This paper provides the research of the infl uence of geometric and kinematic parameters on the microstructure and mechanical properties of welded joint of aluminum alloy AlSi1MgMn (6082-T6) obtained through the Friction Stir Welding (FSW) process. The experiment parameters were welding speed, rotation speed, angle of pin slope, pin diameter and shoulder diameter. On the obtained welded workpieces the dynamic testing on the impact toughness, and determination of microstructural zones were carried out

D. Pavletić

M. Janjić

M. Vukčević

N. Šibalić

V. Mandić

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29 METALURGIJA 51 (2012) 1, 29-33M. JANJIĆ, M. VUKČEVIĆ, V. MANDIĆ, D. PAVLETIĆ, N. ŠIBALIĆMICROSTRUCTURAL EVOLUTION DURING FRICTION STIR WELDING OF AlSi1MgMn ALLOYRecevied – Prispjelo: 2010-12-21Acctepted – Prihvaćeno: 2011-02-10Original Scientifi  c Paper – Izvorni znanstveni radISSN 0543-5846METABK 51(1) 29-33 (2012)UDC – UDK 621.791.1:669.715:620.184/.186=111This paper provides the research of the infl  uence of geometric and kinematic parameters on the microstructure and mechanical properties of welded joint of aluminum alloy AlSi1MgMn (6082-T6) obtained through the Friction Stir Welding (FSW) process. The experiment parameters were welding speed, rotation speed, angle of pin slope, pin di-ameter and shoulder diameter. On the obtained welded workpieces the dynamic testing on the impact toughness, and determination of microstructural zones were carried out.Key words: friction stir welding - FSW, shoulder, pin, impact toughness, microstructureMikrostrukturni razvitak AlSi1MgMn slitine tijekom zavarivanja trenjem. U radu se istražuje uticaj geometrij-skih i kinematskih parametara na mikrostrukturu i mehanička svojstva zavarenog spoja od aluminijske slitine AlSi-1MgMn (6082-T6) dobivene postupkom zavarivanja trenjem (ZT). Parametri provedenih pokusa su brzina zavariva-nja, kutna brzina okretanja alata, kut nagiba trna, promjer trna i promjer čela alata. Kod dobivenih zavarenih uzoraka izvedena su dinamička ispitivanja na udarnu žilavosti i određene su mikrostrukturne zone.Ključne riječi: zavarivanje trenjem - ZT, čelo alata, trn, udarna žilavost, mikrostrukturaINTRODUCTIONTools used in the FSW process are cylindrical and consisted of two concentric parts, which rotate at high speed. Part of the tool with larger diameter is called the shoulder, while the part with smaller diameter is called the pin. Tool and workpieces that are welded are shown in Figure 1 [1-4].AlSi1MgMn alloy belongs to the group hardly weld-ed alloys by conventional methods due to poor harden-ing and high porosity in the welding zone, so the effect occurs due to dissolution and coarsening of hardening phases. For that reason, welding of aluminum alloy us-M. Janjić, M. Vukčević, N. Šibalić, Faculty of Mechanical Engineering, University of Montenegro, Podgorica, Montenegro. V. Mandić, Mechani-cal Engineering Faculty, University of Kragujevac, Kragujevac, Repub-lic of Serbia. D. Pavletić, Faculty of Engineering, University of Rijeka, Rijeka, Croatiaing Friction Stir Welding process represents a major challenge for researchers.Microstructural delineation of the four zones of the cross section of welded joint is shown in Figure 2 [1].EXPERIMENTAL PROCEDUREIn order to determine the inﬂ  uence of geometric pa-rameters of the tool on the microstructural changes and the impact toughness, the experimental research of weld-ing of aluminum alloy AlSi1MgMn, thickness of 7,8 mm was carried out. The family of tools where geometrical parameters were varied was adopted for welding of alu-minum alloy sheet. The general image of the family of tools for the FSW process is shown in Figure 3.Based on preliminary researches, the multifactor or-thogonal plan with varying of factors on two levels, and repetition in the central point of plan n0=4 times is adopted. For input values, factors of the welding regime are adopted: X1=v mm/min (welding speed), X2=ω rpm Figure 1 Scheme of FSW processFigure 2   Various microstructural regions in the transverse cross section of friction stir welded material. A - unaff  ected material or parent metal, B - heat aff  ected zone - HAZ, C - thermo-mechanically aff  ected zone - TMAZ, D - “weld nugget” zone - NZ [1]30   METALURGIJA 51 (2012) 1, 29-33M. JANJIĆ et al.:  MICROSTRUCTURAL EVOLUTION DURING FRICTION STIR WELDING OF AlSi1MgMn ALLOY(rotation speed of tool) and geometrical factors of tools: X3=αo (angle of pin slope), X4=d mm (diameter of the pin) and X5=D mm (diameter of the shoulder). Levels of variation of input factors are adopted and given in Table 1 [3].Table 1 Levels of variation of input factors [3]Input factors Upper level Lower level Basic levelX1 200 80 125,00X2 1 000 630 800,00X3 5 3 3,87X4 7 5 5,92X5 28 25 26,46Based on the adopted values of X3, X4 and X5, the set of nine tools is made, and shown in Figure 4.The process of conducting the experimental re-searches is shown in Figure 5, and Figure 6 presents the wel  ded workpieces for the 36th point of the experimen-tal plan.DETERMINATION OF THE IMPACT TOUGHNESS The welded joint obtained by using FSW process presents a complex and heterogeneous structure and the critical place in the welded workpiece, when it comes to industrial use in welded construction. Therefore, in most cases, the safety of welded joints or welded struc-tures is determined according to the mechanical proper-ties of welded joint, for that reason the paper includes the dynamic impact testing and determination of impact toughness of the material ρ. Sampling of the welded workpieces was performed. Specimens are cut from the direction normal to the direction of welding, y - trans-verse direction, so that all microstructural zones are lo-cated on the tested specimen.Toughness testing of welded material is performed by impact load on the Charpy pendulum, with a maxi-mum initial available energy of 150 J.When impact occurs, the specimen is exposed to bending. The toughness of the welded material and shock resistance in bending is the work spend to break the specimen reduced to a unit of the specimen cross section:  AF= ρ  J/cm2 (1) The specimen was made according to standards MEST EN 10045-1:1993 [5], (Figure 7). A narrow specimen with a V-notch was adopted.The value of energy consumed for the fracture of the specimen made of unaffected material is 42 J and im-pact toughness is ρ=70 J/cm2.The minimum values of impact toughness occur in the 17th and 18th point of the experimental plan: ρ17=13,33 J/cm2 and ρ18=10 J/cm2. The greatest value of impact Figure 3   Tool with accepted dimensions and parameters D, d and αFigure 4 Set of tools made according plan of experimentFigure 5   Research site: 1 - auxiliary equipment,2 - workpieces of aluminum alloy AlSi1MgMn, 3 - tool for FSWFigure 6 Welded workpieces [3]Figure 7 The specimen for testing of the impact toughness [5]31 METALURGIJA 51 (2012) 1, 29-33M. JANJIĆ et al.:  MICROSTRUCTURAL EVOLUTION DURING FRICTION STIR WELDING OF AlSi1MgMn ALLOYtough  ness is obtained for the central point or the 35th point of experimental plan: ρ35=66,66 J/cm2. All obtained values of impact toughness are lower than values obtai-ned by the unaffected material, which is ρom=70 J/cm2.DETERMINATION OF MICROSTRUCTURAL ZONESThe process of metallographic, mechanical speci-mens preparation is done through a series of successive operations: cutting (sampling), roughing, mounting, grinding and polishing.Chemical analysis of specimens obtained from cuts of workpieces welded by FSW process, is carried out with the appropriate reagent. The reagent is prepared by measuring of 1,3 ml 40 % HF hydrochloric acid in the gauge and gently mixed with 200 ml of distilled water into a suitable glass container. In this way, the reagent is ready for use.Figure 8 shows the specimen for 28th point of the experimental plan, which is chemically treated and ready for testing.Examination of macrostructure and microstructure is carried out by methods of light microscopy. Micro-structural images of specimens are recorded on the met-allographic microscope by inverse-type brands “META-VAL” optics “CARL ZEISS JENA” featured with “SAM-SUNG” camera, model BW-4302. Figure 9 shows the metallographic microscope with camera, and recording patterns of the specimen. Metallographic microscope has the ability to zoom up to 500 times, and the camera has the ability to record up to 750 times of magniﬁ  ca-tion.Method of light microscopy was used for providing recordings for all specimens, for all points of the ex-perimental plan in elected positions. The aim of metal-lographic research is to identify the various defects that occur during the FSW, as well as the identiﬁ  cation of microstructural changes.Figure 10 shows the pattern of microstructural analy-sis harmonized with international researchings of FSW processes, and Figures 11, 12 and 13 shows the macro-structure of the specimen with the positions of micro-structural zones.Characteristic structural zones of FSW can be clearly indentiﬁ   ed from recorded images of macrostructure, those zones are: unaffected material or parent metal, heat affected zone - HAZ, thermo-mechanically affected zone - TMAZ, and so called “weld nugget” zone - NZ. The images clearly show defects that are present for certain points of the experimental plan. That is so called “tunnel” effect on the advancing side, as a result of insufﬁ  cient transport material around the pin. This defect is one of the density errors, which can be detected by radiographic im-ages, which are mostly incessant. This error can be avoided by proper choice of geometrical parameters of tools and kinematic parameters of the process.Based on the macrostructure images for certain spe-ciﬁ  c positions, images of microstructure are recorded, which provide a clearer view of the observed structure of welded joints, as well as the grain size.Figure 14 shows the microstructure of the unaffected material.Figure 15 shows the microstructure of heat affected zone - HAZ, while ﬁ  gure 16 shows the microstructure of the transition between the heat affected zone and the Figure 8   Display of the treated specimen for point of the experimental plan No. 28Figure 9   Metallographic microscope used to capture the structure of welded jointsFigure 10 Scheme of microstructural analysisFigure 11 Macrostructure of the specimen No. 1Figure 12 Macrostructure of the specimen No. 30Figure 13 Macrostructure of the specimen No. 3632   METALURGIJA 51 (2012) 1, 29-33M. JANJIĆ et al.:  MICROSTRUCTURAL EVOLUTION DURING FRICTION STIR WELDING OF AlSi1MgMn ALLOYzone of thermo-mechanical effects for the center point of the experimental plan No 35.When examining the microstructure, one can clearly identify the transition between the zone that was affect-ed with deformation from tools and the unaffected ma-terial zone. The material which was deformed by FSW process, shows a well-turbulent grain structure, as well as materials in the vicinity of the heat affected zone - HAZ, which gives the proper arrangement of grains. In the mixing zone, a very ﬁ  ne recrystallized grains are present, due to the large deformation of material and high temperature during the FSW process. Microstruc-tures within the “weld nugget” zone indicates dynamic recrystallized grains, which are much smaller than with the unaffected material, where the larger size grains are present. Dynamic recrystallized “weld nugget” zone is shown in Figure 17, for the center point of the experi-mental plan No 35.In the heat affected zone - HAZ oriented grains are also present, which are assumed to be caused by the in-ﬂ  uence of residual stress from the process of FSW.Thermo-mechanical affected zone - TMAZ depends on the size of the shoulder diameter and it is delineated with a “dot - dash” line in Figures 11, 12 and 13.In the center of the stirred zone the “weld nugget” zone occurs, or a mixture of materials from two work-pieces of aluminum alloy 6082-T6. This zone is easily identiﬁ  ed. It is delineated with the “dotted” line in Fig-ures 11, 12 and 13. This is the zone which suffered the greatest deformations and it is distinguished by ﬁ  ne-grain recrystallized materials from other zones, namely that is the zone where the pin has passed.Details of the structure caused by the pin rotation should be observed in all specimens. The details are presented in Figure 18.CONCLUSIONSFSW welding process is relatively new and current technology. It is characterized by complex processes that take place in the welding zone, conditioned with the effects of many inﬂ  uential and interrelated factors, such as tool geometry, welding regime factors (kine-matic factors), the properties of materials to be welded etc.The experimental researches carried out successful welding of aluminum alloys AlSi1MgMn with the thick-ness of 7,8 mm, using the FSW process and it was de-Figure 14 Microstructure of the unaff  ected metalx 300Figure 15 HAZ microstructurex 200Figure 16 The microstructure of HAZ – TMAZ x 100Figure 17 Microstructure of the “weld nugget” zonex 10033 METALURGIJA 51 (2012) 1, 29-33M. JANJIĆ et al.:  MICROSTRUCTURAL EVOLUTION DURING FRICTION STIR WELDING OF AlSi1MgMn ALLOYterminated that the quality of weld depends greatly upon tool dimensions (shoulder diameter, pin diameter, and the angle of the pin slope) as well as regimes of welding (welding speed and rotation speed).When testing the impact toughness, the smallest val-ues are obtained with specimens where the “tunnel” ef-fect is present, and all obtained values of impact tough-ness are smaller than the values obtained by testing the unaffected material. Based on performed analysis, we may conclude that the best impact toughness values are obtained in the central points of the experimental plan.Testing the macrostructure of FSW specimens, the existance of several microstructural zones was deter-mined: Unaffected Material Zone, Heat Affected Zone - HAZ, Thermo-Mechanically Affected Zone - TMAZ and “Nugget” Zone - NZ. Microstructure obtained on advancing and retreating side are quite different: on the advancing side, the sharp border between the thermo-mechanically affected zone and stirred zone has appea-red, and on the retreating side there is a continuous change between the two regions.REFERENCES[1]  R. Mishra, M. Mahoney: Introduction, Friction Stir Wel-ding and Processing, R. Mishra, M. Mahoney (ured.), sv. 1, ASM International, 2007. str. 1-5.[2]  T. Watanabe, H. Takayama, A. Yanagisawa: Journal of Ma-terials Processing Technology, 178(2006), 342-349.[3]  M. Vukcevic, M. Janjic, M. Plancak, N. Sibalic: Steel Re-search International, 81(2010)9, 1080-1083.[4]  I. Kalemba, M. Kopyscianski, S. Dymek: Steel Research International, 81(2010)9, 1088-1091.[5]  MEST EN 10045-1:1993. Mehanička ispitivanja metala - ispitivanje udarom po Šarpiju - Dio 1: Metoda ispitivanja.Note:   The responsible translator for English language is Aleksandra Mišurović, Podgorica, MontenegroFigure 18   Microstructure of specimen No. 9, resulted after the infl  uence of the pin

Microstructural evolution during friction stir welding of AlSi1MgMn alloy

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