Joint Efficiency

Joint efficiency of the welded material means the ratio of ultimate tensile strength (UTS) of the joint to the UTS of its base material. It gives an important perception for the joint quality and the mechanical property of the joint. Thus, tensile tests of the produced

T-butt joints were carried out along the skin and stringer directions. Figure 18 shows the joint efficiencies along the skin and stringer of the AA2024 and AA7075 T-butt joints produced using the welding parameters of 400, 600 and 800 rpm rotational speeds, and a constant travel speed of 50 mm/min. For the skin tensile test (perpendicular to welding direction), Figure 18a shows no significant difference in the efficiency values between joints welded at 400 and 600 rpm, while the rotational speed increases from 600 to 800 rpm the efficiency decreases from 83.40 % to reach 72.02 % of the base metal AA2024-T4. Tensile properties in the direction of stringer (perpendicular to welding direction) are given in Figure 18b. It can be seen that the joint efficiencies along stringer increases with increasing tool rotational speeds. The T-butt joints produced at 800 rpm displays the highest joint efficiency of 52.30% of the base metal AA2024-T4. It can be noted from Figure 18a,b, that the joint efficiencies of all produced T-butt joints along the stringer are lower than those of all joints along the skin. This trend is in good agreement with the results obtained by Cui et al. [37] for the similar AA6061-T4 T-butt joints (C-series). Furthermore, they reported that all T-joint efficiencies showed tendency to enhancement with increasing the rotational speeds at constant travel speed.

**Figure 18.** Joint efficiencies against rotational speeds for the T-butt joints of 2024-7075 Al alloys; (**a**) tensile tested along the skin and (**b**) along the stringer directions.

It can be noted that or the joint welded at 800 rpm the joint efficiency along the skin direction is the lowest and for the stringer direction is the highest as shown in Figure 18a,b can be due to two reasons. First, the reduction in the hardness relative to the other two revolution speeds as shown in Figure 7, due to the increased heat input. Second, the reduced skin thickness due to the pressing effect of the shoulder (Figure 5), this is not the case for tension in the stinger direction. Moreover, at 800 rpm the pressing by the shoulder and the increased softening has generated larger fillet than the other two lower rotation speeds (400 and 600 rpm). This large fillet represents more support for the stringer leading to the highest joint efficiency.

The authors are still not satisfied with the attained joint efficiency especially in the skin direction, so that further welding trials should be performed with other welding parameters to overcome the reduction in the skin thickness due to the applied pressure by the tool shoulder.
