What Is 3D Printing Titanium?
3D printing with metals is still a new process; however, as the machines and materials continue to drop in price it will become more adopted by different industries for similar use cases. One of the most desired industrial quality metals is titanium due to its lightweight, high strength, resistance to corrosion, melting point properties. It also a great material for osseointegration or formation of bone tissue on an implant surface.
Direct Metal Laser Melting
(DMLM) employs a high-powered laser to fuse layers of powdered metal into three-dimensional solid parts. The advantage is that the more complex the component becomes, the more economical the process becomes. Below are graphics showing the process
Electron-beam fabrication doesn’t require a bed of material since it melts a feed of titanium wire in successive layers to create the desired structure. This process is similar to FDM of plastics printing in that the energy source of the laser heats up the material in a freeform manner. The scalability of this process is also similar to the FDM process in that the size of the printed product is a function of vacuum build space and the diameter of the wire. A typical deposition rate is 2500 cm3/hr (150 in3/hr) and the three parameters that affect the build time are translation
speed, wire feed rate, and laser power. There also is a trade-off between high deposition rates and the quality of the grain microstructures. The quality of microstructures affect the tensile properties and the resistance to fatigue, fracture, and crack propagation.
Taken from Wikipedia, this table shows current plastic and metal 3D printing processes:
|Extrusion||Fused deposition modeling (FDM)||Thermoplastics (e.g. PLA, ABS), eutectic metals, edible materials|
|Granular||Direct metal laser sintering (DMLS)||Almost any metal alloy|
|Electron beam melting (EBM)||Titanium alloys|
|Selective heat sintering (SHS)
|Selective laser sintering (SLS)||Thermoplastics, metal powders, ceramic powders|
|Powder bed and inkjet head 3d printing, Plaster-based 3D printing (PP)||Plaster|
|Laminated||Laminated object manufacturing (LOM)||Paper, metal foil, plastic film|
|Light polymerised||Stereolithography (SLA)||photopolymer|
|Digital Light Processing (DLP)||liquid resin|
Design Guidelines for Metal Printing
With 3D printing, there can be a fair amount of trial and error; however, there are some common design obstacles documented and understood. It’s important for engineers to know about this obstacles even before designing the CAD models. There is also variation in different 3D metal printers; the laser spot size and translation speed are essential to knowing.
The design obstacles that should be understood are part orientation, porosity, density, and deformation.
3D printing creates parts with anisotropic properties, which means they have different mechanical properties in different build directions. Some of the available free gcode software can let you change the print shape of each layer such as honeycomb, linear, star, triangle, and more. The parts have higher tensile strength in the X and Y direction than they do in the Z direction. Hence, part orientation needs to be considered before printing, and the designer must pay attention to where certain areas will sustain stress.
Quick side note on holes and overhangs
The general rule of thumb is to not design a gap or hole under 0.5 mm as well as overhangs should not extent more than 0.5mm (with no support structure) and its good practice to use chambers (or at least fillets) on overhanging features. Support can be used to create elaborate overhangs and holes, but the designer must be mindful of the moments and stresses on those features after the support is removed.
While 3D printed structures are optimized for their weight, the designer must be mindful of porosity leading to cracks and accelerated fatigue. This occurs when very small cavities form within the body of a part as it is being printed. The 3D printing process itself or the powder used can cause the small cavities. The particles may not fuse correctly when there is not enough powder or when the laser intensity is too low; however, spatter ejection can occur when too much power is used.
The best way to avoid these problems to buy certified material and tune the machine parameters of power, spot size, and spot shape for a given material and print job.
The porosity is inversely related to density; for load-bearing applications, a density of above 99 percent is required resist fatigue or cracking under pressure. There is a tradeoff between density and build time; the bigger particles reduce the build time, and the density of the part, as well as, the smaller particles increase the build time and density. Volume flow rate also affects the density of the part. A constant volume flow rate will allow for an evenly distributed part; a variable flow rate will increase the porosity of the part.
Cracks in structure expand exponential, that why porosity and density are so important with powder-based processes. It is hard to predict the creep or deformation in a non even distributed part.
If the part in purposed for functional use, the effects of expansion and contraction must be anticipated. Expansion and contraction can be due to high stress or temperatures that cause the residual stress to exceed the tensile strength of the part during and after the build.
During the build, the highest magnitude of stress is found at the edge of the material and the build plate. The hot material is going to curl away from the build plate which creates a tensile stress at the edge and a compressive stress in the middle. Even if the part remains adhered to the build part, the stress can deform the part due to the unloading of the stress. A substrate is added to the top layer of the build plate to reduce this effect, but the high-level way to reduce this temperature-educed stress is to reduce the temperature range of the material. Besides modifying the laser length and distribution, the material can be heated before its hit with the laser to reduce the temperature difference of operating process. The support structure and proper part orientation can help reduce the deformation by strengthening the part during the build; however, the excess support material can absorb the heat needed to fuse the particles together (increasing the porosity of the material). Engineers can optimize the shape and build process of the support structure for each build by formulas or the method of trial by error and most of the metal printing research has been focused on reducing deformation of the printed material.
Now having talked about the problems with metal printing, it’s important to highlight the advantages of creating a metal printed part.
Advantages of creating a metal printed part
Metal printing’s benefits over traditional manufacturing techniques include,
- The process doesn’t require special tooling parts, and one machine can make many different parts
- Create part with features that cannot be cast or otherwise machined and can reduce the number of parts and fasteners in assemblies
- Parts can be optimally designed for each use case to minimize material and maximizing yield strength
- Compared to plastic printers, prototypes can now be functional hardware made out of the same material as production components. Currently available alloys include 17-4 and 15-5 stainless steel, maraging steel, cobalt chromium, Inconel 625 and 718, aluminum AlSi10Mg, and titanium Ti6Al4V
With all of these advantages, why don’t more companies implement these systems?
The need for volume and speed is greater than the need for complexity and optimal design. Molding and subtraction processes have already been streamlined to crack out hundreds of parts before a 3D printer can make one. Traditional manufacturing also have a much greater build area compared to 3D printers
The 3D printing services are facing enormous inertia with traditional manufacturing; companies already have processes and training done for traditional machines. There is a learning curve that will have to be overcome before 3D metal printing can reach its full potential. Metal printing is much more complex and requiring more design training than plastic printing.
The Future of Metal Printing
Metal printing’s current target is creating low volume complex parts that need to be strong and lightweight. For other parts, other manufacturing processes are more cost-effective and faster.
There will be hybrid additive/subtractive machines (such as DMLS and CNC machining) as to print a rough draft of the product and refine it with the subtractive process as well as create complex geometries on subtractive parts. The key to metal printing’s adoption will depend on widespread design knowledge and cost of 3D printing.