How to Get the Most Out of 3D Metal Printing
The impact of metal 3D printing on the manufacturing landscape has become significantly more measurable within the last decade. Material developments and concentrated research into producing fully dense additive metal parts has led to recent widespread adoption of the technology for end-use production. This tutorial will cover metal 3D printing design constraints and freedoms, available metal materials, quality and process controls, surface finishes, and heat treatment methods that truly empower metals to overcome conventional manufacturing challenges and deliver incredible parts impossible to achieve through other production means.
Step 1: Materials and Alloys Available for Metal 3D Printing
- Stainless steel
- Titanium (pure)
- Titanium alloys
- Aluminum alloys
- Nickel-based alloys
- Cobalt chrome alloys
- Copper-based alloys
Step 2: Design Affects Performance
Direct Metal Laser Sintering (DMLS) parts are built anchored to the build platform. A buffer zone of supports (~6mm) helps achieve clean part removal via machining. As mentioned earlier, DMLS uses a very powerful laser to melt powdered metals. The laser is powerful enough to cause delicate, unsupported features to burn and distort. Burn occurs when a downward facing unsupported surface of a part cannot survive the laser. The next three steps explain how to factor in supports and angles to minimize support structures and optimize your design for DMLS manufacturing.
Step 3: Angles
Support structures are automatically generated through STL editors and then manually refined within the part file prior to building. Support material is the same metal as the final part and can require extensive labor to remove. With DMLS, features below 45 degrees and delicate unsupported features will experience burn and therefore require support structures. 60 degree angles are ideal and will not suffer from ablation.
Consider the simple design below. In the imagine below, the open box design (left) is intended to allow ample cooling for interior components. It has a large and downward facing surface. Due to the nature of metal 3D printing, its downward facing surface will warp or burn out without extensive support structures (shown right in blue). this simple design might work well for machining, but it isn’t taking advantage of DMLS.
One solution to minimize supports: exchange the flat downward facing surfaces for an angled, saw-blade configuration. In the imagine below, the saw-blade design reduces supports by offering a surface with degree variances that aid in the sintering process. the new design cuts support removal time in half which reduces overall build time. In the next step, we’ll show you how to further optimize the design for metal 3D printing.
Step 4: Self-Supporting Features
DMLS is fundamentally minimalistic in material usage. Compared to machining, which subtracts from a pre- existing block of metal, DMLS forms parts one layer at a time, welding material only where it is needed, leaving excess metal powder untouched. Such a manufacturing approach encompasses unique design opportunities.
For example, the redefined self-supporting structure in the imagine below may appear more involved than our original model, but to DMLS, the imagine below is as easy to execute as our original open box design because it’s a matter of melting or not melting material. In fact, because the columns in the imagine below eliminate the need for support material, it is easier for DMLS to produce when factoring in labor. Additionally, because the original intent of the square frame was to provide interior objects breathing for cooling, the simple addition of columns maintains this function while enhancing the structural integrity of the design. Therefore this seemingly more complex column configuration results in a stronger part by taking advantage of DMLS capabilities.
Step 5: No-Access Features
While DMLS is favored because it can build no-access features, enclosed designs may still require support structures which can become trapped within a part. However, a few simple design rules for DMLS can actualize critical internal features without the addition of supports.
The imagine above shows a cross-sectional view of a part with sharp internal angles (left). These internal angles require supports that become trapped inside the part (right). One solution for this design is to change the angled features into feature gradations. Rather than an abrupt change from surface to overhanging feature, allowing these features a fluid movement at gradations above 45 degrees eliminates trapped supports that would be difficult or impossible to remove.
In the imagine below, we’ve incorporated a fillet to smooth out the abrupt angle in the center, and a chamber at the top. Now, the design achieves internal no-access features machining or casting would be hard pressed to imitate while reducing material consumption and eliminating internal support structures.
Step 6: Now, Post-Processing
The above design tips will influence the strength of your part, especially when it comes to malleability, by complementing the post-process stress and heat treatments DMLS parts must undergo to achieve full density. Stress relief and heat treatments aid in a DMLS part’s ability to perform excellently under testing and avoid the cracking or fracturing of the metal throughout its lifetime. Stratasys Direct Manufacturing has carefully studied DMLS heat treatment measures internally, as detailed in the four steps.
Step 7: Preventing Fractures in AM Parts & Improving Density
Fractures or cracks in a DMLS part occur due to three factors: 1.) internal stress exceeding the yield strength of the material; 2.) stress risers in the design or 3.) long-term component use. Cracks are easily removed during post-processing, but fractures can be prevented from occurring during the lifespan of a component through heat treatment processes.
Step 8: Stress Relief
All DMLS parts are subjected to stress relief prior to removal from the build platform to avoid deformation. Stress relief cycles at Stratasys Direct Manufacturing vary from alloy to alloy but are typically performed at 1950°f for 1.4 hours and air cooled at room temperature. Depending on the metal alloy and part design, stress relief may be performed at 1725 – 1850°f and then cooled. Through stress relief the metal returns to an annealed state.
Step 9: HIP, SHT and PHT
Secondary heat treatments such as HIP (hot isostatic pressing), SHT (solution heat treatment), and PHT (precipitation hardening treatments) can result in stronger parts with properties closer to wrought metals. These treatments have repeatedly proven to enhance density from ~95.5% as-built to 100% density. At Stratasys Direct Manufacturing, we subjected our Inconel 625 and Inconel 718 to heat treatment processes to measure the effectiveness of heat treatments on 3D printed metals and determine whether a 3D printed metal can achieve industry relevant mechanical properties. We built 20 tensile bars in both materials and recorded their properties. Then, we machined the bars to ASTM standards and subjected the bars to thermal conditioning. Conditioning included HIP SHT, and PHT treatments. We discovered DMLS metals can be brought to the same standards of AMS specfied aerospace alloys through HIP SHT and PHT treatments.
Step 10: Finishing
On top of these strengthening treatments, DMLS parts can be hand polished to result in ideal surface finishes and machined to meet critical engineering tolerances. The surface of a DMLS part as-built is ~350 Ra μinch. Glass bead blasting will smooth surfaces to 98-236 Ra μinch while a tumbled finish will further ameliorate surfaces to 32-124 Ra μinch. Hand polishing is more ideal for one-off unique parts that require a very specific surface quality.
Step 11: The Future of DMLS
DMLS is the ideal alternative to complex designs that machining or casting can’t achieve. It offers the mechanical properties of aerospace standard materials and the design freedom of 3D printing. As further industries adapt to the technology, we’ll see metal 3D printing overtake healthcare through medical devices and tools, aerospace through consolidation and lightweight solutions, energy through fine-tuned complex mechanisms and many other industries. It may transform applications we haven’t even dreamed up just yet.