Joining of hybrid AA6063-6SiCp-3Grpcomposite and AISI 1030 steel by friction welding

N.Rjesh Jesudoss Hynes Associte Professor,M.Vivek Prhu Assistnt Professor,P.Ngrj Senior Professor

aDepartment of Mechanical Engineering,Mepco Schlenk Engineering College,Sivakasi 626005,Tamil Nadu,India

bDepartment of Mechanical Engineering,Velammal College of Engineering and Technology,Madurai 625009,Tamil Nadu,India

Joining of hybrid AA6063-6SiCp-3Grpcomposite and AISI 1030 steel by friction welding

N.Rajesh Jesudoss Hynes Associate Professora,*,M.Vivek Prabhu Assistant Professorb,P.Nagaraj Senior Professora

aDepartment of Mechanical Engineering,Mepco Schlenk Engineering College,Sivakasi 626005,Tamil Nadu,India

bDepartment of Mechanical Engineering,Velammal College of Engineering and Technology,Madurai 625009,Tamil Nadu,India

1.Introduction

The welding of composites and ceramics has gathered attention among engineers and scientists in the last decade.It is effectively possible through solid state processes like friction welding and diffusion bonding.In particular,friction welding provides impressive results in joining metals and composites for automotive,ship building and electronic applications[1].Friction welding processenables the efficient combination of metals and ceramics by harnessing the frictional heat induced by the faying surfaces.A series of thermo-mechanical stages such as generation of heat by friction and bonding due to the upsetting,results in a unique characteristic weld[2].The fusion welding techniques for joining dissimilar metals are easily subjected to failure due to their brittle intermetallic compound formation.Moreover,the differences in their melting point results in inappropriate intermetallic structure,poor properties and inferior weld strength[3].

For this study,the hybrid aluminium matrix composite was developed through Stir Casting technique.The SiC present in the hybrid matrix composite increases the thermo-physical and mechanical properties[4]of the composite such as low co-efficient of thermal expansion,higher strength,good specific modulus and so on.The inclusion of graphite[5],a solid lubricant guarantees the usage of hybrid matrix composite in applications demanding high strength and wear resistance.But,there are many other potential problems in welding metal matrix composites with non-metallic reinforcements[6].The differences in their density can lead to solidification effects such as segregation of the reinforcements in the molten metal.Also,it gives rise to non-uniform packing density which makes conventional techniques not suitable for joining metal matrix composites.The other issues are undesirable chemical reactions between the molten metal and the reinforcements during the extended exposure during fusion welding.Being a solid state welding technique friction welding could be employed in crucial situations of joining of dissimilar materials[7].Morikawa et al.[8]friction welded Stud joints of dissimilar aluminium alloys.Process parameters were varied and joint strength was evaluated and it is found that the tensile strength increased with increase of friction time and friction pressure.Ashfaq et al.[9]studied the influence of alternate designs of faying surfaces such as providing taper(external and internal)and smoother surfaces on the strength of friction welded Stainless Steel/Aluminium Alloy joints.It was concluded that external taper fayed surfaces could result in improved joint strength.

Moreover,in friction welding process,it has been reported that use of metal interlayer increases the strength of the dissimilar joints by Noh et al.[10].Sassani et al.claimed that nearly 40%of the joint strength could be increased when a compatible interlayer is used[11].The bonding gets better with the use of metallic interlayer[10],since they provide superior flow of the metal during the process.The application of metallic interlayer is considered as essential to produce a strong bond between ceramics and metals.The presence of interlayer is found to be crucial in increasing the strength of the welded joint.It is also discovered that the joints without interlayer have lower strength and lower impact toughness.Moreover,the interlayers act as an active diffusion barrier in preventing the movement of carbon resulting in premature failures[12].

Hence,in the present study,joining feasibility of aluminium hybrid metal matrix composite and AISI 1030 steel is studied by friction stud welding technique using aluminium sheet as interlayer.The micro structural aspects were studied by SEM and EDX analysis and mechanical properties of the dissimilar joints are evaluated.In addition,the current work deals with the study of interaction of various process parameters in determining the impact strength and axial shortening distance of the welded joint.

2.Materials and methods

In the present work,investigation on joining of Hybrid AA6063-6SiCp-3Grpcomposite to AISI 1030 steel has been carried out using AA1100 interlayer.The composition of AA6063,hybrid composite,AISI 1030 steel and AA1100 interlayer are given in Tables 1-4 respectively.The Aluminium Hybrid metal matrix composite has been developed using stir casting process.Aluminium alloy 6063 is chosen as the matrix material whereas Silicon Carbide(40μm,average size)and Graphite(60μm,average size)are selected as the particle reinforcements for the development of HMC.The selection of Aluminium alloy as the matrix material is due to the optimal presence of magnesium,which affords the coalescence during the welding process.

2.1.Stir casting of hybrid composite

The composition of hybrid composite is given in Table 2.The aluminium alloy rods are allowed to melt in the furnace at 760°C(Fig.3).The SiC particles(40μm,average size)and Graphite(60μm,average size)particles are pre-heated at a temperature of 250°C for about 2-3 h[14].The reinforcements are added into molten metal at a steady rate of 2 g for every 30 s.After the addition,the molten metal mixture is vigorously stirred by a mechanical stirrer rotating at 450 rpm to avoid agglomeration of the particles.The stirred molten metal is maintained at a temperature of 750°C and it is poured into the mould.The hybrid matrix composite is allowed to cool and solidify at normal room temperature.

2.2.Experimentation

Experiments were carried out in an in-house made friction stud welding machine developed from a conventional lathe(Type 141-A141:PSG,Coimbatore).The Fig.4 gives the schematic representation of the experimental setup used for joining the dissimilar materials.A Programmable Logic Controller is employed to control the parameters in the friction stud welding process.The desired welding conditions can be set using the PLC.The AA6063-6SiCp-3Grphybrid composite specimen(Fig.1)is placed in the chuck and the AISI 1030 specimen(Fig.2)is held by a stud holder which is connected to the pneumatic actuator of the pneumatic drive.While the hybrid composite specimen rotates in the chuck,friction pressure is imparted by the pneumatic actuator.Pneumatic actuator pushes the AISI 1030 steel specimen against the rotating workpiece and frictional heating takes occurs at the interface of the specimen.Then,an upsetting pressure is imparted by the pneumatic drive to complete the process.

Experiments were conducted using Design of Experiments(DOE)technique to ensure robust design(Fig.5).The Table 5 given below provides the details of the selection of parameters and their levels in the experimentation.The design of experiments is done varying four variables at three levels as shown in Table 5.Based on L9 orthogonal array of Taguchi's method,nine experimental trials were conducted.The experimental results obtained during experimentation are given in Table 6.A digital vernier calliper(Model No.CD-12C,Mitutoya Corporation,Japan)is used to find out the axial shortening distance in the welded specimens.Friction stud welded AISI 1030/AA6063-6SiCp-3Grpjoints during the experimentation are shown in Figs.6 and 7.

Mechanical testing(Charpy impact test)is carried out to evaluate the strength of the welded joints at room temperature.10X10X50 mm impact test specimen is prepared with a"V"notch of 2 mm depth and a 45°groove.For micro structural examination,the specimen was sectioned perpendicular to the weld surface and mechanically polished by using emery papers and etched with ferric chloride solution.The prepared specimen undergoes micro structural evaluation using SEM,(Hitachi,SU1510-Japan),EDX analysis and micro hardness measurement.Variations in micro hardness across the welded joint,is studied using Vickers hardness tester.Using a load of 5 kg and a holding time of 20 s,indents were made on the welded specimen.The indenter is then removed fromthe surface of the specimen and the indentation dimensions were measured using an optical microscope.

Table 1 Composition of AA 6063.

Table 2 Composition of hybrid Composite.

Table 3 AISI 1030 composition.

Table 4 AA1100 interlayer composition.

Fig.1.Dimension of hybrid composite specimen to be held in chuck.

Fig.2.Dimension of AISI 1030 steel specimen to be fixed in holder.

3.Results and discussions

3.1.Micro hardness measurement

Fig.8 shows the micro hardness profile across the welded specimen pertaining to.Micro hardness value is high at the weld interface and it changes when moving towards the base materials.In both steel side and hybrid composite side,it decreases and then reaches a constant value same as that of parent materials.This change in hardness values indicates the presence of three different zones in the heat affected area of the welded joint.

In the fully plasticized zone,maximum hardness value is 205Hv.This is due to the plastic deformation caused due to the upsetting pressure during the friction stud welding process.Fine dynamically recrystalized grains increase hardness as well as strength of the joint according to Hall petch's equation[15].A dip in the hardness value indicates the presence of partially deformed zone.Further decrease and constant values refer to the third region which is the unaffected zone.Here,hardness value is same as that of the parent materials.

Fig.3.Stir casting setup.

3.2.Micro structural analysis

Fig.9(experiment no.VP-6)shows the micrograph of the joint taken by scanning electron microscopy.While examining the SEM micrograph of the welded specimen,following three regions were observed as shown in Fig.12.

.Fully plasticized deformed region(FPDR)at the interface

.Partially deformed region(PDR)on both the sides

.Unaffected regions(UR)in both the sides

Dynamic recrystallisation has occurred in the fully plasticized deformed region and there is greater reduction in the size of dynamic recrystallisation grains.In the partially deformed region,recrystallisation has taken place and reductions in grain size were observed.Microstructure of the particles in HMC side has been altered to a great extent than the changes that took place at the AISI 1030 steel side.This is due to the low friction time and friction pressure selected in the experimentation.The SiC particles close to the weld zone were reduced into the size of ultra-fine grains.This is due to the breaking up of the SiC particles during friction and upset pressure.These ultra-fine grains found at the interfacial region resulted in increase of micro hardness value at the weld interface.The size of the ultra fine grains are of the range of 300-600 nm.SiC particles appear as light gray spots in the hybrid composite near weld interface and reinforced graphite particles appear dark black in the micrograph.White inclusion shown in Fig.9 refers to aluminium oxide formed at the weld interface.EDS(Energy-Dispersive Spectroscopy)of the welded interface is shown in the Fig.10(experiment no.VP-6).Presence of intermetallic phase was confirmed and the chemical composition of the interface region is sensed by EDS which is presented in Table 11.It is found that Fe2Al5is formed as an intermetallic compound in the interfacial region.

Fig.4.Schematic representation Friction Welding experimental setup.

Fig.5.Experimentation using DOE.

Table 5 Selection of parameters and levels in experimentation.

Table 6 Tabulation of experimental results.

The impact tested specimen is examined to identify the nature of the failure.Fig.11 shows the SEM micrograph taken at the fractured region at AISI 1030 steel surface.The observed white braids and cracks could be attributed to brittle fracture[13].The fracture happened in the side of the hybrid composite;veryclose to the interface shows the presence of remnants of hybrid composite in the steel surface(Fig.11).The HMC fragments sticking to the AISI 1030 steel surface is clearly seen in the investigated specimen as shown in the Fig.11(experiment no.VP-6).Fig.13 shows good dispersion ofSiC and Graphite particles in the developed composite.

Fig.6.Friction welded AISI 1030/AA6063-6SiCp-3Grpjoint.

Fig.7.Dissimilar AISI 1030/AA6063-6SiCp-3Grpjoints made during welding trials.

Fig.8.Micro hardness profile of welded AISI 1030/AA6063-6SiCp-3Grpjoint(Experiment No.VP-10).

Fig.9.SEM Micrograph at the interface of AISI 1030/AA6063-6SiCp-3Grpjoint(Experiment No.VP-6).

Fig.10.EDS analysis at point"+"in the interfacial region(Experiment No.VP-6).

3.3.Statistical analysis

ANOVA tool is used for analysing the statistical data obtained from the experiments.It evaluates the significance of the response variables by examining the means of the response variables at different levels.It tests the hypothesis whether the means of two or more populations are equal or not.The null hypothesis states that all population means are equal while the alternative hypothesis states that at least one is different.In this study,the contributing parameters for axial shortening and impact strength are identified using ANOVA.

Fig.11.SEM image of the fractured region at AISI 1030 side(Experiment No.VP-6).

Fig.12.SEM image showing various zones at a magnification of 2000X(Experiment No.VP-6).

Fig.13.SEM image showing dispersion of SiC and Graphite particles in AA6063-6SiCp-3Grpcomposite.

3.3.1.Analysis of variance of axial shortening distance

The ANOVA table for axial shortening distance is shown below in Table 7.The percentage contribution of factors for axialshortening distance is given in pie chart as shown in Fig.14.It is observed that rotational speed is the most significant factor with 71%of contribution and interlayer sheet thickness provides the least impact on axial shortening distance.When the rotational speed increases heat input increases due to the stirring action.The softened HMC material flows out as a flash covering the steel and more amount of material is consumed resulting in increase of axial shortening distance.

Table 7 ANOVA for Axial Shortening Distance using adjusted SS for tests.

Fig.14.Percentage contribution of process parameters to axial shortening distance.

Fig.15.Main effect plots for axial shortening distance(AX).

The main effect plots for axial shortening are shown in the Fig.15.The steeper the slope of the line,the greater is the magnitude of the main effect.The main effect plots for the rotational speed,friction time,friction pressure and sheet thickness are placed in together in one graph to compare their relative magnitudes.Interaction plots are studied to understand whether the effect of one factor depends on the level of the other factor.Interaction plots are used to visualize possible interactions.Parallel lines in an interaction plot indicate no interaction.The greater the difference in slope between the lines,higher is the degree of interaction.However,the interaction plot does not give information if the interaction is statistically significant.The interaction plot for axial shortening is given in Fig.16.

Fig.16.Interaction plots for axial shortening distance(AX).

3.3.2.Analysis of variance for impact strength

The ANOVA table for impact strength is shown Table 8.The percentage contribution of factors for impact strength is given in pie chart format is shown in the Fig.17.It is observed that the rotational speed and interlayer sheet thickness are the significant factors and the friction time provides the least influence on impact strength of the welded joints.Aluminium interlayer increases the wetting ability at the weld interface and offers superior bonding properties.Besides it reduces the heat affected zone.But selection of interlayer with optimum thickness of 5 mm gives food results.

The main effect plots for upset are given in Fig.18.From the Fig.18,it is observed that impact strength is influenced to a maximum level by rotational speed and sheet thickness.Friction time and friction pressure have the least effect on impact strength.The interaction plot for impact strength is given in Fig.19.

3.3.3.Regression analysis for axial shortening

The regression equation for axial shortening is

where AX-axial shortening,RS-rotational speed,FT-friction time,FP-friction pressure,ST-sheet thickness.

Regression coefficients of weld parameters vs.axial shortening are given in Table 9.The normal probability plot for axial shortening distance is given Fig.20.

3.3.4.Regression analysis for impact strength

The regression equation for impact strength is

where IS-impact strength,RS-rotational speed,FT-friction time,FP-friction pressure,ST-sheet thickness.Regression coefficients of weld parameters vs.impact strength are given inTable 10.The normal probability plot for impact strength is given Fig.21.

Table 8 ANOVA for Impact Strength using adjusted SS for tests.

Fig.17.Percentage contribution of process parameters to impact strength.

Fig.18.Main effect plots for impact strength(IS).

Fig.19.Interaction plots for impact strength(IS).

Table 9 Regression coefficients of weld parameters vs.axial shortening distance.

Fig.20.Normal probability plot for axial shortening distance.

Table 10 Regression coefficients of weld parameters vs.impact strength.

4.Conclusions

1)From the experimental results,it was observed that the specimen no.VP-9 has the highest impact strength(351.30 kJ/m2)and the specimen no.VP-11 has the lowest impact strength(17 kJ/m2).

2)Rotational speed and interlayer sheet thickness contribute about 39%and 36%respectively in determining the impact strength of the welded joints.0.5 mm interlayer sheet is recommended for efficient joints.

3)It is observed that rotational speed is the most significant factor with 71%of contribution in determining axial shortening distance.When the rotational speed increases heat input increases due to the stirring action.The softened HMC material flows outas a flash covering the steel and more material is consumed resulting in increase of axial shortening distance.

Table 11 EDX analysis data at the interface.

Fig.21.Normal probability plot for impact strength.

4)Micro hardness profile shows increase in hardness value at the fully plasticized deformed zone of the interfacial region.This is due to the plastic deformation caused by upsetting pressure.

5)Micro structural examinations reveal three separate zones namely fully plasticized zone,partially deformed zone and unaffected base material zone.Ultra fine dynamically recrystallized grains of about 341 nm are observed at the fully plasticized zone.

6)EDX analysis confirms the presence of intermetallic compound at the joint interface.It is identified as Fe2Al5,which has orthorhombic structure.Increase in the micro hardness could also be attributed to the presence of Fe2Al5(Table 11).

7)Based on the experimental results,regression model has been developed to predict impact strength and axial shortening distance with reasonable accuracy.

8)In specific,the present work shows that SiC and graphite reinforced aluminium hybrid composite can be welded to AISI 1030 steel successfully using friction stud welding which is a novel variant of friction welding process.

Acknowledgement

The authors gratefully acknowledge the financial support of this work by SERB of Department of Science&Technology,New Delhi.(Vide Letter No.:SERB/F/1452/2013-2014 dated 10.06.2013).

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A R T I C L E I N F O

Article history:

26 December 2016

in revised form

5 May 2017

Accepted 24 May 2017

Available online 27 May 2017

Metal matrix composite

Welding

Mechanical properties

Scanning Electron Microscopy

Joining of metals and aluminium hybrid metal matrix composites has significant applications in aviation,ship building and automotive industries.In the present work,investigation is carried out on Friction Welding of AISI 1030 steel and hybrid AA6063-6SiCp-3Grpcomposite,that are difficult to weld by fusion welding technique.Silicon carbide and graphite particle reinforced AA6063 matrix hybrid composite was developed successfully using stir casting method and the joining feasibility of AISI1030 steel with AA6063-6SiCp-3Grphybrid composite was tried out by friction stud welding technique.During friction stage of welding process,the particulates(SiC&Graphite)used for reinforcement,tend to increase the viscosity and lead to improper mixing of matrix and reinforcement.This eventually results in lower strength in dissimilar joints.To overcome this difficulty AA1100 interlayer is used while joining hybrid composite to AISI 1030 steel.Experimentation was carried out using Taguchi based design of experiments(DOE)technique.Multiple regression methods were applied to understand the relationship between process parameters of the friction stud welding process.Micro structural examination reveals three separate zones namely fully plasticized zone,partially deformed zone and unaffected base material zone.Ultra fine dynamically recrystallized grains of about 341 nm were observed at the fully plasticized zone.EDX analysis confirms the presence of intermetallic compound Fe2Al5at the joint interface.According to the experimental analysis using DOE,rotational speed and interlayer sheet thickness contribute about 39%and 36%respectively in determining the impact strength of the welded joints.It is found that joining with 0.5 mm interlayer sheet provides efficient joints.Developed regression model could be used to predict the axial shortening distance and impact strength of the welded joint with reasonable accuracy.

?2017 The Authors.Published by Elsevier Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

*Corresponding author.

E-mail addresses: findhynes@yahoo.co.in (N.Rajesh Jesudoss Hynes),vivekprabhu.mech@gmail.com (M.Vivek Prabhu),pnagaraj@mepcoeng.ac.in(P.Nagaraj).

Peer review under responsibility of China Ordnance Society.