Influence Of The Processing Parameters On Microstructure And Mechanical Properties Of AZ31 Magnesium Alloys During Friction Stir Processing

AZ31 magnesium alloy is the widely used wrought magnesium alloy.It has a very good development prospect in automotive lightweighting.Friction stir processing as a new kind of solid phase connection technology,laies the foundation for the application and research of magnesium alloy.Now research on friction stir processing of magnesium alloy is generally under different stir processing tools,reduction and tilt,to respectively discuss the organization and performance of material under a single or a few processing parameters.It makes the results difficult to unity,and is complex to analysis the organization and performance.Therefore,this article selects the AZ31 magnesium alloy as the research object,uses same stir processing tools,reduction and tilt angle,and only changes rotating speed and advancing speed.The article explores the influence of the change of the friction stir processing parameters on the microstructure and mechanical properties of processing area in the rotating speed is 1200-1600 r/min,and the advancing speed is 200-400 mm/min.The main results are as follows:(1)Through metallographic microstructure,we know the microstructure of welding nugget zone is fine and uniform equiaxed grains,and its grain size decreases from top to bottom at the ND direction.The microstructure of the thermal mechanical affected zone is not uniform,and the grain size is different.The grains of heat affected zone grow up obviously.When only increasing rotating speed,the average grain size will be bigger than welding nugget zone,thermal mechanical affected zone and heat affected zone.When only increasing advancing speed,welding nuclear area,the average grain size will be smaller of thermal mechanical affected zone and heat affected zone.(2)Research of the evolution rule on the microstructure and texture of materials under different processing parameters shows: on the ND direction,surface of shoulder affected zone(SFZ)has a layer of thin section.It has different crystal orientation with middles of SFZ.When the rotating speed is 1600 r/min,and the speed is 300 mm/min,the difference of the crystal orientation is largest,and the grain size of SFZ surface is smallest.On the ND direction,when n/V increases,the grain size of SFZ surface increases,the grains increase on the <0001> directon,the small angle grain boundary decreases,and C axis will be perpendicular to the TD direction,moving from ND direction to WD direction.When n/V increases,the grain size of middle of SFZ increases,most grains are on the <01-10> or <-12-10> direction,the small angle grain boundary increases,and C axis is perpendicular to the TD direction,with disobvious deflection.When n/V increases,the grain size of SZ increases,most grains are on the <0001> direction,the small angle grain boundary increases,and C axis is perpendicular to the TD direction,moving from ND direction to WD direction.When n/V increases,the grain size of SWZ increases,grains are mainly on the <01-10> or <-12-10> direction,the small angle grain boundary increases,and C axis is perpendicular to the TD direction.On the TD direction,when n/V increases,the grain size of TMAZ near SZ of advancing side increases,and there are different crystal orientations.And the small angle grain boundary increases.When n/V increases,the grain size of TMAZ near SZ of retreating side increases and most grains are on the <01-10> or <-12-10> direction,at the same time with a small amount grains of <0001> direction.And the small angle grain boundary increases.From TMAZ to SZ,C axis has the characterstic of tilting to the ND direction.When n/V increases,C axis turns more sharply.(3)Through the microhardness test,we know that the microhardness curve is inversed "V" shape for different processing parameters and different layers,and SZ has a highest hardness.When only increasing rotational speed,microhardness decreases.When only increasing advancing speed,microhardness increases.By tensile test,it shows that compared with the parent metal,yield strength,tensile strength and elongation of materials after friction stir processing decrease.When rotating speed is 1600 r/min,and advancing speed is 300 mm/min,tensile strength and elongation of the tensile samples are best.Under different processing parameters,the fracture position of the tensile sample position is always near TMAZ of the advancing side.When rotational speed is 1400 r/min,advancing speed is 300 mm/min,fracture of tensile sample is the "?" shape.For other tensile sample,angle of the fracture surface and the TD direction is about 45°.There are a lot of tearing ridges and fine dimples and some cleavage plane on the fracture of the sample.it is mixed fracture.