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One significant advantage is that this process creates metal parts from the investment casting pro- cess, using the intended alloy, but without the need for tooling. Mul- tiple iterations can be evaluated in order to converge on a solution or as touch-and-feel prototypes to anticipate and relieve downstream process risk. Additional pieces can be cast to develop machining processes, establish assembly methods, per- form functional qualification testing, and deliver low rate initial produc- tion quantities. Tis SLA to casting process was pioneered by Raytheon (formerly Texas Instruments Defense Systems & Electronics Group) in the late 1980s and has remained a key enabler to rapid development of complex castings.

Alloy and Process Selection Knowing the configuration was

generally castable with the baseline features, it was time to pin down the requirements for material strength, alloy, and soundness. Analysis showed the housing was not stress critical, except for the region adjacent to the mounting lugs (Fig. 5). Strength requirements in this

“high-stress” region led to the selec- tion of alloy A357 and particularly its beryllium-free version, now designated F357. Overall strength was specified at a cost-effective level, but mechanical property requirements and soundness were upgraded in the “high-stress” region with integrally-attached tensile specimens required to validate the strength of each heat treat lot. Sound- ness, as verified by nondestructive testing, was established per AMS2175, “Classification and Inspection of Cast- ings.” Fluorescent penetrant inspection per ASTM E1417 was required for 100% of the items produced, but radio- graphic inspection was specified on a sampling basis to provide cost-effective process control of key variables in melt- ing, feeding, and solidification. Another key element of foundry

processing was proper control and performance of in-process welding to correct discontinuities detected by inspections. Specification AMS2694, “In Process Welding of Castings,” was

30 | MODERN CASTING July 2013

Another area of functional Figure 3. The mechanical concept is shown in two views.

included in the technical data pack- age to require a documented welding procedure, qualified welders, matching filler composition, and appropriate post-weld processes and inspections. A lively discussion always sur-

rounds the appropriate use of in- process welding of castings. Tests show welds in most aluminum casting alloys result in mechanical properties equal to, or better than, the adjacent parent metal. Debate generally includes categorization of the root cause for these types of casting discontinuities and aspects of metalcasting process improve- ments that can ultimately reduce (but not completely eliminate) the need for weld authorization. Designers consider the ultimate

impact of quality welds on functional performance of the product, including analysis of fatigue loads/life appli- cable to the service environment. As a rule of thumb, castings categorized as Class 1 and Class 2 per AMS2175 require a more careful analysis prior to weld authorization, sometimes limited within critical areas. Class 3 and Class 4 castings are rarely weld-critical and generally do not include restrictions other than compliance with AMS2694.

concern was integrity of the circuit card guides. Tese required a high precision of location for mating to the motherboard and also a straight and coplanar face to pro- vide adequate surface area contact for heat sinking. Te solution to the design side involved use of a sub-datum structure for dimensioning and tolerances of these card guides. Dimensional tolerances were held tight relative to the circuit card net- work but allowed to float more gener- ously with respect to the mechanical superstructure of the housing. Clever processing and tooling within the metalcasting operations ensured the required tolerances were attained on each casting. A series of cost tradeoffs were

performed to determine which features to cast net versus which features would be machined. Opti- cal alignment features, precision mounts, and gasketed cover seal sur- faces required machining, sometimes with dimensional precision on the order of +/- 0.001 in. and better. It is not enough for these features

to comply under constraint—dimen- sional requirements for precision features must be validated in the “free state,” unconstrained and after temperature cycling simulations of the in-service environment. These demand close coordination between the designer, the metalcaster and the machine shop to ensure end item requirements are achieved in the most cost-effective manner. Special processes are added to the foundry operations and also to the machine shop operations to enhance dimen- sional stability and ensure piece by piece product consistency. Fea- tures and requirements that move upstream into the net-cast configu- ration add to the casting unit price and tooling requirements, but reduce the total cost of ownership for the end item.

Ensuring Quality First article process qualification

Figure 4. The pattern was produced with stereolithography.

included dimensional compliance of the casting, validation of mechani- cal properties, fluorescent penetrant

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