Achieving an excellent strength–ductility synergy in Zircaloy-4 by FSW with rapid cooling

Abstract

Zircaloy-4 plates of 2 mm in thickness were successfully joined by friction stir welding with rapid cooling. The processing temperatures were strictly controlled below the α/β phase transformation point. The stir zone (SZ) has an ultra-refined grain structure with high dislocation density and abundant twin boundaries. Grain subdivision and discontinuously dynamic recrystallisation contributed most to the grain refinement. A good match of strength and uniform elongation of 895 MPa and 16.8%, respectively, was achieved in the SZ by using liquid nitrogen cooling. This work provides an effective strategy to enhance the strength of friction stir welded Zircaloy-4 joint without ductility loss.

Structure–properties’ modification of 70/30 brass by large-load and low-speed friction stir processing

“…Figure 1 shows the microstructural characteristics of the LL-FSP 70/30 brass observed by SEM/EBSD. Oxides and porosities disappeared since the SZ underwent a relatively low peak temperature [10] (Figure 1(a)). The SZ was characterised by recrystallised grains, and the average grain size was determined to be 0.5 µm, which is highly refined compared with 24.7 µm of the base material (Figure 1(b)).…”

“…These defects significantly deteriorated the strength and the ductility of the SZ [8,9]. In our recent study, large-load and low-speed FSW which are newly developed by the authors was adopted to the joining of pure Cu [10]. A sound SZ without any defects can be obtained even the welded joint underwent a relatively low peak temperature.…”

Effect of rapid cooling on the microstructure and properties of fine-grained 7075 aluminium alloy under friction stir welding

“…This was because the friction between the shoulder and the upper surface of the BM provided heat input, but plastic deformation did not occur, which affected the grain growth behaviour within the upper surface. However, with a decrease in heat from the top to the bottom, the grain growth trend gradually weakened; this resulted in the inclined arrangement of the grain size [28].…”

Effects of intense cooling on microstructure and properties of friction-stir-welded Ti–6Al–4V alloy

“…The maximum hardness reaches approximately 351.3 HV. As we all know, the values of hardness are related to grain sizes for polycrystal, which can be described by the Hall-Petch relation [24], HV = HV 0 + kd −1/2 , where HV is the hardness of a polycrystalline metal, d is the grain size, and HV 0 and k are constants. The Hall-Petch relation predicts that the increase in hardness is due to the decreasing grain sizes in the SZ.…”

“…With regard to high-melting-point alloys, Garg et al [21], Stewart [22] and Baillie et al [23] demonstrated the feasibility of different kinds of steels underwater FSW. Furthermore, Xu et al [24] sprayed liquid CO 2 and liquid nitrogen towards the weld of Zircaloy-4 during the welding process. The ultra-refined grain structure and increased hardness in the SZ were attained under the rapid cooling condition and the tensile strength of joint using liquid nitrogen was bigger than those under other cooling conditions.…”