Melt Pool to Macro-Scale: Size-Dependent LPBF 316L Properties

It has proven difficult to predict how parts made with Laser Powder Bed Fusion (LPBF) will perform under load. This is because their as-built properties and mechanical behavior depend on their processing history and their overall size. This means using standard material data for structural analysis just leads to very poor predictions.

This research describes a computational framework that creates a direct link between the LPBF process for 316L Stainless Steel, the part's size, and its effective mechanical properties.

Our framework uses a new spatially integrated line heat source model (SILHS) that's about twice as fast as traditional models. This speed allows us to run large-scale studies on how size affects the part. We also developed a new "process-informed" homogenization scheme that uniquely uses the full residual stress field as the starting point for virtual mechanical tests.

Our simulations show a distinct core-shell stress architecture: a compressively stressed bulk core (p > +100 MPa) and a tensile Boundary Effect Zone (BEZ) (p < -150 MPa).

We found that this strong size effect is driven by the volume of this tensile BEZ. In small parts, it makes up 45.5% of the volume, but it drops to just 17.7% in larger parts.

This dramatic size difference leads to equally dramatic performance. The effective Ultimate Tensile Strength jumps by over 35%, from around 550 MPa in small coupons to a stable value of 750 MPa in larger parts.

The tensile BEZ is also the most probable location for fatigue initiation. The stress field acts as a significant mean stress, and applying a compressive service load can decrease the Smith-Watson-Topper fatigue parameter by over 33%.

Classical ISMs fail to capture this core-shell architecture or its size dependence. Our validated framework now provides the size-dependent, anisotropic data needed for accurate structural analysis of AM parts.