Svetsaren nr 2. 2000 - Esab

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Figure 4 (left) Horizontal hardness profile across friction stir weld in AA 6082.Figure 5 (right) Horizontal hardness profile across friction stir weld in AA 5083 measured 1.7 mm from the root face.The hardness profile was measured 2.5 mm from the root face and shows hardness minima in the thermomechanicallyaffectedzone.Alloy AA 5083 AA 6082Plate thickness (mm) 15 10 10 6 6 10 10 5 5Welding speed (cm/min) 4.6 6.6 9.2 9.2 13.2 26.4 37.4 53 75Tensile strength (MPa) 318 344 331 312 303 226 236 254 254Table 2. Transverse tensile properties of friction stir welds.tion. Typical grain sizes were 30 m along the rolling directionand 20 m across. In AA 6082, the grain size wasnon-homogeneous (Fig. 3a), due to partial recrystallization.Recrystallized grains were about 10 m in size,whereas the size of non-recrystallized grains typicallyvaried between 50 m and 150 m.The recrystallization of the nugget zone during frictionstir welding effectively wiped out any trace of theprevious grain structure. The nugget zone in both materialsconsisted of fine, equiaxed grains with a grain sizeof about 10 m (Fig. 3b).The transition between the nuggetand the thermomechanically-affected zone wasclearly visible in the AA 6082 alloy, as shown in Fig. 3c.This figure also illustrates that the “ring contrast“ is notdue to grain size differences. The contrast instead appearsto be related to variations in grain orientationand possibly to the degree of relative disorientationbetween adjacent grains.3.3 HardnessThe hardness of unaffected base material was approximately75 HV1 and was practically constant across theweld, both horizontally and vertically, in AA 5083(Fig. 4).The horizontal hardness profile across the weld in AA6082 (Fig. 5) had a significantly different appearance. Unaffectedbase material is harder (about 110 HV1) andthere is a decrease in hardness towards the weld, with aminimum of about 60-65 HV1 in the thermomechanicallyaffectedzone.The hardness of the nugget zone itself is typically70-75 HV1. A slight tendency towards decreasinghardness towards the top surface of the weld was noted atthe centre of the weld. The location of isohardness curves,corresponding to 85 HV1, approximately corresponds tothe width of the tool shoulder at the top side and becomesnarrower towards the root side.Microhardness measurements (HV25g) across thenugget zone in AA 6082 did not show any systematicvariations that could be correlated to the ring pattern(see also [5]). Nor did measurements in rings andbetween rings reveal any differences in hardness.3.4 Tensile propertiesThere is a considerable difference in the transversestrength of friction stir welds in the two alloys (Table 2).The transverse strength was between 303 and 344 MPafor AA 5083, while AA 6082 had a transverse strengthin the range of 226 to 254 MPa.An interesting pattern was found when examiningthe location of the fracture. For welds in AA 5083, thefracture in most cases was close to the centre of theweld and the fracture surface was generally inclined8 • Svetsaren nr 2 • 2000
Figure 6 (left). Results of fatigue testing of FSW welds in base material AA 5083. All values are well above the design curve.Figure 7 (right). Fatigue properties of FSW weldments in AA 6082 base material. All experimental values are above thedesign curve.about 45 degrees. The fracture surface was close to thecentre of the weld on the root side, but the original jointline never appeared to be the initiation point. In AA6082, the fracture was with few exceptions close towhere the outer edge of the tool shoulder had touchedthe top side. The fracture surface was inclined, with thefracture surface closer to the weld centre at the rootside but still displaced a few mm.3.5 Fatigue propertiesIn Figures 6 and 7, the fatigue properties of FSW weldmentsare presented and compared with design curves[8]. In most samples, the fracture was initiated in thebase material, or in the centre of the weld. In only a fewsamples, the fractures started in the weld metal/basematerial transition region. All the tested samplesshowed very good fatigue behaviour. There are someindications that a lower welding speed results in higherresistance in the weld, although this has to be confirmedby further testing. Welds in base material AA 5083showed better fatigue properties, compared with weldsin AA 6082. The scatter was also larger for welds in AA6082 than for welds in AA 5083.4.Discussion4.1 MicrostructureSpecial attention was paid to the annual rings seen in thenugget zone. A similar pattern has previously been observedin aluminium welded using pulsed TIG. This ringpattern is due to varying grain size caused by a periodicchange in cooling rate. However, no difference in grainsize was noted between rings and areas between rings infriction stir welds (Fig.3c). Nor was any difference in particledistribution detected [6]. The absence of hardnessdifferences between rings and areas between rings alsosupports the assumption that the ring structure is not associatedwith precipitation.A likely explanation is thereforethat the movement of the rotating, profiled pin-toolthrough the material results in periodic variations instrain. This produce variations in grain orientation, or inthe relative orientation of adjacent grains, resulting indifferences in etching response. However, further investigationsare needed to verify this hypothesis.4.3 Mechanical propertiesWelded AA 5083 had a tensile strength close to that ofmaterial in annealed condition, whereas the weldstrength of AA 6082 was between that typical of coldagedand heat-treated material. From measuredstrength levels it would therefore be expected that fracturewould occur in the “most annealed region“ in AA5083 welds. This is in good agreement with the fractureusually taking place in the fully-annealed, equiaxed microstructurein the centre of the nugget zone. It was difficultto predict where to expect fracture in AA 6082from strength comparisons. However, there was a clearcorrelation between fracture path and the measuredline of lowest hardness.Svetsaren nr 2 • 2000 • 9
Page 1 and 2: A WELDING REVIEW PUBLISHED BY ESAB
Page 3 and 4: The advancement of aluminiumwithin
Page 5 and 6: Transportation and containersFor re
Page 7: atio R (=min stress/max stress) was
Page 11 and 12: Equipment for aluminium weldingBy K
Page 13 and 14: Figure 5. The principleof friction
Page 15 and 16: Figure 2a: Shielding gas: argon + 3
Page 17 and 18: High quality aluminium welding - a
Page 19 and 20: is widely used within the automotiv
Page 21 and 22: Simple bending tests on 6000-alloys
Page 23 and 24: nets Using a hammer, chisel and ton
Page 25 and 26: Because of their hygroscopic tenden
Page 27 and 28: Reduce set-up time, eliminate guess
Page 29 and 30: This project was conceived by the P
Page 31 and 32: A century of aluminium—a product
Page 33 and 34: Figure 6. Equipment for FSW of prof
Page 35 and 36: Figure 4- are prototyping a number
Page 37 and 38: Porosity is a result of hydrogen ga
Page 39 and 40: propagation sensitivity. Ductility

Svetsaren nr 2. 2000 - Esab

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