint interface.
Optimization of process parameters by Hybrid Fuzzy-Taguchi-grey relational analysis technique in dissimilar Al/Cu FSW.
Effect of position of high melting point plate with respect to tool rotation, tool offset and tool rotational speed on metallurgical properties and assessment of different mechanical properties of Al/Cu dissimilar joints.
Improvement of mechanical and metallurgical properties of Al/Cu dissimilar FSW joint using third material at the joint interface.
Introduction
Development of joint configurations for effective joining Al alloy with thickness difference between the workpieces for improved mechanical and microstructural properties.
1.4 Contribution of the Thesis
The important contributions of this research work are summarized as follows:
A detailed experimental investigation is carried out to evaluate the influence of process parameters on weld properties of Mg alloy. Optimized parameter range for multiple weld quality characteristics is also proposed using grey-Taguchi technique.
Al and Zn foil as alloying elements are introduced for the first time for reduction of brittleness of the Mg weld zone that leads to improved ductility with enhanced mechanical and metallurgical properties of the FSW joint.
Newly proposed hybrid Fuzzy-Grey-Taguchi technique has been successfully used for multi-weld quality optimization of Al/Cu dissimilar joint.
Defect free joints with improved mechanical properties are achieved in Al/Cu dissimilar joint by positioning hard Cu plate on the advancing side and with proper tool offset towards soft Al alloy.
This is the first attempt, in butt joint configuration, for controlling IMCs formation to enhance mechanical and metallurgical properties in Al/Cu dissimilar FSW using Ni, Ti and Zn as an interlayer (third) material.
Three new joint configurations are proposed and evaluated for welding plates (or structures) with dissimilar thickness using FSW.
1.5 Outlines of the Thesis
The contents of the thesis focus on the enhancement of mechanical properties of the FSW similar and dissimilar metal. It covers optimization of the considered process parameters and their effects on the mechanical and microstructural properties. The principal aim of this work is to enhance the mechanical properties of similar and dissimilar welds by addition third material at the joint interface. Third material behaves as an alloying element in case of similar FSW. In case of dissimilar FSW alloying elements behaves as third material control the IMCs formation and enhances the
Chapter 1 mechanical properties. The contents of the thesis are divided into nine chapters and they are as follows:
Chapter 1 provides general introduction and brief background to FSW process.
Chapter 2 has been devoted to give a brief literature review on effect of process parameters on the mechanical and microstructural properties of similar and dissimilar FSWed joints and effect of third material on the weld quality. Research work in the area of dissimilar thickness joining is also discussed.
Chapter 3 elaborates the procedures, methods or techniques implemented for conducting the welding experiments. This chapter also outlines different techniques used for analyzing and testing experimental specimens.
Chapter 4 presents optimization of process parameters and effect of individual process
parameters on mechanical and microstructural properties of FSWed Mg alloy.
Chapter 5 deals with the effect of alloying element on mechanical and microstructural properties of FSWed Mg alloy joint.
Chapter 6 deals with optimization of process parameters using hybrid fuzzy-grey- Taguchi technique and effect of individual process parameters on mechanical and microstructural properties of Al/Cu dissimilar joints.
Chapter 7 investigates effect of third material on mechanical and microstructural properties of Al/Cu dissimilar FSW.
Chapter 8 dedicated to study the effects of process parameters, single pass and double
pass welding and joint configurations on the weld quality of dissimilar thickness plates.
Chapter 9 displays the concise conclusions of the research work with relevant future scope of work.
Finally the thesis is end with complete references and the fuzzy rules base, described in Appendix I.