The Research Of Process And Properties Of Magnesium Alloys And Steel Friction Stir Welding

Magnesium alloys is one of the lightest structure material which is widely used inautomotive、aerospace and communication fields，and the steel is the widely usedstructure material at currently，thus，it is inevitable to face the dissimilar metal joiningproblem between magnesium alloys and steels in project fields．Due to the greatdifferences in physical and chemical properties of the two kinds of metals，it is difficultto obtain a good joint by using the conventional fusion welding methods，and themagnesium will be lossed easily during fusion welding process．As a new joiningtechnology，friction stir welding (FSW) has a great advantage to solve the joiningproblem of dissimilar materials with high performance differences．And FSW is an idealweld method for joining dissimilar materials which has a great significance to realizinga well joining between the magnesium alloys and steels．In this paper，the4mm thick AZ31B magnesium alloy and Q235steel wereselected as experimental materials for friction stir lap welding and butt welding．Theimpact of process parameters on the weld formation and the joint performance wereinvestigated．The transition of microstructure of joints was observed by opticalmicroscope，and the elements distribution of the interface and interface characteristicsof magnesium and steel were examined by scanning electron microscopy (SEM)，X-rayenergy dispersive spectroscopy (EDS) and electron probe microanalysis．The results show that，the butt welding and lap welding of the magnesium and steelwere success welded by friction stir welding, For the friction stir butt welding，when therotating speed was600rpm，welding speed was60mm/min，the pin offset is1mmtoward steel and steel was put on return side，the well weld was formed and theconnection of magnesium/steel was tight．The upper part of the weld nugget isconstituted of magnesium，the grains of this part are coarse compared with the basematerial，the lower part of the weld nugget has larger number of steel particles，Thegrains of heat-affected zone (HAZ) on the magnesium-side was growing significantly；The microstructure of the thermo-mechanically affected zone (TMAZ) on the steel-sideup to down was non-uniformed ferrite side plate+proeutectoid ferrite+pearlite，non-uniformed ferrite side plate+pearlite，then fine acicular ferrite and pearlite alongthe thickness direction．The microstructure of the HAZ on the steel side and the base metal are constituted of proeutectoid ferrite and pearlite．The interface shape betweennugget and steel change from irregular to straight line along the thickness direction，theMg atoms diffused into steel side within short distance close to the interface，but the Featoms had not diffused into magnesium side；The hardness of the cross section of jointwas showing a high-(non-uniformed)-low distribution，the hardness of some area innugget is quite high，the maximum hardness value reaches to324.7HV．When rotatingspeed was600rpm，welding speed was60mm/min，the pin offset was1mm towardsteel，the maximum tensile strength is about176.5MPa，which was about67.9%of thetensile strength of magnesium matrix．The tensile strength of joint is non-uniformed，themaximum tensile strength appeared in the central of joints，and it is higher than theoverall joints tensile strength，which was about75.3%of the tensile strength ofmagnesium matrix，the tensile strength of the upper and the lower part of joint waslower than the overall joints tensile strength．The joint usually fractured along theinterface between the weld nugget and steel．In the lap welding，when the plunge depth of shoulder was0.4mm，rotating speed was600rpm，welding speed was60mm/min, the formation of joint is the best，the interfacebetween weld nugget and steel like a“pincer”．The microstructure of the TMAZ on thesteel side is including of ferrite side plate and pearlite，the grains was refine compared tothe base material，the HAZ on the steel side is not obvious．Compared with magnesiummatrix，the grains in magnesium side were growing significantly，the grains of weldnugget near the interface between magnesium and steel were growing mostly．Thehardness of the cross section of joint is distributing uniformly，the maximum tensileshear force of the joint is6.39kN...