Luan Guohong

EFFECT OF WELDING PARAMETERS ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF FRICTION STIR WELDED AlCuLi ALLOY JOINTS

Friction stir welding(FSW) of a novel AlCuLi alloy was conducted to investigate the effect of welding parameters on the microstructure and mechanical properties of the joints.The fine and equiaxed dynamically recrystallized grains were observed in the nugget zone(NZ).As the rotation rate increased,the size of the grains in the NZ increased.However,with increasing the welding speed,the size of the grains in the NZ decreased slightly.TEM analyses indicated that most of the precipitates in the NZ dissolved into the matrix during FSW and some coarse precipitates formed during subsequent cooling process.Moreover,many coarse precipitates were observed in the heat affected zone(HAZ) due to the FSW thermal cycle.At a low welding speed of 80 mm/min,the ultimate tensile strength of the joints increased as the rotation rate increased,and could reach up to 442 MPa which was 87% of that of the base metal.All of the joints failed in the lowest hardness zone of the HAZ.At a high welding speed of 200 mm/min,some defects resulting from insufficient material flow were observed on the fracture surfaces.At a low rotation rate,the joints failed along the defects in the NZ and exhibited a low strength.As the rotation rate increased,the size and number of the defects decreased.Therefore, the effect of the defects on the strength of the joints was significantly reduced,and a joint efficiency of 84%was obtained.

MICROSTRUCTURES AND MECHANICAL PROPERTIES OF BANDED TEXTURES OF FRICTION STIR WELDED 7075-T6 ALUMINUM ALLOY

Friction stir welding (FSW) is a ideal method for joining light metal materials, especially for AA2000 and AA7000 series aluminum alloys. In the welding field, many aspects of FSW have been studied quite extensively and comprehensively. However, the origin of the banded texture is still a subject of current research. One of the phenomena that strike researchers today is the occurrence of banded texture on the top surface in FSW joints. Furthermore, many waves also emerged in shoulder affected zone (SAZ) after a joint being polished and etched. The correlation between the banded textures and the waves is not clear. In this paper, 1.6 mm thick 7075-T6 aluminum alloy sheets were friction stir welded, microstructures and hardness profile of banded textures on the top surface of the joint were investigated. The results of metallographic observation showed that alternately light and dark waves presented in SAZ, the light waves coincided with valleys of the banded textures, whereas the dark waves coincided with peaks. SEM and TEM analyses revealed that compared with the valleys, the peaks were characterized by more secondary phase particles and small grains, resulting in higher hardness at the peaks.

Material flow behaviour of 2024-T351 aluminium alloys friction stir welding

Copper strips (0.1mm thick) were used as markers to investigate the metal flow during friction stir welding. After welding, radiography and metallurgical sectioning were used to investigate the marker movement. The scope of this investigation is to evaluate the effect of shoulder and probe on the material flow and oxide in different welding parameters. The results of this study indicate that the shoulder produces a grind effect and the probe produces an effective fragmentation and dispersion, on the other hand, with the increase of heat input, the effect of fragmentation and dispersion is decreased.