| Aluminum matrix composites have been widely applied in aerospace,nuclear power and automotive industries due to their excellent performances,such as low density,high stiffness,high hardness,low coefficient of thermal expansion,good wear resistance and thermal conductivity.However,joining aluminum matrix composites by traditional fusion welding methods brings about pores,particles segregation and brittle phases,which were detrimental to the mechanical properties of the joint.Friction stir welding(FSW)is a novel solid state welding technique and an ideal welding method for aluminum matrix composites,since FSW can effectively eliminate the problems associated with the fusion welding.However,severe wear of stirring tool always occurred during FSW of aluminum matrix composites,due to the presence of the hard reinforcing particles.Besides,the poor ductility resulted in the a narrow range of welding parameters,which limits the wide application of the aluminum matrix composites.In this study,SiCp/6092Al composite was subjected to FSW under a wide range of welding parameters,and the mechanisms affecting mechanical properties and high cycle fatigue properties of the FSW joints were clarified.Meanwhile,the dissimilar FSW of SiCp/6092Al composite and commercial 6061 aluminum alloy was investigated,the effect of the tool offset to the aluminum alloy side on the tool wear,and the effect of welding parameters on the microstructure and mechanical properties were systematically studied.The main research works had been carried out:T6-state 17vol.%SiCp/6092Al plates were conducted by FSW at different welding speeds by a hard wearing cermet tool.The effect of welding speeds on the microstructure and mechanical properties of FSW joint was investigated.The results showed that almost no obvious defects were observed in the SiCp/6092AI FSW joints at the rotational rate of 1000 r/min,as the welding speed increased from 50 mm/min to 800 mm/min.However,the tunnel micro-defect was produced in the nugget zone on the advancing side at a high welding speed of 1200 mm/min.The hardness and tensile strength of FSW joints were gradually enhanced with the welding speed increased.Specially,as the welding speed increased to 800 mm/min,the FSW joint exhibited the highest tensile strength of 355 MPa,which reached 72.4%of the base material.When the rotational rate increased to 2000 r/min,a defect-free FSW joint was obtained at a ultra high welding speed of 2000 mm/min.Microstructural observation indicated that the coarsening of the precipitates was greatly inhibited in the heat affected zone of the FSW joint at high welding speed,due to the significantly reduced peak temperature and duration at high temperature.Therefore,prominent enhancement of the ultimate tensile strength was achieved at this high welding speed,the joint efficiency could reach up to 75.3%of the base material.The dissimilar FSW was performed on the T6-state 17 vol.%SiCp/6092 Al composite and 6061-T6 aluminum alloy by using a common steel tool.The effect of tool offset on tool wear and the influence of welding parameters on the microstructure and mechanical properties of heterogeneous FSW joints were studied.The results showed that tool wear could be effectively reduced when the tool was offsetted toward the 6061 aluminum alloy side.Almost no tool wear was observed with the pin offset on the 6061 aluminum alloy side completely.Furthermore,the pin offset had little effect on the size of the nugget zone and the mechanical properties of the joints.As the rotational rate increased,the mixing effect between the two materials was better.When the rotational rate was 1200 r/min,an aluminum alloy band was formed between the nugget zone and the thermal-mechanically affected zone on the advancing side,which significantly weakened the local interfacial bonding.As the welding speed increased,the interfacial bonding was not weakened even if the mixing effect became worse,while the hardness and tensile strength of the joints were gradually increased.When the rotational rate was 800 r/min and the welding speed was 800 mm/min,the joints exhibited the highest tensile strength of 233 MPa,which was 79.8%of the 6061 aluminum alloy.The high cycle fatigue performances of SiCp/6092Al composite FSW joints and dissimilar FSW joints between SiCp/6092Al composite and 6061 aluminum alloy were studied,and the key factors affecting the high cycle fatigue performances were clarified.The results showed that increasing the welding speed could enhance the hardness and tensile strength of the SiCp/6092Al FSW joints,but the fatigue properties were not improved for the joints with unpolished surfaces.The result of the three-dimension surface topography showed that a large surface roughness was achieved on the surface of the joint at the high welding speed,resulting in the lower fatigue limit compared to that of the joint at the low welding speed.For the specimens with polished surfaces,the fatigue limit was improved by 40?65 MPa compared to that of the specimens with unpolished surfaces.In this case,a high fatigue limit of 205 MPa was obtained in the joint at the high welding speed of 800 mm/min,and all the specimens failed at the lowest hardness zone and nearby.For dissimilar FSW joints between SiCp/6092Al composite and 6061 aluminum alloy,the poor interface joining in the swirl zone led to the relative low and discrete fatigue life at the rotational rate of 800 r/min and the welding speed of 800 mm/min.The poor interface joining of could be improved significantly when the rotational rate increased to 1200 r/min,and a stable fatigue performance was acquired.In this case,the fatigue strength at high welding speed(800 mm/min)was 10 MPa higher than that at low welding speed(100 mm/min).The effects of tool size,pin length and welding parameters on the "Hook" defects and mechanical properties of the friction stir lap welded joints of SiCp/6092Al composites were studied.The results showed that all the tensile shear specimens fractured along the "Hook"defect by using various tool sizes and pin lengths.Using the smaller tool with the shorter pin,the "Hook" defect was weakened and a higher tensile shear load was achieved.Furthermore,the "Hook" defect became severer,accompanied with the reduced tensile shear load as the rotational rate increased.Meanwhile,the "Hook" defect was weakened significantly,accompanied by the enhancement of the tensile shear load as the welding speed increased.In particular,when the welding speed was increased to 1000 mm/min,almost no "Hook" defect was observed.As a result,a rather high tensile shear load of 7.79 kN was obtained at this high welding speed.Then,the joint fractured along the original lap interface of joints rather than from the "Hook" defect. |
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