Influences of tool traverse speed on tensile properties of air cooled and water cooled friction stir welded AA2519-T87 aluminium alloy joints

The fusion welding of AA2519-T87 aluminium alloy is generally not preferred due to hot cracking and alloy segregation problems. To overcome these shortcomings, friction stir welding (FSW), a solid state joining process is employed. However, the thermal cycles involved in FSW process leads to coarsening and dissolution of the strengthening precipitates in the matrix of the alloys. To minimize these metallurgical changes and enhance the joint properties, under water friction stir welding (UWFSW) process can be adopted. Tool traverse speed is one of the important process parameters controlling the heat generation in FSW. The influence of this parameter in conventional FSW (air cooled) is well known, but the influence of tool traverse speed in underwater FSW (water cooled) is not yet fully understood. Hence, this investigation was undertaken to study the effect of tool traverse speed on the stir zone characteristics and the resultant tensile properties of the FSW and UWFSW joints. It was found that the UWFSW joint made using the higher tool traverse speed of 30 mm/min exhibited superior tensile properties. 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).