CRITERIA FOR SUCCESS Prominent in the steel casting’s success were the follow-
ing manufacturing engineering elements integrated into the casting design for overall specifi cation compliance: • Conversion from carbon or low alloy steel (both of which suffer from very poor fl uid 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, tough- ness, wear resistance and fl uidity in its liquid state.
• Choice of the investment casting process for the 17-4 PH al- loy, poured into a hot mold with a metal delivery system de- signed to provide pouring pressure and mold atmosphere venting suffi cient to enable cast walls 0.08 in. (2 mm) thick.
• Integration of the metal delivery geometry with 17-4 PH solidifi cation gradients to provide properly placed sources of liquid to feed solidifi cation 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 fi rst prototype iteration.
• Designing the solidifi cation gradients in the metal de- livery system to preclude the need for HIP in eventual
the casting and performed 100% die penetrant. T e 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 restriction, the profi le tolerance allowed enough adjustment to make the casting usable for machining to ensure form, fi t and function in the gun system and pos- sible fi ring test sample. T e collabora- tion achieved an approved prototype, on the fi rst attempt, and delivered a fi nished casting in 110 days, thus demonstrating a lightweight, high performance steel casting can replace a 15-piece fabrication.
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 solidifi ca- tion and heat treatment.
• Combining the tie bar geometry with the geometric dimensioning and tolerancing (GD&T) zones required for dimensional compliance.
• Evaluation of high alloy steel contraction rates in the investment shell mold tooling design to make a best estimate for centering critical as-cast dimensions in the required GD&T zones.
• Use of rapid prototyping/rapid tooling technologies to create the investment shell mold tooling via stereolithog- raphy (SLA) to save the lead time required for a tradition- al wax die and avoid the additional dimensional centering variable of wax contraction.
• Creation of a dimensional checking/straightening fi xture directly from the as-cast solid model via selective laser sin- tering of nylon powder. The resulting nylon fi xture is tough enough to withstand processing of prototype quantities.
The tray casting guides the projectile into the cannon’s breech.
Jul/Aug 2013 | METAL CASTING DESIGN & PURCHASING | 31
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