Elsevier

Materials & Design (1980-2015)

Volume 62, October 2014, Pages 118-123
Materials & Design (1980-2015)

Technical Report
Ultrasonic evaluation of friction stud welded AA 6063/AISI 1030 steel joints

https://doi.org/10.1016/j.matdes.2014.05.017Get rights and content

Highlights

Abstract

Friction stud welding is a promising technique in many applications related to oil and gas industries. It is used to attach grating to offshore oil platforms in areas where arc welding is not permitted because of the risk of causing a fire or explosion. Attachment of anodes inside seawater discharge pipelines in a gas processing plant is performed by this process. This solid state joining process permits metal combinations such as welding of aluminum studs to steel which would be problematic with arc welding because of the formation of thick and brittle inter-metallic compounds. In the present work, AA 6063 is joined to AISI 1030 steel using friction stud welding machine. Properties that are of interest to manufacturing applications such as Young’s modulus, longitudinal velocity, bulk modulus and shear modulus are evaluated by means of an ultrasonic flaw detector. At the interface of the joint, there is an increase of 4.4%, 1.8%, 1.15% and 4.42% is observed for the properties Young’s modulus, longitudinal velocity, bulk modulus and shear modulus respectively. This is due to the formation of intermetallic compound and increase in hardness at the interfacial region. Energy Dispersive X-ray analysis confirms the presence of FeAl as the intermetallic compound. Scanning Electron Microscope evaluation shows the presence of an unbound zone at the center of the inner region which is due to the minimum rotational speed and low axial load experienced at that point. In the unbound zone, there is an incomplete bond between dissimilar metals and it is detrimental to joint strength. Optimum value of friction time and usage of pure aluminum interlayer during the friction stud welding process hinders the formation of unbound zone and enhances the quality of AA 6063/AISI 1030 joints. The tensile strength of AA 6063/AISI 1030 joint was found to be 36.64% and 78.15% lower than that of AA 6063 and AISI 1030 respectively. Reasons for lower joint strength are attributed to low frictional pressure, non-optimal rotational speed and friction time, improper mechanical mixing, presence of oxide layer and formation of brittle intermetallic compound at the joint interface.

Introduction

Friction stud welding is a solid state welding process that produces a weld between work piece and stud under compressive force and plastically displaces material from faying surfaces. The principle of operation of this process is transformation of mechanical energy into heat energy. In friction stud welded joint, the heat affected zone is small and there is no defect of slag inclusion, undercut, etc.

Friction stud welding has been used to attach grating to offshore oil platforms in areas where arc welding is not permitted because of the risk of causing a fire or explosion [1]. Specific recent applications of friction stud welding include:

  • Retrofitting anodes in floating, production, storage and offloading (FPSO) oil storage tank in a Zone 1 area.

  • Retrofitting equipment in Zone 1 areas on offshore platforms.

  • Attachment of anodes inside seawater discharge pipelines in a gas processing plant [2].

The welding of aluminum to low carbon steel is of particular interest, since the resulting products join the very different but favorable properties of each component, namely, the high thermal conductivity and low density of aluminum, and the low thermal conductivity and the high tensile strength of steels [3]. The demand for aluminum/steel joints has therefore increased in many areas including oil and gas processing plants, cryogenic applications, spacecraft, high vacuum chambers and cooking utensils owing to their superior properties. In these structures, aluminum has been partially replaced by stainless steel. In this case, it is necessary to join stainless steel to aluminum alloys. The earlier application of aluminum/low carbon steel friction welding which has resulted in considerable cost saving, in the production of down hanger assemblies. These consist of a mild steel billet joined to aluminum alloy bar, for use in aluminum smelters [4].

In aerospace engineering, the concept of damage-tolerance deals with the establishment of maximum permissible flaw sizes. In order to ensure that flaws of that critical size can be detected with adequate confidence, both during production and in the field, nondestructive evaluation is carried out [5]. Eliminating scrap and waste that result from destructive testing, NDT can generate worthwhile cost reductions. Case depth of hardened parts is measured using ultrasonics to avoid “saw cut” sectioning followed by manual measurement of case depth. Likewise, the thicknesses of the various layers in plastic fuel tanks are measured ultrasonically in place of sectioning, staining, and measuring under a microscope [6]. A major reality in automotive plants is the need to minimize costs, including the cost of inspections. In European plants, incorporating intelligence into instrumentation is illustrated by developments in nondestructive spot-weld inspection [7]. In friction stir welding of aluminum alloys non-destructive testing techniques are currently employed [8].

Ultrasonic testing is used for detecting flaws, characteristics and discontinuities in the internal structure of materials like steel, welds and alloys. During ultrasonic testing, no damage is caused to the material inspected, thus falls in the Nondestructive testing group. Short ultrasonic wave pulses are used in the process of ultrasonic testing. These wave pulses are in a frequency range of 0.1–15 MHz and can sometimes reach up to 50 MHz. The testing comprises of several ultrasonic inspection equipments or functional units like receiver, instrumentation transducers, calibration standards and display devices. Ultrasonic inspection finds its application in evaluating materials used in aerospace, automotive, transportation industries and oil and gas industries.

Nagy and Adler [9] evaluated solid state bond microstructure using ultrasonic principle. Using ultrasonic technique, Lienert et al. [10] conducted investigation on the microstructure of Aluminum based metal matrix composites. In the present work, dissimilar metals aluminum alloy AA 6063 and AISI 1030 steel are welded by friction stud welding process and the resulting properties were investigated. The purpose of choosing ultrasonic testing is to determine various mechanical properties like longitudinal modulus, bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio in dissimilar welded joints. From the obtained data, mechanical properties were found using standard formulae [11] and variations of the properties at different locations of the AA 6063/AISI 1030 joint could be determined.

Section snippets

Experimental procedure

During experimentation, AA 6063 studs and AISI 1030steel were joined together in a in house made friction stud welding machine. The compositions of AA 6063 and AISI 1030 are given in Table 1 and Table 2. The welding parameters used in the experimentation are given in Table 3. The specimens for experimentation were machined down AISI 1030 and AA 6063 rods to the geometry shown in Table 4. The AISI 1030 steel component is held in the chuck and aluminum alloy is fixed to the stud holder. Fig. 1

Results and discussion

Fig. 4, Fig. 5, Fig. 6 show the recorded characteristic curves obtained by transmitting ultrasonic waves with 3 MHz frequency using ultrasonic flaw detector at AA 6063 side, AISI 1030 and interface respectively. The computed results of ultrasonic evaluation are given in Table 5.

At a distance of 15 mm away from the weld line, highest value of Young’s modulus is found in the AISI 1030 steel side. Lowest value of Young’s modulus is found at a distance of 15 mm away from the weld line in the AA 6063

Conclusions

In the present work, dissimilar metals AA 6063 and AISI 1030 are joined successfully using friction stud welding technique. Using ultrasonic principle, the material properties of the welded joint are evaluated. Young’s modulus, longitudinal velocity, bulk modulus and shear modulus are computed from the recorded measurements. During the evaluation of the joint properties, there is an increase of 4.4%, 1.8%, 1.15% and 4.42% is observed at the AA 6063 side for Young’s modulus, longitudinal

Acknowledgment

Authors gratefully acknowledge the financial support of this work by SERB of Department of Science & Technology, New Delhi under Fast Track Scheme for Young Scientists. (Vide Letter No.: SERB/F/1452/2013–2014 dated 10.06.2013).

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