Characteristics of FSW and UWFSW joints of AA2519-T87 aluminium alloy: Effect of tool rotation speed

Abstract

AA2519-T87 aluminium alloy is a recently developed armour grade aluminium alloy used for fabricating light combat vehicles in the armour application. Joining of this alloy using fusion welding resulted with solidification defects like porosity, hot cracking, etc. To overcome these shortcomings, friction stir welding (FSW) process can be used. Even though the joining of materials occurs below the melting temperature, the temperature is adequate to coarsen and dissolute the precipitates in the thermo-mechanical affected zone (TMAZ) and heat affected zone (HAZ). To enhance the joint properties, under water friction stir welding (UWFSW) process can be adopted. Tool rotational speed and tool travel speed are the two important parameters will control the heat generation in FSW. The influence of these parameters in conventional FSW is well known, but the influence of these parameters in underwater FSW is not yet fully understood. Hence a comparative study is undertaken to investigate the effect of tool rotation speed on the stir zone characteristics and the resultant tensile properties of the FSW and UWFSW joints. From this investigation, it is found that the UWFSW joint made using the lower tool rotation speed of 1200rpm exhibited superior tensile properties than FSW joints. This may be attributed to the lower heat generation, higher grain boundary strengthening, high volume fraction of precipitates and narrowing of the lower hardness distribution region (LHDR).