| Aluminum alloys with low density and high specific strength are prone to asubstitute of steel in automobile industry for fuel efficiency and energyconservation. The reliable joints of Al/Steel are needed to satisfy thedevelopment of automobile industry. However, the joining technologies of thesetwo materials are limited due to their different physical and thermal properties.Brittle intermetallic compounds (IMCs) were easily formed during the weldingprocess which seriously affected the durability of the joint. Recently, friction stirlap welding (FSLW) of Al/steel has been becoming a research hotspot. In thisthesis, AA6082-T6and QSTE340TM steel were friction stir lap welded in thispaper to investigate its joint microstructures, mechanical properties and theformation mechanism of IMCs, and reveal the relationship betweenmicrostructure, IMC and mechanical property.With pin length of2.9mm,3.1mm,3.3mm and process parameters as the tiltangle of the rotating tool with respect to the vertical axis of the aluminum plateof2.5degree, the shoulder plunge depth of0.2mm, the rotating speed of1000rpm and the traverse speed of50mm/min, defect-free FSLW joints wereobtained. The pin length of tools has a significant effect on mechanical propertyof Al/steel friction stir lap welded joint, with the pin length of3.1mm, the failureload can reach5233N, much higher than that of2.9mm and3.3mm. The failurelocations were all located at the interface region of the lap joints, fracture revealsdissociation. This results were analyzed as follows: effective bearing area andthe microstructure of IZ determines the mechanical of aluminum/steel lap joint,with pin length of2.9mm, the effective bearing area was small and the transitionin chemical composition at the interface became sudden, while with pin length of3.3mm,the microstructure of IZ was complicated and a heavy IMC wasformed,which severely reduced the mechanical property of jointsThe microstructure of weld joints includes weld nugget zone,thermo-mechanically affected zone, heat-affected zone, the interface zone ofaluminum/steel and fine grain zone at steel side. The interface zone revealsfeature of hook-like structures, laminate structure and fragment of Al and steel. The hook-like structures were Fe and the laminate structure was FeAl3-FeAl-Fecomposites. Brittle IMCs such as FeAl3, FeAl and Fe3Al were formed at IZ, thusIZ became the weakest part of Al/Fe joints. At fine grain zone of steel, grainswere extended, twisted, and refined due to a strong stirring action. With thedistance increase from the IZ, the degree of grain refinement significantlyreduced greatly, its hardness distribution was organization correspondence withmicrostructure feature.Double-pass FSLW method was introduced to join Al and steel, the twoWNZ were overlapped, and interaction region were obtained. Besides asignificant change in IZ. The two IZs were not overlapped. The hook-likestructures, laminate structure and fragment of Al and steel reduced. No IMCswere detected. When the offset distance of the two passes were5.8mm and8.0mm, the maximum tensile shear load were increased by nearly35.5%and56.8%, respectively. When the offset distance of8.0mm, fracture location shiftedfrom the interfacial region to the interface of heat affected zones at the junctionof aluminum plate. Its fractures still revealed dissociation, which was due to thetwo weld heat affected zones superimposed on each other, second-phase particlesprecipitated, and heat affected zone became weak. Due to the effective bondingarea increased at IZ and microstructure morphology of IZ was optimized.Finally, a new type of rotating tool was developed. The effective bearingarea of the joint increased and the joint defects reduced. The tensile shear load ofthe joint increased by37%, while the fracture location and fracture morphologydid not change. |
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