Challenges in Failure Analysis of AM Inserts for Die Casting Applications

Aliya, Debbie

The North American Die Casting Association (NADCA) funded a two year study in Failure Analysis of Additively Manufactured (Laser Powder Bed Fusion) inserts for die casting applications. Forged steel is less prone to cracking than as printed AM steel components, but AM allows freedom to create curved passages for water cooling, which allows much faster solidification of the product, and concurrent higher production rates.

Generally, the inserts are used in the as-printed, or printed and heat treated condition, rather than the Hot Isostatic Pressed (HIP) condition that finds use in the aerospace industry, among others. The As-Printed components generally have many more discontinuities than those that are fully densified after printing. The damaging discontinuities that were characterized during the NADCA study included bumpy sheet like voids, parallel to the build direction, resulting from incomplete bonding between columns of material, free surfaces perpendicular to the build direction, often rounded inclusions of composition much richer in some alloying elements than the nominal composition, and “downskins” (surfaces facing downward during printing) with extremely rough surfaces exhibiting colonies of “clingons,” or stalagtite or icicle shaped protrusions, inviting initiation sites for stress corrosion cracking or fatigue cracks.

Major challenges included “calibration” of the “eye/mind system” of the fractographer, necessary to understand meso-fractographic features on the crack surfaces, and the extremely complex geometries created by the asymmetric shapes of the inserts, necessary to understand the stress state and related likely crack initiation positions. The dynamic nature of the stress state, associated with the rapid thermal cycling due to contact with molten aluminum on the outer surfaces and the water cooling on the inner surfaces, revealed additional challenges related to the skill sets of those who prepare dynamic FEA models of the die casting tools.