the intended alloy, but without the need for tooling. Multiple itera- tions 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 pro- cesses, establish assembly methods, perform functional qualification testing, and deliver low rate initial production quantities. Tis SLA to casting process was pioneered by Raytheon (formerly Texas Instru- ments Defense Systems & Electronics Group) in the late 1980s and has re- mained a key enabler to rapid develop- ment 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 selection of alloy A357 and particularly its beryllium-free version, now designated F357. Overall strength was specified at a cost-effective level, but mechanical property require- ments and soundness were upgraded in the high stress region with integrally- attached tensile specimens required to validate the strength of each heat treat lot. Soundness, as verified by nonde- structive testing, was established per AMS2175, “Classification and Inspec- tion of Castings.” Fluorescent penetrant inspection per ASTM E1417 was required for 100% of the items produced, but radiographic inspection was specified on a sampling basis to provide cost- effective process control of key variables in melting, 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 included in the technical data pack- age to require a documented welding procedure, qualified welders, matching filler composition, and appropriate
Jul/Aug 2013 | METAL CASTING DESIGN & PURCHASING | 23
Designers consider the ultimate Figure 3. The mechanical concept is shown in two views.
impact of quality welds on func- tional performance of the prod- uct, including analysis of fatigue loads/life applicable 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
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.
Class 4 castings are rarely weld-critical and generally do not include restric- tions other than compliance with AMS2694. Another area of functional concern was integrity of the circuit card guides. Tese required a high precision of lo- cation for mating to the motherboard and also a straight and coplanar face to provide 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. Di- mensional tolerances were held tight
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