Characterizing material transitions in large-scale Additive Manufacturing

Abstract

Integrating Multiple Materials (MM) into large-scale Additive Manufacturing (AM) is a key for various industrial applications wishing to incorporate site-specific properties into geometrically complex designs that are difficult to manufacture with traditional techniques. Printing with multiple materials is typically accomplished by using layers as natural material boundaries, but having the capability to switch between materials within a single layer without pausing would further expand MM possibilities. This study used Cincinnati Incorporated’s Big Area Additive Manufacturing (BAAM) system to explore material transitions with a novel dual-hopper that enables in-situ material blending of a pelletized feedstock. Constructing MM and functionally graded material (FGM) structures requires depositing a specific material composition at a specific geometric location to achieve a desired performance. Accurately implementing this with the BAAM’s blended extrusion system requires a thorough understanding of the transition between distinct material compositions. This study characterizes a step-change transition between neat acrylonitrile butadiene styrene (ABS) and carbon fiber-reinforced ABS. Three distinct techniques were compared for analyzing the fiber content, and the transition zone between materials was characterized as a function of transition direction. The transition process was consistent to within 0.7 wt% carbon fiber variation between different layers and prints. The transition between materials was found to be directionally dependent, with ABS to CF/ABS having a transition length of 3.5 m compared to 3.2 m for CF/ABS to ABS. Furthermore, the transition from Material A to Material B was found to be repeatable with a possible variance in transition length of 0.3 m.

Introduction

The advantage of Additive Manufacturing (AM) traditionally lies in the ability to construct complex geometries [1], [2]. However, most AM systems are restricted to depositing a single material, leading to a performance entirely dependent upon geometry. Recently, newer AM systems have been adapted to deposit multiple materials within a given part, allowing for deliberate placement of each material [3], [4], [5], [6], [7]. Through construction of Multi-Material (MM) and Functionally Graded Material (FGM) structures, these advanced systems enable manufacturing of complex geometries with site-specific properties. Typically, MM structures use a Multi-Material Additive Manufacturing (MMAM) system, which utilizes more than one material without pre-mixing, pre-compositing, or non-AM post-processing treatments [8]. FGMs, on the other hand, do not necessarily require an MMAM system. Instead, FGMs have a defined spatial variation in density (single-material) or composition (MM) that achieves a desired functional performance [9]. FGM structures can vary in either a stepwise or continuous manner. A stepwise FGM consists of multiple regions that differ in composition or microstructure, have discrete boundaries, and are intentionally arranged to achieve a gradient over the entire structure. In contrast, a continuous FGM lacks discrete interfaces and instead has a controlled, smooth gradient in composition or microstructure to attain the desired functionality [10].

MM printing is compatible with a variety of processes, all of which generally assign different materials to different layers for ease of construction. However, bonding dissimilar materials can be difficult, often resulting in structural weak points that are vulnerable to delamination. Traditional sandwich panel designs that are 3D printed typically suffer from fractures at the material interface, regardless of the MMAM method. Vu et al. evaluated sandwich structures with a T-Peel geometry made with a Stratasys PolyJet Connex 350 and observed fractures favored the material interfaces [3]. While aligning the interfaces perpendicular to the build direction increased their fracture resistance, interface failure was still common [11]. Brischetto et al. experienced similar behavior on a multi-nozzle Fused Filament Fabrication (FFF) system. Exchanging acrylonitrile butadiene styrene (ABS) for polylactic acid (PLA) in the face sheets yielded a weaker sandwich structure despite maintaining the same PLA honeycomb core. Once again, delamination between the dissimilar materials was largely responsible [4]. Roger and Krawczak demonstrated that simply adding a filler can influence bonding and delamination tendencies. An interpenetrating arrangement preventing the material interface from occupying a single plane improved mechanical performance, but the single-material counterpart still showed a greater resistance to delamination [12]. Kim et al. also investigated interface location and number by placing multiple interfaces in both the print and z-directions (across layers). While z-direction boundaries decreased void formations, boundaries in the build-direction resulted in voids that delaminated during testing [13].

MMAM processes have routinely utilized layers as inherent discrete boundaries when implementing stepwise FGMs. In a continuation of previous studies, Vu et al. assessed the effect of a stepwise FGM core on a sandwich structure performance. Resistance to delamination in T-Peel testing demonstrated an increased resistance to peel and fracture compared to the typical single-material cores [11]. To balance structural flexibility and rigidity, researchers implemented a similar stepwise FGM approach for construction of a robot shell. Numerous material changes gradually increased the elastic modulus from 1 MPa to 1 GPa, allowing it to survive impacts that shattered a rigid version while limiting the elastic deformation that made the flexible version inoperable [5]. While boundary-representation approaches struggled to place MM within the same layer, a new FGM design enabled this function via volumetric placement. It successfully demonstrated control of porous regions and differing materials [6]. One in-depth study used voxel-based inkjet printing techniques to demonstrate the strength gains provided by continuous FGMs structures compared to stepwise variants [14], while another utilized sequential MMAM to construct FGMs ranging in size from millimeters to meters [15]. Other bioinspired FGMs have targeted medical usage in orthopedic implants [16], [17]. Regardless of the process or materials, there are a wide variety of applications and interests in using MMAM to create FGM structures with unique properties [18], [19], [20]. In each case, FGMs functioned as a bridge between dissimilar materials, enabling regional properties while limiting typical MM defects such as delamination. As such, it is critical to understand the options available to AM for FGMs and to explore the best production methods at all available scales. The goals of this study were to conduct the first investigation of in-situ blending of discrete materials using LS-ME, to demonstrate continuous transition from one material to another during printing, and to discuss the application of this technique to FGM printing.