Double-sided Friction Stir Welding Of Magnesium Alloy Thick Plates

As a new solid phase connection method,friction stir welding has a series of incomparable advantages,and it has made fruitful achievements in magnesium alloys welding.While magnesium alloys are the lightest metal structure material,there are plenty of industrial demands for large scale magnesium alloys.However,present studies basically concentrating on magnesium alloy sheet,thick plates of more than 20 mm magnesium alloy are rarely studied for welding.This paper welds AZ80 magnesium alloy plates of 30 mm and 40 mm in thickness with double-sided friction stir welding,studying the welding process,joint microstructure,mechanical properties and corrosion properties.The purpose is to explore the feasibility of magnesium alloy thick plate friction stir welding,and to explore dynamic re-crystallization nucleation mechanism of nugget zone and the corrosion properties of welded joints.With a rotation speed of 360r/min and a welding speed of 40mm/min,the 30 mm thick AZ80 magnesium alloy was successfully jointed by two-sided friction stir welding,and joint's tensile strength is 294 MPa,83% of the base material.Nugget zone of the joint are uniformly fine re-crystallized grains,and is a single phase structure of ?-Mg because?-Mg17Al12 phase is almost completely dissolved.Grains in heat affected zone(HAZ)are similar to the base metal except ?-Mg17Al12 phase partial dissolution.Base metal has not changed,as a two-phase structure of ?-Mg and ?-Mg17Al12.TEM shows that nugget zone dynamic re-crystallization nucleation mechanisms are in two ways: cellular nucleation and bulging nucleation.40mm-thick AZ80 magnesium alloy was welded with a rotation speed of 160r/min and a welding speed of 80mm/min as well as a rotation speed of 180r/min and a welding speed of80mm/min,using double-sided friction stir welding.No significant differences are found in the microstructure of two kinds of welded joints.Both joint has a uniform fine-grain nugget zone,a larger grain-size shoulder zone than nugget zone and a HAZ with a portion of grains growing up.Micro-hardness of two kinds of joints horizontally presents “W-shaped” distribution.The base metal has the highest micro-hardness and thermal-mechanical affected zone(TMAZ)has the lowest.In the thickness direction,the joint of A parameter shows an “M-shaped”distribution while B parameter's joint presents an “?”shape.The tensile strength of the joint under A parameter is 320 MPa,90% of the base material.Under B parameters,tensile strength of joints is 311 MPa,87% of the base material.And tensile fractures are both in the forward side of the TMAZ.When the strain rate rises from 10-3s-1 to 103s-1,base metal,nugget zone and HAZ all show a strain rate effect.When the strain rate is higher than 103s-1,base metal,nugget zone and HAZ behave without strain rate effect.In a similar strain rate,base material shows the highest fracture strength and nugget zone is lowest.Nugget zone shows the highest fracture strain and the most energy absorption while the base material owns a minimum.Nucleus grains are deformed due to the high-speed impact,by contrast,there are many twin crystals in the grains of HAZ and base metal.Slippage is the main mechanism of plastic deformation in nugget zone,and the heat-affected zone and base material have a slip and twinning concomitant mechanism.Electrochemical corrosion and weight loss test of AZ80 magnesium alloy FSW joints show that the best corrosion resistance is the base material,followed by heat affected zone and the worst nugget zone.Morphology of corrosion weight loss of the base metal appears an inter-granular characteristic,HAZ corrodes in a lateral spreading along Mg17Al12,and nugget zone shows a filamentous corrosion developing in depth extensions of the substrate.