Scripta Materialia,Vol.36,No.I,pp.69-75,1997 Pergamon Elsevier Science Ltd Copyright199 Acta Metallurgica Inc. Printed in the USA.All rights reserved 1359-6462/97$17.00+.00 PIⅡS1359-6462(96)00344-2 EFFECTS OF FRICTION STIR WELDING ON MICROSTRUCTURE OF 7075 ALUMINUM C.G.Rhodes,M.W.Mahoney,W.H.Bingel, R.A.Spurling and C.C.Bampton Rockwell Science Center,Thousand Oaks,CA 91360 (Received April 15,1996) (Accepted July 30,1996) Introduction Friction stir welding is a relatively new technique developed by The Welding Institute (TWI)for the joining of aluminum alloys(1).The technique,based on friction heating at the faying surfaces of two pieces to be joined,results in a joint created by interface deformation,heat,and solid-state diffusion (2-4). Fusion welding of commercial aluminum alloys is difficult at best.For instance,using arc welding techniques,an alloy like 2024 has only limited weldability and the 7XXX series are not recommended for welding(5).Some Al alloys can be resistance welded,but the surface preparation is extensive,with surface oxide being a major problem.Friction stir welding,on the other hand,can be used to join most Al alloys,and surface oxide is no deterrent to the process.No special cleaning techniques are required prior to welding. Although the work piece does heat up during friction stir welding,the temperature does not reach the melting point.The friction-stir-welded joint,then,does not have the dendritic structure typical of a fusion-weld joint,with its attendant serious degradation of mechanical properties. Details of the friction stir welding process are available in the literature(1,2).Basically,a non- consumable tool with a specially designed rotating probe is entered into the abutting edges of the sheets or plates to be welded.Once entered,the rotating tool produces frictional heat and plastic de- formation in the workpiece.The tool,or the part,is then translated along the joint to complete the joining process. In evaluating friction stir welding,critical issues (beyond a sound joint)include microstructure control and localized mechanical property variations.A serious problem with fusion welding,even when a sound weld can be made,is the complete alteration of microstructure and the attendant loss of mechanical properties.Being a solid-state process,friction stir welding has the potential to avoid sig- nificant changes in microstructure and mechanical properties. The objective of this study was to evaluate the microstructural changes effected by friction stir welding of 7075 Al.The effects on tensile properties are reported in a separate paper(6). 69
ScriDta Materialia. Vol. 36. No.]. DD. 69-15. 1997 Perga.mon PI1 S1359-6462(96)00344-2 , . . . Elsevier Science Ltd Copyright 0 1996 Acta Metallurgica Inc. Printed in the USA. All rights reserved 1359-6462/97 $17.00 + .OO IEFFECTS OF FRICTION STIR WELDING ON MICROSTRUCTURE OF 7075 ALUMINUM C.G. Rhodes, M.W. Mahoney, W.H. Bingel, R.A. Spurling and C.6. Bampton Rockwell Science Center, Thousand Oaks, CA (Received April 15, 1996) (Accepted July 30, 1996) 91360 Introduction Friction stir welding is a relatively new technique developed by The Welding Institute (TWI) for the joining of aluminum alloys( 1). The technique, based on friction heating at the faying surfaces of two pieces to be joined, results in a joint created by interface deformation, heat, and solid-state diffusion (2-4). Fusion welding of commercial aluminum alloys is difficult at best. For instance, using arc welding techniques, an alloy like 2024 has only limited weldability and the 7XXX series are not recommended for welding(5). Some Al alloys can be resistance welded, but the surface preparation is extensive, with surface oxide being a major problem. Friction stir welding, on the other hand, can be used to join most Al alloys, and surface oxide is no deterrent to the process. No special cleaning techniques are required prior to welding. Although the work piece does heat up during friction stir welding, the temperature does not reach the melting point. The friction-stir-welded joint, then, does not have the dendritic structure typical of a fusion-weld joint, with its attendant serious degradation of mechanical properties. Details of the friction stir welding process are available in the literature(l,2). Basically, a nonconsumable tool with a specially designed rotating probe is entered into the abutting edges of the sheets or plates to be welded. Once entered, the rotating tool produces frictional heat and plastic deformation in the workpiece. The tool, or the part, is then translated along the joint to complete the joining process. In evaluating friction stir welding, critical issues (beyond a sound joint) include microstructure control and localized mechanical property variations. A serious problem with fusion welding, even when a sound weld can be made, is the complete alteration of microstructure and the attendant loss of mechanical properties. Being a solid-state process, friction stir welding has the potential to avoid significant changes in microstructure and mechanical properties. The objective of this study was to evaluate the microstructural changes effected by friction stir welding of 707fi Al. The effects on tensile properties are reported in a separate paper(6). 69
70 FRICTION STIR WELDING Vol.36,No.I TABLE I Nominal Composition of 7075Al Mg wt.pcl. 007 0.23 93 2.4 29 07 0.12 Experimental Procedure The alloy selected was 6.35 mm(0.25 in.)gauge 7075-T6 Al plate which was butt welded using the friction stir technique.Nominal composition is listed in Table 1. Detailed weld parameters,such as tool design and tool rotation speed,are proprietary to TWI's group-sponsored project members.It can be noted,however,that the workpiece travel speed for this study was 5 in/min;using this technique,the travel speed can be up to~12 in/min,making it compara- ble to other welding techniques. Transmission electron microscopy was performed using a Philips CM30 electron microscope.Thin foil samples were made by electropolishing in a nitric acid/methanol solution followed by ion mill cleaning. Results A macroscopic view of a friction stir welded joint is shown in Figure 1.The weld has an elliptical cross section with a zone around the weld nugget that has undergone extensive deformation.Micro- hardness measurements across the joint indicate only small variations from the base metal. Base Metal The parent 7075-T6 plate exhibits an elongated matrix grain morphology,Figure 2.The typical Fe- containing and Si-containing constituent particles are evident by light (LM)and scanning electron microscopy(SEM),but the strengthening precipitates can be discerned only by transmission electron microscopy(TEM),Figure 3. 5 mm Figure 1.Cross section of friction stir weld in 7075 Al
70 FRICTION STIR WELDING Vol. 36, No. 1 TABLE 1 Nominal Composition of 7075Al ( Al I %n I hlg I cu I yoo7 I 56 1 2.5 I I.6 ‘J3.M 2.4 I 2.9 I 0.7 I Cr I 0.23 I nt2 Experimental Procedure The alloy selected was 6.35 mm (0.25 in.) gauge 7075T6 Al plate which was butt welded using the friction stir technique. Nominal composition is listed in Table 1. Detailed weld parameters, such as tool design and tool rotation speed, are proprietary to TWI’s group-sponsored project members. It can be noted, however, that the workpiece travel speed for this study was 5 in/min; using this technique, the travel speed can be up to -12 in/mm making it comparable to other welding techniques. Transmission electron microscopy was performed using a Philips CM30 electron microscope. Thin foil samples were made by electropolishing in a nitric acid/methanol solution followed by ion mill cleaning. Results A macroscopic view of a friction stir welded joint is shown in Figure 1. The weld has an elliptical cross section with a zone around the weld nugget that has undergone extensive deformation. Microhardness measurements across the joint indicate only small variations from the base metal. Base Metal The parent 7075-T6 plate exhibits an elongated matrix grain morphology, Figure 2. The typical Fecontaining and Si-containing constituent particles are evident by light (LM) and scanning electron microscopy (SEM), but the strengthening precipitates can be discerned only by transmission electron microscopy (TEM), Figure 3. Figure 1. Cross section of friction stir weld in 7075 Al
Vol.36,No.I FRICTION STIR WELDING 71 0.5μm Figure 2.Microstructure of parent metal Figure 3.Parent metal,showing distribution of 50-75nm strengthening precipitates. There is some disagreement in the literature as to the exact composition of the strengthening pre- cipitates in 7075 Al(7-9)and the temperatures at which they form.Wert(7)has reported that the pre- cipitates formed on aging at 400C are a solid solution of the isomorphous phases MgZn and MgAICu,which he describes as Mg(Zn2,AlCu).This phase has the C14 (hexagonal)crystal structure with lattice parameters a=0.519nm and c=0.850nm.Lorimer(8),on the other hand,reported that the precipitate forming above 190C is Mgz(Al,Zn)49,having a bcc crystal structure with a=1.416nm. Lorimer further reported that eta phase(MgZn2)forms in the temperature range of 120-190C. In the present study,there are two populations of strengthening precipitates:one group at 50-75 nm and the other group at 10-20 nm,Figure 4.The larger precipitates tend to be oriented in the rolling direction of the plate.Convergent-beam electron diffraction was inconclusive as to the exact structure of the larger(50-75nm)precipitates.Some patterns could be indexed as Mgs2(Al,Zn)49,whereas others were indexed as Mg(Zn2,AlCu).Quite possibly both types of precipitates are present,although such an 200nm 200nm Figure 4.Dark field TEM showing 10-20nm precipitates in parent metal. Figure 5.Dislocations in parent metal
Vol. 36, No. I FRICTION STIR WELDING 71 Figure 2. hGcrostructure of parent metal. Figure 3. Parent metal, showing distribution of 50-75nm strengthening precipitates. There is some disagreement in the literature as to the exact composition of the strengthening precipitates in 7075 Al(7-9) and the temperatures at which they form. Wert(7) has reported that the precipitates formed on aging at 400°C are a solid solution of the isomorphous phases MgZrb and MgAlCu, which he describes as Mg(Znz,AICu). This phase has the Cl4 (hexagonal) crystal structure with lattice parameters a = 0.5 19nm and c = 0.85Onm. Lorimer(8), on the other hand, reported that the precipitate forming above 190°C is MgS2(A1,Zn&, having a bee crystal structure with a = 1.4 16nm. Lorimer further :reported that eta phase (MgZnz) forms in the temperature range of 120-190°C. In the present study, there are two populations of strengthening precipitates: one group at 50-75 nm and the other group at lo-20 nm, Figure 4. The larger precipitates tend to be oriented in the rolling direction of the Iplate. Convergent-beam electron diffraction was inconclusive as to the exact structure of the larger (50-75nm) precipitates. Some patterns could be indexed as Mgjz(Al,Zn)49, whereas others were indexed as Mg(Zns,AlCu). Quite possibly both types of precipitates are present, although such an Figure 4. Dark field TEM showing lo-20nm precipitates in parent metal. Figure 5. Dislocations in parent metal
72 FRICTION STIR WELDING Vol.36,No.1 um I um Figure 6.Equiaxed grains in weld nugget,backscattered Figure 7.Showing particle distribution and grain size in clectron image. weld nugget. analysis was beyond the scope of this study.The smaller(10 nm)precipitates were too small for dif- fraction analysis,although their reaction in dark field was identical to the larger precipitates. A third population of precipitates was found at grain boundaries.These were in the 30-40nm size group and were generally elongated particles.No diffraction study was done to identify these particles, although previous studies(8)would suggest these are also the MgZna type. The dislocation density is modest,comprising fairly loose tangles,Figure 5.The T6 treatment has clearly provided recovery from the cold rolled condition without inducing recrystallization.Some dipoles have developed,but virtually no loops are present,reflecting the temperature of the T6 treat- ment. Oμ网 0.5m Figure 9.Elongated grain structure in mechanically af- Figure 8.Dislocations in weld nugget. fected zone
72 FRICTION STIR WELDING Vol. 36, No. 1 Figure 6. Equiaxed grains in weld nugget, backscattel electron image. .ed Figure 7. Showing particle distribution and grain size in weld nugget. analysis was beyond the scope of this study. The smaller (10 nm) precipitates were too small for diffraction analysis, although their reaction in dark field was identical to the larger precipitates. A third population of precipitates was found at grain boundaries. These were in the 30-40nm size group and were generally elongated particles. No diffraction study was done to identify these particles, although previous studies(S) would suggest these are also the MgZn2 type. The dislocation density is modest, comprising fairly loose tangles, Figure 5. The T6 treatment has clearly provided recovery from the cold rolled condition without inducing recrystallization. Some dipoles have developed, but virtually no loops are present, reflecting the temperature of the T6 treatment. Figure 8. Dislocations in weld nugget. Figure 9. Elongated grain structure in mechanically fected zone. af-
Vol.36,No.I FRICTION STIR WELDING 73 200n Figure 10.Structure in mechanically affected zone. Figure 11.Dark field TEM illustrating distribution of 50- 70nm and 20nm precipitates in mechanically affected zone. Weld Nugget The weld nugget is characterized by concentric flow lines as,shown in Figure 1.These flow lines apparently represent plastic deformation increments that develop as the rotating tool pin moves through the joint. Unlike the parent metal,the weld nugget has a recrystallized,fine equiaxed grain structure on the order of 2-4 um in diameter,Figure 6.Electron diffraction indicates the grain boundaries to be high- angle,discounting the prospect that these are sub-grains.A high density of randomly oriented intra- granular precipitates are 60-80 nm in size and tend to be disks or plates,Figure 7.As was the case for the parent metal,diffraction patterns from these precipitates can be indexed as either Mg2(Al,Zn)49 or Mg(Zn2,AlCu).These appear to be the same type of precipitates as the 50-75nm particles oriented (stringered)in the rolling direction in the parent metal.This precipitate redistribution from stringered to random suggests that these particles have gone into solution and re-precipitated during the joining process.There are also precipitates at the grain boundaries,indexable as Mg32(Al,Zn)49.Constitutent particles persist in the weld nugget in a distribution similar to that found in the parent metal. In contrast to the parent metal,the dislocation density in the weld nugget is quite low.Individual dislocations extend between particles,but no tangles occur,Figure 8. There are none of the very fine(10 nm)intragranular precipitates seen in the parent metal.The recrystallization of the weld nugget grains and the redistribution of the precipitates indicate that the temperature excursion during joining was above the solution temperature for the hardening precipi- tates,but below the melting temperature of the alloy.A likely temperature is somewhere between 450 and 480C(7-9).Apparently,cooling rates were such that the larger precipitates could nucleate and grow,but that the finer ones could not nucleate,i.e.,the cooling curve intersects the t-t-t curve at a temperature well above the nose. Mechanically Affected Zone The transition one between the parent metal and the weld nugget is characterized by a highly de- formed structure,Figure 9.The elongated grains characteristic of the parent metal have been deformed
Vol. 36, No. 1 FRICTION STIR WELDING 73 Figure 10. Structure in mechanically affected zone. Figure 11. Dark field TEM illustrating distribution of 50- 70nm and 2Onm precipitates in mechanically affected zone. Weld Nugget The weld nugget is characterized by concentric flow lines as, shown in Figure 1. These flow lines apparently represent plastic deformation increments that develop as the rotating tool pin moves through the joint. Unlike the parent metal, the weld nugget has a recrystallized, fme equiaxed grain structure on the order of 2-4 l.urt in diameter, Figure 6. Electron diffraction indicates the grain boundaries to be highangle, discounting the prospect that these are sub-grains. A high density of randomly oriented intragranular precipitates are 60-80 mn in size and tend to be disks or plates, Figure 7. As was the case for the parent metal, diMaction patterns from these precipitates can be indexed as either MBz(A1,Zn)49 or Mg(Znl,AICu). These appear to be the same type of precipitates as the 50-75nm particles oriented (stringered) in the rolling direction in the parent metal. This precipitate redistribution from stringered to random suggests that these particles have gone into solution and re-precipitated during the joining process. There are also precipitates at the grain boundaries, indexable as Mg32(A1,Zn)49. Constitutent particles persist in the weld nugget in a distribution similar to that found in the parent metal. In contrast to the parent metal, the dislocation density in the weld nugget is quite low. Individual dislocations extsend between particles, but no tangles occur, Figure 8. There are none of the very fine (10 nm) intragranular precipitates seen in the parent metal. The recrystallization of the weld nugget grains and the redistribution of the precipitates indicate that the temperature excursion during joining was above the solution temperature for the hardening precipitates, but below the melting temperature of the alloy. A likely temperature is somewhere between 450 and 480°C (7-9). Apparently, cooling rates were such that the larger precipitates could nucleate and grow, but that the finer ones could not nucleate, i.e., the cooling curve intersects the t-t-t curve at a temperature well above the nose. Mechanically Affected Zone The transition :zone between the parent metal and the weld nugget is characterized by a highly deformed structure, Figure 9. The elongated grains characteristic of the parent metal have been deformed
74 FRICTION STIR WELDING Vol.36,No.I in a flowing pattern around the weld nugget.TEM reveals that these grains have not recrystallized,as occurred in the weld nugget,Figure 10. The larger(50-75 nm)hardening precipitates appear not to have been significantly altered,either in size or morphology,during the joining process,Figure 11.The smaller precipitates,on the other hand, have coarsened during welding.They are disks having diameters on the order of 20 nm and thicknesses of about 6 nm,Figure 11.Precipitate-free zones are present around many of the larger matrix precipi- tates and around all of the grain-boundary precipitates. The dislocation density is quite low,with individual dislocations extending between particles,pri- marily in the PFZ near grain boundaries.There is no evidence of the tangles that existed in the parent metal. Discussion Friction stir welding has been shown to be a viable solid-state joining process for 7075 Al.Although there are a weld nugget and a heat affected zone,the microstructural changes were found to be less drastic than those occuring in a fusion weld. The most significant changes in microstructure in the 7075 Al examined in this study were the re- crystallization of the elongated grains and the solution of the fine(10-20nm)hardening precipitates in the weld nugget.A thermocouple placed in front of the rotating tool recorded a temperature of~425C. However,no thermocouple could be applied to the trailing portion of the weld where the temperature is higher,so an accurate measure of nugget temperature was not made.The subtle changes in micro- structure provide an estimate of the temperature excursion.The solution of all precipitates and re- precipitation of the strengthening precipitates(Mgs2(Al,Zn)49 or Mg(Zn2,AlCu))indicate the tempera- ture has exceeded 400C and was likely closer to 480C in the weld nugget.The sizes and distributions of the two populations of hardening precipitates in the mechanically affected zone indicate that that zone remained below 400C. The concentric flow lines observed in the weld nugget likely have an influence on the recrystalliza- tion of the matrix grains and on the precipitation of the strengthening particles.Details of these flow lines are currently being studied and will be reported in a subsequent paper. Summary and Conclusions 7075 Al plate has been successfully joined by friction stir techniques.Unlike fusion welding,this is a solid-state process with no evidence of melting.The weld is characterized by a recrystallized nugget having a 2-4 um grain size.The dislocation density in the nugget is lowered from that in the parent metal;strengthening precipitates appear to have been solutionized during the welding proce,with the larger ones re-precipitating on cooling.The process,thus,provides a method for joining an unweldable aluminum alloy without introducing a cast microstructure. Acknowledgments We are pleased to acknowledge the assistance of E.Litwinski of Rockwell Space Systems Division for the preparation of samples and weld support.This work was funded by Rockwell International I.R. and D
74 FRICTION STIR WELDING Vol. 36, No. 1 in a flowing pattern around the weld nugget. TEM reveals that these grains have not recrystallized, as occurred in the weld nugget, Figure 10. The larger (50-75 nm) hardening precipitates appear not to have been significantly altered, either in size or morphology, during the joining process, Figure 11. The smaller precipitates, on the other hand, have coarsened during welding. They are disks having diameters on the order of 20 nm and thicknesses of about 6 nm, Figure 11. Precipitate-free zones are present around many of the larger matrix precipitates and around all of the grain-boundary precipitates. The dislocation density is quite low, with individual dislocations extending between particles, primarily in the PFZ near grain boundaries. There is no evidence of the tangles that existed in the parent metal. Discussion Friction stir welding has been shown to be a viable solid-state joining process for 7075 Al. Although there are a weld nugget and a heat affected zone, the microstructural changes were found to be less drastic than those occuring in a fusion weld. The most significant changes in microstructure in the 7075 Al examined in this study were the recrystallization of the elongated grains and the solution of the fine (lo-20nm) hardening precipitates in the weld nugget. A thermocouple placed in front of the rotating tool recorded a temperature of -425°C. However, no thermocouple could be applied to the trailing portion of the weld where the temperature is higher, so an accurate measure of nugget temperature was not made. The subtle changes in microstructure provide an estimate of the temperature excursion. The solution of all precipitates and reprecipitation of the strengthening precipitates (Mgz(Al,Zn)49 or Mg(Znn,AICu)) indicate the temperature has exceeded 400°C and was likely closer to 480°C in the weld nugget. The sizes and distributions of the two populations of hardening precipitates in the mechanically affected zone indicate that that zone remained below 4OO’C. The concentric flow lines observed in the weld nugget likely have an influence on the recrystallization of the matrix grains and on the precipitation of the strengthening particles. Details of these flow lines are currently being studied and will be reported in a subsequent paper. Summary and Conclusions 7075 Al plate has been successfully joined by friction stir techniques. Unlike fusion welding, this is a solid-state process with no evidence of melting. The weld is characterized by a recrystallized nugget having a 2-4 pm grain size. The dislocation density in the nugget is lowered from that in the parent metal; strengthening precipitates appear to have been solutionized during the welding proci , with the larger ones re-precipitating on cooling. The process, thus, provides a method for joining an unweldable aluminum alloy without introducing a cast microstructure. Acknowledgments We are pleased to acknowledge the assistance of E. Litwinski of Rockwell Space Systems Division for the preparation of samples and weld support. This work was funded by Rockwell International I.R. and D
Vol.36,No.I FRICTION STIR WELDING 75 References 1.W.M.Thomas et al.:'Friction Stir Butt Welding,'International Patent Appl.No.PCT/GB92/02203 and GB Patent Appl. No.9125978.8,Dec.1991.U.S.Patent No.5,460,317. 2.C.Dawes and W.Thomas,TWI Bulletin 6,124 [Nov./Dec.1995]. 3.M.Ellis and M.Strangwood,TWI Bulletin 6,138 [Nov/Dec 1995]. 4. C.J.Dawes and W.M.Thomas,Welding Joumal,75(3),41 (1996) 5. Metals Handbook,Ninth Edition,Vol 6,p.373,ASM,Metals Park,OH(1983). 6.M.W.Mahoney,et al.,Script.Met.,to be published. 7.J.A.Wert,Scripta Met.15,445(1981). 8. G.W.Lorimer,Precipitation Processes in Solids,K.C.Russell and H.I.Aaronson,eds.,p.87,Met.Soc.AIME,Warren- dale,PA(1978). 9. R.H.Brown and L.A.Willey,Aluminum,vol.1.Properties,Physical Metallurgy,and Phase Diagrams,K.R.Van Horn, ed.,p.31,ASM,Metals Park,OH(1967)
Vol. 36, No. 1 FRICTION STIR WELDING 75 2. 3. 4. 5. 6. 7. 8. 9. References W. M. Thomas et al.: ‘Friction Stir Butt Welding,’ International Patent Appl. No. PCT/GB92/02203 and GB Patent Appl. No. 9125978.8, Dec. 1991, U.S. Patent No. 5460,317. C. Dawes and W. Thomas, TWl Bulletin 6, 124 [Nov./Dee. 19951. M. Ellis and M. Strangwood, TWI Bulletin 6, 138 [Nov/Dec 19951. C.J. Dawes and W.M. Thomas, Welding Journal, 75 (3), 41(1996). Metals Handbook, Ninth Edition, Vol6, p. 373, ASM, Metals Park, OH (1983). M.W. Mahoney, et al., Script. Met., to be published. J.A. Wert, Scrllpta Met. 15,445 (1981). G.W. Lorimer, Precipitation Processes in Solids, K.C. Russell and H.I. Aaronson, eds., p. 87, Met. Sot. AIME, Warrendale, PA (1978). R.H. Brown and L.A. Willey, Aluminum, vol. 1. Properties, Physical Metallurgy, and Phase Diagrams, K.R. Van Horn, ed., p. 31, ASM, Metals Park, OH (1967)
