Fused filament fabrication (FFF) is one of the most widely-used polymer-based additive manufacturing technologies

with advantages of high raw material utilization

simple working principle

and low cost. However

nowadays FFF technique tends to be used to fabricate prototypes rather than final products

especially in industrial area

since it produces polymer parts with weak interlayer interfaces and therefore anisotropic mechanical properties due to its later-by-layer manufacturing principle. During FFF process

neighbour layers weld at the interface through surface wetting

molecular diffusion and entanglement

highly depending on the thermal profiles experienced by the deposited polymer strands. To this end

the

in situ

external field-assisted FFF techniques have been developed

aiming at promoting the welding behaviours between adjacent layers during the FFF process

with the help of thermal and force fields and their combination external fields

which in turn improve the mechanical properties of the FFF parts. This paper firstly summarizes the state-of-art of the modeling and experiments devoted to exploring the interlayer welding behaviours of amorphous and semicrystalline polymers during FFF process. Then

various thermal

force and their combination external field-assisted FFF techniques are presented

from aspects of the principles and their effects on the mechanical properties of the FFF parts. Finally

the challenges and prospects of the thermal/force-assisted FFF technology are discussed from the perspectives of materials

processes

and devices

and the development and application of FFF technique are emphatically analyzed.