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        Influenc of tool pin profil and rotational speed on the formation of friction stir welding zone in AZ31 magnesium alloy

        2018-08-18 07:02:10DrUgender
        Journal of Magnesium and Alloys 2018年2期

        Dr.S.Ugender

        Department of Mechanical Engineering,Holy Mary Institute of Technology and Science,Hyderabad,India-501301

        Abstract In this investigation,the effect of friction stir welding(FSW)parameters such as tool pin profiles rotational speed and welding speed on the mechanical properties of tensile strength,hardness and impact energy of magnesium alloy AZ31 was studied.The experiments were carried out as per Taguchi parametric design concepts and an L9 orthogonal array was used to study the influenc of various combinations of process parameters.Statistical optimization technique,ANOVA,was used to determine the optimum levels and to f nd the significanc of each process parameter.The results indicate that rotational speed(RS)and transverse speed(TS)are the most significan factors,followed by tool pin profil(PF),in deciding the mechanical properties of friction stir welded magnesium alloy.In addition,mathematical models were developed to establish relationship between different process variables and mechanical properties.

        Keywords:Friction stir welding;Tool pin profile Mechanical properties,Taguchi orthogonal array,Analysis of variance(ANOVA).

        1.Introduction

        As one of the lightest metallic materials for structural applications,magnesium alloys have been increasingly used in various industries,in particular automotive.Weight reduction in transportation industry is one of the most crucial pathways to increase fuel economy and reduce pollution.To further expand the application of magnesium alloys,more effective welding and joining techniques are required.Conventionally tungsten inert gas(TIG)and metal inert gas(MIG)processes are the main welding methods for magnesium castings.With the introduction of friction stir welding(FSW)technique in 1991,this relatively new solid state joining process may provide significan potential for magnesium alloy welding since it can reduce or eliminate some solidification-relate welding defects such as cracks,porosity.To date,however,most of the research and development work has concentrated on friction stir welding of aluminum alloys.

        Friction stir welding(FSW)is capable of joining magnesium alloys without melting and thus it can eliminate problems related to the solidification As FSW does not require any f ller material,the metallurgical problems associated with it can also be eliminated and good quality weld can be obtained[1].FSW involves complex material movement and plastic deformation.Welding parameters,tool geometry,and joint design exert significan effect on the material f ow pattern and temperature distribution,thereby influencin the microstructural evolution of material.Of these factors tool geometry is the one of the most influentia aspect of process development[2].The design of the shoulder and of the pin is very important for the quality of the weld.The pin of the tool generates the heat and stirs the material being welded but the shoulder also plays an important part by providing additional frictional heat as well as preventing the plasticized material from escaping from the weld region.

        Fig.1.The schematic diagram of AZ31B Mg alloy plates used for FSW.

        The recent studies,however,have been restricted to Al alloys so that a few data have been published about friction stir welding of magnesium alloys and other materials[3–6].Magnesium alloys are potential candidate to replace aluminum alloys in many structural applications due to some of their unique properties.Magnesium alloys have a low density,high strength-to-weight ratio and good castability[7].They are also considered as the advanced materials for energy conservation and environmental pollution regulation.However,the joining of magnesium alloy plates prepared from FSW.This technique prevents many of the metallurgical problems that occur with conventional fusion welding such as distortion,shrinkage,porosity and splatter.Furthermore,helps in obtaining improved mechanical properties.However,FSW is used extensively for joining of aerospace materials such as AA 6061 aluminum alloy,AA 2014 aluminum alloy and AZ31B magnesium alloy,which may be crucial for these applications,is still limited in general investigation[8].Magnesium alloys are easily oxidized in welding zone because of their high chemical reactivity even at moderate temperature.Therefore,a lot of weld defects can be generated when Mg alloys are welded by conventional fusion welding methods.The purpose of this study is to evaluate the mechanical properties of friction stir welding for AZ31 magnesium alloy.Hence in this investigation effect of tool pin profile tool rotation speed and tool travel speed on tensile strength and hardness of the friction stir welded AZ31 magnesium alloy joint have been analyzed with Taguchi’s design concept(L9 orthogonal array).Initially an overview of experimental procedure has been discussed in brief.Result and discussion has been included in subsequent sections.The study indicates that tool rotational speed is the main process parameter that has the highest statistical influenc on mechanical properties.However,other parameters such as welding speed and tool pin profil also have significan effect on mechanical properties.

        2.Experimental design and procedure

        The Design-Expert,statistical software along with Taguchi design with L9 orthogonal array composed of 3 columns and 3 rows were employed to optimize the friction stir welding parameters(Table 3).The selected friction stir welding parameters for this study were:tool pin profile rotational speed,and welding speed.The Taguchi method was applied to the experimental data and the signal to noise ratio(S/N)for each level of process parameters is measured based on the S/N analysis.Regardless of the category of the quality characteristic,a higher S/N ratio corresponds to a better quality characteristic.Therefore,the optimal level of the process parameters is the level with the highest S/N ratio[9].A detailed analysis of variance(ANOVA)framework for assessing the significanc of the process parameters is also provided.The optimal combination of the process parameters can be then predicted.Finally,a model for hardness values based on FSW parameters is offered by a design expert.

        In the present investigation,the base material employed in this study is 5mm thick AZ31 magnesium alloy.It has been found that rotational speed has a significan influenc on grain refinemen of material.The schematic diagram of AZ31 Mg alloy plate and the schematic diagram of tool geometry used for FSW are shown in Figs.1 and 2.Various rotational speeds such as 900rpm,1120rpm and 1400rpm were used in this study.The tools were made with a nominal shoulder diameter of 18mm,pin diameter of 6mm and pin length of 4.8mm.The optimum welding parameters and materials which give better mechanical properties of Mg AZ31 alloy are 1120rpm,welding 40mm/min and stainless steel.Friction stir welding was carried out on AZ31 magnesium alloy plates having dimensions of 240mm(l)×60mm(w)×5mm(h)in butt joint configuration The chemical composition and mechanical properties of base metal are tabulated in Tables 1 and 2,respectively.

        Fig.2.The schematic diagram of various tool pin profiles

        Table 1 Chemical composition(wt%)of base metal AZ31B magnesium alloy.

        Table 2 Mechanical properties of base metal AZ31B magnesium alloy.

        Table 3 DOE-experimental levels and factors.

        Fig.3.Schematic sketch of tensile specimen.

        Three different non-consumable tool pin profile (taper threaded,taper cylindrical and straight cylindrical)were used to fabricate the joints with a constant tool made of stainless steel tool.It should be noted that length of the pin is the same in each case while surface area of the pin in contact with abutting base metal plates is different and depends upon tool pin profile The process is carried out on a vertical milling machine(VMM)(Make HMT FM-2,10hp,3000rpm).During the welding tool tilt angle was kept constant at 2°.FSW welding was carried out with different tool geometry,tool rotational speed and tool travel speed using Taguchi orthogonal array Design of experiments technique.For the present study Taguchi orthogonal array design of experiment with three factors at three levels was used.The following variables have been chosen as independent variables:tool pin profile tool rotational speed and welding speed.All the factors and their levels are tabulated in Table 3.

        Fig.4.The schematic sketch of charpy impact specimen.

        Tensile test and Charpy impact specimens were prepared as per ASTM E8M-04 using CNC Milling machine(Figs.3 and 4).Two tensile specimens were prepared for each combination of process parameters.Then average values of these two tests have been reported.Tensile tests have been carried out on 400KN,electro-mechanical controlled Universal Testing Machine.The specimen is loaded at a rate of 1.5kN/min as per ASTM specifications FSW welding was carried out with different tool pin profile tool rotational speed and tool travel speed using Taguchi orthogonal array design of experiments technique.Other process parameters like downward force and tilt angle etc.were kept constant.Weldments prepared by friction stir welding processes were visually inspected for their soundness.After FSW,the microstructure of the base material and processed samples was examined by optical microscope.The specimens for microstructural analysis were sectioned to the required size and then polished using different grades of emery papers and etched with a standard reagent made of 4.2g picric acid,10ml acetic acid,10ml diluted water,and 70ml ethanol.Microstructural analysis was carried out using a light optical microscope and SEM.

        Microhardness tests were carried out at the cross section of nugget zone(NZ)of welded joints normal to the FSW direction,sample were tested with a load of 15 g and duration of 15 s using a Vickers digital micro-hardness tester.

        2.1.Experimental design using Taguchi’s method

        In this study trial experiments were conducted by varying the tool pin profile(taper with threaded,taper cylindrical and straight cylindrical)and keeping the other parameters constant to fin the working range of rotational speeds.Feasible levels of the process parameters were chosen in such a way that the welded joints should be free from defects.

        2.2.FSW process parameters

        The friction stir welding parameters such as the tool pin profile tool rotational speed,the welding speed play a major role in influencin microstructure,and therefore the mechanical properties.In the present investigation,three process parameters namely,tool pin profile tool rotational speed the welding speed are considered.Pilot experiments were carried out using 5mm thick rolled sheet of AZ31 magnesium to determine the working range of FSW process parameters.

        When the tool rotational speed is lower than 900rpm,worm hole defect is observed due to insufficien heat generation and insufficien metal f lling whereas a tunnel defect was found due to excessive heat generation when the rotational speed is higher than 1400rpm.When the traverse speed is lower than 25mm/min,pin holes are observed due to excessive heat generation and a tunnel defect is found due to insufficien heat input caused by inadequate f ow of metal,when the transverse speed is greater than 40mm/min.Defect free surface was obtained,for a tool pin profile of taper threaded,taper cylindrical and straight cylindrical.Based on the above experiments,the range of process parameters are fi ed as 900–1400rpm for rotational speed,25–75mm/min for transverse speed and taper threaded,taper cylindrical and straight cylindrical.

        The quality characteristics such as ultimate tensile strength(UTS),yield strength(YS),percentage of elongation(%EL),microhardness and impact toughness(IT)of AZ31 magnesium alloys were evaluated for all the trials and then statistical analysis of variance was carried out.Based on analysis of variance,the contribution of each element to the quality characteristic is evaluated.The optimum combinations of process parameters were predicted and verified

        3.Results and discussions

        3.1.Microstructure

        The optical micrographs of all(Exp.1–9)the AZ31 magnesium alloys were examined and presented in Fig.5.It is observed that the sample at the optimum condition(i.e.A1B2C2)resulted in uniform distribution of the grains in the welding zone.In fusion welding of magnesium alloys,defectslikeporosity,hot cracks,etc.deterioratetheweld quality and joint properties.Usually,friction stir welded joints are free from these defects since no melting takes place during welding and the metals are joined in the solid state itself due to the heat generated by the friction and fl w of metal by the stirring action.However,FSW joints are prone to other defects like pinhole,tunnel defect,piping defect,kissing bond,cracks,etc.due to improper fl w of metal and insufficien consolidation of metal in the FSW region.Various pin profile like taper threaded,tapered cylindrical and straight cylindrical are used.Out of these fla pin profile have a greater eccentricity[10]which allows incompressible material to move around the pin profile Dynamic orbit is concerned with the FSW process and related to the eccentricity of the rotating pin[11].The pins are designed to disrupt faying of the contacting surfaces and causes deformational or frictional heating.They shear the material in front of the tool and move behind it.In addition the depth of deformation and tool travel speed are governed by the tool pin design.The microstructure with straight at lower rotational speed of 900rpm showed larger grains in the SZ due to lesser generation of heat causing inadequate moving of the metal,resultantly poor consolidation of the stirred material.The microstructure at higher rotational speed of 1120rpm revealed a mixture of coarse and fin grains.As the speed of the tool increased,the heat input of the joint enhances which it causes inferior tensile strength because of increase in temperature[12–14]and also observed that,at higher rotational speed(1120rpm)excessive flas is formed at the extreme ends due to higher heat input.The joint fabricated with welding speed(40mm/min)have fin grains in the stir zone(SZ)compared to other joints.The existence of well equiaxed grains in the SZ might be the reason for improved tensile strength of the joints compared to the other joints.

        Fig.6.SEM micrographs of magnesium alloys(Exp.1–9).

        3.2.Fractography

        SEM fractographs of fractured tensile specimen and friction stir welded AZ31 magnesium alloy are shown in Fig.6.The base metal exhibits elongated dimples and friction stir welded joint exhibits fine dimples on the fractured surface.Medium and lower values of quality characteristics were analyzed by SEM to reveal the fracture surface and the results are presented in Fig.6.It is difficul to deform magnesium at room temperature because of insufficien number of slip systems.Magnesium alloys need to be heated to initiate required slip systems in order to plastically deforms.Therefore,in the present study,samples were heated to 300°C and friction stir welding was performed.Also,for subsequent passes,the samples were again reheated to 300°C before each pressing.During the reheating of the welded samples,recrystallization and grain growth can occur as observed in the 2nd sample.The grain refinemen in the 2nd sample was more compared with the 3rd and 5th sample but compared with the 2nd sample,no improvement was observed.For the 4th pass sample,there is a grain refinemen compared with the 3rd sample,but similar to the 2nd sample.However,few large grains of size appeared in 4th sample.But the fraction of fine grains in 5th pass sample is more when compared with all the other samples.Therefore,it can be understood that reheating the samples for successive welded passes has a significan effect on microstructure evolution in AZ31 magnesium alloy.This is due to dispersion of very large number of Al and Mn particles which severely limits the movement of dislocations and decreases the ductility significantl.

        3.3.Mechanical properties

        Mechanical properties such as UTS,YS,%EL,microhardness and impact strength were evaluated and presented in Table 4.Regression analysis is used to evaluate the data on all the properties of AZ31 magnesium alloys.These developed regression equations are used to predicting the UTS,YS,%EL,microhardness,and impact strength within the factorial space exploited.

        Table 4 Mechanical properties of magnesium alloy.

        Table 5 Regression equations for the mechanical properties of AZ31B Magnesium alloys.

        3.3.1.Development of regression models

        The regression model commonly used is represented by Y=f(A,B and C).Y denotes the performance characteristics and A,B and C are the process parameters.The general regression model consisting of only linear and quadratic effects is given by Eq.(1),

        Where β0,β1...β6are regression coefficient of process parameters andεis the experimental error.The developed regression equations and correlation coefficient for the observed properties are summarized in Table 5.High correlation coefficien indicates good relationship between the process parameters and the observed property data.

        The coefficien of correlation(R2)is define as the ratio of explained variation to the total variation and measures the degree of the f tting to the model.When it approaches to unity,the developed model fit the actual data with given confidence Here all models have higher values of R2,i.e.above 95%,which means that the regression model provides an excellent explanation of relationship between parameters and responses.All these models are statistically significan at 95%confidenc level.

        3.3.2.Optimization and validation of process parameters performance characteristics

        The optimization of tool pin profile welding speed and rotational speed using Taguchi method permits evaluation of the effects of individual elements independent of other elements on the identifie quality characteristics i.e.UTS,YS,%EL,microhardness and impact strength.The influenc of each tool pin profil and rotational speed can be evaluated by determining the S/N ratio for each factor at each level.The main effect plots for various responses are shown in Figs.7–11.From the main effect plots analysis,the optimum parametric combinations for better mechanical properties are obtained and summarized in Table 6.

        Fig.7.S/N ratio response graph for UTS.

        The predicted values for various responses at optimum condition are calculated using the predicted S/N ratio(ηopt)Eq.(2)

        whereηjmis the mean S/N ratio of optimum level and j is the number of process parameters that affect the response.For validation of the optimum results,experiments are con-ducted at optimum condition and the results are presented in Table 6.It is observed that experimental values were closer to the optimum values.

        Table 6 Validation of the optimum results.

        3.3.3.Analysis of variance(ANOVA)

        In order to fin the effect of process parameters on various responses ANOVA is performed and the results are presented in Table 7.The calculated F-values of the ANOVA for various responses determine the relative significance of different process parameters.Results of ANOVA revealed that the rotational speed and welding speed were significantl effect on all the quality characteristics.It is clear from the ANOVA(Table 7)that the tool pin profil has less percentage of contribution on the mechanical properties compared to rotational speed and welding speed.

        3.3.4.Effect of process parameters on microhardness

        Fig.11 shows the main effects plot for microhardness.It is revealed that increasing the rotational speed increases the microhardness and then gradually decreases.The joints fabricated with the rotational speed of 1120rpm,welding speed of 40mm/min,tool pin made of taper threaded recorded higher hardness in the stir zone,and this is also one of the reasons for superior tensile properties of these joints compared to other joints.There are two main reasons for the improved hardness of stir zone.Since the grain size of stir zone is much fine than that of base metal,grain refinemen plays an important role in material strengthening.Further increasing the rotational speed decreases the microhardness.This is due to the high heat generation that causes material softening which results in decrease in the microhardness.This softening of the nugget zone was a result of in coarsening and/or dissolution of strengthening precipitates in the magnesium alloys.The optimum microhardness value was obtained at the optimum condition of 1120rpm,welding speed of 40mm/min and tool pin made of taper threaded.This is due to the factthat at 1120rpm,higher rotational speed supplied sufficien heat input to make the improved mechanical properties by the softenened metal and rotated with FSW tool which results in well partition and allocation in the stir zone.

        Table 7 ANOVA analysis results for various responses.

        Fig.8.S/N ratio response graph for YS.

        Fig.9.S/N ratio response graph for%of EL.

        3.3.5.Effect of process parameters on tensile properties

        Figs.7–9 shows the main effects plot for UTS,YS and%EL.It is revealed that the increasing the rotational speed increases and gradually decreases the UTS,YS and%EL.However the earlier mentioned the formation of fin equiaxed grains and uniformly distributed very fin strengthening precipitates in the weld region which resulted in material softening.Actually the softening of material will improve the%EL.It is also observed that the increasing the welding speed of 40mm/min,initially decreases then after increases the UTS,YS and%EL.This is due to increases the interface area between the taper threaded tool and magnesium alloy due to low softening of plastic deformation,which results in high tensile properties.

        Fig.10.S/N ratio response graph for%of IS.

        Fig.11.S/N ratio response graph for Microhardness.

        4.Conclusion

        In the present work,an attempt has been made to study the effect of FSW process parameters on the mechanical properties of friction stir processed AZ31 magnesium alloy.The following important conclusions are derived from this investigation.

        1.AZ31 magnesium alloy was successfully friction stir welding without any macro level defect under the following range of process parameters:tool pin profil made of taper threaded,taper cylindrical and straight cylindrical,tool rotational speed of 900?1400rpm,and welding speed of 25–75mm/min.

        2.By meansof Analysis of Variance(ANOVA),the important parameters influencin the weld mechanical properties are tool pin profile and the combination of rotational speed and welding speed.

        3.The same optimum condition(A1B2C2 i.e.,tool pin profil at taper thread,rotational speed 1120rpm and welding speed of 40mm/min)is experiential in ultimate tensile strength,yield strength,percentage of elongation,impact strength and microhardness of AZ31 magnesium alloy.

        4.Regression models were developed to predict the quality characteristics(UTS,YS,%EL,Impact toughness and microhardness)within the selected range of process parameters(tool pin profile rotational speed and welding speed).The results are validated through ANOVA.

        5.ANOVA analysis test was conducted to determine the significanc of each FSW process parameter on the mechanical properties.It is found that tool rotational speed has the highest statistical influenc on tensile strength and hardness,whereas,for impact energy,transverse speed.

        6.Regression models were developed to predict the mechanical properties for various tool pin profiles rotational speeds and welding speeds without requiring experimental tests.The validity of the model developed was proved with an experimental test.

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