Comparative evaluation of the effect of the substrate thickness and inherent process defects on the static and fatigue performance of FSW and adhesive-bonded overlap-joints in an AA6016 alloy
Introduction
Aluminum alloys have long been at the center of attention of transport design. Thanks to their lightness, these materials allow designers to satisfy design criteria that foresee weight reduction ensuring fuel saving. This synergy also makes it possible to reduce polluting emissions allowing environmental protection [1].
However, their metallurgical characteristics, along with consequent distortions due to thermal flow, often make welding difficult to be performed via conventional electric-arc technologies [2]. To date, Resistance Spot Welding (RSW) is the predominant process in the automotive sector for sheet metal joining, thanks to advantages such as high operating speeds and ease of automation or robotization. However, achieving defect-free, structurally sound and reliable welds is not so easy. The complexity of RSW derives from the need to find a right balance between electric current and pressure between the parts to be joined, to obtain correct fusion of the weld nugget and limited geometric alterations of the surfaces on which the electrodes act [3].
Over time, many studies have been focused on mechanical joining methods, such as riveting and clinching [4]. Notably, riveting makes it possible to obtain very similar joints to common spot welds with completely comparable manufacturing times. Nevertheless, even its limitations are similar to those of RSW, being mainly related to the necessity of proper access from both sides of the joint and, very often, need of sealing [5].
In this scenario, joining technologies for lightweight-structure manufacturing must be an effective tool to increase joint performance. This aspect is all the more important in transport applications, in which components are typically subjected to both static and dynamic stresses, and, therefore, the mechanical behavior of the joints requires an in-depth investigation.
Among all the applicable joining technologies, Adhesive Bonding (AB) and Friction Stir Welding (FSW) allow manufacturing of aluminum-alloy overlap joints in an effective and continuous way. The former is a highly studied method today thanks to the possibility of joining dissimilar materials without bringing about distortions, even when the pieces to be joined are small in thickness (such as those commonly used in the transport field). Furthermore, AB allows sealing and insulation of the joined parts, with obvious advantages in terms of corrosion resistance of the components. Compared to mechanical fasteners or spot welds, AB increases the joint stiffness since it produces a continuous bond rather than a punctual contact, resulting in more uniform stress distribution over a wider area, and minimizing the presence of portions on the joint affected by stress concentrations. This aspect, in principle, remarkably contributes to longer fatigue-life of the joints, with respect to those fabricated via RSW or riveting processes. However, several aspects – firstly, surface preparation and environmental aging [[6], [7], [8]] – also have to be considered for better estimation of the fatigue behavior of the AB joints. Limitations of the bonding process have to be found in the intrinsic toxicity of adhesives and solvents, that leads to considerable safety concerns [9]; for this reason, managing the various phases of the process is not always easy and requires an in-depth preparation of the personnel involved.
On the other hand, FSW is a consolidated and widespread technology for joining aluminum-alloy parts or dissimilar materials. FSW is based on friction heating at the facing surfaces of the two parts to be welded, resulting in a joint created as a synergy of interface deformations, heat, and solid-state diffusion. Since welding is carried out at temperatures that are lower than the melting point of the material, many problems of conventional fusion welding techniques are avoided. Indeed, before FSW, many materials were considered as non-weldable or, in case of dissimilar materials, they could not be combined preserving features such as good mechanical properties, low distortions, or good surface finishing.
Butt- or overlap-joints are the most common joint configurations in FSW. In fact, by its very nature, such process lends itself very well to butt welding, since the tool action is perfect for mixing two materials placed side by side on the same plane. Nevertheless, an overlap configuration is a preferred choice when joint fabrication is possible only operating from one of the two sides of the coupling. In this condition, the junction is a mixed, continuous portion at the interface between the overlapped sheets. A typical disadvantage of this configuration lies in the presence of notch effects due to hook defects at the edge of the weld [10]. The effect of the process defects on FSW-joint performance was investigated by Guo et al. [11], who performed fatigue tests on two aluminum series, manufacturing both butt- and overlap- FSW joints, in which some defects were intentionally inserted. The authors observed that internal defects, such as kissing-bonds, generally reduced the joint fatigue life more than those on the external surface (such as toe-flash defects due to rotation of the friction tool). Furthermore, comparing the two configurations, overlap joints resulted in a lower fatigue life than butt-joints1 .
In recent years, the possible combination of welding and adhesive-bonding has been progressively evaluated, in order to merge the advantages of the two processes, minimizing the respective limitations. The most typical combinations concern RSW and structural adhesives, and such hybrid joints have resulted promising to attain higher strength, stiffness and energy absorption, compared to joints manufactured with the two technologies separately [12,13]. In contrast, the hybrid fabrication of FSW-AB joints is still difficult owing to the weld thermal wave that affects the adhesive, deteriorating it and compromising the overall functionality of the joint [14,15].
To the best of the authors’ knowledge, in no study were FSW and AB systematically compared in order to highlight the conditions that make adoption of one or the other technology suitable, especially focusing on joint geometry. Hence, in this work, the authors propose a comparative analysis between FSW and AB technologies, evaluating the static and dynamic performance of lap joints in an AA6016 aluminum alloy. Sheets of two thicknesses were used, and particular attention was paid to understanding which of the two processes was preferable on the basis of their industrial applicability.
Section snippets
Materials
In this investigation, an extensive experimental campaign was carried out on an AA6016 aluminum-alloy material, usually adopted in automotive parts construction, as well as for aeronautic or naval parts thanks to its high workability, weldability and corrosion resistance. This was provided in the form of rolled sheets of dimensions 100 mm × 300 mm, having thicknesses of 1.2 or 3 mm. The base material used was a precipitation hardening aluminum alloy containing magnesium and silicon as principal
Preliminary static characterization and determination of the overlap for the AB joints
To compare the two processes, five identical FSW specimens (N = 5), 25 mm in width each, were first cut from the original welded plates, and then tested for tensile strength (Fig. 2a). It not being possible to uniquely determine the effective resistant area of the FSW joint, a normalization of the load values obtained from tensile testing was carried out with respect to the specimen width ( = 25 mm), that was the only known dimension of the joining area.
At first, the 1.2-mm-thickness case was
Conclusions
A comparison between Friction Stir Welding (FSW) and Adhesive Bonding (AB) was proposed by considering their respective application to an AA6016 aluminum alloy. An overlap-joining configuration was adopted and the influence of two different substrate thicknesses was investigated. From a preliminary evaluation of the static behavior of the joints, it was observed that a comparison of the two processes was possible, for both thin (1.2 mm) and thick (3.0 mm) substrates, adopting an overlap length
Declaration
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
References (21)
- et al.
Joining techniques for aluminum spaceframes used in automobiles. Part II - adhesive bonding and mechanical fasteners
J Mater Process Technol
(2000) - et al.
Fatigue resistance of an aluminium one-component polyurethane adhesive joint for the automotive industry: effect of surface roughness and adhesive thickness
Int J Adhes Adhes
(2018) - et al.
Effect of surface treatment on the shear strength of aluminium adhesive single-lap joints for automotive applications
Int J Adhes Adhes
(2016) Adhesive bonding
Mater Des
(1988)- et al.
Friction stir welding of lap joints: influence of process parameters on the metallurgical and mechanical properties
Mater Sci Eng A
(2009) - et al.
Effect of quality control parameter variations on the fatigue performance of aluminum friction stir welded joints
Int J Fatigue
(2019) Weldbonding-a hybrid method of assembly
Met Finish
(2013)- et al.
On adhesive properties of nano-silica/epoxy bonded single-lap joints
Mater Des
(2016) - et al.
Incredible improvement in fatigue resistance of friction stir welded 7075-T651 aluminum alloy via surface mechanical rolling treatment
Int J Fatigue
(2019) - et al.
Impacts of vehicle weight reduction via material substitution on life-cycle greenhouse gas emissions
Environ Sci Technol
(2015)
Cited by (6)
The relationship between microstructures and mechanical properties in friction stir lap welding of titanium alloy
2023, Materials Chemistry and PhysicsCitation Excerpt :As a solid state joining technology, friction stir welding (FSW) can lead to tight joint by the material flows and the recrystallization caused by the stirring effect from the rotating tool [1–5]. FSW can be suitable for the joining of aluminum alloy, titanium alloy, magnesium alloy, steel and dissimilar metals [6–15]. The dislocation density in as welded state is one of the key factors determining the final mechanical properties in friction stir welding (FSW) [16,17].
Investigation of the fastening behavior of self-tapping plastic joints with various supporting ribs
2022, Journal of Manufacturing ProcessesCitation Excerpt :The three major assembly methods employed for joining polymers are mechanical fastening, adhesive bonding, and welding [3,4]. The adhesive joint produces a uniform bond in a wide area and can be applied to dissimilar materials, but it requires surface preparation and the adhesives and solvents have intrinsic toxicity [5]. The welded joint, especially with laser, is widely used to assemble the polymers because it is environmentally friendly, fast, and flexible [6].
Effect of microstructure evolution on corrosion behavior of 2195 Al-Li alloy friction stir welding joint
2022, Materials CharacterizationCitation Excerpt :Friction stir welding (FSW) as a newly solid-state connection method is invented by The Welding Institute (TWI) in 1991 [4]. Compared to the conventional welding methods, FSW, as a solid welding method, can avoid a lot of hot defects such as porosity, crack and burning loss of Li elements [5–7]. However, FSW, as similar as other welding methods, would also cause the change of microstructures during the welding process [8,9].