Modern Machine Shop

AUG 2018

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Page 42 of 316

ADDITIVE INSIGHTS MMS AUGUST 2018 40 Understanding Industrial 3D Printing Machinability of Additively Manufactured Support Structures TIMOTHY W. SIMPSON | COLUMNIST What happens when you cut away thin-walled supports on your AM part? FIGURE 1: If you have been reading my column, you are aware of the need for and the challenges associ- ated with support structures in additive manu- facturing (AM) processes. Support structures are essential for minimizing distortion during the build, but they are difficult to remove because they are made of the same metal as your part. Despite the need to remove support structures after the build is complete, there are surprisingly few resources published about their machinability. Most studies have focused on the material micro- structure and mechanical properties of AM parts instead of determining the best way to machine away support structures encountered in AM. Many people think that the same strategies used to machine stainless steel, titanium and nickel-based alloys readily translate to structures made with AM, but that may not be the case. Some studies have been conducted of the machinability of solid AM parts made from Ti-6Al-4V and 17-4 stainless steel, but there are subtle differences that are important to keep in mind when dealing with support structures. If you are being asked to machine support structures, keep in mind that the material you are machining away may not be what you think it is. This depends on how the material was heat-treated (or not), as the AM part may have a different microstructure, and therefore, a different response when it is machined. Furthermore, in most cases, the support structures are not fully dense, meaning they are intentionally porous to save build time and material usage. Finally, the support structures themselves can trap loose powder depending on the geometry that is used. This adds uncertainty when machining support structures away from an AM part. To gain some insight into this, I worked with two of my machin- ing colleagues at Penn State, Professor Edward De Meter and Professor Guha Manogharan, and a team of graduate students to conduct experi- ments to study the machinability of Inconel 718 support structures. FIGURE 2: (a) Bent walls at cut ter exit (b) Powder trapped inside suppor ts (c) Chips from suppor t structures (d) Chips from fully dense metal 0.6 mm 1 mm 1.78 mm Wire cut sur face at build plate inter face 25.4 mm 25.4 mm 12.5 mm 2 mm Thin-walled suppor ts Solid material (a) Machining specimens (b) E xperimental test set up (c) Orientation of feed relative to suppor ts Y X Feed direction S Y X (feed direction) (ma ximum force on the workpiece) Z

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