Metal 3D Printing: The advantages of 3D printing metals

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Henrik Metsäranta

METAL 3D PRINTING

The advantages of 3D printing metals

Faculty of Engineering and Natural Sciences Bachelor’s Thesis May 2021

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ABSTRACT

Henrik Metsäranta: Metal 3D Printing Bachelor’s Thesis

Tampere University

Bachelor's Programme in Engineering Sciences May 2021

This thesis explores metal 3D printing technologies, its advantages, and its applications. The most popular methods of 3D printing metallic materials are powder bed fusion (PBF) methods, direct energy deposition (DED), and binder jetting. The text discusses the main differences and similarities of these 3D printing methods. Direct metal laser deposition (DMLS) and selective laser melting (SLM) use a laser to sinter and melt metal powder, respectively. Electron beam melting (EBM) uses an electron beam to melt metal powder, and it is mainly used to process brittle metals.

DED deposits and melts the metal simultaneously; it can use powder or wire as the feed material.

Finally, binder jetting consists of fusing layers of metal powder together with a binder material.

Binder jetting requires lots of post processing but is very cost efficient.

The main advantages of metal 3D printing are the almost unlimited design possibilities and one-step manufacturing. Being able to design a component with a computer, then directly manufacture the component with a 3D printer has led to rapid prototyping. Miniature and fully functional prototypes can be produced in a matter of days. The freedom of design has led to the production of newly designed parts. This includes parts with entirely new design, as well as parts, which were previously assembled from several components. Many components are being redesigned, espe- cially to achieve a light weight.

Metal 3D printing is being utilized mostly by the medical and automotive industries. The medical industry uses it to produce customized implants and new medical tools. The automotive industry uses it for prototyping, producing light weight components, and producing rare spare parts.

In general, metal 3D printing is used to produce expensive, specialized, and low-volume parts. In the future, metal 3D printing may become less expensive, which can open new possibilities. How- ever, due to the high cost, metal 3D printing will not replace most conventional methods of production.

Keywords: Metal 3D printing, design, applications

The originality of this thesis has been checked using the Turnitin OriginalityCheck service.

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LIST OF FIGURES

Figure 1. Basic principle of modern 3D printing. [2] ... 2

Figure 2. 3D printing market size and future predictions. [4] [adapted from 3D Hubs, The 3D Printing Trends Report Q1 2019] ... 3

Figure 3. A visualization of a STL file. [6] ... 5

Figure 4. Schematic diagram of a simple powder bed delivery system. [11] ... 7

Figure 5. Comparison of microstructures of SLS and SLM produced parts. [12] ... 8

Figure 6. Schematic diagram of an EBM system. [17] ... 11

Figure 7. Schematic of a binder jetting system [18]. ... 12

Figure 8. Schematic of a DED system [8]... 14

Figure 9. DMLS produced satellite antenna. [24] ... 16

Figure 10. Scanning electron microscope picture of 316L stainless steel: (a) fresh powder, (b) 16 times recycled powder. ... 21

Figure 11. 3D printed orthopaedic hip replacement. [31] ... 22

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LIST OF SYMBOLS AND ABBREVIATIONS

AM Additive manufacturing

CAD Computer aided design

DED Direct energy deposition

DMLS Direct metal laser sintering

EBM Electron beam melting

PBF Powder bed fusion

RP Rapid prototyping

SLM Selective laser melting

SLS Selective laser sintering

STL Standard triangle language

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1. INTRODUCTION

Additive manufacturing (AM), rapid prototyping (RP) and 3D printing are terms, which describe additive forms of production. They are often used interchangea- bly, and even though they are similar terms, they do differ. AM is a broad term, and covers a large variety of technological methods, including RP and 3D printing. AM covers the construction of any physical 3D objects by consecutively add- ing layers of material on top of each other, without a physical model to build on.

Technically, it could be as simple as building a brick wall. First, the brick wall is designed in a way that it will endure, then it is built from a stable base to a complete wall. Simple AM methods can be traced far back in history due to the broad- ness of the term. What makes AM unique is the initial approach in production.

Rather than using subtractive methods of production, or using a mold, AM methods construct the product directly, without molding or subtracting material.

This approach to manufacturing has provided exciting new opportunities for designing and fabricating parts. Some of the most innovational aspects of AM is producing 3D printed organs for humans. A 3D printed bladder has even been successfully transplanted. Although 3D printing is providing possibilities for producing new materials and objects, it is also changing the way current designs are being changed. Metals are a group of materials, which is often overlooked when considering AM. This text has a look at how and why metal 3D printing is being utilized in the world. What are the advantages of 3D printing metals? If the advantages are big, could 3D printing replace conventional methods of production in the future? These are questions that this text will attempt to answer by looking into methods of 3D printing metals, the design and mechanical aspects, and the current applications of 3D printed metals.

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2. ADDITIVE MANUFACTURING

The concept of a modern additive layer by layer approach is shown in figure 1, which contains an illustration of a system with a laser power supply. This is an example of a 3D printing method. Material is added in thin layers and is then melted using a laser beam. The material is added layer by layer to form the shape going from rectangular base on the left to a more complex end shape on the right.

Figure 1 contains a somewhat simple shaped object in construction; however, it illustrates the basic concept of 3D printing extremely well. The same concept ap- plies to more complex objects.

Figure 1. Basic principle of modern 3D printing. [1]

Modern AM methods are usually referred to as RP or 3D printing. They consist of fabricating a physical object from a virtual design. The virtual model is first created on a computer, then it is built using a flexible automated system. The difference between RP and 3D printing is very small, and the terms can often be used synonymously. RP was initially a method of producing prototypes for products in a fast manner, so that large investments in producing prototypes could be avoided. Even though RP machines were expensive, it was usually cheaper to produce a prototype using RP technology rather than invest in a full line of production to produce the prototype. RP technology was exceedingly expensive, but as the technology further developed over time and prices decreased, the term 3D printer was born. 3D printing is essentially focused on using cheaper machines

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with more limited functionality than RP. 3D printing is considered to be a production method, whereas RP is only applicable for constructing prototypes. Nowa- days, RP has come so far that 3D printers are affordable for private citizens. In In the past, AM technology was used by top-level scientists, researchers, and corporate research and development departments. [2]

Manufacturers like Phrozen, Creality and Monoprice provide budget 3D printers for private citizens, with prices below 500 USD. This shows that the 3D printing market has surpassed just industrial uses and is also targeting civilians.

The 3D printing market is growing extremely fast. According to the 3D printing trends report of 2019, the market is showing exponential growth, as shown in figure 2. The report includes predictions for the next few years, as well as dis- playing an average annual growth of 24.7 %. This indicates that the 3D printing market has shown steady growth and is extremely likely to keep growing at a fast rate. [3]

Figure 2. 3D printing market size and future predictions. [3] [adapted from 3D Hubs, The 3D Printing Trends Report Q1 2019]

It is important to note that figure 2 represents the entire 3D printing market, whereas this text will discuss 3D printing of metals. However, it is safe to assume that 3D printing of metals also shows steady growth.

The process of 3D printing essentially consists of the following five steps. [2]

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1. Creating a computer aided design (CAD) model of the desirable object.

2. Converting the CAD model into an STL file.

3. Slicing the STL file into two-dimensional layers.

4. Constructing the object using an AM method.

5. Post-processing.

CAD involves creating a model of the desired object using computer software.

Initially, CAD was limited to a 2D design, and was basically just a drawing board used to eliminate repetitive drawing processes. However, nowadays, CAD almost always includes a 3D model, as well as a 2D model for convenience. Many different CAD software exist, which can be used to create the virtual model, but they all include similar steps in designing the object.

The CAD model must be converted into an STL file. STL is a file type developed by 3D Systems, which was the first company to bring AM technology to the market [4]. The file type “STL” was named after the stereolithography technology used in additive manufacturing. However, backronyms have been invented such as “standard triangle language”. Since then, the STL file format has been made publicly available and is the standard file format used in CAD software. It utilizes triangles to describe the CAD model. In figure 3, an object has been converted to STL format. Each triangle contains coordinates for the three points, as well as a vector describing the outward side of the triangle [4]. Information about each triangle is stored in the STL file, which can be read by AM machines. Before manufacturing, the STL file is sliced into 2D layers, viewed from the top, so that the file is easier to use for printing.

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Figure 3. A visualization of a STL file. [5]

Once the STL file is sliced, the object can be constructed using an AM method.

Usually, the constructed object will require some type of post-processing such as heat-treatment or machining. However, if the object is used for RP, it will most likely not need to be processed.

Now that the basic principles and terminology of 3D printing have been explained, different 3D printing technologies will be explored. Specifically, this text will ex- amine the most common 3D printing methods for metal materials, their advantages and disadvantages, and their applications.

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3. METAL 3D PRINTING METHODS

Metal 3D printing technologies are constantly developing, including new innova- tions, which continue to shape how 3D printing of metals is approached. Cur- rently, the most popular methods of 3D printing metals are powder bed fusion methods, direct energy deposition, and binder jetting [6]. Other AM methods for metals include metal extrusion, sheet lamination and ultrasonic AM. Each of these methods contain unique advantages and disadvantages. In most cases, the advantages are specific to certain industries and products, which is why metal 3D printing is already utilized in several industries [7]. In some cases, it has re- placed traditional methods of production entirely. Whereas others have adapted 3D printing to complement their production strategies. Powder bed fusion (PBF) methods are the most widely utilized methods for 3D printing of metals [6]. This text will explore different PBF methods, as well as binder jetting and direct energy deposition. We will discuss the basic concepts, similarities and differences between the technologies, as well as the advantages and disadvantages of each method.

3.1 Powder Bed Fusion Methods

PBF is a 3D printing method consisting of several subcategories. PBF works by selectively melting or sintering metallic powder using a high-power energy source [8]. The most common PBF methods are selective laser melting (SLM), direct metal laser sintering (DLMS) and electron beam melting (EBM).

PBF methods utilize a powder bed. A powder bed system uses powdered material as the raw material. Figure 4 contains a schematic diagram of a powder bed system. The system uses a roller or a scraper to deliver the powder onto the powder bed. The laser or electron beam then sinters or melts only a selected part of the powder layer. This process of selective sintering or melting is repeated layer by layer to construct the final component. As a result, the constructed component is surrounded by excess metal powder, which has not been melted. The component is shown in figure 1 with solid black, surrounded by grey dots repre- senting unused metal powder. This unused powder can be reused to a certain

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extent; however, it should be used with caution because the material is not the same as newly produced powder. [9] Powder surrounding the constructed component will also absorb heat from the energy system. As a result, it will undergo microscopic changes, which can affect the properties of the powder. Therefore, the resulting powder is not the same as newly acquired powder from a manufac- turer.

Figure 4. Schematic diagram of a simple powder bed delivery system. [10]

3.1.1 Direct Metal Laser Sintering (DMLS) and Selective Laser Melting (SLM)

Selective laser sintering (SLS) is a 3D printing method used for a variety of materials, including metals. However, for metals, the method is called direct metal laser sintering (DMLS). DMLS is a name created by the brand EOS and has become widely used in the metal 3D printing industry. DMLS and selective laser melting (SLM) are very similar methods of metal 3D printing. DMLS uses a laser to heat the metal powder to a certain temperature, which allows the grains to fuse together via sintering. In contrast, SLM uses higher temperatures to completely melt and fuse the powder grains together. Figure 5 displays the differences in microstructures of DMLS and SLM produced metal components. The sintered part shows an interconnected metal structure, but contains voids, because the metal powder grains were not completely melted. The melted part shows a more uniform microstructure without empty spaces between grains. As a result, the

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structure of the sintered part is porous and does not have as strong mechanical properties. For example, parts produced via sintering exhibit a lower Young’s modulus and tensile strength than parts produced with SLM. Therefore, DMLS is not used as much and SLM has become the more prominent method. However, sintering is still used for many applications, which do not have such strong mechanical requirements. An advantage of sintering is that parts can be easily manufactured from alloys, which contain materials with different melting points. Ma- terials with high melting points can be easily added to the alloy because the material does not need to be heated to the melting point.

Figure 5. Comparison of microstructures of SLS and SLM produced parts. [11]

The SLM process consists of depositing a thin layer of the metallic powder onto the substrate plate or the previously applied layer of powder. The layer thickness varies depending on the desired results, but typical thickness for one layer is 20 µm – 100 µm [7]. The thickness of the layer will affect the melting of the grains, therefore affecting how much power is required to melt the metal powder. The thickness of the layers will also influence the mechanical properties of the final product. For example, parts constructed with a 30 µm layer thickness show higher strength and lower elongation at the breaking point than those built with a 50 µm layer thickness [12]. After applying a layer of powder, the laser selectively melts the desired parts of the metal powder particles and fuses them together. The laser beam power varies with a range from 20 W – 1 kW [7]. The required power depends on the melting point of the material, powder size and layer thickness.

The data for the areas of metal powder, which are to be melted is obtained from the sliced STL file of the designed part. The STL file is utilized so that the 3D printing system has sufficient information to melt the desired areas, which form

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the layer. The entire process is performed in a pressurized inert gas environment, such as nitrogen or argon [6]. At high temperatures, metals can undergo oxidation with relative ease. Therefore, inert gas is used so that the metal does not react with oxygen or other gases in the air. The choice of gas depends on the metal material, as different metals can react differently in the same environment.

Another factor, which can be controlled is the temperature of the substrate plate.

The substrate plate may be heated to somewhere between 200 ⁰C – 500 ⁰C [6].

The temperature of the substrate plate will affect the cooling rate of the material, ultimately affecting the microstructure. Therefore, the substrate plate is heated if a lower cooling rate is desired. This could be in the case of brittle, high temperature materials, that would otherwise crack during faster cooling.

SLM is one of the most adaptable PBF methods due to its large variety of materials available for use. SLM can utilize aluminium, titanium, iron, nickel, cobalt, and copper alloys, as well as their composites [6]. Mechanical properties can also be slightly modified by fine tuning different parameters [13]. For example, SLM can achieve a high cooling rate. A higher cooling rate results in smaller grain size and less material phases present in the product. Materials with a smaller grain size generally have superior hardness, tensile strength and elongation when compared to a larger grain size.

The largest advantages of SLM are the large variety of materials available, and the ability to fine tune mechanical properties. However, SLM is a slow and time- consuming process due to the process itself, as well as time spent optimizing the parameters. To combat the slow process, two lasers can be used simultaneously to increase the melting speed. The scanning speed can also be increased to a certain extent; however, increasing the scanning speed too much can result in insufficient melting. Therefore, laser power, layer thickness and scanning speed are carefully optimized to ensure sufficient melting, without slowing down the process too much. Usually, SLM produced parts will also require post-processing.

Heat treatment can be used to alter the microstructure of the product. For example, surface hardness, strength and ductility can be increased using heat treatment methods. Usually, the surface of the part also requires post-processing. Any

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un-melted powder must be removed and cleaned from the surface. This can be done either mechanically or chemically. One factor affecting the surface quality of SLM produced parts is the particle size of the metal powder [6]. SLM uses relatively large powder particles (30-120 µm), which results in a rough surface.

Smaller particle sizes could be used; however, that would affect the flow of the metal powder in a way that the layers are not spread in a uniform manner [14].

Layer thickness also affects the surface roughness. A smaller layer thickness produces a finer surface roughness. Therefore, parts produced with a larger layer thickness will require more extensive post-processing. The powder, which is not used in the final product can possibly be reused, depending on much it has been altered during the SLM process [15]. The material can be recycled; however, changes in microstructure may occur in the powder rendering it unusable. If the powder has begun to melt, it should not be recycled. If the temperature of the powder has changed only slightly during the process, it can be reused.

In practice, constructing a part, which does not contain any defects, is difficult.

SLM is a delicate process, which contains many sensitive parameters. All the parameters must be carefully controlled, to ensure a high-quality product. Poorly controlled parameters can lead to failing during the process or a low-quality product.

3.1.2 Electron Beam Melting (EBM)

EBM is a 3D printing method which is comparable to SLM. The main difference is that while SLM uses a laser to melt and fuse the metal powder, EBM utilizes an electron beam as its power source. The process, however, is similar. EBM also uses a powder bed system and melts the metal powder. A simple schematic of an EBM system is displayed in figure 6. The rake delivers the powder from the powder hopper to the powder bed. The electron beam then selectively melts parts of the powder bed to create the desired object. This schematic also shows a heat shield, which prevents heat from the electron beam and the melted material from escaping. As a result, the cooling rate of EBM is lower than with SLM. The powder bed stays at temperatures usually above 870 K [6]. This is to ensure sufficient melting, and to achieve a slow cooling rate. After the build is complete, it is usually left to cool overnight because of the slow cooling rate [6]. Due to the slow cooling

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rate, the process is slow and therefore very expensive. Another difference is that the EBM process takes place in a vacuum chamber. This is because the electrons would lose their kinetic energy if they collided with the particles in the air. SLM does not need to be completed in vacuum because the laser does not lose its power while travelling through air. As a result of the vacuum, there is no risk of oxidation.

Figure 6. Schematic diagram of an EBM system. [16]

EBM has more process parameters than SLM. Additional parameters include beam power, beam focus, beam diameter, beam line spacing, plate temperature, pre-heat temperature, contour strategies, and scan strategy [6]. The optimization of EBM parameters is more difficult when compared to SLM because there are more parameters to optimize [6]. As a result, fewer materials are used with EBM.

Alloys with readily volatile components like Zn, Mg, Pb and Bi should be avoided because they can evaporate during the printing process [6]. Some materials that can be used with EBM are Ti grade 2, Ti6Al4V, Inconel 718 and CoCrMo.

An advantage of the slow cooling rate is that EBM can employ brittle materials much better than SLM. Brittle materials are generally to be avoided with SLM

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because of the high cooling rate. Brittle materials usually have poor thermal ex- pansion behavior, i.e., when they are cooled rapidly, the internal stress from so- lidification may cause cracks in the material. [6]

3.2 Binder Jetting

Binder jetting is a quite unique method of 3D printing because it does not use heat to melt the material. Instead, it uses liquid binder agent to bind the layers of material together. Figure 7 contains a schematic of a binder jetting system. The system utilizes a powder bed system, like PBF methods. Once a layer of material is delivered, the inkjet printhead spreads liquid binder selectively. Next, the build platform moves down, and the next layer is applied.

Figure 7. Schematic of a binder jetting system [17].

Parameters and materials

The binder jetting process allows a good choice of materials. It can handle Al-, Cu-, Fe-, Ni-, and Co alloys as well as ceramic materials like glass, sand, and graphite [6]. In theory almost any material can be used because the powder layers are connected using a binding agent rather than with heat. Unless the material reacts with the binding agent, there should be no problem.

Parts produced with binder jetting often have a porous structure because the powder is not melted. Like DMLS produced parts, they have inferior mechanical properties when compared to SLM and EBM produced parts. Therefore, binder

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jetting cannot be used to produce parts with strong mechanical requirements.

However, the mechanical properties of a part can be modified by adjusting the ratio of powder to liquid binder. The amount of powder material and binding agent will affect how the powder fuses together. More binding agent will cause the layers to stick together better but will result in a more porous structure. Coarse powders can be used with binder jetting because the powder does not need to be melted. Coarse powder is cheaper to produce than fine powder. Therefore, using coarse powder will reduce the cost of raw materials. Additionally, around 95% of the unused powder in the powder bed is recycled [18]. Overall, binder jetting is a very cost-effective method of 3D printing because of the cheaper raw materials, fast printing speed and low power requirements. [6]

One of the main advantages of binder jetting is the printing speed. Parts can be manufactured faster because it does not melt the product and requires no cooling.

However, parts produced with binder jetting require lots of post-processing. After printing, the parts need curing, de-powdering, sintering, infiltration, annealing, and finishing [6]. The post-processing can take longer than the printing process and is dependent on the material in question. The binding agent is typically a wax, such as carnauba, paraffin, or a special polyethylene wax [19]. The binding agent can be burned off during sintering. Once the produced part is sintered and the binding agent removed, we are left with an interconnected structure similar to that of DMLS. If improved mechanical properties are desired, infiltration can be employed to fill the voids in the structure. For example, liquid bronze can be used to infiltrate the voids [19]. This increases the density and mechanical properties of the final product.

3.3 Direct Energy Deposition

Direct energy deposition (DED) is a 3D printing method, which simultaneously melts the surface of the part and delivers more powder material. A simple DED system is shown in figure 8. The laser passes through a lens and the deposition head. At the same time powder is being supplied by the deposition head. The powder is delivered using a pressurized inert carrier gas like nitrogen or argon.

At the deposition head, the powder is melted by the laser beam and directly deposited onto the build plate or previous layer. Powder feed DED is performed in

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an inert gas environment to prevent the oxidation of the metal. The gas in the chamber is the same gas used for delivering the metallic powder.

Figure 8. Schematic of a DED system [7].

A wire can also be used as the feed stock instead of powder. The wire is fed to the deposition head, where it is melted by the laser beam and deposited on the surface. The process is performed in a pressurized inert gas environment or under vacuum. Wire feed DED is essentially computer-controlled welding. It is clas- sified as a 3D printing method because it is a modern additive manufacturing method. Using a wire feed requires more energy because powder melts more readily than a wire. [20]

The advantages of using a wire feed system are the higher deposition rate, and therefore, faster printing speed. Metal wire is also cheaper than metal powder, and thus material waste is more acceptable with a wire DED process.

DED can employ a wide variety of materials including Ti alloys, steels, Al alloys, Inconel, NiCu and refractory metals. In addition, the materials used for DED are cheaper than the materials used in PBF processes. Another advantage of using DED is the high manufacturing speed. Therefore, DED can be a much cheaper production method than PBF methods. There is also less material waste in DED

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than with PBF methods. This is because with PBF, there is a lot of unused powder. In theory, DED only uses the powder it needs to construct the part, because it is directly deposited to the surface. However, excess material is often used because the final product must be finished. Some material might be subtracted from the surface to produce the final size and shape of the product. [21]

One disadvantage of DED is the limited geometrical capability and low resolution, when compared to other 3D printing methods. Powder DED produces a higher resolution than wire DED. Therefore, powder DED can produce a part closer to the final product, reducing machining and material waste. Since wire DED has a lower resolution, more machining is required. [20]

The microstructure of powder DED produced parts is like that of SLM produced parts. This is because of the high cooling rate. A high cooling rate produces a smaller average grain size with a uniform structure due to melting. The mechanical properties are also similar to those of parts produced with the SLM process.

Wire DED produces a different type of microstructure than powder DED. The deposited layers are thicker and there are clear fusion lines between the layers.

Since the layers are much thinner with powder DED, there is a more uniform microstructure with less defined fusion lines. The areas with fusion lines are more vulnerable to cracking. Therefore, wire DED should not be employed if the produced part is under strong mechanical stress. [22]

Overall, powder DED produces parts with a higher resolution, better microstructure, mechanical properties, as well as less waste. However, wire DED is a faster and more cost-effective method.

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4. ADVANTAGES OF METAL 3D PRINTING

There are many advantages of using 3D printing for producing metal components. One of the most beneficial aspects of using 3D printing technology is the almost endless design and customization possibilities. Another advantage of 3D printing metals is the possibility to create light weight parts by producing components, which have porous internal structures. This chapter will explore the main advantages of metal 3D printing.

4.1 Design and Customization

3D printing of metals allows the manufactured parts to have extremely complex geometry and design, which would not be possible or feasible using conventional methods of production. As shown in figure 9, which contains a picture of a satellite antenna produced via DMLS, very complex structures can be constructed using 3D printing. This satellite antenna contains net-like structure on the bottom part and rectangular tubing on the top. The object could be produced as one solid part, rather than several parts being produced, and connected with, for example, welding. 3D printing enables accurate manufacturing of complex parts. In contrast, this part would be seemingly impossible to manufacture using conventional methods of production.

Figure 9. DMLS produced satellite antenna. [23]

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Complex parts are being produced using 3D printing because of the extensive design possibilities. In addition to parts with complicated design, customized parts can also be easily produced. For example, the medical industry utilizes metal 3D printing for producing customized implants for patients [24]. This is because the implants can be designed and produced more rapidly than with conventional methods of production, like machining.

4.2 Physical and Mechanical Properties

In addition to the complex geometry, which can be produced by metal 3D printing, 3D printed parts can also obtain good mechanical properties. It is important to carefully choose a suitable 3D printing method for the part in question. The method must be compatible with the desired material. In some cases, the desired material can define the method of production. For example, titanium aluminide, used for aerospace applications, offers good heat and oxidation resistance but has low ductility [25]. Due to its brittle nature, it should be produced with EBM because of the slow cooling rate. The material choice will have the most profound effect on the final mechanical properties. However, the chosen 3D printing method will also affect the properties of the fabricated part.

Components produced with metal 3D printing methods can achieve mechanical properties comparable to the mechanical properties of its counterparts produced with conventional methods of production. In a study comparing SLM manufactured and cold-rolled 316 stainless steel, it was found that the yield strength of the SLM manufactured steel was 45% higher [26]. This shows that a higher yield strength can be achieved with 3D printing. However, ultimate tensile strength was 9% lower and elongation was over 60% lower [26]. This can be due to common defects like micropores, cavities and insufficient fusion [26]. Even though the elongation was greatly reduced, the 3D printed material still showed ductile behavior. A large reduction in the elongation indicates that some mechanical properties of 3D printed parts can be inferior to those produced with conventional methods of production. However, the mechanical properties achieved depend greatly on the 3D printing method and the post processing. In general, components produced with SLM, EBM or DED can have mechanical properties comparable to their conventionally produced counterparts. However, some properties

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can show a decrease. Therefore, the mechanical requirements of the manufactured component should be carefully considered before choosing a method of production. Additionally, components produced with DMLS and binder jetting will usually have lower mechanical properties because of the porous structure. Infil- tration can be used to fill the voids and improve the mechanical properties. An- other method of improving the mechanical properties is alloying. Using high strength alloys will result in a stronger component.

Table 1 contains some mechanical properties for a 3D printed alloy and a die cast alloy. The chemical compositions of these alloys are very similar, which is why they are being compared. It should be noted that the mechanical properties of the 3D printed object can be different in the XY plane when compared to the Z- axis. This is because of the additive layer by layer approach, which results in fused layers. Therefore, the Z-axis consists of layers, whereas the XY-axis consists of fused particles next to each other. The areas where the layers are fused together are susceptible to breakage.

Table 1. Comparison of mechanical properties of a 3D printed AlSi10Mg alloy and a die cast A360 alloy. [27]

AlSi10Mg (3D printed alloy) A360 (Die cast alloy)

Yield Strength XY: 230 MPa

Z: 230 MPa 165 MPa

Tensile Strength XY: 345 MPa

Z: 350 MPa 317 MPa

Young’s Modulus XY: 70 GPa

Z: 60 GPa 71 GPa

Elongation at break XY: 12 %

Z 11% 3.5 %

Hardness 119 HBW 75 HBW

Fatigue Strength 97 MPa 124 MPa

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Overall, the yield strength, tensile strength, elongation at break, and hardness are higher for the 3D printed alloy. The Young’s modulus has comparable values for the alloys. Finally, the fatigue strength for the 3D printed alloy is clearly inferior.

This can be attributed to typical defects and voids formed with 3D printing, as well as the high surface roughness. The high surface roughness allows cracks to form and advance more quickly. The increased yield strength and hardness are the major advantages of 3D printing metals. The most prominent disadvantage is the decrease in fatigue strength.

In many cases, 3D printing is utilized because it can produce components with porous and net-like structures. One advantage of producing porous materials is the reduced density. A component, which contains pores in the structure will weigh less than a uniformly solid component. Even though the porosity usually results in poorer mechanical properties, they may still be sufficient for the in- tended use. The possibility of fabricating porous materials using DMLS or binder jetting has led to the production of light weight components. Many industries have adapted metal 3D printing for this reason. They can produce the same components as before with a lower overall weight, which can have its advantages.

One disadvantage of metal 3D printing is the poor surface quality. Processes, which use large powder particles produce a rough surface [6]. A high layer thickness will also result in a rougher surface quality. If a smooth surface is required, small powder particles and a small layer thickness should be used. Additionally, finishing can be used to achieve a better surface quality. Many conventional methods of production also require surface treatment; thus, it is not a problem unique to 3D printing. Poor surface quality can negatively affect the fatigue strength.

Overall, metal 3D printing can achieve good mechanical and physical properties.

It is not used to achieve very high mechanical properties, but rather, good mechanical properties with complex design or light weight. Most applications of metal 3D printing utilize the possibility of producing light weight components.

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4.3 Waste Material

3D printing is an AM method, and therefore it is associated with minimal waste material. When compared to subtractive methods, which always remove excess material, AM methods do not produce as much waste. However, some waste is produced by the unused powder in 3D printing methods. Most of the unused powder in PBF methods can be recycled and used again to a certain extent. The powder absorbs heat during the manufacturing process, which can affect the properties of the powder. If the powder has undergone too much temperature change, it should not be used for production. When comparing virgin powder with recycled powder in an EBM process, it was found that the recycled powder has undergone changes in particle size distribution, morphology, and an increase in the oxygen content [28]. This is because the powder absorbs some oxygen with each process, and the morphology will also change due to the environment. This results in changes in the properties of the manufactured object, which should be considered when recycling powder. For applications that may not require such high mechanical properties, such as prototyping, it is acceptable to use recycled powder.

In a study [29] determining the effects of powder recycling in the binder jetting process, there was a 22% increase in the number of coarse particles, as well as an 18.2% decrease in the number of small particles. The powder was recycled up to 16 times, which is a high degree of recycling. The hardness and yield strength of parts built using virgin powder and recycled powder were compared.

The hardness for the virgin powder and the recycled powder were 155 HV and 165 HV, respectively [29]. This shows a slight increase in hardness. The yield strengths were 206 MPa and 192 MPa, respectively [29]. Thus, the yield strength decreased slightly because of using recycled powder. This shows that parts built using virgin powder exhibit a higher yield strength. However, the yield strength has only slightly decreased, and in many applications, the decrease would not be so significant.

Figure 10 shows the difference in virgin powder and recycled powder. (a) shows 316L stainless steel virgin powder and (b) shows powder of the same material, which has been recycled 16 times. On average, the particle size has increased in the recycled powder. This negatively affects the mechanical properties of the

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manufactured object. The fresh powder contains small spherical particles, whereas the recycled powder also shows more deformed particles. The uneven particle size distribution and morphology affects the flowability and spreading of the powder, ultimately affecting the density and porosity of the manufactured part.

Increased porosity and lower density negatively affect the mechanical properties of the object. Overall, powder can be reused for most applications, but the possible decrease in mechanical properties should be noted. [29]

Figure 10. Scanning electron microscope picture of 316L stainless steel: (a) fresh powder, (b) 16 times recycled powder.

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5. APPLICATIONS

Several industries, such as the medical industry, automotive industry, and aerospace industry have adapted 3D printing of metals as a primary method of production. Usually, the parts produced have complex design and are produced in small quantities. Another reason 3D printing is utilized is to manufacture light- weight components by producing parts with a porous microstructure.

5.1 Medical Industry

The medical industry uses 3D printing to produce customized implants. Every- body has a somewhat unique anatomy. Therefore, implants must be designed so that they are suitable for the patient. CAD allows customized implants to be designed with relative ease. Implants are designed with anatomical data from the patient, obtained through various medical scans. The implants can also be produced as one solid part, even if the design is complicated. Another advantage of producing implants with 3D printing is the ability to produce light-weight parts with porous, net-like structure. Figure 10 shows an orthopedic hip replacement socket, which has been produced with 3D printing. The component has a complex outer texture and smooth inside. These implants are designed according to the patients’ anatomy and manufactured with 3D printing. The medical industry has adopted 3D printed implants because they are light-weight, durable, and can be personalized. [24]

Figure 11. 3D printed orthopaedic hip replacement. [30]

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The medical industry also produces surgical tools with metal 3D printing. Many surgical tools are manufactured in only one size and handle type. 3D printing has enabled these tools to be customized and updated according to surgeons’ needs.

In addition to customization, new surgical tools have been produced with 3D printing. For example, endocon GmbH has developed a new tool for hip cup removal.

As a result, the procedure time has been reduced from 30 min to 3 min. This new tool also carries less risk damaging surrounding bone and tissue. These tools are manufactured from a stainless steel alloy. Due to improved materials, the rejection rate of the surgery has decreased from 30% to only 3%. The rejection rate is the rate at which the body rejects a transplant. Finally, the manufacturing costs have been reduced by 45% when compared to the traditional tools. This is a massive improvement in terms of surgical outcome and cost reduction. This shows that the medical industry can hugely benefit from metal 3D printing new tools with improved capabilities. [31]

The future of 3D printing in the medical industry is almost certain. It will grow due to the major advantages it can provide when designing customized transplants.

Even though metal 3D printing is still quite expensive, most people are ready to pay for their health.

5.2 Automotive Industry

Car manufacturers have several applications for 3D printing metals. One purpose is for prototyping. Every new car model begins with a prototype. Miniature models and full-scale prototypes can be manufactured without fabricating every part in- dividually. 3D printing enables manufacturers to produce fully functional prototypes from CAD models. Part prototypes can also be created and delivered to manufacturers with ease. Additionally, rare car parts can be produced without molds or extensive production lines. It is difficult to find replacement parts for classic cars and limited-edition models. Rather than starting up an old factory to produce replacement parts, they can be manufactured using 3D printing. 3D printing will always be more cost effective than traditional methods of production when the production quantity is low. Producing spare parts with 3D printing could also reduce storage space requirements because they can be produced in a short

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time frame. Rather than having a large inventory of spare parts, they could be produced when needed. [32]

The automotive industry is turning to 3D printing to produce light-weight parts.

This leads to lighter overall cars and thus greater fuel efficiency. The weight of cars has a huge impact on their mileage. By producing parts with a porous structure using aluminium alloys, cars can be made significantly lighter. Another weight reducing component is improved design. Components can be designed and manufactured so that the crucial areas of the component are strong, and mass is deducted from less important areas. The weight of components can be reduced by as much as 80%. These parts are just as reliable as traditionally produced components. Overall, the automotive industry greatly benefits from 3D printing metal components. In the future, we may see cars produced entirely with 3D printing. XEV has an electric car model, the LSEV, which is mostly produced with 3D printing. However, the car has its limitations with a top speed of 69 km/hr, and a 145 km range. The technology is still very slow and costly; however, it is constantly improving. [32]

Overall, metal 3D printing is being utilized in applications which require low-volume parts, customized parts, parts with complex design, or parts with light-weight requirements. One problem with metal 3D printing is the high cost. The industries, which have adapted metal 3D printing are large industries, which can invest in the process. Another possibility is where the technology reduces the environmen- tal impact, like with cars. AM producers and consumers are aware of the high cost, which is why lots of research and work is put into reducing the cost of producing 3D printed parts. In the near future, 3D printing will remain expensive, but over time, it will get cheaper, and therefore more industries will adapt it.

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6. CONCLUSIONS

There are several methods of 3D printing metallic materials, each with their own advantages and disadvantages. Table 2 shows a comparison of the major advantages and disadvantages of each method. Note that porous structure is listed as a disadvantage, but in some cases a porous structure is desirable to achieve reduced weight.

Table 2. A comparison of the major advantages and disadvantages of different metal 3D printing methods.

3D Printing Method Advantages Disadvantages

DMLS Cost effective Porous structure

High surface roughness

SLM Good mechanical properties

Low surface roughness Slow and expensive

EBM

High density

Good mechanical properties Suitable for brittle materials

Slow and expensive High surface roughness

Binder Jetting Fast

Very cost effective

Porous structure Low precision

DED Minimal post processing Zero waste

High surface roughness Low precision

In general, there are multiple advantages of 3D printing metals. The most prominent one is the ability to produce parts with extremely complex design. This has led to new components being designed and manufactured, as well as combining multiple components into one part. As a result, less components and assembly are required. One design limitation is the size of manufactured components. 3D

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printers are still limited to relatively small components. Another advantage of the one-step manufacturing is the ability to customize components easily. Once the object is designed with CAD, it can be constructed using 3D printing. This allows fast design and production, which can lead to made to order components. 3D printing has also enabled the production of strong and light weight parts.

Metal 3D printing is being largely utilized by the medical and automotive industries. The medical industry uses it to provide light weight, customized medical implants, as well as produce new tools for surgery. The automotive industry uses it to produce prototypes, spare parts and light weight components, improving fuel efficiency. Overall, 3D printing is cost efficient at low volumes, but very expensive at high volumes. Therefore, it is still limited to low volume production. In this sense, I do not believe metal 3D printing will replace conventional methods of production in the near future. There would have to be major developments in the costs and the production times. However, technology is constantly developing, and more industries are incorporating the 3D printing of metals. This shows that metal 3D printing is replacing some methods of production, as well as providing new possibilities for design and manufacturing.

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Metal 3D Printing: The advantages of 3D printing metals

Henrik Metsäranta