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Friction Stir Welding of SiCp/2009AI Composites

Author: WangDong
Tutor: MaZongYi
School: University of Science and Technology of China
Course: Materials Processing Engineering
Keywords: discontinuously reinforced aluminium matrix composite friction stirwelding microstructure mechanical properties local corrosion resistance
CLC: TG453.9
Type: PhD thesis
Year: 2014
Downloads: 41
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Abstract

Friction stir welding (FSW), as a solid-state joining technique, is considered a promising welding method for joining the discontinuously reinforced aluminium matrix composites (AMCs) to avoid the drawbacks of the fusion welding. However, severe wear of the steel tool occurred during FSW due to the presence of hard ceramic reinforcements. This not only reduced the lifetime of the tool, but also limited the welding parameters of the AMCs. Furthermore, the factors determining the properties of the joints were not well understood. In this study, an ultra-hard cermet tool was used to join SiCp/2009A1plates, which have been widely applied in industry. The aims are to investigate possibility of joining SiCp/2009A1in various heat treatment conditions, to elucidate the effects of shape and dimension of the welding tools on the microstructure and mechanical properties of the FSW joints, and to clarify the mechanisms affecting mechanical properties and corrosion properties of the FSW jointsHot-rolled, solutionized and naturally aged15vol.%SiCp/2009A1plates were successfully joined by FSW using the ultra-hard cermet tool. In the FSW joints of hot-rolled composite, part of Al2CU was dissolved into the aluminum matrix in the nugget zone (NZ) due to intensely plastic deformation and high temperature during FSW. The undissovled Al2Cu particles remained in the NZ and coarsened during the cooling process after FSW. The ultimate tensile strength (UTS) of the as-welded joint is only321MPa and failed in the BM zone due to the low strength of the BM. For both solutionized and naturally aged composites, a similar welding behavior was observed. In the NZ of both samples, the grain size and the distribution of the coarse Al2Cu phases and the hardness values were similar. In the heat affected zone, two low hardness zones (LHZs) were observed for both samples. The first LHZ adjacent to the NZ (LHZ I) had the lowest hardness. Both samples had the similar hardness in this zone.For the joint of solutionized composites, the LHZ far away from the NZ (LHZ II) had a higher hardness and was closer to the NZ.The ultimate tensile strength of both the samples was similar and reached83%of T4-tempered base metal.The effects of shape and dimension of the welding tools on the microstructure and mechanical properties of the FSW joints were investigated. The welding tools with cylindrical pin was used to join17vol.%SiCp/2009Al. For a pin design with three inclined flats on the outer surface and the triangular end surface, increasing the degree of the inclined flats and the diameter of the shoulder is beneficial to the material flow and the disruption of the oxides on the butt interface during FSW. However, the strength of the joints achieved with these tools was not very good. Especially, with low heat input, the strength of joints re-treated to a T4temper was lower than that of the T4-temperaed base material. Only with the high heat input (rotation rate is1000rpm and welding speed is50mm/min), the stregnth of the joints re-treated to a T4temper were similar to that of the T4-temperaed base material. The effects of the rotation rate, welding speed, and the shoulder plunge depth on the microstructure and mechanical properties by the threaded pin with coniform shape. This tool promoted the materials deformation during FSW. The sound joints could be achieved at a welding speed of100mm/min and tool rotation rates of600-2000rpm. The strength of the joints was85%of the BM and failed in the LHZ I. The strength of the joints was not changing with the tool rotation rate increasing. Furthermore, the17vol.%SiCp/2009Al plates could be successfully joined at high welding speed with increasing the shoulder plunge depth during welding process. At a tool rotation rate of1000rpm, the strength of the joints increased as the welding speed increased. The strength of the joint at a welding speed of600mm/min reached92%of the base material. At low welding speeds of50to200mm/min, two low hardness zones were observed in the heat affected zones and the tensile samples failed in the LHZ I. At welding speeds of300to600mm/min, the hardness values of the NZ and HAZ were similar. The joints failed in the NZ or HAZ randomly.The severe plastic deformation during FSW resulted in the variation of the size and shape of the SiC particles in the NZ. The yield strength of the NZ decreased slightly due to the variation of the SiC particles. Large compound particles on the interfaces were broken off during FSW, whereas the amorphous layer and small compound particles remained on the interfaces. The dynamically recrystallized Al grains nucleated on the surface of fractured SiC particles during FSW, forming nano-sized grains around the SiC particles. The clean interfaces were beneficial to the load transfer from Al matrix to SiC particles and then increased the UTS of the NZ. The microstructure and mechanical properties of the joints achieved at high welding speed (800mm/min) were different from that at low welding speed. The NZ exhibited the lowest hardness due to the formation of both0phases-100nm in size and S phases-20nm in size. In the heat affected zone, only part of the clusters dissolved into the Al matrix. The UTS of the joints reached97%of the based material and failed in the NZ.The local corrosion resistance of2009A1,17vol.%SiCp/2009A1and the FSW joints of17vol.%SiCp/2009A1were investigated. Since the segregation of the intermetallic particles in the matrix, some corrosion rings formed on the surface of the2009Al, when the samples was immersed in the57g NaCl+10ml H2O2+1L H2O solution. The severe intergranular corrosion and pitting corrosion occurred in the rings and penetrated into the matrix. For the composite, the presence of the SiC/Al interface inhibited the formation of the corrosion ring in the surface of the sample. Although the intergranular corrosion and pitting corrosion would also occur in the composite, the corrosion became homogenously compared to that in the2009A1. At low welding speed (50mm/min), the intergranular corrosion occurred in the NZ and LHZ I due to the coarse precipitates on the grain boundaries. Meanwhile, the Al2Cu phases on the SiC/Al interface and within the grains also improved the anodic dissolution of the Al matrix. Therefore, the corrosion potential and electrochemical impedance of the NZ and LHZ I decreased compared to those of the based material. At high welding speed (800mm/min), the small precipitates on the grain boundaries accelerated the intergranular corrosion. The severe intergranular corrosion resulted in the exfoliation corrosion on the bottom of the NZ.

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