1. Introduction
Additive manufacturing technology (AM) is based on the manufacture of parts or products by adding layers of material. It was originally developed to produce rapid prototypes. Today, this technology has many applications in different industrial sectors [
1].
One of the AM techniques that is receiving more attention lately is the binder jetting (BJ) technique [
2]. This technique is based on joining powder by means of a binder [
3]. The process starts by spreading a layer of powder on a platform using a roller. Then, micro-droplets of a liquid binder are selectively injected through a print-head. This creates a two-dimensional layer where the powders that receive the binder bond with each other. The platform then descends a distance equivalent to the thickness of the layer and the process is repeated according to the slicing software. Finally, a three-dimensional part called the “green part” is obtained, which is usually post-treated to improve the final properties [
4,
5].
The applications of this technique are present in diverse sectors from aeronautics [
6] to biomedical [
7,
8,
9]. However, much effort is being made to use the BJ technique in the foundry industry, as it reduces time and cost [
10]. Chen et al. [
11] reviewed the techniques used in 3D printing for ceramic materials and concluded that the BJ technique is the most suitable for manufacturing moulds or cores. Although metal casting dates back to ancient times, the evolution of materials and technology drives the further development of knowledge about this industry [
12].
Current market requirements make the traditional foundry industry uncompetitive when low quantities are requested [
13]. However, AM has proven to be useful for making small or single series castings as well as for validating prototypes that are then manufactured on a mass scale. The BJ technique offers several advantages, such as design freedom, minimum material waste, lower tooling costs and improved supply chain efficiency [
14]. Almaghariz et al. [
15] identified the breakeven point where 3D printing is cost-effective for castings, considering the level of complexity. They demonstrated that AM fast casting technology is very efficient for manufacturing up to 45 units for simple parts and up to 1000 units for extremely complex parts. After these amounts, traditional casting methods start to become more cost-effective.
Mould design has few limitations when using the BJ technique. Shangguan et al. [
16] and Kang et al. [
17] manufactured casting moulds created by 3D printing reinforced with rib or lattice structures. This new concept of design improved the heat transfer and improved the control of cooling. In addition, the new designs made lighter moulds. They concluded that rapid and uniform cooling improves production efficiency and reduces deformation, residual stress and casting defects. Deng et al. [
18] developed 3D printed moulds with internal hollow structures to control local cooling in the castings. They observed that forcing air through the internal ducts around the part cavity improved its cooling. On the other hand, the internal hollows in the feed system functioned as insulators, which allowed the metal to take longer to solidify, improving the filling.
Sama et al. [
19] developed unconventional design rules in the filling system to reduce surface turbulence, oxide films and air entrapment, improving the performance of the foundry. In another study, the same authors [
20] investigated new designs of sprues for moulds manufactured by the BJ technique, concluding that the use of a conical helical sprue reduces the turbulence of the metal and reduces the volume of casting defects, improving the resistance to bending of the obtained parts.
Wang et al. [
21] redesign a casting part using topological optimization. The new casting part improved the safety factor by 30% and reduced its weight by 50% compared to the original casting. Although this study did not consider the impact of other aspects of castings such as surface roughness and errors due to the solidification process.
The inherent design freedom of the BJ technique and its use for small series does not allow the application of a trial and error method for optimizing geometries, because it would not be economically viable. This has encouraged the need to use numerical simulation software to evaluate the effectiveness of the geometries, and to predict the flow behaviour during the filling and solidification process inside the mould.
Yang et al. [
22] developed a mathematical model using the Navier-Stokes equation for the pouring and solidification stages; they validated the simulation results, analysing the casting defects. Behera et al. [
23] generated a numerical simulation model to analyse the thermal stresses generated in a casting mould. The data results from the simulation, provided an accurate prediction of defects in the mould and the casting. Sun et al. [
24] simulated a low-pressure casting process to analyse the origin of the defects found in the castings, confirming that they are due to gas entrainment.
Although the BJ-technique has been successfully proved in the foundry industry, it still presents challenges to overcome. The composition, size and shape of the powder are very important parameters to achieve high quality parts. Zhao et al. [
25] observed that the particle size of the powder directly affects roughness of castings, concluding that if the size of the mould particles is reduced, the average roughness decreases and, therefore, the roughness of the casting is reduced. In addition, Du et al. [
26] observed that by increasing the number of fine particles in the powder, the surface roughness of the cast iron decreases as fine particles settle between larger particles, reducing the roughness of the mould.
Another very important aspect is powder compaction. There is a general lack of densification during the printing. The low density and the porosity created during the manufacturing is transferred to the green part and the final properties of the moulds [
27]. Hodder et al. [
28] analysed the mould quality created by AM using local sand as raw material. They concluded that traditional foundry sand can be used inside a 3D printer, but the use of this type of powder generates less optimal moulds. The analysis revealed that dimensional quality and roughness were worse for this type of technique compared to traditional techniques, due to the low powder compaction and the bleeding effect.
Another major issue is adhesive agents. Trombetta et al. [
29] have defined a print parameter called “binder level” as the percentage of the space without powder that is filled with binder. It is related to the saturation level and does not depend on the apparent density of the layer. This way, the optimum binder level can be obtained.
The mechanical properties and roughness of the part depend on the quantity of binder. An insufficient quantity of binder cannot join the powder strongly and the printing process will fail. On the other hand, an excessive quantity of binder can cause gross printed lines and/or bleeding effect. This phenomenon deteriorates dimensional accuracy [
30].
Bleeding effect is defined as the undesirable migration of the binder on a particle scale outside the intended geometry. Bleeding effect can be controlled by improving binder evaporation, however this can leave pockets in the structure, reducing the mechanical strength. Although the strength of the parts is expected to improve when increasing saturation, this is not always true [
31]. Therefore, the amount of binder must be handled carefully when a casting mould is manufactured. Excessive binder may create gases during the casting process that cause erosion in the mould structure or defects in the castings. Snelling et al. [
32] manufactured binder injection moulds, using ZCast
® and ExOne silica powder. They concluded that moulds created with ZCast
® material produced lower quality castings due to gas defects associated with a high level of saturation.
Binders such as furan are predominantly used in foundry applications, but they generate toxic and carcinogenic gases, harmful to operators and the environment. These aspects lead to important limitations, therefore it is necessary to investigate alternative materials for safer and environmentally friendly casting [
33]. Rodriguez et al. [
34] demonstrated that the manufacture of moulds using the BJ technique with alternative materials can be an option for the use of furan resins.
Vacuum Suction Casting (VSC) is commonly used to avoid gas entrapment defects caused by the binder evaporation or the mould calcination. This process consists of generating a negative pressure in the mould, capable of extracting or compressing the gas generated during the casting process.
Molten metal must remain in a liquid state long enough for the vacuum effect to act. In addition, the pressure difference operates as an injector that forces the metal into the mould, reducing the filling time and avoiding turbulences. Compared to conventional casting technique, vacuum suction casting can obtain high quality parts and improve the qualification rate.
One of the mainly used alloys in the casting industry are Al-Si alloys due to their good mechanical properties, low density and high corrosion resistance. In addition, the casting capacity of these alloys allows the manufacture of components with complex geometries. However, traditional casting techniques have metallurgical properties with imperfections, due to either gas entrapment, shrinkage or the casting technique itself. Some authors have studied the application of vacuum techniques to increase the properties of aluminium alloys. Szklarz et al. [
35] investigated the effect of vacuum on the microstructure and the electrochemical response for a specimen as-cast 2017 aluminium alloy. They concluded that by applying vacuum techniques, a more homogeneous microstructure and better corrosion resistance in chlorides is obtained. Liu et al. [
36] studied the filling behaviour for an A356 alloy as a function of the depressurization rate, and observed that a high depressurization rate caused the speed at the gate to remain unchanged.
Cao et al. [
37] investigated the effect of the T6 treatment on the microstructure and the mechanical properties of a die-cast Al-Si-Cu alloy with three degrees of vacuum. The same authors [
38] also studied the influence of different degrees of vacuum on AlSi9Cu3 alloy castings parts created by vacuum-assisted high-pressure injection (HPDC). In both studies, they concluded that as absolute pressure decreases, average porosity and pore sizes are reduced and tensile strength and elongation are significantly improved.
Vacuum suction is a technique commonly used in the industry to improve the properties of cast parts. The purpose of this research is to demonstrate the improvement of properties in moulds manufactured by AM, being the first known study in which the vacuum suction technique is applied to moulds manufactured by BJ. To demonstrate the improvements of the vacuum suction technique, two moulds were manufactured using the BJ technique and filled using the different techniques (gravity and vacuum assisted pouring). The casting parts were compared and it was possible to quantify the improvement that the vacuum offers to the casting parts and the feasibility of using vacuum processes in the BJ technique.
3. Results
3.1. Results of Simulation
The results of the simulation made it possible to know the velocity profile, to avoid a high turbulent flow of the liquid metal during the filling process. Campbell [
40] defined the maximum filling speed as 0.5 m/s; increasing this speed value can lead to gas entrainment and oxide films. The colour map in
Figure 7 shows the velocity profile of both moulds during the filling process. The vacuum-assisted technique has a more controlled filling, avoiding the possible defects of gas entrapment, inclusions or oxides.
The flow behaviour of the liquid metal inside the gravity-pouring mould was progressive (
Figure 7a), with moderate turbulence and a relatively homogeneous velocity profile. Although the filling velocity remained at values very close to 0.5 m/s at some points, this velocity was exceeded, creating waves in the early stages of the filling process. This phenomenon was due to small differences in the velocities at the gates. These differences were in turn imposed by the metallostatic pressure of the sprue.
The waves did not cease until the sprue was saturated with liquid metal. From that moment on, the metallostatic pressure acted uniformly on each gate. The waves disappeared to give way to a progressive filling as shown in
Figure 7a. In addition, the velocity in the gates area remained stable throughout the filling process, providing moderate turbulence that minimized the risk of gas entrainment. Finally, the filling time was 5.2 s.
The behaviour of the liquid flow for the vacuum-assisted mould was more stable and homogeneous (
Figure 7b). In the early stages of the filling process, the liquid entered the mould cavity at a high velocity; this was due to the vertical pouring configuration of the mould. The initial jet impact generated a turbulence, which was absorbed by the overflow. A slight wave effect was produced, but this effect was mitigated by the suction forces applied by the vacuum. Subsequently, a uniform filling process was obtained. The filling time in this case was 1.9 s.
The vacuum-assisted technique showed a more controlled filling, avoiding the possible defects of gas entrapment, inclusions or oxides.
The velocity profile of the gate cross-section during the filling process was obtained using the simulation software.
Figure 8 shows the different velocities at the gates during the filling process. This velocity was divided in two stages: (i) up to 15% of filling (the metallic liquid was not stabilized because the gate has not been saturated) and (ii) up to 100% of moulds filling (the metal liquid was stabilized at the gate).
For the gravity-pouring mould, in stage (i), a strong increase in velocity was observed, which descended rapidly, with the highest value being 0.56 m/s. The velocity fluctuations generated the waves previously described.
In contrast, the vacuum-assisted technique presented two phases within the first stage. In the first phase, a sudden increase in velocity was observed. The liquid entrance was placed at the top of the mould; therefore, the gravity force acted in favour of the filling. In the second phase, the filling cup was saturated with liquid metal and the velocity gradually increased to the optimum filling velocity.
Stage (ii) represents the filling process when the metal liquid was stabilized at the gate. As shown in
Figure 8, the gravity-pouring mould showed a filling with fluctuations over an average value of 0.48 m/s. The initial source effect dragged these fluctuations during the filling process. In addition, a damped harmonic state was observed in which the flow-generated waves decreased in amplitude as the filling process was completed. This is consistent with
Figure 7a, where waves generated by the source effect tended to decrease and the flow became more stable. High runner pressures and large areas in the cavity result in higher gate velocities [
49,
50]. Even so, only in specific cases did the average velocity exceed the critical value of 0.5 m/s. Therefore, there was low risk of gas entrapment.
For the stage (ii) in the vacuum-assisted technique, a constant gate velocity of 0.43 m/s was observed. The filling velocity only depends on the gravity force, the vacuum force and the gate area. These three parameters were constant during the process and therefore the velocity was constant in this second filling stage.
3.5. Porosity
Porosity of castings directly affects the mechanical properties of the castings. Large pores reduce the loading area and more easily induce crack propagation.
To evaluate the volumetric porosity of the parts, it was necessary to calculate the bulk density of the parts using the following expression [
38]:
where:
ρp and
ρw are the specimen density and water density, respectively;
m1 and
m2 are the mass of the specimen in air and water, respectively. Finally, the following expression was used to calculate the volumetric porosity:
where:
ρr is the actual density of aluminium ENAB46000 (2760 kg/m
3) according to EN1706 [
53].
Table 5 shows the volumetric porosity results for the different part zones in both techniques.
Volumetric porosity increased with the zone. This was due to the vertical filling in both moulds. The density of the bubbles entrapped was lower than the metallic liquid, so the bubbles ascended and accumulated in the upper part of the casting. The vacuum pressure reduced the porosity of the parts in each zone, resulting in an average porosity 2.212% lower than in the gravity-pouring part.
To analyse porosity in depth, a study of the number, size and shape of the pores was carried out.
Figure 12 shows the statistical distribution of the pores according to their size for each zone and location. Pore size was evaluated using Feret′s maximum diameter.
The following can be deduced: (i) the pore size is in general between 10 and 14 μm for the gravity-pouring casting and between 0 and 14 μm for the vacuum-assisted casting. Therefore, the vacuum pressure decreases the pore size. (ii) The number of pores diminished in all areas with the vacuum-assisted casting. (iii) For both parts, the number of pores increased in Zones II and III, as explained in the previous paragraph. On the other hand,
Figure 12 reveals that the number of pores increased on the outside of the parts regardless of the zone. The reason is that the gas entrapped by the metal liquid could not be evacuated before solidification. Preheating of the mould is recommended to avoid this phenomenon.
Figure 13 shows the pores’ morphology. Circularity of pores was calculated according to the following expression:
where:
C is the circularity of the pore;
a is the area occupied by the pore and
p is the perimeter of the pore.
Circularity in the vacuum-assisted part increased in all zones and locations. Therefore, it can be stated that when the size of the pores decreases, the circularity increases. In contrast, the larger pores had more irregular shapes, due to the defects associated with contraction by solidification, among others. The circularity of these particles was very low. Finally, the trend in morphology is very similar in all the analysed samples. For the gravity-pouring part, the circularity is grouped around 0.3; this indicates sharper and more irregular pores. However, for the vacuum-assisted part, the circularity tends to be higher (0.6), indicating more rounded pores.
Figure 14 shows the morphology of pores in every zone in the parts: (i) Pores with sharp corners associated with shrinkage (
Figure 14a); (ii) Oval-shaped pores with less sharp corners, resulting from a combination of shrinkage and gas entrapment (
Figure 14b) and (iii) rounded-shaped pores associated with gas entrapment (
Figure 14c). These three types of pores appeared in all specimens [
24,
34,
38].
Table 6 shows the summary of the porosity results for each technique, zone and location of specimens. The application of vacuum results in a significant reduction in the average pore size (47.3%). Meanwhile, the maximum diameter, the minimum diameter and the number of pores also decrease with the application of vacuum.
Large pores (>55 μm) have very irregular shapes and a random distribution. The number of large pores decreased up to 78% for the vacuum-assisted part, demonstrating that the application of vacuum pressure significantly reduces the size of the large pores. However, it was not sufficient to eliminate small pores (<55 μm).
The combination of volumetric porosity and average pore size results shows that the gravity-poured part (4.03% and 22.82 μm) is significantly larger than the vacuum assisted part (1.82% and 12.85 μm).
A high degree of vacuum during the casting is beneficial for reducing the entrapment of gases in the molten metal. Therefore, the choice of the correct value can significantly reduce the gas porosity and improve the mechanical properties of the casting part.