Processability of metal-filament through polymer FDM machine

Mariangela Quarto. Department of Management, Information and Production Engineering, University of Bergamo, Via Pasubio 7/ b, 24044, Dalmine (BG), Italy Corresponding author: mariangela.quarto@unibg.it Mattia Carminati. Department of Management, Information and Production Engineering, University of Bergamo, Via Pasubio 7/b, 24044, Dalmine (BG), Italy Gianluca D’Urso. Department of Management, Information and Production Engineering, University of Bergamo, Via Pasubio 7/b, 24044, Dalmine (BG), Italy Claudio Giardini. Department of Management, Information and Production Engineering, University of Bergamo, Via Pasubio 7/b, 24044, Dalmine (BG), Italy Giancarlo Maccarini. Department of Management, Information and Production Engineering, University of Bergamo, Via Pasubio 7/b, 24044, Dalmine (BG), Italy


Intr Introduction oduction
The term Additive Manufacturing (AM) describes an innovative technology capable to manufacture parts layer upon layer as opposed to subtractive and formative manufacturing technologies [1]. In the early stages, it was mainly used for prototyping, while today it is widely used for manufacturing functional parts for several application fields, such as aerospace [2], automotive [3], energy [4] and medical [5]. Compared to traditional technologies, the AM allows to have wide freedom of component design [6] and speed up of product development process. It is a resource-efficient process that minimizes material waste, adding material only where it is necessary, for obtaining lighter components [7]. In addition to the AM technologies for polymers, new processes capable of printing components by adding metal powders layer by layers were developed, providing high-resolution hollow parts with physical and mechanical properties comparable to those obtained by traditional technologies [8,9]. Metal AM processes spend a lot of energy for sintering the metal powders generating a high cost of equipment and great expenditure of time. These characteristics suggest the need to introduce an affordable metal AM fabrication method in terms of investments, processes, activities and ESAFORM 2021. MS13 (Additive Manufacturing), 10.25518/esaform21.2114 2114/1 maintenance. In this way, it would be possible to expand the applications to no high added value products. A possible answer to this need could be found in the arrangement of Fused Deposition Modelling (FDM) process, which is one of the most widely used for manufacturing polymeric parts because of its simplicity and low costs investment. Today some particular filaments have been developed, combining polymer with metal particles. These are a homogenous mixture of multi-component binder systems and sinterable metal powders and several studies were already carried out in this field [10][11][12][13][14]. For example, in [13] the quality of filaments loaded with the same content vol% of two different stainless-steel powders, 316L and 17-4PH, was investigated to ensure correct printing. Gibert et al. used martensitic stainless steel AISI 630 powders to design an extrusion-based system with 5-axes control of the worktable and with parallel kinematics [14]. The type of filler particles resulted to affect the processability and the tensile strength of different filaments. Besides, the morphological characteristics of the powders (e.g. size and shape) can influence the physical and mechanical properties of the feedstock materials [10].
The polymeric system consists of a main binder component in the largest amount, a backbone (second binder) used to hold together the structure of the part, avoiding the spreading of metal particles, and additives to prevent agglomeration and phase separation [15] [16]. The binder fraction is removed from the printed part through a debinding step that uses solvents and/or thermal debinding [17]. On the other side, the sintering step provides the interparticle bonding that leads to near full densification, through a thermal cycle performed below the melting temperature [18]. The introduction of polymer filaments highly loaded with metal particles allows metal AM to evolve significantly, permitting to metal FDM to become a cost-efficient option for manufacturing metal parts because of lower equipment costs and faster buildup rates.
In this paper, a commercial polymeric FDM printer was used in combination with metal-filament to verify the possibility of fitting a low-cost machine for printing metal components. The conducted analysis allowed to verify if the main relevant parameters of FDM printing process affect the physical and dimensional response of the produced parts.
This study aims to identify which factors affect the final AM products in terms of shrinkage percentage, along X, Y and Z directions, and bulk density ( ). The experimental tests were carried out a low-cost 3D printer using a metalpolymer composite filament. Once it is defined if the selected process parameters affect in some way the indicators, it will be possible to identify the specific values of the shrinkage useful for oversizing CAD models. In the same way, it is possible to set-up the process parameters in order to obtain a satisfactory density. The novelty of this work is related to the possibility of producing parts for a non-critical environment with low-cost equipment.
2 Experiments and methods 2 Experiments and methods

E Equipment and mat quipment and materials erials
Samples were fabricated by means of an Ultimaker S5 printer, using a filament with a diameter of 2.85 mm provided by BASF and called Ultrafuse 316L. This is an innovative metal filament made up of AISI 316L powder (90 wt%), characterized by high ductility and corrosion resistance, and polyoxymethylene (POM) and polyolefin for easy printing.
The direct drive extruder of the printer was equipped with a hardened steel nozzle CC0.6 (supplied by Ultimaker) with a diameter of 0.6 mm. After the printing phase, the polymeric fraction has to be removed from the so-called green-part through debinding and sintering processes ( Fig. 1). At the end of these post-printing thermal treatments, the samples theoretically reach their final properties close to those of the monolithic AISI 316L parts. These treatments were performed by an external service. to enhance the bonding of the first layer and to avoid the warpage phenomena) were considered as fixed parameters.  Where indicates the dimension (X, Y, Z), Dgp and Dpp are the dimensions of the green-part and the post-processing-part respectively. The was calculated as the ratio between the weight and the geometrical volume of the sample (2).
In particular, ℎ is the volume of the post-processing-part after thermal treatments, calculated using the dimensions estimated through the CMM, and indicates the post-processing weight, measured using a precision balance.

P Por orosity e osity ev valuation aluation
The FDM process can generate porosity into the internal structure; for this reason, the bulk density was compared with the density of monolithic AISI 316L ( confidence interval equal to 95% was taken into account.

R Results and discussion esults and discussion
The outlier analysis allows to identify and delete the data from the dataset generating an updated version of it. In The residuals demonstrated, in all cases, to be normally distributed and randomly scattered with an average value near to zero. The ANOVA results show that the single parameters affect, in each case in a different way, the indicators, while only a few 3-way interactions affect the indicators (Table 3). In particular, the bulk density is not affected by the , as a single parameter but it plays an important role from the interaction point of view, especially as regards its interaction with the infill type and the layer thickness. Each shrinkage is affected differently; in general, speed is the factor that affects all shrinkage directions, but, while shrinkage along X-axis shows an effect derived from layer thickness, the shrinkage along Z-axis shows also an effect due to the , which influences also the adhesion of the layers along the growth direction.
Considering the material involved in this process, the optimal results are represented by obtaining the highest bulk density, close to the monolithic AISI 316L (8 g/cm 3 ). The main effects plot (Fig. 3) shows the parameters combination satisfying this requirement: = Line infill, = 20 mm/s and ℎ = 0.1 mm, regardless of the value. This is also confirmed by the graph reported in Fig. 4. Where the average values and the standard deviation (of 4 runs) of bulk density are reported as a function of process parameters combination. Both Fig. 4 and Fig. 5 show a low standard deviation indicating stability and repeatability of the process.
Processability of metal-filament through polymer FDM machine 2114/6 T   Table 3).  This study introduced the application of a metal filament on a low-cost FDM machine, permitting a faster and less expensive process than the existing metal AM technology. Both the bulk density and the shrinkage were affected by the printing parameters, moreover, it was found that shrinkage along X and Y directions has a similar behavior (about 16.40%), while the effects due to the thermal treatments was more critical along the Z-axis in terms of both shrinkage value and scatter. This is mainly due to the effect of the layer thickness and the interaction between the temperature and the infill type. The best combination was found for a line infill, ℎ=0.1 and =20 / which gives rise to a shrinkage along Z-axis equal to 20%. Therefore, a print characterized by a low speed of material feeding and growth (low layer thickness) leads to better results in terms of density.
Considering these evidences, the use of a metal filament in FDM process is a promising way of making non-critical metal AM parts and deserves further investigations, also thanks to its cost-efficiency. In particular, it was shown that it is possible to convert a commercial FDM printer, typically used for polymeric materials, into a printer for metal filament by setting the machine with a nozzle with higher wear resistance. This may represent a sustainable solution for both the economical aspect and the simplicity of production of parts having complex geometry.