The shell was cleaned to remove ash residue from the sintered SLA material, sealed and wrapped with k-wool insulation in the locations on the draw- ing provided by HAB to ensure proper solidifica- tion. The ceramic shell was heated in an oven and the castings poured to HAB- specified shell and metal temperatures. Upon shakeout and
cleanup, the casting exhib- ited some minor misruns and nonfills along the rim of the 0.08-in. walls. These areas were weld repaired and cleaned, and the casting was sent to hot isostatic pressing (HIP) to help ensure metal quality and meet the nondestructive testing (NDT) requirements. After heat treatment, Acme Castings removed the tie bars and set the casting up for straightening. Upon placing
The rim of the casting features 0.08-in. walls.
the casting in the printed inspection fixture, the metalcaster discovered the casting had hardly moved at all and minimal straightening was required. After straightening, the casting was inspected in the fixture using go/no-go gages and then sent to final heat treatment. An outside lab performed NDT X-rays to Grade
B in critical areas and Grade C in the balance of the cast- ing and performed 100% die penetrant. Te entire casting passed except for a minor amount of ash residue that fell into a machined area. Because this was to be machined off, no repair was required. Finally, Acme performed
100% coordinate-measuring machine (CMM) dimensional inspection. While the casting did not pass every dimen- sion due to shell restriction, the profile tolerance allowed enough adjustment to make the casting usable for machin-
ing to ensure form, fit and func- tion in the gun system and possible firing test sample. Te collaboration achieved an approved prototype, on the first attempt, and delivered a finished casting in 110 days, thus demonstrating a lightweight, high performance steel casting can replace a 15-piece fabrication.
New tanks and cannons for the U.S. Army are required to be lighter and more agile. February 2014 MODERN CASTING | 47
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