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straightening, to the casting model to minimize the distortion during the quench. The next issue involved facilitating part inspection dur- ing the straightening process. The team decided to design an inspec- tion fixture using go/no-go gages on critical features. A conventional metal inspection fixture was deemed too costly with too great a leadtime for a single casting. As an alterna- tive, the check fixture was designed to use the conventional metal go/ no-go gages. HAB created the gat- ing design using CAD modeling, but did not perform verification with solidification software due to the high cost and long lead time estimates received. Instead, the com- pany relied on its extensive tooling experience and ability to produce a sound casting. After inspection, HAB shipped


the SLA patterns to Acme Castings, along with a gating design model which included fully dimensioned gating drawings and all wax gate bars cut to size and identified. The metalcaster gated the part accord- ing to HAB’s design, dipped the ceramic, and fired the shell. The shell was cleaned to remove ash resi- due from the sintered SLA material,


The tray casting guides the projectile into the cannon’s breech.


sealed, and wrapped with k-wool insulation in the locations on the drawing provided by HAB to ensure proper solidification. 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 exhibited 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 nondestruc- tive testing (NDT) requirements. After heat treatment, Acme Cast- ings removed the tie bars and set the casting up for straightening. Upon placing the casting in the printed


CRITERIA FOR SUCCESS


Prominent in the steel casting’s success were the following manufacturing engineering elements integrated into the cast- ing design for overall specification compliance: • Conversion from carbon or low alloy steel (either of which suffer from very poor fluid life and limited ability to form thin cast walls) to a martensitic high alloy steel. 17-4 PH in the H1100 aged condition was selected for strength, toughness, wear resistance, and fluidity in its liquid state.


• Choice of the investment casting process for the 17-4 PH alloy, poured into a hot mold with a metal delivery system designed to provide pouring pressure and mold atmosphere venting sufficient to enable cast walls 0.08 in. (2 mm) thick


• Integration of the metal delivery geometry with 17-4 PH solidification gradients to provide properly placed sources of liquid to feed solidification shrinkage at the required Grade B and C integrity levels


• Provision for hot isostatic pressing (HIP) of the rough casting to assure the Grade B and C integrity levels were achieved in the first prototype iteration.


• Designing the solidification gradients in the metal delivery sys- tem to preclude the need for HIP in eventual production castings


• Further integration of the metal delivery system with“tie- bars,” which are braces designed to resist warping and twisting of a thin, rangy casting shape during solidification and heat treatment.


• Combining the tie bar geometry with the geometric dimen- sioning and tolerancing (GD&T) zones required for dimen- sional compliance


• Evaluation of high alloy steel contraction rates in the investment shell mold tooling design to make a “best estimate” for center- ing critical as-cast dimensions in the required GD&T zones.


• Use of rapid prototyping/rapid tooling technologies to cre- ate the investment shell mold tooling via stereolithography (SLA) to save the leadtime required for a traditional wax die and avoid the additional dimensional centering vari- able of wax contraction


• Creation of a dimensional checking/straightening fixture directly from the as-cast solid model via selective laser sintering of nylon powder. The resulting nylon fixture is tough enough to withstand processing of prototype quantities.


July 2013 MODERN CASTING | 25


inspection fixture, the metalcaster discovered the casting had hardly moved at all and minimal straight- ening was required. After straight- ening, 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 casting and performed 100% die penetrant. The 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 dimension due to shell restric- tion, the profile tolerance allowed enough adjustment to make the casting usable for machining to ensure form, fit, and function in the gun system and possible firing test sample. The collaboration achieved an approved prototype, on the first attempt, delivered as a finished casting in 110, thus demonstrating a lightweight, high performance steel casting can replace a 15-piece fabrication.


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