Increased fracture toughness of additively manufactured amorphous thermoplastics via thermal annealing
Graphical abstract
Introduction
Fused filament fabrication (FFF) is the most prolific additive manufacturing technology, in terms of number of users, parts, printers, and printer manufacturers worldwide [1]. In this technique, thermoplastic filament feedstock is mechanically forced into a heated nozzle on a robotic gantry, which deposits the molten thermoplastic line-by-line to build a three-dimensional part. FFF is highly popular because of its low cost and simple feedstocks. However, the mechanical properties of FFF parts are still insufficient for most engineering applications, resulting in FFF being used most commonly for artistic renderings or parts with non-critical structural requirements.
Parts fabricated using FFF are weakest at the inter-laminar interface between printed layers. Inter-laminar failures have been observed under a variety of testing conditions including tension [[2], [3], [4], [5], [6], [7], [8], [9]], flexure [10], torsion [7], and compression [11,12]. Therefore, inter-laminar bond strength of additively manufactured (AM) parts frequently dictates failure of the structure, regardless of the anticipated loading. A variety of methods have been employed to increase the inter-laminar bond strength of FFF parts including: varying print parameters (i.e. raster speed, printing temperature, material deposition rate, and infill density) [[13], [14], [15]], using adaptive height algorithms [16,17], using dual extrusion through the Z-direction [16], adding plasticizers to the filament [18], adding low molecular weight species [19], adding composite fillers [20], or using microwave irradiation [21]. While minor improvements in mechanical performance were observed using these techniques, rarely do these properties approach the mechanical strength and toughness of similarly shaped materials made through more traditional means like injection molding.
Strength development during melt-bonding of thermoplastic interfaces has been described in various ways [[22], [23], [24], [25]]. Here, the present study summarizes the process as four critical and sequential stages: (1) interface heating, enabling local polymer flow and molecular mobility; (2) intimate contact, or close physical association of the two adherend surfaces; (3) molecular diffusion across the interface; and (4) cooling of the interface to below the glass transition temperature (Tg). During FFF, the polymer is heated to T > Tg and then extruded onto previously-deposited polymer that has already cooled below Tg, driving the polymer-polymer interface temperature rapidly above and below Tg over a time period of only a few seconds or less [10,26]. This heat transfer process is very sensitive to printing and boundary conditions, including raster path history, deposition speed, and chamber geometry [10,26,27]. The speed and complexity of the interfacial thermal history during FFF presents a significant challenge for consistently achieving complete intimate contact and molecular diffusion, leading to the common observation of low inter-laminar toughness.
The objective of the present work is to provide a detailed understanding of the development of inter-laminar fracture toughness by subjecting amorphous FFF parts to a post-print thermal anneal at temperatures of only ∼25–75 °C above Tg, for periods of hours. The use of lower temperature and longer times, compared to the FFF deposition process, allows for greater control of the interface time-temperature history and relevant strength-development mechanisms. Single Edge Notch Bend (SENB) fracture specimens made of acrylonitrile-butadiene-styrene (ABS), fabricated through the FFF process, were printed and thermally annealed for varying times and temperatures. Fracture properties of annealed SENB samples were then evaluated using a modified compliance check fracture testing technique capable of accommodating samples with differing fracture behaviors. Results of fracture testing were related to estimated timescales for heat transfer, intimate contact, and molecular diffusion. A variety of imaging techniques, including optical microscopy, scanning electron microscopy, and X-ray computed tomography were used to investigate the changes in the materials' micro- and macro-structure during the annealing process.
Section snippets
Differential scanning calorimetry
To evaluate critical polymer transition temperatures, heat flow as a function of temperature of the ABS filament (ABS M30; Stratasys Inc.; Eden Prairie, MN) was measured using differential scanning calorimetry (DSC) according to ASTM D3418 [28]. Approximately 10 mg of material was placed into an aluminum DSC pan (TA Instruments; New Castle, DE) and hermetically sealed. A dynamic heat/cool/heat cycle was then performed on the sample between temperatures of 50 °C and 300 °C at a rate of 10 °C/min
Example of unstable vs. stable fracture response
Example load vs. displacement curves for two different SENB samples undergoing compliance check fracture testing are provided in Fig. 3. Fig. 3a depicts the load vs. displacement behavior of a sample with unstable (brittle) crack propagation, as indicated by the sudden drop in load at approximately 0.75 mm of cross-head displacement. Conversely, Fig. 3b depicts the load vs. displacement behavior of a sample with stable crack propagation, as indicated by the cresting of each re-loading curve
Differential scanning calorimetry
Heat flow as a function of temperature for ABS M30 filament is depicted in Fig. 5. Glass transition temperature was measured on the second heating cycle and was found to be approximately 103 °C. As is typical for amorphous thermoplastics like ABS, no true melt peak is observed. During the first cycle, an exothermic peak is present near 230 °C, which has been previously attributed to oxidation of the butadiene phase [43]. Annealing temperatures are highlighted in this figure and all fall outside
Fracture properties of printed and annealed vs injection molded SENB specimens
Injection molded ABS SENB samples have been previously evaluated using a similar sample geometry and testing conditions [53]. Authors report toughness values in the range of 4000–6000 J/m2, markedly less than the values of ≈7000 J/m2 observed here for some of the annealing conditions studied. It is hypothesized that the increase in fracture toughness is associated with the porous structure of annealed FFF parts, which may aid in toughening the material by blunting the crack during propagation.
Conclusions
In this work, fracture performance of isothermally annealed, additively manufactured polymers was investigated. Through sintering and reptation between inter-laminar layers via thermal annealing, AM polymers demonstrated tremendous increases in inter-laminar toughness, going to levels beyond even those of injection molded parts previously seen in the literature. Time-scales of the mechanisms governing the healing process during annealing were examined. Reptation times determined from
Acknowledgements
This research was supported in part by an appointment to the Postgraduate Research Participation Program at the U.S. Army Research Laboratory administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and USARL. Authors gratefully acknowledge Mr. David Gray for his assistance with mechanical testing and Mr. Larry Long for assistance with fabrication of the annealing fixture. Authors declare no competing interests.
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