Improving mechanical properties of FSWed AA6061-T6 joint by controlling microstructural changes through utilization of stationary shoulder tool in presence of Al2O3 nanoparticles and external cooling

Abstract

A novel cooling-assisted stationary shoulder friction stir welding (SSFSW) was employed, using Al2O3 nanoparticles, to achieve high-strength joints in AA6061-T6. The approach resulted in improved mechanical properties, with the optimal joint achieving an efficiency of 91%, representing a substantial increase compared to the 77% efficiency achieved in submerged FSW with rotational shoulder (RFSW). This was accomplished through narrower weld zones, finer grain structure, maintained strengthening precipitates, and more symmetrical temperature and material flow fields. In contrast to RFSW, SSFSW samples exhibited a nugget zone with a grain structure in the nanometer range (900 nm) and a higher density of strengthening precipitates. The underwater SSFSW prevented weakening in the heat-affected zone by reducing the heat input and increasing the cooling rate. As a result, the minimum hardness shifted from the heat affected zone to its boundary with the thermo-mechanically affected zone. The addition of nanoparticles significantly contributed to joint strengthening, and the specimen prepared from the stir zone of the SSFSW-optimum sample achieved a tensile strength of 494 MPa. The primary mechanism of joint strengthening in SSFSW was grain boundary hardening, while quench hardening was the primary mechanism in RFSW. Additionally, the Orowan hardening mechanism had a more significant contribution in SSFSW due to the higher concentration of strengthening precipitates that were retained during the process.

Differences in Achilles tendon mechanical properties between professional ballet dancers and collegiate athletes utilizing shear wave elastography

Abstract

Purpose

To report normative stiffness parameters obtained using shear wave elastography in dorsiflexion from the Achilles tendons in asymptomatic professional ballet dancers and compare them with college-level athletes.

Methods

An Institutional Review Board (IRB)-approved study consists of 28 professional ballet dancers and 64 asymptomatic collegiate athletes. The athletes were further subdivided into runner and non-runner disciplines. Shear wave elastography (SWE) measurements were made in maximum ankle dorsiflexion position.

Results and discussion

Forty-eight (52%) males and 44 (48%) females were examined with an overall mean age of 22.2 (± 3.8 years). There were no significant SWE differences between dominant and non-dominant legs in both groups and comparing spin vs. non-spin leg of ballet dancers (p > 0.05). Ballet dancers had significantly higher short-axis velocity values than runners and non-runners (2.34 m/s increase and 2.79 m/s increase, respectively, p < 0.001). Long-axis velocity was significantly higher in ballet dancers compared to non-runners (by 0.80 m/s, p < 0.001), but was not different between ballet dancers and runners (p > 0.05). Short-axis modulus was significantly higher in dancers compared to runners and non-runners (by 135.2 kPa and 159.2 kPa, respectively, p < 0.001). Long-axis modulus (LAM) was not significantly different in ballet dancers when compared to runners.

Conclusion

Asymptomatic professional ballet dancers exhibit greater short-axis tendon stiffness compared to athletes and greater long-axis tendon stiffness compared to non-runners but similar to runners. The functional benefit from elevated short-axis stiffness in dancers is not clear but may be related to greater axial loading and adaptations of the tendon matrix.