Study of Bonding Formation between the Filaments of PLA in FFF Process

Fused filament fabrication (FFF) is an additive manufacturing (AM) process that provides physical objects commonly used for modeling, prototyping and production applications. The major drawback of this process is poor mechanical property due to the porous structure of final parts. This process requires careful management of coalescence phenomenon. In this paper, the major influencing factors during the FFF processing of poly(lactic acid) (PLA) were investigated experimentally and with a numerical model. It has been shown that the polymer temperature has a significant effect on the rheological behavior of PLA, especially on the adhesion of the filaments. An experimental set-up has been placed in the machine to have the cyclic temperature of the filament. A variation of the polymer temperature influences process parameters such as feed rate, temperature of the nozzle and temperature of the platform. The results showed that the amount of polymeric coalescence (neck growth) rises when increasing the feed rate, the nozzle temperature, and the platform temperature. A model to predict the neck growth is proposed. It predicts a lower amount of neck growth value than obtained experimentally. This difference has been explained as the effect of other phenomena, such as polymer relaxation time, pressure of the nozzle and especially cyclic temperature which is not taken into account in the model

Influence of gas atmosphere (Ar or He) on the laser powder bed fusion of a Ni-based alloy

The gaseous atmosphere plays a major role in the quality of the manufactured parts in Laser Powder Bed Fusion (L-PBF) by protecting the metal from high temperature oxidation. If argon and nitrogen are the most commonly used gases, helium has almost never been considered as a possible candidate as a chemically inert shielding gas. To provide a better understanding of the influence of the gas atmosphere on the process stability, a comparative study of L-PBF manufacturing under argon and helium atmospheres has been carried out, considering a nickel-based alloy Inconel® 625 and a single bead configuration. To this end, in-situ process measurements were carried out on a dedicated experimental setup. The melt pool behaviour, the expansion of the vapour plume and the amount of spatters were evaluated with high-speed imaging for the two gases considered, together with the final L-PBF bead dimensions. Results were also compared to single fusion beads carried out in an industrial L-PBF machine for a comparable range of volume energy densities. The influence of the shielding atmosphere on L-PBF single beads was as follows: (1) dimensions of beads were shown to be constant whatever the gas; (2) fewer and smaller spatters were produced under helium atmosphere, especially for high volume energy densities. Physical mechanisms were then discussed to understand those specific effects

Deterministic defect generation in selective laser melting: parametric optimization and control

International audienceSelective Laser Melting (SLM) is a powder based additive manufacturing process where parts are made layer-by-layer from a 3D file (STL). The complexity of a part is not a barrier in SLM, and thus the process opens new doors to design and elaborate intricate and complex shapes. This study focuses on the deterministic defect/pore generation SLM –i.e. leaving precisely unmelted powder zones in an SLM part on purpose. In order to get a high accuracy and part reproducibility, a thorough parametric optimization is required, especially for the laser contour scan, which determines both the roughness outside a part and the geometry inside a deterministic defect. Different sets of parameters must be used for the contour and hatch scans, in order to obtain a minimum porosity amount in the solid area while getting a minimal deformation around the pores. The deterministic pore generation gives new tools to characterize and develop new NDT techniques or simulate casting defects. Those pores could also be integrated in novel multifunctional materials, to implement damage detection (structural health monitoring) or to attain better thermal properties while still keeping good mechanical performances

An experimental and numerical investigation of the behaviour of AA5083 aluminium alloy in presence of the Portevin–Le Chatelier effect

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