Characteristics and Formation Mechanisms of Welding Defects in Underwater Friction Stir Welded Aluminum Alloy

Abstract

Underwater friction stir welding (FSW) has been demonstrated to be a promising method for strength improvement of heat-treatable aluminum alloy joints. However, when improper welding parameters are utilized, welding defects, such as voids can be produced in the joints, leading to dramatically deteriorated mechanical properties. Thus to obtain high-quality underwater joints, it is necessary to understand the variables that promote the formation of these defects. In this study, the characteristics of welding defects in underwater joints were examined, and the formation mechanisms of the defects were investigated by analyzing the material flow patterns during underwater FSW. The results indicated that welding defects can occur at both low-and high-rotation speeds (HRS). The defects formed at HRS can be divided into two types according to the welding speed. When a HRS and a low welding speed are chosen, the material beneath the tool shoulder tends to be extruded into the pin stirred zone (PSZ) after flowing back to the advancing side. This results in a turbulent flow condition, creating void defects in the PSZ. When a high welding speed is coupled with the HRS, a large amount of material from the thermo-mechanically affected zone is dragged into the pin hole, which causes the material of the shoulder stirred zone to fill the pin hole in a downward flow direction. This leads to turbulent flow in PSZ, and creates voids or even groove defects in the as-welded joints.

Effect of Pin Length on Hook Size and Joint Properties in Friction Stir Lap Welding of 7B04 Aluminum Alloy

“…This flow pattern constitutes the shoulder driven flow. The rest of pin-sheared material not extruded into shoulder concave undergoes circle and upward flow due to the shearing of pin threads (Ref 17,18), and this flow pattern constitutes the pin driven flow, as illustrated in Fig. 8.…”

Effect of Forced Air Cooling on the Microstructures, Tensile Strength, and Hardness Distribution of Dissimilar Friction Stir Welded AA5A06-AA6061 Joints

“…They found that the HAZ of the joint disappeared, and the strength and hardness were improved. Other studies also indicated that a forced cooling medium is beneficial for improving the mechanical performances of FSW aluminium alloy joints [20][21][22][23].…”

Force measurement-based discontinuity detection during friction stir welding

“…However, it does not facilitate the detection or estimation of the discontinuity sizes. Zhang et al, (2012) examined the characteristics of discontinuities in underwater friction stir welds and investigated the formation mechanism of the discontinuities. After welding, joints were cross-sectioned perpendicular to the weld direction, and the weld cross-sections were imaged via light microscopy.…”