CHAPTER 3 SCOPE AND OBJECTIVES

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1 35 CHAPTER 3 SCOPE AND OBJECTIVES 3.1 MOTIVATION TO THE PRESENT WORK Limiting the emission of green house gases as well as the reduction of fuel consumption is an urgent area of research that needs to be solved in the near future especially in transportation industries. More efficient processes and smart design and the use of light weight materials are the ways to address these problems. Despite the fact that there is always a competition among various material groups such as light metals, steels and polymers for use in relevant light-weight structures depending on economic aspects, materials and component requirements, magnesium alloys still offer a promising potential. With regard to their intrinsic characteristics like low density, promising mechanical properties and their high recyclability, magnesium alloys found more implementation in relevant applications. Wrought magnesium alloys are usually welded more easily than certain cast alloys. The preferred welding processes for magnesium alloys are Gas Metal Arc Welding (GMAW) and Gas Tungsten Arc Welding (GTAW) due to their comparatively easier applicability and better economy. Mechanical joining processes such as riveting and bolt bonding are dominant in the connection of wrought magnesium alloy components as an integrated part. However, the limited strength of the joints restricts the applications of magnesium parts. Magnesium alloys can be also joined using laser beam welding, electron beam welding and solid state welding processes. Shielding

2 36 the welding region by inert gas or flux is needed to prevent the fire hazard and risk. Preheating is needed in welding magnesium alloys because of the degree of joint restraint and metal thickness. A laser beam is a preferred method for welding magnesium because of low heat input, elevated speed and limited deformation; however, the tendency of developing porosity must be considered. Recently developed friction stir welding is also an equally good process for welding magnesium alloys. Problems in fusion welding of magnesium alloys such as solidification cracking, liquation cracking and porosity are eliminated with friction stir welding due to its solid state nature of process. Significant research is still needed on magnesium joining to achieve future goals to reduce the vehicle mass and the amount of greenhouse gases released during the process From the literature review (presented in Chapter 2), it is understood that reported works on welding of magnesium alloys are very few in numbers compared to aluminium alloys. Most of the investigations on FSW work of magnesium alloys were carried out on AZ31 grade magnesium alloy which contain 3% aluminium and 1% zinc. However, there is very little information available on friction stir welding of AZ61A grade magnesium alloy which contains 6% aluminium and 1 % zinc. The higher volume percentage of aluminium in AZ61A grade magnesium alloys may cause some difference in friction stir welded joint characteristics. Very few research works have been published so far on the effect of FSW parameters and tool pin profiles on mechanical and metallurgical properties of AZ61A magnesium alloy. A detailed comparison has not yet been reported on mechanical and metallurgical properties of PCGTAW and FSW joints of AZ61A magnesium alloy. Hence, in this investigation, a systematic study has been made to evaluate the mechanical and metallurgical properties of FSW joints of AZ61A magnesium alloy.

3 SCOPE Wrought magnesium alloys are currently used to a very limited extent, due to lack of suitable alloys and some technological restrictions imposed by the hexagonal crystal structure of magnesium. The development of new grades of alloys and manufacturing techniques such as welding play an important role in exploiting the new field of applications. Weldability of magnesium alloys has recently been investigated with a variety of processes, particularly gas tungsten arc welding, laser beam welding and friction stir welding. Of all commercial magnesium alloys, those with aluminum as the primary alloying element are most weldable using any of these three processes. With arc welding processes, lack of weld penetration is generally a limitation of the magnesium alloys. Even though these magnesium alloys are easily weldable by fusion welding processes, the welded joint strength is poor. Weld fusion zones typically exhibit coarse grains because of the prevailing thermal conditions during the weld metal solidification. This often results in inferior weld mechanical properties and poor resistance to hot cracking. Further, in fusion welding of magnesium alloys, the defects such as slag inclusion, solidification cracks deteriorate the weld quality and joint properties. GTAW of magnesium alloys produced some defects such as porosity and hot crack, which deteriorate their mechanical properties. FSW is capable of joining magnesium alloys without melting and thus it can eliminate problems related to solidification. Usually, friction stir welded joints are free from these defects since there is no melting during welding and the metals are joined in the solid state itself due to the heat generated by the friction and flow of metal by the stirring action. However, FSW joints are prone to other defects like pin hole, tunnel defect, piping defect, kissing bond, cracks due to improper flow and insufficient consolidation of metal in the FSP region. Hence, the FSW process parameter

4 38 must be optimized to get high quality joints. In this investigation, the effect of process parameters on Friction Stir Welding of AZ61A magnesium alloy were studied and results were compared with the mechanical and metallurgical properties of PCGTAW joints. 3.3 OBJECTIVES Although the development of FSW technology to make complex welds is proceeding at an extremely rapid pace, primarily due to the efforts of the industry, understanding of the formation of defects that occur during the welding process and of the weld mechanical properties has been slow. By keeping all the above aspects in mind, the present investigation has been carried out to attain the following objectives. To study the effect of FSW process parameters such as tool rotational speed, welding speed and axial force on tensile properties of AZ61A magnesium alloy joints; To study the effect of FSW tool parameters such as tool pin profiles, tool shoulder diameter and pin diameters on tensile properties of AZ61A magnesium alloy joints; To study the effect of pulsed current GTAW (PCGTAW) process parameters such as peak current, base current, pulse on time and pulse frequency on tensile properties of AZ61A magnesium alloy joints; To develop empirical relationships to predict tensile strength of FSW and PCGTAW joints of AZ61A magnesium alloy incorporating respective process parameters;

5 39 To optimize the FSW and PCGTAW process parameters to attain maximum tensile strength using Response Surface Methodology (RSM); To compare the metallurgical characteristics and mechanical properties of FSW and PCGTAW joints of AZ61A magnesium alloy.