Impact of thermoplastic polyurethane (TPU) as a plasticiser-free binder on additive manufacturing of magnesium alloys: A comparison with polypropylene-polyethylene copolymers

Abstract

This study presents a breakthrough in screw-based material extrusion (SBME) of magnesium (Mg) alloys by introducing a plasticiser-free thermoplastic polyurethane (TPU) binder system that achieves a 70 vol% powder loading, the highest reported for material extrusion of any solid material. TPU performance was compared with polypropylene–polyethylene copolymer (PPcoPE), the current state-of-the-art binder for Mg alloys. Unlike conventional multi-component systems, the single-backbone TPU eliminates plasticisers, preventing phase separation and enabling stable extrusion at high powder fractions. The absence of plasticisers also suggests that solvent debinding can be bypassed, saving time, cost, and energy while avoiding solvent reactions with Mg. Rheological analysis revealed a unique viscosity-reduction mechanism in TPU, where powder particles disrupt polymer chain entanglements. This effect improves flow at high loadings, while TPU’s cohesive melting profile ensures uniform extrusion and prevents nozzle clogging, unlike the four-step melting of PPcoPE. Preliminary sintering studies between 500 and 700 °C for up to 64 h showed no densification for both binders, probably due to carbonate and oxide surface layers (detected via EDS, XRD, and OPTIR) on Mg-5Ca after debinding, suggesting a change in Mg alloy composition for higher sinterability. These findings highlight TPU’s potential for record-high powder loadings in next-generation powder-based additive manufacturing.