Titanium vs. aluminum: which metal to choose for 3D printing?
Metals are one of the most popular materials used in additive manufacturing processes today. Unsurprisingly, their exceptional properties make them ideal for the most demanding applications in terms of performance and resistance. In this article, we will focus on the two main metals used in 3D printing: titanium and aluminum. These are mainly used in processes such as laser powder bed fusion (L-PBF) or concentrated energy deposition (DED). They are mainly available in powder form, especially in industrial environments. We will compare their similarities and differences in order to better understand their properties and applications, and to understand the advantages they offer in the manufacturing process.
Production and characteristics of titanium and aluminum
Titanium.
Titanium is a material that does not exist as an element in nature and must be extracted from minerals such as rutile (TiO2) or ilmenite (FeTiO3). The extraction of pure titanium is a complex process involving several steps. The most widely used method for producing pure titanium is the Kroll method, developed by American chemist William J. Kroll in 1940. The method involves the reduction of titanium dioxide (TiO2) with chlorine gas (Cl2) to produce titanium tetrachloride (TiCl4), which is then reduced with magnesium (Mg). While the Croll method is effective in producing pure titanium, it is an expensive process that requires a great deal of energy. In addition, titanium's high reactivity makes it difficult to obtain pure metal, so samples with 99.9% purity are considered commercially pure. This is why it is often combined with other elements to form alloys.
Titanium has many properties that make it versatile and useful in many industries. As mentioned earlier, it is usually used in alloy form, but due to its high biocompatibility, pure extracted titanium is used in certain applications, such as the medical industry. Its main characteristics are high mechanical strength, low density, excellent corrosion resistance and high rigidity.
Development
The main titanium alloys used in 3D printing are:
Titanium 6Al-4V, Grade 5: This is the most important and common. It is used in additive manufacturing due to its high strength and durability. The alloy is made of titanium, aluminum and vanadium and can withstand high temperatures and corrosive environments.
Titanium 6Al-4V, Grade 23: Biocompatible and commonly used in medical implants and prostheses.
Titanium Beta 21S: Stronger than conventional titanium alloys, it is also more resistant to oxidation and deformation. It is well suited for orthopedic implants and aerospace engine applications. beta titanium is popular in orthodontics.
Cp-Ti (pure titanium), grades 1 and 2: Titanium has a wide range of applications in the medical industry due to its biocompatibility with the human body.
TA15: This is an alloy composed almost entirely of titanium with the addition of aluminum and zirconium. Components made from this alloy are extremely strong and resistant to high temperatures, making them ideal for the manufacture of robust components in airplanes and engines. They are also lightweight relative to their strength.
Aluminum
Aluminum is a metal that strikes the perfect balance between weight and strength. In addition to being corrosion-resistant, it is also weldable. In its pure state it is very rare, so we will use it in alloy form and with metals that can improve its physical and mechanical properties, such as silicon and magnesium. As with titanium, two successive industrial processes make it possible to obtain the material in its pure state. In the first process, known as the Bayer method, alumina is obtained from bauxite ore. The ore is washed, crushed, dissolved in caustic soda and filtered to obtain pure aluminum hydroxide. It is then heated to obtain alumina powder. In a second process, called the Hall-Héroult process, the alumina is electrolytically reduced to obtain pure aluminum. Most beneficiation plants are built near mines to reduce ore transportation costs.
As mentioned above, aluminum alloys are more common than pure aluminum alloys and are used in many industrial applications. In addition, they have a very good strength/weight ratio and very good fatigue and corrosion resistance. They are also recyclable, thermally conductive, electrically conductive and have low toxicity.
The main alloys used in aluminum 3D printing are:
AISi10Mg: This is the most common alloy formed from silicon and magnesium. It allows for the creation of strong, complex parts and is used in the manufacture of a variety of objects such as housings, engine parts and production tools.
Al2139: The strongest aluminum alloy, it is ideal for industries such as automotive due to its light weight, strength and resistance to chemicals. It has been used by organizations such as the U.S. Air Force, Mercedes-Benz and Airbus. The beauty of this material is that it is specifically designed for additive manufacturing and is superior to many other alloys on the market.
Al 7000 Series: A well-known family of powder alloys with high tensile and low temperature strength.
Al 6061 and Al 7075: Recently, 3D manufacturers have achieved very good results with these two alloys. 6061 has a lower tensile strength and hardness than 7075. on the other hand, 7075 has a better impact resistance and warpage than aluminum 6061.
A201.1: It is part of the 200 series of copper-aluminum alloys and is known to be very durable. However, they are difficult to cast. These alloys are recommended for applications where strength-to-weight ratio is critical, such as transportation and aerospace.
If we compare these two metals, what is the difference?
In terms of strength-to-weight ratio, titanium is ideal when high strength and robustness are required, which is why it is used in medical components and even satellite parts. On the other hand, while aluminum is not as durable as titanium, it is lighter and cheaper. As far as thermal properties are concerned, aluminum is perfect for applications that require high thermal conductivity. Titanium, on the other hand, is ideal for applications in high-temperature environments, such as aerospace engine components, due to its high melting point. Both aluminum and titanium have excellent corrosion resistance. However, titanium has better biocompatibility than aluminum, which is why it is widely used in the medical field.
Material shapes and 3D technologies used
Shape
In most cases, titanium and aluminum are in powder form, although they can also be in wire form, such as the titanium or aluminum filaments offered by Virtual Foundry or even Nanoe.
To 3D print parts using these metals, the alloy powder must first be obtained, which is done using two main techniques: plasma atomization or gas atomization. Plasma (ionized gas) atomization is a process that uses high temperatures, energy and heat sources, an inert medium (such as argon), and high velocity to atomize metal. The process produces high quality wear resistant powders. On the other hand, gas atomization uses air, argon or helium as gases to break up a stream of molten material. This is a very effective process that is widely used to produce fine, spherical metal powders. The technology used to create the metal powder is important because it significantly affects the final properties of the part.
3D technology used
In order to use titanium in 3D printing, a variety of processes can be used, such as laser powder bed fusion (L-PBF), DED or powder bonding. For processes related to aluminum, in addition to those already mentioned, there is another process, such as cold spraying, also known as cold painting.
In the L-PBF additive manufacturing process, a laser beam is used to heat the powdered metal layer by layer to the melting point and build the object. Titanium melts at very high temperatures (1,600°C), so the thermal and mechanical effects of the material need to be analyzed before 3D printing. Aluminum melts at a much lower temperature (around 630°C), but aluminum has high reflectivity and thermal conductivity. Another interesting aspect of aluminum additive manufacturing is that it will form a natural oxide layer, which other metals will later form around their edges, meaning that the presence of this thin layer on aluminum slows down the process.
Regarding DED, this is a very similar process to the previous one, but here the material is melted while it is being deposited through the nozzle and can be fabricated in powder or wire form. Typically, this technology allows for higher production speeds and lower costs per volume.
In the case of binder jetting, the material is in the form of an unmelted powder, but in order to make the particles adhere to each other, the binder is jetted to specific locations on the layer by means of a "print head". A sintering step is also required after printing. The parts that come out of the 3D printer are very fragile and porous, and need to be heat-treated to obtain the final mechanical properties.
In the cold spraying process, we also find metallic materials in powder form, but since there is no need to melt or fuse it in this case, cold spraying helps to avoid deformation due to heat and does not require a protective atmosphere.
Post-treatment
In order to obtain optimum results, one or more post-treatment steps are necessary. There are no specific differences in the post-treatment of titanium and aluminum, so the following steps apply to both materials. Since titanium and aluminum are often used in applications subject to mechanical stress, microblasting and shot peening are very useful. In the first method, small metal or ceramic balls are projected onto the surface of the part to produce a controlled deformation of the surface layer of the part. This improves the adhesion of subsequent coatings and reduces the likelihood of cracks, fractures, etc. Shot peening removes only the surface material, which improves the aesthetics of the part, removes dirt and corrosion, and prepares the surface for subsequent coatings.
Another option is to combine metal printing with traditional manufacturing methods. cnc machining is a suitable post-process for this purpose as it ensures tight tolerances and the required surface finish. Especially with DED technology, the surface of 3D printed parts is very rough because the metal melts directly during the extrusion process. This is why CNC machining is always required to obtain a smooth and well-defined surface.
Solution annealing is a heat treatment option that involves heating the printed part to a high temperature and cooling it quickly to change the microstructure, which improves the material's ductility, i.e., its ability to deform under load before fracturing. Generally, this process results in better mechanical properties and is primarily used for aluminum parts.
Sintering is also necessary when aluminum and titanium are used in so-called indirect 3D printing processes, such as FDM or powder bonding. After the printing phase, the part must go through a degreasing process in order to separate the binder polymer from the metal. The part is then heated in a sintering furnace to a certain temperature slightly below the melting temperature, which consolidates the final object. This results in a very low porosity of the part, as the cavities where the binder is located are closed during the process, leading to compression.
Areas of application
The aerospace industry has found tremendous benefits in using titanium additive manufacturing. It is an ideal material for the manufacture of aerospace components such as jet engines and gas turbines as it significantly reduces the weight of structures subjected to high stress. An example of the use of titanium in additive manufacturing is Boeing's collaboration with Norsk Titanium to manufacture large structural components for the 787 Dreamliner. The technology used in the process is DED, which is said to be 50-100 times faster than powder systems and uses 25-50% less titanium than forging, potentially saving up to $3 million per airplane.
If titanium is currently being used in space exploration through 3D printing, the use of aluminum in industry has increased exponentially. For example, Boeing uses aluminum alloys coated with nanoparticles to produce 3D printed parts during the cooling phase. This allows for extremely strong aluminum alloys to be welded without breaking when hot. The manufactured parts are lighter, allowing airplanes to use fuel efficiently and fly longer distances on the same amount of fuel.
While the high price of titanium in the automotive sector may prevent its widespread use, we could see an increase in titanium employment in the sector, especially in the luxury car segment. Currently, 3D printing is used to create parts where weight/performance ratio is critical. For example, Bugatti used SLM technology to print the brake calipers for its titanium braking system in just 45 hours, which is claimed to be 40% lighter than milled aluminum brake calipers made the traditional way. Despite its lightness, the titanium component also ensures its elasticity and temperature resistance. On the other hand, aluminum is much more common in the automotive industry. Porsche used 3D printing to create high-performance aluminum pistons for its flagship 911 model, the GT2 RS. Thanks to this technology, the 700-horsepower twin-turbo engine can gain up to 30 horsepower and increase its efficiency. In addition, Porsche produced an all-aluminum 3D-printed housing for the electric drivetrain in 2020, which successfully passed all of the company's quality and load tests.
Finally, titanium is a very interesting material for the medical industry due to its high strength, corrosion resistance, and biocompatibility, which makes it ideal for orthopedic and dental implants.3D printing creates porous structures that mimic the texture of bone, which aids in the rapid healing and growth of bones and tissues. TrabTech in Turkey uses titanium to create trabecular implants, such as hip joints. Aluminum is not as commonly used in the medical industry as titanium, but it can be used in orthopedic and dental applications.
