Metal 3D printing is becoming increasingly popular with diverse applications in many industries such as energy, aerospace, automotive, healthcare, etc. Not only printing parts to make prototypes or even for direct use, 3D printing can also be applied to repair damaged parts after a period of operation in harsh environments. For each application's requirements, the industrial sector in which the materials used for 3D printing will also be different. In this article, we will focus on learning about the two metals most used in metal 3D printing: titanium and aluminum. These are two materials mainly used in L-PBF and DED technology.

MANUFACTURING PROCESS AND PROPERTIES OF TITANIUM AND ALUMINUM

Titanium production process

Titanium is a material that does not exist in nature as an element but must instead be extracted from minerals such as rutile (TiO2) or ilmenite (FeTiO3). Mining pure titanium is a complex process involving many steps. The most widely used method for producing pure titanium is the Kroll method, developed by American chemist William J. Kroll in 1940. This method involves reducing titanium dioxide (TiO2) with gas chlorine (Cl2) to produce titanium tetrachloride (TiCl4), which is then reduced with magnesium (Mg). Although the Kroll method is effective in producing pure titanium, it is an expensive and energy-intensive process. Additionally, titanium's high reactivity makes it difficult to obtain as a pure metal, to the point that a sample with 99.9% purity is considered commercially pure titanium. Therefore, it is often used in combination with other elements to form alloys.

Source: PyroGenesis Additive

Titanium has many properties that make it very versatile and useful in a number of fields. Because pure Titanium is expensive to produce, the metal is often used in alloy form, but pure extracted titanium is used in certain applications, such as in the medical industry, due to its high biocompatibility. Its main characteristics are high mechanical strength, low density, excellent corrosion resistance and high hardness.

The main titanium alloys used in 3D printing include:

🔹 Titanium 6Al-4V, grade 5: The most important and most popular titanium alloy. It is used in additive manufacturing because of its high strength and durability. This alloy is composed of titanium, aluminum and vanadium and can withstand high temperatures and corrosive environments.

🔹 Titanium 6Al-4V, grade 23: Often used in medical implants and prosthetics thanks to its biocompatibility.

🔹 Titanium Beta 21S: Stronger than conventional titanium alloys, it is also more resistant to oxidation and deformation. It is perfect for orthopedic implant and aerospace engine applications. Beta titanium is highly valued in orthodontics.

🔹 Cp-Ti (pure titanium), grade 1, 2: This alloy is used in the medical industry with many applications due to titanium's biocompatibility with the human body.

🔹 TA15: An alloy made almost entirely of titanium, with added aluminum and zirconium. Components made from this alloy are very durable and resistant to high temperatures, these characteristics are the reasons why TA15 is ideal for making parts used in aircraft and engines. 3D printing with TA15 will create parts that have the same strength as usual but are significantly lighter in weight.

Aluminum production process

Aluminum is a metal used a lot in 3D printing. It offers a great compromise between lightness and durability. In addition to being corrosion resistant, aluminum can also be used for welding. Like Titanium, it is very rare to find aluminum in its pure state. It is therefore used in alloy form, with other metals such as silicon and magnesium to improve its physical and mechanical properties. For aluminum, two consecutive industrial processes make it possible to obtain the material in its pure state. In the first process, called the Bayer process, aluminum oxide is obtained from bauxite ore. The ore is washed and crushed, dissolved in caustic soda and filtered to obtain pure aluminum hydroxide. It is then heated to obtain aluminum oxide powder. In the second process, known as the Hall-Heroult process, electrolytic reduction of aluminum oxide is carried out to obtain pure aluminum. Most processing plants are built near mines to reduce ore transportation costs.

Source: xtra GmbH

As mentioned above, aluminum alloys are more popular than their pure form and are used in many industrial applications. Additionally, they have a very good strength-to-weight ratio and are very resistant to fatigue and corrosion. They are also easily recyclable, conduct heat and electricity, and have low toxicity.

Main alloys used in aluminum 3D printing:

🔹 AISi10Mg: This is the most common alloy formed from silicon and magnesium. It can be used to produce solid and complex parts and is used to create various objects such as shells, engine parts and production tools.

🔹 Al2139: The most durable aluminum alloy, ideal for industries such as automobiles due to its light weight, durability and high chemical resistance. It has been used by organizations such as the US Air Force, Mercedes-Benz and Airbus. The beauty of this material is that it is specifically designed for additive manufacturing and outperforms many other alloys on the market.

🔹 Al 7000 series: This is a famous powder alloy line, with high tensile strength and low temperature resistance.

🔹 Al 6061 & Al 7075: Recently, these two alloys produce better 3D printed parts than other alloys. Al 6061 has lower tensile strength and hardness than 7075. On the other hand, Al 7075 has better impact resistance and less deformation than 6061 aluminum.

🔹 A201.1: It is part of the 200 series of aluminum-copper alloys, which are known to be very durable. However, they are difficult to cast. These alloys are recommended for applications where the strength-to-weight ratio is important, such as in transportation and aerospace.

The difference between the two materials

In terms of strength-to-weight ratio, titanium is the ideal choice when high strength is required, which is why it is used for medical parts or even in satellite parts. On the other hand, although aluminum is less durable than titanium, it is much lighter and more affordable. In terms of thermal properties, aluminum is perfect for applications requiring high thermal conductivity while titanium is ideal for applications in high temperature environments due to its high melting point, such as aerospace engine parts universe. Both aluminum and titanium have excellent corrosion resistance. However, titanium is more biocompatible than aluminum, which is why it is widely used in the medical field.

MATERIAL FORM AND COMPATIBILITY WITH 3D PRINTING TECHNOLOGY

Material form

In most cases, titanium and aluminum come in powder form, although they can also come in wire form, such as titanium or aluminum filaments offered by Virtual Foundry or Nanoe. To 3D print parts with these metals, the alloy powder must first be obtained, which is done using two main techniques: plasma atomization or gas atomization. Plasma atomization (ionized gas) is a process that uses high temperatures, energy and heat sources, an inert medium such as argon, and high velocities to atomize metals. This process produces high-quality, abrasion-resistant powder. On the other hand, gas atomization uses air, argon or helium as the gas to fragment the molten material stream. This is a very efficient process, widely used to produce fine spherical metal powders. The technique used to produce metal powder is important because it greatly influences the final properties of the 3D printed part.

3D technology used

Titanium can be used in a variety of 3D printing processes, including laser powder bed fusion (L-PBF), DED, and binder jetting (Binder Jetting). For aluminum, it can not only participate in the above processes but can also participate in another technology, which is Cold Spray.
In L-PBF, a laser beam is used to heat each layer of powdered metal to the melting point and build the object layer by layer. Titanium melts at very high temperatures (1,600°C), so the thermal and mechanical effects of the material will need to be analyzed before 3D printing. The melting temperature of aluminum is much lower (about 630°C), but aluminum has high reflectivity and thermal conductivity. Another interesting aspect of additive manufacturing with aluminum is that it forms a natural oxide layer, which other metals do later on their edges, which means the presence of This thin layer on aluminum also slows down the printing process.

For DED, the process is quite similar to L-PBF, but here the material is melted when deposited by the nozzle and can be in powder or filament form. Typically, this technology delivers higher production rates and lower costs per unit volume.

Source: RamLab

In the case of Binder Jetting, where the material is in the form of a powder that is not thermally bonded, the binder is sprayed onto the layer at specific locations using a multi-nozzle inkjet printer. After printing, the printed product will need to undergo post-solidification treatment. When leaving the 3D printer, the parts are fragile and porous and will need to be heat treated to achieve their final mechanical properties.

Source: ExOne

In the Cold Spray process, the metal material used is also in powder form, but since in this case the metal powder does not need to be melted, cold spraying avoids thermal deformation. and the procedure does not need to be performed in a sealed chamber with a special gas for protection.

Source: Titomic

Read more about cold spray technology here.

POST-PROCESSING

To achieve optimal results, the metal part needs to undergo one or more post-processing steps. Post-processing for aluminum and titanium does not differ too much, so the following steps apply to both materials. Because titanium and aluminum are commonly used in applications Because of mechanical stress, shot blasting is the first choice. Small metal or ceramic particles are blown onto the surface of the part to create controlled deformation of the part's surface layer. This method improves the adhesion of subsequent coatings and reduces the likelihood of cracks, breaks, and other problems. Shot blasting removes only the top layer of material, which improves the aesthetic appearance of the part, removes dirt and corrosion, and perfects the surface for subsequent coatings.

Source: FacFox

Another option is to combine metal 3D printing with traditional manufacturing methods. CNC machining is a suitable post-processing process for this purpose as it ensures tight tolerances and the desired surface finish. With DED technology in particular, 3D printed parts have very rough surfaces due to the metal being melted directly during the extrusion process. Therefore, CNC machining is a necessary step to achieve a smooth and clear surface.

Solution annealing is a heat treatment option. The metal part will be heated to a high temperature and cooled rapidly to change the microstructure, which improves the material's ductility, or ability to deform under load before breaking. In general, parts post-processed using this method tend to have better mechanical properties. This method is mainly used for aluminum parts.

Sintering is also a required post-processing method for aluminum and titanium when they are used in indirect 3D printing processes, such as FDM or Binder Jetting. After the printing stage, the parts must undergo a de-adhesive process to separate the polymers from the metal binder. The parts are then heated to a certain temperature in a sintering furnace to cure the final object.

APPLICATION

Aerospace is the industry that uses titanium metal 3D printing the most. It is an ideal material for parts used in aerospace, such as jet engines and gas turbines, because metal 3D printed products can significantly reduce their weight. . An example of a titanium metal 3D printing application is Boeing's collaboration with Norsk Titanium to produce large-sized components for the 787 Dreamliner. The technology used in this process is DED, which is 50 to 100 times faster than other powder-based printing technologies and uses 25 to 50 percent less material than forging. This helps the company save up to $3 million in production costs for each aircraft.

Although the high price of titanium may be an obstacle to the widespread use of this material in the automotive sector, we can see titanium being used more and more in this sector, especially in luxury segment. Currently, 3D printing is used to create parts with high weight/performance ratio requirements. For example, Bugatti printed brake calipers for titanium brake systems using SLM technology in just 45 hours. The product is claimed to be 40 percent lighter than conventionally manufactured aluminum brake calipers. Despite being significantly lighter in weight, the titanium upper retains its elasticity and heat resistance.