Page 15 - 3D Metal Printing Spring 2016

Insights in Metal Printing 3D
placement of material was determined using topological optimiza- tion—the use of mathe- matics to optimize the strength-to-weight ratio of a design. It results in using the minimum amount of material for given functional parame- ters. Using topology opti- mization, the weight of the cylinder head dropped from 11.2 to
4.2 lb.
While the cylinder-
head design represents
an impressive engineer-
ing achievement, this
level of design complexity
does not come easily or
quickly. Part of the success came from the capabilities of the design software used—Selective Space Structures software from netfabb GmBH, used to develop the organic shape and lattice structures.
CFD, topology optimization and lattice-generation software can be expensive and difficult to use. And they require a sub- stantial investment in training, coupled with time-consuming trial and error—often beyond the reach of small companies.
Design Challenges
AM indeed offers interesting design opportunities, but it also introduces unique design challenges. The mathematical- ly optimized shapes generated by the software may not always be feasible to build. For example, it’s important to know the minimum possible wall thickness or the smallest hole that the AM process can produce. Overhanging features also must be supported by extra structures during the AM build process; the supports must later be removed, an impor- tant design consideration.
Thermal stresses also present another challenge, particular- ly during the metal powder-bed fusion process. Here, thermal stresses can lead to warping when the parts are cut away from the built plate. To reduce warping, designers often add anchors to secure the parts and their features to the build plate. Howev- er, adding too many anchors creates additional work for their removal; too few anchors will allow curling and warping. The optimum design often requires compromise between reality and the theoretically optimum shapes, although the level of compromise typically is much less than that found with con-
ventional manufacturing processes such as metal casting.
Powder removal repre- sents another critical design consideration. With the cylinder head described above, powder can fill all of the internal cavities and holes. When removing the parts from the build chamber, the technician must remove all of this excess powder. Designers, therefore, should include drain holes and exit paths that will prevent powder from being trapped inside the part. This requires careful consideration of the size
and location of the holes and paths, and then adding them to the CAD model. Often, these holes need to be plugged, adding time and cost.
Most designers and engineers have not yet received formal education and training on designing for AM. Consequently, most companies considering AM for production applications are encountering it for the first time. And while some compa- nies have developed tribal knowledge amongst a small group of pioneers within their company, typically these pioneers rep- resent a limited resource. Bottom line: The demand for educa- tion and training on designing for AM exceeds the supply.
Training the Aerospace Industry
To meet the need for education in one segment of the aero- space industry, Wohlers Associates has conducted two formal classes on designing for AM, for NASA Marshall Space Flight Center. A four-day class focused on hands-on learning with advanced methods of design, including part consolidation, topology optimization and lattice and mesh structures. The second class, three days, focused mostly on metal AM.
Additive manufacturing offers many opportunities to improve the performance and weight of a design. Using special software tools and techniques, designers can achieve major improvements impossible with conventional manufacturing methods. Since design freedom is much greater with AM than with conventional processes, we believe that AM will find increased use for developing entirely new types of parts and products, including automobile-engine parts, that will be superior to their predecessors in many ways. 3DMP
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Fig. 2—Engine cylinder head with interior lattice framework, courtesy of FIT