Material extrusion additive manufacturing of metal (metal MEX), which is one of the 3D printing processes, has gained more interests because of its simplicity and economics. Metal MEX process is similar to the conventional metal injection moulding (MIM) process, consisting of feedstock preparation of metal powder and polymer binders, layer-by-layer 3D printing (metal MEX) or injection (MIM) to create green parts, debinding to remove the binders and sintering to create the consolidated metallic parts.
1. Introduction
From the ISO/ASTM 52900, additive manufacturing (AM), usually known as 3D-printing, is a process of joining materials to make parts from 3D model data, usually layer by layer, as opposed to subtractive manufacturing and formative manufacturing methodologies
[1]. This process has become increasingly popular for various material fabrications, such as ceramic, polymer and metal
[2,3,4,5][2][3][4][5]. Many metal AM processes, such as powder bed fusion (PBF), direct energy deposition (DED) and materials extrusion (MEX) can successfully fabricate various metals, e.g., stainless steel
[6,7,8[6][7][8][9],
9], titanium alloys
[10[10][11][12][13],
11,12,13], nickel alloys
[14[14][15][16][17][18],
15,16,17,18], cobalt
[19,20][19][20] and aluminium alloys
[21,22,23,24,25][21][22][23][24][25]. AM can also provide a high degree of freedom, lightweight design with almost unlimited shape, complexity and a varied range of sizes depending on the printing process
[26]. In addition, the AM parts are not only limited to prototyping, but can be applied in various technologies, including modelling, pattern-making, tool-making and end-use parts productions with very high growth rates
[27]. Hence, AM parts can be served in many industries, e.g., biomedical, aerospace and energy applications
[3,28][3][28]. Among the several techniques of metal AM, metal MEX utilises low-cost equipment with simplicity and safety, as neither loose metal powder nor a high-power source is required when compared to other common metal AM processes, i.e., laser powder bed fusion (LPBF) and electron beam powder bed fusion (EPBF)
[9,29][9][29]. During the last decade, this metal MEX process has attracted more attention due to the as-mentioned advantages and the familiarities with conventional polymer 3D printing, which is the metal-fused filament fabrication process (FFF), usually called fused deposition modelling (FDM).
Figure 1 shows the number of publications relating to metal MEX per year and the cumulative number.
The nature of metal MEX is very similar to the conventional metal injection moulding (MIM)
[124,125][124][125]. The overall MIM and metal MEX processing steps are presented in
Figure 2a,b,d,e and
Figure 2a,c,d,e, respectively. The MIM process starts with the mixing of sinterable metal powder with suitable polymeric binders and then granulating the metal-binder mixture into feedstock
Figure 2a. The feedstock is subsequently injected into a mould to create the injected part, commonly called a “green part” (
Figure 2b). The polymeric binders are then removed by solvent (optional) and thermal debinding (
Figure 2d) before the debound parts are sintered in a controlled atmosphere, e.g., H
2, N
2, Ar or vacuum atmosphere, to densify the parts (
Figure 2e). During sintering, necks are formed to bond between adjacent powder particles, consolidation takes place and voids are closed. This causes shrinkage of the sintered part, which in theory should be uniform. However, in practice, the uniformity of shrinkage depends on several factors, e.g., the homogeneity of feedstock and the resultant green parts, geometry, gravity and friction between the parts and sintering tray. Typical MIM shrinkage lies within the range of 12–20%
[125,126,127][125][126][127]. Hence, the mould cavity needs to be oversized to compensate for the shrinkage. After sintering, the density of the MIMed specimen can reach up to 99% of the theoretical density. Hot isostatic pressing (HIP) can be applied, if high mechanical property and density are required. For the metal MEX, instead of forming the green part by the injection moulding process, it is printed layer by layer (the process in
Figure 2b is replaced by that in
Figure 2c) with various forms of feedstock, i.e., granule, bar and filament, depending on the printer. After printing, the subsequent debinding and sintering steps (
Figure 2d,e) may be slightly different from the MIM process due to the differences in compositions of binders and the metal powder fraction (usually named “solid loading”), metal powder size and its distribution. The shrinkage of the sintered metal MEX part is generally higher than for MIM parts because the metal MEX feedstock usually has higher binder content (lower solid loading) than MIM so that the metal MEX feedstock is printable and can be easily handled. Therefore, dimensions of the CAD model need to be carefully compensated to acquire the required dimension after sintering. The sintered density and mechanical properties of the metal MEX part are theoretically lower than those of MIM due to the voids between deposited paths generated during printing
[8]. Thereby, the print strategy, which can generate not only such voids but also deflection and incomplete weld in polymer 3D-print parts
[128,129[128][129][130],
130], needs to be carefully controlled for metal MEX before progressing to the debinding and sintering.