Microstructure and Mechanical Property of 5000 and Mechanical Property of 5000 Series Aluminum Stud
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Microstructure and Mechanical Property of 5000 Series Aluminum Stud Joint
with Zinc Insert Using Friction Welding*1
Taichi Nishida1;*2, Tomo Ogura1, Mitsuo Fujimoto2 and Akio Hirose1
1Division of Materials and Manufacturing Science, Osaka University, Suita 565-0871, Japan2Kawasaki Heavy Industries Ltd., Kobe 650-8670, Japan
Friction stud welding with zinc insert was applied to 5000 series aluminum alloys. A cone-shaped A5056 stud bolt was successfullyfriction-welded onto A5083 plate at low torque using zinc insert compared to that without zinc insert. This is considered to be contributed by aeutectic reaction between aluminum and zinc, which can effectively remove the oxide film and promote the joining. With bigger torque andlonger welding time, the amount of residual zinc was reduced and high strength was achieved. On the other hand, a twist break between thestirred zone and the stud bolt increased during friction and this causes the decrease of tensile strength of the joint after a certain amount of torque.The results of micro-tensile test showed that strength at the edge of joint interface was higher than that at the center area regardless of zinc insert.From the experimental results, the joining process of the present friction stud welding classified into five processes, i.e. wear of stud bolt, start ofjoining, increase of tensile strength, decrease of tensile strength, and fracture during friction. [doi:10.2320/matertrans.L-MZ201124]
(Received October 1, 2010; Accepted January 28, 2011; Published April 20, 2011)
Keywords: aluminum stud joint, friction welding, zinc insert, micro-tensile test
5000 series aluminum alloys are widely used in thestructures that require low-temperature properties andcorrosion resistance.1) The joining of the stud bolt in thesealloys is mainly performed by arc stud welding.2,3) However,quality of the joints depends on workers skill, and theremoval of the oxide film on the aluminum surface isnecessary before welding.4) Recently, friction stud weldinghas been applied to overcome these problems in arc studwelding.2,5) This method is a modified friction welding, andcan achieve steady quality of joints because welding onlyproceeds by prearranged parameters. Furthermore, the oxidefilm at the interface of the joint is efficiently removed duringfriction stage.68) To remove the oxide film easier, the studbolt has a corn shaped tip, and zinc insert is applied betweenthe stud bolt and the base material.2,912) Therefore, thejoining process of the friction stud joint is considered to bequite unique and distinctive compared with other conven-tional friction welding.1315)
To optimize the welding parameters in friction studwelding, it is necessary to precisely understand the joiningprocess and its mechanism. In the present study, micro-structures at the interface of stud joints were observed,and tensile test was conducted to evaluate the mechanicalproperties with varying friction torque. Moreover, micro-tensile test was also conducted to evaluate the mechanicalproperties at each region of the interface in micro-scalearea. To clarify the effect of zinc insert on the joiningprocess and the joint properties, the joints without zincinsert were also evaluated. From the obtained results,joining process of the present friction stud welding wasdiscussed.
2. Experimental Procedure
The corn shaped A5056 stud bolt with 8 mm in diameterwas friction welded onto A5083 plate (designated as basematerial) using a direct drive welding machine. The zincprimer with 100 mm thick was applied between the stud boltand the base material. During the welding, the rotation speedand the friction load were set to 6000 rpm and 1800 N,respectively. The friction torque was varied from 0.8 to4.0 Nm and 1.0 to 3.0 Nm in the joint with and without zincinsert, respectively. The friction torque was monitored usinga load cell through an A/D converter with a sampling timeof 0.01 s. The rotation of the stud bolt stops and goes ontoupset stage when the friction torque reaches at a setup value.Upset load and upset time were set to 2000 N and 3.2 s,respectively.
For the microstructural observation, the joint was cutvertically, mechanically polished, and etched with Kellersetchant or Tuckers etchant. Microstructural observation wasperformed using a stereomicroscope (SZX7, OLYMPUSInc.), optical microscope (OPTIPHOT-100, NIKON Inc.)and scanning electron microscope (SEM; S-3000H,HITACHI Inc.). Tensile test (5505, INSTRON Inc.) andmicro-tensile test were performed to evaluate the mechanicalproperties of the friction stud joint with a cross-head speedof 5:00 102 mm/s and 1.00 mm/s, respectively. Theshapes of the specimens are shown in Fig. 1.
3.1 Microstructures of friction stud jointsFigure 2 shows a typical image of the interface of the
friction stud joint without zinc insert (Torque: 2.0 Nm). Alens shaped stirred zone was observed between the stud boltand the base material (inside dotted lines). The stirred zonebecame larger with bigger friction torque. In this study, thejoint interface was designated as three areas, i.e. center area,middle area and edge area. The center area and the edge area
*1The Paper Contains Partial Overlap with the ICAA12 Proceedings by
USB under the Permission of the Editorial Committee.*2Graduate Student, Osaka University
Materials Transactions, Vol. 52, No. 5 (2011) pp. 960 to 966Special Issue on Aluminium Alloys 2010#2011 The Japan Institute of Light Metals
mean the center of the rotated stud bolt and the edge of thejoint interface, respectively. The middle area is located in themiddle of the center and the edge area.
Figure 3 shows a typical image of the interface of thefriction stud joint with zinc insert (Torque: 2.0 Nm). Residualzinc was clearly observed between the stirred zone and thebase material at the center and the middle area (Fig. 3(b)).Although the residual zinc decreased with increasing frictiontorque, it could not be completely exhausted from the joint
interface in the present welding conditions. A break betweenthe stud bolt and the stirred zone (Fig. 3(d), designated astwist break) was observed at the edge area. When a frictiontorque increased, this twist break increased along theboundary between the stud bolt and the stirred zone, wherethe microstructures of the stud bolt were much plasticallydeformed.
3.2 Tensile test and fracture surface observationFigure 4 shows tensile strength of each joint. In the joints
without zinc insert, strength was obtained with the torque of1.0 Nm and increased with friction torque to 3.0 Nm. In thejoint with zinc insert, joining could not be achieved with thetorque of 0.8 Nm. Joining was achieved from the torque of1.0 Nm and this strength was higher than that of the jointwithout zinc insert. Tensile strength increased with frictiontorque until 3.0 Nm, similar to the joint without zinc insert.After showing maximum strength at 3.0 Nm, strength of thejoint began to decrease gradually with larger friction torque,and joining could not achieved again when the torque was4.0 Nm.
SEM images of the fracture surface of the joints are shownin Fig. 5. Three types of fracture morphology, i.e. shearfracture (Fig. 5(a)), ductile fracture (Fig. 5(b)) and interfacialfracture (Fig. 5(c)) were observed. The shear fracture wasseen where twist break was located between the stud bolt andthe stirred zone as seen in Fig. 2. This break is considered to
t =0.4unit : mm
15 15 30
Fig. 1 Dimension of the specimens for (a) tensile test and (b) micro-tensile
Middle Center Stud bolt
Fig. 2 Optical microstructure of the cross section appearance of the friction stud joint without zinc insert. (Torque 2:0 Nm).
(b) (c) (d)
200m 200m 200m
Middle Center Edge
Twist breakResidual zinc
Fig. 3 Optical microstructures of the friction stud joint with zinc insert. (Torque 2:0 Nm) (a) Cross section appearance and (b)(d)magnified view of each area.
Microstructure and Mechanical Property of 5000 Series Aluminum Stud Joint with Zinc Insert Using Friction Welding 961
occur at the initiating during the friction stage because thefracture surface was flat and traces of rubbing were seen.Ductile fracture occurred at base material when the joiningwas achieved at the joint interface. Interfacial fracturebetween the stirred zone and the base material showedlaminar surface along the rotation direction.
The appearance of the fracture surface and its cross sectionof each joint was given in Figs. 68, respectively. In the jointwithout zinc insert, the fracture at the torque of 1.0 Nm wasmainly interfacial fracture (Figs. 6 and 7). At a torque of3.0 Nm, the joined area was larger than that of 1.0 Nmbecause the joining progressed with larger friction torque.The fracture mode was partly changed to ductile fracture.
In the joints with zinc insert, the surface of the joint withthe torque of 0.8 Nm was shiny as shown in Fig. 6. In thejoints with the torque of 1.0 and 3.0 Nm, as shown in Fig. 8,while the area of the interfacial fracture decreased, the ductilefractured region and twist break region increased as thejoining progressed. The twist break which generated from theedge of the joint interface increased into center, and finallythe joint fractured during the friction. This is the reason whythe joint strength decreases from the torque 3.0 to 4.0 Nm.
The obtained result indicates that zinc inse