Shadow and contact masks for abrasive slurry jet micro-machining of planar areas with uniform depth

Abrasive slurry jets can be used as cost-effective and versatile means for the controlled depth micro-milling of planar areas. Erosion-resistant masks can be incorporated in the process to mill complex patterns with enhanced precision. However, because of the unpredictable interaction between the jet and mask, an undesirable erosion and mask under-etch can occur in the machined pocket near the mask edges. This paper investigates how mask configuration affects this interaction, and presents a novel method using non-contact shadow masks that virtually eliminates the undesirable effects. A high-pressure slurry jet setup was used to mill square pockets in Al 6061-T6 using both contact and shadow masks made from SS304 of varying thickness and in various configurations. Experimentally validated computational fluid dynamics (CFD) coupled with Lagrangian particle tracing was used to predict the severity of the undesirable erosion as well as its underlying mechanisms in the various scenarios. For contact masks, thicker masks led to more undesirable erosion because of the greater possibility for the flow to separate and recirculate, resulting in a locally high abrasive mass flux near the mask edge. The undesirable effects occurred to a lesser degree when shadow masks were used because there was no such flow separation. Although the flow streamlines compressed near the shadow mask edge yielding a locally high erosive efficacy, this could be mitigated by using a high mask to surface standoff that allowed the jet to spread before striking the surface. With the shadow mask in this configuration, the undesirable erosion was virtually eliminated, allowing for the micro-fabrication of planar areas at a far more uniform depth and reduced under-etch than when using contact masks. • Effect of mask thickness on undesirable erosion in ASJ pocket milling investigated. • A shadow masking technique was introduced for AWJ pocket milling. • Flawless planar areas were fabricated using shadow masking. • A possible mechanism for undesirable erosion in shadow masking was identified.