Design and testing of an innovative 3D-printed metal-composite junction

Abstract

This paper describes an innovative 3D-printed beam-based lightweight structure that is used to increase the adhesion strength of metal-composite joints without damaging the composite fibers. It is conceived as the interface between the two parts to be joined: by filling the voids of this structure with resin, a mechanical interlocking effect can be generated to enhance the mechanical properties of the junction. A dedicated design workflow was defined to explore different types of 3D beam-based structures, starting from the analysis of the main failure modes of this type of junction. Tensile tests were performed on both polymeric and metal samples to validate the effectiveness of this interlocking strategy. Results demonstrated an increase in the adhesion strength relative to standard adhesive joints. A possible practical implementation is also discussed: a new type of insert is presented for application in metal-to-polymer composite joints. Finally, such a beam-based joining approach also represents an innovative application in the field of design for additive manufacturing.

Introduction

Multiple technical solutions and approaches are already available nowadays, as discussed in [1], to manufacture metal-composite junctions. They are focused on creating high-strength interfaces [2] among the elements to be joined because junctions usually represent the weakest parts of structures. This issue, together with the possibility to develop lightweight and performant solutions, has lead in years to the development of new or improved joining strategies, especially in the transportation and aerospace sectors [3], [4], [5], [6], [7].

Although there is still significant interest in adhesive joints [8], [1], one of the most promising approaches for increasing the adhesion strength at the composite-metal interface is to perform ad hoc surface modifications of the metal part. Such modifications can be achieved using, for example, the arc-welding [2], [9] and the laser-powder bed fusion (L-PBF) [10] processes. In both cases, the objective is to create metallic protrusions/pins. The adhesion strength of the composite-metal interface is thus a combination of two effects: (1) the adhesive bond between the metal surfaces and the composite resin and (2) the mechanical interlocking effect generated by these metallic protrusions on the fibers of the composite and the metal part [11]. As already demonstrated in the literature, junction life and mechanical behavior are strongly influenced by the shape of these protrusions [12]. Moreover, this pin-based solution has the side effect of damaging the composite fibres.

This paper presents an innovative concept: the beam-based junction. Metallic pins are not placed on the surfaces of the metal part of the junction, but they are used to create the part of the metal component that should act as the interface with the composite. A metal additive manufacturing (AM) technology can be used to build a beam-based structure that is capable of generating a mechanical interlocking effect between the metal part and the resin of the composite. This approach does not damage the composite fibres, and it can be used for both short and long fibres. Moreover, such a joining strategy does not exclude the possibility of adding extra-pins to hook the composite fibres (e.g., see [2], [10]), using the same AM technology.

The use of metal AM technologies also provides further benefits. The higher roughness values, which usually characterize, for example, the L-PBF process [13], could be beneficial for increasing the metal-composite adhesion strength [14], especially when adhesive solutions based on interlocking mechanisms are explored [15]. Further, the design freedom allowed by these technologies enables the tuning of the geometry of the interface with respect to technical and even aesthetic design requirements.

An additional element of novelty of this study is the approach implemented to develop the beam-based joint from both the design and prototyping point of view. Such an approach is based on the following considerations. The mechanical interlocking effect between the resin and the beam-based interface is mainly due to the geometry of this interface. To design the interface, we first performed the analysis and formalization of its working conditions to identify the main design inputs and variables. These working conditions were formalised through the modelling of the main failure modes that could affect the junction. This formalisation allowed us to make the design variables explicit and derive a design workflow that was used to explore different junction shapes. This design workflow was not conceived as an optimization process but rather as a rational strategy to drive the exploration of the design space. Besides, since the mechanical properties of the materials are one of the input parameters for the workflow, the initial exploration of the beam-based structure effectiveness was performed by using polymer 3D-printed samples. These polymer-based samples were manufactured using the Fused Deposition Modeling (FDM) technology. The metal and composite materials were initially replaced with two low-cost alternatives also characterized by a low chemical affinity to avoid the introduction of unwanted additional variables during these initial tests. Once the effectiveness of the junction modelling was validated thanks to the polymer-based samples, the second round of tests was performed using L-PBF 3D-printed samples. Finally, this study also represents an innovative functional application of beam-based, or more generally, lattice structures. It is worth emphasizing that, for greater rigor, in this paper, we will mainly use the expression beam-based structure instead of lattice structure, because the implemented design process differs from the classical workflow followed for designing these structures, which usually begins with the selection of the unit cell [16].

The paper is structured as follows: Section 2 discusses the background of this research; Section 3 details the design and the implemented experimental approach; Section 4 summarizes the experimental results; Section 5 discusses these results and describes a new concept, namely, the insert, as a possible application of this new type of beam-based joint; and Section 6 concludes the paper.