Joint quality of self-piercing riveting (SPR) and mechanical behavior under the frictional effect of various rivet coatings

Highlights

• Friction properties of rivet coatings influences performance of SPR joints.

• Various rivet coatings need optimization of rivet setting parameters.

• Higher friction coefficient of rivet coating contributes to higher joint strength.

• Deformation of rivet coating alters friction properties at rivet/sheet interface.

Abstract

This study investigates the frictional influence of different rivet coatings on the joint quality and strength of self-piercing riveted aluminum alloys. A mechanically plated Zn-Sn-Al (Almac®) and an electroplated Zn–Ni coating were applied to the steel rivets to examine the frictional effect of rivet coatings. The friction properties of the rivet coatings were evaluated using a micro-mechanical surface tester. The results demonstrated that the friction properties of the rivet coatings remarkably influenced both the joint quality after riveting and the mechanical behavior of the joints. Under a similar riveting condition, owing to the lower friction coefficient, the Zn-Ni-coated rivets experienced deeper rivet head penetration and larger interlocking in terms of joint quality compared to those of Almac®-coated rivets with a higher friction coefficient. However, under a similar joint quality with respect to the appropriate riveting parameters, the joints with Almac®-coated rivets exhibited higher lap-shear and cross-tensile loads owing to the higher frictional force at the rivet/sheet interface compared to the joints with Zn-Ni-coated rivets. The surface profiles of the coatings after applying an external load using a micro-mechanical surface tester revealed a larger plastic deformation and subsequently higher delamination of the Almac® coating during the riveting process, which altered the friction properties at the rivet/sheet interface and thereby contributed to the higher lap-shear and cross-tensile loads. A numerical analysis also elucidated a significant effect of the friction coefficient at the rivet/sheet interface on the joint quality and lap-shear behavior, which is consistent with the experimental results.

Introduction

Aluminum is the material of choice for lightweight automotive manufacturing as it allows a weight saving of up to 50 % over conventional steels within reasonable cost and without compromising safety [1,2]. The weight reduction of vehicles aims to improve the fuel economy and emission control [3,4]. The application of aluminum alloys is highly promising for car body parts such as roof, doors, and hood. The highly formable 5000 series alloys for inner panels and the heat-treatable 6000 series alloys for outer panels are extensively used. However, the joining of aluminum alloys using traditional fusion welding is complicated due to its low melting point, surface oxide layer, high thermal conductivity, high hydrogen solubility, and high solidification shrinkage [5,6]. Self-piercing riveting (SPR) has recently been a promising joining technology for aluminum alloys in automotive and aircraft manufacturing [[7], [8], [9], [10]]. SPR can join multiple layers, as well as varying thicknesses of materials [11,12]; it does not require any predrilled holes or surface pre-treatment. Furthermore, SPR has the advantages of a high joining speed and automation flexibility [13]. Compared with traditional spot welds, the SPR joints of aluminum alloys have better tensile and peel strengths, along with a superior fatigue life [14]. However, the strength of SPR joints is influenced by several factors including the strength and thickness of sheet materials [15], rivet geometry [16], die geometry [17], specimen configuration [18], and friction at sheets overlap region and rivet/sheet interface. A previous simulation study reported that the displacement in the load–displacement curve of the peel test increased with an increase in the friction coefficient at the rivet/sheet interface [19].

Joint quality (geometry) is a key factor in achieving a reliable and robust SPR joint. Researchers and manufacturers have set several quality criteria to produce defect-free joints; three major joint quality aspects are the rivet head height, interlock distance, and remaining thickness of the bottom material [20]. The head height is important for the strength, corrosion behavior, and cosmetic appearance of the joints. A proud head (rivet head protrudes out of the top sheet) reduces the joint strength by forming a gap between the rivet head and top sheet, while a flush head (rivet head penetrates into the top sheet) reduces the joint strength by damaging the top sheet around the rivet head [21]. Furthermore, the degradation of the top sheet reduces the local corrosion resistance [22]. The interlock distance determines the locking strength between the rivet and bottom sheet. The riveted sheets can be easily separated under loading conditions because of an insufficient interlock distance. The remaining thickness of the bottom material is essential for joint strength and corrosion performance. An inadequate thickness of the remaining bottom material reduces the strength and corrosion resistance by initiating cracks in the joint button of the bottom sheet. However, many factors can influence the joint quality, including the sheet material strength [23], hardness [24], thickness [[23], [24], [25]], pre-straining of the sheet materials [26], stack orientations [27], diameter and pressure of the blank holder [28], rivet geometry [16], die geometry [25,29], and friction at several locations of the joint. A previous simulation work stated that a higher friction coefficient at the rivet/sheet interface required a slightly higher setting force for riveting [30].

The surface condition of the material significantly influences the friction properties. Previous studies evaluated the effect of surface roughness of coated and uncoated sheets on the quality and strength of SPR joints [31,32]. Rivets are also essentially coated for better lubrication during rivet setting, and for better corrosion protection of the joints. Mechanically plated Zn-Al-Sn (commercially known as Almac®) and Zn–Sn coatings, as well as electroplated Zn–Ni coatings, are widely used for rivet applications [33]. The difference between the formation mechanisms of mechanically plated and electroplated coatings result in different friction properties [34]. The friction properties at the rivet/sheet interface can be influenced by the type of rivet coating used. Although the degree of friction at the rivet/sheet interface is an important factor for joint quality and strength, studies have not yet investigated the quality and mechanical behavior of SPR joints under the frictional effect of various rivet coatings. Furthermore, it is difficult to measure the friction at the rivet/sheet interface using in-situ methods due to several reasons including the fact that the friction occurs locally, interface geometries are complex, and the plastic deformation of the rivet coatings during the riveting process can change the friction properties. Considering these circumstances, the aim of this study was to evaluate the friction and deformation behavior of different rivet coatings, followed by the subsequent influence on the joint quality and mechanical behavior of the aluminum alloy SPR joints. The friction properties and deformation behavior of the rivet coatings were evaluated using a micro-mechanical surface tester. In addition to the experimental investigations, a numerical analysis using various friction coefficients at the rivet/sheet interface was also performed to evaluate the effect of friction on the quality and lap-shear behavior of the joints.