P. Xue

Achieving high property friction stir welded aluminium/copper lap joint at low heat input

Abstract Aluminium and copper plates with 3 mm thickness were successfully friction stir lap welded at a lower rotation rate of 600 rev min−1 using a larger pin 8 mm in diameter. Good metallurgical bonding on the Al/Cuinterface was achieved due to the formation of a thin, continuous and uniform Al–Cu intermetallic compound layer. Furthermore, many Cu particles consisting of pure Cu and intermetallic compound layers were generated at the lower part of the nugget zone, forming a composite structure with increased hardness. A lower rotation rate resulted in a decrease in annealing softening in the heat affected zone (HAZ), and a larger diameter pin increased the Al–Cu bonding area. These factors resulted in that the friction stir welded lap joint exhibited a high failure load of 2680 N with failure in the HAZ on the aluminium side.

Effect of Rotation Rate on Microstructures and Mechanical Properties of FSW Mg-Zn-Y-Zr Alloy Joints

Friction stir welding (FSW) of Mg-Zn-Y-Zr plates with 6 mm in thickness was successfully carried out under a wide range of rotation rates of 600-1200 r/min with a constant traverse speed of 100 mm/min. After FSW, the coarse grains in the parent material (PM) were changed into fine equiaxed recrystallized grains at the nugget zone (NZ). Furthermore, the coarse Mg-Zn-Y particles (W-phase) were broken up and dispersed homogenously into the Mg matrix. With increasing rotation rates, the size of the W-phase particles at the NZ significantly decreased, but the recrystallized grain size tended to increase. The hardness values of the NZs for all the FSW joints were higher than those of the PM, and the lowest hardness values were detected in the heat affected zone (HAZ). The fracture occurred in the thermo-mechanical affected zone (TMAZ) on the advancing side for all the FSW joints in the tensile test, due to the incompatibility of the plastic deformation between the NZ and TMAZ caused by remarkably different orientation of grains and W-phase particles. The strength of FSW joint reaches 90% of that of its PM.

Realising equal strength welding to parent metal in precipitation-hardened Al–Mg–Si alloy via low heat input friction stir welding

ABSTRACT Two millimetres thick Al–Mg–Si (6061Al-T6) alloy plates were friction stir welded at various welding conditions. Under a low rotation rate of 400 rev min–1 with rapid water cooling, the softening zone in the joint disappeared and a nanostructure with an average grain size of 80 nm was obtained in the stir zone (SZ). Therefore, a weld with equal strength to the parent metal (PM) was successfully achieved with the fracture occurring in the PM. Further, the average microhardness and ultimate tensile strength (UTS) of the SZs increased with the decreasing rotation rate and increasing cooling speed. The average microhardness and UTS of the SZ with nanostructure reached up to 134 HV and 505 MPa, respectively; though the initial strengthened precipitates disappeared. This work provides an effective strategy of achieving high property joints and enhancing the mechanical properties of precipitation-hardened Al alloys.

Material flow and void defect formation in friction stir welding of aluminium alloys

ABSTRACT An ingenious experimental programme by combining artificially thickened oxide layer as marker material and ‘stop-action’ welding were used to study the material flow and defect formation in friction stir welding of aluminium alloys. The results showed that material flow around the pin on the advancing side (AS) was severer than that on the retreating side (RS) and the fastest velocity of material flow in the middle stir zone (SZ) was 43.9 mm s−1. Moreover, the material under the RS shoulder included extruded metal only and the material under the AS shoulder included extruded and rotated metal. Lastly, instantaneous void occurrence and insufficient inflow material were reasons for the preferential formation of void defects in the top SZ on the AS.

Microstructural and Mechanical Evolution of a Low Carbon Steel by Friction Stir Processing

A low carbon steel (Grade A) was subjected to friction stir processing (FSP), and the effect of FSP on the microstructure and mechanical properties was investigated systematically. It was found that two distinct zones called stir zone (SZ) and heat-effected zone (HAZ) were formed during FSP. The SZ and HAZ consist mainly of ferrite, widmanstatten ferrite, ferrite+cementite aggregates, and martensite. FSP considerably refined the microstructure of the steel by means of dynamic recrystallization mechanism and formed a volumetric defect-free basin-like processed region. The ferritic grain size of the steel decreased from 25 µm in the coarse-grained state to about 3 µm in the fine-grained state, and the grains formed were separated mostly by high angle of misorientation with low density of dislocations. This microstructural evolution brought about a considerable increase in both hardness and strength values without a considerable decrease in ductility. Ultrafine-grained microstructure formed around and just beneath the pin increased the hardness of the steel from 140 Hv0.3 to about 245 Hv0.3. However, no hardness uniformity was formed throughout the processed zone due to the changes in deformation- and temperature-induced microstructure. Both yield and tensile strength values of processed zone increased from 256 and 435 MPa to about 334 and 525 MPa, respectively.

Achieving superior low temperature and high strain rate superplasticity in submerged friction stir welded Ti-6AI-4V alloy

The superplastic forming of Ti alloy welds has great application prospects in producing integrated components. However, the nugget zone (NZ) of the Ti alloy welds, produced by fusion welding or conventional friction stir welding (FSW), consists of lamellar microstructure, which exhibits either low superplasticity or high superplastic temperautre and low strain rate. As a result, the NZ plays a leading role in hindering the superplastic forming of the whole welds. In this study, submerged friction stir welding (SFSW) was conducted in Ti-6Al-4V alloy for the first time, and a defectfree weld with the NZ consisting of a strip microstructure was obtained. The NZ exhibited a low-temperature superplasticity at 600°C, which was the lowest superplastic temperature ever reported in the Ti alloy welds. Besides, at 800°C, the NZ showed high strain rate (3×10−2 s−1) superplasticity and a largest elongation of 615% at 1×10−3 s−1. Compared to conventional FSW joints, the NZ of SFSW joint exhibited a much lower flow stress and a decrease in optimal superplastic temperature by 100°C. This is mainly attributed to the easy globularization of the strip microstructure, enhancing the ability of grain/phase boundary sliding.摘要钛合金焊接接头超塑成型用于生产整体构件具有广泛应用前景. 熔焊或常规搅拌摩擦焊(FSW)通常得到具有片层组织的焊核, 从而导致过低超塑性、 或过高超塑温度以及过低应变速率, 成为影响接头整体成型的关键. 本研究首次采用水下FSW(SFSW)对Ti-6A1-4V进行焊接, 得到焊核为条带组织的无缺陷接头. 焊核在600°C下仍具有超塑性, 是目前实现钛合金焊接头超塑性的最低温度. 此外, 焊核可在800°C下和高应变速率(3×10−2 s—1)下实现超塑性, 并在1×10−3 s−1下获高达615%的延伸率. 与常规FSW相比, SFSW焊核的最佳超塑温度下降了100°C且流变应力大幅下降, 其优异超塑性能主要是由于条带组织在超塑变形中极易球化, 提高了晶界/相界滑移能力的结果.

Effect of Processing Parameters on Plastic Flow and Defect Formation in Friction-Stir-Welded Aluminum Alloy

The effect of processing parameters on material flow and defect formation during friction stir welding (FSW) was investigated on 6.0-mm-thick 2014Al-T6 rolled plates with an artificially thickened oxide layer on the butt surface as the marker material. It was found that the “S” line in the stir zone (SZ) rotated with the pin and stayed on the retreating side (RS) and advancing side (AS) at low and high heat inputs, respectively. When the tool rotation rate was extremely low, the oxide layer under the pin moved to the RS first and then to the AS perpendicular to the welding direction, rather than rotating with the pin. The material flow was driven by the shear stresses produced by the forces at the pin–workpiece interface. With increases of the rotation rate, the depth of the shoulder-affected zone (SAZ) first decreased and then increased due to the decreasing shoulder friction force and increasing heat input. Insufficient material flow appeared in the whole of the SZ at low rotation rates and in the bottom of the SZ at high rotation rates, resulting in the formation of the “S” line. The extremely inadequate material flow is the reason for the lack of penetration and the kissing bonds in the bottom of the SZ at extremely low and low rotation rates, respectively.