Elsevier

Materials & Design

Volume 93, 5 March 2016, Pages 224-231
Materials & Design

Interface formation and properties of friction spot welded joints of AA5754 and Ti6Al4V alloys

https://doi.org/10.1016/j.matdes.2015.12.170Get rights and content

Highlights

  • Interface formation and properties of friction spot welded joints of AA5754 and Ti6Al4V were investigated.

  • The joint interface was mainly composed of a TiAl3 intermetallic layer

  • Minimizing the brittle intermetallic phase was demonstrated to be critical to achieve high strength Ti/Al dissimilar joints

  • Reaction controlled mechanism were successfully used to express the TiAl3 growth kinetics

Abstract

This study aims to understand the influence of dwell time on the microstructure of the interface and lap shear strength of friction spot welded joints of AA5754 and Ti6Al4V alloys. The interface reaction layer consists mainly of TiAl3 intermetallic compound. It was noticed that dwell time significantly influences the diffusion process during the friction spot welding, thereby modifying the thickness of the interface and thus affecting the mechanical performance of the joints. Minimizing or optimizing the brittle TiAl3 phase was demonstrated to be the key issue to achieve high strength Ti/Al dissimilar joints. The intermetallic compound growth kinetics was also examined showing that an incubation time of approximately 2.7 s is necessary before it nucleates at the Ti/Al interface and grows laterally to form a continuous layer. Afterwards, the TiAl3 layer grows toward the Al during thickening with a corresponding growth rate of k = 2.92 × 10 7 m/s. These results are critical to understand the microstructure–properties relationship and contribute with additional improvements on the joint performance.

Introduction

The efficient use of resources and energy is one of the main challenges for modern production and product design in automotive and aerospace industries. In particular, the demand of multi-material structures made of titanium and aluminum has significantly increased as they could reduce weight and cost (in the case of aluminum alloys) and improve strength and corrosion resistance (in the case of titanium alloys) [1]. Therefore, the joining of Al alloys and Ti alloys is an emergent problem to be solved for future industrial application. Mainly owing to the extensive formation of brittle intermetallic phases at the interface, it is difficult to weld these materials by means of fusion welding [2], [3]. Minimizing or optimizing Ti–Al intermetallic phases by using alternative welding methods have become the key issue to achieve a robust Ti/Al dissimilar joint.

Friction spot welding (FSpW), also known as refill friction stir spot welding, is a solid state technique and a potential candidate for joining dissimilar materials, especially for structural transport applications, due to its advantageous lower processing temperature over conventional fusion welding and geometry compatibility to replace rivets and fasteners. FSpW uses a non-consumable tool that consists of two rotating parts, a probe and a sleeve, and a stationary clamping ring. Initially the sheets are clamped together by the clamping ring against a backing anvil and both pin and sleeve start to rotate producing frictional heat on the upper sheet surface. On the second stage, probe and sleeve move in opposite directions; one is plunged into the material while the other moves upwards, creating a cylindrical cavity to accommodate the plasticized material. After reaching a pre-set plunge depth and subsequently dwell time the process is reversed and both tool elements retract back to the surface of the upper sheet. The last stage is marked by the removal of the welding head from the work pieces resulting in a free keyhole connection with minimum material loss. A schematic illustration of the FSpW process is shown in Fig. 1.

Previous studies in FSpW of dissimilar materials reported that the mechanical properties of the joints are mainly affected by the thickness and morphology of the intermetallic compounds (IMCs) formed at the interfacial area [4]. However, it was also shown that the formation and growth of the intermetallic phases can be effectively controlled by process related temperature–time cycles through the optimization of the process parameters. This trend was recently confirmed for FSpW applied to the materials proposed in this work, AA5754 and Ti6Al4V alloys [5]. The authors systematically investigated the influence of important process parameters on the lap shear strength of the joints. The results demonstrated that the joining time seems to be the physical variable mostly affecting the diffusion process during the interface formation, and consequently the mechanical performance of the joints. However, still a lack of understanding about details of the interface and the kinetics of IMC formation during the process, which can elucidate the microstructure–properties relationship and assist with further improvements on the joint performance.

In the current study, dissimilar AA5754 and Ti6Al4V joints were produced by FSpW employing different dwell times and used to study the characteristic and growth kinetics of the IMCs formed at the joint interface. The results aim to provide theoretical guidance to understand the influence of the exposure time on the interface and mechanical properties of the welded joints.

Section snippets

Experimental procedure

In this investigation overlap single joints of 2-mm-thick 5754 Al alloy and 2.5-mm-thick Ti–6Al–4V were produced by FSpW. Sheets were initially cut to 100 mm long and 25.4 mm wide samples. The specimens were prepared in lap-shear configuration with 25.4 mm overlap. The sleeve plunge variant of the FSpW process was carried out using a RPS 100 machine (Harms & Wende, Germany), capable of applying axial forces and rotational speeds up to 15 kN and 3000 rpm respectively. The tool system comprised a 14.5 

Results and discussions

The corresponding lap shear test results of the welds are presented in Fig. 2. For comparison purposes, the highest LSS data resulting from similar AA5754 FSpW welds, using the same tool dimensions, are also presented in Fig. 2. It can be noticed from Fig. 2 that the joint cannot be consolidated in the sample welded without a dwell time, probably due to the insufficient amount of heat input to assist atomic diffusion. Meanwhile, a maximum LSS of 7.4 kN was obtained at the weld produced with a

Conclusions

Interface formation and mechanical performance under shear loading of friction spot welded joints of AA5754 and Ti6Al4V alloys were investigated. The following conclusions can be drawn from this work:

  • 1.

    A maximum average failure load of 7.4 kN was obtained for the produced joints using a dwell time of 2 s, which is comparable to the failure load of the similar AA5754 Al alloy overlap joint. Lap shear strength was observed to be mainly dependent on the interface thickness, which is significantly

Acknowledgments

The authors would like to acknowledge the financial support of CNPq (248614/2013-4) (National Council for Scientific and Technological Development — Brazil) and FAPESP (14/16791-0) (São Paulo Research Foundation — Brazil).

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