MELTED FILM FORMATION AND CRACKING IN Al 2024 FRICTION STIR SPOT WELDS *A.P. Gerlich1, D. Lim1, T. Shibayanagi2
1
University of Alberta
Dept. of Chemical and Materials Engineering 536 CME Building
Edmonton, CANADA T6G 2G6 (*Corresponding author: [email protected])
2
Osaka University
Joining and Welding Research Institute, 11–1 Mihogaoka, Ibaraki-shi,
Osaka, JAPAN 567–0047
ABSTRACT
Friction stir spot welding of Al 2024-T3 alloy is examined in a weld on plate configuration. The temperature profiles at the tool pin and shoulder suggest that local melting of Al2CuMg phases contained in the base material may occur. The stir zone microstructures indicate that Cu-rich films are formed in the stir zone when tool rotation speeds >1500 RPM are used, and that cracking occurs along these films. It is suggested that the formation of melted films may account for surface defects produced when high tool rotation speeds are used during Al 2024 friction stir seam welding.
INTRODUCTION
The friction stir welding process was developed by TWI, Abington, UK in 1991 for joining aluminum alloys [1]. The two overlapping sheets are joined at a single location during friction stir spot welding and the rotating tool is withdrawn leaving a keyhole depression. A stir zone comprising dynamically recrystallized material forms adjacent to the periphery of the rotating tool in a cycle time typically ranging from 1 to 5 seconds during spot welding [2]. Various aspects of the process have already been examined, including the tool penetration process [3], factors determining energy utilization [4,5], stir zone formation and joint mechanical properties during friction stir spot welding of a range of Al-alloy and Mg-alloy base materials [6,7].
It previous work, Gerlich et al. [2,8] showed that the strain rate at the contact interface during friction stir spot welding of Al 7075-T6 and Al 2024-T3 drastically decreased when tool rotation speeds increased from 1500 to 3000 RPM. It was suggested that the reduction in strain rate is associated with tool slippage caused by spontaneous melting of second-phase particles (η, S and T phases) in the base materials [2,8], however supporting microstructural evidence was not presented. There has also been evidence that suggests local melting may occur during friction stir spot welding of Al 6061-T6 Al2O3 composite material [9], AZ31 magnesium alloy [10], and in dissimilar Al 6111/AZ91 spot welds [11]. The proposition that local melting occurs during friction stir seam welding is a particularly controversial issue, since the tool rotation speeds are much lower in comparison to spot welding operations. In spite of this, evidence of local melting has been found in Al 2024-T3 friction stir seam welds [12], and in dissimilar Al 1050/AZ31 friction stir welds [13].
Since the stir zone temperatures produced during friction stir spot welding increase with tool rotation speed, the present paper reports the testing output during Al 2024 friction stir spot welding using tool rotational speed settings from 750 to 3000 RPM. This range corresponds with the tool rotation speed settings that promoted tool slippage during friction stir spot welding of Al 2024 alloy, and SEM microscopy is used for the detailed examination of stir zone microstructure. In order to determine if the melting temperature of second phase particles is exceeded during welding, the stir zone temperature is measured by locating thermocouples within the welding tool itself at the locations 200 μm from the tip of the rotating pin and midway between the pin and tool shoulder peripheries.
EXPERIMENTAL
Table 1 shows the chemical compositions of the Al 2024 base material employed during all tests, and welds were produced in 6.3 mm thick monolithic plate material.
Table 1 – Base material compositions, in wt%.
Alloy Al Cr Cu Fe Mg Mn Si Ti Zn Al 2024-T3 bal. <0.01 4.30 0.15 1.42 0.62 0.07 0.03 0.07
The tool geometry comprised a 10 mm diameter shoulder, a pin diameter of 4 mm and a pin length of 2.2 mm. The rotating pin had a simple threaded profile. The tool comprised H13 steel (0.35 wt.% C, 5 wt.% Cr, 1.5 wt.% Mo, 1wt.% V, 0.4% Mn), heat-treated and tempered to a final hardness of 46-48 HRC. A plunge rate of 2.5 mm/s was used during all temperature measurements and the specified pin penetration depth during all spot welding trials was 2.2 mm which corresponds with the point when the tool shoulder contacts the upper surface of the sheet being spot welded. When the rotating pin was fully-penetrated the spot welding cycle was extended by incorporating a dwell time of four seconds. The tool rotational speed remained constant while axial force and torque decreased during the four second long dwell period and due to machine compliance, the rotating tool penetrated up to 0.2 mm into the upper surface of the Al-alloy section.
The influence of tool rotational speed variations from 750 to 3000 RPM on the peak temperature in the stir zone was investigated. The thermal cycle during friction stir spot welding was measured by embedding 0.25 mm diameter K-type thermocouples at the location 0.2 mm from the tip of the rotating pin and 1.3 mm from the outer periphery of the tool shoulder. During temperature measurements the thermocouple junction was always in direct contact with dynamically recrystallized material formed
during the spot welding operation. A detailed description of the temperature measurement set-up has been provided elsewhere [2]. All temperature measurements were repeated at least three times at each welding parameter setting. The variation in temperature during the 4 second dwell period was < 7.4ºC during repeat testing. The error ranges in the peak temperature represent one standard deviation above and below the average.
During metallographic examination using SEM the test samples were polished using 1 µm diamond compound, without etching to avoid removal of second-phase particles. Microstructures were examined using a Hitachi S-4500 field emission scanning electron microscope, and EDX analysis was performed on select areas. Samples examined using polarized light microscopy were electrolytically-etched at 30V with 3% HBF4 for 2 minutes.
RESULTS Temperature Output
The tool torque and temperature values during friction stir spot welding of Al 2024-T3 alloy using a tool rotational speed of 2250 RPM are shown in Figure 1. Similar output was found during friction stir spot welding using tool rotational speeds from 750 to 3000 RPM. In Figure 1 the dwell period initiates 1.25 s after process initiation and coincides with the point when the rotating pin is fully-penetrated into the test sample and the peak torque (or power input) reaches its highest value. When the tool rotational speed increases from 750 to 3000 RPM the heating rate during the tool penetration stage in spot welding (from 0 to 1.25 seconds in Figure 1) increases from 254 to 448 ºC/s at the tip of the rotating pin, and from 491 to 653 ºC/s midway across the tool shoulder.
Figure 1 – Tool torque and temperature output during friction stir spot welding of Al 2024-T3 material using a rotational speed of 2250 RPM.
The average peak temperatures measured at the tip of the rotating pin increased from 420 to 497ºC when the tool rotational speed increased from 750 to 3000 RPM, see Table 2. The peak shoulder temperatures measured at the tool shoulder are consistently lower than those measured close to the tip of the rotating pin, and vary from 406 to 449 ºC when the tool rotational speed increases from 750 to 3000 RPM, see Table 2. It is also readily apparent that the amount of scatter in temperature measurements made close to the tip of the rotating pin and at the tool shoulder greatly increases when tool rotational speeds of <1500 RPM are used.
Table 2 – Peak tool temperatures during friction stir spot welding of Al 2024-T3 alloy RPM Pin temperature, ºC Shoulder temperature, ºC
750 420 ± 27.2 425 ± 31.9 950 434 ± 16.6 406 ± 19.1 1125 439 ± 9.0 433 ± 9.2 1500 477 ± 2.4 423 ± 3.4 2250 493 ± 1.0 444 ± 2.2 3000 497 ± 7.2 449 ± 8.8
Stir Zone Microscopy
The microstructure of as-received Al 2024 base material consisted of pancake-shaped grains aligned in the rolling direction with second-phase particles at the grain boundary regions. The secondary phases primarily comprised Al2Cu, as well as Al2CuMg and particles containing Cu, Fe, and Mn (found using EDX analysis). The presence of Al2CuMg particles was confirmed using EDX, and these are well known to be present in Al 2024-T3 base material [8].
Figure 2(a) shows a transverse section through a spot weld produced using a tool rotational speed of 1500 RPM. The size and number of second phase particles was depleted within the stir zone. The details of the boundary between the stir zone and thermo-mechanically affected zone (TMAZ) in Region A is shown using polarized light in Figure 2(b). The stir zone consisted of dynamically recrystallized fine grains which could not be resolved using optical microscopy. The pancake shaped grains in the base material have been heavily deformed in the TMAZ zone and micro-bands have formed in this region adjacent to the stir zone. These microstructural features were observed in all spot welded joints.
(a)
(b)
Figure 2 – (a) Optical micrograph of friction stir spot welded Al 2024-T3 using 1500 RPM. (b) Polarized light micrograph showing detail of Region A. The boundary between the stir zone and
thermo-mechanically affected zone is indicated by a dashed line. Region A
Figure 3 shows a transverse section through a spot weld produced using a tool rotational speed of 2250 RPM. The stir zone dimensions slightly increased compared to friction stir spot weld produced using 1500 (see Figure 2). Cracking was observed in the stir zone in the location beneath the tool shoulder in spot welds made using a tool rotational speed of 2250 RPM; see Figures 3 and 4. The interior of the cracks had a smooth, rippled morphology typically observed when liquation cracking occurs in welded joints, see Figure 4. The cracked regions were associated with elongated films rich in Cu and were decorated with voids, see Figure 5. Numerous thin films having high Cu contents were also observed outlining grain boundary regions.
Figure 3 – Optical micrograph of friction stir spot welded Al 2024-T3 using 2250 RPM which contained cracks in the stir zone. The boundary between the stir zone and thermo-mechanically affected zone is
indicated by a dashed line.
Figure 4 – SEM micrograph of cracked region and interior of crack surface in friction stir spot welded Al 2024-T3 using 3000 RPM.
Figure 5 – SEM micrograph near cracked region with melted films in friction stir spot welded Al 2024-T3 using 3000 RPM. EDX analysis of Location B was: 69.0% Al, 29.6% Cu, 1.4% Mg, in wt%.
Cracks
Evidence of melted films and cracking was only found in spot welds produced using tool rotational speeds of 2250 and 3000 RPM. There was no evidence of cracking or the formation of thin films rich in Cu at grain boundary regions in spot welds made using tool rotational speeds of ≤1500 RPM. Elliptical-shaped second-phase particles observed in the stir zones of spot welds made using lower rotational speed settings were elongated and aligned in a direction parallel to the stir zone extremity, see Figure 6.
Figure 6 – SEM micrograph of second phase particles aligned in the stir zone of friction stir spot welded Al 2024-T3 using 1500 RPM.
DISCUSSION Stir Zone Temperature and Local Melting
The peak temperature in the stir zone during friction stir spot welding exceeded 493ºC at the location close to the tip of the rotating pin when using tool rotational speeds ≥2250 RPM, see Table 2. This temperature is very close to the solidus temperature of Al 2024 base material (502ºC [14]). The average peak temperatures measured by the thermocouple located close to the tip of the rotating pin are from 5 to 53ºC higher than those measured at the tool shoulder for the range of tool rotational speed settings investigated in the present study. Lower temperatures are measured at the tool shoulder since contact with the upper surface of the Al 2024 sheet only occurs late in the friction stir spot welding cycle [3]. This accounts for the lag in measured temperature output from the tool shoulder region and the observation that the highest heating rate (653ºC/s) is recorded at the tool shoulder following initiation of the dwell period in friction stir spot welding, see Figure 1.
The scatter in measured temperature values increases in friction stir spot welds made using tool rotational speeds <1500 RPM, see Table 2. Since the axial force and torque determine the energy input during friction stir spot welding [3,4] test-to-test scatter results from small changes in the amount of shoulder penetration into the surface of the upper sheet and the amount of material entrapped beneath the tool shoulder during tool penetration [15]. Small changes in axial force may influence the stir temperature values found during Al 2024 friction stir spot welding.
Huang and Kou [16] found evidence of liquid film formation in Al 2024 fusion welded joints when the temperature exceeded 548ºC and a eutectic comprising Al and Al2Cu formed. However, there was no evidence of cracking associated with melting of Fe and Mn-bearing particles such as Al12Si(Fe,Cu,Mn)3, since these particles have melting points well above the solidus temperature of Al 2024 alloy [16,17]. Table 2 shows that the stir zone temperatures measured in the stir zone during Al 2024 friction stir spot welding are much less than those required for the formation of an Al + Al2Cu eutectic.
alignment direction Weld surface
Formation of Melted Films and Cracking
Although Yan et al. [18] suggested that melted eutectic films formed in Al 2524 friction stir seam welds the stir zone temperature was not measured and no information concerning the chemical composition of the suggested melted phases was presented. Also, Yamamoto et al. [19] proposed that cracking observed in the stir zone close to its extremity in AZ91 friction stir spot welds emanated from melted eutectic films, which formed in the TMAZ region. When the stir zone increased in width during the dwell period in spot welding grain boundary regions were penetrated by melted eutectic material and liquid penetration induced cracking (LPI) resulted. However, there is no evidence of melted film formation in the TMAZ region immediately adjacent to the stir zone extremity and therefore, LPI cracking cannot explain cracking in the stir zone during Al 2024 friction stir spot welding. However, the melting point of the Al2CuMg phase (490ºC [20]) is exceeded in the stir zones of Al 2024 friction stir spot welds made using tool rotational speeds of 2250 and 3000 RPM. In addition, there was no evidence of cracking or elongated films at grain boundary regions in friction stir spot welds made using tool rotational speeds ≤1500 RPM and in each case, the stir zone temperature is less than 490ºC. Since the melted films rich found in Al 2024 friction stir spot welds are rich in Cu and Mg (see Figures 5), it is proposed that cracking during spot welding is caused by spontaneous melting of Al2CuMg particles when the stir zone temperature reaches 490ºC [20].
Since the heating rate during friction stir spot welding of Al 2024 is extremely rapid second-phase particles from the base material have insufficient time to dissolve before their melting temperatures are reached. It has been shown that during the Al 2024 spot welding only Al2CuMg particles which are <200 nm can be completely dissolved during the dwell period, and a similar situation occurs in Al 7075 and Mg AZ91 alloys [2,7,8,19]. Su et al. [21] have confirmed that material from the locations beneath the tool shoulder and the bottom of the rotating pin is incorporated at the top of the pin thread and is moved downwards via the thread on the rotating pin and into the growing stir zone during the dwell period in friction stir spot welding [21,22]. It is suggested that undissolved Al2CuMg particles are aligned during material flow (see Figure 6) and melt spontaneously when the temperature reaches 490ºC in the stir zone during Al 2024 friction stir spot welding. Following melting, liquid is squeezed out from compressively-loaded regions between grains so that melted films are formed [23]. Preferential formation of melted Al2CuMg films at grain boundary regions and triple junctions will also be driven by the reduction of surface energy at these locations [24].
It has been suggested that a defect known as surface galling during friction stir seam welding will limit the welding speed during the process [25]. It is proposed that melting of Al2CuMg particles during friction stir seam welding of Al 2024 alloy will lead to the cracking of material under the tool shoulder and produce galling on the weld surface. Once cracking under the tool shoulder occurs during seam welding, the material may be ejected away as the tool is traversed across the surface. This may lead to the formation of galling defects which have been typically reported on the surface of seam welds when high tool rotation speeds are used in conjunction with low travel speeds [25,26]. Surface galling defects are commonly observed in friction stir seam welded Mg alloys [27], and it has also been shown that there is a much greater tendency for local melting of eutectics and cracking in these materials [28].
CONCLUSIONS
The temperature and microstructures produced during friction stir spot welding of Al 2024-T3 were examined using tool rotational speed settings from 750 to 3000 RPM. Rapid heating limits dissolution of Al2CuMg particles contained in the as-received Al 2024-T3 base material so that spontaneous melting occurs when the stir zone temperature during friction stir spot welding exceeds 490ºC. SEM examination revealed that the stir zone contained melted films of Al2CuMg eutectic material which facilitates cracking. Melted material is incorporated into the stir zone and is squeezed out from compressively-loaded regions between grains forming films, which crack when tensile strains are applied during tool retraction. It is suggested that the formation melted films during friction stir seam welding may lead to galling on the surface of the weld, which limits the tool rotation speed which may be applied during seam welding of Al 2024 alloy.
ACKNOWLEDGMENTS
The authors acknowledge the financial support from the Natural Sciences and Engineering Research Council of Canada during this project.
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