Study On Process And Mechanical Properties Of Friction Sir Welded Dissimilar Al-steel Joints

Dissimilar joints between aluminum and steel are increasingly being used in industry applications like shipbuilding,aerospace,automobile and other fields.Joining Al and steel by conventional fusion-welding is rather complicated and difficult,as a result of the huge differences in their physical,chemical and mechanical properties.As a new solid-state joining process,friction stir welding(FSW)has been attained more and more attention in the joining of dissimilar Al-steel.In this thesis,5182 Al and DP1180 steel plates with thickness of 1.2 mm were subjected to FSW.A systematic study of the FSW parameters of Al-steel butt and lap joints was investigated.The aim of this work is to obtain the key parameters for sound Al-steel joints by the process optimization,and investigate the microstructure characteristics of intermetallic compounds(IMC)and illustrate the interface forming mechanism.At the same time,the effects of the microstructure of interface on mechanical properties were also analyzed in detail.The main results are summarized as follows:1.2 mm thick Al-steel sheets were selected for the research of FSW butt-joints.The steel plate is located on the advancing side(AS)while aluminum plate located on the retreating side(RS).When the pin offset was 1 mm in steel side,the formability of joints could be the best.The heat input in the FSW process can be controlled by adjusting two process parameters of rotational speed and welding speed.Sound butt joints with the same strength to the base metal can be obtained by using a low heat input parameters of 400 rpm-50 mm/min.Under this parameter,good metallurgical combination between two materials was achieved as a thin,uniform and continuous IMC generating in Al-steel interface.The IMC on the interface and nugget zone(NZ)of FSW Al-steel butt joints was investigated in detail by transmission electron microscopy(TEM),and Al-Fe reaction mechanism was described.NZ with onion ring structure exhibited a mixture of the Almatrix and steel particles.The Al-Fe IMC layer generated around the steel particles,and the IMC was characterized as Al5Fe2 phase by electron diffraction analysis.Some layed structures were observed nearby the interface,which increase the thickness of the IMC layer,causing the decrease of the mechanical properties.TEM investigations revealed in detail that the Al-steel interface consisted of mixed layers at steel side and the IMC layer with a thickness of 250 nm.In the mixed layer,a banded-like structure with lighter and darker parts was observed,which were identified as steel matrix(~500 nm thick)and the thin IMC stripes(~50 nm thick).Mixed layer exhibited elongated grains whose main elements were Fe.The IMC in Al-steel interface of FSW butt joints were also identified as the Al5Fe2 phase.Adjacent to the IMC layer,ultra-fine grains of Al were observed.Using different size stir tools,1.2 mm thick Al-steel sheets were selected for the research of FSW lap joints.The effects of the tool size,plunge depth and heat input on the microstructure and mechanical properties of Al-steel lap joints were investigated.The steel plate was located above the Al plate.When the pin was pressed about 0.2 mm into the steel plate and the steel plate was located in the AS,the lap joints with best properties could be attained at the parameters of 800rpm-50 mm/min by using the large size tool.Under this parameter,all the joints exhibited the same strength with the base metal and the fracture locations were at the Al side.NZ distibuted lots of steel particles,which wrapped around with thick IMC layers.Hook defects were obviously weakened under the control which only slightly humped in the AS.A continuous and uniform IMC layer formed at the Al-steel interface,whose composition was identified as Al4 Fe phase by EDS.