Milenko Perovic 1 , Sebastian Balos 2 , Drazan Kozak 3 , Darko Bajic 4 , Tomaz Vuherer 5 1 Chamber of Economy of Montenegro, Podgorica, Montenegro; 2 University of Novi Sad, Faculty of Technical Sciences, Novi Sad, Serbia 3 Josip Juraj Strossmayer University of Osijek, Mechanical Engineering Faculty in Slavonski Brod, Slavonski Brod, Republic of Croatia 4 University of Montenegro, Faculty of Mechanical Engineering, Podgorica, Montenegro ; 5 University of Maribor, Faculty of Mechanical Engineering, Maribor, Slovenia Until the discovery of friction stir welding two and a half decades ago, weldability of thermally strengthened (tempered) aluminium alloys was a serious technological problem. Among the alloys that were not weldable were the alloys of the type 2xxx, and especially type 7xxx. Their high mechanical properties are achieved by precipitation hardening obtained by heat treatment. However, that also means arc welding is difficult, since the precipitates completely disintegrate at temperatures of approximately 4000 o C and are being replaced by a cast microstructure of a much lower strength. When the welding is necessary (usually gas tungsten arc welding ‐ GTAW), consumables of AA5356 type are used containing 4.5‐5.5 % Mg. In this paper, friction stir welding (FSW) was applied on AA7075 alloy, without consumable material, making savings in critical raw materials (CRMs) Alloy type / batch number Content of elements, mas % Zn Mg Cu Mn Cr Zr Tr V B Fe Si EN AW 7075 5.28 2.25 1.58 0.29 0.18 0.14 0.15 0.007 0.003 0.16 0.09 Alloy type / batch number Tensile strenght Rm [MPa] Yield strength Rp 0 , 2 [MPa] Elongation, [%] Hardness HB EN AW 7075 588 506 8 160 Experimental point Tool rotation speed n [min ‐1 ] Welding speed v [mm/min] ET 05 850 80 ET 06 750 60 ET 07 750 80 ET 08 850 60 Friction stir welding (FSW) was applied on AA7075 alloy, without consumable material, making savings in critical raw materials (CRMs). Two rotational speeds were used, 750 and 850 min‐1, and two welding speeds 60 and 80 mm/min. The tool has a reservoir and a whorl pin. Weld instrumented Charpy impact strengths and microstructures were determined. • Intermetallic phases are moved away of the welded joint, • Relation between the number of revolutions of tools and velocity of welding directly influences the value of impact energy which is required for initiation and propagation of the crack, • The asymmetry of the welded joint and changes in metallurgical transformations occurring around the pin and under the shoulder of the tool during its combined moving, influence the value of impact strength in various areas of the welded joint, • The crucial parameter is the rotation speed, which when lower, results in a more convenient microstructure strengthened by the grain boundary mechanism. It was found that the most uniform impact energy values were determined in the specimen welded with the tool operating at 750 min ‐1 and 80 mm/min. In this specimen, 5.53; 5.24 and 5.38 J/cm 2 were obtained, taken from the nugget, retreating and advancing side, respectively. On the other hand, in the specimen welded with 850 min ‐1 and 80 mm/min, 4.06; 4.79 and 4.06 J/cm 2 were obtained. Macro structure of welded joint in point 5 of the experiment with the microstructure of a) nugget zone, b) thermomechanical zone, c) heat affected zone, d) base material Impact strength (energy, crack initiation and crack propagation energy) F‐t and E‐t diagrams taken from the nugget, retreating and advancing side experimental point 05 experimental point 06 experimental point 07 experimental point 08

2 Novi Sad, Faculty of Technical Sciences, Novi Sad, Serbia · Milenko Perovic1, Sebastian Balos2, Drazan Kozak3, Darko Bajic4, TomazVuherer5 1 Chamber of Economy of Montenegro, Podgorica,

Download PDF Report

Upload
others
View
6
Download
0

Embed Size (px)

Citation preview

Milenko Perovic1, Sebastian Balos2, Drazan Kozak3, Darko Bajic4, Tomaz Vuherer5

1Chamber of Economy of Montenegro, Podgorica, Montenegro;2University of Novi Sad, Faculty of Technical Sciences, Novi Sad, Serbia

3Josip Juraj Strossmayer University of Osijek, Mechanical Engineering Faculty in Slavonski Brod, Slavonski Brod, Republic of Croatia

4University of Montenegro, Faculty of Mechanical Engineering, Podgorica, Montenegro ;5University of Maribor, Faculty of Mechanical Engineering, Maribor, Slovenia

Until the discovery of friction stir welding two and ahalf decades ago, weldability of thermally strengthened(tempered) aluminium alloys was a serious technologicalproblem. Among the alloys that were not weldable werethe alloys of the type 2xxx, and especially type 7xxx. Theirhigh mechanical properties are achieved by precipitationhardening obtained by heat treatment. However, that alsomeans arc welding is difficult, since the precipitatescompletely disintegrate at temperatures of approximately4000oC and are being replaced by a cast microstructure ofa much lower strength.When the welding is necessary (usually gas tungsten arcwelding ‐ GTAW), consumables of AA5356 type are usedcontaining 4.5‐5.5 % Mg. In this paper, friction stir welding(FSW) was applied on AA7075 alloy, without consumablematerial, making savings in critical raw materials (CRMs)

Alloy type / batch number

Content of elements, mas %

Zn Mg Cu Mn Cr Zr Tr V B Fe Si

EN AW 70755.28 2.25 1.58 0.29 0.18 0.14 0.15 0.007 0.003 0.16 0.09

Alloy type / batch number

Tensile strenghtRm [MPa]

Yield strength Rp0,2 [MPa]

Elongation,[%]

HardnessHB

EN AW 7075 588 506 8 160

Experimental point

Tool rotation speedn [min‐1]

Welding speedv [mm/min]

ET 05 850 80

ET 06 750 60

ET 07 750 80

ET 08 850 60

Friction stir welding (FSW) was applied onAA7075 alloy, without consumable material,making savings in critical raw materials (CRMs).Two rotational speeds were used, 750 and850 min‐1, and two welding speeds 60 and80 mm/min. The tool has a reservoir and a whorlpin. Weld instrumented Charpy impact strengthsand microstructures were determined.

• Intermetallic phases are moved away of the weldedjoint,

• Relation between the number of revolutions of toolsand velocity of welding directly influences the value ofimpact energy which is required for initiation andpropagation of the crack,

• The asymmetry of the welded joint and changes inmetallurgical transformations occurring around the pinand under the shoulder of the tool during its combinedmoving, influence the value of impact strength invarious areas of the welded joint,

• The crucial parameter is the rotation speed, which whenlower, results in a more convenient microstructurestrengthened by the grain boundary mechanism.

It was found that the most uniform impact energy values were determinedin the specimen welded with the tool operating at 750 min‐1 and 80mm/min. In this specimen, 5.53; 5.24 and 5.38 J/cm2were obtained, takenfrom the nugget, retreating and advancing side, respectively. On the otherhand, in the specimen welded with 850 min‐1 and 80 mm/min, 4.06; 4.79and 4.06 J/cm2 were obtained.

Macro structure of welded joint in point 5 of the experiment with the microstructure of a) nugget zone, b) thermomechanical zone, c) heat affected zone, d) base material

Impact strength (energy, crack initiation and crack propagation energy) F‐t and E‐t diagrams taken from the nugget, retreating and advancing side

experimental point 05 experimental point 06

experimental point 07 experimental point 08