Effect of Microstructural changes on
Mechanical properties of Friction stir welded
Nano SiC reinforced AA6061composite
GOVIND.NANDIPATIa,*, NAGESWARA RAO.DAMERAb,
RAMANAIAH.NALLUc
Asst.Professor, Department of Mechanical Engineering , R.V.R&J.C college of Engineering , Guntur, A.P- 522019, INDIA.
b
Professor, Department of Mechanical Engineering, AUCE, Andhra University, Visakhapatnam, A.P- 530 003, INDIA.
c
Asssociate Professor, Department of Mechanical Engineering, AUCE, Andhra University, Visakhapatnam, A.P- 530 003, INDIA.
Abstract
Aluminum alloys used in aircrafts generally exhibit low weldability on fusion welding techniques. Friction stir welding (FSW) has got a lot of attention as a solid state joining technique and provided an improved way of producing aluminum joints in a faster way. In the present work, nano Silicon carbide(SiC) particles reinforced aluminum AA6061metal matrix composites which find applications in aircrafts are casted and friction stir welded. FSW resulted in significant grain refinement and homogeneous distribution of nano SiC particles. The Microstructural analysis is carried out using optical microscopy(OM) and Scanning Electron Microscopy (SEM).The joint strength is increased compared to the conventional fusion welding techniques. The relationship between mechanical properties [hardness, UTS, Y.S] and microstructure of the welded region are studied.
Key words: Friction stir welding (FSW), metal matrix Nano composites (MMNC), Heat effected zone (HAZ), Stir Zone (SZ).
1. Introduction
In the present work, the heat treated and friction stir welded composites are subjected to Microstructural examination and Mechanical properties of the joints are determined by testing flat tensile specimens prepared as perASTM-E8 standards.
2. Experimental procedure: Aluminum alloys are widely used in light applications.AA6061 is used in those applications recently.
Table.1 Chemical composition of AA6061 % by weight -Base material
Alloy elements
Al Mg Si Fe Cu Mg Cr Ti
%by weight
97.768 0.825 0.711 0.342 0.152 0.023 0.017 0.083
Nano composite plates of Aluminum-AA6061 with varying weight percentages (0.1%, 0.2%, 0.3%, 0.4%, and 0.5%) of nano SiC particulate are produced by casting process. The Nano SiC particles are dispersed in the molten metal by mechanical stirring and ultrasonic cavitation methods. The plates are subjected to heat treatment, solutionised at 5600C for 1hr then water quenched and ageing is done for 12hrs at 1650C. The thickness of the plates are 6mm and friction stir welding is performed along the required line of joint at a tool rotation of 1200rpm and at a traverse speed of 0.7mm/sec. A mild steel tool with a shoulder of 18mm in diameter and a cylindrical pin of 4mm diameter is used. The hardness values are measured along the weld line in the transverse direction using Vicker’s hardness tester.
Fig.1 Tensile specimen used for testing
Transverse tensile specimens are machined from FSW plates in the perpendicular direction to the weld bead. The metallographic samples are cut from FSW zone and from the other region. These samples are ground, rough polished, finished with abrasive powder and finally etched with Keller’s reagent. (Composing 3ml HCl, 2mlHF, 20ml Nitric acid and 190ml of distilled water). The microstructures of welds are examined using Optical microscopy and Scanning electron microscopy. Tensile tests are conducted at room temperature.
3. Results and Discussions:
3.1 Hardness Tests
The hardness tests are conducted on MMNC and heat treated FSW samples. The hardness values are presented in fig.3
Fig.3- Comparison of hardness
In the previous experiments K.V.Jata, et.al [9], reported that the hardness of heat treatable FSW aluminum alloys greatly depend on the precipitate distribution and slightly on the dislocation structures. The fine grain structure in the nugget zone has been usually attributed to dynamic recrystallisation.FSW resulted in partial dissolution of the coarse precipitates and subsequent reprecipitation during natural aging after FSW. FSW of Aluminum alloy composite consists of three zones: the nugget zone (NZ), thermo-mechanically effected zone (TMAZ) and the heat affected zone.
Fig.4 Hardness profile along the weld zone
During heat treatment the refinement of both SiC particles and grains also contributed to the increase of hardness in the NZ. This increase in hardness is attributed to the generation of the fine recrystallised grains and dispersed ternary eutectic phases [12]. However under T6 condition transverse strengths of the composite weld are lower than parent metal [10]. A.H. Feng et.al. [10] reported that the hardness profile of the heat treatable aluminum alloys greatly depend on the precipitation distribution and only slightly on the grain and dislocation structures.FSW
60 70 80 90 100 110 120 130
0 0.2 0.4 0.6
Hardness
value
%wt of nano SiC
MMNC
hardness from FSW zone to MMNC is observed, even with a grain refinement of alloy matrix fig4. It could be probably related to the prevailing effect in the aluminum alloy matrix, induced by frictional heating. The hardness value of 110Hv is observed at 0.5% nano SiC friction stir welded MMNC than the hardness of the parent metal is as 125Hv.The hardness of the stir zone increases with the decrease in frictional heat flow from the tool shoulder to the joint. The hardness minima are observed in both sides of the weld in the HAZ. It is related to the loss of strain hardening compared to the MMNC.
3.2 Tensile tests:
The tensile behavior of the composite for different weight percentages of nano SiC particulate and Friction stir welded condition are studied. The tensile specimens are machined perpendicular to the FSW direction. The dimensions of the specimen are as per the ASTM-E8 (gauge length-28mm and cross sectional area 5x6mm2). The tensile tests on the unwelded composite confirm that addition of ceramic reinforcement to the aluminum matrix alloy, increases tensile strength while reduces elongation to failure[21].The results of tensile tests such as yield strength, tensile strength and percentage of elongation are reported in table.2 and table.3. From the results it can be inferred that the percentage of nano powders influence the tensile properties of the FSW joints. It can also be seen that FSW led to decrease in tensile properties compared to MMNC where as properties increase compared to fusion welds.
During Tensile tests, most of the specimens failed in the FSW region, but exact location of failure is either at the retreating side or at the advancing side which is evident from the fracture surfaces. The increased elongation to failure could be related to the prevailing effect of the over-aging of the aluminum alloy matrix due to coarsening of the intermetallic precipitates. The FSW of butt joint exerted some effect on the tensile and fracture behavior of the welded region [16]. The ductility of the weld was significantly reduced. The percentage of elongation reduced as the weight % of nano particulate increases. The ratio between the tensile strength of a welded joint and tensile strength of unwelded parent MMNC is termed as joint efficiency. The friction stir welded joints showed the efficiencies as Y.S-73% UTS-76%, Elongation-63% of the MMNC. These results can be attributed to the concurrent effects of different Microstructural modifications induced by the FSW process, like refinement of both aluminum matrix grain size and reinforcement particle.; roundness of the particles, overaging of aluminum matrix alloy induced by frictional heating, coarsening of intermetallic precipitates in the FSW zone[22].
Table.2 Mechanical properties of Aluminum MMNC % by weight of
nano Sic particulate
Yield strength(MPa)
Ultimate Tensile strength [MPa]
Elongation (%)
0.1 320 336 39
0.2 335 355 35
0.3 360 390 30
0.4 386 425 28
Fig.5 Stress-Strain diagram for MMNC
Table.3 Mechanical properties of Aluminum MMNC after FSW
Fig-6 Stress Strain diagram for MMNC after FSW
0 100 200 300 400 500
0 0.2 0.4 0.6
Stress in MPa Strain 0.5 0.4 0.3 0.2 0.1 0 50 100 150 200 250 300 350
0 0.1 0.2 0.3
Stress in MPa Strain 0.5 0.4 0.3 0.2 0.1
% by weight of nano Sic particulate
Yield strength(MPa) UltimateTensile strength[MPa]
Elongation(%)
0.1 252 267 35
0.2 265 280 26
0.3 273 298 21
0.4 285 308 18
3.3 Microstructure:
The microstructures of the joints are examined at various locations in and adjacent to the FSW zones. Under optical microscopy grain size and grain boundaries of the MMNC are observed at lower magnification and compared with FSW micrographs. SEM images obtained at the FSW region are presented in figures-9-12.The formation and distribution of the precipitates is clearly seen in fig-9. The main precipitate formed in the 6000 series Al-Mg-Si alloy system is Mg2Si [13].Most of the strengthening precipitates present in the base metal are dissolved during FSW. There is much reduced density of the precipitates, which can be observed after FSW. As the composite is solution treated and aged, precipitates appear to be agglomerated continuously at the grain boundaries. The dispersion of nano particles is clearly seen in SEM images [fig-9-12]. The nano particles occupied the grain boundaries. It is seen that the nano particles are obstructing the growth of the grains. The refinement of the precipitates and grains increase, as the percentage of the nano particles increases. The grain sizes of the microstructure for different percentages of nano particles are shown in the figures. The grain size in HAZ is coarser than that in the nugget zone with precipitates being coarsened. Thus the HAZ is usually the weakest zone of FSW joint [12]. The fine equiaxed grains are seen in the FSW region, implies that dynamic recrystallisation has taken place during friction stir welding due to plastic deformation which has been also observed by (16,17,18). Microscopic investigations reveal that mechanical mixing is the major material flow mechanism in the stir zone [19].
The grain structures surrounding the FSW tool are studied [20], which reveals that sub grains are formed ahead of FSW tool during the processing. The sub grains gradually develop greater disorientations as they approach the tool, while maintaining the same size, producing the final refined grain structure. The final refined grain structures are evolution of microstructures, formed around the pin of the tool. The SiC particles had a large effect on the recrystallisation kinetics. The Dynamic recrystallisation is nucleated in the regions of very high dislocation density between the reinforcements. That is SiC particles provided more nucleation sites for the new recrystallised grains by increasing local strain in the matrix and causing lattice disorientation.Fig-7(a) &7(b) show the optical images of grain boundaries of MMNC and FSW composites at 200 x magnifications. The larger grains are seen for MMNC and smaller grains are seen for Friction stir welded composites. Similarly at 400x magnification images are observed in fig-8.
Scanning Electron microscopy images of various weight % of nano composites are seen clearly in the figures-9, 10, 11, 12. In these figures the SiC nano particles are visible distinctly.
(a) (b)
(a) (b)
Fig.8-optical images of (a) MMNC-400x (b) AfterFSW-400x
(a) (b)
Fig-9 SEM images of 0.2%SiC-20,000x (b) 0.2%SiC-25,000x
(a)
(b)
Fig.11-SEM images of (a) 0.4%SiC-20,000x (b) 0.4% SiC-25,000x
The fine equiaxed grains observed in the nugget zone of FSW composite are very small than those in the MMNC. This indicates that dynamic re-crystallization (DRX) took place in the nugget zone of the FSW composite during the FSW process. Like any re-crystallization process DRX proceeds by nucleation and nucleus growth.
(a)
(b)
Fig.12-SEM images of (a) 0.5%SiC-20,000x (b) 0.5%SiC-25,000x
The particles play an important role in controlling the recrystallised grain size by particle stimulated nucleation. The limited grain growth is attributed to the effective pinning of the SiC particles. FSW can be considered as a hot working process in which severe plastic deformation is imparted to the work piece through the rotating pin and shoulder. The maximum temperatures can exceed 0.8Tm in aluminum alloys (where Tm is the melting temperature of aluminum expressed in K) which is enough to induce partial dissolution of the hardening precipitates. During natural aging after FSW, re-precipitation of the dissolved precipitates occurred in the nugget zone.
4. Conclusions
The influence of FSW on the mechanical properties and microstructure of aluminum metal matrix nano composite is investigated and the following conclusions are drawn.
[ii] The tensile tests have shown reasonable joint efficiencies in terms of ultimate tensile strength but poor efficiency in terms of elongation at the rupture.
[iii] FSW resulted in the generation of fine equiaxed recrystallised grains, the breaking and uniform distribution of the SiC particles in the nugget zone.
[iv] FSW process shows the efficiency as Yield strength 72% and Ultimate tensile strength as 73% compared to the unwelded MMNC.
[v]The microstructures of the friction stir welded samples showed the uniform distribution of nano particles in the weld region and grain refinement is distinctly seen between unwelded and welded region.
Acknowledgements
:
The authors wish to thank Mechanical department- Andhra University, Metals joining laboratory- IIT Madras and Central Research Facility-IIT Kharagpur for providing the Equipment to conduct Experiments.References:
[1] W.M.Thomas, E.D.Nicholas, Friction stir welding for the Transportation industries, Materials and Design vol.18 , 1997, pp 269-273. [2] M.Peel, A.Steuwer,M.Preuss,P.J.Withers, Microstructure,Mechanical properties & residual stresses as afunction of welding speed in
Aluminum AA5083 friction stir welds, Acta Materilia,vol.51, 2003,pp 4791-4801.
[3] P.Staron, M.Kocak, S.W.Williams, A.Wescott, Residual stresses in friction stir welded Al.sheets Phys.B:Condens.Matter vol.350, 2004, ppE491-E493.
[4] G.Pouget, A.P. Reynolds,Residual stress and microstructure effects on fatigue crack growth in AA2050 friction stir welds, International journal of fatigue,vol.30, 2008,pp463-472.
[5] K.Masubuchi, Analysis of welded structures: Residual stresses, Distortion and Their Consequences, Pergamon Press, Oxford,1980. [6] G.Bussu, P.E.Irving, The role of residual stress and heat affected zone properties on fatigue crack propagation in friction stir welded
2024-T351 aluminum joints .International journal of Fatigue,vol.25 issue-1, 2003, pp77-88.
[7] H.Lombard et al., Optimizing FSW process parameters to minimize defects and maximize fatigue life in 5083-H321 aluminum alloy, Vol.75, 2008, pp341-354.
[8] A.Cabello et.al., Comparision of TIG welded and friction stir welded Al-4.5Mg-0.26Sc alloy Journal of Materials processing Technology vol.197, 2008, pp337-343.
[9] K.V.Jata,S.L.Semiatin, ,Continuous Dynamic recrystallisation during friction stir welding of high strength aluminum alloys, Scripta mater.vol.43, 2000, pp743-749
[10] A.H.Feng,B.L.Xiao,Z.Y.Ma, Effect of Microstructural evolution on mechanical properties of friction stir welded AA2009/SiCp composite,
Composites science and Technology xxx-xxx.article in press. 2008.
[11] Tomotake Hirata.et.al. Influence of friction stir welding parameters on grain size and formability in 5083 aluminum alloy, Material science and engineering vol.A456, 2007, pp344-349.
[12] G.M.Xie Z.Y.Ma, L.Geng, R.S.Chen. Development of a fine grained microstructure and the properties of a nugget zone in friction stir welded pure copper, Material Science and Engineering vol. A471, 2007, pp63-68.
[13] K.Elangovan, V.Balasubramanian, Influences of post weld heat treatment on tensile properties of friction stir welded AA6061 aluminum alloy joints, Material characterization, article in press xxx-xxx. 2007,
[14] K.Kumar, Satish V.Kailash , On the role of axial load and the effect of interface position on the tensile strength of a friction stir welded aluminum alloy, Materials and design, vol. 29, 2008,pp791-797.
[15] S.R.Ren,Z.Y.Ma,L.Q.Chen,Effect of initial butt surface on tensile properties and fracture behavior of friction stir welded Al-Zn-Mg-Cu alloy, Materials Science and Engineering A, vol.475,2008, pp293-299.
[16] Colligan K. Material flow behavior during Friction stir welding of Aluminum, Weld Journal, 1999, pp229s-237s.
[17] Barcellona A, Buffa G, Fratini L, Palmeri D,On Microstructural phenomena occurring in friction stir welding of aluminum alloys, Material Processing Technology, vol.177, . 2006, pp340-343.
[18] Amancio-Filho S T,et.al, Preliminary study on the microstructure and mechanical properties of dissimilar friction stir welds in air craft aluminum alloys 2024-T351 and 6056-T4, Journal of Material Processing Technology xxx(2008) xxx-xxx.
[19] R.W.Fonda, J.F.Bingert,K.J.Colligan, Development of grain structure during friction stir welding,Scripta Materilia.vol.51, 2004, pp243-248.
[20] Jian Qing Su et al, Microstructural evolution during ESW/FSP of high strength aluminum alloys, Material science and Engineering vol.A 405, 2005, pp277-286.
[21] L. Ceschini, I.Boromei, G.Minak,A.Morri,F.Tarterini, Microstructure,tensile and fatigue properties of AA6061/20 vol.%Al2 O3p friction stir
