Multi-scale material modeling for additive manufacturing

…for improving and ensuring consistent microstructure and mechanical properties.

Part complexity is not a restraint anymore for a creative design engineer. With the leap of additive techniques from rapid prototyping only to the production of actual end-use parts, a new range of design possibilities have been unlocked. In contrast with the now-called subtractive methods, additive manufacturing (AM) refers to a vast new group of processes producing end-use parts from 3D numerical CAD model by building them layer-by-layer. Switching to additive processes grants the opportunity to manufacture without the need for specific mold tooling design, allowing the production of low volume parts of greater complexity in less time at a fixed cost per part. Among the large number of additive processes currently available for reinforced plastics, selective laser sintering (SLS) and fused deposition modelling (FDM) are the most mature and industrially widespread. In FDM, material filaments are deposited through a moving head while in SLS, powdered material is sintered using a laser as heat source. Both of these technologies are able to process not only classical polymers but also reinforced polymers. A clear interest is increasingly shown in these materials in both automotive and aerospace fields. As light weighting has become a top design priority, the mix of mechanical performance and low density they offer is an obvious choice.

Multi-scale modeling as an answer to challenges in additive manufacturing

While additive manufacturing of reinforced polymers is appealing and increasingly considered for production of actual parts, major obstacles must be overcome by engineers. Dimensional accuracy of the part must obey to strict tolerances that may not be met due to thermally induced part distortion or poor surface roughness. On the material side, an anisotropic material behavior is brought in by the specific 3D printed layered architecture and oriented reinforcements. This process-induced material behavior makes the part mechanical response challenging to predict.

These issues result from the lack of insight into AM materials and process parameters influence. Rapid manufacturing was predominantly used to generate physical parts for visualization purposes. Finished parts were only required to keep their mechanical integrity for handling and demonstration purposes. The focus was not on designing materials tailor-made for a specific purpose nor fine-tuning process parameters, and deposition strategies to reach the best dimensional accuracy and part performance. This lack of process control extends to the qualification process. The production quality of a part will rely on the operator’s skills and experience to constantly adjust process parameters. If cost and time consuming trial-and-error attempts could be avoided by providing reliable sensitivity quantification on process parameters to secure consistent printing conditions, AM would become a robust manufacturing method.

e-Xstream engineering, an MSC software company, is developing simulation tools within the Digimat software suite for SLS and FDM processes modelling. These tools enable engineers to design 3D printed parts that meet all the dimensional and structural requirements while accounting for the chosen printing technology specificities. The proposed approach to additive manufacturing modelling is built along three deeply connected aspects:

• Material engineering: design new materials whose microstructure and physical properties are consistent. Both with the requirements of a specific AM technology and the targeted final application.
• Process simulation: optimize printing parameters to improve dimensional accuracy, surface finish and reduce manufacturing time while predicting residual stresses and strains.
• Evaluate and optimize the performance of 3D printed parts: simulate the mechanical response of as-manufactured parts to improve part shape and manufacturing orientation including real printed dimensions, process-induced defects and local material properties.

Industrial application of 3D printing of polymers with Digimat

The presented workflow has been successfully applied to the Polimotor plenum from the Polimotor 2 project, whose principal material sponsor is Solvay Engineering Plastics, a global leader in advanced polyamide solutions. The Polimotor project aims to open the way for a technological breakthrough in the automotive sector by replacing up to 10 metal parts by plastic materials in the engine Polimotor 2. The studied plenum is produced through selective laser sintering using a Sinterline® Technyl® polyamide 6 (PA6) powder grade reinforced with a 40 percent loading of glass beads.

A multiscale thermomechanical material reinforced plastic is created with Digimat, and subsequently used for both process and structural engineering. The layer-by-layer manufacturing of the plenum is modelled, followed by a static calculation to establish the ultimate pressure loading the part is able to withstand while operating. A low distortion from the targeted shape was predicted, as well as a maximum sustained pressure of 9.1 bars which confirmed the plenum design. In addition, manufacturing guidance was provided by optimizing printing direction such that the ultimate sustainable load could be increased by 40% without adjusting the geometry.

Integrated structural and material design tools are needed to accelerate the adoption of AM by the aerospace and automotive design community and to promote new innovative structural designs needed to save energy and weight. Experimental feedback is not enough to build a reliable production workflow. Process control is the key to reach the desired dimensional and structural requirements of 3D printed part design.