Research On Microstructure And Fatigue Damage Mechanism In 6005A Aluminium Alloy Welded Joints

With the acceleration of industrialization and the dramatic increase in population,the quantity demanded for energy increase.But the extensive use of energy causes a series of environmental problems.The problem of all kinds of transportation energy conservation and emissions reduction becomes the important direction concerned by various countries.The cold metal transfer(CMT)welding method based on melts gas arc welding is a new type of welding technology for splashless and low heat input which is suitable for thin plate,particularly.CMT technology suits for a variety of sheet,such as galvanized steel,aluminum alloy plate,magnesium alloy plate and other materials.In the welding process,CMT could achieve the alternating cold and hot by pulling wire to reduce current,thereby reducing the heat input.In this paper,CMT welding machine manufactured by Fronius Company is used to weld 6005 A aluminum alloy.The effect of microstructure,microhardness,fatigue property,and characteristics of fatigue crack initiation and propagation.The mechanism of fatigue crack initiation and propagation were studied.6005A aluminum alloy welded joint can be divided into three parts: base metal(BM),heat affected zone(HAZ)and welding zone(WZ).The width of WZ and HAZ is 5mm and 10 mm respectively.microstructure in WZ is typical casting structures,gains attach on the edge of the fusion zone and grow into the welding seam,In turn,generate the cellular grains,cellular dendritic grains and equiaxed grains which can't continue to grow too big because of the stirring action of CMT.Grains in BM are narrow strip.In the heat affected zone near the base area,grains' shape is similar to the BM which is narrow strip,but in the area near the WZ the narrow strip grains are coarsening which is blocky.The main phases in BM are ?-Al solid solution and impurity phases Al Fe Si phases.The main phases in WZ are ?-Al solid solution and Al Fe Si phases.There is a softening zone in HAZ where the hardness is 40 HV the lowest value in whole joint and the hardness in WZ is 70 HV.The fatigue limit of BM and welding joint are 82 MPa and 46 MPa,respectively.The fractures in BM samples are all in parallel part due to the machining defects on surface.The fatigue samples of welded joints are fractured in the WZ.Pores in WZ are the main reasons for breaking samples.Stress concentration appears in pole of pores during the cyclic stress which leads peak stress near pores is bigger than real cyclic stress,so samples break from pores.But there are two kinds of crack models near the pores because of different distribution locations of the pores.The microstructure near the fusion line is cellular grain which is long striped and smooth.The polemay be covered by cellular which make the stress concentration in there smaller than stress concentration in the pole with a gap,so the fatigue crack can't initiate in lower cyclic stress.If the pores are in center of WZ where the grains are equiaxed grains with smaller size,the pole couldn't be covered by the grain which make the gap is close to the polar.Then,the cracks near both poles are easy to initiate which lead double crack model.In general,when cyclic stress is small,crack still could initiate from the pore,but no propagation.So it has no influence to the fatigue life of samples.If the pores gather together,the crack initiated from each pores may link each other and reduce the fatigue strength.Secondary phases with Fe element can be found in the WZ which are long striped,and few are blocky.There are cracks near the secondary phase due to the interaction between dislocation and phase.The crack inside the long striped secondary phase is caused by dislocation pile-up in the boundary between secondary phase and matrix.The crack around the blocky secondary phase is caused by the dislocation ring pile-up.During the steady expansion of fatigue crack,the crack tip is constantly being passivated and sharpening under the action of tearing stress,thus forming a Z-shape crack direction.When the cyclic stress on fatigue sample unloads,micro crack closure phenomenon appears in the sample.But,there is no perfect closed,because the fatigue crack in the extension process can produce cylindrical plastic zone around the crack which leads matrix produce plastic deformation.The small deformation makes the upper and lower crack surface are different slightly,finally the crack can't perfect closure.