Investigation of microstructure and mechanical properties of friction stir welde

Kulwnt Singh,Gurhinder Singh,Hrmeet Singh

Abstract Increasing global demands for energy conservation and environmental protection led to the replacement of heavy components with lighter alloys.As magnesium alloys are believed to be unique candidates for lightweight applications and friction stir welding(FSW)is capable of joining magnesium alloys,in the current work,FSW joint of AZ61 Mg alloy has been fabricated.Microstructure and mechanical properties of the joints were evaluated.The elongated grains of the base metal were recrystallized in the stir zone and in transition zone during friction stir welding.The formation of fine grains in the stir zone of the joint is responsible for increase the hardness of the stir zone.The microhardness of base metal is higher than that of thermo-mechanically affected zone but lower than that of stir zone.The tensile strength of the weld was about 82%of the as-received base metal.The joint failed in ductile mode.This ductile failure of joint was due to the uniform deformation of material.

Keywords:Magnesium alloy;Friction stir welding;Stir zone;Microhardness;Microstructure;AZ61.

1.Introduction

The requirement of lightweight components has led to energy conservation and environmental protection[1].Magnesium is the lightest of all the engineering metals,having a density of 1.74 g/cm3[2].Alloying magnesium with aluminum,manganese,rare earths,thorium,zinc or zirconium increases the strength to weight ratio making them important materials for applications where weight reduction is important,and where it is imperative to reduce inertial forces.Because of this property,denser material,not only steels,cast iron and copper base alloys,but even aluminum alloys are replaced by magnesium-based alloys in a variety of automotive and structural applications by magnesium alloys[3,4].The adaptability of magnesium alloys can significantl influenc the overall performance of transportation industries in enhancing the fuel economy and reducing pollution[5].The current state of research and recent global demand has inspired the investigators to explore the feasibility of use of magnesium alloy for lightweight applications[6].Mg alloys are needed to promote wider adoption for improvements in manufacturing and processing,in-service performance,and cost of automotive[7].Even though the wider use of magnesium alloys requires feasible joining methods,there is still a lack of efficien and effective welding techniques for magnesium alloys[8].Magnesium alloys have limited workability at room temperature owing to their hexagonal close-packed structure;they have good formability at high temperature[9].Magnesium alloys are also attractive owing to their electromagnetic interference shielding properties and their recyclability[10].Magnesium alloys may be welded using conventional arc and advanced fusion welding techniques.But,the challenges for fusion welding of Mg stem from its relatively low viscosity and surface tension in the molten state,high oxidation potential,relatively high vapor pressure in the liquid phase,and significantl high hydrogen solubility in the liquid phase relative to the solid phase,which can result in an unstable weld pool,production of spatter,poor surface quality,porosity,solid inclusions,and evaporative losses[11].Friction stir welding is capable of joining magnesium alloys without melting and thus it can eliminate problems related to the solidificatio [12].Amongst all the traditional welding techniques,FSW is an energy efficien and versatile method of joining metals,alloys,and composite[13].As FSW does not require any fille material,the metallurgical problems associated with it can also be eliminated and good quality weld can be obtained[14].This technique was firs used to join aluminum and its alloys[15].Recently it has been used to weld magnesium alloys and other alloys[16–18].FSW has many potential applications in major industries,i.e.shipbuilding,aerospace,automobile,railway and many other industrial applications[19–23].Comprehensive work has been published for the friction stir welding and limited work on magnesium alloy[24,25]has motivated researchers to explore the work and scope of applications.In this work,friction stir welded joints of magnesium alloy AZ61 were investigated using optimized process parameters in earlier research.

Fig.1.Schematic illustrations of friction stir welding Process[27].

2.Working principle of friction stir welding process

In Friction stir welding,a non-consumable rotating tool with a specially designed pin and shoulder is inserted into the abutting edges of plates to fabricate the joint and traversed along the line of the joint[14,15,26].The schematic drawing of friction stir welding process is shown in fig 1.

In this process,the heating is accomplished by friction between the tool and the workpiece and plastic deformation of workpiece[27,28].The localized heating softens the material around the pin before it reaches its melting point and the combination of tool rotation and translation leads to the movement of material from the front of the pin to the back of the pin[29,30].As a result of this process,a joint is produced in ‘solid state’[14,15,31,32].Friction Stir Welding process parameters like tool rotation speed,welding direction,axial force,tool material and tool geometry play a very important role in FSW[33].

3.Experimental procedures

The plates of 4 mm thick AZ61 grade magnesium alloy were used in experimental work.The plates were then cutto the required length of 150 mm and width of 50 mm.The chemical compositions and mechanical properties of the base metal for this paper i.e.,AZ61 magnesium alloy are listed in Table 1.The AZ61 Mg alloy fla plates were cleaned using acetone before the welding for removal of any surface contamination.The initial joint configuratio was obtained by securing the plates in position using mechanical clamps(Fig.2).A square butt configuratio joint of the AZ61 pieces was fabricated by means of friction stir welding technique.

Table 1 Chemical composition of base metal AZ61 magnesium alloy(mass fraction,%).

Fig.2.Arrangement of the AZ61 Mg alloy fla plates in the fixture

A non-consumable tool used to fabricate the joints in present work was left-handed thread tool pin with three flute and the material tool was H-13.The shoulder diameter was 18 mm,keeping pin diameter and pin length of 6 mm and 3.8 mm respectively.Single pass welding procedure was used to fabricate the FSW joints.The welding parameters such as tool rotational speed and traveling speed were 1400 rpm and 25 mm/min,respectively.The process was carried out on a fie-axis milling center Deckel Maho DMU 50T.The specimens required for testing were cut by wire EDM.All samples were prepared as per ASTM(American Society for Testing Materials)standard.Tensile test specimens were prepared in accordance with ASTM E-8M specifications A standard metallurgical procedure was followed in the preparation of the metallurgical specimens.

Fig.3.(a)The plates of AZ61 magnesium alloy used for FSW,(b)square butt joint fabricated by friction stir welding technique,(c)tool used to fabricate the joints,(d)tool after fabrication of joint,(e)tensile test specimen as per ASTM E-8M,(f)tensile test specimens prepared in this investigation.

All polished specimens were etched by dipping in aceticpicral[10 ml acetic acid(99%),4.2-g picric acid,10 ml water,70 ml ethanol(95%)]for about 40 s and followed by drying in the blast of air.The plates of AZ61 magnesium alloy used for FSW,square butt joint fabricated by means of friction stir welding technique,tool used to fabricate the joints in present work,shape of tool after fabrication of joint,tensile test specimen dimensions as per ASTM E-8M specification and tensile specimens prepared in this investigation are shown in Fig.3.

4.Results and discussion

4.1.Macrostructure and microstructure

The weld cross section of the joint was analyzed at low magnificatio using a stereo zoom optical microscope and Fig.4 shows an overall macroscopic cross-sectional image of the friction stir weld zone of AZ61 Mg alloy at a rotation speed 1400 rpm and a welding speed 25 mm/min.The width of the friction stir zone/nugget zone(SZ)was about 6 mm i.e.equal to the diameter of the pin.There was a different macrostructure of the cross section in retreating side and advancing side,which is related to flw fiel of friction stir welds[34].It was found that the joint yielded an elliptical shaped and defect free stir zone.

Fig.4.Overall macroscopic cross sectional image of the friction stir weld zone in of AZ61 alloy.

The microstructure of friction stir zone(SZ)was greatly refine due to dynamic recrystallization and the grains become smaller as going from base metal to TMAZ and Nugget zone/stir zone(SZ)[17,35].The grains in thermomechanically affected zone(TMAZ)were severely deformed.The elongated grains of the base metal were recrystallized in the stir zone and also in transition zone during friction stir welding.

The development of recrystallized grains structure in the stir/nugget zone was due to the severe plastic deformation and heat due to friction introduced by the rotating tool pin and its shoulder during welding process[26,36–37].The high welding speed results these smaller grains in the stir zone are associated to the low heat input.Another reason for the grain size reduction at high welding speed could be the more straining in the material which activates more strain free nucleation sites[17,38].There was a significan difference in grain size of thermo-mechanically affected zone(TMAZ)of advancing side and retreating side.The metal pulled(extruded)from AS undergoes dynamic recrystallization(a characteristic feature of FSW process)and redeposit on the retreating side[15,39]and hence the grains are found comparatively fine in retreating side than the grains in advancing side of thermo-mechanically affected zone(TMAZ).Optical micrographs were taken at different regions across the weld and the micrographs of base metal,the center of welding stir zone(SZ)of joint,advancing side of the interface and retreating side of the interface are shown in Fig.5.

4.2.Tensile properties

The transversal tensile properties(like yield strength,tensile strength,elongation and joint efficien y)of friction stir welded AZ61 magnesium alloy joints were evaluated.For each condition,three specimens were tested and the average of three results is used.The prepared specimens were inspected firstl and the universal testing machine with a maximum fie ton capacity was used for the tensile test.The tensile force was given at the rate of 1 kN/min as per the ASTM guidelines.The yield strength and tensile strength of base metal were 219 MPa and 270 MPa,respectively,and the joint fabricated showed yield strength of 175 MPa,tensile strength of 220 MPa,elongation about 7.2%and efficien y of joint about 82%.The stress vs.strain curve was obtained when the tensile specimen of base metal and welded joint breaks as shown in Fig.6.Fig.7 represents the tensile properties of the base material and the friction stir welded joint.

Fig.5.Optical micrographs of:(a)base metal;(b)centre of stir zone;(c)advancing side of interface;(d)retreating side of interface.

Fig.6.Strain–stress curves of base metal and friction stir welded joint.

Fig.7.Tensile properties of the base material AZ61 Mg alloy and the FSW joint.

4.3.Microhardness

Microhardness of the weld joint was measured along the mid-thickness line of cross section and the values are presented in Fig.8.Microhardness measurements were carried out by Vickers indentation method by applying 100 g load for 10 s as per ASTM specifications Measurements were carried out on three parallel samples.The hardness of base metal AZ61 and stir zone was 70 Hv and 81 Hv,respectively.The hardness of base metal was higher than that of thermomechanically affected zone(TMAZ)but lower than that of stir zone(SZ).

Fig.8.Microhardness measurements across the weld joint of AZ61 Mg alloy.

The hardness of the stir zone was significantl higher than that of the base metal AZ61and the factors responsible for the improved hardness of stir zone are:(i)The grain size of stir zone is much fine than that of base metal,grain refinemen plays an important role in material strengthening.According to the Hall–Petch equation,hardness increases as the grain size decreases.ii)According to the Orowan hardening mechanism,the small particles of intermetallic compounds are also helpful in improvement of hardness[35].As the result,the difference in hardness between different zones is attributed to the grain size of the particular zone[12,34].

From Fig.8;advancing side and retreating side of the interface regions show the minimum hardness values(lower than the hardness of base material).The hardness drop at these zones/regions(having smaller grain size than the base metal)is due to the fact that there is no mechanical deformation(stirring);however the peak temperature reached is enough to soften the material near the stir/nugget zone.This result is in agreement with previously reported results for magnesium as well as aluminum alloys[40,41]

4.4.Fracture surface

Fig.9 shows SEM micrographs of fracture surfaces of the base metal and the friction stir welded joint of AZ61 magnesium alloy after tensile testing.The fractograph regularly contain dimples(red circle)and this confirme that the joint fail in ductile mode[42].This type of fracture surface reflecte ductile fracture that was characterized by cup-like depressions[43,44].The failure of joint was due to the uniform deformation of material.Tensile fracture initiation could have started from a cleavage area between the weld stir/nugget zone and the thermo-mechanically affected zone.This result is in agreement with earlier reported results for magnesium alloys[17,45,46].

Fig.9.Typical SEM micrographs of:(a)base metal,(b)fracture surface of friction stir welded AZ61magnesium alloy specimen after tensile testing.

5.Conclusions

Sound friction stir weld joint of AZ61 Magnesium alloy produced at rotational tool speed 1400 rpm and welding speed 25 mm/s.The joint was investigated by examining welding defects,microstructure,microhardness and tensile properties etc.Some conclusions can be drawn as follows:

1.The width of the friction stir zone was about 6 mm i.e.equal to the diameter of the pin.

2.The elongated grains of the base metal were recrystallized in the stir zone and in transition zone during friction stir welding.The weld nugget had fin and equiaxed grains due to dynamic recrystallization.The grains were found comparatively fine in the retreating side than the grains in the advancing side of thermo-mechanically affected zone.3.The FSW joint shows yield strength 175 MPa,tensile strength 220 MPa,elongation about 7.2%and efficien y of joint about 82%.

4.The formation of fine grains in the stir zone results in improvement in hardness of the joint.The microhardness of base metal is higher than that of thermo-mechanically affected zone but lower than that of stir zone.

5.The uniform deformation of material followed the ductile failure.This uniform deformation in the joint confirm the presence of defect free stir zone.