3D printing as an additive manufacturing technique continues to be classified as an innovative manufacturing method, although this is also debatable. First, let’s go back to the roots of additive manufacturing techniques. The official date of birth of 3D printing is August 8, 1984 (I was after the 4th grade of primary school at that time). On that date, Charles Hull filed his patent application and did not stop there. Charles Hull founded the first company to produce 3D printers – 3D Systems (Currently, 3D Systems cooperates very closely with GF Machining Solutions). Previously, he developed and implemented the first official 3D printing method – Stereolithography (SLA). He was not the first to patent 3D printing. Three weeks earlier, three French people, namely Alain Le Méhauté, Olivier de Witte and Jean Claude André, had filed their patent application, but their employers rejected this application, considering the described additive manufacturing technique as not future-proof. The application was eventually rejected .
The roots of 3D printing go back to the beginning of the seventies of the last century. In 1971, the French Pierre A. L. Ciraud described a method of producing parts of any geometry by adding material in the form of a powder using an energy source. The article describing this method was published on July 5, 1973 (I was born this month) and is considered the foundation of the know-how for the 3D printing method known today as SLS (Selectve Laser Sintering – Selective Laser Sintering) .
Although 3D printing, especially from metals (Fig. 1), has become widespread in the last 10 years, as such a manufacturing technique is not a new technology, but it already has a history. We owe the popularization of 3D printing to the qualitative aspect of technological progress. It is not only a matter of the construction of the printers and laser heads themselves, but also of atomizers for the production of metal powders. Thanks to extensive development work, today we have at our disposal a number of industrial methods of 3D printing from metals.
3D printing – advantages and limitations
Despite many years of history, we are still at the stage of discovering the possibilities of 3D printing from metals. The implementation of 3D metal printing usually requires many test prints to determine the properties of such a part, and thus the production capacity. In the context of metal 3D printing, the following advantages are mentioned:
- The possibility of producing objects with very complex shapes, the production of which using conventional manufacturing techniques (decrement techniques, molding, plastic working) is very difficult or even impossible [2, 4, 5].
- No waste compared to other manufacturing techniques: casting and especially machining. Metal powder not used in 3D printing can be reused. This allows for the reduction of printing costs, which are still high .
- The developed 3D model for printing can be used in any location and on various printers .
- The ability to manufacture ready-made components of small mechanisms.
- Very diverse fields of application – various industries: aviation, automotive, medical, biomedical, artistic.
Until the SARS CoV-2 coronavirus pandemic, the leader of 3D printing was the aviation industry, despite the formal issues with putting into use parts manufactured using 3D printing. Each part for the aviation industry must undergo certification tests. An important aspect conducive to the use of 3D printing in the aviation industry is the production scale, which contributes to the economic justification of the implementation of additive manufacturing techniques. The time-consuming process of 3D printing is not a disadvantage as in the case of the automotive industry.
In the context of 3D printing, the lack of the need for high qualifications on the part of users and operators is also mentioned as an advantage. This is debatable for me. First of all, the issue of designing, as it were, for the use of 3D printing, discussed below. It takes design competence and the ability to jump to an entirely different approach. The planning of metal 3D printing itself is also a bit of a challenge. It turns out that the orientation in the space of the printed part affects its mechanical properties (eg tensile strength) [1, 4]. So, can we talk about the lack of the need for high-level technologists?
Determining the technological capabilities of 3D printing as part of the implementation analysis is not enough to determine whether it is justified or not. Efficiency has a significant impact in this respect, and this, compared to conventional machining methods, leaves much to be desired. The specificity of designing parts for 3D printing should also be taken into account (figures 2 and 3).
In figure 2, on the left, a part of the car steering system machined from a casting as a semi-finished product, and on the right, the same component designed for 3D printing from metal (AlSi10Mg alloy), the weight of which is much lower, while maintaining performance and durability properties. The printout took 1 day, 23 hours and 16 minutes, which took much longer than in the case of traditional techniques.
Figure 3 shows very well the differences between designing parts for technological processes using machining, casting or plastic working, and designing for 3D printing. The printed part (fig. 3 – right) has the same or even better strength properties but is significantly lighter. According to the authors , it is this difference in the approach to design that is one of the limitations. As a result of didactic processes in technical schools and technical colleges, as engineers we are used to designing with subtractive methods or even subconsciously ingrained in mind. In 3D printing , we create a spatial object layer by layer on the basis of a 3D model, whether solid or surface, developed in CAD software.
According to , the disadvantages include the legal issues related to the possibility of manufacturing, at home, parts subject to restrictions, such as the so-called essential elements of the weapon. Some countries, such as the USA, are trying to introduce specific regulations in this regard. From a technological point of view, a serious disadvantage is worse quality factors, i.e. geometric accuracy and surface roughness. It is especially visible in relation to analogous parts machined with the use of subtractive machining.
In terms of quality, it should be assumed that only in some cases it is possible to manufacture the final item without further subtractive processing. The dominant rule is that the object produced by 3D printing from metal must be heat treated after the printing process, and then the important surfaces must be treated with the use of subtractive methods (e.g. machining, EDM, grinding).
A certain number of commercially available metal powders can be considered as a limitation. Although the methods of producing metal powders are also evolving and are becoming more and more available, they must meet very high requirements. It is very important to maintain the specified size and spherical shape of the metallic powder granules.
Metal 3D printing – industrial methods
In this scope, the following methods of 3D printing from metals are considered key:
- DMLS (Direct Metal Laser Sintering);
- EBM (Electro Beam Melting) – an electron beam is used to melt the powder;
- SLS (Selective Laser Sintering);
- SLM (Selective Laser Melting) – selective remelting / fusing of powder materials.
All these methods use various metal powders / granules as the base material. The working table on which the printout is made moves vertically in order to maintain a constant distance of the metallic powder layer from the energy source (laser head). In the first step, a layer of powder is applied to the surface of the table. In the next step, the laser beam scans the 2D contour of the printed object for a given material layer and melts or sintering (joining together) metal powder particles. The table then moves down one layer thickness. Another layer of metal powder is applied (fig. 4). The cycle repeats until the item synthesis is completed. In the case of DMLS and SLM methods, we can talk about technological identity, and the differences in names result from patent restrictions and trademark protection.
According to the authors , 3D printing with the DMLS method is an alternative to casting. At the same time, there is no need to design the form and make it. There is no need to clean the mold after each casting or the need to make another one (depends on the casting method). Contamination may occur in the cast part in the material as opposed to the printed part. In both DMLS and SLM, the working chamber can be filled with inactive gas such as nitrogen or argon [5, 8]. The use of an inert gas avoids the formation of oxides that can adversely affect the mechanical and functional properties of the printed part. Figure 5 shows a 3D printer using the DMLS method produced by GF Machining Solutions – the DMP Flex 350 model, which can be used both in R&D and in production.
In the case of the EBM method, an electron beam is used as the energy source, which requires the use of vacuum chambers, which also helps to avoid material degradation under the influence of the atmospheric environment. The electron beam itself is characterized by high energy density and high energy efficiency. The beam deflection takes place with the use of electromagnetic lenses. Compared to other methods, the EBM method is characterized by a higher scanning speed, better positioning accuracy and a high construction rate, even up to 60 cm³ / h. Objects printed using the EBM method are characterized by a very high density .
3D printing – comparison with other manufacturing techniques
The authors  compared the mechanical properties of Ti6Al4V titanium alloy samples made with the metallurgical method (rolled rod) and the 3D printed one. The rolled bar is characterized by a much lower roughness than the printed sample. The results of the tensile tests showed that the titanium elements produced by the SLM method are characterized by higher values of strength and yield point than elements made with the conventional method. The maximum tensile strength value for the 3D printed sample was 1360 MPa, and its yield point was 1225.7 MPa. In turn, the highest values of these parameters for the sample made of rolled bars were respectively 1126 MPa and 1107 MPa. The obtained data show that the samples made in 3D printing have lower elongation values than those made of a rolled bar. Based on the literature, it can be concluded that the higher the strength parameters, the lower the elongation values. The authors showed that the printed samples have better properties, and the surface roughness of the 3D-printed sample has no effect on the tensile strength. Analyzing other literature items [1, 3, 4], the very orientation of 3D printed samples in the working space has an impact on the mechanical properties.
The authors of the works [1, 3, 4] conducted research on various materials: stainless steel, martensitic aged nickel steel, inconel 718. All these studies confirmed the influence of the orientation of the printed part on its mechanical properties. In , the authors took up the issue of the corrosivity of 3D printed parts subjected to the influence of the marine environment. The 3D printed structural part works in various environmental conditions. One can risk the thesis that the porosity of the printed parts favors the occurrence of corrosion.
As part of the study , the authors printed samples in three positions (vertical, 45 °, horizontal). Then they were subjected to heat treatment at the temperatures of 490ºC, 600ºC and 900ºC. In the next stage, the samples were exposed to salt spray. Of course, one set of samples served as a reference for the rest. The influence of the aggressive environment of the salt mist affected the microstructure and composition (in the surface layer – according to the author). The samples printed horizontally and at an angle of 45° showed higher tensile strength than the samples printed vertically.
In the case of all samples, regardless of the printing direction, subjected to heat treatment at the temperatures of 490ºC and 600ºC, there was a significant decrease in corrosion resistance. During the tests, it was shown that the 3D printed samples showed a lower decrease in tensile strength than the samples mechanically processed from a metallurgical blank. The thesis was confirmed that the printed samples were more susceptible to pitting corrosion.
In the paper  the authors described the research on the mechanical properties of samples printed by DMLS from Inconel 718. This material is used primarily in the aviation industry. Due to their application (e.g. turbojet engines), components made of this material must be characterized by high strength at high operating temperatures, corrosion resistance, low thermal conductivity, high hardness, and a high level of reliability. The authors put together 3D printed, forged and cast samples. As in the article , the authors found the influence of the orientation of the position of the printed part (here the tensile sample) on its mechanical properties, including tensile strength (fig. 6).
The authors  printed samples in 5 positions – three samples in the XY plane (along the X – P0X axis, along the Y – P0Y axis and at an angle of 45º – P0XY), one sample in the axis perpendicular to the XY table plane (sample printed vertically – P90) and a sample tilted at an angle of 45° in all axes – P45XYZ.
After the printing process, 3D printed samples were of course subjected to heat treatment, which is aimed at neutralizing the phases in a state of imbalance and minimizing thermal stresses. The final shape and surface roughness of the printed samples were obtained by machining.
Test results indicated that, overall, printed parts have comparable or better mechanical properties than forged or cast parts. This is because the grain microstructure in the printed parts is finer. The authors made the effort to verify the properties of the samples at the temperatures at which, as a rule, parts of this material work (450 ºC and 650 ºC). All samples printed in the 0_X direction, i.e. horizontally with an axis parallel to the direction of the powder feeder, achieved better results in terms of tensile strength. The tensile strength values ranged from 1440 to 1475 MPa, and the yield strength from 1253 to 1278 MPa. According to other studies, samples printed in the Z (vertical) direction have a tensile strength lower by about 80 MPa and the yield point also lower, by about 60 MPa . Additionally, all samples printed horizontally in the X direction showed very similar deformation results. However, for the sample printed vertically in the Z direction, the measurements showed large differences.
In the article , the authors chose the commonly used 316L stainless steel, which is designated as 1.4404 in accordance with the PN-EN 10027-2 standard. This steel is a corrosion-resistant iron-based alloy and belongs to the group of austenitic steels. This steel has a high elongation capacity and is characterized by being non-magnetic. When using a layered structure, components made of this steel have a certain level of anisotropy, which is reflected in the mechanical properties. The chromium contained in the steel reacts with oxygen and forms a layer invisible to the naked eye on the surface. If this layer is damaged mechanically, it rebuilds automatically. Chromium reacts again with the oxygen in the air. However, it is not recommended to heat treatment of this steel in the temperature range from 427 °C to 816 °C. The consequence is the precipitation of chromium carbide, which in turn leads to the occurrence of intergranular corrosion (decrease in strength and ductility of the material). This steel is suitable for hot forging in the temperature range from 900 °C to 1200 °C. After forging, the steel is cooled in the air.
The stainless steel samples were printed at four different angular positions 0°, 30°, 60° and 90°. Tensile tests confirmed that horizontally printed samples (0° angle) are characterized by the highest yield point and tensile strength. For all 3D printed samples, regardless of the angular orientation of the print, higher yield strengths were demonstrated than for forged samples. The highest fatigue strength was obtained for a sample printed at an angle of 60°. The greatest decrease in fatigue strength occurred for a vertically printed sample (90° angle) .
The authors  focused their attention on the microstructure of the printed samples. It was found, inter alia, what follows:
- The print angle has no significant influence on the average grain value of the microstructure.
- The samples printed at an angle of 60 ° had larger grains and higher tensile strength than the samples printed at an angle of 30°.
- 3D printed specimens were found to be as reliable as forged specimens with a simultaneous higher yield point. However, the highest value of the tensile strength of 3D-printed samples turned out to be lower than that of forged samples.
- The fatigue strength of 3D printed samples is much higher than for forged samples.
- During the tear tests, all samples were damaged as a result of ductile fracture. In the case of printed samples, the presence of incompletely melted grains was found, which played the role of inclusions constituting the nucleus of cracking.
- In the case of a vertically printed sample (angle of 90°), it was found that the lower values of the yield point, tensile strength and fatigue strength result from the parallelism of the stress plane to the layered structure. This sample had sequential delamination.
A lot of similar research has been carried out for various materials, for various metals and their alloys. The selected research papers briefly presented above indicate the enormous potential of 3D printing in the context of mechanical and functional properties. Printed metal components require heat treatment and, in most cases, machining to give the required geometric accuracy and roughness to the essential surfaces. 3D printing of metals against the background of casting, forging, drawing and machining is very promising.
- Ansell T.Y., Ricks J.P., Park C., Tipper C.S., Luhrs C.C., Mechanical properties of 3D-printed maraging steel induced by enviromental exposure, Metals 2020, 10, 218
- Karolewska K., Ligaj B., Comparison analysis of titanium alloy Ti6Al4V produced by metallurgical and 3D printing method, AIP Conference Proceedings 2077, 020025, 2019
- Daňa M., Zetková I., Mach J., Mechanical properties of inconel alloy 718 produced by 3D printing using DMLS, Manufacturing Technology Vol. 18, No. 4, 2018
- Penn R., 3D printing of 316L stainless steel and its effect on microstructure and mechanical properties. Masters of Science in Metallurgical Engineering and Mineral Processing, Montana Tech Library 2017
- Fousová M., Vojtěch D., Kubásek J., Dvorský D., Machová M., 3D printing as an alternative to casting, forging and machining technologies?, Manufacturing Technology, listopad 2015
- Pîrjan A., Petroşanu D-M., The impact of 3D printing technology on the society and economy, Journal of Information System & Operations Management Vol. 7, No.2, 2013
- HISTORIA DRUKU 3D – CZĘŚĆ 1: jak stary jest druk 3D, kto naprawdę jest jego twórcą oraz kto wymyślił jego nazwę?
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Born 1973. In 1993, I graduated from Technical Secondary School No. 1. In 1998, the Faculty of Mechanical Engineering and Automation (now Faculty of Production Engineering) - Warsaw University of Technology. 1997-2000 cutting tools manufacturer at VIS Precise Products Factory S.A. 2004. Unfortunately, this company no longer exists. PhD in gear technology. Production technologies and technological processes are my passion.
8 May 2020
28 June 2019
16 January 2019
6 January 2019