## 1 Introduction  
Additive manufacturing (AM) technologies produce parts layer by layer [1; 2]. This enables complex parts to be fabricated in a single process and endowed with multi-mechanical properties [3]. Therefore, AM has been widely adopted in the aerospace, defense, medical, automotive, and many other industries [4; 5]. AM technologies can be classified in various ways [6]; in ASTM F2792-12, AM are divided into seven categories: material extrusion (ME), cut photopolymerization (VP), material jetting (JJ), binder jetting (BJ), powder-bed fusion (PBF), direct-energy deposition (DED), and lamination (SL) [7]. The first of these, ME, is based on the extrusion of melted polymer. In ME, various thermoplastics can be used, e.g., polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), or polycarbonate (PC) [8; 9; 10]. Because of the simple mechanism involved, the cost of the necessary machinery (often available from open-source projects) is relatively low. In addition, ME is the only AM process with a deposition mechanism suitable for use in the space environment [11]. In the traditional ME process, a polymer filament is extruded from a pellet to fabricate the part. Recently, metal/ceramic filaments have been developed by adding the metal/ceramic powders into the polymer-based extrusion process [12; 13]. After the initial fabrication of a part from metal/ceramic filament, post-processes (especially debinding and sintering) must be performed to remove the polymer and bind the metal powders [14; 15]. Parts are assigned different names at different stages of this fabrication process: immediately after initial fabrication, a part is called \"green part;\" after the debinding process, it is called \"brown part;\" the final metal part after sintering is called \"white part.\" An open-source-based machine can be utilized, and the post-processing is similar to that in metal injection molding (MIM) [16]. The post-processing is well studied. Hence, metal/ceramic ME has been applied to numerous materials, including 316L stainless steel [17], 17-4 PH steel [18], Ti6 Al4V [19], zirconia [20], and alumina [21]. In general ME fabrication, porosity is inevitable; pores appear between layers or strands [22], diminishing the part's mechanical properties. Much research has focused on optimizing the process parameters to reduce porosity [23; 24]. One way of doing this is to reduce gaps between layers or strands: a small layer thickness can reduce inter-layer gaps; increasing the flowrate can reduce the gaps between strands [25; 26]. Another approach is to optimize parameters related to the path generation of the extruder nozzle, such as the infill pattern, infill density, and infill overlap [27; 28; 29; 30]. Finally, post-processing can be utilized. Among various post-processing processes, annealing process is a post-processing that has been widely adopted to improve mechanical properties in polymer. In a composite comprised of fiber and polymer, annealing process brought the increase of the mechanical properties due to the fiber matrix boding due to the reduction of voids [31; 32; 33]. [PERSON] et al. reported that annealing process could help the increase in the tensile strength of a part from ME process with ABS polymer, but after the annealing process, pores still existed [34]. [PERSON] et al. used a low-pressure chamber to control printing environment and found that the low-pressure environment could help the increase in mechanical properties of continuous fiber-reinforced polymer [35]. In metal/ceramic ME, the same problem appears but has not yet been solved. One possible approach is to apply warm isostatic press (WIP), a post-process used in conventional ME. [PERSON] et al. employed WIP to reduce the internal pores of a part fabricated by polymer-based ME [34]; the mechanical properties improved after the WIP process, because there was no obstacle to reducing the pores by flowing the melted polymer.