CASTING ANSWERS & ADVICE
AFS Technical Department, Schaumburg, Illinois
Q A
Is ASTM Level III a good generic requirement for the porosity sever- ity level specifi ed
on a casting drawing?
It is impossible to deter- mine a generic require- ment for all castings. Because castings are produced for a wide va-
riety of applications (from household items with little requirements beyond visual cosmetics to safety critical au- tomotive and airplane components), the porosity level specifi ed by a cast- ing user will depend on the end-use requirements of the component. To determine the integrity level required, casting design engineers defi ne the stress level and service loads antici- pated for the fi nal cast component, as well as the criticality of any potential non-performance. Additionally, it is diffi cult to specify
a single radiographic level for an entire cast part because it is likely that neither the casting mechanical properties nor the in-service stresses will be uniform throughout the com-
Sample Guidelines for X-ray Porosity Standards Aluminum Castings
ponent. In practice, this means that different radiographic levels may need to be applied to different areas of the casting. When applying X-ray standards, designers also can specify the technique used (the specifi c tech- nical settings for the radiographic or fl uoroscopic image) to ensure both coverage and image quality. When creating a casting specifi ca-
tion, casting users and designers tend to over-specify requirements. Some designers may be unfamiliar with the casting process and materials and have uncertainty about quality and consistency, or they may have a desire to “pad” their engineering safety margin. Overly severe specifi cations that mandate quality levels beyond the end-use requirements of the com- ponent can add cost to the fi nal part. While placing a requirement of grade 1, grade 2 or better on the casting drawing might help assure a designer receives no parts from the metalcaster that would have perfor- mance issues, this also could result in a casting specifi cation requiring inspection and validation that add cost but provide no real benefi t. The lowest cost specifi cation is one that
precludes all things that will harm the necessary component performance but permit everything else. A good dialogue between the cast-
ing designer, specifi er and metalcaster will produce the best combination of quality, safety and cost. Casting pro- ducers should negotiate with users to better understand the functional re- quirements and assist in the translation of those needs into appropriate and effective specifi cations. The dialogue is vital in the development of effective gating and risering, which can deter- mine local mechanical properties, and production process. ASTM x-ray levels or void sizes
Companies that produce aluminum engineered prod- ucts and structural components will specify anything from a frame 1 to a frame 4, depending on actual design requirements. Aluminum castings will not gen- erally meet ASTM mechanical property minimums with porosity severity levels greater than a frame 3. A good rule of thumb is for a designer to specify frame 3 if the desire is to meet ASTM property limits for design. This provides a good compromise on cost and performance.
Ductile Iron Castings
Safety critical automotive ductile iron automotive parts (steering knuckles, control arms, brake calipers, etc.) are typically specifi ed as Level II or better, unless oth- erwise designated by the customer. This includes any casting that falls under the National Highway Traffi c Safety Administration defi nition of a component whose failure could result in the loss of vehicle control and potentially life or limb.
should not be accepted arbitrarily as the sole uniform quality criteria. The key issues relating to casting quality are the functional durability and performance reliability of the cast component. For example, cast aluminum automotive wheels (where ASTM E155 is applied) bring with them industry standard durability and performance tests whose results can be correlated for a particular design with an ASTM level. Many manufacturers leave the ra- diographic limit open until the testing process demonstrates what can be accepted safely within the specifi ed performance parameters. This pro- vides for a functional and pragmatic approach to radiographic standard setting. During testing, sample parts can be used to determine the maxi- mum fl aw size (amount of mass loss, location, etc.) that can diminish per- formance and establish a correlation back to x-ray level. Radiography may not always be
the complete solution for defect identifi cation. Ultrasonic inspection has benefi ts, and some types of detri- mental indications may be discovered through surface analysis, such as dye penetrant inspection. As stresses tend to be conveyed over surfaces, discontinuities in a surface may be more important quality-infl uencing factors than internal voids.
MC 42 MODERN CASTING / October 2010
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