Mark Barfoot Mark Barfoot

AM Parts: From Inception to Inspection

October 14, 2019

Creating production parts using metal additive manufacturing (AM) requires consideration of the entire process chain during product development. This journey involves several important steps.

The first step: design phase. Typically it is not cost-effective or appropriate to take an existing CNC-machined or cast part and convert it directly to an additive part. Those parts were designed to meet specific machining or molding rules, and those same rules do not apply to AM. To get the most out of AM, the part should utilize the benefits of additive such as lightweighting and creating topology-optimized models that include material only where needed. AM also enables the integration of several traditionally manufactured part components into the build of one single part, which eliminates assembly time. In addition, features within traditional parts require support structures (i.e., round holes). The removal of these support structures can be a challenge and one of the highest cost drivers of AM parts. A change in design from the outset eliminates the need for support structures. A round hole converted to a teardrop shape requires no support.

The second step: optimize part build. To achieve this, consider adjusting build parameters to ensure optimum part properties for your specific geometry. By maximizing the number of related parts built together on a tray as opposed to building just a single type of part, costs decrease.

The third step: post-processing. Here it is important to ask questions such as, “How should you go about removing the support structures?” “How do you remove trapped powder inside small channels or crevices in your part?” and “Do you need to apply any post finishing (i.e.: sand blasting, vapor polishing) to the part to improve surface finish to meet the end use needs?” For post-processing, consider stress relieving, heat treating and/or hot isostatic pressing (HIP) for reducing residual stresses, improving strength properties and reducing internal porosity. Developing the correct processing parameters for each of these steps is crucial for ensuring that parts do not deform or have less-desirable properties. 

Unlike the traditional CNC-machined part shown on the right, these three 3D printed parts were designed to utilize the benefits of additive manufacturing, such as lightweighting and topology-optimized models that include material only where needed.

Another area often not considered: removal from the build plate. If parts can be built with minimal build-plate contact, removal could be an easy twist off of the plate, though this typically is not practical. More often, bandsaw or wire EDM is needed to remove parts from a build plate. Each method has its pros and cons.

An important part of post-processing to consider: machining to clean up surfaces or dial in tolerances. Most metal AM parts require some level of subtractive machining to ensure that the final parts meet final tolerances and part requirements. Often, grip points, data and more are ignored when developing the original part design. However, inclusion of such factors in the design ensures rigid holds in the CNC while processing the part.

The final consideration: post-build analysis. What nondestructive evaluation (NDE) technique are you going to use to evaluate and certify the part quality? One main NDE technique is x-ray computed tomography (CT), employs x-ray attenuation through a sample to reveal information regarding its inner workings. A variety of characteristics can be evaluated using this technique in an AM part. Porosity analysis can ensure detection of defects and identification of the location and size. Many times, the welds between layers can exhibit lack of fusion, hot cracking or poor penetration, which ultimately will reduce part strength. CT can identify these traits. Many AM parts built with complex geometry can trap powder internally, and CT easily detects this. Variations in part size to computer-aided design geometry and/or cracks also can be detected using CT technology. However, the CT can penetrate only to certain depths of a part. If not considered during part design, CT imaging of the part may not be possible. Other techniques using eddy current, ultrasound and coordinate measuring machines can help, but CT seems to be the largest adopted technique due to its ease of use. Determining the needed frequency of an inspection can save significant project costs. Complete inspection using CT can be costly; developing a plan to pre-qualify the process and only conduct regular process checks may be more cost-effective.

To ensure that a part meets requirements and produces the highest yield through the process, all factors need consideration early on. Too often, a company simply takes an existing part and orders a print from a service bureau that finds a way to print the part. If part use and function are not considered, you may not get the part you expect. 3DMP

Industry-Related Terms: Additive manufacturing, Build plate, Metal additive manufacturing, Porosity, Post-processing, Surface finish
View Glossary of 3D Metal Printing Terms


See also: EWI

Technologies: Management, Post Processing, Software


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