稀有金属(英文版) 2020,39(04),429-435

Microstructure and wear behavior of AA6061/SiC surface composite fabricated via friction stir processing with different pins and passes

Hamidreza Eftekharinia Ahmad Ali Amadeh Alireza Khodabandeh Moslem Paidar

Department of Materials Engineering,University of Islamic Azad,Science and Research Branch

School of Metallurgy and Materials Engineering,University College of Engineering,University of Tehran

Department of Materials Engineering,University of Islamic Azad,South Tehran Branch

作者简介：*Moslem Paidar e-mail:M.paidar@srbiau.ac.ir;

收稿日期：4 September 2015

Microstructure and wear behavior of AA6061/SiC surface composite fabricated via friction stir processing with different pins and passes

Hamidreza Eftekharinia Ahmad Ali Amadeh Alireza Khodabandeh Moslem Paidar

Department of Materials Engineering,University of Islamic Azad,Science and Research Branch

School of Metallurgy and Materials Engineering,University College of Engineering,University of Tehran

Department of Materials Engineering,University of Islamic Azad,South Tehran Branch

Abstract：

In this investigation,the effects of pin geometry and number of passes on macrostructure,microstructure,wear rate and also microhardness profile of 6061-T6 aluminum alloy surface composites fabricated via friction stir processing(FSP) were discussed by reinforcement particles of silicon carbide(SiC).The results show that after each FSP pass,a modify distribution of SiC particles is acquired and the increase in the number of passes reduces the average grain size in stir zone(SZ).Furthermore,it is discovered that pin geometry and pass number play a dominant role in the grain size of SZ and distribution of SiC particles in SZ.It is found that after each FSP pass,wear rate is improved due to the uniform distribution of SiC particles in surface of Al/SiC composite.Additionally,the results show that the square pin and smooth(straight)cylindrical pin have the highest and lowest resistance to wear,respectively.

Keyword：

Friction stir processing; Pin geometry; Silicon carbide; Stir zone; Wear rate;

Received： 4 September 2015

1 Introduction

Friction stir processing (FSP) is variation of friction stir welding (FSW).If so,it has the same principles but cannot modify the microstructure of specific areas to enhance local properties [ 1] .Lately,FSP is developed to be used as an efficient technique with modified microstructure for fabrication of aluminum matrix composites (AMCs) [ 2] .FSP has been progressed as a technique for grain refinement;however,it can be counted as an affective process for fabricating composites [ 3] .Some researchers investigated the effect of processing parameters on mechanical properties and microstructure of FSPed specimens.For instance,Zohoor et al. [ 4] studied the FSP aluminum 5083with nano-sized Cu particles to investigate the influence of these particles on the mechanical and physical properties.They reported that addition of Cu during FSP of aluminum improved the grain size and subsequently caused the improvement in yield tensile strength (YTS),ultimate tensile strength (UTS),elongation and hardness values.Vijayavel et al. [ 5] indicated that FSP resulted in more tensile properties than the base metal.El-Kady and Fathy [ 6] investigated the effect of the amount and size of SiC particles on physical properties.They reported that increasing the amount and also decreasing the size of SiC led to reduction in thermal conductivity.Some researchers investigated the effect of the pin profile on the joint strength and the microstructure of the composite fabricated by FSP technique.For example,Bahrami et al. [ 7] studied the influences of the pin geometry on AA7075/SiC nanocomposite.They revealed that the geometry of pins had major role in the particle distribution.Furthermore,Azizieh et al. [ 8] investigated the effect of the pin profile and FSP pass number on microstructure of AZ31/Al₂O₃ nanocomposites.They indicated that pin geometry and pass number affected fabricated composites.They also indicated that the ceramic particles distribution was improved after each FSP pass and different pin geometry.

Regarding these results and different findings of other researchers,it is important to know that the effect of processing variables must be deeply investigated on physical and mechanical properties of samples made by FSP.In this investigation,the effects of pass number by using threaded cylindrical pin and pin geometry on the variations of friction coefficient and wear rate of 6061-T6 aluminum alloy surface composites fabricated via FSP at the same rotational speed and travel speed were discussed by reinforcement particles of silicon carbide (SiC).Additionally,the effect of pin geometry and the number of passes by using threaded cylindrical pin on microhardness profile was investigated.

2 Experimental

In this study,6061-T6 aluminum alloy sheets with 10 mm in thickness were used.The dimensions of the aluminum alloy sheets were 50 mm×125 mm.According to the results of this study,during the processing,the FSP parameters changed in four conditions included pin geometry (threaded (M6) cylindrical,straight cylindrical,square and triangular) and number of passes (one,two and four).The threaded pin had 1 mm pitch.It should be noted that the process was conducted at a constant shoulder penetration depth,plunge rate,tool rotational speed and travel speed of 0.42 mm,4 mm·min^-1,1250 r·min^-1 and80 mm·min^-1,respectively.The tools were made of H13steel with hardness of about HV 530.Shoulder diameters were 20 mm and pin and side lengths were 6 mm,with a concave profile angled of 8°(Fig.1).

Fig.1 Tool geometries used in this study:(1),threaded cylindrical,(2) non-threaded cylindrical,(3) square and (4) triangular pins (unit:mm)

All pins had the length of 3 mm.After welding,the specimens were polished and then etched by a Weck reagent (4 g KMnO₄,1 g NaOH and 100 ml H₂O) for 10 s.In order to develop the FSP tests,a properly designed clamping fixture was utilized to fix specimens.In addition,the scanning electron microscope (SEM,VEGA TESCAN)equipped with energy dispersive spectroscopy (EDS) and the optical microscope (OM,Olympus CK40) were utilized to determine the micros truc ture and worn surface of samples.Vickers microhardness was performed in the cross section of specimens and perpendicular to the travel direction of the tool with a load of 0.98 N,and then,it was maintained for 15 s with interval of 1.0 mm to investigate the effect of the pin geometry and FSP pass number on microhardness profiles.A groove with width and depth,respectively,of 0.7 and 2.0 mm was machined out to insert SiC particles with 40μm in size.Figure 2 shows transmission electron microscopy (TEM,zeiss-EM10C-80 kV)of the as-received SiC particles.To understand the effects of pin geometry and the number of passes on the friction and wear behavior and microhardness profile of the joints,FSW was carried out under welding condition:rotational speed of 1250 r·min^-1 and travel speed of 80 mm·min^-1.It should be noted that a cylindrical threaded pin was used to investigate the effect of number of passes on the micro structure and other parameters.After that,the intended amounts of SiC particles were contrived into the groove.At first,the groove's gap was closed using a tool with shoulder but no pin (rotational speed of 1000 r·min^-1and travel speed of 50 mm·min^-1) to prevent outpouring of SiC particles.Then,the tool inserted to specimens by pin and composites were produced.At room temperature and under ASTM G99 standard in air,friction and wear behavior were investigated by a pin-on-disk test rig.Pin with 5 mm in diameter with the axis normal to the FSP direction as specimens and the hardened steel disk (steel52,100) as counter disk were used.In Table 1,the wear test(pin-on-disk) conditions are mentioned.By measuring the frictional force with a stress sensor,the coefficient of friction between the disk and the pin specimen was calculated.Before loading,the surface of samples on the test rig and the disk were ground and then were cleaned in acetone by 800 grit SiC paper and then worn surfaces were checked by SEM.It should be noted that the wear rate(mm³·m^-1) was calculated by following equation:

Fig.2 TEM image of as-received SiC particles

  下载原图

Table 1 Wear test conditions (pin-on-disk)

Wear rate=Volume loss/Sliding distance (1)

3 Results and discussion

3.1 Evaluation of macro and microstructures

Figure 3 shows the optical macroscopic and microscopic images of the different regions of FSPed specimen reinforced with SiC particles.Figure 3a shows that the distribution of SiC particles in stir zone (SZ) is completely different.Moreover,SiC particles are dispersed non-uniformly in SZ.It is well known that accumulation of SiC particles in advancing side (AS) is more than those in retreating side (RS) because of the greater displacement of the material in AS.It is further noted that fluidity of thematerial in RS is lower than that in AS.It means that the fluidity of the material decreases in RS due to opposing rotation of the tool with welding direction.In AS because of the parallel direction of rotation of the tool with welding direction,fluidity of material is intensified [ 9] .Figure 3b-e shows the microstructures of different regions produced during FSP of 6061-T6 aluminum alloy include base metal(BM),stir zone (SZ),thermo-mechanic ally affected zone(TMAZ) and heat affected zone (HAZ) [ 7, 10, 11] .

Fig.3 A cross section of FSPed specimens:a macrostructure of joint cross section and microstructures of b BM,c HAZ,d TMAZ and e SZ

According to the results shown in Fig.3e,the very fine equiaxed grains are formed in SZ.This is due to dynamic recrystallization (DRX) [ 3, 12, 13, 14] .In other words,frictional heat and sever plastic deformation lead to creation of DRX and subsequently production of the finer grain size during FSW/FSP [ 7] .Note that the width of SZ in the lower parts beneath pin is somewhat equal to the pin diameter while in upper parts beneath the shoulder it is equal to the shoulder diameter,which should be due to the higher frictional heat generated between shoulder and parent metal.The micros truc ture of TMAZ is shown in Fig.3d.This zone was formed between SZ and the parent metal.Figure 3d shows that the width of TMAZ in AS is smaller than that in RS,because as mentioned before,the heat friction and subsequently temperature produced in AS are more than those produced in RS.Since materials are under less strain and strain rates in TMAZ [ 4] ,it is known that complete DRX does not occur in TMAZ [ 10] .Moreover,the microstructure of HAZ is shown in Fig.3c.It should be pointed that depending on parameters,base metals and the type of powder,HAZ might be or might not be created [ 15] .

Figure 4a-d shows the microstructures of FSPed specimens by threaded cylindrical,non-threaded cylindrical,square and triangular pin tools,respectively.The pin geometry plays a vital role in material flow during FSP/FSW [ 16, 17, 18] .As shown in Fig.4,a change in the pin geometry leads to variation of the grain size in SZ,which may be due to the different material flows.Furthermore,from the comparison between the microstructure of SZ in FSPed samples with threaded and non-threaded cylindrical pins (Fig.4a,b),it can be said that the presence of threads on the cylindrical pin enhances flow and mixing of materials.The absence of threads on the pin restricts the material flow around the pin and subsequently reduces the distribution of SiC particles in SZ.When pin geometry changes to the square profile,a finer grain size is acquired in SZ.This is associated with flat faces of tool,because the square pin profiles make a pulsating stirring action in the flowing material [ 16, 17, 18] that produces finer grain size in SZ.For example,the average grain sizes in SZ by square,threaded cylindrical,non-threaded cylindrical and triangular pin geometry are 3.41,3.94,5.63 and 4.29μm,respectively,which affect the microhardness profile of specimens.It should be pointed out that the change in the pin geometry not only affects the microstructure of FSPed specimens,but also follows the fragmentation and distribution of SiC particles.So it could be inferred that in FSPed specimens by square and non-threaded cylindrical pin profiles,the smallest and largest average grain sizes are acquired,respectively.

Fig.4 OM images of specimen FSPed with different tools:a threaded cylindrical,b non-threaded cylindrical,c square and d triangular pin

Figure 5 shows OM images of SZ of samples prepared by one,two and four passes.According to Fig.5,it is evident that increasing FSP pass number improves average grain size in SZ.It can be attributed to the higher stirring of materials by increasing FSP pass number.In contrary,Asadi et al. [ 12] attributed this to the plastic deformation that induced the production of new recrystallization by increasing FSP pass number.They also attributed it to the fragmentation of SiC particles in SZ.It means that,by increasing FSP pass number,the grain size of SiC particles reduced and those particles distributed separately in a larger area.These particles acted as barriers and restricted the grain growth [ 12] .These results indicate that pin geometry and pass number play a dominant role in grain size of SZ and distribution of SiC particles during FSPed aluminum6061-T6.

3.2 Evaluation of microhardness variation

Vickers microhardness was performed to measure the influence of pin geometry and FSP pass number on the hardness variations of FSPed specimens.The average microhardness value of as-received AA6061-T6 is HV 85.The relationship between the microhardness profile and pin geometry is shown in Fig.6a.Figure 6a shows that the maximum hardness of all samples is observed in AS (distance of about 4 mm from the center line).

On the other hand,it is clear that the hardness profile changes with pin geometry.This can be attributed to the various flows and mixing materials by the pin geometry which induces production of structure with different grain sizes in FSPed specimens.As seen,square pin shows the highest average hardness value while non-threaded(smooth) cylindrical pin shows the lowest average hardness value,which may be attributed to the grain size produced in SZ and also various distributions of SiC particles by square and straight cylindrical pins.These results confirm the presence of finer clusters of SiC particles and finer grain size structure in these specimens.

Figure 6b shows the effect of FSP pass number on the microhardness profile by one,two and four passes with cylindrical threaded pin.Moreover,Fig.6b shows that in addition to pin geometry,the FSP pass number affects the microhardness profile.As seen,increasing FSP pass number leads to the increase in average hardness of SZ due to the decrease in grain size [ 12] .Barmouz et al. [ 10] reported that multi-pass FSP alters the dispersion of the refinement particles in composites.In other words,with the increase in pass number during FSPed specimens,dispersion of SiC particles is homogeneous [ 3, 9, 15] .Owing to the uniform distribution of SiC particles in the specimen made by four passes FSP,a uniform microhardness variation is obtained.

Fig.5 OM images of SZs produced with SiC particles after different passes:a one pass,b two passes and c four passes

Fig.6 Effect of a pin geometry and b FSP pass number on microhardness profile of FSPed specimens

3.3 Wear rate and friction coefficient

Figure 7 shows the effect of FSP pass number on friction coefficient and wear rate.As can be seen,increasing FSP pass number decreases the wear rate but in fact improves the wear resistance.Moreover,increasing FSP pass number decreases the fluctuations of friction coefficient.It is evident from Fig.7 that the average friction coefficient of the specimen fabricated with one pass is somewhat lower than that fabricated with four passes.

Because of the smaller grain size and therefore higher hardness in the composite layers,if SiC particles distribute better,the wear resistance improves [ 12] .As discussed before,with FSP pass number increasing,distribution of the SiC particles in surface is uniformed and the finer grain size is obtained [ 12] .According to these results,the wear rate is significantly lower in the surface of FSPed specimens with SiC particles.Thus,the wear rate is a function of SiC particle distribution in surface.Figure 8 shows SEMimages of worn surface as a function of number of passes using cylindrical threaded pin.As shown in Fig.8a,the wear rate in FSPed specimen by one pass is higher than that by four pass,due to the increase in the SZ hardness by increasing pass number.Decreasing SZ size results in higher hardness in A16061/SiC composites,which causes lower material loss and hence lower wear rates.These results mean that the wear resistance is remarkably improved by increasing passes number.Further,it is observed that more metal is removed during the wear test in one FSP pass (Fig.8).

Fig.7 Effect of FSP pass number on wear rate a and friction coefficient:b one pass,c two passes and d four passes

Figure 9 shows the influence of pin geometry on wear rate by FSP.Figure 9 shows that the square pin and smooth(straight) cylindrical pin have the highest and lowest resistance to wear,respectively.This is due to the enhanced hardness obtained by square pin compared to the straight cylindrical pin.The attained data show that the change in pin geometry causes varying materials flows,which induces breaking and fragmentation of SiC particles in SZ and also improvement in SiC distribution on surface of Al/SiC composite.In addition,the results show that the change in the pin geometry leads to variation of the grain size in SZ and consequently different SiC distribution.According to these results and since square pin produces finer SiC particles and finer grain size in SZ,higher wear resistance is obtained.In other words,the size and distribution of SiC particles in SZ are functions of pin geometry.

Fig.8 SEM images of worn surface as a function of pass number:a one pass and b four passes

Fig.9 Effect of pin profile on wear rate

Fig.10 SEM images of worn surface as a function of pin geometry:a straight cylindrical pin and b square pin

Figure 10 shows the effect of pin geometry on worn surface with one FSP pass.As shown in Fig.10,the wear rate in FSPed specimen by straight cylindrical pin is more than that by square pin.This can be attributed to homogenized distribution of SiC particles.It is also observed that the worn debris at straight cylindrical pin is more than that at the square,which can be attributed to their hardness in surface of composite.These results indicate that the wear rate is significantly affected by different parameters including pin geometry,rotational speed,travel speed and FSP pass number.Additionally,it can be inferred that suitable distribution,finer SiC particles and finer grain size are the main reasons affecting wear surface of FSPed specimens.

4 Conclusion

FSP of 6061-T6 aluminum alloy was investigated by using pin geometry (threaded cylindrical,straight cylindrical,square and triangular) and number of passes (one,two and four).In summary,it is concluded that different regions produced during FSP of aluminum 6061-T6 include HAZ,SZ and TMAZ.In addition,the results show that a change in the pin geometry not only affects the microstructure of FSPed specimens,but also follows the fragmentation of SiC particles.For example,the average grain sizes in SZ by square and triangular pin geometry are 3.41 and 4.29μm,respectively.Moreover,increasing FSP pass number homogenizes SiC particle distribution and improves average grain size in SZ.As a result,the square and straight cylindrical pins have the highest and lowest resistance to wear,respectively.

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